Origin of the Maximum in the Activity Volcano Correlations for MN4 Molecular Catalysts Compared to That for Metallic Electrodes
Authors: Gutierrez Cristian A , Cisternas Ricardo , Zagal Jose H , Oñate Ruben , Ponce Ingrid 
Published: December 2020

A classical reactivity descriptor in electrocatalysis is the binding energy of key intermediates to the active sites. This is well documented for the catalytic activity of metal electrodes [1], alloys and metal oxides and less studied for molecular catalysts [2]. The activity at constant potential plotted versus these descriptors has the shape of a volcano. This is explained by the classical Sabatier principle, which states that there is an optimum “bond strength” (not too strong, not too weak) establishing 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]ad + R(aq) ↔ [RMN4+]ad + e- for oxidations and [MN4]ad + R(aq) + e- ↔ [RMN4-]ad for reductions where MN4 is surface confined macrocyclic complex. If the reaction involves the transfer of several electrons, several adsorbed intermediates can be involved. If the controlling step is the first step, it is expected that for the binding step when Gad = 0 a maximum activity should be observed and the partial coverage of adsorbed intermediate is  = 0.5. This implies an equilibrium constant equal to one for the first step and for the best catalyst. In this work we have tested this hypothesis by studying hydrazine electrooxidation and glutathione in alkaline media using several iron porphyrins and iron phthalocyanines as catalysts immobilized on graphite and carbon nanotubes. Figure A shows the trends in reactivity versus the Fe(III)/(II) redox potential for the oxidation of glutathione describing a typical volcano correlation. However if the currents are divided by the θFe(II), the fraction of active sites calculated at E=-0.3 V vs SCE using the Nernst equation, log(i/ θFe(II),)E vs. E°
Fe(III)/(II) gives a straight line of slope -0.140V/decade. The maximum corresponds to θFe(II) = 0.5 and occurs at a potential equal to that used for comparing the activities. A similar behaviour is observed for the oxidation of hydrazine but the continuation of the straight line is observed at potentials more positive than that of the maximum. Again the maximum is observed at the potential chosen for comparison. Those catalysts having Eo’ Fe(III)/(II) >> θFe(II)) than -0.56V are in the Fe(III) state as predicted by the Nernst equation. That oxidation state Fe(III) is inactive for the reaction as OH- ions are strongly bound to Fe(III), especially in alkaline media. So the falling of the activities in the strong binding side of the volcano can be attributed preferentially to a gradual decline in the number of Fe(II) active sites and not to gradual decrease of the fraction (1-θ ) of empty or available active sites due to occupancy by intermediates. This phenomena has also been observed for ORR and for thiols oxidation and seems to be a unique feature of molecular catalysts compared to metal catalysts even though Schmickler and Santos [3] have shown that some volcano correlations for HER on metals that are in an oxidized form, and then inactive for this reason.
Acknowledgements: This work was supported by Anillo Project ACT 192175, Fondecyt, Projects 1181037 & 1171408 C.G.C is grateful to a Posdoctoral Fellowship from Dicyt-USACH.
References: [1] J.K. Norskov et al., J Catal.,328 (2015) 35 [2] J.H. Zagal & M.T.M.Koper, Angew. Chem., 128 (2016) 14726 [3] E. Santos, W. Schmickler et al. Chem. Phys. Chem., 12 (2011) 2274.

Figure 1