Effect of substitution by cerium on the activity of LaMnO3 perovskite in methane combustion

This study concerns the effect of lanthanum substitution by cerium on the catalytic activity of La1-xCexMnO3 catalysts and its relation to their physico-chemical characteristics. Samples of pure and cerium-substituted lanthanum manganese perovskites. La1-xCexMnO3 with x = 0.1-0.5 and LaCexMnO3 with x = 0.1, 0.2 and 0.3, were prepared by the citrate method and calcined 5 h at 973 or 1073 K. All samples were characterized by XRD, XPS and oxygen TPD and had their specific surface area (SSA) determined by nitrogen adsorption. The catalytic activity was determined, using 0.1 g catalyst, 1% methane in air at a flow rate of 75 ml/min (GHSV = 45,000 ml/g(cat) h). Substitution with cerium affects significantly the physico-chemical properties of individual compositions. It slows the rate of perovskite phase formation, increases the SSA, has an effect on thermal stability and modifies the oxygen desorption characteristics. However, these changes do not correlate in the expected way with changes in activity for methane combustion. Substitution with cerium or addition of cerium over the formal stoichiometry were positive only for x = 0.1 in samples calcined 5 h at 973 K. Higher x values resulted in lower activity. (C) 2002 Elsevier Science B.V. All rights reserved

Methane and propane combustion over lanthanum transition-metal perovskites: role of oxygen mobility

Catalytic hydrocarbon combustion is a technologically important, but still relatively poorly understood reaction. To shed more light on the role of various physical and chemical characteristics of the catalyst on its activity for hydrocarbon combustion, La1-xSrxM1-yM'O-y(3-delta) perovskites (M and M' represent transition metals) were used as a model system. Four representative compositions were prepared and fully characterized by different methods and their activity was determined in methane and propane combustion. Oxygen desorption reflecting oxygen mobility is mainly a function of composition, more or less independently from specific surface area (SSA). On the other hand, the results confirm that SSA is the important factor determining the high activity. Yet, in these oxide type catalysts (perovskites) the loss of activity due to lower SSA, resulting from aging at elevated temperatures, seems, to a large part, be compensated for by fast oxygen mobility assured in oxygen nonstoichiometric compositions. (C) 2003 Elsevier B.V. All rights reserved