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

Two LaFeO3 +/- delta and La2NiO4 +/- delta catalysts characterized by high thermal stability and phase purity have been prepared. Scanning electron mi croscopy analysis showed that they consisted of 20-60 nm spheroids, general ly clustered in one order of magnitude larger particles. Electron paramagne tic resonance (EPR) spectroscopy showed that both samples possess superpara magnetic properties, connected with ca. 20 nm diameter particles. Furthermo re, the EPR patterns were broader in La2NiO4 +/- delta, than in LaFeO3 +/- delta, indicating a greater amount of local disorder in the former. This wa s 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 o n the basis of catalytic properties of the two samples for the flameless co mbustion of methane.