Oxidation 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 structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

Development of stable bimetallic catalysts for carbon dioxide reforming of methane

CuO-CeO2 catalysts have been proposed as a promising candidate catalytic system for CO removal from reformed fuels via selective oxidation. In this work, the performance of CuO-CeO2 catalysts, synthesized via the urea–nitrate combustion method, in the reaction of selective CO oxidation in the presence of H2 has been investigated. The combustion method was found to be a simple and fast route for the synthesis of ultrafine, nanocrystalline CuO-CeO2 catalysts. The influence of the fuel/oxidant (urea/nitrate) ratio and the Cu content on the catalytic properties of CuO-CeO2 catalysts has been studied and optimal values of these parameters have been determined. Compared to CuO-CeO2 catalysts prepared with other techniques, the catalysts prepared via the combustion method exhibited similar catalytic performance, remaining very active and stable, remarkably selective and with good tolerance towards CO2 and H2O.

Selective CO oxidation over CuO-CeO2 catalysts prepared via the urea-nitrate combustion method

The mitigation and utilization of greenhouse gases, such as carbon dioxide and methane, are among the most important challenges in the area of energy research. Dry reforming of CH4 (DRM), which uses both CO2 and CH4 as reactants, is a potential method to utilize the greenhouse gases in the atmosphere. Natural gas containing high concentrations of CO2 and CH4 could therefore be utilized for hydrogen and synthesis gas (syngas) production in the near future, without need for the removal of CO2 from the source gas. Thus, the DRM reaction is a suitable process to convert CH4 and CO2 to syngas, which is a raw material for liquid fuel production, through the Fischer–Tropsch process. Herein, the development of CO2 reforming for syngas production is reviewed, covering process chemistry, catalyst development, and process technologies as well as the potential future direction for this process.

Catalytic Technology for Carbon Dioxide Reforming of Methane to Synthesis Gas

CuO–CeO 2 catalysts were prepared via a urea combustion method and their performance in the oxidation of ethanol, ethyl acetate and toluene was evaluated. XRD, H 2 -TPR and N 2 physisorption were employed in catalyst characterization. The specific surface area of mixed materials was higher than the one of single oxides. In ceria-rich materials, crystalline copper oxide phases are absent and segregation of a CuO phase takes place at atomic Cu/(Cu + Ce) ratios higher than 0.25. The mixed oxides get reduced by H 2 at lower temperatures than the corresponding single oxides and copper ions promote reduction of ceria. Ethanol gets more easily oxidized than ethyl acetate, which in turn gets more easily oxidized than toluene. CuO–CeO 2 catalysts of low copper content produce very low amounts of acetaldehyde during ethanol and ethyl acetate oxidation at all conversion levels. This is augmented by the presence of water in the feed. The specific activity of Cu x Ce 1− x catalysts in the oxidation of ethanol, ethylacetate and toluene (specific rate of volatile organic compound (VOC) consumption) is lower than the one of pure CuO and CeO 2 , i.e. combination of the two phases leads to suppression of intrinsic activity. On the other hand, the specific rate of CO 2 production during ethanol and ethyl acetate oxidation is also lower over CuO–CeO 2 than over CeO 2 , but higher than over CuO. The larger surface area of CuO–CeO 2 catalysts counterbalances their smaller specific activity allowing complete VOC conversion at lower temperatures compared to the single oxides.

VOC oxidation over CuO–CeO2 catalysts prepared by a combustion method

Yttria-doped ceria (YDC) and pure ceria (CeO2), respectively, were deposited on γ-alumina (γ-Al2O3) using the impregnation method; then, copper oxide was also supported on them by employing the impregnation method. For comparison, CuO/γ-Al2O3 catalysts were prepared in this work. The catalysts were characterized by temperature-programmed reduction (TPR) and X-ray diffraction (XRD). For CuO/γ-Al2O3 catalysts, two TPR peaks, namely β and γ, were observed. These have been attributed to the reduction of highly dispersed copper oxide species and bulk-like copper oxide, respectively. For CuO/CeO2/γ-Al2O3 and CuO/YDC/γ-Al2O3 catalysts, four TPR peaks, namely α1, α2, β′ and γ′, could be observed. The α peaks with lower peak temperatures as compared to those of β′ and γ′ peaks have been attributed to the reduction of interface-boundary copper oxide species that contact closely and interact strongly with the supported ceria or YDC. Crystal sizes calculated from XRD measurements confirmed that yttria (Y2O3) addition could lead to crystal growth of ceria and correspondingly enhance the dispersion of the supported copper oxide due to the partition of YDC crystallite. Hence, this work shows that supported YDC and ceria can act bi-functionally as a textural promoter as well as a structural promoter.

TPR and XRD studies of yttria-doped ceria/γ-alumina-supported copper oxide catalyst

Synergistic Catalysis of Carbon Monoxide Oxidation over Copper Oxide Supported on Samaria-Doped Ceria

Carbon dioxide reforming of methane was studied over Ni supported on ceria and yttria-doped ceria (YDC) to seek insight into the effect of catalysis at the metal–support interface. In contrast to the performance of Ni/γ-Al 2 O 3 , which displayed very rapid deactivation with time on stream, the rate over the ceria- or YDC-supported catalysts exhibited exceptional enhancement of activity and coking resistivity. The extent of enhancement decreased with increased yttria loading. The enhancement of activity and coking resistivity of the ceria-based catalysts, coupled with the results of temperature-programmed reduction and hydrogenation, suggested the formation of interfacial active centers and the existence of metal–support interaction, which decreased with increased yttria loading. EPR and XPS results provided evidence that the ability of CeO 2 to lose its lattice oxygen to form Ce 3+ was inhibited with yttria doping. The lattice oxygen of ceria is inferred to have played some positive role in the activation of both CH 4 and CO 2 ; such promotion is attributed in part to the availability of Ce 3+ species at the metal–support interface.

Study of ceria-supported nickel catalyst and effect of yttria doping on carbon dioxide reforming of methane

The selective oxidation of CO in a H2-rich environment was studied over copper oxide supported on samaria-doped ceria (SDC). Activity tests, as well as H2- and CO-temperature programmed reduction (CO-TPR), were carried out to seek insight into the effects of metal/support interaction occurring in the reaction. The interfacial active centers at the metal/support interfaces are inferred to be the major active sites of the reaction. By comparing the selective CO oxidation in Ar and in an atmosphere rich in H2, we found that the conversion of CO was inhibited by H2 at temperatures higher than 90 °C. In the H2-rich atmosphere, the oxidation of CO and H2 picked up considerably at 80 and 130 °C, respectively, due to differences in the reactivity of CO and hydrogen toward interfacial oxygen ions. Once bulk copper oxide is reduced by CO or H2, the metastable states formed on the surface of the bulk CuO may serve as active sites, in addition to interfacial active centers, for the selective CO oxidation in excess H2. CO has higher reactivity toward interfacial oxygen ions. Besides the interfacial active centers, the metastable copper clusters may also contribute to the occurrence of hysteresis in H2 oxidation. Selectivity of the CuO/SDC catalyst results mainly from differences in the reactivity of CO and H2 with these oxygen ions. By preparing SDC supports and CuO/SDC catalysts with different surface areas and dispersions, it was found that, although the better-dispersed catalysts exhibited higher CO conversions, more pronounced hydrogen oxidation was observed with these catalysts, thereby resulting in severer decline in selectivity.

Ta-Jen Huang

Papers

TPR and XRD studies of yttria-doped ceria/γ-alumina-supported copper oxide catalyst

Synergistic Catalysis of Carbon Monoxide Oxidation over Copper Oxide Supported on Samaria-Doped Ceria

Study of ceria-supported nickel catalyst and effect of yttria doping on carbon dioxide reforming of methane

Selective CO oxidation in rich hydrogen over CuO/samaria-doped ceria

Effects of Oxygen Vacancy of Yttria-Stabilized Zirconia Support on Carbon Monoxide Oxidation over Copper Catalyst