Reactivity as a function of the

Since dissociation of a molecule requires a certain minimum ensemble of surface atoms, the reactivity of very small metal clusters tends to increase with particle size. Once a cluster with the proper ensemble of atoms has become available the reactivity decreases, because the ensemble acquires an increasing number of metal atom neighbours. These two basic features, illustrated in Fig. 4.50, may explain the frequently observed maximum in cluster reactivity as a function of the number of metal atoms. 

Finally, other researchers consider the reactivity as a function of the chemical reactivity, dependent of temperature and reactants partial pressure but independent of conversion, and of a structural factor, solely dependent on the de ee of conversion . 

As we mentioned, oxide surfaces are important in the field of nanocatalysis by supported metals. In practical applications, the support has the crucial role of stabilizing small metallic particles, which act as the actual catalysts in a chemical process. Once the oxide surface is sufficiently well characterized, one can deposit small metal clusters and study their reactivity as a function of the support, of the metal, of the size of the cluster, etc. In this way, complex catalytic processes can be divided into a series of substeps, which allow a more detailed microscopic characterization. Despite the fact that only recently well-defined metal clusters have been deposited under controlled conditions on oxide surfaces and thin films, great advances have been obtained in the understanding of the mechanisms of adhesion and growth of the metal particles to the oxide surface. In this process, the role of theory is quite substantial. 

The investigation of the properties of metal clusters under different experimental conditions has provided useful information about the evolution of matter from the gas phase to the solid state. In particular, with respect to the kinetics of reaction, there is a large variation in reactivity as a function of the identity of the metal as well as cluster size. The explanation of these variations provide exciting new challenges. Understanding how the cluster geometry and electronic structure affect chemical reactivity can have an important influence in fields such as thin film coating and catalysis. 

Anionic reactivity as a function of the length of the alkyl chains bonded to the cationic center is shown in Table 8 In the series of hexadecyl-trialkylammonium salts the major increase in reactivity occurs when passing from the trimethyl to the triethyl derivative with higher homologous, the results are nearly the same. Determinations for methyl and ethyl derivatives were obviously limited to anhydrous homogeneous solutions, since these salts behave as typical surfactants under PTC conditions. 

When these results are viewed in terms of the hormonal activity of the substituted insulins, an interesting parallelism is noted between the effects of the substitutions on disulphide bonds reactivity and on the biological activity the disulphide bonds reactivity appears to be affected only by those substitutions which are known to abolish the biological activity of the hormone, when carried beyond a certain level. It was therefore planned to study the changes of biological activity and of -S-S- bonds reactivity as a function of the degree of substitution. This aspect was studied in detail on iodinated and methylated insulins. 

All methyl isomers of norbornene, 1-, 2-, 5-, and 7-methylnorbornene have been reacted in the presence of catalysts based on tungsten, rhenium, ruthenium, osmium and iridium compounds [7]. The polymers corresponded to ring-opened products having various microstructures. Racemic mixtures or pure enantiomers have been used as starting materials. Differences in reactivities as a function of the methyl position (1, 2, 5 or 7) and steric configuration (endo-exo and syn-anti) have been reported. 

The changes of the void reactivity as a function of the seed fuel region radius and ZrHi.7 thickness are shown in Fig. 7.4. In the case of the Super FR, using a 1-cm thick ZrHi.7 layer has an equivalent effect to decreasing the radius of the seed fuel region by 35%. When the layer thickness is above 2 cm, the effect is saturated. 

Non-stoichiometric Zn/Cr and Cu/Cr mixed oxides are one of the principal examples of these unusual solids. They have applications as both solid state gas sensors (5) and catalysts for hydrogenation reactions (of CO to methanol and/or methanol-higher alcohol mixtures, and of many organic molecules) (6-12). These systems have been widely investigated over the last few years, and results obtained show that their peculiar catalytic properties may be associated with the presence of non-stoichiometric phases (with a M /M ratio higher than 0.5, M= metal), in which some of the zinc or copper ions are present in octahedral positions, i.e., with an unusual coordination. However, until now very few data have been reported regarding the changes in structure and reactivity as a function of the composition in ternary systems (for instance Cu/Zn/Cr). 

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