(394f) Structural Stability and Catalytic Activity of Lanthanum-Based Perovskite Oxides

Perovskite-type oxide materials with a general formula La(1-x)A(x)Fe(1-y)Co(y)O(3-delta), where A is an alkaline earth metal, e.g., Sr or Ba, have been extensively used as cathode materials for catalytic reduction of oxygen in solid oxide fuel cells (SOEC), as well as combustion catalysts. Catalytic activity in these materials depends on the two main factors: (i) Their structural stability at temperatures corresponding to a given catalytic process; (ii) The kinetics of oxygen adsorption and reduction at the surface and oxygen migration in the bulk. The structural instability mainly occurs through segregation of the perovskite to metal oxides with the formation of grain boundaries. The reconstruction of the perovskite significantly lowers oxygen diffusivity through these materials (which is dominated by a vacancy mechanism: specifically - an oxygen vacancy diffuses over the oxygen sublattice) or by surface exchange (i.e., by the rate of oxygen desorption from the surface). The kinetics of oxygen adsorption and reduction at the surface may be different in different perovskites and depends on many factors, such as surface structure and chemical composition, presence of different point or extended surface defects, etc. One could expect that some surfaces can easily loose oxygen, i.e., surface oxygen can oxidize other molecules adsorbed at the surface (according to the Mars-Krevelen mechanism), while another surfaces may grab oxygen atoms from adsorbed molecule (reducing them). Therefore, the interplay between the surface oxygen exchange and the diffusion through the perovskite define the type of the preferable surface catalytic reaction (oxidation or reduction). In this presentation we use a combination of density-functional-theory (DFT) calculations, temperature programmed reduction measurements, X-ray diffraction, and catalytic measurements to elucidate the main processes that contribute into the structural stability and catalytic activity of the lanthanum-based perovskite materials. In particular, we investigated the dynamics of the perovskite structure reconstruction with oxygen loss during the regulated increase of the temperature. Sr-doped perovskite structures undergo higher number of phase transformations than Ba-doped structures which is explained by higher mobility of Sr atoms in the perovskite matrix. We also investigated (experimentally and theoretically) the catalytic activity of complex perovskite surfaces for the carbon monoxide oxidation and the reduction of NO. The modeling/calculation studies provide a guidance for experimental measurements suggesting some special combinations of the perovskite chemical composition and surface structure for which one could expect the highest reduction (oxidation) surface activity.