Possible Explanations of Particle Size Effects Experiments versus Models

Possible Explanations of Particle Size Effects Experiments versus Models 

A coherent organization of the welter of data presented in the graphs of this article is no easy task. However, the attempt must be made, knowing that future results will challenge some of the ideas set forth. The generally accepted behavior of the reaction classes is maintained hydrogenations are less structure-sensitive than are most other reactions. For any one 

Among the systems presented in Figs. 9-12, only a few (see Table XV) show structure insensitivity, i.e., no variation of TOF with FE within the apparent precision of the data. This definition is narrower than that used by Boudart (346d). We think that the experimental methods now in use permit significance to be attached to changes of TOF of much less than an order of magnitude. Of course, reference is to the relative values (shapes of curves) for a given study. 

In what follows we shall emphasize certain ideas that arise from our study of the literature  

Studies on both supported small metal particles and naked metal clusters indicate that the TOF tends to zero as the number of atoms per particle tends to unity. This amounts to saying that, in the limit of the smallest particles, all systems show antipathetic structure sensitivity. We have used particles as a general term. They may be crystallites, but as the number of atoms per particle decreases their organization departs in general from that of a macroscopic crystal. These small particles ( 1000 atoms d = 2.5 nm) are called clusters in the literature on naked metal particles (347). However, organometallic chemists may reserve the term cluster to describe a structure of metal atoms at least partly terminated by various ligands. 

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