Mixed metal catalysts reductive deposition

Metal oxide catalysts are extensively employed in the chemical, petroleum and pollution control industries as oxidation catalysts (e.g., oxidation of methanol to formaldehyde, oxidation of o-xylene to phthalic anhydride, ammoxidation of propylene/propane to acrylonitrile, selective oxidation of HjS to elemental sulfur (SuperClaus) or SO2/SO3, selective catalytic reduction (SCR) of NO, with NHj, catalytic combustion of VOCs, etc.)- A special class of metal oxide catalysts consists of supported metal oxide catalysts, where an active phase (e.g., vanadium oxide) is deposited on a high surface area oxide support (e.g., alumina, titania, ziiconia, niobia, ceria, etc.). Supported metal oxide catalysts provide several advantages over bulk mixed metal oxide catalysts for fundamental studies since (1) the number of surface active sites can be controlled because the active metal oxide is 100% dispersed on the oxide support below monolayer coverage,... 

Common metals often form mixed oxides with the support compounds. For that reason common metals are usually used as massive metal catalysts. In the case of massive metal catalysts, a few weight percent of a promoter is added. Some promoters make mixed oxides with the active element and influence the reduction process and the surface area (structural promoter). Others are deposited on the metal surface and have an electronic interaction with the surface (chemical effect). Ammonia activity on Fe is known to be enhanced by adding AI2O3 and K2O. It is believed that AI2O3 stabilizes the high surface area of Fe (structural effect) and K2O promotes the ammonia activity per Fe surface area (chemical effect). The structural effect is well studied on Fe single crystal surfaces, where Fe(lll) is the most active plane and Fe(llO) is the next and Fe(lOO) is the least active plane [93]. Such studies have been expanded to other catalysts such as Re, and will be reviewed in Section 3.2.4.2. 

One of the main advantage of this method is that it allows different possibilities of synthesis of metallic particles deposited on a carbon support (1) synthesis of the catalysts with a controlled atomic ratio by coreduction, which consists in mixing different metal salts before their reduction leading to colloid formation and deposition on carbon (2) synthesis of the catalysts with a controlled atomic ratio by codeposition, which consists in mixing colloids of different... 

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. 

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