EPR investigation of superparamagnetism in nanometric-size LaFeO3 +/-delta and La2NiO4 +/-delta perovskites

Two LaFeO3 +/- delta and La2NiO4 +/- delta catalysts characterized by high thermal stability and phase purity have been prepared. Scanning electron microscopy analysis showed that they consisted of 20-60 nm spheroids, generally clustered in one order of magnitude larger particles. Electron paramagnetic resonance (EPR) spectroscopy showed that both samples possess superparamagnetic properties, connected with ca. 20 nm diameter particles. Furthermore, the EPR patterns were broader in La2NiO4 +/- delta, than in LaFeO3 +/- delta, indicating a greater amount of local disorder in the former. This was attributed to the presence of more oxygen vacancies in the Ni-based than in the Fe-based sample, in agreement with what was elsewhere hypothesized on the basis of catalytic properties of the two samples for the flameless combustion of methane.