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 (Ptδ+). 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.