Coalescence and reactivity of gold-silver bimetallic clusters in cyanide solution

Pulse radiolysis of mixed solutions of two monovalent metals, Ag-I(CN)(2)(-) and Au-I(CN)(2)(-), has permitted the dynamics of the alloying process between gold and silver and the reactions of the alloyed cluster to be followed from the nanosecond to the second time range. The pulse induces simultaneous reduction of the ions into silver and gold atoms. Then, through coalescence and association with excess ions, homologue or different, these coexisting atoms give rise to transient bimetallic alloyed clusters. However, in the interval 2-20 s the clusters are enriched in silver at the expense of gold, the process being assigned to a slow intermetal electron transfer from gold atoms (less noble in the presence of CN-) to silver ions adsorbed on clusters. In the presence of the electron acceptor/donor couple MV2+/MV.+, gold atoms generated in monometallic KAu(CN)(2) solutions act as an electron relay and readily transfer electrons to MV2+ as they are oxidized. In contrast, when silver and gold atoms are generated simultaneously, the transient bimetallic clusters that form behave as a catalytic electron acceptor toward MV.+ above a critical nuclearity, as for pure silver clusters, and grow as large alloyed particles. It is concluded that even in low proportions, silver increases the redox potential of mixed clusters relative to pure gold clusters in the presence of cyanide.