Author Proof PROOF COPY [EEL-12-3465] 003304EEL ECS Electrochemistry Letters, 2 (4) H1-H3 (2013) H1 2162-8726/2013/2(4)/H1/3/$31.00 © The Electrochemical Society Matching the Catalyst Co(II)/(I) Formal Potential of a Macrocyclic Complex to the Reversible Potential of Hydrazine Oxidation for the Highest Activity 1 2 3 Francisco Javier Recio, a Daniela Geraldo, b Paulina Ca ˜ nete, a and Jos´ e Her´ aclito Zagal a, *, z 4 a Facultad de Qu´ ımica y Biolog´ ıa, Departamento de Qu´ ımica de los Materiales, Universidad de Santiago de Chile, Sucursal Matucana, Santiago 9170022, Chile 5 6 b Doctorado en Fisicoqu´ ımica Molecular, Relativistic Molecular Physics (ReMoPh) Group, Universidad Andr´ es Bello, Santiago, Chile 7 8 9 In this work we have re-interpreted volcano correlations for the oxidation of hydrazine catalyzed by CoMN4 catalysts and found that the highest catalytic activity is observed when the formal potential of the catalyst matches the reversible potential of the reaction, i.e. the hydrazine/dinitrogen reversible potential, that is −0.4959 V vs. SCE at pH 13. This clearly shows that the formal potential of the catalysts needs to be “tuned” or to be very close to the reversible potential of the target molecule to undergo an ET process. This is also true for other reactions we are studying. 10 11 12 13 14 © 2013 The Electrochemical Society. [DOI: 10.1149/2.003304eel] All rights reserved. 15 16 Manuscript submitted December 10, 2012; revised manuscript received January 9, 2012. Published 00 0, 2013. 17 Electrocatalysis is present in many processes of technological rel- 18 evance. These electron transfer (ET) reactions involve molecules that 19 for reacting at desired rates, apart from an overpotential, they require 20 the interaction of reactants, intermediates or products with active sites 21 on the electrode surface. 1–3 It is very important to understand the 22 fundamentals of electrocatalysis in order to design better catalytic 23 electrode materials. 1–4 In particular, MN4 macrocyclic complexes like 24 phthalocyanines and porphyrins, when adsorbed on electrode surfaces, 25 catalyze a myriad of electrochemical reactions and in many cases the 26 activity plotted versus the formal potential of the catalyst give volcano 27 correlations. 4 We have shown the importance of “tuning” the formal 28 potential of MN4 macrocyclic complexes for optimizing the electro- 29 catalytic activity of these species for many reactions. 3,4 However in 30 previous work no reference to the reversible potential of the reaction 31 under study has been made. In this work we have re-interpreted data 32 previously published in the literature related to the electrooxidation 33 of hydrazine catalyzed by CoN4 macrocycles adsorbed on graphite 34 electrodes and propose a new mechanism for the reaction. Further, 35 we have found that for Co macrocyclics, in a volcano correlation, 36 the maximum activity is observed for the catalyst that has a formal 37 potential very close or equal to the reversible potential of the hy- 38 drazine/dinitrogen couple. This is also true for Fe macrocyclics. 5 This 39 finding is very important since it suggests that to design a catalyst, 40 its formal potential need to be tuned so to approach the reversible 41 potential of the reaction to be catalyzed under the experimental con- 42 ditions. This seems to be true for other reactions, like the oxidation of 43 L-cysteine catalyzed by CoN4 macrocyclic complexes. 6 44 Discussion 45 The oxidation of hydrazine involves the transfer of 4 electrons to 46 give N 2. Co phthalocyanines and vitamin B 12 (as aquocobalamine), 47 when adsorbed on graphite electrode catalyze this reaction. 4,7–17 The 48 following mechanism has been proposed, 15 assuming that the active 49 species involve Co(II) generated in step (I): 50 [R n PcCo(I)] − ad ⇆ [R n PcCo(II)] ad + e − [I] 51 N 2 H 4 + [R n PcCo(II)] ad → [R n PcCo(I) − − (N 2 H 4 ) + ] ad [II] 52 [R n PcCo(I) − −(N 2 H 4 ) + ] ad +OH − rds → RnPcCo(II) +N 2 H · 3 +H 2 O +e − [III] ∗ Electrochemical Society Active Member. z E-mail: jose.zagal@usach.cl 53 N 2 H · 3 + 3OH − fast → N 2 + 3e − + 3H 2 O [IV] The above mechanism agrees with an order of the reaction close 54 to one for OH − ions, hydrazine and surface concentration of the 55 [RnPcCo(II)] ad catalyst. However, if adduct formation takes place 56 before the rate determining step (reaction II), this will stabilize the 57 hydrazine molecule and will cause an increase in the activation energy, 58 which is the opposite to what is expected from an electrocatalytic 59 process. In this work, we propose that adduct formation probably 60 takes place in a concerted way with the ET process in step (IIa): 61 N 2 H 4 + [R n PcCo(II)] ad + OH − rds →[R n PcCo(I) − N 2 H 3 ] ad + H 2 O + e − [IIa] 62 [R n PcCo(I) − N 2 H 3 ] ad → [RnPcCo(I)] ad + N 2 H · 3 [IIIa] Even though, more evidence is needed for this mechanism, it agrees 63 with the kinetic parameters reported and also very important, it will 64 hypothetically lower the activation energy due to the stabilization of 65 the N 2 H 3 radical which is quickly decomposed in step (IV). 66 Figure 1 shows a series of cyclic voltammograms taken from 67 previous work 15 illustrating the response of some adsorbed CoPc 68 Figure 1. Cyclic voltammograms of the OPG modified with CoPc with dif- ferent substituents on phthalocyanine ligand. Dashed lines show the foot of the wave for N 2 H 4 oxidation after adding 0.05 M hydrazine to the electrolyte 0.1 M pH 13 under nitrogen. Scan rate 0.3 V s −1 for the adsorbed complexes and 0.005 Vs −1 for the foot of the wave of hydrazine oxidation. Electrode area 0.44 cm 2 . Adapted from Fig. 2 in. 15