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 s itu Raman spectroscopy as a function of the specific oxide support (Al 2O3, ZrO2, TiO2, SiO2, Nb2O3, 3% SiO2/TiO2, 3% TiO2/SiO2, and a physic al 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 a t similar to 1030 cm(-1)) and a polymeric species (possessing a Cr=O s tretching frequency at similar to 1005-1010 cm(-1) and a Cr-O-Cr bendi ng 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 sur face chromium oxide species, reflected by the decrease in the Raman in tensity of the Cr=O bonds, demonstrates that the polymeric surface chr omium oxide species is more easily reducible than the monomeric chromi um oxide species on the same support. The extent of reduction of the s urface chromium oxide species strongly depends on the specific oxide s upport (3% TiO2/SiO2 approximate to 3% SiO2/TiO2 approximate to (SiO2 + TiO2) approximate to TiO2 > ZrO2).