The roles of chlorine and sulfur in bimetallic Pt-Re/Al 2O 3 reforming catalyst

Abstract

Factors affecting rhenium reduction and the degree of bimetallic cluster formation were studied for Pt-Re/Al 2O 3catalysts. Temperature programmed reduction (TPR) and X-ray absorption near edge structure (XANES) were used to examine Pt catalyzed reduction of Re following a series of thermal pretreatments and suggest that chloride on the Al 2O 3 support plays a major role in Pt-Re alloying during reduction. A critical review of published data on the role of chloride and sulfur and their interaction with both the precursors and metal cluster to form an active reforming catalyst is also presented. To minimize yield-octane loss via hydrocracking, hydrogenolysis and low activity, the active metal sites of Pt-Re/Al 2O 3 catalysts must be carefully constructed so as to maximize the number of well-dispersed Pt-Re alloy clusters and essentially eliminate the presence of isolated Re atoms. Alloy clusters are formed by the catalytic reduction of Re species by immobile Pt°, and the Re species must be mobile on the alumina surface. Two mobile Re species are believed to exist, a hydrous Re oxide and a Re oxychloride. Pt°-catalyzed reduction of hydrous Re oxide, a wet reduction, is undesirable for two reasons. Water strips chloride from the alumina and can remove Re from Pt-Re alloy clusters. For the preferred dry reduction of a Re oxychloride to be successful, it is critically important that there be adequate Cl to provide Re mobility. Experimental data generated by TPR and XANES support this reasoning. The function of the S added to Pt-Re/Al 2O 3 catalyst is to confine activity of the Pt to hydrogenation- dehydrogenation. It is shown that the amount of irreversibly adsorbed sulfur, S (irr) is an equilibrium value dependent on temperature and H 2S partial pressure. However, there must be enough S (irr) present to sulfide all the Re and about 10%of the Pt at the stoichiometry of Pt 2S and Re 2S. If the S (irr) is too low, either through undersulfiding or overstripping, catalyst performance suffers. Reversibly adsorbed sulfur, S (rev), is also an equilibrium value dependent on temperature and H 2S partial pressure. Activity maintenance of Pt-Re/Al 2O 3 catalysts is decreased by S (rev). As the Re to Pt atomic ratio increases, adverse sensitivity of the catalyst to S (rev) also increases.