Thermodynamic Aspects of Electrocatalytic CO2 Reduction in

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Thermodynamic Aspects of Electrocatalytic CO2 Reduction in Acetonitrile and with an Ionic Liquid as Solvent or Electrolyte Yasuo Matsubara,*,† David C. Grills,*,‡ and Yutaka Kuwahara§ †

Department of Material and Life Chemistry, Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa-ku, Yokohama 221-8686, Japan Chemistry Department, Brookhaven National Laboratory, P.O. Box 5000, Upton, New York 11973-5000, United States § Department of Applied Chemistry and Biochemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan ‡

S Supporting Information *

1. INTRODUCTION The electrochemical reduction of CO2 has been proposed as a key sustainable energy technology to produce C1 feedstocks (e.g., CO) and useful fuels (e.g., formic acid and ultimately methanol) without gasification of hydrocarbons, provided the required electricity can be generated from a renewable energy source (e.g., photovoltaics) or the direct photoexcitation of an electrode material with sunlight.1−6 Thermodynamically speaking, because the formation of multielectron reduced species from CO2 requires less electrochemical potential than the direct one-electron reduction to CO2•− (as shown in eqs 1−6 in Table 1),7−10 the development of multielectron/multiproton redox catalysts has been extensively studied.11,1−6 However, particularly in the case of homogeneous catalysts, not many have been reported that are capable of efficiently producing species beyond the two-electron reduced products (i.e., carbon monoxide (CO) and formic acid (HCO2H)),11 and even just the observation of such a phenomenon remains a challenge. In contrast, many electrocatalysts for the reduction of CO2 to CO have been reported, and their development is now at the stage of improving energy efficiencies by lowering overpotentials, η (see definition of η below) while attempting to retain high current densities (i.e., rates of CO2 reduction). The fact that CO is such an important industrial chemical feedstock, combined with its increasing use as a reagent in Fischer− Tropsch processes for the formation of liquid hydrocarbon fuels,12 is therefore likely to encourage the continued development of CO-producing CO2 reduction electrocatalysts. For a given electrochemical reaction, η is defined as the difference between the applied electrode potential, E and the equilibrium potential for the reaction, Eeq.13,11 Using the Nernst equation, Eeq can be expressed in terms of the standard potential corresponding to the overall reaction, E° and the equilibrium activities, a of the reactants and products. For example, for eq 3, Eeq = E° + (RT/2F) × ln{(aCO2·aH+2)/(aCO· aH2O)}. Research into the electrocatalytic production of CO from CO2 is essentially divided into the direct reduction of CO2 at heterogeneous catalyst surfaces (most commonly bare metal or metal alloy electrodes) and the reduction of CO2 by homogeneous catalysts (typically transition-metal complexes), with important progress having been made in both areas in recent years.3,5,6 In general, although metal electrodes often exhibit a high activity (current density), they can suffer from © XXXX American Chemical Society

low product selectivity, with a number of possible side products being generated such as HCOO−, CH4, CH3OH, C2H5OH, and often H2 from the reduction of protons in aqueous electrolyte solutions. However, this can be overcome somewhat through the use of noble metals, such as silver or gold, and other alloys, which have a higher selectivity for CO.17,18 A 100% selectivity for CO production is the preferred situation because it simplifies product recovery and maximizes the use of the electrons. Many homogeneous catalysts do show a very high selectivity for CO production (Faradaic efficiency, FE is often 100%). Furthermore, homogeneous catalysts are desirable to study in that their active sites are usually well-defined, they are easily tunable, and their reaction mechanisms are easier to unravel. Unfortunately though, their activities are generally lower than those of heterogeneous catalysts, with catalyst decomposition sometimes being a problem, although there have been reports19,20 of homogeneous catalysts that have overcome some of these issues. Comparing catalytic figures of merit such as overpotential, current density, and turnover frequency (TOF) between heterogeneous and homogeneous catalysts is complex, and this topic has recently been discussed in detail.21−24 A comparison of these parameters for some of the most heavily investigated homogeneous catalysts has been provided by Savéant,19 and it can be seen, for example, that TOFs span a wide range from 15 000 s−1, with overpotentials as high as >1 V. In terms of the direct reduction of CO2 at metallic electrodes, significant improvements in catalytic activity have been realized through the use of nanostructured electrode surfaces. Nanostructured materials are so effective because they provide a much greater number of surface active sites compared with bulk metals, and they contain more edge/low-coordinated sites, which are typically the most active. This has led to large enhancements in the rate constant for CO production. Nanostructured catalysts for CO2 reduction have recently been reviewed,6 and a direct comparison of various nanostructured catalysts is also provided in the Supporting Information (SI) of another recent publication.25 A promising approach for lowering the overpotential for CO2 reduction at metal electrodes that has gained increasing attention in recent years, has been to make use of ionic liquids (ILs) as mediators or cocatalysts, either as electrolytes dissolved Received: March 27, 2015

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DOI: 10.1021/acscatal.5b00656 ACS Catal. 2015, 5, 6440−6452

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ACS Catalysis

Table 1. Equilibrium Potentials for Various CO2 Reduction Reactions vs the Standard Hydrogen Electrode (SHE)a equation

reaction

Eeq (V vs SHE)

1 1′ 2 3 3′ 4 5 6

CO2(g) + e− ⇌ CO2•−(aq) CO2(aq) + e− ⇌ CO2•−(aq) CO2(g) + 2H+(aq) + 2e− ⇌ HCO2H(aq) CO2(g) + 2H+(aq) + 2e− ⇌ CO(g) + H2O(l) 3CO2(g) + H2O(l) + 2e− ⇌ CO(g) + 2HCO3−(aq) CO2(g) + 4H+(aq) + 4e− ⇌ H2C(OH)2(aq) + H2O(l) CO2(g) + 6H+(aq) + 6e− ⇌ CH3OH(aq) + H2O(l) CO2(g) + 8H+(aq) + 8e− ⇌ CH4(g) + 2H2O(l)

−1.99b,c −1.90b −0.61 −0.52 −0.56 −0.49d −0.38 −0.24

Standard potentials were taken from, or calculated using thermodynamic data tabulated in ref 7, unless otherwise stated. Conditions: pH 7, 25 °C, 1 atm of gases (g), 1 M solutes (aq), in water as a solvent (l). bRef 14. cCalculated using the Nernst equation, with solubility data taken from ref 15. d Formaldehyde actually exists in the hydrated form (i.e., the diol).16 a

glassy carbon electrode show very high selectivity for CO production in acetonitrile (CH3CN) in the presence of imidazolium ILs at a low reported overpotential of 165 mV40,41 (see later for a discussion of overpotentials in CH3CN/IL mixtures). As in other studies with heterogeneous catalysts, the rate-determining step was shown to be the formation of CO2•−, and thus an interaction between CO2•− and the imidazolium is likely helping to lower the overpotential. In the Bi investigations40,41 and others in CH3CN,38 the imidazolium was assumed to be acting as a proton source for the CO2 reduction reaction. Other researchers found that when [emim][NTf2] (NTf2 = bis(trifluoromethylsulfonyl)imide) was used as a supporting electrolyte in CH3CN, the overpotential for CO2 reduction at a lead electrode was reduced by 0.18 V when compared to tetraethylammonium perchlorate as the electrolyte.42 Furthermore, the IL was again found to alter the mechanism by promoting the formation of CO as a reduction product as opposed to oxalate anion in the absence of IL, with an imidazolium carboxylate complex also being observed as a competing byproduct, likely from the deprotonation of emim+ by CO2•−. Prompted by the success of imidazolium-based ILs as catalytic promoters for the electrocatalytic reduction of CO2 to CO at metal and metal-based electrodes, we recently sought to discover if ILs could have similar beneficial effects for the electrocatalytic reduction of CO2 with a homogeneous catalyst at a carbon electrode.43 We found that when neat [emim][TCB] (TCB = tetracyanoborate) is used as both the solvent and electrolyte for the electrocatalytic reduction of CO2 to CO with the homogeneous catalyst, fac-ReCl(bpy)(CO)3 (bpy =2,2′-bipyridine), the onset potential for catalytic CO2 reduction is reduced by ∼0.45 V, compared to when CH3CN is used as a solvent with tetrabutylammonium hexafluorophosphate ([TBA][PF6]) supporting electrolyte.43 Importantly, we found that the rate constant for the CO2 reduction reaction also increased by a factor of ∼40× in [emim][TCB] compared to in CH3CN. It is clear that the enhancement effect invoked by the imidazolium-based IL is different from enhancement effects previously reported in the presence of alkali or alkaline-earth metals; that is, these mono- or divalent metal cations can either improve the catalytic rate constant44,45 or reduce the catalytic onset potential,46 while the IL can do both simultaneously.43 At this stage, the precise mechanisms of electrochemical CO2 reduction reactions in the presence of ILs have not been fully clarified. In many cases, even the overall reaction has not been elucidated, making it difficult to determine which equilibrium potential should be used in order to deduce an overpotential for a given electrochemical system. Moreover, how these

in a solvent, or in their neat form as a combined solvent/ electrolyte. ILs are defined as salts that are liquid below 100 °C (often being liquid at room temperature), typically being composed of bulky organic cations and inorganic anions.26 They are unique solvents that are emerging as superior alternatives to organic solvents for many applications, including energy,27 catalysis,28,29 and/or electrochemistry.30−33 Combining ILs with nanostructured electrodes has resulted in some of the most effective catalytic systems for CO2 reduction.6 Many different cations can produce ILs, with N-alkyl substituted imidazoliums, quaternary ammoniums, and pyrrolidiniums being very popular, among others. Interestingly though, improvements in CO2 reduction efficiency in the presence of ILs have so far only been reported with imidazoliums. A prominent example was the use of a silver electrode in the presence of 18 mol % 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]) IL as an electrolyte in water. In this system, CO2 was found to be catalytically reduced to CO at an extremely low overpotential of 96%).34 It was assumed that complexation of the imidazolium cation with the initially formed one-electron reduced CO2•− intermediate at the electrode surface results in a substantial lowering of the activation energy for the CO2 reduction process and thus a reduced overpotential. Subsequent investigations provided some spectroscopic evidence for such a [emim]-CO2 intermediate species at the electrode surface.35 In later work with Ag nanoparticles as the electrode material in the presence of [emim][BF4], it was shown that the water content strongly influences the FE for CO production, with the FE increasing to a maximum at ∼90 mol % H2O.36 Remarkably though, H2 production was suppressed by the presence of the IL, even at the high water concentrations, indicating another potential advantage of imidazolium IL additives. In other studies, the effects of electrode geometry on catalytic performance in the presence of ILs were investigated. For example, 5 nm Ag nanoparticles showed superior performance over both smaller and larger particles,37 hierarchical Au islands exhibited superior performance over polycrystalline Au,38 and CO2 reduction at the edge sites of a low-cost MoS2 electrode in water containing 4 mol % [emim][BF4] outperformed even Ag and Au nanoparticles, with the high efficiency being suggested to result from a synergistic effect of the IL lowering the overpotential and the edge sites improving the rate of CO2 reduction.39 IL additives have also been shown to alter CO2 reduction pathways. For example, low-cost Bi-based electrodes are typically selective for formate production in aqueous electrolytes. However, electrodeposited structured films of Bi on a 6441

DOI: 10.1021/acscatal.5b00656 ACS Catal. 2015, 5, 6440−6452

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ACS Catalysis equilibium potentials change in, or in the presence of, an IL has not been investigated. However, it is clear that ILs can significantly improve the efficiency of electrochemical CO2 reduction, and we anticipate that the electrolysis of CO2 in, and in the presence of, ILs will be a promising approach for maximizing energy efficiency in future applications. Therefore, a clarification of the role(s) of imidazolium cations in CO2 reduction processes will be a valuable contribution to the chemistry of CO2 activation in the presence of ILs. For example, in the case of homogeneous catalysts at carbon electrodes, the mechanism that has so far been assumed to occur in heterogeneous metallic electrode systems; that is, a direct interaction of the one-electron reduced electrode-surfaceadsorbed product, CO2•−, with the imidazolium cation, is unlikely because the potentials employed for CO2 reduction with homogeneous catalysts are more positive than the potential required to generate adsorbed CO2•− at a carbon electrode.47 This Viewpoint article consists of two main discussions, as follows: (1) the standard electrode potentials of CO2 reduction to CO when using an imidazolium-based IL as a solvent, or as an electrolyte in CH3CN solvent, are first discussed; and (2) how the IL contributes to CO2 reduction as a cocatalyst when a homogeneous rhenium complex is employed as a catalyst in CH 3 CN is then considered. Both discussions include experimental results so that this article can provide a thermodynamic insight. In the course of our investigations, we also investigated the standard electrode potential for the reduction of CO2 to CO in wet CH3CN, leading to a −0.28 V revision of the previously published19 value. This revision has implications for previously reported overpotentials of electrochemical CO2 reduction in CH3CN.

reaction stoichiometry. The formation of such imidazolium carboxylate species has been well studied, especially in terms of their use as precursors to N-heterocyclic carbenes (NHCs).48−52 Furthermore, as mentioned above an imidazolium carboxylate complex was also observed during electrochemical CO2 reduction in CH3CN in the presence of [emim][NTf2] supporting electrolyte.42 However, to the best of our knowledge, no thermodynamic study for this reaction has been reported. Therefore, we investigated the thermodynamic relationship between CO2 and N-alkyl substituted imidazoliums in CH3CN, including the apparent pKa of CO2 + H2O in CH3CN (eq 10; see SI for an explanation of why this is referred to as an apparent pKa), which ultimately led to a revision of the previously published19 standard electrode potential for the reduction of CO2 to CO in CH3CN, as discussed in the following section. 2.1. Standard Electrode Potential for the Reduction of CO2 to CO in Acetonitrile. The standard electrode potential for the reduction of CO2 to CO in wet CH3CN (eq 8) has previously been estimated19 to be −1.274 V vs Fc+/0 (Fc = ferrocene),53,54 based on the standard electrode potential for eq 9 in dry CH3CN and the apparent pKa for eq 10 in water, combined with the experimental ion transfer Gibbs free energies for the transfer of the corresponding ions from water to CH3CN. This estimation was an important milestone in the chemistry of CO2 reduction. However, the estimation involves a value for the transfer energy of H2O from water to CH3CN, which was determined by means of Density Functional Theory (DFT) calculations, and as such, it is difficult to evaluate the reliability of this value. For example, although the apparent pKa for eq 10 in CH3CN was estimated to be 17.03,19 we have found by 1H NMR measurements that bicarbonate anion in CH3CN was able to completely deprotonate triethylammonium cation (Et3NH+), for which the pKa was measured to be 18.7 in CH3CN,55 forming H2CO3 which decomposes into CO2 and H2O (Figure S12 in the SI). This implies that the apparent pKa for eq 10 is much larger than 17.03, and therefore the actual standard electrode potential for the reaction in eq 8 will be more negative than previously reported.19

2. STANDARD ELECTRODE POTENTIALS FOR THE REDUCTION OF CO2 TO CO IN ACETONITRILE AND WITH AN IMIDAZOLIUM-BASED IL AS SOLVENT OR ELECTROLYTE When a stoichiometric equation is known for an electrochemical process in a given solvent, the overpotential can be deduced by using the corresponding standard electrode potential in that solvent. Because the standard electrode potential in any given solvent can be converted to that in another solvent by taking into account the standard transfer energies of the species, in theory, overpotentials determined in such a way could be compared with each other regardless of the type of system and the nature of the solvent. The equilibrium potentials for CO2 reduction to various C1 compounds have been investigated in water as shown in eqs 1−6. However, despite the fact that many electrocatalysts have been studied in various organic solvents for decades (most often in CH3CN),3,5 as well as in ILs and with ILs as supporting electrolytes in other solvents such as CH3CN,40,43,41,42,38 only two potentials for the reduction of CO2 to CO in two organic solvents (CH3CN and N,N-dimethylformamide) have been estimated.19 While investigating the standard electrode potential for the reduction of CO2 to CO in an imidazolium-based IL as a solvent, and in CH3CN with an IL electrolyte, we realized that depending on the nature of the solution (e.g., pH), carboxylation of the imidazolium in the presence of CO2, as depicted in eq 7 for 1,3-dimethylimidazolium (mmim+), could be a key process that would render the imidazolium as a proton source for the CO2 reduction reaction and therefore affect the

3CO2(g) + H 2O(CH3CN) + 2e− ⇌ CO(g) + 2HCO−3(CH3CN) (8)

CO2(g) +

2H+(CH3CN)

−

+ 2e ⇌ CO(g) + H 2O(CH3CN) (9)

CO2(sol) + H 2O(sol) ⇌

HCO−3(sol)

+

H+(sol)

(10)

To more precisely estimate the standard electrode potential for eq 8 in CH3CN, we determined the apparent pKa of CO2 + H2O in wet CH3CN (eq 10) to be 23.4 ± 0.1 by means of isothermal titration calorimetry (ITC). In this method, while an CH3CN solution of triethylammonium cation containing 1.1 M water56 was titrated into an CH3CN solution containing [TBA][HCO3], CO2, and 1.1 M water, the reaction heat evolved by the titration was recorded and analyzed. Four titrations, each using a different concentration of CO2, were 6442

DOI: 10.1021/acscatal.5b00656 ACS Catal. 2015, 5, 6440−6452

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ACS Catalysis performed. Full details of these experiments are provided in the SI. Our apparent pKa of 23.4 for eq 10 in CH3CN is significantly larger than the previously estimated value19 of 17.03 and very similar to the pKa of acetic acid (23.51) in CH3CN57 (see the SI for details). Using the standard potential of the H+/H2 couple in CH3CN (+0.028 V vs Fc+/0),58 combined with our newly measured apparent pKa for eq 10 and solubility data of gases in CH3CN, we therefore calculated the standard electrode potential for the reduction of CO2 to CO in wet (1 M H2O) acetonitrile (eq 8), resulting in E°eq 8 = −1.55 V vs Fc+/0 or −0.93 V vs SHE (with a salt bridge from KClaq to tetraethylammonium perchlorate (TEAP) in CH3CN). See the SI for full details of this conversion from Fc+/0 in CH3CN to SHE in H2O. Our value for E°eq 8 is 0.28 V more negative than the previously reported19 estimate of E° = −1.274 V vs Fc+/0.53,54 Meanwhile, we also calculated the standard electrode potential for eq 9 in dry CH3CN and obtained E°eq 9 = −0.13 V vs Fc+/0 or +0.49 V vs SHE (with a salt bridge from KClaq to TEAP in CH3CN). Again, see the SI for details of the SHE conversion. We therefore propose that our newly estimated values for the standard electrode potentials of eq 8 (E°eq 8 = −1.55 V vs Fc+/0) and eq 9 (E°eq 9 = −0.13 V vs Fc+/0) are more reliable for calculating the overpotential for a given electrochemical CO2-to-CO reduction system in wet CH3CN ([H2O] = 1 M)56 and dry CH3CN, respectively. 2.2. Standard Electrode Potential for the Reduction of CO2 to CO in Acetonitrile Containing an ImidazoliumBased IL as an Electrolyte. If an imidazolium is used as the cation of an IL in the presence of CO2, formation of the imidazolium carboxylate with the release of a proton (eq 7), has to be considered when one calculates an equilibrium potential for CO2 reduction represented by eq 9, involving protons as a component. Rogers et al. showed that mmim+ can be reversibly converted to mmim-CO2 in the presence of HCO3− and CO2.59 We have now quantified the thermodynamics of this process, finding that pK7 for eq 7 is almost identical to the apparent pKa of CO2 + H2O in CH3CN under certain conditions as discussed below. Thus, once the imidazolium happens to be deprotonated, the deprotonated form can generate the imidazolium carboxylate. Following Rogers’ synthesis of mmim-CO 2,59 others established synthetic routes to various carboxylates by using bicarbonate,51,52 while it was found that some carboxylates can be equilibrated with a bicarbonate salt of the imidazolium,59,51,52 depending on the type of solvent used. Suresh et al. reported a conclusion deduced from DFT calculations that the stability of an imidazolium carboxylate can be described by two parameters: (1) the electronic effect on the heteroaromatic ring and (2) steric hindrance by substituents at the 1- and 3positions of the ring.60 More recently, in a neat IL a heteroconjugation of an imidazolium carboxylate with imidazolium was observed, as shown in eq 11.61

Although the thermodynamics related to these species have not been investigated, we found, for example, that [mmim][BPh4] reacts with [TBA][HCO3] in wet CH3CN ([H2O] = 1.1 M) to form a carboxylate adduct, without observation of the heteroconjugate species (eq 12). pK12 for eq 12 was estimated

to be ca. 0 (at ionic strengths ranging from 0.04 to 0.07 M in CH3CN) by quantifying the amount of each species in solution at equilibrium by 1H NMR spectroscopy when various concentrations of reactants were used. The approximately zero value of pK12 clearly indicates that pK7 for the formation of mmim-CO2 with the release of a proton (eq 7) is comparable to the apparent pKa of CO2 + H2O (eq 10), since pK7 = pK10 + pK12. This is also expected to be the case for the formation of emim-CO2.62 Thus, in terms of the reduction of CO2 to CO, the stoichiometric formula of a system in dry CH3CN containing an imidazolium-based IL as an electrolyte could be represented by eq 13, with a standard electrode potential of

E°eq 13 = −1.55 V vs Fc+/0, which is identical to E°eq 8 in wet CH3CN described above in section 2.1. If the CH3CN/IL solution contains a relatively large amount of H2O, the stoichiometric formula would be better represented by eq 8. In this case, the primary proton source for CO2 reduction is either the imidazolium or CO2 + H2O, but the standard electrode potential would still be E° = −1.55 V vs Fc+/0. This revision of the standard electrode potential for the reduction of CO2 to CO in CH3CN containing an imidazolium-based electrolyte implies that caution must be exercised when claiming extremely low overpotentials for certain CO2 reduction processes in the presence of ILs40,41,38,63 and that the reported overpotentials may not be as low as was originally assumed. 2.3. Standard Electrode Potential for the Reduction of CO2 to CO in an Imidazolium-Based IL as a Solvent. Based on the thermodynamic properties discussed above, some imidazoliums can react with bicarbonate in CH3CN to give the imidazolium carboxylate. Thus, the formation of the carboxylate by reaction with CO2 could render the imidazolium as a proton source during CO2 reduction in CH3CN. Meanwhile, in neat IL, not so many thermodynamic parameters are available, even compared with the amount known in the CH3CN system. However, some research groups have reported a few data sets for benzoic acids,64 pyridines,65 and amines,66 illustrating that their pKas tend to be similar to those in CH3CN. Thus, it seems reasonable to assume that similar behavior to that described above in CH3CN might also be observed in neat ILs (i.e., pK7 might be comparable to the apparent pKa of CO2 + H2O in a neat IL). In that case, the IL would no longer simply be acting as a solvent but would also be participating as one of the reactants in the CO2 reduction reaction. A more complicated case is a wet IL, which is sometimes employed as a solvent medium in electrochemical systems. If the amount of water in the IL is quite small, it is conceivable 6443

DOI: 10.1021/acscatal.5b00656 ACS Catal. 2015, 5, 6440−6452

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ACS Catalysis that the properties of the IL can be discussed on the basis of those of the pure IL. However, when a solvent consists of a large mol % of water in an IL, whether the mixture can be considered as a derivative of water, or of the pure IL, is not always obvious. For example, water containing an IL, for example, 20 mol % [emim][BF4], sounds like it would behave mainly as an aqueous solution, but it is actually prepared by mixing approximately 1.5 L (1.5 kg) of water and 3.0 L (4.0 kg) of the IL. Cox pointed out that expression by weight % or volume % represents the influence of the interaction energies between the components, especially in the case of a mixture of solvents with very different quantities of the two components.67 Assuming water is the solvent in a wet IL, a thermodynamic equation for the reduction of CO2 to CO can be represented by eq 14 (when the carboxylate is assumed not to be involved in the reduction). The standard electrode potential for eq 14 is based on −0.52 V vs SHE for eq 3 and is affected by two quantities: the activities of water as a solvent and protons as a solute in the wet IL. CO2(g) + 2H+(wet IL) + 2e− ⇌ CO(g) + H 2O(liquid in wet IL)

that also binds CO2. These quite interesting reactivities will be important avenues for future research.

3. ROLE(S) OF AN IMIDAZOLIUM-BASED IL IN CO2 REDUCTION WITH A HOMOGENEOUS CATALYST WHEN EMPLOYED AS AN ELECTROLYTE IN ACETONITRILE An obvious question to ask is, if pK7 in CH3CN is comparable to the apparent pKa of CO2 + H2O in CH3CN (eq 10), is there any other role being played by the imidazolium in homogeneous CO2 reduction which leads to the observed catalytic enhancement, beside it acting as a proton source? The answer is “yes” and our current understanding of this role is discussed below. This work is ongoing and other aspects, such as the precise function of the imidazolium carboxylate will also be addressed in future work. In order to explore the role of imidazolium, we have investigated the electrocatalytic reduction of CO2 in dry CH3CN using fac-ReCl(bpy)(CO)3 as a homogeneous catalyst and [emim][TCB] as an electrolyte at various concentrations, balanced with [TBA][PF6] to maintain the ionic strength at a constant value of 1.2 M, in the presence of a 1:1 ratio of benzoic acid (BzOH) to [TBA][BzO], which forms an acid− base buffer at pH 21.5 in CH3CN.57 Under these conditions, the pH of the solution was actually found to be ∼21.0 by means of a glass electrode. Kolthoff previously reported78,79 that the addition of hydrogen bond donors (e.g., water, methanol, etc.) to CH3CN solutions of benzoic acid-benzoate and picric acidpicrate buffers decreased the pH of the solutions. Because emim+ can act as a weak hydrogen bond donor, the slight decrease in pH in the presence of [emim][TCB] is likely caused by such an effect. At a pH of 21.0, the 2-position of the imidazolium is thermodynamically estimated to remain almost fully protonated; that is, the formation of the imidazolium carboxylate by reaction with CO2 will be effectively suppressed. Figure 1 shows linear sweep voltammograms (LSVs) recorded under the above-mentioned conditions in the presence of 1 atm of CO2, with the concentration of emim+ being varied from 0 to 0.1 to 0.5 M while maintaining a constant pH, under two different acid−base buffer concentration regimes. These LSVs show catalytic onset potentials that gradually shift positive as the concentration of emim+ increases. In addition, the catalytic current increases with acid−base buffer concentration (compare c and d), but for a given acid−base buffer concentration, there is also a small additional increase in the intensity of the catalytic wave due to the presence of [emim][TCB] (c → a and d → b), although this is a relatively minor effect. Analysis of two of the LSV’s in Figure 1 using the method of Appel and Helm22 (theoretical basis was developed by Nicholson and Shain80) reveals that there is a positive shift in the potential at which the catalytic current is half its maximum value, from −1.98 V to −1.88 V, upon increasing [emim+TCB−] from 0 to 0.5 M.81 Bulk electrolysis of a CO2-saturated solution containing 2 mM fac-ReCl(bpy)(CO)3, 0.1 M [emim][TCB], and 0.1 M BzOH/BzO− at −1.94 V vs Fc+/0 confirmed that the product of catalysis is CO with a selectivity of >99% and a FE of 100 ± 10%. Analysis of the solution by 1H NMR after the bulk electrolysis also confirmed that no emim + was deprotonated by CO2, as expected. These confirmations render the stoichiometric formula for this electrolysis as eq 15, which is the same type of reaction as eq 9.

(14)

The activity of water can be estimated from a derivation of the vapor pressure of water from Raoult’s law in a water−IL mixture. For example, the activity coefficient of water (γH2O) in water containing 18 mol % of [bmim][BF4] (in which the mole fraction of water (χH2O) is 0.82), is calculated to be 0.76 at 298 K,68 which shifts the potential more positive by 6 mV, i.e., − (RT/2F) ln(γH2O.χH2O). The activity coefficient of H+ (γH+) can be inferred from a pH measurement using a glass electrode, although it should be noted that BF4 anions are known to suffer from hydrolysis to yield hydrofluoric acid with various borates.69 For example, we observed that the pH of an aqueous solution containing 0.1 M HCl and 3.7 M [bmim][BF4] (corresponding to 18 mol %) is 0.6 at 293 K,70 which shifts the potential more positive by 28 mV, that is, (RT/2F) ln(γH+2). Variation of pH values with the concentration of [bmim][BF4] in water has also been reported.71 Further studies of the standard electrode potential of CO2 reduction in such mixed solvent systems containing ILs are expected. The thermodynamic relationship between CO 2 and imidazolium cations discussed above renders two interesting directions of research, as follows: (1) because the imidazolium can behave as an acid in the presence of CO2 due to the formation of a carboxylate of the imidazolium, this reaction could be deliberately inhibited by the presence of a stronger acid, allowing any additional roles of imidazolium cations in the CO2 reduction mechanism to be investigated; and (2) an investigation of the possibility that the imidazolium carboxylate might be involved as an intermediate in the reduction of CO2. In this Viewpoint, the former has been applied in an effort to understand the “true” role of imidazolium beside it acting as a proton source, as discussed in the next section. The latter has already been an active research topic related to the field of organocatalysis.72,73 Regarding the involvement of imidazolium carboxylate in CO2 reduction, few observations have been reported. For example, the formation of methanol by a reaction of the carboxylate with silanes was observed,74 and a theoretical explanation for this observation has been provided.75 In the presence of alkali metals, a type of heteroconjugation of imidazolium carboxylate was found to yield a supramolecular species binding CO2,76 resembling a dicopper supramolecule77

CO2 + 2BzOH + 2e− ⇌ CO + 2BzO− + H 2O 6444

(15)

DOI: 10.1021/acscatal.5b00656 ACS Catal. 2015, 5, 6440−6452

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which would result in initiation of the catalytic cycle at more positive potential. We also tested for any H/D exchange of the proton at the 2position of the imidazolium in this electrocatalytic system by means of deuterium-labeling experiments, because despite its thermodynamic stability (pKa = 23.0 in water84), deuterium exchange of the proton at the 2-position of 1,3-dimethylimidazolium can take place in D2O due to its high kinetic acidity.84 In acetonitrile, we confirmed by 1H NMR that the proton at the 2-position of emim+ reaches a thermal equilibrium mixture with the deuterium of BzOH deuterated at the carboxylic group (BzOD) within 8 h (without performing electrolysis) under the same reaction conditions we used for the electrolysis described above. Therefore, the following reaction pathway could conceivably be possible: the proton at the 2-position is transferred to a CO2-bound intermediate of the rhenium complex and recovered back from a proton of BzOH in the solution (Scheme 1). To test if such a proton exchange takes

Figure 1. Linear sweep voltammograms (LSVs) at constant pH for facReCl(bpy)(CO)3 (0.5 mM) in dry, CO2-saturated (1 atm) CH3CN solution containing (a) 0.5 M and (b) 0.1 M [emim][TCB], 1:1 BzOH:[TBA][BzO] (at a 5× lower concentration than that of [emim][TCB]), and [TBA][PF6] (added to maintain a total ionic strength of the solution of 1.2 M). LSVs (c) and (d) were recorded in the absence of [emim][TCB] under the following buffer conditions, with BzOH concentrations that were slightly modified from those in (a) and (b), respectively, in order to maintain a constant pH for all solutions under investigation: (c) [BzOH] = 0.12 M, [TBA+BzO−] = 0.10 M, and (d) [BzOH] = 0.03 M, [TBA+BzO−] = 0.02 M. A glassy carbon disk (effective diameter: 3 mm) was used as the working electrode at a scan rate of 0.35 V/s at 25 ± 3 °C. A CV recorded under CO2 in the absence of the rhenium complex in the presence of 0.5 M [emim][TCB], 0.1 M BzOH, 0.1 M [TBA][BzO], and 0.6 M [TBA][PF6] is also shown in dashed gray.

Scheme 1. Possible Proton Exchange Mechanisma

a

The scheme above shows a possible proton exchange mechanism that a deuterium-labeling bulk electrolysis experiment suggests does not occur during the time scale of the electrocatalytic reduction of CO2 to CO with fac-ReCl(bpy)(CO)3 as a homogeneous catalyst in CH3CN in the presence of emim+ cations and a BzOH/BzO− acid−base buffer (see text for details). [Re] = Re(bpy)(CO)3.

Interestingly, our observation in CH3CN solution of a positive shift of catalytic onset potential but only a small increase in catalytic current with increasing concentration of emim+ differs from the effect we previously observed in neat [emim][TCB],43 which consisted of a positive shift of onset potential together with a very large enhancement of catalytic current. Further studies in neat [emim][TCB] will be required to understand this difference. A kinetic analysis of the catalytic current intensities in CH3CN with various concentrations of the acid−base buffer and emim+ (see Supporting Information for details) permitted the reaction orders to be determined with respect to BzOH and emim+ for the rate-determining step during the catalytic response. These experiments revealed a reaction order of one for BzOH and a reaction order close to zero for emim+. The fact that the catalytic current is almost independent of [emim+], together with the observation of a positive potential shift of the second reduction wave (i.e., the catalytic wave) as the concentration of emim+ is increased, can be explained by saturation kinetics, which are common for reactions that have a rapid pre-equilibrium step.82 This implies the presence of an initial equilibrium subsequent to the first reduction wave that forms an intermediate species involving emim+ prior to the rate-determining catalytic step, which involves one BzOH. Although the structure of such an intermediate cannot be determined at this stage, we postulate that it may involve a strong hydrogen bonding interaction between the C−H bonds of the imidazolium ring and the twoelectron reduced catalyst, involving an interaction either with the chloride ligand (similar to previously reported83 interactions with chloride anions), or the five-coordinate metal center after chloride dissociation occurs (see below), both of

place during catalysis, we electrolyzed a solution containing BzOD, i.e., 2 mM fac-ReCl(bpy)(CO)3, 0.05 M [emim][TCB], 0.05 M [TBA][PF6], and 0.1 M BzOD/BzO− at −1.97 V vs Fc+/0. Under these conditions, the turnover number of the rhenium catalyst for CO production reached approximately 7 after 2 h of electrolysis, which is a short enough time to observe any deuterium exchange caused by the catalysis without significant interference from the thermal H/D exchange reaction described above. Interestingly, we observed almost no change in the integrals of the 1H NMR signal of the proton at the 2-position after the 2 h of electrolysis (Figure S13 in the SI). This strongly indicates that deuterium exchange by a mechanism such as that depicted in Scheme 1 does not take place during catalysis. Further evidence that a proton exchange mechanism such as that depicted in Scheme 1 is not taking place comes from an additional LSV experiment performed in the presence of the PF6− salt of the protonated form of the Verkade superbase (P), 2,8,9-trimethyl-2,5,8,9-tetraaza-1-phosphabicyclo[3.3.3]undecane ([PH][PF6]). The structure of PH+ is shown in Chart 1. The pKa of PH+ in CH3CN (32.9)85 is very similar to that calculated for emim+ in CH3CN (32.4).86 Thus, if a simple proton exchange mechanism such as that shown in Scheme 1 is occurring, we should observe similar LSVs in the presence of [emim][TCB] and [PH][PF6] electrolytes. However, as shown in Figure S11, the LSV in the presence of [PH][PF6] is almost identical to that in the presence of [TBA][PF6], exhibiting no significant positive shift in onset potential or increase in 6445

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enhancement. This is also in agreement with a recent study87 of heterogeneous CO2 electroreduction at a Ag electrode in N,N-dimethylformamide (DMF) in the presence of various imidazolium and non-imidazolium electrolytes, which showed that the anion has little role in the observed catalytic enhancement. Taking into account all of the above observations, we can conclude the following about the role of [emim][TCB] in the observed enhancement of electrocatalytic CO2 reduction with fac-ReCl(bpy)(CO)3 as a homogeneous catalyst in dry CH3CN in the presence of [emim][TCB] and a BzOH/BzO− acid− base buffer: First, it is clearly the imidazolium cation that is responsible for the observed catalytic enhancement, which manifests itself predominantly as a positive shift of the catalytic onset potential with increasing concentration of emim+, together with a small increase in catalytic current. The catalytic current also increases with the concentration of acid−base buffer, and variable concentration experiments have revealed reaction orders of one for BzOH and close to zero for emim+ for the rate-determining step of catalysis. This led to the conclusion that a specific interaction must be occurring between emim+ and the two-electron reduced catalyst to form an initial intermediate species prior to the ratedetermining step of catalysis. Upon coordination of CO2 to this emim−catalyst intermediate, catalytic CO2 reduction is initiated at a more positive potential than in the presence of conventional electrolytes such as [TBA][PF6]. Thus, emim+ must remain in the second coordination sphere, facilitating the CO2 reduction reaction, which explains the observation of close to a zero-order dependence on emim+ for the CO2 reduction kinetics, while one BzOH molecule is involved in the ratedetermining step. Therefore, what can we postulate as the possible structure of the intermediate in the rate-determining step of the CO2 reduction reaction, which involves the reduced catalyst, CO2, the imidazolium cation, and one molecule of BzOH? During the reduction of CO2 to CO by homogeneous catalysts, coordination of CO2 to the reduced metal center via the carbon atom results in the initial formation of a metallocarboxylate intermediate, [M−CO2]−.88−90 Subsequent steps include protonation to generate the metallocarboxylic acid, M− COOH, and further reduction followed by protonation (or vice versa), involving C−OH bond cleavage/H2O release to form a CO ligand, and CO dissociation steps. Although many of these elementary steps are still under debate,91−96 C−OH bond cleavage appears to be the rate-determining step during the whole catalytic cycle involving a single metal center. Our observations also fit in with this explanation, with the imidazolium cation playing an important role(s). A recent study87 of heterogeneous CO2 electroreduction at a Ag electrode in DMF showed that catalytic enhancements observed in the presence of imidazolium-based salts are due to strong ion pairing between the imidazolium cation and CO2•−. By analogy, we postulate that the imidazolium first interacts with the doubly reduced Re center to stabilize formation of the Re-bound CO2 as the metallocarboxylic acid and then, in the rate-determining step, the imidazolium remains in the coordination sphere interacting with the carboxylic acid moiety, which attacks a proton, e.g., from BzOH when the reaction is carried out in the presence of BzOH/BzO− acid− base buffer, as depicted in Scheme 2. Similar types of interactions between imidazolium cations and reaction intermediates have previously been proposed in other unrelated

Chart 1. Structure of the Protonated Verkade Superbase, PH+

catalytic current like that observed with [emim][TCB] electrolyte, thus ruling out Scheme 1. The results of the experiments discussed above, in which the proton donating ability of the imidazolium cation was suppressed by the presence of an acid−base buffer, confirm that for the electrocatalytic reduction of CO2 to CO by facReCl(bpy)(CO)3, the enhancement of the catalytic efficiency observed in the presence of imidazolium cations is not caused by the thermodynamic acidity of the proton at the 2-position of the imidazolium. Nor is it caused by a proton exchange mechanism involving the 2-position, such as that shown in Scheme 1. In order to confirm that it is indeed the imidazolium cation that causes the observed catalytic enhancement and not the TCB− anion, we performed LSV measurements in the presence of either [emim][TCB], [emim][PF6], or [TBA][PF6] electrolyte under otherwise identical conditions (see Figure 2). This experiment revealed identical catalytic onset potentials and currents for [emim][TCB] and [emim][PF6], which were more positive and slightly higher, respectively, than with [TBA][PF6], thus confirming that it is the imidazolium cation that plays the key role in the observed catalytic

Figure 2. LSVs at constant pH for fac-ReCl(bpy)(CO)3 (0.5 mM) in dry, CO2-saturated (1 atm) CH3CN solution containing [TBA][PF6] (1.08 M), 1:1 BzOH:[TBA][BzO] (both at a concentration of 0.02 M, except where noted), and various electrolytes at 0.1 M concentration as follows: (a) [emim][TCB], (b) [emim][PF6], and (c) [TBA][PF6] ([BzOH] = 0.03 M and [TBA+BzO−] = 0.02 M in order to adjust the pH to be equal to that of the solutions containing the emim+ electrolytes). LSVs were measured using a glassy carbon disk (effective diameter: 3 mm) as a working electrode at a scan rate of 0.35 V/s at 25 ± 3 °C. A CV recorded under CO2 in the absence of the rhenium complex in the presence of 0.1 M [emim][TCB], 0.02 M BzOH, 0.02 M [TBA][BzO], and 1.08 M [TBA][PF6] is also shown in dashed gray. 6446

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a The scheme above shows a possible interaction between an emim+ cation and the reduced catalyst in an initial pre-coordination equilibrium step, as well as a possible interaction between the emim+ and the CO2-bound catalyst in the rate-determining step during electrocatalytic CO2 reduction by fac-ReCl(bpy)(CO)3 in CH3CN in the presence of emim+ and a BzOH/BzO− acid−base buffer.

molecular orbitals (HOMOs) in the form of Kohn−Sham molecular orbitals (MOs), which are a good basis for qualitative interpretation of MOs in terms of their symmetry and shape.107 In structure A, the imidazolium ring lies above the bipyridine ligand due to π−π interactions (or so-called π+−π interactions108,109), while also binding the carbonyl of the carboxylic acid via the proton at its 2-position and its methyl group. In contrast, in structure B, the imidazolium is more centrally located, only anchoring to the carboxylic acid moiety. Thus, there will be more steric hindrance when a BzOH proton is attacked by the carboxylic acid in structure B compared to in structure A. Structure A is calculated to be only slightly more stable than structure B (ΔG = 0.4 kcal/mol, after correction for basis set superposition error (BSSE)), probably due to competition between the π−π interaction and Pauli repulsion. Thus, structures A and B of the metallocarboxylic acid are likely to coexist in solution. The shapes of their HOMOs are almost identical, consisting mainly of pπ orbitals of the carboxylic acid and a dπ orbital of the Re center, which is a typical orbital structure for η1-C-coordinated carbon dioxide to a reduced metal center previously observed in ab initio MO studies.110−113 The characteristic(s) of the imidazolium interaction can be qualitatively illustrated by comparing the energy levels of the HOMOs in the optimized structures of the metallocarboxylic acids in the presence of the imidazolium cation (A and B), with those of the HOMOs for structures containing K+ as a typical conventional cation (C), and no cation as a reference (D), as shown in Figure 4. Similar to A and B, the HOMOs of C and D were also found to contain mainly pπ orbitals of the carboxylic acid and a dπ orbital of the Re center (see Figure S14). Among these HOMOs, the HOMO of D is highest in energy, followed by those of A and B, and then C. The energies, ε of the occupied Kohn−Sham orbitals are approximately equal to relaxed vertical ionization potentials (vIPs) for outer valence orbitals, becoming an exact identity for the HOMO,114−116 and even at a level of nonexact Kohn−Sham potential, approximately linear dependencies of Kohn−Sham orbital energies on vIPs have been observed.107,117 In fact, we also found a linear correlation between ε and vIP of A−D as mentioned below. Thus, if nucleophilic attack on a proton by the hydroxy oxygen atom of the carboxylic acid moiety proceeds under frontier orbital control, this implies that the kinetic proton affinity decreases upon interaction with a conventional cation such as K+, but it decreases to a lesser extent when the reduced catalyst interacts with an imidazolium cation, thus explaining a unique effect of imidazolium on the CO2 reduction reaction. This is also indicated by a comparison of Fukui (or frontier) functions,

processes, such as the catalytic aerobic oxidation of decamethylferrocene28 and the electroreduction of O2.97,98 Although other possibilities exist, such as an interaction between the imidazolium cation and the carbonyl ligands of the Re catalyst, such interactions with peripheral ligands may result in a much weaker catalytic enhancement, and based on the previous prediction87 of strong ion pairing between imidazolium and CO2•− in heterogeneous systems, we suggest that an interaction with the reduced Re-bound CO2 moiety is more likely. To obtain theoretical insight into the interaction between the imidazolium and the reduced catalyst, especially in terms of any differences from interactions with conventional additive cations that have previously been used to enhance CO2 reduction catalysis, such as alkali metal cations,45 we first calculated possible structures between dimethylimidazolium and the metallocarboxylic acid as a model using DFT calculations99 at the level of the PW6B95100,101-D3BJ102,103 functional and madef2-SVP104,105 basis, with the COSMO106 solvation model for CH3CN (see the SI for computational details). Figure 3 shows two such possible structures of the carboxylic acid adduct (A and B) together with their corresponding highest occupied

Figure 3. Two calculated structures (A and B) of the adducts of the metallocarboxylic acid with the dimethylimidazolium cation as a model, together with their Kohn−Sham HOMOs. The structures were optimized by using the PW6B95-D3BJ functional and the ma-def2SVP basis with the COSMO solvation model with CH3CN as the solvent. The HOMOs were calculated at the same level but using the ma-def2-TZVP basis. 6447

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between the cations (the imidazolium or K+) and the carboxylic acid moiety binding to the Re center, but to different extents determined by how much the positive charge is delocalized on the cations. Thus, interaction with the delocalized imidazolium cation results in a smaller decrease in thermodynamic proton affinity of the metallocarboxylic acid adduct than interaction with a conventional cation such as K+. Meanwhile, the calculated vertical electron affinities (vEAs), which are assumed to correlate with the reduction potentials of the adducts, are larger (i.e., more positive reduction potentials) for the species with cations (A−C), than for D which does not involve interaction with a cation. Interestingly, in concert with the energy ordering of the LUMOs (Figure 4), all of which are π* of the bpy moiety (see Figure S14), the vEA for A with the imidazolium (2.82 eV) is larger than that for C with K+ (2.74 eV), despite the positive charge being delocalized in the imidazolium as mentioned above. This effect can be explained by the π−π interaction with the bpy ligand because the vEA for structure B in the absence of π−π interactions (2.67 eV), is smaller than that for structure A and close to that for structure D (2.62 eV). A similar effect was observed in a series of cis,trans-[Re(dmb)(PR3)(PR′3)]+-type complexes (dmb = 4,4′dimethyl-2,2′-bipyridine).120 Therefore, we postulate that the key characteristics of the imidazolium cation are (i) to reduce the impact of the electrolyte cation on the energy levels of, especially, the pπ orbitals of the metallocarboxylic acid moiety by its delocalized positive charge, thus facilitating attack on protons, while (ii) simultaneously enhancing the kinetic and/or thermodynamic electron affinity via π−π interactions between the imidazolium and bipyridine heteroaromatic rings, resulting in catalytic CO2 reduction occurring at a more positive potential than with conventional electrolytes. In future investigations, we will attempt to observe and clarify any specific interactions between the imidazolium cation and catalytic intermediates, such as those postulated in Scheme 2, using techniques such as IR spectroelectrochemistry and timeresolved infrared (TRIR) spectroscopy.

Figure 4. Correlation of the Kohn−Sham orbital energy levels for the HOMOs of structures A−D (horizontal black lines in lower half of the figure; structures of C and D are shown here, while those of A and B are shown in Figure 3). These orbitals consist significantly of the pπ orbitals of the carboxylic acid moiety and a dπ orbital of the Re center, as depicted in Figures 3 and S14. Also shown are the lowest unoccupied molecular orbital (LUMO) energies (horizontal black lines in upper half of the figure). Calculated values of vertical electron affinities (vEAs) and vertical ionization potentials (vIPs) are listed for each species. All structures were optimized at the same level of theory as that used in Figure 3. The energy levels, vIPs, and vEAs were calculated at the same level, but using the ma-def2-TZVP basis.

Figure 5. Fukui functions of structures A and C calculated by a finite difference scheme between the total electron densities in the parent state and the corresponding cationic state. The OH moieties in A and C are highlighted by black arrows. Both functions were obtained at the same level of theory as that used in Figure 3 but using the ma-def2TZVP basis.

4. SUMMARY AND FUTURE OUTLOOK In recent years, it has been shown that imidazolium-based ILs can act as mediators or cocatalysts to enhance the energy efficiency (i.e., reduce the required overpotential) of the electrocatalytic reduction of CO 2 to CO, both with heterogeneous and homogeneous catalysts. In the case of heterogeneous systems, where CO2 is directly reduced at a metallic electrode surface, an interaction between the imidazolium cation and the one-electron reduced CO2•− species is presumed to result in a lowering of the activation energy and thus the overpotential for CO2 reduction.34 In the homogeneous system that was investigated with fac-ReCl(bpy)(CO)3 as a catalyst,43 not only was the overpotential reduced when [emim][TCB] was used as a combined solvent/ electrolyte, but the rate constant for CO2 reduction was also enhanced by a factor of 40× compared to in CH3CN solvent with [TBA][PF6] electrolyte. It is unlikely that a similar interaction between emim+ and surface-adsorbed CO2•− occurs in the homogeneous system because CO2•− is not generated on a carbon electrode at the potential at which catalysis occurs. Precise mechanisms for electrochemical CO2 reduction in, and in the presence of, ILs are still unknown, and the standard electrode potentials for such CO2 reduction processes have not been properly investigated. This prompted us to consider two topics in this Viewpoint: (1) the standard electrode potentials

representing susceptibilities to a nucleophilic attack on H+.118,119 Figure 5 shows Fukui functions, f−(r) of A and C for the nucleophilic attack, calculated by a finite difference scheme to evaluate f−(r) by taking ρN(r) − ρN−1(r), where ρN(r) and ρN−1(r) are total electron densities at position r of a molecule with a total number of electrons, N, and N−1 (i.e., the cation), respectively. It can be seen that the shape of the isosurface at the OH moiety in A is more extended than that in C, indicating that the OH moiety in A should be more likely to nucleophilically attack H+ and complete the catalytic cycle compared to in C. The thermodynamic proton affinities of structures A−D also help to characterize the imidazolium interaction. The calculated vertical ionization potentials (vIPs), the relative ordering of which will correspond to that of the thermodynamic proton affinity (see the SI for details), support this implication as listed at the bottom of Figure 4 where the vIP for structure D is the smallest, followed by structures A and B, and lastly structure C, which corresponds to the energy order of their HOMOs. This phenomenon can be interpreted by coulomb interactions 6448

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locarboxylic acid; and (3) the imidazolium cation remains in the coordination sphere, interacting with the bound carboxylic acid, making the reduction potential of the species more positive than in the presence of conventional electrolytes, while the energy level of the HOMO that consists of pπ orbitals (CO2) and a dπ orbital (Re) remains close to that in the absence of the cation, resulting in little impediment for the carboxylic acid to attack a proton of BzOH in the ratedetermining step. These effects would represent at least two of the beneficial characteristics of the imidazolium cation in the CO2 reduction reaction, while conventional cations (e.g., K+) would not show such useful effects. Although we do not currently have direct spectroscopic evidence for the proposed intermediates depicted in Scheme 2, the possibility of such interactions will be investigated in future work using techniques such as IR spectroelectrochemistry and TRIR spectroscopy. Furthermore, we believe that the theoretical insights we have obtained regarding the beneficial catalyst−imidazolium interactions will aid the development of a second generation of cocatalysts for electrocatalytic CO2 reduction with homogeneous catalysts. It is clear that the presence of imidazolium-based ILs can have a positive influence on the efficiency of electrocatalytic CO2 reduction processes. Although the effect has thus far only been investigated with one homogeneous catalyst, it is likely that it may become a general strategy for reducing the overpotential and improving the rate of CO2 reduction with other homogeneous catalysts. However, it is important that the mechanisms of CO2 reduction involving ILs are properly investigated, allowing appropriate standard electrode potentials to be applied and the precise role(s) of the IL in the CO2 reduction reaction to be determined. Furthermore, to facilitate a fair comparison of catalytic activity between different homogeneous catalysts and the ability to reproduce data among different researchers, it is critical that precise reaction conditions are reported, including the water content of the solution and the concentration of added Brønsted acids (i.e., the pH). This is particularly important when analyzing cyclic voltammetry data, which can be strongly influenced by such factors, as we and others have shown. The work discussed in this Viewpoint represents the beginning of a detailed thermodynamic and kinetic investigation of CO2 reduction with homogeneous electrocatalysts in CH3CN in the presence of IL electrolytes. So far, this has revealed the bifunctionality of the imidazolium-based cation, that is, as a catalyst promoter through interactions with catalytic intermediates, and as a proton donor under appropriate pH conditions. However, the field is still in its infancy, and more work is required to investigate the various functions of many different types of ILs in order to discover highly efficient reactivities for electrochemical CO2 reduction chemistry.

of CO2 reduction to CO when using an imidazolium-based IL as a solvent, or as an electrolyte in CH3CN solvent; and (2) how the imidazolium IL contributes to CO2 reduction as a cocatalyst when a homogeneous rhenium complex is employed as a catalyst in CH3CN. When CO2 reduction is performed in the presence of an imidazolium-based IL, the possible formation of an imidazolium carboxylate (eq 7), which renders the imidazolium as a proton source for CO2 reduction, must be considered. We therefore examined in more detail the thermodynamic relationship between CO2 and N-alkyl substituted imidazoliums in CH3CN, which first necessitated a clarification of the standard electrode potential for the reduction of CO2 to CO in CH3CN. On the basis of new experimental data, we have revised the previously published estimate53,54,19 of the standard electrode potential for this reaction in wet (1 M H2O) CH3CN (eq 8) with a new estimate of E°eq 8 = −1.55 V vs Fc+/0 or −0.93 V vs SHE (with a salt bridge from KClaq to TEAP in CH3CN; see the SI for details of the SHE conversion). We also clarified that the equilibrium constant for the formation of imidazolium carboxylate as a result of the reaction of the proton at the 2position (eq 7) is comparable to the apparent acid dissociation constant, Ka(app) of CO2 + H2O (eq 10, i.e., formation of HCO3−) in CH3CN, meaning that the standard electrode potential for the reduction of CO2 to CO in CH3CN containing imidazolium-based ILs as electrolytes is also estimated to be E° = −1.55 V vs Fc+/0 or −0.93 V vs SHE (with a salt bridge from KClaq to TEAP in CH3CN). This revision of the standard electrode potential in CH3CN in the presence of an imidazolium-based IL implies that some previously reported overpotentials for CO2 reduction processes may not be as low as had originally been assumed, and that caution should be exercised in future investigations when overpotentials are being estimated. For ILs containing water (or water-containing ILs), more work is required to thermodynamically clarify the chemical behaviors of N-alkyl substituted imidazoliums in the presence of CO2 in terms of CO2 reduction chemistry. In the second part of the discussion, we showed that when [emim][TCB] is used as an electrolyte in CH3CN for the electrocatalytic reduction of CO2 to CO with the homogeneous catalyst, fac-ReCl(bpy)(CO)3, in the presence of a BzOH/ BzO− acid−base buffer, the imidazolium cation plays an important role in catalytic enhancement, despite the fact that the buffer suppresses the ability of the imidazolium to serve as a simple proton donor. This enhancement manifests itself predominantly as a positive shift of the catalytic onset potential with increasing concentration of emim+, together with a small increase in catalytic current. The catalytic current also increases with the concentration of acid−base buffer, and variable concentration experiments revealed reaction orders of one for BzOH and close to zero for emim+ for the rate-determining step of catalysis. We further showed that the 2-position of the imidazolium ring does not serve as a proton relay during catalysis and that it is definitely the imidazolium cation that results in the observed effect, and not the TCB− anion. These observations, supported by DFT calculations, have led us to propose the reactivity pathway depicted in Scheme 2, in which (1) the imidazolium cation initially interacts with the two-electron reduced catalyst (shown in Scheme 2 as the fivecoordinate anion, [Re(bpy)(CO)3]−) in a precoordination step; (2) CO2 then binds to the reduced metal center to form the carboxylate followed by protonation giving the metal-

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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/acscatal.5b00656. Experimental and computational details, determination of apparent pKa of CO2 + H2O in wet acetonitrile, standard electrode potential calculations, kinetic analyses of CV data, additional CV data, NMR data for the formation of CO2 by reaction of bicarbonate with triethylammonium, stability test of the proton at the 26449

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position of imidazolium during electrolysis using deuterated buffer, and selected MOs of calculated structures of metallocarboxylic acid adducts (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.M.). *E-mail: [email protected] (D.C.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences under contracts DE-AC02-98CH10886 and DE-SC0012704. Y.M. is supported by the PRESTO project: “Chemical Conversion of Light Energy” of the Japan Science and Technology Agency (JST). Y.K. is supported by the Japan Society for the Promotion of Science (JSPS) through the Strategic Young Researcher Overseas Visits Program for Accelerating Brain Circulation. We thank Drs. Dmitry Polyansky and John Smalley for helpful discussions.

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