In situ DRIFTS study of the effect of structure (CeO2–La2O3) and surface (Na) modifiers on the catalytic and surface behaviour of Pt/γ-Al2O3 catalyst under simulated exhaust conditions

The nature and relative populations of adsorbed species formed on the surface of un-promoted and sodium-promoted Pt catalysts supported either on bare Al₂O₃ or CeO₂/La₂O₃-modified Al₂O₃, were investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) under simulated automobile exhaust conditions (CO + NO + C₃H₆ + O₂) at the stoichiometric point. The DRIFT spectra indicate that interaction of the reaction mixture with the Pt/Al₂O₃ catalyst leads mainly to formation of formates and acetates on the support and carbonyl species on partially positively charged Pt atoms (Pt^δ+). Although enrichment of Al₂O₃ with lanthanide elements (CeO₂ and La₂O₃) does not significantly modify the carboxylate species formed on the support, it causes significant modification of the oxidation state of Pt, as indicated by the appearance of a substantial population of carbonyl species on reduced Pt sites (Pt⁰–CO). This modification of the Pt component is enhanced when Na-promotion is used, leading to formation of carbonyl species only on electron enriched Pt (i.e., fully reduced Pt⁰ sites) and to the formation of NCO on these Pt entities (2180 cm⁻¹). The latter are thought to result from enhanced NO dissociation at Na-modified Pt sites. These results correlate well with observed differences in the catalytic performance of the three different systems.