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

Oxide surfaces

“…This attraction is balanced by the Pauli-repulsion between CO andpredominantly-the O − 2 anions. If the approaching molecule possesses unpaired electrons, as for instance in NO [63,140] or CH 3 [151], there can also be a small chemical contribution to the bond. The large difference in the bonding between CO on metals and on oxides is caused by the fact that the 4s and 4p atomic orbitals (AOs) on the metal, which are needed to construct empty spd-hybrids, which point toward CO and can accept electrons from the doubly occupied 5σ orbital of CO, are not available in metal oxides because there is no space for them among the bulky electronic charge distributions of the O − 2 anions with fully occupied 2p shells.…”

“…The majority of the theoretical treatments of oxide surfaces, in particular if local properties like core ionization, d-d excitations, adsorption processes, or defects are considered, are currently performed by means of cluster calculations, some of them with semi-empirical methods, but many more with density functional or ab initio techniques. In the following we will not give a systematic and complete survey of the existing literature (for this purpose we refer to the recent review by Sauer et al [117] and the workshop reports edited by Pacchioni et al [118,120], but we will discuss the possibilities and limitations of the cluster approach for the example of the neutral NiO(100) surface [63,69,[137][138][139][140]151]. Other cubic oxides (MgO, CaO, CoO) and neutral surfaces of oxides with more complicated crystal structures (TiO 2 (110)) can be treated similarly.…”