Clusters particle size effects

There are families of metal cluster compounds (Fig. 6.40) containing metal clusters surrounded by ligands (Lewis Green, 1982). In small cluster compounds, the electrons are paired, but in large clusters there will be closely spaced electronic levels, as in metal particles. In such clusters, quantum size effects would be expected. Benfield et al (1982) have found intrinsic paramagnetism in H20sio(CO)24 below 70K as expected of an osmium particle of approximate diameter of 10 A the excess paramagnetism increases with cluster size in osmium compounds (Johnson et al, 1985). 

Electron microscopic examination of these catalysts showed that the powders that exhibited Type A behavior had many particles less than 10 nm in diameter, whereas the powders showing Type B characteristics had relatively few of these small particles. 2 similar size effect was reported for Ni(P) catalysts prepared by the reduction of nickel orthophosphate.93 When the catalyst was reduced at temperatures near 400°C, Type A behavior was observed, whereas Type B behavior occurred with catalysts prepared at higher reduction temperatures. Here, too, a particle size effect was proposed to explain these results. Lower temperature reduction gives fine clusters of Ni interspersed with the Ni(P), and it was considered that these small particles were more effective for... 

When the catalytic properties of supported clusters are measured by standard electrochemical methods such as cyclic voltammetry or oxidation transient measurement, only the average properties of the entire distribution of active particles on the electrode surface can be measured. A range of properties of supported nanoparticles, e.g., their geometric structure, their electronic and magnetic properties, as well as their catalytic activity, depends on the size of the particles. Geometric as well as electronic effects have been used to explain particle-size effects in electrocatalysis. 

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... 

Particle size effects are important since they can influence the ratio of different surface facets along with the ratio of step, kink and terrace sites. In addition, as the particle sizes becomes smaller than a critical size, they can take on unique behavior owing to quantum size effects. When a molecule adsorbs there is an attractive interaction between the molecule and the atoms at the catalyst surface. Bonds within the molecule, as well as bonds within the metal cluster, tend to weaken. The overall interaction energy is then the sum of these three terms. Cluster size effects specifically alter the response of the... 

In order to obtain a homogenous and stable latex compound, it is necessary that insoluble additives be reduced in particle size to an optimum of ca 5 )Tm and dispersed or emulsified in water. Larger-size chemical particles form a nucleus for agglomeration of smaller particles and cause localized dispersion instabiHty particles <3 fim tend to cluster with similar effect, and over-milled zinc oxide dispersions are particularly prone to this. Water-soluble ingredients, including some accelerators, can be added directly to the latex but should be made at dilute strength and at similar pH value to that of the latex concentrate. 

As we have seen in the previous chapter, the apparent topography and corrugation of thin oxide films as imaged by STM may vary drastically as a function of the sample bias. This will of course play an important role in the determination of cluster sizes with STM, which will be discussed in the following section. The determination of the size of the metallic nanoparticles on oxide films is a crucial issue in the investigation of model catalysts since the reactivity of the particles may be closely related to their size. Therefore, the investigation of reactions on model catalysts calls for a precise determination of the particle size. If the sizes of the metal particles on an oxidic support are measured by STM, two different effects, which distort the size measurement, have to be taken into account. 

Although the De Broglie wavelength of free electrons is 0.1 nm, the value of an electron in a small crystallite can be much larger because the effective mass of electrons in a small particle is considerably smaller. Energy levels evolve from HOMO and LUMO to those of clusters, Q-sized particles, and finally bulk semiconductor. Figure 7.7 shows the energy levels in bulk- and Q-sized particulate semiconductors. 

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