Redispersion of platinum on alumina support

Abstract

The increase in metal dispersion of several Pt Al 2O 3 catalysts, following a heat treatment in air or oxygen at 500 °C, has been studied as a function of sintering conditions, support pretreatment, and chlorine content. The changes in metal dispersion were followed by hydrogen chemisorption measurements. Heating the catalysts in H 2 at 600 °C resulted in a slow decrease in metal dispersion and simultaneous loss of chlorine was observed, which depended on the water content of the gas stream. A limiting value of dispersion was obtained after prolonged sintering (≥100 hr) and this behavior is consistent with a crystallite migration mechanism. Hydrogen sintered samples were easily redispersed by air treatment at 500 °C, the increase being dependent on the sintering time and the residual level of chlorine. Successive sintering (H 2)-redispersion (air) cycles performed on a given sample indicated a slow decrease in the final dispersion, particularly for heavily sintered samples. This trend was reversed by restoring the original level of chlorine. A metal-support interaction involving the platinum atoms and oxidizing sites on the alumina is postulated to explain these results. Heating the catalysts in air at >580 °C caused a very rapid decrease in metal dispersion, compared to hydrogen, and the presintered catalysts were redispersed by oxygen treatment at 500 °C. In this case the redispersion process was found to depend on time and the final dispersion actually exceeded the original value. These observations suggest that the redispersion of Pt Al 2O 3 catalysts occurs by different mechanisms depending on the sintering atmosphere. The origin of the alumina or the support pretreatment did not influence the redispersion process.