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Electrochimica Acta

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inear versus volcano correlations for the electrocatalytic oxidation ofydrazine on graphite electrodes modified with MN4 macrocyclicomplexes

ederico Tascaa,1,2, F. Javier Recioa,b,c,1,2, Ricardo Venegasa, Daniela A. Geraldob,amie Sancyc, José H. Zagala,2

Facultad de Química y Biología, Departamento de Química de los Materiales Universidad de Santiago de Chile, Casilla 40, Correo 33, Sucursal Matucana,antiago 9170022, ChileDoctorado en Fisicoquimica Molecular, Facultad de Ciencias Exactas, Universidad Andrés Bello. República 275, Santiago, ChileFuerza Aérea de Chile, Academia Politécnica Aeronáutica, José Miguel Carrera 11085, El Bosque, Santiago, Chile

r t i c l e i n f o

rticle history:eceived 17 January 2014eceived in revised form 7 April 2014ccepted 9 April 2014vailable online xxx

eywords:etallophthalocyanineydrazine oxidation

a b s t r a c t

For electrochemical reactions catalyzed by electrodes modified with MN4 macrocyclic transition metalcomplexes, it is commonly accepted that d-band vacancy, surface lattice strain, and eg-orbital filling thetransition metals are essential parameters to take in account for an optimum catalysis. These parametersaffect the formal potential of the catalyst which is related to the free energy of the adsorption of thereacting molecule to a point that volcano correlations have been reported when plotting the activity ofthe catalyst versus the Mz+/M(z-1)+ formal potential of the catalyst (M (II) = Fe, Co). The highest catalyticactivity is achieved when the formal potential of the catalyst is close to −0.5 V vs SCE regardless ofwhether the central metal is Fe or Co. In this work, we review the work done until now on the oxidation

lectrocatalysisolcano correlationsinear free energy correlations

of hydrazine in alkaline medium at metallophthalocyanines modified graphite electrodes and we showthat for some complexes the redox potential of the MN4 macrocyclic transition metal complex can bevaried changing the concentration of the catalyst at the electrode surface. Therefore we show that if logi (normalized for the actual surface concentrations of M(II) active sites (M = Fe, Co) at constant potentialis plotted versus the M(II)/(I), the correlations is linear with a slope close to 2RT/F.

. Introduction

Understanding how electron transfer (ET) occurs at the elec-rode surface is of paramount importance for the design of betteratalysts for sensors and fuel cells. The ET at the electrode surfacenvolves the formation and rupture of bonds of reacting moleculesr intermediates. In accordance to Sabatier′s principle of catalysishe reacting molecules interact to a degree that it is not too weakr not too strong with the atomic centers acting as active bind-ng sites. Various material properties govern the interaction andherefore the reaction rates. For example, d-band vacancy [1], sur-ace lattice strain [2], and eg-orbital filling the transition metals [3].

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uning such electronic and structural material properties, optimi-ing the d-orbital occupancy; and achieving a fine balance betweenhe chemisorptions energy of intermediates and the number of

E-mail addresses: jose.zagal@usach.cl, j zagal@yahoo.com (J.H. Zagal).1 Federico Tasca and Francisco J. Recio contributed equally.2 ISE Member.

ttp://dx.doi.org/10.1016/j.electacta.2014.04.059013-4686/© 2014 Elsevier Ltd. All rights reserved.

© 2014 Elsevier Ltd. All rights reserved.

surface sites are essential requirement for an optimum catalysis[4–8].

The necessity to replace expensive noble metals has led toa class of catalysts comprising MN4 macrocyclic complexes likephthalocyanines and porphyrins. These complexes present manyadvantages: are cheap and easy to synthesize, present good stabil-ity at various pHs, catalyze a myriad of electrochemical reactions,and the tuning of the formal potential of the active site can beachieved by placing appropriate groups on the ligand so to affectthe electron density of the metal center [9–11]. This last character-istic is of extreme importance because the electrocatalysis seemsto be governed by the nature of the central metal ion and thesurrounding ligands [9–12]. The d-electron density on the metalsite determines the progress of the reaction, interacting with theintermediates, while the ligands control the metal ion electronicstructure by relocating the redox potential [9–12]. As a matter of

ear versus volcano correlations for the electrocatalytic MN4 macrocyclic complexes, Electrochim. Acta (2014),

fact the formal potential of the active metal site is directly linkedto the maximum activity of the catalyst for a determinate substrate[9,10,12]. We have demonstrated this concept for a series of reac-tions [5–8,13–16]. In a recent series of papers we have proved the

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oncept also for the oxidation of hydrazine [5,6,8]. Here we summa-ize our findings and moreover we show that if log i (normalizedor the actual surface concentrations of M(II) active sites where

= Co, Fe) at constant potential is plotted versus the M(II)/(I), theorrelations is linear with a slope close to 2RT/F.

Among the many possible reactions catalyzed by MN4 macro-yclic complexes, we are especially interested into the oxidationf hydrazine (N2H4) because of the possibility to use this pow-rful reducing agent in the anode of fuel cells [17,18]. Moreoverydrazine and its derivatives are frequently found in the environ-ent and are used as essential raw materials and/or intermediates

n industrial preparations, such as in the production of pesticides.hey are also suspected of being carcinogenic and mutagenic [19].n spite of its high reactivity, hydrazine presents rather large oxi-ization overpotential at most electrode materials [20], whicheems to be a common phenomenon for reactions involving multi-lectron transfers.

. Experimental

Cobalt phthalocyanine (CoPc), cobalt-hexadecafluoro-phthalo-yanine (16(F)CoPc), perchlorinated phthalocyanine (FePcCl16),nd Fe phthalocyanine (FePc) were obtained from Aldrich andsed as provided. Cobalt-octaethylhexyloxyphthalocyanine8b(EH)CoPc), cobalt-tetrapyridinophorphyrazine (4b(Pyr)CoPz),obalt-tetrapentylopyrrol phthalocyanine (4b(PenPyr)CoPc),obalt-octahydroxyethanothiolphthalocyanine (8b(SC2H4OH)oPc) were synthesized according to the literature [21–23].obalt-tetramethoxyphthalocyanine (8b(MeO)CoPc), Fe-etramethoxyphthalocyanine (FeOMePc) and Fe-tetracarboxy-hthalocyanine (FeTCPc) were donated by Professor A. A.anaka and cobalt-tetrasulfophthalocyanine (4b(SO3)CoPc)as synthesized and purified according to the literature [24]. Fe

etrasulfophthalocyanine (FeTSPc) was synthesized according toethods described in the literature [24]. All other products were

f analytical reagent grade and used as received. The workinglectrode was an ordinary pyrolytic graphite (OPG) disk electroderom Pine Instruments (USA) with a geometrical area of 0.44 cm2.he electrodes were polished before each experiment with 800nd 2400 grit emery paper followed by an ultrasonic extensiveinsing with ultra-pure Milli-Q water to remove solid particles. Thelectrochemical experiments were carried out with a conventionalhree-electrode cell and a BAS-100 potentiostat (USA) or AutolabGST30 potentiostat/galvanostat (Netherlands). Platinum spiralire of 2 cm2 geometric area was used as the counter electrode

nd calomel saturated electrode, SCE, as the reference electrode.lectrolytic solutions were routinely deoxygenated with pureitrogen. All the potential values are given versus the calomel sat-rated electrode SCE. The electrolyte was aqueous a 0.2 M NaOH,olution prepared from double-distilled water and deaerated withltra pure N2. CoPc and 16(F) CoF16Pc were adsorbed on OPGlectrode, respectively, by placing a drop of 0.1 mM solutions ofoPc and 16(F)CoPc complexes in dimethylformamide (DMF) forarious time intervals ranging from 10 to 1200 s on the graphiteurface. After this procedure, the electrodes were rinsed withMF, ethanol and double-distilled water, in order to remove anyxcess of the metal complexes. This procedure avoids the presencef microcrystals on the graphite surface, so the electrochemicalesponse of the electrode can be attributed solely to moleculeshat are adsorbed on the electrode. Adsorption of all complexesas verified by the appearance of typical current peaks in the

Please cite this article in press as: F. Tasca, et al., Linoxidation of hydrazine on graphite electrodes modified withhttp://dx.doi.org/10.1016/j.electacta.2014.04.059

yclic voltammograms of the modified electrodes. The electrodeas stable (i.e. >90% of the original response for electrocatalytic

urrents) for the entire duration of the experiment which couldast up to 12 hours.

Fig. 1. Molecular structures of metalloporphyrins and metallophthalocyanines.

Hydrazine was obtained from Riedel-de Haën and used as pro-vided. NaOH was A.R. grade from Merck.

3. Results and discussion

In spite of the great variety of complexes studied for theoxidation of hydrazine, there are rather few systematic studies[5,6,15,25], or theoretical calculations [26,27], oriented to estab-lish the reactivity indexes for the activity of these metal complexes.The correlation between the M(II)/(I) or M(III)/(II) formal poten-tial of the MN4 catalyst and the electrocatalytic activity have beenestablished, with the formal potential to be used depending on thereaction [5,6,9,13,15,25,28,29]. The correlations have the shape ofa volcano indicating that the formal potential of the catalysts hasto be “tuned” to a specific value for obtaining the highest catalyticactivity. The closest the redox potential of the catalytic redox cou-ple M(II)/(I) or M(III)/(II) of the MN4 to the redox potential of thereaction to catalyze, the highest the activity [5,6,9,13,15,25,28,29].

The tuning can be achieved by using electron-withdrawing orelectron-donating groups on the phthalocyanine ligand [9,29]. Inorder to establish reactivity guidelines for a series of catalysts itis important to compare catalysts that are in different branches ofthe volcano correlation. For most Co and Fe complexes, oxidation ofhydrazine starts at potential more positive than that of the M(II)/(I)redox couple, so we can assume that for both Co and Fe phthalocya-nines the catalytic species is the M(II)Pc. Further, this is clear fromprevious publications that show that at potentials where Fe(III) isformed the reaction becomes strongly inhibited [14,30–33]. Fig. 1-A and 1-B illustrates a series of cyclic voltammograms of CoPcs andFePcs pre-adsorbed on a ordinary pyrolytic graphite electrode in0.2 M NaOH solution. Increasing the electron withdrawing powerof the groups on the ligand causes a shift of the Co(II)/(I) processto more positive potentials. As expected, the same happens forFe(II)/(I) formal potential of Fe-phthalocyanines. Fig. 1-C and 1-Dshows the volcano correlations published previously and clearlyindicates that optimum Co(II)/(I) [32] and optimum Fe(II)/(I) [32,34]formal potentials exist for achieving maximum catalytic activity forthe reaction of oxidation of hydrazine. Further, the optimum formalpotential for both series of catalysts is about the same, i.e. ca. −0.5 V

ear versus volcano correlations for the electrocatalytic MN4 macrocyclic complexes, Electrochim. Acta (2014),

vs SCE. To move the redox potential to higher values than −0.5 V vsSCE causes a decrease on the catalytic activity of the M(II)/(I)Pcs. Inprevious work, the maximum formal potential has been interpretedusing the Sabatier Principle that essentially states that the optimal

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is approximately proportional to �ad/(1-�ad) assuming a Langmuirisotherm with a maximum activity at �ad = 0.5 (2). However, thedrop in activity in Fig. 2-C and D and Fig. 3 in the falling regionmight then be attributed to three factors: i) fewer catalyst sites in

ig. 2. A and B, typical cyclic voltammograms of metallophthalocyanines adsorbed olots for hydrazine oxidation on different metallophthaloyanines and vitamin B12f The Electrochemical Society (C) and Elsevier (D).

atalytic activity is achieved for a catalyst where the adsorptionf the reacting species is not too weak, not too strong. However,s it will be discussed further down, we now provide a differentxplanation for the apex of the volcano correlation.

The oxidation of hydrazine involves the transfer of 4 electronso give N2. In a recent work we proposed the following mechanismor Co catalysts [5]:

RnPcCo(I)]−ad � [RnPcCo(II)]ad + e− (Ia)

2H4 + [RnPcCo(II)]ad + OH− rds−→[RnPcCo(I) − −N2H3]−ad + H2O

(IIa)

RnPcCo(I) − −N2H3]−ad → [RnPcCo(I)]−ad + •N2H3 (IIIa)

N2H3 + 3OH− fast−→N2 + 3e− + 3H2O (IVa)

This mechanism that we proposed in [5] agrees with the kineticarameters reported, involving a Tafel slope of 0.060 V/decade, anrder in hydrazine and in OH− ions equal to one. An expression forhe reaction rate for electrocatalysis of hydrazine is:

= nFko[M(II)Pc]ad[N2H4]aq[OH−]aq(1 − �ad) exp[−ˇ′�Gad/RT]

Please cite this article in press as: F. Tasca, et al., Linoxidation of hydrazine on graphite electrodes modified withhttp://dx.doi.org/10.1016/j.electacta.2014.04.059

(1a)

here [M(II)Pc]ad is the surface concentration of M(II)Pcs at thelectrode potential E, [N2H4]aq is the concentration of hydrazine

nary pyrolytic graphite (OPG) in 0.1 M NaOH N2 saturated at 0.1 V/s. C and D volcanobed on OPG from data in [5,6,8]. Potentials in V vs SCE. Reproduced by permission

in the bulk, (1-�ad) is the surface coverage of M(II) free sites, and�Gad is the free energy of the dissociative adsorption of N2H4 onM(II) in step II with the loss of a proton. The rate of the reaction

ear versus volcano correlations for the electrocatalytic MN4 macrocyclic complexes, Electrochim. Acta (2014),

Fig. 3. Surface concentration effect on the Co(II)/(I) formal potential of CoPc and16(F)CoPc adsorbed on OPG in 0.2 M NaOH N2 saturated solution, data taken from[8]. Potentials in V vs SCE.

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Fig. 4. Volcano correlation for hydrazine oxidation as in Fig. 2A with added dataobtained with different surface concentration of 16(F)CoPc obtained from [8] (opencircles). Potentials in V vs SCE.

Fig. 5. Correlation between log(i/�Fe(II)) (currents divided by the Fe(II) surface con-

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he M(II) active oxidation state at the potential of the electrode ii)o the dumping effect of (1-�ad) that becomes gradually less than.5 in the falling region; iii) Hydrazine coordinates weakly to M(I)nd more strongly to M(II) by lone-pair donation.

In Zagal et al., 2013 [5] we reported that for 16(F)CoPc the footf the catalytic wave for hydrazine oxidation appears at a potentialelow that of the peak of the Co(II)/(I) process. In a following paper8] we reported on the effect of neighboring 16(F)CoPc molecules onhe Co(II)/(I) formal potential. This is illustrated in Fig. 2 that showshe influence of the surface concentration on the Co(II)/(I) formalotential of the catalyst. For Co(II) the formal potential does nothange but it does for 16(F)CoPc and more details can be found thehe previous publication [8]. However, it will be discussed brieflyhat when we decrease the coverage of the electrode surface � with6(F)CoPc, it becomes harder to oxidize the Co center, while withigher concentrations of l6(F)CoPc we facilitate the reduction ofo(II). Therefore the redox potential of Co(II)/(I) varies at differenturface concentrations of this complex. It can be suggested thathis happens because the neighboring molecules act as electron-ithdrawing agents on the Co center in one single molecule of

6(F)CoPc and this issue needs to be investigated in more detailn future work since it is not observed for other Co complexes.

hen comparing catalytic activities of electrodes coated with dif-erent surface concentrations of 16(F)CoPc, as log(i/�) (� is theotal surface concentration of 16(F)CoPc) versus the Co(II)/(I) for-

al potential, it was possible to reproduce the falling region of aolcano correlation obtained by comparing the activity of severaloN4 macrocyclics and reported before [5,6,9,35] as illustrated inig. 3. So 16(F)CoPc serves to incorporate more data point in thealling region of the volcano (Fig. 3) with open circles (data obtainedrom [8]).

For Fe macrocyclics the kinetic parameters are slightly differentompared to Co complexes: the Tafel slope is ca. 0.040 V/decadenstead of 0.060 V/decade, the order in hydrazine is still 1 but therder in OH− is now 2 so a different reaction scheme is proposeds follows:

RnPcFe(I)]−ad � [RnPcFe(II)]ad + e− (Ib)

2H4 + [RnPcFe(II)]ad + 2OH−

→ [RnPcFe(I) − −N2H2]−ad + 2H2O + e−rds (IIb)

RnPcFe(I) − −N2H2]−ad � [RnPcFe(I)]−ad + •N2H2 (IIIb)

N2H2 + 2OH− fast−→N2 + 2H2O + 2e− (IVb)

As in the case of Co macrocyclics step (IVb) can involve morehan one-electron transfer steps but we write it like this just forimplicity. An expression for the reaction rate for the electrocat-lytic oxidation of hydrazine on Fe macrocyclics can be written asollows:

= nFko[Fe(II)Pc]ad[N2H4]aq[OH−]2aq(1 − �ad) exp[ˇ(E − Erds)F/RT]

× exp[−ˇ�Gad/RT] (1b)

here [Fe(II)Pc]ad is the surface concentration of Fe(II)Pcs at thelectrode potential E, [N2H4]aq is the concentration of hydrazine inhe bulk, (1-�ad) is the surface coverage of Fe(II) free sites, Erds ishe reversible potential of the rate determining step II and �Gad ishe free energy of the dissociative adsorption of N2H4 on Fe(II) intep IIb with the loss of a proton as in CoMN4 catalysts. In contrast

Please cite this article in press as: F. Tasca, et al., Linoxidation of hydrazine on graphite electrodes modified withhttp://dx.doi.org/10.1016/j.electacta.2014.04.059

o Co complexes, the rate expression is now potential dependent inwo terms, namely the surface Fe(II) concentration that is given byhe Nernst equation and the potential dependent exponential termf the rate determining step.

centration at -0.56 V) versus the Fe(II)/(I) formal potential of the catalyst. Hydrazineoxidation in 0.05 M N2H4 + 0.1 M NaOH deareated solution). Data taken from [6].Potentials in V vs SCE.

Fig. 4 shows the same data of Fig. 2-D but plotted as (logi/�Fe(II))E versus E◦’ where E◦’is the formal potential of the catalystmeasured from cyclic voltammograms as in Fig. 1 The currents arenow divied by the real Fe(II) surface concentration calculated usingthe Nernst equation as �Fe(II) = �total/[1 + exp-F/RT(E- E◦’)] where�total is the surface coverage of the FePcs estimated from cyclicvoltammograms as in Fig. 1. The volcano shaped curve of Fig. 2Dbecomes now a linear correlation. This correlation is essentially afree-energy linear correlation similar to a Tafel plot but in this casethe driving force of the electrode is constant and the driving forcechanges for each catalyst depending on its formal potential as inredox catalysis. The slope of the linear correlation is 0.090 V/decadewhich is a bit lower than an hypothetical slope of 2RT/F assumingthat the symmetry factor � is different from 0.5, i.e. an asymmet-rical energy barrier. Note that the adsorption step IIB is also the ETrate determining step so � appears in both exponential terms ofthe expression (1b).

The same exercise shown in Fig. 4 for FeN4 catalysts can berepeated for CoN4 macrocyclics, i.e. to divide the currents by theCo(II) surface concentration of the corresponding catalysts at thepotential of the electrode E. This is illustrated in Fig. 6 and including

ear versus volcano correlations for the electrocatalytic MN4 macrocyclic complexes, Electrochim. Acta (2014),

data obtained with 16(F)CoPc at differenmt surface concentrationof this catalyst. Again, a linear correlation is obtained, with a slopeof 0.120 V/decade which shows that in this case the energy barrieris symmetrical with �’ equal to 0.5. Note that for this particular case

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ig. 6. Correlation between log(i/�Co(II)) (currents divided by the Co(II) surface con-entration at -0.40 V) versus the Co(II)/(I) formal potential of the catalyst. Hydrazinexidation in 0.05 M N2H4 + 0.1 M NaOH deareated solution). Data taken from [5].

e use �’. A Bronsted coefficient since the rate determining step is chemical step, not involving an ET from the electrode.

Both linear correlation in Figs. 5 and 6 essentially show thathe bending in the Volcano curves is not due to gradual block-ge of Fe(II) sites by adsorbed reactant species but to a gradualepletion of Fe(II) sites attributed to the external applied potentials.he question one can ask is if the linear correlations in Figs. 5 and 6ill bend for catalyst with hypothetical redox potentials more pos-

tive than those shown in Figs. 5 and 6. The data in these Figs.hows that adsorption of hydrazine on Fe(II) and Co(II) sites is weaknd does not reach values where the active sites become graduallylocked.

. Conclusions

We have found that the Co(II)/(I) formal potential of 16(F)CoPcdsorbed on graphite shifts to more negative values with increas-ng the surface concentration of this catalyst. This is not observedor unsubstituted CoPc [8]. It is not clear why this phenomenons observed for 16(F)CoPc since the exact orientation of these

olecules on the graphite surface is not known. If they were toe forming stacks, instead of monolayer, one would expect thatnly the molecules located at the outermost position in a stackhould exhibit activity, because hydrazine molecules would notave access to 16(F)CoPc molecules located below [36]. In this casehe correlations between (log i)E should not be linear or shouldhow linear behaviour only at low concentrations of 16(F)CoPc. Thiss not observed. However, some interaction between neighbouring6(F)CoPc molecules should be occurring to affect electron den-ity on the Co centre and then cause a shift in the Co(II)/(I) formalotential. These interactions do not occur with CoPc. When com-aring catalytic activities of electrodes coated with different surfaceoncentrations of 16(F)CoPc, as log(i/�) (� is the total surfaceoncentration of 16(F)CoPc) versus the Co(II)/(I) formal poten-ial, it was possible to reproduce the falling region of a volcanoorrelation obtained by comparing the activity of several CoN4acrocyclics and reported before [5,6,9,13,15,25,28,29]. If the same

lot is repeated using the surface concentration of Co(II) active sitest the potential used for the comparisons, i.e. �Co(II), the activity perctive site increases with the driving force of the catalyst, reprodu-ing what can be considered a linear free energy correlation, with

slope close to +0.120 V/decade. This unique linear correlation is

Please cite this article in press as: F. Tasca, et al., Linoxidation of hydrazine on graphite electrodes modified withhttp://dx.doi.org/10.1016/j.electacta.2014.04.059

bserved for several CoN4 macrocyclis at a fixed surface concen-ration plus data obtained for different surface concentration of6(F)CoPc. The same is observed for Fe phthalocyanines, where aolcano correlation becomes linear after correcting the currents by

[

PRESScta xxx (2014) xxx–xxx 5

the real surface concentration of Fe(II) active sites. So the volcano-shaped correlation cannot be attributed to the Sabatier principlewhere the falling side is attributed to a decrease in free active sitesbut to a decreased surface concentration of M(II) sites due to theexternal applied potential. The maxima in the correlation corre-spond to a formal potential of the catalyst that is close but lower tothe potential of the electrode. As expected then, for catalysts havingM(II)/(I) formal potentials more positive than the potential of theelectrode, the surface concentration of M(II) active sites graduallydecreases compared to the total concentration of the MPc complex.The linear correlation found in Figs. 5 and 6 strongly suggest thatadsorption of hydrazine molecules on Fe(II) and Co(II) sites is weak.

Acknowledgements

Work supported by Fondecyt Project 1100773 and by FondecytProject 1130167 and by the Milenium Nucleus of Molecular Engi-neering for Catalysis and Biosensors RC120001 funded by ICM. F.T thanks also the Center for the Development of Nanoscience andNanotechnology (CEDENNA) for financial support.

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15] G. Ochoa, C. Gutierrez, I. Ponce, J. Francisco Silva, M. Paez, J. Pavez, J.H.Zagal, Reactivity trends of surface-confined Co-tetraphenyl porphyrins andvitamin B-12 for the oxidation of 2-aminoethanethiol: Comparison with Co-phthalocyanines and oxidation of other thiols, Journal of ElectroanalyticalChemistry 639 (2009) 88.

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16] C. Linares-Flores, D. Mac-Leod Carey, A. Munoz-Castro, J.H. Zagal, J. Pavez, D.Pino-Riffo, R. Arratia-Perez, Reinterpreting the Role of the Catalyst FormalPotential. The case of Thiocyanate Electrooxidation Catalyzed by CoN4-Macrocyclic Complexes, Journal of Physical Chemistry C 116 (2012) 7091.

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29] M. Villagran, F. Caruso, M. Rossi, J.H. Zagal, J. Costamagna, SubstituentEffects on Structural, Electronic, and Redox Properties of Bis(N-alkyl-2-oxy-1-naphthaldiminato)copper(II) Complexes Revisited - Inequivalence in Solid-and Solution-State Structures by Electronic Spectroscopy and X-ray Diffrac-tion Explained by DFT, European Journal of Inorganic Chemistry (2010)1373.

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