Direct Observation of Oxygen Dissociation on Non-Stoichiometric Metal Oxide Catalysts

摘要

Oxygen dissociation on metal oxides is a key reaction step, limiting the efficiency of numerous technologies. The complexity of the multi-step oxygen reduction reaction (ORR) makes it difficult to investigate the oxygen dissociation step independently. Direct observation of the oxygen dissociation process is described, quantitatively, on perovskites La0.6Sr0.4Co0.2Fe0.8O3- and (La0.8Sr0.2)(0.95)MnO3 +/-, using gas-phase isotope-exchange with a 1:1 O-16(2):O-18(2) ratio. Oxygen transport mechanisms between gas-surface reactions and surface-bulk exchange are deconvoluted. Our findings show that regardless of participation of lattice oxygen, La0.6Sr0.4Co0.2Fe0.8O3- is better at oxygen dissociation than (La0.8Sr0.2)(0.95)MnO3 +/-. Heteroexchange, involving lattice oxygen, dominates on La0.6Sr0.4Co0.2Fe0.8O3-. In contrast, (La0.8Sr0.2)(0.95)MnO3 +/- shows both homoexchange and heteroexchange, with the latter only happening above 600 degrees C. Using a 1:1 isotope mixture, a simple method is presented for separation of the oxygen dissociation step from the overall ORR.