International audienceOxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structure

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Temperature-programmed isotopic exchange of 18O2 with 16O of several oxides was carried out in the 200−900 °C temperature range. The oxides can be ranked according to their maximal rates of exchange obtained at the following temperatures:  CeO2, 410 °C ≫ CeO2-Al2O3, 480 °C ≈ MgO, 490 °C > ZrO2, 530 °C ≫ γ-Al2O3, 620 °C ≫ SiO2, 850 °C. Except on CeO2 and on CeO2-Al2O3 a simple exchange yielding initially 18O16O can be observed. With ceria containing oxides, the reaction occurs in part via a multiple exchange mechanism yielding initially 16O2 which is indicative of the presence of binuclear species (O2, O2 -, or O2 2-) at the ceria surface. Chlorine-free rhodium catalysts supported on these oxides were prepared with metal dispersions between 32 and 89%. The presence of rhodium accelerates considerably the oxygen exchange with the support:  the maximal rates of the exchange can be observed at much lower temperatures, by about 200−300 °C with respect to the bare oxides. This is attributed to a spillover of oxygen from the rhodium particles to the support. Isotopic exchange experiments carried out at temperatures (300−400 °C) at which the direct exchange is negligible allow for calculation of the coefficient of surface diffusion of oxygen on the oxides. At 400 °C, the relative mobility of oxygen (base 100 for γ-Al2O3) is CeO2, 28 100 ≫ MgO, 500 > ZrO2, 280 > CeO2-Al2O3, 180 > γ-Al2O3, 100 ≫ SiO2, 1.7. Oxygen mobility can be paralleled with the surface concentration of basic sites measured by CO2 chemisorption (sites per nm-2):  CeO2, 3.23 > MgO, 1.77 > ZrO2, 1.45 > CeO2-Al2O3, 0.44 > γ-Al2O3, 0.17 > SiO2, ≈ 0. Actually, the basicity of CeO2 cannot alone explain the exceptional mobility of oxygen on this oxide, due to a large part to the presence of oxygen vacancies. Above 400 °C, bulk oxygen diffusion can be observed on CeO2, ZrO2, γ-Al2O3 and CeO2-Al2O3. Ceria possesses a very high internal mobility. The coefficient of bulk diffusion of oxygen in ZrO2 is about two orders of magnitude higher than in γ-Al2O3, which contrasts with the relatively close values of their surface mobility. Except for CeO2, there is a good correlation between this surface mobility and the metal−oxygen bond strength in the oxide crystal.

Ceria has been the subject of thorough investigations, mainly because of its use as an active component of catalytic converters for the treatment of exhaust gases. However, ceria‐based catalysts have also been developed for different applications in organic chemistry. The redox and acid–base properties of ceria, either alone or in the presence of transition metals, are important parameters that allow to activate complex organic molecules and to selectively orient their transformation. Pure ceria is used in several organic reactions, such as the dehydration of alcohols, the alkylation of aromatic compounds, ketone formation, and aldolization, and in redox reactions. Ceria‐supported metal catalysts allow the hydrogenation of many unsaturated compounds. They can also be used for coupling or ring‐opening reactions. Cerium atoms can be added as dopants to catalytic system or impregnated onto zeolites and mesoporous catalyst materials to improve their performances. This Review demonstrates that the exceptional surface (and sometimes bulk) properties of ceria make cerium‐based catalysts very effective for a broad range of organic reactions.

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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.

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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.