Download Hi-Res ImageDownload to MS-PowerPointCite This:J. Phys. Chem. C 2022, 126, 7, 3443-3456

C: Chemical and Catalytic Reactivity at InterfacesFebruary 9, 2022

Oxygen Exchange on Vanadium Pentoxide
Click to copy article linkArticle link copied!

Yuanqing Wang
Yuanqing Wang
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, D-14195 Berlin, Germany
BasCat, UniCat BASF Jointlab, Technische Universität Berlin, D-10623 Berlin, Germany
More by Yuanqing Wang
https://orcid.org/0000-0002-1332-7956
Frank Rosowski
Frank Rosowski
BasCat, UniCat BASF Jointlab, Technische Universität Berlin, D-10623 Berlin, Germany
Heterogeneous Catalysis, BASF SE, Process Research and Chemical Engineering, Carl-Bosch-Straße 38, D-67065 Ludwigshafen, Germany
More by Frank Rosowski
Robert Schlögl
Robert Schlögl
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, D-14195 Berlin, Germany
Max-Planck-Institut für Chemische Energiekonversion, Stiftstrase 34−36, D-45470 Mülheim, Germany
More by Robert Schlögl
Annette Trunschke*
Annette Trunschke
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, D-14195 Berlin, Germany
*Email: [email protected]
More by Annette Trunschke
https://orcid.org/0000-0003-2869-0181

Open PDF Supporting Information (1)

The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2022, 126, 7, 3443–3456

Click to copy citationCitation copied!

https://doi.org/10.1021/acs.jpcc.2c00174

Published February 9, 2022

CC-BY 4.0 .

Abstract

Click to copy section linkSection link copied!

The isotopic exchange of ¹⁸O₂ on polycrystalline V₂¹⁶O₅ was studied by Raman spectroscopy at different temperatures between 300 and 580 °C and in the presence of different mixtures of oxygen with ethane, propane, or n-butane in the gas phase. Supported by DFT calculations, a method was developed to determine which of the three differently coordinated oxygen atoms in the crystal structure of V₂O₅ (vanadyl oxygen O1, 2-fold-coordinated oxygen O2, and three-coordinated oxygen O3) are involved in the exchange with ¹⁸O₂ from the gas phase. Thus, it was found that the band at 994 cm^–1, which is commonly exclusively assigned to a V═¹⁶O1 stretching (A_g) vibration, also contains contributions of an ¹⁶O1–V–¹⁶O2 stretching vibration (B_2g). If only the O1 position is exchanged, the B_2g component shifts to 964.2 cm^–1, while if both O1 and O2 are exchanged, a shift to 953.4 cm^–1 is expected. In contrast, the A_g component shifts only to 955 cm^–1, regardless of whether only the O1 position or all three oxygen atoms are exchanged. On this basis, it was found that oxygen exchange at 573 °C in absence of an alkane involves O1 and O3 atoms, whereas in the presence of propane all three oxygen atoms are exchanged. In the latter case, the overall exchange rate appears to be limited by bulk diffusion. At typical reaction temperatures for the oxidative dehydrogenation of propane between 320 and 430 °C, no exchange occurs in pure oxygen. In presence of ethane or propane, only O1 is partly exchanged possibly at the surface and/or in a near-surface region. Under the typical reaction conditions of oxidative dehydrogenation of propane at 400 °C, there is hardly any variation in the spectra, and the small changes observed after long times on stream only affect O1, which, considering the sensitivity of the measurement method, leaves open whether the Mars–van Krevelen mechanism is indeed the predominant reaction mechanism under the conditions of oxidative dehydrogenation of alkanes on V₂O₅.

This publication is licensed under

CC-BY 4.0 .

License Summary*
You are free to share(copy and redistribute) this article in any medium or format and to adapt(remix, transform, and build upon) the material for any purpose, even commercially within the parameters below:
- Creative Commons (CC): This is a Creative Commons license.
- Attribution (BY): Credit must be given to the creator.
View full license
*Disclaimer
This summary highlights only some of the key features and terms of the actual license. It is not a license and has no legal value. Carefully review the actual license before using these materials.
License Summary*
You are free to share(copy and redistribute) this article in any medium or format and to adapt(remix, transform, and build upon) the material for any purpose, even commercially within the parameters below:
- Creative Commons (CC): This is a Creative Commons license.
- Attribution (BY): Credit must be given to the creator.
View full license
*Disclaimer
This summary highlights only some of the key features and terms of the actual license. It is not a license and has no legal value. Carefully review the actual license before using these materials.

Subjects

1. Introduction

Click to copy section linkSection link copied!

Activation of molecular oxygen on the surface of metal oxides is of fundamental importance in heterogeneous catalysis, (1−3) solid oxide fuel cells, (4) and gas sensor technology. (5) Direct oxidation of inert alkane molecules with gas-phase oxygen over metal oxide catalysts not only is of economic interest but poses a challenge in scientific understanding due to the complexity of the catalysts as well as the reaction networks. (6) The selective oxidation of alkanes requires both the activation of C–H bonds in the alkane molecule and the activation of molecular oxygen. Reactive oxygen species formed in the process of charge transfer between the hydrocarbon and the oxygen molecule include electrophilic or nucleophilic intermediates, respectively. The process of O₂ dissociation and final incorporation of oxygen into the oxide lattice involves the polarized adsorbed O₂ molecule, differently adsorbed charged molecular species (superoxide O₂^– and peroxide O₂^2–), and charged atomic species (O^–) with electrophilic character. The reaction ultimately leads to the incorporation of nucleophilic O^2– ions into the lattice (eq 1), (7)

(1)

where the symbols * stand for a free adsorption site, g for gas phase, s for surface, and l for lattice. It was discussed that surface and volume defects are important for both adsorption and dissociation of oxygen. (8−13)

Control over the occurrence and distribution of the different oxygen species under the reaction conditions is key to achieving high selectivity for certain reaction products and avoiding complete combustion to unwanted CO₂ during alkane oxidation. Different types of oxygen species are required depending on whether an olefin is to be formed by oxidative hydrogen abstraction or whether oxygenates are the desired products. The synthesis of oxygenates containing C–C double bonds requires the simultaneous presence of nucleophilic and electrophilic oxygen species, which can coexist at different locations on the catalyst surface. Ultimately, it is a matter of avoiding total oxidation to carbon dioxide, which is also the product of oxygen incorporation and C–C bond cleavage.

Experimental discrimination of oxygen species actually involved in hydrocarbon oxidation is complicated due to the elevated reaction temperatures that lead to rapid interconversion of the intermediates, which appear in eq 1. (1) Furthermore, the low concentration of active sites, (14,15) the limited sensitivity of suitable spectroscopic techniques such as vibrational (16) and EPR spectroscopies, or the superposition of a multitude of oxygen species in photoelectron spectra of working catalysts (17−19) pose additional challenges. (20,21)

Isotope exchange experiments were used to investigate oxygen exchange between the gas phase and oxides under various conditions. (22,23) Three different types of oxygen exchange reactions are distinguished in the postulated mechanisms. The R⁰ mechanism does not involve lattice oxygen (M–O_L), but gas-phase ¹⁶O₂ exchanges one oxygen atom with gas-phase ¹⁸O₂ under formation of mixed ¹⁶O¹⁸O, while the atomic fraction of the two oxygen isotopes in the gas phase remains constant. In the R¹ and R² exchange mechanisms, one or two ¹⁶O lattice oxygen atoms, respectively, participate under formation of ¹⁶O¹⁸O and ¹⁶O₂, respectively, in the presence of ¹⁸O₂ in the gas phase. Winter, (24,25) Nováková et al., (22,26) and Boreskov (27) derived expressions to distinguish the exchange type based on the exchange rate. V₂O₅ was found to follow a combination of R¹ (in the lower temperature range of 480–540 °C) and R² (in the higher temperature range of 520–600 °C) mechanism. (25) In other works, only the R² mechanism was found above 450 °C. (28,29) Over supported vanadium oxides catalysts the R² activity decreases in the order V₂O₅/TiO₂ > V₂O₅/Al₂O₃ ∼ V₂O₅/SiO₂, which corresponds to the order in which the activity in butane oxidation decreases. (28,30) However, the R² activity cannot be solely attributed to surface vanadia species, because ¹⁷O NMR provided evidence for participation of the oxygen from the support in the exchange mechanism, (31) suggesting that oxygen exchange rate and reactivity are not necessarily linked. Based on density functional theory (DFT) calculations, the involvement of a surface ozonide complex O₃^– in the R¹ mechanism, and a 4-atom complex with the O₂^– and O₂^2– surface species in the R² mechanism over VO_x/TiO₂, respectively, were proposed. (32,33) Some authors indicated that the presence of a reducing gas or the degree of reduction of the oxide has an impact on the oxygen exchange. (29,34)

Kinetic measurements of oxygen exchange, however, do not disclose which lattice oxygen is involved in the exchange. Three different types of oxygen atoms can be distinguished in bulk V₂O₅: (1) vanadyl oxygen O1; (2) 2-fold-coordinated oxygen O2; (3) three-coordinated oxygen O3 (Figure 1). The combination of oxygen exchange with vibrational spectroscopy provides the opportunity to distinguish the type of oxygen that participates in the exchange mechanism. In addition, another disadvantage of the purely kinetic studies of oxygen exchange is eliminated, since the vibrational spectroscopic experiments can usually be carried out in situ and not in vacuum and in the absence of alkanes.

Figure 1. Unit cell of V₂O₅: O1, terminal oxygen; O2, bridging oxygen; O3, chain oxygen.

In early studies, Hirota and Kera et al. investigated the oxygen exchange between gaseous C¹⁸O₂ or ¹⁸O₂ over V₂O₅ by infrared spectroscopy. (35−37) A new band appearing at 962 cm^–1 was ascribed to the V═¹⁸O1 stretching in contrast to V═¹⁶O1 at 1019 cm^–1, however, the low resolution of the infrared spectra complicated the identification of other vibrational modes, which might be simultaneously present.

Raman spectroscopy is a versatile tool in analyzing the interaction of O₂ with metal oxides, because the stretching frequency of molecular oxygen species, which can be measured directly, decreases with decreasing bond order caused by increasing charge transfer from 1556 cm^–1 for the free oxygen molecule (38) to 1200–1000 cm^–1 for superoxide and 1000–800 cm^–1 for peroxide species adsorbed on metal oxide surfaces. (39−41) Furthermore, Raman spectroscopy in combination with theory and isotope exchange experiments has been used to analyze the molecular structure of supported vanadium oxide species. (42−47)

Oyama et al. investigated the oxygen exchange over silica-supported vanadium oxide catalysts with different vanadia loadings that contained mixtures of surface vanadia species characterized by a vanadyl stretching vibration at 1042 cm^–1 and segregated crystalline V₂O₅ particles exhibiting the typical V═¹⁶O1 mode of bulk V₂O₅ at 997 cm^–1 by Raman spectroscopy. (48) The silica-supported vanadyl groups were exchanged at 347 °C, whereas the vanadyl groups in the V₂O₅ crystallites were not affected. The analysis was not trivial because the ¹⁸O substitution of the silica-supported vanadyl species shifted the original V═¹⁶O band at 1042 cm^–1 to 997 cm^–1, the same position as that for the bulk V═¹⁶O1 vibrations in the segregated crystalline V₂O₅ particles.

The shift was confirmed in studies of highly dispersed vanadium oxide species supported on zirconia and silica, respectively, investigating catalysts that do not exhibit spectroscopic features of bulk V₂O₅ particles. (42,43) It is, however, problematic to assign the bands of silica-supported vanadium oxide in the range of 1000–1030 cm^–1 exclusively to vanadium oxide stretching vibrations because of strong coupling of the V–O modes with the modes of Si–O–V interphase bonds, (45) which also complicates the assignment of the bands due to exchange with ¹⁸O₂. (44)

Ono et al. investigated oxygen exchange in the presence of n-butane and propane at 430–520 °C on ill-defined V₂O₅/SiO₂ catalysts that exhibited a Raman spectrum, which was entirely dominated by segregated V₂O₅ crystallites. (49,50) The bands of V₂O₅ at 998 and 703 cm^–1 were shifted to 964 and 685 cm^–1, respectively, upon dosing of ¹⁸O₂. Oxygen atoms at the V–O3 site (703 cm^–1) were found to exchange easier than that at the V═O1 site (998 cm^–1) in the presence of n-butane, while in the presence of propane, exchange at the V═O1 site was easier. The difference was ascribed to differences in crystal orientation, lacking any convincing evidence due to the ill-defined, polycrystalline structure of the catalysts.

Taking all this together, it can be said that Raman spectroscopic studies of oxygen exchange on vanadium pentoxide have so far only been carried out on heterogeneous systems. A clear assignment of the Raman bands was therefore not possible. Thus, the changes in the spectra caused by the oxygen exchange under different conditions were difficult to interpret. This motivated us to investigate crystalline V₂O₅ again in depth with Raman spectroscopy. DFT calculations were used to support the assignment of the Raman modes. The spectral changes resulting from the partial exchange of ¹⁶O atoms on the surface and in the bulk of V₂O₅ with ¹⁸O₂ from the gas phase at different temperatures and in the presence of different gas atmospheres could thus be clearly elucidated. These studies are model investigations for processes on vanadium-containing mixed metal oxides, which are known for their high performance in alkane and electrocatalytic water oxidation. (51−53) The work contributes to a better understanding of oxygen activation and exchange on vanadium pentoxide, which is important for targeted design with respect to surface or bulk properties of oxidation catalysts. In addition, the study provides reference experiments for operando investigations of vanadium oxide-based catalysts.

2. Methods

Click to copy section linkSection link copied!

2.1. Properties of V₂O₅

According to X-ray diffraction (XRD) (Figure S1 in the Supporting Information and Table S1) and X-ray fluorescence spectroscopy (XRF) analysis, the studied V₂O₅ (internal ID: 22055) was phase pure (orthorhombic phase, space group Pmmn (No. 59), point group D_2h, ICSD 60767; for lattice parameters, see Table S1), contained no impurities, and it had a specific surface area of 4.2 m²/g. V₂O₅ was thermally stable under Ar up to 669 °C followed by melting as indicated by an endothermic peak (Figure S2). Melting was accompanied by mass loss due to the evolution of oxygen and sublimation. STEM images (Figure S3) revealed a rod-like shape of the V₂O₅ particles with a size ranging from several hundred nanometers to 3 μm.

2.2. Basic Characterization of V₂O₅

The XRD measurements were performed in Bragg–Brentano geometry on a Bruker AXS D8 Advance II θ/θ diffractometer, using Ni-filtered Cu Kα₁₊₂ radiation and a position sensitive energy dispersive LynxEye silicon strip detector. X-ray fluorescence spectroscopy (XRF) was used for elemental analysis of V₂O₅, applying a Bruker S4 Pioneer X-ray spectrometer. For sample preparation, the mixture of V₂O₅ (0.1 g) and lithium tetraborate (8.9 g, >99.995%, Aldrich) was fused into a disk using an automated fusion machine (Vulcan 2MA, Fluxana). Nitrogen adsorption was performed at −196 °C using the Autosorb-6B analyzer (Quantachrome) after outgassing the material in vacuum for 2 h at 200 °C with an external preparation unit (Autosorb Degasser). All data treatments were performed using the Quantachrome Autosorb software package ASWin. The specific surface area S_BET was calculated according to the multipoint BET method in the p/p_o = 0.05–0.15 pressure range assuming an N₂ cross sectional area of 16.2 Å². Thermogravimetry (TG) and differential thermal analysis (DTA) were carried out on a Netzsch STA449 Jupiter thermanalyzer in 70 N mL/min Ar total flow applying a heating rate of 10 °C/min. Around 80 mg of the sample was placed into the corundum crucible (0.2 mL) without lid. The gas phase analysis during heating was monitored with a quadrupole mass spectrometer (Pfeiffer, QMS200 Thermostar). Evolution of O₂ was recorded by monitoring the change of intensity of the ion currents (m/z = 32). The high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were taken on an aberration-corrected JEOL JEM-ARM200F transmission electron microscope operated at 200 kV. The UV/vis spectrum of V₂O₅ was measured using a Cary 5000 UV–vis spectrometer (Agilent) equipped with a Praying Mantis diffuse reflection accessory (Harrick). The data were obtained in percent reflectance and then converted to the Kubelka–Munk function F(R). Spectralon was used as a white standard in the range between 800 and 250 nm.

2.3. Raman Spectroscopy

In a typical Raman measurement, polycrystalline V₂O₅ was either compressed on a glass slide for ex situ measurements or placed in the sample holder of a Linkam CCR1000 reaction cell (Linkam Scientific Instruments LTD), or a high temperature reaction chamber from Harrick Scientific Products Inc., respectively, for in situ measurements. Except for the experiments presented in Figure 7 using the Harrick Raman chamber, all the in situ measurements were conducted in the Linkam cell. The spectra were measured using a customized multiwavelength Raman spectroscopy system (Figure S4) assembled by S&I Spectroscopy & Imaging GmbH (Warstein, Germany). Nine different lasers (785, 633, 532, 488, 457, 442, 355, 325, and 266 nm) were used as excitation sources. Laser powers were tuned by neutral density filters (785 nm, 0.3 mW; 633 nm, 0.2 mW; 532 and 488 nm, 0.5 mW; 266 nm, 0.3 mW; all other lasers, 0.4 mW) to prohibit sample damage. Exposure times (a few seconds to 600 s) were optimized accordingly with regard to a high signal-to-noise ratio. We were aware that V₂O₅ is sensitive to UV lasers (325 and 266 nm), and long exposure times with relative high laser power could lead to progressive appearance of Raman features around 758, 896, 910, and 1009 cm^–1 (Figure S5). The laser-induced damage is relevant to structural change due to the removal of vanadyl oxygen as evidenced by the disappearance of the Raman band at 994 cm^–1. The entrance slit was set to 100 μm for all measurements. The scattered light was dispersed on 300, 600, or 2400 grooves/mm gratings depending on laser wavelength and resolution requirements, and finally collected by one of the two liquid-nitrogen-cooled CCD detectors (PyLoN:2K and PyLoN:100 from Princeton Instruments). The primary beam was eliminated by using corresponding edge filters. To compare Raman spectra measured by using different excitation lasers, the true Raman scattering intensities were extracted by correcting instrument effects and optical absorption properties of V₂O₅. A standard lamp (deuterium and halogen light source, Ocean Optics DH-2000) with a known emission spectrum was used to correct instrument effects (objectives, filters, gratings, detectors, etc.). In addition, the absorption of light by V₂O₅ was taken into account by the G(R) function that can be calculated from the measured diffuse reflectance spectrum of V₂O₅. (54) A detailed description of the Raman spectroscopy setup and the intensity correction is given in the Supporting Information (Figures S4 and S6–S8).

2.4. Oxygen Isotope Exchange

A 20–50 mg V₂O₅ sample was placed in the Linkam CCR1000 reaction cell unless otherwise mentioned and pretreated by heating the sample at a heating rate of 5 °C/min to the desired temperature under a flow of 20% ¹⁶O₂ (99.999%) balanced by helium (or nitrogen) with a total flow rate of 10 mL/min. Then ¹⁶O₂ was switched to ¹⁸O₂ (97 at% enrichment from the supplier, Linde) to conduct oxygen exchange for 2 h. The fractions of ¹⁶O¹⁶O and ¹⁶O¹⁸O, respectively, that were present as impurities in the ¹⁸O₂ gas cylinder were determined by mass spectrometry and were 1.2% and 2.3%, respectively (Figure S9). Other gases used were propane (Westfalen AG, 99.95%), deuterium labeled propane (Sigma-Aldrich, 99 atom % D), ethane (Westfalen AG, 99.95%), and n-butane (Westfalen AG, 99.95%). Raman spectra were measured continuously throughout the exchange process with each spectrum taking 6 or 8 min (532 nm). There is a second thermocouple inserted into the catalyst bed in the Linkam CCR1000 reaction cell to measure the actual temperature. For the Harrick Raman chamber, the heating element and the thermocouple are separated from the reaction part, and thus a calibrated temperature was given.

2.5. Operando Raman Spectroscopy

A 20 mg V₂O₅ sample was placed in the Linkam CCR1000 reaction cell to perform the operando Raman experiments. V₂O₅ was first heated from room temperature to 420 °C at a heating rate of 5 °C/min under 20% ¹⁶O₂ balanced by helium with a total flow rate of 10 mL/min. After reaching 420 °C, the sample was further heated under various reaction feeds (C₃H₈/¹⁶O₂/He). The effluent gas was connected to a micro-GC (Agilent 490) to perform online gas product analysis. A combination of MS5A (10 m length) and PPQ columns (10 m length), connected to a thermal conductivity detector (TCD), was used to analyze the oxidation products in the gas phase. The conversion of propane X and the product selectivity S were calculated based on the sum of the detected products. Raman measurements were performed simultaneously by using the 532 nm laser during propane oxidation over V₂O₅.

2.6. DFT Calculations

Phonon calculations were performed using Quantum Espresso 5.4.0 (55) to obtain vibrational frequencies. The crystal structure of V₂O₅ (Figure 1, 9012221.cif (56)) was obtained from the Crystallography Open Database (COD, Web site: http://www.crystallography.net/cod/index.php), which was used as the initial input structure. The structure was optimized by variable cell relaxation calculations (Tables S1 and S2). The calculations were carried out by using the Perdew–Burke–Ernzerhof (PBE) functional and ultrasoft pseudopotentials. A van der Waals-inclusive correction (DFT-D) (57,58) was applied to take dispersion forces between layers into account. The plane wave kinetic energy cut off values of 60 and 480 Ry were adopted for V₂O₅ for the wave functions and the charge densities, respectively. A Methfessel–Paxton smearing of 0.01 Ry was used to improve calculation performance. Here, 6 × 6 × 6 Monkhorst–Pack grids of k-point sampling was chosen for V₂O₅. (59)

3. Results and Discussion

Click to copy section linkSection link copied!

3.1. Experimental and Calculated Raman Spectra of V₂O₅

Generally, the Raman spectrum of V₂O₅ can be divided into (i) the high energy range from 1000 to 500 cm^–1 where V–O stretching vibrations arise, (ii) the medium energy range from 500 to 300 cm^–1 where V–O–V bending vibrations arise, and (iii) the low energy range ν ∼ <300 cm^─1 where collective vibrations of (V₂O₅)_n units arise. Based on the symmetry of the crystal structure (space group Pmmn (No. 59), point group D_2h), analysis using the Bilbao crystallographic server (Web site: http://www.cryst.ehu.es/) results in 39 optical modes

in which 21 modes are Raman active

According to the light absorption properties of V₂O₅ in the UV/vis range (Figure 2a), the experimental Raman spectra of V₂O₅ recorded using nine different lasers can be grouped into off-resonance Raman spectra (785 and 633 nm), one preresonance Raman spectrum (532 nm), and resonance Raman spectra (488 to 266 nm). As shown in Figure 2b, 10 distinct bands were resolved experimentally. Note that the band at 102 cm^–1 is not observed with the UV lasers (266, 325, and 355 nm) due to cutting off by the edge filter. The excitation wavelength has no significant effect on the band position. The peak maximum at 1001.4 ± 0.54 cm^–1 of the standard polystyrene was deviating by less than 3 cm^–1 with all lasers (Figure S7). Raman shifts indicated in Figure 2b were determined based on the spectrum of V₂O₅ measured with the 532 nm laser. In addition, six weak bands were observed at 1025, 962, 800, 660, 356, and 232 cm^–1 with the 785 nm laser (Figure S10, parts b and c).

Figure 2. UV/vis spectrum (a) and Raman spectra (b) of V₂O₅ collected at room temperature (*ex situ*). The laser wavelength used for the Raman measurements is shown next to the Raman spectra. The Raman spectra were corrected with respect to instrumental effects, taking into account the known response curve of a white lamp and the absorption spectrum of V₂O₅ (see Figure S8 for details of the correction procedure and Figure S10 for uncorrected spectra); All spectra were normalized to the corresponding maximum band intensity ([0,1]), which differs for the different excitation energies; The band positions indicated were determined for the spectrum measured with the excitation wavelength 532 nm; The intensity ratio of the band at 994 cm^–1 to the band at 144 cm^–1 as a function of excitation wavelength is plotted in part a.

With the increase of photon energy (decrease of excitation wavelength), the intensity ratio of the band at 994 cm^–1 to the band at 144 cm^–1 increases and exhibits a maximum at 355 nm (Figure 2a) suggesting resonance enhancement of the band at 994 cm^–1 by using excitation energies in the UV range. (60) However, the probing depths changes with the excitation wavelength as well. Therefore, the change in the intensity ratio may also reflect structural gradients within the depth profile. UV lasers are more surface sensitive. (61) Consequently, the relative low intensities of bands at lower wavenumbers in the spectra recorded with the UV lasers, which are due to long-range order, can be interpreted in terms of restructuring and formation of a near surface region, which exhibits less long-range order.

Various experimental and theoretical studies contributed to an assignment of the peaks observed in the Raman spectrum of V₂O₅. (62−66) However, some inconsistencies occur in the literature. Therefore, phonon calculations were performed to refine the assignment. The calculated lattice parameters of the optimized structure agree very well with the lattice parameters of the investigated V₂O₅ material determined by XRD (Table S1), indicating that the model represents reality quite well. The atomic coordinates of the optimized calculated V₂O₅ structure are given in Table S2. Experimentally determined peaks measured by using the 532 nm laser are compared with calculation results of the present and a previous study (66) in Table 1 and Figure S11. In Figure S11, the atomic displacement patterns of all calculated Raman active phonon modes are shown. The peak positions calculated in the present work agree well with the experiment. The largest deviation is 14 cm^–1. However, not all calculated modes are resolved experimentally (resolution ∼0.5 to ∼7 cm^–1, please see Figure S6). Each of the six bands found at 197, 283, 303, 481, 700, and 994 cm^–1 is caused by two different calculated modes (Table 1). The most pronounced band at 144 cm^–1 is composed of three different modes calculated at 133.7, 142.5, and 144.4 cm^–1. Two modes calculated at 214.2 and 944.7 cm^–1 were not observed experimentally, most likely due to low Raman intensity.

Table 1. Experimentally Determined (Excitation Wavelength 532 nm) Peaks, Calculated Raman Active Phonon Modes (Unit: cm^–1), and Vibrational Assignment

mode	exptl^a	calcd^b V₂¹⁶O₅¹⁶O1–¹⁶O2–¹⁶O3	calcd^c	calcd^b V₂^16/18O₅¹⁸O1-¹⁶O2-¹⁶O3	calcd^b V₂¹⁸O₅¹⁸O1-¹⁸O2-¹⁸O3	assignment
A_g	102	96.6	107.7	94.7	93.0	T_z translation
	197	192.3	192.8	184.7	181.2	skeleton bending
	303	303.8	294.1	297.4	286.5	R_x libration
	404	400.8	381.7	396.9	383.6	skeleton bending
	481	467.3	527.8	466.3	447.2	O2–V–O3 bending
	527	526.1	542.5	525.4	512.0	O2–V–O3 bending
	994	998.1	1028.9	955.0	955.0	V═O1 stretching
B_1g	144	142.5	167.3	138.6	138.4	R_z libration
	283	280.6	276.7	270.7	270.5	O1–V–O3 bending
	700	691.9	772.3	691.0	653.0	V–O3 stretching
B_2g	144	133.7	146.0	131.4	129.4	R_y libration
	197	191.8	199.8	184.9	183.1	T_x translation, R_y libration
	303	307.1	298.8	299.5	291.3	R_y libration
	356	350.7	361.3	346.2	340.1	skeleton bending
	481	482.4	528.7	480.7	454.4	displacement of O3
	n.d.^d	944.7	1010.3	935.6	900.7	V–O2 stretching
	994	997.6	1030.1	964.2	953.4	O1–V–O2 stretching
B_3g	144	144.4	168.7	140.5	140.2	T_y translation
	n.d.	214.2	218.6	214.9	203.0	R_x libration
	283	281.9	279.0	272.4	271.6	O1–V–O2 bending
	700	691.5	772.4	690.5	652.6	V–O3 stretching

^{^a}

Experimental Raman spectra are shown in Figure 2b, the peak positions were determined using the 532 nm laser.

^{^b}

Calculated Raman active phonon modes, this work

^{^c}

Calculated Raman active phonon modes from reference. (66)

^{^d}

n.d.: not detected.

Table 1 provides the assignment of all experimental modes according to the present DFT calculations. The band at 994 cm^–1 is frequently exclusively attributed to the V═O1 stretching mode. (65) Our results show that the 994 cm^–1 band has noticeable contribution from O1–V–O2 stretching, hence, it involves O1 and O2 atoms.

The band observed at 527 cm^–1 was assigned to V–O2–V symmetric stretching by Abello et al. (63) In contrast, the same band was attributed to V–O3 bond stretching and V–O3–V angle deformation by Clauws et al. (64) and Brázdová et al. (65) Based on the present calculations (Table 1), the mode involves both O2 and O3, and it is related to O2–V–O3 bending, which is in agreement with Zhou et al. (66)

Major inconsistencies exist concerning the involvement of the three different oxygen atoms in the V₂O₅ structure to the bands observed at 404 and 481 cm^–1, which were ascribed to O2 and O3 atoms displacements, respectively, by Abello et al. (63) Clauws et al., however, ascribed the bands to O1 and O2 atoms displacements, respectively. (64) In turn, Brázdová et al. proposed the two bands to O3 and O2 atom displacement, respectively. (65) Here we assign the band at 404 cm^–1 to skeleton bending involving all three oxygen atoms and the vanadium atom (Table 1). The band at 481 cm^–1 originates from O2–V–O3 bending and the displacement of the O3 atom (Table 1).

The four weak and broad bands located at around 1025, 962, 800, and 660 cm^–1, which were clearly observed by using the infrared laser (785 nm) (Figure S10b), have been tentatively assigned to overtones or combination bands in previous studies. (63,64) In contrast, Brázdová et al. (65) assigned the band at 962 cm^–1 to a fundamental band (V–O2–V stretching, B_2g mode). A band ascribed to V–O2 stretching was predicted at 944.7 cm^–1 in our study (Table 1), which, however, deviates considerably from the experimentally observed band at 962 cm^–1. The band at around 1025 cm^–1 may arise from surface VO_x species self-supported on V₂O₅, because VO_x species supported on oxides similarly exhibit a broad feature in the range between 1012 and 1027 cm^–1. (67) The assignment of the band observed at 232 cm^–1 with the 785 nm laser excitation (Figure S10c) remains unclear so far. The closest calculated value of 214.2 cm^–1 is due to a collective vibration of the V₂O₅ lattice.

The assignment of the bands is an important prerequisite for the interpretation of the shift of bands observed during isotope exchange.

3.2. Oxygen Isotope Exchange

Isotope exchange of oxygen on the surface and in the bulk of polycrystalline V₂¹⁶O₅ with ¹⁸O₂ in the gas phase was studied by Raman spectroscopy at different temperatures and chemical potentials. The temperatures of ca. 320, 430, and 570 °C applied in the present study were chosen below, at and above typical reaction temperatures in selective oxidation of alkanes over vanadium oxide based catalysts, respectively. All temperatures are distinctly above the Tammann temperature T_Tam(V₂O₅) of 204 °C (477 K). At the Tammann temperature, T_Tam ≈ 0.5T_melt [K], atoms or ions become sufficiently mobile and migration from the bulk to the surface and surface liquefaction are possible. The exchange was probed in synthetic air and in the presence of alkanes and the type of oxygen atom exchanged was concluded based on the results of the DFT calculations.

3.2.1. Prediction of Raman Shifts in Partially and Fully Exchanged V₂O₅ by DFT Calculations

The changes in the Raman spectrum of V₂O₅ were predicted by DFT calculations for two cases:

(i)	only ¹⁶O1 is exchanged by ¹⁸O
(ii)	all three types of oxygen atoms are exchanged by ¹⁸O

The calculated peak positions for the two cases are listed in Table 1.

(i)	The exchange of the O1 atom will have an impact on the position of the experimentally determined band at 994 cm^–1, which contains contributions of two different phonon modes as explained above. The A_g mode is related to O1 (V═O1 stretching) and the B_2g mode is related to O1 (major contribution) and O2 atoms (minor contribution) in an O1–V–O2 stretching vibration (Table 1). The calculated values of A_g at 998.1 cm^–1 and B_2g at 997.6 cm^–1 in V₂¹⁶O₅ differ slightly and are experimentally not resolved, giving rise to only one experimentally observed band at 994 cm^–1 in V₂¹⁶O₅. However, when ¹⁶O atoms are exchanged by ¹⁸O only at the V═O1 site, the calculated A_g and B_2g modes shift differently by 43.1 cm^–1 to 955.0 cm^–1 and by 33.4 cm^–1 to 964.2 cm^–1, respectively (Table 1). Hence, the appearance of two new peaks at lower energy is expected. It should be possible to resolve these peaks experimentally due to the difference of about 10 cm^–1. In addition, the O1 atom contributes to the B_1g mode with an O1–V–O3 bending vibration (calculated shift at 280.6 cm^–1) and to the B_3g mode with an O1–V–O2 bending vibration (calculated shift at 281.9 cm^–1). The two modes are giving rise to the experimentally observed band of V₂¹⁶O₅ at 283 cm^–1 (Table 1). The calculated Raman shifts are moved by 9.9 and 9.5 cm^–1, respectively, to 270.7 and 272.4 cm^–1, respectively, when just the O1 atom is exchanged. Hence, it is expected that this band will not split into two components in the experimental spectrum and will shift only slightly to lower energy. In particular, when the oxygen exchange is not complete, the shift will be negligible.
(ii)	When all types of oxygen atoms are exchanged to ¹⁸O, all bands are shifted to lower energies and are different from case i. A_g and B_2g modes (994 cm^–1) shift in a similar manner by ca. 44 cm^–1 to 955.0 and 953.4 cm^–1, respectively.

3.2.2. Experimentally Observed Changes in the Raman Spectra

3.2.2.1. Temperature Effect in the Presence of Oxygen Only

No change in the Raman spectrum was detectable when a flow of 20% ¹⁶O₂ in helium was exchanged by a flow of 20% ¹⁸O₂ in helium at 431 °C (Figure 3).

Figure 3. Raman spectra of V₂O₅ measured at room temperature before and after isotopic oxygen exchange; Gray and red spectra denote the cases before and after exchange at 322, 431, and 573 °C, respectively, using ¹⁸O₂ (20% in He) or a mixture of propane and oxygen (C₃H₈ (1%) + ¹⁸O₂ (19%) in He) for 2 h. All the spectra were normalized to [0,1]. The positions of all spectra were aligned with respect to the band at 994 cm^–1. Laser: 532 nm. Heating rate: 5 °C/min. Total flow rate: 10 mL/min.

At 573 °C, however, the peak at 994 cm^–1 lost intensity. Taking into account the results of the DFT calculations of partially (O1) and fully (O1, O2, O3) exchanged V₂O₅, the Raman spectra recorded at room temperature after the isotope exchange experiments at different temperatures were deconvoluted to get insight into the contributing components in a qualitative manner (Figure 4). The deconvolution procedure is explained in detail the Supporting Information. The fitting parameters are provided in Table S3.

Figure 4. Deconvolution of Raman spectra collected at room temperature after treatment at different temperatures and in different gas atmospheres (the same Raman spectra as shown in Figure 3) in the V–O3 (a) as well as V═O1 and O1–V–O2 stretching vibration region (b). Temperatures and gas atmospheres are specified in the right top corner of each section. (c) Corresponding representations of vibrational motions of phonon modes.

Figure 4a, middle, shows the V–O3 stretching vibrations, whereas Figure 4b, middle, shows V═O1 and O1–V–O2 stretching vibrations after exchange in 20% ¹⁸O₂ in helium at 573 °C. Two components at higher energy (features >1000 cm^–1 assigned above to surface VO_x species) become visible now at 1007 and 1029 cm^–1, which might be attributed to V═¹⁸O_(s) and V═¹⁶O_(s), respectively (Figure 4b, middle). The two components appear also after exchange in the presence of propane (see Section 3.2.2.2 and Figure 4b, top). Interestingly, the peak width of V═¹⁶O_(s) species in terms of fwhm (full width at half-maximum) is much larger than that of V═¹⁸O_(s) (∼50 vs ∼15 cm^–1, Table S3). The large peak width in the spectrum before oxygen exchange reflects the heterogeneity of the VO_x species at the surface. This may mean that the reactivity of the energetically different surface species is also different. The relatively small peak width of V═¹⁸O_(s) suggests that only a portion of the V═¹⁶O_(s) species is reactive and has been exchanged. Assuming an isotope ratio of 1.0452, one would expect a band at ∼1050 cm^–1 for this reactive V═¹⁶O_(s) species. The exposure of the V₂O₅ to C₃H₈ + ¹⁶O₂ feed alone does not cause any spectral modification (Figure S12), suggesting that the observed change in the presence of oxygen at high temperature is indeed due to a partial oxygen exchange and not, for example, due to the formation of defects.

In addition, a new band appeared at around 962 cm^–1 (¹⁸O1–V–¹⁶O2) with a shoulder around 954 cm^–1 (V═¹⁸O1) (Figure 4b, middle). The deconvolution of the original spectra is shown in the Supporting Information, Figure S13. The band at 702 cm^–1 moved to 690 cm^–1, suggesting a partial exchange of the O3 atom, which affects the V–O3 stretching vibration (Figure 4a, middle). Shifts of bands below 530 cm^–1 are hardly visible. The observations suggest that oxygen atoms at the V═O1 and V–O3 sites were partially exchanged in the presence of 20% ¹⁸O₂/He at 573 °C.

The temporal evolution of spectra recorded in situ during the experiment at 573 °C is shown in Figure 5a. The gradual increase of the new band around 962 cm^–1 is clearly observed reflecting the increase of the exchange extent. The band positions of original bands are not affected, showing that the extent of exchange (O1 atoms) has no influence on the band position.

Figure 5. *In situ* Raman spectra of V₂O₅ at 573 °C under 20% ¹⁸O₂ in He (a), 1% C₃H₈ + 19% ¹⁸O₂ in He (b), and 1% C₃D₈ + 19% ¹⁸O₂ in He (c) as a function of time. Conditions of exchange experiments are described in the caption of Figure 3. The numbers show the time that had passed since the start of the experiment at the moment of recording. Laser: 532 nm.

3.2.2.2. Oxygen Isotope Exchange in the Presence of Propane

The following experiment explores how the chemical potential of the gas phase influences the oxygen exchange at different temperatures. For this purpose, propane was added to the oxygen flow. The presence of propane, which undergoes oxidation under the applied conditions (propane conversion X = 36% at 573 °C), will change the defect concentration on the surface and in the bulk of the vanadia. A low ratio of propane to oxygen (1:19) was used to prevent severe reduction of V₂O₅, in particular at the high reaction temperatures.

When a flow of C₃H₈:¹⁸O₂:He in a ratio of 1:19:80 was applied at 573 °C, the intensity of the band at 994 cm^–1 decreased significantly and all other bands moved to lower wavenumbers (Figures 3, 4a, 4b(top), and 5b). A new band appeared at around 954 cm^–1. This is different from the case using 20% ¹⁸O₂ in helium (Figures 4a, 4b(middle), and 5a), where the development of a band at 962 cm^–1 with a shoulder around 955 cm^–1 was observed. Specifically, the extent of exchange of O3 atoms is higher than under only oxygen flow as indicated by the larger shift of the position of the peak near 700 cm^–1 and the appearance of a component of full V–¹⁸O3 in this band (Figure 4a(top)). The deconvolution analysis further reveals that the newly formed band at 954 cm^–1 possibly contains three components from different combinations of exchanged O1 and O2 atoms due to V–O1 and O1–V–O2 stretching vibrations (Figure 4b(top)). The presence of a component corresponding to ¹⁸O1–V–^16/18O2 (coupling of O2 atoms) suggests a faster exchange of O1 atoms with ¹⁸O as compared to O2 atoms. Bearing in mind the calculated Raman spectrum of V₂¹⁸O₅ (Table 1), the results suggest that essentially all three types of oxygen atoms were exchanged in the presence of propane at 573 °C. In a control experiment, ¹⁶O₂ instead of ¹⁸O₂ was used (Figure S12). According to that, propane oxidation at this high reaction temperature does not change the structure of V₂O₅ so drastically as to affect the Raman spectrum. Consequently, the change in band positions presented in Figure 3 is due to ¹⁸O incorporation into V₂O₅. These results directly prove that the presence of a reductant accelerates exchange rates. (29,34) Assuming the R² exchange mechanism over V₂O₅, two surface vacancies are required to make exchange happen. (28) It is likely that propane effectively generates vacancies on the surface of V₂O₅ and greatly increases the probability of having two neighboring vacancies. DFT calculations also imply that reduced surfaces of metal oxides are energetically more favorable for oxygen activation than unreduced surfaces. (40) It was pointed out that the formation energy of surface oxygen vacancies is reduced by the presence of surface OH groups due to a weakening of the V–O bond in V–OH. (68) To verify this, deuterium labeled propane was used in the isotopic oxygen exchange experiment. The exchange proceeds indeed at a slightly lower rate in the case of using deuterium labeled propane as compared to normal propane (Figure 5c). The exchange extent at each time step is lower than that using normal propane. The observed isotope effect suggests that hydrogen is involved in the oxygen activation process, possibly by reducing the energy to form oxygen vacancies at the surface.

To further quantify the exchange process, the exchange extent ΔA,

(2)

where A refers to the band area, as a function of time (Figure 5) was analyzed at 573 °C in the different gas atmospheres (Figure 6). The analysis does not allow the differentiation between the three types of oxygen atoms. Instead, the total extent of exchange is estimated. In general, isotopic oxygen exchange comprises surface exchange and bulk diffusion. (69−71) Surface exchange takes place at the interface between the solid and the gas. The propane molecule would also be involved in the surface exchange by reacting with labeled surface oxygen species and generating oxygen vacancies. Bulk diffusion of oxygen can proceed through vacancies or interstitial sites. (72) In the following, the two borderline cases

(3)

and

(4)

where k_ex refers to the surface oxygen exchange coefficient, D is the oxygen diffusion coefficient, and a is the radius of the spherical catalyst particles, have been considered. The assumption of spherical particles is a rather crude simplification in view of the plate-like nature of the V₂O₅ particles (Figure S3). However, a comparison of relative changes is possible. The detailed derivation of eqs 3 and 4 and the fitting parameters (Table S4) are provided in the Supporting Information. As Figure 6 shows, oxygen exchange at 573 °C remains limited to the surface in the absence of propane (Figure 6, black lines and data points). Studies of oxygen exchange on amorphous V₂O₅ at 400–550 °C have shown that, in the absence of a reducing agent, the rate of exchange is controlled by the surface reaction. (73) In the presence of propane, however, the overall exchange rate appears to be limited by the bulk diffusion (Figure 6, red and blue lines and data points). Apparently, propane at 573 °C accelerates oxygen exchange at the surface, so that oxygen diffusion in the bulk becomes a comparatively slow process.

Figure 6. Extent of oxygen exchange as a function of time; The lines are the fitted results using eqs 3 (dashed) and 4 (solid). The experimental data points were taken from the experiments shown in Figure 5. Note that the time used in the graph is the average time taken to record each spectrum.

The main bands positions were not shifted when the same experiment was performed at 431 °C (Figures 3 4a, and 4b(bottom)). The temperature 431 °C is a typical reaction temperature applied in alkane oxidation over V-based catalysts. Just a small intensity difference near 962 cm^–1 was found indicating that the oxygen exchange is just detectable by Raman spectroscopy at this temperature and mainly V═O1 sites were exchanged (Figure 4b, bottom). In contrast, oxygen exchange at the same temperature in pure oxygen could not be detected by Raman spectroscopy. Due to the small extent of exchange at 431 °C, it was not possible to perform a kinetic analysis in analogy to the experiment at 573 °C (Figure 6). However, since oxygen diffusion in the bulk is even more difficult at lower temperatures, it is very likely that this process does not play a role in the lower temperature range.

Finally, the exchange in the presence of propane was also performed at 322 °C. No change in the Raman spectrum was observed under these conditions after 2 h (Figure 3).

The direct comparison of the present in situ Raman experiments with kinetic studies of isotopic oxygen exchange by mass spectroscopy in the past is not possible, as these experiments were carried out in vacuum at low oxygen partial pressures of a few millibar, (28,29) which were lower than or in the order of the magnitude of the equilibrium oxygen partial pressure that develops above V₂O₅ at elevated temperatures. (74−76) Online mass spectra recorded during a temperature programed experiment in the Raman cell in the presence of 20% ¹⁸O₂ in He show that the onset temperature for observing mixed ¹⁶O¹⁸O in the gas phase was ∼424 °C (Figure S9), and for ¹⁶O₂ at ∼450 °C, which is in agreement with the observation that no oxygen exchange was observed by Raman spectroscopy at 431 °C in 20% ¹⁸O₂ in He (Figure 3). The Raman spectroscopy appears a little less sensitive than the mass spectroscopy. Furthermore, an increase in the relative concentration of ¹⁶O atoms in the gas phase oxygen also occurred at ∼424 °C, which confirms that the oxygen exchange between the gas phase and the solid starts at temperatures higher than 420 °C in absence of a reducing agent, because in the case of a full gas-phase R⁰ mechanism a constant atomic ratio of ¹⁶O would be expected. The formation of ¹⁶O¹⁸O further implies that oxygen dissociation (eq 1) on V₂O₅ is reversible, which is different from a previous work, in which no formation of ¹⁶O¹⁸O was found from C₃H₈–¹⁶O₂–¹⁸O₂ mixtures on V¹⁶O_x/Zr¹⁶O₂ at about 427 °C. (77)

3.2.2.3. Influence of the Hydrocarbon Chain Length

The effect of the nature of the alkane on the isotope oxygen exchange was examined at 400 °C applying an alkane:oxygen ratio of 10:5. These conditions correspond to the typical temperature and feed composition used in the oxidative dehydrogenation of alkanes. Ethane, propane, and n-butane were compared. In case of ethane and propane, a small band near 962 cm^–1 occurs after 2 h under these conditions, while all the other bands maintain their positions (Figure 7a). Apparently, mainly a small fraction of V═O1 sites was exchanged. By using n-butane, a new band at 946 cm^–1 is already formed after 2 min (Figure 7b). No change in the Raman spectra was observed after such a short time in the experiments with ethane and propane. As compared to the spectrum obtained under ¹⁶O₂ at the same temperature, the band at 700 cm^–1 (shifted to 693 cm^–1 due to thermal effect) maintains its position as well as other bands. V₂O₅ was fully reduced, however, after only 12 min, and no clear features can be found in the in situ Raman spectra anymore (Figure 7b). The more reducing conditions in n-butane lead most likely to a higher concentration of surface oxygen vacancies. The newly formed band at 946 cm^–1 is attributed either to isotope oxygen exchange (see band at 953.4 cm^–1 for ¹⁸O1–V–¹⁸O2 stretching in Table 1) or to an intermediate (suboxide) during reduction of V₂O₅.

Figure 7. Effect of the nature of the alkane on isotope oxygen exchange. (a) Raman spectra of V₂O₅ measured at room temperature before and after isotopic oxygen exchange experiments under 10% C₃H₈ + 5% ¹⁸O₂ and 10% C₂H₆ + 5% ¹⁸O₂, respectively, at 400 °C. (b) *In situ* Raman spectra of V₂O₅ at 400 °C under ¹⁶O₂ and 10% C₄H₁₀ + 5% ¹⁸O₂ as a function of time. Numbers on the spectra indicate the starting time of measurement. Asterisks represent cut cosmic ray signals. Raman spectra were normalized to [0,1]. Laser: 532 nm. Exchange experiments were conducted at 400 °C for 2 h in the Harrick Raman chamber. Heating rate: 5 °C/min. Total flow rate: 10 mL/min.

4. General Discussion and Conclusions

Click to copy section linkSection link copied!

V₂O₅ was studied using multiwavelength Raman spectroscopy. Reassignment of the experimentally observed active Raman vibrational modes using DFT calculations revealed that the band at 994 cm^–1, which in previous studies was attributed exclusively to the vanadyl oxygen stretching vibration (V═O1), contains additional contributions from O2 atoms. Based on the present combined experimental and theoretical approach, a method was developed to unambiguously determine the nature of the oxygen atom in the crystal structure of V₂O₅ (O1, O2, or O3) exchanging with ¹⁸O₂ in the gas phase. The new assignment makes it possible to determine experimentally which type of oxygen atom can be exchanged under which reaction conditions. The exchange position depends strongly on the temperature and the chemical potential in the gas phase. An overview of the cases studied in the present work is given in Table 2. The conditions investigated are important for the selective and total oxidation of alkanes.

Table 2. Types of Oxygen Atoms in V₂O₅ Exchanged at Different Temperatures and in Different Gas Atmospheres

	gas phase composition (vol %)			temperature of oxygen exchange (°C)
	alkane	O₂	inert	573	431	400	322
O₂/He	0	20	80	O1, O3	n.d.^a	–^b	–
C₂H₆/O₂	10	5	85	–	–	O1	–
C₃H₈/O₂	1	19	80	O1, O2, O3	O1	–	n.d.
C₃H₈/O₂	10	5	85	–	–	O1	–
C₄H₁₀/O₂	10	5	85	–	–	O1, O2^c	–

^{^a}

n.d.: not detectable by Raman spectroscopy.

^{^b}

–: not analyzed.

^{^c}

Observation difficult due to superimposed reduction of V₂O₅

For example, in the presence of propane, but under quite oxidative conditions (1% C₃H₈, 19% O₂), only O1 oxygen atoms are exchanged at 431 °C. This temperature is in the range where selective oxidation reactions of alkanes are typically carried out. Using eq 2, a degree of exchange of ∼6.8% is measured by Raman spectroscopy after 2 h under these conditions. Since the probing depth of Raman spectroscopy is limited and ranges between a few and several hundreds of nanometers depending on the energy of the exciting laser, the overall exchange extent of the entire sample is much smaller than this value. The very low degree of exchange, which was only detectable after prolonged reaction time (2 h), indicates that the oxygen isotope exchange occurs mainly at the surface and/or in a near-surface region under the applied conditions. With increasing temperature in an oxidative atmosphere or in the presence of reducing agents, O3 sites and eventually O2 sites are exchanged. All types of oxygen (O1, O2, and O3) are involved in the exchange at very high temperatures (573 °C) and in the presence of propane in the feed. Under these conditions, a kinetic analysis utilizing spectral changes as a function of time was possible. The analysis shows that oxygen vacancies generated by propane facilitate the incorporation of gaseous oxygen into the lattice of V₂O₅ by accelerating the surface exchange process. Hence, rapid oxygen exchange at 573 °C in the presence of propane appears to be limited by bulk diffusion.

The exchange of oxygen in V₂O₅ becomes energetically increasingly difficult in the following order: O1 < O3 < O2. The terminal vanadyl defects are the easiest to form in agreement with predictions by theory. (9,78) Under typical reaction conditions of propane oxidation (400–450 °C), mainly vanadyl oxygen O1 is involved in the oxygen exchange and the very low exchange extent suggests that the exchange occurs primarily on the surface and/or the near surface region. With a classical feed composition (C₃H₈:O₂ = 2:1) and temperature (400 °C) of the oxidative dehydrogenation of propane, hardly any change is seen in the spectrum and this small change only affects O1. In this temperature range, the working catalyst in the Raman cell produces propene by oxidative dehydrogenation of propane and CO₂ as the product of total oxidation (Figure S14). The selectivity to propene under these conditions and at low conversions, which are typically set for kinetic studies, is between 55 and 60%. The catalyst was working in steady-state (Figure S14), and consequently, no changes in the Raman spectra are observed with time on stream. Also under other reaction conditions, no changes were detected after a total reaction time of 7 h (Figure S14).

In vanadium oxide based systems, it is commonly assumed that the formation of propene and other oxidation products occurs according to the Mars–van Krevelen mechanism, (79) which would involve consumption of surface oxygen atoms by incorporation of surface or lattice oxygen into water or oxygen-containing oxidation products. As far as propane oxidation on vanadium oxide catalysts is concerned, these conclusions are mainly based on kinetic modeling, (77,80) or DFT calculations at 0 K. (81) In the present work, the basis for the assignment of the Raman bands of V₂(¹⁶O,¹⁸O)₅ was provided. The isotope exchange experiments presented here cast doubt on whether the Mars–van Krevelen mechanism prevails under typical conditions of selective oxidation of propane, since a very small change in the spectra was observed only after 2 h of operation at 400 °C, indicating some exchanged O1 atoms (Figure 7a), while the catalyst produced propylene and CO₂ at the steady state long before these changes were observed in the Raman spectra (Figure S14a). However, due to the limited sensitivity of Raman spectroscopy, further systematic operando investigations in combination with mass spectrometry are needed here to obtain more clarity.

Supporting Information

Click to copy section linkSection link copied!

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c00174.

XRD pattern of V₂O₅, TG and DTA analysis of V₂O₅, HAADF-STEM images of V₂O₅, description of multiwavelength Raman spectroscopy, mass spectra of oxygen exchange of V₂O₅, uncorrected Raman spectra of V₂O₅, the atomic displacement patterns of Raman active phonon modes, Raman spectra of V₂O₅ after treatment in the presence of C₃H₈ and ¹⁶O₂ at 573 °C, table of calculated lattice parameters and bond lengths, table of atomic coordinates of the optimized structure, description of the deconvolution of Raman spectra, discussions of surface reaction and bulk diffusion, and operando Raman spectroscopy of V₂O₅ during propane oxidation (PDF)

jp2c00174_si_001.pdf (1.73 MB)

Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

Click to copy section linkSection link copied!

Corresponding Author
- Annette Trunschke - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, D-14195 Berlin, Germany; https://orcid.org/0000-0003-2869-0181; Email: [email protected]
Authors
- Yuanqing Wang - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, D-14195 Berlin, Germany; BasCat, UniCat BASF Jointlab, Technische Universität Berlin, D-10623 Berlin, Germany; Present Address: Materials Genome Institute, Shanghai University, 200444, Shanghai, China; https://orcid.org/0000-0002-1332-7956
- Frank Rosowski - BasCat, UniCat BASF Jointlab, Technische Universität Berlin, D-10623 Berlin, Germany; Heterogeneous Catalysis, BASF SE, Process Research and Chemical Engineering, Carl-Bosch-Straße 38, D-67065 Ludwigshafen, Germany
- Robert Schlögl - Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4−6, D-14195 Berlin, Germany; Max-Planck-Institut für Chemische Energiekonversion, Stiftstrase 34−36, D-45470 Mülheim, Germany
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
Funding
Open access funded by Max Planck Society.
Notes
The authors declare no competing financial interest.

Acknowledgments

Click to copy section linkSection link copied!

We thank Dr. Andrey Tarasov for TG and DTA measurements, Dr. Girgsdies and Jasmin Allan for XRD measurements, Maike Hashagen for BET measurements, Dr. Olaf Timpe for XRF measurements, Dr. Xing Huang for STEM measurements, and Jutta Kröhnert for UV/vis spectrum measurements. We also express thanks for a helpful discussion with Dr. Anna Wernbacher. The DFT calculations were performed using the computational resource of the Computer Support Group (PP&B) at the Fritz-Haber-Institut. This work was conducted in the framework of the BasCat collaboration among BASF SE, the Technical University Berlin, the Fritz-Haber-Institut der Max-Planck-Gesellschaft, and the cluster of excellence “Unified Concepts in Catalysis” (UniCat http://www.unicat.tu-berlin.de).

References

Click to copy section linkSection link copied!

This article references 81 other publications.

1
Panov, G. I.; Dubkov, K. A.; Starokon, E. V. Active Oxygen in Selective Oxidation Catalysis. Catal. Today 2006, 117, 148– 155, DOI: 10.1016/j.cattod.2006.05.019
Google Scholar
1
Active oxygen in selective oxidation catalysis
Panov, Gennady I.; Dubkov, Konstantin A.; Starokon, Eugeny V.
Catalysis Today (2006), 117 (1-3), 148-155CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
A review. The paper reviews literature data related to oxygen species available on the surface of metal oxides and their possible role in selective oxidn. With some reservations, one may conclude that the main concept accepted presently in oxidn. catalysis assumes that selective oxidn. is provided by strongly bonded lattice oxygen, while the full oxidn. is provided by weakly bonded reactive oxygen. Recent studies on the oxidn. mechanism over FeZSM-5 zeolites with N2O give ground to reconsider this concept in order to integrate the earlier suggested radical oxygen idea, which supposes an important role of O- radicals in selective oxidn. This highly reactive oxygen species can be considered as a powerful tool for activation of org. mols.
2
Freund, H.-J. Oxygen Activation on Oxide Surfaces: A Perspective at the Atomic Level. Catal. Today 2014, 238, 2– 9, DOI: 10.1016/j.cattod.2014.05.037
Google Scholar
2
Oxygen activation on oxide surfaces: A perspective at the atomic level
Freund, Hans-Joachim
Catalysis Today (2014), 238 (), 2-9CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
A review; thin oxide films have been used as model supports to unravel the influence of the oxide-metal and oxide-gas interface. We discuss the influence of defects in the oxide lattice and the oxide surface on properties of adsorbed species, in particular the formation of oxide particles from a deposited transition metal onto ceria and of adsorbed oxygen from the gas phase. Here we correlate the structure of the particles, as revealed by a combination of STM and spectroscopic as well as theor. calcns., with their reactive properties. The nature of the involved defects is characterized by adsorption of Au as a way to correlate the influence of various defects of different structure on the Au charge state. In a second case study, we demonstrate the influence of dopants within the supporting oxide on the adsorption of oxygen on a defect free surface. This is shown to be potentially relevant for the activation of methane in oxidative methane coupling reactions, as recently demonstrated.
3
Hwang, J.; Rao, R. R.; Giordano, L.; Katayama, Y.; Yu, Y.; Shao-Horn, Y. Perovskites in Catalysis and Electrocatalysis. Science 2017, 358, 751– 756, DOI: 10.1126/science.aam7092
Google Scholar
3
Perovskites in catalysis and electrocatalysis
Hwang, Jonathan; Rao, Reshma R.; Giordano, Livia; Katayama, Yu; Yu, Yang; Shao-Horn, Yang
Science (Washington, DC, United States) (2017), 358 (6364), 751-756CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
A review. Catalysts for chem. and electrochem. reactions underpin many aspects of modern technol. and industry, from energy storage and conversion to toxic emissions abatement to chem. and materials synthesis. This role necessitates the design of highly active, stable, yet earth-abundant heterogeneous catalysts. In this Review, we present the perovskite oxide family as a basis for developing such catalysts for (electro)chem. conversions spanning carbon, nitrogen, and oxygen chemistries. A framework for rationalizing activity trends and guiding perovskite oxide catalyst design is described, followed by illustrations of how a robust understanding of perovskite electronic structure provides fundamental insights into activity, stability, and mechanism in oxygen electrocatalysis. We conclude by outlining how these insights open exptl. and computational opportunities to expand the compositional and chem. reaction space for next-generation perovskite catalysts.
4
Adler, S. B. Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104, 4791– 4844, DOI: 10.1021/cr020724o
Google Scholar
4
Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes
Adler, Stuart B.
Chemical Reviews (Washington, DC, United States) (2004), 104 (10), 4791-4843CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review of factors governing oxygen redn. in solid oxide fuel cell cathodes, with emphasis on thin-film ceramic-type conductor and conducting electrode fabrication, mass transfer and transport, catalysis (e.g., by platinum), perovskite-type mixed conductors, oxygen diffusion and adsorption, phase boundaries, interfacial effects and interfacial kinetics, transport models, operating regimes, current constriction, microstructure and microstructure control, overpotential, and composite electrolytes and electrodes. Materials emphasized include Pt, YSZ (yttria-stabilized zirconia), lanthanum strontium manganese oxide ((La,Sr)MnO3), and cobalt iron lanthanum strontium oxide ((La,Sr)(Co,Fe)O3).
5
Gurlo, A. Interplay between O₂ and SnO₂: Oxygen Ionosorption and Spectroscopic Evidence for Adsorbed Oxygen. ChemPhysChem 2006, 7, 2041– 2052, DOI: 10.1002/cphc.200600292
Google Scholar
5
Interplay between O2 and SnO2: oxygen ionosorption and spectroscopic evidence for adsorbed oxygen
Gurlo, Alexander
ChemPhysChem (2006), 7 (10), 2041-2052CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Tin dioxide is the most commonly used material in com. gas sensors based on semiconducting metal oxides. Despite intensive efforts, the mechanism responsible for gas-sensing effects on SnO2 is not fully understood. The key step is the understanding of the electronic response of SnO2 in the presence of background oxygen. For a long time, oxygen interaction with SnO2 was treated within the framework of the ionosorption theory. The adsorbed oxygen species were regarded as free oxygen ions electrostatically stabilized on the surface (with no local chem. bond formation). A contradiction, however, arises when connecting this scenario to spectroscopic findings. Despite trying for a long time, there was not any convincing spectroscopic evidence for ionosorbed oxygen species. Neither superoxide ions O2-, nor charged at. oxygen O-, nor peroxide ions O22- were obsd. on SnO2 under the real working conditions of sensors. Also, several findings show that the superoxide ion does not undergo transformations into charged at. oxygen at the surface, and represents a dead-end form of low-temp. oxygen adsorption on reduced metal oxide.
6
Schlögl, R. Heterogeneous Catalysis. Angew. Chem., Int. Ed. 2015, 54, 3465– 3520, DOI: 10.1002/anie.201410738
Google Scholar
6
Heterogeneous catalysis
Schlogl Robert
Angewandte Chemie (International ed. in English) (2015), 54 (11), 3465-520 ISSN:.
A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.
7
Haber, J.; Turek, W. Kinetic Studies as a Method to Differentiate between Oxygen Species Involved in the Oxidation of Propene. J. Catal. 2000, 190, 320– 326, DOI: 10.1006/jcat.1999.2764
Google Scholar
7
Kinetic Studies as a Method to Differentiate between Oxygen Species Involved in the Oxidation of Propene
Haber, Jerzy; Turek, Wincenty
Journal of Catalysis (2000), 190 (2), 320-326CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
Kinetic equations of propene oxidn. along the nondestructive (nucleophilic oxidn. to allylic products) and the destructive (electrophilic oxidn. to degraded oxygenated and total oxidn. products) pathways have been detd. for three types of oxide catalysts: Bi2O3/MoO3, giving only allylic products, Co3O4, showing only total oxidn., and SnO2/MoO3, giving both types of products. In all cases the reaction order of the nucleophilic oxidn. was 1 with respect to propene and 0 with respect to oxygen, whereas that of electrophilic oxidn. was close to 1 with respect to oxygen. The model of surface interactions is discussed in which propene reacts either with surface lattice oxide ions to give nucleophilic oxidn. products or with transient surface oxygen species to give electrophilic oxidn. These transients result from the dynamic equil. between nonstoichiometric transition metal oxides and gas phase oxygen, so that the two kinetic equations are coupled by the equation expressing the equil. of surface oxygen vacancies. The rate consts. of the homomol. isotopic exchange of oxygen may be taken as a measure of the surface concn. of the transient oxygen species. (c) 2000 Academic Press.
8
Rasmussen, M. D.; Molina, L. M.; Hammer, B. Adsorption, Diffusion, and Dissociation of Molecular Oxygen at Defected TiO₂(110): A Density Functional Theory Study. J. Chem. Phys. 2004, 120, 988– 997, DOI: 10.1063/1.1631922
Google Scholar
8
Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110). A density functional theory study
Rasmussen, M. D.; Molina, L. M.; Hammer, B.
Journal of Chemical Physics (2004), 120 (2), 988-997CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
The properties of reduced rutile TiO2(110) surfaces, as well as the adsorption, diffusion, and dissocn. of mol. O are investigated by d. functional theory. The O2 mol. is found to bind strongly to bridging O vacancies, attaining a mol. state with an expanded O-O bond of 1.44 Å. The mol. O also binds (with somewhat shortened bond lengths) to the fivefold coordinated Ti atoms in the troughs between the bridging oxygen rows, but only when vacancies are present somewhere in the surface. In all cases, the magnetic moment of O2 is lost upon adsorption. The expanded bond lengths reveal together with inspection of electron d. and electronic d. of state plots that charging of the adsorbed mol. oxygen is of key importance in forming the adsorption bond. The processes of O2 diffusion from a vacancy to a trough and O2 dissocn. at a vacancy are both hindered by relative large barriers. The presence of neighboring vacancies can strongly affect the ability of O2 to dissoc. The implications of this in connection with diffusion of the bridging O vacancies are discussed.
9
Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J. Oxygen Vacancies in Transition Metal and Rare Earth Oxides: Current State of Understanding and Remaining Challenges. Surf. Sci. Rep. 2007, 62, 219– 270, DOI: 10.1016/j.surfrep.2007.03.002
Google Scholar
9
Oxygen vacancies in transition metal and rare earth oxides. Current state of understanding and remaining challenges
Ganduglia-Pirovano, M. Veronica; Hofmann, Alexander; Sauer, Joachim
Surface Science Reports (2007), 62 (6), 219-270CODEN: SSREDI; ISSN:0167-5729. (Elsevier B.V.)
A review. Defects at transition metal (TM) and rare earth (RE) oxide surfaces, neutral O vacancies in particular, play a major role in a variety of technol. applications. This is the motivation of numerous studies of partially reduced oxide surfaces. We review, discuss, and compare theor. data for structural and electronic properties and energetic quantities related to the formation of O defects at TM and RE oxide surfaces using TiO2, ZrO2, V2O5, and CeO2 as examples. Bulk defects, as far as relevant for comparison with the properties of reduced surfaces, are briefly reviewed. Special attention is given to the fate of the electrons left in the system upon vacancy formation and the ability of state-of-the-art quantum-mech. methods to provide reliable energies and an accurate description of the electronic structure of the partially reduced oxide systems.
10
Cui, Y.; Shao, X.; Baldofski, M.; Sauer, J.; Nilius, N.; Freund, H.-J. Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides. Angew. Chem., Int. Ed. 2013, 52, 11385– 11387, DOI: 10.1002/anie.201305119
Google Scholar
10
Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides
Cui, Yi; Shao, Xiang; Baldofski, Matthias; Sauer, Joachim; Nilius, Niklas; Freund, Hans-Joachim
Angewandte Chemie, International Edition (2013), 52 (43), 11385-11387CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Charge transfer in the presence of dopants is relevant for the adsorption and activation of small mols., such as O2. Scanning tunneling microscopy and DFT calcns. provide evidence for the formation of strongly bound superoxo species on chem. inert, Mo-doped CaO films. This oxygen surface species shows a high propensity to dissoc. Dopants could also be important for the activation of hydrocarbons on inert oxides.
11
McFarland, E. W.; Metiu, H. Catalysis by Doped Oxides. Chem. Rev. 2013, 113, 4391– 4427, DOI: 10.1021/cr300418s
Google Scholar
11
Catalysis by Doped Oxides
McFarland, Eric W.; Metiu, Horia
Chemical Reviews (Washington, DC, United States) (2013), 113 (6), 4391-4427CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
A review; catalysis by doped oxides is discussed.
12
Schwach, P.; Hamilton, N.; Eichelbaum, M.; Thum, L.; Lunkenbein, T.; Schlögl, R.; Trunschke, A. Structure Sensitivity of the Oxidative Activation of Methane over MgO Model Catalysts: II. Nature of Active Sites and Reaction Mechanism. J. Catal. 2015, 329, 574– 587, DOI: 10.1016/j.jcat.2015.05.008
Google Scholar
12
Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism
Schwach, Pierre; Hamilton, Neil; Eichelbaum, Maik; Thum, Lukas; Lunkenbein, Thomas; Schloegl, Robert; Trunschke, Annette
Journal of Catalysis (2015), 329 (), 574-587CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
A series of pure, nanostructured magnesium oxides prepd. by different synthesis techniques that show different initial, but similar steady-state activity in the oxidative coupling of methane (OCM) (Schwach et al., submitted for publication) has been studied by IR and photoluminescence spectroscopy in the dehydroxylated state before the reaction and after catalysis. The abundance of structural defects, in particular mono-at. steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146 cm-1 and Lewis acid/base pairs probed by co-adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation. IR spectroscopy evidences strong polarization of C-H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high no. of mono-at. steps. It is postulated that these sites effectively promote intermol. charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissocn. of one C-H bond in the methane mol. and simultaneous formation of a superoxide species. Heterolytic splitting of C-H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temp. has been proven by the appearance of an EPR signal assocd. with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono-at. steps and the reconstruction of the MgO termination under formation of polar and faceted surfaces.
13
Zasada, F.; Piskorz, W.; Janas, J.; Gryboś, J.; Indyka, P.; Sojka, Z. Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and Their Relevance in ¹⁶O₂/¹⁸O₂ Isotopic Exchange, deN₂O, and deCH₄ Processes─a Theoretical and Experimental Account. ACS Catal. 2015, 5, 6879– 6892, DOI: 10.1021/acscatal.5b01900
Google Scholar
13
Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and their Relevance in 16O2/18O2 Isotopic Exchange, deN2O, and deCH4 Processes-A Theoretical and Experimental Account
Zasada, Filip; Piskorz, Witold; Janas, Janusz; Grybos, Joanna; Indyka, Paulina; Sojka, Zbigniew
ACS Catalysis (2015), 5 (11), 6879-6892CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Periodic spin unrestricted, gradient cor. DFT calcns. joined with atomistic thermodn. modeling and expt. were used to study the structure and stability of various reactive oxygen species (ROS) and oxygen vacancies produced on the most stable terminations of the cobalt spinel (100) surface. The surface state diagram of oxygen in a wide range of pressures and temps. was constructed for coverage varying from ΘO = 1.51 atom·nm-2 to ΘO = 6.04 atom·nm-2. A large variety of the unraveled surface ROS includes diat. superoxo (CoO-O2--CoO), peroxo (CoT-O22--CoO), and spin paired (CoO-O2-CoO) adducts along with monat. metal-oxo (CoT-O+, CoO-O2+) species, where CoT and CoO stand for the tetrahedral and octahedral cobalt surface centers, resp. There are also two kinds of peroxo species assocd. with surface oxygen ions connected with 3CoO or 2CoO and 1CoT cations ((O2O,1T-O)2- and (O3O-O)2-), resp.). The results revealed that in the oxygen pressure range of typical catalytic reactions (pO2/p° from ∼0.01 to 1), the most stable stoichiometric (100)-S surface accommodates the CoT-O22--CoO and CoO-O2-CoO adducts at temps. below 250-300 °C. In the temp. from 250 to 300 °C and from 550 to 700 °C, it is covered by the O species assocd. with the exposed tetrahedral cobalt sites (CoT-O+) or remains in a bare state. In more reducing conditions (T > 550-700 °C), the (100)-S facet is readily defected due to trigonal oxygen (O2O,1T) release and formation of surface oxygen vacancies. The reactivity of surface ROS was tested in 16O2/18O2 isotopic exchange, N2O decompn., and oxidn. of CH4 and CO model reactions, carried over Co3O4 and Co318O4 nanocryst. samples with the predominant (100) faceting revealed by high angle angular dark field STEM examn. The CoO-O2+ adducts assocd. with octahedral cobalt sites, as well as the peroxo (O2O,1T-O)2- and (O3O-O)2- surface species being thermodynamically unstable are involved in surface oxygen recombination processes, probed by 16O2/18O2 exchange and N2O decompn. It was shown that at low temps. CO is oxidized by the suprafacial CoO-O2-CoO and CoT-O2-CoO diat. oxygen, whereas in CH4 activation, the highly reactive cobalt-oxo species (CoT-O+) are involved. Above 600 °C at pO2/p° = 0.01, due to the onset of oxygen vacancy formation, the suprafacial methane oxidn. gradually changes into the intrafacial Mars-van Krevelen scheme. The constructed surface phase diagram was used for rationalization of the obtained catalytic data, allowing delineation of the specific role of the chem. state of the cobalt spinel surface in the investigated processes, as well as the range of the corresponding temps. and oxygen pressures. It also provides a convenient background for mol. understanding of remarkable activity of Co3O4 in many other catalytic redox reactions.
14
Hävecker, M.; Wrabetz, S.; Kröhnert, J.; Csepei, L.-I.; Naumann d’Alnoncourt, R.; Kolen’ko, Y. V.; Girgsdies, F.; Schlögl, R.; Trunschke, A. Surface Chemistry of Phase-Pure M1Movtenb Oxide During Operation in Selective Oxidation of Propane to Acrylic Acid. J. Catal. 2012, 285, 48– 60, DOI: 10.1016/j.jcat.2011.09.012
Google Scholar
There is no corresponding record for this reference.
15
Amakawa, K. How Strain Affects the Reactivity of Surface Metal Oxide Catalysts. Angew. Chem., Int. Ed. 2013, 52, 13553– 13557, DOI: 10.1002/anie.201306620
Google Scholar
15
How Strain Affects the Reactivity of Surface Metal Oxide Catalysts
Amakawa, Kazuhiko; Sun, Lili; Guo, Chunsheng; Haevecker, Michael; Kube, Pierre; Wachs, Israel E.; Lwin, Soe; Frenkel, Anatoly I.; Patlolla, Anitha; Hermann, Klaus; Schloegl, Robert; Trunschke, Annette
Angewandte Chemie, International Edition (2013), 52 (51), 13553-13557CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepd. by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm-2). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temp.-programmed redn. and a steep increase in the catalytic activity obsd. in metathesis of propene and oxidative dehydrogenation of propane at 8% of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calcns., NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.
16
Davydov, A. A. Molecular Spectroscopy of Oxide Catalyst Surfaces; John Wiley & Sons Ltd.: Chichester, U.K., 2003.
Google Scholar
There is no corresponding record for this reference.
17
Wernbacher, A. M.; Kube, P.; Hävecker, M.; Schlögl, R.; Trunschke, A. Electronic and Dielectric Properties of MoV-Oxide (M1 Phase) under Alkane Oxidation Conditions. J. Phys. Chem. C 2019, 123, 13269– 13282, DOI: 10.1021/acs.jpcc.9b01273
Google Scholar
17
Electronic and dielectric properties of MoV-oxide (M1 Phase) under alkane oxidation conditions
Wernbacher, Anna M.; Kube, Pierre; Haevecker, Michael; Schloegl, Robert; Trunschke, Annette
Journal of Physical Chemistry C (2019), 123 (21), 13269-13282CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Isostructural orthorhombic oxides of the general formula (Mo,V,Te,Sb,Nb,Ta)Ox are an important class of solids, which are interesting as catalysts for oxidn. of light alkanes. The authors investigated relations between the electronic properties of MoV-oxide (orthorhombic M1 phase) and its catalytic performance in the oxidn. of ethane, propane, and n-butane. Operando cond. and permittivity measurements were performed and complemented by near-ambient-pressure XPS. In contrast to the n-type MoVTeNb-oxide, MoV-oxide showed p-type semiconducting behavior. The cond. of the sample adapted sensitively to the surrounding atm., not only to alkane chain lengths but also to reactant conversion levels. However, no measurable change in band bending depending on the alkane chain length was obsd., indicating that the gas-phase-dependent surface potential barrier, which controls the charge transfer between reactants and catalyst, is less pronounced or missing in dry alkane oxidn. feeds. The addn. of steam in propane oxidn. led to a decrease of its cond. and work function. Steam significantly influenced the surface layer on MoV-oxide, resulting in an enrichment of covalently bonded V5+ species and surface hydroxylation. A small change in the surface potential barrier induced by wet propane oxidn. feed can contribute to a modification of the bulk-surface charge transfer and improved selectivity to acrylic acid.
18
Wernbacher, A. M.; Eichelbaum, M.; Risse, T.; Cap, S.; Trunschke, A.; Schlögl, R. Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts. J. Phys. Chem. C 2019, 123, 8005– 8017, DOI: 10.1021/acs.jpcc.8b07417
Google Scholar
18
Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts
Wernbacher, Anna M.; Eichelbaum, Maik; Risse, Thomas; Cap, Sebastien; Trunschke, Annette; Schloegl, Robert
Journal of Physical Chemistry C (2019), 123 (13), 8005-8017CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The elec. cond. and its real and imaginary permittivity parameters were studied over 2 V-contg. catalysts for the selective oxidn. of n-butane to maleic anhydride. Parameter variation under isothermal conditions allowed detn. of multiple steady-state conditions for catalytic performance and charge carrier dynamics. One sample was the n-type semiconductor V2O5-x with low selectivity, and the other sample was the p-type semiconductor vanadyl pyrophosphate (VPP) with high selectivity for the target product. Well-resolved cond. parameters supported by in situ UV-visible studies allowed correlations between performance and charge carrier dynamics. A concept for interpreting the trends is presented, and consequences for further anal. work as well as for material design are derived.
19
Koch, G. Surface Conditions That Constrain Alkane Oxidation on Perovskites. ACS Catal. 2020, 10, 7007– 7020, DOI: 10.1021/acscatal.0c01289
Google Scholar
19
Surface Conditions That Constrain Alkane Oxidation on Perovskites
Koch, Gregor; Haevecker, Michael; Teschner, Detre; Carey, Spencer J.; Wang, Yuanqing; Kube, Pierre; Hetaba, Walid; Lunkenbein, Thomas; Auffermann, Gudrun; Timpe, Olaf; Rosowski, Frank; Schloegl, Robert; Trunschke, Annette
ACS Catalysis (2020), 10 (13), 7007-7020CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The crystal structure of perovskites can incorporate a wide variety of cations, which makes this class of materials so interesting for studies of links between solid-state chem. and catalysis. Perovskites are known as typical total combustion catalysts in hydrocarbon oxidn. reactions. The fundamental question that we investigate here is whether surface modifications of perovskites can lead to the formation of valuable reaction products in alkane oxidn. We studied the effect of segregated two-dimensional surface nanostructures on selectivity to propene in the oxidative dehydrogenation of propane. Manganese-based perovskites AMnO3 (A = La, Sm) were prepd. by combustion and hydrothermal synthesis. Bulk and surface structures were investigated by X-ray diffraction, temp.-programmed redn., aberration-cor. scanning transmission electron microscopy (STEM), multiwavelength Raman, and ambient-pressure XPS (AP-XPS) in combination with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Surface oxygen species responsible for C-H activation were distinguished by AP-XPS on the basis of a rigorous in situ anal. of the O 1s spectra recorded under a broad range of reaction conditions. Signals at 529.2, 530.1, 530.9, 531.2, and 531.8 eV were attributed to lattice O, defect-affected O, surface O, oxygen in carbonates, and hydroxyl groups, resp. Operando AP-XPS revealed crit. surface features, which occur under catalyst operation. The catalyst performance depends on the synthesis technique and the reaction conditions. In presence of a two-dimensional MnOx surface phase, addn. of steam to the feed resulted in an increase in selectivity to the partial oxidn. product propene to practically relevant values. The selectivity increase is related to the presence of Mn in a low oxidn. state (2+/3+), an increased concn. of hydroxyl groups, and a higher abundance of adsorbed activated oxygen species on the catalyst surface. The surface anal. of a working catalyst highlights the importance of the termination layer of polycryst. perovskites as a genuine property implemented by catalyst prepn. Such a termination layer controls the chem. properties and reactivity of perovskites. The information provides input for the development of realistic models that can be used by theory to predict functional properties.
20
Che, M.; Tench, A. J. Characterization and Reactivity of Mononuclear Oxygen Species on Oxide Surfaces. Adv. Catal. 1982, 31, 77– 133, DOI: 10.1016/S0360-0564(08)60453-8
Google Scholar
20
Characterization and reactivity of mononuclear oxygen species on oxide surfaces
Che, M.; Tench, A. J.
Advances in Catalysis (1982), 31 (), 77-133CODEN: ADCAAX; ISSN:0065-2342.
A review on the formation, stability, and reactivity of O- on the surfaces of oxide catalysts, characterization of O- by ESR and optical spectroscopy, and the reactivity of surface O2- species. 193 Refs.
21
Che, M.; Tench, A. J. Characterization and Reactivity of Molecular Oxygen Species on Oxide Surfaces. Adv. Catal. 1983, 32, 1– 148, DOI: 10.1016/S0360-0564(08)60439-3
Google Scholar
21
Characterization and reactivity of molecular oxygen species on oxide surfaces
Che, M.; Tench, A. J.
Advances in Catalysis (1983), 32 (), 1-148CODEN: ADCAAX; ISSN:0065-2342.
A review with over 470 refs.
22
Nováková, J. Isotopic Exchange of Oxygen ¹⁸O between the Gaseous Phase and Oxide Catalysts. Catal. Rev. 1971, 4, 77– 113, DOI: 10.1080/01614947108075486
Google Scholar
There is no corresponding record for this reference.
23
Choi, S. O.; Penninger, M.; Kim, C. H.; Schneider, W. F.; Thompson, L. T. Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La_1–xSr_xCoO₃ Perovskite Catalysts. ACS Catal. 2013, 3, 2719– 2728, DOI: 10.1021/cs400522r
Google Scholar
23
Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La1-xSrxCoO3 Perovskite Catalysts
Choi, Sang Ok; Penninger, Michael; Kim, Chang Hwan; Schneider, William F.; Thompson, Levi T.
ACS Catalysis (2013), 3 (12), 2719-2728CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
NO oxidn. rates over La1-xSrxCoO3 (x = 0-0.3) perovskite catalysts are reported as a function of Sr doping in the absence and presence of NO2 in the feed. Sr substitution is found to increase the rate of oxidn. and to diminish the inhibitory influence of NO2. Temp. programmed desorption and isotopic exchange (TPIE) expts. were used to identify surface species and oxygen exchange processes expected to correlate with NO oxidn. activity. Oxygen exchange in the LaCoO3 perovskites occurred primarily through a heteroexchange process that was enhanced by doping with Sr. D. functional theory (DFT) calcns. were used to further investigate the oxygen exchange processes on (100) facets of undoped and doped LaCoO3. Vacancy formation is predicted to be more facile on CoO2-terminated than LaO-terminated surfaces. The Sr dopant segregates to the LaO-terminated surface and diminishes oxygen bonding consistent with the TPIE results. The results suggest a model in which multiple oxygen species contribute to low- and high-temp. oxygen exchange.
24
Winter, E. R. S. The Reactivity of Oxide Surfaces. Adv. Catal. 1958, 10, 196– 241, DOI: 10.1016/S0360-0564(08)60408-3
Google Scholar
24
Reactivity of oxide surfaces
Winter, E. R. S.
Advances in Catalysis (1958), 10 (), 196-241CODEN: ADCAAX; ISSN:0360-0564.
Reviews with references. Cf. C.A. 51, 16014f.
25
Winter, E. R. S. Exchange Reactions of Oxides. Part IX. J. Chem. Soc. A: Inorg., Phys., Theor. 1968, 2889– 2902, DOI: 10.1039/j19680002889
Google Scholar
25
Exchange reactions of oxide. IX
Winter, Edgar R. S.
Journal of the Chemical Society [Section] A: Inorganic, Physical, Theoretical (1968), (12), 2889-902CODEN: JCSIAP; ISSN:0022-4944.
The kinetics of exchange of 18O between enriched O gas and 38 inorg. oxide and 1 oxy-acid salt has been examd. in detail. Most of the exchange reactions occur by a dissociative at. mechanism confined to the surface layer of O ions. For these there is a strong compensation effect between A0 and E in the rate expression: E shows a systematic fall with increasing size of the unit-crystal cell and related crystal parameters: the plots sep. the oxide into structure-dependent groups. The slow stage is the desorption of O. The oxide PbO, PdO, AgO, and CuO exchange the surface layer by a mol. reactions which also occurs to an appreciable extent, together with the at. mechanism, on a no. of other oxide. Na2WO4, V2O5, MoO3, and WO3 exchange the whole of the bulk O with the gas phase by a combination of both mechanisms. SiO2 and GeO2 are inactive. Possible mechanisms for the two main reactions are discussed.
26
Klier, K.; Nováková, J.; Jíru, P. Exchange Reactions of Oxygen between Oxygen Molecules and Solid Oxides. J. Catal. 1963, 2, 479– 484, DOI: 10.1016/0021-9517(63)90003-4
Google Scholar
26
Exchange reactions of oxygen between oxygen molecules and solid oxides
Klier, K.; Novakova, J.; Jiru, P.
Journal of Catalysis (1963), 2 (6), 479-84CODEN: JCTLA5; ISSN:0021-9517.
A thoretical treatment of the kinetics of isotopic exchange reactions between O mols. and a solid oxide is developed. Good agreement with expt. was found for changes in concn. of 18O18O, 16O18O, and 16O16O with time over MgO.
27
Boreskov, G. K. The Catalysis of Isotopic Exchange in Molecular Oxygen. Adv. Catal. 1965, 15, 285– 339, DOI: 10.1016/S0360-0564(08)60556-8
Google Scholar
There is no corresponding record for this reference.
28
Doornkamp, C.; Clement, M.; Gao, X.; Deo, G.; Wachs, I. E.; Ponec, V. The Oxygen Isotopic Exchange Reaction on Vanadium Oxide Catalysts. J. Catal. 1999, 185, 415– 422, DOI: 10.1006/jcat.1999.2490
Google Scholar
28
The Oxygen Isotopic Exchange Reaction on Vanadium Oxide Catalysts
Doornkamp, C.; Clement, M.; Gao, X.; Deo, G.; Wachs, I. E.; Ponec, V.
Journal of Catalysis (1999), 185 (2), 415-422CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
The reactivity of lattice oxygen of vanadium oxide catalysts was studied with the oxygen isotopic exchange reaction. The reactivity of pure V2O5 is compared with the reactivity of Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, δ-VOPO4, and (VO)2P2O7. According to their behavior in the oxygen exchange reaction, two types of vanadium oxide catalysts could be distinguished. The first type of catalysts only showed exchange activity in the R2 exchange mechanism and the second type showed activity in both the R1 and R2 exchange mechanisms (mechanisms in which, resp., one or two oxygen atoms of the gas phase mol. are exchanged with oxygen atoms of the metal oxide). The catalysts which belong to the first group are bulk V2O5 and δ-VOPO4 and the catalysts which belong to the second group are Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, and (VO)2P2O7. If only the R2 mechanism is obsd. then diffusion of lattice oxygen is probably faster than when both the R1 and R2 mechanisms are obsd. The activity of the supported vanadium oxide catalysts in the oxygen exchange reaction is dependent on the support. The reactivity order is V2O5/TiO2>V2O5/Al2O3∼V2O5/SiO2. (c) 1999 Academic Press.
29
Doornkamp, C.; Clement, M.; Ponec, V. The Isotopic Exchange Reaction of Oxygen on Metal Oxides. J. Catal. 1999, 182, 390– 399, DOI: 10.1006/jcat.1998.2377
Google Scholar
29
The Isotopic Exchange Reaction of Oxygen on Metal Oxides
Doornkamp, C.; Clement, M.; Ponec, V.
Journal of Catalysis (1999), 182 (2), 390-399CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
Two methods have been used to det. the individual rate consts. of the isotopic exchange reaction of oxygen on metal oxides. The best way to det. the three rate consts. is to use the kinetic model of Klier and co-workers and fit the equations to the exptl. data. The method of Tsuchiya and co-workers can be used to check the results obtained by the method of Klier and co-workers. The three rate consts., R0, R1, and R2, of the three different exchange mechanisms are detd. for the period IV metal oxides at various temps. The rate consts. are correlated with parameters characterizing the oxide, i.e. the position of the metal element in the periodic table and the av. metal-oxygen bond strength. (c) 1999 Academic Press.
30
Wachs, I. E.; Jehng, J.-M.; Deo, G.; Weckhuysen, B. M.; Guliants, V. V.; Benziger, J. B.; Sundaresan, S. Fundamental Studies of Butane Oxidation over Model-Supported Vanadium Oxide Catalysts: Molecular Structure-Reactivity Relationships. J. Catal. 1997, 170, 75– 88, DOI: 10.1006/jcat.1997.1742
Google Scholar
30
Fundamental studies of butane oxidation over model-supported vanadium oxide catalysts: molecular structure-reactivity relationships
Wachs, Israel E.; Jehng, Jih-Mirn; Deo, Goutam; Weckhuysen, Bert M.; Guliants, V. V.; Benziger, J. B.; Sundaresan, S.
Journal of Catalysis (1997), 170 (1), 75-88CODEN: JCTLA5; ISSN:0021-9517. (Academic)
The oxidn. of n-butane to maleic anhydride was investigated over a series of model-supported vanadia catalysts where the vanadia phase was present as a two-dimensional metal oxide overlayer on the different oxide supports (TiO2, ZrO2, CeO2, Nb2O5, Al2O3, and SiO2). No correlation was found between the properties of the terminal V=O bond and the butane oxidn. turnover frequency (TOF) during in situ Raman spectroscopy study. Furthermore, neither the n-butane oxidn. TOF nor maleic anhydride selectivity was related to the extent of redn. of the surface vanadia species. The n-butane oxidn. TOF was essentially independent of the surface vanadia coverage, suggesting that the n-butane activation requires only one surface vanadia site. The maleic anhydride TOF, however, increased by a factor of 2-3 as the surface vanadia coverage was increased to monolayer coverage. The higher maleic anhydride TOF at near monolayer coverages suggests that a pair of adjacent vanadia sites may efficiently oxidize n-butane to maleic anhydride, but other factors may also play a contributing role (increase in surface Bronsted acidity and decrease in the no. of exposed support cation sites). Varying the specific oxide support changed the n-butane oxidn. TOF by ca. 50(Ti > Ce > Zr ∼ Nb > Al > Si) as well as the maleic anhydride selectivity. The maleic anhydride selectivity closely followed the Lewis acid strength of the oxide support cations, Al > Nb > Ti > Si > Zr > Ce. The addn. of acidic surface metal oxides (W, Nb, and P) to the surface vanadia layer was found to have a beneficial effect on the n-butane oxidn. TOF and the maleic anhydride selectivity. The creation of bridging V-O-P bonds had an esp. pos. effect on the maleic anhydride selectivity.
31
Klug, C. A.; Kroeker, S.; Aguiar, P. M.; Zhou, M.; Stec, D. F.; Wachs, I. E. Insights into Oxygen Exchange between Gaseous O₂ and Supported Vanadium Oxide Catalysts Via ¹⁷O NMR. Chem. Mater. 2009, 21, 4127– 4134, DOI: 10.1021/cm802680x
Google Scholar
31
Insights into Oxygen Exchange Between Gaseous O2 and Supported Vanadium Oxide Catalysts via 17O NMR
Klug, Christopher A.; Kroeker, Scott; Aguiar, Pedro M.; Zhou, Min; Stec, Donald F.; Wachs, Israel E.
Chemistry of Materials (2009), 21 (18), 4127-4134CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
Vanadium oxide ref. compds., KVO3 and V2O5, and supported vanadium oxide catalysts (Al2O3, TiO2, and SiO2) were investigated using magic angle sample spinning 17O NMR. All samples were 17O-enriched using gas-solid exchange. Extn. of chem. shift and quadrupolar coupling information for the model compds. KVO3 and V2O5 was performed via the simulation of MAS spectra obtained in one-pulse expts. and the observations were consistent with their known bulk structures. For the supported vanadia catalysts, it was found that the oxygen exchange process is dominated by 17O signal from the catalyst oxide supports. Spectra obtained via rotor-synchronized spin echoes revealed addnl. wide lines for Al2O3 and TiO2 supported vanadia catalysts that arise from 17O in the surface vanadia species of the catalysts. Addnl. 17O-51V TRAPDOR (TRAnsfer of Populations in DOuble Resonance) expts. support this assignment. The wide lines suggest that the local environments of the 17O nuclei assocd. with the dehydrated surface vanadia species are extremely heterogeneous and fall in the range of oxygen in singly (V=O) and/or doubly coordinated environments (V-O-V or V-O-Support). The relatively small total amt. of 17O assocd. with the surface vanadia species contrasts with oxygen exchange models which commonly assume only the surface vanadium oxide layer is involved. These results demonstrate that the isotopic exchange of mol. O2 with supported metal oxide catalysts, esp. supported vanadia catalysts, is a much more complex process than originally perceived.
32
Avdeev, V. I.; Bedilo, A. F. Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VO_x/TiO₂. J. Phys. Chem. C 2013, 117, 2879– 2887, DOI: 10.1021/jp311322b
Google Scholar
32
Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VOx/TiO2
Avdeev, Vasilii I.; Bedilo, Alexander F.
Journal of Physical Chemistry C (2013), 117 (6), 2879-2887CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Detailed mol. mechanisms of oxygen isotopic exchange over VOx/TiO2 catalyst following the R0, R1, and R2 mechanisms were studied using periodic DFT anal. of possible pathways by the CI-NEB method. The electronic structures of surface VOx species formed on the VOx/TiO2 model surface after interaction of mol. oxygen with fully oxidized O=V5+-O-V5+=O sites and reduced V3+-O-V3+ sites were analyzed. We found a no. of metastable surface structures that are potential intermediates in the exchange reaction pathways. We present evidence that adsorption of two gas-phase oxygen mols. on a reduced V3+-O-V3+ site leads to the formation of a superoxide complex, followed by its transformation into a peroxide complex with low activation energy about E* = 0.04 eV (0.92 kcal/mol). Subsequent transformation of this surface superoxide-peroxide species follows the Langmuir-Hinshelwood mechanism without participation of lattice oxygen along the R0 reaction pathway. We demonstrate that adsorption of mol. oxygen on fully oxidized O=V5+-O-V5+=O sites results in the formation of either monodentate V<(O3) or bidentate V<(O3)>V surface ozonide species. Their subsequent transformations result in oxygen isotopic exchange following the R1 or R2 mechanisms with the activation energies in the range of 1.44 to 1.64 eV for the R1 mechanism and 1.81 eV for the R2 one. These processes follow the Eley-Rideal mechanism with participation of one or two lattice oxygen atoms, correspondingly.
33
Avdeev, V. I.; Bedilo, A. F. Electronic Structure of Oxygen Radicals on the Surface of VO_x/TiO₂ Catalysts and Their Role in Oxygen Isotopic Exchange. J. Phys. Chem. C 2013, 117, 14701– 14709, DOI: 10.1021/jp404921d
Google Scholar
33
Electronic Structure of Oxygen Radicals on the Surface of VOx/TiO2 Catalysts and Their Role in Oxygen Isotopic Exchange
Avdeev, Vasilii I.; Bedilo, Alexander F.
Journal of Physical Chemistry C (2013), 117 (28), 14701-14709CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The electronic structure of oxygen radicals formed by adsorption of gas-phase oxygen on partially reduced sites of supported vanadium oxide catalyst V4+Ox/TiO2 has been studied by periodic DFT. The unpaired electron d. in the radicals is transferred from the paramagnetic V4+(3d1) ion to the adsorbed oxygen atoms resulting in the formation of surface oxygen radicals: at. O-, superoxide O2-, and ozonide O3-. These radical species exhibit higher reactivity compared to the surface oxygen species stabilized on fully oxidized diamagnetic V5+(3d0) ions. Oxygen isotopic exchange over O- radicals has been investigated by the climbing image nudged elastic band (CI-NEB) method. We show that mol. oxygen can exchange with the lattice oxygen of the surface paramagnetic radicals V5+O- with low activation energy of about 14 kcal/mol, close to the value exptl. obsd. for some heterolytic R1 oxygen exchange reactions on vanadia catalysts. The obtained data suggest that O- radicals formed as short-lived intermediates at elevated temps. are likely to be the active sites of the oxygen exchange following the R1 mechanism. The properties of oxygen radicals and their possible role in catalytic oxidn. processes taking place over bulk and supported metal oxide catalysts are discussed. It is suggested that oxygen radicals can be the active species in catalytic oxidn. reactions.
34
Koranne, M. M.; Goodwin, J. G.; Marcelin, G. Oxygen Involvement in the Partial Oxidation of Methane on Supported and Unsupported V₂O₅. J. Catal. 1994, 148, 378– 387, DOI: 10.1006/jcat.1994.1218
Google Scholar
34
Oxygen involvement in the partial oxidation of methane on supported and unsupported V2O5
Koranne, Manoj M.; Goodwin, James G., Jr.; Marcelin, George
Journal of Catalysis (1994), 148 (1), 378-87CODEN: JCTLA5; ISSN:0021-9517.
O exchange of O2 with V2O5/SiO2, V2O5/Al2O3, V2O5, SiO2, and Al2O3 was studied using steady-state isotopic transient kinetic anal. It was found that bulk V2O5, V2O5/SiO2, V2O5/Al2O3, and Al2O3 exhibited some oxygen exchange capability, whereas SiO2 exhibited negligible oxygen exchange capability under the conditions used. Oxygen exchange reactions were also studied over supported vanadia catalysts under steady-state oxidn. reaction conditions. O exchange of O2 with V2O5/SiO2 increased significantly in the presence of methane. This is attributed to a redox reaction making the surface more active for O exchange in the presence of the methane. The involvement of catalyst oxygen in the formation of the products HCHO, CO, and CO2 was demonstrated. However, an estn. of the contribution of the lattice oxygen in the formation of the products is complicated by the presence of secondary O exchange. The total amt. of 16O exchanged with the feed O2 and the products indicates that the oxygen assocd. with silica or the vanadia-silica interface is also involved in the exchange process. In general, O exchange behavior of various product species with V2O5/Al2O3 was found to be similar to that with V2O5/SiO2. However, unlike V2O5/SiO2, the O exchange of O2 with V2O5/Al2O3 did not increase significantly in the presence of methane. This was attributed to the differences in the interactions of vanadia with silica and alumina. It is speculated that the O assocd. with highly dispersed tetrahedral surface vanadia is involved in a primary oxidn. reaction, whereas O assocd. with the bulk-like vanadia and with the support is involved either in secondary oxidn. reactions of HCHO or CO or in secondary O exchange of various O contg. product species. Caution must be taken in making any conclusions about the source of reactive oxygen during partial oxidn. on oxide catalysts based on isotopic oxygen studies due to the ease of secondary oxygen exchange.
35
Kera, Y.; Teratani, S.; Hirota, K. Infrared Spectra of Surface V═O Bond of Vanadium Pentoxide. Bull. Chem. Soc. Jpn. 1967, 40, 2458– 2458, DOI: 10.1246/bcsj.40.2458
Google Scholar
35
Infrared spectra of surface vanadium-oxygen double bond of vanadium pentoxide
Kera, Yoshiya; Teratani, Shousuke; Hirota, Kozo
Bulletin of the Chemical Society of Japan (1967), 40 (10), 2458CODEN: BCSJA8; ISSN:0009-2673.
V2O5 was treated for 24 hrs. at 490° with a mixt. of CO2 and O, both contg. 60 at. % 18O. The ir spectra of V2O5 taken before and after treatment show a new absorption max. at 962 cm.-1 after treatment which increases in intensity with increased 18O concn. in the CO2 and did not appear in V2O5 treated with 18O alone. The concn. of 18O in CO2 diminished while that in O remained const. This new band is attributed to the V:18O stretching band in agreement with calcd. values, and supports the presence of exchangeable O in the surface V:O bond.
36
Hirota, K.; Kera, Y.; Teratani, S. Carbon Monoxide Oxidation with an Oxygen Tracer over a Vanadium Pentoxide Catalyst. J. Phys. Chem. 1968, 72, 3133– 3141, DOI: 10.1021/j100855a010
Google Scholar
36
Carbon monoxide oxidation with an oxygen tracer over a vanadium pentoxide catalyst
Hirota, Kozo; Kera, Yoshiya; Teratani, Shousuke
Journal of Physical Chemistry (1968), 72 (9), 3133-41CODEN: JPCHAX; ISSN:0022-3654.
The oxidn. of CO with gaseous O on a powd. V2O5 catalyst was studied at 345-410° by using 18O (about 3 atom %) as the tracer. When the lattice O of the catalyst was partially substituted by concd. 18O before the expt., the percentage of 18O in the produced CO2 changed gradually during the oxidn., in accordance with the 18O concn. in the catalyst, even though the percentage of 18O in oxygen and in CO was practically invariant. When a mixt. of O and CO2, both contg. about 60 atom % of 18O, was brought into contact with the catalyst at 370°, only the O in CO2 was exchangeable with the lattice O. The exchange rate of CO2 was increased by the presence of CO. The oxidn. of CO on V2O5 was explained on the basis of the oxidn.-redn. mechanism. The relative rate of each elementary step and the surface intermediates during the reaction are discussed, and a detailed reaction scheme is proposed.
37
Kera, Y.; Hirota, K. Infrared Spectroscopic Study of Oxygen Species in Vanadium Pentoxide with Reference to Its Activity in Catalytic Oxidation. J. Phys. Chem. 1969, 73, 3973– 3981, DOI: 10.1021/j100845a070
Google Scholar
37
Infrared spectroscopic study of oxygen species in vanadium pentoxide with reference to its activity in catalytic oxidation
Kera, Yoshiya; Hirota, Kozo
Journal of Physical Chemistry (1969), 73 (11), 3973-81CODEN: JPCHAX; ISSN:0022-3654.
To clarify the nature of the exchangeable O in V2O5 catalyst during the oxidn. reaction, an ir technique combined with the 18O tracer technique w as applied to the samples isotopically substituted by treatment with 18 O labeled CO2. The bonds appearing at 1019 and 818 cm-1 can be identified to be the V:O and V-O-V groups, resp., because the corresponding shift of both bonds appeared by isotopic substitution. Exchange expts. between gaseous CO2 and V2O5 were carried out at 290, 370, 410, and 450°, and the 18O balance was investigated, taking the intensity of the above ir bands into account. Thus, the behavior of the O species at the surface and in the bulk could be detd. (a) Direct exchange of the oxygen on the surface V:O groups with gaseous O is very rapid at the initial stage of reaction. (b) Oxygen exchange between V:O and V-O-V groups may occur rapidly near the lattice dislocations on the (010) surface as well as on other surfaces. (c) The exchange within the V-O-V net plane is the easiest of all the processes, so that it becomes predominant in the reaction even at the intermediate stage. (d) The exchange between the V:O and V-O-V groups on the surfaces is important from the standpoint of catalytic oxidn.
38
Fletcher, W. H.; Rayside, J. S. High Resolution Vibrational Raman Spectrum of Oxygen. J. Raman Spectrosc. 1974, 2, 3– 14, DOI: 10.1002/jrs.1250020102
Google Scholar
38
High resolution vibrational Raman spectrum of oxygen
Fletcher, William H.; Rayside, John S.
Journal of Raman Spectroscopy (1974), 2 (1), 3-14CODEN: JRSPAF; ISSN:0377-0486.
The fundamental vibrational band of O2 was examd. with a resoln. of 0.05 cm-1 for the Q branch and 0.40 cm-1 for the S and O branches. All lines of the Q branch were clearly resolved except Q(1) and Q(3). Calcd. mol. parameters agree with those previously reported from microwave and electronic spectra. Line width measurements made in the Q branch and on 3 S branch lines, using a resoln. of 0.05 cm-1, are in fair agreement with previously measured and calcd. line widths for pure rotational Raman lines.
39
Pushkarev, V. V.; Kovalchuk, V. I.; d’Itri, J. L. Probing Defect Sites on the CeO₂ Surface with Dioxygen. J. Phys. Chem. B 2004, 108, 5341– 5348, DOI: 10.1021/jp0311254
Google Scholar
39
Probing Defect Sites on the CeO2 Surface with Dioxygen
Pushkarev, Vladimir V.; Kovalchuk, Vladimir I.; D'Itri, Julie L.
Journal of Physical Chemistry B (2004), 108 (17), 5341-5348CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
In situ Raman spectroscopy of adsorbed dioxygen was used to characterize electron defects on the surface of nanocryst. cerium oxide that was partially reduced with H2 and CO. Via 16O/18O isotope substitution, the bands in the range of 1135-1127 and 877-831 cm-1 were assigned to the O-O stretching vibration of dioxygen species bound to one- and two-electron defects on the CeO2 surface to form superoxide (O2-) and peroxide (O22-) species, resp. A band at 357 cm-1 was attributed to the cerium-oxygen vibration of the adsorbed superoxides, O2-, whereas the bands at 538 and 340 cm-1 were assigned to the asym. and sym. cerium-oxygen vibrations of the surface peroxides, O22-, resp. The dynamics of the defect annihilation that results from surface reoxidn. by adsorbed dioxygen species during temp.-programmed expts. allowed peroxide species adsorbed on isolated and aggregated two-electron defects to be distinguished. A general approach to investigate the reactivity of different surface dioxygen species toward reductants was demonstrated using CO oxidn. as a probe reaction.
40
Choi, Y. M.; Abernathy, H.; Chen, H.-T.; Lin, M. C.; Liu, M. Characterization of O₂–CeO₂ Interactions Using in Situ Raman Spectroscopy and First-Principle Calculations. ChemPhysChem 2006, 7, 1957– 1963, DOI: 10.1002/cphc.200600190
Google Scholar
40
Characterization of O2-CeO2 interactions using in situ Raman spectroscopy and first-principle calculations
Choi, Y. M.; Abernathy, Harry; Chen, Hsin-Tsung; Lin, M. C.; Liu, Meilin
ChemPhysChem (2006), 7 (9), 1957-1963CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
Interactions between O2 and CeO2 are examd. exptl. using in situ Raman spectroscopy and theor. using d.-functional slab-model calcns. Two distinct O bands appear at 825 and 1131 cm-1, corresponding to peroxo- and superoxo-like species, resp., when partially reduced CeO2 is exposed to 10% O2. Periodic d.-functional theory (DFT) calcns. aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO2. The O2 adsorption energies on unreduced CeO2 surfaces are endothermic (0.91 < ΔEads < 0.98 eV), while those on reduced surfaces are exothermic (-4.0 < ΔEads < -0.9 eV), depending on other relevant surface processes such as chemisorption and diffusion into the bulk. Partial redn. of surface Ce4+ to Ce3+ (together with formation of O vacancies) alters geometrical parameters and, accordingly, leads to a shift in the vibrational frequencies of adsorbed O species compared to those on unreduced CeO2. Also, the location of O vacancies affects the formation and subsequent dissocn. of O species on the surfaces. DFT predictions of the energetics support the exptl. observation that the reduced surfaces are energetically more favorable than the unreduced surfaces for O adsorption and redn.
41
Wu, Z.; Dai, S.; Overbury, S. H. Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts. J. Phys. Chem. C 2010, 114, 412– 422, DOI: 10.1021/jp9084876
Google Scholar
41
Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts
Wu, Zili; Dai, Sheng; Overbury, Steven H.
Journal of Physical Chemistry C (2010), 114 (1), 412-422CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The mol. structure of SiO2-supported V oxide (VOx) catalysts over wide range of surface VOx d. (0.0002-8 V/nm2) was studied in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV-visible diffuse reflectance spectroscopy. Resonance Raman scattering is clearly obsd. using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the 3 visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm-1) whose assignments were controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V=O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of 2 different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quant. estn. shows that the 2 different monomeric VOx species coexist on SiO2 surface from very low VOx loadings and transform to cryst. V2O5 at VOx loadings above the monolayer. It is postulated that 1 of the 2 monomeric VOx species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The 2 VOx species show similar redn.-oxidn. behavior and may both participate in redox reactions catalyzed by VOx/SiO2 catalysts. This study demonstrates the advantages of multiwavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal-oxide catalysts.
42
Weckhuysen, B. M.; Jehng, J.-M.; Wachs, I. E. In Situ Raman Spectroscopy of Supported Transition Metal Oxide Catalysts: ¹⁸O₂–¹⁶O₂ Isotopic Labeling Studies. J. Phys. Chem. B 2000, 104, 7382– 7387, DOI: 10.1021/jp000055n
Google Scholar
42
In Situ Raman Spectroscopy of Supported Transition Metal Oxide Catalysts: 18O2-16O2 Isotopic Labeling Studies
Weckhuysen, Bert M.; Jehng, Jih-Mirn; Wachs, Israel E.
Journal of Physical Chemistry B (2000), 104 (31), 7382-7387CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
The isothermal isotopic exchange reaction of 18O2 with 16O of CrO3, MoO3, Nb2O5, WO3, V2O5, and Re2O7 supported on ZrO2 has been investigated with in situ laser Raman spectroscopy. Isotopic exchange of the oxygen atoms of the supported transition metal oxides with 18O2 is difficult and requires several successive redn.-18O2 reoxidn. cycles at relatively high temps. The Raman spectroscopy data reveal that all the supported transition metal oxides are present as a monooxo species on ZrO2. This finding is consistent with the shifts calcd. from the isotopic ratios for a simple diat. oscillator, with the corresponding IR spectra of the same catalysts and with the vibrational frequencies of several monooxo ref. compds. On this basis, coordination models of the mol. structures are proposed for CrO3/ZrO2, MoO3/ZrO2, Nb2O5/ZrO2, WO3/ZrO2, V2O5/ZrO2, and Re2O7/ZrO2 catalysts under dehydrated conditions.
43
Lee, E. L.; Wachs, I. E. In Situ Raman Spectroscopy of SiO₂-Supported Transition Metal Oxide Catalysts: An Isotopic ¹⁸O-¹⁶O Exchange Study. J. Phys. Chem. C 2008, 112, 6487– 6498, DOI: 10.1021/jp076485w
Google Scholar
43
In Situ Raman Spectroscopy of SiO2-Supported Transition Metal Oxide Catalysts: An Isotopic 18O-16O Exchange Study
Lee, Edward L.; Wachs, Israel E.
Journal of Physical Chemistry C (2008), 112 (16), 6487-6498CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
The mol. structures of dehydrated group 5-7 transition metal oxides (V2O5, Nb2O5, CrO3, MoO3, WO3, Re2O7) supported on SiO2 were investigated with time-resolved 18O-16O exchange in situ Raman spectroscopy measurements. The supported group 5-7 dehydrated surface transition metal oxides were exclusively present as isolated species on SiO2 because of the absence of bridging M-O-M vibrations. The SiO2-supported group 5 (VOx and NbOx) surface metal oxides exhibit band splitting into two Raman vibrations (M:16O and M:18O), which is consistent with monoxo surface O:M(-O-Si)3 species. The SiO2-supported group 6 (CrOx, MoOx, and WOx) surface metal oxides consist of both monoxo O:M(-O-Si)4 and dioxo (O:)2M(-O-Si)2 structures. The dioxo surface species give rise to triplet band splitting corresponding to M(:16O)2, M(:18O)2, and 18O:M:16O. Identification of the intermediate surface 18O:M:16O structure was guided by recent DFT calcns. The SiO2-supported group 7 (ReOx) metal oxide system exclusively contains trioxo surface (O:)3Re-O-Si species that give rise to quadruplet band splitting (Re(:16O)3, 18O:Re(:16O)2, (18O:)2Re:16O, and (18O:)3Re) during isotopic oxygen exchange. Excellent prediction was also achieved for the isotopic shifts for the completely 18O-exchanged surface metal oxide structures with a simple diat. oscillator model. The isotopic exchange studies reveal, for the first time, the exact no. of Raman bands for surface monoxo, dioxo, and trioxo metal oxide structures, their positions, and their band splitting characteristics during isotopic 18O-16O exchange.
44
Moisii, C.; van de Burgt, L. J.; Stiegman, A. E. Resonance Raman Spectroscopy of Discrete Silica-Supported Vanadium Oxide. Chem. Mater. 2008, 20, 3927– 3935, DOI: 10.1021/cm800095g
Google Scholar
44
Resonance Raman Spectroscopy of Discrete Silica-Supported Vanadium Oxide
Moisii, Cristina; van de Burgt, Lambertus J.; Stiegman, A. E.
Chemistry of Materials (2008), 20 (12), 3927-3935CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
V oxide deposited as discrete oxovanadium groups, [(-O)3V=O], in transparent SiO2 xerogels were studied by resonance Raman spectroscopy. Spectra were collected at 351 and 257 nm excitation into two distinct absorption bands of the oxovanadium site. Three new bands assocd. with vibrations of the V oxide site were obsd. at 496, 568, and 720 cm-1. From these addnl. modes and the previously known vibrations at 1064, 1033, and 923 cm-1 an empirical force field was detd. from which a normal-mode anal. of the primary stretching vibrations of the V oxo group was carried out. This anal. indicates that for most of the obsd. bands the interfacial Si-O-V stretches are the primary component, and in fact, only the weak band at 923 cm-1 was dominated by the terminal V=O stretch. Shifts in the band positions with 18O isotopic enrichment are in general agreement with the normal-mode anal., also, the enrichment indicates that the bridging groups are generally quite labile to substitution.
45
Magg, N. Vibrational Spectra of Alumina- and Silica-Supported Vanadia Revisited: An Experimental and Theoretical Model Catalyst Study. J. Catal. 2004, 226, 88– 100, DOI: 10.1016/j.jcat.2004.04.021
Google Scholar
45
Vibrational spectra of alumina- and silica-supported vanadia revisited: An experimental and theoretical model catalyst study
Magg, Norbert; Immaraporn, Boonchuan; Giorgi, Javier B.; Schroeder, Thomas; Baumer, Marcus; Dobler, Jens; Wu, Zili; Kondratenko, Evgenii; Cherian, Maymol; Baerns, Manfred; Stair, Peter C.; Sauer, Joachim; Freund, Hans-Joachim
Journal of Catalysis (2004), 226 (1), 88-100CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
Oxide-supported vanadia particles were prepd. via evapn. of vanadium metal in an oxygen ambient. As support oxides, we have employed thin, well-ordered alumina and silica films grown on top of NiAl(110) and Mo(112) surfaces. According to our anal., the vanadia particles exhibit very similar morphol. on both supports but differ in the extent of particle-support interactions. It is shown that these differences in the vanadia-support interface region strongly affect the CO adsorption behavior of the particles. The measured vibrational spectra of the model systems are interpreted on the basis of DFT calcns. for model compds. and surface models for both the vanadia/silica and the vanadia/alumina system. The combined information is then compared with Raman spectra of real catalytic materials such as vanadia supported over δ-Al2O3 and mesoporous SiO2 (MCM-41) taken at different laser wavelengths. A consistent interpretation is developed, which shows that the accepted interpretation of vibrational spectra from vanadia catalysts must be revised.
46
Döbler, J.; Pritzsche, M.; Sauer, J. Vibrations of Silica Supported Vanadia: Variation with Particle Size and Local Surface Structure. J. Phys. Chem. C 2009, 113, 12454– 12464, DOI: 10.1021/jp901774t
Google Scholar
46
Vibrations of Silica Supported Vanadia: Variation with Particle Size and Local Surface Structure
Dobler, Jens; Pritzsche, Marc; Sauer, Joachim
Journal of Physical Chemistry C (2009), 113 (28), 12454-12464CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
D. functional theory is applied to mol. models and embedded cluster models for vanadia on three different cryst. silica supports that span the range of local structures found in amorphous silica. For monomeric vanadyl sites, three different types of vibrational modes exist. Contrary to previous assignments, the V-O-Si in-phase modes occur at highest frequencies, 1086-1020 cm-1, whereas the vanadyl modes are found between 1047-1013 cm-1. The V-O-Si out-of-phase modes have the lowest frequencies in the 962-873 cm-1 range. For dimeric and polymeric sites, the frequencies are similar, but an addnl. V-O-V mode is found between 870 and 770 cm-1 and not in the 940-900 cm-1 range as assumed earlier. The vibrational frequencies of cluster and embedded cluster models are in general agreement, except that the embedded clusters predict a strong mixing of vanadia modes with modes of the silica support. This mixing changes the normal modes but has only a small effect on the frequencies. Vanadia on very special support models, like a periodic SiO2 slab consisting of hexagonal prisms, shows distinctly different frequencies with 50 cm-1 blue-shifted V-O-Si frequencies.
47
Nitsche, D.; Hess, C. Normal Mode Analysis of Silica-Supported Vanadium Oxide Catalysts: Comparison of Theory with Experiment. Catal. Commun. 2014, 52, 40– 44, DOI: 10.1016/j.catcom.2014.04.008
Google Scholar
47
Normal mode analysis of silica-supported vanadium oxide catalysts: Comparison of theory with experiment
Nitsche, David; Hess, Christian
Catalysis Communications (2014), 52 (), 40-44CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
The vibrational structure of silica-supported vanadium oxide species has been studied by normal mode anal. using polyhedral oligomeric silsesquioxanes (POSSs) to describe the silica support. The anal. reveals that the vanadium oxide-related vibrational bands are characterized by significant contributions of several force consts. Their discussion in terms of single bond-unit designators is therefore not adequate. The consideration of the silica support is shown to be of importance for the vibrational spectrum. The theor. results are fully consistent with exptl. data for a silica-supported vanadium oxide catalyst with a vanadium coverage of 0.7 atoms/nm2.
48
Oyama, S. T.; Went, G. T.; Lewis, K. B.; Bell, A. T.; Somorjai, G. A. Oxygen Chemisorption and Laser Raman Spectroscopy of Unsupported and Silica-Supported Vanadium Oxide Catalysts. J. Phys. Chem. 1989, 93, 6786– 6790, DOI: 10.1021/j100355a041
Google Scholar
48
Oxygen chemisorption and laser Raman spectroscopy of unsupported and silica-supported vanadium oxide catalysts
Oyama, S. Ted; Went, Gregory T.; Lewis, Kenneth B.; Bell, Alexis T.; Somorjai, Gabor A.
Journal of Physical Chemistry (1989), 93 (18), 6786-90CODEN: JPCHAX; ISSN:0022-3654.
An O chemisorption method was developed for measuring the active surface area of supported and unsupported V2O5 following redn. in H. To achieve complete redn. of the V2O5 surface without reducing the bulk, redn. must be carried out at 640 K. O uptakes of unsupported samples reduced at close to this temp. yield an O atom site d. of 3.2 × 1018 m-2, a value near that expected for a monolayer. The same O chemisorption technique was applied to SiO2-supported V2O5. Laser Raman spectroscopy confirms that, near 640 K, O chemisorbs primarily at the surface of the dispersed V2O5, but does not exchange with the bulk of the oxide. For very low wt. loadings, a limiting stoichiometry of one adsorbed O atom per V atom is obtained. This stoichiometry is used to calc. dispersions of 93-50% for supported V2O5 samples of 0.3-9.8% wt. loading.
49
Ono, T.; Tanaka, Y.; Takeuchi, T.; Yamamoto, K. Characterization of K-Mixed V₂O₅ Catalyst and Oxidative Dehydrogenation of Propane on It. J. Mol. Catal. A Chem. 2000, 159, 293– 300, DOI: 10.1016/S1381-1169(00)00218-1
Google Scholar
49
Characterization of K-mixed V2O5 catalyst and oxidative dehydrogenation of propane on it
Ono, Takehiko; Tanaka, Yuhmo; Takeuchi, Takayoshi; Yamamoto, Kouji
Journal of Molecular Catalysis A: Chemical (2000), 159 (2), 293-300CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
The structure of K contg. V2O5 catalysts has been studied by XRD and spectroscopic methods. The particles of V2O5 were oriented sharply to the direction perpendicular to b axis. The spacings of (010) and (200) planes were slightly contracted by the presence of K ions. V2O5 bronze seemed to be less Raman-active. IR spectra gave the structural information of K-V2O5 at low content of K. The IR bands at 1023 and 830 cm-1 of V2O5 shifted to 1000 and 785 cm-1, resp. This suggests that K ions are present at some micro space of V2O5 crystal. The K-V2O5 catalyst oriented to (010) plane exhibited high selectivity (ca. 80%) to C3H6 in the oxidn. of C3H8 while the activity decreased with the increase in K content. Oxygen ions of oriented V2O5 were exchanged with 18O by the redn. with C3H8 and reoxidn. with 18O2. Raman spectra's anal. of the catalysts exchanged with 18O suggests that V=O species are responsible for oxidative dehydrogenation of C3H8 to C3H6.
50
Ono, T.; Numata, H. Characteristic Features of Raman Band Shifts of Vanadium Oxide Catalysts Exchanged with the ¹⁸O Tracer and Active Sites for Reoxidation. J. Mol. Catal. A Chem. 1997, 116, 421– 429, DOI: 10.1016/S1381-1169(96)00425-6
Google Scholar
50
Characteristic features of Raman band shifts of vanadium oxide catalysts exchanged with the 18O tracer and active sites for reoxidation
Ono, Takehiko; Numata, Hideo
Journal of Molecular Catalysis A: Chemical (1997), 116 (3), 421-429CODEN: JMCCF2; ISSN:1381-1169. (Elsevier)
The oxide oxygen ions of V2O5 catalyst were exchanged with 18O tracer by a redn.-oxidn. method and by a catalytic oxidn. of n-butane using 18O2. The Raman band shifts of the V2O5 exchanged with 18O by the methods were examd. The band at 700 cm-1 was shifted to lower frequencies more preferentially than the band of V:O oxygen at 998 cm-1. Applying the correlation between the Raman bands and stretching modes as described in the literature, the positions of oxide ions and anion vacancies for redn. and reoxidn. were estd. The anion vacancies corresponding to the V-O species in the V square with 1.88 and 2.02 Å distances seem to be active sites for oxygen insertion. The similar conclusions were obtained for Mo contg. V2O5 catalyst.
51
Browne, M. P.; Sofer, Z.; Pumera, M. Layered and Two Dimensional Metal Oxides for Electrochemical Energy Conversion. Energy Environ. Sci. 2019, 12, 41– 58, DOI: 10.1039/C8EE02495B
Google Scholar
51
Layered and two dimensional metal oxides for electrochemical energy conversion
Browne, Michelle P.; Sofer, Zdenek; Pumera, Martin
Energy & Environmental Science (2019), 12 (1), 41-58CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
A review. The oxygen evolution and redn. reactions are two extremely important reactions in terms of energy applications. Currently, the Oxygen Evolution Reaction (OER) hinders the efficient running of electrolyzer devices which convert water into mol. H2. This H2 can subsequently be used in a H2/O2 fuel cell for the renewable generation of electricity with only H2O as a byproduct. However, this fuel cell process is not economy feasible due to the sluggish kinetics of the Oxygen Redn. Reaction (ORR) at the device cathode, even with expensive state-of-the-art electrocatalytic materials. As of late, the amt. of interest in the OER and ORR, from research labs. from all over the globe, has risen rapidly in order to find cheap and efficient catalysts to replace the expensive platinum based catalysts currently used in the two aforementioned energy conversion/generation technologies. Layered transition metal oxides, based on the cheap transition metal oxides Mn, Co, Ni and Fe have been reported as viable catalysts for the OER and ORR. Layered structures have an added advantage over non-layered materials as the surface area can be increase by means of exfoliation, with potential for tailoring electrocatalytic activity. It has been shown that the fabrication process and post-synthetic treatments, e.g. anion exchange or exfoliation, of these materials can alter the catalytic activity of these materials. Here we summarise various fabrication methods and modifications utilized in literature to tailor the performance of layered transition metal and hydroxide based catalysts for the ORR and OER toward that of the state-of-the-art materials for these technologies.
52
Wachs, I. E. Recent Conceptual Advances in the Catalysis Science of Mixed Metal Oxide Catalytic Materials. Catal. Today 2005, 100, 79– 94, DOI: 10.1016/j.cattod.2004.12.019
Google Scholar
52
Recent conceptual advances in the catalysis science of mixed metal oxide catalytic materials
Wachs, Israel E.
Catalysis Today (2005), 100 (1-2), 79-94CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
A review. The catalysis science of mixed metal oxides (supported metal oxides, mol. sieves and bulk mixed metal oxides) has undergone dramatic paradigm changes over the past 25 years as new characterization techniques became available (X-ray absorption spectroscopy (EXAFS/XANES/soft XANES), Raman, solid-state NMR, HR-TEM, UV-vis DRS and LEISS) to catalysis researchers. The major advantages offered by these spectroscopic improvements are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as cryst. nanophases and (2) their ability to collect information under various environmental conditions. Application of these spectroscopic techniques to the investigation of mixed metal oxide catalysts have provided new fundamental insights into the electronic and mol. structures of mixed metal oxide catalytic active sites and how they control the catalytic activity and selectivity characteristics. The most significant discovery has been that amorphous metal oxide phases are always present and are the catalytic active sites for many applications of mixed metal oxide catalysts. This has resulted in a significant paradigm shift as to how mixed metal oxide catalytic materials function for different applications. This article reviews the instrumental advances and the resulting conceptual advances that have evolved over the past 25 years in the catalysis science of mixed metal oxide catalysts.
53
Grant, J. T.; Venegas, J. M.; McDermott, W. P.; Hermans, I. Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts. Chem. Rev. 2018, 118, 2769– 2815, DOI: 10.1021/acs.chemrev.7b00236
Google Scholar
53
Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts
Grant, Joseph T.; Venegas, Juan M.; McDermott, William P.; Hermans, Ive
Chemical Reviews (Washington, DC, United States) (2018), 118 (5), 2769-2815CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidns. of C1-C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidns. of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidns., regardless of the metal oxide, follow Mars-van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidn. steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidn. step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.
54
Kuba, S.; Knözinger, H. Time-Resolved in Situ Raman Spectroscopy of Working Catalysts: Sulfated and Tungstated Zirconia. J. Raman Spectrosc. 2002, 33, 325– 332, DOI: 10.1002/jrs.815
Google Scholar
54
Time-resolved in situ Raman spectroscopy of working catalysts: sulfated and tungstated zirconia
Kuba, S.; Knozinger, H.
Journal of Raman Spectroscopy (2002), 33 (5), 325-332CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
The spectra of time-dependent in situ Raman expts. on sulfated and tungstated zirconia catalysts during the isomerization reaction of n-pentane are presented and discussed. A purpose-made in situ Raman cell which was used for this application is described. The choice of appropriate exptl. parameters, namely laser power and laser wavelength, is discussed. During the expts. the activity and selectivity of the catalysts were detd. simultaneously by online gas chromatog. The sulfated zirconia catalyst shows structural changes in the sulfate region during the reaction. One of two different types of initially present sulfate species is eroded during the reaction, presumably due to a redn. to H2S. The activity shows a typical induction period followed by a fast deactivation. No coke formation is obsd. Since fast deactivation occurs, coke formation cannot be the only reason for the deactivation of the catalyst. The tungstated catalyst shows strong darkening after initiation of the reaction with increasing time-onstream (TOS). This leads to the disappearance of the catalyst bands. However, coke formation indicated by a broad band at 1590 cm-1 can be obsd. Since darkening has a strong effect on Raman intensities, the time evolution of the obsd. bands is not obtained correctly. We propose a new method to correct for the effect of the darkening. The change of the diffuse reflectance of the catalyst is detd. by the variation of the intensity of laser plasma lines. Based on an approx. equation proposed by Waters which correlates the Raman intensities and the diffuse reflectance of a sample, a correction factor for the spectra is obtained. After the intensity correction the spectra indicate that most of the coke is formed in the first few minutes of the reaction followed by a const. formation rate with TOS.
55
Giannozzi, P. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502, DOI: 10.1088/0953-8984/21/39/395502
Google Scholar
55
QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
Giannozzi Paolo; Baroni Stefano; Bonini Nicola; Calandra Matteo; Car Roberto; Cavazzoni Carlo; Ceresoli Davide; Chiarotti Guido L; Cococcioni Matteo; Dabo Ismaila; Dal Corso Andrea; de Gironcoli Stefano; Fabris Stefano; Fratesi Guido; Gebauer Ralph; Gerstmann Uwe; Gougoussis Christos; Kokalj Anton; Lazzeri Michele; Martin-Samos Layla; Marzari Nicola; Mauri Francesco; Mazzarello Riccardo; Paolini Stefano; Pasquarello Alfredo; Paulatto Lorenzo; Sbraccia Carlo; Scandolo Sandro; Sclauzero Gabriele; Seitsonen Ari P; Smogunov Alexander; Umari Paolo; Wentzcovitch Renata M
Journal of physics. Condensed matter : an Institute of Physics journal (2009), 21 (39), 395502 ISSN:.
QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
56
Shklover, V.; Haibach, T.; Ried, F.; Nesper, R.; Novák, P. Crystal Structure of the Product of Mg²⁺ Insertion into V₂O₅ single Crystals. J. Solid State Chem. 1996, 123, 317– 323, DOI: 10.1006/jssc.1996.0186
Google Scholar
56
Crystal structure of the product of Mg2+ insertion into V2O5 single crystal
Shklover, V.; Haibach, T.; Ried, F.; Nesper, R.; Novak, P.
Journal of Solid State Chemistry (1996), 123 (2), 317-323CODEN: JSSCBI; ISSN:0022-4596. (Academic)
Chem. (by interaction with a dibutylmagnesium soln.) and electrochem. (in MeCN soln. of Mg perchlorate) insertion of Mg2+ into the single crystals of V2O5 was performed. The morphol. change of V2O5 crystals as a result of the Mg2+ insertion was studied by SEM. The wavelength dispersive electron probe microanal. clearly showed Mg (at least) at the surface of intercalated V2O5. Based on the single crystal x-ray diffraction study of intercalated V2O5 (orthorhombic, space group Pmn21, a 11.544(6), b 4.383(3), c 3.574(2) Å, Z = 4) the location of a small amt. of Mg (∼1%) in the bulk V2O5 may be suggested, with [6 + 4] O atoms surrounding Mg. The resulting Mg-O sepns. essentially exceed the accepted values for the Mg-O distances in crystals with hexacoordinated Mg atoms, which may be correlated with the structural and electrochem. properties of Mg2+-inserted V2O5.
57
Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787– 1799, DOI: 10.1002/jcc.20495
Google Scholar
57
Semiempirical GGA-type density functional constructed with a long-range dispersion correction
Grimme, Stefan
Journal of Computational Chemistry (2006), 27 (15), 1787-1799CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
A new d. functional (DF) of the generalized gradient approxn. (GGA) type for general chem. applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C6·R-6. A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common d. functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on std. thermochem. benchmark sets, for 40 noncovalently bound complexes, including large stacked arom. mols. and group II element clusters, and for the computation of mol. geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for std. functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean abs. deviation of only 3.8 kcal mol-1. The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the av. CCSD(T) accuracy. The basic strategy in the development to restrict the d. functional description to shorter electron correlation lengths scales and to describe situations with medium to large interat. distances by damped C6·R-6 terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chem. method for large systems where dispersion forces are of general importance.
58
Barone, V.; Casarin, M.; Forrer, D.; Pavone, M.; Sambi, M.; Vittadini, A. Role and Effective Treatment of Dispersive Forces in Materials: Polyethylene and Graphite Crystals as Test Cases. J. Comput. Chem. 2009, 30, 934– 939, DOI: 10.1002/jcc.21112
Google Scholar
58
Role and effective treatment of dispersive forces in materials: polyethylene and graphite crystals as test cases
Barone, Vincenzo; Casarin, Maurizio; Forrer, Daniel; Pavone, Michele; Sambi, Mauro; Vittadini, Andrea
Journal of Computational Chemistry (2009), 30 (6), 934-939CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
A semiempirical addn. of dispersive forces to conventional d. functionals (DFT-D) has been implemented into a pseudopotential plane-wave code. Test calcns. on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are cor. for the basis set superposition error. By applying the DFT-D/plane-wave approach a substantial agreement with expts. is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by std. local and semilocal d. functionals.
59
Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188– 5192, DOI: 10.1103/PhysRevB.13.5188
Google Scholar
There is no corresponding record for this reference.
60
Kim, H.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. Resonance Raman and Surface- and Tip-Enhanced Raman Spectroscopy Methods to Study Solid Catalysts and Heterogeneous Catalytic Reactions. Chem. Soc. Rev. 2010, 39, 4820– 4844, DOI: 10.1039/c0cs00044b
Google Scholar
60
Resonance Raman and surface- and tip-enhanced Raman spectroscopy methods to study solid catalysts and heterogeneous catalytic reactions
Kim, Hacksung; Kosuda, Kathryn M.; Van Duyne, Richard P.; Stair, Peter C.
Chemical Society Reviews (2010), 39 (12), 4820-4844CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
A review. Resonance Raman (RR) spectroscopy has several advantages over the normal Raman spectroscopy (RS) widely used for in situ characterization of solid catalysts and catalytic reactions. Compared with RS, RR can provide much higher sensitivity and selectivity in detecting catalytically-significant surface metal oxides. RR can potentially give useful information on the nature of excited states relevant to photocatalysis and on the anharmonic potential of the ground state. In this crit. review a detailed discussion is presented on several types of RR exptl. systems, 3 distinct sources of so-called Raman (fluorescence) background, detection limits for RR compared to other techniques (EXAFS, PM-IRAS, SFG), and 3 well-known methods to assign UV-vis absorption bands and a band-specific unified method that is derived mainly from RR results. In addn., the virtues and challenges of surface-enhanced Raman spectroscopy (SERS) are discussed for detecting mol. adsorbates at catalytically relevant interfaces. Tip-enhanced Raman spectroscopy (TERS), which is a combination of SERS and near-field scanning probe microscopy and has the capability of probing mol. adsorbates at specific catalytic sites with an enormous surface sensitivity and nanometer spatial resoln., is also reviewed.
61
Harima, H. Raman Scattering Characterization on SiC. Microelectron. Eng. 2006, 83, 126– 129, DOI: 10.1016/j.mee.2005.10.037
Google Scholar
61
Raman scattering characterization on SiC
Harima, Hiroshi
Microelectronic Engineering (2006), 83 (1), 126-129CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
A review. Raman scattering is a powerful non-contact and non-destructive characterization tool for SiC polytypes for both the lattice and electronic properties. Here, I will briefly review 2 recent Raman expts. on SiC; metal/SiC interface reactions probed by visible lasers and ion-implantation damages probed by deep UV lasers. These studies utilize the opposite aspects of the probe laser, i.e. deep and shallow penetration depth into SiC.
62
Gilson, T. R.; Bizri, O. F.; Cheetham, N. Single-Crystal Raman and Infrared Spectra of Vanadium(V) Oxide. J. Chem. Soc., Dalton Trans. 1973, 291– 294, DOI: 10.1039/dt9730000291
Google Scholar
62
Single-crystal Raman and infrared spectra of vanadium pentoxide
Gilson, T. R.; Bizri, O. F.; Cheetham, N.
Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1973), (3), 291-4CODEN: JCDTBI; ISSN:0300-9246.
The oriented single-crystal Raman and ir spectra of V2O5 were detd. and absorption frequencies were compared with those calcd. by using a simple transferred force field. Departures from previously established oxide-group frequencies were due to the relative lightness of V.
63
Abello, L.; Husson, E.; Repelin, Y.; Lucazeau, G. Vibrational Spectra and Valence Force Field of Crystalline V₂O₅. Spectrochim. Acta, Part A 1983, 39, 641– 651, DOI: 10.1016/0584-8539(83)80040-3
Google Scholar
63
Vibrational spectra and valence force field of crystalline vanadium pentoxide
Abello, L.; Husson, E.; Repelin, Y.; Lucazeau, G.
Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (1983), 39A (7), 641-51CODEN: SAMCAS; ISSN:0584-8539.
A complete vibrational study of cryst. V2O5 was performed. Polarized Raman spectra at 300 and 100 K on a single crystal and IR absorption spectra on powder samples were recorded. An assignment in terms of factor group symmetry species is given. A normal coordinate anal. using a complete force field was performed. All the normal modes of vibration are described in terms of potential energy distribution and in terms of cartesian displacements. Some modes characteristic of specific bonds are discussed. Finally some low frequency modes were found to derive, at a 1st approxn., from acoustic modes and some dispersion curves were derived from the simple model of the linear chain.
64
Clauws, P.; Broeckx, J.; Vennik, J. Lattice Vibrations of V₂O₅. Calculation of Normal Vibrations in a Urey-Bradley Force Field. Phys. Status Solidi B 1985, 131, 459– 473, DOI: 10.1002/pssb.2221310207
Google Scholar
64
Lattice vibrations of vanadium(V) oxide. Calculation of normal vibrations in a Urey-Bradley force field
Clauws, P.; Broeckx, J.; Vennik, J.
Physica Status Solidi B: Basic Research (1985), 131 (2), 459-73CODEN: PSSBBD; ISSN:0370-1972.
A normal coordinate anal. of the vibrational spectrum of cryst. V2O5 was carried out with the assumption of a Urey-Bradley force field. The calcd. frequencies were adjusted to 29 exptl. IR and Raman frequencies by an automatic force const. refinement program. Anal. of the potential energy distribution and the at. displacements allows the classification of the modes into 9 types of O vibrations and 3 types of chain modes. A discussion is given of effective charges and IR intensities. An assignment of the IR spectrum of polycryst. V2O5 is added.
65
Brázdová, V.; Ganduglia-Pirovano, M. V.; Sauer, J. Periodic Density Functional Study on Structural and Vibrational Properties of Vanadium Oxide Aggregates. Phys. Rev. B 2004, 69, 165420, DOI: 10.1103/PhysRevB.69.165420
Google Scholar
65
Periodic density functional study on structural and vibrational properties of vanadium oxide aggregates
Brazdova, Veronika; Ganduglia-Pirovano, M. Veronica; Sauer, Joachim
Physical Review B: Condensed Matter and Materials Physics (2004), 69 (16), 165420/1-165420/14CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
We present periodic d.-functional calcns. within the generalized gradient approxn. (Perdew-Wang 91) on structures and vibrational properties of different V oxide aggregates, namely, bulk V2O5 and its (001) surface, as well as thin V oxide films supported by α-Al2O3. V is differently coordinated by O in the different systems. The calcd. vibrational frequencies of bulk V2O5 are in good agreement with obsd. IR and Raman frequencies, for stretching modes the rms deviation is 40 cm-1. The calcns. for the V2O5(001) surface suggest modifications of previous assignments of high-resoln. electron-energy-loss spectroscopy (HREELS) data. In agreement with HREELS, vanadyl frequencies shift to higher wave nos. on surface formation. The calcd. frequencies for bulk Al2O3 are systematically lower than the obsd. IR data (by about 30 cm-1). Models for V2O3 supported on Al2O3 are obtained when in the outermost layers of Al2O3(0001) slabs Al is replaced by V. These films do not show vibrations above 930 cm-1. Oxygen adsorption on top of the vanadium sites on these supported films creates very stable vanadyl groups with binding energies of about 450 kJ/mol (1/2 O2). Bond distances, vibrational frequencies, and oxygen binding energies are compared with those of vanadyl groups at the V2O5(001) surface and in (V2O5)n clusters (n = 2,4). The relevance of the findings for expts. on vanadia particles supported on Al2O3 is discussed.
66
Zhou, B.; He, D. Raman Spectrum of Vanadium Pentoxide from Density-Functional Perturbation Theory. J. Raman Spectrosc. 2008, 39, 1475– 1481, DOI: 10.1002/jrs.2025
Google Scholar
66
Raman spectrum of vanadium pentoxide from density-functional perturbation theory
Zhou, Bo; He, Deyan
Journal of Raman Spectroscopy (2008), 39 (10), 1475-1481CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
We present ab initio calcn. within the framework of the d.-functional theory (DFT) on band structure and vibrational properties of bulk V2O5. The structure of V2O5 comes from optimization of the exptl. data with lattice parameters fixed. The band structure of the optimized structure has been calcd., and the result fits the exptl. data very well and also gives similar results as those calcd. by other methods. The phonon eigen-wavenumbers of the Γ- point of V2O5 bulk have been calcd. ab initio in d.-functional perturbation theory (DFPT). The calcd. vibrational wavenumbers are in good agreement with obsd. IR and Raman wavenumbers, and the predictive full phonon dispersion of bulk V2O5 has also been obtained. Further we calcd. the Raman spectrum of vanadium pentoxide (V2O5) powder sample using the obtained Raman susceptibility. Calcd. and measured intensities show overall good agreement.
67
Wu, Z.; Kim, H.-S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV–Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies. J. Phys. Chem. B 2005, 109, 2793– 2800, DOI: 10.1021/jp046011m
Google Scholar
67
On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV-Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies
Wu, Zili; Kim, Hack-Sung; Stair, Peter C.; Rugmini, Sreekala; Jackson, S. David
Journal of Physical Chemistry B (2005), 109 (7), 2793-2800CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
Vanadia species on aluminas (δ- and γ-Al2O3) with surface VOx d. in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temp.-programmed redn. in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymd. VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm-1, obsd. on dehydrated V/δ-, γ-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidn. reactions catalyzed by supported vanadia. Two types of frequency shifts of the V:O stretching band (1013-1035 cm-1) have been obsd. in the Raman spectra of V/Al2O3: a shift as a function of surface VOx d. and a shift as a function of excitation wavelength. The shift of the V:O band to higher wavenumbers with increasing surface VOx d. is due to the change of VOx structure. The V:O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymd. VOx species while visible Raman is more sensitive to highly polymd. VOx species and cryst. V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.
68
Hermann, K.; Witko, M.; Druzinic, R.; Tokarz, R. Hydrogen Assisted Oxygen Desorption from the V₂O₅(010) Surface. Top. Catal. 2000, 11/12, 67– 75, DOI: 10.1023/A:1027206705195
Google Scholar
68
Hydrogen assisted oxygen desorption from the V2O5(010) surface
Hermann, K.; Witko, M.; Druzinic, R.; Tokarz, R.
Topics in Catalysis (2000), 11/12 (1-4), 67-75CODEN: TOCAFI; ISSN:1022-5528. (Baltzer Science Publishers)
Vanadium oxide surfaces are known to play an active role as catalysts in hydrocarbon oxidn. reactions where oxygen from different surface sites participates in the reaction. Due to the ubiquity of hydrogen in these systems, reaction steps involving (temporary) hydrogenation are possible and may influence the overall reaction scheme. This work examines structural and energetic consequences of hydrogen interacting with different oxygen sites at the V2O5(010) surface where the local surface environment is modeled by embedded clusters. The electronic structure and equil. geometries of the clusters are obtained by d. functional theory (DFT) using gradient cor. functionals (RPBE) for exchange and correlation. Hydrogen is found to stabilize preferentially near oxygen sites forming surface OH and H2O species with binding energies of 0.5-2.3 eV per H atom depending on the site and species. Hydrogen adsorption weakens the binding of the surface oxygen with its vanadium neighbors considerably where the weakening is larger for H2O than for OH formation as evidenced by bond order analyses and results of the binding energetics. Thus, the studies suggest strongly that the presence of hydrogen at the oxide surface facilitates oxygen removal and, therefore, contributes to the enhanced yield of oxygenated products near vanadia based surfaces.
69
Huang, Y.-L.; Pellegrinelli, C.; Wachsman, E. D. Reaction Kinetics of Gas–Solid Exchange Using Gas Phase Isotopic Oxygen Exchange. ACS Catal. 2016, 6, 6025– 6032, DOI: 10.1021/acscatal.6b01462
Google Scholar
69
Reaction Kinetics of Gas-Solid Exchange Using Gas Phase Isotopic Oxygen Exchange
Huang, Yi-Lin; Pellegrinelli, Christopher; Wachsman, Eric D.
ACS Catalysis (2016), 6 (9), 6025-6032CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The oxygen transport kinetics of heterogeneous gas-solid exchange has been investigated on the basis of a two-step reaction mechanism, linking surface catalysis to solid-state self-diffusion, via gas phase isotopic oxygen exchange on the mixed ionic electronic conductor (MIEC) La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and electronic conductor (La0.8Sr0.2)0.95MnO3±x (LSM). The catalytic activity of LSCF is higher than that of LSM toward the elementary step of oxygen dissocn., likely caused by a higher vacancy concn. The apparent activation energy for surface exchange of LSCF is lower than values obtained from bulk characterization techniques. The diffusion coeff. (D) for LSM at different temps. shows a huge deviation from literature values, and an alternate exchange mechanism has been proposed. The fast transport pathway is attributed to the substoichiometry of LSM in the near surface region. These results have significant implications for the improvement of the oxygen redn. reaction for the design of higher-performance materials and the importance and limitations of isotope exchange exptl. design.
70
Huang, Y.-L.; Pellegrinelli, C.; Sakbodin, M.; Wachsman, E. D. Molecular Reactions of O₂ and CO₂ on Ionically Conducting Catalyst. ACS Catal. 2018, 8, 1231– 1237, DOI: 10.1021/acscatal.7b03467
Google Scholar
70
Molecular Reactions of O2 and CO2 on Ionically Conducting Catalyst
Huang, Yi-Lin; Pellegrinelli, Christopher; Sakbodin, Mann; Wachsman, Eric D.
ACS Catalysis (2018), 8 (2), 1231-1237CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The presence of CO2, an unavoidable component in air and fuel environments, is known to cause severe performance degrdn. in oxide catalysts. Understanding the interactions between O2, CO2, and ion-conducting oxides is crit. to developing energy-conversion devices. Here, surface reaction kinetics of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) with the presence of both O2 and CO2 is detd. using gas-phase isotope exchange. BSCF actively reacts with CO2, and the incorporation of oxygen from CO2 to the lattice of BSCF is directly obsd. as low as 50 °C. Above 200 °C, the reaction between CO2 and the BSCF surface dominates and is independent of the oxygen partial pressure. In addn., CO2 competes with O2 for binding to vacancy sites, forming surface intermediate species. Surprisingly, these surface intermediate species offer oxygen to exchange with oxygen in gaseous O2 and CO2, inhibiting the interactions between O2 and the solid surface. This work provides fundamental insight into functioning oxide catalysts, and the results can be applied to the design of improved oxide catalysts.
71
Huang, Y.-L.; Pellegrinelli, C.; Wachsman, E. D. Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides. ACS Catal. 2017, 7, 5766– 5772, DOI: 10.1021/acscatal.7b01096
Google Scholar
71
Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides
Huang, Yi-Lin; Pellegrinelli, Christopher; Wachsman, Eric D.
ACS Catalysis (2017), 7 (9), 5766-5772CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The high activity of oxide catalysts toward the oxygen redn. reaction (ORR) attracts unwanted interactions with other gaseous oxygen-contg. species in air. Understanding the interaction between oxygen-contg. species, mainly water and carbon dioxide, and oxides is important for many energy applications. However, the oxygen self-exchange process and the high-temp. operating conditions limit the investigation of these concurrent reactions. Here we report a direct observation of the effects of water and carbon dioxide on dissocn. rates of ionically conducting catalysts, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and (La0.8Sr0.2)0.95MnO3±δ(LSM), using gas-phase isotope exchange. The concurrent heterogeneous reactions of oxygen and other oxygen-contg. species on oxide catalysts can either promote or hinder oxygen dissocn. rates, depending on the participation of lattice oxygen. LSCF appears to be much more active in exchange with these oxygen-contg. species, while LSM shows relatively little exchange. Oxygen-contg. species exhibit site-blocking effects and inhibit the reaction on LSCF. In contrast, water and CO2 promote the oxygen dissocn. rate on LSM, likely due to the prominence of homoexchange, where intermediate surface species play an important role. Our study provides insights into the reaction mechanism of oxygen dissocn. and the effect of coexisting ambient air oxygen species.
72
Tilley, R. J. Understanding Solids: The Science of Materials; Wiley: 2013.
Google Scholar
There is no corresponding record for this reference.
73
Cameron, W. C.; Parkas, A.; Litz, L. M. Exchange of Isotopic Oxygen between Vanadium Pentoxide, Gaseous Oxygen and Water. J. Phys. Chem. 1953, 57, 229– 238, DOI: 10.1021/j150503a022
Google Scholar
73
Exchange of isotopic oxygen among vanadium pentoxide, gaseous oxygen, and water
Cameron, W. C.; Farkas, A.; Litz, L. M.
Journal of Physical Chemistry (1953), 57 (), 229-38CODEN: JPCHAX; ISSN:0022-3654.
The exchange of O18 (1.3 to 1.6%) was measured by mass spectrometric analyses in the systems V2O5-O (I), V2O5-H2O (II), and V2O5-H2O-O (III) in the temp. range 400-550°. The rate of exchange in I increases with decreasing particle size of the V2O5. In the case of Alundum-supported V2O5 catalysts, the support does not participate in the exchange, and the reaction proceeds according to a surface-reaction-controlled mechanism which is 1st order with time, zero order with O pressure, and with an activation energy of 45 kcal./mole. The kinetics of the O exchange in I with fresh amorphous V2O5 microspheres is controlled by a surface reaction also. On heat-treatment, the microspheres crystallize, and the exchange indicates a diffusion-controlled mechanism. The rate of the O exchange in II (supported V2O5) is 20 to 30 times as rapid as in I. The rate of exchange in III is similar to that in I, indicating that in this instance the latter reaction is rate controlling. In the case of the surface-reaction-controlled exchange, the fast diffusion of the O in the bulk of the V2O5 particles may be due to lattice imperfections. The 1st-order kinetics found in I is compatible with an apparent zero-order with respect to O pressure if the O is strongly adsorbed. It is proposed that the activation process in the exchange of O atoms involves either the dissocn. of O mols. or the loosening of V-O bonds.
74
Milan, E. F. The Dissociation Pressure of Vanadium Pentoxide. J. Phys. Chem. 1929, 33, 498– 508, DOI: 10.1021/j150298a002
Google Scholar
74
The dissociation pressure of vanadium pentoxide
Milan, E. F.
Journal of Physical Chemistry (1929), 33 (), 498-508CODEN: JPCHAX; ISSN:0022-3654.
Pure V2O5 was prepd. from ammonium vanadate and its dissocn. pressure was measured over the temp. range 700-1125°. V2O5 dissociates into V2O4 and O2 at temps. only slightly above its m. p. Small amts. of V2O4 have an enormous effect on dissocn. pressures. Pressure-temp., pressure-compn. and temp.-compn. diagrams are plotted.
75
Nasu, N. The Dissociation Pressure of Vanadium Pentoxide. J. Chem. Soc. Jpn. 1935, 56, 666– 669, DOI: 10.1246/nikkashi1921.56.6_666
Google Scholar
There is no corresponding record for this reference.
76
Spitsyn, B. N.; Maidanovskaya, L. L. The Thermal Dissociation of Vanadium Pentoxide. Zh. Fiz. Khim. 1959, 33, 180– 183
Google Scholar
76
Thermal dissociation of vanadium pentoxide
Spitsyn, B. V.; Maidanovskaya, L. G.
Zhurnal Fizicheskoi Khimii (1959), 33 (), 180-3CODEN: ZFKHA9; ISSN:0044-4537.
The V2O5 thermal dissocn. was studied by the Mǎidanovskaya and Bruns method (ibid. 13, 239(1939)) in which V2O5 is used as catalyst for vapor-phase oxidn. The dissocn. kinetics was described by the Erofeev equation (C.A. 41, 4027c) with which the activation energy of the process were detd. (8.6 kcal./ mole). The amt. of O evolved agreed with data on the high- and low-temp. V2O5 dissocn. The x-ray diffraction measurements of V2O5 remained unchanged between 20 and 460° whereas the chem. reaction proceeding on a single crystal distorted its structure.
77
Chen, K.; Khodakov, A.; Yang, J.; Bell, A. T.; Iglesia, E. Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts. J. Catal. 1999, 186, 325– 333, DOI: 10.1006/jcat.1999.2510
Google Scholar
77
Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts
Chen, Kaidong; Khodakov, Andrei; Yang, Jun; Bell, Alexis T.; Iglesia, Enrique
Journal of Catalysis (1999), 186 (2), 325-333CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
Kinetic anal. and isotopic tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixts. on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-contg. products, as expected if lattice oxygens were required for the activation of C-H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8-18O2-16O2 reactant mixts. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8-C3D8-O2 reactants undergo oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that C-H bond activation steps are also irreversible. Normal kinetic isotopic effects (kC-H/kC-D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and propene combustion steps (1.7 and 2.2, resp.). These data are consistent with kinetically relevant steps involving the dissocn. of C-H bonds in propane and propene. C3H6-D2O and C3D6-H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These isotopic tracer results are consistent with a Mars-van Krevelen redox mechanism involving two lattice oxygens in irreversible C-H bond activation steps. The resulting alkyl species desorb as propene and the remaining O-H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the obsd. water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates. (c) 1999 Academic Press.
78
Ma, W. Y.; Zhou, B.; Wang, J. F.; Zhang, X. D.; Jiang, Z. Y. Effect of Oxygen Vacancy on Li-Ion Diffusion in a V₂O₅ Cathode: A First-Principles Study. J. Phys. D Appl. Phys. 2013, 46, 105306, DOI: 10.1088/0022-3727/46/10/105306
Google Scholar
78
Effect of oxygen vacancy on Li-ion diffusion in a V2O5 cathode: a first-principles study
Ma, W. Y.; Zhou, B.; Wang, J. F.; Zhang, X. D.; Jiang, Z. Y.
Journal of Physics D: Applied Physics (2013), 46 (10), 105306, 8 pp.CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
The energy barriers of lithium-ion mobility in a V2O5 cathode are calcd. using the nudged elastic band method. The low activation energy of the hopping pathway along the 〈0 1 0〉 direction (paralleling with the b-axis, the shortest lattice parameter) indicates that V2O5 has a one-dimensional diffusion pattern at the initial stage of charging and discharging. At a temp. of 300 K, the estd. diffusion coeff. is 2.520 × 10-8 cm2 s-1, which is consistent with the range of diffusivity of the previous reported fast ionic conductors. A systematic investigation of the effect of oxygen vacancy on Li-ion diffusion suggests that the bridging oxygen O(2) plays a very pos. role in the Li-ion diffusion along the 〈0 1 0〉 direction and the activation energy is reduced from 0.340 to 0.215 eV, while the existence of O(1) and O(3) vacancies can hinder the lithium diffusion along this direction due to the increase in the activation energy, resp. The oxygen vacancies make the energy barriers of the other two potential diffusion pathways reduced to some extent, which is still very large compared with the energy barrier of diffusion along the b-axis.
79
Mars, P.; van Krevelen, D. W. Oxidations Carried out by Means of Vanadium Oxide Catalysts. Chem. Eng. Sci. 1954, 3, 41– 59, DOI: 10.1016/S0009-2509(54)80005-4
Google Scholar
79
Oxidations carried out by means of vanadium oxide catalysts
Mars, P.; van Krevelen, D. W.
Chemical Engineering Science (1954), 3 (Spec. Suppl.), 41-59CODEN: CESCAC; ISSN:0009-2509.
The oxidations of C6H6, PhMe, naphthalene, and anthracene were studied in a vanadium oxide catalyst fluidized bed. The partial pressure of O was varied between 80 and 760 mm. and of the aromatic hydrocarbons between 1 and 30 mm. Both the O and the hydrocarbon concentrations affect the rate. The formula describing the reactions was derived by assuming two successive reactions, the reaction between the O on the surface and the adsorbed hydrocarbon, and the subsequent reoxidation of the V. The oxidation of SO2 is also discussed.
80
Routray, K.; Reddy, K. R. S. K.; Deo, G. Oxidative Dehydrogenation of Propane on V₂O₅/Al₂O₃ and V₂O₅/TiO₂ Catalysts: Understanding the Effect of Support by Parameter Estimation. Appl. Catal. A Gen. 2004, 265, 103– 113, DOI: 10.1016/j.apcata.2004.01.006
Google Scholar
80
Oxidative dehydrogenation of propane on V2O5/Al2O3 and V2O5/TiO2 catalysts: understanding the effect of support by parameter estimation
Routray, Kamalakanta; Reddy, K. R. S. K.; Deo, Goutam
Applied Catalysis, A: General (2004), 265 (1), 103-113CODEN: ACAGE4; ISSN:0926-860X. (Elsevier Science B.V.)
In this paper, the effect of the oxide support for supporting the vanadium oxide phase is studied by estg. the reaction parameters for the oxidative dehydrogenation of propane. To achieve this objective, several V2O5/Al2O3 and V2O5/TiO2 catalysts were synthesized by an incipient-wetness-impregnation technique. The supported vanadium oxide catalysts were characterized and the surface area, monolayer coverage and reducibility were detd. The surface area of the catalyst samples was not significantly affected with supported vanadium oxide loading. It was obsd. that the catalysts contain only molecularly dispersed vanadium oxide species below monolayer coverage, and molecularly dispersed and cryst. V2O5 above monolayer coverage. TPR studies revealed that the V2O5/Al2O3 samples were more difficult to reduce relative to the V2O5/TiO2 samples. The monolayer or near-monolayer catalysts, 10% V2O5/Al2O3 and 4% V2O5/TiO2, were selected for detailed kinetic anal. A Mars-van Krevelen (MVK) model contg. eight parameters was chosen for this purpose. The parameters were estd. using a genetic algorithm (GA), which optimizes a suitable objective function for a non-linear multi-response system. From the parameters estd., it was detd. that a similar catalytic cycle occurs independent of the oxide support. However, the rate at which the catalytic cycle occurs appears to be much faster on the more reducible titania support compared to the rate on the less reducible alumina support. The degree of redn. varies along the length of the reactor and depends on the support. Thus, the support has a significant effect on the reaction parameters for the oxidative dehydrogenation of propane over supported vanadium oxide catalysts.
81
Alexopoulos, K.; Reyniers, M.-F.; Marin, G. B. Reaction Path Analysis of Propane Selective Oxidation over V₂O₅ and V₂O₅/TiO₂. J. Catal. 2012, 289, 127– 139, DOI: 10.1016/j.jcat.2012.01.019
Google Scholar
81
Reaction path analysis of propane selective oxidation over V2O5 and V2O5/TiO2
Alexopoulos, Konstantinos; Reyniers, Marie-Francoise; Marin, Guy B.
Journal of Catalysis (2012), 289 (), 127-139CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
The selective oxidn. of propane on the vanadyl and bridging oxygen sites of the fully oxidized (0 0 1) V2O5 surface and of an epitaxial vanadia monolayer supported on (0 0 1) TiO2 anatase is analyzed using periodic d. functional theory (DFT). Both the oxidative dehydrogenation leading to propene and the formation of oxygenated products, n-propanol, i-propanol, propanal and acetone, are studied. Selective oxidn. proceeds via a Mars-van Krevelen redox mechanism, and its elementary steps on the vanadia surface are identified. Propane chemisorption preferentially occurs through a secondary C-H bond activation via a direct hydrogen abstraction by a lattice oxygen. Supporting a vanadia monolayer on titania strongly enhances the C-H bond activation as compared to unsupported V2O5, yielding a lower activation energy and a more exothermic propane chemisorption. In accordance with exptl. observations, the calcns. show that the titania support not only modifies the activity of the vanadia monolayer but it also affects the selectivity of the catalyst, favoring the formation of propene compared to the formation of i-propanol and acetone. The vanadyl oxygen is overall the most active site on V2O5 and V2O5/TiO2, while the bridging oxygen is more selective towards propane dehydrogenation.

Cited By

Click to copy section linkSection link copied!

Explore this article's citation statements on scite.ai

This article is cited by 11 publications.

Sudha Arumugam, Ranjithvel Mohanasundaram, Parasuraman Swaminathan. Salt Electrolytes for Vanadium Pentaoxide-Based Electrochromic Devices. ACS Applied Engineering Materials 2025, 3 (4) , 901-913. https://doi.org/10.1021/acsaenm.5c00040
Daniyal Kiani, Israel E. Wachs. The Conundrum of “Pair Sites” in Langmuir–Hinshelwood Reaction Kinetics in Heterogeneous Catalysis. ACS Catalysis 2024, 14 (13) , 10260-10270. https://doi.org/10.1021/acscatal.4c02813
Gregor Koch, Giulia Bellini, Frank Girgsdies, Michael Hävecker, Spencer J. Carey, Olaf Timpe, Thomas Götsch, Thomas Lunkenbein, Gudrun Auffermann, Andrey Tarasov, Robert Schlögl, Annette Trunschke. Structural Analysis and Redox Properties of Oxygen-“Breathing” A(Mn1–xCux)O3 (A = La, Pr) Perovskites. Chemistry of Materials 2024, 36 (11) , 5388-5404. https://doi.org/10.1021/acs.chemmater.4c00136
Bapuso M. Babar, Umesh D. Babar, Abhijeet J. Kale, Tushar T. Bhosale, Udayraj T. Pawar, Prakash M. Kadam, Sandip V. Nipane, Nishad G. Deshpande, Pramod S. Patil, Laxman D. Kadam. Polyvinylpyrrolidone influenced reduced graphene oxide modified vanadium pentoxide nano rods for NO2 detection and supercapacitor application. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2025, 710 , 136239. https://doi.org/10.1016/j.colsurfa.2025.136239
Valentina Paolucci, Maria Basso, Vittorio Ricci, Elena Colusso, Mattia Cattelan, Enrico Napolitani, Alessandro Martucci, Carlo Cantalini. Pulsed-laser annealed amorphous/crystalline a-V2O5/VO2 heterostructure for selective NO2 and H2 sensing. Sensors and Actuators B: Chemical 2025, 428 , 137235. https://doi.org/10.1016/j.snb.2025.137235
Peng Zhang, Jingyuan Fan, Yuanqing Wang, Yuying Dang, Saskia Heumann, Yuxiao Ding. Insights into the role of defects on the Raman spectroscopy of carbon nanotube and biomass-derived carbon. Carbon 2024, 222 , 118998. https://doi.org/10.1016/j.carbon.2024.118998
Jose Alirio Mendoza Mesa, Sven Robijns, Iqtidar Ali Khan, Marco G. Rigamonti, Max L. Bols, Michiel Dusselier. Support effects in vanadium incipient wetness impregnation for oxidative and non-oxidative propane dehydrogenation catalysis. Catalysis Today 2024, 430 , 114546. https://doi.org/10.1016/j.cattod.2024.114546
V.M. Dzhagan, M. Ya Valakh, O.F. Isaieva, V.O. Yukhymchuk, O.A. Stadnik, O. Yo Gudymenko, P.M. Lytvyn, O.A. Kulbachynskyi, V.S. Yefanov, B.M. Romanyuk, V.P. Melnik. Raman fingerprints of different vanadium oxides as impurity phases in VO2 films. Optical Materials 2024, 148 , 114894. https://doi.org/10.1016/j.optmat.2024.114894
O. Chukova, S. G. Nedilko, T. Voitenko, R. Minikayev, W. Paszkowicz, V. Stasiv, Y. Zhydachevskyy, A. Suchocki. Luminescence mechanisms in the 2V2O5–xLi2O–(98 − x)B2O3 glass matrices developed for creation of glass–ceramic materials. Journal of Materials Science: Materials in Electronics 2023, 34 (7) https://doi.org/10.1007/s10854-023-10026-4
Sanja J. Armaković, Aleksandra Jovanoski Kostić, Andrijana Bilić, Maria M. Savanović, Nataša Tomić, Aleksandar Kremenović, Maja Šćepanović, Mirjana Grujić-Brojčin, Jovana Ćirković, Stevan Armaković. Photocatalytic Activity of the V2O5 Catalyst toward Selected Pharmaceuticals and Their Mixture: Influence of the Molecular Structure on the Efficiency of the Process. Molecules 2023, 28 (2) , 655. https://doi.org/10.3390/molecules28020655
Yuan Gao, Mingxin Jiang, Liuqingqing Yang, Zhuo Li, Fei-Xiang Tian, Yulian He. Recent progress of catalytic methane combustion over transition metal oxide catalysts. Frontiers in Chemistry 2022, 10 https://doi.org/10.3389/fchem.2022.959422

Download PDF

Get e-Alerts

The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2022, 126, 7, 3443–3456

Click to copy citationCitation copied!

https://doi.org/10.1021/acs.jpcc.2c00174

Published February 9, 2022

CC-BY 4.0 .

Article Views

Altmetric

Citations

Learn about these metrics

Article Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.

Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.

The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated.

Abstract
High Resolution Image
Download MS PowerPoint Slide
Figure 1
Figure 1. Unit cell of V₂O₅: O1, terminal oxygen; O2, bridging oxygen; O3, chain oxygen.
High Resolution Image
Download MS PowerPoint Slide
Figure 2
Figure 2. UV/vis spectrum (a) and Raman spectra (b) of V₂O₅ collected at room temperature (ex situ). The laser wavelength used for the Raman measurements is shown next to the Raman spectra. The Raman spectra were corrected with respect to instrumental effects, taking into account the known response curve of a white lamp and the absorption spectrum of V₂O₅ (see Figure S8 for details of the correction procedure and Figure S10 for uncorrected spectra); All spectra were normalized to the corresponding maximum band intensity ([0,1]), which differs for the different excitation energies; The band positions indicated were determined for the spectrum measured with the excitation wavelength 532 nm; The intensity ratio of the band at 994 cm^–1 to the band at 144 cm^–1 as a function of excitation wavelength is plotted in part a.
High Resolution Image
Download MS PowerPoint Slide
Figure 3
Figure 3. Raman spectra of V₂O₅ measured at room temperature before and after isotopic oxygen exchange; Gray and red spectra denote the cases before and after exchange at 322, 431, and 573 °C, respectively, using ¹⁸O₂ (20% in He) or a mixture of propane and oxygen (C₃H₈ (1%) + ¹⁸O₂ (19%) in He) for 2 h. All the spectra were normalized to [0,1]. The positions of all spectra were aligned with respect to the band at 994 cm^–1. Laser: 532 nm. Heating rate: 5 °C/min. Total flow rate: 10 mL/min.
High Resolution Image
Download MS PowerPoint Slide
Figure 4
Figure 4. Deconvolution of Raman spectra collected at room temperature after treatment at different temperatures and in different gas atmospheres (the same Raman spectra as shown in Figure 3) in the V–O3 (a) as well as V═O1 and O1–V–O2 stretching vibration region (b). Temperatures and gas atmospheres are specified in the right top corner of each section. (c) Corresponding representations of vibrational motions of phonon modes.
High Resolution Image
Download MS PowerPoint Slide
Figure 5
Figure 5. In situ Raman spectra of V₂O₅ at 573 °C under 20% ¹⁸O₂ in He (a), 1% C₃H₈ + 19% ¹⁸O₂ in He (b), and 1% C₃D₈ + 19% ¹⁸O₂ in He (c) as a function of time. Conditions of exchange experiments are described in the caption of Figure 3. The numbers show the time that had passed since the start of the experiment at the moment of recording. Laser: 532 nm.
High Resolution Image
Download MS PowerPoint Slide
Figure 6
Figure 6. Extent of oxygen exchange as a function of time; The lines are the fitted results using eqs 3 (dashed) and 4 (solid). The experimental data points were taken from the experiments shown in Figure 5. Note that the time used in the graph is the average time taken to record each spectrum.
High Resolution Image
Download MS PowerPoint Slide
Figure 7
Figure 7. Effect of the nature of the alkane on isotope oxygen exchange. (a) Raman spectra of V₂O₅ measured at room temperature before and after isotopic oxygen exchange experiments under 10% C₃H₈ + 5% ¹⁸O₂ and 10% C₂H₆ + 5% ¹⁸O₂, respectively, at 400 °C. (b) In situ Raman spectra of V₂O₅ at 400 °C under ¹⁶O₂ and 10% C₄H₁₀ + 5% ¹⁸O₂ as a function of time. Numbers on the spectra indicate the starting time of measurement. Asterisks represent cut cosmic ray signals. Raman spectra were normalized to [0,1]. Laser: 532 nm. Exchange experiments were conducted at 400 °C for 2 h in the Harrick Raman chamber. Heating rate: 5 °C/min. Total flow rate: 10 mL/min.
High Resolution Image
Download MS PowerPoint Slide
References
This article references 81 other publications.
1. 1
  Panov, G. I.; Dubkov, K. A.; Starokon, E. V. Active Oxygen in Selective Oxidation Catalysis. Catal. Today 2006, 117, 148– 155, DOI: 10.1016/j.cattod.2006.05.019
  1
  Active oxygen in selective oxidation catalysis
  Panov, Gennady I.; Dubkov, Konstantin A.; Starokon, Eugeny V.
  Catalysis Today (2006), 117 (1-3), 148-155CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
  A review. The paper reviews literature data related to oxygen species available on the surface of metal oxides and their possible role in selective oxidn. With some reservations, one may conclude that the main concept accepted presently in oxidn. catalysis assumes that selective oxidn. is provided by strongly bonded lattice oxygen, while the full oxidn. is provided by weakly bonded reactive oxygen. Recent studies on the oxidn. mechanism over FeZSM-5 zeolites with N2O give ground to reconsider this concept in order to integrate the earlier suggested radical oxygen idea, which supposes an important role of O- radicals in selective oxidn. This highly reactive oxygen species can be considered as a powerful tool for activation of org. mols.
2. 2
  Freund, H.-J. Oxygen Activation on Oxide Surfaces: A Perspective at the Atomic Level. Catal. Today 2014, 238, 2– 9, DOI: 10.1016/j.cattod.2014.05.037
  2
  Oxygen activation on oxide surfaces: A perspective at the atomic level
  Freund, Hans-Joachim
  Catalysis Today (2014), 238 (), 2-9CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
  A review; thin oxide films have been used as model supports to unravel the influence of the oxide-metal and oxide-gas interface. We discuss the influence of defects in the oxide lattice and the oxide surface on properties of adsorbed species, in particular the formation of oxide particles from a deposited transition metal onto ceria and of adsorbed oxygen from the gas phase. Here we correlate the structure of the particles, as revealed by a combination of STM and spectroscopic as well as theor. calcns., with their reactive properties. The nature of the involved defects is characterized by adsorption of Au as a way to correlate the influence of various defects of different structure on the Au charge state. In a second case study, we demonstrate the influence of dopants within the supporting oxide on the adsorption of oxygen on a defect free surface. This is shown to be potentially relevant for the activation of methane in oxidative methane coupling reactions, as recently demonstrated.
3. 3
  Hwang, J.; Rao, R. R.; Giordano, L.; Katayama, Y.; Yu, Y.; Shao-Horn, Y. Perovskites in Catalysis and Electrocatalysis. Science 2017, 358, 751– 756, DOI: 10.1126/science.aam7092
  3
  Perovskites in catalysis and electrocatalysis
  Hwang, Jonathan; Rao, Reshma R.; Giordano, Livia; Katayama, Yu; Yu, Yang; Shao-Horn, Yang
  Science (Washington, DC, United States) (2017), 358 (6364), 751-756CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
  A review. Catalysts for chem. and electrochem. reactions underpin many aspects of modern technol. and industry, from energy storage and conversion to toxic emissions abatement to chem. and materials synthesis. This role necessitates the design of highly active, stable, yet earth-abundant heterogeneous catalysts. In this Review, we present the perovskite oxide family as a basis for developing such catalysts for (electro)chem. conversions spanning carbon, nitrogen, and oxygen chemistries. A framework for rationalizing activity trends and guiding perovskite oxide catalyst design is described, followed by illustrations of how a robust understanding of perovskite electronic structure provides fundamental insights into activity, stability, and mechanism in oxygen electrocatalysis. We conclude by outlining how these insights open exptl. and computational opportunities to expand the compositional and chem. reaction space for next-generation perovskite catalysts.
4. 4
  Adler, S. B. Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104, 4791– 4844, DOI: 10.1021/cr020724o
  4
  Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes
  Adler, Stuart B.
  Chemical Reviews (Washington, DC, United States) (2004), 104 (10), 4791-4843CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review of factors governing oxygen redn. in solid oxide fuel cell cathodes, with emphasis on thin-film ceramic-type conductor and conducting electrode fabrication, mass transfer and transport, catalysis (e.g., by platinum), perovskite-type mixed conductors, oxygen diffusion and adsorption, phase boundaries, interfacial effects and interfacial kinetics, transport models, operating regimes, current constriction, microstructure and microstructure control, overpotential, and composite electrolytes and electrodes. Materials emphasized include Pt, YSZ (yttria-stabilized zirconia), lanthanum strontium manganese oxide ((La,Sr)MnO3), and cobalt iron lanthanum strontium oxide ((La,Sr)(Co,Fe)O3).
5. 5
  Gurlo, A. Interplay between O₂ and SnO₂: Oxygen Ionosorption and Spectroscopic Evidence for Adsorbed Oxygen. ChemPhysChem 2006, 7, 2041– 2052, DOI: 10.1002/cphc.200600292
  5
  Interplay between O2 and SnO2: oxygen ionosorption and spectroscopic evidence for adsorbed oxygen
  Gurlo, Alexander
  ChemPhysChem (2006), 7 (10), 2041-2052CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Tin dioxide is the most commonly used material in com. gas sensors based on semiconducting metal oxides. Despite intensive efforts, the mechanism responsible for gas-sensing effects on SnO2 is not fully understood. The key step is the understanding of the electronic response of SnO2 in the presence of background oxygen. For a long time, oxygen interaction with SnO2 was treated within the framework of the ionosorption theory. The adsorbed oxygen species were regarded as free oxygen ions electrostatically stabilized on the surface (with no local chem. bond formation). A contradiction, however, arises when connecting this scenario to spectroscopic findings. Despite trying for a long time, there was not any convincing spectroscopic evidence for ionosorbed oxygen species. Neither superoxide ions O2-, nor charged at. oxygen O-, nor peroxide ions O22- were obsd. on SnO2 under the real working conditions of sensors. Also, several findings show that the superoxide ion does not undergo transformations into charged at. oxygen at the surface, and represents a dead-end form of low-temp. oxygen adsorption on reduced metal oxide.
6. 6
  Schlögl, R. Heterogeneous Catalysis. Angew. Chem., Int. Ed. 2015, 54, 3465– 3520, DOI: 10.1002/anie.201410738
  6
  Heterogeneous catalysis
  Schlogl Robert
  Angewandte Chemie (International ed. in English) (2015), 54 (11), 3465-520 ISSN:.
  A heterogeneous catalyst is a functional material that continually creates active sites with its reactants under reaction conditions. These sites change the rates of chemical reactions of the reactants localized on them without changing the thermodynamic equilibrium between the materials.
7. 7
  Haber, J.; Turek, W. Kinetic Studies as a Method to Differentiate between Oxygen Species Involved in the Oxidation of Propene. J. Catal. 2000, 190, 320– 326, DOI: 10.1006/jcat.1999.2764
  7
  Kinetic Studies as a Method to Differentiate between Oxygen Species Involved in the Oxidation of Propene
  Haber, Jerzy; Turek, Wincenty
  Journal of Catalysis (2000), 190 (2), 320-326CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
  Kinetic equations of propene oxidn. along the nondestructive (nucleophilic oxidn. to allylic products) and the destructive (electrophilic oxidn. to degraded oxygenated and total oxidn. products) pathways have been detd. for three types of oxide catalysts: Bi2O3/MoO3, giving only allylic products, Co3O4, showing only total oxidn., and SnO2/MoO3, giving both types of products. In all cases the reaction order of the nucleophilic oxidn. was 1 with respect to propene and 0 with respect to oxygen, whereas that of electrophilic oxidn. was close to 1 with respect to oxygen. The model of surface interactions is discussed in which propene reacts either with surface lattice oxide ions to give nucleophilic oxidn. products or with transient surface oxygen species to give electrophilic oxidn. These transients result from the dynamic equil. between nonstoichiometric transition metal oxides and gas phase oxygen, so that the two kinetic equations are coupled by the equation expressing the equil. of surface oxygen vacancies. The rate consts. of the homomol. isotopic exchange of oxygen may be taken as a measure of the surface concn. of the transient oxygen species. (c) 2000 Academic Press.
8. 8
  Rasmussen, M. D.; Molina, L. M.; Hammer, B. Adsorption, Diffusion, and Dissociation of Molecular Oxygen at Defected TiO₂(110): A Density Functional Theory Study. J. Chem. Phys. 2004, 120, 988– 997, DOI: 10.1063/1.1631922
  8
  Adsorption, diffusion, and dissociation of molecular oxygen at defected TiO2(110). A density functional theory study
  Rasmussen, M. D.; Molina, L. M.; Hammer, B.
  Journal of Chemical Physics (2004), 120 (2), 988-997CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
  The properties of reduced rutile TiO2(110) surfaces, as well as the adsorption, diffusion, and dissocn. of mol. O are investigated by d. functional theory. The O2 mol. is found to bind strongly to bridging O vacancies, attaining a mol. state with an expanded O-O bond of 1.44 Å. The mol. O also binds (with somewhat shortened bond lengths) to the fivefold coordinated Ti atoms in the troughs between the bridging oxygen rows, but only when vacancies are present somewhere in the surface. In all cases, the magnetic moment of O2 is lost upon adsorption. The expanded bond lengths reveal together with inspection of electron d. and electronic d. of state plots that charging of the adsorbed mol. oxygen is of key importance in forming the adsorption bond. The processes of O2 diffusion from a vacancy to a trough and O2 dissocn. at a vacancy are both hindered by relative large barriers. The presence of neighboring vacancies can strongly affect the ability of O2 to dissoc. The implications of this in connection with diffusion of the bridging O vacancies are discussed.
9. 9
  Ganduglia-Pirovano, M. V.; Hofmann, A.; Sauer, J. Oxygen Vacancies in Transition Metal and Rare Earth Oxides: Current State of Understanding and Remaining Challenges. Surf. Sci. Rep. 2007, 62, 219– 270, DOI: 10.1016/j.surfrep.2007.03.002
  9
  Oxygen vacancies in transition metal and rare earth oxides. Current state of understanding and remaining challenges
  Ganduglia-Pirovano, M. Veronica; Hofmann, Alexander; Sauer, Joachim
  Surface Science Reports (2007), 62 (6), 219-270CODEN: SSREDI; ISSN:0167-5729. (Elsevier B.V.)
  A review. Defects at transition metal (TM) and rare earth (RE) oxide surfaces, neutral O vacancies in particular, play a major role in a variety of technol. applications. This is the motivation of numerous studies of partially reduced oxide surfaces. We review, discuss, and compare theor. data for structural and electronic properties and energetic quantities related to the formation of O defects at TM and RE oxide surfaces using TiO2, ZrO2, V2O5, and CeO2 as examples. Bulk defects, as far as relevant for comparison with the properties of reduced surfaces, are briefly reviewed. Special attention is given to the fate of the electrons left in the system upon vacancy formation and the ability of state-of-the-art quantum-mech. methods to provide reliable energies and an accurate description of the electronic structure of the partially reduced oxide systems.
10. 10
  Cui, Y.; Shao, X.; Baldofski, M.; Sauer, J.; Nilius, N.; Freund, H.-J. Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides. Angew. Chem., Int. Ed. 2013, 52, 11385– 11387, DOI: 10.1002/anie.201305119
  10
  Adsorption, Activation, and Dissociation of Oxygen on Doped Oxides
  Cui, Yi; Shao, Xiang; Baldofski, Matthias; Sauer, Joachim; Nilius, Niklas; Freund, Hans-Joachim
  Angewandte Chemie, International Edition (2013), 52 (43), 11385-11387CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Charge transfer in the presence of dopants is relevant for the adsorption and activation of small mols., such as O2. Scanning tunneling microscopy and DFT calcns. provide evidence for the formation of strongly bound superoxo species on chem. inert, Mo-doped CaO films. This oxygen surface species shows a high propensity to dissoc. Dopants could also be important for the activation of hydrocarbons on inert oxides.
11. 11
  McFarland, E. W.; Metiu, H. Catalysis by Doped Oxides. Chem. Rev. 2013, 113, 4391– 4427, DOI: 10.1021/cr300418s
  11
  Catalysis by Doped Oxides
  McFarland, Eric W.; Metiu, Horia
  Chemical Reviews (Washington, DC, United States) (2013), 113 (6), 4391-4427CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  A review; catalysis by doped oxides is discussed.
12. 12
  Schwach, P.; Hamilton, N.; Eichelbaum, M.; Thum, L.; Lunkenbein, T.; Schlögl, R.; Trunschke, A. Structure Sensitivity of the Oxidative Activation of Methane over MgO Model Catalysts: II. Nature of Active Sites and Reaction Mechanism. J. Catal. 2015, 329, 574– 587, DOI: 10.1016/j.jcat.2015.05.008
  12
  Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism
  Schwach, Pierre; Hamilton, Neil; Eichelbaum, Maik; Thum, Lukas; Lunkenbein, Thomas; Schloegl, Robert; Trunschke, Annette
  Journal of Catalysis (2015), 329 (), 574-587CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
  A series of pure, nanostructured magnesium oxides prepd. by different synthesis techniques that show different initial, but similar steady-state activity in the oxidative coupling of methane (OCM) (Schwach et al., submitted for publication) has been studied by IR and photoluminescence spectroscopy in the dehydroxylated state before the reaction and after catalysis. The abundance of structural defects, in particular mono-at. steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146 cm-1 and Lewis acid/base pairs probed by co-adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation. IR spectroscopy evidences strong polarization of C-H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high no. of mono-at. steps. It is postulated that these sites effectively promote intermol. charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissocn. of one C-H bond in the methane mol. and simultaneous formation of a superoxide species. Heterolytic splitting of C-H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temp. has been proven by the appearance of an EPR signal assocd. with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono-at. steps and the reconstruction of the MgO termination under formation of polar and faceted surfaces.
13. 13
  Zasada, F.; Piskorz, W.; Janas, J.; Gryboś, J.; Indyka, P.; Sojka, Z. Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and Their Relevance in ¹⁶O₂/¹⁸O₂ Isotopic Exchange, deN₂O, and deCH₄ Processes─a Theoretical and Experimental Account. ACS Catal. 2015, 5, 6879– 6892, DOI: 10.1021/acscatal.5b01900
  13
  Reactive Oxygen Species on the (100) Facet of Cobalt Spinel Nanocatalyst and their Relevance in 16O2/18O2 Isotopic Exchange, deN2O, and deCH4 Processes-A Theoretical and Experimental Account
  Zasada, Filip; Piskorz, Witold; Janas, Janusz; Grybos, Joanna; Indyka, Paulina; Sojka, Zbigniew
  ACS Catalysis (2015), 5 (11), 6879-6892CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Periodic spin unrestricted, gradient cor. DFT calcns. joined with atomistic thermodn. modeling and expt. were used to study the structure and stability of various reactive oxygen species (ROS) and oxygen vacancies produced on the most stable terminations of the cobalt spinel (100) surface. The surface state diagram of oxygen in a wide range of pressures and temps. was constructed for coverage varying from ΘO = 1.51 atom·nm-2 to ΘO = 6.04 atom·nm-2. A large variety of the unraveled surface ROS includes diat. superoxo (CoO-O2--CoO), peroxo (CoT-O22--CoO), and spin paired (CoO-O2-CoO) adducts along with monat. metal-oxo (CoT-O+, CoO-O2+) species, where CoT and CoO stand for the tetrahedral and octahedral cobalt surface centers, resp. There are also two kinds of peroxo species assocd. with surface oxygen ions connected with 3CoO or 2CoO and 1CoT cations ((O2O,1T-O)2- and (O3O-O)2-), resp.). The results revealed that in the oxygen pressure range of typical catalytic reactions (pO2/p° from ∼0.01 to 1), the most stable stoichiometric (100)-S surface accommodates the CoT-O22--CoO and CoO-O2-CoO adducts at temps. below 250-300 °C. In the temp. from 250 to 300 °C and from 550 to 700 °C, it is covered by the O species assocd. with the exposed tetrahedral cobalt sites (CoT-O+) or remains in a bare state. In more reducing conditions (T > 550-700 °C), the (100)-S facet is readily defected due to trigonal oxygen (O2O,1T) release and formation of surface oxygen vacancies. The reactivity of surface ROS was tested in 16O2/18O2 isotopic exchange, N2O decompn., and oxidn. of CH4 and CO model reactions, carried over Co3O4 and Co318O4 nanocryst. samples with the predominant (100) faceting revealed by high angle angular dark field STEM examn. The CoO-O2+ adducts assocd. with octahedral cobalt sites, as well as the peroxo (O2O,1T-O)2- and (O3O-O)2- surface species being thermodynamically unstable are involved in surface oxygen recombination processes, probed by 16O2/18O2 exchange and N2O decompn. It was shown that at low temps. CO is oxidized by the suprafacial CoO-O2-CoO and CoT-O2-CoO diat. oxygen, whereas in CH4 activation, the highly reactive cobalt-oxo species (CoT-O+) are involved. Above 600 °C at pO2/p° = 0.01, due to the onset of oxygen vacancy formation, the suprafacial methane oxidn. gradually changes into the intrafacial Mars-van Krevelen scheme. The constructed surface phase diagram was used for rationalization of the obtained catalytic data, allowing delineation of the specific role of the chem. state of the cobalt spinel surface in the investigated processes, as well as the range of the corresponding temps. and oxygen pressures. It also provides a convenient background for mol. understanding of remarkable activity of Co3O4 in many other catalytic redox reactions.
14. 14
  Hävecker, M.; Wrabetz, S.; Kröhnert, J.; Csepei, L.-I.; Naumann d’Alnoncourt, R.; Kolen’ko, Y. V.; Girgsdies, F.; Schlögl, R.; Trunschke, A. Surface Chemistry of Phase-Pure M1Movtenb Oxide During Operation in Selective Oxidation of Propane to Acrylic Acid. J. Catal. 2012, 285, 48– 60, DOI: 10.1016/j.jcat.2011.09.012
  There is no corresponding record for this reference.
15. 15
  Amakawa, K. How Strain Affects the Reactivity of Surface Metal Oxide Catalysts. Angew. Chem., Int. Ed. 2013, 52, 13553– 13557, DOI: 10.1002/anie.201306620
  15
  How Strain Affects the Reactivity of Surface Metal Oxide Catalysts
  Amakawa, Kazuhiko; Sun, Lili; Guo, Chunsheng; Haevecker, Michael; Kube, Pierre; Wachs, Israel E.; Lwin, Soe; Frenkel, Anatoly I.; Patlolla, Anitha; Hermann, Klaus; Schloegl, Robert; Trunschke, Annette
  Angewandte Chemie, International Edition (2013), 52 (51), 13553-13557CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Highly dispersed molybdenum oxide supported on mesoporous silica SBA-15 has been prepd. by anion exchange resulting in a series of catalysts with changing Mo densities (0.2-2.5 Mo atoms nm-2). X-ray absorption, UV/Vis, Raman, and IR spectroscopy indicate that doubly anchored tetrahedral dioxo MoO4 units are the major surface species at all loadings. Higher reducibility at loadings close to the monolayer measured by temp.-programmed redn. and a steep increase in the catalytic activity obsd. in metathesis of propene and oxidative dehydrogenation of propane at 8% of Mo loading are attributed to frustration of Mo oxide surface species and lateral interactions. Based on DFT calcns., NEXAFS spectra at the O-K-edge at high Mo loadings are explained by distorted MoO4 complexes. Limited availability of anchor silanol groups at high loadings forces the MoO4 groups to form more strained configurations. The occurrence of strain is linked to the increase in reactivity.
16. 16
  Davydov, A. A. Molecular Spectroscopy of Oxide Catalyst Surfaces; John Wiley & Sons Ltd.: Chichester, U.K., 2003.
  There is no corresponding record for this reference.
17. 17
  Wernbacher, A. M.; Kube, P.; Hävecker, M.; Schlögl, R.; Trunschke, A. Electronic and Dielectric Properties of MoV-Oxide (M1 Phase) under Alkane Oxidation Conditions. J. Phys. Chem. C 2019, 123, 13269– 13282, DOI: 10.1021/acs.jpcc.9b01273
  17
  Electronic and dielectric properties of MoV-oxide (M1 Phase) under alkane oxidation conditions
  Wernbacher, Anna M.; Kube, Pierre; Haevecker, Michael; Schloegl, Robert; Trunschke, Annette
  Journal of Physical Chemistry C (2019), 123 (21), 13269-13282CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Isostructural orthorhombic oxides of the general formula (Mo,V,Te,Sb,Nb,Ta)Ox are an important class of solids, which are interesting as catalysts for oxidn. of light alkanes. The authors investigated relations between the electronic properties of MoV-oxide (orthorhombic M1 phase) and its catalytic performance in the oxidn. of ethane, propane, and n-butane. Operando cond. and permittivity measurements were performed and complemented by near-ambient-pressure XPS. In contrast to the n-type MoVTeNb-oxide, MoV-oxide showed p-type semiconducting behavior. The cond. of the sample adapted sensitively to the surrounding atm., not only to alkane chain lengths but also to reactant conversion levels. However, no measurable change in band bending depending on the alkane chain length was obsd., indicating that the gas-phase-dependent surface potential barrier, which controls the charge transfer between reactants and catalyst, is less pronounced or missing in dry alkane oxidn. feeds. The addn. of steam in propane oxidn. led to a decrease of its cond. and work function. Steam significantly influenced the surface layer on MoV-oxide, resulting in an enrichment of covalently bonded V5+ species and surface hydroxylation. A small change in the surface potential barrier induced by wet propane oxidn. feed can contribute to a modification of the bulk-surface charge transfer and improved selectivity to acrylic acid.
18. 18
  Wernbacher, A. M.; Eichelbaum, M.; Risse, T.; Cap, S.; Trunschke, A.; Schlögl, R. Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts. J. Phys. Chem. C 2019, 123, 8005– 8017, DOI: 10.1021/acs.jpcc.8b07417
  18
  Operando Electrical Conductivity and Complex Permittivity Study on Vanadia Oxidation Catalysts
  Wernbacher, Anna M.; Eichelbaum, Maik; Risse, Thomas; Cap, Sebastien; Trunschke, Annette; Schloegl, Robert
  Journal of Physical Chemistry C (2019), 123 (13), 8005-8017CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The elec. cond. and its real and imaginary permittivity parameters were studied over 2 V-contg. catalysts for the selective oxidn. of n-butane to maleic anhydride. Parameter variation under isothermal conditions allowed detn. of multiple steady-state conditions for catalytic performance and charge carrier dynamics. One sample was the n-type semiconductor V2O5-x with low selectivity, and the other sample was the p-type semiconductor vanadyl pyrophosphate (VPP) with high selectivity for the target product. Well-resolved cond. parameters supported by in situ UV-visible studies allowed correlations between performance and charge carrier dynamics. A concept for interpreting the trends is presented, and consequences for further anal. work as well as for material design are derived.
19. 19
  Koch, G. Surface Conditions That Constrain Alkane Oxidation on Perovskites. ACS Catal. 2020, 10, 7007– 7020, DOI: 10.1021/acscatal.0c01289
  19
  Surface Conditions That Constrain Alkane Oxidation on Perovskites
  Koch, Gregor; Haevecker, Michael; Teschner, Detre; Carey, Spencer J.; Wang, Yuanqing; Kube, Pierre; Hetaba, Walid; Lunkenbein, Thomas; Auffermann, Gudrun; Timpe, Olaf; Rosowski, Frank; Schloegl, Robert; Trunschke, Annette
  ACS Catalysis (2020), 10 (13), 7007-7020CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The crystal structure of perovskites can incorporate a wide variety of cations, which makes this class of materials so interesting for studies of links between solid-state chem. and catalysis. Perovskites are known as typical total combustion catalysts in hydrocarbon oxidn. reactions. The fundamental question that we investigate here is whether surface modifications of perovskites can lead to the formation of valuable reaction products in alkane oxidn. We studied the effect of segregated two-dimensional surface nanostructures on selectivity to propene in the oxidative dehydrogenation of propane. Manganese-based perovskites AMnO3 (A = La, Sm) were prepd. by combustion and hydrothermal synthesis. Bulk and surface structures were investigated by X-ray diffraction, temp.-programmed redn., aberration-cor. scanning transmission electron microscopy (STEM), multiwavelength Raman, and ambient-pressure XPS (AP-XPS) in combination with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Surface oxygen species responsible for C-H activation were distinguished by AP-XPS on the basis of a rigorous in situ anal. of the O 1s spectra recorded under a broad range of reaction conditions. Signals at 529.2, 530.1, 530.9, 531.2, and 531.8 eV were attributed to lattice O, defect-affected O, surface O, oxygen in carbonates, and hydroxyl groups, resp. Operando AP-XPS revealed crit. surface features, which occur under catalyst operation. The catalyst performance depends on the synthesis technique and the reaction conditions. In presence of a two-dimensional MnOx surface phase, addn. of steam to the feed resulted in an increase in selectivity to the partial oxidn. product propene to practically relevant values. The selectivity increase is related to the presence of Mn in a low oxidn. state (2+/3+), an increased concn. of hydroxyl groups, and a higher abundance of adsorbed activated oxygen species on the catalyst surface. The surface anal. of a working catalyst highlights the importance of the termination layer of polycryst. perovskites as a genuine property implemented by catalyst prepn. Such a termination layer controls the chem. properties and reactivity of perovskites. The information provides input for the development of realistic models that can be used by theory to predict functional properties.
20. 20
  Che, M.; Tench, A. J. Characterization and Reactivity of Mononuclear Oxygen Species on Oxide Surfaces. Adv. Catal. 1982, 31, 77– 133, DOI: 10.1016/S0360-0564(08)60453-8
  20
  Characterization and reactivity of mononuclear oxygen species on oxide surfaces
  Che, M.; Tench, A. J.
  Advances in Catalysis (1982), 31 (), 77-133CODEN: ADCAAX; ISSN:0065-2342.
  A review on the formation, stability, and reactivity of O- on the surfaces of oxide catalysts, characterization of O- by ESR and optical spectroscopy, and the reactivity of surface O2- species. 193 Refs.
21. 21
  Che, M.; Tench, A. J. Characterization and Reactivity of Molecular Oxygen Species on Oxide Surfaces. Adv. Catal. 1983, 32, 1– 148, DOI: 10.1016/S0360-0564(08)60439-3
  21
  Characterization and reactivity of molecular oxygen species on oxide surfaces
  Che, M.; Tench, A. J.
  Advances in Catalysis (1983), 32 (), 1-148CODEN: ADCAAX; ISSN:0065-2342.
  A review with over 470 refs.
22. 22
  Nováková, J. Isotopic Exchange of Oxygen ¹⁸O between the Gaseous Phase and Oxide Catalysts. Catal. Rev. 1971, 4, 77– 113, DOI: 10.1080/01614947108075486
  There is no corresponding record for this reference.
23. 23
  Choi, S. O.; Penninger, M.; Kim, C. H.; Schneider, W. F.; Thompson, L. T. Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La_1–xSr_xCoO₃ Perovskite Catalysts. ACS Catal. 2013, 3, 2719– 2728, DOI: 10.1021/cs400522r
  23
  Experimental and Computational Investigation of Effect of Sr on NO Oxidation and Oxygen Exchange for La1-xSrxCoO3 Perovskite Catalysts
  Choi, Sang Ok; Penninger, Michael; Kim, Chang Hwan; Schneider, William F.; Thompson, Levi T.
  ACS Catalysis (2013), 3 (12), 2719-2728CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  NO oxidn. rates over La1-xSrxCoO3 (x = 0-0.3) perovskite catalysts are reported as a function of Sr doping in the absence and presence of NO2 in the feed. Sr substitution is found to increase the rate of oxidn. and to diminish the inhibitory influence of NO2. Temp. programmed desorption and isotopic exchange (TPIE) expts. were used to identify surface species and oxygen exchange processes expected to correlate with NO oxidn. activity. Oxygen exchange in the LaCoO3 perovskites occurred primarily through a heteroexchange process that was enhanced by doping with Sr. D. functional theory (DFT) calcns. were used to further investigate the oxygen exchange processes on (100) facets of undoped and doped LaCoO3. Vacancy formation is predicted to be more facile on CoO2-terminated than LaO-terminated surfaces. The Sr dopant segregates to the LaO-terminated surface and diminishes oxygen bonding consistent with the TPIE results. The results suggest a model in which multiple oxygen species contribute to low- and high-temp. oxygen exchange.
24. 24
  Winter, E. R. S. The Reactivity of Oxide Surfaces. Adv. Catal. 1958, 10, 196– 241, DOI: 10.1016/S0360-0564(08)60408-3
  24
  Reactivity of oxide surfaces
  Winter, E. R. S.
  Advances in Catalysis (1958), 10 (), 196-241CODEN: ADCAAX; ISSN:0360-0564.
  Reviews with references. Cf. C.A. 51, 16014f.
25. 25
  Winter, E. R. S. Exchange Reactions of Oxides. Part IX. J. Chem. Soc. A: Inorg., Phys., Theor. 1968, 2889– 2902, DOI: 10.1039/j19680002889
  25
  Exchange reactions of oxide. IX
  Winter, Edgar R. S.
  Journal of the Chemical Society [Section] A: Inorganic, Physical, Theoretical (1968), (12), 2889-902CODEN: JCSIAP; ISSN:0022-4944.
  The kinetics of exchange of 18O between enriched O gas and 38 inorg. oxide and 1 oxy-acid salt has been examd. in detail. Most of the exchange reactions occur by a dissociative at. mechanism confined to the surface layer of O ions. For these there is a strong compensation effect between A0 and E in the rate expression: E shows a systematic fall with increasing size of the unit-crystal cell and related crystal parameters: the plots sep. the oxide into structure-dependent groups. The slow stage is the desorption of O. The oxide PbO, PdO, AgO, and CuO exchange the surface layer by a mol. reactions which also occurs to an appreciable extent, together with the at. mechanism, on a no. of other oxide. Na2WO4, V2O5, MoO3, and WO3 exchange the whole of the bulk O with the gas phase by a combination of both mechanisms. SiO2 and GeO2 are inactive. Possible mechanisms for the two main reactions are discussed.
26. 26
  Klier, K.; Nováková, J.; Jíru, P. Exchange Reactions of Oxygen between Oxygen Molecules and Solid Oxides. J. Catal. 1963, 2, 479– 484, DOI: 10.1016/0021-9517(63)90003-4
  26
  Exchange reactions of oxygen between oxygen molecules and solid oxides
  Klier, K.; Novakova, J.; Jiru, P.
  Journal of Catalysis (1963), 2 (6), 479-84CODEN: JCTLA5; ISSN:0021-9517.
  A thoretical treatment of the kinetics of isotopic exchange reactions between O mols. and a solid oxide is developed. Good agreement with expt. was found for changes in concn. of 18O18O, 16O18O, and 16O16O with time over MgO.
27. 27
  Boreskov, G. K. The Catalysis of Isotopic Exchange in Molecular Oxygen. Adv. Catal. 1965, 15, 285– 339, DOI: 10.1016/S0360-0564(08)60556-8
  There is no corresponding record for this reference.
28. 28
  Doornkamp, C.; Clement, M.; Gao, X.; Deo, G.; Wachs, I. E.; Ponec, V. The Oxygen Isotopic Exchange Reaction on Vanadium Oxide Catalysts. J. Catal. 1999, 185, 415– 422, DOI: 10.1006/jcat.1999.2490
  28
  The Oxygen Isotopic Exchange Reaction on Vanadium Oxide Catalysts
  Doornkamp, C.; Clement, M.; Gao, X.; Deo, G.; Wachs, I. E.; Ponec, V.
  Journal of Catalysis (1999), 185 (2), 415-422CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
  The reactivity of lattice oxygen of vanadium oxide catalysts was studied with the oxygen isotopic exchange reaction. The reactivity of pure V2O5 is compared with the reactivity of Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, δ-VOPO4, and (VO)2P2O7. According to their behavior in the oxygen exchange reaction, two types of vanadium oxide catalysts could be distinguished. The first type of catalysts only showed exchange activity in the R2 exchange mechanism and the second type showed activity in both the R1 and R2 exchange mechanisms (mechanisms in which, resp., one or two oxygen atoms of the gas phase mol. are exchanged with oxygen atoms of the metal oxide). The catalysts which belong to the first group are bulk V2O5 and δ-VOPO4 and the catalysts which belong to the second group are Li0.33V2O5, V2O5/TiO2, V2O5/Al2O3, V2O5/SiO2, and (VO)2P2O7. If only the R2 mechanism is obsd. then diffusion of lattice oxygen is probably faster than when both the R1 and R2 mechanisms are obsd. The activity of the supported vanadium oxide catalysts in the oxygen exchange reaction is dependent on the support. The reactivity order is V2O5/TiO2>V2O5/Al2O3∼V2O5/SiO2. (c) 1999 Academic Press.
29. 29
  Doornkamp, C.; Clement, M.; Ponec, V. The Isotopic Exchange Reaction of Oxygen on Metal Oxides. J. Catal. 1999, 182, 390– 399, DOI: 10.1006/jcat.1998.2377
  29
  The Isotopic Exchange Reaction of Oxygen on Metal Oxides
  Doornkamp, C.; Clement, M.; Ponec, V.
  Journal of Catalysis (1999), 182 (2), 390-399CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
  Two methods have been used to det. the individual rate consts. of the isotopic exchange reaction of oxygen on metal oxides. The best way to det. the three rate consts. is to use the kinetic model of Klier and co-workers and fit the equations to the exptl. data. The method of Tsuchiya and co-workers can be used to check the results obtained by the method of Klier and co-workers. The three rate consts., R0, R1, and R2, of the three different exchange mechanisms are detd. for the period IV metal oxides at various temps. The rate consts. are correlated with parameters characterizing the oxide, i.e. the position of the metal element in the periodic table and the av. metal-oxygen bond strength. (c) 1999 Academic Press.
30. 30
  Wachs, I. E.; Jehng, J.-M.; Deo, G.; Weckhuysen, B. M.; Guliants, V. V.; Benziger, J. B.; Sundaresan, S. Fundamental Studies of Butane Oxidation over Model-Supported Vanadium Oxide Catalysts: Molecular Structure-Reactivity Relationships. J. Catal. 1997, 170, 75– 88, DOI: 10.1006/jcat.1997.1742
  30
  Fundamental studies of butane oxidation over model-supported vanadium oxide catalysts: molecular structure-reactivity relationships
  Wachs, Israel E.; Jehng, Jih-Mirn; Deo, Goutam; Weckhuysen, Bert M.; Guliants, V. V.; Benziger, J. B.; Sundaresan, S.
  Journal of Catalysis (1997), 170 (1), 75-88CODEN: JCTLA5; ISSN:0021-9517. (Academic)
  The oxidn. of n-butane to maleic anhydride was investigated over a series of model-supported vanadia catalysts where the vanadia phase was present as a two-dimensional metal oxide overlayer on the different oxide supports (TiO2, ZrO2, CeO2, Nb2O5, Al2O3, and SiO2). No correlation was found between the properties of the terminal V=O bond and the butane oxidn. turnover frequency (TOF) during in situ Raman spectroscopy study. Furthermore, neither the n-butane oxidn. TOF nor maleic anhydride selectivity was related to the extent of redn. of the surface vanadia species. The n-butane oxidn. TOF was essentially independent of the surface vanadia coverage, suggesting that the n-butane activation requires only one surface vanadia site. The maleic anhydride TOF, however, increased by a factor of 2-3 as the surface vanadia coverage was increased to monolayer coverage. The higher maleic anhydride TOF at near monolayer coverages suggests that a pair of adjacent vanadia sites may efficiently oxidize n-butane to maleic anhydride, but other factors may also play a contributing role (increase in surface Bronsted acidity and decrease in the no. of exposed support cation sites). Varying the specific oxide support changed the n-butane oxidn. TOF by ca. 50(Ti > Ce > Zr ∼ Nb > Al > Si) as well as the maleic anhydride selectivity. The maleic anhydride selectivity closely followed the Lewis acid strength of the oxide support cations, Al > Nb > Ti > Si > Zr > Ce. The addn. of acidic surface metal oxides (W, Nb, and P) to the surface vanadia layer was found to have a beneficial effect on the n-butane oxidn. TOF and the maleic anhydride selectivity. The creation of bridging V-O-P bonds had an esp. pos. effect on the maleic anhydride selectivity.
31. 31
  Klug, C. A.; Kroeker, S.; Aguiar, P. M.; Zhou, M.; Stec, D. F.; Wachs, I. E. Insights into Oxygen Exchange between Gaseous O₂ and Supported Vanadium Oxide Catalysts Via ¹⁷O NMR. Chem. Mater. 2009, 21, 4127– 4134, DOI: 10.1021/cm802680x
  31
  Insights into Oxygen Exchange Between Gaseous O2 and Supported Vanadium Oxide Catalysts via 17O NMR
  Klug, Christopher A.; Kroeker, Scott; Aguiar, Pedro M.; Zhou, Min; Stec, Donald F.; Wachs, Israel E.
  Chemistry of Materials (2009), 21 (18), 4127-4134CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  Vanadium oxide ref. compds., KVO3 and V2O5, and supported vanadium oxide catalysts (Al2O3, TiO2, and SiO2) were investigated using magic angle sample spinning 17O NMR. All samples were 17O-enriched using gas-solid exchange. Extn. of chem. shift and quadrupolar coupling information for the model compds. KVO3 and V2O5 was performed via the simulation of MAS spectra obtained in one-pulse expts. and the observations were consistent with their known bulk structures. For the supported vanadia catalysts, it was found that the oxygen exchange process is dominated by 17O signal from the catalyst oxide supports. Spectra obtained via rotor-synchronized spin echoes revealed addnl. wide lines for Al2O3 and TiO2 supported vanadia catalysts that arise from 17O in the surface vanadia species of the catalysts. Addnl. 17O-51V TRAPDOR (TRAnsfer of Populations in DOuble Resonance) expts. support this assignment. The wide lines suggest that the local environments of the 17O nuclei assocd. with the dehydrated surface vanadia species are extremely heterogeneous and fall in the range of oxygen in singly (V=O) and/or doubly coordinated environments (V-O-V or V-O-Support). The relatively small total amt. of 17O assocd. with the surface vanadia species contrasts with oxygen exchange models which commonly assume only the surface vanadium oxide layer is involved. These results demonstrate that the isotopic exchange of mol. O2 with supported metal oxide catalysts, esp. supported vanadia catalysts, is a much more complex process than originally perceived.
32. 32
  Avdeev, V. I.; Bedilo, A. F. Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VO_x/TiO₂. J. Phys. Chem. C 2013, 117, 2879– 2887, DOI: 10.1021/jp311322b
  32
  Molecular Mechanism of Oxygen Isotopic Exchange over Supported Vanadium Oxide Catalyst VOx/TiO2
  Avdeev, Vasilii I.; Bedilo, Alexander F.
  Journal of Physical Chemistry C (2013), 117 (6), 2879-2887CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Detailed mol. mechanisms of oxygen isotopic exchange over VOx/TiO2 catalyst following the R0, R1, and R2 mechanisms were studied using periodic DFT anal. of possible pathways by the CI-NEB method. The electronic structures of surface VOx species formed on the VOx/TiO2 model surface after interaction of mol. oxygen with fully oxidized O=V5+-O-V5+=O sites and reduced V3+-O-V3+ sites were analyzed. We found a no. of metastable surface structures that are potential intermediates in the exchange reaction pathways. We present evidence that adsorption of two gas-phase oxygen mols. on a reduced V3+-O-V3+ site leads to the formation of a superoxide complex, followed by its transformation into a peroxide complex with low activation energy about E* = 0.04 eV (0.92 kcal/mol). Subsequent transformation of this surface superoxide-peroxide species follows the Langmuir-Hinshelwood mechanism without participation of lattice oxygen along the R0 reaction pathway. We demonstrate that adsorption of mol. oxygen on fully oxidized O=V5+-O-V5+=O sites results in the formation of either monodentate V<(O3) or bidentate V<(O3)>V surface ozonide species. Their subsequent transformations result in oxygen isotopic exchange following the R1 or R2 mechanisms with the activation energies in the range of 1.44 to 1.64 eV for the R1 mechanism and 1.81 eV for the R2 one. These processes follow the Eley-Rideal mechanism with participation of one or two lattice oxygen atoms, correspondingly.
33. 33
  Avdeev, V. I.; Bedilo, A. F. Electronic Structure of Oxygen Radicals on the Surface of VO_x/TiO₂ Catalysts and Their Role in Oxygen Isotopic Exchange. J. Phys. Chem. C 2013, 117, 14701– 14709, DOI: 10.1021/jp404921d
  33
  Electronic Structure of Oxygen Radicals on the Surface of VOx/TiO2 Catalysts and Their Role in Oxygen Isotopic Exchange
  Avdeev, Vasilii I.; Bedilo, Alexander F.
  Journal of Physical Chemistry C (2013), 117 (28), 14701-14709CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The electronic structure of oxygen radicals formed by adsorption of gas-phase oxygen on partially reduced sites of supported vanadium oxide catalyst V4+Ox/TiO2 has been studied by periodic DFT. The unpaired electron d. in the radicals is transferred from the paramagnetic V4+(3d1) ion to the adsorbed oxygen atoms resulting in the formation of surface oxygen radicals: at. O-, superoxide O2-, and ozonide O3-. These radical species exhibit higher reactivity compared to the surface oxygen species stabilized on fully oxidized diamagnetic V5+(3d0) ions. Oxygen isotopic exchange over O- radicals has been investigated by the climbing image nudged elastic band (CI-NEB) method. We show that mol. oxygen can exchange with the lattice oxygen of the surface paramagnetic radicals V5+O- with low activation energy of about 14 kcal/mol, close to the value exptl. obsd. for some heterolytic R1 oxygen exchange reactions on vanadia catalysts. The obtained data suggest that O- radicals formed as short-lived intermediates at elevated temps. are likely to be the active sites of the oxygen exchange following the R1 mechanism. The properties of oxygen radicals and their possible role in catalytic oxidn. processes taking place over bulk and supported metal oxide catalysts are discussed. It is suggested that oxygen radicals can be the active species in catalytic oxidn. reactions.
34. 34
  Koranne, M. M.; Goodwin, J. G.; Marcelin, G. Oxygen Involvement in the Partial Oxidation of Methane on Supported and Unsupported V₂O₅. J. Catal. 1994, 148, 378– 387, DOI: 10.1006/jcat.1994.1218
  34
  Oxygen involvement in the partial oxidation of methane on supported and unsupported V2O5
  Koranne, Manoj M.; Goodwin, James G., Jr.; Marcelin, George
  Journal of Catalysis (1994), 148 (1), 378-87CODEN: JCTLA5; ISSN:0021-9517.
  O exchange of O2 with V2O5/SiO2, V2O5/Al2O3, V2O5, SiO2, and Al2O3 was studied using steady-state isotopic transient kinetic anal. It was found that bulk V2O5, V2O5/SiO2, V2O5/Al2O3, and Al2O3 exhibited some oxygen exchange capability, whereas SiO2 exhibited negligible oxygen exchange capability under the conditions used. Oxygen exchange reactions were also studied over supported vanadia catalysts under steady-state oxidn. reaction conditions. O exchange of O2 with V2O5/SiO2 increased significantly in the presence of methane. This is attributed to a redox reaction making the surface more active for O exchange in the presence of the methane. The involvement of catalyst oxygen in the formation of the products HCHO, CO, and CO2 was demonstrated. However, an estn. of the contribution of the lattice oxygen in the formation of the products is complicated by the presence of secondary O exchange. The total amt. of 16O exchanged with the feed O2 and the products indicates that the oxygen assocd. with silica or the vanadia-silica interface is also involved in the exchange process. In general, O exchange behavior of various product species with V2O5/Al2O3 was found to be similar to that with V2O5/SiO2. However, unlike V2O5/SiO2, the O exchange of O2 with V2O5/Al2O3 did not increase significantly in the presence of methane. This was attributed to the differences in the interactions of vanadia with silica and alumina. It is speculated that the O assocd. with highly dispersed tetrahedral surface vanadia is involved in a primary oxidn. reaction, whereas O assocd. with the bulk-like vanadia and with the support is involved either in secondary oxidn. reactions of HCHO or CO or in secondary O exchange of various O contg. product species. Caution must be taken in making any conclusions about the source of reactive oxygen during partial oxidn. on oxide catalysts based on isotopic oxygen studies due to the ease of secondary oxygen exchange.
35. 35
  Kera, Y.; Teratani, S.; Hirota, K. Infrared Spectra of Surface V═O Bond of Vanadium Pentoxide. Bull. Chem. Soc. Jpn. 1967, 40, 2458– 2458, DOI: 10.1246/bcsj.40.2458
  35
  Infrared spectra of surface vanadium-oxygen double bond of vanadium pentoxide
  Kera, Yoshiya; Teratani, Shousuke; Hirota, Kozo
  Bulletin of the Chemical Society of Japan (1967), 40 (10), 2458CODEN: BCSJA8; ISSN:0009-2673.
  V2O5 was treated for 24 hrs. at 490° with a mixt. of CO2 and O, both contg. 60 at. % 18O. The ir spectra of V2O5 taken before and after treatment show a new absorption max. at 962 cm.-1 after treatment which increases in intensity with increased 18O concn. in the CO2 and did not appear in V2O5 treated with 18O alone. The concn. of 18O in CO2 diminished while that in O remained const. This new band is attributed to the V:18O stretching band in agreement with calcd. values, and supports the presence of exchangeable O in the surface V:O bond.
36. 36
  Hirota, K.; Kera, Y.; Teratani, S. Carbon Monoxide Oxidation with an Oxygen Tracer over a Vanadium Pentoxide Catalyst. J. Phys. Chem. 1968, 72, 3133– 3141, DOI: 10.1021/j100855a010
  36
  Carbon monoxide oxidation with an oxygen tracer over a vanadium pentoxide catalyst
  Hirota, Kozo; Kera, Yoshiya; Teratani, Shousuke
  Journal of Physical Chemistry (1968), 72 (9), 3133-41CODEN: JPCHAX; ISSN:0022-3654.
  The oxidn. of CO with gaseous O on a powd. V2O5 catalyst was studied at 345-410° by using 18O (about 3 atom %) as the tracer. When the lattice O of the catalyst was partially substituted by concd. 18O before the expt., the percentage of 18O in the produced CO2 changed gradually during the oxidn., in accordance with the 18O concn. in the catalyst, even though the percentage of 18O in oxygen and in CO was practically invariant. When a mixt. of O and CO2, both contg. about 60 atom % of 18O, was brought into contact with the catalyst at 370°, only the O in CO2 was exchangeable with the lattice O. The exchange rate of CO2 was increased by the presence of CO. The oxidn. of CO on V2O5 was explained on the basis of the oxidn.-redn. mechanism. The relative rate of each elementary step and the surface intermediates during the reaction are discussed, and a detailed reaction scheme is proposed.
37. 37
  Kera, Y.; Hirota, K. Infrared Spectroscopic Study of Oxygen Species in Vanadium Pentoxide with Reference to Its Activity in Catalytic Oxidation. J. Phys. Chem. 1969, 73, 3973– 3981, DOI: 10.1021/j100845a070
  37
  Infrared spectroscopic study of oxygen species in vanadium pentoxide with reference to its activity in catalytic oxidation
  Kera, Yoshiya; Hirota, Kozo
  Journal of Physical Chemistry (1969), 73 (11), 3973-81CODEN: JPCHAX; ISSN:0022-3654.
  To clarify the nature of the exchangeable O in V2O5 catalyst during the oxidn. reaction, an ir technique combined with the 18O tracer technique w as applied to the samples isotopically substituted by treatment with 18 O labeled CO2. The bonds appearing at 1019 and 818 cm-1 can be identified to be the V:O and V-O-V groups, resp., because the corresponding shift of both bonds appeared by isotopic substitution. Exchange expts. between gaseous CO2 and V2O5 were carried out at 290, 370, 410, and 450°, and the 18O balance was investigated, taking the intensity of the above ir bands into account. Thus, the behavior of the O species at the surface and in the bulk could be detd. (a) Direct exchange of the oxygen on the surface V:O groups with gaseous O is very rapid at the initial stage of reaction. (b) Oxygen exchange between V:O and V-O-V groups may occur rapidly near the lattice dislocations on the (010) surface as well as on other surfaces. (c) The exchange within the V-O-V net plane is the easiest of all the processes, so that it becomes predominant in the reaction even at the intermediate stage. (d) The exchange between the V:O and V-O-V groups on the surfaces is important from the standpoint of catalytic oxidn.
38. 38
  Fletcher, W. H.; Rayside, J. S. High Resolution Vibrational Raman Spectrum of Oxygen. J. Raman Spectrosc. 1974, 2, 3– 14, DOI: 10.1002/jrs.1250020102
  38
  High resolution vibrational Raman spectrum of oxygen
  Fletcher, William H.; Rayside, John S.
  Journal of Raman Spectroscopy (1974), 2 (1), 3-14CODEN: JRSPAF; ISSN:0377-0486.
  The fundamental vibrational band of O2 was examd. with a resoln. of 0.05 cm-1 for the Q branch and 0.40 cm-1 for the S and O branches. All lines of the Q branch were clearly resolved except Q(1) and Q(3). Calcd. mol. parameters agree with those previously reported from microwave and electronic spectra. Line width measurements made in the Q branch and on 3 S branch lines, using a resoln. of 0.05 cm-1, are in fair agreement with previously measured and calcd. line widths for pure rotational Raman lines.
39. 39
  Pushkarev, V. V.; Kovalchuk, V. I.; d’Itri, J. L. Probing Defect Sites on the CeO₂ Surface with Dioxygen. J. Phys. Chem. B 2004, 108, 5341– 5348, DOI: 10.1021/jp0311254
  39
  Probing Defect Sites on the CeO2 Surface with Dioxygen
  Pushkarev, Vladimir V.; Kovalchuk, Vladimir I.; D'Itri, Julie L.
  Journal of Physical Chemistry B (2004), 108 (17), 5341-5348CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  In situ Raman spectroscopy of adsorbed dioxygen was used to characterize electron defects on the surface of nanocryst. cerium oxide that was partially reduced with H2 and CO. Via 16O/18O isotope substitution, the bands in the range of 1135-1127 and 877-831 cm-1 were assigned to the O-O stretching vibration of dioxygen species bound to one- and two-electron defects on the CeO2 surface to form superoxide (O2-) and peroxide (O22-) species, resp. A band at 357 cm-1 was attributed to the cerium-oxygen vibration of the adsorbed superoxides, O2-, whereas the bands at 538 and 340 cm-1 were assigned to the asym. and sym. cerium-oxygen vibrations of the surface peroxides, O22-, resp. The dynamics of the defect annihilation that results from surface reoxidn. by adsorbed dioxygen species during temp.-programmed expts. allowed peroxide species adsorbed on isolated and aggregated two-electron defects to be distinguished. A general approach to investigate the reactivity of different surface dioxygen species toward reductants was demonstrated using CO oxidn. as a probe reaction.
40. 40
  Choi, Y. M.; Abernathy, H.; Chen, H.-T.; Lin, M. C.; Liu, M. Characterization of O₂–CeO₂ Interactions Using in Situ Raman Spectroscopy and First-Principle Calculations. ChemPhysChem 2006, 7, 1957– 1963, DOI: 10.1002/cphc.200600190
  40
  Characterization of O2-CeO2 interactions using in situ Raman spectroscopy and first-principle calculations
  Choi, Y. M.; Abernathy, Harry; Chen, Hsin-Tsung; Lin, M. C.; Liu, Meilin
  ChemPhysChem (2006), 7 (9), 1957-1963CODEN: CPCHFT; ISSN:1439-4235. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Interactions between O2 and CeO2 are examd. exptl. using in situ Raman spectroscopy and theor. using d.-functional slab-model calcns. Two distinct O bands appear at 825 and 1131 cm-1, corresponding to peroxo- and superoxo-like species, resp., when partially reduced CeO2 is exposed to 10% O2. Periodic d.-functional theory (DFT) calcns. aid the interpretation of spectroscopic observations and provide energetic and geometric information for the dioxygen species adsorbed on CeO2. The O2 adsorption energies on unreduced CeO2 surfaces are endothermic (0.91 < ΔEads < 0.98 eV), while those on reduced surfaces are exothermic (-4.0 < ΔEads < -0.9 eV), depending on other relevant surface processes such as chemisorption and diffusion into the bulk. Partial redn. of surface Ce4+ to Ce3+ (together with formation of O vacancies) alters geometrical parameters and, accordingly, leads to a shift in the vibrational frequencies of adsorbed O species compared to those on unreduced CeO2. Also, the location of O vacancies affects the formation and subsequent dissocn. of O species on the surfaces. DFT predictions of the energetics support the exptl. observation that the reduced surfaces are energetically more favorable than the unreduced surfaces for O adsorption and redn.
41. 41
  Wu, Z.; Dai, S.; Overbury, S. H. Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts. J. Phys. Chem. C 2010, 114, 412– 422, DOI: 10.1021/jp9084876
  41
  Multiwavelength Raman Spectroscopic Study of Silica-Supported Vanadium Oxide Catalysts
  Wu, Zili; Dai, Sheng; Overbury, Steven H.
  Journal of Physical Chemistry C (2010), 114 (1), 412-422CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The mol. structure of SiO2-supported V oxide (VOx) catalysts over wide range of surface VOx d. (0.0002-8 V/nm2) was studied in detail under dehydrated conditions by in situ multiwavelength Raman spectroscopy (laser excitations at 244, 325, 442, 532, and 633 nm) and in situ UV-visible diffuse reflectance spectroscopy. Resonance Raman scattering is clearly obsd. using 244 and 325 nm excitations, whereas normal Raman scattering occurs using excitation at the 3 visible wavelengths. The observation of strong fundamentals, overtones, and combinational bands due to selective resonance enhancement effect helps clarify assignments of some of the VOx Raman bands (920, 1032, and 1060 cm-1) whose assignments were controversial. The resonance Raman spectra of dehydrated VOx/SiO2 show a V=O band at a smaller Raman shift than that in visible Raman spectra, an indication of the presence of 2 different surface VOx species on dehydrated SiO2 even at submonolayer VOx loading. Quant. estn. shows that the 2 different monomeric VOx species coexist on SiO2 surface from very low VOx loadings and transform to cryst. V2O5 at VOx loadings above the monolayer. It is postulated that 1 of the 2 monomeric VOx species has pyramidal structure and the other is in the partially hydroxylated pyramidal mode. The 2 VOx species show similar redn.-oxidn. behavior and may both participate in redox reactions catalyzed by VOx/SiO2 catalysts. This study demonstrates the advantages of multiwavelength Raman spectroscopy over conventional single-wavelength Raman spectroscopy in structural characterization of supported metal-oxide catalysts.
42. 42
  Weckhuysen, B. M.; Jehng, J.-M.; Wachs, I. E. In Situ Raman Spectroscopy of Supported Transition Metal Oxide Catalysts: ¹⁸O₂–¹⁶O₂ Isotopic Labeling Studies. J. Phys. Chem. B 2000, 104, 7382– 7387, DOI: 10.1021/jp000055n
  42
  In Situ Raman Spectroscopy of Supported Transition Metal Oxide Catalysts: 18O2-16O2 Isotopic Labeling Studies
  Weckhuysen, Bert M.; Jehng, Jih-Mirn; Wachs, Israel E.
  Journal of Physical Chemistry B (2000), 104 (31), 7382-7387CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
  The isothermal isotopic exchange reaction of 18O2 with 16O of CrO3, MoO3, Nb2O5, WO3, V2O5, and Re2O7 supported on ZrO2 has been investigated with in situ laser Raman spectroscopy. Isotopic exchange of the oxygen atoms of the supported transition metal oxides with 18O2 is difficult and requires several successive redn.-18O2 reoxidn. cycles at relatively high temps. The Raman spectroscopy data reveal that all the supported transition metal oxides are present as a monooxo species on ZrO2. This finding is consistent with the shifts calcd. from the isotopic ratios for a simple diat. oscillator, with the corresponding IR spectra of the same catalysts and with the vibrational frequencies of several monooxo ref. compds. On this basis, coordination models of the mol. structures are proposed for CrO3/ZrO2, MoO3/ZrO2, Nb2O5/ZrO2, WO3/ZrO2, V2O5/ZrO2, and Re2O7/ZrO2 catalysts under dehydrated conditions.
43. 43
  Lee, E. L.; Wachs, I. E. In Situ Raman Spectroscopy of SiO₂-Supported Transition Metal Oxide Catalysts: An Isotopic ¹⁸O-¹⁶O Exchange Study. J. Phys. Chem. C 2008, 112, 6487– 6498, DOI: 10.1021/jp076485w
  43
  In Situ Raman Spectroscopy of SiO2-Supported Transition Metal Oxide Catalysts: An Isotopic 18O-16O Exchange Study
  Lee, Edward L.; Wachs, Israel E.
  Journal of Physical Chemistry C (2008), 112 (16), 6487-6498CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  The mol. structures of dehydrated group 5-7 transition metal oxides (V2O5, Nb2O5, CrO3, MoO3, WO3, Re2O7) supported on SiO2 were investigated with time-resolved 18O-16O exchange in situ Raman spectroscopy measurements. The supported group 5-7 dehydrated surface transition metal oxides were exclusively present as isolated species on SiO2 because of the absence of bridging M-O-M vibrations. The SiO2-supported group 5 (VOx and NbOx) surface metal oxides exhibit band splitting into two Raman vibrations (M:16O and M:18O), which is consistent with monoxo surface O:M(-O-Si)3 species. The SiO2-supported group 6 (CrOx, MoOx, and WOx) surface metal oxides consist of both monoxo O:M(-O-Si)4 and dioxo (O:)2M(-O-Si)2 structures. The dioxo surface species give rise to triplet band splitting corresponding to M(:16O)2, M(:18O)2, and 18O:M:16O. Identification of the intermediate surface 18O:M:16O structure was guided by recent DFT calcns. The SiO2-supported group 7 (ReOx) metal oxide system exclusively contains trioxo surface (O:)3Re-O-Si species that give rise to quadruplet band splitting (Re(:16O)3, 18O:Re(:16O)2, (18O:)2Re:16O, and (18O:)3Re) during isotopic oxygen exchange. Excellent prediction was also achieved for the isotopic shifts for the completely 18O-exchanged surface metal oxide structures with a simple diat. oscillator model. The isotopic exchange studies reveal, for the first time, the exact no. of Raman bands for surface monoxo, dioxo, and trioxo metal oxide structures, their positions, and their band splitting characteristics during isotopic 18O-16O exchange.
44. 44
  Moisii, C.; van de Burgt, L. J.; Stiegman, A. E. Resonance Raman Spectroscopy of Discrete Silica-Supported Vanadium Oxide. Chem. Mater. 2008, 20, 3927– 3935, DOI: 10.1021/cm800095g
  44
  Resonance Raman Spectroscopy of Discrete Silica-Supported Vanadium Oxide
  Moisii, Cristina; van de Burgt, Lambertus J.; Stiegman, A. E.
  Chemistry of Materials (2008), 20 (12), 3927-3935CODEN: CMATEX; ISSN:0897-4756. (American Chemical Society)
  V oxide deposited as discrete oxovanadium groups, [(-O)3V=O], in transparent SiO2 xerogels were studied by resonance Raman spectroscopy. Spectra were collected at 351 and 257 nm excitation into two distinct absorption bands of the oxovanadium site. Three new bands assocd. with vibrations of the V oxide site were obsd. at 496, 568, and 720 cm-1. From these addnl. modes and the previously known vibrations at 1064, 1033, and 923 cm-1 an empirical force field was detd. from which a normal-mode anal. of the primary stretching vibrations of the V oxo group was carried out. This anal. indicates that for most of the obsd. bands the interfacial Si-O-V stretches are the primary component, and in fact, only the weak band at 923 cm-1 was dominated by the terminal V=O stretch. Shifts in the band positions with 18O isotopic enrichment are in general agreement with the normal-mode anal., also, the enrichment indicates that the bridging groups are generally quite labile to substitution.
45. 45
  Magg, N. Vibrational Spectra of Alumina- and Silica-Supported Vanadia Revisited: An Experimental and Theoretical Model Catalyst Study. J. Catal. 2004, 226, 88– 100, DOI: 10.1016/j.jcat.2004.04.021
  45
  Vibrational spectra of alumina- and silica-supported vanadia revisited: An experimental and theoretical model catalyst study
  Magg, Norbert; Immaraporn, Boonchuan; Giorgi, Javier B.; Schroeder, Thomas; Baumer, Marcus; Dobler, Jens; Wu, Zili; Kondratenko, Evgenii; Cherian, Maymol; Baerns, Manfred; Stair, Peter C.; Sauer, Joachim; Freund, Hans-Joachim
  Journal of Catalysis (2004), 226 (1), 88-100CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
  Oxide-supported vanadia particles were prepd. via evapn. of vanadium metal in an oxygen ambient. As support oxides, we have employed thin, well-ordered alumina and silica films grown on top of NiAl(110) and Mo(112) surfaces. According to our anal., the vanadia particles exhibit very similar morphol. on both supports but differ in the extent of particle-support interactions. It is shown that these differences in the vanadia-support interface region strongly affect the CO adsorption behavior of the particles. The measured vibrational spectra of the model systems are interpreted on the basis of DFT calcns. for model compds. and surface models for both the vanadia/silica and the vanadia/alumina system. The combined information is then compared with Raman spectra of real catalytic materials such as vanadia supported over δ-Al2O3 and mesoporous SiO2 (MCM-41) taken at different laser wavelengths. A consistent interpretation is developed, which shows that the accepted interpretation of vibrational spectra from vanadia catalysts must be revised.
46. 46
  Döbler, J.; Pritzsche, M.; Sauer, J. Vibrations of Silica Supported Vanadia: Variation with Particle Size and Local Surface Structure. J. Phys. Chem. C 2009, 113, 12454– 12464, DOI: 10.1021/jp901774t
  46
  Vibrations of Silica Supported Vanadia: Variation with Particle Size and Local Surface Structure
  Dobler, Jens; Pritzsche, Marc; Sauer, Joachim
  Journal of Physical Chemistry C (2009), 113 (28), 12454-12464CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  D. functional theory is applied to mol. models and embedded cluster models for vanadia on three different cryst. silica supports that span the range of local structures found in amorphous silica. For monomeric vanadyl sites, three different types of vibrational modes exist. Contrary to previous assignments, the V-O-Si in-phase modes occur at highest frequencies, 1086-1020 cm-1, whereas the vanadyl modes are found between 1047-1013 cm-1. The V-O-Si out-of-phase modes have the lowest frequencies in the 962-873 cm-1 range. For dimeric and polymeric sites, the frequencies are similar, but an addnl. V-O-V mode is found between 870 and 770 cm-1 and not in the 940-900 cm-1 range as assumed earlier. The vibrational frequencies of cluster and embedded cluster models are in general agreement, except that the embedded clusters predict a strong mixing of vanadia modes with modes of the silica support. This mixing changes the normal modes but has only a small effect on the frequencies. Vanadia on very special support models, like a periodic SiO2 slab consisting of hexagonal prisms, shows distinctly different frequencies with 50 cm-1 blue-shifted V-O-Si frequencies.
47. 47
  Nitsche, D.; Hess, C. Normal Mode Analysis of Silica-Supported Vanadium Oxide Catalysts: Comparison of Theory with Experiment. Catal. Commun. 2014, 52, 40– 44, DOI: 10.1016/j.catcom.2014.04.008
  47
  Normal mode analysis of silica-supported vanadium oxide catalysts: Comparison of theory with experiment
  Nitsche, David; Hess, Christian
  Catalysis Communications (2014), 52 (), 40-44CODEN: CCAOAC; ISSN:1566-7367. (Elsevier B.V.)
  The vibrational structure of silica-supported vanadium oxide species has been studied by normal mode anal. using polyhedral oligomeric silsesquioxanes (POSSs) to describe the silica support. The anal. reveals that the vanadium oxide-related vibrational bands are characterized by significant contributions of several force consts. Their discussion in terms of single bond-unit designators is therefore not adequate. The consideration of the silica support is shown to be of importance for the vibrational spectrum. The theor. results are fully consistent with exptl. data for a silica-supported vanadium oxide catalyst with a vanadium coverage of 0.7 atoms/nm2.
48. 48
  Oyama, S. T.; Went, G. T.; Lewis, K. B.; Bell, A. T.; Somorjai, G. A. Oxygen Chemisorption and Laser Raman Spectroscopy of Unsupported and Silica-Supported Vanadium Oxide Catalysts. J. Phys. Chem. 1989, 93, 6786– 6790, DOI: 10.1021/j100355a041
  48
  Oxygen chemisorption and laser Raman spectroscopy of unsupported and silica-supported vanadium oxide catalysts
  Oyama, S. Ted; Went, Gregory T.; Lewis, Kenneth B.; Bell, Alexis T.; Somorjai, Gabor A.
  Journal of Physical Chemistry (1989), 93 (18), 6786-90CODEN: JPCHAX; ISSN:0022-3654.
  An O chemisorption method was developed for measuring the active surface area of supported and unsupported V2O5 following redn. in H. To achieve complete redn. of the V2O5 surface without reducing the bulk, redn. must be carried out at 640 K. O uptakes of unsupported samples reduced at close to this temp. yield an O atom site d. of 3.2 × 1018 m-2, a value near that expected for a monolayer. The same O chemisorption technique was applied to SiO2-supported V2O5. Laser Raman spectroscopy confirms that, near 640 K, O chemisorbs primarily at the surface of the dispersed V2O5, but does not exchange with the bulk of the oxide. For very low wt. loadings, a limiting stoichiometry of one adsorbed O atom per V atom is obtained. This stoichiometry is used to calc. dispersions of 93-50% for supported V2O5 samples of 0.3-9.8% wt. loading.
49. 49
  Ono, T.; Tanaka, Y.; Takeuchi, T.; Yamamoto, K. Characterization of K-Mixed V₂O₅ Catalyst and Oxidative Dehydrogenation of Propane on It. J. Mol. Catal. A Chem. 2000, 159, 293– 300, DOI: 10.1016/S1381-1169(00)00218-1
  49
  Characterization of K-mixed V2O5 catalyst and oxidative dehydrogenation of propane on it
  Ono, Takehiko; Tanaka, Yuhmo; Takeuchi, Takayoshi; Yamamoto, Kouji
  Journal of Molecular Catalysis A: Chemical (2000), 159 (2), 293-300CODEN: JMCCF2; ISSN:1381-1169. (Elsevier Science B.V.)
  The structure of K contg. V2O5 catalysts has been studied by XRD and spectroscopic methods. The particles of V2O5 were oriented sharply to the direction perpendicular to b axis. The spacings of (010) and (200) planes were slightly contracted by the presence of K ions. V2O5 bronze seemed to be less Raman-active. IR spectra gave the structural information of K-V2O5 at low content of K. The IR bands at 1023 and 830 cm-1 of V2O5 shifted to 1000 and 785 cm-1, resp. This suggests that K ions are present at some micro space of V2O5 crystal. The K-V2O5 catalyst oriented to (010) plane exhibited high selectivity (ca. 80%) to C3H6 in the oxidn. of C3H8 while the activity decreased with the increase in K content. Oxygen ions of oriented V2O5 were exchanged with 18O by the redn. with C3H8 and reoxidn. with 18O2. Raman spectra's anal. of the catalysts exchanged with 18O suggests that V=O species are responsible for oxidative dehydrogenation of C3H8 to C3H6.
50. 50
  Ono, T.; Numata, H. Characteristic Features of Raman Band Shifts of Vanadium Oxide Catalysts Exchanged with the ¹⁸O Tracer and Active Sites for Reoxidation. J. Mol. Catal. A Chem. 1997, 116, 421– 429, DOI: 10.1016/S1381-1169(96)00425-6
  50
  Characteristic features of Raman band shifts of vanadium oxide catalysts exchanged with the 18O tracer and active sites for reoxidation
  Ono, Takehiko; Numata, Hideo
  Journal of Molecular Catalysis A: Chemical (1997), 116 (3), 421-429CODEN: JMCCF2; ISSN:1381-1169. (Elsevier)
  The oxide oxygen ions of V2O5 catalyst were exchanged with 18O tracer by a redn.-oxidn. method and by a catalytic oxidn. of n-butane using 18O2. The Raman band shifts of the V2O5 exchanged with 18O by the methods were examd. The band at 700 cm-1 was shifted to lower frequencies more preferentially than the band of V:O oxygen at 998 cm-1. Applying the correlation between the Raman bands and stretching modes as described in the literature, the positions of oxide ions and anion vacancies for redn. and reoxidn. were estd. The anion vacancies corresponding to the V-O species in the V square with 1.88 and 2.02 Å distances seem to be active sites for oxygen insertion. The similar conclusions were obtained for Mo contg. V2O5 catalyst.
51. 51
  Browne, M. P.; Sofer, Z.; Pumera, M. Layered and Two Dimensional Metal Oxides for Electrochemical Energy Conversion. Energy Environ. Sci. 2019, 12, 41– 58, DOI: 10.1039/C8EE02495B
  51
  Layered and two dimensional metal oxides for electrochemical energy conversion
  Browne, Michelle P.; Sofer, Zdenek; Pumera, Martin
  Energy & Environmental Science (2019), 12 (1), 41-58CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
  A review. The oxygen evolution and redn. reactions are two extremely important reactions in terms of energy applications. Currently, the Oxygen Evolution Reaction (OER) hinders the efficient running of electrolyzer devices which convert water into mol. H2. This H2 can subsequently be used in a H2/O2 fuel cell for the renewable generation of electricity with only H2O as a byproduct. However, this fuel cell process is not economy feasible due to the sluggish kinetics of the Oxygen Redn. Reaction (ORR) at the device cathode, even with expensive state-of-the-art electrocatalytic materials. As of late, the amt. of interest in the OER and ORR, from research labs. from all over the globe, has risen rapidly in order to find cheap and efficient catalysts to replace the expensive platinum based catalysts currently used in the two aforementioned energy conversion/generation technologies. Layered transition metal oxides, based on the cheap transition metal oxides Mn, Co, Ni and Fe have been reported as viable catalysts for the OER and ORR. Layered structures have an added advantage over non-layered materials as the surface area can be increase by means of exfoliation, with potential for tailoring electrocatalytic activity. It has been shown that the fabrication process and post-synthetic treatments, e.g. anion exchange or exfoliation, of these materials can alter the catalytic activity of these materials. Here we summarise various fabrication methods and modifications utilized in literature to tailor the performance of layered transition metal and hydroxide based catalysts for the ORR and OER toward that of the state-of-the-art materials for these technologies.
52. 52
  Wachs, I. E. Recent Conceptual Advances in the Catalysis Science of Mixed Metal Oxide Catalytic Materials. Catal. Today 2005, 100, 79– 94, DOI: 10.1016/j.cattod.2004.12.019
  52
  Recent conceptual advances in the catalysis science of mixed metal oxide catalytic materials
  Wachs, Israel E.
  Catalysis Today (2005), 100 (1-2), 79-94CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
  A review. The catalysis science of mixed metal oxides (supported metal oxides, mol. sieves and bulk mixed metal oxides) has undergone dramatic paradigm changes over the past 25 years as new characterization techniques became available (X-ray absorption spectroscopy (EXAFS/XANES/soft XANES), Raman, solid-state NMR, HR-TEM, UV-vis DRS and LEISS) to catalysis researchers. The major advantages offered by these spectroscopic improvements are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as cryst. nanophases and (2) their ability to collect information under various environmental conditions. Application of these spectroscopic techniques to the investigation of mixed metal oxide catalysts have provided new fundamental insights into the electronic and mol. structures of mixed metal oxide catalytic active sites and how they control the catalytic activity and selectivity characteristics. The most significant discovery has been that amorphous metal oxide phases are always present and are the catalytic active sites for many applications of mixed metal oxide catalysts. This has resulted in a significant paradigm shift as to how mixed metal oxide catalytic materials function for different applications. This article reviews the instrumental advances and the resulting conceptual advances that have evolved over the past 25 years in the catalysis science of mixed metal oxide catalysts.
53. 53
  Grant, J. T.; Venegas, J. M.; McDermott, W. P.; Hermans, I. Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts. Chem. Rev. 2018, 118, 2769– 2815, DOI: 10.1021/acs.chemrev.7b00236
  53
  Aerobic Oxidations of Light Alkanes over Solid Metal Oxide Catalysts
  Grant, Joseph T.; Venegas, Juan M.; McDermott, William P.; Hermans, Ive
  Chemical Reviews (Washington, DC, United States) (2018), 118 (5), 2769-2815CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
  Heterogeneous metal oxide catalysts are widely studied for the aerobic oxidns. of C1-C4 alkanes to form olefins and oxygenates. In this review, we outline the properties of supported metal oxides, mixed-metal oxides, and zeolites and detail their most common applications as catalysts for partial oxidns. of light alkanes. By doing this we establish similarities between different classes of metal oxides and identify common themes in reaction mechanisms and research strategies for catalyst improvement. For example, almost all partial alkane oxidns., regardless of the metal oxide, follow Mars-van Krevelen reaction kinetics, which utilize lattice oxygen atoms to reoxidize the reduced metal centers while the gaseous O2 reactant replenishes these lattice oxygen vacancies. Many of the most-promising metal oxide catalysts include V5+ surface species as a necessary constituent to convert the alkane. Transformations involving sequential oxidn. steps (i.e., propane to acrylic acid) require specific reaction sites for each oxidn. step and benefit from site isolation provided by spectator species. These themes, and others, are discussed in the text.
54. 54
  Kuba, S.; Knözinger, H. Time-Resolved in Situ Raman Spectroscopy of Working Catalysts: Sulfated and Tungstated Zirconia. J. Raman Spectrosc. 2002, 33, 325– 332, DOI: 10.1002/jrs.815
  54
  Time-resolved in situ Raman spectroscopy of working catalysts: sulfated and tungstated zirconia
  Kuba, S.; Knozinger, H.
  Journal of Raman Spectroscopy (2002), 33 (5), 325-332CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
  The spectra of time-dependent in situ Raman expts. on sulfated and tungstated zirconia catalysts during the isomerization reaction of n-pentane are presented and discussed. A purpose-made in situ Raman cell which was used for this application is described. The choice of appropriate exptl. parameters, namely laser power and laser wavelength, is discussed. During the expts. the activity and selectivity of the catalysts were detd. simultaneously by online gas chromatog. The sulfated zirconia catalyst shows structural changes in the sulfate region during the reaction. One of two different types of initially present sulfate species is eroded during the reaction, presumably due to a redn. to H2S. The activity shows a typical induction period followed by a fast deactivation. No coke formation is obsd. Since fast deactivation occurs, coke formation cannot be the only reason for the deactivation of the catalyst. The tungstated catalyst shows strong darkening after initiation of the reaction with increasing time-onstream (TOS). This leads to the disappearance of the catalyst bands. However, coke formation indicated by a broad band at 1590 cm-1 can be obsd. Since darkening has a strong effect on Raman intensities, the time evolution of the obsd. bands is not obtained correctly. We propose a new method to correct for the effect of the darkening. The change of the diffuse reflectance of the catalyst is detd. by the variation of the intensity of laser plasma lines. Based on an approx. equation proposed by Waters which correlates the Raman intensities and the diffuse reflectance of a sample, a correction factor for the spectra is obtained. After the intensity correction the spectra indicate that most of the coke is formed in the first few minutes of the reaction followed by a const. formation rate with TOS.
55. 55
  Giannozzi, P. Quantum Espresso: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502, DOI: 10.1088/0953-8984/21/39/395502
  55
  QUANTUM ESPRESSO: a modular and open-source software project for quantum simulations of materials
  Giannozzi Paolo; Baroni Stefano; Bonini Nicola; Calandra Matteo; Car Roberto; Cavazzoni Carlo; Ceresoli Davide; Chiarotti Guido L; Cococcioni Matteo; Dabo Ismaila; Dal Corso Andrea; de Gironcoli Stefano; Fabris Stefano; Fratesi Guido; Gebauer Ralph; Gerstmann Uwe; Gougoussis Christos; Kokalj Anton; Lazzeri Michele; Martin-Samos Layla; Marzari Nicola; Mauri Francesco; Mazzarello Riccardo; Paolini Stefano; Pasquarello Alfredo; Paulatto Lorenzo; Sbraccia Carlo; Scandolo Sandro; Sclauzero Gabriele; Seitsonen Ari P; Smogunov Alexander; Umari Paolo; Wentzcovitch Renata M
  Journal of physics. Condensed matter : an Institute of Physics journal (2009), 21 (39), 395502 ISSN:.
  QUANTUM ESPRESSO is an integrated suite of computer codes for electronic-structure calculations and materials modeling, based on density-functional theory, plane waves, and pseudopotentials (norm-conserving, ultrasoft, and projector-augmented wave). The acronym ESPRESSO stands for opEn Source Package for Research in Electronic Structure, Simulation, and Optimization. It is freely available to researchers around the world under the terms of the GNU General Public License. QUANTUM ESPRESSO builds upon newly-restructured electronic-structure codes that have been developed and tested by some of the original authors of novel electronic-structure algorithms and applied in the last twenty years by some of the leading materials modeling groups worldwide. Innovation and efficiency are still its main focus, with special attention paid to massively parallel architectures, and a great effort being devoted to user friendliness. QUANTUM ESPRESSO is evolving towards a distribution of independent and interoperable codes in the spirit of an open-source project, where researchers active in the field of electronic-structure calculations are encouraged to participate in the project by contributing their own codes or by implementing their own ideas into existing codes.
56. 56
  Shklover, V.; Haibach, T.; Ried, F.; Nesper, R.; Novák, P. Crystal Structure of the Product of Mg²⁺ Insertion into V₂O₅ single Crystals. J. Solid State Chem. 1996, 123, 317– 323, DOI: 10.1006/jssc.1996.0186
  56
  Crystal structure of the product of Mg2+ insertion into V2O5 single crystal
  Shklover, V.; Haibach, T.; Ried, F.; Nesper, R.; Novak, P.
  Journal of Solid State Chemistry (1996), 123 (2), 317-323CODEN: JSSCBI; ISSN:0022-4596. (Academic)
  Chem. (by interaction with a dibutylmagnesium soln.) and electrochem. (in MeCN soln. of Mg perchlorate) insertion of Mg2+ into the single crystals of V2O5 was performed. The morphol. change of V2O5 crystals as a result of the Mg2+ insertion was studied by SEM. The wavelength dispersive electron probe microanal. clearly showed Mg (at least) at the surface of intercalated V2O5. Based on the single crystal x-ray diffraction study of intercalated V2O5 (orthorhombic, space group Pmn21, a 11.544(6), b 4.383(3), c 3.574(2) Å, Z = 4) the location of a small amt. of Mg (∼1%) in the bulk V2O5 may be suggested, with [6 + 4] O atoms surrounding Mg. The resulting Mg-O sepns. essentially exceed the accepted values for the Mg-O distances in crystals with hexacoordinated Mg atoms, which may be correlated with the structural and electrochem. properties of Mg2+-inserted V2O5.
57. 57
  Grimme, S. Semiempirical Gga-Type Density Functional Constructed with a Long-Range Dispersion Correction. J. Comput. Chem. 2006, 27, 1787– 1799, DOI: 10.1002/jcc.20495
  57
  Semiempirical GGA-type density functional constructed with a long-range dispersion correction
  Grimme, Stefan
  Journal of Computational Chemistry (2006), 27 (15), 1787-1799CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
  A new d. functional (DF) of the generalized gradient approxn. (GGA) type for general chem. applications termed B97-D is proposed. It is based on Becke's power-series ansatz from 1997 and is explicitly parameterized by including damped atom-pairwise dispersion corrections of the form C6·R-6. A general computational scheme for the parameters used in this correction has been established and parameters for elements up to xenon and a scaling factor for the dispersion part for several common d. functionals (BLYP, PBE, TPSS, B3LYP) are reported. The new functional is tested in comparison with other GGAs and the B3LYP hybrid functional on std. thermochem. benchmark sets, for 40 noncovalently bound complexes, including large stacked arom. mols. and group II element clusters, and for the computation of mol. geometries. Further cross-validation tests were performed for organometallic reactions and other difficult problems for std. functionals. In summary, it is found that B97-D belongs to one of the most accurate general purpose GGAs, reaching, for example for the G97/2 set of heat of formations, a mean abs. deviation of only 3.8 kcal mol-1. The performance for noncovalently bound systems including many pure van der Waals complexes is exceptionally good, reaching on the av. CCSD(T) accuracy. The basic strategy in the development to restrict the d. functional description to shorter electron correlation lengths scales and to describe situations with medium to large interat. distances by damped C6·R-6 terms seems to be very successful, as demonstrated for some notoriously difficult reactions. As an example, for the isomerization of larger branched to linear alkanes, B97-D is the only DF available that yields the right sign for the energy difference. From a practical point of view, the new functional seems to be quite robust and it is thus suggested as an efficient and accurate quantum chem. method for large systems where dispersion forces are of general importance.
58. 58
  Barone, V.; Casarin, M.; Forrer, D.; Pavone, M.; Sambi, M.; Vittadini, A. Role and Effective Treatment of Dispersive Forces in Materials: Polyethylene and Graphite Crystals as Test Cases. J. Comput. Chem. 2009, 30, 934– 939, DOI: 10.1002/jcc.21112
  58
  Role and effective treatment of dispersive forces in materials: polyethylene and graphite crystals as test cases
  Barone, Vincenzo; Casarin, Maurizio; Forrer, Daniel; Pavone, Michele; Sambi, Mauro; Vittadini, Andrea
  Journal of Computational Chemistry (2009), 30 (6), 934-939CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
  A semiempirical addn. of dispersive forces to conventional d. functionals (DFT-D) has been implemented into a pseudopotential plane-wave code. Test calcns. on the benzene dimer reproduced the results obtained by using localized basis set, provided that the latter are cor. for the basis set superposition error. By applying the DFT-D/plane-wave approach a substantial agreement with expts. is found for the structure and energetics of polyethylene and graphite, two typical solids that are badly described by std. local and semilocal d. functionals.
59. 59
  Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188– 5192, DOI: 10.1103/PhysRevB.13.5188
  There is no corresponding record for this reference.
60. 60
  Kim, H.; Kosuda, K. M.; Van Duyne, R. P.; Stair, P. C. Resonance Raman and Surface- and Tip-Enhanced Raman Spectroscopy Methods to Study Solid Catalysts and Heterogeneous Catalytic Reactions. Chem. Soc. Rev. 2010, 39, 4820– 4844, DOI: 10.1039/c0cs00044b
  60
  Resonance Raman and surface- and tip-enhanced Raman spectroscopy methods to study solid catalysts and heterogeneous catalytic reactions
  Kim, Hacksung; Kosuda, Kathryn M.; Van Duyne, Richard P.; Stair, Peter C.
  Chemical Society Reviews (2010), 39 (12), 4820-4844CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  A review. Resonance Raman (RR) spectroscopy has several advantages over the normal Raman spectroscopy (RS) widely used for in situ characterization of solid catalysts and catalytic reactions. Compared with RS, RR can provide much higher sensitivity and selectivity in detecting catalytically-significant surface metal oxides. RR can potentially give useful information on the nature of excited states relevant to photocatalysis and on the anharmonic potential of the ground state. In this crit. review a detailed discussion is presented on several types of RR exptl. systems, 3 distinct sources of so-called Raman (fluorescence) background, detection limits for RR compared to other techniques (EXAFS, PM-IRAS, SFG), and 3 well-known methods to assign UV-vis absorption bands and a band-specific unified method that is derived mainly from RR results. In addn., the virtues and challenges of surface-enhanced Raman spectroscopy (SERS) are discussed for detecting mol. adsorbates at catalytically relevant interfaces. Tip-enhanced Raman spectroscopy (TERS), which is a combination of SERS and near-field scanning probe microscopy and has the capability of probing mol. adsorbates at specific catalytic sites with an enormous surface sensitivity and nanometer spatial resoln., is also reviewed.
61. 61
  Harima, H. Raman Scattering Characterization on SiC. Microelectron. Eng. 2006, 83, 126– 129, DOI: 10.1016/j.mee.2005.10.037
  61
  Raman scattering characterization on SiC
  Harima, Hiroshi
  Microelectronic Engineering (2006), 83 (1), 126-129CODEN: MIENEF; ISSN:0167-9317. (Elsevier B.V.)
  A review. Raman scattering is a powerful non-contact and non-destructive characterization tool for SiC polytypes for both the lattice and electronic properties. Here, I will briefly review 2 recent Raman expts. on SiC; metal/SiC interface reactions probed by visible lasers and ion-implantation damages probed by deep UV lasers. These studies utilize the opposite aspects of the probe laser, i.e. deep and shallow penetration depth into SiC.
62. 62
  Gilson, T. R.; Bizri, O. F.; Cheetham, N. Single-Crystal Raman and Infrared Spectra of Vanadium(V) Oxide. J. Chem. Soc., Dalton Trans. 1973, 291– 294, DOI: 10.1039/dt9730000291
  62
  Single-crystal Raman and infrared spectra of vanadium pentoxide
  Gilson, T. R.; Bizri, O. F.; Cheetham, N.
  Journal of the Chemical Society, Dalton Transactions: Inorganic Chemistry (1972-1999) (1973), (3), 291-4CODEN: JCDTBI; ISSN:0300-9246.
  The oriented single-crystal Raman and ir spectra of V2O5 were detd. and absorption frequencies were compared with those calcd. by using a simple transferred force field. Departures from previously established oxide-group frequencies were due to the relative lightness of V.
63. 63
  Abello, L.; Husson, E.; Repelin, Y.; Lucazeau, G. Vibrational Spectra and Valence Force Field of Crystalline V₂O₅. Spectrochim. Acta, Part A 1983, 39, 641– 651, DOI: 10.1016/0584-8539(83)80040-3
  63
  Vibrational spectra and valence force field of crystalline vanadium pentoxide
  Abello, L.; Husson, E.; Repelin, Y.; Lucazeau, G.
  Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (1983), 39A (7), 641-51CODEN: SAMCAS; ISSN:0584-8539.
  A complete vibrational study of cryst. V2O5 was performed. Polarized Raman spectra at 300 and 100 K on a single crystal and IR absorption spectra on powder samples were recorded. An assignment in terms of factor group symmetry species is given. A normal coordinate anal. using a complete force field was performed. All the normal modes of vibration are described in terms of potential energy distribution and in terms of cartesian displacements. Some modes characteristic of specific bonds are discussed. Finally some low frequency modes were found to derive, at a 1st approxn., from acoustic modes and some dispersion curves were derived from the simple model of the linear chain.
64. 64
  Clauws, P.; Broeckx, J.; Vennik, J. Lattice Vibrations of V₂O₅. Calculation of Normal Vibrations in a Urey-Bradley Force Field. Phys. Status Solidi B 1985, 131, 459– 473, DOI: 10.1002/pssb.2221310207
  64
  Lattice vibrations of vanadium(V) oxide. Calculation of normal vibrations in a Urey-Bradley force field
  Clauws, P.; Broeckx, J.; Vennik, J.
  Physica Status Solidi B: Basic Research (1985), 131 (2), 459-73CODEN: PSSBBD; ISSN:0370-1972.
  A normal coordinate anal. of the vibrational spectrum of cryst. V2O5 was carried out with the assumption of a Urey-Bradley force field. The calcd. frequencies were adjusted to 29 exptl. IR and Raman frequencies by an automatic force const. refinement program. Anal. of the potential energy distribution and the at. displacements allows the classification of the modes into 9 types of O vibrations and 3 types of chain modes. A discussion is given of effective charges and IR intensities. An assignment of the IR spectrum of polycryst. V2O5 is added.
65. 65
  Brázdová, V.; Ganduglia-Pirovano, M. V.; Sauer, J. Periodic Density Functional Study on Structural and Vibrational Properties of Vanadium Oxide Aggregates. Phys. Rev. B 2004, 69, 165420, DOI: 10.1103/PhysRevB.69.165420
  65
  Periodic density functional study on structural and vibrational properties of vanadium oxide aggregates
  Brazdova, Veronika; Ganduglia-Pirovano, M. Veronica; Sauer, Joachim
  Physical Review B: Condensed Matter and Materials Physics (2004), 69 (16), 165420/1-165420/14CODEN: PRBMDO; ISSN:0163-1829. (American Physical Society)
  We present periodic d.-functional calcns. within the generalized gradient approxn. (Perdew-Wang 91) on structures and vibrational properties of different V oxide aggregates, namely, bulk V2O5 and its (001) surface, as well as thin V oxide films supported by α-Al2O3. V is differently coordinated by O in the different systems. The calcd. vibrational frequencies of bulk V2O5 are in good agreement with obsd. IR and Raman frequencies, for stretching modes the rms deviation is 40 cm-1. The calcns. for the V2O5(001) surface suggest modifications of previous assignments of high-resoln. electron-energy-loss spectroscopy (HREELS) data. In agreement with HREELS, vanadyl frequencies shift to higher wave nos. on surface formation. The calcd. frequencies for bulk Al2O3 are systematically lower than the obsd. IR data (by about 30 cm-1). Models for V2O3 supported on Al2O3 are obtained when in the outermost layers of Al2O3(0001) slabs Al is replaced by V. These films do not show vibrations above 930 cm-1. Oxygen adsorption on top of the vanadium sites on these supported films creates very stable vanadyl groups with binding energies of about 450 kJ/mol (1/2 O2). Bond distances, vibrational frequencies, and oxygen binding energies are compared with those of vanadyl groups at the V2O5(001) surface and in (V2O5)n clusters (n = 2,4). The relevance of the findings for expts. on vanadia particles supported on Al2O3 is discussed.
66. 66
  Zhou, B.; He, D. Raman Spectrum of Vanadium Pentoxide from Density-Functional Perturbation Theory. J. Raman Spectrosc. 2008, 39, 1475– 1481, DOI: 10.1002/jrs.2025
  66
  Raman spectrum of vanadium pentoxide from density-functional perturbation theory
  Zhou, Bo; He, Deyan
  Journal of Raman Spectroscopy (2008), 39 (10), 1475-1481CODEN: JRSPAF; ISSN:0377-0486. (John Wiley & Sons Ltd.)
  We present ab initio calcn. within the framework of the d.-functional theory (DFT) on band structure and vibrational properties of bulk V2O5. The structure of V2O5 comes from optimization of the exptl. data with lattice parameters fixed. The band structure of the optimized structure has been calcd., and the result fits the exptl. data very well and also gives similar results as those calcd. by other methods. The phonon eigen-wavenumbers of the Γ- point of V2O5 bulk have been calcd. ab initio in d.-functional perturbation theory (DFPT). The calcd. vibrational wavenumbers are in good agreement with obsd. IR and Raman wavenumbers, and the predictive full phonon dispersion of bulk V2O5 has also been obtained. Further we calcd. the Raman spectrum of vanadium pentoxide (V2O5) powder sample using the obtained Raman susceptibility. Calcd. and measured intensities show overall good agreement.
67. 67
  Wu, Z.; Kim, H.-S.; Stair, P. C.; Rugmini, S.; Jackson, S. D. On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV–Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies. J. Phys. Chem. B 2005, 109, 2793– 2800, DOI: 10.1021/jp046011m
  67
  On the Structure of Vanadium Oxide Supported on Aluminas: UV and Visible Raman Spectroscopy, UV-Visible Diffuse Reflectance Spectroscopy, and Temperature-Programmed Reduction Studies
  Wu, Zili; Kim, Hack-Sung; Stair, Peter C.; Rugmini, Sreekala; Jackson, S. David
  Journal of Physical Chemistry B (2005), 109 (7), 2793-2800CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
  Vanadia species on aluminas (δ- and γ-Al2O3) with surface VOx d. in the range 0.01-14.2 V/nm2 have been characterized by UV and visible Raman spectroscopy, UV-visible diffuse reflectance spectroscopy (UV-Vis DRS), and temp.-programmed redn. in hydrogen. It is shown that the alumina phase has little influence on the structure and reducibility of surface VOx species under either dehydrated or hydrated conditions. Three similar types of dispersed VOx species, i.e., monovanadates, polyvanadates, and V2O5, are identified on both aluminas under dehydrated conditions. Upon hydration, polymd. VOx species dominate on the surfaces of the two aluminas. The broad Raman band at around 910 cm-1, obsd. on dehydrated V/δ-, γ-Al2O3 at all V loadings (0.01-14.2 V/nm2), is assigned to the interface mode (V-O-Al) instead of the conventionally assigned V-O-V bond. The direct observation of the interface bond is of significance for the understanding of redox catalysis because this bond has been considered to be the key site in oxidn. reactions catalyzed by supported vanadia. Two types of frequency shifts of the V:O stretching band (1013-1035 cm-1) have been obsd. in the Raman spectra of V/Al2O3: a shift as a function of surface VOx d. and a shift as a function of excitation wavelength. The shift of the V:O band to higher wavenumbers with increasing surface VOx d. is due to the change of VOx structure. The V:O stretching band in dispersed vanadia always appears at lower wavenumber in UV Raman spectra than in visible Raman spectra for the same V/Al2O3 sample. This shift is explained by selective resonance enhancement according to the UV-Vis DRS results. It implies that UV Raman has higher sensitivity to isolated and less polymd. VOx species while visible Raman is more sensitive to highly polymd. VOx species and cryst. V2O5. These results show that a multiwavelength excitation approach provides a more complete structural characterization of supported VOx catalysts.
68. 68
  Hermann, K.; Witko, M.; Druzinic, R.; Tokarz, R. Hydrogen Assisted Oxygen Desorption from the V₂O₅(010) Surface. Top. Catal. 2000, 11/12, 67– 75, DOI: 10.1023/A:1027206705195
  68
  Hydrogen assisted oxygen desorption from the V2O5(010) surface
  Hermann, K.; Witko, M.; Druzinic, R.; Tokarz, R.
  Topics in Catalysis (2000), 11/12 (1-4), 67-75CODEN: TOCAFI; ISSN:1022-5528. (Baltzer Science Publishers)
  Vanadium oxide surfaces are known to play an active role as catalysts in hydrocarbon oxidn. reactions where oxygen from different surface sites participates in the reaction. Due to the ubiquity of hydrogen in these systems, reaction steps involving (temporary) hydrogenation are possible and may influence the overall reaction scheme. This work examines structural and energetic consequences of hydrogen interacting with different oxygen sites at the V2O5(010) surface where the local surface environment is modeled by embedded clusters. The electronic structure and equil. geometries of the clusters are obtained by d. functional theory (DFT) using gradient cor. functionals (RPBE) for exchange and correlation. Hydrogen is found to stabilize preferentially near oxygen sites forming surface OH and H2O species with binding energies of 0.5-2.3 eV per H atom depending on the site and species. Hydrogen adsorption weakens the binding of the surface oxygen with its vanadium neighbors considerably where the weakening is larger for H2O than for OH formation as evidenced by bond order analyses and results of the binding energetics. Thus, the studies suggest strongly that the presence of hydrogen at the oxide surface facilitates oxygen removal and, therefore, contributes to the enhanced yield of oxygenated products near vanadia based surfaces.
69. 69
  Huang, Y.-L.; Pellegrinelli, C.; Wachsman, E. D. Reaction Kinetics of Gas–Solid Exchange Using Gas Phase Isotopic Oxygen Exchange. ACS Catal. 2016, 6, 6025– 6032, DOI: 10.1021/acscatal.6b01462
  69
  Reaction Kinetics of Gas-Solid Exchange Using Gas Phase Isotopic Oxygen Exchange
  Huang, Yi-Lin; Pellegrinelli, Christopher; Wachsman, Eric D.
  ACS Catalysis (2016), 6 (9), 6025-6032CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The oxygen transport kinetics of heterogeneous gas-solid exchange has been investigated on the basis of a two-step reaction mechanism, linking surface catalysis to solid-state self-diffusion, via gas phase isotopic oxygen exchange on the mixed ionic electronic conductor (MIEC) La0.6Sr0.4Co0.2Fe0.8O3-x (LSCF) and electronic conductor (La0.8Sr0.2)0.95MnO3±x (LSM). The catalytic activity of LSCF is higher than that of LSM toward the elementary step of oxygen dissocn., likely caused by a higher vacancy concn. The apparent activation energy for surface exchange of LSCF is lower than values obtained from bulk characterization techniques. The diffusion coeff. (D) for LSM at different temps. shows a huge deviation from literature values, and an alternate exchange mechanism has been proposed. The fast transport pathway is attributed to the substoichiometry of LSM in the near surface region. These results have significant implications for the improvement of the oxygen redn. reaction for the design of higher-performance materials and the importance and limitations of isotope exchange exptl. design.
70. 70
  Huang, Y.-L.; Pellegrinelli, C.; Sakbodin, M.; Wachsman, E. D. Molecular Reactions of O₂ and CO₂ on Ionically Conducting Catalyst. ACS Catal. 2018, 8, 1231– 1237, DOI: 10.1021/acscatal.7b03467
  70
  Molecular Reactions of O2 and CO2 on Ionically Conducting Catalyst
  Huang, Yi-Lin; Pellegrinelli, Christopher; Sakbodin, Mann; Wachsman, Eric D.
  ACS Catalysis (2018), 8 (2), 1231-1237CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The presence of CO2, an unavoidable component in air and fuel environments, is known to cause severe performance degrdn. in oxide catalysts. Understanding the interactions between O2, CO2, and ion-conducting oxides is crit. to developing energy-conversion devices. Here, surface reaction kinetics of Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) with the presence of both O2 and CO2 is detd. using gas-phase isotope exchange. BSCF actively reacts with CO2, and the incorporation of oxygen from CO2 to the lattice of BSCF is directly obsd. as low as 50 °C. Above 200 °C, the reaction between CO2 and the BSCF surface dominates and is independent of the oxygen partial pressure. In addn., CO2 competes with O2 for binding to vacancy sites, forming surface intermediate species. Surprisingly, these surface intermediate species offer oxygen to exchange with oxygen in gaseous O2 and CO2, inhibiting the interactions between O2 and the solid surface. This work provides fundamental insight into functioning oxide catalysts, and the results can be applied to the design of improved oxide catalysts.
71. 71
  Huang, Y.-L.; Pellegrinelli, C.; Wachsman, E. D. Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides. ACS Catal. 2017, 7, 5766– 5772, DOI: 10.1021/acscatal.7b01096
  71
  Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides
  Huang, Yi-Lin; Pellegrinelli, Christopher; Wachsman, Eric D.
  ACS Catalysis (2017), 7 (9), 5766-5772CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The high activity of oxide catalysts toward the oxygen redn. reaction (ORR) attracts unwanted interactions with other gaseous oxygen-contg. species in air. Understanding the interaction between oxygen-contg. species, mainly water and carbon dioxide, and oxides is important for many energy applications. However, the oxygen self-exchange process and the high-temp. operating conditions limit the investigation of these concurrent reactions. Here we report a direct observation of the effects of water and carbon dioxide on dissocn. rates of ionically conducting catalysts, La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) and (La0.8Sr0.2)0.95MnO3±δ(LSM), using gas-phase isotope exchange. The concurrent heterogeneous reactions of oxygen and other oxygen-contg. species on oxide catalysts can either promote or hinder oxygen dissocn. rates, depending on the participation of lattice oxygen. LSCF appears to be much more active in exchange with these oxygen-contg. species, while LSM shows relatively little exchange. Oxygen-contg. species exhibit site-blocking effects and inhibit the reaction on LSCF. In contrast, water and CO2 promote the oxygen dissocn. rate on LSM, likely due to the prominence of homoexchange, where intermediate surface species play an important role. Our study provides insights into the reaction mechanism of oxygen dissocn. and the effect of coexisting ambient air oxygen species.
72. 72
  Tilley, R. J. Understanding Solids: The Science of Materials; Wiley: 2013.
  There is no corresponding record for this reference.
73. 73
  Cameron, W. C.; Parkas, A.; Litz, L. M. Exchange of Isotopic Oxygen between Vanadium Pentoxide, Gaseous Oxygen and Water. J. Phys. Chem. 1953, 57, 229– 238, DOI: 10.1021/j150503a022
  73
  Exchange of isotopic oxygen among vanadium pentoxide, gaseous oxygen, and water
  Cameron, W. C.; Farkas, A.; Litz, L. M.
  Journal of Physical Chemistry (1953), 57 (), 229-38CODEN: JPCHAX; ISSN:0022-3654.
  The exchange of O18 (1.3 to 1.6%) was measured by mass spectrometric analyses in the systems V2O5-O (I), V2O5-H2O (II), and V2O5-H2O-O (III) in the temp. range 400-550°. The rate of exchange in I increases with decreasing particle size of the V2O5. In the case of Alundum-supported V2O5 catalysts, the support does not participate in the exchange, and the reaction proceeds according to a surface-reaction-controlled mechanism which is 1st order with time, zero order with O pressure, and with an activation energy of 45 kcal./mole. The kinetics of the O exchange in I with fresh amorphous V2O5 microspheres is controlled by a surface reaction also. On heat-treatment, the microspheres crystallize, and the exchange indicates a diffusion-controlled mechanism. The rate of the O exchange in II (supported V2O5) is 20 to 30 times as rapid as in I. The rate of exchange in III is similar to that in I, indicating that in this instance the latter reaction is rate controlling. In the case of the surface-reaction-controlled exchange, the fast diffusion of the O in the bulk of the V2O5 particles may be due to lattice imperfections. The 1st-order kinetics found in I is compatible with an apparent zero-order with respect to O pressure if the O is strongly adsorbed. It is proposed that the activation process in the exchange of O atoms involves either the dissocn. of O mols. or the loosening of V-O bonds.
74. 74
  Milan, E. F. The Dissociation Pressure of Vanadium Pentoxide. J. Phys. Chem. 1929, 33, 498– 508, DOI: 10.1021/j150298a002
  74
  The dissociation pressure of vanadium pentoxide
  Milan, E. F.
  Journal of Physical Chemistry (1929), 33 (), 498-508CODEN: JPCHAX; ISSN:0022-3654.
  Pure V2O5 was prepd. from ammonium vanadate and its dissocn. pressure was measured over the temp. range 700-1125°. V2O5 dissociates into V2O4 and O2 at temps. only slightly above its m. p. Small amts. of V2O4 have an enormous effect on dissocn. pressures. Pressure-temp., pressure-compn. and temp.-compn. diagrams are plotted.
75. 75
  Nasu, N. The Dissociation Pressure of Vanadium Pentoxide. J. Chem. Soc. Jpn. 1935, 56, 666– 669, DOI: 10.1246/nikkashi1921.56.6_666
  There is no corresponding record for this reference.
76. 76
  Spitsyn, B. N.; Maidanovskaya, L. L. The Thermal Dissociation of Vanadium Pentoxide. Zh. Fiz. Khim. 1959, 33, 180– 183
  76
  Thermal dissociation of vanadium pentoxide
  Spitsyn, B. V.; Maidanovskaya, L. G.
  Zhurnal Fizicheskoi Khimii (1959), 33 (), 180-3CODEN: ZFKHA9; ISSN:0044-4537.
  The V2O5 thermal dissocn. was studied by the Mǎidanovskaya and Bruns method (ibid. 13, 239(1939)) in which V2O5 is used as catalyst for vapor-phase oxidn. The dissocn. kinetics was described by the Erofeev equation (C.A. 41, 4027c) with which the activation energy of the process were detd. (8.6 kcal./ mole). The amt. of O evolved agreed with data on the high- and low-temp. V2O5 dissocn. The x-ray diffraction measurements of V2O5 remained unchanged between 20 and 460° whereas the chem. reaction proceeding on a single crystal distorted its structure.
77. 77
  Chen, K.; Khodakov, A.; Yang, J.; Bell, A. T.; Iglesia, E. Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts. J. Catal. 1999, 186, 325– 333, DOI: 10.1006/jcat.1999.2510
  77
  Isotopic Tracer and Kinetic Studies of Oxidative Dehydrogenation Pathways on Vanadium Oxide Catalysts
  Chen, Kaidong; Khodakov, Andrei; Yang, Jun; Bell, Alexis T.; Iglesia, Enrique
  Journal of Catalysis (1999), 186 (2), 325-333CODEN: JCTLA5; ISSN:0021-9517. (Academic Press)
  Kinetic anal. and isotopic tracer studies were used to identify elementary steps and their reversibility in the oxidative dehydrogenation of propane on VOx/ZrO2 catalysts with VOx surface densities between 1.6 and 6 VOx/nm2. Competitive reactions of C3H6 and CH313CH2CH3 showed that CO forms via secondary combustion of propene intermediates. CO2 formed via this reaction and also via the direct combustion of propane. Reactions of 18O2/C3H8 mixts. on supported V216O5 led to the preferential initial appearance of lattice 16O atoms in all oxygen-contg. products, as expected if lattice oxygens were required for the activation of C-H bonds. Isotopically mixed O2 species were not detected during reactions of C3H8-18O2-16O2 reactant mixts. Therefore, dissociative O2 chemisorption steps are irreversible. Similarly, C3H8-C3D8-O2 reactants undergo oxidative dehydrogenation without forming C3H8-xDx mixed isotopomers, suggesting that C-H bond activation steps are also irreversible. Normal kinetic isotopic effects (kC-H/kC-D=2.5) were measured for primary oxidative dehydrogenation reactions. Kinetic isotope effects were slightly lower for propane and propene combustion steps (1.7 and 2.2, resp.). These data are consistent with kinetically relevant steps involving the dissocn. of C-H bonds in propane and propene. C3H6-D2O and C3D6-H2O cross exchange reactions occur readily during reaction; therefore, OH recombination steps are reversible and nearly equilibrated. These isotopic tracer results are consistent with a Mars-van Krevelen redox mechanism involving two lattice oxygens in irreversible C-H bond activation steps. The resulting alkyl species desorb as propene and the remaining O-H group recombines with neighboring OH groups to form water and reduced V centers. These reduced V centers reoxidize by irreversible dissociative chemisorption of O2. The application of pseudo-steady-state and reversibility assumptions leads to a complex kinetic rate expression that describes accurately the obsd. water inhibition effects and the kinetic orders in propane and oxygen when surface oxygen and OH groups are assumed to be the most abundant surface intermediates. (c) 1999 Academic Press.
78. 78
  Ma, W. Y.; Zhou, B.; Wang, J. F.; Zhang, X. D.; Jiang, Z. Y. Effect of Oxygen Vacancy on Li-Ion Diffusion in a V₂O₅ Cathode: A First-Principles Study. J. Phys. D Appl. Phys. 2013, 46, 105306, DOI: 10.1088/0022-3727/46/10/105306
  78
  Effect of oxygen vacancy on Li-ion diffusion in a V2O5 cathode: a first-principles study
  Ma, W. Y.; Zhou, B.; Wang, J. F.; Zhang, X. D.; Jiang, Z. Y.
  Journal of Physics D: Applied Physics (2013), 46 (10), 105306, 8 pp.CODEN: JPAPBE; ISSN:0022-3727. (IOP Publishing Ltd.)
  The energy barriers of lithium-ion mobility in a V2O5 cathode are calcd. using the nudged elastic band method. The low activation energy of the hopping pathway along the 〈0 1 0〉 direction (paralleling with the b-axis, the shortest lattice parameter) indicates that V2O5 has a one-dimensional diffusion pattern at the initial stage of charging and discharging. At a temp. of 300 K, the estd. diffusion coeff. is 2.520 × 10-8 cm2 s-1, which is consistent with the range of diffusivity of the previous reported fast ionic conductors. A systematic investigation of the effect of oxygen vacancy on Li-ion diffusion suggests that the bridging oxygen O(2) plays a very pos. role in the Li-ion diffusion along the 〈0 1 0〉 direction and the activation energy is reduced from 0.340 to 0.215 eV, while the existence of O(1) and O(3) vacancies can hinder the lithium diffusion along this direction due to the increase in the activation energy, resp. The oxygen vacancies make the energy barriers of the other two potential diffusion pathways reduced to some extent, which is still very large compared with the energy barrier of diffusion along the b-axis.
79. 79
  Mars, P.; van Krevelen, D. W. Oxidations Carried out by Means of Vanadium Oxide Catalysts. Chem. Eng. Sci. 1954, 3, 41– 59, DOI: 10.1016/S0009-2509(54)80005-4
  79
  Oxidations carried out by means of vanadium oxide catalysts
  Mars, P.; van Krevelen, D. W.
  Chemical Engineering Science (1954), 3 (Spec. Suppl.), 41-59CODEN: CESCAC; ISSN:0009-2509.
  The oxidations of C6H6, PhMe, naphthalene, and anthracene were studied in a vanadium oxide catalyst fluidized bed. The partial pressure of O was varied between 80 and 760 mm. and of the aromatic hydrocarbons between 1 and 30 mm. Both the O and the hydrocarbon concentrations affect the rate. The formula describing the reactions was derived by assuming two successive reactions, the reaction between the O on the surface and the adsorbed hydrocarbon, and the subsequent reoxidation of the V. The oxidation of SO2 is also discussed.
80. 80
  Routray, K.; Reddy, K. R. S. K.; Deo, G. Oxidative Dehydrogenation of Propane on V₂O₅/Al₂O₃ and V₂O₅/TiO₂ Catalysts: Understanding the Effect of Support by Parameter Estimation. Appl. Catal. A Gen. 2004, 265, 103– 113, DOI: 10.1016/j.apcata.2004.01.006
  80
  Oxidative dehydrogenation of propane on V2O5/Al2O3 and V2O5/TiO2 catalysts: understanding the effect of support by parameter estimation
  Routray, Kamalakanta; Reddy, K. R. S. K.; Deo, Goutam
  Applied Catalysis, A: General (2004), 265 (1), 103-113CODEN: ACAGE4; ISSN:0926-860X. (Elsevier Science B.V.)
  In this paper, the effect of the oxide support for supporting the vanadium oxide phase is studied by estg. the reaction parameters for the oxidative dehydrogenation of propane. To achieve this objective, several V2O5/Al2O3 and V2O5/TiO2 catalysts were synthesized by an incipient-wetness-impregnation technique. The supported vanadium oxide catalysts were characterized and the surface area, monolayer coverage and reducibility were detd. The surface area of the catalyst samples was not significantly affected with supported vanadium oxide loading. It was obsd. that the catalysts contain only molecularly dispersed vanadium oxide species below monolayer coverage, and molecularly dispersed and cryst. V2O5 above monolayer coverage. TPR studies revealed that the V2O5/Al2O3 samples were more difficult to reduce relative to the V2O5/TiO2 samples. The monolayer or near-monolayer catalysts, 10% V2O5/Al2O3 and 4% V2O5/TiO2, were selected for detailed kinetic anal. A Mars-van Krevelen (MVK) model contg. eight parameters was chosen for this purpose. The parameters were estd. using a genetic algorithm (GA), which optimizes a suitable objective function for a non-linear multi-response system. From the parameters estd., it was detd. that a similar catalytic cycle occurs independent of the oxide support. However, the rate at which the catalytic cycle occurs appears to be much faster on the more reducible titania support compared to the rate on the less reducible alumina support. The degree of redn. varies along the length of the reactor and depends on the support. Thus, the support has a significant effect on the reaction parameters for the oxidative dehydrogenation of propane over supported vanadium oxide catalysts.
81. 81
  Alexopoulos, K.; Reyniers, M.-F.; Marin, G. B. Reaction Path Analysis of Propane Selective Oxidation over V₂O₅ and V₂O₅/TiO₂. J. Catal. 2012, 289, 127– 139, DOI: 10.1016/j.jcat.2012.01.019
  81
  Reaction path analysis of propane selective oxidation over V2O5 and V2O5/TiO2
  Alexopoulos, Konstantinos; Reyniers, Marie-Francoise; Marin, Guy B.
  Journal of Catalysis (2012), 289 (), 127-139CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
  The selective oxidn. of propane on the vanadyl and bridging oxygen sites of the fully oxidized (0 0 1) V2O5 surface and of an epitaxial vanadia monolayer supported on (0 0 1) TiO2 anatase is analyzed using periodic d. functional theory (DFT). Both the oxidative dehydrogenation leading to propene and the formation of oxygenated products, n-propanol, i-propanol, propanal and acetone, are studied. Selective oxidn. proceeds via a Mars-van Krevelen redox mechanism, and its elementary steps on the vanadia surface are identified. Propane chemisorption preferentially occurs through a secondary C-H bond activation via a direct hydrogen abstraction by a lattice oxygen. Supporting a vanadia monolayer on titania strongly enhances the C-H bond activation as compared to unsupported V2O5, yielding a lower activation energy and a more exothermic propane chemisorption. In accordance with exptl. observations, the calcns. show that the titania support not only modifies the activity of the vanadia monolayer but it also affects the selectivity of the catalyst, favoring the formation of propene compared to the formation of i-propanol and acetone. The vanadyl oxygen is overall the most active site on V2O5 and V2O5/TiO2, while the bridging oxygen is more selective towards propane dehydrogenation.
Supporting Information
Supporting Information
The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c00174.
- XRD pattern of V₂O₅, TG and DTA analysis of V₂O₅, HAADF-STEM images of V₂O₅, description of multiwavelength Raman spectroscopy, mass spectra of oxygen exchange of V₂O₅, uncorrected Raman spectra of V₂O₅, the atomic displacement patterns of Raman active phonon modes, Raman spectra of V₂O₅ after treatment in the presence of C₃H₈ and ¹⁶O₂ at 573 °C, table of calculated lattice parameters and bond lengths, table of atomic coordinates of the optimized structure, description of the deconvolution of Raman spectra, discussions of surface reaction and bulk diffusion, and operando Raman spectroscopy of V₂O₅ during propane oxidation (PDF)
- jp2c00174_si_001.pdf (1.73 MB)
Terms & Conditions

Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Oxygen Exchange on Vanadium PentoxideClick to copy article linkArticle link copied!

The Journal of Physical Chemistry C

License Summary*

Abstract

License Summary*

License Summary*

License Summary*

1. Introduction

Figure 1

2. Methods

2.1. Properties of V2O5

2.2. Basic Characterization of V2O5

2.3. Raman Spectroscopy

2.4. Oxygen Isotope Exchange

2.5. Operando Raman Spectroscopy

2.6. DFT Calculations

3. Results and Discussion

3.1. Experimental and Calculated Raman Spectra of V2O5

Figure 2

3.2. Oxygen Isotope Exchange

3.2.1. Prediction of Raman Shifts in Partially and Fully Exchanged V2O5 by DFT Calculations

3.2.2. Experimentally Observed Changes in the Raman Spectra

3.2.2.1. Temperature Effect in the Presence of Oxygen Only

Figure 3

Figure 4

Figure 5

3.2.2.2. Oxygen Isotope Exchange in the Presence of Propane

Figure 6

3.2.2.3. Influence of the Hydrocarbon Chain Length

Figure 7

4. General Discussion and Conclusions

Supporting Information

Terms & Conditions

Author Information

Acknowledgments

References

Cited By

The Journal of Physical Chemistry C

License Summary*

Article Views

Altmetric

Citations

Recommended Articles

Abstract

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

References

Supporting Information

Supporting Information

Terms & Conditions

Oxygen Exchange on Vanadium Pentoxide
Click to copy article linkArticle link copied!

2.1. Properties of V₂O₅

2.2. Basic Characterization of V₂O₅

3.1. Experimental and Calculated Raman Spectra of V₂O₅

3.2.1. Prediction of Raman Shifts in Partially and Fully Exchanged V₂O₅ by DFT Calculations