Metal colloid synthesis catalysts

Supports are in most cases used to prevent the sintering of metal particles. This is also the role of the otherwise inert promoters (e.g. AI2O3 in Fe). Small metal particles can be prepared as colloids (see e.g. ref. 31) or dispersed on a support. Only the second way and the fused metal technique have been applied to the Fischer-Tropsch synthesis catalysts. 

The formation and stabilization of noble metal colloids in the aqueous phase are widely known. Platinum and palladium are most widely used in hydrogenation of C=C bonds but some results have been described with rhodium. Generally, surfactants are investigated as stabilizers for the preparation of rhodium nanoparticles for biphasic catalysis in water. In many cases, ionic surfactants, such as ammonium salts, which provide sufficiently hydrophilic character to maintain the catalytic species within the aqueous phase, are used. The obtained micelles constitute interesting nanoreactors for the synthesis of controlled size nanoparticles due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained and can be easily used as catalysts. 

Initial reports for the synthesis of bi-metallic, nanosized PtRu catalysts involved a number of steps. A well-known synthesis of bi-metallic carbon-supported Pt/Ru catalysts of high surface area (ca. 80 m g Pt + Ru) is the Watanabe method [9]. They first formed colloidal Pt02 from platinic acid (H2PtCl6) with the reduction of NaHSOs and oxidative decomposition with H2O2 at a fixed pH of 5, according to the following, possible reaction scheme [10] ... 

Xin and co-workers modified the alkaline EG synthesis method by heating the metal hydroxides or oxides colloidal particles in EG or EG/water mixture in the presence of carbon supports, for preparing various metal and alloy nanoclusters supported on carbon [20-24]. It was found that the ratio of water to EG in the reaction media was a key factor influencing the average size and size distribution of metal nanoparticles supported on the carbon supports. As shown in Table 2, in the preparation of multiwalled carbon nanotube-supported Pt catalysts... 

Surfactants are well known as stabilizers in the preparation of metal nanoparticles for catalysis in water. Micelles constitute interesting nanoreactors for the synthesis of controlled-size nanoparticles from metal salts due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained which can be easily used as catalysts. 

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