CHROMIA ON SILICA AND ALUMINA CATALYSTS - A THERMOANALYTICAL AND SPECTROSCOPIC INVESTIGATION OF THERMAL GENESIS OF THE CATALYSTS

CrOx/SiO2 and CrOx/gamma-Al2O3 catalysts were obtained by calcination at 600-degrees-C for 5 h of precursor materials consisting of chromia loaded on supports at various concentrations (0.5-10 mole% Cr2O3). The materials were prepared using different parent compounds [viz. Cr(NO3)3 . 9H2O, Cr2O3 . 5H2O gel and CrO3]. Chromium nitrate and trioxide were impregnated onto the supports from aqueous solutions, whereas the gel was loaded by a coating procedure. The thermal genesis of the catalysts was explored by means of thermogravimetric and differential thermal analyses. Thermal events encountered were explained by correlation with X-ray diffraction, infrared absorption and UV/VIS diffuse reflectance data obtained for intermediate and final solid products. A thermogravimetric method is proposed to characterize the chemical reaction at the precursor/support interface during the catalyst genesis. The most prominent result of these analyses is the predominance of alpha-Cr2O3 particles on SiO2 and the formation of dispersed chromates on gamma-Al2O3, irrespective of the type of precursor used. This distinct behaviour is thought to be closely related to the different contents, populations and chemistries of surface hydroxyl groups on SiO2 and gamma-Al2O3.