Ceria metals

The next libraries were designed step-wise on the basis of knowledge gained from previous ones. As such, the second, third and fourth libraries were based on platinum-ceria, metal oxide-ceria and copper-ceria formulas, respectively, each of them using a similar DoE model for their design [18]. 

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. 

B. Harrison, A. Diwell and C. Hallett, Promoting platinum metals by ceria metal-support interactions in autocatalysts, Platinum Met. Rev., 32, 73-83 (1988). 

Penner S, Wang D, Su DS, RupprechterG, Podloucky R, SchloglR, HayekK(2003) Platinum nanocrystals supported by sUica, alumina and ceria metal-support interaction due to high-temperature reduction in hydrogen. Surf Sci 532 276-280... 

CeOg nanopartides deposited on Rh (111) Pt(111) Cu (111) and Au(lll) are much easier to reduce than extended surfaces of bulk CeOg. Oxygen vacancies can be produced by annealing the supported ceria nanopartides in vacuum at elevated tempera-tures or by direct reaction with CO or The reducibility of the ceria/metal systems can be an important property since bulk ceria exhibits a low reactivity towards many molecules and is chemically active only after the generation of O vacancies. Thus, in the ceria/metal systems one could couple the special reactivity of the oxide nanopartides to the reactivity of the metal to obtain highly active catalysts. 

In addition to platinum and related metals, the principal active component ia the multiflmctioaal systems is cerium oxide. Each catalytic coaverter coataias 50—100 g of finely divided ceria dispersed within the washcoat. Elucidatioa of the detailed behavior of cerium is difficult and compHcated by the presence of other additives, eg, lanthanum oxide, that perform related functions. Ceria acts as a stabilizer for the high surface area alumina, as a promoter of the water gas shift reaction, as an oxygen storage component, and as an enhancer of the NO reduction capability of rhodium. 

Active heterogeneous catalysts have been obtained. Examples include titania-, vanadia-, silica-, and ceria-based catalysts. A survey of catalytic materials prepared in flames can be found in [20]. Recent advances include nanocrystalline Ti02 [24], one-step synthesis of noble metal Ti02 [25], Ru-doped cobalt-zirconia [26], vanadia-titania [27], Rh-Al203 for chemoselective hydrogenations [28], and alumina-supported noble metal particles via high-throughput experimentation [29]. 

These results lead us to the conclusion that nano-sized metallic gold and strong interaction between gold and ceria contribute enhanced catalytic activity for CO oxidation in the absence and presence of water vapor. 

Granger, P., Delannoy, L., Lecomte, J.J. et al. (2002) Kinetics of the CO + NO Reaction over Bimetallic Platinum-Rhodium on Alumina Effect of Ceria Incorporation into Noble Metals, J. Catal., 207, 202. 

We shall mainly consider, in the present chapter, non-precious transition metals, but the model can be extended to precious metals presenting an oxidation state higher than zero [10,11], such as Rhx+, Pdx+, Ptx+ and tix+. The model also applies to some oxides alone, such as ceria (Ce02) [19] or mixed oxides such as ceria-zirconia (CeZr02) able to present redox properties and oxygen vacancies during catalytic reactions. 

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. 

Equations 16 and 17 imply that 02 adsorption is not dissociative, which is coherent with the kinetic data. However, 02 should be dissociated in further steps of the surface reaction. On ceria, new sites for 02 activation are created at the metal/support interface or in the vicinity of metal particles. As CO and 02 do not compete with the same sites, the rate equation becomes ... 

Two regions can be seen on the reaction profiles the low-temperature domain corresponding to CO oxidation (limited at a 40% conversion) and the high temperature one where the CO having not been oxidized react with water (WGS domain). For every reaction, Pt is the best metal and ceria acts as a promoter. However, an exceptional increase of conversion can be observed in WGS when the metals are deposited on CeA. 

For environmental reasons, reaction (Eqn. 21) (NO -> N2) should be promoted, N20 having a dramatic greenhouse gas effect. The different steps of reaction (Eqn. 23) have been investigated in detail, mainly by FTIR spectroscopy [61-63], One of the possible intermediate is isocyanate. NCO species could be formed on the metal and migrate on the support, which may explain the large differences observed when Rh is supported on different oxides (alumina, silica, zirconia, ceria-alumina, etc.). However, the main step should be the dissociative adsorption of NO ... 

The promotion by reduced ceria could be due to a spillover phenomenon of O species from metal to support. In fact, this is not sufficient to explain all the results of Mullins and Overbury. An exposure of the Rh/CeOx surface to water leads to a re-oxidation accompanied by a hydroxylation of the support while the metal surface is left unchanged. In fact, it seems that preferential orientation of Rh surface on reduced ceria may also explain the specific role of CeOx surface. This is consistent with the fact that NO dissociation occurs at lower temperatures on Rh (110) and on Rh (100) than on Rh (111) [83,84],... 

However, reduced ceria is able, alone, to dissociate NO. Martinez-Arias et al. [85] have first investigated by electron paramagnetic resonance (EPR) and FTIR spectroscopies NO reaction on ceria pre-outgassed at different temperatures and showed the role of superoxides differentially coordinated in the formation of hyponitrites species further decomposed into NzO. Later Haneda et al. [86] have demonstrated that reduced ceria and reduced praseodymium oxide dissociate NO even though the presence of a noble metal (Pt) significantly increases the formation of N2 or N20. The main results of this study are summarized in Table 8.9. 

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