A 4,t";

Smface Science 277 11992)377-388 North-Holland

Theoretical studies of adsorbate binding and ionization of methoxy on the Cu(111) surface K. Hermann and C. Meyer l:ritz-Haber-lnstitut der MPG and Freie Universitdt Berlin, Berlin, German}'

Received 8 November 1991; accepted for publication 30 June 1992

Model studies on small CunOCH3, n = 1, 2, 3 clusters are used to determine the detailed chemisorptive interaction of the methoxy (OCH 3) radical with the Cu(111) surface as well as surface-induced shifts of the adsorbate valence ionization potentials. The calculations are based on the ab initio H a r t r e e - F o c k - L C A O method using all-electron Gaussian basis sets. The model results from energy optimizations for various lateral adsorption sites are consistent with experimental findings which suggest that OCH~ stabilizes at a 3-fold central site on the C u ( l l l ) surface. The O C H 3 - m e t a l interaction is characterized by considerable Cu to OCH 3 charge transfer filling the OCH 3 2e lone pair orbitals while OCH 3 to Cu charge transfer involving the OCH 3 5a i orbital is small. As a result, the adsorbate becomes negatively charged in the presence of the surface. The calculated adsorbate binding energies (E B = 1.7 eV for Cu3OCH 3 with OCH 3 at the central and bridge site, E~ = 1.3 eV at the on top site) are found to be rather large containing ionic and covalent contributions of about the same magnitude. Calculations on valence hole states of Cu3OCH 3 corresponding to ionization of OCH3-type cluster orbitals are performed. The results show differential shifts of the OCH 3 5a t and 2e derived ionization potentials (IP) compared to the free radical values which are explained by the electrostatic adsorbate-subst ra te interaction and the charge distributions of the orbitals. The reduction of the 5a t - l e I P separation due to the chemisorptive interaction is consistent with experimental data from photoemission on the OCH~/Cu( I11 ) adsorbate system.

Surface Science 277 (1992) 389-394 North-Holland

The interaction of amine bases on the Lewis acid sites of aluminum oxide - a theoretical study Pipsa Hirva and Tapani A. Pakkanen Department of Chemistry, University of Joensuu, P.O. Box 111, SF-80101 Joensuu, Finland

Received 25 January 1992; accepted for publication 30 June 1992

In the present work amine interactions with a luminum oxide surfaces are modeled with quan tum chemical methods. The results from our valence calculations with a minimal basis are compared with s tandard all-electron calculations with different basis sets. The tetrahedral and octahedral Lewis acid sites are considered as active sites for ammonia and pyridine adsorption. The interaction energies are overestimated, since small cluster models cannot represent the effects of neighboring acid-base sites. However, qualitative information on the adsorption on different sites can be obtained. The results show higher acidity and thus higher amine interaction energies for the tetrahedral Lewis acid sites compared to the octahedral ones. The adsorption energies are similar for both ammonia and pyridine indicating that the amine bases are capable of displacing water from the surface.

Surface Science 277 (1992) 395-406 North-Holland

Model calculations for surfaces and chemisorption processes of zinc-chalcogenide thin films by an ab initio cluster approach Katalin Tdth 1, Tapani A. Pakkanen *, Pipsa Hirva and Juha Muilu Department of Chemistry, University of Joensuu, SF-80101 Joensuu, Finland

Received 25 February 1992; accepted for publication 7 July 1992

A49

Chemisorption on zinc-chalcogenide surfaces and the electronic structure of these surfaces were studied by using valence SCF calculations based on stepwise density matrix approximations. Closed-ring cluster models used in describing the surface properties of ZnSe and ZnTe thin films seem suitable in the case of ZnS as well. The zinc-ended (111) surface was found more stable than the selenium-ended surface, but the opposite behaviour was observed in the case of ZnTe, agreeing with experimental results. Relaxation studies predict that the top atomic layer of the polar (111) surface is situated closer to the bulk. Studies of zinc chemisorption on ZnSe and ZnTe clusters show that the zinc atom as an adsorbate is quite mobile on these surfaces. Adsorption of an open Se n chain with an odd "n" is energetically preferred because 6-atom rings may possibly be formed. Detailed analyses were carried out for zinc-chloride adsorption on small zinc-chalcogenide clusters. Results show that adsorption energies decrease with increasing lattice constant and decreasing fractional ionicity of the substrate crystals.

Surface Science 277 (1992)407-413 North-Holland

Non-empirical cluster model calculations of the adsorption of H20 on Ni( l l l ) I.I. Zakharov, V.I. Avdeev and G.M. Zhidomirov Institute of Catalysis, Nocosibirsk 630090, Russian Federation

Received 12 March 1992; accepted for publication 5 June 1992

H20 adsorption on a Ni(l l l ) surface is calculated by an ab initio method taking into account four valence GVB-pairs of water molecules. Ni( l l l ) is simulated by the surface duster Nilo, calculated with a modified effective potential (MEP). The calculated structure of an adsorbed H20 monomer with an inclined molecular axis (a -~ 350-60 °) and an adsorption energy Ead s -- 50-70 kJ/mol is in agreement with the experimental results. The critical effect of the d-orbitals of the oxygen atom on the structure of the adsorbed H20 monomer, based on these calculations, is recognized.

Molecular complexes of H20 and H2S with Li are also calculated. For the first time a quasi-tetrahedral structure is obtained for these complexes.

Surface Science 277 (1992) 414-428 North-Holland

Kinetic phase transitions in dimer-dimer surface reaction models studied by means of mean-field and Monte Carlo methods Alberto Maltz Departamento de Matemdticas, Facultad de Ciencias Exactas, Unicersidad Nacional de La Plata, Casilla de correo 172, 1900 La Plata, Argentina

and

Ezequiel V. Albano Instituto de lncestigaciones Fisicoquknicas Te6ricas y Aplicadas (INIFTA), Facultad de Ciencias Exactas, Unit'ersidad Nacional de La Plata, Sucursal 4, Casilla de correo 16, 1900 La Plata, Argentina

Received 23 January 1992; accepted for publication 24 June 1992

Dimer-dimer surface reaction models inspired in the catalytic oxidation of hydrogen on transition metal surfaces - which may apply to generic bimolecular reactions of the type (½)A 2 + B 2 ~ B2A - are studied by means of a mean-field approach and Monte Carlo simulations. The models exhibit irreversible (kinetic) phase transitions (IPT) from a reactive stationary regime with B2A production to poisoned states where the surface becomes saturated by the reactants. The critical points at which IPTs take place are determined. The critical behavior of relevant quantities, such as the rate o'f B2A production and the coverage with the reactants, is studied and the corresponding critical exponents are evaluated. Good qualitative agreement is found when comparing mean-field and Monte Carlo results.