Catalytic and redox properties of nano-sized La0.8Sr0.2Mn1-xFexO3-δ mixed oxides synthesized by different routes

The La0.8Sr0.2Mn1-x,FexO3-6 (LSMF) samples of perovskite-type structure, with the value of x ranging from 0 to 1.0 were synthesized by using two different methods, viz. nitrate and solid-state routes, and by calcining at different temperatures. Efforts were directed to identify the microstructural and morphological properties responsible to the substitution-induced modification in catalytic behavior of these materials. The XRD studies showed that the incorporation of iron resulted in single-phase samples of orthorhombic symmetry instead of the rhombohedral symmetry of the parent LSM perovskite. The Mossbauer spectra revealed that iron existed in trivalent state but in three distinctive coordinative environments. At one of these sites, iron was found to be paramagnetic in nature and the concentration of these species decreased with the increasing iron content. On the other hand, XPS results revealed that the valence state of 'B' site cations, Fe and Mn, was different at the surfaces of the sample as compared to the bulk. No significant segregation of an individual metal was noticed at the surface layer. At the same time, the particles of Fe-containing samples were smaller in size as compared to LSM and it was found to have a direct impact on the lowered reduction temperature and the enhanced catalytic activity of LSMF samples. The results of our study reveal that in addition to the oxygen ion vacancies generated in the lattice due to multiple oxidation states of 'B' site cations, the symmetry around a substituent cation and the subtle changes in particle morphology may also play an important role in deciding the catalytic behavior of the LSMF perovskites. (c) 2005 Elsevier B.V. All rights reserved.