CO Chemisorption Energy Research Articles

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO‐O Complex**

AbstractThe catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO‐inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO‐poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near‐ambient pressure X‐ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light‐off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO‐Pt‐O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.

Catalytic Oxidation of CO on a Curved Pt(111) Surface: Simultaneous Ignition at All Facets through a Transient CO-O Complex*.

The catalytic oxidation of CO on transition metals, such as Pt, is commonly viewed as a sharp transition from the CO-inhibited surface to the active metal, covered with O. However, we find that minor amounts of O are present in the CO-poisoned layer that explain why, surprisingly, CO desorbs at stepped and flat Pt crystal planes at once, regardless of the reaction conditions. Using near-ambient pressure X-ray photoemission and a curved Pt(111) crystal we probe the chemical composition at surfaces with variable step density during the CO oxidation reaction. Analysis of C and O core levels across the curved crystal reveals that, right before light-off, subsurface O builds up within (111) terraces. This is key to trigger the simultaneous ignition of the catalytic reaction at different Pt surfaces: a CO-Pt-O complex is formed that equals the CO chemisorption energy at terraces and steps, leading to the abrupt desorption of poisoning CO from all crystal facets at the same temperature.

Role of different Pd/Pt ensembles in determining CO chemisorption on Au-based bimetallic alloys: A first-principles study

Using spin-polarized density functional calculations, we investigate the role of different Pd/Pt ensembles in determining CO chemisorption on Au-based bimetallic alloys through a study of the energetics, charge transfer, geometric and electronic structures of CO on various Pd/Pt ensembles (monomer/dimer/trimer/tetramer). We find that the effect of Pd ensembles on the reduction of CO chemisorption energy is much larger than the Pt ensemble case. In particular, small-sized Pd ensembles like monomer show a substantial reduction of CO chemisorption energy compared to the pure Pd (111) surface, while there are no significant size and shape effects of Pt ensembles on CO chemisorption energy. This is related to two factors: (1) the steeper potential energy surface (PES) of CO in Pd (111) than in Pt (111), indicating that the effect of switch of binding site preference on CO chemisorption energy is much larger in Pd ensembles than in Pt ensembles, and (2) down-shift of d-band in Pd ensembles/up-shift of d-band in Pt ensembles as compared to the corresponding pure Pd (111)/Pt (111) surfaces, suggesting more reduced activity of Pd ensembles toward CO adsorption than the Pt ensemble case. We also present the different bonding mechanism of CO on Pd/Pt ensembles by the analysis of orbital resolved density of state.

Tolerance of platinum clusters to CO poisoning induced by molybdenum doping

The influence of a single Mo dopant atom on the CO adsorption on isolated cationic platinum clusters in the gas phase, Ptn+ (13≤n≤24), has been investigated. The sticking probability of the first CO molecule to bare Ptn+ clusters is estimated to be close to unity and is not notably changed upon Mo doping. The adsorption probability of the second CO molecule, however, shows a significant reduction for Ptn−1Mo+ compared to Ptn+, reaching a maximal reduction of as much as 80% for Pt19Mo+. As a result, the average number of CO molecules adsorbed on Ptn+ with 19≤n≤24, and surviving on the time scale of the experiment, decreases by about 10–15% upon substitution of a single Pt atom by a single Mo atom. A statistical analysis of the unimolecular dissociation of the cluster CO complexes suggests that the lower sticking probability for the second CO molecule for Mo-doped species is related to a reduction of the CO chemisorption energy. Electron transfer from Mo to Pt resulting in a lowering of the Pt 5d vacancies and a downshift of the 5d-center is likely responsible for this reduced CO binding energy.

Multiscale quantum/atomistic coupling using constrained density functional theory

Multiscale coupling of a quantum mechanical (QM) domain to a coarser-scale material description in a larger surrounding domain should yield forces and energies in the QM domain that are the same as would be achieved in a QM simulation of the entire system. Here, such a coupling is achieved by using constrained density functional theory (DFT) in which the quantum mechanical interaction between the domains is captured via a constraint potential arising from an imposed constraint on the charge density in a boundary region between the two domains. The implementation of the method, including the construction of the constraint charge density and the calculation of the constraint potential, is presented. The method is applied to problems in three different metals (Al, Fe, and Pd) and is validated against periodic DFT calculations. The method reproduces the QM charge density and magnetic moments of bulk materials, produces a reasonable edge dislocation core structure for Fe, and also gives accurate vacancy formation energy for Al and chemisorption energy on a flat Pd surface. Finally, the method is used to study the chemisorption energy of CO on a stepped Pd surface. In general, the method can mitigate fictitious interactions between surface steps and other extended defects, and accommodate long-range deformation fields, and thus improves upon periodic DFT calculations.

Quantum size effects on chemisorption properties: CO on ultrathin Cu films from first principles

We address, by means of ab-initio calculations, the origin of the correlation that has been observed experimentally between the chemisorption energy of CO on nanoscale Cu(001) supported films and quantum-size effects. The calculated chemisorption energy shows systematic oscillations, as a function of film thickness, with a periodicity corresponding to that of quantum-well states at the surface-Brillouin-zone center crossing the Fermi energy. We explain this trend based on the oscillations, with film thickness, of the decay length on the vacuum side of the quantum-well states at the Fermi energy. Contrary to previous suggestions, we find that the actual oscillations with film thickness of the density of states per atom of the film at the Fermi energy cannot account for the observed trend in the chemisorption energy.

Density-functional calculations of graphene interfaces with Pt(1 1 1) and Pt(1 1 1)/Ru ML surfaces

Density-functional calculations were performed to examine the interface between graphene and a Pt(1 1 1) or Pt(1 1 1)/Ru ML surface. Interfacial Ru atoms were found to make the interface more stable and carbon vacancies on graphene enhanced the interaction between graphene and multi-layer metal surfaces. The CO chemisorption energy on metal surfaces on graphene was calculated to clarify the effect of the catalyst carrier.

A DFT study of hydrogen and carbon monoxide chemisorption onto small gold clusters

DFT calculations have been performed to study the interaction of small gold clusters, Au 1–Au 13, with H and CO. An odd–even oscillation of the H chemisorption energy was observed, with H adsorption onto odd-numbered clusters being more favourable. This correlates with the smaller HOMO–LUMO gaps for these clusters. For CO chemisorption, the overlap between CO orbitals and cluster frontier orbitals is important. As a result, there is no simple relationship between CO chemisorption energy and the frontier orbital energies of Au n clusters.

Adsorption of CO on Co(0001) and Pt–Co(0001) surfaces: an experimental and theoretical study

CO adsorption on Co(0001) and Pt submonolayer deposits on Co(0001) at room temperature have been investigated by combining the surface techniques of low-energy electron diffraction and X-ray and UV photoelectron spectroscopy. The influence of bimetallic system formation on the CO adsorption was studied. CO is molecularly adsorbed on both surfaces. The saturation coverage under ultrahigh vacuum conditions corresponds to a well-ordered ( 3 × 3 )R30° structure in the presence of Pt. The CO uptake on Pt–Co(0001) was found to be lowered in comparison with Co(0001) as the platinum coverage increased between 0 to 0.6 ML. However, CO is adsorbed both on the Pt and Co areas. It is shown that CO is located in the top Pt sites, with an adsorption energy reduced by 38% with respect to the pure Pt(111) surface. This result is in good agreement with our theoretical results of CO chemisorption energy on a pseudomorphic Pt overlayer supported by Co(0001). A decreased Pt density of states at the Fermi level and a high binding energy shift of the d-band center in comparison with the pure metal was observed both experimentally and theoretically.

A density functional theory study on the interaction between chemisorbed CO and S on Rh(111)

Density functional theory calculations are carried out for Rh(111)-p(2×2)-CO, Rh(111)-p(2×2)-S, Rh(111)-p(2×2)-(S+CO), Rh(111)-p(3×3)-CO, Rh(111)-p(3×3)-S and Rh(111)-p(3×3)-(S+CO), aiming to shed some light on the S poisoning effect. Geometrical structures of these systems are optimized and chemisorption energies are determined. The presence of S does not significantly influence the geometrical structure and chemisorption energy of CO and vice versa, which strongly suggests that the interaction between CO and S on the Rh(111) surface is mainly short-range in nature. The long range electronic effect for the dramatic attenuation of the CO methanation activity by sulfur is likely to be incorrect. It is suggested that an ensemble effect may be dominant in the catalytic deactivation.

THEORETICAL STUDY OF CO ADSORPTION ON Ni(111), Pt(111) AND Pt/Ni(111) SURFACES

CO adsorption on a pseudomorphic Pt overlayer supported by Ni (111) has been studied with the use of extended Huckel calculations. Experimental information on the pure Pt (111) and Ni (111) single crystals was employed to select a consistent parameter set for our bimetallic system. This gives a good description of the chemisorption bond changes between the various systems considered in our study. The CO chemisorption energy on Pt/Ni (111) was found to be lowered in comparison with Pt (111) and Ni (111), in good agreement with experimental data on Pt-rich Pt–Ni surface alloys. This observation could be justified by the electronic changes of the Pt states (valence band broadening and decreasing density at the Fermi level). Indeed, they induce, in comparison with the pure substrates, a repulsion between Pt and CO although the 2π* population of the chemisorbed molecule increases. This points to the necessity of going beyond arguments based on an analysis of the 5σ donation and 2π* backdonation for a complete description of the chemisorption bond.

Electronic structure of a Pt(111) [sbnd]Ge surface alloy and adsorbed CO

Angle-integrated photoemission spectra are observed for Pt(111) and a Pt(111) Ge surface alloy. The spectra for Pt(111) Ge show d band filling compared with those for Pt(111). Although the chemisorption energy of CO is noticeably reduced by alloy formation, the binding energies of the occupied levels for adsorbed CO are almost the same as those on pure Pt(111). The results are discussed by the d band filling due to s-d hybridization of Ge s electrons and the Pt d band and by two-level hybridization of the 5σ and 2π states with the modified d band.

CO chemisorption at metal surfaces and overlayers.

A database of ab initio calculations of the chemisorption energy of CO over Ni(111), Cu(111), Ru(0001), Pd(111), Ag(111), Pt(111), Au(111), $\mathrm{Cu}{}_{3}$Pt(111), and some metallic overlayer structures is presented. The trends can be reproduced with a simple model describing the interaction between the metal $d$ states and the CO $2{\ensuremath{\pi}}^{*}$ and 5 $\ensuremath{\sigma}$ states, renormalized by the metal $\mathrm{sp}$ continuum. Our model rationalizes the results by Rodriguez and Goodman [Science 257, 897 (1992)] showing a strong correlation between the CO chemisorption energy and the surface core level shift.

On the Kinetics of Co Methanation on Nickel Surfaces

A microkinetic model for CO methanation on nickel based on CO dissociation and stepwise hydrogenation of surface carbon is presented. Very good agreement with the methanation rates measured on Ni(100) by Goodman et al. (J. Catal. 63, 226 (1980)) and on nickel foils by Polizzotti and Schwarz (J. Catal. 77, 1 (1982)) is obtained by assuming that the rate-controlling step is hydrogenation of surface methylidyne and by treating the coverage of reactive carbon, ΘC, as a parameter. For not too low H2:CO ratios it seems to be a good approximation to keep ΘC constant. The validity of this approximation may be related to the observations that only a small part of the surface carbon is reactive and that the rate depends on previous treatments of the nickel surface supposed to create special sites. It is shown that previously published macrokinetic models, which can be tested, do not give rates in agreement with the single crystal and foil results. Analysis of the present model shows that the overall activation energy, Ea, of the reaction is mainly determined by the chemisorption energy of CO in the temperature range where Ea is almost constant. The order of the H2 dependence derived from the kinetic model is close to 1 in the entire range of reaction conditions considered, while the order of the CO dependence is −1 at low temperatures increasing at the highest temperatures to between −0.12 and −0.06 depending on the total pressure.

Modification of the electronic structure of Pd by U films: chemisorption of CO

X-Ray and ultraviolet photoelectron spectroscopies (XPS and UPS), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) have been used to study the coadsorption of CO and U on polycrystalline Pd. We investigated the modification of the electronic structure of Pd at low U coverages and the mode of CO chemisorption (dissociative vs. associative) as a function of surface U concentration. U adsorption results in the narrowing of the Pd 4d band. At low U coverages the density of states (DOS) at the Fermi level decreases and the U 5f electrons are localized. The CO saturation coverage at room temperature decreases with increasing U surface concentration, while CO adsorption at –165 °C is less affected. This is attributed mainly to a decrease of CO chemisorption energy but also to blocking of Pd adsorption sites by U. A. decrease in the heat of chemisorption for CO is explained by a change of the electronic structure of Pd by U. At low dosage U itself loses most of its reactivity, probably because of U—Pd solid-state bonding and below a critical U surface concentration CO dissociation becomes an activated process. High-temperature reaction between CO and U-covered surfaces leads to the partial transformation of U into a surface oxycarbide. In the presence of this compound some CO is chemisorbed strongly on the surface and is stable even at 300 °C, in contrast to CO on Pd metal. A similar effect has also been observed for classical promoters such as the alkali metals.

The chemisorption energy of CO on a disordered binary alloy

The one-dimensional tight-binding model and the one-electron chemisorption theory are used to calculate the chemisorption energy of CO on a NiCu disordered binary alloy surface, in which the coherent-potential approximation and the technique of complex-energy-plane integration are adopted. The change of surface segregation due to chemisorption is also investigated. The chemisorption of the CO/NiCu system can change the surface component concentration drastically, thereby influencing the chemisorption energy. The result shows that the chemisorption of the system strongly constrains surface segregation, resulting in chemisorption properties more like those of CO/Ni rather than those of CO/Cu.

Density functional calculation of the vibrational stretching mode of CO coadsorbed with ammonia on palladium clusters

Vibrational analysis of the shift of the stretching mode of carbon monoxide coadsorbed with ammonia on model palladium clusters is presented. The CO molecule is adsorbed on the threefold site while NH 3 is adsorbed on an on-top site on the opposite side of the cluster. The electronic structure calculations are made by using a local density approximation. The vibrational analysis involves a two-dimensional potential depending on the CO and MeC distances. Several effects are analysed; firstly the addition of electrons to the cluster shifts the CO vibration to lower frequencies. This is explained as, primarily, a chemical effect; a small charge transfer to the adsorbate is observed and this is confirmed by the decrease of the binding energy of CO on the surface. Second, the effect of coadsorbed ammonia is to weaken the CO bond and to increase the CO chemisorption energy. The stretching mode frequency ω co is decreasing by about 20 cm −1 depending slightly on the cluster size while the MeCO beating mode has its frequency slightly increased. Changes in work function, σ-d and d-π ∗ backdonation, core electron energy of C, O and N atoms, as well as Mulliken population analyses are discussed and the link with the vibrational behavior of both studied vibrations is made. It is concluded that the CO is influenced indirectly by the ammonia which polarizes the cluster and hence increases the donation-backdonation between CO and the cluster although there is no charge transfer from NH 3 to the CO molecule through the palladium surface. It is shown in all cases that the anharmonicity contribution and the coupling of the stretching with the beating of the CO on the surface are small.

CO and O chemisorption on disordered binary alloys

One-dimensional tight-binding model and the one-electron chemisorption theory are used to calculate the chemisorption energy of CO or O on Ni x Cu 1-x surface under the average T-matrix approximation. It is found that the chemisorption of CO and O on Ni x Cu 1-x are strengthened with the increase of bulk Ni concentration. The Cu segragation to the NiCu alloy surface may weaken the chemisorption.

CO AND O CHEMISORPTION ON DISORDERED BINARY ALLOY NiCu

One-dimensional tight-binding model and the one-electron chemisorption theory are used to calculate the chemisorption energy of CO or O on NixCu1-x surface under the average T-matrix approximation. It is found that the chemisorption of CO and O on NixCu1-x are strengthened with the increase of bulk Ni concentration. The Cu segragation to the NiCu alloy surface may weaken the chemisorption.

Effects of chemisorbed and substitutional 0, I, and II Ge, Sn, and Pb on CO adsorption on Pt(111): molecular orbital theory

Methanol oxidation rates on Pt anodes are increased by surface Sn adatoms apparently because of faster oxidation of CO, a surface poison, to CO 2. In an ASED-MO study using cluster models, Sn as well as Ge and Pb adatoms are found to reduce the chemisorption energy of CO at adjacent sites of Pt(111). Oxidation of these adatoms by OH only slightly reduces their effect on CO adsorption but oxidation by O nearly cancels it. When the atoms in their reduced and oxidized forms are placed substitutionally in the Pt surface they have very small effects on CO adsorption energies. The weakening caused by Sn adatoms stems in part from a CO4σ-Sn 5s closed-shell repulsion through the Pt surface atoms. The through-space repulsion is negligible. The weakening of the CO adsorption bond to Pt by Sn adatoms has been observed in recent high vacuum studies. During electrochemical oxidation of adsorbed CO this weakening may play a role, but other effects, such as the influence of Sn on the presence of active oxygen, need further examination.