Low CO Coverage Research Articles

Mechanistic Implications of Low CO Coverage on Cu in the Electrochemical CO and CO2 Reduction Reactions.

Electrochemical CO or CO2 reduction reactions (CO(2)RR), powered by renewable energy, represent one of the promising strategies for upgrading CO2 to valuable products. To design efficient and selective catalysts for the CO(2)RR, a comprehensive mechanistic understanding is necessary, including a comprehensive understanding of the reaction network and the identity of kinetically relevant steps. Surface-adsorbed CO (COad) is the most commonly reported reaction intermediate in the CO(2)RR, and its surface coverage (θCO) and binding energy are proposed to be key to the catalytic performance. Recent experimental evidence sugguests that θCO on Cu electrode at electrochemical conditions is quite low (∼0.05 monolayer), while relatively high θCO is often assumed in literature mechanistic discussion. This Perspective briefly summarizes existing efforts in determining θCO on Cu surfaces, analyzes mechanistic impacts of low θCO on the reaction pathway and catalytic performance, and discusses potential fruitful future directions in advancing our understanding of the Cu-catalyzed CO(2)RR.

Multi-perspective analysis of CO poisoning in high-temperature proton exchange membrane fuel cell stack via numerical investigation

To comprehensively understand the CO poisoning effect on high-temperature PEMFC stack (HT-PEMFCs), a 3-D numerical HT-PEMFCs model is developed. The crucial anodic kinetic parameters are obtained from experimental data to improve the accuracy of the model. Then, multi-perspective analysis is conducted on different HT-PEMFC stacks with 1, 3, and 5 cells working at 160 °C with 3 mol% CO in H2. Besides, parametric simulations are conducted on the three-cell stack. It is found that the CO and H2 coverage varies between cells in the stack due to the temperature distribution difference, and the stack with more cells as well as the middle-cell of multi-cell stack have better performance, higher H2 coverage, lower CO coverage due to higher internal temperature. However, too high local temperature not only reduce the CO coverage but reduce the H2 coverage and increase the anode overpotential. The resistance to CO poisoning can be improved by increasing operating temperature, but when CO2 and water vapor are introduced into the anode, the H2 coverage decreases and the CO coverage increases due to the decrease of H2 concentration. The study clearly demonstrated that high performance and high CO resistance can be achieved by carefully regulating the operating conditions.

Geometric Modulation of Local CO Flux in Ag@Cu2 O Nanoreactors for Steering the CO2 RR Pathway toward High-Efficacy Methane Production.

The electroreduction of carbon dioxide (CO2 RR) to CH4 stands as one of the promising paths for resourceful CO2 utilization in meeting the imminent "carbon-neutral" goal of the near future. Yet, limited success has been witnessed in the development of high-efficiency catalysts imparting satisfactory methane selectivity at a commercially viable current density. Herein, a unique category of CO2 RR catalysts is fabricated with the yolk-shell nanocell structure, comprising an Ag core and a Cu2 O shell that resembles the tandem nanoreactor. By fixing the Ag core and tuning the Cu2 O envelope size, the CO flux arriving at the oxide-derived Cu shell can be regulated, which further modulates the *CO coverage and *H adsorption at the Cu surface, consequently steering the CO2 RR pathway. Density functional theory simulations show that lower CO coverage favors methane formation via stabilizing the intermediate *CHO. As a result, the best catalyst in the flow cell shows a high CH4 Faraday efficiency of 74 ± 2% and partial current density of 178 ± 5 mA cm- 2 at -1.2 VRHE , ranking above the state-of-the-art catalysts reported today for methane production. These findings mark the significance of precision synthesis in tailoring the catalyst geometry for achieving desired CO2 RR performance.

Transient behavior and reaction mechanism of CO catalytic ignition over a CuO–CeO2 mixed oxide

This study focuses on the variation in activity-controlling factors during CO catalytic ignition over a CuO–CeO2 catalyst. The activity for CO combustion follows the decreasing order of CuO–CeO2 > CuO > CeO2. Except for inactive CeO2, increasing temperature induces CO ignition to achieve self-sustained combustion over CuO and CuO–CeO2. However, CuO provides enough copper sites to adsorb CO, and abundant active lattice oxygen, thus obtaining a higher hot zone temperature (208.3°C) than that of CuO–CeO2 (197.3 °C). Catalytic ignition triggers a kinetic transition from the low-rate steady-state regime to a high-rate steady-state regime. During the induction process, Raman, X-ray photoelectron spectroscopy (XPS), CO temperature-programmed desorption (CO-TPD) and infrared (IR) spectroscopy results suggested that CO is preferentially adsorbed on oxygen vacancies (Cu+-[Ov]-Ce3+) to yield Cu+-[CO]-Ce3+ complexes. Because of the self-poisoning of CO, the adsorbed CO and traces of adsorbed oxygen react at a relative rate, which is entirely governed by the kinetics on the CO-covered surface and the heat transport until the pre-ignition regime. Nonetheless, the Cu+-[CO]-Ce3+ complex is a major contributor to CO ignition. The step-response runs and kinetic models testified that after ignition, a kinetic phase transition occurs from a CO-covered surface to an active lattice oxygen-covered surface. During CO self-sustained combustion, the rapid gas diffusivity and mass transfer is beneficial for handling the low coverage of CO. The active lattice oxygen of CuO takes part in CO oxidation.

Mitigation of CO poisoning on functionalized palladium monolayer supported on titanium carbide

The adsorption and catalytic oxidation of CO on monolayer Pd adsorbed on the TiC(001) surface (PdML/TiC(001)) are studied by density functional theory calculations. The PdML/TiC(001) exhibits weaker CO adsorption at low CO coverages and the moderate O2 adsorption compared to Pt(111), hence promoting the catalytic activity of CO oxidation and CO tolerance. The CO oxidation reaction on PdML/TiC(001) has surmountable energy barriers (0.37 eV) via Eley-Rideal (ER) mechanisms, indicating the critical role of Pd monolayer modification in lowering the reaction barrier of the rate-limiting step and improving the catalytic activity. This work would contribute to design the effective CO-tolerance catalyst based on the surface modification of transition metal carbides.

Constraining CO coverage on copper promotes high-efficiency ethylene electroproduction

The availability of inexpensive industrial CO gas streams motivates efficient electrocatalytic upgrading of CO to higher-value feedstocks such as ethylene. However, the electrosynthesis of ethylene by the CO reduction reaction (CORR) has suffered from low selectivity and energy efficiency. Here we find that the recent strategy of increasing performance through use of highly alkaline electrolyte—which is very effective in CO2RR—fails in CORR and drives the reaction to acetate. We then observe that ethylene selectivity increases when we constrain (decrease) CO availability. Using density functional theory, we show how CO coverage on copper influences the reaction pathways of ethylene versus oxygenate: lower CO coverage stabilizes the ethylene-relevant intermediates whereas higher CO coverage favours oxygenate formation. We then control local CO availability experimentally by tuning the CO concentration and reaction rate; we achieve ethylene Faradaic efficiencies of 72% and a partial current density of >800 mA cm−2. The overall system provides a half-cell energy efficiency of 44% for ethylene production. The electrocatalytic upgrading of CO to higher-value feedstocks provides a promising route to multicarbon products. Here, the authors show that high ethylene selectivity can be achieved by constraining CO availability on copper, with an ethylene Faradaic efficiency of 72% and a partial current density of >800 mA cm−2.

Electronic Effects Determine the Selectivity of Planar Au-Cu Bimetallic Thin Films for Electrochemical CO2 Reduction.

Au–Cu bimetallic thin films with controlled composition were fabricated by magnetron sputtering co-deposition, and their performance for the electrocatalytic reduction of CO2 was investigated. The uniform planar morphology served as a platform to evaluate the electronic effect isolated from morphological effects while minimizing geometric contributions. The catalytic selectivity and activity of Au–Cu alloys was found to be correlated with the variation of electronic structure that was varied with tunable composition. Notably, the d-band center gradually shifted away from the Fermi level with increasing Au atomic ratio, leading to a weakened binding energy of *CO, which is consistent with low CO coverage observed in CO stripping experiments. The decrease in the *CO binding strength results in the enhanced catalytic activity for CO formation with the increase in Au content. In addition, it was observed that copper oxide/hydroxide species are less stable on Au–Cu surfaces compared to those on the pure Cu surface, where the surface oxophilicity could be critical to tuning the binding strength of *OCHO. These results imply that the altered electronic structure could explain the decreased formation of HCOO– on the Au–Cu alloys. In general, the formation of CO and HCOO– as main CO2 reduction products on planar Au–Cu alloys followed the shift of the d-band center, which indicates that the electronic effect is the major governing factor for the electrocatalytic activity of CO2 reduction on Au–Cu bimetallic thin films.

First-principles descriptors of CO chemisorption on Ni and Cu surfaces.

A comprehensive analysis of low coverage CO adsorption on Ni and Cu low-index miller surfaces - (100), (110), and (111) - over all the possible adsorption sites is presented. Systems are theoretically studied within an accurate adsorption model using RPBE density functional calculations to obtain electronic and geometrical structure predictions along with their corresponding adsorption energy computations. Based on the surface- and site-dependent comparisons of the adsorption mechanisms, we were able to grasp trends that point to the factors that determine the final C-O structure upon adsorption. The resulting C-O bond length is found to be directly dependent on structural parameters, such as depth of adsorption and metal-adsorbate bonding distances, and the quantity of charge transferred from the surface to the CO molecule. Those factors are collated into a formula that defines the final C-O bond: the "C-O formula". For each adsorption site, the final C-O bond length is calculated using this formula and compared with the DFT predictions, where consistent matching results are obtained. Deeper analysis of the adsorbed C-O molecule is also presented from a molecular orbital level. Density of states (DOS) charts were exploited to investigate the perturbations in the 3σ and 1π orbitals that hold the internal C-O bond. From this analysis, a consistent link between the degree of destabilization of the orbitals and the final C-O bond length is found, obtaining a more profound understanding of the final adsorbate structure. Energetically, adsorptions on Cu and Ni surfaces are compared within the Blyholder-Nilsson and Petersson (B-NP) model. The frontier (5σ and 2π*) orbital energies relative to the d-band center of the metal surfaces are displayed, which implicitly defines the adsorption energy. The controversial repulsive nature of the σ-interaction proposed in the NP model has been tested by tracking the charge redistribution within the metallic states, including the broad sp-states. The nature of σ-interaction is, however, found to be dependent on the substrate type; repulsive σ-interaction is concluded for Ni, while for Cu, a rather dual nature is found, including both partial repulsive and attractive behavior, with a dominant and overall repulsive nature, in agreement with the NP model. The degree of σ repulsion/attraction is also found to be dependent on the metal coordination. Finally, spin-polarized DFT calculations were repeated for Ni surfaces and compared with the previous Ni results without spin-polarization. The reported results confirmed the absence of correlation between adsorption energetics and the final adsorbate structure, and verified the factors presented in the "C-O formula" as the main descriptors for the adsorbate structure.

Weakening the strength of CO binding on subsurface alloyed Pt(111)

The interaction of CO with Pt/3d/Pt(111) subsurface alloys (3d: Ni, Co, Fe) was investigated by combination of vibrational spectroscopy and temperature programmed desorption. The binding strength of CO is significantly reduced on Pt/Fe/Pt(111) and Pt/Co/Pt(111) and is sorted in the following order: Pt>Ni>Co≈Fe. Our analysis shows that the 3d admetal is stable in the subsurface region and CO is linearly bonded to the atop sites of the Pt(111) surface. At high CO coverage, compressed structures are obtained. At low CO coverage, the desorption activation energy of CO (∼143 kJ/mol for Pt(111)) drops to 124 kJ/mol for Pt/Ni/Pt(111), and goes further down to 111 and 110 kJ/mol for Pt/Co/Pt(111) and Pt/Fe/Pt(111), respectively. The enhancement in the surface activity is attributed to the modified backdonation to the antibonding states of adsorbed CO due to the downshifted 5d-band center of Pt upon embedding 3d metals in the subsurface region.

Insights into Self-Poisoning during Catalytic Hydrogenation on Platinum Surfaces Using ATR-IR Spectroelectrochemistry

Attenuated total reflection infrared (ATR-IR) spectroscopy has been combined with electrochemical methods to investigate molecular decomposition and self-poisoning processes on platinum surfaces under the conditions of catalytic hydrogenation. In aqueous 0.1 M H2SO4 the α-keto ester ethyl pyruvate (EP) is found to decompose on polycrystalline platinum electrodes to yield surface-adsorbed CO, but the observed behavior is highly dependent on the electrode potential, a parameter intimately linked to the surface-adsorbed hydrogen coverage. In the potential range −0.2 to −0.4 V (vs mercury/mercurous sulfate electrode) where the hydrogen coverage is negligible, CO is readily produced at the platinum surface along with other molecular fragments but the decomposition process becomes inhibited at high EP solution concentrations. At −0.5 V only very low coverages of CO are observed due to competing hydrogen adsorption at Pt(100) step sites which most favor EP decomposition. At more negative potentials, during the onset of catalytic EP hydrogenation, CO is generated rapidly but other intermediates or products are not observed in the ATR-IR spectra. Together these observations suggest two different mechanisms of EP decomposition, the first occurring directly upon EP adsorption and the second occurring after a single hydrogen atom transfer under hydrogen rich conditions. This ability to control substrate decomposition by tuning the surface hydrogen coverage may be used as a potential route to mitigating catalyst poisoning and deactivation during hydrogenation reactions.

CO oxidation over supported Pt clusters at different CO coverage

CO adsorption and oxidation over supported Pt14 with different CO coverage on TiO2(110) surface were investigated using density functional theory (DFT) calculations and thermodynamic analysis. According to the phase diagram, Pt14/TiO2(110) and 11CO@Pt14/TiO2(110) were chosen to represent the low and high CO coverage of Pt clusters, respectively. Our study shows that the high coverage of CO can induce the structural change of supported Pt clusters and weaken the interaction between Pt clusters and TiO2 support. The CO adsorption and oxidation mechanism depends on the CO coverage, which is determined by the experimental reactant composition, pressure, and temperature. At low CO coverage, the dissociated oxygen is active specie to form CO2 by reacting with CO. At high coverage, the molecular oxygen can directly react with CO via the formation of OOCO intermediate. Our proposed mechanisms provide useful information for understanding the CO oxidation over Pt clusters with different CO coverage. © 2016 Wiley Periodicals, Inc.

Morphology control of K2O promoter on Hägg carbide (χ-Fe5C2) under Fischer–Tropsch synthesis condition

The structure and stability of seventeen facets of the Hägg carbide phase (χ-Fe5C2) under the consideration of K2O promotion have been analyzed utilizing density functional theory approach and ab initio atomistic thermodynamics. On the basis of different interaction strengths of these facets with K2O promoter, the morphology of the χ-Fe5C2 phase under the variation of K2O content has been predicated. At the surface Fe/K atomic ratio of 30, the morphology of the χ-Fe5C2 phase can be modified apparently. Among the most exposed (111), (100), (1 1 1¯), (510) and (4¯ 1 1) facets, the (1 1 1¯), (510) and (4¯ 1 1) facets have more open surface structures as well as provide more surface Fe atoms and less carbon atoms. However, all these mostly exposed facets do not favor CO direct dissociation at low CO coverage.

CO adsorption over Pd nanoparticles: A general framework for IR simulations on nanoparticles

CO vibrational spectra over catalytic nanoparticles under high coverages/pressures are discussed from a DFT perspective. Hybrid B3LYP and PBE DFT calculations of CO chemisorbed over Pd4 and Pd13 nanoclusters, and a 1.1nm Pd38 nanoparticle, have been performed in order to simulate the corresponding coverage dependent infrared (IR) absorption spectra, and hence provide a quantitative foundation for the interpretation of experimental IR spectra of CO over Pd nanocatalysts. B3LYP simulated IR intensities are used to quantify site occupation numbers through comparison with experimental DRIFTS spectra, allowing an atomistic model of CO surface coverage to be created. DFT adsorption energetics for low CO coverage (θ→0) suggest the CO binding strength follows the order hollow>bridge>linear, even for dispersion-corrected functionals for sub-nanometre Pd nanoclusters. For a Pd38 nanoparticle, hollow and bridge-bound are energetically similar (hollow≈bridge>atop). It is well known that this ordering has not been found at the high coverages used experimentally, wherein atop CO has a much higher population than observed over Pd(111), confirmed by our DRIFTS spectra for Pd nanoparticles supported on a KIT-6 silica, and hence site populations were calculated through a comparison of DFT and spectroscopic data. At high CO coverage (θ=1), all three adsorbed CO species co-exist on Pd38, and their interdiffusion is thermally feasible at STP. Under such high surface coverages, DFT predicts that bridge-bound CO chains are thermodynamically stable and isoenergetic to an entirely hollow bound Pd/CO system. The Pd38 nanoparticle undergoes a linear (3.5%), isotropic expansion with increasing CO coverage, accompanied by 63 and 30cm−1 blue-shifts of hollow and linear bound CO respectively.

A comparative study of CO adsorption on tetrahexahedral Pt nanocrystals and interrelated Pt single crystal electrodes by using cyclic voltammetry and in situ FTIR spectroscopy.

This study focuses on CO adsorption at tetrahexahedral Pt nanocrystals (THH Pt NCs) by using cyclic voltammetry and in situ FTIR spectroscopy. Since the electrochemically prepared THH Pt NCs in this study are enclosed by {730} facets which could be considered by a subfacet configuration of 2{210} + {310}, we have also studied CO adsorption on the interrelated Pt(310) and Pt(210) single crystal electrodes as a comparison. Cyclic voltammetry results demonstrated that CO adsorbs dominantly on the (100) sites of THH Pt NCs at low CO coverage (θ(CO)≤ 0.135), while on both (100) and (110) sites at higher CO coverage. On ordered Pt(310) and Pt(210), i.e. they were flame annealed and then cooled in H(2) + Ar, CO adsorption also illustrates relative priority on (100) sites at low CO coverage; while at high CO coverage or on oxygen-disordered Pt(310) and Pt(210) when they were cooled in air after flame annealing, the adsorption of CO presents a weak preference on (100) sites of Pt(310) and even no preference at all on (100) sites of Pt(210). In situ FTIR spectroscopic studies illustrated that CO adsorption on THH Pt NCs yields anomalous infrared effects (AIREs), which are depicted by the Fano-like IR feature on a dense distribution (60 μm(-2)) and the enhancement of abnormal IR absorption on a sparse distribution (22 μm(-2)) of THH Pt NCs on glassy carbon substrate. Systematic investigation of CO coverage dependence of IR features revealed that, on THH Pt NCs, the IR band center (ν(COL)) of linearly bonded CO (COL) is rapidly shifted to higher wavenumbers along with the increase of CO coverage to 0.184, yielding a fast linear increase rate with a high slope (dν(COL)/dθ(IR)(CO) = 219 cm(-1)); when θ > 0.184, the increase of ν(COL) with θCO slows down and deviates drastically from linearity. In contrast, the ν(COL) on the ordered Pt(310) electrode maintains a linear increase with θ(IR)(CO) for the whole range of θ(IR)(CO) variation, and gives a much smaller increase rate of slope 74.3 cm(-1). The significant differences in CO adsorption behavior on THH Pt NCs and on interrelated Pt single crystal planes demonstrated clearly the unique properties of nanoparticles enclosed by high-index facets.

Adsorption configurations of carbon monoxide on gold monolayer supported by graphene or monolayer hexagonal boron nitride: a first-principles study

Using density functional theory with a semiempirical van der Waals approach proposed by Grimme, the adsorption behavior of carbon monoxide on a gold monolayer supported by graphene or monolayer hexagonal boron nitride has been investigated. Based on the changes in the Dirac cone of graphene and a Bader charge analysis, we observe that the Au(111) monolayer gains a small charge from graphene and monolayer h-BN. The adsorbed CO molecule adopts similar adsorption configurations on Au(111)/graphene and Au(111)/h-BN with Au-C distance 2.17−2.50 A and Au-C-O angle of 123.9°–139.6°. Moreover, we found that for low CO coverages, bonding to the gold surface is surprisingly energy-favorable. Yet the CO adsorption binding energy diminishes at high coverage due to the repulsive van der Waals interactions between CO molecules.

Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm(-1) on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm(-1) below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti(4+) sites. At a higher density of defects, adsorption on Ti(3+) sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti(4+) sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm(-1). These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.

On the apparent lack of preferential site occupancy and electrooxidation of CO adsorbed at low coverage onto stepped platinum surfaces

We report time evolution studies of low coverage CO adsorption (surface hydrogen site blocking < 40%) and oxidative stripping on stepped Pt(776) and Pt(554) surfaces. It was observed that there is no preferential site occupancy for CO adsorption on step or terrace. It is proposed that CO adsorption onto these surfaces is a random process, and after CO adsorption there is no appreciable shift from CO-(111) to CO-(110) sites. This implies that after adsorption, CO molecules either have a very long residence time, or that the diffusion coefficient is much lower than previously thought. After CO electrooxidation the sites released included both terrace (111) and step (110) orientations. For surface hydrogen site blocking > 40%, the lateral interactions might play a role in the preferential CO site occupancy.

Adsorption of CO onNi3Al(111)investigated using high-resolution photoemission spectroscopy and density functional theory

The adsorption of CO on ${\text{Ni}}_{3}\text{Al}(111)$ has been studied using high-resolution photoemission spectroscopy and density functional theory. Despite the fact that CO binds to Ni dominated sites only at this surface, CO adsorption induces a shifted contribution in the $\text{Al}\text{ }2p$ core-level spectra. This contribution moves toward higher binding energy upon increasing CO coverage. The calculations give $\text{Al}\text{ }2p$ core-level binding energy shifts in good agreement with the experimental values and show that adsorption of CO in the Ni sites induces core-level binding energy shifts for nearby Al atoms located in the two outermost surface layers. The surface Al atoms relax inward upon CO adsorption. At low CO coverage only one peak is observed in the $\text{C}\text{ }1s$ spectra. This contribution is assigned to CO adsorbed in Ni threefold hollow sites. The calculations predict that CO adsorbs in the hollow sites for coverages up to 0.50 ML with a strong preference for the hcp site above a second layer Al atom at low coverage. At higher CO coverage, an additional contribution appears in the $\text{C}\text{ }1s$ spectra whereas the other contribution shifts toward higher binding energies. The theoretical results suggest that this behavior is originating from the occupation of Ni on top and Ni bridge sites in addition to hollow sites.

A computational study of H 2 dissociation and CO adsorption on the Pt ML/WC(0 0 0 1) surface

We studied computationally the relative stability of Pt ML/WC(0 0 0 1) [pseudomorphic monolayer of Pt(1 1 1) on WC(0 0 0 1)] interfacial structures using a density functional slab model approach. The work of adhesion was calculated for six different interfacial structures, taking into account both W- and C-terminations of the carbide. The results show that the optimal interfacial structure of Pt ML/WC(0 0 0 1) is the W-terminated WC(0 0 0 1) with Pt atoms adhesion on the hcp site (W-hcp). The nature of metal/carbide bonding for the W-hcp interfacial geometry was determined on the basis of the partial density of states (PDOS). Adsorption of atomic hydrogen and dissociation of the hydrogen molecule on the W-hcp Pt ML/WC(0 0 0 1) was investigated and compared to that on Pt(1 1 1). It is found that the most favorable H 2 dissociation channels need similar activation energies of 5.28 and 4.93 kJ/mol on Pt ML/WC(0 0 0 1) and Pt(1 1 1), respectively, with the release of considerable reaction energies. Furthermore, adsorption of CO on the W-hcp Pt ML/WC(0 0 0 1) and Pt(1 1 1) was also investigated. The results indicate that Pt ML/WC(0 0 0 1) is much less susceptible to CO poisoning than Pt(1 1 1), especially at the low coverage of CO.

Reaction Kinetics and Polarization Modulation Infrared Reflection Absorption Spectroscopy Investigations of CO Oxidation over Planar Pt-Group Model Catalysts

Microscopic and spectroscopic techniques at near-atmospheric pressures have been used in recent years to investigate CO oxidation over Pt-group metals. New insights have been obtained that allow broadening of the understanding of this reaction beyond the ultrahigh vacuum regime where it is well-understood. However, new issues also have arisen that need clarification. In this paper, we review recent work in our laboratory, using polarization modulation infrared reflection absorption spectroscopy (PM-IRAS) and reaction kinetics measurements from ultrahigh vacuum to near-atmospheric pressures. These studies reveal a continuity of this reaction with respect to pressure over Pt, Pd, and Rh; that is, Langmuir-Hinshelwood kinetics is exhibited over a wide pressure range with no apparent "pressure gap". The difference between Ru(0001) and other noble metals is well-understood with respect to higher oxygen binding energies and reduced CO inhibition. It is concluded that for all Pt-group metals the most active phase is one saturated with chemisorbed oxygen and with low CO coverages. The significance of oxide phases under most industrially relevant catalytic conditions suggested recently in the literature is not consistent with the experimental data.