A QUANTITATIVE DIFFUSE REFLECTANCE SPECTROSCOPY STUDY OF SUPPORTED CHROMIUM CATALYSTS

The diffuse reflectance spectra of chromium supported on alumina, on silica, and on silica-alumina have been investigated after calcination, CO reduction, and recalcination. A method for quantifying the Cr6+, Cr3+, and Cr2+ concentrations was developed. After calcination at 550-degrees-C, the chromate:dichromate ratio is infinity, 0.56, and 2.18 on respectively alumina, silica, and silica-alumina. Reduction results in the formation of three new species: octahedral Cr3+, octahedral Cr2+, and pseudotetrahedral Cr2+. On silica and silica-alumina, the relative proportions of these three species depend on the reduction temperature. On alumina, only octahedral Cr3+ and small amounts of octahedral Cr2+ were observed after reduction. Recalcination reestablishes the chromate: dichromate ratio; however, this regeneration is complete only on silica. On alumina and silica-alumina, about one-third and two-thirds respectively of the total Cr content remain as Cr 3+. The sensitivity of Cr6+ to reduction is greater on silica than on alumina and silica-alumina. This is indicated by the increase of the Cr2+:Cr3+ ratio from alumina over silica-alumina to silica.