Effects of oxidation/reduction and oxychlorination/reduction cycles on CO adsorption by Pt-Re/Al2O3 catalysts

Infrared spectra of CO adsorbed on Pt-Re/Al2O3 after individual steps in sequences of oxidation, oxychlorination, and reduction treatments are compared with corresponding spectra for Pt/Al2O3 and Re/Al2O3, and with H-2 chemisorption data. Chloro-Pt and oxychloro-Pt complexes were formed independently of the presence of Re during oxychlorination and contributed to a Cl-dependent enhanced dispersion of Pt in Pt-Re/Al2O3. After a given treatment the dispersions of Pt in Pt/Al2O3 and Pt-Re/Al2O3 were similar but the Hz uptakes were considerably decreased by the addition of Re. In general, reduced Pt-Re/Al2O3 contained very small bimetallic particles with Re surface enrichment. Small ensembles of high coordination Pt-0 atoms were retained, although Re atoms may have been located at both ensemble and edge sites. Surface sintering after repeated oxidation/reduction cycles led to an enhanced coverage of bimetallic particles by Re, the effect being reversed by oxychlorination/reduction cycles. (C) 2000 Academic Press.