Mixed metals sequential precipitation

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. 

Colloidal Pt/RuO c- (C5 0.4nm) stabilized by a surfactant was prepared by co-hydrolysis of PtCU and RuCls under basic conditions. The Pt Ru ratio in the colloids can be between 1 4 and 4 1 by variation of the stoichiometry of the transition metal salts. The corresponding zerovalent metal colloids are obtained by the subsequent application of H2 to the colloidal Pt/Ru oxides (optionally in the immobilized form). Additional metals have been included in the metal oxide concept [Eq. (10)] in order to prepare binary and ternary mixed metal oxides in the colloidal form. Pt/Ru/WO c is regarded as a good precatalyst especially for the application in DMECs. Main-group elements such as A1 have been included in multimetallic alloy systems in order to improve the durability of fuel-cell catalysts. PtsAlCo.s alloyed with Cr, Mo, or W particles of 4—7-nm size has been prepared by sequential precipitation on conductant carbon supports such as highly disperse Vulcan XC72 [70]. Alternatively, colloidal precursors composed of Pt/Ru/Al allow... 

The above general remarks apply also to mixed precipitates of metal hydroxides as well as to silica—hydrated metal oxide mixed gels. As would be ejq)ected, products obtained by simultaneous precipitation of two (or more) compounds differ from those obtained by mechanical mixing of finished gels both of these types of gels are in turn different from mixed gels produced by sequential precipitation in the same solution. Finally, we should mention the so-called chalky silica gels (see p. 1656). 

Titration of a solution containing salts of the metal cations to be precipitated with a basic solution, usually an alkaline metal hydroxide or carbonate [98, 99] is one of the simplest methods to prepare LDHs. A sequential precipitation of the metal hydroxides (forming particles with the less soluble hydroxide in the nuclei and an external shell formed by the hydroxide of the divalent cation), could be expected, but it is generally found that a true mixed hydroxide is formed it seems that precipitation of M(OH)j induces the simultaneous precipitation of MCOH). ... 

There is a common misconception that coprecipitation generates a mix that is homogeneous on an atomic level. However, most coprecipitation reactions precipitate the metal ions sequentially, not simultaneously. Also, while atomically mixed batches would form crystals at much lower temperatures than mixtures of coarser particles, the coprecipitated batches seldom lower the reaction temperature by more than 100 Celsius degrees. 

Alloy catalysts are prepared either by co-precipitation with Pt or by sequential deposition. One widely used method is sequential deposition of alloying elements by carbo thermal process on Pt loaded carbon support. However, carbo thermal method is suitable for non-noble metals and does not yield high purity alloys. This is due to the presence of large quantity of unalloyed metal and the need to be Cleaned by soaking in acid. The other method involves formation of various types of complexes of Pt and the alloy metal to form a mixed colloid Sol . This mixed colloid when adsorbed on the carbon support yields highly uniform alloy distribution. 

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