Metal nanoparticles synthesis silica-supported

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

Transition metal nanoparticles supported on different substrates are used as catalysts for different reactions, such as hydrogenations and enantioselective-synthesis of organic compounds, oxidations and epoxidations, reduction, and decomposition [24,25], Among the supports that have been applied in the preparation of supported transition metal nanoparticles are active carbon, silica, titanium dioxide, and alumina. 

Zeolites have ordered micropores smaller than 2nm in diameter and are widely used as catalysts and supports in many practical reactions. Some zeolites have solid acidity and show shape-selectivity, which gives crucial effects in the processes of oil refining and petrochemistry. Metal nanoclusters and complexes can be synthesized in zeolites by the ship-in-a-bottle technique (Figure 1) [1,2], and the composite materials have also been applied to catalytic reactions. However, the decline of catalytic activity was often observed due to the diffusion-limitation of substrates or products in the micropores of zeolites. To overcome this drawback, newly developed mesoporous silicas such as FSM-16 [3,4], MCM-41 [5], and SBA-15 [6] have been used as catalyst supports, because they have large pores (2-10 nm) and high surface area (500-1000 m g ) [7,8]. The internal surface of the channels accounts for more than 90% of the surface area of mesoporous silicas. With the help of the new incredible materials, template synthesis of metal nanoclusters inside mesoporous channels is achieved and the nanoclusters give stupendous performances in various applications [9]. In this chapter, nanoclusters include nanoparticles and nanowires, and we focus on the synthesis and catalytic application of noble-metal nanoclusters in mesoporous silicas. 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

The synthesis of silicalite-1 as described earher does not yield an active catalyst per se, but rather a framework structure aldn to the well-known zeolite ZSM-5, which is heavily applied as solid add in, for example, petrochemical industries for the production of transportation fuels and bulk chemicals. Other examples of support oxides are, for example, amorphous silica and alumina. These are most regularly functionalized in a set of steps leading to the deposition of metal or metal oxide nanoparticles. This procedure is schematically shown in Figure 12.4, and explained later, with direct links to possible in situ characterization studies. 

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