Growth and reactivity of silver clusters in cyanide solution

Pulse radiolysis was used to investigate the growth and reactivity dynamics of silver clusters in the presence of the cyanide ligands, Ag-n,Ag-CN-, and of an electron acceptor/donor, the methyl viologen redox couple whose potential is E-0(MV2+/MV.+) -0.41 V-NHE. The absorbance of the MV.+ radical, produced by the same pulse as the silver atoms, is at first constant during an induction time delay, and then decays due to a catalytic electron transfer toward supercritical silver clusters. Correlated growth of the Ag-n,Ag-CN- absorbance is also observed. Another process of reverse electron transfer from subcritical clusters to MV2+, concomitant with the transfer from MV.+ to supercritical clusters, for the first time clearly appears to also occur in the kinetics. Through a numerical simulation model, including coalescence reactions between atoms or aggregates, catalytic electron transfer from MV.+ toward clusters above a critical size, and corrosion of subcritical aggregates by MV2+ we derive the critical number n, and the rate constants of the mechanism. The presence of CN- causes a slowing of both the coalescence and electron transfer from MV.+ reactions. We conclude that n(c) = 5-6 and hence that the reference redox potential in the presence of cyanide ligands corresponds to the couple E-o(Ag-6-7,Ag-CN-/Ag-6-7,Ag-CN-) approximate to -0.4 V-NHE.