Thermal and spectroscopic studies of chromium chromate hexahydrate - a likely composition for redox surfaces of calcined chromia catalysts

Invoked by the possibility that chromium chromate species might constitute the surface composition of calcined chromia catalysts, a commercial Cr-2(Cr O4)(3). 6H(2)O compound was examined by thermogravimetry and differential s canning calorimetry in different reactive gas atmospheres. The principle go al was a proper assessment of its thermal and chemical stabilities with the hope of making an objective judgment on current environmental reservations about industrial applications of calcined chromia catalysts, owing to vola tility of Cr-IV-O species. On the basis of the thermal analysis results, th e chromate compound was calcined at some selected temperatures in the range from 150 degrees C to 1000 degrees C. The solid products were, then, subje cted to X-ray powder diffractometry and spectroscopies of infrared absorpti on and UV-Vis diffuse reflectance. The results have shown the parent bulk c hromate to dehydrate completely near 300 degrees C and to decompose into ch romate-covered alpha-Cr2O3 particles at 450-1000 degrees C, encompassing fo rmation of noncrystalline polychromate (Cr1+xO4+3x2-) and nonstoichiometric gamma-Cr2O3+x, bulk phases. in the presence of hydrogen atmosphere, the de hydration is intercepted by an immediate reduction, however the eventual pr oduct (alpha-chromia) remained chromate covered to 1000 degrees C. The high -temperature thermal and chemical stabilities thus revealed for the dispers ed chromates an attributed to electronic interactions with nearby Cr-III-O species dwelled in a crystalline lattice. (C) 1999 Elsevier Science B.V. Al l rights reserved.