Co chemisorption on PtCu surfaces III. CO on PtCu(111)

Selected thermal desorption and valence band photoemission data on the chemisorption of CO on PtCu(111) surfaces are presented. The main objective is to make a comparison with CO chemisorption on an annealed (1 × 3) reconstructed Pt_0.98Cu_0.02(110) surface. The (111) alloy surfaces are unreconstructed (1 × 1) surfaces, with average near-surface Cu concentrations ranging from ? 7.5% to ? 20% as indicated by the Cu 920 eV Auger signal. It is observed that the effect of alloying Pt(111) with Cu is to progressively lower the desorption peak temperature and hence the free energy of CO desorption from Pt sites. A second observation is that the energy distribution of the Cu 3d-derived states is little affected by CO adsorption on Cu sites at 155 K. Both these results offer a contrast to the results for CO/Pt_0.98Cu_0.02(110) reported earlier.     

Nature of versatile chemisorption on TiC(111) and TiN(111) surfaces

Density-functional calculations on the polar TiX(111) (X = C, N) surfaces show (i) for clean surfaces, strong Ti3d-derived surface resonances (SR’s) at the Fermi level and X2p-derived SR’s deep in the upper valence band and (ii) for adatoms in periods 1-3, pyramidic trends in atomic adsorption energies, peaking at oxygen (9 eV). A concerted-coupling model, where adatom states couple to both kinds of SR’s in a concerted way, describes the adsorption. The chemisorption versatility and the general nature of the model indicate ramifications and predictive abilities in, e.g., growth and catalysis.     

Embedded cluster calculations for hydrogen chemisorption on the (111) surfaces of fcc Co,Ni, Cu,Rh, Pd and Ag

We present a muffin tin based calculation on (TM)₃H, (TM)₇H and (TM)₁₉H clusters embedded at the surface of an effective jellium-like medium whose potential is treated in scattering length approximation. We consider the changes occurring when the d-like perturbation of the TM muffin tins is switched on. The broad chemisorption-induced resonance seen for H on the effective jellium surface is narrowed and shifted down in energy. Furthermore the occupation of this resonance is increased from about 1.1 electrons to about 1.4 (on 3d metals) or 1.8 (on 4d metals), due to d-like states dropping down from the d band to form a relatively welldefined “bonding state”. An antibonding state containing about 0.4 electrons is formed at the top of the d band. The results are compared with other calculations and with photoemission data. Implications for the metal-hydrogen distance and (for Ni) the demagnetizing effect of hydrogen chemisorption are discussed. We use the change in total single particle energy when the d-like perturbation is switched on to estimate trends in chemisorption energy along the 3d and 4d series. In the 3d case experimental data is available on the difference in chemisorption energy between Ni and Cu which is in reasonable agreement with our estimate.     

Pyridine chemisorption on silicon and germanium (111) surfaces

The chemisorption of pyridine molecules on cleaved Si(111) and Ge(111) surfaces was investigated by ultraviolet photoemission spectroscopy with synchrotron radiation. Evidence was found that the chemisorption process strongly affects all the three highest occupied molecular levels, i.e. the n-level and the two π-levels la₂ and 2b₂. This result was used to rule out a chemisorption geometry with the aromatic ring parallel to the surface. In the most likely chemisorption geometry the ring is tilted with respect to the substrate and the n-orbital plays a leading role in the formation of chemisorption bonds.     

The co-adsorption of oxygen and hydrogen on Rh(111)

The co-adsorption of oxygen and hydrogen on Rh(111) at temperatures below 140 K has been studied by thermal desorption mass spectrometry, Auger electron spectroscopy, and lowenergy electron diffraction. The co-adsorption phenomena observed were dependent upon the sequence of adsorption in preparing the co-adsorbed overlayer. It has been found that oxygen extensively blocks sites for subsequent hydrogen adsorption and that the interaction splits the hydrogen thermal desorption into two states. The capacity of the oxygenated Rh(111) surface for hydrogen adsorption is very sensitive to the structure of the oxygen overlayer, with a disordered oxygen layer exhibiting the lowest capacity for hydrogen chemisorption. Studies with hydrogen pre-adsorption indicate that a hydrogen layer suppresses completely the formation of ordered oxygen superstructures as well as O₂ desorption above 800 K. This occurs with only a 20% reduction in total oxygen coverage as measured by Auger spectroscopy.     

Chlorine chemisorption and surface chloride formation on Au(111)

The adsorption-desorption properties of the gold(111)-chlorine system have been investigated. Thermal desorption experiments following chlorine adsorption at 298 K indicated two desorption processes: the high temperature peak (ΔH = 217 kJmol^?1) showed desorption of equal numbers of molecukr and atomic chlorine species, while the lower temperature peak (ΔH = 140 kJmol^?1) was due to the desorption of Cl₂ only. Chlorine adsorption led to a maximum work function change of +0.5 V and electron-stimulated desorption proceeded with constant cross-section (1.4 × 10¹⁸ cm²), until all chlorine had been removed from the surface These observations are consistent with the immediate formation of a surface chloride (AuCl₃) during chlorine adsorption on Au(111) at 298 K without the intervention of an initial adsorbed overlayer.     

Angle-resolved photoemission study of no chemisorption on Ag(111) surface

An angle-resolved photoemission study using synchrotron radiation has been carried out to investigate the adsorption of NO on Ag(111). At 150 K, NO is adsorbed molecularly on the Ag(111) surface. The NO molecule is found to have an upright orientation from a study using incidence angle dependent measurements and symmetry selection rules. The upright orientation is also confirmed from the study of the shape resonance of the 5σ level. Photon induced dissociation of the chemisorbed NO was observed and is briefly discussed.     

Theoretical study of oxygen chemisorption on (111) and (100) silicon surfaces

The chemisorption of atomic oxygen on (111) and (100) silicon surfaces has been studied by the MNDO method using a cluster approach. The results show that, for both surfaces, chemisorption occurs preferentially on bridge positions, but chemisorption on top positions can play a significant role especially for the (111) surface.     

The interaction of oxygen with the Rh(111) surface a

The adsorption of oxygen on Rh(111) at 100 K has been studied by TDS, AES, and LEED. Oxygen adsorbs in a disordered state at 100 K and orders irreversibly into an apparent (2 × 2) surface structure upon heating to T? 150 K. The kinetics of this ordering process have been measured by monitoring the intensity of the oxygen $\text{(1,}\text{1}\text{2}\text{) LEED}$ beam as a function of time with a Faraday cup collector. The kinetic data fit a model in which the rate of ordering of oxygen atoms is proportional to the square of the concentration of disordered species due to the nature of adparticle interactions in building up an island structure. The activation energy for ordering is $\text{13.5 ± 0.5}\text{kcal}\text{mole}$. At higher temperatures, the oxygen undergoes a two-step irreversible disordering (T? 280 K) and dissolution (T?400K) process. Formation of the high temperature disordered state is impeded at high oxygen coverages. Analysis of the oxygen thermal desorption data, assuming second order desorption kinetics, yields values of 56 ± 2 kcal/ mole and 2.5 ± 10^?3 cm² s^?1 for the activation energy of desorption and the pre-exponential factor of the desorption rate coefficient, respectively, in the limit of zero coverage. At non-zero coverages the desorption data are complicated by contributions from multiple states. A value for the initial sticking probability of 0.2 was determined from Auger data at 100 K applying a mobile precursor model of adsorption.     

The geometrical and vibrational properties of the Rh(111) surface

Low-energy electron diffraction (LEED) data have been used to characterize the clean Rh(111) surface. The surface geometry, the degree of surface relaxation, and the Debye temperature have been determined. In the Debye temperature measurement, specular LEED beam intensities were monitored as a function of temperature over a range of electron energies from approximately 30 to 1000 eV. It was found that the bulk Debye temperature is 380 ± 23 K, and the normal component of the Debye temperature at the lowest electron energy used is 197 ± 12 K. The Rh(111) surface relaxation has been determined both by a convolution-transform analysis and by dynamical calculations. Within experimental error, neither expansion nor contraction of the topmost layer has been detected. The results of the convolution-transform analysis of specular beams at two angles of incidence and of a nonspecular beam at normal incidence suggest an expansion of the topmost layer of 3 ± 5% of the bulk layer spacing. In agreement with this, comparisons between the results of the dynamical calculation and experimental data for five nonspecular beams at normal incidence suggest that the surface layer relaxes by 0 ± 5%. In addition, the dynamical calculations indicate that the topmost layer maintains an fcc structure.     

Co desorption and adsorption on Pt(111)

The kinetics of the desorption of CO from a Pt(111) crystal between 419 and 505 K is reported using a Low-Energy Molecular-Beam-Scattering (LEMS) technique with a helium probe beam and a CO dosing beam. The resulting first-order Arrhenius rate constant is $\text{k = 2.7 × 10}{}^{13}\text{exp(?31.1}\text{kcal}\text{mole}\text{· RT) s}{}^{\mathrm{?1}}$. We also report a study of the equilibriumadsorbed CO between 400 and 600 K using LEMS. These results, fitted to a Temkin isotherm model, indicate that the adsorption energy decreases linearly with surface coverage with the average value equal to $\text{31.1 + 1.2}\text{kcal}\text{mole}$ over the coverage range 0 < θ ? 0.5. The average harmonic oscillator frequency of the adsorbed CO molecules is 191 ± 76 cm^?1.     

A computational DFT analysis of OH chemisorption on catalytic Pt(111) nanoparticles

In this article, various energies and geometries of pure platinum nanoparticles and those of platinum nanoparticles with adsorbed OH were investigated. Fourteen different platinum clusters of 3–40 atoms were studied using spin-unrestricted density functional theory (DFT) with a double numerical plus polarization basis set. This range of sizes gave enough information for establishing the general trends that were the primary goal of the study. Three different shapes of platinum clusters were presented, and the effect of cluster size on binding energy, total energy, and HOMO–LUMO energy gap was investigated. Subsequently, same calculations were performed for those selected Pt clusters with OH adsorbate on the surface. The results show that the stability of both the pure clusters and the clusters with adsorbed OH molecule increases with an increase of cluster size. This fact indicates that direct influence of the size of Pt cluster on the reaction rate is possible, and the understanding of how cluster size would affect binding energy is important. As expected, the effect of cluster size on total energy of molecule was shown to be a linear function independent of cluster type. We also found that optimized (stable) Pt clusters were bigger in size than that of the initial clusters, or clusters with bulk geometry.     

Energy barriers for interlayer diffusion in Pt/Pt(111) and Rh/Rh(111) homoepitaxy: Small islands

We performed detailed molecular statics calculations of energy barriers for various adatom movements in the vicinity of small island on (111) surfaces of Pt and Rh using Rosato-Guillope-Legrand interatomic potential. We found that for both systems the exchange processes are energetically favorable in comparison with direct jumps of an adatom. We observed that Ehrlich-Schwoebel barriers for exchange decrease significantly on small islands in comparison to single step on the surface. A comparison of both materials is presented. Presented at the VIII-th Symposium on Surface Physics, Třešt’ Castle, Czech Republic, June 28 – July 2, 1999. Financial support for this work was provided by the COST project P3.80.     

Self-limited growth of a thin oxide layer on Rh(111)

The oxidation of the Rh(111) surface at oxygen pressures from 10(-10) mbar to 0.5 bar and temperatures between 300 and 900 K has been studied on the atomic scale using a multimethod approach of experimental and theoretical techniques. Oxidation starts at the steps, resulting in a trilayer O-Rh-O surface oxide which, although not thermodynamically stable, prevents further oxidation at intermediate pressures. A thick corundum like Rh2O3 bulk oxide is formed only at significantly higher pressures and temperatures.     

Angle-resolved photoemission study of Pd(111) and Co/Pd(111)

Electronic structure of epitaxial Co films on Pd(111) has been investigated as a function of the Co overlayer thickness, by using angle-resolved photoemission spectroscopy (ARPES). It is found that experimental band structures for Pd(111) and work functions for Co/Pd(111) agree reasonably well with the results of band structure calculations. Compared to Pd(111), valence band ARPES spectra of Co/Pd(111) support the existence of the induced magnetic polarization of Pd atoms near the interface, suggesting large hybridization between Co 3d and Pd 4d states near the interface.     

The hydrogen chemisorption on TiC(111) surface studied by high resolution electron energy loss spectroscopy

At room temperature, hydrogen chemisorbs dissociatively on the three-fold hollow site of the TiC(111) surface with a sticking probability of ～ 0.5. The frequency of the stretching vibration of H is 1009 cm^-1, and the effective charge e¹ of hydrogen has been estimated to be 0.08 from the cross section of dipole scattering.     

Geometry of hydrogen chemisorption on Ni(111) analyzed by low energy electron diffraction

A LEED analysis predicts that hydrogen chemisorbed in a half monolayer on Ni(111) arranges in an overlayer with graphite-layer geometry of (2×2) unit cell, adsorbing in both types of threefold hollow sites with indistinguishable Ni-H bond lengths of 1.84+-0.06 Å, corresponding to an overlayer-substrate spacing of 1.15+-0.1 Å.