Other Ceria Based Catalysts

There is not much work on Ce02 based catalysts containing elements other than Mn or precious metals. Imamura et al. reported that Co/Ce composite oxide with 13 mol% of Ce [Co/Ce(87/13)] is active for the oxidation of ammonia, although details are not known [25]. 

Hocevar et al. investigated the action of Ce1.xCuxO2.s8 (0 05 x 0.20) for phenol oxidation (reaction temperature 150°C) encouraged by the fact that this kind of catalyst is active in the vapor-phase oxidation of organic pollutants and also by the fact that de Leitenburg et al. [33] used Ce02-Zr02-Cu0 catalyst in the wet-oxidation of acetic acid [58]. The activity and stability of this catalyst depend much 

However, there is much that remains unknown and in particular it is not known how the above phenomenon (Eq. 14.12) reflects the catalyst performance. Fig. 14.10 shows the change in the rate constant k (mol s ) of the oxidation of phenol with the change in the Cu content. The rate constant (k) linearly increases with the Cu content regardless of the calcination temperature of the catalysts. Thus, the active component in CCi Cup g catalysts is Cu. In the subsequent work, they reported that sol-gel technique is better than co-precipitation method, and high dispersion of copper oxide phase on the cerium oxide causes high catalytic activity. They emphasized the importance of the combination of the mixed valence state of Ce and Ce caused by an incorporation of Cu, which causes the reversible addition and removal of oxygen in this inherently defect structure. Here, the defect structure in CeOj is also emphasized. However, their discussion is rather sophisticated and their claim on the inherent function of this catalyst is difficult to be understood. The clear point is that copper in this catalyst is considerably active and durable in the wet-oxidation condition due to the effect of CeO,. 

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Ceria-promoted Precious Metal Catalysts J4.2.4. Other Ceria Based Catalysts ... 

Other methods The traditional methods, such as gas condensation, sol-gel methods, precipitation with postheating methods, and mechanical milling methods are widely used for the synthesis of nano-crystalline ceria materials and ceria based catalysts (Fu et al., 2001 ... 

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. 

In addition, FBC [21] technology provides constant and continuously fresh ceria-based catalyst to the soot layer. This explains why, unlike many other catalyst-based DPF-regeneration technologies, the ceria-based fuel-borne catalyst is relatively insensitive to fuel-sulfur levels and is able to function with fuel containing over 2,500ppm of sulfur, as demonstrated in marine and stationary applications. However, in the presently described automotive application, the permissible level of sulfur is limited by the sulfur-sensitivity of other components of the complete DPF system. 

The active phase of CuO-CeOg WGS catalysts was also investigated. The results of in situ time-resolved XRD (Fig. 6.5) and XAS (not shown) point to a complete reduction of the CuO to metallic Cu before the catalysts become active for the WGS. This reduction of the CuO is consistent with the behavior observed for CuO/ZnO catalysts and other Cu-based catalysts.As in the case of AuOx-CeOg, the active phase of CuO-CeOg catalysts contains nanoparticles of the noble metal disperse d on a partially reduced ceria support. The same is valid for Pt-CeOg and Pd-CeOg catalysts. 

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

On the other hand, cerium has been shown to be an effective oxygen reservoir, enhancing the activity of many oxidation catalysts. Due to this property, cerium oxide is also considered to potentially enhance the thioresistance of the catatysts. This aspect is of great practical importance, since the use of palladium catalysts is hindered by the poisonous effect of sulphur compoimds, often present in off gases. Most works dealing with ceria-zirconia catalysts have been carried out with catalysts prepared by coprecipitation methods, whereas in this work an ahemative procedure, based on the incipient wetness technique is used to incorporate ceria to the zirconia support. The aim is to maintain the advantages of zirconia supports, especially the thermal stability. 

Early reports on this reaction were focused on iron oxide-based catalysts, supported on different supports alumina, zeolite, and active carbon, among others. Later, hydrotalcite-like compounds, and the topical V-Mg-0 catalysts were explored. Park and his group, one of the most active in this field, first reported the use of zirconia as a catalyst, " and later on that of ceria-zirconia-based catalysts and vanadium-antimony oxide-based catalysts. Publications up to 2007 have been subject of several reviews. Therefore, the most relevant feamres will be revised hereinafter, as well as more recent publications, and the reader is invited to get further details in the aforementioned reviews. 

In this chapter, two catalyzed reactions with practical relevance have been selected to provide an overview of the interaction of nitrogen oxides (NgO, NO, and NO2) with ceria-based materials. The first is the catalytic decomposition of NgO on Rh/ceria catalysts, which is relevant for N2O pollution control, and the other is the ceria-catalyzed oxidation of NO to NO2, which is involved in soot combustion in diesel exhausts. 

Usually catalysts for NO removal must deal with NO + O2 mixtures, as in the de-NOx process for diesel vehicles or in coal-combustion power plants. In contrast to NgO, NO s interaction with ceria-based materials (and many other catalysts) in Og-rich streams does not lead to the decomposition of the molecule but to oxidation to NOg. This process involves NO chemisorption and the formation of several surface nitrogen species, oxidation of these surface species, and desorption of NOg. In addition, NOg can be re-adsorbed on the catalyst after release. All these processes are discussed in this section. 

The performance of acceptor-doped ceria membranes, composite ceria-based membranes, and membranes with redox active dopants was reviewed in Sections 12.6.4.2, 12.6.4.3, 12.6.4.4, and 12.6.4.5, respectively. Variations in sample geometry, membrane thickness, as well as test conditions make comparison of performance somewhat difficult. Also for some studies, catalysts have been added to the surface of the samples to enhance the flux, whereas in other studies the as-prepared surface is used in the experiments. However, in particular, studies of thin-film membranes, both the... 

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