Noble metal colloids, synthesis

Antonietti M, Groehn F, Hartmann J, Bronstein L (1997) Nonclassical shapes of noble-metal colloids by synthesis in microgel nanoreactors. Angew Chem Int Ed Engl 36 2080-2083... 

The formation and stabilization of noble metal colloids in the aqueous phase are widely known. Platinum and palladium are most widely used in hydrogenation of C=C bonds but some results have been described with rhodium. Generally, surfactants are investigated as stabilizers for the preparation of rhodium nanoparticles for biphasic catalysis in water. In many cases, ionic surfactants, such as ammonium salts, which provide sufficiently hydrophilic character to maintain the catalytic species within the aqueous phase, are used. The obtained micelles constitute interesting nanoreactors for the synthesis of controlled size nanoparticles due to the confinement of the particles inside the micelle cores. Aqueous colloidal solutions are then obtained and can be easily used as catalysts. 

Antonietti M, Wenz E, Bronstein LM, Seregina MS (1995) Synthesis and characterization of noble metal colloids in block copolymer micelles. Adv Mater 7 1000-1005... 

In this chapter, we intend to introduce to the reader some of the most popular preparation methods of noble metal colloids, but do not intend by any means to thoroughly review the scientific literature on metal colloid synthesis. We have chosen examples of synthesis in aqueous and organic solvents, and differentiate between spherical and anisotropic nanoparticles (nanorods and nanoprisms). Additionally, in section 3 we shall describe one of the most popular recent procedures to assemble metal colloids into nanostructured materials (layer-by-layer assembly), as well as the properties of the resulting structures. 

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. 

The alkaline EG synthesis method described in this chapter is highly efficient for preparing colloidal solutions of small and narrowly distributed unprotected noble metal or alloy nanoclusters with high metal concentration. 

A new class of heterogeneous catalyst has emerged from the incorporation of mono- and bimetallic nanocolloids in the mesopores of MCM-41 or via the entrapment of pro-prepared colloidal metal in sol-gel materials [170-172], Noble metal nanoparticles containing Mex-MCM-41 were synthesized using surfactant stabilized palladium, iridium, and rhodium nanoparticles in the synthesis gel. The materials were characterized by a number of physical methods, showed that the nanoparticles were present inside the pores of MCM-41. They were found to be active catalysts in the hydrogenation of cyclic olefins such as cyclohexene, cyclooctene, cyclododecene, and... 

In these experimental conditions the precipitation of cj>, and the reduction of Co(II) and Ni(II) species occur simultaneously, and then < >, is not able to act as a reservoir for these species. Thus, sodium hydroxide was added in a large excess to the reaction medium in the synthesis of CoNi monodisperse particles in order to provide an excess of hydroxide ions (8,30). This method can be extended to obtain nickel-noble metal alloys. Thus PVP-protected Ni-Pd colloids were obtained by Toshima et al. (31) for different Ni/Pd ratios. 

A comprehensive study on the sonochemical synthesis of colloidal solutions of noble metals was conducted by Grieser and coworkers [32-34]. The 515 kHz ultrasound-initiated reduction of AuCl4 to Au (0) was examined as a function of the concentration of various surface-active solutes [32]. The amount of AuCU reduced in the presence of ethanol, 1-propanol, and 1-butanol was found to be dependent on the surface excess of the alcohol at the gas/solution interface, i.e., the relative concentration of the alcohol at the gas/solution interface compared to the bulk solution concentration. The efficiency of reduction of AuCl4 in the presence of the surfactants sodium dodecyl sulfate or octaethylene glycol monodecyl ether was found to be related to the monomer concentration of the surfactant in solution. 

In a few studies sonochemistry was used to coat polymers with nanosized par-tides [48-50]. Of these three reports one [50] dealt with metals, more specifically with noble metals (Pt, Pd, and Au). In this research, metal colloids are adsorbed to the surface of neutral functionalized polystyrene microspheres, PSMS. The authors report on the synthesis and characterization of catalytically important noble monometallic colloids using various chemical and sonochemical methods. These metal colloids are then adsorbed onto suitably functionalized PSMS. The metal-immobilized microspheres are reacted with a linker such as 4-mercaptobutyl phosphonic acid and subsequently used to grow multilayers. 

By this time, substantial experience in the synthesis of low-valence noble metal complexes in the form of non-crystalline, colloid-like samples has been accumulated. For instance, a series of amorphous, high-molecular weight palladium complexes, which had remarkable catalytic capability, had been obtained starting from low-nuclearity Pd(I) clusters and Pd(II) complexes. ... 

The asymmetric hydrogenation of prochiral ketones is often an important step in the industrial synthesis of fine and pharmaceutical products. Several noble metal nanoparticles have been investigated for asymmetric catalysis of prochiral substrates but platinum colloids have been the most widely studied and relevant enantiomeric excesses have been reported (>95%). Nevertheless, the enantioselec-tive hydrogenation of ethyl pyruvate catalyzed by PVP-stabilized rhodium nanocluster modified by cinchonidine and quinine was reported by Li and coworkers (Scheme 11.7) [68]. 

Although gold colloids were known and studied very early [3.2], the synthesis technique has been much studied and greatly improved during the past few decades. The two-phase approach permits the fabrication of thiol-covered noble-metal nanoparticles [3.141]. The thiol layer prevents coalescence of the metal particles, thus stabilizing the colloid and preserving the size distribution. Motivated by the self-ordering properties of layers of nanoparticles with a narrow size distribution. 

Min B-K, Choi S-D (2004) Role of CaO as crystallite growth inhibitor in SnO. Sens Actuators B 99 288-296 Nakao Y (1995) Noble metal solid sols in poly(methyl) methacrylate. J Colloid Interface Sci 171 386-391 Nayral C, Viala E, Colhere V, Fau P, Senocq F, Maisonnat A, Chaudret B (2000) Synthesis and use of a novel SnO nanomaterial for gas sensing. Appl Surf Sci 164 219-226... 

Photocatalysis experiments were conducted using an established semiconductor system and for decoration with noble metals, combining it with size-selected clusters. The general synthesis and measurement approach is depicted in Fig. 3.17. Colloidal semiconductor CdS nano rods are spin coated onto conducting ITO glass substrates... 

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