Investigating Oxygen Surface Exchange Kinetics of La[sub 0.8]Sr[sub .20]MnO[sub 3-δ] and La[sub 0.6]Sr[sub 0.4]Co[sub 0.2]Fe[sub 0.8]O[sub 3-δ] Using an Isotopic Tracer

Abstract

An in situ method of investigating the kinetics and mechanisms of oxygen surface exchange on (LSM) and (LSCF) was developed by combining temperature programmed desorption, reaction, and isothermal switching with an isotopic tracer, . The gas exchange behavior is separated into surface- and bulk-originated oxygen and compared to behavior predicted by chemical kinetics. A two-step mechanism is proposed consisting of dissociative adsorption of oxygen onto the surface followed by adsorbed oxygen incorporation into the material; different dependences are obtained depending on which step is rate limiting. The rate-limiting step in LSCF is dissociative adsorption, whereas LSM is mostly incorporation limited at and becomes more dissociative adsorption limited at . The change in mechanism may be due to a phase transition and/or formation of oxygen vacancies. Overall, LSM is less catalytically active toward oxygen exchange than LSCF, with exchange onset occurring at in LSM and in LSCF. Above , the activation energy for oxygen exchange in LSM and LSCF was found to be 0.90 and , respectively.