Catalytic CO Oxidation over La1‐xSrxMnO3 at High CO Partial Pressures

Abstract

AbstractThe catalytic CO oxidation over polycrystalline LaMnO3 exhibits a bistability at high CO concentration. Under atmospheric pressure, the regime of bistability extends from 200 to 220°C and widens with increasing CO concentration. A systematic study of the solid solution system La1‐xSrxMnO3 which crystallizes in the perovskite structure, shows that the width of the bistable regime decreases with an increasing amount of strontium substitution and no bifurcation was observed for x > 0.2. A model is proposed to explain the bistability, based on a CO‐driven reconstruction of the reduced surface, leading to pairs of Mn2 ions with a Mn‐Mn distance comparable to the spacing in the metal. These pairs provide reactive sites for CO oxidation and oxygen chemisorption. Such metal‐metal pairs are not found in the perovskite lattice but are a structural feature of the closely related hexagonal 4‐layered packing which is the normal crystal structure of SrMnO3. The rate of change back to the “oxidized” surface is found to correlate with the lattice oxygen mobility. With low oxygen mobility, the transition from the active into the less active state becomes slow and hysteresis may result. Strontium substitution increases the number of defects in the oxygen lattice and leads ultimately to an oxygen deficiency (reductive nonstoichiometry). The catalytic activity of the system peaks at x = 0.2, indicating the influence of both the oxygen mobility and the oxygen nonstoichiometry on the catalytic oxidation reaction.