The role of support oxygen in the epoxidation of propene over gold–titania catalysts investigated by isotopic transient kinetics

Abstract

Transient kinetic experiments were performed on gold–titania catalysts for the epoxidation of propene using hydrogen and oxygen to investigate the reaction mechanism. A ‘classical’ 1 wt% gold on titania catalyst was studied, as well as a 1 wt% gold on Ti-SBA-15 catalyst. Steady-State Isotopic Transient Kinetic experiments using oxygen-18 were performed, which provide information on the types and quantities of species present on the catalyst during reaction, as well as on possible reaction pathways. The isotopic transients of the reaction products take place on a time scale of minutes, while the isotopic switch of the unreacted feed gases proceeds in a matter of seconds. This indicates that the oxygen pool on the catalysts, either in the form of adsorbed products or in the form of oxygen available for the reaction, is large in comparison to the product formation rates. The results indicate that support oxygen is playing a role in the reaction mechanism, but this could not be determined conclusively. For the water produced as a side product, it could be determined that support oxygen is ending up in the water; however, this can be explained by an exchange after its formation. The size of the oxygen pool in the neighborhood of gold, calculated based on the size of the TEM visible gold particles, is smaller than the amount of oxygen that ends up in the carbon containing oxidation products, which indicates that most likely part of the gold is present in sub-nanometer particles, not visible by TEM.