Stabilization by polymers

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. 

Sp(t), the area of polymer particles stabilized by polymer end groups rather than soap, might in the general case be important but it is very difficult to obtain an expression for it. Aj(t) on the other hand, the area of monomer droplets, is usually neglected as being quite a few orders of magnitude less than Ap(t). 

A new method to synthesis nanoparticles of group VIII-X elements has been developed by Choukroun et al. [178] by reduction of metalUc precursors with CP2V. Mono- and bimetallic colloids of different metals (stabilized by polymers) have been prepared in this way (Fe, Pd, Rh, Rh/Pd). These colloids are then used as catalysts in various reactions such as hydrogenation of CC, CO, NO or CN multiple bonds, hydroformylation, carbonylation, etc. 

For example, the aggregated structures of the solutions containing polymer-metal complexes and the colloidal dispersions of metal nanoparticles stabilized by polymers have been analyzed quantitatively (64). SAXS analyses of colloidal dispersions of Pi, Rh, and Pt/Rh (1/1) nanoparticles stabilized by PVP have indicated that spatial distributions of metal nanoparticles in colloidal dispersions are different from each other. The superstructure (greater than 10.0 nm in diameter), with average size highly dependent on the metal element employed, is proposed. These superstructures are composed of several fundamental clusters with a diameter of 2.0-4.0 nm, as shown in Figure 9.1.13 for PVP-stabilized Pt nanoparticles. 

Biffis et al. compared the catalytic performance of polymer gel immobilized Au NPs and Au/AC, with similar sized Au NPs, for alcohol oxidations in water [172]. Gold stabilized by polymer gel is advantageous over Au/AC for the use of hydrophobic substrates such as 1-octanol and 1-phenylethanol in aqueous media, although lower selectivity was obtained in some cases. For instance, polymer microgel supported Au NPs gave 1-octanoic acid by the oxidation of 1-octanol with a selectivity of 84% at 59% conversion, whereas Au/AC gave a selectivity of 93% at 65% conversion. 

Figure 12.10 Droplet stabilized by polymer (left) and by adsorbed solid particles (right). The contact angles of the solid particles with the continuous phase should be smaller than 90°.

Colloidal metal clusters, which offer a high surface area for better activity, have been stabilized by polymers. Thus, a homogeneous dispersion of a cobalt-modified platinum cluster was stabilized by a coordinating polymer, poly(/V-vinyl-2-pyrrolidone).74 Addition of the cobalt(II) [or iron(III)] doubled the activity and increased the selectivity from 12 to 99% when the catalyst was used to reduce cin-namaldehyde (5.23). 

A general thermodynamic formalism for steric stabilization has been presented by Everett (1978c). This is an elaboration of that set forth by Ash et al. (1973), which will be discussed in some detail in Section 17.5 in connection with the stabilization by polymers free in solution. As Everett (1978c) has pointed out, the procedure propounded by Ash et al. (1973) is inapplicable to steric stabilization because equilibrium is not maintained between adsorbed polymer and polymer in the bulk solution. An alternative approach must therefore be devised. 

Fig. 12.4. The distance dependence of the steric interaction energy for two spheres of radius stabilized by polymer layers with different segment density distribution functions I, exponential 2, constant 3, Gaussian 4, radial Gaussian (after Smitham and Napper, 1979).

This section describes C-C coupling by soluble palladium colloids, i.e., palladium nanoparticles dipersed in a liquid phase, the particles being colloidally stabilized by polymers. As they do not differ fundamentally either in terms of selectivity and activity in catalysis or in physical properties, e.g., those relevant for catalyst recovery, palladium colloids stabilized by low molecular weight surfactants and ligands are also included. 

Table 1 Examples of C-C coupling reactions catalyzed by palladium colloids stabilized by polymers or surfactants. 

The derivation of the preceding equations requires the system to be in equilibrium that is, it must be fully relaxed. It implies that adsorption of the various components i should reach their equilibrium values at any distance when the plates change their separation. This requiranent is usually not met for adsorbed polymers (cf. Chapter 15) and, hence. Equation 20.13 is not applicable to steric stabilization by polymers that are fixed at the approaching surfaces. 

The most topical theoretical problems for the formation of clusters and nanoparticles in polymeric matrices involve questions about structural-morphological and spatial organization at the local, molecular, and supramolecular levels. Among such problems are the thermodynamic peculiarities of cluster and nanoparticle formation. An analysis has recently been performed using macrochelates. The most important problem seems to be the nanoparticle stabilization by polymer monolayers and LBFs. A insufficient munber of studies have been carried out on this topic. Thus the natme of the adhesion at the interfaces formed still remains obscure. 

Colloidal dispersions can be very well stabilized by polymers attached to the particle surfaces [17]. Here we consider polymer chains that are in a good solvent . This means that the chains are swollen and repel each other. As two colloidal particles, protected with attached polymers, approach each other the local osmotic pressure increases dramatically due to steric hindrance of the polymer chains on both particles. This competition between the chains for the same volume leads to a repulsive interaction, as was realized already by Fischer [42]. 

The liquid-junction semiconductor electrodes stabilized by polymer-coating can be used for photochemical conversion systems. The stabilized n-CdS coated with PP which incorporates RUO2 as catalyst was used for visible-light-induced water cleavage Photochemical diodes were fabricated by coating CdS with PP and polystyrene films, the latter containing metal dispersions such as Pt, Rh and RUO2 as a catalyst (Fig. 35). 

The freeze fracture method has been used to study the structure of colloidal particles in water-oil mixtures stabilized by polymer emulsifiers. Microemulsions consisting of water, toluene and graft copolymer composed of a polystyrene backbone and a poly(ethylene oxide) graft were deposited onto a small gold plate, quenched in liquid nitrogen in equilibrium with its own solid phase [436]. Replicas of the fractured surfaces were washed with tetrahydrofuran, which showed the micellar structure of the copolymers. A similar method was used for the preparation of polystyrene polymer latexes for TEM study of the size distribution [437]. In this case, the frozen droplet was microtomed, with a cold knife at -100 to -120°C, etched for up to 90 s and then a platinum-carbon replica was prepared. Etching was found to be unnecessary and a potential cause of error. The remaining latex was dissolved away before examination of the replica. Such replicas can reveal the size distribution and structure of the latex particles. 

Abstract Silver, palladium, and rhodium nanoparticles were prepared by heterogeneous nucleation in the interlamellar space of a layered kaolinite support. Disaggregation of the lamellae of nonswelling kaolinite was achieved by intercalation of dimethyl sulfoxide. The kaolinite was suspended in various metal precursor solutions and the adsorbed metal cations were reduced with NaBH4 The diameter of the silver particles (4-10 nm) prepared in this way depends on the initial Ag" " concentration. Palladium and rhodium particles were stabilized by polymers and by the lamellae of kaolinite. The effect of the molecular mass and the concentration of the polymers on the size of the particles... 

Fig. 19 Face-selective adsorption of ions or low molar mass additives (a), steric particle stabilization by polymers (b), and face-selective adsorption and particle stabilization by DHBC (c). Figure reproduced from [57] with permission of the Royal Society of Chemistry...

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