Homogeneous Molecular Catalysis of Electrochemical Reactions

Article pubs.acs.org/JACS

Homogeneous Molecular Catalysis of Electrochemical Reactions: Catalyst Benchmarking and Optimization Strategies Cyrille Costentin* and Jean-Michel Savéant* Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d’Electrochimie Moléculaire, Unité Mixte de Recherche UniversitéCNRS No. 7591, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France S Supporting Information *

ABSTRACT: Modern energy challenges currently trigger an intense interest in catalysis of redox reactionselectrochemical and photochemicalparticularly those involving small molecules such as water, hydrogen, oxygen, proton, carbon dioxide. A continuously increasing number of molecular catalysts of these reactions, mostly transition metal complexes, have been proposed, rendering necessary procedures for their rational benchmarking and fueling the quest for leading principles that could inspire the design of improved catalysts. The search of “volcano plots” correlating catalysis kinetics to the stability of the key intermediate is a popular approach to the question in catalysis by surface-active sites, with as foremost example the electrochemical reduction of aqueous proton on metal surfaces. We discussed here for the first time, on theoretical and experimental grounds, the pertinence of such an approach in the field of molecular catalysis. This is the occasion to insist on the virtue of careful mechanism assignments. Particular emphasis is put on the interest of expressing the catalysts’ intrinsic kinetic properties by means of catalytic Tafel plots, which relate kinetics and overpotential. We also underscore that the principle and strategies put forward for the catalytic activation of the above-mentioned small molecules are general as illustrated by catalytic applications out of this particular field.

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INTRODUCTION Modern energy and environmental challenges currently trigger an intense interest in catalysis of redox reactions, particularly oxidation or reduction of small molecules such as water, hydrogen, oxygen, proton, carbon dioxide, involving mostly transition metal complexes as catalysts.1−7 In view of the rapidly increasing number of such molecules proposed as molecular catalysts of these reactions, a first task has been to devise procedures for their rational benchmarking.8,9 Another important issue is the quest of leading principles that could inspire the design of improved catalysts. In this respect, analogy with catalysis of electrochemical reactions by surface-active sites, commonly called “electrocatalysis”, points to the idea that the stability of the intermediate formed in the initial phase of the reaction might be the main factor of catalytic efficiency. In electrocatalysis, the kinetics of the reaction is measured by the current exchange density. Increasing the stability of the intermediate is expected to have two contradictious effects. One is to lower the kinetic barrier for the formation of the intermediate. The other tells us that a large stabilization of the intermediate would slow down its conversion into products and the ensuing regeneration of the catalytic surface. This common sense observationmore formally known as Sabatier principle10has led to the idea that compromising these conflicting two trends should lead to a maximum of the characteristic rate constant. The best so-optimized system stands at the apex of © 2017 American Chemical Society

the volcano that one should be able to contemplate upon plotting the exchange current vs the adsorption free energy of the intermediate within a family of electrocatalysts of a given reaction. These notions have been put forward a long time ago11,12 but are still attracting active attention and discussion in very recent times.13−15 As far as molecular catalysis is concerned, application of similar or equivalent notions are scarce and brief. One may find in Figure 5 of ref 16 under the heading “two-step chemical catalysis”, a subliminal attempt to apply the Sabatier principle to molecular catalysis of electrochemical reaction. The only merit of the triple-well potential energy profile shown there is to draw attention to the problem. A brief but specific allusion to “volcano plots” appears in the concluding prospects of a recent study of molecular catalysis of dioxygen reduction,17 which sounded to our ears as an encouragement for a deeper analysis of this general problem. The aim of the present contribution is accordingly to analyze in details the possible occurrence of volcano plots in the field of molecular catalysis of electrochemical reactions as well as to provide a critical assessment of their usefulness for inspiring the conception of new catalysts. The former of these tasks is a test of the diagnostic character of the appearance of a volcano plot. The latter consists in examining whether other expressions of Received: March 22, 2017 Published: May 1, 2017 8245

DOI: 10.1021/jacs.7b02879 J. Am. Chem. Soc. 2017, 139, 8245−8250

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Journal of the American Chemical Society

electron transfer from Q to A. The only thermodynamic descriptor of this simple reaction is its driving force, namely, F(E0cat − E0AC). Looking for volcano plots in a similar manner as in electrocatalysis then amounts to evaluate the electrochemical kinetic response as a function of F(E0cat − E0AC). To do this, it is necessary to specify how the electrochemical kinetic response is recorded. In homogeneous molecular catalytic reactiondiffusion systems under consideration, the most valuable practical situations are those in which the reaction is so fast that “pure kinetic” conditions are fulfilled, where Q has reached a steady state by mutual compensation of catalytic reaction and catalyst diffusion and, at the same time, the substrate concentration is large enough for its concentration to remain the same from the bulk of the solution to the electrode surface.16,18 Using cyclic voltammetry, as we do throughout this contribution, the current−potential responses are S-shaped and the most appropriate expression of the kinetics is the turnover frequency, TOF = Nproduct/Nmax active cat, where Nproduct is the number of moles produced in a unit of time and Nmax active cat, the maximal number of moles of active catalyst, leading to

the thermodynamics and kinetics of the catalytic process may be more appropriate to compare catalysts and to inspire strategies for their improvement. In this purpose, we will start with the analysis of simple oneelectron schemes involving either a one-step or a two-step catalytic reaction in the quest of the reasons that may cause a volcano plot to occur. It will thus been shown that volcanoes can appear upon plotting the kinetics of the reaction against a single thermodynamic descriptor, even in the case of an extremely simple reaction scheme. The next analysis concerns two-step mechanisms and discusses the role of relative stability of intermediates as a governing factor of the catalysis efficiency in line with the application of the Sabatier principle. This will give us the occasion to discuss the possible occurrence of “scaling relationships” in molecular catalysis, as briefly alluded to at the end of a recent study of molecular catalysis of dioxygen reduction.17 We will then show that breaking such scaling relationships may be a valuable strategy in the search of more efficient catalysts. These analyzes will be illustrated with two recent experimental examples, involving the catalysis of CO2 and O2 reduction, respectively and also with an older study in the organic field, namely, the reductive conversion of vicinal dihalides into the corresponding olefins. On these theoretical and experimental grounds, we will then discuss the usefulness of the volcano plot approach and examine whether ampler mechanism analyses and expression of the catalytic properties by means of catalytic Tafel plots should be preferred.

TOF =

kCA0 F F 0 0 ⎤ 1 + exp − RT η exp⎡⎣ RT (Ecat )⎦ − EAC

(

)

(1)

(C0A:

bulk concentrations of substrate). This relationship shows that the kinetic response is a tradeoff between the equilibrated formation of Q via exp[(F/ RT)(E0AC − E0cat)] and the kinetic of the homogeneous step via kC0A. It should be emphasized that the variation of the electrode potential, E, does not cause a change of its driving force, which remains equal to F(E0cat − E0AC). Shifting the electrode potential in the negative direction merely induces an increase of the concentration ratio between reduced and oxidized forms of the catalyst leading to an increase of catalytic current according to the Nernst law if the catalyst redox couple is fast as is usually the case. For relating the rate constant of the catalytic reaction to its driving force we may use a linear approximationa homogeneous version of the classical Butler−Volmer law of electrode outersphere electron transfers (pp 31, 32 in ref 18) which introduces a driving force-independent transfer coefficient, α and k0 the standard rate constant of the reaction, i.e., the rate constant at zero driving force:

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RESULTS AND DISCUSSION Looking for Volcano Plots in the Kinetics of a Basic One-Electron-One-Step Molecular Catalytic Process. The reaction mechanism depicted in Scheme 1 for the electroScheme 1

0 0 k = k0exp[−α(F /RT )(Ecat − EAC )]

with 0 < α < 1 (2)

Eq 2 is a linear approximation of the quadratic Hush−Levich− Marcus (HLM) classically applicable homogeneous outersphere electron transfers (pp 43, 44 in ref 18). From eqs 1 and 2, the turnover frequency is then expressed by eq 3.

chemical reduction of substrate A into product C corresponds to the fast (Nernstian) equilibrated electron transfer leading to the active form of the catalyst, Q, followed by the irreversible homogeneous reaction of Q with A, leading to the product C and regenerating the oxidized form of the catalyst, P (the reasonings and terminology are for reductions here and throughout the paper. They should be easily transposed for oxidations). As shown in the energy diagram of Scheme 1, the overall driving force of the process is the overpotential, η = E0AC − E, E0AC being the standard potential of the A/C couple and E the electrode potential. In this scheme, there is no intermediate between the active form of the catalyst, Q, in its reaction with the substrate A, en route to the formation of the product C and the regeneration of the oxidized form of the catalyst, P. Breaking or formation of bonds possibly occurring during this reaction are consequently viewed as being concerted with the

TOF = k0CA0 1 0 0 ⎡ αF(Ecat0 − EAC ⎡ (α − 1)F(Ecat0 − EAC )⎤ )⎤ Fη + − exp⎢⎣ exp exp ⎥ ⎢ ⎥⎦ ⎦ ⎣ RT RT RT

{

(

)

}

(3)

There are various ways of representing the kinetics vs thermodynamic correlations for this type of simple reaction mechanism, starting from the most classical one, which simply plots the catalytic rate constant or equivalently, the maximal value of the TOF, TOFmax obtained for η → ∞, i.e., from the value corresponding to the plateau of the S-shaped catalytic 8246


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Looking for Volcano Plots in the Kinetics of OneElectron-Two-Step Molecular Catalytic Processes. We address now reaction schemes more directly related to the Sabatier principle in which the catalytic reaction goes through an intermediate as pictured in Scheme 2. These reactions are

wave under pure kinetic conditions, versus the standard potential of the catalyst as shown in Figure 1a. This type of

Scheme 2

Figure 1. Reaction Scheme 1. (a) Variation of the catalytic rate constant (or of TOFmax) with its driving force (α = 0.5). (b) Catalytic Tafel plots. (c) Volcano plots. Colored dots and curves are for (F/ RT)(E0cat − E0AC) −1 (blue), −2 (green), −3 (red), −4 (1) (magenta), −5 (orange). (c) Black curves: from bottom to top (F/RT)η = 1, 2, 3, 4, 5.

mechanism and this type of representation correspond to situations in which the direct electrode electron transfer, A + e − → C and the mediating solution electron transfer Q + A → P + C are both simple outersphere processes, corresponding to the so-call “redox catalysis”. In the application of the HLM model the homogeneous electron donors reacting with the substrate must also have quasi constant intrinsic properties, as is the case, e.g., for bulky aromatic ion radical, whose solvent intrinsic reorganization energy changes little in the series (see the reductive conversion of vicinal dihalides into the corresponding olefins taken as experimental example latter on) The results reported in Figure 1a may be reorganized as catalytic Tafel plots, which relates the turnover frequency to the overpotential η (= E0AC − E) for a series of values of the catalyst standard potential (Figure 1b). As compared to Figure 1a, there is no additional information besides the fact that electrode electron transfer in the catalyst couple is assumed to be fast enough to obey the Nernst law. However, introduction of the electrode potential, E, into the game, through the overpotential, allows the selection of appropriate conditions for establishing the desired compromise between energy expense and rate of substrate conversion. Comparison between catalytic Tafel plots thus offers a rational way of benchmarking catalysts both in terms of overpotential and turnover frequency. Noteworthy in this respect is the possible crossing of catalytic Tafel plots, indicating that a given catalyst may have a better TOF than another one at a given overpotential while the order may be reversed at another overpotential. This observation directly leads to a third representation of the kinetics−thermodynamic relationships as volcano plots similarly to the case of electrocatalysis. This is shown in Figure 1c where the TOF is plotted against the catalyst standard potential, indeed showing maxima (volcano apexes) in the zones where the catalytic Tafel plots cross each other. In closing this section, it should be emphasized that the observation of volcano plots has no extraordinary character since it may happen in the framework of the simplest catalytic mechanism. It is also worth noting that, even in this simple case, the comparison between catalyst may not be involve a single descriptor: k0 and α should be taken into account in addition to E0cat.

relevant to “chemical catalysis”as opposed to “redox catalysis”in which the formation of a transient adduct between the catalyst and the substrate is thought as a way for boosting catalysis.16 The thermodynamics of the global reaction and of the catalytic process are defined in Scheme 2. There are now two thermodynamic descriptors, which may be taken as E0cat − E0AC and ΔG01, the standard free energy of formation of the intermediate B. We are looking for the way in which the rate constants of the various reactions (the k’s in Scheme 2) and the catalytic response depend on these parameters and on the overpotential defined as in the preceding section. The very fact that two thermodynamic descriptors are now involved makes the search of volcano plots sound as a losing battle since their occurrence is based on the existence of a single thermodynamic descriptor. However, within a given family of catalysts, it may happen that the two descriptors are linked together, ideally through a linear free energy relationship. We will illustrate the existence of such relationships in the section devoted to experimental examples. For the moment, we assume that the following relationship holds: 0 0 ΔG10 /F = β1(Ecat − EAC ) + C1

and therefore 0 0 ΔG20 /F = (1 − β1)(Ecat − EAC ) − C1

where the coefficients β1 and C1 are constants within the investigated family of catalysts. As in the preceding case, we express the kinetic response by means of the turnover frequency, TOF = Nproduct/Nmax active cat, where Nproduct is the number of moles produced in a unit of time and Nmax active cat, the maximal number of moles of active catalyst. We describe the rate constants of each step by the following linear activation-driving force relationships, introducing the transfer coefficients α1 and α2. 8247


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Journal of the American Chemical Society ⎛ α ΔG 0 ⎞ k1C 0 = k10exp⎜ − 1 1 ⎟ RT ⎠ ⎝

In view of the above assumptions, the main parameter in the list is the catalyst standard potential, E0cat − E0AC, which measures the stabilization of the catalytic intermediates and is, as such, the linchpin of the possible existence of volcano plots. The rather cumbersome expressions of AIrr‑nS and BIrr‑nS as a function of the parameters in the subscript list are given and established in the SI, noting that TOF tends toward a limit upon increasing the overpotential:

and thus ⎤ ⎡ α1β F 0 k1 0 )⎥ = exp⎢ − 1 (Ecat − EAC ⎦ ⎣ RT k1*

with TOFmax =

⎛ FC ⎞ k1* = k10exp⎜ −α1 1 ⎟ ⎝ RT ⎠

1 1 k1CA0

+

1

=

⎡ (1 − α )β F ΔG 0 ⎤ 1 1 1 ⎥ k −1 = k10exp⎢ ⎢⎣ ⎥⎦ RT

1 k2 1

0 − E0 ) ⎤ ⎡ α1β F(Ecat k1*CA0 exp⎢− 1 RT AC ⎥

⎣

⎦

+

1 0 − E0 ) ⎤ ⎡ α2(1 − β1)F(Ecat AC k 2*exp⎢− ⎥ RT ⎣ ⎦

As in the preceding simple scheme (Scheme 1), at large overpotential, the initial Nernstian equilibrium is so displaced in favor of Q that no matter the thermodynamic stability of Q, the overall kinetic is simply given by the homogeneous process and no volcano plot is expected as long as β1 remaining constant within a family of catalysts and 0 < β1 < 1. The expressions of AIrr‑nS and BIrr‑nS have allowed the drawing of Figures 2b and 2c, which associate examples of catalytic Tafel plots and volcano plots for the same set of reaction parameters. It is indeed observed that there is no volcano plot at large overpotential and, as in the preceding simple scheme, at low overpotential, the initial Nernstian equilibrium is less displaced in favor of Q and there is a trade-off between the equilibrium

and thus ⎡ (1 − α1)β F ⎤ k −1 0 0 1 = exp⎢ − EAC (Ecat )⎥ RT k −*1 ⎣ ⎦ ⎡ FC ⎤ k −*1 = exp⎢(1 − α1) 1 ⎥ ⎣ RT ⎦ ⎛ α ΔG 0 ⎞ k 2 = k 20exp⎜ − 2 2 ⎟ RT ⎠ ⎝

and thus ⎡ α2(1 − β )F ⎤ k2 0 0 1 = exp⎢ − − EAC (Ecat )⎥ RT k 2* ⎣ ⎦

with

⎛ FC ⎞ k 2* = k 20exp⎜α2 1 ⎟ ⎝ RT ⎠ k01 and k02 are the standard rate constants of the two steps, i.e., the rate constants at zero driving force (ΔG01 = 0, ΔG02 = 0). C0 is a normalizing concentration. k*1 and k*2 are the rate constants of the two steps not at zero driving force but for E0cat − E0AC = 0. There are thus four kinetic parameters k1*C0, k2* and α1 and α2 Normalizing TOF vs k*1 C0 or k*2 leads to only three kinetic parameters that we will also considered as constant within the family of catalysts. With all these tools in hand, we will represent the variations of the turnover frequency in the same three ways as in the preceding section. This will be done for two different kinetic regimes successively in the framework of Scheme 2. In the (IrrnS) regime, the first and second steps are irreversible and the intermediate B is not necessarily at steady-state (may accumulate). In the (Rev-S) regime, the first step is reversible, the second step is irreversible and the intermediate B is at steady-state. The (Irr-nS) Regime. As shown in the Supporting Information (SI), the current-overpotential response is given by eq 4 in which the list in subscript indexes of the coefficients of AIrr‑nS and BIrr‑nS define the parameters on which the kinetic responses depend and emphasizes their large number. A[(IrrE‐0nS− E 0 ), β and α , α ,(k */ k *C 0 )] TOF cat AC 1 2 2 1 A 1 = Fη Irr nS ‐ k1*CA0 B[(E 0 − E 0 ), β and α , α ,(k */ k *C 0 )] + exp⎡⎣ − RT ⎤⎦ cat

AC

1

1

2

2

1

A

Figure 2. Reaction Scheme 2, with k2*/k1*C0A = 1, β1 = 1, α1 = α2 = 0.5. (a, a′) Variation of the TOFmax with the driving force of the catalytic reaction corresponding to Scheme 2. (b, b′) Catalytic Tafel plots. (c,c′) Volcano plots. Colored dots and curves are for (F/RT)(E0cat − E0AC) −1 (blue), −2 (green), −3 (red), −4 (1) (magenta), −5 (orange). (c,c′) Black curves: from bottom to top (F/RT)η = 1, 2, 3, 4, 5.

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contradictious effects on the catalysts’ performances: what is gained in terms of overpotential is lost in terms of turnover frequency and vice versa. It follows that breaking this correlation is a way of designing catalysts whose efficiency goes beyond this limitation. One example of the success of this strategy can be found in the same domain, where the introduction of substituents that induce through-space (electrostatic or H-bonding) effects on top of the usual through-structure effects. Catalysis of the Reduction of Dioxygen in Acetonitrile and DMF by the FeIII/II Couple of a Series of Diversely Substituted Iron Porphyrins. This is the second experimental example. It again involves a multielectron-multistep. Several steps of the reaction sequence may involve linear free energy correlation, although protonation of the FeIIIO•− 2 is the most likely candidate. The main kinetic data are presented in Figure 4 under the same three forms as in

Nernstian reaction and the overall kinetic of the homogeneous steps leading to possible volcano plots. The (Rev-S) Regime. The same analysis has been drawn, mutatis mutandis, for the (Rev-S) regime, leading to eq 5 and to ‐S A[(Rev 0 0 Ecat − EAC ), β1 and α1, α2 ,(k 2*/ k1*CA0 )] TOF = Rev ‐ S B[(E 0 − E 0 ), β and α , α ,(k */ k *C 0 )] + exp[−(F /RT )η] k 2* cat

AC

1

1

2

2

1

A

(5)

Figure 2a′−c′. In view of the large number of governing parameters, Figure 2 represents nothing else than examples. They however show that, in the framework of a more realistic reaction scheme for homogeneous molecular catalysis of electrochemical reactions than the simple scheme analyzed previously, appearance of volcano plots is actually rooted to the very same trade-off. At the same time and also because of the large number of governing parameters, these volcano plots, in spite of their spectacular character, are a little part of the story in the quest of optimized catalyst. Patient step-by-step analysis, making vary all possible experimental parameters is a better route to reliably establishing mechanisms.

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EXPERIMENTAL EXAMPLES

The above analyses are now illustrated by three experimental examples taken from literature. Catalysis of the CO2-to-CO Conversion by the FeI/Fe0 Couple of Substituted Iron Porphyrins. The first one concerns a family of diversely substituted iron porphyrins, which catalyze the CO2-to-CO conversion in N,N′-dimethylformamide (DMF) in the presence of 1 M PhOH at the level of their FeI/Fe0 couple.19 Depending of the overpotential at which the catalysis kinetics is observed (Figure 3), a volcano does or does not appear. In this case Figure 4. Catalysis of the reduction of dioxygen in DMF + 20 mM DMF-H+ by the FeIII/II couple of the indicated iron porphyrins (from the data in ref 17, potentials in V, TOF in s−1). (c) From bottom to top: η = 0.5, 1. the other cases.17 Although the variations of the catalytic rate constant with the substituents are not very large, the presence of volcanoes on the plot is discernible. Reductive Conversion of Vicinal Dihalides into the Corresponding Olefins. Among several similar processes, catalysis of the reaction depicted in Scheme 3 has been the object of particularly

Scheme 3

Figure 3. Catalysis of the CO2-to-CO conversion at the FeI/Fe0 couple of the four indicated iron porphyrins in DMF + 1 M PhOH (from the data in ref 19, potentials in V, TOF in s−1). (c) From bottom to top: η = 0.2, 0.4, 0.6.

detailed studies involving several families of catalysts (Figure 5). Among them, electrogenerated anion radicals (solid circles in Figure 5) have been the occasion an extended examination of “redox catalysis”,16 i.e., catalysis by outersphere reagents and testing of the attending HLM model of homogeneous outersphere electron transfer. The activation-driving force relationship (lower solid line in Figure 5a)strictly speaking a parabolais close to a 1/0.12 V−1 slope− straight line. The TOFmax value at zero-driving force essentially reflects the intrinsic electron transfer properties of the substrate since the reorganization energy of the (largely delocalized) anion radicals is small. The two other families (open circles−dotted line and solid square−dashed-dotted line) stand much over the outersphere line.

the reaction sequence is more complex than in the preceding theoretical analysis. It indeed involves a two-electron reaction in which electron transfers are associated with H-bonding, proton transfer and cleavage of one of the C−O bond of CO2 and CO release. A careful analysis of the cyclic voltammetric responses recorded as a function of all possible operational parameters allowed the description of the reaction mechanism and the assignment of a linear free energy correlation for each of the four steps of the catalytic process. This correlation is the reason that, in this case, substitution exerts 8249



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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02879. Analysis of the various physical-mathematical problems and derivation of the attending equations (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected]

Figure 5. Catalysis of the reduction of Scheme 3 reaction in DMF by anion radicals of aromatic hydrocarbons and of H2TPP, ZnTPP, CuTPP (solid circles and solid lines), FeII/I porphyrins (open circles and dotted lines), CoII/I porphyrins (solid squares and dashed-dotted lines), from the data in ref 20, potentials in V, TOF in s−1. (a) Variation of the maximal TOF with the catalyst standard potential. (b) Catalytic Tafel plots.

ORCID

Jean-Michel Savéant: 0000-0003-1651-3153 Notes

The authors declare no competing financial interest.

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They are typical illustrations of “chemical catalysis”,16 whose first step consists in the formation of a transient adduct between the active form of the catalyst and the substrate. The very fact that the activationdriving force lines are parallel to the outersphere line indicates that the various steps of the catalytic process under linear free energy correlation (with, globally, αβ ≈ 0.5), which also result in much more favorable intrinsic electron transfer properties in each family (through the values of the constant C or equivalently of k*). We did not represent volcano plots for this reaction because they do no bring about further insights beyond the careful analysis of the reaction kinetics and mechanisms, which mobilized all resources of the application of cyclic voltammetry. It is worth noting also that reactions of the same vein, stereo chemical evidence additionally helped to establish the fine details of the reaction mechanisms. Although volcano plots are not of much help in the present case, catalytic Tafel plots (Figure 5b) are worth drawing since they provide a quick comparison of the catalysts both in terms of overpotentials and driving forces.

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REFERENCES

(1) Hoffert, M. I.; Caldeira, K.; Jain, A. K.; Haites, E. F.; Harvey, L. D. D.; Potter, S. D.; Schlesinger, M. E.; Schneider, S. H.; Watts, R. G.; Wigley, T. M. L.; Wuebbles, D. J. Nature 1998, 395, 881−884. (2) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15729−15735. (3) Gray, H. B. Nat. Chem. 2009, 1, 7−7. (4) Nocera, D. G. Inorg. Chem. 2009, 48, 10001−10017. (5) Abbott, D. Proc. IEEE 2010, 98, 42−66. (6) Chu, S.; Majumdar, A. Nature 2012, 488, 294−303. (7) Artero, V.; Fontecave, M. Chem. Soc. Rev. 2013, 42, 2338−2356. (8) Costentin, C.; Drouet, S.; Robert, M.; Saveant, J.-M. J. Am. Chem. Soc. 2012, 134, 11235−11242. (9) Costentin, C.; Passard, G.; Savéant, J.-M. J. Am. Chem. Soc. 2015, 137, 5461−5467. (10) Sabatier, P. La Catalyse En Chimie Organique; Librairie Polytechnique: Paris et Liège, 1920. (11) Parsons, R. Trans. Faraday Soc. 1958, 54, 1053−1063. (12) Gerischer, H. Bull. Soc. Chim. Belg. 1958, 67, 506−527. (13) Koper, M. T. M. J. Solid State Electrochem. 2013, 17, 339−344. (14) Quaino, P.; Juarez, F.; Santos, E.; Schmickler, W. Beilstein J. Nanotechnol. 2014, 5, 846−854. (15) Zeradjanin, A. R.; Grote, J.-P.; Polymeros, G.; Mayrhofer, K. J. J. Electroanalysis 2016, 28, 2256−2269. (16) Savéant, J.-M. Chem. Rev. 2008, 108, 2348−2378. (17) Pegis, M. L.; McKeown, B. A.; Kumar, N.; Lang, K.; Wasylenko, D. J.; Zhang, X. P.; Raugei, S.; Mayer, J. M. ACS Cent. Sci. 2016, 2, 850−856. (18) Savéant, J.-M. Elements of Molecular and Biomolecular Electrochemistry: An Electrochemical Approach to Electron Transfer Chemistry; John Wiley & Sons: Hoboken, NJ, 2006; pp 106−118. (19) Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M. J. Phys. Chem. C 2016, 120, 28951−28960. (20) Lexa, D.; Savéant, J.-M.; Schafer, H. J.; Su, K.-B.; Vering, B.; Wang, D. L. J. Am. Chem. Soc. 1990, 112, 6162−6177. (21) Costentin, C.; Saveant, J.-M., submitted. (22) Costentin, C.; Dridi, H.; Savéant, J.-M. J. Am. Chem. Soc. 2014, 136, 13727−13734. (23) Mondal, B.; Rana, A.; Sen, P.; Dey, A. J. Am. Chem. Soc. 2015, 137, 11214−11217.

CONCLUSION

May volcanoes show up in homogeneous molecular catalysis upon plotting the turnover frequency of the catalytic reaction against the stabilization free energy of the initial intermediate? The answer is both yes and no. It depends on the reaction mechanism and on the value of the overpotential at which the kinetics are looked at. The main drawback of the volcano plot approach is, here as in the case of electrocatalysis reactions,14,15,21 the implicit assumption that the stability of one of the reaction intermediates is the only (or at least the most overwhelming) factor governing the catalysis kinetics, which is not likely to be the case in practice. It follows that more extensive mechanistic analyses, investigating all the effects of the maximal number of parameters should be privileged over the magic quest for volcanoes. Organizing the resulting kinetics data by means of catalytic Tafel plots8 is a convenient way of obtaining the identity card of each member of a catalyst family and for comparisons. A recessive and largely frustrating issue in the search of catalysis mechanisms is the spectrochemical identification of key intermediates. This seems to be a losing battle as catalytic reactions of interest are fast and involve steady-state intermediates. However, even fast catalytic reactions may involve intermediates that are not at steady state and may accumulate to a certain extent opening the possibility of spectroscopic characterization.22,23 8250


Homogeneous Molecular Catalysis of Electrochemical Reactions

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