Activity particle size, effect

Structure sensitivity is not a single phenomenon but can often be regarded as a set of independent, but interlinked mechanisms. At the very beginning it is important to realize that the term particle size effect (PSE) not only refers to the size of active component particles but moreover comprises effects deriving from peculiarities in their morphology, that is, their shape and structure. 

As was demonstrated in the preceding sections, structure-sensitivity phenomena are mostly confined to particle size regimes smaller than 3-4 nm. A process of industrial relevance was investigated by de Jong et al. [127] in their study on cobalt particle size effects in the Fischer-Tropsch reaction. Earlier works noted distinct drop in activity for Co particles smaller than lOnm and ascribed this phenomenon to either a partial oxide or carbide formation which should be enhanced for particles in this size regime [128-139]. In order to avoid similar effects, de Jong used... 

The results can be understood in terms of the influence of an intrinsic particle size effect (independent of the support) and a support-induced particle size effect. For both reactions and both supported metals, the intrinsic effect manifests itself in a decrease in activity with decreasing particle size below about 3 nm. 

Panagiotopoulou, P., Christodoulakis, A., Kondarides, D.I., and Boghosian, S. 2006. Particle size effects on the reducibility of titanium dioxide and its relation to the water-gas shift activity of Pt/Ti02 catalysts. J. Catal. 240 114—25. 

Zheng et al. investigated the NH3 splitting over Ru catalysts supported on differently treated CNTs. They observed a negative particle size effect, i.e., the H2 formation rate decreases at higher dispersion of the active component [37]. ft was also shown that... 

The ventilated and perfused human lung lobe was used as described by Linder and co-workers [74], A twofold difference in the appearance of drug and metabolites in the perfusate was found for the two formulations. Small fractions of the applied dose of BDP were immediately detectable in the perfusate and the amount of the major metabolite, beclomethasone-17-propionate (17-BMP), increased over the experimental period. These observations were similar to the clinical observations that BDP is detected rapidly in the plasma after inhalation and that the appearance of the active metabolite 17-BMP occurs rapidly. The kinetic differences between the formulations were explained on the basis of particle size effects with the conclusion that the discriminatory value of this system to examine the lung pharmacokinetics of inhaled medicines in the absence of systemic effects such as hepatic metabolism was apparent. 

It has been already mentioned in passing that indications exist in the literature showing that the 3C isomerization can take place by formation of at least two different 3C complexes, having different activation energies of isomerization, different particle size effects, different responses to alloying, etc. (157, 195-198). The suggestions presented above offer a choice of different complexes for further speculations. However, a definitive description of isomerization mechanisms under different conditions (H2 pressure, temperature, etc.) and with different catalysts (pure metals, alloys, etc.) is not yet possible. 

In view of this complexity and the evident interrelation between reaction and self-poisoning, it is actually less surprising that the literature does not offer a clear picture of the particle size effects in the HC reactions. Hydro-genolysis of ethane by Pt and lr reveals a decrease in TON by a factor of 10-15, when the dispersion D is increased (antipathic relation) from %0.1 to 0.8 (231). An early paper on ethane hydrogenolysis by Ni reports a strong and a later paper reports a less pronounced variation of activity with D. However, in contrast to Pt and lr, this variation is always a sympathetic one (232, 233). The same holds true for Rh (234). 

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. 

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