Nanosized metal particles

Templates made of surfactants are very effective in order to control the size, shape, and polydispersity of nanosized metal particles. Surfactant micelles may enclose metal ions to form amphiphilic microreactors (Figure 11a). Water-in-oil reverse micelles (Figure 11b) or larger vesicles may function in similar ways. On the addition of reducing agents such as hydrazine nanosized metal particles are formed. The size and the shape of the products are pre-imprinted by the constrained environment in which they are grown. 

Schematic representation of carbon filaments of different structure produced by metal-catalyzed decomposition of methane, (a) Platelet structure, (b) "herringbone" structure, and (c) ribbon structure. MP denotes a nanosized metal particle.

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). 

Unmodified poly(ethyleneimine) and poly(vinylpyrrolidinone) have also been used as polymeric ligands for complex formation with Rh(in), Pd(II), Ni(II), Pt(II) etc. aqueous solutions of these complexes catalyzed the hydrogenation of olefins, carbonyls, nitriles, aromatics etc. [94]. The products were separated by ultrafiltration while the water-soluble macromolecular catalysts were retained in the hydrogenation reactor. However, it is very likely, that during the preactivation with H2, nanosize metal particles were formed and the polymer-stabilized metal colloids [64,96] acted as catalysts in the hydrogenation of unsaturated substrates. 

Nan-metallic Clusters, Quasi-metallic Clusters, and Nanosized Metallic Particles... 

Significant new insight has been gained into the formation of small clusters and nanosized metallic particles [501,502]. This fundamental information is not only inherently fascinating, but it is vitally important for the construction of new generations of advanced nanostructured materials. Evolution of nanosized metallic particles from non-metallic clusters and the chemistries of these species will, therefore, be discussed in the following sections. 

Transition from non-metallic clusters consisting of only a few atoms to nanosized metallic particles consisting of thousands of atoms and the concomitant conversion from covalent bond to continuous band structures have been the subject of intense scrutiny in both the gas phase and the solid state during the last decade [503-505]. It is only recently that modern-day colloid chemists have launched investigations into the kinetics and mechanisms of duster formation and cluster aggregation in aqueous solutions. Steady-state and pulse-radiolytic techniques have been used primarily to examine the evolution of nanosized metallic particles in metal-ion solutions [506-508]. 

Fig. 4.9 Schematic cross-sectional view of a semiconductor (n-Si) electrode coated with sparsely scattered nanosized metal particles...

Abstract. IR pyrolysis of PAN and PAN based composites yields ordered graphitelike structure as well as several carbon nanostructures. Metal-carbon nanocomposites, in which the nanosized metal particles were introduced into the structure of carbon matrix in the course of IR pyrolysis of composite-precursor on the basis of PAN and metal (Gd, Pt, Ru, Re) compounds were prepared. The carbon phase of metal-carbon nanocomposites was shown to include different types of nano structured carbon particles. Bamboo-like CNT were observed in the structure of pyrolized at 910 and 1000°C composite-precursor based on PAN and GdCl3. At T=1200°C the solid carbon spheres with diameter in the range of 50-360 nm and octahedral carbon particles with the size in the range of 300-350 nm were observed. These nanostructured particles consist of carbon only or they include Gd nanoparticles incapsulated in carbon shell. IR pyrolysis of composite-precursor based on PAN as well as H2PtCl6 and RuC13 or NH4Re04 (Pt Ru(Re)=10 l) allows the preparation of Pt-Ru and Pt-Re alloys nanoparticles with 2[Pg.577]

Including into initial PAN solution metal compounds provides the formation of metal-carbon nanocomposites. The nanosized metal particles were introduced into the structured carbon matrix in the course of IR pyrolysis of composite-precursor on the basis of PAN and compounds of corresponding metals. In this way carbon composites containing nanosized Gd particles (4Efficient reduction of metal takes place in the course of IR pyrolysis of composite-precursor with participation of hydrogen, which is released in dehydrogenation of main polymeric chain of PAN. 

Mulvaney P (1996) Surface plasmon spectroscopy of nanosized metal particles. Langmuir 12 788-800... 

Mulvaney, P. (1996). Surface Plasmon Spectroscopy of Nanosized Metal Particles. Langmuir 12 788-800. 

Properties of the metal particles under study depends on the synthesis conditions such as availability of stabilizing medium, its composition, the type of reducer and temperature. Nanosized metal particles possess lyophobic properties. We have established a possibility to use the arabinogalactan polysaccharide for their stabilization in the aqueous medium. It efficiently reduces and stabilizes nanoparticles of silver and other noble metals. 11 is expected also that the stabilizer controls the electronic characteristics of metal nanoparticles. Our results below have confirmed this assumption. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

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. 

Many electrochemical studies relate to mesoscopic (i.e., nanosized) metal particles [147]. In one study, the nucleation and growth processes of Ag particles... 

The properties of a microemulsion will to a great extent depend on the nature of the surfactant. Surfactants may be non-ionic, anionic or cationic. Previous studies have shown that a suitable system for the preparation of metallic nanoparticles consists of a non-ionic surfactant such as pentaethyleneglycol-dodecylether, hexane and water (Figure 2). A water-soluble precursor can be added to the system and thus, a reasonable amount of nanosized metal particles may be obtained. In some particular cases, systems based on ionic surfactants such as AOT or cationic surfactants such as cetyltriammonium bromide (CTAB) will give a lower solubility of the metal precursor. ... 

Compounds. Model Systems for Nanosized Metal Particles... 

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