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C: Chemical and Catalytic Reactivity at Interfaces

The Catalytic Reactivity of Alloys; Ethanol and Formic Acid Decomposition on Cu–Pd(110)
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  • Michael Bowker*
    Michael Bowker
    Catalysis Hub, RCAH, Rutherford Appleton Laboratory, Harwell Oxford Campus, Didcot OX11 0QX, United Kingdom
    Max Planck- Cardiff Centre on the Fundamentals of Heterogeneous Catalysis FUNCAT, Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0001-5075-1089
  • Richard Holroyd
    Richard Holroyd
    Chemistry Department, University of Reading, Reading RG6 6AH, United Kingdom
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  • Neil Perkins
    Neil Perkins
    Chemistry Department, University of Reading, Reading RG6 6AH, United Kingdom
    Element Six, Campus, Harwell, Fermi Avenue, Didcot OX11 0QR, United Kingdom
    More by Neil Perkins
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The Journal of Physical Chemistry C

Cite this: J. Phys. Chem. C 2022, 126, 37, 15703–15709
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https://doi.org/10.1021/acs.jpcc.2c04881
Published September 12, 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 effect of alloying Cu and Pd on the reactivity pattern for formic acid and for ethanol has been examined. The electronic structure of the material is strongly affected by the alloying, with the d-band lowered in energy and filled, compared with Pd alone. Hence the reactivity would be expected to be strongly affected by the alloying. This appears to be the case for formic acid decomposition, whose decomposition temperature in temperature-programmed desorption is shifted by alloying and is between the temperatures for the individual components (at 350 K, compared with 250 and 470 K for Pd and Cu, respectively). However, when a different molecule is chosen as the probe of surface reactivity, namely, ethanol, we come to a very different conclusion. Here the individual reactivity patterns for the two elemental components of the alloy are seen, namely, dehydrogenation on the Cu (to produce acetaldehyde) and decarbonylation on Pd (to methane and CO). There are effects of alloying on destabilizing the former pathway and stabilizing the latter, but the major conclusion from this work is that it is not average electronic structure that dictates reactivity but the individual atomic nature of the surface components. Only monodentate adsorbates truly probe this behavior.

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Published as part of The Journal of Physical Chemistry virtual special issue “Cynthia Friend Festschrift”.

Introduction

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A perennial problem in catalysis has been the establishment of the relative importance of ligand (or electronic) versus ensemble (or geometric) effects on metal reactivity. Here we will show that, for an alloy surface comprising both Cu and Pd atoms in a well-ordered arrangement, what determines the surface chemistry is predominantly the chemical nature of the individual atoms. This contrasts with the fact that this system is a near-ideal alloy, with good atom mixing and an exothermic enthalpy of mixing of 0.17 eV/atom (1) and with greatly changed electronic structure compared with the component metals. (2−7) The d-band is shifted away from the Fermi level compared with pure Pd, implying filling of the d-band, but is closer to the Fermi level than for pure Cu.
The probe molecules we have used to illustrate this contention are two quite different entities, namely, a simple alcohol, ethanol, and the simplest carboxylic acid, formic acid. The results from the two molecules are very different in terms of their reactivity modification by alloying. However, we contend that this only relates to the fact that ethanol forms an alkoxy species as an intermediate, singly bonded to atoms in the surface through the oxygen atom, (8−10) whereas the acid is bidentate, bonding as a formate species to two adjacent surface atoms; see schematic Figure 1. (11) The latter has been shown to be the case on monometallic Cu(110), for instance, with both oxygen atoms appearing to be in identical environments (12−16) and in a number of other recent publications for a variety of Cu surfaces, from single crystals of different orientation (17−20) to catalysts. (21,22)

Figure 1

Figure 1. Schematic models of alkoxy and formate bonding on a Cu surface. Reproduced from Bowker. (11)

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We have carried out kinetic and surface analysis studies on well-defined Cu–Pd alloys, (23−26) as have others. (3,5,27−33) The 1:1 CuPd(110) surface used here is a body-centered cubic structure, and the surface structure is illustrated in Figure 2, as described by a number of authors. (6,38) The preparation of such a surface needs care, as detailed by Laboda-Cackovic et al., (27−31) including avoidance of the order–disorder transition of the sample, which occurs above 770 K, which could incur loss of the sample integrity. The surface consists of a nearly close-packed unit cell with one metal atom central and the other metal comprising the corners of the cell, so that a variety of sites of Pd–Pd, Cu–Cu, and Pd–Cu are available. Details of the preparation are given below and in the Supporting Information.

Figure 2

Figure 2. Structure of the 1:1 CuPd(110) surface. Cu atoms in red, Pd in blue.

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Here we intend only to highlight the effects of alloying on reactivity, manifested mainly in temperature-programmed desorption (TPD) experiments with ethanol and formic acid. The question we wish to address is the following─does alloying produce a homogeneous surface of quite different reactivity from the individual atoms or a surface for which adjacent atoms are modified by the second alloying metal, or is the reactivity just a mix of the two properties?

Methods

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The experiments were carried out in a ultrahigh-vacuum (UHV) system described in the Supporting Information but, most importantly for this paper, containing facilities for dosing of gases into the vacuum system and for temperature-programmed desorption and molecular beam measurements, (34) using mass spectrometry. This machine operated at a base pressure of 2 × 10–10 Torr and was equipped with Auger, low energy electron diffraction (LEED), and a mass spectrometer for TPD and molecular beam measurements. The crystal was mounted on 0.15 mm diameter tungsten wires, suspended between two 2 mm tungsten support rods on the manipulator, and the sample could be cooled to 90 K and heated to the desired treatment temperature, measured by an attached thermocouple. It was cleaned, following the methods of Laboda-Cackovic et al., (27−31) by cycles of argon ion bombardment (current density 2–3 μA cm–2) followed by annealing to 750 K and monitoring of the Cu/Pd Auger ratio. Checks of top surface layer composition were also done by following CO TPD spectra. More details about the treatments can be found in the Supporting Information.

Results and Discussion

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Formic acid adsorbs dissociatively at this temperature on Cu(110) (12,34−36) and Pd(110) (37) resulting in the production of a formate species on the surface. Figure 3 shows the result for TPD after adsorption on the alloy surface at 185 K, and the peak decomposition temperatures for Cu(110) (12,34−36) and Pd(110) (37) are also shown. The first peak of hydrogen evolution at 280 K is due to the recombination of H atoms, which are dissociated from the formic acid at lower temperature. The coincident desorption of H2 and CO2 at 350 K is rate-limited by the formate decomposition, not by the desorption energy of CO2 or H2, which, as individual adsorbates, desorb from the surface at lower temperatures. This TPD indicates that the alloy is homogeneous, since the formate has very different stability, intermediate between that of the component metals and has only one state; it is much reduced in stability compared with Cu(110) (36) (by −120 K) and much increased in stability compared with Pd(110) (37) (by +100 K). This agrees with the calculations of Yuan and Zhang who also find that the heat of adsorption of formate on this surface is intermediate between those on Cu and Pd. (38) This is then also in line with the changed electronic structure, which is intermediate, though perhaps more like Cu, in that the d-band is filled. (1−5) Furthermore, there is only one desorption state. Thus, these findings appear to support the idea of a new material whose different electronic structure leads to the very different stability seen in Figure 3. However, in what follows we will show that this is very misleading by using a more definitive probe molecule.

Figure 3

Figure 3. Desorption after adsorbing formic acid on the CuPd(110) surface, showing formate decomposition at 350 K, which yields coincident desorption of H2 and CO2. Also illustrated are the peak decomposition temperatures for Pd(110) (36) and Cu(110). (34,35)

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Figure 4a shows the desorption and decomposition of ethanol from Pd(110) (39,40) and Cu(110), (9) these reactions having been reported in detail previously and are only shown here for comparison with the results for the alloy surface shown in Figure 4b. The important features to note in Figure 4a are
1.

Cu shows only the dehydrogenation of ethanol, as reported originally by Madix and co-workers (8−10) producing acetaldehyde and hydrogen from the surface in a very selective manner; there are no other products. The desorption temperature of the acetaldehyde is ∼310 K.

2.

Pd shows only decarbonylation, yielding methane and CO as carbon-containing products into the gas phase in a very selective manner. The desorption temperature for the methane is ∼290 K. (39,40)

Figure 4

Figure 4. (a) Ethanol TPD from Pd and Cu, showing the decomposition products and very different reaction pathways. The figure is compiled from data for Pd(110) (36) and for Cu(110). (35) (b) TPD after adsorption of ethanol on the Cu–Pd alloy surface at 180 K. The product pattern is generally similar to that from the two individual components of the alloy, showing both dehydrogenation (ethanal desorption at 270 K) and decarbonylation (methane desorption at 310 K).

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Figure 4b shows the desorption from the Cu–Pd(110) alloy surface. It is quite clear that this looks like a combination of the desorption from the monometallic surfaces shown above; that is, it displays both the dehydrogenation typical of Cu and the decarbonylation typical of Pd, even though these elements are intimately mixed at the surface. Thus, it appears that the individual surface atoms behave in a way typical of the element, and we can state that individual atomic character dominates the reactivity for such unidentate molecules. Alloying effects are secondary, but real. The most notable effect is perhaps on the CO desorption, which is shifted to lower temperature by ∼100 K compared to Pd(110) as shown in Figure 4b and is at a similar peak temperature to that found by Hammoudeh et al. for CO adsorption alone (27) and as suggested by theory; (5) it is much more strongly held than on Cu, where desorption occurs typically at ∼200 K. (41) A similar shift is found by Jeroro et al. (42) for CO adsorption on 0.5 ML of Cu deposited on Pd(111) and also from methanol decomposition on that surface. In Figure 3b the methane peak is shifted to higher temperature (by ca. 40 K), while the acetaldehyde desorption is shifted to lower temperature (by ca. −40 K), corresponding with changes in activation energy for the decomposition of the surface intermediates, which produce them of only ∼9 kJmol–1 or roughly 10% of the reaction activation energy (using the Redhead equation (43)). These shifts are in the direction that might intuitively be expected; that is, Pd is alloyed with Cu, and so the intermediate that produces methane (the methyl group (39,40)) is stabilized by the presence of the less reactive metal, and the peak is shifted to higher temperature. On the other hand, Cu is alloyed with Pd, and so the ethoxy intermediate is destabilized by the more reactive metal, dehydrogenation to acetaldehyde proceeds at a higher rate, and hence the peak appears at a lower temperature than for the pure Cu metal.
This combined behavior is also seen in thermal molecular beam experiments, where both products are seen to evolve from the surface (the details of these measurements are given in the Supporting Information). In this case we had to predose the crystal with oxygen to observe measurable sticking of the ethanol molecule on the surface (i.e., greater than ∼0.02). As can be seen in Figure 5 the sticking probability is hugely increased by the predosed oxygen atoms to ∼0.6. Both methane and acetaldehyde desorb into the gas phase if the reaction is carried out above 310 K. At this reaction temperature, 343 K, both products, methane and acetaldehyde, can desorb, but CO is left adsorbed on the surface. So, the drop in sticking and product formation is a combination of removal of preadsorbed oxygen (as water) and blockage of the surface by CO. Although both products desorb it might be expected that acetaldehyde would evolve first (from the TPD of Figure 4b), but clearly methane evolves first. That it evolves immediately is not a surprise, since 343 K is above the decomposition temperature peak, but why is acetaldehyde delayed? This is possibly due to a higher local sticking probability of ethanol at the Pd sites. However, more likely is that, for some reason, the Cu sites are effectively blocked by the presence of the oxygen, and some support for this can be seen in Figure 5, since ethanol begins to evolve and peaks after the water evolution peak, which appears to closely follow the methane evolution.

Figure 5

Figure 5. Molecular beam raw reactor data for ethanol in the beam reacting with oxygen predosed CuPd surface at 343 K surface temperature. The beam is opened to hit the surface at 0.5 min.

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The TPD result for ethanol is somewhat surprising since it is well-known that Cu–Pd represents one of those alloy systems where the electronic structure is strongly modified by the alloying. (1−7,44) Thus, the alloy appears to be group 11-like in nature; that is, there is low electron density at the Fermi level typical of those elements, and the d-electron states are positioned below the Fermi level. This certainly appears very different from the situation for Pd itself and might be taken to imply some charge transfer to d-states of the Pd, as shown by a number of authors. However, it must be noted that Weightman et al. have proposed net charge transfer to be very low between Cu and Pd and that most of the electronic structure change is intra-atomic in nature, due to s → d transfer within the Pd valence levels. (45)
An important development to describe surface reactivity has been the d-band model developed by Norskov et al., (46−48) for which the position of the d-band with, respect to the Fermi level, and the bandwidth are considered to be major descriptors for alloy reactivity, as espoused also by earlier workers in catalysis (see, e.g., Dowden (49)). The data presented here appear to contrast with what we may predict from such models, since clearly the d-band is significantly altered compared with either individual metal, whereas the identity of the reactive behavior of those components is essentially maintained. Thus, we consider that the ensemble effect, rather than a ligand effect, dominates the reactivity here. It must be noted that, for some other systems too, the d-band model does not seem to apply well, as described further below. Norskov and Hammer themselves suggest that the ligand effect band model fails for certain classes of alloys, including CuPd alloys. (50)
Why do we claim that the ensemble effect is important, rather than the electronic effects? The evidence from formic acid decomposition appears to counter this proposal; in particular, the stability of the formate, seen in Figure 3, is almost exactly between that of the two metals, it is very much shifted in peak temperature from the individual metals, and there appears to be only one type of formate. However, this seems not to be due to an electronic effect, as shown above for the ethoxy, but is probably due either to the fact that the two oxygen atoms in the formate bond to two different metals─one to the Cu and one to the Pd─or that the transition state involves heteroatom interaction. Yuan et al. (38) calculate similar binding energies for bidentate formate on this CuPd surface for Pd–Pd, Pd–Cu, and Cu–Cu, but with higher binding to the latter (−2.54 eV) compared with Pd–Pd (−2.47 eV). This is a difference of only ∼7 kJ/mol. Note that it has already been shown earlier by scanning tunnelling microscopy (STM) that under these conditions of treatment (see methodology in the Supporting Information) this is an ordered alloy with alternating Cu and Pd (23,24) in the surface layer, although this was for a thin-layer material that was produced by depositing Pd onto Cu(110). It would be very useful to have a detailed study of a bulk CuPd crystal carried out by high-resolution STM, especially to resolve the exact binding sites of ethoxy and formate species, but such bulk alloy materials are not easy to obtain and require the detailed preparation methods described in the Supporting Information. Alternatively well-ordered thin films can be produced, but most workers currently focus on single-site catalysts (e.g., dilute layers of Pd in Cu (32)), and again, well-ordered mixed layers can be rather difficult to achieve in that way. Note that Ji et al. (51) have also used aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) to atomically resolve the surface on large CuPd nanoparticles of the same body-centered cubic (bcc) bulk structure as used here.
The ensemble then has very different stability for the intermediate since the two elements are involved in formate bonding. Further support for this view comes from our earlier work on an alloy with much lower Pd content. In that case the surface structure of the alloy is different (with a lower bulk Pd content of only 15%); the surface layer is completely Cu, (25,26,52,53) and we should have identified only electronic effects due to the high level of Pd that is present in the second layer (half a monolayer in a p(2 × 1) structure), to which every surface Cu atom is attached. The shift in formate stability was very small: it was destabilized compared with Cu alone, but only by ∼17 K, (52) because it is only bonded to surface Cu atoms, and the electronic effect from the adjacent sublayer Pd is therefore evidently very small, if it is present at all, since the lattice is slightly expanded compared with Cu and may be the entire cause of this small difference. Similarly, working with dilute Pd/Cu catalysts Qiu et al. (54) found that the effect of alloying on Pd was mainly geometric, with little evidence of an electronic component. Jeroro et al. (42) compared the behavior of PdCu and PdZn and, in contrast to the latter alloy, found little evidence of an electronic effect on adsorption of CO and methanol, considering that the changes that were seen were almost entirely due to ensemble effects. Further Qiu et al. (54) examined the effect of alloying on CO2 hydrogenation (reverse water gas shift mainly) on CuPd alloys and concluded that changes in behavior were dominated by geometric, ensemble effects.
We note, however, that the effect on CO desorption is rather marked. The CO is much more strongly held than on Cu and more weakly than on Pd. However, we would contend that this again is due to the fact that CO binds multiply to the surface and therefore bonds to both Cu and Pd. The exact binding site is not known for this alloy, but it is interesting to note that, for CO adsorption on a mixed-alloy thin-layer material, Hager et al. (33) conclude that both ligand and electronic effects play a role. However, Sakong et al. (6) consider that changes in CO binding are mainly due to changes in the nature of the Pd ensembles available where threefold Pd sites bind strongly, but when diluted by Cu they are weakened by reduced ensemble sizes, leaving only terminal sites at very dilute Pd concentrations.
These results appear counter to much of what we consider to be known about alloy catalysis─that is, it is the average electronic structure that is crucial for reactivity. Norskov et al. relate reactivity to the position and width of the d-band maximum, (46−48) and their conclusions appear quite reasonable for a number of systems. In the case here, however, the electronic structure of these Cu–Pd alloys is considerably modified, as shown by valence-band studies, for instance. (1−7) Thus, by the arguments of Norskov et al., and of many others, the stability of intermediates and the reactivity pattern must be significantly altered in a homogeneous manner relating to the d-band maximum position, yet apparently, they are not, and the effects strongly depend on the nature of binding of the adsorbate itself and the nature of the metals involved, as described above. How are we to view this then? Is it possible that the electronic structure, as revealed by ultraviolet photoelectron spectroscopy (UPS) and valence-band measurements, does not give a good representation of the surface with respect to reactivity? The answer to the latter question must be, at least in this instance, yes.
So, returning to the questions posed in the introduction, namely, (i) does alloying produce a homogeneous surface of quite different reactivity from the individual atoms, or (ii) a surface for which adjacent atoms are electronically modified by the second alloying metal or (iii) is the reactivity just a mix of the two properties? Clearly (i) is eliminated, and (iii) is the main observation here; that is, both monometallic Cu and Pd atom reactive characteristics are observed. However, considering (ii), their reactivity is indeed modified somewhat by the presence of the other metal, but this could either be due to electronic modification or, we feel more likely, due to the fact that the bonded state or transition state has to bond to both species during the decomposition process to form products. We propose that the ensemble effect is the dominant factor in the reactivity of CuPd.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c04881.

  • More detailed information about the equipment used for the experiments (section 1), sample preparation (section 2, Figures) and the molecular beam system and methodology (section 3) (PDF)

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

Author Information

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  • Corresponding Author
    • Michael Bowker - Catalysis Hub, RCAH, Rutherford Appleton Laboratory, Harwell Oxford Campus, Didcot OX11 0QX, United Kingdom;  Max Planck- Cardiff Centre on the Fundamentals of Heterogeneous Catalysis FUNCAT, Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, United Kingdom;  Orcidhttps://orcid.org/0000-0001-5075-1089;  Email: [email protected]
  • Authors
    • Richard Holroyd - Chemistry Department, University of Reading, Reading RG6 6AH, United Kingdom
    • Neil Perkins - Chemistry Department, University of Reading, Reading RG6 6AH, United Kingdom;  Element Six, Campus, Harwell, Fermi Avenue, Didcot OX11 0QR, United Kingdom
  • Author Contributions

    M.B. wrote the main draft and was the supervisor. Experimental work was contributed from R.H. and N.P., together with contributions to the text and figures.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors wish to acknowledge support from the EPSRC International Centre-to-Centre Project New trimetallic nanoparticles as catalysts for the conversion of carbon dioxide to renewable fuels (EPSRC EP/S030468/1), and to Cardiff University and the Max Planck Centre for Fundamental Heterogeneous Catalysis (FUNCAT) for financial support.

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    Different adsorption modes of trichloroethene on the PdCu alloy were investigated. With application of ab initio periodic d. functional theory, some insights about these adsorption modes were revealed. The 2 different (110) terminations of the Cu3Pd alloy were employed as models for the surface region of the Cu50Pd50 alloy because of Cu segregation. They are based on the regular phase of the face-centered cubic (fcc) structure. The first model shows a mixed (Pd/Cu = 1) top layer, whereas the other one is Cu-terminated. The anal. of the position of the center of the d-band projected on the Pd and Cu atoms of both surfaces indicated that the Pd atom in the alloy has similar reactivity to the pure metal surface, whereas the opposite trend has been found for the Cu atoms. This was also confirmed by the adsorption energy, calcd. for the distinct modes. Trichlorethene prefers to interact with the Pd atoms on the mixed PdCu surface. Both di-σ and π modes are the most stable configurations among all studied. On the other hand, the adsorption via the chlorine atoms is the preferred on the Cu-terminated surface. The interaction of the C:C bond of trichloroethene on the Pd atoms is similar to the one of the ethene mol. The main difference between the adsorption of these 2 mols. is due to the extra Cl interactions with the Cu atoms. The di-σ configurations are the precursors of the dissocn. reaction on the mixed surface.
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  6. 6
    Sakong, S.; Mosch, C.; Groß, A. CO adsorption on Cu–Pd alloy surfaces: ligandversus ensemble effects. Phys. Chem. Chem. Phys. 2007, 9, 2216,  DOI: 10.1039/B615547B
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    CO adsorption on Cu-Pd alloy surfaces: ligand versus ensemble effects
    Sakong, Sung; Mosch, Christian; Gross, Axel
    Physical Chemistry Chemical Physics (2007), 9 (18), 2216-2225CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The CO adsorption on ordered Cu-Pd alloy surfaces and surface alloys was studied using d. functional theory (DFT) within the framework of the generalized gradient approxn. (GGA). On the surface alloys, the CO adsorption energy at the top sites decreases with increasing concn. of the more reactive metal Pd. This surprising ligand effect is caused by the effective compressive strain induced by the larger size of the Pd atoms. However, at the most favorable adsorption sites the CO binding becomes stronger with increasing Pd concn. which is caused by an ensemble effect related to the availability of higher coordinated adsorption sites. At the surfaces of the bulk alloys, the trends in the adsorption energy as a function of the Pd concn. are less clear because of the strong Pd-Cu interaction and the absence of effective strain effects.
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  7. 7
    Sun, C. Q.; Wang, Y.; Nie, Y. G.; Mehta, B. R.; Khanuja, M.; Shivaprasad, S. M.; Sun, Y.; Pan, J. S.; Pan, L. K.; Sun, Z. Interface quantum trap depression and charge polarization in the CuPd and AgPd bimetallic alloy catalysts. Phys. Chem. Chem. Phys. 2010, 12, 3131– 3135,  DOI: 10.1039/b922677j
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    Interface quantum trap depression and charge polarization in the CuPd and AgPd bimetallic alloy catalysts
    Sun, Chang Q.; Wang, Yan; Nie, Yan Guang; Mehta, B. R.; Khanuja, M.; Shivaprasad, S. M.; Sun, Yi; Pan, Ji Sheng; Pan, L. K.; Sun, Zhuo
    Physical Chemistry Chemical Physics (2010), 12 (13), 3131-3135CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The ability of a catalyst to accept or donate charge is the key to the process of catalytic reaction. However, the detn. of the catalytic nature of a specimen as yet remains a great challenge. Here we report an effective yet simple method for this purpose based on the tight binding theory considerations and XPS monitoring of the evolution of valence and core electrons upon alloy formation. Firstly, we measured the valence and core band charge d. of the constituent elements of Cu, Ag, and Pd and then the resp. states upon alloy formation. A subtraction of the resultant spectrum of the alloy by the composed elemental spectra gives the residual that shows clearly the occurrence of charge trapping or polarization. We found that the valence and the core electrons of the CuPd alloy shift pos. to deeper energies, opposite to the occurrences in the AgPd alloy. Findings clarify for the first time that CuPd serves as an acceptor due to quantum trapping and the AgPd as a donor because of charge polarization, which also explain why AgPd and CuPd perform very differently as important catalysts.
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  8. 8
    Wachs, I. E.; Madix, R. J. The Oxidation Of Ethanol On Cu(110) And Ag(1 10) Catalysts. Applications of Surface Science 1978, 1, 303– 328,  DOI: 10.1016/0378-5963(78)90034-X
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    The oxidation of ethanol on copper(110) and silver(110) catalysts
    Wachs, Israel E.; Madix, Robert J.
    Applications of Surface Science (1977-1985) (1978), 1 (3), 303-28CODEN: ASUSDD; ISSN:0378-5963.
    The programmed reaction spectroscopy of EtOD on 18O2-preoxidized Ag (110) or Cu (110) single crystal surfaces indicated that the ability of the surfaces to dissociatively chemisorb the EtOD was enhanced by surface O and that EtOD was selectively oxidized by adsorption at 180 K to give adsorbed EtO- and D218O. The Ag (110) surface was more active than the Cu (110) surface for the dehydrogenation of EtOD to MeCHO and H; the recombination of EtO- with H to give EtOH was also obsd. At high EtOD concns. surface EtO+D2 (I) was obsd. at 180 K. Decompn. of I gave CH2:CH2, D2O, and H. EtO- was less stable than MeO- on both surfaces.
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  9. 9
    Bowker, M.; Madix, R. J. XPS, UPS and thermal desorption studies of alcohol adsorption on Cu(110); II. Higher Alcohols, Surface Sci. 1982, 116, 549,  DOI: 10.1016/0039-6028(82)90364-8
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    9
    XPS, UPS and thermal desorption studies of alcohol adsorption on copper(110). II. Higher alcohols
    Bowker, M.; Madix, R. J.
    Surface Science (1982), 116 (3), 549-72CODEN: SUSCAS; ISSN:0039-6028.
    The adsorption of EtOH, n- and i-PrOH, and ethylene glycol on Cu was studied.. These mols. form stable alkoxy species on the surface, i.e., the alc. dissocd. at the OH group. In contrast to MeOH, the higher alcs. react further with the surface, dehydrogenating to yield the corresponding aldehydes or ketones in the gas phase. Ethylene glycol reacts to form the most strongly bound intermediates, decompg. at ∼390 K to produce glyoxal, with little evidence of monoaldehyde formation or C-C bond breakage. The influence of preadsorbed O on these reactions is to generally increase the amt. of alkoxy formed on the surface by enhancing the degree of dissociative adsorption. The alkoxide decompn. peaks are shifted to slightly higher temps. and considerably broadened in such expts. The decompn. peak temps. of the surface alkoxides correlate fairly well with literature values of the α C-H bond strength, which is weaker in i-PrOH than MeOH. XPS showed broad O(1s) spectra for the mols. adsorbed at 140 K, probably due to H-bonding effects in the adlayer, with peak emissions at ∼533 eV. When the surface was warmed to 250 K, the O(1s) spectra narrow to close to instrumental linewidths, with a concomitant shift to a lower binding energy at ∼531 eV. C(1s) spectra show little change between the adsorbed alc. and alkoxy species. The UPS showed low-temp. spectra similar to that for the gas phase, but the highest occupied orbitals (essentially O(2p) orbitals), showed a chemisorption bonding shift of several tenths of an eV. UPS for these mols. has considerable less utility than for MeOH, because of masking of possible orbital shifts during chem. changes on the surface by the presence of overlapping emissions.
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  10. 10
    Wang, Z.-T.; Xu, Y.; El-Soda, M.; Lucci, F. R.; Madix, R. J.; Friend, C. M.; Sykes, E. C. H. Surface Structure Dependence of the Dry Dehydrogenation of Alcohols on Cu(111) and Cu(110). J. Phys. Chem. C 2017, 121, 12800– 12806,  DOI: 10.1021/acs.jpcc.7b02957
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    10
    Surface Structure Dependence of the Dry Dehydrogenation of Alcohols on Cu(111) and Cu(110)
    Wang, Zhi-Tao; Xu, Yunfei; El-Soda, Mostafa; Lucci, Felicia R.; Madix, Robert J.; Friend, Cynthia M.; Sykes, E. Charles H.
    Journal of Physical Chemistry C (2017), 121 (23), 12800-12806CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    The nonoxidative dehydrogenation of alcs. is considered as an important method to produce aldehydes for the chem. industry and hydrogen gas. However, current industrial processes are oxidative, meaning that the aldehydes are formed along with water, which, in addn. to being less energy efficient, poses sepn. problems. Herein the prodn. of aldehydes from methanol and ethanol on clean and dry Cu(111) and Cu(110) surfaces was investigated in order to understand the catalytic anhyd. dehydrogenation of alcs. Both ethanol and methanol preferentially react under ultrahigh vacuum conditions at surface defects to yield acetaldehyde and formaldehyde, resp., in the absence of surface oxygen and water. The amt. of alkoxide reaction intermediates measured by scanning tunneling microscopy, and aldehyde and hydrogen products detected by temp. programmed reaction, are increased by inducing more defects in the Cu substrates with Ar ion sputtering. This work also reveals that the Cu model surfaces are not poisoned by the reaction and exhibit 100% selectivity for alc. dehydrogenation to aldehyde and hydrogen.
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  11. 11
    Bowker, M. Methanol Synthesis from CO2 Hydrogenation. ChemCatChem. 2019, 11, 4238– 4246,  DOI: 10.1002/cctc.201900401
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    11
    Methanol synthesis from CO2 hydrogenation
    Bowker, Michael
    ChemCatChem (2019), 11 (17), 4238-4246CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. In the future we will be phasing out the use of fossil fuels in favor of more sustainable forms of energy, esp. solar derived forms such as hydroelec., wind and photovoltaic. However, due to the variable nature of the latter sources which depend on time of day, and season of the year, we also need to have a way of storing such energy at peak prodn. times for use in times of low prodn. One way to do this is to convert such energy into chem. energy, and the principal way considered at present is the prodn. of hydrogen. Although this may be achieved directly in the future via photocatalytic water splitting, at present it is electrolytic prodn. which dominates thinking. In turn, it may well be important to store this hydrogen in an energy dense liq. form such as methanol or ammonia. In this brief review it is emphasized that CO2 is the microscopic carbon source for current industrial methanol synthesis, operating through the surface formate intermediate, although when using CO in the feed, it is CO which is hydrogenated at the global scale. However, methanol can be produced from pure CO2 and hydrogen using conventional and novel types of catalysts. Examples of such processes, and of a demonstrator plant in construction, are given, which utilize CO2 (which would otherwise enter the atm. directly) and hydrogen which can be produced in a sustainable manner. This is a fast-evolving area of science and new ideas and processes will be developed in the near future.
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  12. 12
    Bowker, M.; Madix, R. J. XPS, UPS and thermal desorption studies on the reactions of formaldehyde and formic acid with the Cu(110) surface. Surf. Sci. 1981, 102, 542,  DOI: 10.1016/0039-6028(81)90045-5
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    12
    XPS, UPS and thermal desorption studies of the reactions of formaldehyde and formic acid with the copper(110) surface
    Bowker, M.; Madix, R. J.
    Surface Science (1981), 102 (2-3), 542-65CODEN: SUSCAS; ISSN:0039-6028.
    Formaldehyde adsorbs on the clean Cu (110) surface in a weakly bound state which interacts with the metal primarily through its most weakly bound orbital, the in-plane orbital composed mainly of O(2p). When O is preadsorbed on the surface, the XPS spectrum clearly shows the formation of 2 distinct species when formaldehyde is subsequently adsorbed. One species is formaldehyde as on the clean surface and the other is H2CO2. The latter species decomps. at ∼230 K, leaving the formate, HCOO, which then decomps. at 500 K, to produce H2 and CO2. The formate species is also produced by the adsorption of DCOOH on Cu(110) at 400 K. At 140 K, formic acid adsorbs molecularly but UPS indicates that its mol. structure is already strongly perturbed by adsorption; dissocn. of the acid H takes place during heating around 250 K. Oxygen preadsorption increases the amt. of formate produced from formic acid, abstracting the acid H atoms to produce H2O.
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  13. 13
    Nakano, H.; Nakamura, I.; Fujitani, T.; Nakamura, J. Synthesis and Decomposition of Formate Species. J. Phys. Chem. B 2001, 105, 1355– 65,  DOI: 10.1021/jp002644z
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    13
    Structure-Dependent Kinetics for Synthesis and Decomposition of Formate Species over Cu(111) and Cu(110) Model Catalysts
    Nakano, Haruhisa; Nakamura, Isao; Fujitani, Tadahiro; Nakamura, Junji
    Journal of Physical Chemistry B (2001), 105 (7), 1355-1365CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The kinetics and mechanism of formate synthesis by hydrogenation of CO2 (CO2 + 1/2H2 → HCOOa) and the formate decompn. into CO2 and H2 (HCOOa → CO2 + 1/2H2) over Cu(111) and Cu(110) surfaces were studied by in-situ IR reflection-absorption spectroscopy (IRAS) using a high-pressure reactor (∼1 atm). The reaction rates and the apparent activation energy of the formate synthesis were measured for Cu(111) and Cu(110), indicating that the formate synthesis on Cu was found to be structure-insensitive. The pressure dependence of CO2 and H2 on the initial formation rate of formate suggested an Eley-Rideal type mechanism, in which a gaseous CO2 mol. directly reacts with an adsorbed hydrogen atom on Cu. This is analogous to the well-known mechanism of formate synthesis by organometallic catalysts, in which CO2 is inserted into a Cu-hydride bond. The reaction rates and the activation energy of the decompn. were measured for Cu(111) and Cu(110). It was found that the formate decompn. on Cu was structure-sensitive in contrast to the formate synthesis. The promotional effect of coexisting H2 upon the rate of formate decompn. by 17 times at max. was incidentally found only on Cu(111). Interestingly, the increase in the decompn. rate was due to an increase in the preexponential factor of the rate const. for the formate decompn. with the activation energy being const. Furthermore, the decompn. kinetics of the formate prepd. by adsorption of formic acid on O/Cu(111) was identical with the H2-promoted decompn. kinetics of the synthesized formate. The difference in the decompn. kinetics was ascribed to the ordered structure of formate based on the previous STM results, in which a chainlike structure of formate was obsd. for the synthesized formate, whereas no formate chain was obsd. for the formate prepd. by adsorption of formic acid on O/Cu(111). The unique character of both the decompn. kinetics and the structure of formate obsd. only for Cu(111) was discussed from the viewpoint of the mass transport of copper atoms creating added formate chains.
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  14. 14
    Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, M. A. An iras study of formic acid and surface formate adsorbed on Cu(110). Surf. Sci. 1983, 133, 589,  DOI: 10.1016/0039-6028(83)90021-3
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    14
    An IRAS study of formic acid and surface formate adsorbed on copper(110)
    Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, A. M.
    Surface Science (1983), 133 (2-3), 589-604CODEN: SUSCAS; ISSN:0039-6028.
    The adsorption and decompn. of formic acid on Cu(110) was studied by IR reflection-absorption spectroscopy and temp.-programmed desorption. A broad double peak at ∼1635 cm-1 is characteristic of the carbonyl stretch ν(C:O) of the acid monolayer after adsorption at 120 K. Deprotonation occurs at ∼270 K to give a surface formate species. The estd. formate coverage θsat = 0.25 ± 0.05 and the reaction probability for deprotonation is estd. The sym. O-C-O vibration νs(COO) (1348-1358 cm-1) and the C-H stretch vibration ν(CH) (2891-2900 cm-1) are given as a function of formate coverage. A vibrational band corresponding to ν(CH) (2891-2900 cm-1) is given as a function of formate coverage. A vibrational band corresponding to νa(COO) is not obsd. under any exptl. conditions; the formate species is thus thought to have C2v or Cs(1) symmetry. A combination band νcomb at 2950 cm-1 is obsd. Its components (νa(COO) and δ(CH)) are not obsd. individually, since they have dynamic dipoles parallel to the surface. The combination band shifts at low coverages to lower frequencies, which is due to lateral interactions between parallel dynamic dipoles, and has an intensity comparable to that of the neighboring C-H stretch, which may be due to a Fermi resonance.
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  15. 15
    Sexton, B. A.; Hughes, A. E.; Avery, N. R. A spectroscopic study of the adsorption and reactions of methanol, formaldehyde and methyl formate on clean and oxygenated Cu (110) surfaces. Surf. Sci. 1985, 155, 366,  DOI: 10.1016/0039-6028(85)90423-6
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    15
    A spectroscopic study of the adsorption and reactions of methanol, formaldehyde and methyl formate on clean and oxygenated copper(110) surfaces
    Sexton, B. A.; Hughes, A. E.; Avery, N. R.
    Surface Science (1985), 155 (1), 366-86CODEN: SUSCAS; ISSN:0039-6028.
    A clean Cu surface is relatively unreactive, but adsorbed O readily attacks the hydroxyl proton and formyl C atoms to generate intermediate MeO and radicals. MeOCHO is split into 2 intermediates, MeO and HCO2. The condensed multilayer at 90 K, the weakly bound mol. monolayer states prior to dissocn. of reaction, and the reactive intermediates at higher temps. were analyzed by electron energy-loss, UPS, and thermal-desorption spectroscopies. MeOCHO coordinates to Cu via the carbonyl lone-pair orbital and MeOH via the O lone-pair orbital. There is no evidence for MeOCHO synthesis by dimerization of CH2O (the Tischenko reaction) or dehydrogenation of MeOH on the clean Cu(110) surface.
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  16. 16
    Crapper, M. D.; Riley, C. E.; Woodruff, D. P.; Puschmann, A.; Haase, J. Determination of the adsorption structure for formate on Cu (110) using SEXAFS and NEXAFS. Surf. Sci. 1986, 171, 1,  DOI: 10.1016/0039-6028(86)90558-3
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    16
    Determination of the adsorption structure for formate on copper(110) using SEXAFS and NEXAFS
    Crapper, M. D.; Riley, C. E.; Woodruff, D. P.; Puschmann, A.; Haase, J.
    Surface Science (1986), 171 (1), 1-12CODEN: SUSCAS; ISSN:0039-6028.
    Near-edge x-ray absorption fine structure (NEXAFS) is used to establish the C-O bond length (1.25 ± 0.05 Å), the O-C-O angle (124 ± 15°) and the azimuthal orientation (along [1‾10]) of the mol. plane of the formate catalytic intermediate adsorbed on Cu(110). Extended x-ray absorption fine structure (EXAFS) is then used to det. the Cu-O bond length (1.98 ± 0.07 Å) and the adsorption site (atop Cu atoms in the top (ridge) rows, with O atoms near short bridge sites). The problems of obtaining reliable quant. results from NEXAFS are discussed. The adsorption site and bond length, which agrees well with that for anhyd. Cu formate, is contrasted with the much longer bond length found for this species adsorbed on Cu(100) by Stoehr et al. (1985).
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  17. 17
    Shiozawa, Y.; Koitaya, T.; Mukai, K.; Yoshimoto, S.; Yoshinobu, J. The Roles of Step- Site and Zinc in Surface Chemistry of Formic Acid on Clean and Zn-Modified Cu(111) and Cu(997) Surfaces Studied by HR-XPS, TPD, and IRAS. J. Chem. Phys. 2020, 152, 044703  DOI: 10.1063/1.5132979
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    17
    The roles of step-site and zinc in surface chemistry of formic acid on clean and Zn-modified Cu(111) and Cu(997) surfaces studied by HR-XPS, TPD, and IRAS
    Shiozawa, Yuichiro; Koitaya, Takanori; Mukai, Kozo; Yoshimoto, Shinya; Yoshinobu, Jun
    Journal of Chemical Physics (2020), 152 (4), 044703CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The adsorption, desorption, and decompn. of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resoln. XPS, temp. programmed desorption, and IR reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid mols. adsorbed at 83 K were dissocd. to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH mols. were dissocd. into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH mols. were already dissocd. into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid mols. were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissocn. at low temp. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissocn. of formic acid. The desorption peak maxima of the formate decompn. products (CO2 and H2) on Zn-Cu(997) were shifted to higher temps. than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface. (c) 2020 American Institute of Physics.
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  18. 18
    Shiozawa, Y.; Koitaya, T.; Mukai, K.; Yoshimoto, S.; Yoshinobu, J. Quantitative Analysis of Desorption and Decomposition Kinetics of Formic Acid on Cu(111): The Importance of Hydrogen Bonding between Adsorbed Species. J. Chem. Phys. 2015, 143, 234707,  DOI: 10.1063/1.4937414
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    18
    Quantitative analysis of desorption and decomposition kinetics of formic acid on Cu(111): The importance of hydrogen bonding between adsorbed species
    Shiozawa, Yuichiro; Koitaya, Takanori; Mukai, Kozo; Yoshimoto, Shinya; Yoshinobu, Jun
    Journal of Chemical Physics (2015), 143 (23), 234707/1-234707/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Quant. anal. of desorption and decompn. kinetics of formic acid (HCOOH) on Cu(111) was performed by temp. programmed desorption (TPD), XPS, and time-resolved IR reflection absorption spectroscopy. The activation energy for desorption is estd. to be 53-75 kJ/mol by the threshold TPD method as a function of coverage. Vibrational spectra of the first layer HCOOH at 155.3 K show that adsorbed mols. form a polymeric structure via the hydrogen bonding network. Adsorbed HCOOH mols. are dissocd. gradually into monodentate formate species. The activation energy for the dissocn. into monodentate formate species is estd. to be 65.0 kJ/mol at a submonolayer coverage (0.26 mols./surface Cu atom). The hydrogen bonding between adsorbed HCOOH species plays an important role in the stabilization of HCOOH on Cu(111). The monodentate formate species are stabilized at higher coverages, because of the lack of vacant sites for the bidentate formation. (c) 2015 American Institute of Physics.
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  19. 19
    Chutia, A.; Silverwood, I. P.; Farrow, M. R.; Scanlon, D. O.; Wells, P. P.; Bowker, M.; Parker, S. F.; Catlow, C. R. A. Adsorption of formate species on Cu(h,k,l) low index surfaces – a combined DFT and INS study. Surf. Sci. 2016, 653, 45– 54,  DOI: 10.1016/j.susc.2016.05.002
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    19
    Adsorption of formate species on Cu(h,k,l) low index surfaces
    Chutia, Arunabhiram; Silverwood, Ian P.; Farrow, Matthew R.; Scanlon, David O.; Wells, Peter P.; Bowker, Michael; Parker, Stewart F.; Catlow, C. Richard A.
    Surface Science (2016), 653 (), 45-54CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    We report a d. functional theory study on the relative stability of formate species on Cu(h,k,l) low index surfaces using a range of exchange-correlation functionals. We find that these functionals predict similar geometries for the formate mol. adsorbed on the Cu surface. A comparison of the calcd. vibrational transition energies of a perpendicular configuration of formate on Cu surface shows an excellent agreement with the exptl. spectrum obtained from inelastic neutron spectroscopy. From the calcns. on adsorption energy we find that formate is most stable on the Cu(110) surface as compared to Cu(111) and Cu(100) surfaces. Bader anal. shows that this feature could be related to the higher charge transfer from the Cu(110) surface and optimum charge d. at the interfacial region due to bidirectional electron transfer between the formate and the Cu surface. Anal. of the partial d. of states finds that in the -5.5 eV to -4.0 eV region, hybridization between O p and the non-axial Cu dyz and dxz orbitals takes place on the Cu(110) surface, which is energetically more favorable than on the other surfaces.
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  20. 20
    Li, S.; Scaranto, J.; Mavrikakis, M. On the Structure Sensitivity of Formic Acid Decomposition on Cu Catalysts. Top. Catal. 2016, 59, 1580– 1588,  DOI: 10.1007/s11244-016-0672-1
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    20
    On the Structure Sensitivity of Formic Acid Decomposition on Cu Catalysts
    Li, Sha; Scaranto, Jessica; Mavrikakis, Manos
    Topics in Catalysis (2016), 59 (17-18), 1580-1588CODEN: TOCAFI; ISSN:1022-5528. (Springer)
    Catalytic decompn. of formic acid (HCOOH) has attracted substantial attention since HCOOH is a major byproduct in biomass reforming, a promising hydrogen carrier, and also a potential low temp. fuel cell feed. Despite the abundance of exptl. studies for vapor-phase HCOOH decompn. on Cu catalysts, the reaction mechanism and its structure sensitivity is still under debate. Self-consistent, periodic d. functional theory calcns. were performed on three model surfaces of copper-Cu(111), Cu(100) and Cu(211), and both the HCOO (formate)-mediated and COOH (carboxyl)-mediated pathways were studied for HCOOH decompn. The energetics of both pathways suggest that the HCOO-mediated route is more favorable than the COOH-mediated route on all three surfaces, and that HCOOH decompn. proceeds through two consecutive dehydrogenation steps via the HCOO intermediate followed by the recombinative desorption of H2. On all three surfaces, HCOO dehydrogenation is the likely rate detg. step since it has the highest transition state energy and also the highest activation energy among the three catalytic steps in the HCOO pathway. The reaction is structure sensitive on Cu catalysts since the examd. three Cu facets have dramatically different binding strengths for the key intermediate HCOO and varied barriers for the likely rate detg. step-HCOO dehydrogenation. Cu(100) and Cu(211) bind HCOO much more strongly than Cu(111), and they are also characterized by potential energy surfaces that are lower in energy than that for the Cu(111) facet. Coadsorbed HCOO and H represents the most stable state along the reaction coordinate, indicating that, under reaction conditions, there might be a substantial surface coverage of the HCOO intermediate, esp. at under-coordinated step, corner or defect sites. Therefore, under reaction conditions, HCOOH decompn. is predicted to occur most readily on the terrace sites of Cu nanoparticles. As a result, the authors hereby present an example of a fundamentally structure-sensitive reaction, which may present itself as structure-insensitive in typical varied particle-size expts.
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  21. 21
    Kattel, S.; Ramírez, P. J.; Chen, J. G.; Rodriguez, J. A.; Liu, P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts. Science 2017, 355, 1296– 1299,  DOI: 10.1126/science.aal3573
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    Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts
    Kattel, Shyam; Ramirez, Pedro J.; Chen, Jingguang G.; Rodriguez, Jose A.; Liu, Ping
    Science (Washington, DC, United States) (2017), 355 (6331), 1296-1299CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    The active sites over com. copper/zinc oxide/aluminum oxide (Cu/ZnO/Al2O3) catalysts for carbon dioxide (CO2) hydrogenation to methanol, the Zn-Cu bimetallic sites or ZnO-Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x-ray photoemission spectroscopy, d. functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both exptl. and theor. results agree that ZnCu undergoes surface oxidn. under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.
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  22. 22
    Yang, Y.; Mims, C. A.; Disselkamp, R. S.; Mei, D.; Kwak, J.-H.; Szanyi, J.; Peden, C. H. F.; Campbell, C. T. Isotope Effects in Methanol Synthesis and the Reactivity of Copper Formates on a Cu/SiO2 Catalyst. Catal. Lett. 2008, 125, 201– 208,  DOI: 10.1007/s10562-008-9592-4
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    22
    Isotope Effects in Methanol Synthesis and the Reactivity of Copper Formates on a Cu/SiO2 Catalyst
    Yang, Y.; Mims, C. A.; Disselkamp, R. S.; Mei, D.; Kwak, Ja-Hun; Szanyi, J.; Peden, C. H. F.; Campbell, C. T.
    Catalysis Letters (2008), 125 (3-4), 201-208CODEN: CALEER; ISSN:1011-372X. (Springer)
    Here we investigate isotope effects on the catalytic methanol synthesis reaction and the reactivity of copper-bound formate species in CO2-H2 atmospheres on Cu/SiO2 catalysts by simultaneous IR and MS measurements, both steady-state and transient. Studies of isotopic variants (H/D, 12C/13C) reveal that bidentate formate dominates the copper surface at steady state. The steady-state formate coverages of HCOO (in 6 bar 3:1 H2:CO2) and DCOO (in D2:CO2) are similar and the steady-state formate coverages in both systems decrease by ∼80% from 350 K to 550 K. Over the temp. range 413 K-553 K, the steady-state methanol synthesis rate shows a weak H/D isotope effect (1.05 ± 0.05) with somewhat higher activation energies in H2:CO2 (79 kJ/mol) than D2:CO2 (71 kJ/mol) over the range 473 K-553 K. The reverse water gas shift (RWGS) rates are higher than methanol synthesis and also shows a weak pos. H/D isotope effect with higher activation energy for H2/CO2 than D2/CO2 (108 vs. and 102 kJ/mol) The reactivity of the resulting formate species in 6 bar H2, 6 bar D2 and 6 bar Ar is strongly dominated by decompn. back to CO2 and H2. H2 and D2 exposure compared to Ar do not enhance the formate decompn. rate. The decompn. profiles on the supported catalyst deviate from first order decay, indicating distributed surface reactivity. The av. decompn. rates are similar to values previously reported on single crystals. The av. activation energies for formate decompn. are 90 ± 17 kJ/mol for HCOO and 119 ± 11 kJ/mol for DCOO. By contrast to the catalytic reaction rates, the formate decompn. rate shows a strong H/D kinetic isotope effect (H/D ∼8 at 413 K), similar to previously obsd. values on Cu(110).
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  23. 23
    Reilly, J.; Barnes, C.; Price, N.; Bennett, R.; Poulston, S.; Stone, P.; Bowker, M. ’The Growth, Mechanism, Thermal Stability and Reactivity of Pd Mono- amd Multi layers on Cu(110). J. Phys. Chem. B 1999, 103, 6521,  DOI: 10.1021/jp990156p
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    23
    The Growth Mechanism, Thermal Stability, and Reactivity of Palladium Mono- and Multilayers on Cu(110)
    Reilly, J. P.; Barnes, C. J.; Price, N. J.; Bennett, R. A.; Poulston, S.; Stone, P.; Bowker, M.
    Journal of Physical Chemistry B (1999), 103 (31), 6521-6532CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
    The room-temp. growth of palladium (Pd) on Cu(110) was studied by XPS, scanning tunneling microscopy (STM), LEED, and temp.-programmed desorption (TPD). XPS signal vs. deposition time plots rule out a simple layer-by-layer growth mechanism. STM/LEED indicates formation of regions of (2 × 1) overlayer at low Pd coverages (θPd < 1 ML), with considerable disorder as monolayer deep pits and islands. Higher Pd coverages give a granular film consisting of densely packed, flat-topped Pd clusters of av. size 75 × 150 Å and with largely a rectangular shape. The favored growth mechanism is of multilayered Pd islands above a mixed (2 × 1) CuPd interface of two to three at. layers thickness. The thermal stability of the Pd/Cu(110) system was studied with XPS peak intensity vs. annealing temp. plots that indicate that bulk intermixing takes place rapidly between 500 and 600 K. The Pd 3d5/2 XPS peak widths narrow, suggesting the formation of a largely homogeneous CuPd surface alloy. STM indicates that heating to 500 K leaves the Pd clusters in a largely unaltered morphol. with no sign of Ostwald ripening, whereas annealing to 600 K leads to considerable changes in topog. The granular structure of the Pd film is disrupted, leading to a surface with irregularly shaped flat domains sepd. by mono-at. steps. High temp. (720 K) annealing leads to further flattening and appearance of regular parallel lines in STM images. The spacing of these lines varies with Pd loading, and they are assigned to strain due to lattice mismatch between the capping copper monolayer and the underlying mixed CuPd alloy. The reactivity of the Pd/Cu(110) surface was probed by dosing formic acid and monitoring formate decompn. High Pd coverages lead to a substantial destabilization of the formate relative to clean Cu(110), which is assigned to formate adsorption on mixed CuPd sites.
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  24. 24
    Bennett, R.; Poulston, S.; Reilly, J.; Stone, P.; Barnes, C.; Bowker, M. Morphology of Pd Multilayers on Cu(110). Surf. Sci. 1999, 433–435, 816– 821
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    24
    Morphology of Pd multilayers on Cu(110)
    Bennett, R. A.; Poulston, S.; Price, N. J.; Reilly, J. P.; Stone, P.; Barnes, C. J.; Bowker, M.
    Surface Science (1999), 433-435 (), 816-821CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
    The growth of Pd films deposited on a Cu(110) single crystal surface and the resulting morphol. of the film structure has been followed by STM for deposition at 310 K. For ∼1 ML films an ordered (2 × 1) surface alloy is formed which is also visible in LEED. The surface morphol. is, however, not flat but has roughened step edges, raised islands and smaller trenches. For increased coverages of Pd the growth shifts to the formation of rectangular multilayered Pd islands. Upon annealing, AES shows a preferential segregation of Cu to the surface region that accompanies a flattening of the surface morphol. in STM. Annealing thick films (>4 ML) to 603 K produces a surface morphol. with larger flat terraces and small islands that have rough, non-crystallog. aligned step edges. With further annealing to 723 K these surfaces also display large-scale "banding" in the STM images that we attribute to stress relaxation between the CuPd alloy and Cu.
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    Bowker, M.; Newton, M.; Francis, S.; Gleeson, M.; Barnes, C. The Surface Structure of a Low Pd Cu/Pd-(110) Crystal Alloy. Surf. Rev. Lett. 1994, 1, 569– 571,  DOI: 10.1142/S0218625X94000680
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    The surface structure of a low Pd Cu/Pd-(110) crystal alloy
    Bowker, M.; Newton, M.; Francis, S. M.; Gleeson, M.; Barnes, C.
    Surface Review and Letters (1994), 1 (4), 569-71CODEN: SRLEFH; ISSN:0218-625X. (World Scientific)
    X-ray photoelectron diffraction studies of this alloy surface have been carried out and indicate that there is a significant expansion of the lattice in the near-surface region due to the high concn. of Pd in layer 2. Preliminary single scattering calcns. lend support to this proposal for the surface structure, and place this expansion in the subsurface mainly between layers 2 and 3.
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    Newton, M.; Francis, S.; Bowker, M. The use of forward focused photoelectrons in analysis of an alloy surface: Cu/Pd (85/15) {110}. J. Physics, Cond. Matter 1991, 3, S139,  DOI: 10.1088/0953-8984/3/S/023
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    Hammoudeh, A.; Mousa, M.S.; Loboda-Cackovic, J. Interaction of CO with clean and oxygen covered PdCu (110) single crystal alloy. Vacuum 1999, 54, 239,  DOI: 10.1016/S0042-207X(98)00471-0
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    Interaction of CO with clean and oxygen covered PdCu(110) single crystal alloy
    Hammoudeh, A.; Mousa, M. S.; Loboda-Cackovic, J.
    Vacuum (1999), 54 (1-4), 239-243CODEN: VACUAV; ISSN:0042-207X. (Elsevier Science Ltd.)
    Compositional changes that may occur upon CO chemisorption on PdCu (110) single crystal alloy have been studied by means of work function change measurements using the Kelvin probe technique. Furthermore, various surfaces of this alloy with different surface compns. were investigated with respect to their reactivity in the CO oxidn. applying the same technique. It was found that CO chemisorption, in the case of Cu-rich surfaces, induces the segregation of Pd to the surface. High CO pressures (10-4-10-3 mbar) also cause the surface segregation of still unidentified contamination characterized by an AES signal at 180 eV. In the CO oxidn. expts. the PdCu alloy showed lower reactivities compared to pure Pd, even though the outermost layer was entirely composed of Pd atoms. The reactivity was found to decrease further with increasing Cu concns. The lower reactivity of the PdCu alloy was attributed to the charge transfer from Pd to Cu obsd. in this system resulting in decreasing the adsorption enthalpy (and probably the sticking coeff.) of CO on "modified" Pd of the alloy surface.
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    Loboda-Cackovic, J.; Mousa, M.; Block, J. H. Surface analysis of the PdCu (110) single crystal alloy at different segregation rates. Vacuum 1995, 46, 89,  DOI: 10.1016/0042-207X(95)87000-8
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    Surface analysis of the PdCu(110) single crystal alloy at different segregation rates
    Loboda-Cackovic, J.; Mousa, M. S.; Block, J. H.
    Vacuum (1995), 46 (2), 89-96CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
    The PdCu(110) plane, with Pd:Cu = 1:1 in the bulk, was prepd. to have various surface compns. by applying cycles of argon ion sputtering (ion energy of ≤750 eV with ion current densities ≤1 μA/cm2) and annealing temps. 420 < T < 820 K. With this prepn. method different compns. of the top surface layers (TL) were obtained: (i) exclusively Pd atoms; (ii) compn. of various Cu/Pd ratios, including the bulk ratio; and (iii) exclusively Cu atoms. The surface region (SR) of ∼4 layers depth analyzed by AES, and the CO TDS used for TL-characterization, allowed the av. ratio of Cu/Pd in the next three subsurface layers (3SSL) to be evaluated. LEED enabled the detn. of the surface structure after each step. The high temp. treatment of the surface followed by low temp. sputtering (TSP) and TAN below that of Cu segregation (TSEG) ∼550 K, produced a smooth Pd-rich surface. On the other hand, a large no. of sputtering and annealing cycles without prior high TAN produced a rough surface, enabling the Cu atoms lying beneath the TL to be accessible for surface reactions. A diagram describing LEED structures can be drawn from the results which present the large variety of surface structures for the clean PdCu(110) single crystal surface depending on the TSP and TAN. In addn. to this diagram the SR Cu/Pd ratio for various TSP and TAN is given for slow and fast annealing modes at long and short pretreatment periods. Three phase diagrams comparing exptl. surface properties for each of the SR, TL and 3SSL with bulk compn. are presented.
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    Loboda-Cackovic, J. Properties of PdCu (110) single crystal alloy surfaces: Temperature-induced processes in the surface microstructure. Vacuum 1996, 47, 1405,  DOI: 10.1016/S0042-207X(96)00228-X
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    Properties of PdCu(110) single crystal alloy surfaces: temperature-induced processes in the surface microstructure
    Loboda-Cackovic, J.
    Vacuum (1996), 47 (12), 1405-1411CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
    The Pd-Cu(110) alloy single crystal surfaces, with a 1:1 at. concn. ratio in the bulk, were prepd. with various compns. Cu/Pd ratios in the surface regions, measured ∼4 layers thick and ranged from 0.3 to 2. Different compns. of the top surface layers were obtained, from exclusively Pd atoms, over mixts. of Pd and Cu atoms, to exclusively Cu atoms. The behavior of video LEED spot intensities and profiles during specimen heating and cooling was measured. The Pd-Cu(110) surface consists of domains contg. 10-20 lattice cells depending on the prepn. procedure. The surface microstructure influences the temp.-induced processes at the surface, such as surface roughening and partial surface disordering starting at ∼550 and ∼700 K, resp. The domain grain boundaries play an important role in the surface roughening process. At higher temps., the domain size distribution, aside from grain boundaries, causes partial surface disordering. The two Cu segregation processes, starting at ∼550 and ∼700 K, are the vol. counterparts to these surface processes.
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    Mousa, M.; Loboda-Cackovic, J.; Block, J. H. Characterization of PdCu (110) single crystal surface compositions during CO chemisorption. Vacuum 1995, 46, 117,  DOI: 10.1016/0042-207X(94)E0002-G
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    Characterization of PdCu(110) single crystal surface compositions during CO chemisorption
    Mousa, M. S.; Loboda-Cackovic, J.; Block, J. H.
    Vacuum (1995), 46 (2), 117-25CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
    The PdCu(110) single crystal alloy surfaces (bulk Pd/Cu = 1:1) were prepd. by sputtering and annealing at different temps. Surfaces with Cd/Pd contents ranging from ∼100% Pd to ∼100% Cu at the top surface layer could be obtained. AES with surface region (SR) anal. of ∼4 layers, LEED and TDS of CO with top surface layer (TL) anal. were the methods used for characterization. LEED of clean surfaces prepd. at low temps. (<550 K) showed the Pd(1×2) structure. By treating the alloy at higher temps. (>550 K), Cu segregation occurs and the structure changes to (1×1) and then to Cu(1×2). For a Pd-rich surface with Cu/Pd(SR) ≤0.1, CO TDS showed five desorption states, three of them appearing at 450, 400 and 340 K resp., thus resembling those from CO/pure Pd(110). The other two new CO-Pd binding states appear at 250 and 190 K, which are characteristic for the allow surface. The appearance of the 250 K max. indicates a ligand effect in CO-desorption. At Cu/Pd(SR) of ≥0.3 the CO desorption from Pd bridge-sites is absent in the spectra. The other two high temp. desorption sites shift to lower temps. (by ∼50 K) indicating the ensemble effect. With increasing Cu concn. in the TL, TDS show an addnl. peak at 160 K. Another fascinating property of this alloy is the possibility of prepg. the TL with ∼100% Pd, while in the next three subsurface layers (3SSL) the Cu/Pd ratio ranges from 0.1 to 2. For the Pd-rich TL, the CO initial sticking coeff. increases with increasing Cu/Pd ratio in the 3SSL. Detailed anal. of all these results revealed that the bcc. lattice model is essential for the PdCu(110) alloy surface at Cu/Pd(SR) > 0.2.
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    Loboda-Cackovic, J.; Block, J. H. Properties of PdCu (110) single crystal alloy surfaces: reversible changes of domain sizes. Vacuum 1995, 46, 1449,  DOI: 10.1016/0042-207X(95)00169-7
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    Properties of PdCu(110) single crystal alloy surfaces: reversible changes of domain sizes
    Loboda-Cackovic, J.; Block, J. H.
    Vacuum (1995), 46 (12), 1449-53CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
    The compn. of PdCu(110) single crystal alloy surfaces (bulk Pd/Cu = 1:1) can be changed by sputtering and annealing at different temps. A survey of surface structures and compns. in equil. shows the influence of Cu segregation. Samples with Cu/Pd ratios in the surface region (≃4 layers) from 0.3 to 1.3 and with (1 × 2) and (1 × 1) surface structures were prepd. Surfaces with ∼100% Pd atoms and mixed compns. of Pd and Cu atoms (down to ∼70% Pd) at the top surface layer were studied. Methods used for characterization were video LEED for structure anal., Auger electron spectroscopy for the surface region of ∼4 layers depth and CO thermal desorption spectroscopy for compn. anal. of the top surface layer. Domain extents along [10] and [01] crystallog. directions, of surfaces with different compns., changed reversibly when measured during sample heating and cooling cycles. The change proceeds in a manger that depends on the sizes of domains, on the cryst. amt. and on the amt. of Cu in the surface region. The surface domains contain a small no. of lattice cells (∼9 to ∼18) and a large amt. of atoms in the disordered intercryst. areas.
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    Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 2012, 335, 1209– 12,  DOI: 10.1126/science.1215864
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    Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations
    Kyriakou, Georgios; Boucher, Matthew B.; Jewell, April D.; Lewis, Emily A.; Lawton, Timothy J.; Baber, Ashleigh E.; Tierney, Heather L.; Flytzani-Stephanopoulos, Maria; Sykes, E. Charles H.
    Science (Washington, DC, United States) (2012), 335 (6073), 1209-1212CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Facile dissocn. of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resoln. scanning tunneling microscopy, the authors show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissocn. at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.
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    Hager, T.; Rauscher, H.; Behm, R. J. Interaction of CO with PdCu surface alloys supported on Ru (0 0 0 1). Surf. Sci. 2004, 558, 181– 94,  DOI: 10.1016/j.susc.2004.04.001
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    Interaction of CO with PdCu surface alloys supported on Ru(0 0 0 1)
    Hager, T.; Rauscher, H.; Behm, R. J.
    Surface Science (2004), 558 (1-3), 181-194CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
    The adsorption and desorption of CO on monolayer PdCu surface alloys on a Ru(0 0 0 1) substrate and, for comparison, on Pd/Ru(0 0 0 1), and Cu/Ru(0 0 0 1) monolayer films, were studied by temp. programmed desorption and IR reflection absorption spectroscopy, aiming at a detailed understanding of the different effects controlling the chem. properties of bimetallic surfaces, geometric ensemble effects, electronic ligand effects and strain induced effects. All of these effects are present in these surfaces, acting in a cooperative, synergetic way. Strain effects lead to a destabilization of the Pd-CO and a stabilization of the Cu-CO interaction as compared to CO adsorption on the resp. bulk substrates. Geometric ensemble effects are imposed by the highly disperse distribution of the Cu and Pd atoms, reducing the no. of otherwise favorable Pd3 sites, and electronic ligand effects lead to an increasing stability of the Pd-CO bond with increasing Cu content.
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    Bowker, M.; Pudney, P. D. A.; Barnes, C. A simple molecular beam system for surface reactivity studies. J. Vac. Sci. Technol. 1990, A8, 816– 820,  DOI: 10.1116/1.576924
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    Ying, D.; Madix, R. J. Thermal desorption study of formic acid decomposition on a clean Cu (110) surface. J. Catal. 1980, 61, 48,  DOI: 10.1016/0021-9517(80)90338-3
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    Thermal desorption study of formic acid decomposition on a clean copper (110) surface
    Ying, David H. S.; Madix, Robert J.
    Journal of Catalysis (1980), 61 (1), 48-56CODEN: JCTLA5; ISSN:0021-9517.
    The catalytic decompn. of HCO2H was examd. on a (110)-oriented Cu single-crystal surface. Dehydrogenation of HCO2H was the sole decompn. path obsd. The parent mol. dissocd. to form a stable formate species (HCO2(a)) as the surface intermediate. The HCO2- species was bonded to the surface Cu sites through the two O atoms. The decompn. of the HCO2- species was the rate-eliminating step governing the rates of the formation. of the CO2 and H2 products. The rate const. was detd. A comparison of the catalytic properties of the Cu(110) surface with those of the carburized Ni(110) surface showed strong chem. similarities between the 2 surfaces.
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    Bowker, M.; Rowbotham, E.; Leibsle, F.; Haq, S. The Adsorption and Decomposition of Formic Acid on Cu(110). Surf. Sci. 1996, 349, 97,  DOI: 10.1016/0039-6028(95)01069-6
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    The adsorption and decomposition of formic acid on Cu{110}
    Bowker, M.; Rowbotham, E.; Leibsle, F. M.; Haq, S.
    Surface Science (1996), 349 (2), 97-110CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
    The reactive adsorption of formic acid (HCOOH) on O-dosed Cu (110) was studied by using a mol. beam system, TPD, LEED, and STM. At low temp., the reaction is strongly O-coverage dependent. All coverages result in high reaction probability (0.8 at room temp.) for formic acid and for < 0.25 monolayers of O, there is complete O clean-off, leaving formate on the surface in a c(2×2) structure. At higher coverages, the situation is more complex, with some O remaining coadsorbed with the formate. The 2 adsorbates are then mainly phase sepd. into islands of c(6×2) O and (3×1) formate. The 2 phases mutually compress each other due to pressure at the phase boundaries. The reaction stoichiometry is 2:1 formic acid:O atoms in this temp. range. At higher temps. (> 450 K), the formate itself is unstable and decomps. during adsorption which results in a change of stoichiometry of the reaction; 1 mol. of formic acid removes an O atom as H2O, and H evolution ceases. There is a range of temp. between 350 and 420 K for which the reaction becomes very difficult, and the reaction probability drops to ∼ 0.1. This is due to rapid compression of much of the O adlayer into the unreactive c(6×2) structure by small amts. of formate. The reaction proceeds through a highly mobile, weakly held, "precursor" state on the surface, which is able to seek out the active sites on the surface, which are low in coverage at high levels of O. These active sites are the terminal O atoms in the O islands (in the [001] direction), which are only present at step edges or phase boundaries at 0.5 monolayers coverage of O.
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    Aas, N.; Li, Y.; Bowker, M. The adsorption and decomposition of formic acid on clean and oxygen dosed Pd(110). J. Phys. Cond. Matter 1991, 3, S281,  DOI: 10.1088/0953-8984/3/S/044
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    Yuan, D.; Zhang, Y. Theoretical investigations of HCOOH decomposition on ordered Cu-Pd alloy surfaces. Appl. Surf. Sci. 2018, 462, 649– 658,  DOI: 10.1016/j.apsusc.2018.08.048
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    Theoretical investigations of HCOOH decomposition on ordered Cu-Pd alloy surfaces
    Yuan, Dingwang; Zhang, Yong
    Applied Surface Science (2018), 462 (), 649-658CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    Formic acid (HCOOH) decompn. is studied on four ordered Cu-Pd compds. (B2-type CuPd, L10-type CuPd, L12-type Cu3Pd, and L12-type CuPd3) through systematic d. functional calcns. The crystal structures and at. compns. play a significant role to control activity and selectivity of catalysts. The O-H bond dissocn. barrier of HCOOH is less than that of the C-H bond scission, and the most high selectivity for O-H/C-H dissocn. is found on the single Pd atom alloyed in Cu lattice of L12 Cu3Pd(1 1 1) surface. However, the contiguous Pd atoms greatly facilitate hydrogen prodn. from HCOOH, and the barriers are 0.44 and 0.73 eV for the dehydrogenation reactions on of HCOOH → HCOO + H → CO2 + 2H on L10 CuPd(1 1 1) surface. Though the CO formation pathway is energetically favorable for COOH decompn. on the ordered Cu-Pd surfaces except Pd-rich ordered CuPd3(1 1 1) surface, the adsorbed CO can be easily removed due to the weaker interaction compared with pure Pd surfaces. The calcd. results indicate that L10 CuPd and L12 CuPd3 alloys are effective catalysts for formic acid decompn. with high catalytic selectivity and activity.
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    Bowker, M.; Holroyd, R.; Sharpe, R.; Corneille, J.; Francis, S.; Goodman, D. W. Molecular Beam Studies of Ethanol Oxidation on Pd(110). Surf. Sci. 1997, 370, 113– 24,  DOI: 10.1016/S0039-6028(96)00959-4
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    Molecular beam studies of ethanol oxidation on Pd(110)
    Bowker, M.; Holroyd, R. P.; Sharpe, R. G.; Corneille, J. S.; Francis, S. M.; Goodman, D. W.
    Surface Science (1997), 370 (2-3), 113-124CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
    The adsorption and decompn. of ethanol on Pd(110) has been studied by use of a mol. beam reactor and temp. programmed desorption. It is found that the major pathway for ethanol decompn. occurs via a surface ethoxy to a Me group, carbon monoxide and hydrogen adatoms. The Me groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decomp. (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temp. (450 K) hydrogen peak is obsd. in TPD, resulting from the decompn. of stable hydrocarbon fragments. A competing pathway also exists which involves C-O bond scission of the ethoxy, probably caused by a crit. ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decompn. pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide resp. Below 250 K, some ethanol can form acetate which decomps. around 400 K to produce carbon dioxide and hydrogen.
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    Holroyd, R.; Bennett, R.; Jones, I.; Bowker, M. High Resolution XPS Study of the Ethanol Oxidation Reaction on Pd(110). J. Chem. Phys. 1999, 110, 8703– 13,  DOI: 10.1063/1.478777
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    High-resolution x-ray photoelectron spectroscopy study of the ethanol oxidation reaction on Pd(110)
    Holroyd, R. P.; Bennett, R. A.; Jones, I. Z.; Bowker, M.
    Journal of Chemical Physics (1999), 110 (17), 8703-8713CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The adsorption, decompn., and oxidn. of ethanol on Pd(110) has been studied using high-resoln. XPS (XPS) and temp.-programmed XPS. The decompn. pathways of ethanol on the clean surface (to methane, hydrogen and carbon monoxide; and to methane, hydrogen, and carbon and oxygen adatoms) previously studied using mol. beam and thermal desorption spectroscopy were confirmed by this study. The presence of an overlayer of oxygen did not significantly alter the major or minor decompn. pathways obsd. on the clean surface, except for the prodn. of water and, at temps. above 380 K, carbon dioxide as oxidn. products. It also resulted in the formation of acetate, which was first seen during temp.-programmed desorption as coincident carbon dioxide and hydrogen desorption, and was confirmed by XPS. Two C 1s peaks, one assigned to the Me carbon in acetate and the other to the carboxylate carbon, developed simultaneously during TPXPS. The disappearance of these peaks in XPS occurred at a similar temp. (400 K) to that seen during temp.-programmed desorption.
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    Zakeri, K. H.; Dashti, A. Monte Carlo simulation of temperature-programmed desorption CO/Cu(110) and CO2/Cu(100) systems. Surf. Rev. Lett. 2004, 11, 137– 143,  DOI: 10.1142/S0218625X04006037
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    Monte Carlo simulation of temperature-programmed desorption CO/Cu(110) and CO2/Cu(100) systems
    Zakeri, Kh.; Dashti, A.
    Surface Review and Letters (2004), 11 (2), 137-143CODEN: SRLEFH; ISSN:0218-625X. (World Scientific Publishing Co. Pte. Ltd.)
    We have studied the kinetics and mechanism of desorption of CO from the Cu(110) surface using a new Monte Carlo simulation and putting emphasis on high order lateral interaction. According to our simulated TPD spectra, for β = 10 K/s the max. desorption rate occurs at Tm = 218.6 K. Furthermore, anal. of simulated TPD spectra of CO desorption shows that it is strongly lateral-interactive and results an activation energy of CO desorption from Cu(110) that is Ed = 66.6 Kj/mol. These simulated results are compared with other reported results and show excellent agreement. After that we have investigated the kinetics and mechanism of desorption of CO2 from the Cu(100) surface using a Monte Carlo simulation. According to our simulated TPD spectra, for β = 0.5 K/s the max. desorption rate occurs at Tm = 89.7 K. Anal. of simulated TPD spectra of CO2 desorption shows that it is not strongly lateral-interactive and results in an activation energy of CO desorption from Cu(100) that is Ed = 25.2 Kj/mol. Finally, the CO/Cu(110) system is compared with the CO2/Cu(100) system.
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    Jeroro, E.; Hyman, M. P.; Vohs, J. M. Ensemble vs. electronic effects on the reactivity of two-dimensional Pd alloys: a comparison of CO and CH3OH adsorption on Zn/Pd(111) and Cu/Pd(111). Phys. Chem. Chem. Phys. 2009, 11, 10457– 10465,  DOI: 10.1039/b913220a
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    Ensemble vs. electronic effects on the reactivity of two-dimensional Pd alloys: a comparison of CO and CH3OH adsorption on Zn/Pd(111) and Cu/Pd(111)
    Jeroro, Eseoghene; Hyman, Matthew P.; Vohs, John M.
    Physical Chemistry Chemical Physics (2009), 11 (44), 10457-10465CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The adsorption of CO and CH3OH on two-dimensional PdCu alloys on Pd(111) was studied using temp.-programmed desorption (TPD) and high-resoln. electron energy loss spectroscopy (HREELS), and compared to results previously obtained for analogous PdZn alloys on Pd(111). Cu addn. to the Pd(111) surface was found to alter the preferred adsorption sites for CO from threefold to bridge Pd sites and decrease the activity for the dehydrogenation of CH3OH. However, the effect of Cu was much less dramatic than that obsd. for Zn on Zn-modified surfaces. Preliminary DFT calcns. also show that Cu causes less perturbation of the electronic structure of nearby Pd sites relative to Zn. The exptl. results for the surface PdCu alloys indicate that Cu predominantly has an ensemble effect on reactivity while more significant long-range electronic interactions in addn. to ensemble effects appear to be important for PdZn.
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    Vacuum (1962), 12 (), 203-11CODEN: VACUAV; ISSN:0042-207X.
    For small pumping speeds the desorption rate (by flashing chemisorbed gases from a filament) is proportional to the 1st derivative of pressure with time for very fast sweep rates. At high pumping speeds the desorption rate is proportional to the pressure. The activation energy of desorption (cal./mole) can be obtained from the temp. (Tp) at which the desorption rate is a max. (for 1st order reaction) and from a change of Tp with surface coverage (for 2nd order reaction), the order being established by the shape of exptl. curves. Curves are shown of the desorption rate of Ar from W for various values of bombarding ion energy and of H adsorbed on polycryst. W at 80°K. for various values of adsorption time.
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    Yuan, D.; Cai, L.; Xie, T.; Liao, H.; Hu, W. Selective hydrogenation of acetylene on Cu–Pd intermetallic compounds and Pd atoms substituted Cu(111) surfaces. Phys. Chem. Chem. Phys. 2021, 23, 8653,  DOI: 10.1039/D0CP05285J
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    Selective hydrogenation of acetylene on Cu-Pd intermetallic compounds and Pd atoms substituted Cu(111) surfaces
    Yuan, Dingwang; Cai, Li; Xie, Tuanping; Liao, Heting; Hu, Wangyu
    Physical Chemistry Chemical Physics (2021), 23 (14), 8653-8660CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The selective hydrogenation of acetylene was studied on the ordered Cu-Pd intermetallic compds. (L10-type CuPd, L12-type Cu3Pd, and L12-type CuPd3) and Pd-modified Cu(111) surfaces through first-principles calcns. The catalytic selectivity and activity of Cu-Pd alloy catalysts are closely related to the crystal structure and compn. of Cu-Pd intermetallic compds. and the size of Pd ensembles of Cu-based dil. alloy surface for the selective hydrogenation of acetylene to ethylene. Significantly, we found that the ordered Cu-Pd alloy surface contg. isolated Pd atoms (i.e., L12-type Cu3Pd(111) surface) is highly efficient for the selective hydrogenation reaction of C2H2 + H2 → C2H4. The contiguous Pd atom ensembles (Pd dimer and trimer) are catalytically active towards C2H2 + H → C2H3 and C2H3 + H → C2H4 reactions than the single Pd atom on a Pd-decorated Cu(111) surface. However, the small Pd ensembles on Cu(111) present a low chem. activity for H2 dissocn. compared with the ordered Cu-Pd intermetallic compds. Our theor. results provide a strategy of crystal phase and compn. control for enhancing the selectivity and activity of Cu-Pd catalysts towards acetylene selective hydrogenation.
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    Weightman, P.; Wright, H.; Waddington, S. D.; van der Marel, D.; Sawatzky, G. A.; Diakun, G. P.; Norman, D. Local lattice expansion around Pd impurities in Cu and its influence on the Pd density of states: An extended x-ray-absorption fine-structure and Auger study. Phys. Rev. B 1987, 36, 9098,  DOI: 10.1103/PhysRevB.36.9098
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    Local lattice expansion around palladium impurities in copper and its influence on the palladium density of states: an extended x-ray-absorption fine-structure and Auger study
    Weightman, P.; Wright, H.; Waddington, S. D.; Van der Marel, D.; Sawatzky, G. A.; Diakun, G. P.; Norman, D.
    Physical Review B: Condensed Matter and Materials Physics (1987), 36 (17), 9098-106CODEN: PRBMDO; ISSN:0163-1829.
    The d. of states of Pd in Cu given by impurity calcns. is incompatible with the obsd. profile of Pd Auger transitions. EXAFS expts. showed that there is a local expansion of the lattice around Pd impurities in Cu but not in Ag. An anal. of the influence of the local lattice expansion on the electronic structure of CuPd alloys showed that it led to a redn. in the intensity of the Pd d. of states at the bottom of the band and yielded a Pd d. of states in agreement with the obsd. Auger profile.
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    Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Effect of strain on the reactivity of metal surfaces. Phys. Rev. Lett. 1998, 81, 2819,  DOI: 10.1103/PhysRevLett.81.2819
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    Physical Review Letters (1998), 81 (13), 2819-2822CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
    Self-consistent d. functional calcns. for the adsorption of O and CO, and the dissocn. of CO on strained and unstrained Ru(0001) surfaces are used to show how strained metal surfaces have chem. properties that are significantly different from those of unstrained surfaces. Surface reactivity increases with lattice expansion, following a concurrent up-shift of the metal d states. Consequences for the catalytic activity of thin metal overlayers are discussed.
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    Hammer, B.; Nørskov, J. K. Theoretical surface science and catalysis – calucxlations and concepts. Adv. Catal. 2000, 45, 71,  DOI: 10.1016/S0360-0564(02)45013-4
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    A review, with 146 refs., is given on how calculational methods in close conjunction with expts. can be used to develop some useful concepts to describe and understand adsorption and reactions on surfaces. The application of d. functional theory to calc. adsorption properties, reaction pathways, and activation energies for surface chem. reactions is reviewed. Particular emphasis is placed on developing concepts that can be used to understand and predict variations in reactivity from one transition metal to the next or the effects of alloying, surface structure, and adsorbate-adsorbate interactions on the reactivity. Most examples discussed are concerned with the catalytic properties of transition metal surfaces, but the calculational approach and the concepts developed to understand trends in reactivity for metals can also be used for sulfide and oxide catalysts. (c) 2000 Academic Press.
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    Electronic factors determining the reactivity of metal surfaces
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    Surface Science (1995), 343 (3), 211-20CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
    Based on d. functional theory calcns. of H2 dissocn. on Al(111), Cu(111), Pt(111) and Cu3Pt(111) we present a consistent picture of some key phys. properties detg. the reactivity of metal and alloy surfaces. The four metal surfaces are chosen to represent metals with no d-bands, with filled d-bands and with d-states at the Fermi level. We show that electronic states in the entire valence band of the metal surface are responsible for the reactivity, which consequently cannot be understood solely in terms of the d. of states at the Fermi level nor in terms of the empty d-states above it. Rather we suggest that trends in reactivities can be understood in terms of the hybridization energy between the bonding and anti-bonding adsorbate states and the metal d-bands (when present), and we demonstrate that a simple frozen potential based est. of the hybridization energy correlates well with the calcd. variation of the barrier height for the different metal surfaces.
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    Greeley, J.; Nørskov, J. K. A general scheme for the estimation of oxygen binding energies on binary transition metal surface alloys. Surf. Sci. 2005, 592, 104– 111,  DOI: 10.1016/j.susc.2005.07.018
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    A general scheme for the estimation of oxygen binding energies on binary transition metal surface alloys
    Greeley, Jeff; Norskov, Jens K.
    Surface Science (2005), 592 (1-3), 104-111CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
    A simple scheme for the estn. of oxygen binding energies on transition metal surface alloys is presented. A d-band center model of the alloy surfaces is a convenient and appropriate basis for this scheme; variations in chem. compn., strain effects, and ligand effects are all incorporated into the binding energy anal. through this parameter. With few exceptions, the agreement of the results from the simple model with full DFT calcns. on hundreds of binary surface alloys is remarkable. The scheme should therefore provide a fast and effective method for the estn. of oxygen binding energies on a wide variety of transition metal alloys.
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    Ji, Y.; Chen, Z.; Wei, R.; Yang, C.; Wang, Y.; Xu, J.; Zhang, H.; Guan, A.; Chen, J.; Sham, T-k Selective CO-to-acetate electroreduction via intermediate adsorption tuning on ordered Cu–Pd sites Nature. Catalysis 2022, 5, 251– 258,  DOI: 10.1038/s41929-022-00757-8
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    Selective CO-to-acetate electroreduction via intermediate adsorption tuning on ordered Cu-Pd sites
    Ji, Yali; Chen, Zheng; Wei, Ruilin; Yang, Chao; Wang, Yuhang; Xu, Jie; Zhang, Hao; Guan, Anxiang; Chen, Jiatang; Sham, Tsun-Kong; Luo, Jun; Yang, Yaoyue; Xu, Xin; Zheng, Gengfeng
    Nature Catalysis (2022), 5 (4), 251-258CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
    Electrochem. redn. of carbon monoxide (CO) has recently emerged as a potential approach for obtaining high-value, multicarbon products such as acetate, while the activity and selectivity for prodution of acetate have remained low. Herein, we develop an atomically ordered copper-palladium intermetallic compd. (CuPd) composed of a high d. of Cu-Pd pairs that feature as catalytic sites to enrich surface *CO coverage, stabilize ethenone as a key acetate path intermediate and inhibit the hydrogen evolution reaction, thus substantially promoting acetate formation. The CuPd electrocatalyst enables a high Faradaic efficiency of 70 ± 5% for CO-to-acetate electroredn. and a high acetate partial c.d. of 425 mA cm-2. Under membrane electrode assembly conditions, the CuPd electrocatalyst demonstrated a 500 h CO-to-acetate conversion at 500 mA cm-2 with a stable acetate Faradaic efficiency of ∼50%.
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    Newton, M.; Francis, S. M.; Li, Y.; Law, D.; Bowker, M. Cu-Pd alloy surfaces I: Cu/Pd (85/15) {110}. surface structure and reactivity. Surf. Sci. 1991, 259, 45,  DOI: 10.1016/0039-6028(91)90522-T
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    Copper-palladium alloy surfaces. I. Copper/palladium [85:15]{110}, surface structure and reactivity
    Newton, Mark A.; Francis, Stephen M.; Li, Yongxue; Law, Donald; Bowker, Michael
    Surface Science (1991), 259 (1-2), 45-55CODEN: SUSCAS; ISSN:0039-6028.
    Both ordered and disordered phases of Cu:Pd[85:15]{110} were studied by using LEED, XPS, and UPS. Low-energy ion scattering (LEIS) also was employed to study the ordered surface. In the case of the ordered phase, these techniques were used to ascertain the structural and electronic influence of the Pd on this surface in comparison to that obsd. for Cu{110}. In addn., temp.-programmed desorption (TPD) was employed to det. the chem. reactivity of this surface towards the decompn. of adsorbed HCOOH and DCOOH. This is compared to the same reaction on Cu{110} and a kinetic isotope effect is noted in the decompn. of the 2 acids. Segregation of the Cu component of the alloy is obsd. upon the thermally induced transition from disordered to ordered surface phases, leaving the surface layer essentially free of Pd, while the second layer is enriched in Pd to ∼50% compn.
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    Newton, M.; Francis, S. M.; Bowker, M. Cu-Pd alloy surfaces II: Equilibrium surface compositions of dilute Cu/Pd alloys from a simple segregation model. Surf. Sci. 1991, 259, 56,  DOI: 10.1016/0039-6028(91)90523-U
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    Copper-palladium alloy surfaces. II. Equilibrium surface compositions of dilute palladium/copper alloys from a simple segregation model
    Newton, M. A.; Francis, S. M.; Bowker, M.
    Surface Science (1991), 259 (1-2), 56-64CODEN: SUSCAS; ISSN:0039-6028.
    The compn. of the CuPd[85:15](110) alloy selvedge region was modeled by using a quasichem. bond-breaking model of surface segregation. The results of these calcns. are compared to those obtained from exptl. studies of this surface, and it is shown that despite the neglect of size mismatch of solute and solvent atoms and the formal neglect of the ordering of the selvedge, the results for the top layer Pd concn. are compared to those made by a more comprehensive, but general theory of segregation. Comparisons to the analogous Cu3Au alloy surfaces and the Pt in Cu case are made, as are predictions about equil. selvedge compns. of other dil. Pd in Cu alloys.
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  54. 54
    Qiu, R.; Ding, Z.; Xu, Y.; Yang, Q.; Sun, K.; Hou, R. CuPd bimetallic catalyst with high Cu/Pd ratio and its application in CO2 hydrogenation. Appl. Surf. Sci. 2021, 544, 148974,  DOI: 10.1016/j.apsusc.2021.148974
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    54
    CuPd bimetallic catalyst with high Cu/Pd ratio and its application in CO2 hydrogenation
    Qiu, Rui; Ding, Ziluo; Xu, Yamei; Yang, Qiuchen; Sun, Kening; Hou, Ruijun
    Applied Surface Science (2021), 544 (), 148974CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
    The surface and catalytic properties Cu-Pd bimetallic catalyst with a high Cu/Pd ratio of 33.5 was investigated for the hydrogenation of CO2 and was compared with the corresponding monometallic catalysts. The catalysts were synthesized by the incipient-wetness impregnation method, characterized by N2-physisorption, XRD, TPR, CO pulse titrn., N2O chemisorption, CO2-TPD, and FT-IR of adsorbed CO, and were evaluated in a fixed-bed reactor. The interaction between Cu and Pd results in a better dispersion of both Cu and Pd atoms and facilitates the redn. properties. Phase sepn. of Pd1Cu3 alloy and Cu appears over the bimetallic catalyst, while at some part of the catalysts, isolated Pd sites exist over the Cu-rich particles. Both geometric and electronic effects are present over the bimetallic catalyst. The catalytic performances are found to be similar over Cu/SiO2 and Cu-Pd/SiO2 when normalized to the surface Cu sites. Specifically, CuPd/SiO2 shows enhanced activity than Cu/SiO2 mainly due to the geometric effect, whereas the electronic effect is weak for the catalytic performance.
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Cite this: J. Phys. Chem. C 2022, 126, 37, 15703–15709
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https://doi.org/10.1021/acs.jpcc.2c04881
Published September 12, 2022

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  • Abstract

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    Figure 1

    Figure 1. Schematic models of alkoxy and formate bonding on a Cu surface. Reproduced from Bowker. (11)

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    Figure 2

    Figure 2. Structure of the 1:1 CuPd(110) surface. Cu atoms in red, Pd in blue.

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    Figure 3

    Figure 3. Desorption after adsorbing formic acid on the CuPd(110) surface, showing formate decomposition at 350 K, which yields coincident desorption of H2 and CO2. Also illustrated are the peak decomposition temperatures for Pd(110) (36) and Cu(110). (34,35)

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    Figure 4

    Figure 4. (a) Ethanol TPD from Pd and Cu, showing the decomposition products and very different reaction pathways. The figure is compiled from data for Pd(110) (36) and for Cu(110). (35) (b) TPD after adsorption of ethanol on the Cu–Pd alloy surface at 180 K. The product pattern is generally similar to that from the two individual components of the alloy, showing both dehydrogenation (ethanal desorption at 270 K) and decarbonylation (methane desorption at 310 K).

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    Figure 5

    Figure 5. Molecular beam raw reactor data for ethanol in the beam reacting with oxygen predosed CuPd surface at 343 K surface temperature. The beam is opened to hit the surface at 0.5 min.

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  • References

    This article references 54 other publications.

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      The c(2×2) surface phase obtained from deposition of submonolayer amts. of Pd onto a clean Cu{001} surface is proven to have a top layer consisting of an ordered 50%-50% mixt. of Pd and Cu atoms, and no ordered content in deeper layers, analogous to the structure found for Cu{001}c(2×2)-Au. The structure of the top layer is almost planar, with the Pd atoms located 0.02 ± 0.03 Å outwards from the Cu atoms, and an interlayer distance from the Cu atoms to 2nd (100% Cu) layer equal to the Cu{001} bulk interlayer spacing (1.807 Å). Angle-resolved photoemission spectra show that the valence band of the surface alloy is characterized by a well-defined Cu 3d-derived band and by the appearance of features due to Pd. The narrower width of the obsd. photoemission peaks compared with the valence band of Cu-rich Cu-Pd alloys studied by others confirms the ordered-alloy nature of the Cu{001}c(2×2)-Pd surface phase.
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      Tourillon, G.; Cassuto, A.; Jugnet, Y.; Massardier, J.; Bertolini, J. C. Buta-1,3-diene and but-1-ene chemisorption on Pt(111), Pd(111), Pd(110) and Pd50Cu50(111) as studied by UPS, NEXAFS and HREELS in relation to catalysis. J. Chem. Soc., Faraday Trans. 1996, 92, 4835,  DOI: 10.1039/ft9969204835
      4
      1,3-Butadiene and 1-butene chemisorption on Pt(111), Pd(111), Pd(110) and Pd50Cu50(111) as studied by UPS, NEXAFS and HREELS in relation to catalysis
      Tourillon, G.; Cassuto, A.; Jugnet, Y.; Massardier, J.; Bertolini, J. C.
      Journal of the Chemical Society, Faraday Transactions (1996), 92 (23), 4835-4841CODEN: JCFTEV; ISSN:0956-5000. (Royal Society of Chemistry)
      Chemisorption of both 1,3-butadiene (I) and 1-butene (II) on Pt(111), Pd(111), Pd(110) and Pd50Cu50(111) samples was characterized by near-edge x-ray absorption fine structure (NEXAFS), UPS and high-resoln. electron energy loss spectroscopy (HREELS). I hydrogenation, studied on the same surfaces of Pd and Pd50Cu50, displays a very good selectivity in butenes and a higher activity compared to Pt(111). The Pd activity greatly depends on the surface cryst. orientation and is also influenced by alloying effects. The origin of these effects was sought in differences of chemisorption modes of I and II on the various surfaces: at 95 K, I and II are physisorbed on the different Pd-based single crystals. On Pt(111), I is π-bonded while II is di-σ-bonded. At 300 K, II either dehydrogenates into I on Pd(110) and Pd50Cu50(111) or very probably transforms into butylidyne on Pt(111) and on Pd(111) by analogy to C2H4 adsorbed on these (111) surfaces. I leads to a di-σ mode on Pt(111) and to a di-π one on the different Pd surfaces. The NEXAFS expts. reveal that the π1*-π2* splitting variations [2.0 eV on Pd(111), ∼2.4 eV on Pd(110) and Pd50Cu50(111), ∼2.7 eV on the condensed multilayer] agree with a decrease in the hydrocarbon-substrate interaction in the order Pd(111) > Pd(110) > Pd50Cu50(111). The activity would therefore obey the reverse sequence in agreement with the reactivity results.
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      Barbosa, L.; Loffreda, D.; Sautet, P. Chemisorption of Trichloroethene on the PdCu Alloy (110) Surface: A Periodical Density Functional Study. Langmuir 2002, 18, 2625– 2635,  DOI: 10.1021/la011113e
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      Chemisorption of trichloroethene on the PdCu alloy (110) surface. A Periodical Density Functional Study
      Barbosa, L. A. M. M.; Loffreda, D.; Sautet, P.
      Langmuir (2002), 18 (7), 2625-2635CODEN: LANGD5; ISSN:0743-7463. (American Chemical Society)
      Different adsorption modes of trichloroethene on the PdCu alloy were investigated. With application of ab initio periodic d. functional theory, some insights about these adsorption modes were revealed. The 2 different (110) terminations of the Cu3Pd alloy were employed as models for the surface region of the Cu50Pd50 alloy because of Cu segregation. They are based on the regular phase of the face-centered cubic (fcc) structure. The first model shows a mixed (Pd/Cu = 1) top layer, whereas the other one is Cu-terminated. The anal. of the position of the center of the d-band projected on the Pd and Cu atoms of both surfaces indicated that the Pd atom in the alloy has similar reactivity to the pure metal surface, whereas the opposite trend has been found for the Cu atoms. This was also confirmed by the adsorption energy, calcd. for the distinct modes. Trichlorethene prefers to interact with the Pd atoms on the mixed PdCu surface. Both di-σ and π modes are the most stable configurations among all studied. On the other hand, the adsorption via the chlorine atoms is the preferred on the Cu-terminated surface. The interaction of the C:C bond of trichloroethene on the Pd atoms is similar to the one of the ethene mol. The main difference between the adsorption of these 2 mols. is due to the extra Cl interactions with the Cu atoms. The di-σ configurations are the precursors of the dissocn. reaction on the mixed surface.
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      Sakong, S.; Mosch, C.; Groß, A. CO adsorption on Cu–Pd alloy surfaces: ligandversus ensemble effects. Phys. Chem. Chem. Phys. 2007, 9, 2216,  DOI: 10.1039/B615547B
      6
      CO adsorption on Cu-Pd alloy surfaces: ligand versus ensemble effects
      Sakong, Sung; Mosch, Christian; Gross, Axel
      Physical Chemistry Chemical Physics (2007), 9 (18), 2216-2225CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The CO adsorption on ordered Cu-Pd alloy surfaces and surface alloys was studied using d. functional theory (DFT) within the framework of the generalized gradient approxn. (GGA). On the surface alloys, the CO adsorption energy at the top sites decreases with increasing concn. of the more reactive metal Pd. This surprising ligand effect is caused by the effective compressive strain induced by the larger size of the Pd atoms. However, at the most favorable adsorption sites the CO binding becomes stronger with increasing Pd concn. which is caused by an ensemble effect related to the availability of higher coordinated adsorption sites. At the surfaces of the bulk alloys, the trends in the adsorption energy as a function of the Pd concn. are less clear because of the strong Pd-Cu interaction and the absence of effective strain effects.
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    7. 7
      Sun, C. Q.; Wang, Y.; Nie, Y. G.; Mehta, B. R.; Khanuja, M.; Shivaprasad, S. M.; Sun, Y.; Pan, J. S.; Pan, L. K.; Sun, Z. Interface quantum trap depression and charge polarization in the CuPd and AgPd bimetallic alloy catalysts. Phys. Chem. Chem. Phys. 2010, 12, 3131– 3135,  DOI: 10.1039/b922677j
      7
      Interface quantum trap depression and charge polarization in the CuPd and AgPd bimetallic alloy catalysts
      Sun, Chang Q.; Wang, Yan; Nie, Yan Guang; Mehta, B. R.; Khanuja, M.; Shivaprasad, S. M.; Sun, Yi; Pan, Ji Sheng; Pan, L. K.; Sun, Zhuo
      Physical Chemistry Chemical Physics (2010), 12 (13), 3131-3135CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The ability of a catalyst to accept or donate charge is the key to the process of catalytic reaction. However, the detn. of the catalytic nature of a specimen as yet remains a great challenge. Here we report an effective yet simple method for this purpose based on the tight binding theory considerations and XPS monitoring of the evolution of valence and core electrons upon alloy formation. Firstly, we measured the valence and core band charge d. of the constituent elements of Cu, Ag, and Pd and then the resp. states upon alloy formation. A subtraction of the resultant spectrum of the alloy by the composed elemental spectra gives the residual that shows clearly the occurrence of charge trapping or polarization. We found that the valence and the core electrons of the CuPd alloy shift pos. to deeper energies, opposite to the occurrences in the AgPd alloy. Findings clarify for the first time that CuPd serves as an acceptor due to quantum trapping and the AgPd as a donor because of charge polarization, which also explain why AgPd and CuPd perform very differently as important catalysts.
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    8. 8
      Wachs, I. E.; Madix, R. J. The Oxidation Of Ethanol On Cu(110) And Ag(1 10) Catalysts. Applications of Surface Science 1978, 1, 303– 328,  DOI: 10.1016/0378-5963(78)90034-X
      8
      The oxidation of ethanol on copper(110) and silver(110) catalysts
      Wachs, Israel E.; Madix, Robert J.
      Applications of Surface Science (1977-1985) (1978), 1 (3), 303-28CODEN: ASUSDD; ISSN:0378-5963.
      The programmed reaction spectroscopy of EtOD on 18O2-preoxidized Ag (110) or Cu (110) single crystal surfaces indicated that the ability of the surfaces to dissociatively chemisorb the EtOD was enhanced by surface O and that EtOD was selectively oxidized by adsorption at 180 K to give adsorbed EtO- and D218O. The Ag (110) surface was more active than the Cu (110) surface for the dehydrogenation of EtOD to MeCHO and H; the recombination of EtO- with H to give EtOH was also obsd. At high EtOD concns. surface EtO+D2 (I) was obsd. at 180 K. Decompn. of I gave CH2:CH2, D2O, and H. EtO- was less stable than MeO- on both surfaces.
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      Bowker, M.; Madix, R. J. XPS, UPS and thermal desorption studies of alcohol adsorption on Cu(110); II. Higher Alcohols, Surface Sci. 1982, 116, 549,  DOI: 10.1016/0039-6028(82)90364-8
      9
      XPS, UPS and thermal desorption studies of alcohol adsorption on copper(110). II. Higher alcohols
      Bowker, M.; Madix, R. J.
      Surface Science (1982), 116 (3), 549-72CODEN: SUSCAS; ISSN:0039-6028.
      The adsorption of EtOH, n- and i-PrOH, and ethylene glycol on Cu was studied.. These mols. form stable alkoxy species on the surface, i.e., the alc. dissocd. at the OH group. In contrast to MeOH, the higher alcs. react further with the surface, dehydrogenating to yield the corresponding aldehydes or ketones in the gas phase. Ethylene glycol reacts to form the most strongly bound intermediates, decompg. at ∼390 K to produce glyoxal, with little evidence of monoaldehyde formation or C-C bond breakage. The influence of preadsorbed O on these reactions is to generally increase the amt. of alkoxy formed on the surface by enhancing the degree of dissociative adsorption. The alkoxide decompn. peaks are shifted to slightly higher temps. and considerably broadened in such expts. The decompn. peak temps. of the surface alkoxides correlate fairly well with literature values of the α C-H bond strength, which is weaker in i-PrOH than MeOH. XPS showed broad O(1s) spectra for the mols. adsorbed at 140 K, probably due to H-bonding effects in the adlayer, with peak emissions at ∼533 eV. When the surface was warmed to 250 K, the O(1s) spectra narrow to close to instrumental linewidths, with a concomitant shift to a lower binding energy at ∼531 eV. C(1s) spectra show little change between the adsorbed alc. and alkoxy species. The UPS showed low-temp. spectra similar to that for the gas phase, but the highest occupied orbitals (essentially O(2p) orbitals), showed a chemisorption bonding shift of several tenths of an eV. UPS for these mols. has considerable less utility than for MeOH, because of masking of possible orbital shifts during chem. changes on the surface by the presence of overlapping emissions.
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    10. 10
      Wang, Z.-T.; Xu, Y.; El-Soda, M.; Lucci, F. R.; Madix, R. J.; Friend, C. M.; Sykes, E. C. H. Surface Structure Dependence of the Dry Dehydrogenation of Alcohols on Cu(111) and Cu(110). J. Phys. Chem. C 2017, 121, 12800– 12806,  DOI: 10.1021/acs.jpcc.7b02957
      10
      Surface Structure Dependence of the Dry Dehydrogenation of Alcohols on Cu(111) and Cu(110)
      Wang, Zhi-Tao; Xu, Yunfei; El-Soda, Mostafa; Lucci, Felicia R.; Madix, Robert J.; Friend, Cynthia M.; Sykes, E. Charles H.
      Journal of Physical Chemistry C (2017), 121 (23), 12800-12806CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      The nonoxidative dehydrogenation of alcs. is considered as an important method to produce aldehydes for the chem. industry and hydrogen gas. However, current industrial processes are oxidative, meaning that the aldehydes are formed along with water, which, in addn. to being less energy efficient, poses sepn. problems. Herein the prodn. of aldehydes from methanol and ethanol on clean and dry Cu(111) and Cu(110) surfaces was investigated in order to understand the catalytic anhyd. dehydrogenation of alcs. Both ethanol and methanol preferentially react under ultrahigh vacuum conditions at surface defects to yield acetaldehyde and formaldehyde, resp., in the absence of surface oxygen and water. The amt. of alkoxide reaction intermediates measured by scanning tunneling microscopy, and aldehyde and hydrogen products detected by temp. programmed reaction, are increased by inducing more defects in the Cu substrates with Ar ion sputtering. This work also reveals that the Cu model surfaces are not poisoned by the reaction and exhibit 100% selectivity for alc. dehydrogenation to aldehyde and hydrogen.
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    11. 11
      Bowker, M. Methanol Synthesis from CO2 Hydrogenation. ChemCatChem. 2019, 11, 4238– 4246,  DOI: 10.1002/cctc.201900401
      11
      Methanol synthesis from CO2 hydrogenation
      Bowker, Michael
      ChemCatChem (2019), 11 (17), 4238-4246CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. In the future we will be phasing out the use of fossil fuels in favor of more sustainable forms of energy, esp. solar derived forms such as hydroelec., wind and photovoltaic. However, due to the variable nature of the latter sources which depend on time of day, and season of the year, we also need to have a way of storing such energy at peak prodn. times for use in times of low prodn. One way to do this is to convert such energy into chem. energy, and the principal way considered at present is the prodn. of hydrogen. Although this may be achieved directly in the future via photocatalytic water splitting, at present it is electrolytic prodn. which dominates thinking. In turn, it may well be important to store this hydrogen in an energy dense liq. form such as methanol or ammonia. In this brief review it is emphasized that CO2 is the microscopic carbon source for current industrial methanol synthesis, operating through the surface formate intermediate, although when using CO in the feed, it is CO which is hydrogenated at the global scale. However, methanol can be produced from pure CO2 and hydrogen using conventional and novel types of catalysts. Examples of such processes, and of a demonstrator plant in construction, are given, which utilize CO2 (which would otherwise enter the atm. directly) and hydrogen which can be produced in a sustainable manner. This is a fast-evolving area of science and new ideas and processes will be developed in the near future.
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    12. 12
      Bowker, M.; Madix, R. J. XPS, UPS and thermal desorption studies on the reactions of formaldehyde and formic acid with the Cu(110) surface. Surf. Sci. 1981, 102, 542,  DOI: 10.1016/0039-6028(81)90045-5
      12
      XPS, UPS and thermal desorption studies of the reactions of formaldehyde and formic acid with the copper(110) surface
      Bowker, M.; Madix, R. J.
      Surface Science (1981), 102 (2-3), 542-65CODEN: SUSCAS; ISSN:0039-6028.
      Formaldehyde adsorbs on the clean Cu (110) surface in a weakly bound state which interacts with the metal primarily through its most weakly bound orbital, the in-plane orbital composed mainly of O(2p). When O is preadsorbed on the surface, the XPS spectrum clearly shows the formation of 2 distinct species when formaldehyde is subsequently adsorbed. One species is formaldehyde as on the clean surface and the other is H2CO2. The latter species decomps. at ∼230 K, leaving the formate, HCOO, which then decomps. at 500 K, to produce H2 and CO2. The formate species is also produced by the adsorption of DCOOH on Cu(110) at 400 K. At 140 K, formic acid adsorbs molecularly but UPS indicates that its mol. structure is already strongly perturbed by adsorption; dissocn. of the acid H takes place during heating around 250 K. Oxygen preadsorption increases the amt. of formate produced from formic acid, abstracting the acid H atoms to produce H2O.
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    13. 13
      Nakano, H.; Nakamura, I.; Fujitani, T.; Nakamura, J. Synthesis and Decomposition of Formate Species. J. Phys. Chem. B 2001, 105, 1355– 65,  DOI: 10.1021/jp002644z
      13
      Structure-Dependent Kinetics for Synthesis and Decomposition of Formate Species over Cu(111) and Cu(110) Model Catalysts
      Nakano, Haruhisa; Nakamura, Isao; Fujitani, Tadahiro; Nakamura, Junji
      Journal of Physical Chemistry B (2001), 105 (7), 1355-1365CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The kinetics and mechanism of formate synthesis by hydrogenation of CO2 (CO2 + 1/2H2 → HCOOa) and the formate decompn. into CO2 and H2 (HCOOa → CO2 + 1/2H2) over Cu(111) and Cu(110) surfaces were studied by in-situ IR reflection-absorption spectroscopy (IRAS) using a high-pressure reactor (∼1 atm). The reaction rates and the apparent activation energy of the formate synthesis were measured for Cu(111) and Cu(110), indicating that the formate synthesis on Cu was found to be structure-insensitive. The pressure dependence of CO2 and H2 on the initial formation rate of formate suggested an Eley-Rideal type mechanism, in which a gaseous CO2 mol. directly reacts with an adsorbed hydrogen atom on Cu. This is analogous to the well-known mechanism of formate synthesis by organometallic catalysts, in which CO2 is inserted into a Cu-hydride bond. The reaction rates and the activation energy of the decompn. were measured for Cu(111) and Cu(110). It was found that the formate decompn. on Cu was structure-sensitive in contrast to the formate synthesis. The promotional effect of coexisting H2 upon the rate of formate decompn. by 17 times at max. was incidentally found only on Cu(111). Interestingly, the increase in the decompn. rate was due to an increase in the preexponential factor of the rate const. for the formate decompn. with the activation energy being const. Furthermore, the decompn. kinetics of the formate prepd. by adsorption of formic acid on O/Cu(111) was identical with the H2-promoted decompn. kinetics of the synthesized formate. The difference in the decompn. kinetics was ascribed to the ordered structure of formate based on the previous STM results, in which a chainlike structure of formate was obsd. for the synthesized formate, whereas no formate chain was obsd. for the formate prepd. by adsorption of formic acid on O/Cu(111). The unique character of both the decompn. kinetics and the structure of formate obsd. only for Cu(111) was discussed from the viewpoint of the mass transport of copper atoms creating added formate chains.
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    14. 14
      Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, M. A. An iras study of formic acid and surface formate adsorbed on Cu(110). Surf. Sci. 1983, 133, 589,  DOI: 10.1016/0039-6028(83)90021-3
      14
      An IRAS study of formic acid and surface formate adsorbed on copper(110)
      Hayden, B. E.; Prince, K.; Woodruff, D. P.; Bradshaw, A. M.
      Surface Science (1983), 133 (2-3), 589-604CODEN: SUSCAS; ISSN:0039-6028.
      The adsorption and decompn. of formic acid on Cu(110) was studied by IR reflection-absorption spectroscopy and temp.-programmed desorption. A broad double peak at ∼1635 cm-1 is characteristic of the carbonyl stretch ν(C:O) of the acid monolayer after adsorption at 120 K. Deprotonation occurs at ∼270 K to give a surface formate species. The estd. formate coverage θsat = 0.25 ± 0.05 and the reaction probability for deprotonation is estd. The sym. O-C-O vibration νs(COO) (1348-1358 cm-1) and the C-H stretch vibration ν(CH) (2891-2900 cm-1) are given as a function of formate coverage. A vibrational band corresponding to ν(CH) (2891-2900 cm-1) is given as a function of formate coverage. A vibrational band corresponding to νa(COO) is not obsd. under any exptl. conditions; the formate species is thus thought to have C2v or Cs(1) symmetry. A combination band νcomb at 2950 cm-1 is obsd. Its components (νa(COO) and δ(CH)) are not obsd. individually, since they have dynamic dipoles parallel to the surface. The combination band shifts at low coverages to lower frequencies, which is due to lateral interactions between parallel dynamic dipoles, and has an intensity comparable to that of the neighboring C-H stretch, which may be due to a Fermi resonance.
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    15. 15
      Sexton, B. A.; Hughes, A. E.; Avery, N. R. A spectroscopic study of the adsorption and reactions of methanol, formaldehyde and methyl formate on clean and oxygenated Cu (110) surfaces. Surf. Sci. 1985, 155, 366,  DOI: 10.1016/0039-6028(85)90423-6
      15
      A spectroscopic study of the adsorption and reactions of methanol, formaldehyde and methyl formate on clean and oxygenated copper(110) surfaces
      Sexton, B. A.; Hughes, A. E.; Avery, N. R.
      Surface Science (1985), 155 (1), 366-86CODEN: SUSCAS; ISSN:0039-6028.
      A clean Cu surface is relatively unreactive, but adsorbed O readily attacks the hydroxyl proton and formyl C atoms to generate intermediate MeO and radicals. MeOCHO is split into 2 intermediates, MeO and HCO2. The condensed multilayer at 90 K, the weakly bound mol. monolayer states prior to dissocn. of reaction, and the reactive intermediates at higher temps. were analyzed by electron energy-loss, UPS, and thermal-desorption spectroscopies. MeOCHO coordinates to Cu via the carbonyl lone-pair orbital and MeOH via the O lone-pair orbital. There is no evidence for MeOCHO synthesis by dimerization of CH2O (the Tischenko reaction) or dehydrogenation of MeOH on the clean Cu(110) surface.
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    16. 16
      Crapper, M. D.; Riley, C. E.; Woodruff, D. P.; Puschmann, A.; Haase, J. Determination of the adsorption structure for formate on Cu (110) using SEXAFS and NEXAFS. Surf. Sci. 1986, 171, 1,  DOI: 10.1016/0039-6028(86)90558-3
      16
      Determination of the adsorption structure for formate on copper(110) using SEXAFS and NEXAFS
      Crapper, M. D.; Riley, C. E.; Woodruff, D. P.; Puschmann, A.; Haase, J.
      Surface Science (1986), 171 (1), 1-12CODEN: SUSCAS; ISSN:0039-6028.
      Near-edge x-ray absorption fine structure (NEXAFS) is used to establish the C-O bond length (1.25 ± 0.05 Å), the O-C-O angle (124 ± 15°) and the azimuthal orientation (along [1‾10]) of the mol. plane of the formate catalytic intermediate adsorbed on Cu(110). Extended x-ray absorption fine structure (EXAFS) is then used to det. the Cu-O bond length (1.98 ± 0.07 Å) and the adsorption site (atop Cu atoms in the top (ridge) rows, with O atoms near short bridge sites). The problems of obtaining reliable quant. results from NEXAFS are discussed. The adsorption site and bond length, which agrees well with that for anhyd. Cu formate, is contrasted with the much longer bond length found for this species adsorbed on Cu(100) by Stoehr et al. (1985).
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    17. 17
      Shiozawa, Y.; Koitaya, T.; Mukai, K.; Yoshimoto, S.; Yoshinobu, J. The Roles of Step- Site and Zinc in Surface Chemistry of Formic Acid on Clean and Zn-Modified Cu(111) and Cu(997) Surfaces Studied by HR-XPS, TPD, and IRAS. J. Chem. Phys. 2020, 152, 044703  DOI: 10.1063/1.5132979
      17
      The roles of step-site and zinc in surface chemistry of formic acid on clean and Zn-modified Cu(111) and Cu(997) surfaces studied by HR-XPS, TPD, and IRAS
      Shiozawa, Yuichiro; Koitaya, Takanori; Mukai, Kozo; Yoshimoto, Shinya; Yoshinobu, Jun
      Journal of Chemical Physics (2020), 152 (4), 044703CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The adsorption, desorption, and decompn. of formic acid (HCOOH) on Cu(111), Cu(997), Zn-Cu(111), and Zn-Cu(997) were systematically studied by high-resoln. XPS, temp. programmed desorption, and IR reflection absorption spectroscopy. On the clean Cu(111) surface, 13% of formic acid mols. adsorbed at 83 K were dissocd. to form bidentate formate species by heating at 300 K; however, on the Zn-Cu(111) surface, only 4% of adsorbed HCOOH mols. were dissocd. into the bidentate formate species. On the contrary, 13% of adsorbed HCOOH mols. were already dissocd. into monodentate formate species on Cu(997) even at 83 K and 17% of adsorbed formic acid mols. were transformed to bidentate formate species by heating at 300 K, indicating that the stepped Cu surface has higher reactivity for HCOOH dissocn. at low temp. On the Zn-Cu(997) surface, 20% of formic acid became bidentate formate species in contrast to the case with Zn-Cu(111). Thus, the Zn deposited Cu step surface shows special activity for adsorption and dissocn. of formic acid. The desorption peak maxima of the formate decompn. products (CO2 and H2) on Zn-Cu(997) were shifted to higher temps. than those on Cu(997). Zn on Cu surfaces plays an important role in the stabilization of formate species, which probably leads to the decrease in the activation barrier for hydrogenation on the Zn-Cu alloyed surface. (c) 2020 American Institute of Physics.
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    18. 18
      Shiozawa, Y.; Koitaya, T.; Mukai, K.; Yoshimoto, S.; Yoshinobu, J. Quantitative Analysis of Desorption and Decomposition Kinetics of Formic Acid on Cu(111): The Importance of Hydrogen Bonding between Adsorbed Species. J. Chem. Phys. 2015, 143, 234707,  DOI: 10.1063/1.4937414
      18
      Quantitative analysis of desorption and decomposition kinetics of formic acid on Cu(111): The importance of hydrogen bonding between adsorbed species
      Shiozawa, Yuichiro; Koitaya, Takanori; Mukai, Kozo; Yoshimoto, Shinya; Yoshinobu, Jun
      Journal of Chemical Physics (2015), 143 (23), 234707/1-234707/7CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Quant. anal. of desorption and decompn. kinetics of formic acid (HCOOH) on Cu(111) was performed by temp. programmed desorption (TPD), XPS, and time-resolved IR reflection absorption spectroscopy. The activation energy for desorption is estd. to be 53-75 kJ/mol by the threshold TPD method as a function of coverage. Vibrational spectra of the first layer HCOOH at 155.3 K show that adsorbed mols. form a polymeric structure via the hydrogen bonding network. Adsorbed HCOOH mols. are dissocd. gradually into monodentate formate species. The activation energy for the dissocn. into monodentate formate species is estd. to be 65.0 kJ/mol at a submonolayer coverage (0.26 mols./surface Cu atom). The hydrogen bonding between adsorbed HCOOH species plays an important role in the stabilization of HCOOH on Cu(111). The monodentate formate species are stabilized at higher coverages, because of the lack of vacant sites for the bidentate formation. (c) 2015 American Institute of Physics.
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    19. 19
      Chutia, A.; Silverwood, I. P.; Farrow, M. R.; Scanlon, D. O.; Wells, P. P.; Bowker, M.; Parker, S. F.; Catlow, C. R. A. Adsorption of formate species on Cu(h,k,l) low index surfaces – a combined DFT and INS study. Surf. Sci. 2016, 653, 45– 54,  DOI: 10.1016/j.susc.2016.05.002
      19
      Adsorption of formate species on Cu(h,k,l) low index surfaces
      Chutia, Arunabhiram; Silverwood, Ian P.; Farrow, Matthew R.; Scanlon, David O.; Wells, Peter P.; Bowker, Michael; Parker, Stewart F.; Catlow, C. Richard A.
      Surface Science (2016), 653 (), 45-54CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      We report a d. functional theory study on the relative stability of formate species on Cu(h,k,l) low index surfaces using a range of exchange-correlation functionals. We find that these functionals predict similar geometries for the formate mol. adsorbed on the Cu surface. A comparison of the calcd. vibrational transition energies of a perpendicular configuration of formate on Cu surface shows an excellent agreement with the exptl. spectrum obtained from inelastic neutron spectroscopy. From the calcns. on adsorption energy we find that formate is most stable on the Cu(110) surface as compared to Cu(111) and Cu(100) surfaces. Bader anal. shows that this feature could be related to the higher charge transfer from the Cu(110) surface and optimum charge d. at the interfacial region due to bidirectional electron transfer between the formate and the Cu surface. Anal. of the partial d. of states finds that in the -5.5 eV to -4.0 eV region, hybridization between O p and the non-axial Cu dyz and dxz orbitals takes place on the Cu(110) surface, which is energetically more favorable than on the other surfaces.
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    20. 20
      Li, S.; Scaranto, J.; Mavrikakis, M. On the Structure Sensitivity of Formic Acid Decomposition on Cu Catalysts. Top. Catal. 2016, 59, 1580– 1588,  DOI: 10.1007/s11244-016-0672-1
      20
      On the Structure Sensitivity of Formic Acid Decomposition on Cu Catalysts
      Li, Sha; Scaranto, Jessica; Mavrikakis, Manos
      Topics in Catalysis (2016), 59 (17-18), 1580-1588CODEN: TOCAFI; ISSN:1022-5528. (Springer)
      Catalytic decompn. of formic acid (HCOOH) has attracted substantial attention since HCOOH is a major byproduct in biomass reforming, a promising hydrogen carrier, and also a potential low temp. fuel cell feed. Despite the abundance of exptl. studies for vapor-phase HCOOH decompn. on Cu catalysts, the reaction mechanism and its structure sensitivity is still under debate. Self-consistent, periodic d. functional theory calcns. were performed on three model surfaces of copper-Cu(111), Cu(100) and Cu(211), and both the HCOO (formate)-mediated and COOH (carboxyl)-mediated pathways were studied for HCOOH decompn. The energetics of both pathways suggest that the HCOO-mediated route is more favorable than the COOH-mediated route on all three surfaces, and that HCOOH decompn. proceeds through two consecutive dehydrogenation steps via the HCOO intermediate followed by the recombinative desorption of H2. On all three surfaces, HCOO dehydrogenation is the likely rate detg. step since it has the highest transition state energy and also the highest activation energy among the three catalytic steps in the HCOO pathway. The reaction is structure sensitive on Cu catalysts since the examd. three Cu facets have dramatically different binding strengths for the key intermediate HCOO and varied barriers for the likely rate detg. step-HCOO dehydrogenation. Cu(100) and Cu(211) bind HCOO much more strongly than Cu(111), and they are also characterized by potential energy surfaces that are lower in energy than that for the Cu(111) facet. Coadsorbed HCOO and H represents the most stable state along the reaction coordinate, indicating that, under reaction conditions, there might be a substantial surface coverage of the HCOO intermediate, esp. at under-coordinated step, corner or defect sites. Therefore, under reaction conditions, HCOOH decompn. is predicted to occur most readily on the terrace sites of Cu nanoparticles. As a result, the authors hereby present an example of a fundamentally structure-sensitive reaction, which may present itself as structure-insensitive in typical varied particle-size expts.
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    21. 21
      Kattel, S.; Ramírez, P. J.; Chen, J. G.; Rodriguez, J. A.; Liu, P. Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts. Science 2017, 355, 1296– 1299,  DOI: 10.1126/science.aal3573
      21
      Active sites for CO2 hydrogenation to methanol on Cu/ZnO catalysts
      Kattel, Shyam; Ramirez, Pedro J.; Chen, Jingguang G.; Rodriguez, Jose A.; Liu, Ping
      Science (Washington, DC, United States) (2017), 355 (6331), 1296-1299CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      The active sites over com. copper/zinc oxide/aluminum oxide (Cu/ZnO/Al2O3) catalysts for carbon dioxide (CO2) hydrogenation to methanol, the Zn-Cu bimetallic sites or ZnO-Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x-ray photoemission spectroscopy, d. functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both exptl. and theor. results agree that ZnCu undergoes surface oxidn. under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.
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    22. 22
      Yang, Y.; Mims, C. A.; Disselkamp, R. S.; Mei, D.; Kwak, J.-H.; Szanyi, J.; Peden, C. H. F.; Campbell, C. T. Isotope Effects in Methanol Synthesis and the Reactivity of Copper Formates on a Cu/SiO2 Catalyst. Catal. Lett. 2008, 125, 201– 208,  DOI: 10.1007/s10562-008-9592-4
      22
      Isotope Effects in Methanol Synthesis and the Reactivity of Copper Formates on a Cu/SiO2 Catalyst
      Yang, Y.; Mims, C. A.; Disselkamp, R. S.; Mei, D.; Kwak, Ja-Hun; Szanyi, J.; Peden, C. H. F.; Campbell, C. T.
      Catalysis Letters (2008), 125 (3-4), 201-208CODEN: CALEER; ISSN:1011-372X. (Springer)
      Here we investigate isotope effects on the catalytic methanol synthesis reaction and the reactivity of copper-bound formate species in CO2-H2 atmospheres on Cu/SiO2 catalysts by simultaneous IR and MS measurements, both steady-state and transient. Studies of isotopic variants (H/D, 12C/13C) reveal that bidentate formate dominates the copper surface at steady state. The steady-state formate coverages of HCOO (in 6 bar 3:1 H2:CO2) and DCOO (in D2:CO2) are similar and the steady-state formate coverages in both systems decrease by ∼80% from 350 K to 550 K. Over the temp. range 413 K-553 K, the steady-state methanol synthesis rate shows a weak H/D isotope effect (1.05 ± 0.05) with somewhat higher activation energies in H2:CO2 (79 kJ/mol) than D2:CO2 (71 kJ/mol) over the range 473 K-553 K. The reverse water gas shift (RWGS) rates are higher than methanol synthesis and also shows a weak pos. H/D isotope effect with higher activation energy for H2/CO2 than D2/CO2 (108 vs. and 102 kJ/mol) The reactivity of the resulting formate species in 6 bar H2, 6 bar D2 and 6 bar Ar is strongly dominated by decompn. back to CO2 and H2. H2 and D2 exposure compared to Ar do not enhance the formate decompn. rate. The decompn. profiles on the supported catalyst deviate from first order decay, indicating distributed surface reactivity. The av. decompn. rates are similar to values previously reported on single crystals. The av. activation energies for formate decompn. are 90 ± 17 kJ/mol for HCOO and 119 ± 11 kJ/mol for DCOO. By contrast to the catalytic reaction rates, the formate decompn. rate shows a strong H/D kinetic isotope effect (H/D ∼8 at 413 K), similar to previously obsd. values on Cu(110).
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    23. 23
      Reilly, J.; Barnes, C.; Price, N.; Bennett, R.; Poulston, S.; Stone, P.; Bowker, M. ’The Growth, Mechanism, Thermal Stability and Reactivity of Pd Mono- amd Multi layers on Cu(110). J. Phys. Chem. B 1999, 103, 6521,  DOI: 10.1021/jp990156p
      23
      The Growth Mechanism, Thermal Stability, and Reactivity of Palladium Mono- and Multilayers on Cu(110)
      Reilly, J. P.; Barnes, C. J.; Price, N. J.; Bennett, R. A.; Poulston, S.; Stone, P.; Bowker, M.
      Journal of Physical Chemistry B (1999), 103 (31), 6521-6532CODEN: JPCBFK; ISSN:1089-5647. (American Chemical Society)
      The room-temp. growth of palladium (Pd) on Cu(110) was studied by XPS, scanning tunneling microscopy (STM), LEED, and temp.-programmed desorption (TPD). XPS signal vs. deposition time plots rule out a simple layer-by-layer growth mechanism. STM/LEED indicates formation of regions of (2 × 1) overlayer at low Pd coverages (θPd < 1 ML), with considerable disorder as monolayer deep pits and islands. Higher Pd coverages give a granular film consisting of densely packed, flat-topped Pd clusters of av. size 75 × 150 Å and with largely a rectangular shape. The favored growth mechanism is of multilayered Pd islands above a mixed (2 × 1) CuPd interface of two to three at. layers thickness. The thermal stability of the Pd/Cu(110) system was studied with XPS peak intensity vs. annealing temp. plots that indicate that bulk intermixing takes place rapidly between 500 and 600 K. The Pd 3d5/2 XPS peak widths narrow, suggesting the formation of a largely homogeneous CuPd surface alloy. STM indicates that heating to 500 K leaves the Pd clusters in a largely unaltered morphol. with no sign of Ostwald ripening, whereas annealing to 600 K leads to considerable changes in topog. The granular structure of the Pd film is disrupted, leading to a surface with irregularly shaped flat domains sepd. by mono-at. steps. High temp. (720 K) annealing leads to further flattening and appearance of regular parallel lines in STM images. The spacing of these lines varies with Pd loading, and they are assigned to strain due to lattice mismatch between the capping copper monolayer and the underlying mixed CuPd alloy. The reactivity of the Pd/Cu(110) surface was probed by dosing formic acid and monitoring formate decompn. High Pd coverages lead to a substantial destabilization of the formate relative to clean Cu(110), which is assigned to formate adsorption on mixed CuPd sites.
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    24. 24
      Bennett, R.; Poulston, S.; Reilly, J.; Stone, P.; Barnes, C.; Bowker, M. Morphology of Pd Multilayers on Cu(110). Surf. Sci. 1999, 433–435, 816– 821
      24
      Morphology of Pd multilayers on Cu(110)
      Bennett, R. A.; Poulston, S.; Price, N. J.; Reilly, J. P.; Stone, P.; Barnes, C. J.; Bowker, M.
      Surface Science (1999), 433-435 (), 816-821CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
      The growth of Pd films deposited on a Cu(110) single crystal surface and the resulting morphol. of the film structure has been followed by STM for deposition at 310 K. For ∼1 ML films an ordered (2 × 1) surface alloy is formed which is also visible in LEED. The surface morphol. is, however, not flat but has roughened step edges, raised islands and smaller trenches. For increased coverages of Pd the growth shifts to the formation of rectangular multilayered Pd islands. Upon annealing, AES shows a preferential segregation of Cu to the surface region that accompanies a flattening of the surface morphol. in STM. Annealing thick films (>4 ML) to 603 K produces a surface morphol. with larger flat terraces and small islands that have rough, non-crystallog. aligned step edges. With further annealing to 723 K these surfaces also display large-scale "banding" in the STM images that we attribute to stress relaxation between the CuPd alloy and Cu.
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    25. 25
      Bowker, M.; Newton, M.; Francis, S.; Gleeson, M.; Barnes, C. The Surface Structure of a Low Pd Cu/Pd-(110) Crystal Alloy. Surf. Rev. Lett. 1994, 1, 569– 571,  DOI: 10.1142/S0218625X94000680
      25
      The surface structure of a low Pd Cu/Pd-(110) crystal alloy
      Bowker, M.; Newton, M.; Francis, S. M.; Gleeson, M.; Barnes, C.
      Surface Review and Letters (1994), 1 (4), 569-71CODEN: SRLEFH; ISSN:0218-625X. (World Scientific)
      X-ray photoelectron diffraction studies of this alloy surface have been carried out and indicate that there is a significant expansion of the lattice in the near-surface region due to the high concn. of Pd in layer 2. Preliminary single scattering calcns. lend support to this proposal for the surface structure, and place this expansion in the subsurface mainly between layers 2 and 3.
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    26. 26
      Newton, M.; Francis, S.; Bowker, M. The use of forward focused photoelectrons in analysis of an alloy surface: Cu/Pd (85/15) {110}. J. Physics, Cond. Matter 1991, 3, S139,  DOI: 10.1088/0953-8984/3/S/023
      There is no corresponding record for this reference.
    27. 27
      Hammoudeh, A.; Mousa, M.S.; Loboda-Cackovic, J. Interaction of CO with clean and oxygen covered PdCu (110) single crystal alloy. Vacuum 1999, 54, 239,  DOI: 10.1016/S0042-207X(98)00471-0
      27
      Interaction of CO with clean and oxygen covered PdCu(110) single crystal alloy
      Hammoudeh, A.; Mousa, M. S.; Loboda-Cackovic, J.
      Vacuum (1999), 54 (1-4), 239-243CODEN: VACUAV; ISSN:0042-207X. (Elsevier Science Ltd.)
      Compositional changes that may occur upon CO chemisorption on PdCu (110) single crystal alloy have been studied by means of work function change measurements using the Kelvin probe technique. Furthermore, various surfaces of this alloy with different surface compns. were investigated with respect to their reactivity in the CO oxidn. applying the same technique. It was found that CO chemisorption, in the case of Cu-rich surfaces, induces the segregation of Pd to the surface. High CO pressures (10-4-10-3 mbar) also cause the surface segregation of still unidentified contamination characterized by an AES signal at 180 eV. In the CO oxidn. expts. the PdCu alloy showed lower reactivities compared to pure Pd, even though the outermost layer was entirely composed of Pd atoms. The reactivity was found to decrease further with increasing Cu concns. The lower reactivity of the PdCu alloy was attributed to the charge transfer from Pd to Cu obsd. in this system resulting in decreasing the adsorption enthalpy (and probably the sticking coeff.) of CO on "modified" Pd of the alloy surface.
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    28. 28
      Loboda-Cackovic, J.; Mousa, M.; Block, J. H. Surface analysis of the PdCu (110) single crystal alloy at different segregation rates. Vacuum 1995, 46, 89,  DOI: 10.1016/0042-207X(95)87000-8
      28
      Surface analysis of the PdCu(110) single crystal alloy at different segregation rates
      Loboda-Cackovic, J.; Mousa, M. S.; Block, J. H.
      Vacuum (1995), 46 (2), 89-96CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
      The PdCu(110) plane, with Pd:Cu = 1:1 in the bulk, was prepd. to have various surface compns. by applying cycles of argon ion sputtering (ion energy of ≤750 eV with ion current densities ≤1 μA/cm2) and annealing temps. 420 < T < 820 K. With this prepn. method different compns. of the top surface layers (TL) were obtained: (i) exclusively Pd atoms; (ii) compn. of various Cu/Pd ratios, including the bulk ratio; and (iii) exclusively Cu atoms. The surface region (SR) of ∼4 layers depth analyzed by AES, and the CO TDS used for TL-characterization, allowed the av. ratio of Cu/Pd in the next three subsurface layers (3SSL) to be evaluated. LEED enabled the detn. of the surface structure after each step. The high temp. treatment of the surface followed by low temp. sputtering (TSP) and TAN below that of Cu segregation (TSEG) ∼550 K, produced a smooth Pd-rich surface. On the other hand, a large no. of sputtering and annealing cycles without prior high TAN produced a rough surface, enabling the Cu atoms lying beneath the TL to be accessible for surface reactions. A diagram describing LEED structures can be drawn from the results which present the large variety of surface structures for the clean PdCu(110) single crystal surface depending on the TSP and TAN. In addn. to this diagram the SR Cu/Pd ratio for various TSP and TAN is given for slow and fast annealing modes at long and short pretreatment periods. Three phase diagrams comparing exptl. surface properties for each of the SR, TL and 3SSL with bulk compn. are presented.
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    29. 29
      Loboda-Cackovic, J. Properties of PdCu (110) single crystal alloy surfaces: Temperature-induced processes in the surface microstructure. Vacuum 1996, 47, 1405,  DOI: 10.1016/S0042-207X(96)00228-X
      29
      Properties of PdCu(110) single crystal alloy surfaces: temperature-induced processes in the surface microstructure
      Loboda-Cackovic, J.
      Vacuum (1996), 47 (12), 1405-1411CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
      The Pd-Cu(110) alloy single crystal surfaces, with a 1:1 at. concn. ratio in the bulk, were prepd. with various compns. Cu/Pd ratios in the surface regions, measured ∼4 layers thick and ranged from 0.3 to 2. Different compns. of the top surface layers were obtained, from exclusively Pd atoms, over mixts. of Pd and Cu atoms, to exclusively Cu atoms. The behavior of video LEED spot intensities and profiles during specimen heating and cooling was measured. The Pd-Cu(110) surface consists of domains contg. 10-20 lattice cells depending on the prepn. procedure. The surface microstructure influences the temp.-induced processes at the surface, such as surface roughening and partial surface disordering starting at ∼550 and ∼700 K, resp. The domain grain boundaries play an important role in the surface roughening process. At higher temps., the domain size distribution, aside from grain boundaries, causes partial surface disordering. The two Cu segregation processes, starting at ∼550 and ∼700 K, are the vol. counterparts to these surface processes.
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    30. 30
      Mousa, M.; Loboda-Cackovic, J.; Block, J. H. Characterization of PdCu (110) single crystal surface compositions during CO chemisorption. Vacuum 1995, 46, 117,  DOI: 10.1016/0042-207X(94)E0002-G
      30
      Characterization of PdCu(110) single crystal surface compositions during CO chemisorption
      Mousa, M. S.; Loboda-Cackovic, J.; Block, J. H.
      Vacuum (1995), 46 (2), 117-25CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
      The PdCu(110) single crystal alloy surfaces (bulk Pd/Cu = 1:1) were prepd. by sputtering and annealing at different temps. Surfaces with Cd/Pd contents ranging from ∼100% Pd to ∼100% Cu at the top surface layer could be obtained. AES with surface region (SR) anal. of ∼4 layers, LEED and TDS of CO with top surface layer (TL) anal. were the methods used for characterization. LEED of clean surfaces prepd. at low temps. (<550 K) showed the Pd(1×2) structure. By treating the alloy at higher temps. (>550 K), Cu segregation occurs and the structure changes to (1×1) and then to Cu(1×2). For a Pd-rich surface with Cu/Pd(SR) ≤0.1, CO TDS showed five desorption states, three of them appearing at 450, 400 and 340 K resp., thus resembling those from CO/pure Pd(110). The other two new CO-Pd binding states appear at 250 and 190 K, which are characteristic for the allow surface. The appearance of the 250 K max. indicates a ligand effect in CO-desorption. At Cu/Pd(SR) of ≥0.3 the CO desorption from Pd bridge-sites is absent in the spectra. The other two high temp. desorption sites shift to lower temps. (by ∼50 K) indicating the ensemble effect. With increasing Cu concn. in the TL, TDS show an addnl. peak at 160 K. Another fascinating property of this alloy is the possibility of prepg. the TL with ∼100% Pd, while in the next three subsurface layers (3SSL) the Cu/Pd ratio ranges from 0.1 to 2. For the Pd-rich TL, the CO initial sticking coeff. increases with increasing Cu/Pd ratio in the 3SSL. Detailed anal. of all these results revealed that the bcc. lattice model is essential for the PdCu(110) alloy surface at Cu/Pd(SR) > 0.2.
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    31. 31
      Loboda-Cackovic, J.; Block, J. H. Properties of PdCu (110) single crystal alloy surfaces: reversible changes of domain sizes. Vacuum 1995, 46, 1449,  DOI: 10.1016/0042-207X(95)00169-7
      31
      Properties of PdCu(110) single crystal alloy surfaces: reversible changes of domain sizes
      Loboda-Cackovic, J.; Block, J. H.
      Vacuum (1995), 46 (12), 1449-53CODEN: VACUAV; ISSN:0042-207X. (Elsevier)
      The compn. of PdCu(110) single crystal alloy surfaces (bulk Pd/Cu = 1:1) can be changed by sputtering and annealing at different temps. A survey of surface structures and compns. in equil. shows the influence of Cu segregation. Samples with Cu/Pd ratios in the surface region (≃4 layers) from 0.3 to 1.3 and with (1 × 2) and (1 × 1) surface structures were prepd. Surfaces with ∼100% Pd atoms and mixed compns. of Pd and Cu atoms (down to ∼70% Pd) at the top surface layer were studied. Methods used for characterization were video LEED for structure anal., Auger electron spectroscopy for the surface region of ∼4 layers depth and CO thermal desorption spectroscopy for compn. anal. of the top surface layer. Domain extents along [10] and [01] crystallog. directions, of surfaces with different compns., changed reversibly when measured during sample heating and cooling cycles. The change proceeds in a manger that depends on the sizes of domains, on the cryst. amt. and on the amt. of Cu in the surface region. The surface domains contain a small no. of lattice cells (∼9 to ∼18) and a large amt. of atoms in the disordered intercryst. areas.
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    32. 32
      Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Isolated metal atom geometries as a strategy for selective heterogeneous hydrogenations. Science 2012, 335, 1209– 12,  DOI: 10.1126/science.1215864
      32
      Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations
      Kyriakou, Georgios; Boucher, Matthew B.; Jewell, April D.; Lewis, Emily A.; Lawton, Timothy J.; Baber, Ashleigh E.; Tierney, Heather L.; Flytzani-Stephanopoulos, Maria; Sykes, E. Charles H.
      Science (Washington, DC, United States) (2012), 335 (6073), 1209-1212CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Facile dissocn. of reactants and weak binding of intermediates are key requirements for efficient and selective catalysis. However, these two variables are intimately linked in a way that does not generally allow the optimization of both properties simultaneously. By using desorption measurements in combination with high-resoln. scanning tunneling microscopy, the authors show that individual, isolated Pd atoms in a Cu surface substantially lower the energy barrier to both hydrogen uptake on and subsequent desorption from the Cu metal surface. This facile hydrogen dissocn. at Pd atom sites and weak binding to Cu allow for very selective hydrogenation of styrene and acetylene as compared with pure Cu or Pd metal alone.
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    33. 33
      Hager, T.; Rauscher, H.; Behm, R. J. Interaction of CO with PdCu surface alloys supported on Ru (0 0 0 1). Surf. Sci. 2004, 558, 181– 94,  DOI: 10.1016/j.susc.2004.04.001
      33
      Interaction of CO with PdCu surface alloys supported on Ru(0 0 0 1)
      Hager, T.; Rauscher, H.; Behm, R. J.
      Surface Science (2004), 558 (1-3), 181-194CODEN: SUSCAS; ISSN:0039-6028. (Elsevier Science B.V.)
      The adsorption and desorption of CO on monolayer PdCu surface alloys on a Ru(0 0 0 1) substrate and, for comparison, on Pd/Ru(0 0 0 1), and Cu/Ru(0 0 0 1) monolayer films, were studied by temp. programmed desorption and IR reflection absorption spectroscopy, aiming at a detailed understanding of the different effects controlling the chem. properties of bimetallic surfaces, geometric ensemble effects, electronic ligand effects and strain induced effects. All of these effects are present in these surfaces, acting in a cooperative, synergetic way. Strain effects lead to a destabilization of the Pd-CO and a stabilization of the Cu-CO interaction as compared to CO adsorption on the resp. bulk substrates. Geometric ensemble effects are imposed by the highly disperse distribution of the Cu and Pd atoms, reducing the no. of otherwise favorable Pd3 sites, and electronic ligand effects lead to an increasing stability of the Pd-CO bond with increasing Cu content.
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    34. 34
      Bowker, M.; Pudney, P. D. A.; Barnes, C. A simple molecular beam system for surface reactivity studies. J. Vac. Sci. Technol. 1990, A8, 816– 820,  DOI: 10.1116/1.576924
      There is no corresponding record for this reference.
    35. 35
      Ying, D.; Madix, R. J. Thermal desorption study of formic acid decomposition on a clean Cu (110) surface. J. Catal. 1980, 61, 48,  DOI: 10.1016/0021-9517(80)90338-3
      35
      Thermal desorption study of formic acid decomposition on a clean copper (110) surface
      Ying, David H. S.; Madix, Robert J.
      Journal of Catalysis (1980), 61 (1), 48-56CODEN: JCTLA5; ISSN:0021-9517.
      The catalytic decompn. of HCO2H was examd. on a (110)-oriented Cu single-crystal surface. Dehydrogenation of HCO2H was the sole decompn. path obsd. The parent mol. dissocd. to form a stable formate species (HCO2(a)) as the surface intermediate. The HCO2- species was bonded to the surface Cu sites through the two O atoms. The decompn. of the HCO2- species was the rate-eliminating step governing the rates of the formation. of the CO2 and H2 products. The rate const. was detd. A comparison of the catalytic properties of the Cu(110) surface with those of the carburized Ni(110) surface showed strong chem. similarities between the 2 surfaces.
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    36. 36
      Bowker, M.; Rowbotham, E.; Leibsle, F.; Haq, S. The Adsorption and Decomposition of Formic Acid on Cu(110). Surf. Sci. 1996, 349, 97,  DOI: 10.1016/0039-6028(95)01069-6
      36
      The adsorption and decomposition of formic acid on Cu{110}
      Bowker, M.; Rowbotham, E.; Leibsle, F. M.; Haq, S.
      Surface Science (1996), 349 (2), 97-110CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
      The reactive adsorption of formic acid (HCOOH) on O-dosed Cu (110) was studied by using a mol. beam system, TPD, LEED, and STM. At low temp., the reaction is strongly O-coverage dependent. All coverages result in high reaction probability (0.8 at room temp.) for formic acid and for < 0.25 monolayers of O, there is complete O clean-off, leaving formate on the surface in a c(2×2) structure. At higher coverages, the situation is more complex, with some O remaining coadsorbed with the formate. The 2 adsorbates are then mainly phase sepd. into islands of c(6×2) O and (3×1) formate. The 2 phases mutually compress each other due to pressure at the phase boundaries. The reaction stoichiometry is 2:1 formic acid:O atoms in this temp. range. At higher temps. (> 450 K), the formate itself is unstable and decomps. during adsorption which results in a change of stoichiometry of the reaction; 1 mol. of formic acid removes an O atom as H2O, and H evolution ceases. There is a range of temp. between 350 and 420 K for which the reaction becomes very difficult, and the reaction probability drops to ∼ 0.1. This is due to rapid compression of much of the O adlayer into the unreactive c(6×2) structure by small amts. of formate. The reaction proceeds through a highly mobile, weakly held, "precursor" state on the surface, which is able to seek out the active sites on the surface, which are low in coverage at high levels of O. These active sites are the terminal O atoms in the O islands (in the [001] direction), which are only present at step edges or phase boundaries at 0.5 monolayers coverage of O.
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    37. 37
      Aas, N.; Li, Y.; Bowker, M. The adsorption and decomposition of formic acid on clean and oxygen dosed Pd(110). J. Phys. Cond. Matter 1991, 3, S281,  DOI: 10.1088/0953-8984/3/S/044
      There is no corresponding record for this reference.
    38. 38
      Yuan, D.; Zhang, Y. Theoretical investigations of HCOOH decomposition on ordered Cu-Pd alloy surfaces. Appl. Surf. Sci. 2018, 462, 649– 658,  DOI: 10.1016/j.apsusc.2018.08.048
      38
      Theoretical investigations of HCOOH decomposition on ordered Cu-Pd alloy surfaces
      Yuan, Dingwang; Zhang, Yong
      Applied Surface Science (2018), 462 (), 649-658CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      Formic acid (HCOOH) decompn. is studied on four ordered Cu-Pd compds. (B2-type CuPd, L10-type CuPd, L12-type Cu3Pd, and L12-type CuPd3) through systematic d. functional calcns. The crystal structures and at. compns. play a significant role to control activity and selectivity of catalysts. The O-H bond dissocn. barrier of HCOOH is less than that of the C-H bond scission, and the most high selectivity for O-H/C-H dissocn. is found on the single Pd atom alloyed in Cu lattice of L12 Cu3Pd(1 1 1) surface. However, the contiguous Pd atoms greatly facilitate hydrogen prodn. from HCOOH, and the barriers are 0.44 and 0.73 eV for the dehydrogenation reactions on of HCOOH → HCOO + H → CO2 + 2H on L10 CuPd(1 1 1) surface. Though the CO formation pathway is energetically favorable for COOH decompn. on the ordered Cu-Pd surfaces except Pd-rich ordered CuPd3(1 1 1) surface, the adsorbed CO can be easily removed due to the weaker interaction compared with pure Pd surfaces. The calcd. results indicate that L10 CuPd and L12 CuPd3 alloys are effective catalysts for formic acid decompn. with high catalytic selectivity and activity.
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    39. 39
      Bowker, M.; Holroyd, R.; Sharpe, R.; Corneille, J.; Francis, S.; Goodman, D. W. Molecular Beam Studies of Ethanol Oxidation on Pd(110). Surf. Sci. 1997, 370, 113– 24,  DOI: 10.1016/S0039-6028(96)00959-4
      39
      Molecular beam studies of ethanol oxidation on Pd(110)
      Bowker, M.; Holroyd, R. P.; Sharpe, R. G.; Corneille, J. S.; Francis, S. M.; Goodman, D. W.
      Surface Science (1997), 370 (2-3), 113-124CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
      The adsorption and decompn. of ethanol on Pd(110) has been studied by use of a mol. beam reactor and temp. programmed desorption. It is found that the major pathway for ethanol decompn. occurs via a surface ethoxy to a Me group, carbon monoxide and hydrogen adatoms. The Me groups can either produce methane (which they do with a high selectivity for adsorption below 250 K) or can further decomp. (which they do with a high selectivity for adsorption above 350 K) resulting in surface carbon. If adsorption occurs above 250 K a high temp. (450 K) hydrogen peak is obsd. in TPD, resulting from the decompn. of stable hydrocarbon fragments. A competing pathway also exists which involves C-O bond scission of the ethoxy, probably caused by a crit. ensemble of palladium atoms at steps, defects or due to a local surface reconstruction. The presence of oxygen does not significantly alter the decompn. pathway above 250 K except that water and, above 380 K, carbon dioxide are produced by reaction of the oxygen adatoms with hydrogen adatoms and adsorbed carbon monoxide resp. Below 250 K, some ethanol can form acetate which decomps. around 400 K to produce carbon dioxide and hydrogen.
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    40. 40
      Holroyd, R.; Bennett, R.; Jones, I.; Bowker, M. High Resolution XPS Study of the Ethanol Oxidation Reaction on Pd(110). J. Chem. Phys. 1999, 110, 8703– 13,  DOI: 10.1063/1.478777
      40
      High-resolution x-ray photoelectron spectroscopy study of the ethanol oxidation reaction on Pd(110)
      Holroyd, R. P.; Bennett, R. A.; Jones, I. Z.; Bowker, M.
      Journal of Chemical Physics (1999), 110 (17), 8703-8713CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The adsorption, decompn., and oxidn. of ethanol on Pd(110) has been studied using high-resoln. XPS (XPS) and temp.-programmed XPS. The decompn. pathways of ethanol on the clean surface (to methane, hydrogen and carbon monoxide; and to methane, hydrogen, and carbon and oxygen adatoms) previously studied using mol. beam and thermal desorption spectroscopy were confirmed by this study. The presence of an overlayer of oxygen did not significantly alter the major or minor decompn. pathways obsd. on the clean surface, except for the prodn. of water and, at temps. above 380 K, carbon dioxide as oxidn. products. It also resulted in the formation of acetate, which was first seen during temp.-programmed desorption as coincident carbon dioxide and hydrogen desorption, and was confirmed by XPS. Two C 1s peaks, one assigned to the Me carbon in acetate and the other to the carboxylate carbon, developed simultaneously during TPXPS. The disappearance of these peaks in XPS occurred at a similar temp. (400 K) to that seen during temp.-programmed desorption.
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    41. 41
      Zakeri, K. H.; Dashti, A. Monte Carlo simulation of temperature-programmed desorption CO/Cu(110) and CO2/Cu(100) systems. Surf. Rev. Lett. 2004, 11, 137– 143,  DOI: 10.1142/S0218625X04006037
      41
      Monte Carlo simulation of temperature-programmed desorption CO/Cu(110) and CO2/Cu(100) systems
      Zakeri, Kh.; Dashti, A.
      Surface Review and Letters (2004), 11 (2), 137-143CODEN: SRLEFH; ISSN:0218-625X. (World Scientific Publishing Co. Pte. Ltd.)
      We have studied the kinetics and mechanism of desorption of CO from the Cu(110) surface using a new Monte Carlo simulation and putting emphasis on high order lateral interaction. According to our simulated TPD spectra, for β = 10 K/s the max. desorption rate occurs at Tm = 218.6 K. Furthermore, anal. of simulated TPD spectra of CO desorption shows that it is strongly lateral-interactive and results an activation energy of CO desorption from Cu(110) that is Ed = 66.6 Kj/mol. These simulated results are compared with other reported results and show excellent agreement. After that we have investigated the kinetics and mechanism of desorption of CO2 from the Cu(100) surface using a Monte Carlo simulation. According to our simulated TPD spectra, for β = 0.5 K/s the max. desorption rate occurs at Tm = 89.7 K. Anal. of simulated TPD spectra of CO2 desorption shows that it is not strongly lateral-interactive and results in an activation energy of CO desorption from Cu(100) that is Ed = 25.2 Kj/mol. Finally, the CO/Cu(110) system is compared with the CO2/Cu(100) system.
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    42. 42
      Jeroro, E.; Hyman, M. P.; Vohs, J. M. Ensemble vs. electronic effects on the reactivity of two-dimensional Pd alloys: a comparison of CO and CH3OH adsorption on Zn/Pd(111) and Cu/Pd(111). Phys. Chem. Chem. Phys. 2009, 11, 10457– 10465,  DOI: 10.1039/b913220a
      42
      Ensemble vs. electronic effects on the reactivity of two-dimensional Pd alloys: a comparison of CO and CH3OH adsorption on Zn/Pd(111) and Cu/Pd(111)
      Jeroro, Eseoghene; Hyman, Matthew P.; Vohs, John M.
      Physical Chemistry Chemical Physics (2009), 11 (44), 10457-10465CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The adsorption of CO and CH3OH on two-dimensional PdCu alloys on Pd(111) was studied using temp.-programmed desorption (TPD) and high-resoln. electron energy loss spectroscopy (HREELS), and compared to results previously obtained for analogous PdZn alloys on Pd(111). Cu addn. to the Pd(111) surface was found to alter the preferred adsorption sites for CO from threefold to bridge Pd sites and decrease the activity for the dehydrogenation of CH3OH. However, the effect of Cu was much less dramatic than that obsd. for Zn on Zn-modified surfaces. Preliminary DFT calcns. also show that Cu causes less perturbation of the electronic structure of nearby Pd sites relative to Zn. The exptl. results for the surface PdCu alloys indicate that Cu predominantly has an ensemble effect on reactivity while more significant long-range electronic interactions in addn. to ensemble effects appear to be important for PdZn.
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    43. 43
      Redhead, P. A. Thermal Desorption of Gases. Vacuum 1962, 12, 203,  DOI: 10.1016/0042-207X(62)90978-8
      43
      Thermal desorption of gases
      Redhead, P. A.
      Vacuum (1962), 12 (), 203-11CODEN: VACUAV; ISSN:0042-207X.
      For small pumping speeds the desorption rate (by flashing chemisorbed gases from a filament) is proportional to the 1st derivative of pressure with time for very fast sweep rates. At high pumping speeds the desorption rate is proportional to the pressure. The activation energy of desorption (cal./mole) can be obtained from the temp. (Tp) at which the desorption rate is a max. (for 1st order reaction) and from a change of Tp with surface coverage (for 2nd order reaction), the order being established by the shape of exptl. curves. Curves are shown of the desorption rate of Ar from W for various values of bombarding ion energy and of H adsorbed on polycryst. W at 80°K. for various values of adsorption time.
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    44. 44
      Yuan, D.; Cai, L.; Xie, T.; Liao, H.; Hu, W. Selective hydrogenation of acetylene on Cu–Pd intermetallic compounds and Pd atoms substituted Cu(111) surfaces. Phys. Chem. Chem. Phys. 2021, 23, 8653,  DOI: 10.1039/D0CP05285J
      44
      Selective hydrogenation of acetylene on Cu-Pd intermetallic compounds and Pd atoms substituted Cu(111) surfaces
      Yuan, Dingwang; Cai, Li; Xie, Tuanping; Liao, Heting; Hu, Wangyu
      Physical Chemistry Chemical Physics (2021), 23 (14), 8653-8660CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The selective hydrogenation of acetylene was studied on the ordered Cu-Pd intermetallic compds. (L10-type CuPd, L12-type Cu3Pd, and L12-type CuPd3) and Pd-modified Cu(111) surfaces through first-principles calcns. The catalytic selectivity and activity of Cu-Pd alloy catalysts are closely related to the crystal structure and compn. of Cu-Pd intermetallic compds. and the size of Pd ensembles of Cu-based dil. alloy surface for the selective hydrogenation of acetylene to ethylene. Significantly, we found that the ordered Cu-Pd alloy surface contg. isolated Pd atoms (i.e., L12-type Cu3Pd(111) surface) is highly efficient for the selective hydrogenation reaction of C2H2 + H2 → C2H4. The contiguous Pd atom ensembles (Pd dimer and trimer) are catalytically active towards C2H2 + H → C2H3 and C2H3 + H → C2H4 reactions than the single Pd atom on a Pd-decorated Cu(111) surface. However, the small Pd ensembles on Cu(111) present a low chem. activity for H2 dissocn. compared with the ordered Cu-Pd intermetallic compds. Our theor. results provide a strategy of crystal phase and compn. control for enhancing the selectivity and activity of Cu-Pd catalysts towards acetylene selective hydrogenation.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmtVSqtbc%253D&md5=04131b9e841b57f4385fa547033791af
    45. 45
      Weightman, P.; Wright, H.; Waddington, S. D.; van der Marel, D.; Sawatzky, G. A.; Diakun, G. P.; Norman, D. Local lattice expansion around Pd impurities in Cu and its influence on the Pd density of states: An extended x-ray-absorption fine-structure and Auger study. Phys. Rev. B 1987, 36, 9098,  DOI: 10.1103/PhysRevB.36.9098
      45
      Local lattice expansion around palladium impurities in copper and its influence on the palladium density of states: an extended x-ray-absorption fine-structure and Auger study
      Weightman, P.; Wright, H.; Waddington, S. D.; Van der Marel, D.; Sawatzky, G. A.; Diakun, G. P.; Norman, D.
      Physical Review B: Condensed Matter and Materials Physics (1987), 36 (17), 9098-106CODEN: PRBMDO; ISSN:0163-1829.
      The d. of states of Pd in Cu given by impurity calcns. is incompatible with the obsd. profile of Pd Auger transitions. EXAFS expts. showed that there is a local expansion of the lattice around Pd impurities in Cu but not in Ag. An anal. of the influence of the local lattice expansion on the electronic structure of CuPd alloys showed that it led to a redn. in the intensity of the Pd d. of states at the bottom of the band and yielded a Pd d. of states in agreement with the obsd. Auger profile.
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    46. 46
      Mavrikakis, M.; Hammer, B.; Nørskov, J. K. Effect of strain on the reactivity of metal surfaces. Phys. Rev. Lett. 1998, 81, 2819,  DOI: 10.1103/PhysRevLett.81.2819
      46
      Effect of strain on the reactivity of metal surfaces
      Mavrikakis, M.; Hammer, B.; Norskov, J. K.
      Physical Review Letters (1998), 81 (13), 2819-2822CODEN: PRLTAO; ISSN:0031-9007. (American Physical Society)
      Self-consistent d. functional calcns. for the adsorption of O and CO, and the dissocn. of CO on strained and unstrained Ru(0001) surfaces are used to show how strained metal surfaces have chem. properties that are significantly different from those of unstrained surfaces. Surface reactivity increases with lattice expansion, following a concurrent up-shift of the metal d states. Consequences for the catalytic activity of thin metal overlayers are discussed.
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    47. 47
      Hammer, B.; Nørskov, J. K. Theoretical surface science and catalysis – calucxlations and concepts. Adv. Catal. 2000, 45, 71,  DOI: 10.1016/S0360-0564(02)45013-4
      47
      Theoretical surface science and catalysis - calculations and concepts
      Hammer, B.; Norskov, J. K.
      Advances in Catalysis (2000), 45 (), 71-129CODEN: ADCAAX; ISSN:0065-2342. (Academic Press)
      A review, with 146 refs., is given on how calculational methods in close conjunction with expts. can be used to develop some useful concepts to describe and understand adsorption and reactions on surfaces. The application of d. functional theory to calc. adsorption properties, reaction pathways, and activation energies for surface chem. reactions is reviewed. Particular emphasis is placed on developing concepts that can be used to understand and predict variations in reactivity from one transition metal to the next or the effects of alloying, surface structure, and adsorbate-adsorbate interactions on the reactivity. Most examples discussed are concerned with the catalytic properties of transition metal surfaces, but the calculational approach and the concepts developed to understand trends in reactivity for metals can also be used for sulfide and oxide catalysts. (c) 2000 Academic Press.
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    48. 48
      Hammer, B.; Norskov, J. K. Electronic factors determining the reactivity of metal surfaces. Surf. Sci. 1995, 343, 211,  DOI: 10.1016/0039-6028(96)80007-0
      48
      Electronic factors determining the reactivity of metal surfaces
      Hammer, B.; Noerskov, J. K.
      Surface Science (1995), 343 (3), 211-20CODEN: SUSCAS; ISSN:0039-6028. (Elsevier)
      Based on d. functional theory calcns. of H2 dissocn. on Al(111), Cu(111), Pt(111) and Cu3Pt(111) we present a consistent picture of some key phys. properties detg. the reactivity of metal and alloy surfaces. The four metal surfaces are chosen to represent metals with no d-bands, with filled d-bands and with d-states at the Fermi level. We show that electronic states in the entire valence band of the metal surface are responsible for the reactivity, which consequently cannot be understood solely in terms of the d. of states at the Fermi level nor in terms of the empty d-states above it. Rather we suggest that trends in reactivities can be understood in terms of the hybridization energy between the bonding and anti-bonding adsorbate states and the metal d-bands (when present), and we demonstrate that a simple frozen potential based est. of the hybridization energy correlates well with the calcd. variation of the barrier height for the different metal surfaces.
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    49. 49
      Dowden, D. A.; Reynolds, P. W. Some reactions over alloy catalysts. Disc. Faraday Soc. 1950, 8, 184– 190,  DOI: 10.1039/df9500800184
      There is no corresponding record for this reference.
    50. 50
      Greeley, J.; Nørskov, J. K. A general scheme for the estimation of oxygen binding energies on binary transition metal surface alloys. Surf. Sci. 2005, 592, 104– 111,  DOI: 10.1016/j.susc.2005.07.018
      50
      A general scheme for the estimation of oxygen binding energies on binary transition metal surface alloys
      Greeley, Jeff; Norskov, Jens K.
      Surface Science (2005), 592 (1-3), 104-111CODEN: SUSCAS; ISSN:0039-6028. (Elsevier B.V.)
      A simple scheme for the estn. of oxygen binding energies on transition metal surface alloys is presented. A d-band center model of the alloy surfaces is a convenient and appropriate basis for this scheme; variations in chem. compn., strain effects, and ligand effects are all incorporated into the binding energy anal. through this parameter. With few exceptions, the agreement of the results from the simple model with full DFT calcns. on hundreds of binary surface alloys is remarkable. The scheme should therefore provide a fast and effective method for the estn. of oxygen binding energies on a wide variety of transition metal alloys.
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    51. 51
      Ji, Y.; Chen, Z.; Wei, R.; Yang, C.; Wang, Y.; Xu, J.; Zhang, H.; Guan, A.; Chen, J.; Sham, T-k Selective CO-to-acetate electroreduction via intermediate adsorption tuning on ordered Cu–Pd sites Nature. Catalysis 2022, 5, 251– 258,  DOI: 10.1038/s41929-022-00757-8
      51
      Selective CO-to-acetate electroreduction via intermediate adsorption tuning on ordered Cu-Pd sites
      Ji, Yali; Chen, Zheng; Wei, Ruilin; Yang, Chao; Wang, Yuhang; Xu, Jie; Zhang, Hao; Guan, Anxiang; Chen, Jiatang; Sham, Tsun-Kong; Luo, Jun; Yang, Yaoyue; Xu, Xin; Zheng, Gengfeng
      Nature Catalysis (2022), 5 (4), 251-258CODEN: NCAACP; ISSN:2520-1158. (Nature Portfolio)
      Electrochem. redn. of carbon monoxide (CO) has recently emerged as a potential approach for obtaining high-value, multicarbon products such as acetate, while the activity and selectivity for prodution of acetate have remained low. Herein, we develop an atomically ordered copper-palladium intermetallic compd. (CuPd) composed of a high d. of Cu-Pd pairs that feature as catalytic sites to enrich surface *CO coverage, stabilize ethenone as a key acetate path intermediate and inhibit the hydrogen evolution reaction, thus substantially promoting acetate formation. The CuPd electrocatalyst enables a high Faradaic efficiency of 70 ± 5% for CO-to-acetate electroredn. and a high acetate partial c.d. of 425 mA cm-2. Under membrane electrode assembly conditions, the CuPd electrocatalyst demonstrated a 500 h CO-to-acetate conversion at 500 mA cm-2 with a stable acetate Faradaic efficiency of ∼50%.
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    52. 52
      Newton, M.; Francis, S. M.; Li, Y.; Law, D.; Bowker, M. Cu-Pd alloy surfaces I: Cu/Pd (85/15) {110}. surface structure and reactivity. Surf. Sci. 1991, 259, 45,  DOI: 10.1016/0039-6028(91)90522-T
      52
      Copper-palladium alloy surfaces. I. Copper/palladium [85:15]{110}, surface structure and reactivity
      Newton, Mark A.; Francis, Stephen M.; Li, Yongxue; Law, Donald; Bowker, Michael
      Surface Science (1991), 259 (1-2), 45-55CODEN: SUSCAS; ISSN:0039-6028.
      Both ordered and disordered phases of Cu:Pd[85:15]{110} were studied by using LEED, XPS, and UPS. Low-energy ion scattering (LEIS) also was employed to study the ordered surface. In the case of the ordered phase, these techniques were used to ascertain the structural and electronic influence of the Pd on this surface in comparison to that obsd. for Cu{110}. In addn., temp.-programmed desorption (TPD) was employed to det. the chem. reactivity of this surface towards the decompn. of adsorbed HCOOH and DCOOH. This is compared to the same reaction on Cu{110} and a kinetic isotope effect is noted in the decompn. of the 2 acids. Segregation of the Cu component of the alloy is obsd. upon the thermally induced transition from disordered to ordered surface phases, leaving the surface layer essentially free of Pd, while the second layer is enriched in Pd to ∼50% compn.
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    53. 53
      Newton, M.; Francis, S. M.; Bowker, M. Cu-Pd alloy surfaces II: Equilibrium surface compositions of dilute Cu/Pd alloys from a simple segregation model. Surf. Sci. 1991, 259, 56,  DOI: 10.1016/0039-6028(91)90523-U
      53
      Copper-palladium alloy surfaces. II. Equilibrium surface compositions of dilute palladium/copper alloys from a simple segregation model
      Newton, M. A.; Francis, S. M.; Bowker, M.
      Surface Science (1991), 259 (1-2), 56-64CODEN: SUSCAS; ISSN:0039-6028.
      The compn. of the CuPd[85:15](110) alloy selvedge region was modeled by using a quasichem. bond-breaking model of surface segregation. The results of these calcns. are compared to those obtained from exptl. studies of this surface, and it is shown that despite the neglect of size mismatch of solute and solvent atoms and the formal neglect of the ordering of the selvedge, the results for the top layer Pd concn. are compared to those made by a more comprehensive, but general theory of segregation. Comparisons to the analogous Cu3Au alloy surfaces and the Pt in Cu case are made, as are predictions about equil. selvedge compns. of other dil. Pd in Cu alloys.
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    54. 54
      Qiu, R.; Ding, Z.; Xu, Y.; Yang, Q.; Sun, K.; Hou, R. CuPd bimetallic catalyst with high Cu/Pd ratio and its application in CO2 hydrogenation. Appl. Surf. Sci. 2021, 544, 148974,  DOI: 10.1016/j.apsusc.2021.148974
      54
      CuPd bimetallic catalyst with high Cu/Pd ratio and its application in CO2 hydrogenation
      Qiu, Rui; Ding, Ziluo; Xu, Yamei; Yang, Qiuchen; Sun, Kening; Hou, Ruijun
      Applied Surface Science (2021), 544 (), 148974CODEN: ASUSEE; ISSN:0169-4332. (Elsevier B.V.)
      The surface and catalytic properties Cu-Pd bimetallic catalyst with a high Cu/Pd ratio of 33.5 was investigated for the hydrogenation of CO2 and was compared with the corresponding monometallic catalysts. The catalysts were synthesized by the incipient-wetness impregnation method, characterized by N2-physisorption, XRD, TPR, CO pulse titrn., N2O chemisorption, CO2-TPD, and FT-IR of adsorbed CO, and were evaluated in a fixed-bed reactor. The interaction between Cu and Pd results in a better dispersion of both Cu and Pd atoms and facilitates the redn. properties. Phase sepn. of Pd1Cu3 alloy and Cu appears over the bimetallic catalyst, while at some part of the catalysts, isolated Pd sites exist over the Cu-rich particles. Both geometric and electronic effects are present over the bimetallic catalyst. The catalytic performances are found to be similar over Cu/SiO2 and Cu-Pd/SiO2 when normalized to the surface Cu sites. Specifically, CuPd/SiO2 shows enhanced activity than Cu/SiO2 mainly due to the geometric effect, whereas the electronic effect is weak for the catalytic performance.
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  • Supporting Information

    Supporting Information

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