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/Al 2 O 3 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 ≲ 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 B); (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 CO 2 production for D ≳ 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.