CONTROL OF REACTIVITY AT CARBON ELECTRODE SURFACES

Electron transfer kinetics and adsorption were examined at several carbon surfaces, including basal plane highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC). The objective was an understanding of how surface structure affects reactivity. HOPG basal plane exhibited much slower electron transfer rates than GC for all systems studied, and the observed rate constants were several orders of magnitude lower than those predicted from Marcus theory. In addition, HOPG did not adsorb several quinones which showed strong adsorption to GC under similar conditions. Any disturbance to the ordered basal plane (such as near defects) greatly increased electron transfer rates and adsorption. GC exhibited both fast kinetics and strong adsorption, provided the surface was clean. For the case of aquated EU(2+/3+), Fe-2+/3+ and V-2+/3+, mild oxidation of the GC surface greatly increases the observed rate, apparently via an inner-sphere catalytic route involving surface oxides. The results lead to the conclusion that both adsorption and electron transfer are suppressed at HOPG owing to the low density of electronic states near the Fermi level. Disorder at defects or in GC increases the density of states and causes the carbon to behave more like a metal. Activation of EU(2+/3+), Fe-2+/3+ and V-2+/3+ by oxidation is consistent with formation of a surface oxide complex via displacement of ligated water.