AB-INITIO CALCULATIONS FOR THE ADSORPTION OF SMALL MOLECULES ON METAL-OXIDE SURFACES .3. ADSORPTION OF H AND CH3 RADICALS ON NIO(100)

Quantum chemical ab initio calculations have been performed for the adsorption of H atoms and CH3 radicals on the NiO(100) surface. The Ni2+ adsorption site has been modeled by a cluster containing one Ni2+ and five O2- ions, embedded in a Madelung field of point charges +/-2. Three different levels of accuracy have been employed: SCF, CASSCF, and the multi-configuration coupled electron pair approximation (MC-CEPA) for the inclusion of correlation effects. While H and CH3 are bound to the O2- ions on the (100) surface only by very weak van der Waals' forces, the bond to the Ni2+ ions is a genuine chemical bond in the case of the H adsorption and a partly electrostatic and partly chemical bond for the CH3 adsorption. The bonding between Ni2+ and CH3 is slightly strengthened if geometry relaxations of the adsorbate (bending of the CH bonds away from the surface) and the substrate (moving the Ni2+ ion out of the surface) are allowed. The calculated binding energies and equilibrium distances are 0.44 eV at R(Ni-H) = 2.86 a(0) and 0.25 eV at R(Ni-C) = 5.0 a(0) for the H and CH3 adsorptions, respectively.