Catalysis of CO2 Electrochemical Reduction by Protonated Pyridine

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Catalysis of CO Electrochemical Reduction by Protonated Pyridine and Similar Molecules. Useful Lessons from a Methodological Misadventure Cyrille Costentin, Jean-Michel Savéant, and Cedric Tard ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00008 • Publication Date (Web): 02 Feb 2018 Downloaded from http://pubs.acs.org on February 2, 2018

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ACS Energy Letters

Catalysis of CO2 Electrochemical Reduction by Protonated Pyridine and Similar Molecules. Useful Lessons from a Methodological Misadventure Cyrille Costentin,1,2* Jean-Michel Savéant1* and Cédric Tard3* 1

Université Paris Diderot, Sorbonne Paris Cité, Laboratoire d'Electrochimie Moléculaire, Unité

Mixte de Recherche Université - CNRS N° 7591, Bâtiment Lavoisier, 15 rue Jean de Baïf, 75205 Paris Cedex 13, France. 2

Present address: Department of Chemistry and Chemical Biology, Harvard University, 12 Oxford Street, Cambridge, Massachusetts 02138, United States.

3

LCM, CNRS, Ecole Polytechnique, Université Paris-Saclay, 91128 Palaiseau Cedex, France.

AUTHOR INFORMATION Cyrille Costentin : [email protected], Jean-Michel Saveant : [email protected], Cédric Tard [email protected].

ABSTRACT. Electrochemical reduction of carbon dioxide is a particularly important issue in energy and environment contemporary challenges. It mobilized for several decades many researchers in the fields of catalysis by low-valent transition metal complexes. In this context, the report that a molecule as simple as protonated pyridine could catalyze the 6e‒ + 6H+ conversion

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of CO2 into methanol triggered quite a lot of interest and amazement, even if the Faradaic yield was relatively modest (20%) and a precious metal (Pt) has to be used as electrode material. Successive investigations of the same system or of systems involving molecules similar to protonated pyridine produced divergent results. The amount of data available at present from several research groups is sufficiently large to try to take stock of the situation. This concerns first and foremost a critical examination of the nature and Faradaic yields of electrolysis products. It also deals with the extensive use in these researches of a non-destructive electrochemical technique – cyclic voltammetry – as a footprint of the existence of the catalysis process and a way to access its mechanism. As shown here, serious methodological problems arise in this connection, whose discussion may serve as general guidelines for a safer future use of cyclic voltammetry. The same type of methodological questions also result from the theoretical computations that have abundantly accompanied experimental reports.

TOC GRAPHICS

Among modern energy challenges,1,2 the electrochemical reduction of CO2 takes a particularly important place in the various ways of converting solar energy into fuels or, more generally to store solar energy into chemical bonds.3,4,5 CO2 is an inert molecule toward electron injection: its


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conversion into the corresponding anion radical in the liquid phase requires a high energy, as indicated for example by a standard potential as negative as –1.97 V vs. SHE in N,N′dimethylformamide from a measurement carried out on a mercury electrode.6,7 The latter electrode represents the best approximation of an outersphere electrode, meaning that the electrode material is not involved in the reaction.8 This is not the case for a number of other metals with which substantial currents may be obtained at much less negative potentials, thus manifesting their “electrocatalytic” properties toward CO2 reduction. In other words, the electrode material participates in the reaction. Quite extensive studies of various electrode materials have been performed in this connection.9 Another important aspect of CO2 reduction is its multielectron – multistep character involving coupling of electron transfer with proton transfer leading myriad products. This is illustrated, even if only at the thermodynamic level, by figure 1 for a series of possible CO2 reduction products.

FIGURE 1. Variation of the apparent standard potential with pH (Pourbaix diagrams) for the reductive conversion of CO2 into the various products listed in the right-hand side. Figure adapted from reference 10. Homogeneous catalysis of the electrochemical reduction of CO2 has also been considerably developed by means of electrogenerated low-valent transition metal complexes, with a wealth of ingenuity and sophistication in the choice of metals and ligands.11,12,13,14,15,16,17,18,19,20,21,22


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In this framework, considerable interest was triggered by the puzzling report that a molecule as simple as protonated pyridine could catalyze the formation of methanol in sizeable amount (20% Faradaic yield) according to a 6e‒ + 6H+ process,23,24 even if a precious metal electrode (Pt and Pd) has to be used and the Faradaic yield remains modest. At first, the nature of the electrode material did not draw most of the attention. Focus was rather on the seminal role lent to the radical formed by injection of one electron into the pyridinium molecule within homogeneous reaction sequences such as the one shown in Scheme 1. Similar reports involving similar molecules and similar reactions sequences rapidly appeared. In all cases, cyclic voltammetry (CV) 25,26 was used extensively to investigate the reaction mechanism to the point in some instances of skipping electrolyses and product identification. In spite of the known achievements of the method in this domain,26 one is also amazed by the degree of detail and precision reached in the CV deciphering of mechanisms as the one shown in Scheme 1. Quite divergent reports have been issued concerning the reality of methanol production and the description of what actually happens when CO2 is reduced at a platinum electrode in the presence of protonated pyridine and similar molecules. It is time to take stock of these controversies, starting with the main question, namely is methanol really formed and if not, what are the reaction products. The fact that cyclic voltammetry and theoretical calculations (mostly DFT) have been massively employed at this occasion raises methodological issues that ought to be discussed as having a general scope well beyond the particular reaction considered here. Combining these various approaches in the analysis of the electrochemical reduction of CO2 on platinum in the presence of protonated pyridine allows the delineation of a reaction mechanism in which the reduction of protonated pyridine on platinum yields hydrogen, just as any other weak acid. This is also the case for CO2 through its hydration to carbonic acid, although CO2


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is itself reduced to CO, along a self-poisoning reaction, which does not transpire among the bulk electrolysis products.

Scheme 1. Overall proposed mechanism for the pyridinium-catalyzed reduction of CO2 to the various products formic acid, formaldehyde, and methanol. Figure reproduced with permission from reference 24, scheme 1.

Is methanol really formed upon reducing CO2 at a platinum electrode in the presence of protonated pyridine or similar molecules in water?

Scheme 2. Putative catalytic molecules.


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Pyridine An initial report on “hydrogenated” palladium electrode in the presence of protonated pyridine (pyH+) indicates formation of methanol in significant Faradaic yield (up to 30%).23 This reaction is reminiscent of previous reports of the reduction of CO2 on carbon-palladium composite electrodes 27,28 (a mixture of Pd with carbon coated with methylviologen 27), where however formate, rather than methanol, was the reaction product. Fewer details were given for the preparative scale electrolyses carried out under similar conditions on a platinum electrode, where the yields are reported 24 to be similar to those obtained on the “hydrogenated” palladium electrode, the products however being now formate (10%) and methanol (20%). Neither NMR spectra, nor gas chromatograms were shown. Successive attempts to repeat the above results concerning pyridine and platinum did not detect any sizable formation of either methanol or formate under the same galvanostatic conditions as in reference 24 as well as in a number of potentiostatic experiments at various electrolysis potentials.

29,30,31 1

H-NMR, gas chromatography and ionic chromatography were used as detection

techniques. H2 was the only electrolysis product. Formation of methanol with 14% Faradaic efficiency has been reported upon electrolysis on a platinum electrode of CO2 in the presence of protonated pyridine.32a However, detection and quantitation of methanol were carried out only by means of gas chromatography (GC) with doubts on peak retention time identification and measurement.32b With the exception of a few percent Faradaic yield at the beginning of electrolysis, the lack of significant formation of methanol and formate upon electrolysis at a Pt electrode of CO2 in the presence of PyH+ has been more recently confirmed.33


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However, a recent study aiming at investigating the role of dihydropyridines in the CO2 reduction on platinum reports formation of ca. 30% of methanol on Pt electrode in the presence of pyridinium though no formate was detected as opposed to reference 23b.

34a

The electrolyses

were carried out in CH3CN/H2O (40% v/v) mixtures rather than in pure H2O making the identification and quantitation of methanol by means of 1H-NMR and GC quite uncertain.34b Analogous molecules A series of substituted pyridines were examined recently with Faradaic yields of methanol up to 40%.35 As previously, neither 1H-NMR spectra nor GC chromatograms were provided. It is also worth noting that the CV simulations and DFT calculations reproducing these results, and thus supposed to reinforce their credibility are basically incorrect as discussed later on. A Faradaic yield of 4% has been reported when pyridazine (Scheme 2) is used instead of pyridine.32 The same analytical problems as already mentioned above 32b remain here. Besides pyridine and pyridazine a series of aromatic N-heterocycles and dihydropyridines (figure 1 in reference 34) were tried as catalysts for CO2-to-methanol conversion in the previous CH3CN/H2O (40% v/v) mixture with the same unconvincing results as for pyridine under the same conditions.34 With pyridoxine (Scheme 2) a small amount of methanol (5% Faradaic yield) was detected by means of GC upon electrolysis in similar conditions as for the other attempts using pyridine.36 Biomimetic 2e‒+H+ is often viewed as a potential hydride donor. This led to an interesting attempt in which 6,7-dimethyl-4-hydroxy-2-mercaptopteridine (Scheme 2) was reported as a catalyst of the electrochemical conversion of CO2 to methanol and formate, based on cyclic voltammetric, 13C-NMR, IR, and GC analyses.37 After checking electrolysis at the reported potential


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and at a more negative potential to speed up the reaction, it appears, on 1H-NMR and gas chromato-graphic grounds, that there is neither catalysis nor methanol or formate production.38 1HNMR (with H2O presaturation) brings about an unambiguous answer to the eventual production of methanol and formate, much more so than

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C-NMR, which may lead to erroneous conclu-

sions when no internal standard is used as in the abovementioned paper.37 IR analysis is even less conclusive. Use of a GC technique with sufficient sensitivity confirmed the lack of methanol formation.38 Purity of the ingredients. A word of caution CO2 may also be converted into methanol at semi-conductor electrodes under illumination (such as GaAs and InP).39,40 The light energy is thus utilized to decrease the overpotential of the reaction. Quite high Faradaic yields, sometime as high as 100%, are reported. However, a particularly striking observation is that the Faradaic yield may pass from close to 100% to close to 0 upon purification of the solvent water and of the supporting electrolyte, as shown in figure 2.

FIGURE 2. Faradaic efficiency for CH3OH on n-GaAs in CO2 saturated 0.2M Na2SO4. Open symbols: reagent grade salt/distilled water. Solid symbols: 99.999% salt/1.7×107 Ω.cm water. 1017 donors/cm3. Figure adapted from reference 40, figure 7. Copyright 1984, Electrochemical Society.


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This observation calls for extreme care in the purification of all ingredients of the system and may explain, besides analytical pitfalls, some of the discrepancies observed in previous reports of methanol yields (particular attention should be paid to metallic impurities that may serve as catalysts of CO2 reduction) In summary, answering the question raised in this section title it may be concluded that there is no compelling evidence that pyridine or similar molecules are able to catalyze to any significant extent the electrochemical conversion of CO2 to methanol on platinum electrodes. What can be derived from cyclic voltammetry? Uses and abuses General Among the non-destructive electrochemical techniques, cyclic voltammetry is a particularly efficient tool for mechanism and kinetic analysis of electrochemical reactions.26 The currentpotential responses obtained from cyclic linear scanning of the available electrode potential domain are indeed a reflection of the reactions taking place at the electrode surface and at its close vicinity associated with reactant transport in the bathing solution. A series of simplifying conditions should be fulfilled in order to decipher the association of these electrochemical reactions and associated chemical events, so as to establish the reaction mechanism: (i) The ratio of the working electrode surface area (usually of the order of a few mm2) over the solution volume (usually of the order of cm3) should be small enough for the electrolysis taking place during the scan to consume a negligible amount of the reaction (making the technique “non-destructive”, i.e. repeatable a lot of times using the same solution); (ii) The solution contains a supporting electrolyte in sufficient concentration (usually of the order of 0.1 – 1 M) as compared to the reactant concentration (usually of the order of a few mM) for two purposes. One is to minimize the solution resistance so as to ensure a good definition of the working electrode potential vs. the


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reference electrode with the help of a sufficiently fast potentiostat possibly helped by a positive feedback compensation device. This is not too much of a problem in the present case since we are dealing with aqueous solutions, which are not very resistive. The other purpose is to minimize the contribution of migration vs. diffusion in the mass transport of charged reactants; (iii) Diffusion is the only mode of reactant mass transport, which implies that the solution and electrode are immobile so as to avoid convection and also that the scan rate is high enough for avoiding the interference of natural convection. We will be back to this point in a more quantitative manner further on. A sufficiently high scan rate will also minimize the constrained diffusion effect due to the small size of the electrode. If these three conditions are fulfilled, diffusion can be considered as being linear and semi-infinite, which greatly simplifies the analysis of the CV responses. On this basis, CV has been extensively used in the mechanism analysis and kinetic characterization of electrochemical reactions from simple to very complex reaction schemes and with a special mention for catalytic processes.26,41,42

Major inconsistencies in the simulations of the CV responses of CO2-pyridine-Pt reacting systems Testing a reaction mechanism such as that shown in Scheme 1 involves simulation of the CV responses expected for this mechanism with proper adjustment of the kinetic and thermodynamic constants. It seems pretty obvious that all the reactions that contribute to the current leading to all the reaction products must then be taken into account. It is striking that the simulations accompanying the report of methanol formation 24 do not obey this rule. Figure 3 shows such simulations that were carried out with the reaction mechanism involving reactions (1) – (4) in Scheme


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3. As far as the reduction of CO2 is concerned, the final product (reaction 4) is the protonated anion radical of CO2, not methanol not even formate. The further reduction of this one-electron product into formate and the four-electron reduction of formate into methanol, as depicted in Scheme 1 have simply been ignored. If only for this reason, such simulations thus appear irrelevant. Other pitfalls will be addressed in the next section.

FIGURE 3. CVs for the reduction of pyridinium at a Pt disk electrode in aqueous CO2-saturated solution at pH 5.3 with 0.5 M KCl as supporting electrolyte. Scan rates from bottom to top: 1, 5, 10, 50, and 100 mV/s. a: Comparison of experimental (solid) and simulated (dash) from reactions 1-4 in Scheme 3, with pK = 5.17, E0 = –0.58 V vs. SCE, α = 0.64, kS = 0.010 cm s-1, kHy =2.95 M-1s-1, kCO2 =2.95 M-1s-1. Figure adapted from reference 24, figure 4.

Scheme 3. Overall proposed mechanism for the pyridinium reduction in the presence of CO2 from reactions 1-4 in reference 24. Scheme adapted from reference 24.


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Does the reduction of PyH+ on platinum involve the intermediacy of the PyH• radical? The PyH• radical arising from the reduction of PyH+ plays a crucial role in the mechanisms supposed to convert CO2 into methanol (Scheme 1). The reduction of PyH+ in the absence of CO2 was thus investigated with the aim of checking the occurrence of this initial step and obtaining the characteristics of the PyH+/PyH• couple to be used in the simulation of the Scheme 1 mechanism).24 The CV responses (figure 4a) were thus simulated according to the catalytic mechanism depicted by reactions (1) – (3) in Scheme 3.

FIGURE 4. CVs for the reduction of pyridinium at a Pt disk electrode in aqueous solution at pH 5.3 with 0.5 M KCl as supporting electrolyte. Scan rates from bottom to top: 1, 5, 10, 50, and 100 mV/s. a: Comparison of experimental (solid) and simulated (dash) from reactions 1-3 in Scheme 3 under hypothetical linear semi-infinite diffusion conditions, with pK = 5.17, E0 = – 0.58 V vs. SCE, α = 0.64, kS = 0.010 cm s-1, kHy =2.95 M-1s-1 (figure adapted from reference 24, figure 1). b: Simulation from 1-2 in Scheme 3 with the same characteristics as in a but taking into account natural convection as an additional mode of mass transport, characterized by a steady-state diffusion layer thickness δ = 0.05 cm.43 The current-potential curve at the highest scan rate looks like a classical chemically reversible CV response with interference of electrochemical electron transfer kinetics, leading to the characteristics recalled in the caption of figure 4a. The idea that reaction (3) could come into play, giving rise to a catalytic process, represented by the regeneration step of Scheme 3, arose from the form of CV responses at very low scan rates. They indeed depart to a certain extent from the apparently reversible chemical reversibility en route toward an S-shaped response characteristic


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ACS Energy Letters

of catalytic processes.25,41,44 Most likely, the use of very low scan rates was suggested by the fact that the passage from the chemically reversible behavior to the catalytic behavior follows the

(

)

decrease of the dimension-less parameter Fv / RTk Hy Ccat .25,41,43 A small value of the catalytic rate constant may thus be hopefully compensated by a low value of the scan rate. However, in doing so, the basic rule that the scan rate is large enough for diffusion to be the only mode of reactant mass transport, avoiding the interference of natural convection, is clearly violated. This is shown in figure 4b where simulations involve only reactions (1) and (2) and in which the change of shape observed at very low scan rate is accounted for by the interference of natural convection. The value of the diffusion layer thickness corresponding to natural convection conditions used in the simulation – 0.05 cm – perfectly matches current laboratory practice. It follows that if the (1) - (3) mechanism in Scheme 3 were to be taken earnestly into consideration, the rate constant, kHy , of the reaction that closes the catalytic loop would appear as vanishingly small. In other words, this "catalytic" mechanism is incompatible with the very existence of H2-evolution catalysis. This casts very serious doubts on the intermediacy of PyH• in the reduction of PyH+. Another overwhelming argument against the (1) - (3) catalytic mechanism in Scheme 3 is that it is does not involve the electrode material, in sharp contradiction with the observation that the reaction takes place at platinum electrodes and not at an inactive electrode such as glassy carbon. The H2 formation route. As to its reduction on platinum, PyH+ behaves just as another Brønsted acid If, as previously demonstrated, PyH• is not an intermediate in the reduction of PyH+ on platinum, what is then the mechanism of the reaction? A close examination of the CV responses (figure 5A,a) reveals the presence of two waves, as is also the case for another weak acid, acetic acid (figure 5A,b). The first wave corresponds to the direct reduction of hydrated protons. The second


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wave corresponds to the reduction of the hydrated protons generated by the rapid dissociation of the acid, whose equilibrium concentrations are shown in figure 5B or, alternatively, to the direct reduction of HA.

FIGURE 5. A: Cyclic voltammetry of a 2 mM solution of pyridine (a,a') and 3 mM or 2 mM of acetic acid (b,b') on a platinum electrode at 0.2 V/s in presence of 0.1 M KNO3, T = 295 K, as a function of pH: a,a': 2.5 (blue), 3 (green), 4 (red). b,b': 3 (blue), 4 (green). a,b: experimental, a',b': 0,ap H+ /1/2H2

simulated with pKPyH+ = 5.2 and pKAcOH = 4.75, 45 E

= –0.15 V vs. NHE, k S ,+ap

H /1/2H2

=

0.03 cm s-1, αap = 0.7. Diffusion coefficients (10-5 cm2 s-1): D + = 9.3, DH 2 = 5, DPyH + = 1, H =1.1, DAcOH = 1.3. DPyr = 0.6, D AcO −

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b: Equilibrium concentrations of hydrated protons

and acid (in mM) in a 2 mM pyridine and acetic acid solutions as a function of pH. pKs are respectively 5.2 and 4.75. Figure adapted from reference 29, figures 1 and 2. Indeed in this "CE" mechanism, 47 depicted in Scheme 4, the mutual conversion of the acid and the hydrated proton are fast as expected for all oxygen acids. One might thus expect to see a sin-


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ACS Energy Letters

gle reversible wave. In fact, the splitting of the CV response into two waves results from the following reasons. When the first wave develops, hydrated protons are consumed without significant variation of HA concentration (figure 5B). Significant displacement of the acid dissociation equilibrium to regenerate hydrated protons takes place once the hydrated proton concentration close to the electrode is small. Consequently, the second wave may correspond to acid reduction through the reduction of the hydrated proton, developing once hydrated protons initially present in solution have been reduced at the level of the first wave. Alternatively, HA may be directly reduced, noting that the fast HA dissociation equilibrium prevents getting mechanistic insights into HA reduction. The mechanism assignment is corroborated by the simulations

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shown in

figures 5Aa' for PyH+ and 5Ab’ for AcOH. This confirms that PyH+ behaves as any other weak acid and that its reduction does not go through a pyridyl radical.48

Scheme 4. Mechanism for the reduction of weak acids (e.g. PyH+ and AcOH) on platinum.

The co-reduction of PyH+ and CO2 on platinum. The actual mechanism. Concerning now the reduction of CO2, it should be noted first that CO2 in water is a source of Brønsted acids (CO3H2, CO3H-) and may therefore contribute by itself to H2 evolution on platinum as sketched in Scheme 5. In this connection, figure 6 summarizes experiments carried out with a CO2 saturated solution at pH 3.9, in the absence of PyH+ or of any other acid. As compared to the other acids, the current is much smaller than expected for complete CO2 reduction at the electrode surface in view


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of the CO2 saturating concentration in water (0.0383 M 49). Also, the wave tends to be S-shaped. These features indicate that the generation of the hydrated proton from CO2 is slower than it was with the previous acids. This is not really surprising since CO2 has to be hydrated before the resulting carbonic acid generates the hydrated protons. The latter reaction is expected to be very fast since it consists in a proton transfer from an oxygen acid as opposed to the CO2 hydration step, which must overcome severe reorganization barriers caused by important changes of bond lengths and angles (see Scheme 5). Simulation of Scheme 5 reaction sequence, assuming that the proton transfer reaction remains at equilibrium (pK = 3.6

49,50

), with the same characteristics of

the hydrated proton reduction as above, knowing that khydr/k-hydr= Khydr = 1.7×10-3, khydr= 3×10-2 s-1, k-hydr= 17.65 s-1, in excellent agreement with literature data

51

49

leads to

(the shoulder in

the experimental oxidation return current is due to underpotential deposition current, which is not taken into account in the simulation)

Scheme 5. Mechanism for the H2 evolution from CO2 at a platinum electrode.


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FIGURE 6. Cyclic voltammetry of a CO2 saturated solution at pH=3.9 on a platinum electrode in presence of 0.1 M KNO3, T = 295 K, as a function of the scan rate (V/s): 0.05 (blue), 0.1 (green), 0.2 (red), 0.5 (gray). Diffusion coefficients (10-5 cm2 s-1): Diffusion coefficients (10-5 cm2 s-1): 0,ap H+ /1/2H2

=1.1, DAcOH = 1.3. 46 E DH+ = 9.3, DH 2 = 5, DPyH + = 1, DPyr = 0.6, D AcO − vs. NHE, k

S ,ap H + /1/2H2

= –0.15 V

= 0.03 cm s-1, αap = 0.7. [CO2] = 0.0383 M. khydr/k-hydr= Khydr = 1.7×10-3;

khydr= 3×10-2 s-1; k-H+/ kH+= 10-3.6; kH+ = 1010 M-1 s-1. Figure adapted from reference 29, figure 3.

The CV responses of CO2 in the presence of PyH+, and, for the sake of comparison, AcOH (figure 7) are simply the superposition of the contributions of the two acids present, PyH+ or AcOH on the one hand and CO2 on the other. Upon scanning to more negative potentials, water itself is reduced (see green line in figure 7a) leading also to formation of H2 which is reoxidized on the reverse scan. This event can be introduced in the simulations resulting from the combination of Schemes 4 and 5 by considering an ad hoc Butler-Volmer kinetics for H2O reduction (see green line in figure 7a’). In summary, the CV responses match an H2-evolution process that involves both protonated pyridine (or acetic acid) and the bicarbonate deriving from the hydration of CO2 with no special behavior of PyH+ and no formation of methanol or formate.52 There is no indication of a mechanism that would involve the intermediacy of adsorbed H atoms p roduced by re duction of PyH+ and the production of formate.53


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6

6

i ( µA)

i ( µA)

4

4

a

2

2

a'

0

0

-2

-2

-4

-4

-6 8

E (Vvs.NHE)

i ( µA)

E (Vvs.NHE)

i ( µA)

6

-6 8 6

4

b

4

b'

2

2

0

0

-2

-2

-4

-4

-6

-6

E (Vvs.NHE)

-8 0.4

0.2

0

-0.2 -0.4 0.4

E (Vvs.NHE) 0.2

0

-8

-0.2 -0.4 -0.6

FIGURE 7. Cyclic voltammetry of a CO2-saturated solution of 3 mM pyridine (a,a') and 3 mM acetic acid (b,b') on a platinum electrode in presence of 0.1 M KNO3, T = 295 K, at pH = 5.15 (a,a’) and 4 (b,b’). Scan rate: 0.1 V/s (a,a’) and 0.2 V/s (b,b’). blue: acid alone; green: CO2 alone (pH = 4.5 in a,a’ and 4 in b,b’); red: acid + CO2. a, b: experimental; a',b': simulations. Simulation parameters: Diffusion coefficients (10-5 cm2 s-1): D + = 9.3, DH 2 = 5, DPyH + = 1, DPyr = 0.6, H

0,ap H+ /1/2H2

46 D AcO − =1.1, DAcOH = 1.3. E

= –0.15 V vs. NHE, k S ,+ap

H /1/2H2

= 0.03 cm s-1, αap = 0.7.

[CO2] = 0.0383 M. khydr/k-hydr= Khydr = 1.7×10-3; khydr= 3×10-2 s-1; k-H+/ kH+= 10-3.6 ; kH+ = 1010 M1

S ,ap s-1. DH = 9.3, k + +

H /1/2H2

= 0.03 cm s-1, k S ,+ap

H /1/2H2

= 0.03 cm s-1, αap = 0.7. [CO2] = 0.0383 M.

khydr/k-hydr= Khydr = 1.7×10-3; khydr= 3×10-2 s-1; k-H+/ kH+= 10-3.6 ; kH+ = 1010 M-1 s-1Protonation rate constants for Pyr and AcO-: 1010 M-1 s-1. Figure reproduced from reference 29, figure 4.

It thus appears that the CO2 molecule is not reduced itself during the catalytic process at platinum, but rather provides, in the presence of water, protons that participate to H2 evolution. We are going to nuance this assertion in the last section of this article, with however no effect on product distribution.


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What can be learned from theoretical calculations? Uses and abuses General As discussed in the following, theoretical calculations have found an extensive playground in the discussion of the mechanisms that could have explained the alleged formation of methanol or formic acid upon reduction of CO2 in the presence of protonated pyridine. It should however be borne in mind that the results may significantly depend on the choice of the computation parameters.54 It follows that the help possibly provided by theoretical computations concerns trends rather than precise quantitative values. Assistance from theoretical calculations in problematic mechanistic proposals Theoretical estimation of the standard potential of the PyH+/PyH• couple, E 0

PyH + +e − /PyH •

ally referred to by the more ambiguous “redox potential” denomination)

55,56,57,58

(usu-

was used

56

to

dismiss the intermediacy of the pyridyl radical in the reduction of PyH+ in the presence of CO2 on platinum. The theoretical value is indeed around –1.15 V vs. SHE instead of ca –0.35 V vs. SHE for the experimental value. There is no way to check experimentally the computed value of as no corresponding one-electron response couple is observable on an inert electrode such as glassy carbon.59 However, the difference between the E 0

PyH + +e − /PyH •

value and the potential

where the reaction take place is so large that there is little doubt that the reaction cannot involve the intermediacy of PyH•.60 This observation led progressively to the idea that “adsorbed states” were involved

56b

to finally point to the crucial role of platinum in the reaction.46,53 Theoretical

calculations were not really necessary to reach this conclusion: comparison between platinum and glassy carbon should have been enough in this purpose. Even though they had the merit to fall in line with this observation, they were not able to lead to the actual reaction mechanism, as established by analysis of the CV responses as depicted in the preceding section.


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Another track was opened by additional theoretical estimations of characteristic potentials, namely the possible intermediacy of p-dihydropyridine (p-DHP, Scheme 2) based on the proximity between the computed value of E 0

Py+2H + +2e − /p -DHP

and the operating potential.56 DHP could

then more likely play the role that was formerly assigned to PyH• in the conversion of CO2 into formate and methanol.56 A similar mechanism involving o-DHP (Scheme 2) has also been envisaged on theoretical computational grounds,61 as well as other dihydropyridine derivatives.62 There is no more experimental support for these refined versions than for the initially proposed PyH• mechanism. Indeed, as discussed previously, the reduction of PyH+ at platinum in the presence and absence of CO2 does not involve the reduction of the pyridine ring but follows exclusively the H2-evolution pathway as for any weak acid. Hydrides are certainly able to react with CO2,63,64 but pyridine hydrides are not formed from PyH+ reduction under the electrochemical platinum electrode conditions.

Theoretical computations as a tool for repairing experimental errors? Recently, theoretical computations have been offered as a means to discriminate between “conflicting experimental results” 65 concerning the electrocatalytic reduction of CO2 to CH3OH on a glassy carbon electrode by the 6,7-dimethyl-4-hydroxy-2-mercaptopteridine.65 In fact, as discussed in a previous section, there are no “conflicting” results. One of these is simply an artefact due to an experimental error (lack of internal reference in NMR experiments). Relying on theoretical calculations for repairing experimental is contrary to the very nitty-gritty of scientific methodology. Indeed, if theoretical calculations may help chemical intuition in the search of mechanistic models, the reliability of experimental data is, per se, independently of any model, the essential item from which sound scientific conclusions can be drawn. The fact that, in the


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present example, the theoretical calculations agree with the correct experimental result simply indicates that the calculation parameters were not badly chosen. In other words, these theoretical calculations are validated by the correct experimental results and not the opposite. Is it really impossible to reduce the CO2 molecule itself on platinum? A paradoxical answer. From the discussion in Section 2, it appears that CO2 is not really reduced but rather participates, following its aquation into carbonic acid, in hydrogen evolution. Actually, the situation is not that straightforward, as revealed by the observation that CO2 reduction is accompanied by an inhibition of the electrode process taking place when it is carried out in the presence of acids such as PyH+ or AcOH. Cyclic voltammetry (figure 8) and in situ infrared spectroscopy were closely associated to investigate and understand the nature and consequences of the inhibition process.30 Constant comparison between the two acids was required to decipher the course of the reaction owing to the fact that the IR responses are perturbed by PyH+ adsorption. It finally appears that

FIGURE 8. Fifty successive cyclic voltammograms on a Pt electrode at 0.1 V s−1 under 1 atm. CO in the presence of 0.4 M K2SO4 of: a: a 10 mM pyridinium/pyridine solution at pH 5.1; b: 10 mM acetic acid/acetate solution at pH 5.08. In each case, the red curves represent the first scan. Figure reproduced from reference 30, figure 2. 2


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inhibition is caused by the reduction of CO2 into CO, whose high affinity with platinum triggers the formation of a Pt−CO film that prevents the reaction process. This reduction was shown to involve underpotential deposited hydrogen atoms. Thus, a paradoxical situation develops in which the high affinity of Pt for CO helps to decrease the overpotential for the reduction of CO2 and therefore blocks the electrode, precluding the reaction to proceed.66 Conclusion Much ado about nothing? What is certain is that protonated pyridine is not this wonderfully simple catalyst of the conversion of CO2 into methanol at a platinum electrode it was dreamt to be. Its reduction on platinum is merely that of a weak Brønsted acid, just as any other weak Brønsted acid as checked with acetic acid. A positive outcome of this misadventure was the establishment that the reduction of CO2 under these conditions also leads to hydrogen evolution, involving the carbonic acids resulting from its (relatively slow) hydration (a similar role of CO2 in H2 evolution in water has recently been invoked in another context

67,68

). The reduction of

both PyH+ (or AcOH) and CO2 is plainly the superposition of the two H2-evolution pathways. In terms of bulk electrolysis, H2 is the only product, as if it was impossible to inject an electron into CO2, starting its bond-breaking transformation. In fact, this is not quite true: this reaction sequence does occur at the platinum producing, neither formate, nor methanol, but CO, which – faithful to its reputation as a platinum poison – blocks the reaction it has triggered. Cyclic voltammetry is a powerful tool for mechanism deciphering, provided however that some precautions are taken. The list and analysis of the precautions that have not been respected may serve as a positive tutorial for a safe use of cyclic voltammetry in the investigation of catalytic processes. Similar considerations may be developed for the use of theoretical computations. They can be of help in picturing and testing predictions based on chemical intuition. Constant checking of the


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congruence with the detail of experimental reality are expected to avoid irrelevant mechanistic adventures.

Biographies Cyrille Costentin is Professor of University Paris Diderot since 2007. He is currently Visiting Scholar at Harvard University.

Jean-Michel Savéant received his education in Paris Ecole Normale Supérieure. He is, since 1985, Professor at the University Paris-Diderot. His current research interests involve all aspects of molecular electrochemistry. He is a member of the French Science Academy and a foreign associate of the National Academy of Sciences of the USA Cédric Tard is a Professor of Chemistry at the Ecole Polytechnique since 2017. He received his PhD in 2005 from the University of East Anglia under the supervision of Prof. Chris Pickett and then joined the group of Prof. Jean-Michel Savéant at the University Paris Diderot. REFERENCES

(1) Gray, H. B.: Powering the planet with solar fuel. Nat. Chem. 2009, 1, 7-7. (2) Nocera, D. G.: The artificial leaf. Acc. Chem. Res. 2012, 45, 767-776. (3) Kondratenko, E. V.; Mul, G.; Baltrusaitis, J.; Larrazabal, G. O.; Perez-Ramirez, J.: Status and perspectives of CO2 conversion into fuels and chemicals by catalytic, photocatalytic and electrocatalytic processes Energy Environ. Sci., 2013, 6, 3112-3135. (4) Aresta, M.; Dibenedetto, A.; Angelini, A.: The changing paradigm in CO2 utilization. J. CO2 Util. 2013, 3-4, 65-73.


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(5) Zhang, L.; Zhao, Z.-J.; Gong, J.: Nanostructured materials for heterogeneous electrocatalytic CO2 reduction and their related reaction mechanisms. Angew. Chem., Int. Ed. Engl. 2017, 56, 11326-11353. (6) Lamy, E.; Nadjo, L.; Saveant, J. M.: Standard potential and kinetic-parameters of electrochemical reduction of carbon-dioxide in dimethylformamide. J. Electroanal. Chem. 1977, 78, 403-407. (7) Kai, T.; Zhou, M.; Duan, Z.; Henkelman, G. A.; Bard, A. J.: Detection of CO2•‒ in the electrochemical reduction of carbon dioxide in DMF by scanning electrochemical microscopy. J. Am. Chem. Soc. 2017, 139, 18552-18557.

(8) Gennaro, A.; Isse, A. A.; Severin, M.-G.; Vianello, E.; Bhugun, I.; Savéant, J.-M. Mechanism of the electrochemical reduction of carbon dioxide at inert electrodes in media of low proton availability.J. Chem. Soc., Faraday Trans., 1996, 92, 3963-3968. (9) Hori, Y.: Electrochemical CO2 reduction on metal electrodes. In Modern Aspects of Electrochemistry; Vayenas, C. G., White, R. E., Gamboa-Aldeco, M. E., Eds.; Springer: New York,

NY, 2008; pp 89-189. (10) Frese Jr, K. W.: Electrochemical and electrocatalytic reactions of carbon dioxide, B. P.Sullivan, K Krist,. H. E Guard; Eds. Elsevier: New York, NY, 1993, Chap. 6. (11) Fisher, B. J.; Eisenberg, R.: Electrocatalytic reduction of carbon dioxide by using macrocycles of nickel and cobalt. J. Am. Chem. Soc. 1980, 102, 7361-7363. (12) Hammouche, M.; Lexa, D.; Savéant, J.-M.; Momenteau, M.: Catalysis of the electrochemical reduction of carbon dioxide by iron("0") porphyrins. J. Electroanal. Chem. 1988, 249, 347351. (13) Windle, C. D.; Perutz, R. N.: Advances in molecular photocatalytic and electrocatalytic CO2 reduction. Coord. Chem. Rev. 2012, 256, 2562-2570.


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(14) Schneider, J.; Jia, H.; Muckerman, J. T.; Fujita, E.: Thermodynamics and kinetics of CO2, CO, and H+ binding to the metal centre of CO2 reduction catalysts. Chem. Soc. Rev. 2012, 41, 2036-2051. (15) Costentin, C.; Robert, M.; Savéant, J.-M.: Current issues in molecular catalysis illustrated by iron porphyrins as catalysts of the CO2-to-CO electrochemical conversion. Acc. Chem. Res. 2015, 48, 2996-3006. (16) Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M.: Dissection of electronic substituent effects in multielectron-multistep molecular catalysis. Electrochemical CO2-to-CO conversion catalyzed by iron porphyrins. J. Phys. Chem. C 2016, 120, 28951-28960. (17) Azcarate, I.; Costentin, C.; Robert, M.; Savéant, J.-M.: Through-space charge interaction substituent effects in molecular catalysis leading to the design of the most efficient catalyst of CO2-to-CO electrochemical conversion. J. Am. Chem. Soc. 2016, 138, 16639-16644. (18) Costentin, C.; Robert, M.; Savéant, J.-M.: Catalysis of the electrochemical reduction of carbon dioxide. Chem. Soc. Rev. 2013, 42, 2423-2436. (19) Berardi, S.; Drouet, S.; Francas, L.; Gimbert-Surinach, C.; Guttentag, M.; Richmond, C.; Stoll, T.; Llobet, A.: Molecular artificial photosynthesis. Chem. Soc. Rev. 2014, 43, 7501-7519. (20) Gottle, A. J.; Koper, M. T. M.: Proton-coupled electron transfer in the electrocatalysis of CO2 reduction: prediction of sequential vs. concerted pathways using DFT. Chem. Sci. 2017, 8, 458-465. (21) Ngo, K. T.; McKinnon, M.; Mahanti, B.; Narayanan, R. P.; Grills, D. C.; Ertem, M. Z.; Rochford, J.: Turning on the protonation-first pathway for electrocatalytic CO2 reduction by manganese bipyridyl tricarbonyl complexes. J. Am. Chem. Soc. 2017, 139, 2604-2618. (22) Roy, S.; Sharma, B.; Pécaut, J.; Simon, P.; Fontecave, M.; Tran, P. D.; Derat, E.; Artero, V.: Molecular cobalt complexes with pendant amines for selective electrocatalytic reduction of carbon dioxide to formic acid. J. Am. Chem. Soc. 2017, 139, 3685-3696.


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(23) Seshadri, G.; Lin, C.; Bocarsly, A. B.: A new homogeneous electrocatalyst for the reduction of carbon dioxide to methanol at low overpotential. J. Electroanal. Chem. 1994, 372, 145150. (24) Cole, E. B.; Lakkaraju, P. S.; Rampulla, D. M.; Morris, A. J.; Abelev, E.; Bocarsly, A. B.: Using a one-electron shuttle for the multielectron reduction of CO2 to methanol: kinetic, mechanistic, and structural insights. J. Am. Chem. Soc. 2010, 132, 11539-11551. (25) Bard, A. J.; Faulkner, L. R. Electrochemical methods: fundamentals and applications, John Wiley & Sons, 2nd edn., 2001. (26) Savéant, J.-M. Elements of molecular and biomolecular electrochemistry: an electrochemical approach to electron transfer chemistry, John Wiley & Sons: Hoboken, NJ, 2006. (27) Bookbinder, D. C.; Lewis, N. S.; Wrighton, M. S.: Heterogeneous one-electron reduction of metal-containing biological molecules using molecular hydrogen as the reductant: synthesis and use of a surface-confined viologen redox mediator that equilibrates with hydrogen. J. Am. Chem. Soc. 1981, 103, 7656-7659.

(28) Stalder, C. J.; Chao, S.; Summers, D. P.; Wrighton, M. S.: Supported palladium catalysts for the reduction of sodium bicarbonate to sodium formate in aqueous solution at room temperature and one atmosphere of hydrogen. J. Am. Chem. Soc. 1983, 105, 6318-6320. (29) Costentin, C.; Canales, J. C.; Haddou, B.; Savéant, J.-M.: Electrochemistry of acids on platinum. Application to the reduction of carbon dioxide in the presence of pyridinium ion in water. J. Am. Chem. Soc. 2013, 135, 17671-17674.; 2014, 136, 17689-17689. (30) Dridi, H.; Comminges, C.; Morais, C.; Meledje, J.-C.; Kokoh, K. B.; Costentin, C.; Savéant, J.-M.: Catalysis and inhibition in the electrochemical reduction of CO2 on platinum in the presence protonated pyridine. New insights into mechanisms and products. J. Am. Chem. Soc. 2017, 139, 13922–13928.


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(31) Dunwell, M.; Yan, Y.; Xu, B.: In situ infrared spectroscopic investigations of pyridinemediated CO2 reduction on Pt electrocatalysts. ACS Catal. 2017, 7, 5410-5419. (32) (a) Portenkirchner, E.; Enengl, C.; Enengl, S.; Hinterberger, G.; Schlager, S.; Apaydin, D.; Neugebauer, H.; Knör, G.; Sariciftci, N. S.: A Comparison of pyridazine and pyridine as electrocatalysts for the reduction of carbon dioxide to methanol. ChemElectroChem 2014, 1, 15431548. (b) As seen in figure 5 of reference 32a, the retention time of the electrolysis product is not quite the same as that of an authentic sample and its putative concentration falls at the extreme end of the calibration curve. (33) Rybchenko, S. I.; Touhami, D.; Wadhawan, J. D.; Haywood, S. K.: Study of pyridinemediated electrochemical reduction of CO2 to methanol at high CO2 pressure. ChemSusChem. 2016, 9, 1660-1669. (34) (a) Giesbrecht, P. K.; Herbert, D. E.: Electrochemical reduction of carbon dioxide to methanol in the presence of benzannulated dihydropyridine additives. ACS Energy Lett. 2017, 2, 549-555. (b) In 1H-NMR, in spite of the difficulties resulting from the double saturation technique required by the CH3CN-H2O mixt solvent it seems that methanol is not formed. In GC the chromatograms of the electrolyzed are looking quite different from that of an authentic methanol sample. (35) Barton Cole, E. E.; Baruch, M. F.; L’Esperance, R. P.; Kelly, M. T.; Lakkaraju, P. S.; Zeitler, E. L.; Bocarsly, A. B.: Substituent effects in the pyridinium catalyzed reduction of CO2 to methanol: further mechanistic insights. Top. Cat. 2015, 58, 15-22. (36) (a) Lee, J. H. Q.; Lauw, S. J. L.; Webster, R. D.: The electrochemical reduction of carbon dioxide (CO2) to methanol in the presence of pyridoxine (vitamin B6). Electrochem. Commun. 2016, 64, 69-73; 2017, 74, 57. (b) We (JMS & CT) tried unsuccessfully to reproduce these results using 1H-NMR detection. (37) Xiang, D.; Magana, D.; Dyer, R. B.: CO2 reduction catalyzed by mercaptopteridine on glassy carbon. J. Am. Chem. Soc. 2014, 136, 14007-14010.


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(38) Saveant, J.-M.; Tard, C.: Attempts to catalyze the electrochemical CO2-to-methanol conversion by biomimetic 2e– + 2H+ transferring molecules. J. Am. Chem. Soc. 2016, 138, 10171021. (39) Canfield, D.; K. W. Frese, J.: Reduction of carbon dioxide to methanol on n- and p- GaAs and p- InP . Effect of crystal face, electrolyte and current density. J. Electrochem. Soc. 1983, 130, 1772-1773.

(40) Frese, K. W.; Canfield, D.: Reduction of CO2 on n‐ GaAs electrodes and selective methanol synthesis. J. Electrochem. Soc. 1984, 131, 2518–2522. (41) Savéant, J.-M.; Vianello, E.: Recherches sur les courants catalytiques en polarographie oscillographique à balayage linéaire de tension. Étude théorique. Adv. Polarogr. 1959, 1, 367374. (42) Costentin, C.; Savéant, J.-M.: Multielectron, multistep molecular catalysis of electrochemical reactions: benchmarking of homogeneous catalysts. ChemElectroChem 2014, 1, 1226-1236. (43) Using the DigiElch software. See: Rudolph, M.: Digital simulations on unequally spaced grids: Part 2. Using the box method by discretisation on a transformed equally spaced grid. J. Electroanal. Chem. 2003, 543, 23-39.

(44) Savéant, J.-M.; Vianello, E.: Potential-sweep chronoamperometry: Kinetic currents for first-order chemical reaction parallel to electron-transfer process (catalytic currents) Electrochim. Acta, 1965, 10, 905-920. (45) Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000; p 8-45 and 8-48. (46) (a) Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000 (b) For ions ref 46a p 5-95 and 5-96. For AcOH and Pyridine: ref 46a p 6-192. For H2 and CO2: ref 46a p 6-191.


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(47) (a) Savéant, J.-M.; Vianello, E.: Potential-sweep chronoamperometry theory of kinetic currents in the case of a first order chemical reaction preceding the electron-transfer process. Electrochim. Acta, 1963, 8, 905-923. (b) Ref. 26, chap. 2, pp. 92.

(48) (a) This falls in line with recent tacit reconversion of former supporters of pyridyl intermediacy.48b (b) Zeitler, E. L.; Ertem, M. Z.; Pander, J. E.; Yan, Y.; Batista, V. S.; Bocarsly, A. B.: Isotopic probe illuminates the role of the electrode surface in proton coupled hydride transfer electrochemical reduction of pyridinium on Pt(111). J. Electrochem. Soc. 2015, 162, H938H944. (49) Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000 p 8-90. (50) Handbook of Chemistry and Physics, 81st ed.; CRC Press: Boca Raton, FL, 2000 p 8-44. (51) Ho, C.; Sturtevant, J. M.: The kinetics of the hydration of carbon dioxide at 25°. J. Biol.Chem. 1963, 238, 3499-3501.

(52) (a) The role of CO2 in H2 evolution through its hydration to carbonic acid has been confirmed by comparison between experiments carried out in water and acetonitrile.52b (b) Peroff, A. G.; Weitz, E.; Van Duyne, R. P.: Mechanistic studies of pyridinium electrochemistry: alternative chemical pathways in the presence of CO2. Phys.Chem. Chem. Phys. 2016, 18, 1578-1586. (53) Ertem, M. Z.; Konezny, S. J.; Araujo, C. M.; Batista, V. S.: Functional role of pyridinium during aqueous electrochemical reduction of CO2 on Pt(111). J. Phys. Chem. Lett 2013, 4, 745748. (54) Liu, C. P.; Liu, T. B.; Hall, M. B.: Influence of the density functional and basis set on the relative stabilities of oxygenated isomers of diiron models for the active site of FeFe hydrogenase. J. Chem. Theor. Comput. 2015, 11, 205-214. (55) Tossell, J. A.: Calculation of the properties of molecules in the pyridine catalyst system for the photochemical conversion of CO2 to methanol. Comp. Theor. Chem. 2011, 977, 123-127.


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(56) Keith, J. A.; Carter, E. A.: Theoretical insights into pyridinium-based photoelectrocatalytic reduction of CO2. J. Am. Chem. Soc. 2012, 134, 7580-7583.; 2013, 135, 73867386. (57) Keith, J. A.; Carter, E. A.: Electrochemical reactivities of pyridinium in solution: consequences for CO2 reduction mechanisms. Chem. Sci. 2013, 4, 1490-1496. (58) Lim, C.-H.; Holder, A. M.; Musgrave, C. B.: Mechanism of homogeneous reduction of CO2 by pyridine: Proton relay in aqueous solvent and aromatic stabilization. J. Am. Chem. Soc. 2012, 135, 142-154. (59) Lebègue, E.; Agullo, J.; Bélanger, D.: Electrochemical behavior of pyridinium and nmethyl pyridinium cations in aqueous electrolytes for CO2 reduction ChemSusChem 2018, 11, 219-228. (60) Refinements of the DFT analysis involving PyH• intermediacy as described in reference 58 clearly do not meet experimental support. (61) Lim, C.-H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B.: Reduction of CO2 to methanol catalyzed by a biomimetic organo-hydride produced from pyridine. J. Am. Chem. Soc. 2014, 136, 16081-16095. (62) Lim, C.-H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B.: Catalytic reduction of CO2 by renewable organohydrides. J. Phys. Chem. Lett 2015, 6, 5078-5092. (63) Knopf, I.; Cummins, C. C.: Revisiting CO2 Reduction with NaBH4 under aprotic conditions: Synthesis and characterization of sodium triformatoborohydride. Organometallics 2015, 34, 1601-1603.

(64) von Wolff, N.; Lefèvre, G.; Berthet, J. C.; Thuéry, P.; Cantat, T.: Implications of CO2 activation by frustrated lewis pairs in the catalytic hydroboration of CO2: A view using N/Si+ frustrated Lewis pairs. ACS Catal. 2016, 6, 4526-4535.


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(65) Lim, C.-H.; Holder, A. M.; Hynes, J. T.; Musgrave, C. B.: Dihydropteridine/pteridine as a 2H+/2e– redox mediator for the reduction of CO2 to methanol: A computational study. J. Phys. Chem. B, 2017, 121, 4158-4167.

(66) Similar conclusions were recently reached independently.31 (67) Mukhopadhyay, T. K.; MacLean, N. L.; Gan, L.; Ashley, D. C.; Groy, T. L.; Baik, M.-H.; Jones, A. K.; Trovitch, R. J.: Carbon dioxide promoted H+ reduction using a bis(imino)pyridine manganese electrocatalyst. Inorg. Chem. 2015, 54, 4475-4482. (68) Narayanan, R.; McKinnon, M.; Reed, B. R.; Ngo, K. T.; Groysman, S.; Rochford, J.: Ambiguous electrocatalytic CO2 reduction behaviour of a nickel bis(aldimino)pyridine pincer complex. Dalton Trans. 2016, 45, 15285-15289.


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Catalysis of CO2 Electrochemical Reduction by Protonated Pyridine

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