EPR analysis of La1-xMxMnO3+y (M = Ce, Eu, Sr) perovskitic catalysts for methane oxidation

A series of catalysts has been analysed before and after their use for methane flameless oxidation with air. In most cases, a g congruent to 2, linearly temperature-broadened, Lorentzian-shaped EPR line was observed at T > T-min, where T-min is the temperature corresponding to the narrowest EPR line, ranging between 244 and 392 K, depending on the nature and concentration of the doping ion. This trend of the EPR linewidth is discussed and attributed to bottlenecked spin relaxation taking place from the Mn4+ ions to the lattice via the Mn3+ ions. With all the samples, T-min increased after catalytic use. This has been attributed to a change of the Curie temperature, T-c, due to an increase of the conduction bandwidth, accompanying the chemical reduction from Mn4+ to Mn3+ during the catalytic process. Therefore, the presence of more Mn4+ is expected to improve thew catalyst activity. In fact, lower light-off temperatures were obtained for increasing concentration of Sr or Eu, which forces Mn3+ to Mn4+, in contrast to the case of Ce4+ as dopant.