Small metal particles particle size effect

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

A remarkable optical effect of the microstructure is quantum confinement in very small (nanosized) particles. This size effect strongly modifies the optical behavior of semiconductors and metals. It is linked to the removal of the translation symmetry in crystalline solids. 

The Curie temperature, or that temperature where ferromagnetic substances become merely paramagnetic, has also been used to study small metal particles. Although magnetic effects are of great use in the study of small particles of ferromagnetic metals, great care must be exercised since chemisorbed species and the oxidation state of surface atoms can have dramatic effects on the experimental results. A few additional comments about electronic properties, as they depend on particle size, are in order. 

Practical metal catalysts frequently consist of small metal particles on an oxide support. Suitable model systems can be prepared by growing small metal aggregates onto single crystal oxide films, a technique whereby the role of the particle size or of the support material may be studied. [37] A quite remarkable example of the variation of the catalytic activity with particle size has recently been found for finely dispersed Au on a Ti02 support, which was revealed to be highly reactive for combustion reactions. [38] On the basis of STM experiments it was concluded that this phenomenon has to be attributed to a quantum size effect determined by the thickness of the gold layers. 

The curves of Fig. 12.17 nicely illustrate the varied optical effects exhibited by small metallic particles in the surface mode region, both those explained by Mie theory with bulk optical constants and those requiring modification of the electron mean free path (see Section 12.1). Absorption by particles with radii between about 26 and 100 A peaks near the Frohlich frequency (XF — 5200 A), which is independent of size. Absorption decreases markedly at longer... 

As Ponec has pointed out, infrared measurements should be more sensitive than UPS in differentiating various types of surface sites, and hence particle size effects. When infrared measurements are made, it is found that the characteristic frequency of the adsorbed CO molecule at a constant surface coverage is dependent on the particle shape.2280 As the particle shape is changed from flat (film or single crystal) to curved (small particles) there is a shift to higher (for Cu) or to lower (for group VIII metals) frequency. Ponec has reviewed possible reasons for these changes in vibrational frequency.208... 

The conclusion is that particle size effects on catalytic activity or selectivity due to variations in the inherent properties of small metal particles (geometric or electronic) are unlikely to be important for particles larger than about 1.5-2.0 nm. If size effects are observed for larger particles it is necessary to consider the nature and origin of such effects. 

One of the changes between small metal particles and bulk metal is the change in conductivity bulk metal is a conductor, small particles are insulators. In chapter 7 the development of a new in situ probe of metallic character in supported metal clusters utilizing X-ray absorption spectroscopy is described. A very strong support effect on the metal-insulator transition with cluster size in supported Pt clusters is found. Pt particles with basic supports show metallic screening for sizes as small as 6A. In contrast, with acidic supports the Pt particles do not show metallic behavior below 10A. 

Small metal particles on the nanometer dimension are of particular interest regarding their solid-state properties as well as their important application as catalysts. Such particles offer a useful model allowing the study of structural effects with relevance for a variety of applications in chemistry and physics. The key point of interest lies in the fact that such particles often possess very different and sometimes novel properties compared to those of bulk materials, i.e., their physical properties (spectroscopic, electronic, magnetic) differ from those of the bulk phase and are particle size-dependent [68]. One such property concerns the variation in the electrochemical redox potential between metals in a dispersed and bulk state, as was shown by Plieth [69], when he demonstrated that the redox potential depended on the radius (r) of a metal particle on nanometer dimension (the radius was assumed to vary from 1 to lOnm) according to the following equation ... 

Small metal particles as model systems for electrocatalysts are crucial to unravel the influence of electronic or geometrical structure on the catalytic activity. The effect of metal particle size on electrochemical reactivity has been proposed to exist for the electro-oxidation of alcohols as well as for the reduction of oxygen [7,75,76], both vital processes that require much deeper understanding for the development and... 

Hache, F., Ricard, D., Flytzanis, C. Optical non linearities of small metal particles Surface-mediated resonance and quantum size effects. J. Opt. Soc. Am. B 3, 1647-1655 (1986)... 

Fig, 6.7. a Site time yield in the processes described in the figure as a function of radius of the metal cluster involved in the catalytic processes. No particle size effect is observed down to small clusters, b Particle size effect for Pt clusters dispersed on Si02 in the recombination reaction of oxygen and hydrogen, c Particle size effect for Fe clusters dispersed on MgO for the ammonia synthesis at atmospheric pressure and 570° K. d Relationship between catalytic activity, dispersion of the metal cluster on support and bulk metal character of the cluster for metals of the group VIII... 

The selection of the carrier is relatively simple. It may be imposed by the type of reaction to be promoted. For instance, if the latter requires a bifunctional catalyst (metal + acid functions), acidic supports such as silica-aluminas, zeolites, or chlorinated aluminas, will be used. On the other hand, if the reaction occurs only on the metal, a more inert support such as silica will be used. In certain cases, other requirements (shock resistance, thermal conductivity, crush resistance, and flow characteristics) may dominate and structural supports (monoliths) have to be used. For the purpose of obtaining small metal particles, the use of zeolites has turned out to be an effective means to control their size. However, the problem of accessibility and acidity appearing on reduction may mask the evidence of the effect of metal particle size on the catalytic properties. 

Particle Size Effect.- As noted earlier, the small supported metal crystallites exhibit an increase in average particle size due to sintering. The type of metal(s) and support, the vapor phase, and the temperature selected play a role in the sintering rates... 

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