Defective surface clusters

FIG. 24. Thermodynamic path for Ag cluster growth on defect surface (extended Hiickel calculation). 

This combined theoretical-experimental approach shows the potentiality of generating defective surfaces to modify the chemical activity of supported nanoaggregates for catalytic applications. It also shows the importance to obtain an atomistic control on the species deposited on the smface for the proper characterization of their activity. Through a specifically designed experimental apparatus which makes it possible to deposit size-selected metal clusters it has become possible to study the properties of a supported cluster as a function of its size. This opens new, unprecedented perspectives for the understanding of size-dependent activity and selectivity of a supported metal catalyst. To reach this ambitious goal theory plays a role which cannot be overlooked. 

Electron microscopic studies in the 1940s proved that supported catalysts possess a crystalline structure, dispelling earlier conjecture of amorphicity. However, practical catalysts are never uniform, exhibiting particle size distribution, lattice defects (Frenkel or Schottky), and dislocations. The following questions then arise Are all lattice surfaces equally active Do surface clusters of particles and surface atoms have comparable activity Does catalytic activity depend on particle size Is there an optimal particle size or distribution These questions remain, in general, still unanswered. However, in recent electrocatalytic studies concern about these effects is shown, following similar concern in conventional heterogeneous catalysis. 

Fig. 5 The number of times a given structure is found in 40 different simulations as a function of cluster size N for Lennard-Jones clusters. Irreg, DH, MIC, s-IC, r-PIC, and d-Cryst mark irregular, decahedral, multishell icosahedral, surface-centered icosahedral, regular polyicosahedral, and defective crystalline clusters, respectively. Reproduced with permission of American Physical Society from ref 15.

Band and cluster calculations of ideal and defect surfaces of d-metal... 

The Characterization and Properties of Small Metal Particles. Y. Takasu and A. M. Bradshaw, Surf. Defect. Prop. Solids p. 401 1978). 2. Cluster Model Theory. R. P. Messmer, in "The Nature of the Chemisorption Bond G. Ertl and T. Rhodin, eds. North-Holland Publ., Amsterdam, 1978. 3. Clusters and Surfaces. E. L. Muetterties, T. N. Rhodin, E. Band, C. F. Brucker, and W. R. Pretzer, Cornell National Science Center, Ithaca, New York, 1978. 4. Determination of the Properties of Single Atom and Multiple Atom Clusters. J. F. Hamilton, in "Chemical Experimentation Under Extreme Conditions (B. W. Rossiter, ed.) (Series, "Physical Methods of Organic Chemistry ), Wiley (Interscience), New York (1978). 

Abstract This review is a summary of supported metal clusters with nearly molecular properties. These clusters are formed hy adsorption or sirnface-mediated synthesis of metal carbonyl clusters, some of which may he decarhonylated with the metal frame essentially intact. The decarhonylated clusters are bonded to oxide or zeolite supports by metal-oxygen bonds, typically with distances of 2.1-2.2 A they are typically not free of ligands other than the support, and on oxide surfaces they are preferentially bonded at defect sites. The catalytic activities of supported metal clusters incorporating only a few atoms are distinct from those of larger particles that may approximate bulk metals. 

Fig. 4 Osmium clusters supported on MgO(OOl) a OssC/MgisOs and b OS5C at a surface point Vs defect site [33] these were represented by density functional theory, and the samples were characterized by EXAFS spectroscopy, transmission electron microscopy, and other techniques [15]...

This prompted us [111 to try to represent CeoMu by clusters of carbon atoms, CisHuMu and C30H12MU, the external atoms being constrained to lie on a part of a spherical surface with the same radius as Ceo- The results were very similar to the CeoMu calculations with partial geometry optimisation to suggest that this adduct did not depend on the full structure but corresponded to a locdised defect , both structurally and electronically. 

Akola, J. and Hakkinen, H. (2006) Density functional study of gold atoms and clusters on a graphite 0001 surface with defects. Physical Review B - Condensed Matter, 74, 165404-1-165404-9. 

Monte Carlo simulations of this system have shown that by sweeping the chemical potential of Cu, a cluster is allowed to grow withiu the hole risiug four atomic layers above the surface. As iu the experimeuts, it is fouud that its lateral exteusiou remaius coufiued to the area defiued by the borders of the origiual defect. 

Over single crystal surfaces with defect sites, vacuum deposition of gold vapor or size-selected gold anion clusters at low temperatures can lead to relatively homogeneous... 

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