CO adsorption energies on metals with correction for high coordination adsorption sites - A density functional study

We investigate the accuracy of carbon monoxide adsorption energies and site preferences for the metals Ag, Al, Au, Co, Cu, Fe, Ir, Mo, Ni, Pd, Pt, Re, Rh, Ru, W, and Zn using the RPBE functional with a recently suggested empirical adsorption energy correction for carbon monoxide based on the internal CO stretch vibrational frequency. We find that when including the correction, the adsorption site preference for six of the metals changes, and all adsorption site predictions become accurate. We also collect a large number of experimental studies for comparison with our calculated adsorption energies. The mean absolute deviation including the correction is found to be less than 0.2 eV, showing that the RPBE functional gives a much better quantitative agreement between experiments and calculations than the PW91 functional.     

Methanol decomposition on platinum (111)

The interaction of methanol with clean and oxygen-covered Pt(111) surfaces has been examined with high resolution electron loss spectroscopy (EELS) and thermal desorption spectroscopy (TDS). On the clean Pt(111) surface, methanol dehydrogenated above 140 K to form adsorbed carbon monoxide and hydrogen. On a Pt(111)-p(2 × 2)O surface, methanol formed a methoxy species (CH₃O) and adsorbed water. The methoxy species was unstable above 170 K and decomposed to form adsorbed CO and hydrogen. Above room temperature, hydrogen and carbon monoxide desorbed near 360 and 470 K, respectively. The instability of methanol and methoxy groups on the Pt surface is in agreement with the dehydrogenation reaction observed on W, Ru, Pd and Ni surfaces at low pressures. This is in contrast with the higher stability of methoxy groups on silver and copper surfaces, where decomposition to formaldehyde and hydrogen occurs. The hypothesis is proposed that metals with low heats of adsorption of CO and H₂ (Ag, Cu) may selectively form formaldehyde via the methoxy intermediate, whereas other metals with high CO and H₂ chemisorption heats rapidly dehydrogenate methoxy species below room temperature.     

A DFT study of the transition metal promotion effect on ethylene chemisorption on Co(0 0 0 1)

Transition metals are often introduced to a catalyst as promoters to improve catalytic performance. In this work, we study the promotion effect of transition metals on Co, the preferred catalytic metal for Fischer–Tropsch synthesis because of its good compromise of activity, selectivity and stability, for ethylene chemisorption using density functional theory (DFT) calculations, aiming to provide some insight into improving the α-olefin selectivity. In order to obtain the general trend of influence on ethylene chemisorption, twelve transition metals (Zr, Mn, Re, Ru, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au) are calculated. We find that the late transition metals (e.g. Pd and Cu) can decrease ethylene chemisorption energy. These results suggest that the addition of the late transition metals may improve α-olefin selectivity. Electronic structure analyses (both charge density distributions and density of states) are also performed and the understanding of calculated results is presented.     

使用金属掺杂SnO2(110)作为表面催化剂的电催化CO2还原反应：通过调控Sn:O原子比例来控制产物

电催化CO₂还原反应可以产生HCOOH和CO,目前该反应是将可再生电力转化为化学能存储在燃料中的最有前景的方法之一. SnO₂作为将CO₂转换为HCOOH和CO的良好催化剂,其反应发生的晶面可以是不同的. 其中(110)面的SnO₂非常稳定,易于合成. 通过改变SnO₂(110)的Sn:O原子比例,得到了两种典型的SnO₂薄膜：完全氧化型(符合化学计量)和部分还原型. 本文研究了不同金属(Fe、Co、Ni、Cu、Ru、Rh、Pd、Ag、Os、Ir、Pt和Au)掺杂的SnO₂(110),发现在CO₂还原反应中这些材料的催化活性和选择性是不同的. 所有这些变化都可以通过调控(110)表面中Sn:O原子的比例来控制. 结果表明,化学计量型和部分还原型Cu/Ag掺杂的SnO₂(110)对CO₂还原反应具有不同的选择性. 具体而言,化学计量型的Cu/Ag掺杂的SnO₂(110)倾向于产生CO(g),而部分还原型的表面倾向于产生HCOOH(g). 此外,本文还考虑了CO₂还原的竞争析氢反应. 其中Ru、Rh、Pd、Os、Ir和Pt掺杂的SnO₂(110)催化剂对析氢反应具有较高的活性,其他催化剂对CO₂还原反应具有良好的催化作用.     

Role of adsorption on surface composition of Pd–Cu nanoparticles

Monte-Carlo (MC) simulation is used to study the role of adsorption of hydrogen, oxygen and carbon monoxide (CO) on the surface composition and surface bond geometry of Pd–Cu nanoparticles. For clean particles the surface is found to be enriched in Cu. But in the presence of adsorbed hydrogen and CO there is a segregation reversal from Cu segregation at low coverage to Pd segregation at high coverage. In the presence of adsorbed oxygen, on the contrary, the extent of Cu segregation increases with coverage. For a 586-atom nanoparticle with 50% Pd in the bulk the corner sites are found to be occupied by Cu atoms up to one monolayer adsorption. But, while the occupancy of 7, 8 and 9-coordinated sites by Cu atoms decreases with increase of H and CO coverage, for oxygen adsorption this occupancy increases with coverage. The relevance of such results in catalysis studies is discussed.     

Growth and sacrificial oxidation of transition metal nanolayers

Growth and oxidation of Au, Pt, Pd, Rh, Cu, Ru, Ni and Co layers of 0.3-4.3 nm thickness on Mo have been investigated with ARPES and AFM. Co and Ni layers oxidize while the Mo remains metallic. For nobler metals, the on top O and oxidation state of subsurface Mo increase, suggesting sacrificial e⁻ donation by Mo. Au and Cu, in spite of their significantly lower surface free energy, grow in islands on Mo and actually promote Mo oxidation. Applications of the sacrificial oxidation in nanometer thin layers exist in a range of nanoscopic devices, such as nano-electronics and protection of e.g. multilayer X-ray optics for astronomy, medicine and lithography.     

Highly active surfaces for CO oxidation on Rh, Pd, and Pt

Studies show that the rate of CO oxidation on Pt-group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O₂/CO ratio above 0.5. The catalytic surfaces, formed at discrete O₂/CO ratios >0.5, exhibit rates 2-3 orders of magnitude greater than those rates observed for stoichiometric reaction conditions and similar reactant pressures or previously in ultrahigh vacuum studies at any reactant conditions and extrapolate to the collision limit of CO in the absence of mass transfer limitations. The O₂/CO ratios required to achieve these so-called “hyperactive” states (where the reaction probabilities of CO are thought to approach unity) for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. Auger spectroscopy and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate 1 ML of surface oxygen. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with no detectable adsorbed CO. In contrast, under stoichiometric O₂/CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and exhibit far less activity for CO oxidation.     

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)-c(2 × 2)-Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)-c(2 × 2)-Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm⁻¹ and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm⁻¹. In contrast, two distinct bands around 2090 cm⁻¹ were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm⁻¹ for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu-CO and Pd-CO. Taking into account the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the ordered checkerboard structures.     

Migration-coalescence of Nanoparticles During Deposition of Au,Ag, Cu,and GaAs on Amorphous SiO2

Nanoparticles formed during the initial period of film growth can migrate, coalesce, and may also melt. Nanoparticles of Au, Ag, Cu, and GaAs ranging from 1 to 15nm in diameter were sputter-deposited on amorphous SiO₂ (a-SiO₂). Transmission electron microscopy was used to analyze the time-dependent change of the dispersion of particles on a thin film. The number density of nanoparticles was nearly constant during the deposition of Ag. For Au, Cu, and GaAs, however, the number density decreased with time during the early deposition period. For example, for Au the number density decreased from 2.8×10¹⁶m^–2 (surface coverage ratio of 0.08) to 1.8×10¹⁶m^–2 (surface coverage ratio of 0.14). The surface coverage increased because the particle size increased as the number density decreased. This decrease suggests that migration followed by coalescence occurred. For Au, although we found evidence of migration of 2-nm particles at 500°C, the migration rate was too slow to account for the results from the deposition experiments. These observations indicate an autocatalytic mechanism that migration followed by coalescence liberates energy by the formation of chemical bonds, heats the coalesced particles, and enhances further migration. The strong dependence of the structure of nanoparticle dispersions on the deposition rate is a direct consequence of the deposition mechanism, which is a nonlinear, kinetically-controlled process.     

Reflection-absorption infrared and thermal desorption studies of carbon monoxide and hydrogen adsorbed on rhodium

Reflection-absorption infrared spectroscopic and thermal desorption techniques have been used to study the interaction of mixtures of carbon monoxide and hydrogen with evaporated rhodium films. For equimolar mixtures near 10^?9 Torr, hydrogen adsorbed much more rapidly, but long exposure times or increases in CO pressures to 10^?6 Torr led to its partial, but never complete, displacement by adsorbed carbon monoxide. Hydrogen desorption spectra taken during the displacement process showed two peaks which was consistent with a cooperative interaction between adsorbed CO and H species. In contrast to previous transmission studies of CO adsorption on small rhodium particles, the present reflection—absorption infrared study of the film system showed a single absorption band at 2075 ±10 cm^?1. While explanations for the discrepancy in terms of particle size effects are possible it is considered more likely that all CO molecules are linearly bound to individual Rh atoms in the present situation. In our work, increases in CO pressure (especially above 10^?6 Torr) were accompanied by an upward frequency shift (from 2065 cm^?1 to 2085 cm^?1) and a narrowing in half width (from 25 to 17 cm^?1). Several possible explanations for the latter unusual effect are discussed.     

Reconstructed (1 1 0) surfaces of FCC transition metals

The energies of the ideal, missing row (MR) and missing column (MC) (1 1 0) surfaces have been calculated by using modified embedded atom method (MEAM) for seven face centered cubic (FCC) transition metals Au, Pt, Ag, Pd, Rh, Cu and Ni. The results, that the MC reconstruction can not be formed for all metals, while the MR reconstruction can be formed naturally for Au and Pt, inductively for Ag, Pd, Rh and Cu and difficultly for Ni, are better than EAM calculated results in comparing with experimental results. In addition to the surface energy explanation, the results are also related to the surface topography and valence electron structure.     

Atomic O and H exposure of C-covered and oxidized d-metal surfaces

Carbon coverage, oxidation and reduction of Au, Pt, Pd, Rh, Cu, Ru, Ni and Co layers of 1.5 nm thickness on Mo have been characterized with ARPES and desorption spectroscopy upon exposure to thermal H and O radicals. We observe that only part of the carbon species is chemically eroded by atomic H exposure, yielding hydrocarbon desorption. Exposure to atomic O yields complete carbon erosion and CO₂ and H₂O desorption. A dramatic increase in metallic and non-metallic oxide is observed for especially Ni and Co surfaces, while for Au and Cu, the sub-surface Mo layer is much more oxidized. Although volatile oxides exist for some of the d-metals, there is no indication of d-metal erosion. Subsequent atomic H exposure reduces the clean oxides to a metallic state under desorption of H₂O. Due to its adequacy, we propose the atomic oxygen and subsequent atomic hydrogen sequence as a candidate for contamination removal in practical applications like photolithography at 13.5 nm radiation.     

Auger spectra of carbon monoxide chemisorbed on Pt(111) and Cu(111)

The Auger spectra of carbon monoxide adsorbed on Pt(111) and Cu(111) are compared. The characteristic features now regarded as a fingerprint of this adsorbed species are observed, even for the weakly adsorbed CO on copper which gives complex X-ray photoelectron spectra. No coverage dependence of the spectra was observed on either substrate. The C lsVV spectrum of CO/Cu(111) is dominated by transitions involving the “screening” electron in the 2π orbital.     

The influence of CO on hydrogen sorption by Pd(111) single crystals

A novel method of calibration based on the solubility data of hydrogen in palladium is applied to both the analysis of thermal desorption spectra of hydrogen from Pd(111) and to studies on the influence of adsorbed carbon monoxide on the behaviour of adsorbed and absorbed hydrogen, at room temperature. For clean Pd(111) (θ_CO < 0.04 ML) the comparison of experimental and theoretical spectra shows that part of the hydrogen originally adsorbed on the surface dissolves. This hydrogen appears at higher temperatures as a diffusion tail. A strong influence of coadsorbed CO on the maximum hydrogen coverage was observed in the low-coverage region where usually the existence of isolated CO molecules on the surface can be expected. For hydrogen sorption comparison of experimental data with those calculated on the basis of the solution of Fick's second law for a plane sheet indicates that this process is solely diffusion controlled. On the other hand, a strong barrier for desorption of dissolved hydrogen is observed at θ_CO = 0.33 ML. It is concluded that the influence of CO on the behaviour of dissolved hydrogen may be consistently explained if there is a CO-induced change in the polarization state of adsorbed hydrogen atoms.     

Comparison between the adsorption properties of Pd(111) and PdCu(111) surfaces for carbon monoxide and hydrogen

Thermal desorption spectroscopy and LEED have been used to investigate the interaction of CO and hydrogen with a Pd_0.75Cu_0.25(111) single crystal surface with surface composition of about Pd_0.7Cu_0.3. The main objective was to make a comparison with the previously studied Pd_0.67Ag_0.33(111) (surface composition Pd_0.1Ag_0.9) and Pd(111) surfaces. In addition, the effect of preadsorbed H on subsequent CO dosage and the effect of adsorbed CO on postdosed hydrogen are described. Marked differences were found in the adsorption behaviour of the three surfaces towards CO and hydrogen. The maximum amount of H and CO that can be adsorbed at 250 K and pressures below 10⁻⁹ mbar is much lower on the PdCu surface than expected on the basis of the surface composition. This effect appears to be caused by a low heat of adsorption of hydrogen and CO and Pd singlet sites. Arguments are presented that singlet Pd sites or isolated Pd atoms in a Cu or Ag matrix are able to trap and dissociate the hydrogen molecule at 250 K. The CO desorption spectra are not influenced by pre- or postexposed hydrogen. Adsorbed CO hampers the uptake of hydrogen upon subsequent exposure to hydrogen. Postdosed CO causes adsorbed H adatoms to move to the bulk (adsorbed H). CO exposure at 250 K results in a very broad desorption plateau between 310 and 425 K with hardly discernable maxima. The results can be explained in terms of the size and relative concentration of the various Pd sites present on the surface (triplet, doublet and singlet sites). It can be concluded that for Pd (111) the heat of adsorption of both CO and H differ appreciably for the triplet, doublet and singlet sites. The effect of site has a larger contribution to the decrease of the heat of adsorption with coverage than the effect of lateral interaction in the adlayer. For Pd(111), PdCu(111) and PdAg(111) the effect of the available Pd sites is the major effect that determines the heat of adsorption, followed by the effect of lateral interaction and for the alloy surfaces the electronic or ligand effect.     

Carbon monoxide adsorption on Pd-deposited Cu(1 1 0) surface: Infrared reflection absorption and temperature programmed desorption studies

We investigated carbon monoxide (CO) adsorption and desorption behaviors on 0.1-nm-, 0.15-nm-, and 0.3-nm-thick-Pd-deposited Cu(1 1 0) surfaces using infrared reflection absorption (IRRAS) and temperature-programmed desorption (TPD) spectroscopic methods. CO was exposed to the 0.1-nm-thick-Pd/Cu(1 1 0) surface at the substrate temperature of 90 K. The IR band attributable to CO bonded to Cu atoms emerged at 2092 cm⁻¹: the band was located at 2100 cm⁻¹ at saturation coverage, with a shoulder at 2110 cm⁻¹. In addition to these bands, weak absorptions attributable to the PdCO bonds appeared at 2050 and 1960 cm⁻¹. With increasing Pd thickness, the Pd related-bands became increasingly prominent. Particularly at the early stage of exposure, the band at 2115 cm⁻¹ became visible. The band at 2117 cm⁻¹ dominated the spectra all through the exposures for the 0.3-nm-thick-Pd surface. The TPD spectra of the surfaces showed two remarkable features at around 220-250 and 320-390 K, ascribable ,respectively, to CuCO and PdCO. The desorption peaks shifted to higher temperatures with increasing Pd thickness. Based on the TPD and IRRAS results, we discuss the adsorption-desorption behaviors of CO on the Pd/Cu(1 1 0) surfaces.     

Surface enhanced Raman scattering (SERS) of chemisorbed species on various kinds of metals and semiconductors

SERS spectra of pyridine adsorbed on various kinds of vacuum evaporated (10^?5 Torr) metals (Ag, Au, Ni, Pd, Pt, Ti and Co) and on single crystals of semiconductors (NiO and TiO₂) were obtained at room temperature. The peak frequencies as shifted from those of free pyridine are assigned to the bands of N-bonded pyridine (chemisorbed pyridine). The λ₀ dependence varied remarkably from metal to metal. The peak frequency and the λ₀ dependence for the pyridine adsorbed on NiO or TiO₂ are in good agreement with those on Ni or Ti, respectively, showing the chemical bonding between the N atom and the Ni or Ti atom. The effects of background and of polarization on the SERS spectra were examined in detail, thus revealing the orientation of the adsorbed molecules. Carbon monoxide chemisorbed on Ag was measured by infrared specular reflection as well as by SERS. The results indicate that chemisorbed species on the same substrate do not always give SERS. The SERS spectra obtained are well interpreted as being due to the mechanism of resonance Raman scattering via charge transfer excitation of the adsorbent-adsorbate interaction.     

The reaction of carbon monoxide with palladium supported on cerium oxide thin films

In this study we probe the reaction of carbon monoxide with Pd nanoparticles supported on cerium oxide thin films. With the use of soft X-ray photoelectron spectroscopy (sXPS), and temperature programmed desorption (TPD) the surface intermediates and pathways leading to reaction products of CO on Pd supported on ceria were investigated. When Pd is supported on the stoichiometric CeO₂ surface (Ce⁺⁴) only the molecular adsorption of CO on Pd is visible (286.4 eV). All of the CO desorbs below 520 K, however a small amount of O exchange between the CO and the ceria was indicated through the acquisition of labeled ¹⁸O from the substrate in the desorbed CO. The Pd nanoparticles are activated on partially reduced CeO_x to promote the dissociation of <10% of the CO as indicated by a C-Pd species (284.4 eV) in sXPS. The C recombines with O from the ceria and desorbs between 600 and 700 K. The majority of the CO does not dissociate, however, and the degree of dissociation does not increase with the degree of ceria reduction. This result is in contrast with Rh nanoparticles supported on ceria where the degree of dissociation increased with the degree of ceria reduction and nearly total dissociation was obtained when the ceria was highly reduced.     

Adsorption of carbon monoxide on silver/palladium alloys

The adsorption of carbon monoxide on epitaxial (100) and (111) planes of Ag/Pd alloys with definite surface compositions has been studied by means of LEED, Auger electron spectroscopy and work function measurements. The formation of ordered adsorbed structures is prevented by even small amounts of silver in the surfaces. The maximum variation of the work function with CO adsorption bears no simple relationship to the surface composition. From measured adsorption isotherms the isosteric heats of adsorption have been evaluated. For CO adsorption on pure Pd planes the adsorption energies E_ad are either constant or decrease slowly up to high coverages, whereas a continuous decrease was observed with the alloys indicating the energetical heterogeneity. The results are discussed on the basis of our knowledge about the nature of the CO chemisorption and about the electronic structure of Ag/Pd alloys.     

The growth and chemisorptive propertles of Ag and Au monolayers on platinum single crystal surfaces: An AES,TDS and LEED study

The growth and chemisorptive properties of monolayer films of Ag and Au deposited on both the Pt(111) and the stepped Pt(553) surfaces were studied using Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). AES studies indicate that the growth of Au on Pt(111) and Pt(553) and Ag on Pt(111) proceeds via a Stranski-Krastanov mechanism, whereas the growth of Ag on the Pt(553) surface follows a Volmer-Weber mechanism. Au dissolves into the Pt crystal bulk at temperatures > 800 K, whereas Ag desorbs at temperatures > 900 K. TDS studies of Ag-covered Pt surfaces indicate that the AgPt bond (283 kJ mol^?1) is ～25 kJ mol^?1 stronger than the AgAg bond (254 kJ mol^?1). On the Pt(553) surface the Au atoms are uniformly distributed between terrace and step sites, but Ag preferentially segregates to the terraces. The decrease in CO adsorption on the Pt crystal surfaces is in direct proportion to the Ag or Au coverage. No CO adsorption could be detected for Ag or Au coverages above one monolayer at 300 K and 10^?8 Torr. The heat of adsorption of CO on Pt is unaltered by the presence of Ag or Au.