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 conten ts have been investigated by using an interactive self-modeling mixtur e analysis. Four pure components are revealed in the spectra of Cr-cat alysts before and after calcination: these are component A with three characteristic bands at 225, 325, and 495 nm, component B with three b ands at 220, 275, and 400 nm, component C absorbing at 565 nm, and com ponent 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 s upport. After GO-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, 35 5, and 475 nn and are due to Cr6+. They decrease with increasing reduc tion 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 3 05 and 540 nm and is possibly due to traces of Cr3+. All these finding s are discussed in relation with previous results obtained by spectral deconvolution. (C) 1997 Academic Press.