Metals, colloidal protection

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]. 

Transition metal colloids can also be prevented from agglomeration by polymers or oligomers [27,30,42,43]. The adsorption of these molecules at the surface of the particles provides a protective layer. In the interparticle space, the mobility of adsorbed molecules should be reduced decreasing the entropy and thus increasing the free energy (Fig. 2). 

Finally, the term steric stabihzation coifid be used to describe protective transition-metal colloids with traditional ligands or solvents [38]. This stabilization occurs by (i) the strong coordination of various metal nanoparticles with ligands such as phosphines [48-51], thiols [52-55], amines [54,56-58], oxazolines [59] or carbon monoxide [51] (ii) weak interactions with solvents such as tetrahydrofuran or various alcohols. Several examples are known with Ru, Ft and Rh nanoparticles [51,60-63]. In a few cases, it has been estab-hshed that a coordinated solvent such as heptanol is present at the surface and acts as a weakly coordinating ligand [61]. 

In some cases shape-control has also been achieved tetra( -octyl)ammonium glycolate transforms Pd(N03)2 predominantly into trigonal Pd particles [186]. Recent work has confirmed that the colloidal protective agents not only prevent particle agglomeration but even provide control of the crystal growth during particle synthesis (see e.g., Ref. [187-191]). The drawbacks of this route are the restriction to noble metal salts and the limited industrial availability of A-(octyl)j RC02. 

Bifunctional spacer molecules of different sizes have been used to construct nanoparticle networks formed via self-assembly of arrays of metal colloid particles prepared via reductive stabilization [88,309,310]. A combination of physical methods such as TEM, XAS, ASAXS, metastable impact electron spectroscopy (MIES), and ultraviolet photoelectron spectroscopy (UPS) has revealed that the particles are interlinked through rigid spacer molecules with proton-active functional groups to bind at the active aluminium-carbon sites in the metal-organic protecting shells [88]. 

Protective Colloids. Another approach in preparing and stabilizing metal colloids is by adsorption of macromolecules on their surfaces. A wide variety of materials have been used including gummy gelatinous liquids,(J 0) albumin,(27) Icelandic moss,(28) latex,(22) polyvinylpyrrolidone, (29) antibodies, ( 30 ) carbowax 20M, ( 31 ) polyvinylpyridine, (31 ) and various polymer-water/oil-water mixtures.( 2) These studies clearly indicate that "steric stabilization of metal colloids is also important (along with electronic stabilization).(33)... 

Whichever method is followed, a protective agent able to induce a repulsive force opposed to the van der Waals forces is generally necessary to prevent agglomeration of the formed particles and their coalescence into bulk material. Since aggregation leads to the loss of the properties associated with the colloidal state, stabilization of metallic colloids - and therefore the means to preserve their finely dispersed state - is a cmcial aspect for consideration during their synthesis. 

Depending on the nature of these protecting shells, the resulting metal colloid may be taken into organic (organosols) or aqueous (hydrosols) medium. Lipophilic protective agents such as... 

When an electric arc is formed under water between nvo metallic tods, panicles of the mclal of colloidal size are formed along with more or less separation of free metal. A protective colloid increases ihe stability. If ihe metal vaporizes and then condenses to the colloidal stale this is strictly speaking a condensation process, if otherwise, a dispersion process. 

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). 

Interest in the application of nanostructured catalysts stems from the unique electronic structure of the nanosized metal particles and their extremely large surface areas. Nanostructured metal colloids can be defined as isolable particles between 1 and 50 nm that are prevented from agglomerating by protecting shells. They can be prepared to be redispersed in both water ( hydrosols ) and organic solvents ( organosols ). Here we hope to provide a synopsis of the wet chemical syntheses of these materials and their application as precursors of electrochemical catalysts. 

The size-selective precipitation (SPP) was predominantly developed by Pileni [50c]. One example (SPP) is monodisperse silver particles (2.3 nm, 0= 15%), which are precipitated from a polydisperse silver colloid solution in hexane by the addition of pyridine in three iterative steps. Recently, Schmid [52a] has reported the two-dimensional crystallization of truly monodisperse AU55 clusters. Chromatographic separation methods have thus far proven unsuccessful because the colloid decomposed after the colloidal protecting shell had been stripped off [42a]. The size-selective ultracentrifuge separation of Pt colloids has been developed by Colfen [52b]. Although this elegant separation method gives truly monodisperse metal... 

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