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 H2 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 H2 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
Pt0 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.