In situ Raman 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 (Al2O3, ZrO2, TiO2, SiO2, Nb2O3, 3% SiO2/TiO2, 3% TiO2/SiO2, and a physical mixture of SiO2 and TiO2) and chromium oxide loading (1-6 wt% CrO3). Two surface chromium oxide species were observed on the TiO2, ZrO2, Al2O3, 3% SiO2/TiO2, and a physical mixture of SiO2 and TiO2 surfaces: a monomeric species (characterized by a Cr = O stretching frequency at similar to 1030 cm(-1)) and a polymeric species (possessing a Cr=O stretching frequency at similar to 1005-1010 cm(-1) and a Cr-O-Cr bending mode at similar to 880 cm(-1)). The SiO2 and 3% TiO2/SiO2 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% TiO2/SiO2 approximate to 3% SiO2/TiO2 approximate to (SiO2 + TiO2) approximate to TiO2 > ZrO2).