Effect of Steam on the Coking and on the Regeneration of Metal Catalysts: A Comparative Study of Alumina-Supported Platinum, Rhenium, Iridium and Rhodium Catalysts.

Abstract

The effect of steam on coking by cyclopentane (H 2 O injected during coking) and on the regeneration (H 2 O injected after coking) of alumina-supported Pt, Re, Ir and Rh catalysts was studied. The mechanism of coke formation on Ir, Re and Rh is similar to the mechanism proposed for Pt (dehydrogenation on the metal, polymerization of cyclopentadiene on the support, desorption of H 2 via a reverse spillover). Nevertheless the large differences observed in the dehydrogenation activities of the four metals parallel significant changes in coking rates (Pt>Rh=Ir> >Re). When injected during coking, steam decreases the amounts of coke deposited on the support. Regenerative treatments in steam of coked Pt and Re catalysts gasify essentially the carbon deposited on the metal. The coke produced on Ir and Rh is very reactive with steam which will then gasify a significant part of the carbon deposited on the support. These results can be explained by the differences, on the coked catalyst, of the coke structure, the coke location and the mobility of the oxygen species, the latter determined by complementary experiments of 18 O/ 16 O isotopic exchange.