REDUCTION OF PD(II) IN AQUEOUS-SOLUTION - STABILIZATION AND REACTIONS OF AN INTERMEDIATE CLUSTER AND PD COLLOID FORMATION

In the gamma-radiolytic reduction of Pd(NH3)4Cl2 in the presence of poly(ethyleneimine), an intermediate cluster is stabilized by the polymer. Pulse radiolysis experiments show that it is slowly formed in the 0.01-1-s range after the primary reduction of Pd(II) to Pd(I) by hydrated electrons and organic radicals has occurred in the 0.1-100-mu-s range. A photochemical method for the preparation of the cluster is also described. The cluster absorbs at 308 and 400 nm and decays within a few days to yield colloidal palladium metal. It can also be transformed into the colloid by free-radical attack (upon prolonged gamma-irradiation) and by photolysis. The cluster is rapidly oxidized by oxygen and hydrogen peroxide. It reduces methyl viologen, reacts with nitrous oxide to yield N2, with hydrogen sulfide to yield PdS + H2, and with cyanide to yield a PdCN complex, and yields colloidal palladium upon reaction with CO. The cluster is not ESR active. The structure of Pd2(2+) is tentatively assigned to it. Titration with H2O2 yielded an absorption coefficient of 2.2 x 10(4) M-1 cm-1. Some chemical reactions of colloidal palladium are also described. It reacts with hydrogen sulfide to yield PdS and with cyanide to form Pd(CN)4(2-); in both reactions hydrogen is generated simultaneously from the aqueous solvent. H-2 is also generated during the UV illumination of colloidal palladium in the presence of poly(ethyleneimine) (quantum yield 10(-3)).