Structural and surface analysis of unsupported and alumina-supported La(Mn,Fe,Mo)O3 perovskite oxides.

A series of bulk and Al(2)O(3)-supported perovskite oxides of the type LaMn(1-x-y)Fe(x)Mo(y)O(3) (x = 0.00-0.90 and y = 0.00-0.09) were synthesized by the citric acid complexation-gelation method followed by annealing in air at 800 degrees C. For all samples, the local environment and the chemical state and concentration of surface species were determined. Mössbauer spectra revealed the only presence of octahedral Fe(3+) ions dispersed in the perovskite structure, however well-crystallized together with a poorly crystalline LaFeO(3) phases were detected for larger substitutions (x = 0.90). A similar picture was obtained for Mo-loaded (y = 0.02 and 0.05) samples but a new phase most likely related to Fe(3+) ions dispersed aside from the perovskite structure was found for larger substitutions (y = 0.09). Together with these structures, supported samples showed the presence of LaFeO(3) nanoparticles. Finally, photoelectron spectroscopy indicated that the chemical state and composition of the samples in the surface region (2-3 nm) approaches that of the bulk. For the unsupported substituted samples, iron (and molybdenum) enters into the perovskite structure while manganese tends to be slightly segregated. Moreover, in supported perovskites, a fraction of Mo and La atoms interact with the alumina surface. All these oxides were active in methane combustion and best performance was recorded for the Fe-rich composition (x = 0.9) in which both Mn(3+) and Mo(3+) ions were in the same proportion (y = 0.05).