Identification of the active species in oxidation reactions on mixed oxide catalysts: Supra-surface or bulk surface species

Abstract

Mixed metal oxides have been used as catalysts for the partial oxidation of ethanol to acetaldehyde. The solids were shown to be active and selective for the reaction at relatively low temperature. Various physical techniques have been used to establish the active sites for this reaction and understand the role of the different elements. The results obtained showed that Brønsted acidic sites were involved in the formation of diethyl ether, acetic acid, and ethyl acetate and that vanadium was the active site, whereas the Fe3+/Fe2+ redox couple did not play any role. The most important outcome of this study is the observed activity dependence on surface V5+ cations content. These surface V5+ cations, which were formed as a result of the evolution of the surface composition of the solid solution in oxidative atmosphere, should not be considered as part of the bulk oxide, but as supra-surface species. When the solid solution is vanadium rich, the substitution mechanism in the bulk structure generates cationic vacancies that inhibited the formation of these surface species likely by blocking their diffusion to the surface. This study underlines the importance of both the supra-surface species and the bulk structure of oxidation catalysts. While it confirms that the formation of supra-surface species occurs in mixed oxides oxidation catalysts, it also demonstrates that this formation strongly depends on the bulk structure.