Catalytically nanoclusters

Volume 103 Semiconductor Nanoclusters - Physical, Chemical, and Catalytic Aspects edited by P.V. Kamat and D. Meisel... 

Attention has been given to the synthesis of bimetallic silver-gold clusters [71] due to their effective catalytic properties, resistance to poisoning, and selectivity [72]. Recently molecular materials with gold and silver nanoclusters and nanowires have been synthesized. These materials are considered to be good candidates for electronic nanodevices and biosensors [73]. 

Finke has reported remarkable catalytic lifetimes for the polyoxoanion- and tetrabutylammonium-stabi-lized transition metal nanoclusters [288-292]. For example in the catalytic hydrogenation of cyclohexene, a common test for structure insensitive reactions, the lr(0) nanocluster [296] showed up to 18,000 total turnovers with turnover frequencies of 3200 h [293]. As many as 190,000 turnovers were reported in the case of the Rh(0) analogue reported recently. Obviously, the polyoxoanion component prevents the precious metal nanoparticles from aggregating so that the active metals exhibit a high surface area [297]. 

Platinum Nanoclusters Size and Surface Structure Sensitivity of Catalytic Reactions... 

Relevance of Metal Nanoclusters Size Control in Gold(O) Catalytic Chemistry... 

The catalytic chemistry of M° depends on the elementary properties of M and on the structure and size of the M° nanoclusters ( quantum dots ) [2]. S may play a role as a reactivity enhancer of M°/S as a whole (co-catalytic role) and/or as a promoter of its catalytic chemoselectivity (promotional role) [3,4]. 

If a mixture of organic substrates are to be catalytically transformed into suitable products upon the action of M nanoclusters present inside a nanostructurally well defined CFP, a reagent size exclusion will take place with uniquely beneficial effects in terms of chemoselectivity [35,36]. 

In the sixties of past century, a few patents issued to Bergbau Chemie [5,48,49] and to Mobil Oil [50-52], respectively described the use of CFPs as supports for catalytically active metal nanoclusters and as carriers for heterogenized metal complexes of catalytic relevance. For the latter catalysts the term hybrid phase catalysts later came into use [53,54], At that time coordination chemistry and organo-transition metal chemistry were in full development. Homogeneous transition metal catalysis was expected to grow in industrial relevance [54], but catalyst separation was generally a major problem for continuous processing. That is why the concept of hybrid catalysis became very popular in a short time [55]. 

Cross-linked functional polymers appear to be suitable supports for catalytically active metal(O) nanoclusters. 

The catalytic activity of the Ru/Sn02 nanocomposite was eight times higher than that of the most effective Ru metal catalyst reported previously [56]. An o-CAN selectivity over 99.9% at a substrate conversion of 100% was obtained over the Ru/Sn02 catalyst. This selectivity was comparable to the result reported for a boride-modified PVP-Ru colloidal catalyst [56,57], and was better than that of the PVP-Ru catalyst with the same Ru nanoparticles. The Sn02 nanoparticles remarkably promoted both the catalytic activity and selectivity of the Ru nanoclusters. An extremely low dechlorination rate of o-CAN in the absence of o-CNB was observed over this catalyst, which was 20-fold lower than that over the PVP-Ru colloidal catalyst, and was 73-fold lower when compared with a Ru/Si02 nanocomposite catalyst. 

It should be mentioned that the structure of carbon supports could have significant influence on the electro-catalytic properties of the nanocomposite catalysts. Recently, Pt/Ru nanoclusters prepared by the alkaline EG method were impregnated into a synthesized carbon support with highly ordered mesoporous. Although the Pt/ Ru nanoclusters can be well dispersed in the pores of this carbon substrate, the long and narrow channels in this material seem not suitable for the application in... 

The emphases of future investigation on these unprotected metal nanoclusters should be mainly placed on (1) further controlling the size, composition and shape of the unprotected metal or alloy nanoclusters (2) better understanding the stabilizing mechanism of the unprotected metal nanoclusters in colloidal solutions prepared by the alkaline EG synthesis method (3) developing novel catalytic and other functional systems for real applications. 

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. 

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