Ferromagnetic systems forming in La(0.9)M(0.1)CoO(3) (M=Ce; Pr) and catalytic performance in the catalytic flameless combustion of methane: an EMR investigation

Two La0.9M0.1CoO3 samples with M = Pr prepared by the traditional sol-gel (SG) method and calcined in air at 800°C for 4h and 8h (samples SG4 and SG8, respectively) have been compared to a sample with the same nominal composition but prepared by the innovative flame-pyrolysis method (sample FP). The activity for the catalytic flameless combustion (CFC) of methane markedly decreased in the order FP > SG4 > SG8, i.e. with increasing particle size, as shown by SEM images. TPR analysis suggested that better catalysts release oxygen at lower temperature. All the as-prepared samples were EPR silent. By contrast, the two better catalysts, i.e. SG4 and FP, showed a ferromagnetic resonance (FMR) pattern after de-oxygenation, as well as after the use for the CFC of methane. This feature was more intense with SG4 than with FP, i.e. when larger particles were composing the catalyst. However, no FMR feature was observed with the much less performing catalyst SG8, though it was composed of particles with the largest size. These results have been compared to those reported with M = Ce [1]. The different spectroscopic response of all these samples could explain their different catalytic performance, also enlightening the catalytic reaction on the base of molecular mechanism.