Nanosized metallic

This study relates to a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase. The water-alcohol method provided more dispersed catalysts than the impregnation method. The Cu particles of about 20 nm showed enhanced stability and higher activity than the particles larger than 40 nm. This was correlated with the distribution of copper particle sizes shown by XRD and TEM. Compared with silver and zinc, copper is better active and stable metal. 

Industrially, the perfluoroalkyl iodides by telomerization are mostly made by a batch system using peroxide initiators. However, the difficulty of mass production, and the production of hydrogen-containing byproducts in the process are disadvantageous [4]. In this study, a continuous process for the preparation of perfluoroalkyl iodides over nanosized metal catalysts in gas phase and the effects of the particle size on the catalytic activities of different the preparation methods and active metals were considered. 

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. 

Supported metal nanoparticles are of great interest in catalysis, because they offer the opportunity to combine the high reactivity and selectivity of the nanosized metals with the easy separation of the catalysts from the reaction mixture and recycling. 

This method ensures the deposition of very reactive metal nanoparticles that require no activation steps before use. We shall review here the following examples of catalytic reactions that are of interest in line chemical synthesis (a) the hydrogenation of substituted arenes, (b) the selective hydrogenation of a, 3-unsaturated carbonyl compounds, (c) the arylation of alkenes with aryl halides (Heck reaction). The efficiency and selectivity of commercial catalysts and of differently prepared nanosized metal systems will be compared. 

The main issue of the book is application of nanosized particles in both homogeneous and heterogeneous catalysis. A variety of reactions catalyzed by metal colloids or supported nanosized metals is discussed. The most intriguing reaction seems to be ethane hydrogenolysis catalyzed by Pt clusters on porous carrier and studied by G. A. Somorjai and his group. Another challenging observation by this group is shape isomerization of Pt metal particles affected by the addition of silver ions. 

The well-defined large bimetallic Au—Ag cluster as [(Ph3P)i2AuigAg2oCli4] can achieve nearly two orders of magnitude of attenuation of high-intensity laser power and, consequently opens the door to a new class of optical materials based on nanosized metal clusters 3269... 

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). 

The materials described in this chapter are denoted in the literature mostly as metal clusters or metal nanoclusters . However, the terminology metal clusters spans various scientific disciplines and has consequently multiple meanings, including plasmonic nanoparticles and various nanosized metallic structures. Therefore alternative names have been given, although they are at the moment supported only by a fraction of the scientific community quantum clusters [26], nanodots [27], metal quantum dots [25] and superatoms [28]. 

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. 

We sincerely thank the Sixth Framework Program Tailored nanosized metal catalysts for improving activity and selectivity via engineering of their structure and local environment N 506621-1 and NATO Science for Peace program SfP -974173. 

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]. 

Charge alteration on the surfaces of nanosized metallic silver particles has been investigated by simultaneously monitoring absorption and conductivity changes during pulse-radiolytic experiments [506]. Pulse radiolysis of a nitrous-oxide-(N20) saturated aqueous solution of 3.0 nm diameter metallic silver particles containing 0.2 M 2-propanol resulted in electron injection to the colloid. NzO functions to double the yield of hydroxyl radicals ( OH) generated in water... 

Fine tuning of the Fermi levels of nanosized metallic and size-quantized quasi-metallic particles by adsorbing (or desorbing) charges, ions, or molecules opens the door to the construction of tailor-made advanced materials [538]. 

Experimental systems used in the chemical generation and in situ optical monitoring of nanosized metallic particulate films are illustrated in Fig. 90. A precursor gas (CO, for example) may be injected in the arrangement shown in the upper part of Fig. 90, while that shown in the lower part of Fig. 90 permits the... 

The experimental set-up used in the generation and in situ monitoring of semiconductor particulate films is identical to that used for nanosized, metallic, particulate films. Evolution of a nanocrystalline particulate film, illustrated by the formation of sulfide semiconductor particulate films (Fig. 110), has been discussed in terms of the following steps [639] ... 

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