Study of the kinetics of electrochemical reactions by thin-layer voltammetry IV. The influence of chemisorbed species on the reactions of platinum complexes at platinum electrodes

Abstract

The rates of the electrode reactions of representative acidoammine platinum (II) and (IV) complexes have been measured in halide solutions using platinum electrodes treated with irreversibly chemisorbed substances, in order to probe the role of the compact layer in these reactions. The chemisorbed substances apparently displaced most of the specifically adsorbed halide, shifted the potentials in the diffuse double layer to more positive values than would be observed otherwise, and sterically hindered the reactions to an extent which depended on the size and shape of the adsorbed species. For instance, adsorbed iodine and 1,3-butadiene accelerated the reactions of anionic platinum(II) complexes and decelerated the reactions of cationic complexes, as would be expected from simple electrostatic considerations, but adsorbed carbon monoxide, ethylene and 1-octene decelerated the reactions of all the complexes studied, apparently because steric hindrance predominated over electrostatic effects for these surfactants. Physically adsorbed surfactants such as alkanes, aromatics and gelatin had relatively little effect on Pt(II) oxidation. The results are consistent with a model in which adsorbed halide enhances electrontransfer between the electrode surface and the reactant in interconversion of Pt(II) and Pt(IV). Although a variety of chemisorbed substances were observed to partially displace adsorbed halide ions, they did not prevent the reactions from occurring—in keeping with the suggestion that a halide-assisted electron transfer path may be involved in which the bridging halide interactis directly with the electrode at points not occupied by non-bridging surfactants, or indirectly through the adsorbed species.