Evaluation of surface and bulk phases during oxychlorination/reduction cycles of Pt-Re catalysts

A 0.3 wt% Pt-0.3 wt% Re/Al2O3 catalyst and monometallic 0.3 wt% Pt reference have been studied by X-ray absorption near-edge structure (XANES), hydrogen chemisorption, and n-heptane reforming, at stages during a series of oxychlorination/reduction cycles. The initial precalcined, chlorine-free system contains separate Pt4+ and Re7+ oxide clusters, which are reduced through a Re intermediate, yielding a Pt-Re alloy Several chemical characteristics of the Re intermediate and the alloyed phase are revealed by the study. Subsequent oxychlorination of the reduced catalysts leads to the disruption of the alloy particles, producing initially Re7+ oxide and Pt4+ oxychloride species, with the latter eventually losing most of its chlorine ligands to form an oxide-like phase at higher treatment temperatures. The presence of chlorine influences the behavior of the subsequent reduction process, in particular, modifying the final properties of the zero-valent alloyed phase. The physical and chemical parameters affecting the Cl-induced changes are discussed on the basis of the XANES study. (C) 1999 Academic Press.