Catalytic oxidation of toluene over Y–Ba–Cu perovskites : activity and X-ray photoelectron spectroscopy studies

Abstract

The catalytic oxidation of toluene over Y–Ba–Cu–O phases has been studied. At low oxygen pressures, low rates and a certain selectivity for benzaldehyde occurs, whereas at high oxygen pressures, high rates with complete selectivity for carbon oxides is obtained. X-Ray photoelectron spectroscopy (XPS) analysis of used catalysts identifies the presence of predominantly Cu^I states when used at conditions showing some selectivity, whereas after use under combustion conditions, Cu^II states are found. The initial activities of freshly charged catalysts with different oxygen contents are different, but are greatly affected by pre-reduction/oxidation treatments, which yield similar activities. Large variations in surface Y–Ba–Cu concentrations with various treatments are observed. In particular, high surface copper concentrations may be obtained after extensive reduction–oxidation cycles. The catalyst is modelled as highly dispersed copper oxide over a support with variable oxygen content. The different behaviour of the reduced and oxidized catalysts can be interpreted by bond valence calculations, suggesting that the selectivity over reduced forms is connected to more nucleophilic oxygen species and that the cations are milder oxidants than for the oxidized forms. The inferior selectivity compared with the ammoxidation of toluene, but comparable activity, suggests the formation of a reactive benzylic intermediate in the first step for both reactions.