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

1999; Elsevier BV; Volume: 329; Issue: 1 Linguagem: Inglês

10.1016/s0040-6031(98)00658-3

1872-762X

Nasr E. Fouad, Samih A. Halawy, M.A. Mohamed, Mohamed I. Zaki,

Metal Extraction and Bioleaching

Invoked by the possibility that chromium chromate species might constitute the surface composition of calcined chromia catalysts, a commercial Cr2(CrO4)3·6H2O compound was examined by thermogravimetry and differential scanning calorimetry in different reactive gas atmospheres. The principle goal 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 volatility of CrIV–O species. On the basis of the thermal analysis results, the chromate compound was calcined at some selected temperatures in the range from 150°C to 1000°C. The solid products were, then, subjected to X-ray powder diffractometry and spectroscopies of infrared absorption and UV–Vis diffuse reflectance. The results have shown the parent bulk chromate to dehydrate completely near 300°C and to decompose into chromate-covered α-Cr2O3 particles at 450–1000°C, encompassing formation of noncrystalline polychromate (Cr1+xO2−4+3x) and nonstoichiometric γ-Cr2O3+x bulk phases. In the presence of hydrogen atmosphere, the dehydration is intercepted by an immediate reduction, however the eventual product (α-chromia) remained chromate covered to 1000°C. The high-temperature thermal and chemical stabilities thus revealed for the dispersed chromates are attributed to electronic interactions with nearby CrIII–O species dwelled in a crystalline lattice.