Volcano correlations between formal potential and Hammett parameters of substituted cobalt phthalocyanines and their activity for hydrazine electro-oxidation

In this work we have examined the effect of the Co(II)/(I) formal potential of cobalt phthalocyanines (CoPcs) on their catalytic activity for the electro-oxidation of hydrazine. The activity of the different phthalocyanines was examined using these complexes adsorbed on graphite electrodes. When activities for the different metal chelates are compared as a plot of log I (at constant potential) versus the Co(II)/(I) formal potential an asymmetric volcano curve is obtained. For some complexes the rate of the reaction increases very sharply with the driving force of the catalyst (measured as its formal potential) and then it decreases for higher driving forces. The same plot is obtained when plotting log I versus the sum of the Hammett parameters of the substituents on the periphery of the phthalocyanine ring. (C) 2002 Published by Elsevier Science B.V.