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 nearedge
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
Pt4C and Re7C 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 Re7C oxide
and Pt4C 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.