Clusters shape and morfology

Related publications:
1) I. Arčon, A. Tuel, A. Kodre, G. Martin, A. Barbier, J. Synchrotron Rad., 8, (2001), p. 575-577 (reprint)
2) A. Barbier, A.Tuel, I. Arčon, A. Kodre, G. Antonin Martin,
Journal of Catalysis Vol. 200 (2001) 106-116

We have shown that different routes of preparation of the metallic cobalt dispersed on microporous silica lead to formation of fcc Co clusters of different average sizes. Assuming a spherical shape of the clusters, the samples Co/SiO2-1, Co/SiO2-2 and Co/SiO2-3 contain clusters with average diameters of 13.5 A, 10.2 A, and 7.2 A, respectively.

Fig. 1. Relative coordination number in the first four neighbor shells as a function of cluster diameter. Left: (dots) - globular fcc polyhedra, (line) - spherical fcc clusters. Right: (sqare) - model globular fcc polyhedra, (cross) - experimental values for the three cluster samples.

The average sizes of the clusters, deduced from the observed reduction of the average number of neighbours are significantly smaller than those obtained by magnetic measurements or TEM. Such observations on catalytic samples have been reported previously by different authors [Dalmon et al., 1983, Shido et al., 1998]. An explanation of the discrepancy, besides the one cited earlier [Shido et al., 1998] which employs specific models of disorder at the cluster surface, can be given by an aggregate morphology of Co particles.

It should be emphasized that the directly determined experimental parameter in EXAFS analysis is the average number of neighbors. It is correlated linearly to the fraction of surface atoms in a cluster, as can easily be shown for large clusters: atoms in the inside contribute the full coordination number of neighbors (12 in fcc lattice), and those at the surface contribute a deficient number (9 in fcc). Thus, the average number of (first) neighbors is a direct measure of the specific surface of the dispersed metal, and (should be) a direct measure of the catalytic activity. The conclusion is largely independent of the cluster shape and size distribution. - The value of the cluster diameter, on the other side, depends critically on the assumptions of uniformity and globular shape. In the other two cases of simple shapes, cylindrical rods and platelets, the extracted size parameter (diameter of rods and thickness of the platelets) would differ for a small numerical factor (2/3 in fcc). However, it would invariably be the small shape parameter. The average number of neighbors is largely insensitive to the large size parameter, i.e. the length of the rods or the width of the platelets. The conclusion can be extended to aggregates of the simple shapes (dendrites of globules, rods or platelets) as long as the attachment area is small. )

Fig. 2. Globular fcc metalic clusters with increasing number of atoms and a corresponding increase in cluster diameter. Colors indicate consecutive shells of atoms.

If globular clusters with perfect fcc structure are formed, as assumed (in the EXAFS model, then EXAFS is shown to give reliable estimate of the average particle size [Borowski, 1997; Frenkel, 1999; Montano et al. 1986]. Larger aggregates are composed of small globular Co crystallites, attached to each other only by a small fraction of their surface, giving the large overall diameter in MM measurements. For example, samples Co/SiO2-3 and Co/SiO2-2 prepared in ammonia possess aggregates with diameters of ca. 44 A and 92 A, respectively, while the corresponding globule sizes are 7.2 and 10.2 A.

Tight aggregates are also possible, whereby the small globular clusters assume the role of domains in polycrystalline materials. The EXAFS estimate of the size refers to the region of short-range order within the domain. A similar model has been previously proposed for Ni/SiO2 catalysts [Dalmon et al., 1983] and MoS2 on oxidic or carbon carriers [Shido et al., 1998]. There is no direct relationship between the size of the domains and those of the large cobalt particles. It is also not clear however what mechanism would produce that particular morphology.

Fig. 3. Characteristic geometrical shapes of nanoclusters


Fig. 4. Relative coordination number in the first four coordination shells as a function of cluster diameter for three characteristic cluster shapes: sphere, cylinder, plate




Last change: 02-Jun-2006