Preparation of Supported Metal Particles

Recent years have witnessed marked progress in the preparation of small metal particles. This has been achieved by the choice of a suitable support, the selection of the appropriate preparation method, or the combination of both. 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

The Various Preparation Methods for Depositing the Metal Precursor or the Metal 

This review begins with the more complex methods and moves on to the simpler ones. In this context, complexity is defined as the number of species involved before finally obtaining the highly dispersed metal particles. Table I summarizes the principal features of the different methods used to prepare the metal particles on a support, which is the common feature of all the methods and thus is not included in the table. They are subdivided into groups when the principles are similar. We could have ordered the preparation methods differently, for instance, using a chemical approach starting from the oxidation state and nuclearity of the metal in the precursor compound, but this would not have served our purpose, specifically, to show the parameters that can possibly influence the behavior of the final metallic state. 

Most methods deal with the formation of metal particles on a support that is preformed since this leads to simpler preparation processes. There is an important route, however, typically used for metal-SiCh and metal-AI2O3 catalysts, which involves (Table I) the coprecipitation in a precursor form (hydroxides, nitrates, carbonates, silicates, etc.) of both the support and the active phase from a solution 37a,b, 38, 41). The advantage is to produce an intimate mixing of metal precursor and support. The precipitate leads on calcination to a support with the active component dispersed throughout the bulk as well as at the surface. After reduction to the final catalyst, it is difficult to obtain metal crystallites of uniform size 42, 43) because of the presence of both the oxides (of the support and of the active metal) and other intermediate compounds [e.g., nickel alumi-nate or silicate for the Ni/Al203 42) and Ni/SiCh 43) systems, respectively] that have different reducibilities. 

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C. Preparation of Supported Metal Particles from Monolayer-Dispersed Oxide... 

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