In SituRaman Spectroscopy of Supported Chromium Oxide Catalysts: Reactivity Studies with Methanol and Butane

The interactions of methanol and butane with supported chromium oxide catalysts under oxidizing and reducing conditions were studied by in situ Raman spectroscopy as a function of the specific oxide support (Al 2 O 3, ZrO 2, TiO 2, SiO 2, Nb 2 O 5, 3% SiO 2 /TiO 2, 3% TiO 2 /SiO 2, and a physical mixture of SiO 2 and TiO 2 ) and chromium oxide loading (1−6 wt% CrO 3 ). Two surface chromium oxide species were observed on the TiO 2, ZrO 2, Al 2 O 3, 3% SiO 2 /TiO 2, and a physical mixture of SiO 2 and TiO 2 surfaces: a monomeric species (characterized by a Cr O stretching frequency at ∼1030 cm -1 ) and a polymeric species (possessing a Cr O stretching frequency at ∼1005−1010 cm -1 and a Cr−O−Cr bending mode at ∼880 cm -1 ). The SiO 2 and 3% TiO 2 /SiO 2 surfaces possess only the monomeric species. The extent of reduction of the surface chromium oxide species, reflected by the decrease in the Raman intensity of the Cr O bonds, demonstrates that the polymeric surface chromium oxide species is more easily reducible than the monomeric chromium oxide species on the same support. The extent of reduction of the surface chromium oxide species strongly depends on the specific oxide support (3% TiO 2 /SiO 2 ≈ 3% SiO 2 /TiO 2 ≈ (SiO 2 + TiO 2 ) ≈ TiO 2 > ZrO 2 ).