Unusual behaviour of perfluorinated cobalt phthalocyanine compared to unsubstituted cobalt phthalocyanine for the electrocatalytic oxidation of hydrazine. Effect of the surface concentration of the catalyst on a graphite surface

We have found that CoPc and 16(F)CoPc when adsorbed on graphite electrode exhibit voltammograms in alkaline solution (0.2M NaOH) that show the typical redox peaks attributed to the Co(II)/(I) reversible. The peak potential for CoPc is independent of surface concentration of the catalyst. In contrast, for 16(F)CoPc the Co(II)/(I) redox process shifts to more negative potentials when the surface concentration of the catalyst increases. In a volcano correlation of log (i/G)E (activity per active site) versus Co (II)/(I) formal potential of catalyst (using several CoN4 chelates) CoPc appears in the ascending portion (activity increases with the Co (II)/(I) redox potential) whereas 16(F)CoPc appears in the region where activity decreases with the redox potential. In a plot of log (i/G)E versus the Co(II)/(I) formal potential of 16(F)CoPc the declining portion of the volcano is reproduced for one single complex. So 16(F)CoPc at different surface concentrations behaves as Co complexes having different redox potential in the declining portion of the volcano plot, when the activity is normalized for the surface concentration. This is not observed for CoPc.