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Influence of Solvent on Selective Catalytic Reduction of Nitrogen Oxides with Ammonia over Cu-CHA Zeolite
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  • Jamal Abdul Nasir*
    Jamal Abdul Nasir
    Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-0474-1610
  • Jingcheng Guan
    Jingcheng Guan
    Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
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  • Thomas W. Keal
    Thomas W. Keal
    Scientific Computing Department, STFC Daresbury Laboratory, Keckwick Lane, Daresbury, WarringtonWA4 4AD, U.K.
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    Orcidhttps://orcid.org/0000-0001-8747-3975
  • Alec W. Desmoutier
    Alec W. Desmoutier
    Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
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  • You Lu
    You Lu
    Scientific Computing Department, STFC Daresbury Laboratory, Keckwick Lane, Daresbury, WarringtonWA4 4AD, U.K.
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    Orcidhttps://orcid.org/0000-0002-7524-4179
  • Andrew M. Beale
    Andrew M. Beale
    Department of Chemistry, Christopher Ingold Building, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
    UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, R92 Harwell, OxfordshireOX11 0FA, U.K.
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    Orcidhttps://orcid.org/0000-0002-0923-1433
  • C. Richard A. Catlow*
    C. Richard A. Catlow
    Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
    UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, R92 Harwell, OxfordshireOX11 0FA, U.K.
    School of Chemistry, Cardiff University, Park Place, CardiffCF10 3AT, U.K.
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-1341-1541
  • Alexey A. Sokol*
    Alexey A. Sokol
    Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0003-0178-1147
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Journal of the American Chemical Society

Cite this: J. Am. Chem. Soc. 2023, 145, 1, 247–259
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https://doi.org/10.1021/jacs.2c09823
Published December 22, 2022

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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The copper-exchanged zeolite Cu-CHA has received considerable attention in recent years, owing to its application in the selective catalytic reduction (SCR) of NOx species. Here, we study the NH3-SCR reaction mechanism on Cu-CHA using the hybrid quantum mechanical/molecular mechanical (QM/MM) technique and investigate the effects of solvent on the reactivity of active Cu species. To this end, a comparison is made between water- and ammonia-solvated and bare Cu species. The results show the promoting effect of solvent on the oxidation component of the NH3-SCR cycle since the formation of important nitrate species is found to be energetically more favorable on the solvated Cu sites than in the absence of solvent molecules. Conversely, both solvent molecules are predicted to inhibit the reduction component of the NH3-SCR cycle. Diffuse reflectance infrared fourier-transform spectroscopy (DRIFTS) experiments exploiting (concentration) modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) identified spectroscopic signatures of Cu-nitrate and Cu-nitrosamine (H2NNO), important species which had not been previously observed experimentally. This is further supported by the QM/MM-calculated harmonic vibrational analysis. Additional insights are provided into the reactivity of solvated active sites and the formation of key intermediates including their formation energies and vibrational spectroscopic signatures, allowing the development of a detailed understanding of the reaction mechanism. We demonstrate the role of solvated active sites and their influence on the energetics of important species that must be explicitly considered for an accurate understanding of NH3-SCR kinetics.

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1. Introduction

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Atmospheric NOx emissions, consisting mainly of NO, NO2, and N2O, are highly damaging to the environment, producing smog and acid rain; they are also “greenhouse gases” and pose a serious hazard to human health both through direct exposure and via the formation of ozone. (1,2) Human activity has had a dramatic impact on NOx levels through fossil fuel combustion, which releases both NO and NO2 into the atmosphere. (3) To mitigate this problem, transition metal catalysis can be harnessed to remove NOx efficiently from exhaust gases through the selective catalytic reduction (SCR) reaction, in which ammonia is used to reduce these species to nitrogen. (4,5) The major exhaust gases from diesel engines are NO (>90%) rather than NO2; therefore, the reduction of NOx occurs mostly via what is known as the “standard SCR” reaction. (6)
Small-pore zeolites containing copper such as Cu-SSZ-13, possessing the chabazite (CHA) topology, have shown outstanding performance in the NH3-SCR reaction. (2,7,8) It has been reported that Cu-CHA has not only several active sites, including Cu+ and Cu2+ ions in NH3-SCR, but also species beyond single ions and with higher Cu-ion nuclearity. (9,10) For low-temperature SCR (<200–250 °C), it has been proposed that Cu ions of higher nuclearity are active, which is, however, not the case for high-temperature SCR where ions of low nuclearity have been proposed. (11) In the NH3-SCR cycle, the reaction follows either a standard SCR (eq 1) or NO-activation pathway (eq 2), which can proceed with the same reduction step as the fast-SCR (eq 3); however, in the “NO-activation” cycle, the rate does not depend on the concentration of NO2, while it is dependent on the concentration of NO2 in the fast SCR. (12)
4NO+4NH3+O2→4N2+6H2O
(1)
3NO+2NH3+O2→2N2+3H2O+NO2
(2)
NO+2NH3+NO2→4N2+3H2O
(3)
Water and ammonia vapors are among the key components of the NOx-containing exhaust gases, and their inevitable presence could lead to adsorption on the transition metal sites, affecting the energetics of the intermediate species. (13,14) Liu et al. (15) investigated the effect of water on NH3-SCR activity over Cu-LTA and found a promoting effect of water on low-temperature SCR activity with a plausible solvated [H2O–Cu–NH3]+ species. Similarly, the experimental study by Yu et al. (16) over Cu-SAPO-34 also showed a promoting effect of water and reported that in its presence, the reducibility of Cu2+ species at high temperature is improved, while NH3 oxidation is inhibited. It is also found that ammonia does not block the formation of nitrates when water is present in the feed, as reported by Lee et al. (17) Moreover, some studies show that enhanced Cu-ion mobility is caused by ligating water and ammonia molecules, leading to better low-temperature NH3-SCR activity. (10,15,18)
To optimize the catalytic process, not only the chemistry of active sites and the intrinsic NH3–NO reaction kinetics need careful attention but also the diffusion of the counterion must be understood, which is strongly affected by the ligation of solvent molecules such as H2O and NH3. Generally, Al sites are responsible for restricting the mobility of counterions, due to electrostatic attraction to the framework. [Cu(NH3)2]+ mobility inside the CHA framework at the sub-second time scale is predicted by Paolucci et al. (10) who found a displacement of 9 Å for [Cu(NH3)2]+ by ab initio metadynamics. Furthermore, employing molecular dynamics (MD) simulation, O’Malley et al. found that the strong coordination of NH3 with Cu2+ in the center of the CHA cage hinders the interaction of other molecules with the Cu sites. (19)
It is possible to split the NH3-SCR redox cycle and separate the oxidation from the reduction step, which is achieved experimentally by switching between NH3 + NO and NO + O2 (19) atmospheres to study the individual half-cycles. (12) The Cu(II)-nitrate species formed as a result of the NO + O2 oxidation process can be converted back to the Cu(I) state under NH3 + NO reductive conditions (Figure 1). Furthermore, NO can react with the nitrate intermediate to generate gaseous NO2 and Cu-nitrite species. Such NO activation, which has been reported for both single Cu-sites and Cu(II)-pair-mediated systems, accounts for Cu(II) reduction and is often considered necessary for the NH3-SCR reaction. (5,20) The formation of NO2 species facilitates the oxidation half-cycle and hence leads to establishing a link to the fast SCR reaction. (21)
To understand how solvated Cu cations can control nitrogen chemistry in CHA zeolite, we investigate reaction mechanisms with both modeling and experimental techniques. For this purpose, we have employed density functional theory (DFT) using a quantum mechanical/molecular mechanical (QM/MM) methodology, as implemented in the ChemShell software, and DRIFTS which allow us to observe the formation and consumption of short-lived intermediates in the catalytic reaction. Hereby, we elucidate the influence of physisorbed solvents on the reactivity of Cu-CHA sites and their effect on the energetics of intermediates. We report a comprehensive study in which the computational analysis provides a clear assignment of all main spectroscopic features of the NH3-SCR catalytic cycle, which are in good agreement with experiment. Thus, our investigation provides new insights into the NH3-SCR reaction by understanding the chemistry of the solvated-Cu-CHA sites and their impact on the key steps in the reaction.

Figure 1

Figure 1. Schematic representation of the catalytic cycle of NH3-SCR of NOx.

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2. Methods

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2.1. QM/MM Methodologies

A hybrid QM/MM approach (22,23) using the ChemShell software (24−26) was employed to study the NH3-SCR process over Cu-CHA. The model involves a quantum mechanical description of a relatively small cluster of ∼200 atoms around the reaction site, embedded in a much larger cluster modeled using a classical molecular mechanics forcefield. This approach is well suited for describing a local active site, as an alternative periodic DFT approach would require a large, supercell calculation to avoid spurious interactions between the periodically repeated reaction sites. It is also more straightforward in a non-periodic model to increase the accuracy of the QM description through the use of hybrid DFT functionals, as in the current study. Furthermore, the description of the electrostatic environment is more efficiently handled in the QM/MM model through a combination of MM atoms and point charges as detailed below.
In our CHA-cluster model, we set an active region, where all constituent atoms are allowed to relax freely, extending to a radius of 15 Å (∼28.3 Bohr) from the chosen center containing the Al site. A central core of 143 framework atoms and a charge-balancing Cu cation (not including hydrogen link atoms) is described at the QM level (Figure 2). The active region is in turn surrounded by a frozen spherical layer with a thickness of ca. 15 Å. The total number of atoms in the model is 6007, of which 700 are active. A judicious choice of the basis set discussed further below allows us to employ moderate basis sets in the QM region; for example, in the nitrate case, there are 2983 and 2797 Cartesian and harmonic basis functions, respectively, for the largest systems of interest (containing intermediate reacting species). The calculations for the QM clusters were performed using the GAMESS-UK package, (27) while for the MM part, the DL_POLY package (28) was employed, with the Hill-Sauer molecular mechanical forcefield, (29) which assumes that the atoms bind to each other by polar covalent bonds. Further we introduced the outer shell of point charges whose values have been fitted to reproduce accurately the electrostatic field in the active region of the infinite CHA-zeolite framework.

Figure 2

Figure 2. QM/MM embedding setup; CHA cluster (left) with a quantum mechanical region containing nitrate species (right). The outermost region contains point charges to ensure that the Madelung potential in the center of the cluster is accurately reproduced. Atom color codes: Cu (brown), Al (green), Si (yellow), O (red), N (blue), and H (white).

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Atoms at the interface between QM and MM regions are connected by bonds that need careful treatment including terminating the QM region by adding hydrogen atoms, forming O-H groups, and use of modified MM charges at the boundary. (23,26) We divided the QM region into two parts and employed a dual basis set strategy: the number of atoms in the innermost QM1 region is ∼28 including the intermediates which are treated with the triplet-ζ basis set def2-TZVP, (30) while the outer QM2 region (115 atoms), which includes the terminating link H atoms, is treated with a smaller split valence with polarization def2-SVP (30) basis set. To build the QM/MM model, it is necessary to remove the classical charges from the QM centers and make sure that the total charge removed from the system is the same as the total charge of the QM region. The guest species of interest in the zeolite and reactant gaseous species were also treated at the QM level using the higher quality def2-TZVP basis set. The coordinates of all atoms in the QM region and active MM region were fully optimized. To find the most favorable location of Cu2+–OH– in Cu-CHA, we performed DFT calculations using a range of exchange–correlation density functionals, as presented in the Supporting Information section, and have selected the B97-2 (31) results as providing the most accurate reaction energies and molecular structures. Therefore, the B97-2 data will be used in the main body of this paper. In particular, we have found that the extraframework Cu ions are more stable in eight-membered rings (8-MR) than 6-MR, by 0.3 eV (cf. a similar finding in ref (32)). Considering that adsorbed molecules can diffuse through the larger channels, all the adsorbed species including the intermediates were placed within (or near) the 8-MR of the CHA cage. The water (O-end) and ammonia (N-end) molecules were placed at a distance of 2 Å from the Cu center with the proton pointing away from the adjacent oxygens and intermediate species to avoid artificial trapping in hydrogen-bonding interactions. We note that hydrogen bonding with framework oxygen can influence chemistry at metal sites by affecting the binding of functionally important Cu–H2O/NH3 units.
For vibrational frequencies at a local active site modeled by the hybrid QM/MM approach, only nuclear displacements of the active sites are included, that is, a frozen phonon approximation from the expanded environment accounting for electrostatic interactions is applied throughout the vibrational calculations. To this aim, we chose an active region that contains atoms around the Al center within three coordination spheres (5T-site), including the atoms of the intermediate species, and calculated normal and localized modes. Full details of the approach used for vibrational frequency calculations are given in the Supporting Information section.

2.2. Choice of the Model Cluster

To perform the QM/MM calculation, we first created a spherical embedded-cluster model (Figure 2) of CHA from the unit cell of siliceous CHA (33) optimized at the MM level using the GULP package. (34) After creating a simple CHA-cluster model, we constructed active sites. For example, in the case of a Brønsted acid site, we replaced one Si with Al and added a charge compensating proton on a neighboring oxygen atom at a site where it is most accessible to facilitate the reaction. The QM region which is contained within the active part of the model includes atoms from the third oxygen shell from the central T-site; as noted, we have added the hydrogen (link atoms) to saturate the terminal oxygen atoms. Furthermore, we modified the same cluster by incorporating copper into the cage of the CHA framework.
We first optimized the purely siliceous CHA cluster using ChemShell and employed the hybrid QM/MM model for zeolites developed by Sherwood. (35) The geometrical parameters obtained for the Si-tetrahedral sites by ChemShell employing two representative DFT functionals, B97-2 (31) and BB1K, (36) were found to be in an accordance with the experimental data (details are in Supporting Information, Tables S1 and S2).

2.3. Catalyst Preparation

Synthesis of SSZ-13 zeolite (Si/Al = 13) was performed as reported earlier following the hydrothermal approach. (37) The N,N,N trimethyladamantammonium hydroxide was used as a structure-directing agent under fluoride media. The protonated zeolite is obtained by calcining the sample in the air, first for 2.5 h at 1 °C min–1 to 120 °C and then for 10 h at 4 °C min–1 to 550 °C. By employing the wet ion-exchange methodology, a typical amount of H-SSZ-13 is then mixed with a copper sulfate solution (50 mL of a 0.1 M solution of CuSO4 per gram of zeolite) under constant heating (80 °C for 2 h) and stirring. The resultant product consists of a well-defined crystal of rhombohedral morphology (2.92 wt % Cu loading) which was washed with water and kept at 80 °C overnight. (38) To perform operando spectroscopic analysis, the pellets of zeolite (8 mm bore, 1.5-tonne pressure) were prepared which were then crushed and sieved to retain a 250–450 μm fraction for experiments.

2.4. Catalyst Characterization

Powder X-ray Diffraction (PXRD) patterns were collected to confirm phase purity and crystallinity on a Rigaku Miniflex diffractometer (Cu Kα1, 1.54056 Å), and samples were loaded onto a flat Teflon sample holder. Diffraction patterns were collected between 5.0 and 50.0° in 0.02° steps. PXRD shows that a highly crystalline pure phase of Cu-SSZ-13 is present after calcination, subsequent ion-exchange, and calcination steps (see Figure S6). Energy-dispersive X-ray (EDX) analysis of Cu-SSZ-13 (see Table S19) shows that Cu-SSZ-13 has a composition of 2.92 wt % Cu with a Si/Al = 13 which represents 75% Cu ions exchanged into available H+ sites. The Brunauer–Emmett–Teller (BET) method was used in the analysis of the total surface area, and the t-plot method was used in the micropore volume (Table S20).

2.5. DRIFTS Operando ME Experiments

DRIFTS spectra were recorded on a Bruker Vertex 70 spectrometer equipped with a liquid-N2 cooled HgCdTe detector and a Praying Mantis mirror unit (Harrick). The spectroscopic cell connected to heated gas supply lines was equipped with a flat CaF2 window (2 mm thick; diameter 25 mm). The outlet of the cell was coupled to a Fourier Transform Infrared (FTIR) spectrometer equipped with a 70 mm path length gas cell heated to 150 °C (Bruker Alpha). The sample was placed in the sample cup of the cell (ca. 30 mg, 57 mm3) after being dried in situ in 10 vol % O2/N2 (100 mL min–1) at 400 °C for 2 h. DRIFTS spectra were collected by accumulating 10 interferograms under 80 kHz scanner velocity (0.9 s per spectrum) and at 4 cm–1 resolution. Solenoid valves were used to repeatedly switch between gases during a concentration modulation excitation experiment which was functioned using OPUS software (Bruker).

3. Results and Discussion

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An NH3-SCR reaction mechanism has been proposed by Janssens et al., who show both standard and fast NH3-SCR of NOx in a complete cycle that can produce the correct stoichiometry for the reaction. (32) In contrast, in our study, the focus is on solvent effects on the reactivity of the Cu-CHA, and, as argued, it provides key insights into the reaction mechanism from accurate hybrid-QM/MM calculations and concentration modulation ME DRIFTS experiments. Hybrid-QM/MM investigations, DRIFTS experiment, and in-depth scrutiny of solvent effects allowed us to explore especially the intermediate–solvent interactions and identify the important species participating in the NH3-SCR event both on the bare and on the solvated sites. The computational mechanistic study focuses on these results, which complement and give insight into the experimental findings.

3.1. Vibrational Study

First, we performed DRIFTS experiments exploiting (concentration) modulation excitation spectroscopy (MES) and phase-sensitive detection (PSD) to determine the evolution of species in response to a stimulus; in this case, the NO flow during a constant stream of 500 ppm NH3 and 10,000 ppm O2 produces N2 akin to the standard NH3-SCR reaction at 250 °C. (38) This approach allows us to observe the formation and consumption of short-lived intermediates in the catalytic reaction, crucially, the detection of species, which had not been previously observed experimentally. The data are shown in Figure 3a. (38) Notable observations included the initial consumption of [Cu2+(OH)]− to form an important intermediate, which has been identified as copper nitrosamine (Cu–N(═O)–NH2) based on the observation of evolving bands in the IR spectrum at 1436 cm–1 (N═Ostr), 1330 cm–1 (N═Ostr), and 1258 cm–1 (N–Hbend). Below 1200 cm–1, we are unable to collect reliable spectroscopic data due to the overlapping of zeolite framework vibrations with other bands. The next species detected in the cycle was a bidentate nitrate (N═Ostr at 1606 cm–1). Note that all bands appear with a different “phase”, or time during the experiment, indicating that the corresponding species are not typically present at the same time. Furthermore, Figure S5 shows the 2400 DRIFTS spectrum collected throughout the course of the modulation experiment in which NO is turned “on/off” repeatedly between 0 and 500 ppm, while the concentration of other reactive components, NH3 and O2, remains constant. Note that no discernible changes can be detected since the continuous presence of NH3 and products of the reaction (H2O) dominate the spectrum, particularly in the region between 2500 and 3500 cm–1.

Figure 3

Figure 3. Spectroscopic signatures: (a) concentration modulation ME DRIFTS experiment with the corresponding phase-resolved spectrum, (b) reaction cycle highlighting the identified species, (c) data obtained from QM/MM calculations for bidentate Cu nitrate (Cu–NO3) and Cu-nitrosamine (Cu–N(═O)–NH2) species for the neutral system, and (d) for the deprotonated and protonated system. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white). The framework SiO2 is shown using a wire framework motif.

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Second, we calculated the harmonic vibrational spectra of selected intermediates. The harmonic values obtained were scaled using vibrational scaling factors (see Supporting Information), which were determined by comparing experimental and computational harmonic values using a representative set of gas-phase molecules. We focused on bands of nitrosamine (N–Ostr and N–Nstr) and nitrate (N–Ostr); the yielded scaling factor to calculate the vibrational frequency of N–Ostr of nitrosamine is 0.915, while for the N–Nstr, it is 0.918. Similarly, the corresponding scaling factor we applied for the N–Ostr of nitrate is 0.943.
As shown in Figure 3b, for the bare Cu-nitrosamine (Cu–N(═O)–NH2) (species G, Figure 3b) species, we found a vibrational stretching band around 1478 cm–1 compared to our experimental value of 1436 cm–1, tentatively ascribed to the N═Ostr mode. Haszeldine and Jander reported the N═Ostr band at 1488 cm–1 which is close to our calculated frequency. (39) Also, bands in the region 1408–1486 cm–1 have been reported by Piskorz and Urbanski (40) and Tarte (41) and assigned to the N═Ostr frequency of nonassociated dialkylnitrosamine (in nonpolar solvent: CCl4). They also reported peaks for N═Ostr between 1346–1265 and 1321–1292 cm–1, which can be broad and strong or of medium strength. Importantly, this can be also seen in our measured DRIFTS spectra [(N═Ostr), 1330 cm–1]. Furthermore, Kedrova et al. have observed the vibrational frequencies of associated nitrosamine and identified both N═Ostr (1470–1495 cm–1) and N–Nstr (1055–1060 cm–1) bands. (42) On physisorption of water and ammonia at Cu sites, we observed the N-Ostr band shift down to 1467 and 1462 cm–1, respectively. The lower frequency spectral features found in our QM/MM calculations are, in particular, due to N–N stretching vibrations of Cu-nitrosamine (Cu–N(═O)–NH2) species. For the bare site, the N–N stretching frequency is calculated as 1124 cm–1 as compared to the reported experimental values in the nitrosodimethylamine (1052 cm–1) and di-N-nitroso-pentamethylenetetramine (1106 cm–1). (40) For the water- and ammonia-solvated sites, the N–Nstr is found at 1123 and 1111 cm–1, respectively. In addition to the N═Ostr and N–Nstr bands for nitrosamine species, we also examined the N–H bend, which is likely to appear in this region; the computed value for the bare site is at 1225 cm–1, whereas for the water- and ammonia-solvated sites, it is found at 1224 and 1210 cm–1, respectively. The reduction of the vibrational frequency of these modes shows the ligand effect as stronger ligands may weaken the bonding.
Next, we investigated a bidentate nitrate species (species B, Figure 3b) with a focus on the N═Ostr stretching mode. Generally, nitrate species have four notable modes. (43,44) Our calculations show that the bare bidentate Cu–NO3 structure possesses the main N═Ostr stretching frequency of 1620 cm–1, while for the physisorbed water and ammonia, it is found at 1609 and 1600 cm–1, respectively. All these three bands are quite close to our experimental DRIFTS value of 1606 cm–1 and to earlier reports (45,46) of the nitrate N═Ostr difference FTIR band in Cu-SSZ-13. In addition, small broad bands that appeared in our experimental findings in the region of 1230–1250 cm–1 could be assigned to the antisymmetric stretch (ν3 (E′)) of the N–O band, as reported by Zapata and García-Ruiz. (43) However, our QM/MM calculation shows this band at relatively lower frequencies for this mode; for instance, it is seen at 1172 cm–1 for the bare site, while for physisorbed water and ammonia sites, it appears at 1192 cm–1 and 1202 cm–1, respectively. Finally, two bands that originated by out-of-plane and in-plane deformation modes for nitrate species are displayed. The out-of-plane deformation band (ν2 (A2″)) is normally located within the range of 800–900 cm–1, while the in-plane band (ν4 (E′)) ranges from 700 to 780 cm–1 in nitrate salts. (43,44) Our calculated out-of-plane deformation occurs at a somewhat higher frequency, as seen in Figure 3b; however, the in-plane deformation has appeared almost in the same region as previously reported. (43)
One of the significant effects of solvation is the possibility of site deprotonation due to a proton transfer to solvents even if only as a transient species. Therefore, we completed this analysis by studying vibrations of a negatively charged intermediate species that would result from such deprotonation by possibly abstracting the proton from the −NH2 group of nitrosamine to form NH4+ ions (see Figure 3d). The calculated N–Ostr (1416 cm–1) band is found at somewhat lower frequencies for the deprotonated nitrosamine NH4+(Cu–N(═O)–NH) species compared to the neutral system (1478 cm–1), while the N–Nstr stretching bands for this species appear at a relatively higher frequency of 1308 cm–1 than the neutral system (1124 cm–1). There is a mixed asymmetric stretch mode of the NH4+ group combined with N–Nstr, which may be the reason that N–Nstr appears at a higher frequency than expected.
Inversely to the deprotonation of nitrosamine species, we also examined the likely protonation of Cu-nitrosamine (Cu–N(═O)–NH2) that generates an −OH site, that is, (Cu–N(═OH)–NH2). The calculated N–Ostr band (1592 cm–1) in this case was found at significantly higher frequencies compared to both deprotonated nitrosamine (1416 cm–1) and neutral system (1478 cm–1). Based on the result obtained, we infer that the experimental DRIFTS data show either neutral or deprotonated nitrosamine rather than protonated species. Moreover, we also carried out a separate vibrational analysis on Cu–(N(═O)–OH)–NH3 (species F, Figure 3b), a postulated species which further decomposes to the (Cu–N(═O)–NH2) species. The N–Ostr band appeared for this intermediate at 1585 cm–1 (Figure S2).
Furthermore, we examine the N═Ostr vibrational mode of bidentate-nitrite (Cu–NO2) (species C, Figures 3b, S3) to determine whether the 1606 cm–1 band in the DRIFTS spectrum originates from the nitrate or nitrite. The computed N═Ostr band of this particular species is, however, found to be significantly lower than the N═Ostr band of the bidentate-nitrate species at 1273 cm–1, while for the solvated H2O and NH3 active sites, it is observed at 1263 and 1260 cm–1, respectively. Notably, the calculated bands are comparable with the experimental values of ca. 1229 cm–1 for the isostructural linear nitrite species reported in refs (47) and (48).
We have performed a similar calculation on (Cu–(NO2)–NH3) (species D, Figures 3b and S4); however, again we found that the NO band for this species is lower than the 1606 cm–1 band in the DRIFTS spectrum assigned to N═Ostr vibration for all three sites. Based on the calculated vibrational modes, we infer that the spectroscopic signatures that appear in the DRIFTS spectrum can be tentatively assigned to the nitrate and nitrosamine species. Furthermore, we do not detect the N–O signatures in the DRIFTS spectrum for the nitrite (species C) and the species D and F in the cycle and even the first Cu–NO(OH) interactions on NO adsorption, indicating that these species are too short-lived to be observed experimentally. Hence, we propose the assignment of the DRIFTS spectral features summarized in Table 1 and shown in Figure 3a.
Table 1. Features Observed in a Concentration ME DRIFTS Experiment’s Time-Resolved Spectrum under SCR Conditions (NO Gas Switch Pulse Sequence)a
DRIFTS (cm–1)vibrational modestrengthwidthcalculated IR modebref
1606N═O stretch (nitrate)mediumsharp1600 (B), 1609 (H), 1620 (N) (45,46)
1436N═O stretch (nitrosamine)strongsharp1478 (B), 1467 (H), 1462 (N) (39−42)
1330N═O stretch (nitrosamine)smallmedium  (41)
1258N–H bend (nitrosamine)smallsharp1225 (B), 1224 (H), 1210 (N) 
1230–1250N–Oanti-symm. stretch (nitrate)smallbroad1172 (B), 1192 (H), 1202 (N) (43,44)
1210N–N stretchsmallmedium1124 (B), 1123 (H), 1111 (N) (40,42)
a

Assignment is based on an analysis of the literature and calculated vibrational spectra of reactant intermediates shown in Figure 3b.

b

B, bare; H, physisorbed H2O; and N, physisorbed NH3.

Furthermore, we examine the O–H and N–Hstr vibrational features for the competing [Cu2+(OH)]+ and Brønsted acid sites under an NH3-SCR environment. In the DRIFTS spectrum (Figure S5), there are multiple vibrational bands in the region between 2500 and 3500 cm–1 as the spectrum is mostly dominated by NH3 and H2O. The most notable feature in the phase-resolved spectrum is, however, that at 3655 cm–1, which is indicative of [Cu2+(OH)]+ species that exhibit a greater fluctuation than those related to silanol groups or bridging hydroxyls, which is also reported by Giordanino et al. (49) Our calculated value (3668 cm–1) is in good accordance with the DRIFTS experimental value and with the reported data. (45) The experimental attribution of this band to a [Cu2+(OH)]+ species was based on its response to changes in gas composition, indicating that this species actively participates in the catalytic mechanism. (38) The vibrational signatures of the likely NH3 adsorption on the Cu2+ active sites are also confirmed by theory and experiment. In the corresponding DRIFTS spectrum, the N–H band which appears at 3332 cm–1 is in good accordance with the computed value of 3336 cm–1. Moreover, we detect the N–H bending features at 1620 cm–1 in the DRIFTS spectrum which is reproduced in our calculations with an accuracy better than 1 cm–1 and agrees well with previous work. (50) To study the competitive reaction pathway between NH3 adsorbed on Cu sites and Brønsted sites, we analyzed the vibrational signatures of the NH3 adsorption on the Brønsted acid site, the interaction which may lead to evolution of NH4+ ions owing to the NH3 protonation over these acid sites. The band intensity grows from ca. 1454 cm–1 in the DRIFTS spectrum (see Figure S5), indicating the consumption of the v(O–H) band associated with the Brønsted acid sites. The position of this band is confirmed by our computed value of 1455 cm–1 and previous work (49) for NH4+ ions. The N–H stretching band of NH4+ ions appears at 3272 cm–1 in the DRIFTS spectrum, which also agrees with the calculated vibrational value at 3278 cm–1.
Our computational analysis provides a clear assignment (Table 2) of all main spectroscopic features of the species presented in Figure 3b.
Table 2. Calculated Vibrational Bands of Key Intermediate Species Presented in the NH3-SCR Catalytic Cycle─See Figure 3b
specieswavelength (cm–1)adescription of IR active mode
Cu–NO3 (species B)1600 (B), 1609 (H), 1620 (N)N═O stretch
Cu–NO2 (species C)1273 (B), 1263 (H), 1260 (N)N═O stretch
(Cu–(NO2)–NH3) (species D)1501 (B), 1481 (H), 1464 (N)N═O stretch
 3445 (B), 3450 (H), 3447 (N)N–H stretch
Cu–(N(═O)–OH)–NH3 (species F)1585 (B), 1579 (H), 1575 (N)N═O stretch
 3313 (B), 3471 (H), 3485 (N)N–H stretch
 3197 (B), 3216 (H), 3226 (N)O–H stretch
(Cu–N(═O)–NH2) (species G)1478 (B), 1467 (H), 1462 (N)N═O stretch
 3482 (B), 3493 (H), 3517 (N)N–H stretch
[Cu2+(OH)]+ adsorbed NH33336N–H stretch
 3668O–H stretch
 1620N–H bend
Brønsted acid site adsorbed NH33279N–H stretch
 1455N–H bend
a

B, bare; H, physisorbed H2O; and N, physisorbed NH3.

3.2. Comparison of Reaction Energetics with Experiment

Next, we assess our computational approach for the overall de-NOx reaction energy that involves NO activation to stable long-lived intermediates and products. To this end, we compare the simulated data with experimental measurements and analyze the results obtained (Tables S3 and S4). First, we calculated the reaction energy for eq 2, an NO-activation pathway using computational parameters described in Section 2.1 (B97-2 (31) functional and the def2-TZVP (30) basis set). The overall experimental ΔfH0° value is −9.01 eV (−869 kJ/mol), which is calculated from the standard enthalpy of formation (Table S5) of the gaseous species involved in this reaction. The calculated theoretical value is −8.37 eV, which is similar to the previous calculations (∼−8.45 eV, estimated from Figure 3 of ref (32)). Thus although the two theoretical estimates are within 0.08 eV of each other, there remains a small but significant deviation from the experimental value.

3.3. Cu Displacement on Solvation

The promoting effect of water on the migration of Cu species suggested that unanchored Cu ions migrate to form active sites which can promote the SCR reaction. (18) In our calculations summarized in Figure 4a, we observed that an increase in water coordination to the Cu-active sites displaces the Cu species away from the CHA framework. As the number of H2O molecules around the Cu(I)-CHA increases, the interaction between the framework and Cu species becomes weaker. In the case of four H2O molecules, the distance between Al and Cu is calculated to be 8.07 Å, consistent with the understanding that water molecules promote the mobility of Cu ions. Similarly, the effect of NH3 solvation during the oxidation part of the SCR reaction is crucial. Some previous reports suggest that the NH3-solvated Cu(I) sites interact weakly with the zeolite framework and move away as a mobile species to react with the O2 and yield an O-bridged Cu(II) dimer; (51) we find an increase in bond length between Al and Cu as the number of NH3 molecules increases, suggesting that an enhanced number of NH3 molecules can detach Cu species from the framework. Ab initio molecular dynamics (AIMD) simulations show that after the adsorption of one NH3 molecule in Cu-SAPO-34 (52) at 298 K, the Cu+ cation is somewhat displaced yet remains coordinated with one of the framework oxygens in an H3N–Cu–O bond, which is also evident from experiments on Cu-SSZ-13. (49) These studies demonstrate that the Cu–O bond in the Cu+–NH3 system is broken by the adsorption of a second NH3 molecule, resulting in the formation of a linear Cu+(NH3)2 complex that can move readily inside the pore. Our QM/MM simulations agree with these findings, as can be observed from Figure 4a. Furthermore, Paolucci et al. (10) used ab initio metadynamics and a supercell with a minimum image distance in excess of 10 Å to evaluate the mobility of CuI(NH3)2 complexes over time scales which are inaccessible to conventional AIMD. Their results supported by experimental observations show a ∼9 Å diffusion length for a Cu ion that penetrates an 8-MR window separating two adjacent CHA cages. In our study, we employed a more detailed model of the solvent behavior in the Cu-SSZ-13 system. Similar to the abovementioned study, we start our analysis with a single-solvent adsorbate molecule simulating both water and ammonia. Next, we introduced stepwise a series of solvent molecules bringing the solvent content to four molecules per cage. Upon adsorption, we observed a pronounced displacement of the Cu ion away from the framework Al site. In the case of water, the calculated distance between Al and Cu is 8.07 Å, while for the solvated Cu–NH3 case, it is 6.16 Å, consistent with the understanding that solvent molecules promote the mobility of Cu ions. In contrast to the report by Paolucci et al., (10) we do not observe the penetration through the 8-MR window, which could be attributed to the different boundary conditions in our simulations. Notably, the Cu ions in our study are not forced to move away from the Al site but displace spontaneously as a result of geometry optimization from their initial position close to the Al site in response to the strong interaction of the polar solvent with both framework and extraframework metal cations.

Figure 4

Figure 4. Interaction of physisorbed (a) water and ammonia with Cu(I)-CHA active sites, and (b) behavior of NO on solvated Cu(I)-CHA site. The model used is shown as an extra framework. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white).

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We also examined the approach of NO toward the solvated Cu(I)-CHA and infer that in the presence of water, the effect of NO is vital, as it restricts, to some extent, the displacement of the Cu-ion species from the framework (Figure 4b). This is apparent from the corresponding separation distances (2.96 Å) between Cu and Al when compared with those when NO is not present, which can prevent Cu(II) dimer formation and promotes the formation of nitrate species. (53,54) In addition, it also suggests that the presence of NO might prevent the mobility of Cu ions in the presence of water. We, however, observed a complete detachment of Cu ions from the framework when NO interacts with three NH3 coordinated sites with a distance of 3.071 Å between Cu and Al (Figure 4b).

3.4. Adsorption Study

We examine first the binding energies of the gaseous reactant molecules both on the Cu(I)-CHA and Cu(II)-CHA sites.
From the results reported in Figure 5, considering the behavior on the Cu(I) site, we note that the adsorption of NO, NH3, and H2O on the Cu+-/CHA site is, as expected, exothermic. The binding energy for NH3, yielding NH3–Cu+, is calculated as −0.84 eV (−81 kJ/mol) (32,55) where the reported average experimental heat of adsorption for ammonia on Cu-Beta (56) and Cu-CHA (57) is nearly −100 kJ/mol. The experimental values, however, should be treated with caution as Cu-exchanged zeolites would still have a significant fraction of strongly adsorbing Brønsted acid sites. (58,59) We have also examined the interaction of ammonia with the Brønsted acid site and found a binding energy of −1.1 eV (−106 kJ/mol), which is appreciably higher than that of the Cu Lewis site and is close to the experimental report. The heat of NH3 adsorption over non-exchanged zeolites H-CHA has been reported to be as high as −145 kJ/mol (obtained using microcalorimetry techniques), (60) which is appreciably stronger and could be related to the Brønsted acid complexes, which warrants a separate investigation.

Figure 5

Figure 5. Reaction adsorption energies of NH3, NO (with both the O- and N-end down), H2O, and O2 on Cu(I)-CHA and Cu(II)–OH/CHA sites. The model used is shown as an extra framework. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white). The framework SiO2 is shown using a wire framework motif.

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Furthermore, the reported studies also suggest that the heat of adsorption decreases with increased ammonia coverage. (57) It is reported that the binding of NH3 on Cu+ strongly influences the interaction of Cu with the framework of CHA; however, depending on the number of NH3 molecules bound and temperature, cation mobility can become possible. (52) In this context, we observed that a single physisorbed NH3 molecule detaches Cu+ from one of the framework O atoms, making a new coordination adduct, H3N–Cu–O, with a distance between Al and Cu of 2.645 Å. H2O also exothermically binds to the Cu(I)-CHA sites with a binding energy of −0.62 eV (−59 kJ/mol), but the monovalent Cu remains attached to the two framework O atoms. Furthermore, Lercher and co-workers employed a periodic PBE + D3 approach and obtained an adsorption energy of −77 kJ/mol for water on the Brønsted acid site of H-MFI, (61) whereas a value of −78 kJ/mol is obtained for the H-CHA by a hybrid MP2/PBE calculation. (62) We also observed that NO weakly adsorbs on bare Cu(I)-/CHA, through the N atom (with a binding energy of only −0.08 eV); however, it is more favorable on solvated ammonia Cu(I) (−1.25 eV) and water Cu(I) (−0.58 eV) sites, where the corresponding experimental heat of adsorption is −65 kJ/mol (−0.70 eV) on Cu-Beta. (56)
Turning now to the Cu(II) sites, NH3 and NO interact favorably with divalent Cu (II)–OH sites. NO binds through the O atom (−OH) (−1.01 eV), suggesting that it preferentially binds to the OH site of Cu(II)–OH, generating an HONO species. The formation of the HONO species has been described earlier. (47,63) In the case of NH3, strong bonding to the Cu(II) site is calculated, with a binding energy of −0.97 eV (−93 kJ/mol), and as seen in Figure 5, the attachment of Cu(II) with the framework O is intact. We also demonstrate that water can interact strongly with the divalent Cu site (with a binding energy of −0.91 eV), suggesting that water can affect the reactivity of active sites.
Based on the interaction of molecules with the active sites, we conclude that the adsorption of NO species with the O-end down is uncompetitive (with a positive energy) on both Cu+ (0.11 eV) and Cu2+ (0.12 eV) sites, suggesting that the N-end down is the only feasible attachment to Cu-CHA sites. We found that O2 preferably binds to Cu(I) sites with a binding energy of −0.18 eV (−17 kJ/mol) as reported before, while on the Cu2+–OH–/CHA site, the adsorption energy is calculated to be positive (0.01 eV), (52) which rules out the possibility of O2 binding to the Cu2+–OH–/CHA, hence, signifying that O2 plays a key role in the reoxidation of the Cu+-site.

3.5. Catalytic Cycle

During the NH3-SCR cycle, molecules including NO, NH3, O2, N2, and H2O are either adsorbed or desorbed from the Cu-CHA site, and the intermediate species are generated at each step (Figure 6). The adsorption of NO and O2 on Cu+-/CHA generates nitrate and nitrite species which undergo decomposition to N2 and H2O; this half cycle of the NH3-SCR is, as noted, known as the oxidation part, which is followed by the reduction of Cu2+ where Cu2+–OH–/CHA reacts with both NO and NH3, with Cu2+ reduced to Cu+ while generating N2 and H2O as a product, as is evident from both experimental and theoretical studies. (52) The calculated adsorption energies of the corresponding gaseous species allow us to obtain the reaction energy landscape. Note that in the DRIFTS spectra, all bands appear with a different “phase” or time during the experiment, indicating that the corresponding species are not typically present at the same time and giving further credence to the cyclical nature of the proposed mechanism shown in the inset of Figure 6.

Figure 6

Figure 6. Potential reaction-energy landscape for NH3-SCR on the activated Cu-CHA site and (black) on the bare site, (green) with physisorbed ammonia and (red) with physisorbed water. Inset right: NO-activated NH3-SCR of the NOx catalytic cycle.

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Considering the NH3-SCR of the NOx cycle, first, without physisorbed solvent molecules, the corresponding reaction energy landscape is shown in Figures 6 (black lines) and S7. The starting point for the chemical reaction is an isolated Cu(I) site that activates the O2 molecule (step I). The energy diagram demonstrates that O2 adsorption is exothermic (−0.18 eV) over the Cu(I)-CHA site which is followed by NO adsorption that generates the Cu–NO3 species with a formation energy of −1.34 eV while simultaneously oxidizing Cu(I) to Cu(II) (step II). It is evident from previous reports that gaseous NO2 can react with the Cu+ site, yielding a bidentate Cu nitrite species (step III), which is often considered the fast-SCR reaction. (64) The reaction of gaseous NO with nitrate (54) is reported to be a two-step process where first NO coordinates with the nitrate species and then decomposes to nitrite with the release of gaseous NO2. To this end, we found a total energy change of +0.17 eV from the nitrate to nitrite step. In the case of the NH3 reaction with the nitrite species, an intermediate (step IV) is formed with a formation energy of −1.24 eV. Furthermore, we found that NH3 can also react with the Cu–HONO site that generates an NH3–Cu–HONO intermediate (step VII) with an H3N–Cu bond length of 2.160 Å and a formation energy of −0.63 eV. Subsequently, it decomposes and leads to the generation of an important intermediate, the Cu-nitrosamine (Cu–N(═O)–NH2) (step VIII), which is considered to play a key role in the NH3-SCR reaction as it involves the formation of the first N–N bond in the N2 product. The QM/MM results agree with the sequence of species identified in our DRIFTS data. The existence of both nitrosamine and nitrate in the DRIFTS spectra is also evident from the calculated QM/MM harmonic vibrational data.

3.5.1. Effect of Water Solvation

Water is one of the main products of NH3-SCR reaction, and therefore, the hydrated state of the active sites cannot be ignored. (65) Taking into account the calculated adsorption energies of H2O on Cu(I)-CHA (−0.62 eV) and Cu(II)-CHA (−0.91 eV) sites, we suggest that the NH3-SCR reaction proceeds differently on solvated Cu-CHA sites. Using the same computational approach, we have studied the effect of water on the formation of the intermediate species; single water molecules are attached to the isolated Cu(I)/Cu(II) active sites to which intermediate species are bound. The corresponding reaction energy landscape is shown in Figures 6 (red lines) and S8. We observe notable differences in the formation energies of some important steps. For example, our results show that without physisorbed water, the formation of nitrate species is less favorable (−1.34 eV) than in the presence of water (−1.69 eV) (step II), suggesting that water can promote the re-oxidation half-cycle, which is also supported by experimental findings as reported by Wan et al., (18) showing that water could markedly enhance NO reduction. In contrast, we found that on the water-occupied −Cu-CHA site, the formation of Cu-nitrosamine (Cu–N(═O)–NH2) (step VIII) is less favorable (−0.11 eV) than in the absence of water (−0.66 eV), suggesting that water can slow down the formation of this species, which can ultimately affect the reduction part of the NH3-SCR cycle. We observe that the effect of water on nitrosamine is more significant (+0.55 eV) than on the nitrate (−0.35 eV), which implies that the impact of water could be a (slight) net negative influence on activity. Commensurate with this, it has also been reported previously that residual H2O or hydrocarbons can block the active sites or alter their activity. (66)

3.5.2. Effect of Ammonia Solvation

Previous experimental evidence showed that preadsorbed NH3 at 300 or 250 °C can enhance NOx reduction; however, low activity of NO reduction was observed below 150 °C. (14) From the calculated binding energies of NH3 for both Cu(I)-CHA (−0.62 eV) and Cu(II)-CHA (−0.91 eV) sites, we can infer that the reactive sites occupied by NH3 molecules can influence the mechanism. We have investigated the influence of ammonia on the formation of the intermediate species, as shown in green lines in the energy profile (Figures 6 and S9). Notably, the formation of Cu bidentate nitrate species (Cu–NO3) on the physisorbed NH3 site is found to be more favorable (−1.59 eV) than with the bare site (−1.34 eV) (step II). In the case of physisorbed water and the bare site, the generation of nitrite under the release of NO2 is found to be endothermic, but for physisorbed ammonia, it is exothermic (step III). In addition, the formation of the important intermediate, Cu-nitrosamine (Cu–N(═O)–NH2) (step VIII), is less favorable on the physisorbed ammonia active sites (−0.31 eV), when compared to the bare site (−0.66 eV), indicating that physisorbed ammonia can block the active sites for the NH3 + NO activation half cycle.

3.6. H2NNO Decomposition

An important step in the NH3-SCR mechanism is the reaction of NH3 with HONO that generates the NH3–Cu–HONO intermediate which eventually leads to the formation of the key intermediate Cu-nitrosamine (Cu–N(═O)–NH2), as shown in Figure 6 (step VIII). The decomposition of H2NNO has been extensively studied and proceeds through the transformation of several important isomers with high activation barriers. (67−69) For example, DFT calculations showed that the energy barrier for the H2NNO decomposition is considerably lowered via proton exchange between the Brønsted acid site and H2NNO over V2O5, (70) the mechanism of which is similar to dehydrogenation of propane over vanadia. (71) A similar study has been conducted over Cu-CHA where the decomposition of H2NNO is investigated on Brønsted acid sites. (72) Such a study has also been reported over Cu-SAPO-34 (73) and ZSM-5 (74,75) using a cluster-based computational approach. As it is found that the solvent can coordinate to the Cu-site, we, therefore, investigated the decomposition pathway of H2NNO on isolated Cu-CHA active sites both in the presence and absence of physisorbed water (Figure 7). The schematic illustrations of isomeric decompositions of H2NNO intermediates are shown in Figure S10. The adduct rearrangement process of H2NNO species starts with 1,3 H-transfer, breaking one N–H bond with the transmission of an H to an adjacent O atom, which leads to the formation of an O–H bond. The H-transfer leads to two isomers cis–trans (II) and trans–cis (III) through the four-membered ring(I). The total energy change from structures (I) to (II) and (III) are calculated as 0.04 and 0.06 eV on the bare site, respectively, while with ligated water, it is −0.33 and −0.46 eV, respectively, suggesting some positive impact of solvated sites on the energetics of this step. From H2NNO to HNNOH, the H-transfer is accompanied by shortening of the N–N bond (from 1.35 to 1.299 Å) with lengthening of the N–O bond (from 1.208 to 1.306 Å) that eventually ends with the termination of respective bonds and generation of H2O and N2 as products. We have noted that the increased coordination of H2O molecules detached Cu–H2NNO from the framework (Figure S11), showing that the solvent can affect the binding of Cu–H2NNO species to the framework.

Figure 7

Figure 7. Calculated potential energy surfaces for H2NNO isomerization (a) without water and (b) in the presence of physisorbed water on the Cu-CHA-sites. The model used is shown as an extra framework. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white).

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3.7. Reactivity of HONO

We also examined the reactivity of Cu–HONO species for NH3, H2O, O2, and NO, which are the key reactants and products in the NH3-SCR reaction. We have found that NO can bind with the Cu(II)–(OH) site forming the Cu–HONO species (20,53,76) that can react with NH3 to yield NH4NO2, a short-lived and unstable species which can decompose to N2 and H2O. The subsequent desorption of HONO is endothermic (energy barrier: 0.145 eV) as previously reported; (76) we, therefore, performed a series of calculations to assess the interaction of other species with the Cu–HONO site, as displayed in Figure S12. Considering the approach of solvent molecules first, we found that the interaction of Cu–HONO with H2O is exothermic, while with ammonia, it is endothermic. The binding energy of H2O is calculated to be −0.39 eV with a bond distance between O and Cu of 2.155 Å. In the case of NH3, the binding energy is 0.99 eV with a bond distance between N and Cu of 2.160 Å. The interaction of dioxygen with the Cu–HONO site is in the superoxo (O2–) mode with an average bond distance of 2.127 Å between O and Cu. We also note the interaction of NO with Cu–HONO, forming a NO–Cu–HONO adduct.

4. Summary and Conclusions

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The present study has aimed to confirm the identity of key intermediates in the NH3-SCR reaction and elucidate the role of physisorbed solvents, as the Cu-CHA active sites will be affected by solvent molecules. The main catalytically active sites that facilitate the adsorption of species are both monovalent- and divalent-copper sites that can drive the NH3-SCR to generate important intermediate species including nitrates and nitrites. By understanding the parallels between the water and ammonia interactions with active sites, we find that an increase in solvent coordination to the Cu-active sites liberates the Cu species away from the CHA framework. DRIFTS data showed the formation and consumption of short-lived intermediates in the catalytic reaction, crucially, the detection of important bands for both nitrosamine and bidentate nitrate species, which is in accordance with the calculated frequencies by QM/MM calculation, giving further credence to the proposed mechanism. Our computational analysis provides a clear assignment of all main spectroscopic features of the NH3-SCR catalytic cycle, which are in good agreement with experiment. To understand the role of solvents on the kinetics of the NH3-SCR cycle, the adsorption of gaseous species and the formation of intermediates and their spectroscopic signatures on the solvated active sites have been investigated which can help tune the rational design of important reaction steps. From the potential energy landscapes, we observed that the formation of nitrate species is energetically more favorable on solvated active sites than on the bare site, suggesting that solvents can promote the re-oxidation part of the NH3-SCR cycle. This effect is more significant in the case of water than ammonia. In contrast to the potential benefits seen on the oxidative part, both water and ammonia are found to inhibit the reduction part of SCR since the formation of important intermediates such as Cu-nitrosamine is relatively less favorable on solvated active sites than on the bare sites, suggesting that solvent can slow down the reduction part of the NH3-SCR cycle. This finding explains why there is some debate concerning the effect of water and ammonia on the reaction since it seems to affect some parts of the cycle positively and others negatively. In addition, solvating ammonia species were also found to accelerate the oxidation part of the cycle, which is also apparently evident from experimental data showing that preadsorbed NH3 at 250–300 °C can enhance the NOx reduction. Our study provides additional understanding, helping to unravel the influence of solvents on the energetics of the active sites, and provides guidance for optimizing the NH3-SCR process.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.2c09823.

  • Additional computational details; benchmarking studies; vibrational calculations and scaling study; characterization details of prepared Cu-CHA zeolite; potential reaction-energy landscape for all three active sites; and binding energy study of HONO species (PDF)

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Author Information

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  • Corresponding Authors
    • Jamal Abdul Nasir - Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.;  Orcidhttps://orcid.org/0000-0002-0474-1610;  Email: [email protected]
    • C. Richard A. Catlow - Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.;  UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, R92 Harwell, OxfordshireOX11 0FA, U.K.;  School of Chemistry, Cardiff University, Park Place, CardiffCF10 3AT, U.K.;  Orcidhttps://orcid.org/0000-0002-1341-1541;  Email: [email protected]
    • Alexey A. Sokol - Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.;  Orcidhttps://orcid.org/0000-0003-0178-1147;  Email: [email protected]
  • Authors
    • Jingcheng Guan - Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
    • Thomas W. Keal - Scientific Computing Department, STFC Daresbury Laboratory, Keckwick Lane, Daresbury, WarringtonWA4 4AD, U.K.;  Orcidhttps://orcid.org/0000-0001-8747-3975
    • Alec W. Desmoutier - Department of Chemistry, Kathleen Lonsdale Materials Chemistry, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.
    • You Lu - Scientific Computing Department, STFC Daresbury Laboratory, Keckwick Lane, Daresbury, WarringtonWA4 4AD, U.K.;  Orcidhttps://orcid.org/0000-0002-7524-4179
    • Andrew M. Beale - Department of Chemistry, Christopher Ingold Building, University College London, 20 Gordon Street, LondonWC1H 0AJ, U.K.;  UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, R92 Harwell, OxfordshireOX11 0FA, U.K.;  Orcidhttps://orcid.org/0000-0002-0923-1433
  • Author Contributions

    The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

  • Funding

    We would like to acknowledge the financial support for Jamal Abdul Nasir’s study from the Project Management Unit (PMU) Higher Education Department of KPK, Pakistan, and the financial support from the EPSRC.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors wish to acknowledge support from the EPSRC grants EP/R001847/1 and EP/W014580/1 and the UK Catalysis Hub funded by EPSRC grants EP/R026939/1 and EP/R026815/1. We also wish to acknowledge the MCC grant EP/R029431/1 and computational support provided by CoSeC, the Computational Science Centre for Research Communities, through the MCC. We also thank the UCL research computing facilities, the ISIS materials characterisation laboratory for access to the X-ray diffractometer, I. Lezcano-Gonzalez for preparing the Cu-SSZ-13 samples and A. G. Greenaway, A. Marberger, and D. Ferri for the collection and processing of the DRIFTS data.

Abbreviations

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SCR

selective catalytic reduction

QM/MM

quantum mechanical/molecular mechanical

DFT

density functional theory

CHA

chabazite

DRIFTS

Diffuse Reflectance Infrared Fourier-Transform Spectroscopy

FTIR

Fourier transform infrared

MD

molecular dynamics

AIMD

ab initio molecular dynamics

PXRD

powder X-ray diffraction

EDX

energy-dispersive X-ray

BET

Brunauer–Emmett–Teller

MES

modulation excitation spectroscopy

PSD

phase-sensitive detection

References

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    Recent advances in automotive catalysis for NOx emission control by small-pore microporous materials
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    Chemical Society Reviews (2015), 44 (20), 7371-7405CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the no. of lean-NOx redn. technologies, selective catalytic redn. (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx redn., the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx redn. was spurred by the discovery of the very high activity of Cu-SSZ-13 (and the isostructural Cu-SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temps. In this review we summarize the key discoveries of the past ∼5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and prepn. of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterization work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.
  3. 3
    Horowitz, L. W.; Jacob, D. J. Global impact of fossil fuel combustion on atmospheric NOx. J. Geophys. Res. Atmos. 1999, 104, 23823– 23840,  DOI: 10.1029/1999jd900205
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    Global impact of fossil fuel combustion on atmospheric NOx
    Horowitz, Larry W.; Jacob, Daniel J.
    Journal of Geophysical Research, [Atmospheres] (1999), 104 (D19), 23823-23840CODEN: JGRDE3 ISSN:. (American Geophysical Union)
    Fossil fuel combustion is the largest global source of NOx to the troposphere. This source is concd. in polluted continental boundary layers, and the extent to which it impacts tropospheric chem. on a global scale is uncertain. We use a global three-dimensional model of tropospheric chem. and transport to study the impact of fossil fuel combustion on the global distribution of NOx during northern hemisphere summer. In the model, we tag fossil fuel NOx and its reservoir NOy species in order to det. the relative contribution of fossil fuel combustion to NOx concns. in different regions of the world. Our model includes a detailed representation of NOx-O3-nonmethane hydrocarbon (NMHC) chem., which is necessary to properly simulate the export of reactive nitrogen, including org. nitrates such as peroxyacyl nitrates (PANs), from the continental boundary layer. We find that fossil fuel combustion accounts for over 40% of NOx concns. in the lower and middle troposphere throughout the extratropical northern hemisphere. PANs are shown to provide an important mechanism for transporting NOx from source regions to the remote troposphere, accounting for over 80% of the fossil fuel NOx in the lower troposphere over most of the ocean. Sources in the United States are found to contribute about half of the fossil fuel NOx over the North Atlantic Ocean. Emissions from China, which are expected to increase rapidly in the coming decades, currently account for about half of the fossil fuel NOx over the western North Pacific Ocean; the influence of these emissions extends into the tropics. Because of this tropical influence, emissions from China have more potential than emissions in the United States to perturb the global oxidizing power of the atm.
  4. 4
    Damma, D.; Ettireddy, P. R.; Reddy, B. M.; Smirniotis, P. G. A review of low temperature NH3-SCR for removal of NOx. Catalysts 2019, 9, 349,  DOI: 10.3390/catal9040349
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    A review of low temperature NH3-SCR for removal of NOx
    Damma, Devaiah; Ettireddy, Padmanabha R.; Reddy, Benjaram M.; Smirniotis, Panagiotis G.
    Catalysts (2019), 9 (4), 349CODEN: CATACJ; ISSN:2073-4344. (MDPI AG)
    A review. The importance of the low-temp. selective catalytic redn. (LT-SCR) of NOx by NH3 is increasing due to the recent severe pollution regulations being imposed around the world. Supported and mixed transition metal oxides have been widely investigated for LT-SCR technol. However, these catalytic materials have some drawbacks, esp. in terms of catalyst poisoning by H2O or/and SO2. Hence, the development of catalysts for the LT-SCR process is still under active investigation throughout seeking better performance. Extensive research efforts have been made to develop new advanced materials for this technol. This article critically reviews the recent research progress on supported transition and mixed transition metal oxide catalysts for the LT-SCR reaction. The review covered the description of the influence of operating conditions and promoters on the LT-SCR performance. The reaction mechanism, reaction intermediates, and active sites are also discussed in detail using isotopic labeling and in situ FT-IR studies.
  5. 5
    Brandenberger, S.; Kröcher, O.; Tissler, A.; Althoff, R. The State of the Art in Selective Catalytic Reduction of NOxby Ammonia Using Metal-Exchanged Zeolite Catalysts. Catal. Rev. 2008, 50, 492– 531,  DOI: 10.1080/01614940802480122
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    The State of the Art in Selective Catalytic Reduction of NOx by Ammonia Using Metal-Exchanged Zeolite Catalysts
    Brandenberger, Sandro; Krocher, Oliver; Tissler, Arno; Althoff, Roderik
    Catalysis Reviews - Science and Engineering (2008), 50 (4), 492-531CODEN: CRSEC9; ISSN:0161-4940. (Taylor & Francis, Inc.)
    A review. An overview is given of the selective catalytic redn. of NOx by ammonia (NH3-SCR) over metal-exchanged zeolites. The review gives a comprehensive overview of NH3-SCR chem., including undesired side-reactions and aspects of the reaction mechanism over zeolites and the active sites involved. The review attempts to correlate catalyst activity and stability with the prepn. method, the exchange metal, the exchange degree, and the zeolite topol. A comparison of Fe-ZSM-5 catalysts prepd. by different methods and research groups shows that the prepn. method is not a decisive factor in detg. catalytic activity. It seems that decreased turnover frequency (TOF) is an oft-neglected effect of increasing Fe content, and this oversight may have led to the mistaken conclusion that certain prodn. methods produce highly active catalysts. The available data indicate that both isolated and bridged iron species participate in the NH3-SCR reaction over Fe-ZSM-5, with isolated species being the most active.
  6. 6
    Koebel, M.; Elsener, M.; Kleemann, M. Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines. Catal. Today 2000, 59, 335– 345,  DOI: 10.1016/s0920-5861(00)00299-6
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    Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines
    Koebel, M.; Elsener, M.; Kleemann, M.
    Catalysis Today (2000), 59 (3-4), 335-345CODEN: CATTEA; ISSN:0920-5861. (Elsevier Science B.V.)
    Urea-SCR, the selective catalytic redn. using urea as reducing agent, has been investigated for about 10 yr in detail and today is a well established technique for DeNOx of stationary diesel engines. It is presently also considered as the most promising way to diminish NOx emissions originating from heavy duty vehicles, esp. trucks. The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the redn. of the required catalyst vol. while still maintaining a high selectivity for the SCR reaction over a wide temp. range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Addnl. problems include the control strategy for urea dosing, the high f.p. of urea, and the long term stability of the catalyst.
  7. 7
    Deka, U.; Juhin, A.; Eilertsen, E. A.; Emerich, H.; Green, M. A.; Korhonen, S. T.; Weckhuysen, B. M.; Beale, A. M. Confirmation of isolated Cu2+ ions in SSZ-13 zeolite as active sites in NH3-selective catalytic reduction. J. Phys. Chem. C 2012, 116, 4809– 4818,  DOI: 10.1021/jp212450d
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    7
    Confirmation of Isolated Cu2+ Ions in SSZ-13 Zeolite as Active Sites in NH3-Selective Catalytic Reduction
    Deka, Upakul; Juhin, Amelie; Eilertsen, Einar A.; Emerich, Hermann; Green, Mark A.; Korhonen, Satu T.; Weckhuysen, Bert M.; Beale, Andrew M.
    Journal of Physical Chemistry C (2012), 116 (7), 4809-4818CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    NH3-Selective Catalytic Redn. (NH3-SCR) is a widely used technol. for NOx redn. in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and lab. techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temps. (with low activity), XAFS and XRD data revealed a conformational change in the local geometry of the copper from a planar form toward a distorted tetrahedron as a result of a preferential interaction with NH3. This process appears necessary for activity, but results in a stymieing of activity at low temps. At higher temps., the Cu2+ possess a local coordination state akin to that seen after calcination.
  8. 8
    Ma, L.; Cheng, Y.; Cavataio, G.; McCabe, R. W.; Fu, L.; Li, J. Characterization of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts with hydrothermal treatment for NH3-SCR of NOx in diesel exhaust. Chem. Eng. J. 2013, 225, 323– 330,  DOI: 10.1016/j.cej.2013.03.078
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    8
    Characterization of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts with hydrothermal treatment for NH3-SCR of NOx in diesel exhaust
    Ma, Lei; Cheng, Yisun; Cavataio, Giovanni; McCabe, Robert W.; Fu, Lixin; Li, Junhua
    Chemical Engineering Journal (Amsterdam, Netherlands) (2013), 225 (), 323-330CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts were fully formulated washcoat cordierite monoliths purchased from the major catalyst suppliers and were hydrothermally aged at 600, 750, and 850 °C in simulated exhaust gases contg. water. Their catalytic activities were tested in selective catalytic redn. (SCR) of NOx with ammonia. The microstructure of the zeolite supports, identity of copper species, acidity and reactant adsorption ability were investigated in detail using various characterization methods. The results showed that hydrothermal treatment of Cu-SSZ-13 and Cu-SAPO-34 catalysts had a significant influence on the physicochem. properties and reactant adsorption abilities of the catalysts, which resulted in different catalytic performances in NH3-SCR. The hydrothermal treatment drastically decreased the surface area and pore vol. of both the Cu-SSZ-13 and Cu-SAPO-34 catalysts. TEM results indicated that obvious aggregation of Cu-SSZ-13 and Cu-SAPO-34 occurred while some CuO particulates migrated from isolated Cu2+ species were formed during the aging test. Hydrothermal treatment of the Cu-SSZ-13 and Cu-SAPO-34 catalysts caused the migration of Cu2+, a decrease in acidity and dealumination. The change in the coordination environment of Cu2+ affected NOx adsorption and activation on the catalyst surface. Both the copper sites and the acidity might be the main factor for the NH3-SCR reaction on Cu-CHA catalyst.
  9. 9
    Marberger, A.; Petrov, A. W.; Steiger, P.; Elsener, M.; Kröcher, O.; Nachtegaal, M.; Ferri, D. Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13. Nat. Catal. 2018, 1, 221– 227,  DOI: 10.1038/s41929-018-0032-6
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    Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13
    Marberger, Adrian; Petrov, Andrey W.; Steiger, Patrick; Elsener, Martin; Krocher, Oliver; Nachtegaal, Maarten; Ferri, Davide
    Nature Catalysis (2018), 1 (3), 221-227CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
    Practical catalysts often operate under dynamic conditions of temp. variations and sudden changes of feed compn. that call for understanding of operation and catalyst structure under analogous exptl. conditions. For instance, the copper-exchanged small-pore SSZ-13 catalyst used currently in the selective catalytic redn. of harmful nitrogen oxides from the exhaust gas of diesel-fuelled vehicles operates under recurrent ammonia dosage. Here, we report the design of unsteady state expts. that mimic such a dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation, we were able to capture an ammonia inhibition effect on the rate-limiting copper re-oxidn. at low temp. The practical relevance of this observation was demonstrated by optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and an increase in nitrogen oxide conversion at low temp.
  10. 10
    Paolucci, C.; Khurana, I.; Parekh, A. A.; Li, S.; Shih, A. J.; Li, H.; Di Iorio, J. R.; Albarracin-Caballero, J. D.; Yezerets, A.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F.; Gounder, R. Dynamic multinuclear sites formed by mobilized copper ions in NO x selective catalytic reduction. Science 2017, 357, 898– 903,  DOI: 10.1126/science.aan5630
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    Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction
    Paolucci, Christopher; Khurana, Ishant; Parekh, Atish A.; Li, Sichi; Shih, Arthur J.; Li, Hui; Di Iorio, John R.; Albarracin-Caballero, Jonatan D.; Yezerets, Aleksey; Miller, Jeffrey T.; Delgass, W. Nicholas; Ribeiro, Fabio H.; Schneider, William F.; Gounder, Rajamani
    Science (Washington, DC, United States) (2017), 357 (6354), 898-903CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Copper ions exchanged into zeolites are active for the selective catalytic redn. (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temp. rate dependence on copper (Cu) volumetric d. is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calcns. to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)-mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the vol. that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.
  11. 11
    Millan, R.; Cnudde, P.; van Speybroeck, V.; Boronat, M. Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations. JACS Au 2021, 1, 1778– 1787,  DOI: 10.1021/jacsau.1c00337
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    11
    Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations
    Millan, Reisel; Cnudde, Pieter; van Speybroeck, Veronique; Boronat, Mercedes
    JACS Au (2021), 1 (10), 1778-1787CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
    The mobility of the copper cations acting as active sites for the selective catalytic redn. of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temp. and feed compn. Herein, the migration of [Cu(NH3)2]+ complexes between two adjacent cavities of the chabazite structure, including other reactant mols. (NO, O2, H2O, and NH3), in the initial and final cavities is investigated using ab initio mol. dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH3 or NO in the initial cavity, since both reactants form with [Cu(NH3)2]+ stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O2 modifies strongly the interaction of NO with Cu+. At low temps., we observe NO detachment from Cu+ and increased mobility of the [Cu(NH3)2]+ complex, while at high temps., NO reacts spontaneously with O2 to form NO2. The present simulations give evidence for recent exptl. observations, namely, an NH3 inhibition effect on the SCR reaction at low temps., and transport limitations of NO and NH3 at high temps. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temps., the former involving dimeric Cu(NH3)2-O2-Cu(NH3)2 species and the latter occurring by direct NO oxidn. to NO2 in one single cavity.
  12. 12
    Janssens, T. V.; Falsig, H.; Lundegaard, L. F.; Vennestrøm, P. N.; Rasmussen, S. B.; Moses, P. G.; Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lamberti, C.; Bordiga, S.; Godiksen, A.; Mossin, S.; Beato, P. A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia. ACS Catal. 2015, 5, 2832– 2845,  DOI: 10.1021/cs501673g
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    12
    A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia
    Janssens, Ton V. W.; Falsig, Hanne; Lundegaard, Lars F.; Vennestroem, Peter N. R.; Rasmussen, Soeren B.; Moses, Poul Georg; Giordanino, Filippo; Borfecchia, Elisa; Lomachenko, Kirill A.; Lamberti, Carlo; Bordiga, Silvia; Godiksen, Anita; Mossin, Susanne; Beato, Pablo
    ACS Catalysis (2015), 5 (5), 2832-2845CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    For the first time, the std. and fast selective catalytic redn. (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable mols. only. The std. SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidn. of the catalyst by NO + O2 and a redn. by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the redn., and finally, oxidn. by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown exptl. for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), ESR, and Fourier transform IR spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the std. SCR cycle. The activation energy calcd. by d. functional theory (DFT) indicates that the oxidn. of an NO mol. by O2 to a bidentate nitrate ligand is rate-detg. for std. SCR. Finally, the role of a nitrate/nitrite equil. and the possible influence of Cu dimers and Bronsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidn. to NO2.
  13. 13
    Akter, N.; Chen, X.; Parise, J.; Boscoboinik, J. A.; Kim, T. Effects of copper loading on NH3-SCR and NO oxidation over Cu impregnated CHA zeolite. Korean J. Chem. Eng. 2018, 35, 89– 98,  DOI: 10.1007/s11814-017-0268-x
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    13
    Effects of copper loading on NH3-SCR and NO oxidation over Cu impregnated CHA zeolite
    Akter, Nusnin; Chen, Xianyin; Parise, John; Boscoboinik, Jorge Anibal; Kim, Taejin
    Korean Journal of Chemical Engineering (2018), 35 (1), 89-98CODEN: KJCHE6; ISSN:0256-1115. (Springer)
    Cu/CHA catalysts with various Cu loadings (0.5 wt%-6.0 wt%) were synthesized via incipient wetness impregnation. The catalysts were applied to the selective catalytic redn. (SCR) of NO with NH3 and NO oxidn. reaction. XRD and N2 adsorption-desorption data showed that CHA structure was maintained with the incorporation of Cu, while sp. surface areas decreased with increasing Cu loading. At intermediate Cu loading, 4 wt%, the highest NH3-SCR activity was obsd. with ~ 98% N2 selectivity from 150°C to 300°C. Small amts. of water, 2%, slightly increased NO conversion in addn. to the remarkable N2O and NO2 redn. at high temp. Water effects are attributed to the improved Cu ion reducibility and mobility. NO oxidn. results provided no relation between NO2 formation and SCR activity. Physicochem. properties, NO conversion, N2 selectivity, and activation energy data showed that impregnated samples' mol. structure and catalytic activity are comparable to the conventional ion-exchanged (IE) samples ones.
  14. 14
    Wallin, M.; Karlsson, C.-J.; Skoglundh, M.; Palmqvist, A. Selective catalytic reduction of NOx with NH3 over zeolite H-ZSM-5: influence of transient ammonia supply. J. Catal. 2003, 218, 354– 364,  DOI: 10.1016/s0021-9517(03)00148-9
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    14
    Selective catalytic reduction of NOx with NH3 over zeolite H-ZSM-5: influence of transient ammonia supply
    Wallin, Mikaela; Karlsson, Carl-Johan; Skoglundh, Magnus; Palmqvist, Anders
    Journal of Catalysis (2003), 218 (2), 354-364CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
    The effect of ammonia supply on the selective catalytic redn. of NOx over zeolite H-ZSM-5 was investigated using step response expts. between 200 and 500°. For inlet NO:NO2 ratios >1, the activity for NOx redn. transiently increased when NH3 was removed from the feed. For NO:NO2 ratios ≤1, the NOx redn. however decreased. By pulsing NH3 to the feed, the activity for NO redn. was enhanced up to five times compared to continuous supply of ammonia. For NO:NO2 ratios exceeding one, also the selectivity towards N2O formation was lower with transient ammonia supply. Temp. programmed reaction expts. with preadsorbed NH3 showed highest initial NOx redn. activity when ammonia had been adsorbed at 300 or 250° compared to 200°. A min. in NO redn. was obsd. at 130° independent of the ammonia adsorption temp. For NO:NO2 ratios >1, the results strongly indicate that NO oxidn. is the rate detg. step in the ammonia selective catalytic redn. (NH3-SCR) reaction over H-ZSM-5.
  15. 15
    Liu, Y.; Xue, W.; Seo, S.; Tan, X.; Mei, D.; Liu, C.-j.; Nam, I.-S.; Hong, S. B. Water: A promoter of ammonia selective catalytic reduction over copper-exchanged LTA zeolites. Appl. Catal., B 2021, 294, 120244,  DOI: 10.1016/j.apcatb.2021.120244
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    15
    Water: A promoter of ammonia selective catalytic reduction over copper-exchanged LTA zeolites
    Liu, Yang; Xue, Wenjuan; Seo, Seungwan; Tan, Xuechao; Mei, Donghai; Liu, Chang-jun; Nam, In-Sik; Hong, Suk Bong
    Applied Catalysis, B: Environmental (2021), 294 (), 120244CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Neg. effects of water on the hydrothermal stability of ammonia (NH3) selective catalytic redn. (SCR) catalysts are well known. Thus, developing water-resistant, zeolite-based transition metal catalysts like copper (Cu) for mitigating nitrogen oxides from mobile sources is very challenging. Here we show that water markedly facilitates the migration of copper ions from sterically inaccessible sod cage to the readily accessible lta one through six-membered rings within a copper-exchanged high-silica Cu-LTA zeolite. We also show that this ubiquitous mol. promotes the low-temp. SCR activity of the Cu-LTA catalyst with a plausible reaction [H2O-Cu-NH3]+ intermediate. We anticipate that water might play a beneficial role in nitrogen oxides removal over metal-contg. zeolites for cleaner air via tuning the active sites and reaction pathways.
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    Yu, T.; Wang, J.; Shen, M.; Wang, J.; Li, W. The influence of CO2 and H2O on selective catalytic reduction of NO by NH3 over Cu/SAPO-34 catalyst. Chem. Eng. J. 2015, 264, 845– 855,  DOI: 10.1016/j.cej.2014.12.017
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    16
    The influence of CO2 and H2O on selective catalytic reduction of NO by NH3 over Cu/SAPO-34 catalyst
    Yu, Tie; Wang, Jun; Shen, Meiqing; Wang, Jianqiang; Li, Wei
    Chemical Engineering Journal (Amsterdam, Netherlands) (2015), 264 (), 845-855CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
    The effect of CO2 and H2O addn. on selective catalytic redn. of NO by NH3 (NH3-SCR) over Cu/SAPO-34 catalysts was confirmed and we tried to study their influence in this research. Two Cu/SAPO-34 samples with the equiv. Cu loading were utilized, and their acidities were adjusted by the Si amts. in SAPO-34 supports. NH3-SCR, NH3-TPD, NH3 oxidn. and kinetic anal. were conducted to evaluate the role of CO2 and H2O. NH3-SCR results revealed that CO2 in the inlets presented less impact on NO conversion, while the presence of H2O enhanced NO conversion during the whole temp. range. NH3-TPD results showed H2O improved the acidity of Bronsted acid sites and the reducibility of Cu2+ species, both of which benefited the NH3-SCR activity. In addn., the presence of H2O at high temp. inhibited NH3 conversion and side-products during NH3 oxidn. process. Kinetic anal. of NH3-SCR and NH3 oxidn. at high temp. were further conducted to est. the variation of apparent activation energy (Ea) for the presence of water vapor, and finally the relation between the variation of acidity, Cu2+ species reducibility and the SCR mechanism over Cu/SAPO-34 sample at different temp. ranges was proposed, resp.
  17. 17
    Lee, H.; Song, I.; Jeon, S. W.; Kim, D. H. Mobility of Cu Ions in Cu-SSZ-13 Determines the Reactivity of Selective Catalytic Reduction of NOx with NH3. J. Phys. Chem. Lett. 2021, 12, 3210– 3216,  DOI: 10.1021/acs.jpclett.1c00181
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    17
    Mobility of Cu ions in Cu-SSZ-13 determines the reactivity of selective catalytic reduction of NOx with NH3
    Lee, Hwangho; Song, Inhak; Jeon, Se Won; Kim, Do Heui
    Journal of Physical Chemistry Letters (2021), 12 (12), 3210-3216CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    Selective catalytic redn. of NOx with NH3 (NH3-SCR) in Cu-SSZ-13 has been proposed to have a unique homogeneous-like mechanism governed by the spatial proximity of mobile Cu ions. Among factors that det. the proximity, the effect of ion d. on the SCR reaction is well established; however, it has not been verified how the different mobility of the Cu ion influences the SCR reaction. Herein, we try to reveal the mobility-dependent SCR reaction by controlling the Cu species with different ion mobilities in Cu-SSZ-13. Since the reaction kinetics is governed by the diffusion of Cu ions, the Cu ion mobility dets. the reactivity of the Cu-SSZ-13. In terms of this correlation, enhanced ion mobility leads to improved NH3-SCR activity. These findings help understand the behavior of Cu ions in Cu-SSZ-13 under a catalytic reaction and provide insights to design rational catalysts by tuning the ion mobility.
  18. 18
    Wan, Y.; Yang, G.; Xiang, J.; Shen, X.; Yang, D.; Chen, Y.; Rac, V.; Rakic, V.; Du, X. Promoting effects of water on the NH3-SCR reaction over Cu-SAPO-34 catalysts: transient and permanent influences on Cu species. Dalton Trans. 2020, 49, 764– 773,  DOI: 10.1039/c9dt03848e
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    18
    Promoting effects of water on the NH3-SCR reaction over Cu-SAPO-34 catalysts: transient and permanent influences on Cu species
    Wan, Yuyi; Yang, Guangpeng; Xiang, Jinyao; Shen, Xiaoqiang; Yang, Dafei; Chen, Yanrong; Rac, Vladislav; Rakic, Vesna; Du, Xuesen
    Dalton Transactions (2020), 49 (3), 764-773CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    Cu-SAPO-34 catalysts with varied Cu loadings were synthesized through ion exchange to study the influence of water on the NH3-SCR reaction. The catalytic activities were evaluated by selective catalytic redn. of NO under a reactant feed in the presence/absence of water. Transient expts. were designed to study the response of NO conversion to the presence of water. H2-TPR and DFT calcns. were performed to study the reducibility of Cu species. NH3-TPD and XPS were conducted to reveal the migration of Cu species. Water could remarkably improve NO redn. activities and the promoting effect is more significant on the catalyst with low Cu loading. Both transient and permanent influences were found in this promoting phenomenon. For the transient influence, water was proved to accelerate the re-oxidn. half-cycle. Moreover, water can enhance the promoting effect of the SCR feed on the migration of Cu species. These unanchored Cu ions migrate to defect sites to form active sites, which lead to a permanent influence of water.
  19. 19
    O’Malley, A. J.; Hitchcock, I.; Sarwar, M.; Silverwood, I. P.; Hindocha, S.; Catlow, C. R. A.; York, A. P.; Collier, P. Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process. Phys. Chem. Chem. Phys. 2016, 18, 17159– 17168,  DOI: 10.1039/c6cp01160h
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    Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process
    O'Malley, Alexander J.; Hitchcock, Iain; Sarwar, Misbah; Silverwood, Ian P.; Hindocha, Sheena; Catlow, C. Richard A.; York, Andrew P. E.; Collier, P. J.
    Physical Chemistry Chemical Physics (2016), 18 (26), 17159-17168CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The diffusion of NH3 in com. NH3-SCR catalyst Cu-CHA was measured and compared with H-CHA using quasielastic n scattering (QENS) and mol. dynamics (MD) simulations to assess the effect of counterion presence on NH3 mobility in automotive emission control relevant zeolite catalysts. QENS expts. obsd. jump diffusion with a jump distance of 3 Å, giving similar self-diffusion coeff. measurements for both Cu- and H-CHA samples, in the range of ∼5-10 × 10-10 m2 s-1 over the measured temp. range. Self-diffusivities calcd. by MD were within a factor of 6 of those measured exptl. at each temp. The activation energies of diffusion were also similar for both studied systems: 3.7 and 4.4 kJ mol-1 for the H- and Cu-chabazite, resp., suggesting that counterion presence has little impact on NH3 diffusivity on the timescale of the QENS expt. An explanation is given by the MD simulations, which showed the strong coordination of NH3 with Cu2+ counterions in the center of the chabazite cage, shielding other mols. from interaction with the ion, and allowing for intercage diffusion through the 8-ring windows (consistent with the exptl. obsd. jump length) to carry on unhindered.
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    Hu, W.; Selleri, T.; Gramigni, F.; Fenes, E.; Rout, K. R.; Liu, S.; Nova, I.; Chen, D.; Gao, X.; Tronconi, E. On the Redox Mechanism of Low-Temperature NH 3 -SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle. Angew. Chem. 2021, 133, 7273– 7280,  DOI: 10.1002/ange.202014926
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    Ruggeri, M. P.; Nova, I.; Tronconi, E. Experimental study of the NO oxidation to NO 2 over metal promoted zeolites aimed at the identification of the standard SCR rate determining step. Top. Catal. 2013, 56, 109– 113,  DOI: 10.1007/s11244-013-9937-0
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    Experimental study of the NO oxidation to NO2 over metal promoted zeolites aimed at the identification of the standard SCR rate determining step
    Ruggeri, Maria Pia; Nova, Isabella; Tronconi, Enrico
    Topics in Catalysis (2013), 56 (1-8), 109-113CODEN: TOCAFI; ISSN:1022-5528. (Springer)
    Steady state and transient kinetic runs devoted to the comparative anal. of NO oxidn. and std. SCR reactions over com. Cu- and Fe-promoted zeolite catalysts are herein presented with the aim to clarify whether NO oxidn. to NO2 is the rate detg. step (rds) of the std. SCR reaction. It is found that this statement seems questionable in the light both of the herein collected exptl. results and of scattered evidence from the literature.
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    Catlow, C. R. A.; Buckeridge, J.; Farrow, M. R.; Logsdail, A. J.; Sokol, A. A. Quantum Mechanical/Molecular Mechanical (QM/MM) Approaches. In Handbook of Solid State Chemistry; Dronskowski, R., Kikkawa, S., Stein, A., Eds.; Wiley-VCH, 2017; Vol. 5, pp 647– 680.
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    Sherwood, P.; de Vries, A. H.; Collins, S. J.; Greatbanks, S. P.; Burton, N. A.; Vincent, M. A.; Hillier, I. H. Computer simulation of zeolite structure and reactivity using embedded cluster methods. Faraday Discuss. 1997, 106, 79– 92,  DOI: 10.1039/a701790a
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    23
    Computer simulation of zeolite structure and reactivity using embedded cluster methods
    Sherwood, Paul; De Vries, Alex H.; Collins, Simon J.; Greatbanks, Stephen P.; Burton, Neil A.; Vincent, Mark A.; Hillier, Ian H.
    Faraday Discussions (1997), 106 (Solid State Chemistry: New Opportunities from Computer Simulations), 79-92CODEN: FDISE6; ISSN:0301-7249. (Royal Society of Chemistry)
    The use of bare cluster models to understand the nature of zeolite-substrate interactions may be improved to take account of the environment of the Broensted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves generating a specialized correction potential by fitting a non-periodic array of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic array of at. charges to the potential of the infinite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM cluster is embedded. Such embedded cluster calcns., employing a T3 cluster, with electron correlation at the d. functional theory level, are described, to model the interaction of water at a Bronsted acid site. Structures of the water-zeolite complex, and assocd. vibrational frequencies and 1H NMR shifts are calcd. and compared with calcns. of bare clusters of varying size and with exptl. data. We then describe a mixed quantum mech.-mol. mech. (QM-MM) model derived by combining charges from the second model with a std. aluminosilicate force field. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbon cracking reaction; the methyl-shift reaction of a propenium ion coordinated to the acid site.
  24. 24
    Metz, S.; Kästner, J.; Sokol, A. A.; Keal, T. W.; Sherwood, P. C hem S hell-a modular software package for QM / MM simulations. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2014, 4, 101– 110,  DOI: 10.1002/wcms.1163
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    ChemShell-a modular software package for QM/MM simulations
    Metz, Sebastian; Kaestner, Johannes; Sokol, Alexey A.; Keal, Thomas W.; Sherwood, Paul
    Wiley Interdisciplinary Reviews: Computational Molecular Science (2014), 4 (2), 101-110CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
    ChemShell is a modular computational chem. package with a particular focus on hybrid quantum mech./mol. mech. (QM/MM) simulations. A core set of chem. data handling modules and scripted interfaces to a large no. of quantum chem. and mol. modeling packages underpin a flexible QM/MM scheme. ChemShell has been used in the study of small mols., mol. crystals, biol. macromols. such as enzymes, framework materials including zeolites, ionic solids, and surfaces. We outline the range of QM/MM coupling schemes and supporting functions for system setup, geometry optimization, and transition-state location (including those from the open-source DL-FIND optimization library). We discuss recently implemented features allowing a more efficient treatment of long range electrostatic interactions, X-ray based quantum refinement of crystal structures, free energy methods, and excited-state calcns. ChemShell has been ported to a range of parallel computers and we describe a no. of options including parallel execution based on the message-passing capabilities of the interfaced packages and task-farming for applications in which a no. of individual QM, MM, or QM/MM calcns. can performed simultaneously. We exemplify each of the features by brief ref. to published applications.
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    Lu, Y.; Farrow, M. R.; Fayon, P.; Logsdail, A. J.; Sokol, A. A.; Catlow, C. R. A.; Sherwood, P.; Keal, T. W. Open-Source, python-based redevelopment of the ChemShell multiscale QM/MM environment. J. Chem. Theor. Comput. 2018, 15, 1317– 1328,  DOI: 10.1021/acs.jctc.8b01036
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    Sherwood, P.; de Vries, A. H.; Guest, M. F.; Schreckenbach, G.; Catlow, C. R. A.; French, S. A.; Sokol, A. A.; Bromley, S. T.; Thiel, W.; Turner, A. J. QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis. J. Mol. Struct.: THEOCHEM 2003, 632, 1– 28,  DOI: 10.1016/s0166-1280(03)00285-9
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    QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis
    Sherwood, Paul; de Vries, Alex H.; Guest, Martyn F.; Schreckenbach, Georg; Catlow, C. Richard A.; French, Samuel A.; Sokol, Alexey A.; Bromley, Stefan T.; Thiel, Walter; Turner, Alex J.; Billeter, Salomon; Terstegen, Frank; Thiel, Stephan; Kendrick, John; Rogers, Stephen C.; Casci, John; Watson, Mike; King, Frank; Karlsen, Elly; Sjovoll, Merethe; Fahmi, Adil; Schafer, Ansgar; Lennartz, Christian
    Journal of Molecular Structure: THEOCHEM (2003), 632 (), 1-28CODEN: THEODJ; ISSN:0166-1280. (Elsevier Science B.V.)
    The work of the European project QUASI (Quantum Simulation in Industry, project EP25047) which has sought to develop a flexible QM/MM scheme and to apply it to a range of industrial problems is described. A no. of QM/MM approaches were implemented within the computational chem. scripting system, ChemShell, which provides the framework for deploying a variety of independent program packages. This software was applied in several large-scale QM/MM studies which addressed the catalytic decompn. of N2O by Cu-contg. zeolites, methanol synthesis reaction catalyzed by Cu clusters supported on ZnO surfaces, and the modeling of enzyme structure and reactivity.
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    Guest, M. F.; Bush, I. J.; Van Dam, H. J. J.; Sherwood, P.; Thomas, J. M. H.; Van Lenthe, J. H.; Havenith, R. W. A.; Kendrick, J. The GAMESS-UK electronic structure package: algorithms, developments and applications. Mol. Phys. 2005, 103, 719– 747,  DOI: 10.1080/00268970512331340592
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    The GAMESS-UK electronic structure package: Algorithms, developments and applications
    Guest, Martyn F.; Bush, Ian J.; Van Dam, Huub J. J.; Sherwood, Paul; Thomas, Jens M. H.; Van Lenthe, Joop H.; Havenith, Remco W. A.; Kendrick, John
    Molecular Physics (2005), 103 (6-8), 719-747CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
    A description of the ab initio quantum chem. package GAMESS-UK is presented. The package offers a wide range of quantum mech. wavefunctions, capable of treating systems ranging from closed-shell mols. through to the species involved in complex reaction mechanisms. The availability of a wide variety of correlation methods provides the necessary functionality to tackle a no. of chem. important tasks, ranging from geometry optimization and transition-state location to the treatment of solvation effects and the prediction of excited state spectra. With the availability of relativistic ECPs and the development of ZORA, such calcns. may be performed on the entire Periodic Table, including the lanthanides. Emphasis is given to the DFT module, which has been extensively developed in recent years, and a no. of other, novel features of the program. The parallelization strategy used in the program is outlined, and detailed speedup results are given. Applications of the code in the areas of enzyme and zeolite catalysis and in spectroscopy are described.
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    Smith, W.; Yong, C.; Rodger, P. DL_POLY: Application to molecular simulation. Mol. Simul. 2002, 28, 385– 471,  DOI: 10.1080/08927020290018769
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    DL_POLY: application to molecular simulation
    Smith, W.; Yong, C. W.; Rodger, P. M.
    Molecular Simulation (2002), 28 (5), 385-471CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
    A review. DL_POLY is a general-purpose mol. dynamics simulation package, which was developed by Daresbury Lab. in the mid-1990s for the mol. simulation community in the United Kingdom. The package now has a world-wide user base and applications in many areas of mol. simulation. In this article we briefly review the history and design of the package and highlight some recent applications in the areas of; liqs. and solns.; spectroscopy; ionic solids; mol. crystals; polymers; glasses; membranes; proteins; solid and liq. interfaces; catalysis; liq. crystals; intercalation and clathrates; and novel systems. The strengths and weaknesses of the code and its future in the near term are also discussed.
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    Hill, J. R.; Sauer, J. Molecular mechanics potential for silica and zeolite catalysts based on ab initio calculations. 1. Dense and microporous silica. J. Phys. Chem. 1994, 98, 1238– 1244,  DOI: 10.1021/j100055a032
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    Molecular mechanics potential for silica and zeolite catalysts based on ab initio calculations. 1. Dense and microporous silica
    Hill, Joerg R.; Sauer, Joachim
    Journal of Physical Chemistry (1994), 98 (4), 1238-44CODEN: JPCHAX; ISSN:0022-3654.
    A consistent force field for the simulation of aluminum-free zeolite structures is presented. The parameters are derived from results of ab initio calcns. on mol. models which represent typical structural elements of zeolites: SiO4 tetrahedra connected to chains (disilicic and trisilicic acid), rings ([SiO(OH)2]n, n = 3-6), and cages ([SiO3/2(OH)]n, n = 8, 12, 24). These calcns. used a "double zeta + polarization/triple zeta + polarization" basis set. The structures predicted by means of the force field obtained are compared with the results of direct ab initio calcns. of the model mols. and with obsd. structures of dense and microporous silica. The conclusion is reached that it is possible to derive an accurate and transferable force field for mols. and solids solely based on ab initio data for mols.
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    Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305,  DOI: 10.1039/b508541a
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    Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy
    Weigend, Florian; Ahlrichs, Reinhart
    Physical Chemistry Chemical Physics (2005), 7 (18), 3297-3305CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 mols. representing (nearly) all elements-except lanthanides-in their common oxidn. states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, d. functional theory and correlated methods, for which we had chosen Moller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
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    Wilson, P. J.; Bradley, T. J.; Tozer, D. J. Hybrid exchange-correlation functional determined from thermochemical data and ab initio potentials. J. Chem. Phys. 2001, 115, 9233– 9242,  DOI: 10.1063/1.1412605
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    Hybrid exchange-correlation functional determined from thermochemical data and ab initio potentials
    Wilson, Philip J.; Bradley, Thomas J.; Tozer, David J.
    Journal of Chemical Physics (2001), 115 (20), 9233-9242CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Multiplicative potentials, appropriate for adding to the non-multiplicative fractional orbital exchange term in the Kohn-Sham equations, are detd. from correlated ab initio electron densities. The potentials are examd. graphically and are used in conjunction with conventional thermochem. data to det. a new hybrid exchange-correlation functional, denoted B97-2. Calcns. using B97-2 are compared with those from (a) the B97-1 functional [J. Chem. Phys. 109, 6264 (1998)], which has the same functional form and fraction of orbital exchange, but was fitted to just thermochem. data; and (b) the widely used B3LYP functional [J. Chem. Phys. 98, 5648 (1993)]. B97-2 atomization energies are close to those from B97-1; total electronic energies and ionization potentials are less accurate, but remain an improvement over B3LYP. Mol. structures from all three functionals are comparable. Static isotropic polarizabilities improve from B3LYP to B97-1 to B97-2; the B97-2 functional underestimates exptl. values, which is consistent with the neglect of zero-point vibrational corrections. NMR shielding consts. - detd. as the conventional second deriv. of the electronic energy - improve from B3LYP to B97-1 to B97-2. Shieldings detd. directly from these DFT electron densities using the recently proposed MKS approach [Chem. Phys. Lett. 337, 341 (2001)] are two to three times more accurate than the conventional shieldings, and exhibit an analogous improvement across the three functionals. Classical reaction barriers for sixteen chem. reactions improve significantly from B3LYP to B97-1 to B97-2. The introduction of multiplicative potentials into semi-empirical hybrid functional development therefore appears beneficial.
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    Janssens, T. V.; Falsig, H.; Lundegaard, L. F.; Vennestrøm, P. N.; Rasmussen, S. B.; Moses, P. G.; Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lamberti, C.; Bordiga, S.; Godiksen, A.; Mossin, S.; Beato, P. A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia. ACS Catal. 2015, 5, 2832– 2845,  DOI: 10.1021/cs501673g
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    A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia
    Janssens, Ton V. W.; Falsig, Hanne; Lundegaard, Lars F.; Vennestroem, Peter N. R.; Rasmussen, Soeren B.; Moses, Poul Georg; Giordanino, Filippo; Borfecchia, Elisa; Lomachenko, Kirill A.; Lamberti, Carlo; Bordiga, Silvia; Godiksen, Anita; Mossin, Susanne; Beato, Pablo
    ACS Catalysis (2015), 5 (5), 2832-2845CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    For the first time, the std. and fast selective catalytic redn. (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable mols. only. The std. SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidn. of the catalyst by NO + O2 and a redn. by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the redn., and finally, oxidn. by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown exptl. for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), ESR, and Fourier transform IR spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the std. SCR cycle. The activation energy calcd. by d. functional theory (DFT) indicates that the oxidn. of an NO mol. by O2 to a bidentate nitrate ligand is rate-detg. for std. SCR. Finally, the role of a nitrate/nitrite equil. and the possible influence of Cu dimers and Bronsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidn. to NO2.
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    Dent, L.; Smith, J. Crystal structure of chabazite, a molecular sieve. Nature 1958, 181, 1794– 1796,  DOI: 10.1038/1811794b0
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    Crystal structure of chabazite, a molecular sieve
    Dent, L. S.; Smith, J. V.
    Nature (London, United Kingdom) (1958), 181 (), 1794-6CODEN: NATUAS; ISSN:0028-0836.
    By Fourier methods a new structure for the zeolite chabazite was detd. which explains the x-ray intensities, the sorption, and epitaxial integrowth with the same properties of the mineral gmelinite. The space group is R‾3m, a = 9.40 A., α = 94°18', Z = 2.
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    Gale, J. D.; Rohl, A. L. The general utility lattice program (GULP). Mol. Simul. 2003, 29, 291– 341,  DOI: 10.1080/0892702031000104887
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    The General Utility Lattice Program (GULP)
    Gale, Julian D.; Rohl, Andrew L.
    Molecular Simulation (2003), 29 (5), 291-341CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
    The General Utility Lattice Program (gulp) has been extended to include the ability to simulate polymers and surfaces, as well as adding many other new features, and the current status of the program is fully documented. Both the background theory is described, as well as providing a concise review of some of the previous applications in order to demonstrate the range of its use. Examples are presented of work performed using the new compatibilities of the software, including the calcn. of Born effective charges, mech. properties as a function of applied pressure, calcn. of frequency-dependent dielec. data, surface reconstructions of calcite and the performance of a linear-scaling algorithm for bond-order potentials.
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    Sherwood, P.; de Vries, A. H.; Collins, S. J.; Greatbanks, S. P.; Burton, N. A.; Vincent, M. A.; Hillier, I. H. Computer simulation of zeolite structure and reactivity using embedded cluster methods. Faraday Discuss. 1997, 106, 79– 92,  DOI: 10.1039/a701790a
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    Computer simulation of zeolite structure and reactivity using embedded cluster methods
    Sherwood, Paul; De Vries, Alex H.; Collins, Simon J.; Greatbanks, Stephen P.; Burton, Neil A.; Vincent, Mark A.; Hillier, Ian H.
    Faraday Discussions (1997), 106 (Solid State Chemistry: New Opportunities from Computer Simulations), 79-92CODEN: FDISE6; ISSN:0301-7249. (Royal Society of Chemistry)
    The use of bare cluster models to understand the nature of zeolite-substrate interactions may be improved to take account of the environment of the Broensted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves generating a specialized correction potential by fitting a non-periodic array of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic array of at. charges to the potential of the infinite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM cluster is embedded. Such embedded cluster calcns., employing a T3 cluster, with electron correlation at the d. functional theory level, are described, to model the interaction of water at a Bronsted acid site. Structures of the water-zeolite complex, and assocd. vibrational frequencies and 1H NMR shifts are calcd. and compared with calcns. of bare clusters of varying size and with exptl. data. We then describe a mixed quantum mech.-mol. mech. (QM-MM) model derived by combining charges from the second model with a std. aluminosilicate force field. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbon cracking reaction; the methyl-shift reaction of a propenium ion coordinated to the acid site.
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    Zhao, Y.; Lynch, B. J.; Truhlar, D. G. Development and assessment of a new hybrid density functional model for thermochemical kinetics. J. Phys. Chem. A 2004, 108, 2715– 2719,  DOI: 10.1021/jp049908s
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    Development and Assessment of a New Hybrid Density Functional Model for Thermochemical Kinetics
    Zhao, Yan; Lynch, Benjamin J.; Truhlar, Donald G.
    Journal of Physical Chemistry A (2004), 108 (14), 2715-2719CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    A new hybrid Hartree-Fock-d. functional model called the Becke88-Becke95 1-parameter model for kinetics (BB1K) was optimized against a database of three forward barrier heights, three reverse barrier heights, and three energies of reaction for the reactions in the BH6 representative barrier height database. We then assessed the newly developed BB1K method against a saddle point geometries database, a database of 42 barrier heights, the AE6 representative atomization energy database, a mol. geometries data set, and a set of 13 zero point energies. The results show that BB1K can give excellent saddle point geometries and barrier heights, and its performance for calcg. atomization energies is 40% better than MPW1K. Using a mean mean unsigned error criterion that equally wts. the errors in barrier heights and in bond energies, the new BB1K method outperforms all other DFT and hybrid DFT methods by a large margin, and we therefore conclude that it is the best d. functional-type method for thermochem. kinetics.
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    Greenaway, A. G.; Lezcano-Gonzalez, I.; Agote-Aran, M.; Gibson, E. K.; Odarchenko, Y.; Beale, A. M. Operando Spectroscopic Studies of Cu–SSZ-13 for NH3–SCR deNOx Investigates the Role of NH3 in Observed Cu (II) Reduction at High NO Conversions. Top. Catal. 2018, 61, 175– 182,  DOI: 10.1007/s11244-018-0888-3
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    Operando Spectroscopic Studies of Cu-SSZ-13 for NH3-SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions
    Greenaway, Alex G.; Lezcano-Gonzalez, Ines; Agote-Aran, Miren; Gibson, Emma K.; Odarchenko, Yaroslav; Beale, Andrew M.
    Topics in Catalysis (2018), 61 (3-4), 175-182CODEN: TOCAFI; ISSN:1022-5528. (Springer)
    The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic redn. by ammonia (NH3) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH3-SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH3-SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH3-SCR of NOx occurring at Cu six ring sites within the zeolite.
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    Greenaway, A. G.; Marberger, A.; Thetford, A.; Lezcano-González, I.; Agote-Arán, M.; Nachtegaal, M.; Ferri, D.; Kröcher, O.; Catlow, C. R. A.; Beale, A. M. Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNOx by modulation excitation IR spectroscopy. Chem. Sci. 2020, 11, 447– 455,  DOI: 10.1039/c9sc04905c
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    Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNOx by modulation excitation IR spectroscopy
    Greenaway, Alex G.; Marberger, Adrian; Thetford, Adam; Lezcano-Gonzalez, Ines; Agote-Aran, Miren; Nachtegaal, Maarten; Ferri, Davide; Krocher, Oliver; Catlow, C. Richard A.; Beale, Andrew M.
    Chemical Science (2020), 11 (2), 447-455CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    The small pore zeolite Cu-SSZ-13 is an efficient material for the std. selective catalytic redn. of nitrogen oxides (NOx) by ammonia (NH3). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analyzed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the expts. were performed under cyclic Cu+/Cu2+ redox, Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS) expts. provide spectroscopic evidence for previously postulated intermediates Cu-N(=O)-NH2 and Cu-NO3 in the NH3-SCR deNOx mechanism and for the role of [Cu2+(OH-)]+. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.
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    Haszeldine, R. N.; Jander, J.
    Journal of Chemical Physics (1955), 23 (), 979-80CODEN: JCPSA6; ISSN:0021-9606.
    cf. preceding abstr. The amt. of EtONO in equil. with AmOH, EtOH, and AmONO (presumably at room temp.) is less than 10%. In liquid dimethylnitrosamine H bonding or assocn. occurs, but it apparently does not occur in CCl4 soln.
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    Piskorz, M.; Urbanski, T. Ultraviolet and infrared spectra of some nitrosamines. Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1963, 11, 607– 613
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    Ultraviolet and infrared spectra of some nitrosamines
    Piskorz, M.; Urbanski, T.
    Bulletin de l'Academie Polonaise des Sciences, Serie des Sciences Chimiques (1963), 11 (11), 607-13CODEN: BAPCAQ; ISSN:0001-4095.
    Nitrosamines contain 2 absorption bands at about 230 mμ, assigned to N-NO2 and at 345-374 mμ :N-N:0 ⇔ N⊕:N-0⊕ The N:O stretch for nonassocd. mols. is at 1486-1408 cm.-1 and at 1346-1265 and 1321-1292 cm.-1 for assocd. mols., the N-N stretch is at 1106-1052 cm.-1 and the C-N stretch at 1526 and 1118 cm.-1
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    Tarte, P. Recherches spectroscopiques sur les composés nitrosés. Bull. Soc. Chim. Belg. 1954, 63, 525– 541,  DOI: 10.1002/bscb.19540630907
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    Spectroscopic investigation on nitroso compounds
    Tarte, P.
    Bulletin des Societes Chimiques Belges (1954), 63 (), 525-41CODEN: BSCBAG; ISSN:0037-9646.
    cf. Haszeldine and Jander, C.A. 48, 5646i; 49, 1533d. The visible and infrared spectra of some nitroso compds. have been studied. Chloronitroso compds. of the type, R1R2C(NO)Cl, where R1 and R2 are alkyl, have two infrared bands in the 1600 region (bands given in cm.-1) attributed to the valence vibrations of N::O in the two rotational isomers, as these compds. are dimeric, except CCl3NO which is monomeric and shows only one peak at 1616. Absorption due to C:N valence vibration appears about 1100. The visible spectrum indicates that the valence vibration of N::O in the excited state falls at 1400. The nitroso-dimer compds., (R3CNO)2, have an abnormally low N::O absorption frequency at 1200-1300, which lowering is attributed to N::O assocn. The spectra of dialkyl-nitrosamines in liquid phase have 2 bands, one at 1320 for assocd. N::O and the other at 1440 for unassocd. N::O. In the vapor phase, a single N::O absorption is seen at 1490. These show N:N vibration at 1050-1100 and N:N::O deformation at 650.
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    Kedrova, T.; Gafurov, R.; Sogomonyan, E.; Eremenko, L. Spectroscopic study of (nitroalkyl)nitrosamines. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1979, 28, 944– 947,  DOI: 10.1007/bf00963302
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    Zapata, F.; García-Ruiz, C. The discrimination of 72 nitrate, chlorate and perchlorate salts using IR and Raman spectroscopy. Spectrochim. Acta, Part A 2018, 189, 535– 542,  DOI: 10.1016/j.saa.2017.08.058
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    The discrimination of 72 nitrate, chlorate and perchlorate salts using IR and Raman spectroscopy
    Zapata, Felix; Garcia-Ruiz, Carmen
    Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2018), 189 (), 535-542CODEN: SAMCAS; ISSN:1386-1425. (Elsevier B.V.)
    Inorg. oxidizing energetic salts including nitrates, chlorates and perchlorates are widely used in the manuf. of not only licit pyrotechnic compns., but also illicit homemade explosive mixts. Their identification in forensic labs. is usually accomplished by either capillary electrophoresis or ion chromatog., with the disadvantage of dissocg. the salt into its ions. On the contrary, vibrational spectroscopy, including IR and Raman, enables the non-invasive identification of the salt, i.e. avoiding its dissocn. This study focuses on the discrimination of all nitrate, chlorate and perchlorate salts that are com. available, using both Raman and IR spectroscopy, with the aim of testing whether every salt can be unequivocally identified. Besides the visual spectra comparison by assigning every band with the corresponding mol. vibrational mode, a statistical anal. based on Pearson correlation was performed to ensure an objective identification, either using Raman, IR or both. Pos., 25 salts (out of 72) were unequivocally identified using Raman, 30 salts when using IR and 44 when combining both techniques. Neg., some salts were undistinguishable even using both techniques demonstrating there are some salts that provide very similar Raman and IR spectra.
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    Volod’ko, L.; Huoah, L. T. The vibrational spectra of aqueous nitrate solutions. J. Appl. Spectrosc. 1968, 9, 1100– 1104,  DOI: 10.1007/bf01260968
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    Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; Thibault-Starzyk, F. Probing zeolites by vibrational spectroscopies. Chem. Soc. Rev. 2015, 44, 7262– 7341,  DOI: 10.1039/c5cs00396b
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    Probing zeolites by vibrational spectroscopies
    Bordiga, Silvia; Lamberti, Carlo; Bonino, Francesca; Travert, Arnaud; Thibault-Starzyk, Frederic
    Chemical Society Reviews (2015), 44 (20), 7262-7341CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Bronsted and Lewis acidity and surface basicity are treated in detail. The role of probe mols. and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely det. the IR absorption coeffs. is given, making IR a quant. technique. The thermodn. parameters of the adsorption process that can be extd. from a variable-temp. IR study are described. Finally, cutting-edge space- and time-resolved expts. are reviewed. All aspects are discussed by reporting relevant examples. When available, the theor. literature related to the reviewed exptl. results is reported to support the interpretation of the vibrational spectra on an at. level.
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    Negri, C.; Hammershøi, P. S.; Janssens, T. V.; Beato, P.; Berlier, G.; Bordiga, S. Investigating the Low Temperature Formation of Cu II -(N,O) Species on Cu-CHA Zeolites for the Selective Catalytic Reduction of NO x. Chem.─Eur. J. 2018, 24, 12044– 12053,  DOI: 10.1002/chem.201802769
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    Investigating the Low Temperature Formation of CuII-(N,O) Species on Cu-CHA Zeolites for the Selective Catalytic Reduction of NOx
    Negri, Chiara; Hammershoi, Peter S.; Janssens, Ton V. W.; Beato, Pablo; Berlier, Gloria; Bordiga, Silvia
    Chemistry - A European Journal (2018), 24 (46), 12044-12053CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of CuII-(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic redn. of NOx with NH3 (NH3-SCR). In particular, we investigated the reaction of NO and O2 at low temp. (200 and 50°C) on a series of Cu-CHA zeolites with different compn. (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidn. part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200°C. At lower temp. a mixt. of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO+, whose amt. was found to be proportional to the zeolite Bronsted site concn. Nitrates were found to mainly form with CuII ions stabilized by one neg. framework charge (Z), Z-[Cu(OH]I or Z-[Cu(O2]I, without involvement of Z2-CuII ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH3. Finally, water was found to replace preformed chelating copper nitrates and deplete NO+ (though with different kinetics) at both temps., while favoring the presence of monodentate ones.
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    Hadjiivanov, K. I. Identification of neutral and charged N x O y surface species by IR spectroscopy. Catal. Rev. 2000, 42, 71– 144,  DOI: 10.1081/cr-100100260
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    Identification of neutral and charged NxOy surface species by IR spectroscopy
    Hadjiivanov, Konstantin I.
    Catalysis Reviews - Science and Engineering (2000), 42 (1 & 2), 71-144CODEN: CRSEC9; ISSN:0161-4940. (Marcel Dekker, Inc.)
    Review of the available IR spectral data and the assignments on the NxOy adspecies, paying special attention to oxide surfaces, with 268 refs. The IR spectral performance of the NxOy species obsd. on oxide surfaces [N2O, NO-, NO, (NO)2, N2O3, NO+, NO2- (different nitro and nitrito anions), NO2, N2O4, N2O5, NO2+, and NO3- (bridged, bidentate, and monodentate nitrates)] is considered. The spectra of related compds. (N2, H-, and C-contg. nitrogen oxo species, C-N species, NHx species) are also briefly discussed. Some guidelines for spectral identification of NxOy adspecies are proposed and the transformation of the nitrogen oxo species on catalyst surfaces are regarded.
  48. 48
    Yao, X.; Ma, K.; Zou, W.; He, S.; An, J.; Yang, F.; Dong, L. Influence of preparation methods on the physicochemical properties and catalytic performance of MnO -CeO2 catalysts for NH3-SCR at low temperature. Chin. J. Catal. 2017, 38, 146– 159,  DOI: 10.1016/s1872-2067(16)62572-x
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    Influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for NH3-SCR at low temperature
    Yao, Xiaojiang; Ma, Kaili; Zou, Weixin; He, Shenggui; An, Jibin; Yang, Fumo; Dong, Lin
    Chinese Journal of Catalysis (2017), 38 (1), 146-159CODEN: CJCHCI ISSN:. (Science Press)
    This work examd. the influence of prepn. methods on the physicochem. properties and catalytic performance of MnOx-CeO2 catalysts for selective catalytic redn. of NO by NH3 (NH3-SCR) at low temp. Five different methods, namely, mech. mixing, impregnation, hydrothermal treatment, co-pptn., and a sol-gel technique, were used to synthesize MnOx-CeO2 catalysts. The catalysts were characterized in detail, and an NH3-SCR model reaction was chosen to evaluate the catalytic performance. The results showed that the prepn. methods affected the catalytic performance in the order: hydrothermal treatment > sol-gel > co-pptn. > impregnation > mech. mixing. This order correlated with the surface Ce3+ and Mn4+ contents, oxygen vacancies and surface adsorbed oxygen species concn., and the amt. of acidic sites and acidic strength. This trend was related to redox interactions between MnOx and CeO2. The catalyst formed by a hydrothermal treatment exhibited excellent physicochem. properties, optimal catalytic performance, and good H2O resistance in NH3-SCR reaction. This was attributed to incorporation of Mnn+ into the CeO2 lattice to form a uniform ceria-based solid soln. (contg. Mn-O-Ce structures). Strengthening of the electronic interactions between MnOx and CeO2, driven by the high-temp. and high-pressure conditions during the hydrothermal treatment also improved the catalyst characteristics. Thus, the hydrothermal treatment method was an efficient and environment-friendly route to synthesizing low-temp. denitrification (deNOx) catalysts.
  49. 49
    Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lazzarini, A.; Agostini, G.; Gallo, E.; Soldatov, A. V.; Beato, P.; Bordiga, S.; Lamberti, C. Interaction of NH3 with Cu-SSZ-13 catalyst: a complementary FTIR, XANES, and XES study. J. Phys. Chem. Lett. 2014, 5, 1552– 1559,  DOI: 10.1021/jz500241m
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    Interaction of NH3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study
    Giordanino, Filippo; Borfecchia, Elisa; Lomachenko, Kirill A.; Lazzarini, Andrea; Agostini, Giovanni; Gallo, Erik; Soldatov, Alexander V.; Beato, Pablo; Bordiga, Silvia; Lamberti, Carlo
    Journal of Physical Chemistry Letters (2014), 5 (9), 1552-1559CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    In the typical NH3-SCR temp. range (100-500°), ammonia is one of the main adsorbed species on acidic sites of Cu-SSZ-13 catalyst. The study of adsorbed ammonia at high temp. is a key step for the understanding of its role in the NH3-SCR catalytic cycle. Different spectroscopic techniques were employed to investigate the nature of the different complexes occurring upon NH3 interaction. In particular, FTIR spectroscopy revealed the formation of different NH3 species, i.e., (i) NH3 bonded to copper centers, (ii) NH3 bonded to Bronsted sites, and (iii) NH4+·nNH3 assocns. XANES and XES spectroscopy allowed us to get an insight into the geometry and electronic structure of Cu centers upon NH3 adsorption, revealing for the first time in Cu-SSZ-13 the presence of linear Cu+ species in Ofw-Cu-NH3 or H3N-Cu-NH3 configuration.
  50. 50
    Lezcano-Gonzalez, I.; Deka, U.; Arstad, B.; Van Yperen-De Deyne, A.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M. Determining the storage, availability and reactivity of NH3within Cu-Chabazite-based Ammonia Selective Catalytic Reduction systems. Phys. Chem. Chem. Phys. 2014, 16, 1639– 1650,  DOI: 10.1039/c3cp54132k
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    Determining the storage, availability and reactivity of NH3 within Cu-Chabazite-based ammonia selective catalytic reduction systems
    Lezcano-Gonzalez, I.; Deka, U.; Arstad, B.; Van Yperen-De Deyne, A.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M.
    Physical Chemistry Chemical Physics (2014), 16 (4), 1639-1650CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Three different types of NH3 species can be simultaneously present on Cu2+-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Redn. (NH3-SCR) systems. These include ammonium ions (NH4+), formed on the Bronsted acid sites, [Cu(NH3)4]2+ complexes, resulting from NH3 coordination with the Cu2+ Lewis sites, and NH3 adsorbed on extra-framework Al (EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4+ ions react very slowly in comparison to NH3 coordinated to Cu2+ ions and are likely to contribute little to the std. NH3-SCR process, with the Bronsted groups acting primarily as NH3 storage sites. The availability/reactivity of NH4+ ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu2+, accompanied by a remarkable enhancement in the low temp. activity. The presence of EFAl species could also have a pos. influence on the reaction rate of the available NH4+ ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.
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    Paolucci, C.; Khurana, I.; Parekh, A. A.; Li, S.; Shih, A. J.; Li, H.; Di Iorio, J. R.; Albarracin-Caballero, J. D.; Yezerets, A.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F.; Gounder, R. Dynamic multinuclear sites formed by mobilized copper ions in NO x selective catalytic reduction. Science 2017, 357, 898– 903,  DOI: 10.1126/science.aan5630
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    Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction
    Paolucci, Christopher; Khurana, Ishant; Parekh, Atish A.; Li, Sichi; Shih, Arthur J.; Li, Hui; Di Iorio, John R.; Albarracin-Caballero, Jonatan D.; Yezerets, Aleksey; Miller, Jeffrey T.; Delgass, W. Nicholas; Ribeiro, Fabio H.; Schneider, William F.; Gounder, Rajamani
    Science (Washington, DC, United States) (2017), 357 (6354), 898-903CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Copper ions exchanged into zeolites are active for the selective catalytic redn. (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temp. rate dependence on copper (Cu) volumetric d. is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calcns. to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)-mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the vol. that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.
  52. 52
    Millan, R.; Cnudde, P.; Hoffman, A. E.; Lopes, C. W.; Concepción, P.; van Speybroeck, V.; Boronat, M. Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3-SCR-NOx) Conditions. J. Phys. Chem. Lett. 2020, 11, 10060– 10066,  DOI: 10.1021/acs.jpclett.0c03020
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    Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3-SCR-NOx) Conditions
    Millan, Reisel; Cnudde, Pieter; Hoffman, Alexander E. J.; Lopes, Christian W.; Concepcion, Patricia; van Speybroeck, Veronique; Boronat, Mercedes
    Journal of Physical Chemistry Letters (2020), 11 (23), 10060-10066CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    The dynamic nature of the copper cations acting as active sites for selective catalytic redn. of nitrogen oxides with ammonia is investigated using a combined theor. and spectroscopic approach. Ab initio mol. dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800-1000 cm-1 region of the IR spectra. Bands assocd. with the lattice asym. T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to exptl. follow the dynamic reorganization of the active sites at operating conditions.
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    Gao, F.; Mei, D.; Wang, Y.; Szanyi, J.; Peden, C. H. Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis. J. Am. Chem. Soc. 2017, 139, 4935– 4942,  DOI: 10.1021/jacs.7b01128
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    Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis
    Gao, Feng; Mei, Donghai; Wang, Yilin; Szanyi, Janos; Peden, Charles H. F.
    Journal of the American Chemical Society (2017), 139 (13), 4935-4942CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Active centers in Cu/SSZ-13 selective catalytic redn. (SCR) catalysts have been recently identified as isolated Cu2+ and [CuII(OH)]+ ions. A redox reaction mechanism has also been established, where Cu ions cycle between CuI and CuII oxidn. states during SCR reaction. While the mechanism for the redn. half-cycle (CuII → CuI) is reasonably well-understood, that for the oxidn. half-cycle (CuI → CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temp. std. NH3-SCR, supplemented by DFT calcns., as strong evidence that the low-temp. oxidn. half-cycle occurs with the participation of two isolated CuI ions via formation of a transient [CuI(NH3)2]+-O2-[CuI(NH3)2]+ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calcns., and the simulated energy barrier and rate consts. are consistent with exptl. findings. Significantly, the low-temp. std. SCR mechanism proposed here provides full consistency with low-temp. SCR kinetics.
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    Chen, L.; Falsig, H.; Janssens, T. V.; Grönbeck, H. Activation of oxygen on (NH3Cu NH3)+ in NH3-SCR over Cu-CHA. J. Catal. 2018, 358, 179– 186,  DOI: 10.1016/j.jcat.2017.12.009
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    Activation of oxygen on (NH3-Cu-NH3)+ in NH3-SCR over Cu-CHA
    Chen, Lin; Falsig, Hanne; Janssens, Ton V. W.; Groenbeck, Henrik
    Journal of Catalysis (2018), 358 (), 179-186CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
    Cu-CHA materials are efficient catalysts for NH3-SCR of NOx in O excess. A crucial step in the reaction is O (O2) activation, which still is not well understood. D. functional theory calcns. in combination with ab initio thermodn. and mol. dynamics are here used to study O2 dissocn. on Cu(NH3)+2 species, which are present under NH3-SCR conditions. Direct dissocn. of O2 is facile over a pair of Cu(NH3)+2 complexes whereas dissocn. on a single Cu(NH3)+2 species is unlikely due to a high activation energy. The presence of NO promotes O dissocn. on both single and pairs of Cu(NH3)+2 complexes. Nitrites and nitrates are easily formed as O2 dissocs., and NO adsorption over nitrates leads to facile formation of NO2. The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH3-SCR.
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    Moreno-González, M.; Millán, R.; Concepción, P.; Blasco, T.; Boronat, M. Spectroscopic Evidence and Density Functional Theory (DFT) Analysis of Low-Temperature Oxidation of Cu+ to Cu2+ NO x in Cu-CHA Catalysts: Implications for the SCR-NO x Reaction Mechanism. ACS Catal. 2019, 9, 2725– 2738,  DOI: 10.1021/acscatal.8b04717
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    Spectroscopic Evidence and Density Functional Theory (DFT) Analysis of Low-Temperature Oxidation of Cu+ to Cu2+NOx in Cu-CHA Catalysts: Implications for the SCR-NOx Reaction Mechanism
    Moreno-Gonzalez, Marta; Millan, Reisel; Concepcion, Patricia; Blasco, Teresa; Boronat, Mercedes
    ACS Catalysis (2019), 9 (4), 2725-2738CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Despite the intense study on the NH3-SCR-NOx reaction mechanism catalyzed by small pore Cu-CHA zeolites, neither the rate-detg. step of the process nor the exact nature of the active sites under reaction conditions are clearly established. In situ EPR and IR techniques combined with DFT calcns. are applied to the study of the oxidn. half-cycle of the NH3-SCR-NOx reaction on Cu-SSZ-13 and Cu-SAPO-34 catalysts. EPR and IR spectroscopies unambiguously show that Cu+ is oxidized to Cu2+ at room temp. in the presence of the reaction mixt. (NO, O2, and NH3) or NO and O2, producing adsorbed NO2, nitrites, and nitrates. Several pathways are proposed from DFT calcns. to oxidize Cu+ cations placed in the plane of the 6R ring units of SSZ-13 and SAPO-34 to Cu2+, either by NO2 alone or by a mixt. of NO and O2, with activation energy barriers <70 kJ mol-1. A reaction mechanism invoking the formation of nitrate/nitrite intermediates on Cu cations attached to the zeolite framework can be operational in the low-temp. region (T < 350°). Different intermediates, nitrites vs. nitrates, are preferentially stabilized, depending on the catalyst compn., silicoaluminophosphate vs. aluminosilicate.
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    Wilken, N.; Kamasamudram, K.; Currier, N. W.; Li, J.; Yezerets, A.; Olsson, L. Heat of adsorption for NH3, NO2 and NO on Cu-Beta zeolite using microcalorimeter for NH3 SCR applications. Catal. Today 2010, 151, 237– 243,  DOI: 10.1016/j.cattod.2010.02.002
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    Heat of adsorption for NH3, NO2 and NO on Cu-Beta zeolite using microcalorimeter for NH3 SCR applications
    Wilken, Norman; Kamasamudram, Krishna; Currier, Neal W.; Li, Junhui; Yezerets, Aleksey; Olsson, Louise
    Catalysis Today (2010), 151 (3-4), 237-243CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    Micro-calorimetry is a powerful technique to measure the heat of adsorption (ΔH), producing values which are very important when developing kinetic models. The method provides a way to det. these parameters independently. For kinetic models describing NH3 selective catalytic redn. (SCR), it is crit. to accurately describe NH3 and NOx storage to simulate rapid transients occurring in expts. This work measured ΔH of NH3, NO2, and NO on Cu-β zeolites. An NH3 temp.-programmed desorption (TPD) expt. was conducted at 150° using the micro-calorimeter to observe an exotherm when introducing NH3 due to adsorption. This resulted in an av. ΔH of -100 kJ/mol. Good reproducibility was obsd. when using a second sample, resulting in -97 kJ/mol. To assess the coverage dependence of ΔH, an NH3 step-wise expt. was conducted. First, the catalyst was exposed to NH3 at 500°, resulting in the adsorption of strongly bound NH3 and obtaining a ΔH of -110 kJ/mol; thereafter, the catalyst was cooled in Ar and at 400° and NH3 was re-introduced. Since this temp. was lower, the NH3 which adsorbed was weaker. The procedure was repeated at 300, 200, and 100°, resulting in a coverage-dependent activation energy for NH3 desorption (assuming 0 activation for adsorption) according to this formula: NH3 desorption energy = 120.0(1 - 0.38θNH3), where θNH3 is the NH3 coverage on the surface. NO and NO2 adsorption/desorption were assessed using NO and NO2 TPD expts., resp. For the NO2 TPD expt., ∼3 NO2 were stored for each NO produced, corresponding to the disproportionation mechanism. This resulted in ΔH of -65 kJ/mol-NO2 consumed. The NO TPD expt. showed only small amts. of NO were adsorbed.
  57. 57
    Olsson, L.; Wijayanti, K.; Leistner, K.; Kumar, A.; Joshi, S. Y.; Kamasamudram, K.; Currier, N. W.; Yezerets, A. A multi-site kinetic model for NH3-SCR over Cu/SSZ-13. Appl. Catal., B 2015, 174–175, 212– 224,  DOI: 10.1016/j.apcatb.2015.02.037
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    A multi-site kinetic model for NH3-SCR over Cu/SSZ-13
    Olsson, Louise; Wijayanti, Kurnia; Leistner, Kirsten; Kumar, Ashok; Joshi, Saurabh Y.; Kamasamudram, Krishna; Currier, Neal W.; Yezerets, Aleksey
    Applied Catalysis, B: Environmental (2015), 174-175 (), 212-224CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    In this study, we have developed a kinetic model for ammonia-SCR over a well-characterized Cu/SSZ-13 catalyst. It was found that a three-site model was needed in order to describe the ammonia temp. programmed desorption (TPD) with adsorption at 50 and 150 °C as well as ammonia oxidn., and NH3-SCR up to 600 °C. Based on literature studies, where detailed characterization of Cu/SSZ-13 have been conducted using several exptl. techniques, we suggest the following phys. interpretation of the S1 and S2 sites in the model. The S1 sites are assocd. with copper located in a six-membered ring, possibly slightly distorted due to interactions with water and ammonia while, the S2 sites represent copper in the large cages or CuxOy species. In addn., ammonia is also stored on Br.ovrddot.onsted acid sites, but in order not to complicate the model further, it was lumped together in the S1 and S2 sites. Finally, S3 sites have been added in order to describe the large amt. of physisorbed ammonia at low temp. This three-site model was capable of adequately describing the ammonia TPD expts. with the initial temp. of 50 and 150 °C. The heats of adsorption of ammonia on the S1 and S2 sites were detd. using micro-calorimeter expts. Further, the main SCR reaction in the model occurs on S1 sites and the main ammonia oxidn. reaction on S2 sites. However, due to the complex behavior assocd. with ammonia oxidn., where the conversion slightly decreased when the temp. was increased from 350 to 400 °C, an ammonia oxidn. reaction occurring at low temp. with low rate needed to be introduced on S1. In a similar way, an added step was needed for ammonia-SCR on S2, which occurred at high temp. where the ammonia coverage on S1 was low resulting in low conversion. To summarize, the three-site model developed was capable of well describing the ammonia storage and release, ammonia oxidn. as well as SCR and N2O formation across a broad temp. interval (100-600 °C).
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    Krossner, M.; Sauer, J. Interaction of Water with Brønsted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1:1 and 2:1 Surface Complexes. J. Phys. Chem. 1996, 100, 6199– 6211,  DOI: 10.1021/jp952775d
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    Interaction of Water with Broensted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1:1 and 2:1 Surface Complexes
    Krossner, Mariann; Sauer, Joachim
    Journal of Physical Chemistry (1996), 100 (15), 6199-211CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    The adsorption of one and two water mols. on cluster models of Broensted acid sites of zeolite catalysts has been investigated by ab initio quantum chem. methods at the Hartree-Fock SCF (HF), at the second-order Moeller-Plesset perturbation theory (MP2), and at the d. functional theory (DFT) levels. Among the two possible structures of the 1:1 adsorption complex, the water H-bonded to the zeolitic OH group (neutral complex) and the hydroxonium ion attached to the neg. charged zeolite surface (ion pair), only the former is a min. The ion pair complex is a transition structure for the proton transfer from one lattice oxygen to a neighboring one via the adsorbed water. However, the energy difference between both structures is only a few kJ/mol. For the neutral 1:1 adsorption complex we predict an av. shift of the three protons involved of 7-8 ppm; the obsd. shifts are 6-7 ppm for one water mol. per site. The vibrational frequencies calcd. for the ion pair structure do not permit an interpretation of the obsd. IR spectrum. For the neutral structure (MP2) we predict frequencies of 1317 and 1022 cm-1 for the zeolitic in-plane and out-of-plane modes, resp., while the zeolitic OH stretching mode is strongly red-shifted down to 2740-2850 cm-1. These data support a recent interpretation of the IR spectrum which explains the obsd. triplet of bands as a result of Fermi resonance between the strongly perturbed zeolitic OH stretch and the OH bending overtones. The MP2 calcns. for the neutral complex also provide a complex assignment of the peaks obsd. by inelastic neutron scattering for water on H-mordenite. Inclusion of electron correlation proves crucial, and comparison of MP2 and DFT (gradient cor. functionals) methods is made. While energy differences are very similar, the DFT approach yields by far too large frequency shifts for OH donor groups in H bonds. When a second water mol. is added (2:1 complex), both the neutral and the ion pair structure prove to be local min. on the potential energy surface. The adsorption energy is found to drop by 25%, and the ion pair structure becomes the more stable one. Predictions are made on how the vibrational spectra and the 1H NMR chem. shifts change.
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    Jobic, H.; Tuel, A.; Krossner, M.; Sauer, J. Water in interaction with acid sites in H-ZSM-5 zeolite does not form hydroxonium ions. A comparison between neutron scattering results and ab initio calculations. J. Phys. Chem. 1996, 100, 19545– 19550,  DOI: 10.1021/jp9619954
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    Water in Interaction with Acid Sites in H-ZSM-5 Zeolite Does Not Form Hydroxonium Ions. A Comparison between Neutron Scattering Results and ab Initio Calculations
    Jobic, Herve; Tuel, Alain; Krossner, Mariann; Sauer, Joachim
    Journal of Physical Chemistry (1996), 100 (50), 19545-19550CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
    Inelastic neutron scattering (INS) was used to study the adsorption of H2O (at different concns.) in HZSM 5. INS is the only vibrational technique where the intensities can be calcd. with reasonable accuracy from at. displacements. This feature was used to simulate the INS spectra of the 2 possible structures resulting from H2O interaction with the Broensted acid sites of the zeolite: H-bonded H2O or hydroxonium ion. The at. displacements for the 2 structures are derived from recent ab initio MP2 calcns. (M. Krossner and J. Sauer, 1996). The comparison between exptl. and calcd. INS spectra confirms that the 1st H2O mol. is attached to the acid site via 2 H bonds (in agreement with the conclusion of Krossner and Sauer). Hydroxonium ions are not found in HZSM 5; however, this protonated species might be present in zeolites with a different structure.
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    Parrillo, D.; Lee, C.; Gorte, R. Heats of adsorption for ammonia and pyridine in H-ZSM-5: evidence for identical Brønsted-acid sites. Appl. Catal., A 1994, 110, 67– 74,  DOI: 10.1016/0926-860x(94)80106-1
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    Heats of adsorption for ammonia and pyridine in H-ZSM-5: evidence for identical Bronsted-acid sites
    Parrillo, D. J.; Lee, C.; Gorte, R. J.
    Applied Catalysis, A: General (1994), 110 (1), 67-74CODEN: ACAGE4; ISSN:0926-860X.
    The authors used microcalorimetry to examine the stoichiometric adsorption complexes formed by NH3 and pyridine at the Broensted acid sites in 3 samples of HZSM 5 which vary widely in Al content and synthesis methods. The heats of adsorption for both NH3 and pyridine are const. up to a coverage of 1 mol./Al, independent of Si/Al ratio, with values of 145 kJ/mol for NH3 and 200 ± 5 kJ/mol for pyridine. These results suggest that the Broensted acid sites in unsteamed HZSM 5 are independent of sample and equal in concn. to the framework Al content.
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    Mei, D.; Lercher, J. A. Mechanistic insights into aqueous phase propanol dehydration in H-ZSM-5 zeolite. AIChE J. 2017, 63, 172– 184,  DOI: 10.1002/aic.15517
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    Mechanistic insights into aqueous phase propanol dehydration in H-ZSM-5 zeolite
    Mei, Donghai; Lercher, Johannes A.
    AIChE Journal (2017), 63 (1), 172-184CODEN: AICEAC; ISSN:0001-1541. (John Wiley & Sons, Inc.)
    Aq. phase dehydration of 1-propanol over H-ZSM-5 zeolite was investigated using d. functional theory (DFT) calcns. The water mols. in the zeolite pores prefer to aggregate via the hydrogen bonding network and be protonated at the Bronsted acidic sites (BAS). Two typical configurations, i.e., dispersed and clustered, of water mols. were identified by ab initio mol. dynamics simulations of the mimicking aq. phase H-ZSM-5 unit cell with 20 water mols. per unit cell. DFT calcd. Gibbs free energies suggest that the dimeric propanol-propanol, the propanol-water, and the trimeric propanol-propanol-water complexes are formed at high propanol concns. in aq. phase, which provide a kinetically feasible dehydration reaction channel of 1-propanol to propene. The calcn. results indicate that the propanol dehydration via the unimol. mechanism becomes kinetically discouraged due to the enhanced stability of the protonated dimeric propanol and the protonated water cluster acting as the BAS site for alc. dehydration. © 2016 American Institute of Chem. Engineers AIChE J, 2016.
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    Tuma, C.; Sauer, J. A hybrid MP2/planewave-DFT scheme for large chemical systems: proton jumps in zeolites. Chem. Phys. Lett. 2004, 387, 388– 394,  DOI: 10.1016/j.cplett.2004.02.056
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    A hybrid MP2/planewave-DFT scheme for large chemical systems. Proton jumps in zeolites
    Tuma, Christian; Sauer, Joachim
    Chemical Physics Letters (2004), 387 (4-6), 388-394CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
    We present an embedding scheme to introduce local corrections at post Hartree-Fock level to d. functional theory (DFT) calcns. As a first application we study proton jump reactions in the zeolite HSSZ-13 and show that energy barriers and rate consts. are significantly changed by second-order Moller-Plesset perturbation theory (MP2) corrections to plane wave based DFT calcns. Electronic energy barriers increase from 68 to 81 kJ/mol (dry zeolite), and from 22 to 30 kJ/mol (hydrated zeolite). The predicted heats of adsorption of one water mol. onto the Bronsted acidic sites O1 and O2 are 73 and 69 kJ/mol, resp.
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    Gao, F.; Kwak, J. H.; Szanyi, J.; Peden, C. H. Current understanding of Cu-exchanged chabazite molecular sieves for use as commercial diesel engine DeNO x catalysts. Top. Catal. 2013, 56, 1441– 1459,  DOI: 10.1007/s11244-013-0145-8
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    Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts
    Gao, Feng; Kwak, Ja Hun; Szanyi, Janos; Peden, Charles H. F.
    Topics in Catalysis (2013), 56 (15-17), 1441-1459CODEN: TOCAFI; ISSN:1022-5528. (Springer)
    A review. Selective catalytic redn. (SCR) of NOx with ammonia using metal-exchanged mol. sieves with a chabazite structure has recently been commercialized on diesel vehicles. One of the commercialized catalysts, i.e., Cu-SSZ-13, has received much attention for both practical and fundamental studies. For the latter, the particularly well-defined structure of this zeolite is allowing long-standing issues of the catalytically active site for SCR in metal-exchanged zeolites to be addressed. In this review, recent progress is summarized with a focus on two areas. First, the tech. significance of Cu-SSZ-13 as compared to other Cu ion-exchanged zeolites (e.g., Cu-ZSM-5 and Cu-beta) is highlighted. Specifically, the much enhanced hydrothermal stability for Cu-SSZ-13 compared to other zeolite catalysts is addressed via performance measurements and catalyst characterization using several techniques. The enhanced stability of Cu-SSZ-13 is rationalized in terms of the unique small pore structure of this zeolite catalyst. Second, the fundamentals of the catalytically active center; i.e., the chem. nature and locations within the SSZ-13 framework are presented with an emphasis on understanding structure-function relations. For the SCR reaction, traditional kinetic studies are complicated by intra-cryst. diffusion limitations. However, a major side reaction, non-selective ammonia oxidn. by oxygen, does not suffer from mass-transfer limitations at relatively low temps. due to significantly lower reaction rates. This allows structure-function relations that are rather well understood in terms of Cu ion locations and redox properties. Finally, some aspects of the SCR reaction mechanism are addressed from in situ spectroscopic studies.
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    Bendrich, M.; Scheuer, A.; Hayes, R.; Votsmeier, M. Unified mechanistic model for Standard SCR, Fast SCR, and NO 2 SCR over a copper chabazite catalyst. Appl. Catal., B 2018, 222, 76– 87,  DOI: 10.1016/j.apcatb.2017.09.069
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    Unified mechanistic model for Standard SCR, Fast SCR, and NO2 SCR over a copper chabazite catalyst
    Bendrich, M.; Scheuer, A.; Hayes, R. E.; Votsmeier, M.
    Applied Catalysis, B: Environmental (2018), 222 (), 76-87CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    Mechanistic proposals for the different selective catalytic redn. (SCR) sub-reactions were integrated into one surface reaction mechanism which describes the main SCR reactions (std. SCR, fast SCR, NO2 SCR), transient effects due to NO3- storage, and prodn. of the N2O byproduct over a copper chabazite catalyst. The mechanism was parameterized to steady-state and transient expts., and was shown to predict catalyst behavior during a driving cycle without any kinetic parameters re-fitting. A dual site approach was used, where site 1 accounted for adsorbed NH3 which forms on Bronsted acid sites and Cu2+; site 2 was a copper ion (Cu2+-OH) where NO2- and NO3- were adsorbed. All main SCR reactions proceeded by a reaction between NH3 and NO2- (NH4NO2 pathway) to produce N2; NO2- were also the linking species between std. SCR and NO oxidn. reactions. Reactions between NO3- and NH3 to produce NH4NO3 were also included, along with NH4NO3 decompn. pathways (i.e., by NO addn. to feed). Also, a global reaction occurring between adsorbed NH3 and gaseous NO2 to produce N2 at low temps. (<250°) was added to account for an obsd. reaction occurring on the Cu-free zeolite. This mechanism was used to analyze the importance of NO3- formation during a std. driving cycle. Although a significant amt. of inhibitive NH4NO3 was modeled to form during low temp. fast and NO2 SCR steady-state expts., almost no NH4NO3 was predicted to form during the driving cycle, thereby allowing for higher reaction activity than predicted, based on steady-state data. From modeling and catalyst testing perspectives, this showed the importance of capturing catalyst transient behavior rather than only steady-state conditions, since steady-state is not necessarily reached during practical driving scenarios.
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    Borfecchia, E.; Lomachenko, K.; Giordanino, F.; Falsig, H.; Beato, P.; Soldatov, A.; Bordiga, S.; Lamberti, C. Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction. Chem. Sci. 2015, 6, 548– 563,  DOI: 10.1039/c4sc02907k
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    Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction
    Borfecchia, E.; Lomachenko, K. A.; Giordanino, F.; Falsig, H.; Beato, P.; Soldatov, A. V.; Bordiga, S.; Lamberti, C.
    Chemical Science (2015), 6 (1), 548-563CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Cu-SSZ-13 is a highly active NH3-SCR catalyst for the abatement of harmful nitrogen oxides (NOx, x = 1, 2) from the exhausts of lean-burn engines. The study of Cu-speciation occurring upon thermal dehydration is a key step for the understanding of the enhanced catalytic properties of this material and for identifying the SCR active sites and their redox capability. Herein, we combined FTIR, X-ray absorption (XAS) and emission (XES) spectroscopies with DFT computational anal. to elucidate the nature and location of the most abundant Cu sites in the activated catalyst. Different Cu species have been found to be dominant as a function of the dehydration temp. and conditions. Data anal. revealed that the dehydration process of Cu cations is essentially completed at 250 °C, with the formation of dehydrated [CuOH]+ species hosted in close proximity to 1-Al sites in both d6r and 8r units of the SSZ-13 matrix. These species persist at higher temps. only if a certain amt. of O2 is present in the gas feed, while under inert conditions they undergo virtually total "self-redn." as a consequence of an OH extra-ligand loss, resulting in bi-coordinated bare Cu+ cations. Synchrotron characterization supported by computational anal. allowed an unprecedented quant. refinement of the local environment and structural parameters of these Cu(II) and Cu(I) species.
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    Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; Thibault-Starzyk, F. Probing zeolites by vibrational spectroscopies. Chem. Soc. Rev. 2015, 44, 7262– 7341,  DOI: 10.1039/c5cs00396b
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    Probing zeolites by vibrational spectroscopies
    Bordiga, Silvia; Lamberti, Carlo; Bonino, Francesca; Travert, Arnaud; Thibault-Starzyk, Frederic
    Chemical Society Reviews (2015), 44 (20), 7262-7341CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Bronsted and Lewis acidity and surface basicity are treated in detail. The role of probe mols. and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely det. the IR absorption coeffs. is given, making IR a quant. technique. The thermodn. parameters of the adsorption process that can be extd. from a variable-temp. IR study are described. Finally, cutting-edge space- and time-resolved expts. are reviewed. All aspects are discussed by reporting relevant examples. When available, the theor. literature related to the reviewed exptl. results is reported to support the interpretation of the vibrational spectra on an at. level.
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    Soyer, S.; Uzun, A.; Senkan, S.; Onal, I. A quantum chemical study of nitric oxide reduction by ammonia (SCR reaction) on V2O5 catalyst surface. Catal. Today 2006, 118, 268– 278,  DOI: 10.1016/j.cattod.2006.07.033
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    A quantum chemical study of nitric oxide reduction by ammonia (SCR reaction) on V2O5 catalyst surface
    Soyer, Sezen; Uzun, Alper; Senkan, Selim; Onal, Isik
    Catalysis Today (2006), 118 (3-4), 268-278CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    The reaction mechanism for selective catalytic redn. (SCR) of NO by NH3 on (0 1 0) V2O5 surface represented by a V2O9H8 cluster was simulated by d. functional theory (DFT) calcns. performed at the B3LYP/6-31G** level. Computations indicated the SCR reaction consisted of 3 main parts. In part 1, NH3 activation on V2O5 was examd. NH3 was adsorbed on Bronsted acidic V-OH site as NH4+ species by a non-activated process with an exothermic relative energy difference of 28.65 kcal/mol. Lewis acidic NH3 interactions, also considered, were energetically unfavorable. Thus, it was concluded the SCR reaction on (0 1 0) V2O5 surface was favorably initiated by the Bronsted acidic NH3 adsorption. In part 2, the interaction of NO with pre-adsorbed NH3 species eventually formed nitrosamide (NH2NO) species. The rate-limiting step for this part and for the total SCR reaction was identified as NH3NHO formation with a high activation barrier (43.99 kcal/mol); however, it is cautioned that an approx. transition state was obtained for this step. For part 3, gas phase decompn. of NH2NO and decompn. of this species on the catalyst surface were both considered. Gas phase decompn. of NH2NO had high activation barriers vs. NH2NO decompn. on the V2O9H8 cluster surface. NH2NO decompn. on this cluster was achieved by a push-pull H transfer mechanism between active V=O and V-OH groups.
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    Theoretical characterization of the reaction NH2 + NO → products
    Walch, Stephen P.
    Journal of Chemical Physics (1993), 99 (7), 5295-300CODEN: JCPSA6; ISSN:0021-9606.
    The potential energy surface for NH2 + NO was characterized using complete active space self-consistent-field (CASSCF)/deriv. calcns. to det. the stationary point geometries and frequencies, followed by internally contracted CI (ICCI) calcns. to det. the energetics. Prodn. of N2 + N2O involves a complex mechanism. The initially formed NH2NO undergoes a 1,3-hydrogen shift to give an HNNOH isomer (with the substituents trans about the NN bond and cis about the NO bond), which undergoes subsequent cis-trans isomerizations about the NN and NO bonds before decompg. to N2 + H2O. The saddle point for prodn. of N2 + H2O has an approx. rectangular arrangement of one H atom, the two N atoms, and the O atom. This process does not involve a barrier with respect to NH2 + NO. Formation of HN2 + OH can occur from any of the isomers of HNNOH with no barrier, but the overall process is endothermic by 0.7 kcal/mol. The results obtained in this work are qual. the same as previous work, but both the stationary point geometries and energies should be more reliable, because of the use of larger basis sets and more extensive inclusion of electron correlation effects.
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    Sun, D.; Schneider, W. F.; Adams, J. B.; Sengupta, D. Molecular Origins of Selectivity in the Reduction of NOx by NH3. J. Phys. Chem. A 2004, 108, 9365– 9374,  DOI: 10.1021/jp049079a
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    Molecular Origins of Selectivity in the Reduction of NOx by NH3
    Sun, Donghai; Schneider, William F.; Adams, James B.; Sengupta, Debasis
    Journal of Physical Chemistry A (2004), 108 (43), 9365-9374CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The fundamental principle underlying the selective catalytic redn. (SCR) of NOx to N2 is promotion of reactions of reductant with NOx over competing and thermodynamically preferred reactions with a large excess of O2. A similar competition between NOx and O2 exists in the non-catalytic, thermal redn. of NOx with NH3. In this work, d. functional theory calcns. elucidated the origin of the remarkable selectivity in thermal deNOx. Thermal deNOx is initiated by converting NH3 into the active reductant, NH2 radical. NH2 radical reacts with NO at rates typical of gas-phase radical reactions to produce a relatively strongly bound H2NNO adduct which readily rearranges and decomps. to N2 and water. In contrast, NH2 radical reacts exceedingly slowly with O2: the H2N-OO adduct is weakly bound and more readily falls apart than reacts with products. The pronounced discrimination of NH2 radical against reaction with O2 is unusual behavior for a radical, but can be understood by comparing the electronic structures of the H2NNO and H2NOO radical adducts. These 2 key elements of thermal deNOx (reductant activation and kinetic inhibition of reactions with O2) are similarly essential to successful catalytic lean NOx redn. and are important to consider when evaluating and modeling NOx SCR.
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    Anstrom, M.; Topsøe, N.-Y.; Dumesic, J. Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts. J. Catal. 2003, 213, 115– 125,  DOI: 10.1016/s0021-9517(02)00031-3
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    Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts
    Anstrom, Mark; Topsoe, Nan-Yu; Dumesic, J. A.
    Journal of Catalysis (2003), 213 (2), 115-125CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
    D. functional theory (DFT) calcns. were carried out on a vanadium oxide cluster contg. four vanadium atoms to probe the mechanism of the selective catalytic redn. (SCR) of NO with ammonia. The interaction of ammonia with Bronsted acid sites on this V4-cluster leads to the formation of NH4 species bonded to two vanadyl (V:O) groups, with a bonding energy of -110 kJ/mol. This adsorbed NH4 species reacts with NO in a series of steps to form an adsorbed NH2NO species, which subsequently undergoes decompn. to form N2, H2O, and a reduced vanadium oxide cluster (V4-H). The latter reaction occurs via a series of hydrogen-transfer steps by a "push-pull" mechanism with adjacent V:O and V-OH groups on the vanadium oxide cluster. The rate limiting process in this conversion of NO and NH3 to give N2, H2O, and V4-H involves the reaction of an adsorbed NH3NHO adduct to form NH2NO species. The transition state of this step may be stabilized through hydrogen bonding with surrounding vanadia and/or titania moieties.
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    Gilardoni, F.; Bell, A. T.; Chakraborty, A.; Boulet, P. Density Functional Theory Calculations of the Oxidative Dehydrogenation of Propane on the (010) Surface of V2O5†. J. Phys. Chem. B 2000, 104, 12250– 12255,  DOI: 10.1021/jp001746m
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    Density Functional Theory Calculations of the Oxidative Dehydrogenation of Propane on the (010) Surface of V2O5
    Gilardoni, Francois; Bell, Alexis T.; Chakraborty, Arup; Boulet, Pascal
    Journal of Physical Chemistry B (2000), 104 (51), 12250-12255CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    D. functional theory and the calcns. of O nucleophilicity were applied to an anal. of the oxidative dehydrogenation (ODH) of propane on the (010) surface of V2O5. These calcns. show that the energetically preferred initial step is the dissociative adsorption of propane to form i-propoxide and hydroxyl species. Two V:O groups [O(1)] bonded by a V-O-V bridge are required. One of the vanadyl groups attacks the β-C atom of propane and is converted to a V-OCH2Me2 species, whereas the other vanadyl group is converted into a V-OH group. The activation barrier for this process is 9.4 kcal/mol. Dissociative adsorption to form an n-propoxide can also occur, but the activation barrier for this process is 14.5 kcal/mol. Propene and H2O are formed via a concerted process in which an H atom of one of the Me groups of i-propoxide reacts with an O(3)H group. Exploration of alternative pathways for this step reveals that neither O(1, 2, 3), O(1)H, nor O(2)H are sufficiently reactive. These findings are in good qual. agreement with exptl. observations concerning the mechanism and kinetics of propane ODH.
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    Chen, L.; Janssens, T. V.; Vennestrøm, P. N.; Jansson, J.; Skoglundh, M.; Grönbeck, H. A complete multisite reaction mechanism for low-temperature NH3-SCR over Cu-CHA. ACS Catal. 2020, 10, 5646– 5656,  DOI: 10.1021/acscatal.0c00440
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    A Complete Multisite Reaction Mechanism for Low-Temperature NH3-SCR over Cu-CHA
    Chen, Lin; Janssens, Ton V. W.; Vennestroem, Peter N. R.; Jansson, Jonas; Skoglundh, Magnus; Groenbeck, Henrik
    ACS Catalysis (2020), 10 (10), 5646-5656CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The dynamic character of the active centers has made it difficult to unravel the reaction path for NH3-assisted selective catalytic redn. (SCR) of nitrogen oxides over Cu-CHA. Herein, we use d. functional theory calcns. to suggest a complete reaction mechanism for low-temp. NH3-SCR. The reaction is found to proceed in a multisite fashion over ammonia-solvated Cu cations Cu(NH3)2+ and Bronsted acid sites. The activation of oxygen and the formation of the key intermediates HONO and H2NNO occur on the Cu sites, whereas the Bronsted acid sites facilitate the decompn. of HONO and H2NNO to N2 and H2O. The activation and reaction of NO is found to proceed via the formation of nitrosonium (NO+) or nitrite (NO2-) intermediates. These low-temp. mechanisms take the dynamic character of Cu sites into account where oxygen activation requires pairs of Cu(NH3)2+ complexes, whereas HO-NO and H3N-NO coupling may occur on single complexes. The formation and sepn. of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured kinetic data and provides a solid basis for future improvements of the low-temp. NH3-SCR reaction.
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    Mao, Y.; Wang, Z.; Wang, H.-F.; Hu, P. Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34. ACS Catal. 2016, 6, 7882– 7891,  DOI: 10.1021/acscatal.6b01449
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    Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34
    Mao, Yu; Wang, Ziyun; Wang, Hai-Feng; Hu, P.
    ACS Catalysis (2016), 6 (11), 7882-7891CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Metal-exchanged CHA-type (SAPO-34 and SSZ-13) zeolites are promising catalysts for selective catalytic redn. (SCR) of NOx by NH3. However, an understanding of the process at the mol. level is still limited, which hinders the identification of its mechanism and the design of more efficient zeolite catalysts. In this work, modeling the reaction over Cu-SAPO-34, a periodic d. functional theory (DFT) study of NH3-SCR was performed using the hybrid functional (HSE06) with the consideration of van der Waals (vdW) interactions. A mechanism with a low N-N coupling barrier is proposed to account for the activation of NO. The redox cycle of Cu2+ and Cu+, which is crucial for the SCR process, is identified with detailed analyses. In addn., the decompn. of NH2NO is shown to readily occur on the Bronsted acid site by a hydrogen push-pull mechanism, confirming the collective efforts of Bronsted acid and Lewis acid (Cu2+) sites. The special electronic and structural properties of Cu-SAPO-34 are demonstrated to play an essential role in the reaction, which may have general implications on the understanding of zeolite catalysis.
  74. 74
    Li, J.; Li, S. New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study. Phys. Chem. Chem. Phys. 2007, 9, 3304– 3311,  DOI: 10.1039/b700161d
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    New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study
    Li, Jun; Li, Shuhua
    Physical Chemistry Chemical Physics (2007), 9 (25), 3304-3311CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    D. functional theory calcns. were carried out to investigate the reaction mechanism of selective catalytic redn. of nitrogen oxides by ammonia in the presence of oxygen at the Bronsted acid sites of H-form zeolites. The Bronsted acid site of H-form zeolites was modeled by an aluminosilicate cluster contg. five tetrahedral (Al, Si) atoms. A low-activation-energy pathway for the catalytic redn. of NO was proposed. It consists of two successive stages: first NH2NO is formed in gas phase, and then it is decompd. into N2 and H2O over H-form zeolites. In the first stage, the formation of NH2NO may occur via two routes: (1) NO is directly oxidized by O2 to NO2, and then NO2 combines with NO to form N2O3, which reacts with NH3 to produce NH2NO; (2) when NO2 exceeds NO in the content, NO2 assocs. with itself to form N2O4, and then N2O4 reacts with NH3 to produce NH2NO. The second stage was suggested to proceed with low activation energy via a series of synergic proton transfer steps catalyzed by H-form zeolites. The rate-detg. step for the whole redn. of NOx is identified as the oxidn. of NO to NO2 with an activation barrier of 15.6 kcal mol-1. This mechanism was found to account for many known exptl. facts related to selective catalytic redn. of nitrogen oxides by ammonia over H-form zeolites.
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    Brüggemann, T. C.; Keil, F. J. Theoretical investigation of the mechanism of the selective catalytic reduction of nitric oxide with ammonia on H-form zeolites. J. Phys. Chem. C 2008, 112, 17378– 17387,  DOI: 10.1021/jp806674d
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    Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitric Oxide with Ammonia on H-Form Zeolites
    Bruggemann, Till C.; Keil, Frerich J.
    Journal of Physical Chemistry C (2008), 112 (44), 17378-17387CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The selective catalytic redn. of NO with ammonia in the presence of oxygen has been investigated on a portion of the H-ZSM5 framework which contains 5T atoms by using d. functional theory, representing H-form zeolites. The mechanism was subdivided into three parts: (1) the oxidn. of NO to NO2, (2) the formation of an intermediate (NH2NO), and (3) the decompn. of this species to nitrogen and water. For the second step, three different pathways were studied, differing in the NOx species initially present on the active site: (i) two NO mols. form N2O2, (ii) NO2 and NO form N2O3, and (iii) two NO2 mols. form N2O4. For steps 1 and 2, the crossing of potential energy surfaces was considered for the transition of single mols. to adsorbed clusters. For all three parts of the mechanism, the energy profile of the heterogeneously catalyzed reaction is favorable, as compared to that of the corresponding homogeneous reaction. Due to the strong adsorption of ammonia on the acid site, it is likely that the rate-detg. step of the overall reaction is the oxidn. of NO to NO2 caused by blocking of the active site by NH3. As far as we have investigated the reaction mechanism of the selective catalytic redn. of NO with NH3 in this work, the results are in agreement with the exptl. literature.
  76. 76
    Usberti, N.; Gramigni, F.; Nasello, N. D.; Iacobone, U.; Selleri, T.; Hu, W.; Liu, S.; Gao, X.; Nova, I.; Tronconi, E. An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst. Appl. Catal., B 2020, 279, 119397,  DOI: 10.1016/j.apcatb.2020.119397
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    An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst
    Usberti, Nicola; Gramigni, Federica; Nasello, Nicole Daniela; Iacobone, Umberto; Selleri, Tommaso; Hu, Wenshuo; Liu, Shaojun; Gao, Xiang; Nova, Isabella; Tronconi, Enrico
    Applied Catalysis, B: Environmental (2020), 279 (), 119397CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
    The reactivity of Lewis and Bronsted ammonia in the redn. half-cycle (RHC) of the NH3-SCR low temp. redox mechanism was studied over a model Cu-CHA catalyst by transient kinetic tests involving reductive NO pulses. The CuII sites were reduced according to a 1:1:1:1 molar ratio with NO and NH3 conversion and N2 formation. The ammonia coordinated to Cu sites (Lewis ammonia) was preferentially consumed prior to that stored on the Bronsted acid sites. The catalyst was effectively re-oxidized by O2 in He at 150°C even when the Cu-coordinated ammonia was depleted. A redox kinetic model assuming NO activation by CuII to a gaseous mobile intermediate (HONO) which reacts first with Lewis-NH3 and then with Bronsted-NH3 was successfully fitted to our transient data assuming the CuII redn. rate to be second order in the CuII sites. This suggests a possible role of CuII dimeric complexes in the RHC of Std. SCR.

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  • Abstract

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    Figure 1

    Figure 1. Schematic representation of the catalytic cycle of NH3-SCR of NOx.

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    Figure 2

    Figure 2. QM/MM embedding setup; CHA cluster (left) with a quantum mechanical region containing nitrate species (right). The outermost region contains point charges to ensure that the Madelung potential in the center of the cluster is accurately reproduced. Atom color codes: Cu (brown), Al (green), Si (yellow), O (red), N (blue), and H (white).

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    Figure 3

    Figure 3. Spectroscopic signatures: (a) concentration modulation ME DRIFTS experiment with the corresponding phase-resolved spectrum, (b) reaction cycle highlighting the identified species, (c) data obtained from QM/MM calculations for bidentate Cu nitrate (Cu–NO3) and Cu-nitrosamine (Cu–N(═O)–NH2) species for the neutral system, and (d) for the deprotonated and protonated system. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white). The framework SiO2 is shown using a wire framework motif.

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    Figure 4

    Figure 4. Interaction of physisorbed (a) water and ammonia with Cu(I)-CHA active sites, and (b) behavior of NO on solvated Cu(I)-CHA site. The model used is shown as an extra framework. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white).

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    Figure 5

    Figure 5. Reaction adsorption energies of NH3, NO (with both the O- and N-end down), H2O, and O2 on Cu(I)-CHA and Cu(II)–OH/CHA sites. The model used is shown as an extra framework. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white). The framework SiO2 is shown using a wire framework motif.

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    Figure 6

    Figure 6. Potential reaction-energy landscape for NH3-SCR on the activated Cu-CHA site and (black) on the bare site, (green) with physisorbed ammonia and (red) with physisorbed water. Inset right: NO-activated NH3-SCR of the NOx catalytic cycle.

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    Figure 7

    Figure 7. Calculated potential energy surfaces for H2NNO isomerization (a) without water and (b) in the presence of physisorbed water on the Cu-CHA-sites. The model used is shown as an extra framework. Color codes: Cu (brown), Al (green), O (red), N (blue), and H (white).

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  • References

    This article references 76 other publications.

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      Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines
      Koebel, M.; Elsener, M.; Kleemann, M.
      Catalysis Today (2000), 59 (3-4), 335-345CODEN: CATTEA; ISSN:0920-5861. (Elsevier Science B.V.)
      Urea-SCR, the selective catalytic redn. using urea as reducing agent, has been investigated for about 10 yr in detail and today is a well established technique for DeNOx of stationary diesel engines. It is presently also considered as the most promising way to diminish NOx emissions originating from heavy duty vehicles, esp. trucks. The paper discusses the fundamental problems and challenges if urea-SCR is extended to mobile applications. The major goal is the redn. of the required catalyst vol. while still maintaining a high selectivity for the SCR reaction over a wide temp. range. The much shorter residence time of the exhaust gas in the catalyst will lead to higher secondary emissions of ammonia and isocyanic acid originating from the reducing agent. Addnl. problems include the control strategy for urea dosing, the high f.p. of urea, and the long term stability of the catalyst.
    7. 7
      Deka, U.; Juhin, A.; Eilertsen, E. A.; Emerich, H.; Green, M. A.; Korhonen, S. T.; Weckhuysen, B. M.; Beale, A. M. Confirmation of isolated Cu2+ ions in SSZ-13 zeolite as active sites in NH3-selective catalytic reduction. J. Phys. Chem. C 2012, 116, 4809– 4818,  DOI: 10.1021/jp212450d
      7
      Confirmation of Isolated Cu2+ Ions in SSZ-13 Zeolite as Active Sites in NH3-Selective Catalytic Reduction
      Deka, Upakul; Juhin, Amelie; Eilertsen, Einar A.; Emerich, Hermann; Green, Mark A.; Korhonen, Satu T.; Weckhuysen, Bert M.; Beale, Andrew M.
      Journal of Physical Chemistry C (2012), 116 (7), 4809-4818CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      NH3-Selective Catalytic Redn. (NH3-SCR) is a widely used technol. for NOx redn. in the emission control systems of heavy duty diesel vehicles. Copper-based ion exchanged zeolites and in particular Cu-SSZ-13 (CHA framework) catalysts show both exceptional activity and hydrothermal stability for this reaction. In this work, we have studied the origin of the SCR activity of Cu-SSZ-13 as evidenced from a combination of synchrotron-based and lab. techniques. Synchrotron-based in situ XAFS/XRD measurements were used to provide complementary information on the local copper environment under realistic NH3-SCR conditions. Crucial then to the catalytic activity of Cu-SSZ-13 is the local environment of the copper species, particularly in the zeolite. Cu-SSZ-13 contains mononuclear Cu2+ species, located in the face of the double-6-ring subunit of the zeolite after calcination where it remains under reaction conditions. At lower temps. (with low activity), XAFS and XRD data revealed a conformational change in the local geometry of the copper from a planar form toward a distorted tetrahedron as a result of a preferential interaction with NH3. This process appears necessary for activity, but results in a stymieing of activity at low temps. At higher temps., the Cu2+ possess a local coordination state akin to that seen after calcination.
    8. 8
      Ma, L.; Cheng, Y.; Cavataio, G.; McCabe, R. W.; Fu, L.; Li, J. Characterization of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts with hydrothermal treatment for NH3-SCR of NOx in diesel exhaust. Chem. Eng. J. 2013, 225, 323– 330,  DOI: 10.1016/j.cej.2013.03.078
      8
      Characterization of commercial Cu-SSZ-13 and Cu-SAPO-34 catalysts with hydrothermal treatment for NH3-SCR of NOx in diesel exhaust
      Ma, Lei; Cheng, Yisun; Cavataio, Giovanni; McCabe, Robert W.; Fu, Lixin; Li, Junhua
      Chemical Engineering Journal (Amsterdam, Netherlands) (2013), 225 (), 323-330CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      Cu-SSZ-13 and Cu-SAPO-34 zeolite catalysts were fully formulated washcoat cordierite monoliths purchased from the major catalyst suppliers and were hydrothermally aged at 600, 750, and 850 °C in simulated exhaust gases contg. water. Their catalytic activities were tested in selective catalytic redn. (SCR) of NOx with ammonia. The microstructure of the zeolite supports, identity of copper species, acidity and reactant adsorption ability were investigated in detail using various characterization methods. The results showed that hydrothermal treatment of Cu-SSZ-13 and Cu-SAPO-34 catalysts had a significant influence on the physicochem. properties and reactant adsorption abilities of the catalysts, which resulted in different catalytic performances in NH3-SCR. The hydrothermal treatment drastically decreased the surface area and pore vol. of both the Cu-SSZ-13 and Cu-SAPO-34 catalysts. TEM results indicated that obvious aggregation of Cu-SSZ-13 and Cu-SAPO-34 occurred while some CuO particulates migrated from isolated Cu2+ species were formed during the aging test. Hydrothermal treatment of the Cu-SSZ-13 and Cu-SAPO-34 catalysts caused the migration of Cu2+, a decrease in acidity and dealumination. The change in the coordination environment of Cu2+ affected NOx adsorption and activation on the catalyst surface. Both the copper sites and the acidity might be the main factor for the NH3-SCR reaction on Cu-CHA catalyst.
    9. 9
      Marberger, A.; Petrov, A. W.; Steiger, P.; Elsener, M.; Kröcher, O.; Nachtegaal, M.; Ferri, D. Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13. Nat. Catal. 2018, 1, 221– 227,  DOI: 10.1038/s41929-018-0032-6
      9
      Time-resolved copper speciation during selective catalytic reduction of NO on Cu-SSZ-13
      Marberger, Adrian; Petrov, Andrey W.; Steiger, Patrick; Elsener, Martin; Krocher, Oliver; Nachtegaal, Maarten; Ferri, Davide
      Nature Catalysis (2018), 1 (3), 221-227CODEN: NCAACP; ISSN:2520-1158. (Nature Research)
      Practical catalysts often operate under dynamic conditions of temp. variations and sudden changes of feed compn. that call for understanding of operation and catalyst structure under analogous exptl. conditions. For instance, the copper-exchanged small-pore SSZ-13 catalyst used currently in the selective catalytic redn. of harmful nitrogen oxides from the exhaust gas of diesel-fuelled vehicles operates under recurrent ammonia dosage. Here, we report the design of unsteady state expts. that mimic such a dynamic environment to obtain key mechanistic information on this reaction. Through the combination of time-resolved X-ray absorption spectroscopy and transient experimentation, we were able to capture an ammonia inhibition effect on the rate-limiting copper re-oxidn. at low temp. The practical relevance of this observation was demonstrated by optimization of the ammonia dosage on a catalyst washcoat on cordierite honeycomb, resulting in lower ammonia consumption and an increase in nitrogen oxide conversion at low temp.
    10. 10
      Paolucci, C.; Khurana, I.; Parekh, A. A.; Li, S.; Shih, A. J.; Li, H.; Di Iorio, J. R.; Albarracin-Caballero, J. D.; Yezerets, A.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F.; Gounder, R. Dynamic multinuclear sites formed by mobilized copper ions in NO x selective catalytic reduction. Science 2017, 357, 898– 903,  DOI: 10.1126/science.aan5630
      10
      Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction
      Paolucci, Christopher; Khurana, Ishant; Parekh, Atish A.; Li, Sichi; Shih, Arthur J.; Li, Hui; Di Iorio, John R.; Albarracin-Caballero, Jonatan D.; Yezerets, Aleksey; Miller, Jeffrey T.; Delgass, W. Nicholas; Ribeiro, Fabio H.; Schneider, William F.; Gounder, Rajamani
      Science (Washington, DC, United States) (2017), 357 (6354), 898-903CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Copper ions exchanged into zeolites are active for the selective catalytic redn. (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temp. rate dependence on copper (Cu) volumetric d. is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calcns. to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)-mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the vol. that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.
    11. 11
      Millan, R.; Cnudde, P.; van Speybroeck, V.; Boronat, M. Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations. JACS Au 2021, 1, 1778– 1787,  DOI: 10.1021/jacsau.1c00337
      11
      Mobility and Reactivity of Cu+ Species in Cu-CHA Catalysts under NH3-SCR-NOx Reaction Conditions: Insights from AIMD Simulations
      Millan, Reisel; Cnudde, Pieter; van Speybroeck, Veronique; Boronat, Mercedes
      JACS Au (2021), 1 (10), 1778-1787CODEN: JAAUCR; ISSN:2691-3704. (American Chemical Society)
      The mobility of the copper cations acting as active sites for the selective catalytic redn. of nitrogen oxides with ammonia in Cu-CHA catalysts varies with temp. and feed compn. Herein, the migration of [Cu(NH3)2]+ complexes between two adjacent cavities of the chabazite structure, including other reactant mols. (NO, O2, H2O, and NH3), in the initial and final cavities is investigated using ab initio mol. dynamics (AIMD) simulations combined with enhanced sampling techniques to describe hopping events from one cage to the other. We find that such diffusion is only significantly hindered by the presence of excess NH3 or NO in the initial cavity, since both reactants form with [Cu(NH3)2]+ stable intermediates which are too bulky to cross the 8-ring windows connecting the cavities. The presence of O2 modifies strongly the interaction of NO with Cu+. At low temps., we observe NO detachment from Cu+ and increased mobility of the [Cu(NH3)2]+ complex, while at high temps., NO reacts spontaneously with O2 to form NO2. The present simulations give evidence for recent exptl. observations, namely, an NH3 inhibition effect on the SCR reaction at low temps., and transport limitations of NO and NH3 at high temps. Our first principle simulations mimicking operating conditions support the existence of two different reaction mechanisms operating at low and high temps., the former involving dimeric Cu(NH3)2-O2-Cu(NH3)2 species and the latter occurring by direct NO oxidn. to NO2 in one single cavity.
    12. 12
      Janssens, T. V.; Falsig, H.; Lundegaard, L. F.; Vennestrøm, P. N.; Rasmussen, S. B.; Moses, P. G.; Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lamberti, C.; Bordiga, S.; Godiksen, A.; Mossin, S.; Beato, P. A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia. ACS Catal. 2015, 5, 2832– 2845,  DOI: 10.1021/cs501673g
      12
      A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia
      Janssens, Ton V. W.; Falsig, Hanne; Lundegaard, Lars F.; Vennestroem, Peter N. R.; Rasmussen, Soeren B.; Moses, Poul Georg; Giordanino, Filippo; Borfecchia, Elisa; Lomachenko, Kirill A.; Lamberti, Carlo; Bordiga, Silvia; Godiksen, Anita; Mossin, Susanne; Beato, Pablo
      ACS Catalysis (2015), 5 (5), 2832-2845CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      For the first time, the std. and fast selective catalytic redn. (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable mols. only. The std. SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidn. of the catalyst by NO + O2 and a redn. by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the redn., and finally, oxidn. by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown exptl. for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), ESR, and Fourier transform IR spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the std. SCR cycle. The activation energy calcd. by d. functional theory (DFT) indicates that the oxidn. of an NO mol. by O2 to a bidentate nitrate ligand is rate-detg. for std. SCR. Finally, the role of a nitrate/nitrite equil. and the possible influence of Cu dimers and Bronsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidn. to NO2.
    13. 13
      Akter, N.; Chen, X.; Parise, J.; Boscoboinik, J. A.; Kim, T. Effects of copper loading on NH3-SCR and NO oxidation over Cu impregnated CHA zeolite. Korean J. Chem. Eng. 2018, 35, 89– 98,  DOI: 10.1007/s11814-017-0268-x
      13
      Effects of copper loading on NH3-SCR and NO oxidation over Cu impregnated CHA zeolite
      Akter, Nusnin; Chen, Xianyin; Parise, John; Boscoboinik, Jorge Anibal; Kim, Taejin
      Korean Journal of Chemical Engineering (2018), 35 (1), 89-98CODEN: KJCHE6; ISSN:0256-1115. (Springer)
      Cu/CHA catalysts with various Cu loadings (0.5 wt%-6.0 wt%) were synthesized via incipient wetness impregnation. The catalysts were applied to the selective catalytic redn. (SCR) of NO with NH3 and NO oxidn. reaction. XRD and N2 adsorption-desorption data showed that CHA structure was maintained with the incorporation of Cu, while sp. surface areas decreased with increasing Cu loading. At intermediate Cu loading, 4 wt%, the highest NH3-SCR activity was obsd. with ~ 98% N2 selectivity from 150°C to 300°C. Small amts. of water, 2%, slightly increased NO conversion in addn. to the remarkable N2O and NO2 redn. at high temp. Water effects are attributed to the improved Cu ion reducibility and mobility. NO oxidn. results provided no relation between NO2 formation and SCR activity. Physicochem. properties, NO conversion, N2 selectivity, and activation energy data showed that impregnated samples' mol. structure and catalytic activity are comparable to the conventional ion-exchanged (IE) samples ones.
    14. 14
      Wallin, M.; Karlsson, C.-J.; Skoglundh, M.; Palmqvist, A. Selective catalytic reduction of NOx with NH3 over zeolite H-ZSM-5: influence of transient ammonia supply. J. Catal. 2003, 218, 354– 364,  DOI: 10.1016/s0021-9517(03)00148-9
      14
      Selective catalytic reduction of NOx with NH3 over zeolite H-ZSM-5: influence of transient ammonia supply
      Wallin, Mikaela; Karlsson, Carl-Johan; Skoglundh, Magnus; Palmqvist, Anders
      Journal of Catalysis (2003), 218 (2), 354-364CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
      The effect of ammonia supply on the selective catalytic redn. of NOx over zeolite H-ZSM-5 was investigated using step response expts. between 200 and 500°. For inlet NO:NO2 ratios >1, the activity for NOx redn. transiently increased when NH3 was removed from the feed. For NO:NO2 ratios ≤1, the NOx redn. however decreased. By pulsing NH3 to the feed, the activity for NO redn. was enhanced up to five times compared to continuous supply of ammonia. For NO:NO2 ratios exceeding one, also the selectivity towards N2O formation was lower with transient ammonia supply. Temp. programmed reaction expts. with preadsorbed NH3 showed highest initial NOx redn. activity when ammonia had been adsorbed at 300 or 250° compared to 200°. A min. in NO redn. was obsd. at 130° independent of the ammonia adsorption temp. For NO:NO2 ratios >1, the results strongly indicate that NO oxidn. is the rate detg. step in the ammonia selective catalytic redn. (NH3-SCR) reaction over H-ZSM-5.
    15. 15
      Liu, Y.; Xue, W.; Seo, S.; Tan, X.; Mei, D.; Liu, C.-j.; Nam, I.-S.; Hong, S. B. Water: A promoter of ammonia selective catalytic reduction over copper-exchanged LTA zeolites. Appl. Catal., B 2021, 294, 120244,  DOI: 10.1016/j.apcatb.2021.120244
      15
      Water: A promoter of ammonia selective catalytic reduction over copper-exchanged LTA zeolites
      Liu, Yang; Xue, Wenjuan; Seo, Seungwan; Tan, Xuechao; Mei, Donghai; Liu, Chang-jun; Nam, In-Sik; Hong, Suk Bong
      Applied Catalysis, B: Environmental (2021), 294 (), 120244CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Neg. effects of water on the hydrothermal stability of ammonia (NH3) selective catalytic redn. (SCR) catalysts are well known. Thus, developing water-resistant, zeolite-based transition metal catalysts like copper (Cu) for mitigating nitrogen oxides from mobile sources is very challenging. Here we show that water markedly facilitates the migration of copper ions from sterically inaccessible sod cage to the readily accessible lta one through six-membered rings within a copper-exchanged high-silica Cu-LTA zeolite. We also show that this ubiquitous mol. promotes the low-temp. SCR activity of the Cu-LTA catalyst with a plausible reaction [H2O-Cu-NH3]+ intermediate. We anticipate that water might play a beneficial role in nitrogen oxides removal over metal-contg. zeolites for cleaner air via tuning the active sites and reaction pathways.
    16. 16
      Yu, T.; Wang, J.; Shen, M.; Wang, J.; Li, W. The influence of CO2 and H2O on selective catalytic reduction of NO by NH3 over Cu/SAPO-34 catalyst. Chem. Eng. J. 2015, 264, 845– 855,  DOI: 10.1016/j.cej.2014.12.017
      16
      The influence of CO2 and H2O on selective catalytic reduction of NO by NH3 over Cu/SAPO-34 catalyst
      Yu, Tie; Wang, Jun; Shen, Meiqing; Wang, Jianqiang; Li, Wei
      Chemical Engineering Journal (Amsterdam, Netherlands) (2015), 264 (), 845-855CODEN: CMEJAJ; ISSN:1385-8947. (Elsevier B.V.)
      The effect of CO2 and H2O addn. on selective catalytic redn. of NO by NH3 (NH3-SCR) over Cu/SAPO-34 catalysts was confirmed and we tried to study their influence in this research. Two Cu/SAPO-34 samples with the equiv. Cu loading were utilized, and their acidities were adjusted by the Si amts. in SAPO-34 supports. NH3-SCR, NH3-TPD, NH3 oxidn. and kinetic anal. were conducted to evaluate the role of CO2 and H2O. NH3-SCR results revealed that CO2 in the inlets presented less impact on NO conversion, while the presence of H2O enhanced NO conversion during the whole temp. range. NH3-TPD results showed H2O improved the acidity of Bronsted acid sites and the reducibility of Cu2+ species, both of which benefited the NH3-SCR activity. In addn., the presence of H2O at high temp. inhibited NH3 conversion and side-products during NH3 oxidn. process. Kinetic anal. of NH3-SCR and NH3 oxidn. at high temp. were further conducted to est. the variation of apparent activation energy (Ea) for the presence of water vapor, and finally the relation between the variation of acidity, Cu2+ species reducibility and the SCR mechanism over Cu/SAPO-34 sample at different temp. ranges was proposed, resp.
    17. 17
      Lee, H.; Song, I.; Jeon, S. W.; Kim, D. H. Mobility of Cu Ions in Cu-SSZ-13 Determines the Reactivity of Selective Catalytic Reduction of NOx with NH3. J. Phys. Chem. Lett. 2021, 12, 3210– 3216,  DOI: 10.1021/acs.jpclett.1c00181
      17
      Mobility of Cu ions in Cu-SSZ-13 determines the reactivity of selective catalytic reduction of NOx with NH3
      Lee, Hwangho; Song, Inhak; Jeon, Se Won; Kim, Do Heui
      Journal of Physical Chemistry Letters (2021), 12 (12), 3210-3216CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      Selective catalytic redn. of NOx with NH3 (NH3-SCR) in Cu-SSZ-13 has been proposed to have a unique homogeneous-like mechanism governed by the spatial proximity of mobile Cu ions. Among factors that det. the proximity, the effect of ion d. on the SCR reaction is well established; however, it has not been verified how the different mobility of the Cu ion influences the SCR reaction. Herein, we try to reveal the mobility-dependent SCR reaction by controlling the Cu species with different ion mobilities in Cu-SSZ-13. Since the reaction kinetics is governed by the diffusion of Cu ions, the Cu ion mobility dets. the reactivity of the Cu-SSZ-13. In terms of this correlation, enhanced ion mobility leads to improved NH3-SCR activity. These findings help understand the behavior of Cu ions in Cu-SSZ-13 under a catalytic reaction and provide insights to design rational catalysts by tuning the ion mobility.
    18. 18
      Wan, Y.; Yang, G.; Xiang, J.; Shen, X.; Yang, D.; Chen, Y.; Rac, V.; Rakic, V.; Du, X. Promoting effects of water on the NH3-SCR reaction over Cu-SAPO-34 catalysts: transient and permanent influences on Cu species. Dalton Trans. 2020, 49, 764– 773,  DOI: 10.1039/c9dt03848e
      18
      Promoting effects of water on the NH3-SCR reaction over Cu-SAPO-34 catalysts: transient and permanent influences on Cu species
      Wan, Yuyi; Yang, Guangpeng; Xiang, Jinyao; Shen, Xiaoqiang; Yang, Dafei; Chen, Yanrong; Rac, Vladislav; Rakic, Vesna; Du, Xuesen
      Dalton Transactions (2020), 49 (3), 764-773CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      Cu-SAPO-34 catalysts with varied Cu loadings were synthesized through ion exchange to study the influence of water on the NH3-SCR reaction. The catalytic activities were evaluated by selective catalytic redn. of NO under a reactant feed in the presence/absence of water. Transient expts. were designed to study the response of NO conversion to the presence of water. H2-TPR and DFT calcns. were performed to study the reducibility of Cu species. NH3-TPD and XPS were conducted to reveal the migration of Cu species. Water could remarkably improve NO redn. activities and the promoting effect is more significant on the catalyst with low Cu loading. Both transient and permanent influences were found in this promoting phenomenon. For the transient influence, water was proved to accelerate the re-oxidn. half-cycle. Moreover, water can enhance the promoting effect of the SCR feed on the migration of Cu species. These unanchored Cu ions migrate to defect sites to form active sites, which lead to a permanent influence of water.
    19. 19
      O’Malley, A. J.; Hitchcock, I.; Sarwar, M.; Silverwood, I. P.; Hindocha, S.; Catlow, C. R. A.; York, A. P.; Collier, P. Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process. Phys. Chem. Chem. Phys. 2016, 18, 17159– 17168,  DOI: 10.1039/c6cp01160h
      19
      Ammonia mobility in chabazite: insight into the diffusion component of the NH3-SCR process
      O'Malley, Alexander J.; Hitchcock, Iain; Sarwar, Misbah; Silverwood, Ian P.; Hindocha, Sheena; Catlow, C. Richard A.; York, Andrew P. E.; Collier, P. J.
      Physical Chemistry Chemical Physics (2016), 18 (26), 17159-17168CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The diffusion of NH3 in com. NH3-SCR catalyst Cu-CHA was measured and compared with H-CHA using quasielastic n scattering (QENS) and mol. dynamics (MD) simulations to assess the effect of counterion presence on NH3 mobility in automotive emission control relevant zeolite catalysts. QENS expts. obsd. jump diffusion with a jump distance of 3 Å, giving similar self-diffusion coeff. measurements for both Cu- and H-CHA samples, in the range of ∼5-10 × 10-10 m2 s-1 over the measured temp. range. Self-diffusivities calcd. by MD were within a factor of 6 of those measured exptl. at each temp. The activation energies of diffusion were also similar for both studied systems: 3.7 and 4.4 kJ mol-1 for the H- and Cu-chabazite, resp., suggesting that counterion presence has little impact on NH3 diffusivity on the timescale of the QENS expt. An explanation is given by the MD simulations, which showed the strong coordination of NH3 with Cu2+ counterions in the center of the chabazite cage, shielding other mols. from interaction with the ion, and allowing for intercage diffusion through the 8-ring windows (consistent with the exptl. obsd. jump length) to carry on unhindered.
    20. 20
      Hu, W.; Selleri, T.; Gramigni, F.; Fenes, E.; Rout, K. R.; Liu, S.; Nova, I.; Chen, D.; Gao, X.; Tronconi, E. On the Redox Mechanism of Low-Temperature NH 3 -SCR over Cu-CHA: A Combined Experimental and Theoretical Study of the Reduction Half Cycle. Angew. Chem. 2021, 133, 7273– 7280,  DOI: 10.1002/ange.202014926
      There is no corresponding record for this reference.
    21. 21
      Ruggeri, M. P.; Nova, I.; Tronconi, E. Experimental study of the NO oxidation to NO 2 over metal promoted zeolites aimed at the identification of the standard SCR rate determining step. Top. Catal. 2013, 56, 109– 113,  DOI: 10.1007/s11244-013-9937-0
      21
      Experimental study of the NO oxidation to NO2 over metal promoted zeolites aimed at the identification of the standard SCR rate determining step
      Ruggeri, Maria Pia; Nova, Isabella; Tronconi, Enrico
      Topics in Catalysis (2013), 56 (1-8), 109-113CODEN: TOCAFI; ISSN:1022-5528. (Springer)
      Steady state and transient kinetic runs devoted to the comparative anal. of NO oxidn. and std. SCR reactions over com. Cu- and Fe-promoted zeolite catalysts are herein presented with the aim to clarify whether NO oxidn. to NO2 is the rate detg. step (rds) of the std. SCR reaction. It is found that this statement seems questionable in the light both of the herein collected exptl. results and of scattered evidence from the literature.
    22. 22
      Catlow, C. R. A.; Buckeridge, J.; Farrow, M. R.; Logsdail, A. J.; Sokol, A. A. Quantum Mechanical/Molecular Mechanical (QM/MM) Approaches. In Handbook of Solid State Chemistry; Dronskowski, R., Kikkawa, S., Stein, A., Eds.; Wiley-VCH, 2017; Vol. 5, pp 647– 680.
      There is no corresponding record for this reference.
    23. 23
      Sherwood, P.; de Vries, A. H.; Collins, S. J.; Greatbanks, S. P.; Burton, N. A.; Vincent, M. A.; Hillier, I. H. Computer simulation of zeolite structure and reactivity using embedded cluster methods. Faraday Discuss. 1997, 106, 79– 92,  DOI: 10.1039/a701790a
      23
      Computer simulation of zeolite structure and reactivity using embedded cluster methods
      Sherwood, Paul; De Vries, Alex H.; Collins, Simon J.; Greatbanks, Stephen P.; Burton, Neil A.; Vincent, Mark A.; Hillier, Ian H.
      Faraday Discussions (1997), 106 (Solid State Chemistry: New Opportunities from Computer Simulations), 79-92CODEN: FDISE6; ISSN:0301-7249. (Royal Society of Chemistry)
      The use of bare cluster models to understand the nature of zeolite-substrate interactions may be improved to take account of the environment of the Broensted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves generating a specialized correction potential by fitting a non-periodic array of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic array of at. charges to the potential of the infinite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM cluster is embedded. Such embedded cluster calcns., employing a T3 cluster, with electron correlation at the d. functional theory level, are described, to model the interaction of water at a Bronsted acid site. Structures of the water-zeolite complex, and assocd. vibrational frequencies and 1H NMR shifts are calcd. and compared with calcns. of bare clusters of varying size and with exptl. data. We then describe a mixed quantum mech.-mol. mech. (QM-MM) model derived by combining charges from the second model with a std. aluminosilicate force field. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbon cracking reaction; the methyl-shift reaction of a propenium ion coordinated to the acid site.
    24. 24
      Metz, S.; Kästner, J.; Sokol, A. A.; Keal, T. W.; Sherwood, P. C hem S hell-a modular software package for QM / MM simulations. Wiley Interdiscip. Rev.: Comput. Mol. Sci. 2014, 4, 101– 110,  DOI: 10.1002/wcms.1163
      24
      ChemShell-a modular software package for QM/MM simulations
      Metz, Sebastian; Kaestner, Johannes; Sokol, Alexey A.; Keal, Thomas W.; Sherwood, Paul
      Wiley Interdisciplinary Reviews: Computational Molecular Science (2014), 4 (2), 101-110CODEN: WIRCAH; ISSN:1759-0884. (Wiley-Blackwell)
      ChemShell is a modular computational chem. package with a particular focus on hybrid quantum mech./mol. mech. (QM/MM) simulations. A core set of chem. data handling modules and scripted interfaces to a large no. of quantum chem. and mol. modeling packages underpin a flexible QM/MM scheme. ChemShell has been used in the study of small mols., mol. crystals, biol. macromols. such as enzymes, framework materials including zeolites, ionic solids, and surfaces. We outline the range of QM/MM coupling schemes and supporting functions for system setup, geometry optimization, and transition-state location (including those from the open-source DL-FIND optimization library). We discuss recently implemented features allowing a more efficient treatment of long range electrostatic interactions, X-ray based quantum refinement of crystal structures, free energy methods, and excited-state calcns. ChemShell has been ported to a range of parallel computers and we describe a no. of options including parallel execution based on the message-passing capabilities of the interfaced packages and task-farming for applications in which a no. of individual QM, MM, or QM/MM calcns. can performed simultaneously. We exemplify each of the features by brief ref. to published applications.
    25. 25
      Lu, Y.; Farrow, M. R.; Fayon, P.; Logsdail, A. J.; Sokol, A. A.; Catlow, C. R. A.; Sherwood, P.; Keal, T. W. Open-Source, python-based redevelopment of the ChemShell multiscale QM/MM environment. J. Chem. Theor. Comput. 2018, 15, 1317– 1328,  DOI: 10.1021/acs.jctc.8b01036
      There is no corresponding record for this reference.
    26. 26
      Sherwood, P.; de Vries, A. H.; Guest, M. F.; Schreckenbach, G.; Catlow, C. R. A.; French, S. A.; Sokol, A. A.; Bromley, S. T.; Thiel, W.; Turner, A. J. QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis. J. Mol. Struct.: THEOCHEM 2003, 632, 1– 28,  DOI: 10.1016/s0166-1280(03)00285-9
      26
      QUASI: A general purpose implementation of the QM/MM approach and its application to problems in catalysis
      Sherwood, Paul; de Vries, Alex H.; Guest, Martyn F.; Schreckenbach, Georg; Catlow, C. Richard A.; French, Samuel A.; Sokol, Alexey A.; Bromley, Stefan T.; Thiel, Walter; Turner, Alex J.; Billeter, Salomon; Terstegen, Frank; Thiel, Stephan; Kendrick, John; Rogers, Stephen C.; Casci, John; Watson, Mike; King, Frank; Karlsen, Elly; Sjovoll, Merethe; Fahmi, Adil; Schafer, Ansgar; Lennartz, Christian
      Journal of Molecular Structure: THEOCHEM (2003), 632 (), 1-28CODEN: THEODJ; ISSN:0166-1280. (Elsevier Science B.V.)
      The work of the European project QUASI (Quantum Simulation in Industry, project EP25047) which has sought to develop a flexible QM/MM scheme and to apply it to a range of industrial problems is described. A no. of QM/MM approaches were implemented within the computational chem. scripting system, ChemShell, which provides the framework for deploying a variety of independent program packages. This software was applied in several large-scale QM/MM studies which addressed the catalytic decompn. of N2O by Cu-contg. zeolites, methanol synthesis reaction catalyzed by Cu clusters supported on ZnO surfaces, and the modeling of enzyme structure and reactivity.
    27. 27
      Guest, M. F.; Bush, I. J.; Van Dam, H. J. J.; Sherwood, P.; Thomas, J. M. H.; Van Lenthe, J. H.; Havenith, R. W. A.; Kendrick, J. The GAMESS-UK electronic structure package: algorithms, developments and applications. Mol. Phys. 2005, 103, 719– 747,  DOI: 10.1080/00268970512331340592
      27
      The GAMESS-UK electronic structure package: Algorithms, developments and applications
      Guest, Martyn F.; Bush, Ian J.; Van Dam, Huub J. J.; Sherwood, Paul; Thomas, Jens M. H.; Van Lenthe, Joop H.; Havenith, Remco W. A.; Kendrick, John
      Molecular Physics (2005), 103 (6-8), 719-747CODEN: MOPHAM; ISSN:0026-8976. (Taylor & Francis Ltd.)
      A description of the ab initio quantum chem. package GAMESS-UK is presented. The package offers a wide range of quantum mech. wavefunctions, capable of treating systems ranging from closed-shell mols. through to the species involved in complex reaction mechanisms. The availability of a wide variety of correlation methods provides the necessary functionality to tackle a no. of chem. important tasks, ranging from geometry optimization and transition-state location to the treatment of solvation effects and the prediction of excited state spectra. With the availability of relativistic ECPs and the development of ZORA, such calcns. may be performed on the entire Periodic Table, including the lanthanides. Emphasis is given to the DFT module, which has been extensively developed in recent years, and a no. of other, novel features of the program. The parallelization strategy used in the program is outlined, and detailed speedup results are given. Applications of the code in the areas of enzyme and zeolite catalysis and in spectroscopy are described.
    28. 28
      Smith, W.; Yong, C.; Rodger, P. DL_POLY: Application to molecular simulation. Mol. Simul. 2002, 28, 385– 471,  DOI: 10.1080/08927020290018769
      28
      DL_POLY: application to molecular simulation
      Smith, W.; Yong, C. W.; Rodger, P. M.
      Molecular Simulation (2002), 28 (5), 385-471CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
      A review. DL_POLY is a general-purpose mol. dynamics simulation package, which was developed by Daresbury Lab. in the mid-1990s for the mol. simulation community in the United Kingdom. The package now has a world-wide user base and applications in many areas of mol. simulation. In this article we briefly review the history and design of the package and highlight some recent applications in the areas of; liqs. and solns.; spectroscopy; ionic solids; mol. crystals; polymers; glasses; membranes; proteins; solid and liq. interfaces; catalysis; liq. crystals; intercalation and clathrates; and novel systems. The strengths and weaknesses of the code and its future in the near term are also discussed.
    29. 29
      Hill, J. R.; Sauer, J. Molecular mechanics potential for silica and zeolite catalysts based on ab initio calculations. 1. Dense and microporous silica. J. Phys. Chem. 1994, 98, 1238– 1244,  DOI: 10.1021/j100055a032
      29
      Molecular mechanics potential for silica and zeolite catalysts based on ab initio calculations. 1. Dense and microporous silica
      Hill, Joerg R.; Sauer, Joachim
      Journal of Physical Chemistry (1994), 98 (4), 1238-44CODEN: JPCHAX; ISSN:0022-3654.
      A consistent force field for the simulation of aluminum-free zeolite structures is presented. The parameters are derived from results of ab initio calcns. on mol. models which represent typical structural elements of zeolites: SiO4 tetrahedra connected to chains (disilicic and trisilicic acid), rings ([SiO(OH)2]n, n = 3-6), and cages ([SiO3/2(OH)]n, n = 8, 12, 24). These calcns. used a "double zeta + polarization/triple zeta + polarization" basis set. The structures predicted by means of the force field obtained are compared with the results of direct ab initio calcns. of the model mols. and with obsd. structures of dense and microporous silica. The conclusion is reached that it is possible to derive an accurate and transferable force field for mols. and solids solely based on ab initio data for mols.
    30. 30
      Weigend, F.; Ahlrichs, R. Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305,  DOI: 10.1039/b508541a
      30
      Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy
      Weigend, Florian; Ahlrichs, Reinhart
      Physical Chemistry Chemical Physics (2005), 7 (18), 3297-3305CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 mols. representing (nearly) all elements-except lanthanides-in their common oxidn. states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, d. functional theory and correlated methods, for which we had chosen Moller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
    31. 31
      Wilson, P. J.; Bradley, T. J.; Tozer, D. J. Hybrid exchange-correlation functional determined from thermochemical data and ab initio potentials. J. Chem. Phys. 2001, 115, 9233– 9242,  DOI: 10.1063/1.1412605
      31
      Hybrid exchange-correlation functional determined from thermochemical data and ab initio potentials
      Wilson, Philip J.; Bradley, Thomas J.; Tozer, David J.
      Journal of Chemical Physics (2001), 115 (20), 9233-9242CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Multiplicative potentials, appropriate for adding to the non-multiplicative fractional orbital exchange term in the Kohn-Sham equations, are detd. from correlated ab initio electron densities. The potentials are examd. graphically and are used in conjunction with conventional thermochem. data to det. a new hybrid exchange-correlation functional, denoted B97-2. Calcns. using B97-2 are compared with those from (a) the B97-1 functional [J. Chem. Phys. 109, 6264 (1998)], which has the same functional form and fraction of orbital exchange, but was fitted to just thermochem. data; and (b) the widely used B3LYP functional [J. Chem. Phys. 98, 5648 (1993)]. B97-2 atomization energies are close to those from B97-1; total electronic energies and ionization potentials are less accurate, but remain an improvement over B3LYP. Mol. structures from all three functionals are comparable. Static isotropic polarizabilities improve from B3LYP to B97-1 to B97-2; the B97-2 functional underestimates exptl. values, which is consistent with the neglect of zero-point vibrational corrections. NMR shielding consts. - detd. as the conventional second deriv. of the electronic energy - improve from B3LYP to B97-1 to B97-2. Shieldings detd. directly from these DFT electron densities using the recently proposed MKS approach [Chem. Phys. Lett. 337, 341 (2001)] are two to three times more accurate than the conventional shieldings, and exhibit an analogous improvement across the three functionals. Classical reaction barriers for sixteen chem. reactions improve significantly from B3LYP to B97-1 to B97-2. The introduction of multiplicative potentials into semi-empirical hybrid functional development therefore appears beneficial.
    32. 32
      Janssens, T. V.; Falsig, H.; Lundegaard, L. F.; Vennestrøm, P. N.; Rasmussen, S. B.; Moses, P. G.; Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lamberti, C.; Bordiga, S.; Godiksen, A.; Mossin, S.; Beato, P. A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia. ACS Catal. 2015, 5, 2832– 2845,  DOI: 10.1021/cs501673g
      32
      A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia
      Janssens, Ton V. W.; Falsig, Hanne; Lundegaard, Lars F.; Vennestroem, Peter N. R.; Rasmussen, Soeren B.; Moses, Poul Georg; Giordanino, Filippo; Borfecchia, Elisa; Lomachenko, Kirill A.; Lamberti, Carlo; Bordiga, Silvia; Godiksen, Anita; Mossin, Susanne; Beato, Pablo
      ACS Catalysis (2015), 5 (5), 2832-2845CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      For the first time, the std. and fast selective catalytic redn. (SCR) of NO by NH3 are described in a complete catalytic cycle that is able to produce the correct stoichiometry while allowing adsorption and desorption of stable mols. only. The std. SCR reaction is a coupling of the activation of NO by O2 with the fast SCR reaction, enabled by the release of NO2. According to the scheme, the SCR reaction can be divided into an oxidn. of the catalyst by NO + O2 and a redn. by NO + NH3; these steps together constitute a complete catalytic cycle. Furthermore, both NO and NH3 are required in the redn., and finally, oxidn. by NO + O2 or NO2 leads to the same state of the catalyst. These points are shown exptl. for a Cu-CHA catalyst by combining in situ X-ray absorption spectroscopy (XAS), ESR, and Fourier transform IR spectroscopy (FTIR). A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the std. SCR cycle. The activation energy calcd. by d. functional theory (DFT) indicates that the oxidn. of an NO mol. by O2 to a bidentate nitrate ligand is rate-detg. for std. SCR. Finally, the role of a nitrate/nitrite equil. and the possible influence of Cu dimers and Bronsted sites are discussed, and an explanation is offered as to how a catalyst can be effective for SCR while being a poor catalyst for NO oxidn. to NO2.
    33. 33
      Dent, L.; Smith, J. Crystal structure of chabazite, a molecular sieve. Nature 1958, 181, 1794– 1796,  DOI: 10.1038/1811794b0
      33
      Crystal structure of chabazite, a molecular sieve
      Dent, L. S.; Smith, J. V.
      Nature (London, United Kingdom) (1958), 181 (), 1794-6CODEN: NATUAS; ISSN:0028-0836.
      By Fourier methods a new structure for the zeolite chabazite was detd. which explains the x-ray intensities, the sorption, and epitaxial integrowth with the same properties of the mineral gmelinite. The space group is R‾3m, a = 9.40 A., α = 94°18', Z = 2.
    34. 34
      Gale, J. D.; Rohl, A. L. The general utility lattice program (GULP). Mol. Simul. 2003, 29, 291– 341,  DOI: 10.1080/0892702031000104887
      34
      The General Utility Lattice Program (GULP)
      Gale, Julian D.; Rohl, Andrew L.
      Molecular Simulation (2003), 29 (5), 291-341CODEN: MOSIEA; ISSN:0892-7022. (Taylor & Francis Ltd.)
      The General Utility Lattice Program (gulp) has been extended to include the ability to simulate polymers and surfaces, as well as adding many other new features, and the current status of the program is fully documented. Both the background theory is described, as well as providing a concise review of some of the previous applications in order to demonstrate the range of its use. Examples are presented of work performed using the new compatibilities of the software, including the calcn. of Born effective charges, mech. properties as a function of applied pressure, calcn. of frequency-dependent dielec. data, surface reconstructions of calcite and the performance of a linear-scaling algorithm for bond-order potentials.
    35. 35
      Sherwood, P.; de Vries, A. H.; Collins, S. J.; Greatbanks, S. P.; Burton, N. A.; Vincent, M. A.; Hillier, I. H. Computer simulation of zeolite structure and reactivity using embedded cluster methods. Faraday Discuss. 1997, 106, 79– 92,  DOI: 10.1039/a701790a
      35
      Computer simulation of zeolite structure and reactivity using embedded cluster methods
      Sherwood, Paul; De Vries, Alex H.; Collins, Simon J.; Greatbanks, Stephen P.; Burton, Neil A.; Vincent, Mark A.; Hillier, Ian H.
      Faraday Discussions (1997), 106 (Solid State Chemistry: New Opportunities from Computer Simulations), 79-92CODEN: FDISE6; ISSN:0301-7249. (Royal Society of Chemistry)
      The use of bare cluster models to understand the nature of zeolite-substrate interactions may be improved to take account of the environment of the Broensted acid site. We consider two models for introducing the electrostatic effects of the zeolite lattice. The first involves generating a specialized correction potential by fitting a non-periodic array of ca. 60 point charges to the difference between the bare cluster and periodic potentials. The second starts by fitting a periodic array of at. charges to the potential of the infinite lattice and then builds up a classical cluster of ca. 2000 atoms into which the QM cluster is embedded. Such embedded cluster calcns., employing a T3 cluster, with electron correlation at the d. functional theory level, are described, to model the interaction of water at a Bronsted acid site. Structures of the water-zeolite complex, and assocd. vibrational frequencies and 1H NMR shifts are calcd. and compared with calcns. of bare clusters of varying size and with exptl. data. We then describe a mixed quantum mech.-mol. mech. (QM-MM) model derived by combining charges from the second model with a std. aluminosilicate force field. We report preliminary results on the effect of embedding on the energetics of a prototypical hydrocarbon cracking reaction; the methyl-shift reaction of a propenium ion coordinated to the acid site.
    36. 36
      Zhao, Y.; Lynch, B. J.; Truhlar, D. G. Development and assessment of a new hybrid density functional model for thermochemical kinetics. J. Phys. Chem. A 2004, 108, 2715– 2719,  DOI: 10.1021/jp049908s
      36
      Development and Assessment of a New Hybrid Density Functional Model for Thermochemical Kinetics
      Zhao, Yan; Lynch, Benjamin J.; Truhlar, Donald G.
      Journal of Physical Chemistry A (2004), 108 (14), 2715-2719CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      A new hybrid Hartree-Fock-d. functional model called the Becke88-Becke95 1-parameter model for kinetics (BB1K) was optimized against a database of three forward barrier heights, three reverse barrier heights, and three energies of reaction for the reactions in the BH6 representative barrier height database. We then assessed the newly developed BB1K method against a saddle point geometries database, a database of 42 barrier heights, the AE6 representative atomization energy database, a mol. geometries data set, and a set of 13 zero point energies. The results show that BB1K can give excellent saddle point geometries and barrier heights, and its performance for calcg. atomization energies is 40% better than MPW1K. Using a mean mean unsigned error criterion that equally wts. the errors in barrier heights and in bond energies, the new BB1K method outperforms all other DFT and hybrid DFT methods by a large margin, and we therefore conclude that it is the best d. functional-type method for thermochem. kinetics.
    37. 37
      Greenaway, A. G.; Lezcano-Gonzalez, I.; Agote-Aran, M.; Gibson, E. K.; Odarchenko, Y.; Beale, A. M. Operando Spectroscopic Studies of Cu–SSZ-13 for NH3–SCR deNOx Investigates the Role of NH3 in Observed Cu (II) Reduction at High NO Conversions. Top. Catal. 2018, 61, 175– 182,  DOI: 10.1007/s11244-018-0888-3
      37
      Operando Spectroscopic Studies of Cu-SSZ-13 for NH3-SCR deNOx Investigates the Role of NH3 in Observed Cu(II) Reduction at High NO Conversions
      Greenaway, Alex G.; Lezcano-Gonzalez, Ines; Agote-Aran, Miren; Gibson, Emma K.; Odarchenko, Yaroslav; Beale, Andrew M.
      Topics in Catalysis (2018), 61 (3-4), 175-182CODEN: TOCAFI; ISSN:1022-5528. (Springer)
      The small pore zeolite chabazite (SSZ-13) in the copper exchanged form is a very efficient material for the selective catalytic redn. by ammonia (NH3) of nitrogen oxides (NOx) from the exhaust of lean burn engines, typically diesel powered vehicles. The full mechanism occurring during the NH3-SCR process is currently debated with outstanding questions including the nature and role of the catalytically active sites. Time-resolved operando spectroscopic techniques have been used to provide new level of insights in to the mechanism of NH3-SCR, to show that the origin of stable Cu(I) species under SCR conditions is potentially caused by an interaction between NH3 and the Cu cations located in eight ring sites of the bulk of the zeolite and is independent of the NH3-SCR of NOx occurring at Cu six ring sites within the zeolite.
    38. 38
      Greenaway, A. G.; Marberger, A.; Thetford, A.; Lezcano-González, I.; Agote-Arán, M.; Nachtegaal, M.; Ferri, D.; Kröcher, O.; Catlow, C. R. A.; Beale, A. M. Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNOx by modulation excitation IR spectroscopy. Chem. Sci. 2020, 11, 447– 455,  DOI: 10.1039/c9sc04905c
      38
      Detection of key transient Cu intermediates in SSZ-13 during NH3-SCR deNOx by modulation excitation IR spectroscopy
      Greenaway, Alex G.; Marberger, Adrian; Thetford, Adam; Lezcano-Gonzalez, Ines; Agote-Aran, Miren; Nachtegaal, Maarten; Ferri, Davide; Krocher, Oliver; Catlow, C. Richard A.; Beale, Andrew M.
      Chemical Science (2020), 11 (2), 447-455CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      The small pore zeolite Cu-SSZ-13 is an efficient material for the std. selective catalytic redn. of nitrogen oxides (NOx) by ammonia (NH3). In this work, Cu-SSZ-13 has been studied at 250 °C under high conversion using a modulation excitation approach and analyzed with phase sensitive detection (PSD). While the complementary X-ray absorption near edge structure (XANES) spectroscopy measurements showed that the expts. were performed under cyclic Cu+/Cu2+ redox, Diffuse Reflectance IR Fourier Transform Spectroscopy (DRIFTS) expts. provide spectroscopic evidence for previously postulated intermediates Cu-N(=O)-NH2 and Cu-NO3 in the NH3-SCR deNOx mechanism and for the role of [Cu2+(OH-)]+. These results therefore help in building towards a more comprehensive understanding of the reaction mechanism which to date has only been postulated in silico.
    39. 39
      Haszeldine, R.; Jander, J. Further remarks on the spectra of nitrites and nitrosamines. J. Chem. Phys. 1955, 23, 979– 980,  DOI: 10.1063/1.1742168
      39
      The spectra of nitrites and nitrosamines
      Haszeldine, R. N.; Jander, J.
      Journal of Chemical Physics (1955), 23 (), 979-80CODEN: JCPSA6; ISSN:0021-9606.
      cf. preceding abstr. The amt. of EtONO in equil. with AmOH, EtOH, and AmONO (presumably at room temp.) is less than 10%. In liquid dimethylnitrosamine H bonding or assocn. occurs, but it apparently does not occur in CCl4 soln.
    40. 40
      Piskorz, M.; Urbanski, T. Ultraviolet and infrared spectra of some nitrosamines. Bull. Acad. Pol. Sci., Ser. Sci. Chim. 1963, 11, 607– 613
      40
      Ultraviolet and infrared spectra of some nitrosamines
      Piskorz, M.; Urbanski, T.
      Bulletin de l'Academie Polonaise des Sciences, Serie des Sciences Chimiques (1963), 11 (11), 607-13CODEN: BAPCAQ; ISSN:0001-4095.
      Nitrosamines contain 2 absorption bands at about 230 mμ, assigned to N-NO2 and at 345-374 mμ :N-N:0 ⇔ N⊕:N-0⊕ The N:O stretch for nonassocd. mols. is at 1486-1408 cm.-1 and at 1346-1265 and 1321-1292 cm.-1 for assocd. mols., the N-N stretch is at 1106-1052 cm.-1 and the C-N stretch at 1526 and 1118 cm.-1
    41. 41
      Tarte, P. Recherches spectroscopiques sur les composés nitrosés. Bull. Soc. Chim. Belg. 1954, 63, 525– 541,  DOI: 10.1002/bscb.19540630907
      41
      Spectroscopic investigation on nitroso compounds
      Tarte, P.
      Bulletin des Societes Chimiques Belges (1954), 63 (), 525-41CODEN: BSCBAG; ISSN:0037-9646.
      cf. Haszeldine and Jander, C.A. 48, 5646i; 49, 1533d. The visible and infrared spectra of some nitroso compds. have been studied. Chloronitroso compds. of the type, R1R2C(NO)Cl, where R1 and R2 are alkyl, have two infrared bands in the 1600 region (bands given in cm.-1) attributed to the valence vibrations of N::O in the two rotational isomers, as these compds. are dimeric, except CCl3NO which is monomeric and shows only one peak at 1616. Absorption due to C:N valence vibration appears about 1100. The visible spectrum indicates that the valence vibration of N::O in the excited state falls at 1400. The nitroso-dimer compds., (R3CNO)2, have an abnormally low N::O absorption frequency at 1200-1300, which lowering is attributed to N::O assocn. The spectra of dialkyl-nitrosamines in liquid phase have 2 bands, one at 1320 for assocd. N::O and the other at 1440 for unassocd. N::O. In the vapor phase, a single N::O absorption is seen at 1490. These show N:N vibration at 1050-1100 and N:N::O deformation at 650.
    42. 42
      Kedrova, T.; Gafurov, R.; Sogomonyan, E.; Eremenko, L. Spectroscopic study of (nitroalkyl)nitrosamines. Bull. Acad. Sci. USSR, Div. Chem. Sci. 1979, 28, 944– 947,  DOI: 10.1007/bf00963302
      There is no corresponding record for this reference.
    43. 43
      Zapata, F.; García-Ruiz, C. The discrimination of 72 nitrate, chlorate and perchlorate salts using IR and Raman spectroscopy. Spectrochim. Acta, Part A 2018, 189, 535– 542,  DOI: 10.1016/j.saa.2017.08.058
      43
      The discrimination of 72 nitrate, chlorate and perchlorate salts using IR and Raman spectroscopy
      Zapata, Felix; Garcia-Ruiz, Carmen
      Spectrochimica Acta, Part A: Molecular and Biomolecular Spectroscopy (2018), 189 (), 535-542CODEN: SAMCAS; ISSN:1386-1425. (Elsevier B.V.)
      Inorg. oxidizing energetic salts including nitrates, chlorates and perchlorates are widely used in the manuf. of not only licit pyrotechnic compns., but also illicit homemade explosive mixts. Their identification in forensic labs. is usually accomplished by either capillary electrophoresis or ion chromatog., with the disadvantage of dissocg. the salt into its ions. On the contrary, vibrational spectroscopy, including IR and Raman, enables the non-invasive identification of the salt, i.e. avoiding its dissocn. This study focuses on the discrimination of all nitrate, chlorate and perchlorate salts that are com. available, using both Raman and IR spectroscopy, with the aim of testing whether every salt can be unequivocally identified. Besides the visual spectra comparison by assigning every band with the corresponding mol. vibrational mode, a statistical anal. based on Pearson correlation was performed to ensure an objective identification, either using Raman, IR or both. Pos., 25 salts (out of 72) were unequivocally identified using Raman, 30 salts when using IR and 44 when combining both techniques. Neg., some salts were undistinguishable even using both techniques demonstrating there are some salts that provide very similar Raman and IR spectra.
    44. 44
      Volod’ko, L.; Huoah, L. T. The vibrational spectra of aqueous nitrate solutions. J. Appl. Spectrosc. 1968, 9, 1100– 1104,  DOI: 10.1007/bf01260968
      There is no corresponding record for this reference.
    45. 45
      Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; Thibault-Starzyk, F. Probing zeolites by vibrational spectroscopies. Chem. Soc. Rev. 2015, 44, 7262– 7341,  DOI: 10.1039/c5cs00396b
      45
      Probing zeolites by vibrational spectroscopies
      Bordiga, Silvia; Lamberti, Carlo; Bonino, Francesca; Travert, Arnaud; Thibault-Starzyk, Frederic
      Chemical Society Reviews (2015), 44 (20), 7262-7341CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Bronsted and Lewis acidity and surface basicity are treated in detail. The role of probe mols. and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely det. the IR absorption coeffs. is given, making IR a quant. technique. The thermodn. parameters of the adsorption process that can be extd. from a variable-temp. IR study are described. Finally, cutting-edge space- and time-resolved expts. are reviewed. All aspects are discussed by reporting relevant examples. When available, the theor. literature related to the reviewed exptl. results is reported to support the interpretation of the vibrational spectra on an at. level.
    46. 46
      Negri, C.; Hammershøi, P. S.; Janssens, T. V.; Beato, P.; Berlier, G.; Bordiga, S. Investigating the Low Temperature Formation of Cu II -(N,O) Species on Cu-CHA Zeolites for the Selective Catalytic Reduction of NO x. Chem.─Eur. J. 2018, 24, 12044– 12053,  DOI: 10.1002/chem.201802769
      46
      Investigating the Low Temperature Formation of CuII-(N,O) Species on Cu-CHA Zeolites for the Selective Catalytic Reduction of NOx
      Negri, Chiara; Hammershoi, Peter S.; Janssens, Ton V. W.; Beato, Pablo; Berlier, Gloria; Bordiga, Silvia
      Chemistry - A European Journal (2018), 24 (46), 12044-12053CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      In this work, we show the potentiality of operando FTIR spectroscopy to follow the formation of CuII-(N,O) species on Cu exchanged chabazite zeolites (Cu-CHA), active for the selective catalytic redn. of NOx with NH3 (NH3-SCR). In particular, we investigated the reaction of NO and O2 at low temp. (200 and 50°C) on a series of Cu-CHA zeolites with different compn. (Si/Al and Cu/Al ratios), to investigate the nature of the formed copper nitrates, which have been proposed to be key intermediates in the oxidn. part of the SCR cycle. Our results show that chelating bidentate nitrates are the main structures formed at 200°C. At lower temp. a mixt. of chelating and monodentate nitrates are formed, together with the nitrosonium ion NO+, whose amt. was found to be proportional to the zeolite Bronsted site concn. Nitrates were found to mainly form with CuII ions stabilized by one neg. framework charge (Z), Z-[Cu(OH]I or Z-[Cu(O2]I, without involvement of Z2-CuII ones. This evidence, together with the absence of bridging nitrates in samples with high probability for Cu-Cu pairs, indicate that the nitrate ligands are not able to mobilize copper ions, at variance with what recently reported for NH3. Finally, water was found to replace preformed chelating copper nitrates and deplete NO+ (though with different kinetics) at both temps., while favoring the presence of monodentate ones.
    47. 47
      Hadjiivanov, K. I. Identification of neutral and charged N x O y surface species by IR spectroscopy. Catal. Rev. 2000, 42, 71– 144,  DOI: 10.1081/cr-100100260
      47
      Identification of neutral and charged NxOy surface species by IR spectroscopy
      Hadjiivanov, Konstantin I.
      Catalysis Reviews - Science and Engineering (2000), 42 (1 & 2), 71-144CODEN: CRSEC9; ISSN:0161-4940. (Marcel Dekker, Inc.)
      Review of the available IR spectral data and the assignments on the NxOy adspecies, paying special attention to oxide surfaces, with 268 refs. The IR spectral performance of the NxOy species obsd. on oxide surfaces [N2O, NO-, NO, (NO)2, N2O3, NO+, NO2- (different nitro and nitrito anions), NO2, N2O4, N2O5, NO2+, and NO3- (bridged, bidentate, and monodentate nitrates)] is considered. The spectra of related compds. (N2, H-, and C-contg. nitrogen oxo species, C-N species, NHx species) are also briefly discussed. Some guidelines for spectral identification of NxOy adspecies are proposed and the transformation of the nitrogen oxo species on catalyst surfaces are regarded.
    48. 48
      Yao, X.; Ma, K.; Zou, W.; He, S.; An, J.; Yang, F.; Dong, L. Influence of preparation methods on the physicochemical properties and catalytic performance of MnO -CeO2 catalysts for NH3-SCR at low temperature. Chin. J. Catal. 2017, 38, 146– 159,  DOI: 10.1016/s1872-2067(16)62572-x
      48
      Influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for NH3-SCR at low temperature
      Yao, Xiaojiang; Ma, Kaili; Zou, Weixin; He, Shenggui; An, Jibin; Yang, Fumo; Dong, Lin
      Chinese Journal of Catalysis (2017), 38 (1), 146-159CODEN: CJCHCI ISSN:. (Science Press)
      This work examd. the influence of prepn. methods on the physicochem. properties and catalytic performance of MnOx-CeO2 catalysts for selective catalytic redn. of NO by NH3 (NH3-SCR) at low temp. Five different methods, namely, mech. mixing, impregnation, hydrothermal treatment, co-pptn., and a sol-gel technique, were used to synthesize MnOx-CeO2 catalysts. The catalysts were characterized in detail, and an NH3-SCR model reaction was chosen to evaluate the catalytic performance. The results showed that the prepn. methods affected the catalytic performance in the order: hydrothermal treatment > sol-gel > co-pptn. > impregnation > mech. mixing. This order correlated with the surface Ce3+ and Mn4+ contents, oxygen vacancies and surface adsorbed oxygen species concn., and the amt. of acidic sites and acidic strength. This trend was related to redox interactions between MnOx and CeO2. The catalyst formed by a hydrothermal treatment exhibited excellent physicochem. properties, optimal catalytic performance, and good H2O resistance in NH3-SCR reaction. This was attributed to incorporation of Mnn+ into the CeO2 lattice to form a uniform ceria-based solid soln. (contg. Mn-O-Ce structures). Strengthening of the electronic interactions between MnOx and CeO2, driven by the high-temp. and high-pressure conditions during the hydrothermal treatment also improved the catalyst characteristics. Thus, the hydrothermal treatment method was an efficient and environment-friendly route to synthesizing low-temp. denitrification (deNOx) catalysts.
    49. 49
      Giordanino, F.; Borfecchia, E.; Lomachenko, K. A.; Lazzarini, A.; Agostini, G.; Gallo, E.; Soldatov, A. V.; Beato, P.; Bordiga, S.; Lamberti, C. Interaction of NH3 with Cu-SSZ-13 catalyst: a complementary FTIR, XANES, and XES study. J. Phys. Chem. Lett. 2014, 5, 1552– 1559,  DOI: 10.1021/jz500241m
      49
      Interaction of NH3 with Cu-SSZ-13 Catalyst: A Complementary FTIR, XANES, and XES Study
      Giordanino, Filippo; Borfecchia, Elisa; Lomachenko, Kirill A.; Lazzarini, Andrea; Agostini, Giovanni; Gallo, Erik; Soldatov, Alexander V.; Beato, Pablo; Bordiga, Silvia; Lamberti, Carlo
      Journal of Physical Chemistry Letters (2014), 5 (9), 1552-1559CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      In the typical NH3-SCR temp. range (100-500°), ammonia is one of the main adsorbed species on acidic sites of Cu-SSZ-13 catalyst. The study of adsorbed ammonia at high temp. is a key step for the understanding of its role in the NH3-SCR catalytic cycle. Different spectroscopic techniques were employed to investigate the nature of the different complexes occurring upon NH3 interaction. In particular, FTIR spectroscopy revealed the formation of different NH3 species, i.e., (i) NH3 bonded to copper centers, (ii) NH3 bonded to Bronsted sites, and (iii) NH4+·nNH3 assocns. XANES and XES spectroscopy allowed us to get an insight into the geometry and electronic structure of Cu centers upon NH3 adsorption, revealing for the first time in Cu-SSZ-13 the presence of linear Cu+ species in Ofw-Cu-NH3 or H3N-Cu-NH3 configuration.
    50. 50
      Lezcano-Gonzalez, I.; Deka, U.; Arstad, B.; Van Yperen-De Deyne, A.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M. Determining the storage, availability and reactivity of NH3within Cu-Chabazite-based Ammonia Selective Catalytic Reduction systems. Phys. Chem. Chem. Phys. 2014, 16, 1639– 1650,  DOI: 10.1039/c3cp54132k
      50
      Determining the storage, availability and reactivity of NH3 within Cu-Chabazite-based ammonia selective catalytic reduction systems
      Lezcano-Gonzalez, I.; Deka, U.; Arstad, B.; Van Yperen-De Deyne, A.; Hemelsoet, K.; Waroquier, M.; Van Speybroeck, V.; Weckhuysen, B. M.; Beale, A. M.
      Physical Chemistry Chemical Physics (2014), 16 (4), 1639-1650CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Three different types of NH3 species can be simultaneously present on Cu2+-exchanged CHA-type zeolites, commonly used in Ammonia Selective Catalytic Redn. (NH3-SCR) systems. These include ammonium ions (NH4+), formed on the Bronsted acid sites, [Cu(NH3)4]2+ complexes, resulting from NH3 coordination with the Cu2+ Lewis sites, and NH3 adsorbed on extra-framework Al (EFAl) species, in contrast to the only two reacting NH3 species recently reported on Cu-SSZ-13 zeolite. The NH4+ ions react very slowly in comparison to NH3 coordinated to Cu2+ ions and are likely to contribute little to the std. NH3-SCR process, with the Bronsted groups acting primarily as NH3 storage sites. The availability/reactivity of NH4+ ions can be however, notably improved by submitting the zeolite to repeated exchanges with Cu2+, accompanied by a remarkable enhancement in the low temp. activity. The presence of EFAl species could also have a pos. influence on the reaction rate of the available NH4+ ions. These results have important implications for NH3 storage and availability in Cu-Chabazite-based NH3-SCR systems.
    51. 51
      Paolucci, C.; Khurana, I.; Parekh, A. A.; Li, S.; Shih, A. J.; Li, H.; Di Iorio, J. R.; Albarracin-Caballero, J. D.; Yezerets, A.; Miller, J. T.; Delgass, W. N.; Ribeiro, F. H.; Schneider, W. F.; Gounder, R. Dynamic multinuclear sites formed by mobilized copper ions in NO x selective catalytic reduction. Science 2017, 357, 898– 903,  DOI: 10.1126/science.aan5630
      51
      Dynamic multinuclear sites formed by mobilized copper ions in NOx selective catalytic reduction
      Paolucci, Christopher; Khurana, Ishant; Parekh, Atish A.; Li, Sichi; Shih, Arthur J.; Li, Hui; Di Iorio, John R.; Albarracin-Caballero, Jonatan D.; Yezerets, Aleksey; Miller, Jeffrey T.; Delgass, W. Nicholas; Ribeiro, Fabio H.; Schneider, William F.; Gounder, Rajamani
      Science (Washington, DC, United States) (2017), 357 (6354), 898-903CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Copper ions exchanged into zeolites are active for the selective catalytic redn. (SCR) of nitrogen oxides (NOx) with ammonia (NH3), but the low-temp. rate dependence on copper (Cu) volumetric d. is inconsistent with reaction at single sites. We combine steady-state and transient kinetic measurements, x-ray absorption spectroscopy, and first-principles calcns. to demonstrate that under reaction conditions, mobilized Cu ions can travel through zeolite windows and form transient ion pairs that participate in an oxygen (O2)-mediated CuI→CuII redox step integral to SCR. Electrostatic tethering to framework aluminum centers limits the vol. that each ion can explore and thus its capacity to form an ion pair. The dynamic, reversible formation of multinuclear sites from mobilized single atoms represents a distinct phenomenon that falls outside the conventional boundaries of a heterogeneous or homogeneous catalyst.
    52. 52
      Millan, R.; Cnudde, P.; Hoffman, A. E.; Lopes, C. W.; Concepción, P.; van Speybroeck, V.; Boronat, M. Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3-SCR-NOx) Conditions. J. Phys. Chem. Lett. 2020, 11, 10060– 10066,  DOI: 10.1021/acs.jpclett.0c03020
      52
      Theoretical and Spectroscopic Evidence of the Dynamic Nature of Copper Active Sites in Cu-CHA Catalysts under Selective Catalytic Reduction (NH3-SCR-NOx) Conditions
      Millan, Reisel; Cnudde, Pieter; Hoffman, Alexander E. J.; Lopes, Christian W.; Concepcion, Patricia; van Speybroeck, Veronique; Boronat, Mercedes
      Journal of Physical Chemistry Letters (2020), 11 (23), 10060-10066CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      The dynamic nature of the copper cations acting as active sites for selective catalytic redn. of nitrogen oxides with ammonia is investigated using a combined theor. and spectroscopic approach. Ab initio mol. dynamics simulations of Cu-CHA catalysts in contact with reactants and intermediates at realistic operating conditions show that only ammonia is able to release Cu+ and Cu2+ cations from their positions coordinated to the zeolite framework, forming mobile Cu+(NH3)2 and Cu2+(NH3)4 complexes that migrate to the center of the cavity. Herein, we give evidence that such mobilization of copper cations modifies the vibrational fingerprint in the 800-1000 cm-1 region of the IR spectra. Bands assocd. with the lattice asym. T-O-T vibrations are perturbed by the presence of coordinated cations, and allow one to exptl. follow the dynamic reorganization of the active sites at operating conditions.
    53. 53
      Gao, F.; Mei, D.; Wang, Y.; Szanyi, J.; Peden, C. H. Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis. J. Am. Chem. Soc. 2017, 139, 4935– 4942,  DOI: 10.1021/jacs.7b01128
      53
      Selective Catalytic Reduction over Cu/SSZ-13: Linking Homo- and Heterogeneous Catalysis
      Gao, Feng; Mei, Donghai; Wang, Yilin; Szanyi, Janos; Peden, Charles H. F.
      Journal of the American Chemical Society (2017), 139 (13), 4935-4942CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Active centers in Cu/SSZ-13 selective catalytic redn. (SCR) catalysts have been recently identified as isolated Cu2+ and [CuII(OH)]+ ions. A redox reaction mechanism has also been established, where Cu ions cycle between CuI and CuII oxidn. states during SCR reaction. While the mechanism for the redn. half-cycle (CuII → CuI) is reasonably well-understood, that for the oxidn. half-cycle (CuI → CuII) remains an unsettled debate. Herein we report detailed reaction kinetics on low-temp. std. NH3-SCR, supplemented by DFT calcns., as strong evidence that the low-temp. oxidn. half-cycle occurs with the participation of two isolated CuI ions via formation of a transient [CuI(NH3)2]+-O2-[CuI(NH3)2]+ intermediate. The feasibility of this reaction mechanism is confirmed from DFT calcns., and the simulated energy barrier and rate consts. are consistent with exptl. findings. Significantly, the low-temp. std. SCR mechanism proposed here provides full consistency with low-temp. SCR kinetics.
    54. 54
      Chen, L.; Falsig, H.; Janssens, T. V.; Grönbeck, H. Activation of oxygen on (NH3Cu NH3)+ in NH3-SCR over Cu-CHA. J. Catal. 2018, 358, 179– 186,  DOI: 10.1016/j.jcat.2017.12.009
      54
      Activation of oxygen on (NH3-Cu-NH3)+ in NH3-SCR over Cu-CHA
      Chen, Lin; Falsig, Hanne; Janssens, Ton V. W.; Groenbeck, Henrik
      Journal of Catalysis (2018), 358 (), 179-186CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Inc.)
      Cu-CHA materials are efficient catalysts for NH3-SCR of NOx in O excess. A crucial step in the reaction is O (O2) activation, which still is not well understood. D. functional theory calcns. in combination with ab initio thermodn. and mol. dynamics are here used to study O2 dissocn. on Cu(NH3)+2 species, which are present under NH3-SCR conditions. Direct dissocn. of O2 is facile over a pair of Cu(NH3)+2 complexes whereas dissocn. on a single Cu(NH3)+2 species is unlikely due to a high activation energy. The presence of NO promotes O dissocn. on both single and pairs of Cu(NH3)+2 complexes. Nitrites and nitrates are easily formed as O2 dissocs., and NO adsorption over nitrates leads to facile formation of NO2. The results stress the importance of ligand-stabilized Cu species in Cu-CHA catalysts for NH3-SCR.
    55. 55
      Moreno-González, M.; Millán, R.; Concepción, P.; Blasco, T.; Boronat, M. Spectroscopic Evidence and Density Functional Theory (DFT) Analysis of Low-Temperature Oxidation of Cu+ to Cu2+ NO x in Cu-CHA Catalysts: Implications for the SCR-NO x Reaction Mechanism. ACS Catal. 2019, 9, 2725– 2738,  DOI: 10.1021/acscatal.8b04717
      55
      Spectroscopic Evidence and Density Functional Theory (DFT) Analysis of Low-Temperature Oxidation of Cu+ to Cu2+NOx in Cu-CHA Catalysts: Implications for the SCR-NOx Reaction Mechanism
      Moreno-Gonzalez, Marta; Millan, Reisel; Concepcion, Patricia; Blasco, Teresa; Boronat, Mercedes
      ACS Catalysis (2019), 9 (4), 2725-2738CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Despite the intense study on the NH3-SCR-NOx reaction mechanism catalyzed by small pore Cu-CHA zeolites, neither the rate-detg. step of the process nor the exact nature of the active sites under reaction conditions are clearly established. In situ EPR and IR techniques combined with DFT calcns. are applied to the study of the oxidn. half-cycle of the NH3-SCR-NOx reaction on Cu-SSZ-13 and Cu-SAPO-34 catalysts. EPR and IR spectroscopies unambiguously show that Cu+ is oxidized to Cu2+ at room temp. in the presence of the reaction mixt. (NO, O2, and NH3) or NO and O2, producing adsorbed NO2, nitrites, and nitrates. Several pathways are proposed from DFT calcns. to oxidize Cu+ cations placed in the plane of the 6R ring units of SSZ-13 and SAPO-34 to Cu2+, either by NO2 alone or by a mixt. of NO and O2, with activation energy barriers <70 kJ mol-1. A reaction mechanism invoking the formation of nitrate/nitrite intermediates on Cu cations attached to the zeolite framework can be operational in the low-temp. region (T < 350°). Different intermediates, nitrites vs. nitrates, are preferentially stabilized, depending on the catalyst compn., silicoaluminophosphate vs. aluminosilicate.
    56. 56
      Wilken, N.; Kamasamudram, K.; Currier, N. W.; Li, J.; Yezerets, A.; Olsson, L. Heat of adsorption for NH3, NO2 and NO on Cu-Beta zeolite using microcalorimeter for NH3 SCR applications. Catal. Today 2010, 151, 237– 243,  DOI: 10.1016/j.cattod.2010.02.002
      56
      Heat of adsorption for NH3, NO2 and NO on Cu-Beta zeolite using microcalorimeter for NH3 SCR applications
      Wilken, Norman; Kamasamudram, Krishna; Currier, Neal W.; Li, Junhui; Yezerets, Aleksey; Olsson, Louise
      Catalysis Today (2010), 151 (3-4), 237-243CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      Micro-calorimetry is a powerful technique to measure the heat of adsorption (ΔH), producing values which are very important when developing kinetic models. The method provides a way to det. these parameters independently. For kinetic models describing NH3 selective catalytic redn. (SCR), it is crit. to accurately describe NH3 and NOx storage to simulate rapid transients occurring in expts. This work measured ΔH of NH3, NO2, and NO on Cu-β zeolites. An NH3 temp.-programmed desorption (TPD) expt. was conducted at 150° using the micro-calorimeter to observe an exotherm when introducing NH3 due to adsorption. This resulted in an av. ΔH of -100 kJ/mol. Good reproducibility was obsd. when using a second sample, resulting in -97 kJ/mol. To assess the coverage dependence of ΔH, an NH3 step-wise expt. was conducted. First, the catalyst was exposed to NH3 at 500°, resulting in the adsorption of strongly bound NH3 and obtaining a ΔH of -110 kJ/mol; thereafter, the catalyst was cooled in Ar and at 400° and NH3 was re-introduced. Since this temp. was lower, the NH3 which adsorbed was weaker. The procedure was repeated at 300, 200, and 100°, resulting in a coverage-dependent activation energy for NH3 desorption (assuming 0 activation for adsorption) according to this formula: NH3 desorption energy = 120.0(1 - 0.38θNH3), where θNH3 is the NH3 coverage on the surface. NO and NO2 adsorption/desorption were assessed using NO and NO2 TPD expts., resp. For the NO2 TPD expt., ∼3 NO2 were stored for each NO produced, corresponding to the disproportionation mechanism. This resulted in ΔH of -65 kJ/mol-NO2 consumed. The NO TPD expt. showed only small amts. of NO were adsorbed.
    57. 57
      Olsson, L.; Wijayanti, K.; Leistner, K.; Kumar, A.; Joshi, S. Y.; Kamasamudram, K.; Currier, N. W.; Yezerets, A. A multi-site kinetic model for NH3-SCR over Cu/SSZ-13. Appl. Catal., B 2015, 174–175, 212– 224,  DOI: 10.1016/j.apcatb.2015.02.037
      57
      A multi-site kinetic model for NH3-SCR over Cu/SSZ-13
      Olsson, Louise; Wijayanti, Kurnia; Leistner, Kirsten; Kumar, Ashok; Joshi, Saurabh Y.; Kamasamudram, Krishna; Currier, Neal W.; Yezerets, Aleksey
      Applied Catalysis, B: Environmental (2015), 174-175 (), 212-224CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      In this study, we have developed a kinetic model for ammonia-SCR over a well-characterized Cu/SSZ-13 catalyst. It was found that a three-site model was needed in order to describe the ammonia temp. programmed desorption (TPD) with adsorption at 50 and 150 °C as well as ammonia oxidn., and NH3-SCR up to 600 °C. Based on literature studies, where detailed characterization of Cu/SSZ-13 have been conducted using several exptl. techniques, we suggest the following phys. interpretation of the S1 and S2 sites in the model. The S1 sites are assocd. with copper located in a six-membered ring, possibly slightly distorted due to interactions with water and ammonia while, the S2 sites represent copper in the large cages or CuxOy species. In addn., ammonia is also stored on Br.ovrddot.onsted acid sites, but in order not to complicate the model further, it was lumped together in the S1 and S2 sites. Finally, S3 sites have been added in order to describe the large amt. of physisorbed ammonia at low temp. This three-site model was capable of adequately describing the ammonia TPD expts. with the initial temp. of 50 and 150 °C. The heats of adsorption of ammonia on the S1 and S2 sites were detd. using micro-calorimeter expts. Further, the main SCR reaction in the model occurs on S1 sites and the main ammonia oxidn. reaction on S2 sites. However, due to the complex behavior assocd. with ammonia oxidn., where the conversion slightly decreased when the temp. was increased from 350 to 400 °C, an ammonia oxidn. reaction occurring at low temp. with low rate needed to be introduced on S1. In a similar way, an added step was needed for ammonia-SCR on S2, which occurred at high temp. where the ammonia coverage on S1 was low resulting in low conversion. To summarize, the three-site model developed was capable of well describing the ammonia storage and release, ammonia oxidn. as well as SCR and N2O formation across a broad temp. interval (100-600 °C).
    58. 58
      Krossner, M.; Sauer, J. Interaction of Water with Brønsted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1:1 and 2:1 Surface Complexes. J. Phys. Chem. 1996, 100, 6199– 6211,  DOI: 10.1021/jp952775d
      58
      Interaction of Water with Broensted Acidic Sites of Zeolite Catalysts. Ab Initio Study of 1:1 and 2:1 Surface Complexes
      Krossner, Mariann; Sauer, Joachim
      Journal of Physical Chemistry (1996), 100 (15), 6199-211CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      The adsorption of one and two water mols. on cluster models of Broensted acid sites of zeolite catalysts has been investigated by ab initio quantum chem. methods at the Hartree-Fock SCF (HF), at the second-order Moeller-Plesset perturbation theory (MP2), and at the d. functional theory (DFT) levels. Among the two possible structures of the 1:1 adsorption complex, the water H-bonded to the zeolitic OH group (neutral complex) and the hydroxonium ion attached to the neg. charged zeolite surface (ion pair), only the former is a min. The ion pair complex is a transition structure for the proton transfer from one lattice oxygen to a neighboring one via the adsorbed water. However, the energy difference between both structures is only a few kJ/mol. For the neutral 1:1 adsorption complex we predict an av. shift of the three protons involved of 7-8 ppm; the obsd. shifts are 6-7 ppm for one water mol. per site. The vibrational frequencies calcd. for the ion pair structure do not permit an interpretation of the obsd. IR spectrum. For the neutral structure (MP2) we predict frequencies of 1317 and 1022 cm-1 for the zeolitic in-plane and out-of-plane modes, resp., while the zeolitic OH stretching mode is strongly red-shifted down to 2740-2850 cm-1. These data support a recent interpretation of the IR spectrum which explains the obsd. triplet of bands as a result of Fermi resonance between the strongly perturbed zeolitic OH stretch and the OH bending overtones. The MP2 calcns. for the neutral complex also provide a complex assignment of the peaks obsd. by inelastic neutron scattering for water on H-mordenite. Inclusion of electron correlation proves crucial, and comparison of MP2 and DFT (gradient cor. functionals) methods is made. While energy differences are very similar, the DFT approach yields by far too large frequency shifts for OH donor groups in H bonds. When a second water mol. is added (2:1 complex), both the neutral and the ion pair structure prove to be local min. on the potential energy surface. The adsorption energy is found to drop by 25%, and the ion pair structure becomes the more stable one. Predictions are made on how the vibrational spectra and the 1H NMR chem. shifts change.
    59. 59
      Jobic, H.; Tuel, A.; Krossner, M.; Sauer, J. Water in interaction with acid sites in H-ZSM-5 zeolite does not form hydroxonium ions. A comparison between neutron scattering results and ab initio calculations. J. Phys. Chem. 1996, 100, 19545– 19550,  DOI: 10.1021/jp9619954
      59
      Water in Interaction with Acid Sites in H-ZSM-5 Zeolite Does Not Form Hydroxonium Ions. A Comparison between Neutron Scattering Results and ab Initio Calculations
      Jobic, Herve; Tuel, Alain; Krossner, Mariann; Sauer, Joachim
      Journal of Physical Chemistry (1996), 100 (50), 19545-19550CODEN: JPCHAX; ISSN:0022-3654. (American Chemical Society)
      Inelastic neutron scattering (INS) was used to study the adsorption of H2O (at different concns.) in HZSM 5. INS is the only vibrational technique where the intensities can be calcd. with reasonable accuracy from at. displacements. This feature was used to simulate the INS spectra of the 2 possible structures resulting from H2O interaction with the Broensted acid sites of the zeolite: H-bonded H2O or hydroxonium ion. The at. displacements for the 2 structures are derived from recent ab initio MP2 calcns. (M. Krossner and J. Sauer, 1996). The comparison between exptl. and calcd. INS spectra confirms that the 1st H2O mol. is attached to the acid site via 2 H bonds (in agreement with the conclusion of Krossner and Sauer). Hydroxonium ions are not found in HZSM 5; however, this protonated species might be present in zeolites with a different structure.
    60. 60
      Parrillo, D.; Lee, C.; Gorte, R. Heats of adsorption for ammonia and pyridine in H-ZSM-5: evidence for identical Brønsted-acid sites. Appl. Catal., A 1994, 110, 67– 74,  DOI: 10.1016/0926-860x(94)80106-1
      60
      Heats of adsorption for ammonia and pyridine in H-ZSM-5: evidence for identical Bronsted-acid sites
      Parrillo, D. J.; Lee, C.; Gorte, R. J.
      Applied Catalysis, A: General (1994), 110 (1), 67-74CODEN: ACAGE4; ISSN:0926-860X.
      The authors used microcalorimetry to examine the stoichiometric adsorption complexes formed by NH3 and pyridine at the Broensted acid sites in 3 samples of HZSM 5 which vary widely in Al content and synthesis methods. The heats of adsorption for both NH3 and pyridine are const. up to a coverage of 1 mol./Al, independent of Si/Al ratio, with values of 145 kJ/mol for NH3 and 200 ± 5 kJ/mol for pyridine. These results suggest that the Broensted acid sites in unsteamed HZSM 5 are independent of sample and equal in concn. to the framework Al content.
    61. 61
      Mei, D.; Lercher, J. A. Mechanistic insights into aqueous phase propanol dehydration in H-ZSM-5 zeolite. AIChE J. 2017, 63, 172– 184,  DOI: 10.1002/aic.15517
      61
      Mechanistic insights into aqueous phase propanol dehydration in H-ZSM-5 zeolite
      Mei, Donghai; Lercher, Johannes A.
      AIChE Journal (2017), 63 (1), 172-184CODEN: AICEAC; ISSN:0001-1541. (John Wiley & Sons, Inc.)
      Aq. phase dehydration of 1-propanol over H-ZSM-5 zeolite was investigated using d. functional theory (DFT) calcns. The water mols. in the zeolite pores prefer to aggregate via the hydrogen bonding network and be protonated at the Bronsted acidic sites (BAS). Two typical configurations, i.e., dispersed and clustered, of water mols. were identified by ab initio mol. dynamics simulations of the mimicking aq. phase H-ZSM-5 unit cell with 20 water mols. per unit cell. DFT calcd. Gibbs free energies suggest that the dimeric propanol-propanol, the propanol-water, and the trimeric propanol-propanol-water complexes are formed at high propanol concns. in aq. phase, which provide a kinetically feasible dehydration reaction channel of 1-propanol to propene. The calcn. results indicate that the propanol dehydration via the unimol. mechanism becomes kinetically discouraged due to the enhanced stability of the protonated dimeric propanol and the protonated water cluster acting as the BAS site for alc. dehydration. © 2016 American Institute of Chem. Engineers AIChE J, 2016.
    62. 62
      Tuma, C.; Sauer, J. A hybrid MP2/planewave-DFT scheme for large chemical systems: proton jumps in zeolites. Chem. Phys. Lett. 2004, 387, 388– 394,  DOI: 10.1016/j.cplett.2004.02.056
      62
      A hybrid MP2/planewave-DFT scheme for large chemical systems. Proton jumps in zeolites
      Tuma, Christian; Sauer, Joachim
      Chemical Physics Letters (2004), 387 (4-6), 388-394CODEN: CHPLBC; ISSN:0009-2614. (Elsevier Science B.V.)
      We present an embedding scheme to introduce local corrections at post Hartree-Fock level to d. functional theory (DFT) calcns. As a first application we study proton jump reactions in the zeolite HSSZ-13 and show that energy barriers and rate consts. are significantly changed by second-order Moller-Plesset perturbation theory (MP2) corrections to plane wave based DFT calcns. Electronic energy barriers increase from 68 to 81 kJ/mol (dry zeolite), and from 22 to 30 kJ/mol (hydrated zeolite). The predicted heats of adsorption of one water mol. onto the Bronsted acidic sites O1 and O2 are 73 and 69 kJ/mol, resp.
    63. 63
      Gao, F.; Kwak, J. H.; Szanyi, J.; Peden, C. H. Current understanding of Cu-exchanged chabazite molecular sieves for use as commercial diesel engine DeNO x catalysts. Top. Catal. 2013, 56, 1441– 1459,  DOI: 10.1007/s11244-013-0145-8
      63
      Current Understanding of Cu-Exchanged Chabazite Molecular Sieves for Use as Commercial Diesel Engine DeNOx Catalysts
      Gao, Feng; Kwak, Ja Hun; Szanyi, Janos; Peden, Charles H. F.
      Topics in Catalysis (2013), 56 (15-17), 1441-1459CODEN: TOCAFI; ISSN:1022-5528. (Springer)
      A review. Selective catalytic redn. (SCR) of NOx with ammonia using metal-exchanged mol. sieves with a chabazite structure has recently been commercialized on diesel vehicles. One of the commercialized catalysts, i.e., Cu-SSZ-13, has received much attention for both practical and fundamental studies. For the latter, the particularly well-defined structure of this zeolite is allowing long-standing issues of the catalytically active site for SCR in metal-exchanged zeolites to be addressed. In this review, recent progress is summarized with a focus on two areas. First, the tech. significance of Cu-SSZ-13 as compared to other Cu ion-exchanged zeolites (e.g., Cu-ZSM-5 and Cu-beta) is highlighted. Specifically, the much enhanced hydrothermal stability for Cu-SSZ-13 compared to other zeolite catalysts is addressed via performance measurements and catalyst characterization using several techniques. The enhanced stability of Cu-SSZ-13 is rationalized in terms of the unique small pore structure of this zeolite catalyst. Second, the fundamentals of the catalytically active center; i.e., the chem. nature and locations within the SSZ-13 framework are presented with an emphasis on understanding structure-function relations. For the SCR reaction, traditional kinetic studies are complicated by intra-cryst. diffusion limitations. However, a major side reaction, non-selective ammonia oxidn. by oxygen, does not suffer from mass-transfer limitations at relatively low temps. due to significantly lower reaction rates. This allows structure-function relations that are rather well understood in terms of Cu ion locations and redox properties. Finally, some aspects of the SCR reaction mechanism are addressed from in situ spectroscopic studies.
    64. 64
      Bendrich, M.; Scheuer, A.; Hayes, R.; Votsmeier, M. Unified mechanistic model for Standard SCR, Fast SCR, and NO 2 SCR over a copper chabazite catalyst. Appl. Catal., B 2018, 222, 76– 87,  DOI: 10.1016/j.apcatb.2017.09.069
      64
      Unified mechanistic model for Standard SCR, Fast SCR, and NO2 SCR over a copper chabazite catalyst
      Bendrich, M.; Scheuer, A.; Hayes, R. E.; Votsmeier, M.
      Applied Catalysis, B: Environmental (2018), 222 (), 76-87CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      Mechanistic proposals for the different selective catalytic redn. (SCR) sub-reactions were integrated into one surface reaction mechanism which describes the main SCR reactions (std. SCR, fast SCR, NO2 SCR), transient effects due to NO3- storage, and prodn. of the N2O byproduct over a copper chabazite catalyst. The mechanism was parameterized to steady-state and transient expts., and was shown to predict catalyst behavior during a driving cycle without any kinetic parameters re-fitting. A dual site approach was used, where site 1 accounted for adsorbed NH3 which forms on Bronsted acid sites and Cu2+; site 2 was a copper ion (Cu2+-OH) where NO2- and NO3- were adsorbed. All main SCR reactions proceeded by a reaction between NH3 and NO2- (NH4NO2 pathway) to produce N2; NO2- were also the linking species between std. SCR and NO oxidn. reactions. Reactions between NO3- and NH3 to produce NH4NO3 were also included, along with NH4NO3 decompn. pathways (i.e., by NO addn. to feed). Also, a global reaction occurring between adsorbed NH3 and gaseous NO2 to produce N2 at low temps. (<250°) was added to account for an obsd. reaction occurring on the Cu-free zeolite. This mechanism was used to analyze the importance of NO3- formation during a std. driving cycle. Although a significant amt. of inhibitive NH4NO3 was modeled to form during low temp. fast and NO2 SCR steady-state expts., almost no NH4NO3 was predicted to form during the driving cycle, thereby allowing for higher reaction activity than predicted, based on steady-state data. From modeling and catalyst testing perspectives, this showed the importance of capturing catalyst transient behavior rather than only steady-state conditions, since steady-state is not necessarily reached during practical driving scenarios.
    65. 65
      Borfecchia, E.; Lomachenko, K.; Giordanino, F.; Falsig, H.; Beato, P.; Soldatov, A.; Bordiga, S.; Lamberti, C. Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction. Chem. Sci. 2015, 6, 548– 563,  DOI: 10.1039/c4sc02907k
      65
      Revisiting the nature of Cu sites in the activated Cu-SSZ-13 catalyst for SCR reaction
      Borfecchia, E.; Lomachenko, K. A.; Giordanino, F.; Falsig, H.; Beato, P.; Soldatov, A. V.; Bordiga, S.; Lamberti, C.
      Chemical Science (2015), 6 (1), 548-563CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Cu-SSZ-13 is a highly active NH3-SCR catalyst for the abatement of harmful nitrogen oxides (NOx, x = 1, 2) from the exhausts of lean-burn engines. The study of Cu-speciation occurring upon thermal dehydration is a key step for the understanding of the enhanced catalytic properties of this material and for identifying the SCR active sites and their redox capability. Herein, we combined FTIR, X-ray absorption (XAS) and emission (XES) spectroscopies with DFT computational anal. to elucidate the nature and location of the most abundant Cu sites in the activated catalyst. Different Cu species have been found to be dominant as a function of the dehydration temp. and conditions. Data anal. revealed that the dehydration process of Cu cations is essentially completed at 250 °C, with the formation of dehydrated [CuOH]+ species hosted in close proximity to 1-Al sites in both d6r and 8r units of the SSZ-13 matrix. These species persist at higher temps. only if a certain amt. of O2 is present in the gas feed, while under inert conditions they undergo virtually total "self-redn." as a consequence of an OH extra-ligand loss, resulting in bi-coordinated bare Cu+ cations. Synchrotron characterization supported by computational anal. allowed an unprecedented quant. refinement of the local environment and structural parameters of these Cu(II) and Cu(I) species.
    66. 66
      Bordiga, S.; Lamberti, C.; Bonino, F.; Travert, A.; Thibault-Starzyk, F. Probing zeolites by vibrational spectroscopies. Chem. Soc. Rev. 2015, 44, 7262– 7341,  DOI: 10.1039/c5cs00396b
      66
      Probing zeolites by vibrational spectroscopies
      Bordiga, Silvia; Lamberti, Carlo; Bonino, Francesca; Travert, Arnaud; Thibault-Starzyk, Frederic
      Chemical Society Reviews (2015), 44 (20), 7262-7341CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This review addresses the most relevant aspects of vibrational spectroscopies (IR, Raman and INS) applied to zeolites and zeotype materials. Surface Bronsted and Lewis acidity and surface basicity are treated in detail. The role of probe mols. and the relevance of tuning both the proton affinity and the steric hindrance of the probe to fully understand and map the complex site population present inside microporous materials are critically discussed. A detailed description of the methods needed to precisely det. the IR absorption coeffs. is given, making IR a quant. technique. The thermodn. parameters of the adsorption process that can be extd. from a variable-temp. IR study are described. Finally, cutting-edge space- and time-resolved expts. are reviewed. All aspects are discussed by reporting relevant examples. When available, the theor. literature related to the reviewed exptl. results is reported to support the interpretation of the vibrational spectra on an at. level.
    67. 67
      Soyer, S.; Uzun, A.; Senkan, S.; Onal, I. A quantum chemical study of nitric oxide reduction by ammonia (SCR reaction) on V2O5 catalyst surface. Catal. Today 2006, 118, 268– 278,  DOI: 10.1016/j.cattod.2006.07.033
      67
      A quantum chemical study of nitric oxide reduction by ammonia (SCR reaction) on V2O5 catalyst surface
      Soyer, Sezen; Uzun, Alper; Senkan, Selim; Onal, Isik
      Catalysis Today (2006), 118 (3-4), 268-278CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      The reaction mechanism for selective catalytic redn. (SCR) of NO by NH3 on (0 1 0) V2O5 surface represented by a V2O9H8 cluster was simulated by d. functional theory (DFT) calcns. performed at the B3LYP/6-31G** level. Computations indicated the SCR reaction consisted of 3 main parts. In part 1, NH3 activation on V2O5 was examd. NH3 was adsorbed on Bronsted acidic V-OH site as NH4+ species by a non-activated process with an exothermic relative energy difference of 28.65 kcal/mol. Lewis acidic NH3 interactions, also considered, were energetically unfavorable. Thus, it was concluded the SCR reaction on (0 1 0) V2O5 surface was favorably initiated by the Bronsted acidic NH3 adsorption. In part 2, the interaction of NO with pre-adsorbed NH3 species eventually formed nitrosamide (NH2NO) species. The rate-limiting step for this part and for the total SCR reaction was identified as NH3NHO formation with a high activation barrier (43.99 kcal/mol); however, it is cautioned that an approx. transition state was obtained for this step. For part 3, gas phase decompn. of NH2NO and decompn. of this species on the catalyst surface were both considered. Gas phase decompn. of NH2NO had high activation barriers vs. NH2NO decompn. on the V2O9H8 cluster surface. NH2NO decompn. on this cluster was achieved by a push-pull H transfer mechanism between active V=O and V-OH groups.
    68. 68
      Walch, S. P. Theoretical characterization of the reaction NH2+NO→products. J. Chem. Phys. 1993, 99, 5295– 5300,  DOI: 10.1063/1.465972
      68
      Theoretical characterization of the reaction NH2 + NO → products
      Walch, Stephen P.
      Journal of Chemical Physics (1993), 99 (7), 5295-300CODEN: JCPSA6; ISSN:0021-9606.
      The potential energy surface for NH2 + NO was characterized using complete active space self-consistent-field (CASSCF)/deriv. calcns. to det. the stationary point geometries and frequencies, followed by internally contracted CI (ICCI) calcns. to det. the energetics. Prodn. of N2 + N2O involves a complex mechanism. The initially formed NH2NO undergoes a 1,3-hydrogen shift to give an HNNOH isomer (with the substituents trans about the NN bond and cis about the NO bond), which undergoes subsequent cis-trans isomerizations about the NN and NO bonds before decompg. to N2 + H2O. The saddle point for prodn. of N2 + H2O has an approx. rectangular arrangement of one H atom, the two N atoms, and the O atom. This process does not involve a barrier with respect to NH2 + NO. Formation of HN2 + OH can occur from any of the isomers of HNNOH with no barrier, but the overall process is endothermic by 0.7 kcal/mol. The results obtained in this work are qual. the same as previous work, but both the stationary point geometries and energies should be more reliable, because of the use of larger basis sets and more extensive inclusion of electron correlation effects.
    69. 69
      Sun, D.; Schneider, W. F.; Adams, J. B.; Sengupta, D. Molecular Origins of Selectivity in the Reduction of NOx by NH3. J. Phys. Chem. A 2004, 108, 9365– 9374,  DOI: 10.1021/jp049079a
      69
      Molecular Origins of Selectivity in the Reduction of NOx by NH3
      Sun, Donghai; Schneider, William F.; Adams, James B.; Sengupta, Debasis
      Journal of Physical Chemistry A (2004), 108 (43), 9365-9374CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The fundamental principle underlying the selective catalytic redn. (SCR) of NOx to N2 is promotion of reactions of reductant with NOx over competing and thermodynamically preferred reactions with a large excess of O2. A similar competition between NOx and O2 exists in the non-catalytic, thermal redn. of NOx with NH3. In this work, d. functional theory calcns. elucidated the origin of the remarkable selectivity in thermal deNOx. Thermal deNOx is initiated by converting NH3 into the active reductant, NH2 radical. NH2 radical reacts with NO at rates typical of gas-phase radical reactions to produce a relatively strongly bound H2NNO adduct which readily rearranges and decomps. to N2 and water. In contrast, NH2 radical reacts exceedingly slowly with O2: the H2N-OO adduct is weakly bound and more readily falls apart than reacts with products. The pronounced discrimination of NH2 radical against reaction with O2 is unusual behavior for a radical, but can be understood by comparing the electronic structures of the H2NNO and H2NOO radical adducts. These 2 key elements of thermal deNOx (reductant activation and kinetic inhibition of reactions with O2) are similarly essential to successful catalytic lean NOx redn. and are important to consider when evaluating and modeling NOx SCR.
    70. 70
      Anstrom, M.; Topsøe, N.-Y.; Dumesic, J. Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts. J. Catal. 2003, 213, 115– 125,  DOI: 10.1016/s0021-9517(02)00031-3
      70
      Density functional theory studies of mechanistic aspects of the SCR reaction on vanadium oxide catalysts
      Anstrom, Mark; Topsoe, Nan-Yu; Dumesic, J. A.
      Journal of Catalysis (2003), 213 (2), 115-125CODEN: JCTLA5; ISSN:0021-9517. (Elsevier Science)
      D. functional theory (DFT) calcns. were carried out on a vanadium oxide cluster contg. four vanadium atoms to probe the mechanism of the selective catalytic redn. (SCR) of NO with ammonia. The interaction of ammonia with Bronsted acid sites on this V4-cluster leads to the formation of NH4 species bonded to two vanadyl (V:O) groups, with a bonding energy of -110 kJ/mol. This adsorbed NH4 species reacts with NO in a series of steps to form an adsorbed NH2NO species, which subsequently undergoes decompn. to form N2, H2O, and a reduced vanadium oxide cluster (V4-H). The latter reaction occurs via a series of hydrogen-transfer steps by a "push-pull" mechanism with adjacent V:O and V-OH groups on the vanadium oxide cluster. The rate limiting process in this conversion of NO and NH3 to give N2, H2O, and V4-H involves the reaction of an adsorbed NH3NHO adduct to form NH2NO species. The transition state of this step may be stabilized through hydrogen bonding with surrounding vanadia and/or titania moieties.
    71. 71
      Gilardoni, F.; Bell, A. T.; Chakraborty, A.; Boulet, P. Density Functional Theory Calculations of the Oxidative Dehydrogenation of Propane on the (010) Surface of V2O5†. J. Phys. Chem. B 2000, 104, 12250– 12255,  DOI: 10.1021/jp001746m
      71
      Density Functional Theory Calculations of the Oxidative Dehydrogenation of Propane on the (010) Surface of V2O5
      Gilardoni, Francois; Bell, Alexis T.; Chakraborty, Arup; Boulet, Pascal
      Journal of Physical Chemistry B (2000), 104 (51), 12250-12255CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      D. functional theory and the calcns. of O nucleophilicity were applied to an anal. of the oxidative dehydrogenation (ODH) of propane on the (010) surface of V2O5. These calcns. show that the energetically preferred initial step is the dissociative adsorption of propane to form i-propoxide and hydroxyl species. Two V:O groups [O(1)] bonded by a V-O-V bridge are required. One of the vanadyl groups attacks the β-C atom of propane and is converted to a V-OCH2Me2 species, whereas the other vanadyl group is converted into a V-OH group. The activation barrier for this process is 9.4 kcal/mol. Dissociative adsorption to form an n-propoxide can also occur, but the activation barrier for this process is 14.5 kcal/mol. Propene and H2O are formed via a concerted process in which an H atom of one of the Me groups of i-propoxide reacts with an O(3)H group. Exploration of alternative pathways for this step reveals that neither O(1, 2, 3), O(1)H, nor O(2)H are sufficiently reactive. These findings are in good qual. agreement with exptl. observations concerning the mechanism and kinetics of propane ODH.
    72. 72
      Chen, L.; Janssens, T. V.; Vennestrøm, P. N.; Jansson, J.; Skoglundh, M.; Grönbeck, H. A complete multisite reaction mechanism for low-temperature NH3-SCR over Cu-CHA. ACS Catal. 2020, 10, 5646– 5656,  DOI: 10.1021/acscatal.0c00440
      72
      A Complete Multisite Reaction Mechanism for Low-Temperature NH3-SCR over Cu-CHA
      Chen, Lin; Janssens, Ton V. W.; Vennestroem, Peter N. R.; Jansson, Jonas; Skoglundh, Magnus; Groenbeck, Henrik
      ACS Catalysis (2020), 10 (10), 5646-5656CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The dynamic character of the active centers has made it difficult to unravel the reaction path for NH3-assisted selective catalytic redn. (SCR) of nitrogen oxides over Cu-CHA. Herein, we use d. functional theory calcns. to suggest a complete reaction mechanism for low-temp. NH3-SCR. The reaction is found to proceed in a multisite fashion over ammonia-solvated Cu cations Cu(NH3)2+ and Bronsted acid sites. The activation of oxygen and the formation of the key intermediates HONO and H2NNO occur on the Cu sites, whereas the Bronsted acid sites facilitate the decompn. of HONO and H2NNO to N2 and H2O. The activation and reaction of NO is found to proceed via the formation of nitrosonium (NO+) or nitrite (NO2-) intermediates. These low-temp. mechanisms take the dynamic character of Cu sites into account where oxygen activation requires pairs of Cu(NH3)2+ complexes, whereas HO-NO and H3N-NO coupling may occur on single complexes. The formation and sepn. of Cu pairs is assisted by NH3 solvation. The complete reaction mechanism is consistent with measured kinetic data and provides a solid basis for future improvements of the low-temp. NH3-SCR reaction.
    73. 73
      Mao, Y.; Wang, Z.; Wang, H.-F.; Hu, P. Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34. ACS Catal. 2016, 6, 7882– 7891,  DOI: 10.1021/acscatal.6b01449
      73
      Understanding Catalytic Reactions over Zeolites: A Density Functional Theory Study of Selective Catalytic Reduction of NOx by NH3 over Cu-SAPO-34
      Mao, Yu; Wang, Ziyun; Wang, Hai-Feng; Hu, P.
      ACS Catalysis (2016), 6 (11), 7882-7891CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Metal-exchanged CHA-type (SAPO-34 and SSZ-13) zeolites are promising catalysts for selective catalytic redn. (SCR) of NOx by NH3. However, an understanding of the process at the mol. level is still limited, which hinders the identification of its mechanism and the design of more efficient zeolite catalysts. In this work, modeling the reaction over Cu-SAPO-34, a periodic d. functional theory (DFT) study of NH3-SCR was performed using the hybrid functional (HSE06) with the consideration of van der Waals (vdW) interactions. A mechanism with a low N-N coupling barrier is proposed to account for the activation of NO. The redox cycle of Cu2+ and Cu+, which is crucial for the SCR process, is identified with detailed analyses. In addn., the decompn. of NH2NO is shown to readily occur on the Bronsted acid site by a hydrogen push-pull mechanism, confirming the collective efforts of Bronsted acid and Lewis acid (Cu2+) sites. The special electronic and structural properties of Cu-SAPO-34 are demonstrated to play an essential role in the reaction, which may have general implications on the understanding of zeolite catalysis.
    74. 74
      Li, J.; Li, S. New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study. Phys. Chem. Chem. Phys. 2007, 9, 3304– 3311,  DOI: 10.1039/b700161d
      74
      New insight into selective catalytic reduction of nitrogen oxides by ammonia over H-form zeolites: a theoretical study
      Li, Jun; Li, Shuhua
      Physical Chemistry Chemical Physics (2007), 9 (25), 3304-3311CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      D. functional theory calcns. were carried out to investigate the reaction mechanism of selective catalytic redn. of nitrogen oxides by ammonia in the presence of oxygen at the Bronsted acid sites of H-form zeolites. The Bronsted acid site of H-form zeolites was modeled by an aluminosilicate cluster contg. five tetrahedral (Al, Si) atoms. A low-activation-energy pathway for the catalytic redn. of NO was proposed. It consists of two successive stages: first NH2NO is formed in gas phase, and then it is decompd. into N2 and H2O over H-form zeolites. In the first stage, the formation of NH2NO may occur via two routes: (1) NO is directly oxidized by O2 to NO2, and then NO2 combines with NO to form N2O3, which reacts with NH3 to produce NH2NO; (2) when NO2 exceeds NO in the content, NO2 assocs. with itself to form N2O4, and then N2O4 reacts with NH3 to produce NH2NO. The second stage was suggested to proceed with low activation energy via a series of synergic proton transfer steps catalyzed by H-form zeolites. The rate-detg. step for the whole redn. of NOx is identified as the oxidn. of NO to NO2 with an activation barrier of 15.6 kcal mol-1. This mechanism was found to account for many known exptl. facts related to selective catalytic redn. of nitrogen oxides by ammonia over H-form zeolites.
    75. 75
      Brüggemann, T. C.; Keil, F. J. Theoretical investigation of the mechanism of the selective catalytic reduction of nitric oxide with ammonia on H-form zeolites. J. Phys. Chem. C 2008, 112, 17378– 17387,  DOI: 10.1021/jp806674d
      75
      Theoretical Investigation of the Mechanism of the Selective Catalytic Reduction of Nitric Oxide with Ammonia on H-Form Zeolites
      Bruggemann, Till C.; Keil, Frerich J.
      Journal of Physical Chemistry C (2008), 112 (44), 17378-17387CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The selective catalytic redn. of NO with ammonia in the presence of oxygen has been investigated on a portion of the H-ZSM5 framework which contains 5T atoms by using d. functional theory, representing H-form zeolites. The mechanism was subdivided into three parts: (1) the oxidn. of NO to NO2, (2) the formation of an intermediate (NH2NO), and (3) the decompn. of this species to nitrogen and water. For the second step, three different pathways were studied, differing in the NOx species initially present on the active site: (i) two NO mols. form N2O2, (ii) NO2 and NO form N2O3, and (iii) two NO2 mols. form N2O4. For steps 1 and 2, the crossing of potential energy surfaces was considered for the transition of single mols. to adsorbed clusters. For all three parts of the mechanism, the energy profile of the heterogeneously catalyzed reaction is favorable, as compared to that of the corresponding homogeneous reaction. Due to the strong adsorption of ammonia on the acid site, it is likely that the rate-detg. step of the overall reaction is the oxidn. of NO to NO2 caused by blocking of the active site by NH3. As far as we have investigated the reaction mechanism of the selective catalytic redn. of NO with NH3 in this work, the results are in agreement with the exptl. literature.
    76. 76
      Usberti, N.; Gramigni, F.; Nasello, N. D.; Iacobone, U.; Selleri, T.; Hu, W.; Liu, S.; Gao, X.; Nova, I.; Tronconi, E. An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst. Appl. Catal., B 2020, 279, 119397,  DOI: 10.1016/j.apcatb.2020.119397
      76
      An experimental and modelling study of the reactivity of adsorbed NH3 in the low temperature NH3-SCR reduction half-cycle over a Cu-CHA catalyst
      Usberti, Nicola; Gramigni, Federica; Nasello, Nicole Daniela; Iacobone, Umberto; Selleri, Tommaso; Hu, Wenshuo; Liu, Shaojun; Gao, Xiang; Nova, Isabella; Tronconi, Enrico
      Applied Catalysis, B: Environmental (2020), 279 (), 119397CODEN: ACBEE3; ISSN:0926-3373. (Elsevier B.V.)
      The reactivity of Lewis and Bronsted ammonia in the redn. half-cycle (RHC) of the NH3-SCR low temp. redox mechanism was studied over a model Cu-CHA catalyst by transient kinetic tests involving reductive NO pulses. The CuII sites were reduced according to a 1:1:1:1 molar ratio with NO and NH3 conversion and N2 formation. The ammonia coordinated to Cu sites (Lewis ammonia) was preferentially consumed prior to that stored on the Bronsted acid sites. The catalyst was effectively re-oxidized by O2 in He at 150°C even when the Cu-coordinated ammonia was depleted. A redox kinetic model assuming NO activation by CuII to a gaseous mobile intermediate (HONO) which reacts first with Lewis-NH3 and then with Bronsted-NH3 was successfully fitted to our transient data assuming the CuII redn. rate to be second order in the CuII sites. This suggests a possible role of CuII dimeric complexes in the RHC of Std. SCR.

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