Catalytic combustion of methane over LaMnO3 perovskite supported on La2O3 stabilized alumina. A comparative study with Mn3O4, Mn3O4-Al2O3 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.