DIFFUSE-REFLECTANCE SPECTROSCOPY STUDY OF THE THERMAL GENESIS AND MOLECULAR-STRUCTURE OF CHROMIUM-SUPPORTED CATALYSTS

The diffuse reflectance spectra of a series of chromium-supported silica aluminas, with varying SiO2 content, have been investigated before and after drying and after calcination, successive CO reductions, and recalcination. The molecular structure of chromium before drying is influenced by the isoelectric point (IEP) of the supports. It is concluded that the lower the IEP of the support, the higher the amount of surface dichromate. After calcination at 720 degrees C, Cr is anchored to the surface without changing the chromate:dichromate ratio. Reduction results in the formation of octahedral Cr3+, pseudooctahedral and pseudotetrahedral Cr2+. A higher silica content of the support, a higher reduction temperature, and a higher calcination temperature result in a deeper reduction of the supported Cr. Recalcination restores the initial chromate:dichromate ratio on each support; however, small quantities of Cr3+ remain in the samples. This deeper reduction and reversibility can be explained by the low OH content of the supports calcined at 720 degrees C.