CO on Pd(110): determination of the optimal adsorption site

We present ab initio pseudo-potential plane-wave total-energy calculations for the geometric and electronic structure of the CO-covered Pd(110) surface. Our calculations were performed within the local-density approximation (LDA) of density functional theory (DFT). There has been some controversy as to whether CO prefers to adsorb at a bridge or on-top site when exposed to Pd(110). Total energy calculations for a CO monolayer adsorbed at the on-top and bridge adsorption sites revealed the bridge site adsorption to be favored by 0.59 eV per CO molecule. The preferential adsorption of CO to the bridge site was further corroborated by our band-structure calculations, with only the bridge site results being in good agreement with recent inverse photoemission experiments.     

Interpretation of the photoelectron spectra of molecularly adsorbed CO

We have measured the photoemission spectra both for CO adsorbed on Ni and Pd films and for gas phase CO over a wide photon energy range. The comparison of these two sets of measurements lead to the following interpreation of the two observed electronic energy levels of molecularly adsorbed CO. The lower binding energy CO-induced level corresponds to the 1π gas phase level with some admixture of the 5σ level and the higher binding energy level corresponds to the 4σ gas phase level. The hω-dependence of the photoionization cross section is shown to play a predominant role in determining the surface sensitivity.     

The adsorption of carbon monoxide on TiO2(110) supported palladium

Palladium overlayers deposited on TiO₂(110) by metal vapour deposition have been investigated using LEED, XPS and FT-RAIRS of adsorbed CO. Low coverages of palladium (<3 ML) deposited at 300 K adsorb CO exclusively in a bridged configuration with a band (B₁ at 1990 cm⁻¹) characteristic of CO adsorption on Pd(110) and Pd(100) surfaces. When annealed to 500 K, XPS and LEED indicate the nucleation of Pd particles on which CO adsorbs predominantly as a strongly bound linear species which we associate with edge sites on the Pd particles (L* band at 2085 cm⁻¹). Both bridged and linear CO bands are exhibited as increases in reflectivity at the resonant frequency, indicating the retention of small particle size during the annealing process. Palladium overlayers of intermediate coverages (10–20 ML) deposited at 300 K undergo some nucleation during growth, and adsorbed CO exhibits both absorption and transmission bands in the B₁ (1990 cm⁻¹) and B₂ (1940 cm⁻¹) regions. The latter is associated with the formation of Pd(111) facets. Highly dispersed Pd particles are produced on annealing at 500 K. This is evidenced by the dominance of transmission bands for adsorbed CO and a significant concentration of edge sites, which accommodate the strongly bound linear species at 300 K. Adsorption of CO at low temperature also allows the identification of the constituent faces of Pd and the conversion of Pd(110)/(100) facets to Pd(111) facets during the annealing process. High coverages of palladium (100 ML) produce only absorption bands in FT-RAIRS of adsorbed CO associated with the Pd facets, but annealing these surfaces also shows a conversion to Pd(111) facets. LEED indicates that at coverages above 10 ML, the palladium particles exhibit (111) facets parallel to the substrate and aligned with the TiO₂(110) unit cell, and that this ordering in the particles is enhanced by annealing.     

Orientation of adsorbed benzene from angle-resolved photoemission measurements

Angle-resolved photoemission using polarised radiation has been used to confirm the orientation of benzene adsorbed in a (2 × 2) R45° structure on Pd(100). The molecule is found to lie flat on the surface with apparent C_6v local symmetry.     

CO在表面有序合金Cu{001}c(2×2)-Pd上的吸附

用高分辨电子能量损失谱研究CO在有序表面合金Cu{001}c(2×2)-Pd上的吸附位置,在240K,CO仅直立地吸附在Pd原子的顶位;在135K,低覆盖度时,CO仅直立地吸附于Pd原子顶住,覆盖度增加后,CO同时直立地也吸附于Pd和Cu原子的顶位。     

DFT calculations of the electronic structure of SnOx layers on Pd(110)

The electronic structures of Sn and SnO _x layers adsorbed on the Pd(110) surface have been calculated by DFT. In agreement with available results of photoemission studies, it is found that the formation of the oxides induces pronounced changes in the related peak in the DOS. In particular, it is shown that the formation of SnO₂ adsorbed layers leads to a transformation of the Sn 4d spin-orbit doublet into a quadruple peak, while for SnO the peak retains its shape. Due to this feature, the shape of the Sn 4d peak in photoemission spectra can serve as an unambiguous indicator of the degree of oxidation of Sn layers on transition metal surfaces.     

Influence of the substrate structure on the bonding of chemisorbed acetylene to transition metal surfaces

The electronic spectrum of acetylene adsorbed on various transition metals has been measured by ultraviolet photoemission spectroscopy in various laboratories. At low temperatures (T < 150 K), all measurements concur in finding an electron spectrum that differs only moderately from the gas phase spectrum of acetylene. At room temperature, the electron spectrum of acetylene is reported to be similar to the low-temperature form on Ir(100) and Pt(100), but acetylene is reported to form an olefinic surface complex on Pd(111) and Pt(111) surfaces. In order to examine whether the surface structure of the substrate is responsible for the difference, we have measured the electronic spectrum of acetylene adsorbed on the Pd(100) and Ru(0001) surfaces. At 120 K, the spectrum of adsorbed acetylene is again a distorted gas phase spectrum on both surfaces. At 330 K, we find the acetylenic form (with a splitting of 2.5 eV of the σ-orbitals) on Pd(100) and an olefinic form on the basal plane of Ru. We conclude that the olefinic complex is proper to the threefold symmetry of the (111) and (0001) surfaces and the gas-like form is favored on the (100) surfaces of the fcc crystals.     

Angular distribution patterns of photoelectrons from orbitals of CO adsorbed on Ni(100), Pt(111) and Pt(110)

The synchrotron radiation from BESSY has been used to measure the photoemission from CO orbitals adsorbed as ordered overlayers on Ni(100) c(2 × 2), Pt(111) c(4 × 2) and Pt(110) (2 × 1)p2mg. Angular distribution patterns of photoelectrons from CO orbitals were recorded with a display-type analyzer. The data were compared with differential photoionization cross sections calculated for free and oriented molecules. The results demonstrate the upright orientation of CO on Ni(100) and Pt(111), while CO on Pt(110) shows a marked difference which can be explained by assuming that the CO molecules are tilted in the [001] directions of Pt(110), yielding a (2 × 1)p2mg superstructure observed in LEED. The tilt angle is estimated to about 20°. The structure model is supported by the shape resonances of the 4σ (5σ) orbitals of CO/Pt(110) as compared to CO/Pt(111).     

Angle resolved photoemission study of the Ce/Pd(1 1 1) interface

X-ray and UV excitation angle resolved photoemission spectroscopy of ultra-thin films of cerium deposited on a Pd(1 1 1) single-crystal surface has been carried out. Both core level and valence band spectra show a formation of a surface alloy exhibiting d- and f-electron orbital hybridization. An azimuth and polar angle mapping of X-ray excited photoemission intensities results in a surface-geometrical structure information. Mapping of ultra-violet photoelectron intensities as a function of emission angles is used for a band mapping and an electronic structure determination. The d-, f-hybridization occurs even for 0.7 ML of Ce deposited at room temperature. The surface annealing at 260 °C enhances this behavior. A shift of Pd 4d-derived states to higher binding energy in the Ce-Pd systems is observed.     

The chemisorption of CO on Cu(100) studied with angle resolved photoelectron spectroscopy

Using angle resolved UV photoelectron spectroscopy, coupled with the continuum of polarized light available with synchrotron radiation, we make an interpretation of the photoelectron spectrum of CO adsorbed on Cu(100). We point out that the bonding of CO on Cu as observed by photoemission is considerably different from the bonding of CO on Ni. These differences do not seem to be caused by bonding orientation differences, however, as the CO molecular axis is found to be very close to the surface normal, as was the case for CO on Ni(100). No evidence is found for a second phase of CO on Cu(100).     

基于密度泛函理论研究掺杂Pd石墨烯吸附O2及CO

基于第一性原理的密度泛函理论研究了单个O₂和CO气体分子吸附于本征石墨烯和掺杂钯(Pd)的石墨烯的体系, 通过石墨烯掺Pd前后气体分子的吸附能、电荷转移及能带和态密度的计算, 发现掺Pd后气体分子吸附能和电荷转移显著增大, 这是由于Pd的掺杂, 在本征石墨烯能带中引入了杂质能级, 增强了石墨烯和吸附气体分子间的相互作用; 氧化性气体O₂和还原性气体CO吸附对石墨烯体系能带结构和态密度的影响明显不同, 本征石墨烯吸附O₂后, 费米能级附近态密度变大, 掺Pd后在一定程度变小; 吸附还原性的CO后, 石墨烯费米能级附近态密度几乎没有改变, 表明掺杂Pd不会影响石墨烯对CO的气体灵敏度, 但由于CO对石墨烯的吸附能增大, 可以提高石墨烯对还原性气体的气敏响应速度.     

分子取向对CO在Pd(111)面吸附的影响

 用基于密度泛函理论广义梯度近似下的平面波赝势方法计算了在Pd(111)晶面两种不同CO分子取向的吸附结构。计算结果表明，CO分子碳端和氧端靠近Pd(111)面的吸附能分别为-1.75，-0.28 eV，碳端吸附的结构比氧端吸附能力强。因此，分子取向影响CO在Pd(111)面上的吸附，通过控制CO的取向可能减小Pd(111)的吸附进而减弱Pd(111)面CO分子的中毒。     

A study of the infrared spectra of carbon monoxide adsorbed on metals of the copper subgroup and on cobalt,ruthenium, rhodium,and palladium

A study has been made of the infrared spectra of carbon monoxide CO, adsorbed on Cu, Ag, Au, Co, Ru, Rh and Pd at temperatures from –160° C to 200° C and pressures from 10^–5 to 1–10 mm Hg. The spectra of CO adsorbed on Cu, Ag, and Au show one absorption band, characteristic of a surface compound of carbon monoxide, with linear structure. It has been found that in the stable chemosorption of CO on Co, Ru, Rh, and Pd in the range of temperatures and pressures studied, two types of surface compound-linear and bridged-are produced.     

Tailoring confining barriers for surface states by step decoration: CO /vicinal Cu(111)

The influence of CO adsorption on the Shockley type surface state on vicinal Cu(111) surfaces is investigated using angle resolved photoemission. As the steps are decorated with CO the surface state shifts to higher binding energies, which is opposite to the known behavior on flat Cu(111). This is described within a one-dimensional potential model in which clean steps represent repulsive barriers and decorated steps become attractive wells. From the coverage dependence the integrated CO well potential can be quantified. It is U(CO)a = -2.9 eV A on both Cu(332) and Cu(221) surfaces. Density functional calculations reveal that this attractive potential is due to the very local charge transfer from the Cu step atom to the adsorbed molecule.     

Adsorption of CO on clean and oxygen covered Ni(111) surfaces

Adsorption of CO on Ni(111) surfaces was studied by means of LEED, UPS and thermal desorption spectroscopy. On an initially clean surface adsorbed CO forms a √3 × $\text{√3}\text{R30°}$ structure at θ = 0.33 whose unit cell is continuously compressed with increasing coverage leading to a c4 × 2-structure at θ = 0.5. Beyond this coverage a more weakly bound phase characterized by a $\text{√7}\text{2}$ × $\text{√7}\text{2R19°}$ LEED pattern is formed which is interpreted with a hexagonal close-packed arrangement (θ = 0.57) where all CO molecules are either in “bridge” or in single-site positions with a mutual distance of 3.3 Å. If CO is adsorbed on a surface precovered by oxygen (exhibiting an O 2 × 2 structure) a partially disordered coadsorbate 2 × 2 structure with θ_o = θ_co = 0.25 is formed where the CO adsorption energy is lowered by about 4 kcal/mole due to repulsive interactions. In this case the photoemission spectrum exhibits not a simple superposition of the features arising from the single-component adsorbates (i.e. maxima at 5.5 eV below the Fermi level with O_ad, and at 7.8 (5σ + 1π) and 10.6 eV (4σ) with CO_ad, respectively), but the peak derived from the CO 4σ level is shifted by about 0.3 eV towards higher ionization energies.     

Structure and reactivity of Pd doped Ag surfaces

We have performed first principles calculations for clean and Pd doped Ag(1 1 1) and Ag(1 0 0) surfaces, with and without adsorbed O and CO. Our results for the structure of the Pd doped Ag surfaces indicate that Pd atoms are located lower than the surrounding Ag surface atoms. We find that O atoms adsorbed on Pd doped Ag(1 1 1) reside at the fcc hollow sites, the site next to Pd being slightly favored. Moreover, we provide results for O and CO co-adsorption on the clean and Pd doped Ag(1 1 1) surfaces, indicating that Pd can act as an electronic promoter for the CO oxidation reaction.     

X-ray photoelectron spectroscopy (XPS) of adsorbate valence bands

The CO valence levels for a monolayer of CO adsorbed on the basal (001) face of ruthenium have been observed by XPS. The assignment of the observed peaks is discussed.     

Gold at the Si(111)-Pd interface; modification of atomic interdiffusion

We present the results of an angle integrated photoemission (hv = 21.2eV) and Auger investigation of the effect of Au interlayers on the growth of the Si(111)-Pd interface. Gold does not remain totally blocked at the interface with increasing amounts of deposited Pd. As a consequence the same amount of Au (around 6 monolayers) acts as a promotor of Si-Pd mixing at low Pd coverages (below about 8 monolayers) and as an inhibitor at higher Pd coverages. The effect is dependent on the amount of Au so that Au at 10 monolayers is an inhibitor independent of the Pd coverage. The implications on the interface growth mechanism are discussed.     

一氧化碳分子在铁(110)表面上吸附的角分辨紫外光电子谱

本文报道了用角分辨紫外光电子谱对一氧化碳分子在铁(110)表面上吸附的取向进行的研究。实验结果与理论计算比较表明,在低温下(≈150K)较低覆盖度时,一氧化碳分子直立在铁表面上,分子轴线与铁表面法线的夹角不超过10度。 关键词：     

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.