Metal particle size dependence

Titania and silica glass thin films Au, Pt Photoreduction of HAuCl and K.2PtCl4 in ethanol-water in the presence of poly(N-vinyl-2-pyrrolidone) or poly(methyl vinyl ether led to metal particles (sizes depended on solvent composition the smallest, 2.8 nm in diameter, was obtained in 100% alcohol) which were mixed with Ti(i-OC3H7)4 and acetylacetone under N2. Subsequent to 30 minutes of stirring, exposure to moisture produced Ti02-embedded metal particles 74... 

N,N-dimethylacetamide, followed by film casting, and reduction of metal salts by sodium borohydrate.2 The metal particle size depended on metal salt and concentration. 

As illustrated in Figure 10.28 in heterogeneous catalysis for nanosized metal particles, one distinguishes three types of transition-metal particle-size-dependent behavior. When activation of a-type CH bonds is rate controlhng, the rate of reaction normalized per surface atom, TOF (turn over frequency) tends to increase with a decrease in particle size. This is the behavior according to curve II. It is due to the relative increase in the fraction of more reactive coordinatively unsaturated surface atoms. 

The size of metal particles in cogelled catalysts has been examined extensively [24, 28, 30, 32, 33, 39, 40, 109, 152]. Often, metal particles are distributed into two families of different size a majority of small nanoparticles in the range 2-5 nm (small black points in Fig. 7) and a few large particles in the range 10-100 nm (four large black points in Fig. 7). Metal particle size depends on metal nature and loading as well as on many synthesis variables. By optimizing the latter ones, it is however possible to obtain small nanoparticles only with a narrow size distribution (2-3 nm) [40]. 

Metal/polymer nanocomposites were prepared by Chen and co-workers (55) using dispersion of metal chlorides in polyurethane. Both pol5nirethane and metal salts were dissolved in iV,A( -dimethylacetamide, followed by film casting and reduction of the metal salts by sodium borohydrate. The metal particle size depended on the type of metal salt used and on its concentration. 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

General route of the preparation of ME containing nanoparticles consists in the addition of an aqueous metal precursor to the surfactant-containing hydrocarbon forming ME. The size of the final metallic particle will depend... 

In the early work on the thermolysis of metal complexes for the synthesis of metal nanoparticles, the precursor carbonyl complex of transition metals, e.g., Co2(CO)8, in organic solvent functions as a metal source of nanoparticles and thermally decomposes in the presence of various polymers to afford polymer-protected metal nanoparticles under relatively mild conditions [1-3]. Particle sizes depend on the kind of polymers, ranging from 5 to >100 nm. The particle size distribution sometimes became wide. Other cobalt, iron [4], nickel [5], rhodium, iridium, rutheniuim, osmium, palladium, and platinum nanoparticles stabilized by polymers have been prepared by similar thermolysis procedures. Besides carbonyl complexes, palladium acetate, palladium acetylacetonate, and platinum acetylac-etonate were also used as a precursor complex in organic solvents like methyl-wo-butylketone [6-9]. These results proposed facile preparative method of metal nanoparticles. However, it may be considered that the size-regulated preparation of metal nanoparticles by thermolysis procedure should be conducted under the limited condition. 

Such a strong analogy between the electronic behavior of the metallic core of large molecular clusters and small metal particles was already suspected by Basset, Primet et al., who had discovered already in 1975 [21] that the extent of back donation from the core of a metal particle to [NO] (a ligand isoelectronic to CO) adsorbed Pt/alumina catalyst was particle size dependent as if the small particles were behaving as molecular clusters, since the extent of back donation on coordinated CO was shown clearly to be dependent on the size of the cluster in the series of molecular [PtsCCOJ j]" (n = 1-5, etc.) clusters made by Chini s group [22]. 

Returning now to the observed effect of particle size on catalytic activity, van Hardeveld and Hartog 219) have calculated that the relative concentration of C7 sites on octahedral iron crystallites decreases with decreasing particle size and that, in general, the C7 site is not a small-particle surface site. The above correlation of increased catalytic activity with increased C7 site surface concentration thus also explains the observed structure sensitivity (particle size dependence) for this reaction. Finally, this correlation is consistent with results obtained from field electron microscopy of iron (220), single crystal reaction studies on tungsten (also a bee metal) (227), and symmetry considerations (222). 

The velocity of propagation was found to depend strongly on the particle size of the metal for Ti, Zr and Nb, whereas for Ta, such a dependence was not observed. Particle size did not have a significant effect on the total conversion, indicating that to achieve a high conversion, reduction of the metal particle size to very small values was not a prerequisite. However, systems of coarser particles exhibited a narrower ignition region and were more prone to instabilities. 

Since the late 1960s there has been some interest in the concept of a structure-sensitive reaction in heterogeneous catalysis (177, 178). In the case of supported metal catalysts, structure sensitivity is visualized as a dependence of metal particle size and catalytic behavior in a given reaction (activity and selectivity). Almost all of the possible kinds of relationships were reported in the past. Recently, Che and Bennett reviewed this problem (161). Our intention here is not to repeat most of their analysis, rather we shall try to present our view on the general characteristics of palladium versus other platinum metals. 

The structure and stability of small gold particles is a function of the chemical and physical nature of the support on which they reside.7,116,151>152 Jt is clear that the extent of the influence of the support on a metal particle will depend on the fraction of the metal atoms directly in contact with it for particles of the same shape this will increase as the size decreases, but it will also depend on the shape of the particle, which is conditioned by the chemical forces at the interface. In principle the particle shape is determined by the contact angle 0 defined by the equation... 

Gold particle size depends not only on the method of preparation, but also on the conditions used for the subsequent treatment performed to obtain the metal. Indeed, as mentioned in the introduction to this chapter, for most methods gold is in the +3 oxidation state after drying (the so-called as... 

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