CHARACTERIZATION OF CRO3/AL2O3 CATALYSTS UNDER AMBIENT CONDITIONS - INFLUENCE OF COVERAGE AND CALCINATION TEMPERATURE

The CrO3/Al2O3 catalyst system was investigated by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, and BET surface area measurements in order to determine the molecular structures and monolayer coverage of the surface chromium oxide phase under ambient conditions. Up to a surface coverage of 9% CrO3/Al2O3, the chromium oxide is stabilized by the alumina support in the +6 oxidation state after calcination at 120-1050-degrees-C. The molecular structures of the chromium (VI) oxide surface species are a function of the surface coverage and calcination temperature because under ambient conditions the surface structures depend on the net surface pH at point of zero charge of the hydrated oxide surface. Increasing the surface coverage results in a decrease of the net surface pH and formation of more polymerized chromium oxide species. High calcination temperatures (greater-than-or-equal-to 950-degrees-C) cause a reduction of the BET surface area, as well as a phase transformation of gamma-Al2O3 into theta,delta-Al2O3, and also result in an increase in the surface density of the chromium oxide overlayer (more polymerized surface chromium oxide species). Monolayer coverage is reached at ca. 12% CrO3/Al2O3 and crystalline Cr2O3 particles are found on the alumina surface together with surface chromium oxide species at higher loadings. At high calcination temperatures (greater-than-or-equal-to 800-degrees-C), the Cr2O3 Crystalline particles react with the alumina support to form Cr(III) in solid solution with alpha-Al2O3 (corundum). The alpha-Al2O3 lattice is slightly expanded due to the incorporation of chromia.