Water Facilitated Electrochemical Reduction of CO on Cobalt

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Water Facilitated Electrochemical Reduction of CO on Cobalt-Porphyrin Catalysts 2

Kaito Miyamoto, and Ryoji Asahi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019 Downloaded from http://pubs.acs.org on March 27, 2019

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The Journal of Physical Chemistry

Water Facilitated Electrochemical Reduction of CO2 on Cobalt-Porphyrin Catalysts Kaito Miyamoto∗ and Ryoji Asahi Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan E-mail: [email protected] Phone: +81-561-71-7973. Fax: +81-561-63-6279

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Cobalt-porphyrin catalyzed reductive decomposition of CO2 to CO is investigated based on the Koper’s water facilitated CO2 reduction mechanism using simple but accurate protocol based on thermodynamics. In our protocol, accurate predictions of standard redox potentials and free energy differences are achieved by combining strengths of both density functional theory and experimental observations. With the proposed protocol, we found that the proton transfer from H2 O takes place at −0.80 V vs. RHE at pH=3 through a concerted pathway and, as a result, the key intermediate for the CO generation, i.e., [CoP−COOH] – is formed. Since the redox potential of the proton transfer agrees well with experimentally observed CO2 reduction potential, we successfully clarified that H2 O plays an important role in the reductive decomposition of CO2 to CO. This result is valuable not only for understanding the cobalt-porphyrin catalyzed reductive decomposition of CO2 but also as a guide for the development of new catalysts.

INTRODUCTION Electrochemical fixation of carbon dioxide is one of the promising and imperative countermeasures to mitigate global warming and energy storage problems, where CO2 gas is electrochemically converted to fuels and commodity chemicals. 1–3 The key step for this conversion is the activation of thermodynamically stable CO2 . Especially, the conversion to CO is known to be the key and rate determining step to obtain useful chemicals 3–8 and effective catalysts have been pursued to reduce its large overpotential. 2,4–14 Among such catalysts, cobalt porphyrin complexes are considered to be one of the promising candidates since it selectively reduces CO2 to CO with relatively small overpotential in aqueous solutions. 15,16 Recently, drastic improvements in terms of stability in aqueous medium, high selectivity in CO2 reduction, and low overpotential, have been achieved by the immobilization to the graphite electrode 9,17–20 or by using as the building blocks of COF (covalent organic frame2


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works) 8 and MOF (metal organic frameworks). 11 To achieve further improvement, understanding of catalytic effects of cobalt porphyrin complexes is quite important and, therefore, analyses using electronic structure calculations have been carried out. 21–25 The seminal work in this field was done by Leung et al. 21,22 They investigated the catalytic effect of a cobalt porphine (CoP) molecule on reductive decomposition of CO2 in water using the combination of quantum chemistry calculations and ab initio molecular dynamics simulations. Their proposed mechanism starts from the one-electron reduction of CoP (CoP + e – → CoP – ). After the binding of CO2 to the reduced cobalt atom, further reduction takes place ([CoP-CO2 ] – + e – → [CoP−CO2 ]2− ). Then, after the protonation, detachment of OH – moiety occurs ( [CoP−COOH] – → CoP-CO + OH – ). Their mechanism successfully explained the reaction at pH=7 in water. However, it is also known that CO2 reduction takes place in aqueous medium at much lower pH (pH = 3), 9 which cannot be explained by their mechanism. Very recently, Yao et al. proposed an interesting reaction mechanism that starts from the H+ binding to the N site of CoP ([CoP·H])+ . 26 In their mechanism, CO2 binds to Co site after the two-electron reduction of [CoP·H]+ at around −1.23 V vs. SHE. However, it is questionable that the H+ always exists on the N site during the operation since it is consumed to reduce CO2 to CO and the binding between the Co and H+ is much stronger (by 0.7 eV) than that of the N−H+ bond in CoP – (Table S1 in the Supporting Information). Maybe the other main reaction routes also exist. All the above mechanisms assume that H+ plays an important role in the CO2 reduction. Contrary to this, Koper et al. proposed a novel CO2 reduction mechanism in aqueous medium based on their experimental 9 and computational 25 analyses, where H2 O plays a central role in the CO2 reduction. Their reaction mechanism starts from the CO2 binding to the negatively charged cobalt-porphyrin complex ‘M’ i.e., M + CO2 + e− → [M−CO2 ]− .

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(1)


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Then, a proton is transferred from the water molecule and a carboxyl group is created as [M−CO2 ]− + H2 O + e− → [M−COOH]− + OH− .

(2)

Finally, CO gas is released by following the similar reaction proposed by Leung et al. 21,22 that [M−COOH]− → M + CO + OH− .

(3)

The key step in this mechanism is Eq. 2, where the water molecule not the proton becomes the proton source to facilitate CO2 reduction. However, by considering the pH of the pure water which corresponds to the proton concentration of 10−7 mol/l, it is not so easy to imagine that the water molecule has an ability to provide enough protons so that a large amount of CO2 reduction takes place. Therefore further investigations are necessary. The goal of this paper is to shed light on the details of the Koper’s water facilitated CO2 reduction mechanism, i.e., Eqs. 1 - 3, 9,25 using simple but accurate protocol based on thermodynamics, which combines density functional theory (DFT) and experimental data.

COMPUTATIONAL DETAILS In this study, the cobalt porphine (CoP) complex (Fig. 1) is used as the model for the CO2 reduction catalyst, i.e., M=CoP in Eqs. 1 – 3. Gibbs free energies of metal complexes, such as CoP, [CoP−CO2 ] – , and [CoP−COOH] – , are evaluated using DFT where B3P86 27,28 and 6-311++G** 29–31 are used as the exchange-correlation functional and basis set respectively. B3P86 is known to reproduce the geometries of cobalt complexes. 32 The solvation effect is taken into account using the polarizable continuum model. SMD model 33 is selected as the continuum model and default water parameters in Gaussian 09 34 are employed. The validity of the method was confirmed using geometries and one-electron reduction potentials of cobalt tetraphenylporphyrin (CoTPP) and cobalt phthalocyanine (CoPc). As shown in

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Table S2 and S3 in the Supporting Information, the errors in the geometries are less than 0.02 Å and that in the reduction potentials are less than 0.2 V, meaning the results well agree with experimental data.

Figure 1: The geometry of cobalt porphine(CoP). Pink, blue, gray, and white spheres represent Co, N, C, and H atoms, respectively. The spin state of each system is set to be a low-spin state by following the previous reports. 21,24 All the geometries were confirmed to be minima by carrying out frequency calculations. We checked the stability of the wave functions for all the species and confirmed that all the wave functions except for CoP – are stable. As for CoP – , we carried out energy and geometry optimization calculations using the broken symmetry wave functions. 35–39 The thermal corrections to the Gibbs free energy are computed at 298.15 K. All the calculations were performed with Gaussian 09. 34 The standard one-electron redox potential relative to the reversible hydrogen electrode

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(RHE) is computed by ◦ ERHE =−

∆G◦ − E toSHE + 0.0592 × pH − E corr , F

(4)

where F and ∆G◦ are the Faraday constant and the standard Gibbs free energy difference of the target reaction, respectively; E toSHE is the factor to convert reference system from the vacuum level to the standard hydrogen electrode (SHE). We employ the value of 4.28 V 40–42 for E toSHE . The third term of the right hand side is to convert the reference from SHE to RHE. The last term (E corr ) is the parameter to correct the error derived from the computational methods. The effectiveness of this correction is shown in Refs. 32, 43, and 44. Since the errors in the one-electron reduction potentials of CoPc and CoTPP using the present computational condition are −0.17 and −0.19 V respectively, as shown in Table S3 in the Supporting Information, we employ the average value (−0.18 V) as E corr . Here, it is again worth emphasizing that the errors in the one-electron reduction potentials of CoPc and CoTPP (and hence E corr ) are small. In this study, we set pH to 3 since comparable experimental data are available. 9 In order to obtain the standard redox potential or free energy difference in Eq. 2, not only the Gibbs free energies of metal complexes but also an accurate value of a free energy difference between H2 O and OH – , i.e., ∆G◦H2O = G◦ (OH− ) − G◦ (H2 O) is necessary. This value is obtainable from the experimental pKa value of water and the free energy of a single proton in water 45 as ∆G◦H2O = 2.303RT pKa (H2 O) − (Ggas (H+ ) + ∆Gsolv (H+ ) + RT ln 24.46) ,

(5)

where R, T , and pKa (H2 O) denote the gas constant, temperature, and pKa value of water (= 14), 46 and where Ggas (H+ ) (=−6.28 kcal/mol) 45 and ∆Gsolv (H+ ) (= −265.9 kcal/mol) 47 are the free energy of a proton in gas phase and the solvation free energy of a single proton in water respectively. If T is set to be 298.15 K, ∆G◦H2O becomes 12.549 eV. 6


RESULTS AND DISCUSSION In this paper, both sequential and concerted reaction mechanisms, shown in Fig. 2, are investigated for the key step (Eq. 2) with M = CoP, that take place after the formation of [CoP−CO2 ] – (Eq. 1). While the comparisons between the sequential and concerted reaction pathways were made in detail for the proton-coupled electron transfer in the electrochemical reduction of CO2 using cobalt-porphyrin catalysts, 25,48 the present study is different from the previous report in that not the H+ in the aqueous medium but the H2 O molecule becomes the proton source to facilitate the CO2 reduction. Here, it is noteworthy that the [CoP−CO2 ] – formation reaction (Eq. 1) takes place at −0.43 V vs. RHE under the condition that pH = 3 using the present computational condition, which is close to the one-electron reduction potential of CoP (−0.43 V).

[CoIP-CO2]-

+ H 2O

CoIIIP-COOH + OH-

+ e[Co0P-CO2]2-

+ e+ H 2O

[CoIIP-COOH]- + OH-

Figure 2: Possible reaction routes starting from [CoP−CO2 ] – . The solid lines represent the sequential pathways and the dashed line the concerted pathway. At first, we discuss the sequential reactions. In this mechanism, there are two possible reaction routes, i.e., [CoP−CO2 ] – firstly reacts with H2 O and, then, e – comes or vise versa. In case we assume [CoP−CO2 ] – reacts with H2 O first, the reaction route in Eq. 2 is decomposed into two elementary reactions as [CoP−CO2 ]− + H2 O → CoP−COOH + OH−

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(6)


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and the subsequent e – transfer reaction given by (7)

CoP−COOH + e− → [CoP−COOH]− .

The free energy difference of the proton donation reaction (Eq. 6) is shown in Table. 1. The proton donation reaction is the endergonic reaction and the ∆G◦ = 0.50 eV is quite large. Therefore, CO2 reduction does not proceed by this route. Next, the reaction route starting from the electron transfer is considered. Since the redox potential of the reaction: [CoP−CO2 ] – + e – → [CoP−CO2 ]2− is −0.47 V vs. RHE (pH = 3), this reaction occurs at slightly lower potential than the one-electron reduction potential of CoP (−0.43 V, see Table 2). However, the subsequent reaction is endergonic as shown in Table 1. Therefore, CO2 reduction does not proceed by this route. Thus, the water facilitated CO2 reduction does not proceed by the sequential reactions. If enough amount of H+ exists in the electrolyte, CO2 reduction proceeds via the route: [CoP−CO2 ]2− + H+ → [CoP−COOH] – and competes with the direct proton reduction (the H+ reduction to H2 ). However, by considering the experimental results in Ref. 9, which reports that the direct proton reduction is diffusion limitation at pH = 3, CO2 reduction via this route may not be dominant. Table 1: Free energy difference of proton donation reactions from H2 O. reaction [CoP−CO2 ] – + H2 O → CoP−COOH + OH – [CoP−CO2 ]2− + H2 O → [CoP−COOH] – + OH –

∆G◦ (eV) 0.50 0.19

Next, the possibility of the concerted reaction is investigated, where a water molecule and an electron react with [CoP−CO2 ] – in a single step as shown in Fig. 2. The standard equilibrium potentials of the concerted reaction as well as the CoP – /CoP redox system relative to RHE (pH = 3) are summarized in Table 2. It is noteworthy that the redox potential of the CoP – /CoP redox couple is within the range of the one-electron reduction 8


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potentials of typical cobalt porphyrin complexes (−0.19 to −0.44 V relative to RHE at pH = 3). 9,49,50 Using our protocol, E ◦ of the concerted reaction is computed to be −0.67 V. From the experimental analysis, 9 CO2 reduction takes place at around −0.6 V vs. RHE when pH=3, which well agrees with the present result. This agreement suggests that, at pH ≥ 3, the proton supply from H2 O takes place through the concerted reaction: [CoP−CO2 ]− + H2 O + e− → [CoP−COOH]− + OH− .

(8)

Finally, we investigate the effect of the hydrogen bonds on the standard equilibrium potential of the concerted reaction. It is reported that both CoP and CoP – do not form the hydrogen bond with surrounded water molecules while [CoP−CO2 ] – and [CoP−COOH] – forms such bonds at CO2 or COOH moiety. 22 Since E corr in Eq. 4 does not correct errors derived from the hydrogen bonds in E ◦ , we estimate such errors by adding water molecules explicitly. This method is the so-called cluster-continuum method. 51 We added two explicit water molecules in order to investigate the effect of the hydrogen bonds to the standard equilibrium potential of the concerted reaction. Positions of the two water molecules are determined based on the previous report 25 and the detailed geometries are shown in Fig. 3. The number of explicit water molecules is determined based on the previous study 51 which reports that the cluster-continuum method provides pKa values of carboxylic acids with the error of around 0.4, corresponding to the energy error of 0.02 eV, if two explicit water molecules are added to the system. It is also reported that addition of the two water molecules to the system improves the prediction of the pKa values of CoP−COOH and [CoP−COOH] – . 25 Indeed, the computed pKa values of CoP−COOH and [CoP−COOH] – well agree with previously reported values, 22,25 i.e., the average error of less than 0.4 as shown in Table S4 in the Supporting Information. This agreement is interesting by considering the fact that the pKa values are obtained using completely different methods,

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i.e., ab initio molecular dynamics simulations, 22 the method based on the isodesmic protonexchange reaction scheme, 25 and our protocol. The standard equilibrium potential of the concerted reaction using the cluster-continuum model is shown in Table 2. Although the explicit treatment of the hydrogen bonds slightly lowers the redox potential of the concerted reaction (−0.80 V), it still agrees well with experimental CO2 reduction potentials (around −0.60 V). Therefore, we conclude that, at low pH region (pH of around 3), water-facilitated CO2 reduction takes place through concerted reaction (Eq. 8). Table 2: Standard equilibrium potential of the concerted reaction as well as the CoP – /CoP redox system, relative to RHE (pH = 3). reaction CoP +e – → CoP – [CoP−CO2 ] – + H2 O +e – → [CoP−COOH] – + OH – [CoP−CO2 ] – (H2 O)2 + H2 O +e – → [CoP−COOH] – (H2 O)2 + OH –

(a)

E ◦ (V) −0.43 −0.67 −0.80

(b)

Figure 3: The geometries of (a) [CoP−CO2 ] – and (b) [CoP−COOH] – with hydrogen-bonded two water molecules. Pink, blue, gray, red, and white spheres represent Co, N, C, O, and H atoms, respectively.

CONCLUSIONS We investigated the details of the Koper’s water facilitated CO2 reduction mechanism 9,25 using the newly proposed simple but accurate protocol based on thermodynamics, which 10


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utilizes DFT calculations and experimental data. In our protocol, the three major errors, that from the one-electron reduction of CoP, the hydrogen bonds at CO2 or COOH moiety in [CoP−CO2 ] – and [CoP−COOH] – , and the free energy difference between H2 O and OH – , can be reduced substantially. The errors coming from the one-electron reduction potential of CoP and the treatment of the hydrogen bonds are estimated to be the order of 0.01 eV. From the comparison of the three different pathways, we concluded that the water facilitated CO2 reduction takes place through the concerted pathway. We successfully explained, for the first time, the details of the cobalt-porphyrin catalyzed electrochemical reduction of CO2 at low pH region (pH of around 3). The importance of our findings from the point of view of materials design is that the equilibrium potential of Eq. 8 can be controlled by changing the catalyst, meaning it becomes useful guide to design not only the cobalt-porphyrin complexes but also other catalysts, such as MOFs and metal alloys. So far, the binding energies of H and COOH to the catalyst are used as the guide to design new catalysts. 13,14 However, it is difficult to find such catalysts since usually the binding of H+ to the negatively charged catalyst is much stronger than that of CO2 as shown in Ref. 13. With our new guide, i.e., the redox potential of Eq. 8 (or Eq. 2) and its evaluation protocol, we can expect to find broader candidates due to much milder criteria.

Supporting Information Available The following files are available free of charge. • SI_CoP_reduction.pdf: Binding energy between CoP – and H+ ; geometries and oneelectron reduction potentials of CoTPP and CoPc; pKa values of CoP-COOH and [CoP-COOH]− • geometry.txt: Molecular geometries (xyz format) used in the paper.

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Graphical TOC Entry

[CoP-CO2]-

+ H 2O

CoP-COOH + OH-

+ e[CoP-CO2

+ e]2-

+ H 2O

[CoP-COOH]- + OH-

18

Water Facilitated Electrochemical Reduction of CO on Cobalt

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