REDUCTION OF PB2+ IN AQUEOUS-SOLUTION - EARLY STEPS AND COLLOID FORMATION, AND THE ATOM -] METAL TRANSITION

Aqueous solutions of Pb(ClO4)2 containing sodium formate were investigated pulse radiolytically. Pb+ (absorption maximum at 320 nm, epsilon = 2.5 x 10(3) M-1 cm-1) is formed as the first reduction product via the reactions of e(aq)- and CO2- with Pb2+. The bimolecular reaction between two Pb+ ions to yield Pb0 occurs with 2k = 8.2 x 10(9) M-1 s-1. Pb0 has an absorption maximum at 290 nm (epsilon = 1.2 x 10(4) M-1 cm-1) and a weaker one at 660 nm (epsilon = 9.7 x 10(2) M-1 cm-1). During the second-order disappearance of the Pb0 absorption, stronger absorption bands at shorter wavelengths build up, which are attributed to oligomeric precursors and finally metallic particles of lead. The metallic particles have an absorption maximum at 220 nm which is attributed to a plasmon oscillation. The first appearance of this band occurs at a particle agglomeration number slightly smaller than 10; thus, lead is found to possess a giant dipole oscillation band at rather small particle sizes as was first found for silver particles in solution and alkali metal particles in molecular beams.