Nuclearity silver clusters

Ozin GA, Flugues F, Mattar SM, McIntosh DF (1983) Low nuclearity silver clusters in faujasite-type zeolites optical spectroscopy, photochemistry and relationship to the photodimerization of alkanes. J Phys Chem 87 3445-3450... 

The value of the critical nuclearity allowing the transfer from the monitor depends on the redox potential of this selected donor S . The induction time and the donor decay rate both depend on the initial concentrations of metal atoms and of the donor [31,62]. The critical nuclearity corresponding to the potential threshold imposed by the donor and the transfer rate constant value, which is supposed to be independent of n, are derived from the fitting between the kinetics of the experimental donor decay rates under various conditions and numerical simulations through adjusted parameters (Fig. 5) [54]. By changing the reference potential in a series of redox monitors, the dependence of the silver cluster potential on the nuclearity was obtained (Fig. 6 and Table 5) [26,63]. 

Fig. 6 compares the nuclearity effect on the redox potentials [19,31,63] of hydrated Ag+ clusters E°(Ag /Ag )aq together with the effect on ionization potentials IPg (Ag ) of bare silver clusters in the gas phase [67,68]. The asymptotic value of the redox potential is reached at the nuclearity around n = 500 (diameter == 2 nm), which thus represents, for the system, the transition between the mesoscopic and the macroscopic phase of the bulk metal. The density of values available so far is not sufficient to prove the existence of odd-even oscillations as for IPg. However, it is obvious from this figure that the variation of E° and IPg do exhibit opposite trends vs. n, for the solution (Table 5) and the gas phase, respectively. The difference between ionization potentials of bare and solvated clusters decreases with increasing n as which corresponds fairly well to the solvation free energy of the cation deduced from the Born solvation model [45] (for the single atom, the difference of 5 eV represents the solvation energy of the silver cation) [31]. 

Metal oligomers are stabilized at quite small nuclearity when formed at low dose in the presence of polyacrylate PA [85]. From pulse radiolysis of Ag" -PA solutions, it appears that the very slow (10 1 mol sec ) dimerization of Agi" silver oligomers [39,89] (275 and 350 nm) results into a blue-silver clusters ( = 4) absorbing at 292 and 800 nm [90], and stable in air for years (Fig. 3). The 800-nm absorption band is assigned to a cluster-ligand PA interaction [86,91]. Clear images by STM show flat clusters of 0.7 nm with atoms spaced by 0.25 nm (Fig. 3) [85]. Each cluster contains seven atoms (possibly with an eighth atom in the central position). Because only four atoms were reduced, they correspond to the stoichiometry Ag7 (or Agg ). 

Cluster properties, mostly those that control electron transfer processes such as the redox potential in solution, are markedly dependent on their nuclearity. Therefore, clusters of the same metal may behave as electron donor or as electron acceptor, depending on their size. Pulse radiolysis associated with time-resolved optical absorption spectroscopy is used to generate isolated metal atoms and to observe transitorily the subsequent clusters of progressive nuclearity yielded by coalescence. Applied to silver clusters, the kinetic study of the competition of coalescence with reactions in the presence of added reactants of variable redox potential allows us to describe the autocatalytic processes of growth or corrosion of the clusters by electron transfer. The results provide the size dependence of the redox potential of some metal clusters. The influence of the environment (surfactant, ligand, or support) and the role of electron relay of metal clusters in electron transfer catalysis are discussed. 

The increase of the redox potential of a metal cluster in a solvent with its nuclearity is now well established 1-4). The difference between the single atom and the bulk metal potentials is large (more than 2 V, for example, in the case of silver (3)). The size dependence of the redox potential for metal clusters of intermediate nuclearity plays an important role in numerous processes, particularly electron transfer catalysis. Although some values are available for silver clusters (5, 6), the transition of the properties from clusters (mesoscopic phase) to bulk metal (macroscopic phase) is unknown except for the gas phase (7-9). 

Figure 10 shows the nucleaiity dependence of silver cluster redox potential in water together with the data just presented, the previously published values are reported forn = 1 (3), 2 (23), 5 (5), 10 (24), and 11 (25). The E° values for nuclearities n = 1 and n = 2 resulted from thermodynamic calculations. The value for n = 10 was obtained from electron transfer studies where the clusters were the donor and were corroded by HaO. As a function of the... 

The redox potentials of short-lived silver clusters have been determined through kinetics methods using reference systems. Depending on their nuclearity, the clusters change behavior from electron donor to electron acceptor, the threshold being controlled by the reference system potential. Bielectronic systems are often used as electron donors in chemistry. When the process is controlled by critical conditions as for clusters, the successive steps of monoelectronic transfer (and not the overall potential), of which only one determines the threshold of autocatalytical electron transfer (or of development) must be separately considered. The present results provide the nuclearity dependence of the silver cluster redox potential in solution close to the transition between the mesoscopic phase and the bulk metal-like phase. A comparison with other literature data allows emphasis on the influence of strong interaction of the environment (surfactant, ligand, or support) on the cluster redox potential and kinetics. Rela-... 

The transient Ag3 + ion have an intense absorption spectrum with two maxima, at 310 and at 265 nm. Its second-order decay leads to the cluster Ag4 +. Under total reduction conditions the neutral dimer Ag2 is observed at 275 and 310 nm. The optical transitions of low-nuclearity silver oligomers, the rate constants, and the extinction coefficients are derived from adjustment between experimental (Fig. 2, bottom) and calculated absorption spectrum evolution. An even number of atoms favors the high stability of the magic hydrated clusters Ag4 + (275 nm), Agg + and possibly Agi4 + (Fig. 2, top). After a longer time, the plasmon... 

The value of the nuclearity of this critical cluster enabling transfer from the monitor depends on the redox potential of the selected donor, S . We studied electron transfer to silver clusters from the decay of different electron donors,... 

The dependence of cluster potential on nuclearity was obtained by changing the reference potential in a series of redox monitors (Table 5). The redox potentials of hydrated silver clusters are seen to increase with n. The data in Fig. 11 indicate that, at least for the redox properties of silver clusters, the transition between the meso-... 

When silver or gold atoms are generated from Ag CN)2 or Au (CN)2 in the presence of the methylviologen redox couple MV /MV, oxidation of the smallest clusters is also observed, because coalescence in cyanide solutions is slow (Fig. 4) [54,66]. While supercritical silver clusters ( > 6 1) (Table 3) accept electrons from MV with a progressive increase of their nuclearity, the subcritical clusters undergo a progressive oxidation by (Fig. 5). Actually, the reduced ions MV so produced act as an... 

Scheme 3.17 The synthesis of higher nuclearity telluride-tellurolato-bridged silver clusters from AgCI, PRjR, Te(SIMe3)2, and ArTeSiMej.

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