Chemical States and Redox Properties of Mn/CeO2-TiO2 Nanocomposites Prepared by Solution Combustion Route

The catalytically important Mn/CeO2-TiO2 (MCT) solid solutions were synthesized by solution combustion technique using glycine, urea. or PEG as fuel. These samples along with those prepared by coprecipitation and wet impregnation methods were characterized for its structural, textural and redox properties using various physical and spectroscopic techniques, viz., powder X-ray diffraction, high-resolution transmission electron microscopy, temperature-programmed reduction, diffuse reflectance UV-visible, electron paramagnetic resonance, and X-ray photoelectron spectroscopy. The samples prepared by the solution combustion method showed high thermal stability, even at temperatures of 1323 K, whereas the samples synthesized by conventional routes lost its structural integrity at higher temperatures due to fast sintering. The combustion and coprecipitation methods stabilize Mn species both in +2 and +3 oxidation states, while in the MCT sample prepared by wet impregnation, Mn ions are present in +3 state exclusively. The latter possesses highly reversible redox properties, whereas the samples prepared by combustion and coprecipitation techniques show quasireversible redox behavior. On comparing the physicochemical properties of MCT solid solutions with that of Mn/CeO2, we elucidate that Ti incorporation into the ceria matrix alters the redox properties of both cerium and manganese ions. Though the nature of the fuel did not have significant effect on the oxidation states of Mn species stabilized in the solid solutions, the relative concentration of Mn2+ and Mn3+ ions and their existence in the bulk or on the surface of the material did vary with the nature of the fuel used in the combustion process. These variations could be attributed to the intrinsic properties of combustion process, viz., the adiabatic/real flame temperature and mode of combustion and its duration. Employing PEG as fuel in the combustion process, one can obtain thermally stable nanocrystalline MCT samples with high phase purity.