Catalytic combustion of methane over LaMnO 3 perovskite supported on La 2O 3 stabilized alumina. A comparative study with Mn 3O 4, Mn 3O 4-Al 2O 3 spinel oxides

Abstract

Ten and 20 wt % LaMnO3 perovskites supported on La2O3-stabilized γ-Al2O3 were studied for catalytic combustion of methane. A comparison with Mn3O4 and Mn3O4-Al2O3 spinel oxides was also drawn. The catalysts were characterized by microanalysis, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and O2 temperature-programmed desorption (TPD) techniques. Catalytic activity tests were carried out in a fixed-bed reactor at T=300–800°C, space velocity = 40000 h−1, CH4 concentration = 0.4% v/v, O2 concentration = 10% v/v. Both XRD and microanalysis indicated a uniform dispersion of the perovskite phase. The structure of γ-alumina was retained after the treatment at 800°C, the treatment at 1100°C led to the transition to the θ and β phases. TPR measurements suggested the presence of a fraction of Mn4+ in supported perovskites. The possible interaction of manganese with alumina, which stabilizes Mn2+, led to the reduction of the initial average oxidation state of manganese with the perovskite content and the temperature of treatment. O2 desorption in TPD measurements was significant from spinel oxides, whereas negligible from supported perovskites. Supported perovskites gave complete CH4 conversion within 650°C with 100% selectivity to CO2. The activation energy value, evaluated from a methane first-order rate equation, suggested the occurrence of the same reaction mechanism of unsupported LaMnO3. The preexponential factors of the catalysts treated at 800 °C were proportional to the perovskite content, in agreement with a monolayer model. Samples treated at 1100°C showed the same activity not depending on the perovskite content, suggesting that only a fraction of manganese in the 20 wt % LaMnO3 is available for the reaction. This was related to the stabilization of a fraction of Mn2+, probably not involved in the reaction. Spinel oxides catalyze the reaction at lower temperature, giving complete conversion within 600°C with 100% selectivity to CO2. The activation energy was lower than that of supported perovskites. A correlation with the ability to desorb O2 was hypothesized.