Influence of the A-site cation in AMnO_3+x and AFeO_3+x (A = La, Pr, Nd and Gd) perovskite-type oxides on the catalytic activity for methane combustion

Abstract

The effect of rare-earth ions (La, Pr, Nd and Gd) in AMnO_3+x and AFeO_3+x perovskites on the thermal behavior and on the catalytic activity for the deep oxidation of methane has been studied. AMnO_3+x perovskites showed after preparation an oxidative non-stoichiometry. Oxygen desorption analysis revealed for the four manganites different desorption steps occurring between 930 and 1370 K. Stoichiometry was reached after the first desorption step. Heating the samples at temperatures above 1300 K resulted in phase segregation to the simple oxides. AFeO_3+x perovskites were more stable towards thermal decomposition than the Mn-perovskites, showing no oxygen evolution up to 1400 K. The reducibility of these perovskites in hydrogen correlated inversely with the relative effective ionic radii of the trivalent rare-earth cations. Comparative catalytic studies were carried out in a fixed-bed microreactor at atmospheric pressure in the temperature range 600–1200 K. The activities at 770 K, expressed as reaction rates referred to the BET surface area, varied between 1.4 × 10⁻⁷ and 2.9 × 10⁻⁷ mol s⁻¹ m⁻⁷ for the AMnO_3+x, and between 1.1 × 10⁻⁷ and 1.6 × 10⁻⁷ mols⁻¹m⁻²for the AFeO_3+x perovskites.