Thermal phase stability and catalytic properties of Nanostructured TiO2-MgO sol-gel mixed oxides.

Several compositions in the system TiO2-MgO were prepared via sol-gel method. The structural evolution of the samples was followed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and thermal analysis (DTA/TG). In order to study the crystalline phase stability over time, the amorphous fresh gels were subjected to successive thermal treatments from 100 to 1100 degrees C, with a total annealing time of 442 h. Below 350 degrees C, magnesium acetates and oxalates phases were detected; at higher temperatures the binary compounds MgTi2O5, MgTiO3 and Mg2TiO4 appeared over a wide range of compositions. In the titania rich end, brookite, anatase and rutile were detected, these phases easily reacted to form the binary oxides; in contrast, on the rich-end magnesia, periclase showed an overall high stability. The successive thermal treatments over an extended period of time allowed a better control of nanosized crystallite growth; for anatase was 30 nm, below 850 degrees C, and for rutile was 45 nm, between 600 degrees C and 1000 degrees C. Magnesia showed a constant value of 22 nm on the overall studied temperatures. The best catalytic performance for 2-propanol decomposition was achieved by 90Ti/10Mg, annealed at 400 degrees C; this indicates that when magnesium is added to titania, the catalytic activity improves due to the introduction of structural defects and charge deficiencies. For 2-butanol decomposition, the formation of cis and trans isomers indicates the presence of high acidity sites.