Metal particle shape

Figure 12 Effect of metal particle shape on the TOP at 300 K for CO oxidation over Au/Ti02 and Pt/Ti02- Spherical particles corresponds to the sample prepared by impregnation and hemisperical particles by deposition-precipitation method. (Data are presented in Bamwenda, Catal. Lett. 44 (1997) 83.)...

Since a milestone paper in Science in 1996, the group of El-Sayed [238-240] has led the field of synthesis and applications of shape-specific nanoparticles. A classical example of the detailed study of metal-particle shape effects in catalysis was reported by Narayanan and El-Sayed using unsupported polyvinylpyrollidone (PVP) polymer-capped Pt shape-specific colloids (cubes, spheres, and tetrahedra) in a model electron transfer reaction between hexacyanoferrate(III) and thiosulfate... 

Size control has also been reported for the sonochemical decomposition method and y-radiolysis." "" The domain of preparation methods using constrained environments offers control over the metal particle shape by predetermining the size and morphology of the products in nanoscale reaction chambers. 

Clusters are intennediates bridging the properties of the atoms and the bulk. They can be viewed as novel molecules, but different from ordinary molecules, in that they can have various compositions and multiple shapes. Bare clusters are usually quite reactive and unstable against aggregation and have to be studied in vacuum or inert matrices. Interest in clusters comes from a wide range of fields. Clusters are used as models to investigate surface and bulk properties [2]. Since most catalysts are dispersed metal particles [3], isolated clusters provide ideal systems to understand catalytic mechanisms. The versatility of their shapes and compositions make clusters novel molecular systems to extend our concept of chemical bonding, stmcture and dynamics. Stable clusters or passivated clusters can be used as building blocks for new materials or new electronic devices [4] and this aspect has now led to a whole new direction of research into nanoparticles and quantum dots (see chapter C2.17). As the size of electronic devices approaches ever smaller dimensions [5], the new chemical and physical properties of clusters will be relevant to the future of the electronics industry. 

The manufacture of metal in powder form is a complex and highly engineered operation. It is dominated by the variables of the powder, namely those that are closely connected with an individual powder particle, those that refer to the mass of particles which form the powder, and those that refer to the voids in the particles themselves. In a mass of loosely piled powder, >60% of the volume consists of voids. The primary methods for the manufacture of metal powders are atomization, the reduction of metal oxides, and electrolytic deposition (15,16). Typical metal powder particle shapes are shown in Figure 5. 

Fig. 5. Metal powder particle shapes (a) atomized copper (b) sponge iron and (c) atomized iron.

Slip casting of metal powders closely follows ceramic slip casting techniques (see Ceramics). SHp, which is a viscous Hquid containing finely divided metal particles in a stable suspension, is poured into a plaster-of-Paris mold of the shape desired. As the Hquid is absorbed by the mold, the metal particles are carried to the wall and deposited there. This occurs equally in all directions and equally for metal particles of all sizes which gives a uniformly thick layer of powder deposited at the mold wall. 

On a given metallic particle, the repulsive force, E, is dependent on particle mass, AF electrical conductivity. O density, p and shape, s. 

This subject effects designers since many products have the requirement by regulations or otherwise to use recycled plastics. Different methods are used to recycle materials to provide plastics with a continuing life. Method used is influenced by factors such as costs, quantity involved, weight involved, size and shape, complexity of mixed types of plastics, extended of contamination such as metallic particles, continued availability of material, etc. (Recognize that they can also be used as energy sources through incineration that can be combined with production of electricity and/or hot water for example). 

Figure 8.12. Shape of fee metal partieles for different values ofthe metal—support interfaee energy (y/yo as a funetion ofthe ratio y/yo)-For y/yo = 1 there is no energy gain and the particle will behave as a free metal particle, while for y/yo = -1 the particle will try to...

What determines the shape of a metal particle in a vacuum What determines the shape of a metal particle on a substrate ... 

The shape of the nanoparticles depends on numerous parameters such as the nature of the metal and the support, the metal loading. Of the various models of polyhedral metal particles [106], the cubooctaedral structure can be used to represent small metallic particles (Scheme 31). Note that these idealized structures can vary with the nature of chemisorbed species (vide infra) and very subtle atomic rearrangements probably occur during catalytic events. 

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. 

Industrial catalysts are usually composed of inorganic supports and metals on the supports. They are often prepared by heat treatment of metal ions on the support at high temperature sometimes under hydrogen. They have very complex structures. For example, they are the mixtures of metal particles with various sizes and shapes. Metal particles often strongly interact with the inorganic supports, thus resulting in the structure of half balls. 

Before investigating the effect of size, shape, and structure on catalytic behavior, that is, TOFs, a set of five requirements concerning the metal particles has to be met. Besides a monodisperse size distribution, the nanoparticles should be fully reduced, unpoisoned, unperturbed by... 

The reduction of transition metal salts in solution is the most widely practiced method for synthesis of metal colloidal suspensions [7]. In the preparation process, polymer is often used in order to prevent the agglomeration of metal particles as well as to control their size. Ahmadi et al. [5] reported that the concentration of the capping polymer affects the shape of platinum particles obtained by salt reduction. This means that the addition of a... 

For the amino-borane dehydrocoupling using [Rh(l,5-cod)(p-Cl)]2 as starting catalyst, an induction period and a sigmoid-shaped kinetic curve (plot of substrate conversion versus time) were also observed, consistent with metal-particle formation. But, for Ph2PH BH3... 

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. 

---