Metallic clusters potential determination

Cryophotochemical techniques have been developed that (i) allow a controlled synthetic approach to mini-metal clusters 112), Hi) have the potential for "tailor-making small, bimetallic clusters (mini-alloy surfaces) 114,116), Hi) permit the determination of relative extinction-coefficients for naked-metal clusters 149), and iv) allow naked-cluster, cryophotochemical experiments to be conducted in the range of just a few atoms or so 112,150,151). 

Ionization potential of metal clusters is one of the factors affected by cluster size [33]. This study represents the most extensive effort so far to determine the size dependence of IP. The measurements on these clusters showed a decreasing IP with size with apparent oscillatory trend. Even-size particles had a relatively larger IP compared to their odd-size counterparts. The data show oscillatory behavior for small Na clusters with a loss of this oscillation for the larger Na clusters. The IP decreases with cluster size, but even at Nai4 the value 3.5 eV is far from... 

The recent interest in the exploration of electrocatalytic phenomena from first principles can be traced to the early cluster calculations of Anderson [1990] and Anderson and Debnath [1983]. These studies considered the interaction of adsorbates with model metal clusters and related the potential to the electronegativity determined as the average of the ionization potential and electron affinity—quantities that are easily obtained from molecular orbital calculations. In some iterations of this model, changes in reaction chemistry induced by the electrochemical environment... 

The Knudsen effusion method In conjunction with mass spectrometrlc analysis has been used to determine the bond energies and appearance potentials of diatomic metals and small metallic clusters. The experimental bond energies are reported and Interpreted In terms of various empirical models of bonding, such as the Pauling model of a polar single bond, the empirical valence bond model for certain multiply-bonded dlatomlcs, the atomic cell model, and bond additivity concepts. The stability of positive Ions of metal molecules Is also discussed. 

The adiabatic and vertical ionization potentials for Lis are very similar, both being approximately 3.95 eV. This results because of the similar geometries for the 82 state of LisCCsv) and the Ai state of Li3" (D3h). It must be noted, however, that the vertical ionization process for the removal of an electron from linear Lis to give linear Lis leads to the higher ionization potential, 4.39 eV. If both Csv and forms are present in an experiment, a complicated threshold dependence for the ionization process will be observed (j4). The results for Lis exemplify that the fluxional nature of a small metal cluster may complicate the experimental determination of electron affinities and ionization potentials. 

By changing the reference potential in a series of redox monitors, it is then possible to determine the dependence of the cluster potential on the nuclearity. The general trend of increasing redox potential with nuclearity is the same for all metals in solution as it is illustrated in Fig. 2 in the case of E°(AgVAg,) q. However, in gas phase, the variation of the ionization potential IV(Ag ) exhibits the opposite trend versus the nuclearity n. Indeed, since the Fermi potential of the normal hydrogen electrode (NHE) in water is 4.5 eV, and since the solvation free energy of Ag decreases with size as deduced from the Born model, one can explain the two opposite variations with size of F°(Ag /AgJ q and IP (AgJ as illustrated in Fig. 2. 

Fig. 4. Ionisation potentials of Hg, plotted against The bulk value (r" = 0) is at the left, the atomic value (r" = 1) at the right-hand side. The two solid lines starting at (bulk work function) correspond to the two scaling laws proposed for metallic clusters [eqn (7)]. Determined in this experiment O, from ref. (4) and (5) X, from the calculation of ref. (8) and (9). From very large dusters down to R 100 the binding is metallic. After a transition region (hatdied area) one has covalent bonding for 70 < r < 30, after a second transition region van der Waals bonding becomes dominant below R < 13.

In summary, ionisation potentials, dissociation and cohesive energies for mercury clusters have been determined. The mass spectrum of negatively charged Hg clusters is reported. The influence of the transition from van der Waals (n < 13), to covalent (30 < n < 70) to metallic bonding (n > 100) is discussed. A cluster is defined to be metallic , if the ionisation potential behaves like that calculated for a metal sphere. The difference between the measured ionisation potential and that expected for a metallic cluster vanishes rather suddenly around n 100 Hg atoms per cluster. Two possible interpretations are discussed, a rapid decrease of the nearest-neighbour distance and/or the analogue of a Mott transition in a finite system. Electronic correlation effects are strong they make the experimentally observed transitions van der Waals/covalent and covalent/metallic more pronounced than calculated in an independent electron theory. 

Model electrodes with a dehned mesoscopic structure can be generated by a variety of means, e.g., electrodeposition, adsorption from colloidal solutions, and vapor deposition and on a variety of substrates. Such electrodes have relatively well-dehned physico-chemical properties that differ signihcantly from those of the bulk phase. The present work analyzes the application of in-situ STM (scanning tunneling microscopy) and ETIR (Eourier Transformed infrared) spectroscopy in determining the mesoscopic structural properties of these electrodes and the potential effect of these properties on the reactivity of the fuel cell model catalysts. Special attention is paid to the structure and catalytic behavior of supported metal clusters, which are seen as model systems for technical electrocatalysts. 

Figure 1. Principle of the determination of short-lived cluster redox potential by kinetics methods. The reference electron donor, S of a given potential and the metal atoms are generated by a single puke. During cluster coalescence, the redox potential of the couple E°(M -Mn) progressively increases, so that an effective transfer is observed after a critical time when the cluster potential becomes higher than that of the reference, constituting a threshold. Repeatedly, a new adsorption of excess cations, M, onto the reduced cluster, (n xkch (dlows another electron transfer from S with incrementation of nuclearity. The subcritical clusters Mn(n

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