CATALYTIC EFFECT OF UNDER-POTENTIAL DEPOSITED LAYERS ON THE FERROUS FERRIC OUTER-SPHERE ELECTRODE-REACTION

The influence of the electrode metal on the kinetics of outer-sphere redox reactions is still an unsettled question with contradictory results being reported in the literature. Some theoretical and experimental results indicate the complete absence of an effect, while there is also experimental evidence that underpotential deposited (UPD) metal layers can influence the rate of outer-sphere redox reactions. Furthermore, it has been suggested that some of the reported catalytic effect may be experimental artifacts caused by anion adsorption on the electrodes. We have carried out a coupled experimental/theoretical investigation of the effect of UPD-metal layers on the rate of the Fe2+/Fe3+ redox reaction on polycrystalline gold and platinum electrodes in perchloric acid solutions rigorously purified to remove catalytic anions (e.g., chloride ions), with copper, silver, and bismuth UPD layers. Our experimental results indicate clearly that the catalytic effect of UPD-metal layers on outer-sphere redox reactions cannot be explained generally by anion adsorption, since the phenomenon persisted in rigorously purified solutions. We also suggest a possible theoretical explanation of this phenomenon. In a recent study, we found the Fe2+/Fe3+ heterogeneous electron transfer at a gold surface to be non-adiabatic, i.e. electronic coupling contributed significantly to the rate. In the work reported here we have investigated, by means of electronic structure calculations, whether the catalytic effect observed for this reaction at a gold electrode with a UPD-copper monolayer may be due to changes in electronic coupling. We have found that, while the nature of the electronic coupling remains unchanged by the UPD-copper layer, the observed catalytic effect may still be due to a larger electronic coupling because the hydrated ion approaches closer to the UPD-layer-covered surface than the bare surface, thus increasing the electronic coupling and the rate of the reaction.