Perovskite-Type Oxides II. Redox Properties of LaMn1-xCuxO3 and LaCo1-xCuxO3 and Methane Catalytic Combustion

Redox properties of high-surface-area LaM1-xCuxO3 (M=Mn or Co) perovskites prepared by the citrate method were studied by H2 TPR and O2 TPD techniques. We have found that the reduction of Mn4+ occurs in all La–Mn–Cu perovskites at temperatures lower than that of Co3+ in La–Co–Cu samples. The presence of copper affects the reduction of both Mn4+ and Co3+, increasing the temperature of reduction. Furthermore, Mn-based perovskites with a low Cu substitution (x< 0.4) release a great amount of O2 at high temperature. Catalytic activity toward methane combustion was investigated under diluted conditions (0.4% CH4 and 10% O2, N2 as balance) in the temperature range 673–1073 K using a fixed bed reactor with a space velocity of 40,000 N cm3 h-1 g-1. All samples catalyze methane oxidation in the temperature range 673–773 K, giving complete methane conversion below 1073 K and producing 100% CO2. No deactivation phenomena were observed. Substitution with copper decreases catalytic activity. The higher activity of Mn-based perovskites compared with that of the corresponding Co samples with the same composition was attributed to the stronger oxidative properties of LaM1-xCux O3. An apparent activation energy of 23 kcal mol-1 was evaluated on the basis of a first-order kinetic equation for all samples.