Abstract

We have studied the effect of the redox potential of metallophthalocyanines (M-Pcs) adsorbed on graphite on their electrocatalytic activity for the oxidation of 2-mercaptoethanol (ME). This was achieved by: (i) changing the metal in the phthalocyanine (M-Pc where M=Cr, Mn, Fe, Co, Ni and Cu) and (ii) using cobalt phthalocyanines with electron-donor and electron-withdrawing substituents on the macrocyclic ligand. For phthalocyanines of different metals a plot of log k versus the redox potential of the catalyst gives a straight line of slope 0.10 V decade-1 which is close to the value obtained (0.12 V decade-1) from Tafel plots for all M-Pcs investigated. In contrast, when different substituted cobalt phthalocyanines are compared, a plot of log k versus redox potential gives a straight line of negative slope (-0.240 V decade-1) and the rate decreases with driving force. Since ME electrooxidation in aqueous media most likely proceeds via an inner-sphere mechanism and the rate-determining step is the same for all cobalt phthalocyanines investigated, the decrease in rate constant with driving force may be due to a decrease in the electronic coupling between the cobalt center and the sulfur in ME. Preliminary PM3 semi-empirical theoretical calculations of the electronic coupling associated with the interaction of the metal in the phthalocyanine and the ME molecule support this explanation.