Mixed Oxide Supports with 11 Composition

Ceria-zirconia mixed oxide-supported metal nanoparticles are attractive catalysts in low-temperature WGS, CO preferential oxidation (PROX), or three-way catalysis. It was observed that after redox cycles (reduction at 1173K and then oxidation at 823 K), the reducibility of these catalysts was enhanced substantially, which greatly affected their catalytic performance. HAADF studies showed that the pyrochlore-type cation sublattice in the reduced Ce Zr O was retained in the fully oxidized mixed oxide with a Ce Zr Og stoichiometry as far as the oxidation temperature did not exceed 823 K. However, it is not clear whether compositional heterogeneity occurred at the atomic level in Ce Zr Og. Trasobares and coworkers addressed this issue by aberration-corrected STEM, atomic-resolution EELS mapping, and EELS image simulations [66], They synchronously acquired EELS and HAADE signals in... 

For example, Fig. la shows a high-resolution TEM image of a complex catalyst, Pt/CeTbO c, a metal supported on a mixed-metal oxide with a structure and composition the determination of which require elaborate image contrast simulations using various model structures 10). Furthermore, the platinum particles are partly embedded in the oxide support, creating sites with activities... 

After isolation the supported precipitate is washed, dried and usually calcined to produce a supported oxide which is then reduced, commonly in a hydrogen stream. Reduction of these supported oxides generally proceeds more readily than the mixed oxides produced by coprecipitation since there is only a monolayer in which there is a direct interaction of the active component with the support. This monolayer can be considered to be a silicate or aluminate which is more difficult to reduce than the oxide or hydroxide found in the outer metal-containing layers.33 Precipitation-deposition gives catalysts having compositions similar to those produced by sequential precipitation as shown in Fig. 13.2. 

The catalysts derived from HT precursors provide information on the role of the type and amount of active metal, activation procedure and composition of the inert matrix, without any interaction due to side phase segregation or lack of homogeneity. Unlike that claimed previously by Swamy and coworkers [13, 14], the Co/Al (75.0/25.0, as atomic ratio %) mixed oxide shows a very low activity, due to the severe reaction conditions and, mainly, the presence of water, in agreement with previous data for transition metals supported on cubic mixed oxides derived from HT precursors [15]. The addition of a small amount of Rh [i.e., the Rh/Co/Al (0.5/75.0/24.5, as atomic ratio %) catalyst] improves the catalytic performances significantly, giving higher conversions than those of the analogous Rh/Mg/Al catalysts. Even so the level of conversion is still not satisfactory. 

This phenomenon has been observed for Pt (Graham and Shigapov/unpublished), Pd, and Rh supported on ceria-rich mixed oxide as well as for Pd supported on a variety of other materials with composition ranging from zirconia-rich mixed oxide to pure ceria [15,21]. Generally, encapsulation commences when the specific surface area of the support falls below a few m%. The extent to which the noble metal becomes encapsulated depends on additional factors, including metal loading. Table 10.2 lists results obtained for a number of 2 wt% Pd catalysts made with a... 

Transition metal carbides (mainly of W and Mo) have been shown to be effective catalysts in some chemical reactions that are usually catalyzed by noble metals such as Pt and Pd (ref.1). Their remarkable physical properties added to lower cost and better availability could make them good candidates for substitute materials to noble metals in automobile exhaust catalysis. Hence, for this purpose, we have prepared several catalysts of tungsten carbide and W,Mo mixed carbides supported on y alumina with different Mo/W atom ratios. The surface composition has been studied by XPS while the quantitative determination of catalytic sites has been obtained by selective chemisorption of hydrogen and of carbon monoxide. The catalytic performances of these catalysts have been evaluated in the simultaneous conversion of carbon monoxide, nitric oxide and propane from a synthetic exhaust gas. 

Catalysts with perovskitic structure guarantee a good compromise between stability and activity and have a relatively low cost, so they can constitute a valid alternative to supported noble metals, with particular reference to the reactions of partial or total oxidation of hydrocarbons (catalytic combustion). The traditional route used to synthesize perovskites was first introduced by Delmon and co-workers in the late nineteen sixties [1]. It enables to obtain i) mixed oxides over a wide range of composition ii) good control of the stoichiometry iii) an excellent interspersion of the elements in the final product iv) very small grain size materials. 

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