Ab initio calculations for the adsorption of small molecules on metal oxide surfaces. Part 3. Adsorption of H and CH3 radicals on NiO(100)

Abstract

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 RNi-H = 2.86 a0 and 0.25 eV at RNi-C = 5.0 a0 for the H and CH3 adsorptions, respectively.