TAP study on the active oxygen species in the total oxidation of propane over a CuO–CeO 2/γ-Al 2O 3 catalyst

Abstract

The activation of propane both in the absence and presence of gas-phase O 2 or CO 2 over a CuO–CeO 2/γ-Al 2O 3 catalyst is investigated in a TAP reactor between 623 and 873 K. Two different types of oxygen species are involved dependent on the mixture introduced: (1) lattice oxygen from the surface and bulk of the catalyst and (2) weakly adsorbed oxygen species produced either from gas-phase O 2 or CO 2. Both types of oxygen species participate in the total oxidation of propane. If pulses of pure propane are admitted, lattice oxygen from CuO and CeO 2 is consumed, reducing these oxides to a certain extent. Apart from surface lattice oxygen from CuO and CeO 2, also bulk lattice oxygen is involved in the activation of propane, demonstrating the high O mobility within the lattice of these metal oxides. Lattice oxygen of alumina is not available for reaction with propane. Introduction of O 2 reoxidizes the reduced sites on CuO and CeO 2, as expected for a Mars–van Krevelen mechanism. Also CO 2 can replenish O vacancies in the active phase by dissociative adsorption on alumina and ceria. The adsorption on alumina is followed by reverse spillover of reactive O species to the active phase. The second type of oxygen consists of weakly adsorbed oxygen species, produced either from O 2 on both CuO and CeO 2, or from CO 2 on CeO 2, and enhances strongly the catalytic activity. In the presence of both O 2 and CO 2 in the propane feed, the predominant reaction pathways will only involve the two types of active species produced from O 2, and not from CO 2.