Effect of Zn substitutions in YBa2Cu3O6 +x phases on reactivity during the ammoxidation of toluene

Abstract

Zn substitution in Y–Ba–Cu–O phases influences the catalytic ammoxidation of toluene. The activity for formation of both benzonitrile and CO₂ at low and high O₂ pressures, respectively, decreases strongly with increasing Zn content up to 4%. Above this substitution level, the activity, however, increases slightly. Characterisations of the surface by XPS and of the bulk by XRD analysis, indicate the presence of overlayers on the catalysts. In these overlayers, irrespective of the Zn content of catalysts, Cu^I states predominate at low O₂ pressure and Cu^II at high pressure. These states are associated with selective and non-selective properties, respectively. The activity of the catalysts shows a general tendency to increase with surface copper concentrations as found from XPS studies. However, strong deviations from linearity indicate that the activity of the overlayer is greatly influenced by the bulk acting as a support.