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 clust er to be followed from the nanosecond to the second time range. The pu lse induces simultaneous reduction of the ions into silver and gold at oms. Then, through coalescence and association with excess ions, homol ogue or different, these coexisting atoms give rise to transient bimet allic alloyed clusters. However, in the interval 2-20 s the clusters a re enriched in silver at the expense of gold, the process being assign ed to a slow intermetal electron transfer from gold atoms (less noble in the presence of CN-) to silver ions adsorbed on clusters. In the pr esence of the electron acceptor/donor couple MV2+/MV.+, gold atoms gen erated in monometallic KAu(CN)(2) solutions act as an electron relay a nd readily transfer electrons to MV2+ as they are oxidized. In contras t, when silver and gold atoms are generated simultaneously, the transi ent bimetallic clusters that form behave as a catalytic electron accep tor toward MV.+ above a critical nuclearity, as for pure silver cluste rs, and grow as large alloyed particles. It is concluded that even in low proportions, silver increases the redox potential of mixed cluster s relative to pure gold clusters in the presence of cyanide.