Abstract
The interaction of chromium oxide with Al 2O 3, TiO 2 and SiO 2 supports is investigated with Raman spectroscopy. The influence of the nature of the oxide support, calcination temperature and chromium oxide loading upon the molecular state of the supported chromium oxide is determined. The Raman studies reveal that the oxide supports stabilize the chromium oxide as Cr(VI) in tetrahedral coordination at moderate chromium oxide coverages. The surface chromium oxide is present as monomers and dimers on alumina, monomers and possibly dimers on titania, and monomers and polymers (dimers, trimers and tetramers) on silica. On the alumina support, the ratio of dimers/monomers increases with the chromium oxide coverage. On the silica support, the ratios of trimers/dimers and tetramers/dimers also increase with chromium oxide coverage. The surface chromium(VI) oxide species on titania, however, are not stable to elevated calcination temperatures and appear to be converted to a lower chromium oxide oxidation state. The silica support stabilizes the surface chromium(VI) oxide state at elevated calcination temperatures, but the surface chromium oxide polymers are not stable and convert to isolated surface chromium(VI) oxide monomers. Most of these differences are thought to be related to the differing surface-hydroxyl chemistries of Al 2O 3, TiO 2 and SiO 2 supports.
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