Ceria, oxidation catalyst

Although the catalyst did not recover in the reformate stream, the authors did indicate that the carbonate species could be removed by air treatment at temperatures > 400 °C. Ruettinger and coworkers452 reported on the mechanism of aging of Pt/ceria-zirconia mixed oxide catalysts. In that case, the authors followed the activity of a 2% Pt/Ce02-Zr02 catalyst at 228 °C in a feed containing 5.92% CO, 7.4% C02, 31.82% H2, 28.86% N2, and 26% H20, and observed the deactivation data tabulated in Table 86. 

Results for the cell with a Cu—molybdena—YSZ anode, shown in Figure 16b, were very different. Unlike ceria, which is a nonselective oxidation catalyst, molybdena is a selective catalyst for the partial oxidation of propylene to acrolein (CH2=CHCHO) and is used commercially for this process. The primary product at low conversion over the Cu— molybdena—YSZ electrode was acrolein, produced according to the reaction... 

As discussed in previous sections, Cu acts primarily as an electronic conductor within the Cu-based anodes. Because it is a poor catalyst for C—H and C—C bond scission, it is essential to incorporate an oxidation catalyst, ceria, within the anode. While Ni has many attractive properties, its propensity for catalyzing carbon formation prevents its use in dry hydrocarbons at high temperatures. One approach for enhancing the catalytic properties of Cu and stabilizing the tendency of Ni for forming carbon is to use Cu—Ni alloys. Cu—Ni alloys have been used... 

The redox properties of ceria-zirconia mixed oxides are interesting, because these materials find applications as electrolytes for solid oxide fuel cells, supports for catalysts for H2 production, and components in three-way automobile exhaust conversion catalysts. The group of Kaspar and Fornasiero (Montini et al., 2004, 2005) used TPR/TPO-Raman spectroscopy to identify the structural features of more easily reducible zirconia-ceria oxides and the best method for their preparation by suitable treatments. TPR/TPO experiments and Raman spectra recorded during redox cycles demonstrated that a pyrochlore-type cation ordering in Ce2Zr2Og facilitates low temperature reduction. 

The selective oxidation or preferential oxidation of CO in hydrogen-rich stream is another important object for ceria based catalysts. The gas mixture from steam reforming/partial oxidation of alcohols or hydrocarbons, followed by the WGS reaction contains mainly FI2, CO2 and a small portion of CO, H2O, and N2. When such gaseous stream would be taken as input for hydrogen fuel cells, the CO has to be removed to avoid poisoning of the anode electrocatalysts. Ceria based nanomaterials, such as ceria/gold, ceria/copper oxide catalysts exhibit suitable catalytic activities and selectivities for CO PROX process. 

Other oxidation/reduction related reactions are also explored with ceria based catalysts. For example, Murugan and Ramaswamy (2007) reported the oxidative dehydrogenation of ethylbenzene on nanocrystal-line ceria using N2O as the oxidant Concepcion et al. (2004) reported the chemoselective hydrogenation of crotonaldehyde catalyzed by Ft on mesostructured Ce02 NPs embedded within layers of Si02 binder. 

The air pollutants of volatile organic compoimds emitted from many industrial processes and transportation activities could be abated by catalytic combustion processes. Scire et al. reported the catalytic combustion of 2-propanol, methanol, and toluene on ceria-gold catalysts. The catalysts were prepared with coprecipitation and deposition-precipitation methods. The gold significantly enhanced the catalytic activity of ceria for the oxidation of these volatile organic compounds. The supposed reason is that the gold NFs weakened the mobility/reactivity of surface lattice oxygen (Scire et al., 2003). 

Ceria-based catalysts are intensively used because of their high chemical and physical stability, high oxygen mobility and high oxygen vacancy concentrations, which are characteristic of fluorite-type oxides. The possibility of cycling easily between reduced and oxidized states (Ce Ce" ) permits the reversible addition... 

Of major concern for ceria based catalysts is the reducibility of the ceria support. Several papers mention experiments related to this subject. For model surfaces it is possible to quantitatively determine an average oxidation state of the ceria film, i.e. the ratio of Ce / Ce This is most commonly done using XPS of the Ce 3d photoemission features. The 3d spectrum is complicated by satellite features which are well described and provide useful variation with Ce oxidation state. ... 

Even though OSC is an inherently transient phenomenon, it appears that there is a relationship between steady-state reaction rates and OSC [3,17J. For the CO-oxidation, WGS, and steam-reforming reactions, it has been shown that rates can be enhanced by contact between the precious metals and ceria. Furthermore, high-lemperature treatments, which are known to deactivate the OSC properties of pure ceria, also remove the promotional effects associated with ceria [3,20,221, Given that SO2 affects OSC, one should expect SOt to influence the steady-state behavior of ceria-supported catalysts, if OSC is related to these reactions. 

Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38]. For example, ceria films calcined above 1270 K no longer promote the WGS [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. 

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