Potential- and Buffer-Dependent Selectivity for the Conversion of CO2 to CO by a Cobalt Porphyrin-Peptide Electrocatalyst in Water

A semisynthetic electrocatalyst for carbon dioxide reduction to carbon monoxide in water is reported. Cobalt microperoxidase-11 (CoMP11-Ac) is shown to reduce CO2 to CO with a turnover number of up to 32,000 and a selectivity of up to 88:5 CO:H2. Higher selectivity for CO production is favored by a less cathodic applied potential and use of a higher pKa buffer. A mechanistic hypothesis is presented in which avoiding the formation and protonation of a formal Co(I) species favors CO production. These results demonstrate how tuning reaction conditions impact reactivity toward CO2 reduction for a biocatalyst previously developed for H2 production.


■ INTRODUCTION
Carbon dioxide (CO 2 ) is an abundant and attractive feedstock for renewable fuels. Advances in catalysis are crucial for the development of systems for CO 2 utilization, 1,2 therefore attracting significant interest in the chemistry community. 2−6 The inertness and stability of CO 2 present both kinetic and thermodynamic barriers to its activation. 7 The reduction of CO 2 to any stable product is a multi-proton, multi-electron process (for example, see eq 1) with high activation energy, requiring effective catalysts to drive the process at acceptable rates. 8 Molecular catalysts have proven successful in CO 2 reduction reactions (CO 2 RR) and have enabled detailed mechanistic study, 5,8−16 providing insight into the roles of both Brønsted 17−20 and Lewis acids, 21,22 as well as the coordination of electron transfer with both proton transfer and bond breaking and formation. 23 Water is a desirable solvent to use for CO 2 reduction, 5,24−30 yet developing and understanding CO 2 RR catalysis in water brings several challenges: poor solubility of CO 2 ([CO 2 ] = 0.0383 M at 20°C and 1 atm of CO 2 ), 31 pH-dependent equilibria among CO 2 and its hydration products (H 2 CO 3 , HCO 3 − , and CO 3 2− ), and competition from the hydrogen evolution reaction (HER; eq 2). (1) 2e H , 0.41 V, pH 7.0 2 (2) In electrocatalysis, the amount of energy beyond the thermodynamic requirements needed to drive a reaction at a given rate is described by the overpotential. Typically, lowering the overpotential for a given catalyst comes at the expense of slowing catalysis, with the log of the rate exhibiting a linear dependence on overpotential. 32−34 The relationship between overpotential and catalyst selectivity is a less explored topic. Studies of potential-dependent product selectivity are reported for solid (nonmolecular) electrocatalysts, 35−37 but studies of this nature for molecular catalysts are less common. 38−40 This study reports on the effect of applied potential on selectivity for CO vs H 2 production from CO 2 in water by a biomolecular catalyst. We describe a cobalt porphyrin with a covalently attached peptide (CoMP11-Ac; Figure 1), previously described as a catalyst for HER, 41−43 as an active and selective CO 2 -reduction electrocatalyst in water. CoMP11-Ac reaches a turnover number (TON) > 12,000 (at 2 h) for CO 2 reduction to CO at an applied potential of −1.2 V (all potentials here are reported vs Ag/AgCl/KCl (1M) ) with 85% faradaic yield. Our report is notable as a rare demonstration of the use of applied potential to control product selectivity for CO 2 RR by a molecular catalyst. Furthermore, a mechanistic proposal is put forward with the support of observed effects of potential, buffer acid pK a , and CO 2 partial pressure on catalysis.
■ RESULTS AND DISCUSSION Effects of Potential. The CO 2 RR activity of CoMP11-Ac in 50 mM NaHCO 3 (pK a1 6.4) solution saturated with CO 2 was evaluated by cyclic voltammetry (CV) using a hanging mercury drop electrode, as shown in Figure 2. Dip-and-stir experiments 44 reveal that CoMP11-Ac adsorbs to the mercury electrode, indicating that it behaves as an immobilized molecular catalyst (details in Figures S1−S4). A precedent for a system of this nature is that of Ni-cyclam, a CO 2 reduction catalyst also adsorbed onto a mercury electrode. 45,46 Importantly, the activity of particulate cobalt in this reaction is prevented by the use of a mercury electrode, as mercury amalgamates cobalt. 47−49 The CVs of CoMP11-Ac, scanning from 0 to −1.6 V (and the opposite in the return scan), show no measurable current enhancement until −1.2 V when a broad first wave is observed, followed by a second feature of higher current with a half-wave potential (E h ) of ∼ −1.4 V. The catalytic CV current for both features is significantly higher for CO 2 -saturated vs N 2saturated solutions at the same pH, suggesting that CoMP11-Ac may catalyze CO 2 RR. An inverted peak is also observed upon switching the scanning direction, which indicates that the catalyst is partially deactivated at low potentials and reactivated upon scanning anodically. 50 This phenomenon has been described for other molecular catalysts based on cobalt, 51−53 as well as other transition metals. 54−56 To further characterize the activity of this cobalt porphyrinpeptide toward CO 2 RR, we performed controlled potential electrolysis (CPE) experiments at both −1.2 and −1.4 V in 0.5 M NaHCO 3 for 2 h in solutions purged with either CO 2 or N 2 and under 1 atm of the purging gas (Tables 1 and S1, and Figure 3). Under a CO 2 atmosphere, the charge passed with CoMP11-Ac present is comparable at both applied potentials (Figure 3), yet the product distribution is rather different. The faradic efficiency for H 2 (FE (Hd 2 ) ) decreases from 23% at −1.4 V to 4% at −1.2 V, while FE (CO) increases from 61% at −1.4 V to 83% at −1.2 V. Furthermore, the turnover number (TON) for CO production measured after 2 h of CPE increases from 2500 at −1.4 V to 3300 at −1.2 V. In N 2 -saturated NaHCO 3 solution at −1.4 V, CoMP11-Ac produces H 2 with a 76% FE   Data corresponds to the average of at least three individual runs, and the errors correspond to the difference between the average and the replicate of greatest difference from the average. Activity is not reported if it did not exceed three times background in more than one replicate. The pH of the NaHCO 3 solutions after purging with CO 2 was 6.5 ± 0.1 and 8.7 ± 0.3 when purged with N 2 . b Potentials reported vs Ag/AgCl/KCl (1M) . and CO with a 16% FE; CO arises from the reduction of the CO 2 in equilibrium with NaHCO 3 buffer.
There are a few reports of potential-dependent selectivity in molecular CO 2 RR catalysts. 38−40 For example, in a study of a group of Pd complexes, those complexes with more negative reduction potentials favor protonation of the metal to form a hydride (proposed to primarily lead to HER), whereas the complexes with less negative potentials favor protonation of Pd-coordinated CO 2, yielding CO. 39 In a more recent study of Pd molecular catalysts, the authors sought to improve the selectivity for CO 2 -to-CO conversion by increasing the overpotential of HER, which was achieved by installing proximal cations in the second sphere of the catalyst. 40 In our case, we propose that the distinct behavior of CoMP11-Ac arises from a dependence of the CO 2 RR catalytic mechanism upon the applied potential. Because the E Co(II/I) of CoMP11-Ac is estimated to be −1.42 V, 43 the catalytic reduction of CO 2 to CO at −1.2 V must originate from a different catalysis-initiating redox event. We propose that CO 2 binding to the catalyst is coupled to the electroreduction of the catalyst (Scheme 1). This phenomenon where the formation or cleavage of a bond between heavy (non-hydrogen) atoms is coupled to electron and/or proton transfer has been invoked in electrochemical systems before. 23 One example comes from the analysis of the rate-limiting O−O bond cleavage in the electrochemical reduction of aliphatic peroxides. When an allconcerted (coupling of the bond cleavage to both electron and proton transfer) pathway is at play, the CV feature associated with the electrochemically driven O−O bond cleavage was found to be at significantly less negative potential than when a stepwise mechanism is favored. 57 In another example, an intermolecular concerted proton−electron transfer bond cleavage was also found to be the rate-determining step in the catalytic reduction of CO 2 to CO by electrogenerated Fe(0) porphyrins in an aprotic solvent. 58 Finally, the catalytic electroreduction of alkyl cobalt porphyrins is an example where carbon−metal bond breaking/formation and proton transfer are proposed to be concerted. 59 Considering CO 2 reduction by CoMP11-Ac, if a molecule of CO 2 is appropriately prepositioned near the catalyst active site, it could bind the metal center in a manner concerted with electron transfer from the electrode to the Co(II) species ([M] in Scheme 1). Concerted pathways have the advantage of avoiding high-energy intermediates invoked in stepwise pathways. 23,60−64 However, this advantage can be counterbalanced by other kinetic penalties. This is particularly likely in reactions that involve the breaking or formation of bonds between heavy atoms. 23 The low CV current at −1.2 V, relative to the feature at −1.4 V, may be due to this additional kinetic expenditure. Prepositioning the CO 2 molecule prior to binding to the metal center may be enabled or enhanced by conformational changes occurring upon adsorption of the catalyst on the mercury electrode. Similar effects have been found to account for the enhanced CO 2 RR activity of Nicyclam using a mercury working electrode. Adsorption of Ni(cyclam) onto the mercury electrode is proposed to cause a flattening of the ligand, leading to enhanced CO desorption kinetics (often the rate-determining step in CO 2 reduction to CO by molecular catalysts) and diminished catalyst deactivation via CO poisoning. 45,46,65−67 The more cathodic CV feature (at E h = −1.4 V) develops at a potential near the Co(II/I) couple of CoMP11-Ac, suggesting that the dominant reaction mechanism at −1.4 V is initiated by the Co(II/I) reduction of the catalyst. Once the formal Co(I) species is formed, either CO 2 addition or proton transfer from a proton donor HA to the catalyst may occur. Consequently, both CO 2 -to-CO and H 2 -evolution catalysis take place, resulting in lower selectivity for CO 2 reduction at this more cathodic potential (Scheme 2).

Effects of CO 2 Partial Pressure (P COd 2 ).
In the mechanism outlined in Scheme 1 and proposed to be at play at −1.2 V, the catalysis-initiating redox event would entail a Nernstian equilibrium between Co(II)MP11-Ac (M in Scheme 1) and the CO 2 -bound one-electron reduced species, as depicted in eq 3.
Based on the Nernst equation for this process, we expect the half-wave potential (E h ) to shift anodically with increasing partial pressure of CO 2 (P COd 2 ) with a slope of 59.2 mV/decade, as shown in eqs 4 and 5. In these equations, n represents the The CV feature near −1.2 V does not show a clear peak, hindering our ability to accurately determine E h . Instead, we define E i as the potential at which a constant current of 1.5 μA is reached. eq 5 can be then rewritten in terms of E i to obtain eq 6. (Please note that with this approximation, the E°′ term loses any physical meaning.) This approach of using the potential at which a constant current is reached has been employed as a proxy for E h when non-ideal voltammograms are encountered. 68 We apply eq 6 to the voltammograms of CoMP11-Ac collected under different P COd 2 (Figures 4a and S5) achieved using mixtures of CO 2 and N 2 with different known compositions. To avoid deviations between P COd 2 and the concentration of CO 2 in solution, we avoided the use of NaHCO 3 as a buffer and instead used 3-(cyclohexylamino)-1propanesulfonic acid (CAPS pK a 10.4); more information regarding the effects of buffers is provided in the next section. In Figure 4a, we can see that as P COd 2 increases, the onset potential shifts anodically. A plot of E i vs −log(P COd 2 ) ( Figure  4b) shows a slope of ∼66 mV, supporting the proposal that the binding of CO 2 is coupled to the one-electron reduction of the catalyst, as outlined in Scheme 1.

Effects of Proton Donor.
To further test our mechanistic proposals, we varied the pK a of the proton donor HA, which under our experimental conditions, we anticipate being the conjugate acid form of the buffer. 43,44,69 It has been reported that the pK a of the proton donor has a large impact on CO 2reduction catalysis. Relatively strong Brønsted acids lead to fast metal hydride formation and subsequent protonation to yield H 2 , as shown in the lower portion of Scheme 2; to minimize this undesirable pathway, weak Brønsted acids are preferred regardless of whether the catalyst operates in an aprotic or protic solvent. 3,5,17,70 A particularly relevant example is the case of a water-soluble iron-porphyrin catalyst that was shown to evolve only H 2 when using formate (pK a 3.7) buffer, while an equimolar mixture of CO and H 2 was obtained in phosphate-buffered solution (pK a 7.2, H 2 PO 4 − ). 30 In the case of CoMP11-Ac, we have previously reported that the rate of HER in water decreases with increasing buffer pK a due to a slower proton transfer from the buffer acid donor to the formal Co(I) and that such proton transfer to the formal Co(I) species is rate-limiting for buffers of pK a > 7.7. 43 We have also found that the sterics of the proton donor species impact the catalytic CV current arising from HER catalyzed by a cobalt porphyrin mini enzyme in water. 69 When exploring the effects of buffer properties on the CO 2 RR of a Ni(cyclam) electrocatalyst in water, the authors concluded that charge density (i.e., charge and size) of the buffer acid species was the main factor impacting the catalytic activity. 71 With these precedents in mind, we chose to study the CO 2 RR of CoMP11-Ac in the presence of three structurally related buffers as proton donors with different pK a values: 3-(cyclohexylamino)-1-propanesulfonic acid (CAPS, pK a 10.4), 3-(cyclohexylamino)-1-ethanesulfonic acid (CHES, pK a 9.3), and 3-morpholinopropane-1-sulfonic acid (MOPS, pK a 7.2; structures shown in Figure 5).
The CVs of CoMP11-Ac in CO 2 -saturated solutions containing either CAPS, CHES, or MOPS ( Figure 5) exhibit two features, one that peaks around −1.5 V, present also in N 2saturated solution, and a more anodic wave that starts developing near −1.2 V, not present under N 2 . When the weakest Brønsted acid CAPS is present, the addition of CO 2 leads to a significant enhancement in the catalytic CV current ( Figure 5a); as the pK a of the buffer acid decreases, the enhancement seen under CO 2 relative to N 2 becomes less pronounced. For MOPS solutions, the catalytic peak current seen for CoMP11-Ac is similar under both CO 2 and N 2 (Figure 5c). The lower pK a of MOPS is proposed to facilitate proton transfer to the catalyst, 43 yielding a higher CV current (faster catalysis) under both CO 2 and N 2 . This result indicates possible enhancement of H 2 and CO evolution, both of which are impacted by the availability of protons. The catalytic peak currents seen for CoMP11-Ac decrease as the pK a of the buffer present increases. We attribute this trend to the lower acidity of the buffer acid species (i.e., higher pK a ) disfavoring the transfer of protons, thereby slowing catalysis.
To assess whether product distribution is sensitive to the proton donor pK a and the applied potential, we performed CPE experiments at both −1.4 and −1.2 V in solutions containing CAPS, CHES, or MOPS buffers; the results are summarized in Table 2 (see Tables S2−S4 for Figures S6−S8), the total charge passed in the presence of MOPS buffer is significantly higher than in CHES and CAPS. The lower pK a of the conjugate acid form of MOPS allows for more evolved H 2 , leading to a higher charge buildup, consistent with the trends seen in CV above as well as prior work. 43 At −1.4 V in all CAPS, CHES, and MOPS, H 2 is the sole product detected under N 2 , with respective FE values of 83, 92, and 92%. The TON for H 2 is over 40-fold higher for catalysis in the presence of MOPS compared to CAPS. Overall, the charge passed decreases with increasing buffer acid pK a , a finding that is consistent with previous studies of buffer effects on HER by CoMP11-Ac, 43 as well as with similar observations made for other catalysts working in both aqueous and aprotic solvents. 19,44,69,72−75 For CPE experiments on CoMP11-Ac conducted at −1.4 V under CO 2 (Figures S6−S8 and Tables S2−S4), both CO and H 2 are produced with appreciable yields for all three buffers. In CAPS, the FEs for CO and H 2 are 48 and 29%, respectively, while in MOPS, these quantities are 21 and 63%. Thus, the pK a of the buffer is found to impact the product distribution at −1.4 V, with the lowest-pK a buffer MOPS favoring H 2 formation the most. This finding supports the proposed mechanism (Scheme 2) and is consistent with other observations on CO 2 RR in water. 6,17,18,30,53,55,76,77 We propose that the stronger the acid, the more rapidly the Co(I) species is protonated, enhancing the generation of H 2 . Weaker acids (CAPS and CHES) protonate this species more slowly, allowing CO 2 binding to the formal Co(I) active species and leading to more conversion of CO 2 to CO. This model is consistent with previous work on CoMP11-Ac, in which more acidic buffers were found to promote fast HER catalysis and were proposed to protonate the formal Co(I) species more rapidly. 43 When CPEs of CoMP11-Ac are carried out at −1.2 V under N 2 in CAPS and CHES, activity is low, being comparable to the background in CAPS and barely above background for CHES (Figures S6−S8 and Tables S2−S4). In MOPS, at −1.2 V, H 2 is the only product and is detected with 98% FE and TON of 4,900 after 2 hours. This result indicates that −1.2 V is too anodic relative to E (Co(II/I)) to support HER activity unless a relatively acidic proton source (here, MOPS) is present. Previous work on HER by CoMP11-Ac showed that the presence of an acidic proton donor (pK a < 7.7) gives rise to a kinetic shift in the CV, allowing catalysis to occur at −1.2 V. 43 Results under CO 2 reveal a sharp contrast. For CPEs of CoMP11-Ac at −1.2 V under CO 2 , the overall activity is significantly higher than under N 2 ( Figures S6−S8). FE (CO) is nearly the same for CoMP11-Ac in all three buffers: CAPS (88%), CHES (81%), and MOPS (85%), and FE(H 2 ) also is nearly the same in CAPS (5%), CHES (6%), and MOPS (8%) (Tables 2 and S2−S4). The insensitivity of the product  Data shown corresponds to the average of at least three individual runs, and the errors correspond to the difference between the average and the replicate with the greatest difference from the average. Activity is not reported if it did not exceed three times background in more than one replicate. The pH of all MOPS, CHES, and CAPS solutions after purging with CO 2 was 6.5 ± 0.2, and 7.2 ± 0.2 when purged with N 2 . b Potentials reported vs Ag/AgCl/KCl (1M) . distribution at −1.2 V to buffer pK a supports our proposal that in the presence of CO 2 , catalysis is initiated by CO 2 binding coupled to catalyst reduction, which avoids the accumulation of a formal Co(I) species, leading to almost exclusive CO formation regardless of the acidic strength of the proton donor (Scheme 1). In other words, the selectivity-determining step precedes any proton transfer from the buffer acids, favoring the formation of CO irrespective of the proton donor pK a . Also worth highlighting is the TON (CO) of 12,000 achieved at −1.2 V after a 2-h CPE in MOPS under CO 2 , which compares well with other molecular electrocatalysts operating in water. 78−82 An interesting trend seen in the CPEs of CoMP11-Ac in the presence of CO 2 in all four buffers (CAPS, CHES, MOPS, and NaHCO 3 ) is that the catalyst is not only more selective for CO 2 reduction at the less cathodic potential of −1.2 V but also exhibits similar or higher TON (CO) (Tables 1 and 2). We have previously reported that CoMP11-Ac experiences partial deactivation during electrocatalytic HER. 41 This is consistent with the lower FE seen in CPE at the more negative potential (−1.4 V) and with the shape of the CV in which the current rapidly drops after reaching its maximum value between −1.4 and −1.5 V, as well as the inverse peak feature seen in the return scan, which is consistent with reactivation. 50 We propose that enhanced catalyst deactivation is responsible, at least in part, for the lower total charge passed and overall FE at −1.4 V, (particularly when compared to −1.2 V under CO 2 ). The coupled mechanism outlined in Scheme 1 would allow for CO 2 reduction catalysis to occur at potentials at which catalyst deactivation is minimal, yielding the higher charge passed for CoMP11-Ac at −1.2 V under CO 2 . Indeed, the CPE traces of CoMP11-Ac at −1.2 V after 2 hours remain linear, indicating that the catalyst is still active (Figures S6−S8). Furthermore, CoMP11-Ac under CO 2 in CAPS displays minimal deviation from linearity in the charge vs time CPE trace in a 24-h CPE at −1.2 V, yielding a TON (CO) of 9300. The 24-h CPE of CoMP11-Ac under CO 2 in MOPS reveals some loss of activity after ∼6 h, as the CPE trace levels off, yet this more acidic proton donor yields a TON (CO) of 32,000 in 24 hs (Table S5 and Figures S9−S13).
To determine whether the enhanced catalyst deactivation at −1.4 V is responsible for the lower selectivity for CO at this potential, we performed CPE experiments on CoMP11-Ac under CO 2 at −1.4 V for 24 hours ( Figure S11 and Table S5) and compared FE (Hd 2 ) and FE (CO) to the results obtained after the 2-hour CPE under otherwise identical conditions (Tables 2  and S2). The overall FE is lower at 24 h (69%), as expected for a longer bulk electrolysis experiment (attributed to more catalyst degradation), but FE (CO) is similar at 24 h (58 ± 6%) and 2 h (48 ± 10%). Interestingly, FE (Hd 2 ) is lower at 24 h (11 ± 6%) vs 2 h (29 ± 6%), which suggests that the CoMP11-Ac deactivation product is not a more active HER catalyst. Instead, this data suggests that the deactivation product may be generated within the HER mechanism of CoMP11-Ac.

■ CONCLUSIONS
CoMP11-Ac catalyzes the reduction of CO 2 to CO in water with FE (CO) up to 88%, with better selectivity at −1.2 V compared to −1.4 V. The high faradic efficiency for CO production seen in CPE at −1.2 V is proposed to originate from a distinct mechanism initiated by CO 2 addition coupled to the reduction of the catalyst, avoiding accumulation of a formal Co(I) species. The lower selectivity found at −1.4 V is proposed to arise from the Co(II/I) reduction initiating catalysis, as the formal Co(I) species can undergo either CO 2 addition or protonation, where the latter enables HER. Altogether, at the lower applied overpotential, CoMP11-Ac shows higher selectivity toward CO 2 -to-CO conversion as well as enhanced catalyst longevity. These results demonstrate how applied potential and proton donor pK a act together to determine catalyst selectivity. An implication is that these factors may contribute to system selectivity in complex ways, requiring codesign when developing and optimizing catalytic systems.
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