ELECTROCHEMISTRY OF MIXED METAL-CLUSTERS IN AQUEOUS-SOLUTION - REDUCTION OF AG+ BY THE LEAD ATOM

The electron transfer between lead and silver particles in the micro size range, where conventional electrode-potentia considerations are not applicable, is studied by pulse radiolysis. In deaerated solutions containing 1 X 10(-2) M Pb(ClO4)2, 2.5 X 10(-5)-2.0 X 10(-4) M AgClO4, and 1 X 10(-2) M NaCO2H, Pb+ is the first and metallic silver the final product of reduction, and no metallic Pb particles are formed as intermediates. A mechanism is proposed in which the crossover from reduced lead to reduced silver occurs via the mixed lead-silver cluster (PbAg)+, which absorbs weakly at 460 nm. In the reaction of this cluster with Ag+, the Pb component is oxidized and dimeric silver is formed that starts an agglomeration chain leading to larger metallic Ag particles. During these agglomeration steps, intermediate oligomeric silver species, which absorb at 330 and 365 nm, are detected. The results are discussed in the framework of our limited knowledge about the redox potentials of small nonmetallic lead particles.