Defects and Reactivity at Oxide Surfaces: Experimental Aspects of the Interaction of Hydrogen, Co And Co₂ with the Nio{001} Surface

Abstract

This paper summarizes some previous or more recent experimental results on chemical interaction, at moderate temperatures, of H, H₂, CO and CO₂ with the surfaces of nickel oxide, mainly the NiO {001} surface. These results are in a general agreement with theoretical predictions. Nearly perfect surfaces of maximal valency oxides cannot react with molecules. Dissociation of molecules is often required for chemisorption. Conversely, chemical reaction is usually observed either when molecules are predissociated or the surface has defects with an energy level high enough to enable both dissociation and chemisorption. On a nearly perfect, “in situ” cleaved, NiO {001} surface, partially atomized hydrogen was found to be easily adsorbed but CO could react only if traces of hydrogen were present. In contrast CO₂ could be adsorbed reversibly.

Morphology, structure and quality of the ultrathin metallic films grown by reduction of the NiO {001} surface were found to be basically dependent on factors involving either the predissociation state of the gas or the presence of defects on the oxide surface. These two factors directly determine the surface coverage of reducing agents and, therefore, the density of nucleation sites.