Coalescence progressive nuclearity

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. 

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

By such a cascade of oxidation reactions, the nuclearity decreases and clusters are progressively corroded. Note that the process still coexists with the coalescence reactions which, in contrast, result in a nuclearity increase. 

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... 

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