Catalysts polymer supported metal colloids

Polymers play important roles in water photolysis. For multi-electron processes, polymer supported metal colloids or colloidal polynuclear metal complexes are very useful as catalysts. Unstable semiconductors with a small bandgap which photolyse... 

Polymers are attracting much attention as functional materials to construct photochemical solar energy conversion systems. Polymers and molecular assemblies are of great value for a conversion system to realize the necessary one-directional electron flow. Colloids of polymer supported metal and polynuclear metal complex are especially effective as catalysts for water photolysis. Fixation and reduction of N2 or C02 are also attractive in solar energy utilization, although they were not described in this article. If the reduction products such as alcohols, hydrocarbons, and ammonia are to be used as fuels, water should be the electron source for the economical reduction. This is why water photolysis has to be studied first. 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

Note 2 Examples of polymer-supported catalysts are (a) a polymer-metal complex that can coordinate reactants, (b) colloidal palladium dispersed in a swollen network polymer that can act as a hydrogenation catalyst. 

Studies on the immobilization of Pt-based hydrosilylation catalysts have resulted in the development of polymer-supported Pt catalysts that exhibit high hydrosilylation and low isomerization activity, high selectivity, and stability in solventless alkene hydrosilylation at room temperature.627 Results with Rh(I) and Pt(II) complexes supported on polyamides628 and Mn-based carbonyl complexes immobilized on aminated poly(siloxane) have also been published.629 A supported Pt-Pd bimetallic colloid containing Pd as the core metal with Pt on the surface showed a remarkable shift in activity in the hydrosilylation of 1-octene.630... 

Solvated metal atoms can be dispersed in excess organic solvent at low temperature and used as a source of metal particles for the preparation of both unsupported metal powders and supported metal catalysts158,161. Alternatively, metal vapor is condensed into a cold solution of a stabilizing polymer to form crystallites of the order 2-5 nm in diameters159. Equation 17 illustrates the unique activity of a colloidal Pd catalyst in the partial hydrogenation of acenaphthene. 

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]. 

The author s own interest in this area includes new functional polymers for solid phase synthesis [11-13], polymers with molecularly imprinted substrate selectivity [14], polymer-supported transition metal catalysts [15], novel polymers of potential interest for electrocatalysis [16], targeting of colloidal drug carriers [17, 18], molecular composites [19], and biocompatible surfaces [20]. These studies have led to, among other things, a uniquely versatile method of polymer synthesis based on the chemistry of activated acrylates, i.e. polymer synthesis via activated esters. Various aspects of polymers and copolymers of activated (meth)acrylates have also been investigated in this and several other laboratories. 

As described in Section 3 of Chapter 2, multi-electron processes are important for designing conversion systems. Noble metals are most potent catalysts to realize a multi-electron catalytic reaction. It is well known that the activity of a metal catalyst increases remarkably in a colloidal dispersion. Synthetic polymers have often been used to stabilize the colloids. Colloidal platinum supported on synthetic polymers is attracting notice in the field of photochemical solar energy conversion, because it reduces protons by MV to evolve H2 gas.la)... 

The Pti samples (182) were prepared as colloids, protected by a PVP polymer film. Layer statistics according to the NMR layer model (Eqs. 28-30) for samples with x = 0,0.2, and 0.8 are shown in Fig. 63. The metal/ polymer films were loaded into glass tubes and closed with simple stoppers. The NMR spectrum and spin lattice relaxation times of the pure platinum polymer-protected particles are practically the same as those in clean-surface oxide-supported catalysts of similar dispersion. This comparison implies that the interaction of the polymer with the surface platinums is weak and/or restricted to a small number of sites. The spectrum predicted by using the layer distribution from Fig. 63 and the Gaussians from Fig. 48 show s qualitative agreement w ith the observed spectrum for x = 0 (Fig. 64a). 

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