RAMAN-SPECTRA OF CHROMIUM-OXIDE SPECIES IN CRO3/AL2O3 CATALYSTS

Raman spectra are reported of chromium(VI) oxide species in Cr/Al2O3 catalysts, and of aqueous solutions of chromium oxide used for the preparation of these catalysts. The Raman spectra reveal that after impregnation and drying at room temperature under vacuum, the chromium oxide is present as chromate submonolayer at 1 wt.% Cr/Al2O3, as dichromate between 5 and 15 wt.% Cr/Al2O3, as trichromate at 20 wt.% Cr/Al2O3 and as crystalline CrO3 and trichromate at 30 wt.% Cr/Al2O3. The supported chromium oxide species are in wet condition. The chromate in the 1 wt.% Cr/Al2O3 sample is stable upon calcination in air at 823 K. Preparation of the 1 wt.% Cr/Al2O3 catalyst from a Cr(NO3)3 solution gave the same result. It is shown that the molecular structure of the oxides is influenced by the chromium oxide concentration of the impregnation solutions, by the concentration in the catalysts and by the water content of the catalysts, which was varied by exposure to air and by using different laser powers. © 1990.