SMSI effect in some reducible oxides including niobia

Abstract

Niobia is a typical SMSI oxide. High temperature reduction (HTR) exerts a reversible suppression of H 2 chemisorption on Nb 2O 5-supported or Nb 2O 5-promoted Rh or Pd catalysts. The activities of hydrogenolysis of hydrocarbons (a structure-sensitive reaction) are suppressed severely after HTR and those of hydrogenation-dehydrogenation (a structure-insensitive reaction) suffer only mild suppression. The extent of RhNb 2O 5 interaction depends strongly on the preparation conditions, i.e., Nb/Rh atomic ratio, impregnation procedures and calcination temperature. A single phase RhNbO 4 supported on SiO 2 and similar compounds were prepared successfully, and their structural changes during HTR were studied extensively. These systems can be assumed to be a good model for SMSI. The SMSI effect was compared with several catalytic reactions which are supposed to be structure-sensitive. Usually, activities are more or less suppressed by an ensemble effect at the SMSI state, but the enhancement of catalytic activities by SMSI was found on selective hydrogenation of a carbonyl group and hydroformylation of ethylene probably by a ligand effect due to surface decoration. Recently, the role of ZnO was investigated on methanol synthesis over Cu/ZnO/Al 2O 3 catalysts by a combination of oxygen coverage measurements during catalysis, EDX (Energy-dispersive X-ray spectroscopy) observations and XPS studies. They gave evidence for a decoration of the Cu surface with ZnO species, leading to a formation of new active sites, tentatively expressed as Cu +OZn. The fact that SMSI does not necessarily exert only a suppression effect on catalysis will be presented as a review paper of our group.