Impact of Pt dispersion on the elementary steps of CO oxidation by O2 over Pt/Al2O3 catalysts

The impact of Pt dispersion (denoted by D) of Pt/Al2O3 catalysts on the turnover frequency (TOF) of the CO/O-2 reaction and on the elementary steps involved in the catalytic reaction is studied using transient experiments with either a mass or a FTIR spectrometer as a detector. Similar to literature data, it is observed that TOF decreases with an increase in D. The aim of the present study is to correlate this observation with the modifications of the surface elementary steps of two kinetic models, M1 and M2, developed previously for D less than or similar to 0.6. The elementary steps considered are (1) adsorption of CO as a linear CO species (denoted by L and involved in models M1 and M2) and as a bridged CO species (denoted by 13); (2) the oxidation by O-2 of the adsorbed CO species; and (3) the reduction by CO of the strongly adsorbed oxygen species (denoted by O-sads) involved in model M2. It is shown that D has no significant impact on the heat of adsorption of the L CO species and on the mechanism of the reduction of O-sads species by CO. For oxidation of the L CO species by O-2, it is shown that there is an induction period in the CO2 production for D greater than or similar to 0.60 not observed for a lower dispersion. This is explained by considering that the rate of formation of the Pt sites, which adsorb O-2 during oxidation, depends on D. These sites, which represent a small fraction of the Pt-0 sites, are associated with the B CO species. It is shown that the heat of adsorption of the B CO species increases with an increase in D. It is suggested that it is the removal of the B CO species (by desorption and oxidation) that controls the induction duration. (C) 2004 Elsevier Inc. All rights reserved.