Comparative study of geometry and bonding character for methoxy radical adsorption on noble metals

Results are reported of quantum density functional theory (DFT) calculation s for methoxy radical adsorption on the (111) surfaces of copper, silver an d gold. The metallic surfaces are modeled by clusters of seven atoms chosen to simulate the top, bridge, hcp hollow and fee hollow sites. A control te st on the use of these small clusters was performed by running a calculatio n on a 22-atoms cluster. A comparison between the energetics of adsorption on the different sites identifies the fee hollow position as the preferred one for CH3O adsorption on the metal surfaces considered and the methoxy C- O axis is found to be perpendicular to the surface. It is shown how the met hoxy radical binds on the hollow sites of copper, silver and gold metallic surfaces, the three oxygen p orbitals are found to interact strongly with t he nearest neighbour metallic atoms except for the top position where only the p orbital aligned along the direction of the oxygen-metal atom is invol ved. In all cases, there is a charge transfer of approximately 0.6-0.8e fro m the metal atoms to the methoxy radical. The bonding of the methoxy radica l on the surfaces studied has a largely ionic character. The calculated ads orption parameters and vibrational frequencies of the adsorbed species are in good agreement with the available experimental data. (C) 2000 Elsevier S cience B.V. All rights reserved.