Substituent effects on the electrocatalytic reduction of oxygen on quinone-modified glassy carbon electrodes

The reduction of oxygen catalysed by two sulfur-containing derivatives of anthraquinone has been investigated. The quinones were chemically grafted to a glassy carbon electrode surface and the oxygen reduction reaction in alkaline solution followed a two-electron pathway yielding hydrogen peroxide. The mechanism proposed corresponds to an electrochemical-chemical (EC) reaction where the semiquinone radical anion electrochemically formed reacts chemically with oxygen. The influence on reactivity of electron withdrawing groups present in the quinone molecules has been studied and it is shown that there is an unexpected small dependence between reactivity and standard potential of the grafted quinones. It is shown that steric effects appear important in determining their electrocatalytic properties.