Diffuse Reflectance Spectroscopy of Supported Chromium Oxide Catalysts: A Self-Modeling Mixture Analysis

Abstract

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 selfmodeling 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 Cr3C 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 Cr6C. 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 Cr3C and Cr2C, respectively. Pure component H, only present on Cr/SiO2, absorbs at 305 and 540 nm and is possibly due to traces of Cr3C. All these findings are discussed in relation with previous results obtained by spectral deconvolution.