Diffuse reflectance spectroscopy of supported chromium oxide catalysts: A self-modeling mixture analysis

Diffuse reflectance spectra of hydrated, calcined, and reduced chromia/silica-alumina (Cr/SiO₂·Al₂O₃) catalysts with different SiO₂ contents have been investigated by using an interactive self-modeling mixture analysis. Four pure components are revealed in the spectra of Cr-catalysts before and after calcination: these are component A with three characteristic bands at 225, 325, and 495 nm, component B with three bands at 220, 275, and 400 nm, component C absorbing at 565 nm, and component D which absorbs in the region 205-270-350 nm. Components A and B are due to chromate and dichromate, respectively and their relative ratio increases with decreasing SiO₂-content of the support. Component C is assigned to pseudo-octahedral Cr³⁺ and is especially present on SiO₂ after calcination, while component D is a background due to the support. After CO-reduction three (E, F, and G) and four (E, F, G, and H) pure components were extracted from the spectra of Cr/Al₂O₃ and Cr/SiO₂, respectively. Components E and G have absorptions around 225, 355, and 475 nm and are due to Cr⁶⁺. They decrease with increasing reduction temperature. Component F absorbs at 635 nm on Al₂O₃ and at 855 nm on SiO₂. These bands are assigned to pseudo-octahedral Cr³⁺ and Cr²⁺, respectively. Pure component H, only present on Cr/SiO₂, absorbs at 305 and 540 nm and is possibly due to traces of Cr³⁺. All these findings are discussed in relation with previous results obtained by spectral deconvolution.