Polymer-protected metal nanoparticle

Conventional filtration cannot be applied to the separation in purification of metal nanoparticles. If the metal nanoparticles are protected by polymer, however, the membrane filter, which can cut off the pol5mer with certain molecular weight, can be used to separate the polymer protected metal nanoparticles. Free metal nanoparticles which are not protected by polymer can pass through the membrane. Ion filter like cellulose can be used to separate ionic species from the reaction mixtures. 

In the early work on the thermolysis of metal complexes for the synthesis of metal nanoparticles, the precursor carbonyl complex of transition metals, e.g., Co2(CO)8, in organic solvent functions as a metal source of nanoparticles and thermally decomposes in the presence of various polymers to afford polymer-protected metal nanoparticles under relatively mild conditions [1-3]. Particle sizes depend on the kind of polymers, ranging from 5 to >100 nm. The particle size distribution sometimes became wide. Other cobalt, iron [4], nickel [5], rhodium, iridium, rutheniuim, osmium, palladium, and platinum nanoparticles stabilized by polymers have been prepared by similar thermolysis procedures. Besides carbonyl complexes, palladium acetate, palladium acetylacetonate, and platinum acetylac-etonate were also used as a precursor complex in organic solvents like methyl-wo-butylketone [6-9]. These results proposed facile preparative method of metal nanoparticles. However, it may be considered that the size-regulated preparation of metal nanoparticles by thermolysis procedure should be conducted under the limited condition. 

Fig. 3.17 Catalysis process on the surface of polymer-protected metal nanoparticles,...

The polymer-protected metal nanoparticles are usually prepared by in-situ reactions, such as chemical reductions, photolyses, and thermal decompositions of metal sah precursors within the polymer matrix (J-8). 

In this chapter we report some results for several nonionic polymers and cationic polyelectrolytes and their ability to stabilize platinum colloids. Both steric and electrostatic stabilization of the metal colloids can be combined by the use of polyelectrolytes (5). The materials have been examined by transmission electron microscopy (TEM) in order to determine the average particle size, size distribution and particle pe. The catalytic activity of these polymer-protected platinum nanoparticles has been tested by the hydrogenation of cyclohexene, d cyclooctene, and 1-hexene. 

Evaporation of volatile byproducts and solvents is often used to obtain the solid metal nanoparticles. The residue may contain metal nanoparticles and protective reagents. When the nanoparticles are well protected by ligands or polymers, then the solid residues can be dispersed again without coagulation of the particles. When the nanoparticles are not well protected, however, the evaporation often results in aggregation of the nano-particles. 

Dendrimer-protected colloids are capable of adsorbing carbon monoxide while suspended in solution, but upon removal from solution and support on a high surface area metal oxide, CO adsorption was nil presumably due to the collapse of the dendrimer [25]. It is proposed that a similar phenomena occurs on PVP-protected Pt colloids because removal of solvent molecules from the void space in between polymer chains most likely causes them to collapse on each other. Titration of the exposed surface area of colloid solution PVP-protected platinum nanoparticles demonstrated 50% of the total metal surface area was available for reaction, and this exposed area was present as... 

In a more general way, the loading of metal salts into preformed block copolymer micelles has become the most used route for the incorporation of precursors into block copolymer nanostructures because it allows precursor loading with tolerable loading times, it is quite versatile, and it is applicable to a wide variety of precursor/block copolymer/solvent systems. The accordingly synthesized polymer-coated metallic or semiconducting nanoparticles exhibit increased stability, which results in, e.g., protection against oxidation as illustrated by Antonietti et al. [108]. 

The second strategy by which metal nanoparticles can be stabilized is the fixation of molecules by the particles surface atoms. The protecting molecules may consist of polymers, surfactants, or ligands, as they are known from traditional complex chemistry. The use of polymers is for their amphiphilic natiue, that is, they have not only to coordinate to the metal particle, but must simultaneously be solvated by the surrounding fluid. Gelatin, agar, cyclodextrins, cellulose acetate, and cellulose nitrate are used as well as synthetic polymers. Poly(vinylpyrrolidone) (PVP) and poly(vinylalcohol) have turned out to work perfectly in this respect. 

Coram et al. [6] have described the polymer support as a soluble macromolecule or a micellar aggregate that wraps the metal nanoparticle in solution, thus preventing metal sintering and precipitation. It can also be a resin, that is an insoluble material consisting in a bundle of physically and/or chemically cross-linked polymer chains in which the metal nanoparticles are embedded (Figure 11.2). Thus, soluble cross-linked polymers ( microgels ) that can stabilize metal nanoparticles can be prepared in addition, metal colloids protected by soluble linear polymers have been grafted onto insoluble resin supports to yield insoluble catalysts. This chapter is devoted mainly to metal nanoparticles on insoluble resin supports [8]. 

Metal-containing polymers may be produced by various methods, such as chemical reactions of precursors— in particular, reactions of metal salts in polymer solutions, the treatment of polymers with metal vapors, or the polymerization of various metal-monomer systems [1-4], Depending on the metal nature and the polymer structure, these processes lead to organometallic units incorporated into polymer chains, metal-polymer complexes, or metal clusters and nanoparticles physically connected with polymer matrix. Of special interest are syntheses with the use of metal vapors. In this case, metal atoms or clusters are not protected by complexones or solvate envelopes and consequently have specific high reactivity. It should be noted that the apparatus and principles of metal vapor synthesis techniques are closely related to many industrial processes with participation of atomic and molecular species [5]—for example, manufacturing devices for microelectronic from different metals and metal containing precursors [6]. Vapor synthesis methods employ varying metals and... 

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