SITE SELECTION IN ELECTRODE-REACTIONS - BENZOQUINONE HYDROQUINONE REDOX AT SUBMONOLAYER SULFUR-COATED IRIDIUM SURFACES/

The electrode kinetics of the benzoquinone/hydroquinone redox reaction in the unadsorbed state has been studied at a smooth polycrystalline Ir electrode pretreated with sulfur at monolayer and submonolayer cove rages. Kinetic measurements, performed at pH 4 buffered aqueous soluti ons, were based upon thin-layer electrochemical methods. The electrode -transfer rate was found to increase in the order: oxide coated (k deg rees similar to 10(-9) cm s(-1)) < benzoquinone coated (k degrees simi lar to 10(-6) cm s(-1)) < sulfur coated (k degrees similar to 10(-3) c m s(-1)). Even at sulfur coverages as low as 0.1 monolayer, the rate a lready approached Nernstian behavior; this suggests that the electrode -transfer reaction occurred preferentially at the chemisorbed-sulfur s ites. Such sulfur-site selectivity runs counter to the conventional vi ew that chemisorbed sulfur functions as a poison to essentially all el ectrode reactions.