Metal particle nanosize

Size effect and surface effect. Metal particle size is a very important property of supported metal catalysts. The control of metal particle nanosize is of utmost importance for the performance of any industrial catalyst based on supported metals. 

Fig. 3.90 Possible dependence of the fraction of surface atoms working as active sites on metal particle nanosize ...

Dotted orange line No dependence of the fraction of surface atoms working as active sites on metal particle nanosize (structure-insensitive reactions). Full dark blue line Dependence of the fraction of surface atoms working as active sites on metal particle nanosize as in Fig. 3.90 (structure-sensitive reactions)... 

Measurement of metal particle nanosize on the surface of catalyst... 

Physical techniques and chemisorption method can be used to measure the metal particle nanosize on the smface of catalyst. 

For catal3dic purposes, once the correct surface average metal particle nanosize dav is known, it is convenient to calculate the number of total surface metal atoms per catalyst gram (Ns), given by the equation ... 

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. 

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 development of combinations of two or more functional phases is one of the most promising approaches to develop novel materials with specific functional properties. It is known that the immobilization of nanosized noble metal particles... 

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]. 

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