Ceria-based oxide catalysts

2 Ceria-based oxide catalysts 14.2.2.1 CoOJCeOz system 

Of the ceria-based oxide catalysts, mixed ceria cobalt and mixed ceria-copper have been the most extensively investigated. 

Another interesting composite system based on ceria are CuO/CeOg mixed oxides. This system has been widely studied as a replacement for the more expensive noble-metal systems and, with respect to the cobalt system, it has the advantage of being environmentally greener 

Because of the excellent catalytic performance of the ceria-based C03O4 and CuO catalysts, a combination of the two systems was investigated by preparing, through co-precipitation, mixed Cu-Co-Ce-O oxides with different compositions. The mixed oxides had a larger surface area with respect to the Co-Ce-O and Cu-Ce-O composite oxides. Moreover, the addition of cobalt to the Cu-Ce-O catalyst enhanced thermal stability because the particle size was unchanged on calcination at 850 °C. According to the H2-TPR patterns, the peaks due to cobalt oxide reduction and those for copper oxide reduction were reciprocally affected and shifted towards lower temperatures. The interaction between cobalt and 

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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. 

High Resolution Electron Microscopy (HREM) has proven as a very useful technique in the structural characterisation of supported metal catalysts (383-386) in general and, in particular, of noble metal catalysts supported on ceria-based oxides (52,70,72,97,105,109,117,124,135,137,139,144,147,155,171,182-184.194.203,209, 210,218,226,234,235,387) ... 

Develop synthesis methods for highly active water-gas-shift (WGS) catalysts based on platinum supported on nanoscale ceria-based oxides. 

Ceria first, and since the mid 1990s ceria-zirconia mixed oxides, are key components in the formulation of TWCs [18,19]. A variety of functions are attributed to them, those related to their redox properties being particularly relevant [247]. Under the usual TWC operation conditions, the chemical composition of the exhaust gases rapidly oscillates between net reducing and net oxidizing conditions [8]. This implies deviations from the optimum stoichiometric air to fuel (A/F) ratio (A/F = 14.63), and therefore, loss of efficiency in the auto-exhaust catalyst. To attenuate these oscillations, oxygen buffer materials are required, ceria-based oxides constituting the best option at present available [19]. 

Ahfanti, M., Florea, M. and Parvulescu, V. (2007). Ceria-based Oxides as Snpports for LaCo03 Perovskite Catalysts for Total Oxidation of VOC, Appl. Catal. B Environ.,7(l, pp. 400 05. 

More recently, Zhang et al. summarized the catal5dic performance and reaction conditions of topical catalysts for the ODH of ethane with CO2 (Table 26.4). It may be seen that they belong to modifications of the three previously described catalytic systems transition metal (Mn, Fe, Cr) oxides, ceria-based oxides, and gallium oxide catalysts. The latter have received very little attention, probably due to their high cost. Those based in chromium oxide are the more active at the low temperatures (below 700°C), and hence the most widely studied. 

In this section, recent progress in the chemical characterization of conventional cerium-containing oxide-supported gold powder catalysts will be briefly discussed. We will focus our attention on Hg and CO chemisorption studies. There are several reasons justifying this choice. As discussed in Bernal et Hg and CO are by far the most commonly used probe molecules in the characterization of noble metals supported on ceria and closely related mixed oxides. Also very importantly, ceria-based gold catalysts are known to be highly active materials for CO oxida-... 

Ceria is an oxide widely employed in catalysis for its redox properties. It also has weak Lewis acid sites and stronger basic sites, which makes this oxide suitable as a catalyst or a support for many reactions requiring acid-base pairs or basic sites. Combining redox and acid-base properties allows ceria-containing solids to catalyze numerous reactions involved in biomass transformation. The objective of this chapter is to review these reactions stressing the role of ceria-based solid catalysts in the conversion of oxygenated molecules, which are essential compounds in biomass. 

One approach to overcoming the limitations of nickel anodes, which has met with some success, is to augment the oxidation activity of Ni/YSZ cermets through the addition of an oxide-based oxidation catalyst. For example, stable operation on dry methane has been reported at 650°C in an SOFC using an yttria-doped ceria interlayer between the YSZ electrolyte and the Ni/YSZ cermet anode [61]. Ceria is a well-known oxidation catalyst, and might be expected to increase the activity of the anode for the electrochemical oxidation of methane. This approach still requires, however, that the operating temperature be maintained below 700°C to suppress carbon deposition reactions that take place rai nickel. 

Ceria-based OSC compounds may have an impact on oxidation reactions especially when the catalysts are working around the stoichiometry (as this is the case under TW conditions). One of the first systematic studies was reported by Yu Yao [53,54], Most results were obtained in 02 excess (0.5% CO + O.5% 02 or 0.1% HC+ 1% 02). Several series of Pt, Pd and Rh/Al203 of various dispersion, as well as metal foils, were investigated in CO, alkane and alkene oxidation. The effect of metal dispersion in CO and the propane oxidation are shown in Figure 8.5. 

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... 

Catalysts. - Group VIII metals, conventional base metal catalysts (Ni, Co, and Fe) as well as noble metal catalysts (Pt, Ru, Rh, Pd) are active for the SR reaction. These are usually dispersed on various oxide supports. y-Alumina is widely used but a-alumina, magnesium aluminate, calcium aluminate, ceria, magnesia, pervoskites, and zirconia are also used as support materials. The following sections discuss the base metal and noble metal catalysts in detail, focusing on liquid hydrocarbon SR for fuel cell applications. 

For the Selox reaction, the oxidation mechanism may involve oxygen species that are stored in/over the catalyst support, such the ceria-based materials used in three-ways catalytic exhaust systems [16]. Consequently, the oxygen storage capacity (OSC) can also be considered as an objective function, if one assumes or dem-... 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

Related systems It should be noted that specific properties for applications could be enhanced by using solid solutions, doped materials, and composites, instead of pure ceria. For example, ceria-zirconia solid solution is a well known ceria based material for enhanced OSC and high ionic conductivity for solid state fuel cell components. It is also used in the three way catalysts for automobile waste gas cleaning, because of the improved thermal stability, surface area, and reducibility. The synthesis, structure, and properties of ceria-zirconia have been actively studied for a long time. Di Monte and Kaspar et al. presented feature articles on the nanostructured ceria—zirconia-mixed oxides. The studies on phase, structures, as well as the microstructures are discussed and reviewed (Di Monte et al., 2004). 

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. 

Ceria-based systems showed mixed effects for methane oxidation. Composite catalysts of Ag/Ce02 fall apart, forming large silver metal aggregates and deactivating the catalyst system (38). The only system in which silver-modified ceria found any promise is in solid oxide fuel cells utilizing yttira-stabilized zirconia however, the silver-based system was not the optimum one in this case (39). 

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