Surface composition and catalytic activity of La-Fe mixed oxides for methane oxidation

Four La-Fe oxide samples with La/Fe atomic ratio y = 1.02 ∼ 0.68 (denoted as LayFe) were prepared by the citrate method. The samples had a decreased specific surface area with the La/Fe atomic ratio decreasing. XRD pattern proved that the sample La _0.94 Fe is single phase perovskite La _0.94 FeO _3-d . Phase composition of the samples was estimated by the Rietveld refinement method. XPS analyses indicate that La ³⁺ ions are enriched on surface of crystallites for all the samples, and surface carbonate ions are relatively abundant on the samples La _1.02 Fe and La _0.94 Fe. Catalytic activity for methane oxidation per unit surface area of the samples is in the order of La _0.68 Fe > La _0.76 Fe > La _0.94 Fe > La _1.02 Fe both in the presence and in the absence of gaseous oxygen. A reason for this order would be the higher concentration of Fe ³⁺ ion on the surface of the samples La _0.68 Fe and La _0.76 Fe.