Monosized clusters

Structure-sensitive reactions are extensively discussed in the catalytic literature, but careful examination of the published work reveals that on the atomic scale the catalytic materials used in these studies are in general poorly characterized with respect to particle size and structure. Extended X-ray absorption fine structure (EXAFS) has been successfully applied to the study of small particles on supportsand small metal molecules in matrices subject to the caveat that samples of these materials consist of a distribution of particle sizes. Information thus obtained is an average over the entire distribution. Supported, monosized clusters have not yet been used in catalytic studies. However, Woste and coworkers demonstrated in the first experiment where monosized clusters were deposited that Ag4 is the critical cluster... 

Selection of monosized cluster ions by mass spectrometry. 

As discussed earlier, it is now possible to make and study deposits of monosized, highly dispersed, transition metal clusters.(S) In this section we summarize results from the first measurements of the valence and core level photoemission spectra of mass selected, monodispersed platinum clusters. The samples are prepared by depositing single size clusters either on amorphous carbon or upon the natural silica layer of a silicon wafer. We allow the deposition to proceed until about 10 per cent of the surface in a 0.25 cm2 area is covered. For samples consisting of the platinum atom through the six atom duster, we have measured the evolution of the individual valence band electronic structure and the Pt 4f... 

Figure 9.9. Clusters formed in a porous or pigmented body can be described by a hyperbolic function which constitutes a fractal dimension in data space, a) Simulated dispersion of monosized pores which occupy 20 % of the available space, b) Typical clusters which may be found in such a dispersion, displaying orthogonal and diagonal connections, c) Size distribution of the clusters present in (a) presented as a log-log plot.

Under the conditions used by Stober et al. [27], primary particles formed by nucleation, growth, and ripening undergo an ordered aggregation process to form monosized spherical particles. This process represents reaction-limited monomer-cluster aggregation occurring concurrently on two length scales. 

Stripping of the ligands of organometallic cluster compounds by thermolysis [1] or (laser) photolysis [2, 3] may produce the respective monosized (naked) metal clusters, if these highly reactive species can be prevented from aggregation by, e.g., attachment to isolated... 

Formation of monosized supported clusters by the above method seems to be, however, of limited success, apart from an example referred to below [1]. A single cluster size organo-metallic complex will generally lead to a distribution of cluster sizes [7], and clusters (particles) with diameters d up to 20 A (n up to 280) were observed by electron microscopy (EM see abbreviations, p. 320) [1]. For this more general case of cluster production ( supported catalysts ) from organometallic compounds, proceeding probably via migration of atoms, see a recent review on EXAFS studies in catalysis [8] see also an older review by Yermakov and Kuznetsov [9]. 

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