Mechanism of the electrocatalytic reduction of oxygen on metal chelates

Abstract N 4 complexes (tetraarylporphyrins, dibenzotetraazaannulenes, phthalocyanines), N 2 O 2 complexes (Pfeiffer complexes), and N 2 S 2 complexes [diacetyldi(thiophenylhydrazones)] have been studied with respect to their catalytic activity for the electroreduction of oxygen in acid electrolyte. Activity has been found with the N 4 complexes, of which the phthalocyanines are already known as catalysts. Unfortunately we found these metal chelates not to be completely stable when supported by active carbon or carbon black. Especially the phthalocyanines readily disintegrate in acid, but also the dibenzotetraazaannulenes lose their activity within several days of operation. Only certain tetraarylporphyrins were still active after 300 hr. The N 2 S 2 and N 2 O 2 (monomeric and polymeric) complexes did not show any activity at all although the Pfeiffer complexes are known as reversible oxygen carriers. In contrast to these complexes, the N 4 complexes used are not only macrocyclic but also have a conjugated π-electron system, which seems to be a prerequisite for the activation of the oxygen molecule. The mechanism of the activation of the O 2 molecule has been explained on the basis of simple MO considerations, which also provide an explanation for the order of activity of central metal ions—Co > Fe > Ni—and for the effect of the support and of substituents of the ligand on the activity and the stability of the chelate.