Normal modes of CO adsorbed on metal surfaces

A normal mode analysis of experimental data from helium atom scattering (HAS) and electron energy loss spectroscopy (EELS) for vibrations of CO molecules adsorbed in on-top and bridge sites on Ni(100) is presented, using a refined force-constant analysis. The similar case of CO adsorbed on Pt(111) is reinvestigated, revealing a misassignment of normal modes in previous work. Finally, available experimental data for CO on Pt(111) is used to construct a trial potential energy surface.     

On the adsorption and desorption of H2 at metal surfaces

Recent technological developments have made possible measurements of the distribution of internal levels of molecules desorbing from a hot surface. Such measurements provide new information concerning the desorption process and the potential energy surface (PES) that governs it. Associative, or re-combinative desorption is of particular interest because the distributions of internal levels reflect the manner in which the molecular bond is formed as the desorbing species leaves the surface. As the simplest associative desorption systems, H₂ and D₂ adsorbing on and desorbing from metal surfaces deserve special attention and serve as prototypes for systems with a more complex chemistry. In this note I review briefly from the theoretical point of view some features of the interaction of H₂ with metals and their relevance to associative adsorption and dissociative sticking.     

On the interactions of adatoms on metal surfaces

Pariwise interactions of the adatoms and ions adsorbed on metal surfaces has been investigated with the help of the pole approximation in integral equations of the Lippman-Swinger type. These interactions have been shown to depend mainly both on the type of the metal spectrum and on the electron structure of the adatoms. The theory developed in this paper gives a satisfactory explanation of several facts observed in experiments, especially the longrange oscillating interaction between adatoms.     

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.     

The application of SIMS to the study of CO adsorption on polycrystalline metal surfaces

Secondary ion mass spectroscopy (SIMS) was used to study the adsorption of carbon monoxide on polycrystalline nickel, copper, iron, palladium and tungsten foils. The results demonstrate the ability of SIMS to distinguish, qualitatively, between molecular and dissociative adsorption. A correlation between SIMS results and those obtained by infra-red spectroscopy for molecular adsorption is also suggested.     

Chemisorption bonding and bond lengths on transition metal surfaces: Effect of coordination and valency saturation

A smooth correlation between the determined chemisorption bond lengths, for O, S, Se and Te on Ni (001) and for sulphur on Ni (110) and Ni (111) as well, and Pauling's resonating bond ionicity is demonstrated, when the latter are calculated with due regard for the coordination and valency saturation effects. Pauling's bond length-bond number relation is used to provide (i) an independent estimate of the Ni-O bond length on Ni (001), which is found capable of discriminating among the reported values, and (ii) estimates of the bond lengths for O, S, Se and Te on Ni (111) and Ni (110) by using the determined bond lengths on Ni (001). Agreement with the determined values for sulphur is surprisingly good.     

On the bond lengths reported for chemisorption on metal surfaces

The bond lengths and geometrical arrangements reported with multiple-scattering analyses of LEED intensities for chemisorption on metal surfaces are assessed with Pauling's bond length-bond order relation. The approach here is partly empirical and it depends on information on the atomic valencies; the hybridisation model proposed for metals by Altmann, Coulson and Hume-Rothery is also used to guide the allocation of bonding electrons for the purposes of estimating the orders of surface bonds. It is shown that this framework allows estimates of surface bond lengths to within 0.1 Å of the values reported with LEED, and often the correspondence is substantially closer. Other factors that are likely to influence surface bond lengths have been recognised, but refinements to the present analysis should probably await clarifications of uncertainties in the structures determined with LEED including, for some chemisorption systems, consideration of possible displacements of metal atoms.     

Electron spectroscopy applied to the study of reactivity at metal surfaces — A review

Photoelectron spectroscopy (PES) has become an important tool in the study of surfaces and the solid state. This review discusses information obtained on the interaction of gases with metal surfaces. The fundamentals of electron spectroscopy are briefly reviewed and experimental methods outlined. Approaches to the interpretation of both X-ray photoelectron spectroscopy (XPS) and ultra-violet photoelectron spectra (UPS) are discussed. The chemisorption of carbon monoxide is examined in detail, PES distinguishes two situations, where CO retains its molecular identity and where dissociation occurs. Bonding of the molecule is considered, as are factors affecting dissociation. Some XPS investigations of metal oxidation are examined and the significance of the several oxygen 1s peaks observed is discussed. The adsorption and decomposition of organic molecules has been studied primarily by UPS. Several investigations are summarized, and the decomposition of formic acid considered in detail. The application of electron excited Auger electron spectroscopy is briefly discussed.     

On the electronic structure of ferromagnetic transition metal surfaces

The electronic structure at the surface of ferromagnetic Ni and Fe is studied within a rigid exchange splitting model, in the tight-binding approximation. The surface resonances existing in the paramagnetic state are split. The magnetic moment is found to be the same at the surface and in the bulk in agreement with recent experiments. It is shown to be distributed almost spherically, except for (100)Ni where it points towards nearest neighbours.     

On the electronic potential and ionic relaxation at metal surfaces

Some results, obtained by a simple method which gives the main effect of the ions on the surface potential of metals, are advanced. A general expression is given for the change in the surface dipole barrier. The ionic relaxation of (110) and (111) faces of Al are obtained self-consistently, tending to support one of the two current interpretations of LEED diagrams.     

Significant influences of metal reactivity and oxide films at particle surfaces on coating microstructure in cold spraying

Based on large amount of experimental observations, the effects of metal reactivity and oxide films at particle surfaces on coating deposition behavior in cold spraying were presented and discussed. The oxygen contents in as-sprayed Ti, Ti-6Al-4V and Al coatings were higher than those in the corresponding starting powders. The obvious flashing jets outside nozzle exit during deposition of Ti and Ti-6Al-4V were caused by the reaction of the particles with oxygen in the entrained or the adopted air. For Ti and Ti-6Al-4V coatings, their porous structures are predominantly attributed to the surface reactivity (defined as reactivity with oxygen). This surface reaction could be helpful for formation of a metallurgical bonding between the deposited particles. For Al, even though it is more reactive than Ti, the oxide films at Al particle surfaces suppress the surface activity.     

On the influence of size and roughness on the electronic structure of transition metal surfaces

The dependence of surface electronic structure on local and farther environment for transition metals is investigated in the tight binding approximation associated with the moment method. Local densities of states (LDS) on fcc cubooctahedral and on icosahedral clusters of increasing size, from 13 up to 2057 atoms, are studied for corner sites and central sites on differently oriented faces. The general LDS features are determined by the nearest neighbour shell but outstanding details, such as the occurrence of a strong central peak, are dependent on each particular cluster site and dimension. Only for particles larger than one thousand atoms the LDS at the center of (100) and (111) faces look like those of the corresponding infinite surfaces. The importance of symmetry is pointed out by the comparison of LDS at equivalent sites on cubo-octahedra (cubic symmetry) and icosahedra (five-fold symmetry). These results might explain the variation of activity with the catalyser dispersion, as the adsorption energy depends on the relative position of the adatom atomic level and the substrate LDS peaks and Fermi level.     

Adsorption of CO on PtBi2 and PtBi surfaces

Approximate molecular orbital calculations have been applied to explain the low CO poisoning effects observed at PtBi₂ and PtBi electrodes. The bonding patterns for chemisorption of CO on the surfaces of Pt-Bi bulk alloys and pure Pt surfaces are quite similar. The major difference is not induced through much Pt-Bi bonding, but indirectly, by raising the Fermi level of the system, so that the C-O π* levels become practically filled upon interaction. This results in much lower adsorption energies than in the metallic Pt case, in accordance with experimental data. The calculations also imply C-O bond dissociation on the surface, a phenomenon not supported by experiment. CO adsorption at Pt-Pt bridge site (possible only on PtBi) is favored relative to atop chemisorption.