Very small clusters

Finally, the beam — composed mainly of single substrate and solvent molecules and very small clusters — is passed through a heated wire grid, where the last declustering and desolvation occurs, leaving a beam of substrate molecules. 

The presence of both single gold atoms and gold clusters, evident in Figure 3(b), indicates that the deposition flux is largely atomic or very small clusters as expected from the process gas pressure. 

The major disagreement in the data rests in the reactivity of the very small clusters. Originally only the Rice group found clusters smaller than Feg to react. Subsequent studies at Exxon have found Fe4 to react at least as rapidly as Fe. One possible... 

Local coordi native saturation. For the very small cluster the idea oT drawi ng on TTie fi eld o"f stable cluster complexes is very... 

Noble metals (e.g., Pt) can be introduced within the micropores of zeolites by exchange with a complex cation (e.g., Pt(NH3)4 ) followed by calcination and reduction. This mode of introduction generally leads to very small clusters of Pt (high Pt dispersion) located within the micropores. Pt supported on acid zeolites are used as bifunctional catalysts in many commercial processes. The desired transformations involve a series of catalytic and diffusion (D) steps, as shown in n-hexane isomerization over Pt acidic zeolite (Equation 12.1). 

Thus, theoretical calculations can be a very useful tool in mechanistic understanding. As it is now known that the most active Co-Mo-S or SBMS catalysts consist of very small clusters of Mo atoms (as few as seven), it is within the realm of practical computational procedures to completely model the catalyst/S-molecule interactions. Assumptions made about steric crowding around the catalytic site may be quite different for such small clusters as the catalytic site is not an extended planar surface, as discussed in the previous section. Future work in this area should provide new insight into the true limitations in HDS of dialkyldibenzothiophenes. 

Most EXAFS data show that the lattice parameter of a small particle is contracted as compared with the bulk metal. For example, Renouprez and co-workers125,135 observe for Pt encased in Y-zeolite a Pt-Pt distance contracted from 0.277 to 0.270 nm. Similarly, Hamilton et al.123 observe Cu-Cu distances varying from 0.233 for very small clusters, to 0.250 for... 

Theoretical calculations, for example, for the adsorption of H2 on transition metals also show that much smaller clusters are required for chemisorption than for bulk properties.229 230 Also, for CO adsorption, extended Hiickel calculations on chains of Ni atoms showed only minor changes when the chain length exceeded 3 Ni atoms.231 This suggests a very small cluster as compared with the adsorption of halogen (see above). If the calculations are reliable this difference may reflect differences in the mode of bonding of CO and halogens.165... 

In the EXAFS and Electron Energy Loss Spectroscopy (EELS) spectra [11] show no trace of Co oxide, however XAS spectra of the smallest particles present characteristic peaks of CoO bonds in the L3 contribution. The fraction of Co bonded to oxygen atoms is proportional to the fraction of paramagnetic atoms, or very small clusters. However, for (d > 2 nm this... 

In this section we shall try to review available data on catalytic properties of electron-deficient palladium on supported catalysts. The term electron deficient we shall consider to mean very small clusters of Pd on various supports, and/or palladium ions stabilized by virtue of their presence in an appropriate chemical environment. 

The sharp drop in the core-hole electron attraction U compared to the bulk value (AU, Figure 3) around N = 4.5 and 6.5 for the basic and acidic supports respectively is obviously due to the onset of the insulator to metal transition. The size of AU (= 3.5 eV) for the very small clusters is consistent with previous estimates of U(5d5d) = 3-6 eV for the nearly filled 5d band transition metals when efficient metallic screening is shut off. Most interesting is the large difference in the behavior of the core-hole electron attraction for the basic vs. acidic supports. This difference dramatically reveals the importance of the support in determining the nature and onset of the metal-insulator transition. It provides direct experimental evidence for a metal-support interaction. 

Figure 6a shows a diffraction pattern showing the (1 0 0) orientation of the Pd clusters. The good epitaxy is also reflected from the TEM picture (Fig. 6b) that show clusters with a square outline that are all in the same orientation. Figure 6c shows a profile view of such a particle. From these two images it is now possible to reconstruct the actual shape which is a truncated octahedron exhibiting mainly (111) facets and small (1 0 0) truncations on the top and at the edges (Fig. 6d). After coalescence the particles become flatter and they exhibit now a majority of (1 0 0) facets (see Fig. 6e). Very small clusters have a different shape they are truncated half octahedron as shown in Fig. 6f. This different shape has been...

Atomistic theory of nucleation — The theory applies to very small clusters, the size n of which is a discrete variable and the process of nucleus formation must be described by means of atomistic considerations. Thus, the thermodynamic barrier AG ( ) that has to be overcome in order to form an n-atomic nucleus of the new phase is given by the general formula [i-v]... 

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