Gas adsorption properties of stoichiometric and reduced NiO(100)

Abstract

The gas adsorption properties of stoichiometric and surface reduced NiO(100) single crystals are presented in their interactions with CO, ethylene and H 2O at gas pressures of 1×10 -7 to 3×10 -6 Torr and substrate temperatures of 200 to 500 K. At temperatures below 500 K, gas adsorption is sluggish or even completely undetectable, depending upon the degree of surface oxidation and gas of interest. At temperatures of ⪆ 450 K, however, comparatively vigorous reaction occurs between the oxide lattice atoms and the adsorbing gas, which results in surface reduction for CO and ethylene exposures and surface hydroxylation for the case of H 2O exposure. These data suggest that the mechanisms dominating metal oxide catalytic behavior are fundamentally different from those currently proposed for their pure metal counterparts. Significant amounts and rates of gas uptake occur only with a concomitant destruction and/or transformation of the oxide surface, itself. For interactions involving reducing gases, the surface can be described as truly catalytic only from the viewpoint of the metal component, with surface oxygen requiring replenishment either through migration of subsurface oxide or through adsorption of an oxidizing gas.