Relation between defects and catalytic activity of calcium doped LaFeO3 perovskite

Perovskite-type compounds La1 − xCaxFeO3 (x = 0.0, 0.1, 0.2, 0.3, 0.4) were prepared by the amorphous citrate method and tested in the catalytic combustion of methane. The solids were characterized by nitrogen adsorption, XRD, TPR and O2-TPD techniques. It was found that La0.6Ca0.4FeO3 has the highest catalytic activity for methane combustion. Physical properties and catalytic results are interpreted in terms of the defect structure, giving a strong physical–chemical support to the observed results. The higher catalytic activity of the doped perovskites is driven by a fraction of the iron ions in unusual oxidation states (+ 4) which are formed by the partial substitution of lanthanum by calcium. At low substitution levels, x < 0.1, the charge compensation is achieved fundamentally by the oxidation of a part of the iron ions from Fe3+ to Fe4+. At higher doping levels instead, the formation of oxygen vacancies account for the main charge-compensating mechanism. The linear dependence of T50 on the amount of Fe4+(%) suggest that the active sites for the methane combustion are directly related to this unusual oxidation state of iron.