Abstract
The electrooxidation of D-and L-glucose at chiral Pt{321}(r/s) single crystal electrodes modified with Au, Ag and Bi adatoms up to a coverage of one monolayer (ML) is reported. All adatoms investigated are found to selectively decorate kink and step sites. Only at higher coverages is adsorption onto the narrow{111} terrace sites observed for Bi, Ag and Au, consistent with previously reported adsorption behaviour on stepped surfaces vicinal to the{111} plane for chemisorbates exhibiting a lower work function than platinum. However, silver is found to block{111} terrace sites even when Pt step sites are still available on Pt{321}. This behaviour is ascribed to the propensity of silver to undergo place-exchange to form a surface alloy. The selective decoration of chiral kink sites has a profound influence on the voltammetric response of Pt{321} towards glucose electrooxidation. For bismuth adsorption, the electrooxidation current density initially increases and reaches a maximum when bismuth adsorption at{111} terraces commences. This is because the reaction pathways at step/kink sites leading to the formation of adsorbed CO (a surface poison for the clean surface reaction) and other strongly adsorbed intermediates, are either blocked by adsorbed bismuth or their electrooxidation and subsequent removal promoted. Once all step/kink sites are blocked by bismuth, hardly any chiral discrimination can be discerned between r-/s-electrodes towards D-/L-glucose. Silver adsorption does not cause any increase in glucose electrooxidation current density but rather induces a continual attenuation in glucose electrooxidation activity, particular (in contrast to bismuth) electrooxidation current at potentials in excess of 0.35 V. Therefore, unlike for bismuth, the initial glucose adsorption and electrooxidation processes associated with chiral kink sites appear to be preserved even though silver adsorbs at these sites. It is speculated that spontaneous place-exchange of silver with platinum to form a PtAg surface alloy at steps is responsible for this difference in behaviour between silver and bismuth. Finally, gold neither promotes reaction rate nor preserves chiral discrimination and is therefore deduced to act as an inert site blocker (no alloying, no promotion of CO electrooxidation) leading to complete attenuation of glucose electrooxidation current at a coverage of one ML.