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Cerium-zirconium mixed oxides (Ce x Zr (1-x) O 2 ), precalcined at 900 °C in dry air, were supplied by Rhodia Terres Rares as monophasic solid solutions. Introduction of some zirconium atoms in the ceria lattice by isomorphous substitution clearly influences the final properties of these materials as long as the cubic structure of ceria is maintained. Modifications in oxygen storage capacity (OSC measurements), redox properties (CO TPR), and oxygen exchange processes (TPIE) were studied. Ce 0.63 Zr 0.37 O 2 was shown to have the most promising properties with the largest OSC at 400 °C and the highest reactivity in O 2 exchange. All mixed oxides are able to exchange very large amounts of oxygen compared to ceria, implying the participation of bulk oxygen. Furthermore, on Ce x Zr (1-x) O 2 samples, oxygen is predominantly exchanged via a multiple heteroexchange mechanism involving surface dioxygen species as superoxides or peroxides.

Silica-supported bimetallic Pt-Au catalysts prepared via different synthetic routes have been investigated in terms of their structural properties, adsorption of CO, and catalytic activity for the selective catalytic reduction of NO by propylene, the oxidation of propylene in the absence of NO, and the 16 O/ 18 O homoexchange reaction. Catalysts prepared by incipient wetness impregnation from individual Pt and Au precursors exhibited characteristics very similar to those of monometallic Pt catalysts, indicating that in these cases the presence of Au did not affect the catalytic performance of Pt in any significant way. This behavior is consistent with a model in which the two metals remain segregated due to their miscibility gap, and only Pt participates in the adsorption of CO and the reactions under consideration. In contrast, catalysts prepared from aPt 2 Au 4 (C≡C t Bu) 8 organo-bimetallic cluster precursor exhibited different behavior both in terms of CO adsorption and their catalytic activity for the three reactions examined. The combination of the kinetic, spectroscopic, and structural characterization data suggests that in this case Pt and Au remain intimately mixed in the form of Pt-Au bimetallic particles and that the presence of Au in these particles modifies the behavior of Pt.

Oxide-supported noble metal catalysts were tested in the preferential oxidation of carbon monoxide (PROX) reaction in the temperature range between 50 and 300 • C. Both the influence of the noble metal nature (Pt, Ir, Pd), the support physical and chemical properties (redox, acidity, basicity) and the reaction conditions (oxygen stoichiometry) on the catalyst activity and selectivity was evaluated. Platinum and iridium were shown to be the most active and selective catalysts in the whole temperature range compared with palladium. Furthermore, noble metals supported over ceria-based oxides were shown to be active and selective, especially at low temperature. Additionally, it was observed that the higher the molar fraction in ceria in the oxide, the higher the activity and the selectivity in the PROX reaction. Ceria, with the highest oxygen mobility at the oxide surface, was shown to be the best support. Accordingly, on simple oxides (CeO 2 , SiO 2 -Al 2 O 3 ,Al 2 O 3 , SiO 2 ,La 2 O 3 and MgO), the induced mobility of the oxygen atoms at the surface of the support determined elsewhere, well correlated with the basicity of the support, was shown to be one key parameter for the performances of the catalysts in the PROX reaction. Finally, the formation of water (hydrogen oxidation) at high temperature and high oxygen excess was shown to be responsible for the increasing activity of the catalysts in the conversion of CO to CO 2 via the water gas shift reaction (WGSR).

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