The classical reactivity descriptor in electrocatalysis is the binding energy of key reaction intermediates to the active sites of the catalyst. This is well known for the catalytic activity of metal electrodes [1], alloys but less studied for molecular catalysts [2]. The activity at constant E plotted versus this descriptor has the shape of a volcano. According to the Sabatier principle, optimum electrocatalysts require moderate bond energies (not too strong, not too weak) predicting the best catalyst for a given reaction. Essentially one of the key intermediates is the binding of the reacting molecule “R” to the active sites at the rate determining step as: [MN4] + R D [RMN4+] + e for oxidations and [MN4] + R + e D [RMN4-] for reductions where MN4 is macrocyclic complex. If the first step is rate controlling, it is expected that for the binding step DGad = 0 a maximum activity should be observed, and the partial coverage of adsorbed intermediates is q = 0.5 (thermoneutral condition). We have tested this hypothesis by studying the oxygen reaction reaction and hydrazine oxidation in alkaline media using several metalloporphyrins and metallophthalocyanines as catalysts anchored on graphite and carbon nanotube surfaces. Volcano correlations are obtained in both cases but plots of log(TOF)E (TOF = turn over number) give only linear correlations, in contrast to metal electrodes.

Non-classical origin of the maximum electrocatalytic activity in the volcano correlations of MN4 molecular catalysts in contrast to those for metallic electrodes. O2 electro-oxidation and hydrazine electro-oxidation

ACS Spring 2022

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