Theoretical Insights into Electrochemical CO2 Reduction Mechanisms

Letter pubs.acs.org/JPCL

Theoretical Insights into Electrochemical CO2 Reduction Mechanisms Catalyzed by Surface-Bound Nitrogen Heterocycles John A. Keith*,† and Emily A. Carter*,‡ †

Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States Department of Mechanical and Aerospace Engineering, Program in Applied and Computational Mathematics, and Andlinger Center for Energy and the Environment, Princeton University, Princeton, New Jersey, 08544-5263, United States

‡

S Supporting Information *

ABSTRACT: We propose a novel and general reaction mechanism to explain the unique performance of nitrogen-heterocycle-promoted (photo)electrochemical CO2 reduction reactions. This mechanism is based on observations from recent computational and experimental studies of pyridinium-catalyzed CO2 reduction. Herein we report pKas and standard reduction potentials of species adsorbed on GaP photoelectrodes derived from first-principles quantum chemistry computations. We show that on GaP surfaces, proton reduction or pyridinium reduction is energetically unfavorable even at very negative electrode potentials. However, it is thermodynamically favorable to convert a surface-bound pyridine into a 2e− reduced species such as dihydropyridine at less negative applied potentials. Intriguingly, these transient 2e− reduced species share a similar chemical moiety as some biological redox catalysts (e.g., NADH), and their reduction potentials are similar to the thermodynamic redox potentials that would convert CO2 to a variety of products. SECTION: Energy Conversion and Storage; Energy and Charge Transport

however, why or how these heterocycles would catalyze CO2 reduction. Deconvoluting the effects arising from surfaces and from homogeneous solution in electroanalytical measurements is challenging, so electrodes such as glassy carbon and platinum are often used to study so-called homogeneous electrochemical behavior. Bocarsly and co-workers proposed a complete mechanism for the PyH+ process by analyzing data from electrochemical cyclic voltammetry, electrokinetics experiments, digital simulations, and quantum chemistry calculations.12,13 Their mechanism made the assumption that PyH+ has a one-electron reduction potential corresponding to the quasi-reversible reduction occurring at −0.58 V vs the saturated calomel electrode (SCE; all reduction potentials in this study are reported relative to this reference). Yasukouchi, et al. had also attributed similar observed peaks to PyH+ reduction and noted that its reduction was surface dependent in electrolyte solutions in acetonitrile.16 Interestingly, the observed reduction peak appears at more negative potentials when electrodes with higher overpotentials for the hydrogen evolution reaction (HER) are used. Indeed, on glassy carbon electrodes, which are poor catalysts for hydrogen evolution, there is no readily observable reduction peak. This latter observation provided a clue how PyH+ (or ImH+ or other nitrogen-containing molecules) may behave in this chemistry.

(Photo)electrochemical reduction of CO2 promises sustainable utilization of CO2 as a chemical feedstock.1−3 Recent interest in energy independence and renewable energy has driven searches for efficient and sustainable CO2 conversion into fuels and useful chemicals, e.g., CO, formate, and methanol.4−7 One intriguing class of reactions is those where organic molecules appear to promote electrochemical CO2 reduction. For example, nitrogen-containing heterocycles such as pyridinium (PyH+) and imidazolium (ImH+) appear to lower overpotentials (indicating lower reaction barriers) and increase faradaic efficiencies (indicating higher selectivities toward CO2 reduction products) in (photo)electrochemical CO2 conversion reactions.8,9 Masel and co-workers recently reported very high faradaic efficiencies for converting CO2 to CO on Ag electrodes using ionic liquids containing substituted ImH+ cations.8,10 Here, faradaic efficiencies increase from ∼80% to 96% in the presence of the ionic liquid, which is believed to adsorb on the electrode and suppress hydrogen evolution. In related work, Bocarsly and co-workers have published several studies on CO2 reduction cocatalyzed by PyH+ and ImH+.9,11−15 Their 1994 study showed evidence that CO2 electroreduction is catalyzed by PyH+ on hydrogenated Pd electrodes in aqueous electrolytes.11 Their 2008 study reported underpotentials and higher faradaic efficiencies toward methanol using similar PyH+ catalysts on ptype GaP photoelectrodes.9 This result indicated that similar PyH+-catalyzed processes might eventually be developed to carry out sunlight-driven reactions. It is still not entirely clear, © 2013 American Chemical Society

Received: October 4, 2013 Accepted: November 7, 2013 Published: November 7, 2013 4058

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The close similarity of the assumed 1e− reduction potential for PyH+ on Pt (and hydrogenated Pd) electrodes to the sixelectron thermodynamic reduction potential for CO2 + 6 H+ + 6 e− → MeOH + H2O at pH = 5.4 (−0.52 V), suggested that reduced PyH+ functioned as a 1e− donor that facilitated the net transfer of a proton and an electron multiple times to reduce CO2 to formate or methanol.12 The proposed 1e− process is somewhat unconventional, since many molecular CO 2 reduction and HER catalysts are specifically designed to facilitate the transfer of a hydride and a proton to result in net 2e− + 2H+ reductions,17−19 so that stable intermediates could be formed over the course of the reaction. The proposed mechanism of Bocarsly and co-workers was extremely useful as it laid out explicit reaction steps that could be studied with computational quantum chemistry. Several of these theoretical studies have noted that the homogeneous reduction: PyH+ + e− → PyH• actually should occur at potentials far too negative (∼−1.4 V)20−24 to correspond to the observed value of −0.6 V. These theoretical studies prompted new experiments that now bracket the actual reduction potential for PyH+ + e− → PyH• in solution to be somewhere between −0.89 and −1.73 V.15 Thus, the ∼−1.4 V reduction potential predicted previously by theory appears to be reasonable, and the homogeneous PyH+ + e− → PyH• process should therefore be considered highly unlikely to be involved in the reduction of CO2, based on thermodynamic considerations. Theoretical studies have also considered the viabilities of other mechanistic steps for this process. We recently noted that 1,4-(para)- and 1,2-(ortho)-dihydropyridine (p/o-DHP) are intriguing molecules to consider in this chemistry.24 The Py + 2e− + 2H+ → p-DHP process in solution has a calculated thermodynamic reduction potential of −0.72 V. The reduction potential for o-DHP is similar, but it is slightly more negative, −0.82 V. Both values better agree with the observed value from CV measurements on Pt electrodes12 within a conservative margin for error in these calculations (∼0.3 V). Furthermore, our group found that GaP(110) surfaces, which are both nonpolar and the lowest energy low-index planes of GaP, favor water dissociation at sufficiently high coverages of water molecules.25 The thermodynamic driving force for water dissociation on this surface means that the most stable GaP(110) facets should be covered in part with water as well as adsorbed H* and OH* intermediates (* refers to a species bound to a surface site). Thus, GaP photoelectrodes coincidentally share a similar characteristic as other metal electrodes that catalyze HER: they should be covered with H* at negative potentials. Since DHP formation would require the near-simultaneous transfer of 2e− + 2H+, this process would naturally have a very high free energy barrier unless it were catalyzed on a surface covered in H* intermediates. Interestingly, the 2e− potential to convert Py to DHP in solution at pH 5.2 is quite close to the 6e− potential that converts CO2 to methanol at the same pH. For these reasons, we believe a transiently formed DHP-like species on the surface may play the key role that was previously attributed to PyH• in solution,11 namely, the DHP-like species would be the key intermediate that catalyzes CO2 reduction by facilitating proton and electron transfers to CO2 while also residing on the surface and potentially blocking surface reaction sites that would otherwise generate H2. There is additional precedence to consider DHP in this role since it has a similar chemical moiety as the hydride donor in the biologically ubiquitous redox enzyme, nicotinamide adenine dinucleotide (NADH). In

NADH, both the cofactor nicotinamide group (a substituted dihydropyridine species) and the remaining protein are believed to play complementary roles in redox reactions. Here, we envision that transiently formed nitrogen heterocycles play a similar role as enzyme cofactors, while the electrode surface facilitates forming these species while also having a complementary role catalyzing CO2 reduction. However, after proposing DHP as a possible intermediate, the Bocarsly group reported several experimental findings that do not support it being involved in CO2 reduction.15 First, CV experiments with rotating disk voltammetry showed no evidence of a 2e− reduction process (though we note that impedance spectroscopy may be a more definitive means to determine this). Reactions run in D2O solution found no evidence of H/D scrambling at the para-position of Py, a result that would be expected if Py and DHP were ever in thermodynamic equilibrium. Lastly, gels formed when synthesized DHP and CO2 were mixed together and treated with acid, and this led to the release of CO2 with no observed CO2 reduction products. This indicates that DHP in the absence of an electrode will not catalyze CO2 reduction in solution, but it does not exclude DHP playing a complementary role catalyzing CO2 reductions on a surface. The lack of experimental observation of DHP intermediates to date is one indication that Py reduction to DHP is likely not the cause for the observed reduction peak at −0.58 V. A more probable explanation for this reduction was presented earlier this year by Batista and co-workers, who reported that the one-electron reduction potential of PyH+ + 1e− → Py + H* on Pt was also −0.72 V.23 Given the negative applied potentials, the observed reduction peak would then be attributed to Hopd species (H atoms adsorbed at potentials more negative than the thermodynamic potential for the HER) rather than to underpotential deposited H*, i.e., Hupd (H atoms adsorbed at potentials less negative than the thermodynamic potential for the HER).26 Note that the onset of Hopd on Pt electrodes is typically at 0 V vs the standard hydrogen electrode (SHE), or ∼−0.24 V vs the SCE.27 The more negative potentials are observed here due to the Nernst relation because these experiments are maintained at pH 5.2. Indeed, the Bocarsly group carried out investigations showing the observed reduction potential correlates extremely well with the pKa’s of different proton donors.15 Hence, we think this step as proposed by Batista and co-workers is likely in play on metal surfaces. Batista and co-workers also proposed a second mechanistic step where the adsorbed H* species can react with CO2 and another PyH+ to form formate. Although their calculated energies for this process appear feasible, we are skeptical of this second step, since this would imply that any Brønsted acid should catalyze CO2 reduction. By contrast, other studies implicate PyH+ (and ImH+) as having essential roles in CO2 reduction, beyond simple acid−base chemistry.8,10−13 On the other hand, Savéant and co-workers recently reported a similar observation, although their results suggest PyH+ is not catalyzing CO2 reduction, but rather HER.28 To investigate how heterocycles might facilitate electrochemical CO2 reduction, we carried out density functional theory (DFT) calculations using GAMESS-US.29,30 We calculated pKas and reduction potentials of adsorbed species on H-passivated GaP clusters that model GaP(110) surfaces.31 The H-passivation is done to saturate dangling bonds at the subsurface boundaries of the cluster to mimic a semi-infinite crystal surface. Geometry optimizations and vibrational 4059

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frequency calculations used the B3LYP exchange-correlation functional,32,33 6-31G** basis sets on P, O, N, C, and H atoms,34,35 and the Stuttgart effective core potential (ECP) on Ga with its corresponding double-ζ valence basis set.36 At the optimized geometries, electronic energies were recalculated with DFT-B3LYP using the aug-cc-pVDZ basis set37,38 on all atoms except Ga, which used the previously mentioned ECP and basis set. The pKa and redox potential calculations follow calculation schemes outlined in our previous studies.21,24,39 We now briefly summarize previous findings before reporting new pKa and reduction potential data. Our previous work found that Py and DHP bind to GaP(110), chemisorbing via N lone pair donation into the empty orbital on Ga.31 Py binds to this surface by 0.51 eV while p-DHP and o-DHP bind by 0.36 and 0.28 eV, respectively. These binding energies account for the total free energies at 298 K (using ideal gas, rigid rotor, and harmonic oscillator approximations),40 van der Waals dispersion (using the Grimme D-2 scheme),41 and solvation effects (using the CPCM model implemented in GAMESS-US).42 One of our specific goals was to model adsorption energies in the presence of the electrolyte, so we tested the degree to which the PCM solvation model induced polarization of both the bare cluster and the cluster with an adsorbate. Since the induced polarization occurring on the cluster was largely constant whether or not an adsorbate was present, we were confident that spurious solvation energies from the PCM model were not affecting adsorption energies.31 Using a similar calculation scheme, we found that PyH+ does not bind to this surface effectively because PyH+ is strongly stabilized (by ∼−2.5 eV) by desorbing into solution. This result is qualitatively different from the result obtained by Batista and co-workers, who report that PyH+ binds relatively strongly on hydrogenated (100) faces of Pt clusters by 0.43 eV.23 This discrepancy may be due to image charge effects that allow the cation to be attracted to Pt metal much more than to the GaP semiconductor. We also found that adding an extra (unpaired) electron or adsorbing a neutral radical to the cluster model of GaP(110) always results in the excess electron localizing at a surface Ga atom, which then hybridizes to an sp3-like state.31 For example, in calculations where the neutral PyH• is bound to the neutral cluster, the final state is a zwitterion, where PyH+ resides on a negatively charged cluster, with the negative charge localized at the surface near the boundary of the cluster. In periodic slab calculations, we find a similar result except the extra electron is delocalized across the entire surface instead of localized on a single Ga atom.31 Thus, we conclude that on GaP (and likely on other III−V semiconductor surfaces), PyH• is highly unstable and therefore unlikely to form. First, it is not energetically favorable for PyH+ to bind on the surface, and even if it did momentarily, an added extra electron to this system would energetically prefer to remain at the surface, and PyH+ would then eventually desorb since it strongly prefers to be in solution. By using cluster models that permit the use of continuum solvation, we can predict pKas and standard reduction potentials of different species at the surface of GaP(110) using approaches we have employed earlier.21,24,39 Table 1 shows calculated aqueous phase deprotonation energies and their corresponding pKa values. pKas reflect the tendency for species to be in thermodynamic equilibrium in solution at a given pH. For instance, in an aqueous solution of Py and PyH+ maintained at pH 5.3, the

Table 1. Calculated Acidities of Species Adsorbed on GaP(110) Clusters deprotonation reaction

aqueous phase deprotonation energy ΔGaq (eV)

PyH+ → Py + H+

+0.31

H+(Ga24P24H40) → Ga24P24H40 + H+ H2O(Ga24P24H40) → OH−(Ga24P24H40) + H+ PyH+(Ga24P24H40) → Py(Ga24P24H40) + H+ PyH+(Ga24P24H40−) → Py(Ga24P24H40−) + H+

−0.25

calculated pKa value 5.2 (experiment = 5.3) −4.3

+0.82

13.9

−0.29

−5.0

−0.38

−6.4

concentrations of Py and PyH+ will be equal, which in turn means that the Gibbs free energies of both species under these conditions are the same. At higher pH (less acidic conditions) the deprotonated species is favored thermodynamically, while at lower pH (more acidic conditions) the protonated species is favored thermodynamically. Data in Table 1 show that a lone proton residing on GaP(110) would be thermodynamically favorable only in very acidic conditions (pH < −4.3). Likewise, the pKa of water on GaP(110) is found to be roughly two pKa units less than the experimental value (pKa ∼15.7), showing that the chemisorption of H2O on GaP(110) moderately affects the heterolytic bond energy of the O−H bond in H2O. However, the pKas of PyH+ and PyH• are both quite negative (the last row of Table 1 indicates the zwitterionic nature of adsorbed PyH•), indicating that neither species is particularly stable at the GaP(110) surface, as we had already deduced from adsorption energy calculations. We also computed standard reduction potentials for different adsorbates on GaP(110) (Table 2). Adding an electron to the GaP(110) cluster usually results in a similar calculated reduction potential since the extra electron will reside in the Table 2. Standard Reduction Potentials for Relevant Intermediates Using GaP(110) Cluster Models (Potentials Referenced vs the SCE and at pH 5.2)a reduction reactiona −

2 e + 2H → H2 PyH+ + e− → PyH· +

PyH+ + e− → Py + H* Py + 2H+ + 2e− → p-,o-DHP Ga24P24H40 + e− → Ga24P24H40− H+(Ga24P24H40) + e− → H+(Ga24P24H40−) Py(Ga24P24H40) + e− → Py(Ga24P24H40−) PyH+ + Ga24P24H40 + e− → Py + H+(Ga24P24H40−) Py(Ga24P24H40) + 2H+ + 2e− → p-,oDHP(Ga24P24H40)

reduction potential (v) experiment: −0.56b experiment: between −0.89 and −1.73 Vc calculation range: from −1.31 to −1.58d experiment: −0.52c,e calculation: −0.72f −0.55, −0.60g −0.90h −0.73h −0.76h −1.29h −0.63, −0.71h

a Labeled formal charges denote final charge location from calculation. Data for Pt electrodes are also included for comparison. bSHE potential (on Pt electrodes) defined at −0.24 V and then adjusted according to the Nernst relation for pH 5.2. cReference 15, on Pt electrodes. dReferences 20−24, in solution. eReferences 15 and 28. f Reference 23, on Pt electrodes. gReference 24, in solution. hThis work, on GaP(110) electrode models.

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same place in each case, i.e., at a surface Ga atom, as stated earlier. The calculated reduction potentials for adding an electron to the GaP cluster, to an adsorbed proton, or to an adsorbed Py are −0.90, −0.73, and −0.76 V, respectively. Reducing a proton from PyH+ in solution to form Py in solution and an adsorbed H atom on GaP(110) is quite negative, −1.29 V, since the H atom energetically prefers to remain on Py to form PyH+ in solution. We previously reported 2e− reduction potentials for p-DHP and o-DHP in solution calculated from highly accurate CCSD(T)-F12 calculations (−0.72 and −0.82 V, respectively),24 but such calculations are prohibitively expensive for clusters of the size used here. Energies from the DFT-B3LYP calculations used in the present study afforded reduction potentials differing by ∼0.2 V compared to CCSD(T)-F12 results, −0.55 and −0.60 V for p-DHP and o-DHP in solution. Despite the potential 0.2 V errors, these latter values can be directly compared to reduction potentials for p-DHP and o-DHP bound to the surface, −0.63 and −0.71 V, respectively. The presence of the surface causes reduction potentials to be slightly more negative, which we attribute to slightly weaker binding of DHP versus Py to the surface, perhaps due to greater Pauli repulsion between DHP and the surface (since DHP has two more hydrogen atoms than Py). These differences are nevertheless relatively small. We determine the thermodynamic reduction potential for forming DHP from Py on a surface by starting with the homogeneous reduction potential for Py + 2H+ + 2e− → DHP, adding the binding energy of DHP on the surface, and then subtracting the binding energy of Py on the surface. Thus, the difference in binding energies between Py and DHP on a given surface determines how different the reduction potential for Py reduction is on that surface versus Py reduction in solution. Since Py and DHP bind primarily to GaP and Pt via a N lone pair, we expect that the dif ference between these binding energies should be relatively similar whether on GaP or on metal surfaces. Thus, the thermodynamics of DHP formation on Pt should be similar to that for DHP formation on GaP. Based on the results above, the thermodynamics for this process should also be competitive with forming Hopd on Pt surfaces. All of these reduction potentials are also similar to the thermodynamic potential for CO2 reduction to formate and/or methanol. We now reiterate and summarize discussions from above to consider hypothetical reaction sequences that would lead to CO2 reduction products (Scheme 1, which just shows the first reduction, to formic acid, for simplicity). Scheme 1a shows the original homogeneous reaction reported by Bocarsly and coworkers. Based on the high thermodynamic penalty to form PyH• in solution and the high free energy barrier for it to exchange a proton with CO2 (>30 kcal/mol),24 we consider it unfeasible. Scheme 1b shows two alternative reaction mechanisms that would be surface dependent. The first mechanism is analogous to the mechanism proposed by Batista and co-workers, where PyH+ acts as a Brønsted acid that first allows proton reduction to a metal surface and then provides the proton needed to facilitate a coupled hydride-proton transfer to reduce CO2 to formic acid (or formate anions at these pH values). Again, recent experimental work by Bocarsly and co-workers shows that proton reduction on metals is likely occurring at negatively applied potentials leading to the observed reduction at −0.58 V. However, this mechanism only invokes PyH+ as a proton donor, and, in principle, any Brønsted acid would then yield CO2 reduction products, which

Scheme 1. Possible Reaction Mechanisms for PyridiniumCatalyzed CO2 Reduction

is not the case. It remains to be understood why PyH+ would be special. Another thermodynamically feasible route involves the reduced proton on the surface reacting with a surface-bound Py. Note that both H* and Py* adsorbates should be found on both metal and semiconductor surfaces. Since the surfacebound H atom is likely an Hopd species and therefore hydridelike, we denote its partial charge with a minus sign. After a hydride transfer to the Py (via a Langmuir−Hinshelwood mechanism), concomitant proton transfer from the Brønsted acid could form DHP bound to the surface. Based on the calculated reduction potentials shown in Table 2, the DHP mechanism is thermodynamically feasible within our cluster model of GaP(110). Again, since Py and DHP bind to both GaP and Pt in a similar manner (via a N lone pair), the dif ference in binding energies between Py and DHP on Pt(111) should not be significantly different than on GaP(110). Thus, we expect the thermodynamics for forming DHP on a metal electrode would be similarly viable on both materials. We are presently investigating reaction barriers to determine whether this process is kinetically feasible as well. This is a highly promising avenue to consider, because by forming a DHP molecule on the surface (or an imidazoline molecule, in the case of imidazolium-catalyzed reduction processes), an active site would be formed at the surface that would have structural features that may catalyze concomitant hydride and proton transfer to CO2. Specifically, the hydride that would ideally transfer to CO2 would already be removed from the surface, instead situated on the adsorbed molecule (see Supporting Information for an illustration of this intermediate structure). From this position, the H atoms would be more favorably 4061

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No. FA9550-10-1-0572). We thank Prof. Bocarsly and his group for helpful discussions.

positioned to transfer to a nearby CO2 molecule (or other intermediates en route to methanol production) as well, eschewing the high barrier to form carbamate intermediates. Future investigations on the kinetics of these processes would determine which steps would be potential limiting. Lastly, by having hydrides transferring to form surface adsorbates, one is effectively blocking sites where the HER would be taking place, and thereby raising faradaic efficiencies for CO2 reduction. By contrast, the feasibility of the surface-dependent protonation mechanism (middle of Scheme 1) is more difficult to ascertain, since adding an extra electron to the H+(Ga24P24H40) system results in H+(Ga24P24H40−), making it unclear whether the GaP surface model is describing a reduced proton state. In summary, we have shown that the 2e− reduction of Py to DHP should be feasible thermodynamically. Even though several electrochemical experiments have not furnished evidence of DHP formation, considering similar 2e− reduced products on either metal electrode or semiconductor photoelectrode surfaces brings several highly intriguing design factors into harmony. First, the 2e− reduction potential for DHP is similar to that needed to reduce CO2 to a variety of different products, including methanol. Thus, CO2 reduction should also be thermodynamically feasible using these cocatalysts. We envision that other molecules may have similarly aligned reduction potentials for other hydride transfers as well. Second, concerted transfer of two electrons and two protons to form DHP would be most feasible on a surface that had surface hydride adsorbates. Furthermore, by forming DHP, one is effectively blocking sites that would otherwise be evolving H2. This would be a possible explanation for increased faradaic efficiencies for CO2 reduction. The geometric structure of adsorbed DHP on metals and semiconductors opens several possibilities how one may tune this catalysis. One would then do so electronically by a) tuning the reduction potential of the transiently formed intermediates and/or the Lewis acidity of the electrode surface, or structurally by b) using different molecules and surfaces with structural parameters better suited for directional hydride transfers. Finally, steps toward understanding this complementarity of transient electrochemical species on surfaces would also have implications for understanding the complementarity of enzymatic cofactors and their proteins. Thus, study of these processes has biomimetic implications as well.

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ASSOCIATED CONTENT

S Supporting Information *

Tabulated energies for pKa and reduction potential calculations, along with an optimized structure of DHP on GaP(110). This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Air Force Office of Scientific Research through the DOD-MURI program (AFOSR Award 4062

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The Journal of Physical Chemistry Letters

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