Reduction studies on supported metal oxide catalysts

Abstract

A procedure is described for determining the reduction characteristics of metal oxidepromoted catalysts. The method involves circulating a measured volume of hydrogen in a closed system that includes a suitable tube for the catalyst, an absorption tube for removing the water formed, and a manometer for measuring the consumption of hydrogen. Reductions may be conducted either at a definite temperature or the temperature may be increased on a fixed time schedule. In the latter case, regular temperature and pressure readings provide information necessary to plot “reduction profiles” in which the reduction rate is plotted versus temperature (or time). Characteristic curves are obtained for various promoter-support combinations and the areas below the “reduction profile” curves are proportional to the amount of reduction. However, the latter is obtained more precisely by accurate measurement of the hydrogen consumption. The procedure was used for studying the reduction characteristics of supported nickel oxide and chromium oxide catalysts as affected by methods of preparation, kinds of support, and the effects of heat treatment, temperature, and promoter concentrations. The results showed that promoter oxide-support oxide interaction increased the difficulty of reducing the promoter metal oxide. This effect was most pronounced for nickel oxide when supported on alumina and for chromium oxide supported on silica. Coprecipitated catalysts were more difficult to reduce than those prepared by impregnation, apparently because of better distribution and therefore better opportunity for interaction. For the same reason, higher temperatures of catalyst heat treatment resulted in increased difficulty of reduction. The interaction between the metal oxide promoter and the support oxide appears to have an important effect on the catalytic properties of metal oxidepromoted catalysts. This is illustrated by results obtained with chromium oxide—silicaalumina catalysts.