Self-catalysis by catechols and quinones during heterogeneous electron transfer at carbon electrodes

Heterogeneous electron transfer kinetics for several catechols were examined on glassy carbon (GC) electrodes in aqueous solution. Electrode preparations yielded GC surfaces with low levels of oxides or adsorbed impurities, which exhibited strong adsorption of dopamine (DA) and related catechols. Conversely, modification of GC with an organic monolayer suppressed DA adsorption and in many cases prevented electron transfer. By relating catechol adsorption to observed electron transfer, it was concluded that an adsorbed layer of catechol acts as an electrocatalyst for solution-phase redox components. Physisorbed or chemisorbed monolayers of several quinones, including duroquinone, anthraquinone, and dopamine itself, are catalytic toward dopamine oxidation and reduction, but nitrophenyl, trifluoromethylphenyl, and methylene blue monolayers severely inhibit electron transfer. The magnitude of inhibition was affected by electrostatic attraction or repulsion between the surface and the redox system, but the major factor controlling electron-transfer kinetics is not electrostatic in origin. The most plausible mechanism is self-catalysis by an adsorbed quinone, which remained adsorbed during electron transfer to a redox couple in solution. The results are inconsistent with a redox mediation mechanism involving a redox cross-reaction between adsorbed and solution quinone couples. An interaction between the adsorbed and solution quinone species during electron transfer appears to catalyze one or more of the steps in the scheme of squares mechanism for hydroquinone/quinone redox systems. The results explain a variety of observations about catechol and hydroquinone electrochemistry, as well as provide more fundamental insights into quinone electron-transfer mechanisms.