Colloidal metals catalytic applications

Particularly attractive for numerous bioanalytical applications are colloidal metal (e.g., gold) and semiconductor quantum dot nanoparticles. The conductivity and catalytic properties of such systems have been employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors, and DNA sensors. Advances in the application of molecular and biomolecular functionalized metal, semiconductor, and magnetic particles for electroanalytical and bio-electroanalytical applications have been reviewed by Katz et al. [142]. 

Colloidal nanoparticles can be employed as heterogeneous catalyst precursors in the same fashion as molecular clusters. In many respects, colloidal nanoparticles offer opportunities to combine the best features of the traditional and cluster catalyst preparation routes to prepare uniform bimetallic catalysts with controlled particle properties. In general, colloidal metal ratios are reasonably variable and controllable. Further, the application of solution and surface characterization techniques may ultimately help correlate solution synthetic schemes to catalytic activity. 

The major disadvantages of colloidal catalysts studied so far can be attributed to problems in controlling the metal colloid formation (control of particle size, particle size distribution, structure of metal colloids) and stabilization of the prepared particles, which are not yet completely solved. But it is exactly the stability of the nanoparticles, that is decisive for long-term usage during catalytic processes. Moreover for catalytic application, it is extremely important to preserve the large surface of such colloidal systems. 

In summary, the examples given above demonstrate that immobilization of metal salts in a block copolymer micellar system followed by a reduction step is a suitable method to synthesize stable colloids with small particle sizes and narrow size distributions. Moreover, such systems are very interesting for catalytic applications because they offer the possibility of designing tailored catalysts for special demands and can be easily tuned by the choice and combination of different polymer block types and lengths, different types of the metal precursor and of the reduction method used. Additional introduction of further functionalities such as charges or chiral groups could make these catalyst systems even more versatile and effective. 

If the sonolysis is done in the presence of a support or porous host, then colloidal metal particles are formed. These powders have a surface area over a hundred times greater than powders commercially available and are amorphous. Such materials are generally considered for catalytic reactions and not for magnetic applications. 

Copper colloids protected by PVPD with a 3240 degree of polymerization, most effectively catalyze the selective (100%) hydration of acrylonitrile (AN) to acrylamide in water at 80 °C at a molar Cu/AN ratio of 0.017. The acrylamide yield reaches 25.4 mole % within 2h [46]. The reaction is first order with respect to the acrylonitrile concentration down to 47% conversion. The catalytic activity of all other protected colloidal dispersions is also much higher (2.5-8.6 mole % in 2h) than the activity of the copper precipitate which forms by the reduction of copper sulphate by NaBH4 in the absence of a copolymer (0.3 mole % in 2 h). Hirai and Toshima [46] have reviewed the preparation and characterization of polymer-protected colloidal metal catalysts together with their characteristic properties and some of their applications. 

It is dear that colloidal metals not only possess many of the catalytic properties of supported metal catalysts in reactions carried out at temperatures mild enough to sustain colloid stability, they are also amenable to modification and application in ways more reminiscent of molecular homogeneous catalysts. The use of ligand modification with colloidal metal catalysts has hardly been mentioned in the lite-... 

The isolated metal colloids are very stable under argon and useful precursors for catalytic applications. 

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. 

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