The hydrogenation of CN on Pd(111) and Pd(100)

Adsorbed CN may be produced on Pd(111) and Pd(100) surfaces at RT by dissociative adsorption of cyanogen. HREELS measurements show that adsorbed CN forms adsorbed HCN or DCN on these Pd surfaces by reaction with H adsorbed from the residual gas or by dosing with H₂ or D₂. The reaction temperature is slighly lower for Pd(100) than for Pd(111), and the range of temperatures over which the reaction takes place much narrower. The reaction occurs on a time scale easily monitored with HREELS.     

Magnetic properties of (Co Pd ) superstructures on Pd(100) and Pd(111)

The magnetic properties of (Co_nPd_m)_r superstructures on Pd(100) and Pd(111) are evaluated using the fully-relativistic spin-polarized screened Korringa-Kohn-Rostoker method. It is found that only in the case of a Pd(111) substrate such superstructures exhibit perpendicular magnetism, while on a Pd(100) substrate the magnetization is oriented in-plane. Also investigated is the effect of interdiffusion in repeated superstructures. By using the inhomogeneous coherent potential approximation (CPA) for layered systems the effect of ordering into (repeated) superstructures can be described in an ab-initio-like manner. It is found that already small amounts of interdiffusion can be decisive for the actual value of the magnetic anisotropy energy. Received 3 November 1999 and Received in final form 18 January 2000     

Adsorbate band formation: The chemisorption of CO on Pd (100)

In ordered overlayers of adsorbed gases on metal surfaces, high coverage situations can lead to overlap between orbitals on adjacent species and hence to adsorbate band formation. We conclusively demonstrate the existence of this effect in chemisorption by examining the dispersion of the 4α level in the u.v. photoelectron spectrum of the CO/Pd (100) system. The results agree well with a first principles extended tight binding calculation of the two-dimensional band structure.     

Two-dimensional oxide on Pd(111)

The oxidation of Pd(111) leads to an incommensurate surface oxide, which was studied by the use of scanning tunneling microscopy, surface x-ray diffraction, high resolution core level spectroscopy, and density functional calculations. A combination of these methods reveals a two-dimensional structure having no resemblance to bulk oxides of Pd. Our study also demonstrates how the atomic arrangement of a nontrivial incommensurate surface can be solved by molecular dynamics in a case where experimental techniques alone give no solution.     

Pd layers on a W(100) surface

Pd films from zero to several monolayers in thickness on a W(100) surface are studied by AES, LEED, work function change (A0) measurements and TDS. Similar to other metals on W, the adsorption and annealing behaviour differs drastically from that on the W(110) surface but resembles the behaviour of other metals on the W(100) and Mo(100) surface.     

Oxygen adsorption on stepped Pd(100) surfaces

We use scanning tunneling microscopy (STM) and high-resolution core-level spectroscopy (XPS) measurements to study the initial oxidation of vicinal Pd(100) surfaces exhibiting close-packed (111) steps. The XPS data analysis is supported by detailed surface-core level shift calculations based on density-functional theory. Both STM images and the XPS spectra are found to be perfectly consistent with a characteristic zigzag O decoration of the Pd steps predicted by a preceding cluster-expansion based theoretical study [Y. Zhang and K. Reuter, Chem. Phys. Lett. 465, 303 (2008)]. Continued oxygen uptake leads to the additional stabilization of a p(2 × 2)-O overlayer on the Pd(100) terraces, and ultimately to step bunching with the resulting large Pd(100) terraces covered by the PdO(101) surface oxide.     

Strain relief and Pd island shape evolution on the palladium and palladium hydride (100) surface

The mesoscopic relaxation of small Pd islands on Pd(100) and PdH(100) surfaces is investigated on the atomic scale by performing molecular statics calculations. A strong strain and stress inhomogeneity in islands and topmost layers of the substrate is revealed. An unusual size dependence of the shape of islands is discovered.     

Hydrogen chemisorption on Pd(100) studied with he scattering

He scattering from the clean Pd(100) surface yields extremely weak diffraction beams relative to the specular, corresponding to a very small maximum corrugation amplitude of ~ 0.04 Å. Hydrogen adsorption at a temperature of 110 K leads to the formation of a c(2 × 2) ordered phase at a coverage of 0.5 monolayers and a (1 × 1) phase at saturation coverage. The maximum corrugation amplitude of the c(2 × 2)H is ~0.13 Å; surface charge density calculations using overlapping atomic charge densities indicate a normal distance of the hydrogens to the topmost Pd layer d_n ? 0.65–0.70 Å corresponding to a H-Pd bonding distance of ~ 2.05 Å in the fourfold hollow sites. The result that the maximum corrugation amplitude of the (1 × 1) hydrogen phase, with ~ 0.025 Å, even smaller than that of the clean surface may indicate a movement of the hydrogens closer to the topmost metal layer, when the coverage is increased from 0.5 monolayers to saturation.     

Novel interface-mediated metastable oxide phases: vanadium oxides on Pd(111)

In the growth process of ultrathin films of vanadium oxides on Pd(111), a sequence of novel oxide phases with layer-dependent structures and oscillating oxidation states has been detected experimentally and understood theoretically. These phases are interface mediated and metastable with respect to further oxide growth. Transformation into the stable oxide configuration occurs beyond a critical thickness, where energetics combined with kinetic limitations determine the oxide multilayer structure.     

Summary abstract: Vibrational excitations of hydrogen and oxygen on Pd(100)

EMIRS spectra for ¹²CO-¹³CO mixtures on platinum, HSO₄^? and acrylonitrile on gold and water on silver are discussed. These examples illustrate the adsorbate identification, bonding, and orientation information which EMIRS data offer to complement electrochemical Raman data and infrared reflection absorption and EELS results. The advantages of experimental fine control of the electronegativity of the metal are shown with application to studies of coupling mechanisms between adsorbed species.     

The adsorption of CO on Pd(100) studied by IR reflection absorption spectroscopy

High resolution vibrational spectra of the C-O stretching region have been measured for the system CO/Pd(100) at temperatures between 80 and 340 K. LEED and flash desorption studies enabled the absolute coverage for each spectrum to be determined. The C-0 stretching frequency was found to shift linearly with coverage for the three adsorption temperatures 80, 200 and 300 K. At saturation coverage (θ ~ 0.8) the total shift is of the order of 100 cm⁻¹. Experiments with ¹²CO /¹³CO mixtures showed that the contribution of vibrational coupling is ~ 35 cm⁻¹ the rest being due to a static, partly chemical, lateral interaction. The apparent commensurate-incommensurate transition at θ = 0.5 has no significant influence on shift, width or intensity of the absorption band.     

The adsorption and reaction of 2-butanol on clean and oxygen-covered Pd(100)

The surface chemistry of 2-butanol is explored on clean Pd(100), c(2 × 2)-O/Pd(100) and p(2 × 2)-O/Pd(100) surfaces by means of temperature-programmed desorption, reflection–absorption infrared and X-ray photoelectron spectroscopies. 2-Butanol adsorbs molecularly on clean and oxygen-covered Pd(100) below ～ 190 K, but then appears to react to form 2-butoxide species at ～ 200 K. Both 2-butanone and 2-butanol desorb from the clean surface at ～ 226 K, by β-hydride elimination from the 2-butoxide species and rehydrogenation of the 2-butoxide, respectively. In contrast, almost exclusively 2-butanone is formed on oxygen-covered surfaces. Butanone desorbs at ～ 195 K and ～ 260 K from c(2 × 2)-O/Pd(100) with the 195 K peak being the most intense. However, on p(2 × 2)-O/Pd(100), 2-butanone desorbs at ～ 195 K and ～ 295 K, and the latter peak is the most intense. The ～ 195 K, 2-butanone state is proposed to occur due to abstraction by adsorbed atomic oxygen and the change in relative intensity of these features is ascribed to the lower ability of surface hydroxyl groups to facilitate β-hydride elimination on oxygen-covered surfaces. Further heating results in the formation of hydrogen and carbon monoxide and leaves a small amount of carbon deposited on the surface.     

Adsorption of hydrogen and deuterium on potassium-promoted Pd(100) surfaces

The adsorption of H₂ and D₂ has been studied on clean and K-promoted Pd(100) surfaces using thermal desorption, work function changes, ultraviolet photoelectron and Auger spectroscopy. The potassium adlayer significantly lowers the sticking coefficient (from 0.6 to 0.06 at θ_k = 0.2), and the uptake of hydrogen, but increases the desorption energy for H₂ desorption. Calculation showed that each potassium adatom blocks approximately 4–5 adsorption sites for H₂ adsorption. Atomization of hydrogen led to an increase of hydrogen uptake. The adsorption of potassium on the H-covered surface caused a significant decrease in the amount of hydrogen adsorbed on the surface (as indicated by less desorbing hydrogen below 500 K) and promoted the dissolution of H atoms into the bulk of Pd. The dissolved hydrogen was released only above 600–650 K. In the interpetation of the results the extended charge transfer from K-dosed Pd to the adsorbed H atoms and the direct interaction between adsorbed H and K adatoms are taken into account.     

Thermal evolution of acetylene and ethylene on Pd(111)

The thermal evolution of acetylene and ethylene and their deuterated counterparts on a palladium (111) surface has been studied by high-resolution electron energy loss spectroscopy in the temperature range 150–500 K. Analysis of the vibrational spectra indicates that chemisorbed acetylene evolves at 300 K in the presence of surface hydrogen to mainly ethylidyne, CCH₃, and a small amount of residual acetylene. Spectra obtained with and without preadsorbed hydrogen provide evidence for a 〉C CH₂ intermediate in the reaction. Chemisorbed ethylene also evolves to ethylidyne after heating from 150 to 300 K but much of the ethylene desorbs. The high temperature (400–500 K) behavior of C₂H₂ and C₂H₄ involves formation of a CH species. Although a small amount of the CH species may be formed from the dehydrogenation of ethylidyne, it is found that carbon-carbon bond scission of acetylene near 400 K is the dominant mechanism in CH formation.     

小分子在Pd(100)面上的共吸附

本文应用紧束缚模型和单电子理论研究了几种小分子在Pd(100)面上的共吸附特性,结果表明:在Pd(100)面上两个吸附分子(或吸附原子)间的间接相互作用随吸附质与衬底之间的耦合强度的减小呈现振荡性衰减的趋势,并且,当O₂或H₂存在时,NO在Pd(100)面上的解离将受到影响,此外,根据两吸附质之间的间接相互作用,推断出一些吸附层结构,这些结果与实验结果作了比较。 关键词：     

Structure and magnetization of (100) FeCr and FeV films deposited on Pd/Cu/Si(100)

(100) oriented FeCr and FeV films, 1000 Å thick, have been deposited on (100)Si using (100)Pd/Cu seed layers. Three compositions are made for each alloy, 75/25, 50/50 and 25/75, all show lattice spacings corresponding to a body-centered cubic structure. Magnetic measurement shows composition dependent magnetization: both FeCr and FeV with compositions of 75/25 and 50/50 are ferromagnetic, but not the ones with the composition of 25/75. In-plane magnetization along different crystalline axes shows the same crystalline anistropy as that of bulk Fe crystal for both 75/25 structures, with a higher magnetization along the [100] edge than the [110] one. For the 50/50 structures, the [100] edge shows a smaller magnetization at low fields than the [110] one, but exceeds the latter at higher field. The measured magnetizations are less than those from linearly diluted Fe lattices. The correlation between the structures and magnetic properties is discussed.     

Comparative AES and RHEED study of the formation of Pd silicide on clean and oxide covered Si(100) and (111) surfaces

The formation and epitaxial orientation of Pd silicide on clean and native oxide covered Si(100) and (111) surfaces was studied by Auger electron spectroscopy (AES) and reflection high energy electron diffraction (RHEED). Pd was vapor deposited in UHV on to the substrates up to thicknesses of about 6 nm. On clean Si substrates, ultra-thin Pd deposits reacted to form Pd₂Si already at room temperature, as detected by a characteristic splitting of the Si LVV Auger peak. However, a polycrystalline structure with very small crystallite sizes was indicated by diffuse ring patterns in RHEED. When the initial thickness of the Pd deposit exceeded about 3 nm, the diffraction ring pattern of unreacted metal developed. During annealing of room temperature deposits of Pd, the (100) and (111) substrates behaved differently. Larger crystallites formed on Si(100), but the films remained polycrystalline, though textured. On Si(111), virtually perfect epitaxial re-orientation of the silicide was found. When the substrates were initially covered with native oxide of about 2 nm thickness, silicide formation started at about 200°C, resulting in polycrystalline, but strongly textured Pd₂Si. Upon further annealing at temperatures up to 600°C, an additional phase of epitaxially oriented Pd₂Si developed on Si(111), similar to that on clean Si(100). In all experiments, extended annealing at temperatures above 250°C caused segregation of Si to the surface. This was accompanied by the development of an additional peak in the Auger electron spectra at about 313 eV, which we assign to a plasmon loss of δE = 17 eV in the Si overlayer, being excited by Pd Auger electrons of energy 330 eV.