La, Ca and Fe oxide perovskites: preparation, characterization and catalytic properties for methane combustion

摘要

La_(1-x)Ca_xFeO_3(x=0.1, 0.2, 0.3, 0.4, 0.5) perovskites prepared by citrate method and calcined at 1073 K have been investigated as catalysts for methane combustion. The formation of the perovskite structure has been shown by X-ray diffraction (XRD) for all samples. The surface area (SA) values are in the range 3-6m~2 g~(-1) for the samples up to x=0.4, whereas SA is 0.7 m~2 g~(-1) for the specimen with x=0.5. The atomic susceptibility increases with increasing in Ca content. The Fe~(4+)/Fe_(total) ratio has been determined by both redox titration and TPR analysis. The amount of Fe~(4+) enhances with Ca substitution but the Fe~(4+)/Ca~(2+) ratio is quite constant giving rise to an increasing formation of oxygen vacancies to preserve charge neutrality. All perovskites show a similar intrinsic activity in CH_4 combustion with activation energy, E_a, of about 23kcal mol~(-1). A slightly lower value of E_a is shown by LaFeO_3 which, in contrast with Ca substituted samples, exhibits a small cationic defectivity. Methane reaction order lower than one was found for all catalysts.