EPR analysis of La_1-xM_xMnO_3+y (M=Ce, Eu, Sr) perovskitic catalysts for methane oxidation

Abstract

A series of catalysts has been analysed before and after their use for methane flameless oxidation with air. In most cases, a g2, 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 the 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.

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