Oxygen Dissociation Kinetics of Concurrent Heterogeneous Reactions on Metal Oxides

The high activity of oxide catalysts toward the oxygen reduction reaction (ORR) attracts unwanted interactions with other gaseous oxygen-containing species in air. Understanding the interaction between oxygen-containing species, mainly water and carbon dioxide, and oxides is important for many energy applications. However, the oxygen self-exchange process and the high-temperature operating conditions limit the investigation of these concurrent reactions. Here we report a direct observation of the effects of water and carbon dioxide on dissociation rates of ionically conducting catalysts, La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) and (La_0.8Sr_0.2)_0.95MnO_3±δ(LSM), using gas-phase isotope exchange. The concurrent heterogeneous reactions of oxygen and other oxygen-containing species on oxide catalysts can either promote or hinder oxygen dissociation rates, depending on the participation of lattice oxygen. LSCF appears to be much more active in exchange with these oxygen-containing species, while LSM shows relatively little exchange. Oxygen-containing species exhibit site-blocking effects and inhibit the reaction on LSCF. In contrast, water and CO₂ promote the oxygen dissociation rate on LSM, likely due to the prominence of homoexchange, where intermediate surface species play an important role. Our study provides insights into the reaction mechanism of oxygen dissociation and the effect of coexisting ambient air oxygen species.