CO oxidation and methane combustion on LaAl1-xFexO3 perovskite solid solutions

Catalytic CO oxidation and CH4 combustion were studied over LaAl1-xFexO3 (0 less than or equal to x less than or equal to 1) perovskites prepared by citrate method and calcined at 1073 K. The catalysts were characterised by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), and magnetic susceptibility. Redox properties and the content of Fe4+ were derived from temperature programmed reduction (TPR). XRD analysis showed that all the LaAl1-xFexO3 samples are single-phase perovskite-type oxides. A linear variation of the unit cell parameters with x was observed. Optical and magnetic properties are reported and discussed. Particle sizes and surface areas are in the 280-840 Angstrom and 4-6 m(2) g(-1) ranges, respectively. A very low extent of reduction, due to the Fe4+ --> Fe3+ process, was found by TPR. XRD analysis performed after TPR experiments showed that iron reduction does not lead to a loss of the perovskite structure. For CO oxidation all the Fe-containing perovskites are substantially more active than LaAlO3, so pointing to the essential role of the transition metal ion in developing highly active catalysts. Isolated mononuclear iron sites seem to be more active. Methane is totally converted to CO2 within 1020 K on all the catalysts. The activity increases with iron content, that is with the number of active sites. However, the activity per iron ion decreases with the iron content up to x = 0.5, suggesting that the active sites may contain isolated Fe ions bonded to more easily removable oxygen. (C) 2002 Elsevier Science B.V. All rights reserved.