LCAO studies of hydrogen chemisorption on nickel: I. Tight-binding calculations for adsorption on periodic surfaces

The model we have used to study hydrogen chemisorption on nickel surfaces is a tightbinding Extended Hückel method applied to finite (periodic) crystals up to about 250 atoms, the non-orthogonal basis set comprising five 3d orbitals, one 4s orbital and three 4p orbitals per atom. After calculating the band structure of fcc nickel, we have examined, by this model, the effect of the (100), (110) and (111) surfaces on the local density of states and the charge distribution. The results agree closely with moment calculations of the density of states in semi-infinite crystals and with experimental (XPS, UPS and INS) spectra. Extensive studies have been made of the influence of adsorption on the (partial) densities of states in order to illuminate the nature of the chemisorption bond. Particularly, we have concluded that both the 3d electrons and the conduction electrons take part in this bond. Equilibrium positions for adsorption on various sites have been determined and the adsorption energy has been computed and compared with experimental data. We find that the stability of adsorption decreases in the order (110) > (100) > (111) and Atop > Bridge > Centred.     

Cluster studies of CO adsorption: II. CO on small Al clusters

Results for the free-electron-like metal Al (r_s = 2.07 bohr) are compared with previous Li (r_s = 3.25 bohr) results. From an analysis of the various contributions to the total adsorption energy (steric interaction, σ-bonding, π-backbonding) as a function of the CO height above the surface, and the adsorption site, it appears that high conduction electron density leads to strong exchange repulsion. At the top site this effect is partly cancelled by the favourable interaction possibilities with Al 3p functions. The most striking differences with Li are thus the very weak adsorption at the hollow site, and stronger adsorption at the top site.     

Cluster Bethe lattice model studies of chemisorption: H on Ni surface

The tight binding cluster Bethe lattice model formulated earlier for treating chemisorption systems has been used to study a specific system, namely, hydrogen chemisorption on Ni(100) surface. The hydrogen atom occupies the centre hollow position in agreement with the experimental results.     

ESCA studies of oxygen adsorption on nickel

A metallic nickel foil was studied by ESCA. The Ni 2p electron spectrum was characteristic of a mixture of nickel oxide and metallic nickel. Two ox     

Simple models of hydrogen adsorption on germanium

The dependence of work function Δφ on degree of coverage Θ for the Ge(100) and Ge(111) surfaces determined in terms of simple models that include the dipole-dipole interaction of hydrogen adatoms. It is found that experimental dependence Δφ(Θ) for the Ge(111) surface can be explained by taking into account an increase in the adsorption bond length with Θ. The charge of the adatoms as a function of Θ is calculated, and the variation of the surface conductivity of the substrate is estimated.     

Cluster studies of CO adsorption: I. CO on small Li clusters

The various contributions to the adsorption energy of CO such as steric repulsion, σ bonding and π backbonding, are studied as a function of CO-cluster distance, C-O distance, adsorption site and cluster geometry. At the hollow site a double minimum is found in the adsorption energy as a function of CO-cluster distance. The effect of adsorption on the electronic structure of CO is large, in particular at the equilibrium distance close to the surface. Consequences for the C-O stretch frequency, and for the possibility of CO dissociation at the surface, are investigated.     

Two-dimensional models for an electrode with adsorption sites

Through a new method, the following model is solved exactly in the framework of classical equilibrium statistical mechanics of two-dimensional Coulomb systems, for the special value=2 of the coupling constant: the mobile charges of a one-component plasma are attracted by a line of equidistant sticky adsorption sites embedded in a background, the density of which varies in the direction orthogonal to the line. First the general expressions are given for the densities and correlation functions of nonadsorbed and adsorbed particles. Then these results are used to investigate two models of electrodes with localized adsorption: the externally charged hard wall and the impermeable polarized membrane. In each case the influence of the adsorption upon macroscopic features is studied: the potential drop across the interface, the contact theorem, and the Lippmann equation, which involves the surface free energy.     

Dissociative hydrogen adsorption on nickel surfaces: Absence of a magnetocatalytic effect

The temperature dependence of the rate of dissociative hydrogen adsorption on Ni(111) and Ni(110) surfaces was determined by monitoring the rate of HD production from a mixed H₂ and D₂ molecular beam striking the sample surfaces. No anomaly was found in the vicinity of the Curie point T_c if the surfaces were rigorously kept clean. A step-like increase of the rate was, however, observed near T_c if the surfaces were slightly contaminated with carbon which dissolves in the bulk in this temperature range and segregates back upon cooling. The results are discussed in view of the present knowledge of the magnetic properties of nickel surfaces.     

Carbon monoxide and hydrogen chemisorption studies on small supported iron carbide particles

The influence of chemisorption of CO and H₂ has been studied on small iron carbide particles by Mössbauer spectroscopy. The line with is increased by 2–3 times upon evacuation and also the values of isomer shift and magnetic hyperfine field were increased in vacuum. The effect is attributed to the chemisorption induced uniformization of the magnetic exchange interaction among the particles.     

An analytical model of the heat of chemisorption: A perturbation LCAO MO approach

An analytical LCAO MO perturbation model has been developed for treating the heat of chemisorption Q of an adsorbate A monolayer on a transition metal M film. The model combines parameters of the metal band (the Fermi level E_F, band width W = W^occ + W^vac, the d occupancy N_d, density of states n(?), etc.) with those of the local A-M interactions (the adorbital energy ?_A, off-diagonal matrix elements β_AM, etc.). The major cases of A's having lone pair, singly occupied, and vacant adorbitals have been considered, and the analytical expressions for Q as well as some numerical estimations are presented. The relative values of Q seem to be crucially dependent on the ratio β/(?_A ? E_F). The Q vs. N_d plots for the donor and radical A's are rather flat, typically Q decreasing monotonically as N_d increases, but for the acceptor A's the plots are very parameter dependent and show a variety of trends. The results obtained agree with straightforward computations and (scarce) experimental data.     

Cluster calculations of hydrogen in the grain boundaries of nickel

A series of self-consistent field, molecular orbital cluster calculations has been conducted to model the influence of hydrogen on certain structural units in the grain boundaries of nickel. The structural units considered were two Bernal polyhedra, namely the capped trigonal prism and the Archimedean antiprism. Other calculations were performed to simulate interstitial hydrogen and atomic hydrogen chemisorbed on the (111) surface. It was found that hydrogen causes local expansion as an interstitial impurity, in rough quantitative agreement with experiments on dilute solid solutions of hydrogen in nickel. In contrast, it was found that a single hydrogen atom causes a shrinkage of about 1.7% in the bond lengths of the capped trigonal prism and about 2.8% in those of the Archimedean antiprism. Similarly, a hydrogen atom chemisorbed above a nickel atom causes the nearest (111) surface bonds to shorten by about 1.7%. The shrinkage induced in the two Bernal polyhedra appears to be correlated to the low electron density at their centres, and the overall reduction of charge in the vicinity of certain nickel-nickel bonds upon the inclusion of the hydrogen atom.     

Xps studies of donor and acceptor chemisorption of NO and CO on nickel oxide surfaces

Systematic variation on the defect (excess) oxygen concentration in the surface of nickel oxide preheated to 700, 1100 and 1450°C, has been revealed in O(1s) and $\text{Ni(2p}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}\text{)}$ X-ray photoelectron spectra. The magnitude of the surface charge, after evacuation at 500°C, is also directly related to the defect properties. The effect of chemisorption of NO and CO on the free hole concentration of the p-type semiconductor surface has been studied by monitoring the surface charge. NO adsorption, predominantly as negatively charged (electron acceptor) molecules removes much of the surface charge whereas CO adsorption, as CO^δ+ species (donor chemisorption), increases the surface charge on all samples.     

Electron impact desorption of hydrogen on tungsten: II. Hydrogen in mixed adsorption layers

The electron impact desorption (EID) of H⁺ ions and of H₂ molecules from hydrogen coadsorbed with carbon monoxide or oxygen on tungsten has been investigated mass spectrometrically. It is shown that the high EID cross sections for hydrogen on tungsten reported in some earlier investigations must have been due to coadsorbed states. These states have been investigated in some detail with respect to their general adsorptive (relative coverages, sticking coefficients, isosteric heats, and desorption rates) and their EID properties (total and ionic cross sections, threshold energies). The results stress the high specificity of EID for certain (usually weakly bound) adsorption states and its applicability for the investigation of such states even in the presence of other states with much higher populations.     

Cl chemisorption on the Pd(001) surface: A self-consistent LCAO calculation of electronic structure

The total and partial densities of states and work functions for the clean Pd(001) surface and for a c(2×2) Cl overlayer on this surface are calculated using a self consistent Gaussian LCAO technique. The adlayer increases the work function of the clean surface by 0.8 eV and leads to a distinct split off feature in the total density of states. The band structure of the bands composing this feature are compared to the bands of the isolated Cl layer.     

Hydrogen chemisorption on Al,Mg and Na surfaces — calculation of adsorption sites and binding energies

Chemisorption properties for H on Al, Mg and Na surfaces have been computed. Starting with the results from a self-consistent, Kohn-Sham calculation for a H-jellium system, the effects of the substrate pseudopotential lattice have been included by first order perturbation theory. An improved method, namely inclusion of part of the pseudopotentials in a self-consistent way and only the remainder in perturbation theory, has also been used. Binding energies, bond lengths, vibrational frequencies and activation energies have been calculated. It is found that the three substrates behave qualitatively different, as Al is a segregator, Na is an absorber and Mg is in between. Calculated bond lengths agree in general well with the ones for the corresponding molecules. The binding energy is predicted to be below or close to the dissociation energy of H₂ for Al and Na, while it is slightly larger for Mg.     

Optical studies of chemisorption on metals

Optical techniques, particularly ellipsometry, have been highly successful in the study of chemisorption on semiconductors. Such studies on metal surfaces are more difficult because the short screening lengths limit the chemisorption-induced perturbation to a surface-layer only a few ångströms thick. Some success has been achieved using differential reflectance spectroscopy. However this necessitates (1) two independent reflectance measurements or the use of KK analysis to obtain complex optical constants, and (2) assumed or independently-measured values for the thickness of the surface layer in order to completely specify the adsorption-induced changes. The advent of extremely precise in situ modulated ellipsometers has made the optical study of gas-metal interactions less difficult, in particular eliminating the necessity of KK analysis. We briefly describe such a system here, the automated polarization-modulated ellipsometer. When this instrument is coupled with a recently developed approximation technique for obtaining surface-layer thickness and optical constants from only two ellipsometric measurements, the complete optical characterization of many metal-absorbate systems is straightforward. Recent experimental results indicating a chemisorption-induced surface state within 2.4 eV of the Fermi level for oxygen on silver are discussed.     

On the chemisorption of hydrogen on lithium clusters

Ab-initio Hartree-Fock calculations for hydrogen atoms chemisorbed on lithium clusters have been performed using a basis which includes p-functions. No penetration of the hydrogen atom beyond the surface of the cluster was observed. The distance between the hydrogen atom and the nearest lithium atoms of the cluster becomes larger as the coordination number increases.