Surface properties and catalytic performance in CO oxidation of cerium substituted lanthanum-manganese oxides

Perovskite type oxides may potentially replace noble metal catalysts as full oxidation catalysts of hydrocarbons, as present in combustion exhaust streams, due to their high activity and fair hydrothermal stability. Presently we have investigated the catalytic activities of a series of Ce substituted La-Mn perovskites, prepared by coprecipitation, in the oxidation of carbon monoxide. The composition, bulk structure and the surface properties were established using elemental analysis, XRD and XPS. The properties tested include the reducibility and reoxidation behavior, the thermal stability of these perovskites and the oxidation activity. It was observed that Ce is not totally incorporated in the perovskite lattice. For a high degree of substitution, an excess Ce forms a separate CeO2 phase. Simultaneously, an increase in the atomic Mn4+/Mn3+ ratio and a decreasing surface oxygen concentration are observed. This suggests that cation vacancies are created at A (La) sites, resulting in the formation of unsaturated Mn (B) site ions on the surface. The catalytic activity of the La-Mn perovskites systematically changes as a function of the degree of Ce substitution. A plausible explanation for this behavior is the nonstoichiometry induced by Ce substitution, which results in the formation of cation/anion vacancies near the surface. The specific surface area and the atomic surface composition play a less pronounced role in the catalytic activity. (C) 2002 Elsevier Science B.V. All rights reserved.