A series of catalysts has been analysed before and after their use for meth
ane flameless oxidation with air. In most cases, a g congruent to 2, linear
ly 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 attribu
ted 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 cata
lytic use. This has been attributed to a change of the Curie temperature, T
-c, due to an increase of the conduction bandwidth, accompanying the chemic
al reduction from Mn4+ to Mn3+ during the catalytic process. Therefore, the
presence of more Mn4+ is expected to improve thew catalyst activity. In fa
ct, 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.