COADSORPTION OF CO AND CH3O ON NI(100)

The coadsorption of CO and CH3O on Ni(100) is treated using an ab init io embedding theory, modeling the lattice as a 30-atom, three layer cl uster. Self-consistent-held and configuration interaction calculations carried out on a local surface region permit an accurate description of bonding at the surface. The 3d orbitals are explicitly used for six Ni atoms in the local surface region. The calculated adsorption energ y for methoxy at a four-fold site on the Ni(100) surface is 92 kcal mo l(-1) with an O-surface distance of 1.32 Angstrom. The methoxy C-O axi s is nearly perpendicular to the surface, tilted only 5 degrees from t he surface normal. The calculated adsorption energy for methoxy at a f our-fold site with CO coadsorbed at an adjacent four-fold site is 100 kcal mol(-1) for an O-surface equilibrium distance of 1.34 Angstrom. I n this case, the methoxy is found to be tilted with the C-O axis incli ned 25 degrees away from the surface normal. For perpendicular CH3O th ere is a repulsive interaction between the methoxy hydrogens and the C O oxygen. Calculated methoxy C-O stretching frequencies are 1020 cm(-1 ) on Ni(100) and 970 cm(-1) on coadsorption with CO. The methoxy 5a(1) , 1e and 2e orbitals are the orbitals principally involved in bonding with the surface. The dipole moment derivative with respect to the O-s urface distance, d mu/dR, at a tilt angle of 25 degrees is -0.87, indi cating very ionic bonding. Coadsorption of CO causes the methoxy C-O a xis to tilt, increases the adsorption energy of CH3O, and induces chan ge in the charge distribution of methoxy carbon and hydrogens. (C) 199 7 Elsevier Science B.V.