Control preparation of aluminium chromium mixed oxides by sol-gel process

Gels of mixed chromium and aluminium oxides were prepared by hydrolysis and polycondensation of organic precusors of metal. In this work, we try to understand the chemistry involved in the preparation with the aim to better control the formation of final material. Evolution of UV-vis, ESR, IR and Al-27 NMR spectra as a fonction of stirring time of organic metals mixture, shows a modification of Cr(III) and AI(III) ions environment, wich reflect the formation of mixed complex Cr-Al in solution. The synthesis parameters examined in this work are the chromium loading Cr/(Cr+Al)(%), stirring time (Ta) and the hydrolysis ratio k=[CH3COOH]/[Al(O-s-Bu)(3)]. We found that the aging time (Ta) of organic precursors governs the distribution of chromium between the surface and the bulk, and affect the symmetry of surface chromate species. The amount of acetic acid added affects considerably the solid texture and we note an increase of pore size with the K ratio, whereas chromium amount affects the dispersion and chromium oxidation state at the surface.