Facet Dependence of CO2 Reduction Paths on Cu Electrodes - ACS

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Facet Dependence of CO2 Reduction Paths on Cu Electrodes Wenjia Luo, Xiaowa Nie, Michael J. Janik, and Aravind R. Asthagiri ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01967 • Publication Date (Web): 24 Nov 2015 Downloaded from http://pubs.acs.org on November 26, 2015

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Facet Dependence of CO2 Reduction Paths on Cu Electrodes Wenjia Luo,1 Xiaowa Nie,2 Michael J. Janik,*,3 and Aravind Asthagiri*,1 1

William G. Lowrie Department of Chemical & Biomolecular Engineering, The Ohio State

University, Columbus, Ohio 43210, USA 2

State Key Laboratory of Fine Chemicals, PSU-DUT Joint Center for Energy Research, School

of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning Province, P. R. China 3

Department of Chemical Engineering, Pennsylvania State University, University Park, PA

16802, USA Corresponding Author Dr. Aravind Asthagiri Tel: 614-688-8882

Fax: 614-292-3769

E-mail: [email protected]

Fax: 814-865-7846

E-mail: [email protected]

Dr. Michael J. Janik Tel: 814-863-9366 Co-First Author W. Luo and X. Nie contributed equally to this work.

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Abstract Experimental results have shown that CO2 electroreduction is sensitive to the surface morphology of Cu electrodes. We used density functional theory (DFT) to evaluate the thermodynamics and kinetics of CO2 reduction pathways on Cu(100) and Cu(111) with the aim of understanding the experimentally reported differences in CO2 reduction products. Results suggest that the hydrogenation of CO* to hydroxymethylidyne (COH*) or formyl (CHO*) is a key selective step. Cu(111) favors COH* formation, through which methane and ethylene are produced via a common CH2 species under high overpotential (< -0.8 V-RHE). On Cu(100), formation of CHO* is preferred and ethylene formation goes through C-C coupling of two CHO* species followed by a series of reduction steps of the C2 intermediates, under relatively lower overpotential ( -0.4 ~ -0.6 V-RHE ). Further reduction of these C2 intermediates, however, require larger potentials (~ -1.0 V-RHE) and conflicts with the experimentally observed low potential pathway to C2 products on Cu(100). Calculations show that the presence of (111) step sites on the flat (100) terrace can reduce the overpotential for C2 production on Cu electrode, which may be present on Cu(100) due to reconstruction. On Cu(100), a change in CO* coverage from low to high with increasing negative applied potential can trigger a switch from ethylene/ethanol to methane/ethylene as the reduction products by affecting the relative stability of CHO* and COH*.

Keywords: CO2 reduction; Cu electrodes; Cu facet; Cu(100); density functional theory; reaction paths; kinetic barriers


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1. Introduction Photo-catalytic, electrocatalytic, or photo-electrocatalytic reduction of carbon dioxide (CO2) are potential candidate processes for energy storage and synthetic fuel production. These processes generate valuable fuels using energy from renewable sources, such as hydro, solar, or wind.1,2 Among the metals examined in electrokinetic experiments,3 Cu electrocatalysts were found to be uniquely able to reduce CO2 into hydrocarbon products. Methane (CH4) and ethylene (C2H4) are the primary hydrocarbon products on Cu electrodes, which are yielded at reasonable current density and Faradaic efficiencies.3-10 However, the overpotentials required by Cu electrocatalysts are relatively high (~ 1 V)11, which leads to a low energy efficiency. The hydrocarbon/alcohol product distribution is complex (containing primarily C1 and C2 species with minority C3 species) and varies with the electrode potential, but the major products are gas phase molecules (methane and ethylene), which are less useful than liquid fuels such as alcohols. The complexities of the electrochemical environment, the high overpotential, and the inability to control the product selectivity hinder the application and development of CO2 electroreduction. Extensive experiments have shown that the activity and selectivity of CO2 electroreduction can be greatly influenced by the surface morphology of the Cu electrode. Early experiments by Hori and co-workers found that CH4 and C2H4 are the two major products on Cu(111) and Cu(100), however, the Cu(111) facet has a much lower C2/C1 production ratio (0.2) compared with the Cu(100) facet (1.3) under constant current conditions.12-14 More interestingly, the flat Cu(100) is not the facet that most favors C2 over C1 production. Instead, facets that are mostly flat Cu(100) terraces but also contain a small amount of (111) steps, such as the Cu(711) surface, can yield even higher C2/C1 ratios compared with the flat Cu(100) facet13,14. Recent studies have demonstrated that the catalytic performance of Cu could be greatly improved by tailoring the 3 ACS Paragon Plus Environment

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surface morphology of Cu electrodes.15-17 Unfortunately, despite various experimental and theoretical efforts, the principles underlining the relationship between Cu surface structures and activity are still not completely understood. Specifically, the major reaction paths that lead to different products on various Cu facets and their associated thermodynamic and kinetic profiles are not fully clear. Such information would enable the quantitative prediction of the catalytic performance of Cu electrodes based on their surface structures, and would be valuable in guiding the design of highly active CO2 electroreduction catalysts. In our previous publications, we used density functional theory (DFT) to examine the CO2 reduction pathways on the Cu(111) facet18,19. In this study, we aim to delineate the C1 and more complex C2 production paths on the Cu(100) facet. C2 products including ethylene and ethanol are favored on Cu(100)20. Electrokinetic experiments have already generated insights on CO2 reduction mechanisms. For example, adsorbed carbon monoxide (CO*) is a well-known intermediate in CO2 electroreduction reactions. Reduction of CO2 and gaseous CO have similar dependence on the electrode surface structure, as evidenced by the similar product distribution and potential dependence.6,21,22 These experimental results suggest that the rate-limiting step for CO2 electroreduction occurs after CO formation.6,18,19,21,23,24 Electrokinetic experiments by Schouten and co-workers suggested that C2H4 production may have two separate pathways on Cu(100).9,25 One of them shares some common intermediates with the path to CH4 at higher potentials of -0.8 ~ -0.9 V-RHE (more negative potentials will be referred to as “higher” potentials), similar as observed on the Cu(111) electrode. The other path was proposed to go through a CO dimer intermediate, which is unique to the (100) single crystal electrode, and produces only C2H4 at relatively low potentials of only ~ -0.4 V-RHE.9,25 By following this assumption, Calle-Vallejo et al. have used DFT to study the C2 production mechanism on 4 ACS Paragon Plus Environment

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Cu(100) via the CO dimerization path26. However their study is based on thermodynamic calculations and did not consider activation barriers except for the CO dimerization step. But in our previous studies we have shown that activation energies are crucial in determining the most favorable reaction path.18,19 A recent study has reported CO dimerization barriers on Cu(100) and Cu(111)27 and we will discuss these results in the context of our own pathways to C-C bond formation in Section 3.2.1. Therefore if a more accurate description of the CO2 reduction mechanism is desired, both thermodynamic and kinetic factors should be considered extensively for a large number of candidate reaction paths. In this paper, we explore the effect of Cu electrode facet on the dominant path, onset potential, and product selectivity for CO2 reduction. DFT calculations were performed on the Cu(100) facet and compared with a combination of our previous results obtained on Cu(111)18 and additional C2 coupling pathways on Cu(111) reported in this study. Reaction energetics of elementary steps were examined for the electrochemical reduction and non-electrochemical C-C coupling reactions. Solvation effects were also considered by including one explicit water molecule in the computational model. We propose that the hydrogenation of CO* into CHO* and the dimerization of CHO*, instead of the previously proposed CO* dimerization mechanism,26,27 may be the major route of ethylene/ethanol on Cu(100) under low potentials. We found that coverage effects of CO* are important in influencing the product selectivity and may lead to different products at low and high potentials on Cu(100).9,25 While we can find low potential pathways to C2 intermediates on Cu(100), converting these intermediates to C2 products (ethylene or ethanol) requires kinetic barriers much higher than experimentally observed and cannot be resolved by existing proposed pathways in the literature. We show that the introduction of steps on the (100) terrace can reduce the overpotential for C2 product formation, 5 ACS Paragon Plus Environment

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however, future work will be needed to establish if reconstruction of Cu(100) occurs and is the source of low-coordinated sites that promote C2 product formation. Mechanisms proposed in this study, including the facet and coverage dependent COH*/CHO* competition, as well as the possible assistance from a partially reconstructed Cu(100) surface containing small amount of (111) steps, aids the ongoing efforts to understand the elementary steps for CO2 electroreduction on Cu electrodes.

2. Computational methods To examine the electrochemical reduction steps, potential-dependent reaction free energies were determined by the linear free energy method,28 as shown in Equation (1). Potentialdependent kinetic barriers were calculated using the method reported in our previous papers,19,29,30 as described in Equation (2). Explanation of each parameter included in these equations is discussed in Section S1 of the Supporting Information. Essence of this method is that we consider the kinetic barrier to decrease linearly as higher potentials are applied. ∆G = G(AH*) – G(A*) – [½G(H2) – eU]

(1)

Eact(U) = Eact0(U0) + β’(U – U0)

(2)

In the discussion of our DFT calculated barriers, we will refer to the potential when a specific pathway opens up at room temperature. We have assumed that the threshold barrier than can be overcome to give noticiable production rates at room temperature is 0.4 eV.31,32 Others have used a higher barrier of 0.75 eV 27, but, regardless of absolute value the more relevant information is the relative onset potential. All parameters needed to calculate the onset potential of each reaction step are provided in Table S1. For example, for the reaction (1b) listed in Table S1, CO2+H → COOH, the onset potential is calculated by solving the following expression, 0.89 + 6 ACS Paragon Plus Environment

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0.44 (Uonset - (-0.07)) = 0.4, to obtain Uonset = -1.18 eV. The experiments of Koper and coworkers suggest that a C2 pathway can open up on Cu(100) that is 0.4-0.5 V-RHE lower than the high potential methane/ethylene pathway on both Cu(100) and Cu(111).25 Our earlier results on Cu(111) yielded a onset potential of ~ -1.15 V-RHE for the methane/ethylene pathway and, therefore, we would expect a onset potential in the range of ~ -0.7 V-RHE for a C2 pathway on Cu(100).

Electronic structure calculations were performed with Vienna Ab initio Simulation Package (VASP), a plane-wave DFT pseudopotential code.33 The Perdew, Burke, and Ernzerhof (PBE)34 functional within the generalized gradient approximation (GGA)35 was used, indentical to the previous studies for water-metal interfaces and interface reactions.36-39 A plane wave cutoff energy of 400 eV was applied in our calculations. The lattice constant of copper is calculated to be 3.64 Å, which is only 0.6% overestimated compared with the experimenatal value (3.62 Å).40 The Cu(100) surface was modeled by a 3 layer slab, using a 2√2 × 3√2 periodic cell with the bottom 1 layer fixed, which allows sufficient distance between periodic images so that lateral interactions can be minimized. For two reaction steps that require larger surface area, i. e. the dimerization of CO and COH with 2 solvent waters, we have used a 4 × 4 unit cell. We have also increased the number of layers to 4 with the bottom 2 layers fixed, as reported in the Supporting Information (Section S5), but found no substantial difference in calculated reaction energy or kinetic barrier. 16 Å of vacuum space was used to prevent interactions between periodic images of the adsorbates and surfaces. The sampling of the Brillouin zone was performed with a 4 × 3 × 1 Monkhorst-Pack k-point mesh (2 × 2 × 1 k-point mesh for the 4 × 4 unit cells). The force convergence criterion on the free atoms was set to 0.03 eV Å-1. Activation barriers for all


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elementary steps were calculated using the climbing image nudged elastic band (CI-NEB)41 approach. The minimum energy path is examined using 5-10 images, including the initial and final state, during the transition state search. Each transition state was confirmed to have a single imaginary vibrational frequency along the reaction coordinate. Zero point energy, heat capacity, and entropy were computed with standard methods, and then used to convert the electronic energies into free energies at room temperature to allow the comparison with experimental data.25,42 Unless otherwise noted, one explicit water molecule was included in the computational model to incorporate the role of solvation. Although the reaction takes place under a fully solvated environment, in this work we have only considered a single solvent water molecule to reduce the computational cost. A detailed discussion on the accuracy of the single-water solvation model is provided in the Supporting Information (Section S5), but based on our analysis we would estimate for most surface reactions a 0.1 eV decrease in the barrier if a second solvent water molecule were included. Our previous study on Cu(111) showed that electrochemical O-H bond formation reactions occur through H-shuttling via water molecule(s) (a Grotthuss-like mechanism) whereas C-H bond formation occurs through direct transfer of a surface adsorbed H* to the adsorbate with negligible water involvement18. Kinetic barrier calculations, with water solvation inclusion, are essential to determine the dominant reaction path, onset potential, and product selectivity.


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3. Results and discussion 3.1 CO2 reduction to CO* Adsorbed

CO*

has

been

confirmed

as

an

important

intermediate

in

CO2

electroreduction,8,9,15,18,19,43-46 from which the reaction paths lead to different products. We therefore begin our mechanistic evaluation by examining CO* formation from reduction of CO2 on Cu(100). COOH* is found to be the first intermediate in CO2 reduction on various Cu facets,18,19,45,46 which is then reduced to CO* and a water molecule. Optimized structures of the initial, transition, and final states involved in CO2* to COOH* and COOH* to CO* steps on Cu(100) are illustrated in Figure S1. On the (100) facet, the kinetic barrier (Eacto) for CO2 conversion to COOH* is 1.19 eV at a potential (Uo) of -0.01 V-RHE. The elementary reaction free energy is 0.49 eV at 0 V-RHE. For the subsequent CO* formation step, a kinetic barrier of 0.88 eV is found at a potential (Uo) of -0.17 V-RHE. The free energy change for this elementary step is -0.53 eV at 0 V-RHE. Our previous results obtained by examining CO2 reduction to COOH* on Cu(111) with a water bi-layer model showed that the barriers of CO2* and COOH* reduction are sensitive to the number of water molecules included in the model.19 Additional solvent waters further reduce the barriers as much as 0.4 eV on Cu(111).19 To investigate whether similar effects exist for the Cu(100) facet, we have calculated the barriers of these two steps with two solvent water molecules and the results are summarized in Table 1. The first step, CO2* + H* → COOH*, is sensitive to the number of water molecules. For the second step, a one-water model is sufficient to capture the solvation effects.


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Table 1 Potential-dependent kinetic barriers (eV) for CO2* to COOH* and COOH* to CO* steps calculated with the H-shuttling model on the Cu(100) and (111) facets. Reaction Steps CO2* + H* → COOH* COOH* + H* → CO* + H2O*

Facets (100) (111) (100) (111)

Eacto (1 water model) 1.19 1.12 0.88 0.76

a

Eacto (additional solvation) 0.89 0.72 0.84 -

b

b

Eact at 0 VRHE

Onset Potential

0.92 0.77 0.88 0.80

-1.18 -0.64 -0.92 -0.77

Eacto (additional solvation) are barriers with additional solvation correction. For Cu(100), it was calculated by the 2-water solvation model. For CO2* + H* → COOH* on Cu(111), it was calculated with a water bi-layer model as reported in ref19, for COOH* + H* → CO* + H2O* on Cu(111), we have not calculated its barrier with additional solvation since its barrier is already fairly low with the 1-water model. b Eact (0 V-RHE) are barriers at 0 V-RHE calculated from eq. (2). Onset potential is the potential that makes the barrier less than 0.4 eV, as previous described in the Methods section. They are based on the barriers calculated with additional solvation. a

To reduce CO2 to CO*, on Cu(100) an onset potential of -1.18 V-RHE is required to surmount the kinetic barrier (Eact ≤ 0.4 eV as described in the Methods Section

30-32

), while on

Cu(111) an onset potential of -0.77 V-RHE is required. The more negative onset potential on Cu(100) compared with Cu(111) suggests a difference in activity between these two facets. Especially on Cu(100), as will be discussed later, the onset potential of CO2 hydrogenation into CO* is higher than the one to reduce CO* into C2 products. These results would explain why Hori and co-workers did not observe the lower potential peak for ethylene formation in CO2 reduction experiments on copper single crystal electrodes20,47 but Schouten and co-workers observed that peak on the (100) facet, because the latter experiments started with CO.25,48 3.2 Conversion of CO* An overview of the reaction networks examined in this work that starts from CO* on the Cu(100) facet is shown in Figure 1. Parameters for determining the potential-dependent kinetic barriers for all electrochemical reduction steps are summarized in Table S1.


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Figure 1. The overview of the reaction networks on Cu(100) examined starting with CO*. For every reaction step, two numbers are given. The red ones are barriers (Eact) at 0 V-RHE, black numbers are reaction energy (∆G) at 0 V-RHE.

As a comparison, a similar figure showing the CO2 reduction reaction network on Cu(111) based on our previous study18,19 and supplemented with additional exploration of C2 pathways is shown in the Supporting Information (Figure S2). There are multiple ways CO* can be further converted, including CO* dimerization and CO* hydrogenation into CHO* or COH*. This section compares these competitive steps of CO* conversion on Cu(100) and discusses how they can affect the following hydrocarbon production


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pathways. We will also reference the Cu(111) facet to outline important differences in CO* conversion due to the facet structure. 3.2.1 CO* dimerization We first examined the reaction energetics for formation of an adsorbed CO* dimer (OCCO*) through direct coupling of two adsorbed CO* molecules, as illustrated in Figure 1. CalleVallejo and Koper first proposed that the coupling of two CO molecules, mediated by an electron transfer process to form the CO dimer (OC-CO*), could be the rate-limiting step along the major path for C2 species and ethylene production on Cu(100).26 When direct CO dimerization is examined, it is kinetically infeasible due to a substantial barrier (1.22 eV) and a large uphill reaction free energy (1.00 eV) at 0V-RHE. We also considered the solvation effect on CO dimerization by adding two solvent water molecules as described in the Supporting Information (Section S5), however, the barrier only slightly decreased to 1.06 eV. Our results match those of Vallejo and Koper, but they suggested extra stabilization due to electron transfer but the precise barriers were not evaluated.26 For CO* dimerization, we also examined the reaction mode in which C-C coupling and reduction occurs simultaneously as shown in Figure S3. The kinetic barrier (1.52 eV) of this reaction mode is even higher compared with that (1.22 eV) of direct dimerization from two adsorbed CO* without reduction, suggesting that allowing an H to be transferred through water to the adsorbate does not facilitate dimerization. Our findings for direct CO* dimerization are similar to those reported by Montoya and co-workers in a study of C-C coupling kinetics on Cu(211).43 Based on these results we conclude that C-C coupling occurs after initial reduction of CO* to either COH* or CHO*.


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However, a recent study by Montoya et al. argued that the barrier to CO dimerization had been overestimated in previous studies because the reaction system had been represented by a vacuum-metal interface.27 In their calculation, a full water layer was place above the Cu(100) surface and a proton or a metal cation was also included in the unit cell to explicitly charge the surface to mimic surface field effects. With this configuration, they found that the CO dimerization step on Cu(100) has a 0.33 eV barrier at around -1.0 V-SHE (equal to -0.6 V-RHE at pH 7), which suggests that CO dimerization is kinetically feasible at room temperatures at the experimentally observed low potentials (~-0.4 V-RHE). Interestingly, they also found a similar pathway on Cu(111) yielded a barrier of 0.68 eV suggesting a difference in potentials required to obtain C2 intermediates on the two facets.

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Based on this finding they suggested that CO*

dimer may lead to C2 products on Cu(100), however the later reaction steps were not examined. Below we will explore the kinetics of the subsequent reduction of this CO* dimer species and show that when the barriers are examined there are large barrier steps for a CO* dimer (or other C2 intermediates) to be converted to C2 products (i.e. ethylene and ethanol). Our calculated reaction energies and barriers for hydrogenation of the CO dimer are shown in Figure 1. Following CO dimer (OC-CO*) formation, a hydrogenation step can generate either OC-COH* or OC-CHO*. According to our calculation, OC-COH* forms with a barrier of 0.60 eV whereas OC-CHO* would have a barrier of 0.97 eV at 0 V-RHE (not shown in Figure 1 but listed in Table S1). Therefore we suggest that OC-COH* should be the hydrogenation product of CO dimer. C-O bond breaking in OC-COH* to produce OC-C* has a 0.35 eV barrier. Further hydrogenation of OC-C* to OC-CH* and OC-CH2* have high kinetic barriers (1.02 and 1.00 eV, respectively, at 0 V-RHE), but the following step to OHC-CH2* is relatively easy (0.60 eV). The further conversion of OHC-CH2* can lead to ethylene and ethanol, which will be discussed in 13 ACS Paragon Plus Environment

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detail in Section 3.3.1. Nevertheless, the large barriers for hydrogenation of OC-C* present a challenge for any model that proposes that the initial C-C coupling is the rate limiting step. Based on these large barriers one could not expect the low potential pathway experimentally observed by Koper and co-workers on Cu(100).25 We will return to this issue in Section 3.5 and suggest alternatives to how these barriers may be lowered. Although several authors have suggested that CO dimerization may be the major mechanism of C2 production on Cu(100)6,9,26,27, in this study we found that CO hydrogenation into CHO also has a very low kinetic barrier comparable to CO dimerization, which will be discussed in the next section. 3.2.2 CO* reduction to CHO* versus COH* As shown in Figure 1, CO* reduction to CHO* is kinetically favored over reduction to COH* on Cu(100), which is contrary to our findings on Cu(111) (as shown in Figure S2).18,49 On Cu(111), the selectivity determining step for methane/ethylene over methanol was identified to be the preferential formation of COH* over CHO*. On the Cu(111) surface, each Cu atom has 6 nearest neighbors (all with 2.57 Å distance), while on the Cu(100) surface, each Cu atom has only 4 nearest neighbors (with 2.57 Å distance) and 4 second nearest neighbors (with 3.64 Å distance). The Cu(100) is more open compared with Cu(111) since a Cu atom on the Cu(100) surface has lower coordination number compared with that on Cu(111). This difference in surface morphology can alter the structure and stability of the adsorbed intermediates and transition states. The initial, transition, and final states involved in CO* reduction to CHO* and COH* on Cu(100) are shown in Figure 2(a) and (b). The states associated with the Cu(111) calculations are provided in Figure S4 for parallel comparison.


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(a)

(b)

(c)

Figure 2. Optimized structures of the initial, transition, and final states involved in CO* reduction to (a) CHO* with the water-solvated model, and to (b) COH* with the H-shuttling model on the Cu(100) facet. (c) Close up of the H3Oδ+ moiety in the transition state of COH* formation on Cu(100) and Cu(111). Key distances are marked in unit of Å. Distances in (c) refer to H-Cu distances. For CHO* formation, the C-H bond forming reaction requires an H transferred directly from the metal surface to the adsorbed CO* molecule. Within our 1-H2O solvation model, CHO* formation has a much lower barrier on Cu(100) (Eact0 = 0.63 eV) compared with Cu(111) (Eact0 = 0.95 eV). We have also calculated the CHO* formation barriers without the presence of H2O, and on Cu(100) (Eact0 = 0.89 eV) the barrier is still lower than Cu(111) (Eact0 = 1.03 eV)

19

to

form a CHO*. This difference suggests that the lower coordinated surface Cu atoms on Cu(100) better stabilize the C-H bond formation transition state. The introduction of solvation water decreases the CHO* formation barrier by 0.26 eV on Cu(100), while a decrease of only 0.08 eV 15 ACS Paragon Plus Environment

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was observed on Cu(111), suggesting that the solvation effects are more significant on Cu(100) than Cu(111) for the CHO* formation step, which can be visualized in the different transition state structures on Cu(100) and Cu(111). In the transition state configuration on Cu(100), as shown in Figure 2(a), the hydrogen bond formed between CO* and the adjacent H2O provides extra stabilization for the transition state, as reflected by the contraction of the OCO-HH2O atomic distance from 2.09 Å in the initial state to 1.85 Å in the transition state. On Cu(111), as shown in Figure S4(a), the surrounding water does not introduce additional stabilization to the transition state since the OCO-HH2O distance remains the same (2.47 Å) in the initial and transition state. Therefore, the lower barrier of CHO* formation on Cu(100) can be attributed to a combined effect of the more active C-H bond formation site and the different solvation effects on the (100) facet. To produce COH*, the O-H bond forming reaction occurs with an H being shuttled through a water molecule to the adsorbed CO*. In this process, the H2O molecule explicitly participates in the reaction by forming a H3Oδ+ species in the transition state. On both Cu(100) and Cu(111), the highest energy state appears after the formation of the Hsurf-Owater bond but before the formation of the Hwater-CCO bond, as reflected by the large Hwater-CCO distances in the transition states on both Cu(100) (1.64 Å) and Cu(111) (1.84 Å) (see Figure 2(b) and Figure S4(b)). Therefore, we hypothesize that the difference in COH* formation barrier between Cu(100) and Cu(111) must lie in their ability to stabilize the H3Oδ+ high energy state. To provide evidence for this speculation, we have removed the CO moiety from the transition and initial states of COH* formation on both Cu(100) and Cu(111) and performed single point energy calculations. Without the presence of the CO moiety, the H3Oδ+ state is 1.19 and 0.93 eV higher than H* and H2O* adsorbed on the surface of Cu(100) and Cu(111), respectively. The difference (0.26 eV) in the 16 ACS Paragon Plus Environment

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stability of H3Oδ+ closely represents the difference in COH* formation barrier Eact0 (0.22 eV) on Cu(100) and Cu(111). This difference can be better understood from the surface structures of Cu(100) and Cu(111). Although Figure 2(b) and Figure S4(b) show that the H3Oδ+ species have similar adsorption configurations on Cu(100) and Cu(111) (both are above bridge sites with one H pointing down towards the surface), close up pictures of the H3Oδ+ species in the transition states, Figure 2(c), show that the bonding of the H3Oδ+ with the surfaces are different. On Cu(100), only 2 Cu atoms are close to the H3Oδ+ species with H-Cu distances being 1.94 and 2.08 Å. All other Cu atoms are at least 3.2 Å from the H3Oδ+ species. However on Cu(111), in addition to the two closely bonded Cu, the hexagonal arrangement of atoms enables two other Cu atoms to be close to the H3Oδ+ species with distances of 2.62 and 2.81 Å. The Bader charge analysis50 show that, on Cu(100), the H3Oδ+ species has a positive charge of 0.40, while the surface Cu atoms have a positive charge of 0.34 (all negative charges are on the CO molecule). On Cu(111) the charges on the H3Oδ+ species and the Cu atoms are 0.36 and 0.19. These charge differences suggest that the bonding of H3Oδ+ on Cu(111) is more like a covalent bond while on Cu(100) it is more like an ionic bond. Therefore H3Oδ+ on Cu(111) is more stable and that leads to a lower barrier of COH* formation. The relatively less stable H3Oδ+ state reveals an important feature of Cu(100). Since all O-H bond formation steps preferentially proceed through the H-shuttling mechanism, the instability of H3Oδ+ would suggest that on Cu(100) all O-H bond formation steps may have higher barriers compared with Cu(111). As shown in Figure 1 and Figure S2, we found this statement to be generally true since most O-H formation steps have barriers ranging between 0.9 and 1.1 eV on Cu(100) while between 0.5 and 0.7 eV on Cu(111).


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Interestingly, the ability of Cu(111) to stabilize hydronium ions is further reinforced when more solvent water molecules are included in the computational model. When 2 H2O molecules were included, we observed a stable (H5O2δ+ … CO*) structure on Cu(111) (shown in Figure S5) which is a local energy minimum and is only 0.26 eV less stable than the separately adsorbed H2O*, H*, and CO*. Our earlier work also reported similar H5O2δ+ structures on Cu(111) when a water bilayer model was used.19 In contrast, despite our efforts, H5O2δ+ structures were never found to be stable on the Cu(100) surface. The lower stability of H3Oδ+ and instability of H5O2δ+ on Cu (100) suggest that O-H bond formation is generally more favorable due to lower kinetic barriers on Cu(111) compared with Cu(100), and that this conclusion likely still applies if more solvent water molecules were included in the computational model. Based on the kinetics calculation, CO* hydrogenation into CHO* can be opened up under low potentials since its barrier will be lower than 0.44 eV when the potential reaches -0.4 V-RHE. At -0.6 V-RHE, its barrier will be further lowered to 0.33 eV, which is equal to the barrier of CO dimerization calculated by Montoya et al. at the same potential.27 Therefore, in this study we suggest that CHO* formation and subsequent dimerization can also be a route of C2 production. In the next section we will discuss how CHO* can couple and lead to C2 intermediates and then discuss the subsequent reduction of the C2 intermediates to ethylene and ethanol. While in this section we have established that CO* hydrogenation into COH* requires a higher potential than CHO* on Cu(100), in Section 3.4 we will discuss how the applied potential can affect the competition between CHO* and COH* due to coverage effects. These potential-induced coverage effects may play an important role in the experimentally observed changes in CO2 reduction pathways on Cu(100) at low versus high applied potentials. 3.3 Reaction paths to ethylene, ethanol, and methanol through CHO* 18 ACS Paragon Plus Environment

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As illustrated in Figure 1, subsequent conversion of CHO* to coupled C2* intermediates or reduced C1* species is competitive. CHO* can be converted to C2 species such as OC-CHO* or OHC-CHO* through non-electrochemical coupling reactions with a barrier of 0.77 and 0.22 eV, respectively. These results suggest that increasing the degree of hydrogenation of C1 intermediates facilitates C-C coupling kinetics, which is consistent with the results reported by Montoya and co-workers in examining C-C coupling reactions in CO2 electroreduction on Cu(211).43 To produce C1* intermediates, CHO* can be reduced to CH2O* or CHOH* with barriers of 0.49 and 0.98 eV, respectively. Based on these results, the preferred conversion of CHO* is either to OHC-CHO* via C-C coupling or to CH2O* via reduction, and OHC-CHO* is favored at low potentials. Because C-C coupling is a non-electrochemical step, the reduction of CHO* to CH2O* will become more favorable at higher applied potentials. In the subsequent conversion, we classify two major paths to examine C2* and C1* reaction chemistry, as illustrated in Figure 1: (i) the ethylene and ethanol path through OHC-CHO* and (ii) the methanol path through CH2O*, and examine them individually. Within the reaction networks, several branching paths are also examined and separately discussed in the Supporting Information (Section S3). 3.3.1 Ethylene and ethanol formation path The dominant path identified for ethylene production on Cu(100) is illustrated in Figure 1 and the energetics of this path at 0 and -0.4 V-RHE are shown in Figure 3. In this reaction path, adsorbed CO* initially reduces to a CHO* intermediate, followed by the coupling of two CHO* to form an OHC-CHO* species. Subsequently, the coupled OHC-CHO* goes through a series of reduction steps to produce ethylene and ethanol, involving O-H bond forming, C-OH bond breaking, and C-H bond forming processes. In this reaction path, both O-H bond forming and C19 ACS Paragon Plus Environment

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OH bond breaking steps are found to have substantial kinetic barriers (> 0.9 eV at 0 V-RHE). These results imply that the O-containing C2 species are relatively stable on the (100) facet and further reduction of them by breaking the C-O bonds requires higher overpotentials. Once the first C-OH bond is dissociated to produce OHC-CH*, further reduction of these C2 species through C-H bond formation is relatively facile. At 0 V-RHE, the barrier of OHC-CH* reduction to OHC-CH2* is 0.75 eV, and for OHC-CH2* reduction to ethylene oxide (CH2-CH2O*), the barrier is 0.83 eV. We also found that the O-C bond breaking in ethylene oxide is not an easy step, with a high barrier of 1.01 eV, which is contrary to the scheme proposed by Calle-Vallejo and Koper.26 Instead, it is more favorable for ethylene oxide to be reduced to ethylene hydroxide (CH2-CH2OH*) with a barrier of 0.85 eV, from which both ethylene and ethanol can be produced. In the last step involving the reduction of ethylene hydroxide, ethylene formation has a lower barrier (0.39 eV) compared with ethanol formation (0.75 eV), which can also explain why ethylene is the major product (40.4 %) while ethanol is the minor product (9.7 %) in experiments.14


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Figure 3. Dominant path and associated reaction energetics identified for ethylene and ethanol production starting with CO* on the Cu(100) facet at 0 V-RHE, and at -0.4 V-RHE. Energy values are referenced to CO2(g), H+(aq) + e- pair, and a clean Cu(100) surface. By determining the potential-dependent kinetic barriers, the rate limiting step of the ethylene/ethanol production path will be the hydrogenation of CHO-CHO* into CHO-CHOH*. With the 1 H2O solvation model, we found this step having a barrier of 1.06 eV at 0 V-RHE. Although in this study we have only considered 1 H2O solvation model for other steps because of the computational cost, for this step, we have explored the effect of addition solvation by adding 2 H2O molecules. The barrier is slightly decreased to 0.97 eV at 0 V-RHE with the additional H2O molecule present, which suggests that an electrode potential of -1.25 V-RHE is required to open this path at room temperature (to satisfy the Eact < 0.4 eV criteria). Our calculated onset potential on the (100) facet is much higher than the experimental observation in CO reduction, 21 ACS Paragon Plus Environment

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which was reported as -0.4 V-RHE.25 A CO dimer would have to overcome these same limiting reduction steps, and, therefore, regardless of whether CO or CHO is involved in the C-C coupling step, the limiting elementary step is later in the reduction path and this does not represent a low potential path to ethylene/ethanol.25-27 Our calculations in addition to the recent work of Norskov and co-workers27 suggest that there are several plausible pathways to C2 intermediates at relatively low potentials, but transforming these C2 intermediates to C2 products requires breaking C-O bonds through OH bond formation steps that show large barriers on Cu(100). While there are many factors that could be examined to resolve this impasse, in Section 3.5 we present a scenario where the reconstruction of the Cu(100) surface could play a role in reducing the barriers for breaking C-O bonds of the C2 intermediates. Before concluding this section, it is worth comparing with C2 coupling on the Cu(111) surface. As noted above, CO* reduction to COH* is preferred on Cu(111). We have determined the non-electrochemical C-C coupling step on Cu(111) for both COH* and CHO* (see Figure. S2). From Figure S2 we see that CHO* coupling also has low barrier (0.12 eV) on Cu(111) but the coupling of COH (0.80 eV) cannot happen at room temperature. These results should explain why a low potential pathway to C2 products on Cu(111) has not been seen experimentally. Based on this observation, we did not further study the C2 pathways on Cu(111), which is the reason why Figure S2 contains much fewer reaction steps compared with Figure 1. 3.3.2 Methanol path As shown in Figure 1, instead of coupling into OHC-CHO*, CHO* can also be hydrogenated into formaldehyde (CH2O*), with a barrier of 0.49 eV at 0 V-RHE. This barrier will be higher than the one needed to couple two CHO* species until the potential reaches -0.54


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V-RHE, since CHO* hydrogenation is an electrochemical step whereas CHO* coupling is a nonelectrochemical step. Once CH2O* is produced, the further reduction of CH2O* to CH3O* is relatively facile with a kinetic barrier of only 0.64 eV. However, the further reduction of CH3O* to methanol has a larger barrier of 1.03 eV at 0 V-RHE. An electrode potential of -1.12 V-RHE is required to get the barrier surmounted at room temperature. This calculated onset potential is comparable with that for ethylene formation from CHO* coupling, which suggests that at relatively high potentials methanol production would be competitive with ethylene on the Cu(100) facet. However in fact, methanol was either not observed at all

9,19,20,25,42,47

or only in

tiny amount ( ~ 0.02 % )8,51 in electrokinetic experiments. This disagreement between DFT and experiments might result from the neglect of coverage effects and competitive adsorption of key species such as H* and CO*, which will be discussed in the next section to suggest a mechanism for switching from the CHO* to the COH* pathway on Cu(100).

3.4 Coverage effects of CO*, and high potential path to methane and ethylene 3.4.1 CO*/H* competitive adsorption model and influence on COH*/CHO* selectivity In the previous section, we have examined the reaction paths leading to ethanol and ethylene production via the formation of CHO*, and also discussed the possible competition between C2 products and methanol. However, there are several experimental observations that have not been explained. The first one is that when the applied potential is higher than -0.8 VRHE, the major products change from ethylene/ethanol to methane/ethylene.25,48 The second one is the competition between methanol and C2 under high potentials; the production of methanol seems to be kinetically feasible, but methanol was not observed (or only in very small quantities) 23 ACS Paragon Plus Environment

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in experiments under all potentials.25,48 These disagreements between DFT and experiments might result from the neglect of coverage effects. We have recently showed that the competitive adsorption of CO* and H* can be used to explain the unique activity of Cu catalysts over other metals such as Pt and Au for the reduction of CO2 to hydrocarbon products.52 We have applied a similar approach to predict the competitive adsorption of CO* versus H* on Cu(100) with applied potential as shown in Figure 4. The methods used are similar to the ones used before52 and are explained in detail in the Supporting Information (Section S2). The results show that H* adsorbs at higher coverages than CO* at potentials between 0.15 ~ -0.55 V-RHE. At potentials negative of -0.55 V-RHE, CO* coverage dramatically increases while H* coverage sharply drops off. When CO* desorption is considered (the dotted line in Figure 4), the CO* coverage can reach as high as 0.52 ML (if in equilibrium with CO(g) at 0.1 bar) when the potential is more negative than -0.6 V-RHE. Details on obtaining this equilibrium CO* coverage are described in in the Supporting Information (Section S2).


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Figure 4. Relative surface coverage variations of CO*, H*, and vacant sites as a function of electrode potential on Cu(100). The black dashed line denotes the surface coverage of CO* (0.52 ML) that is in equilibrium with 0.1 bar of CO(g)

The high surface coverage of CO* at higher potentials will have a significant influence on the competition between CHO* and COH* as the hydrogenation product of CO*. Table 2 compares the thermodynamic stability of CHO* and COH* if co-adsorbed with different coverages of CO*. Details of the calculations are provided in the Supporting Information (Section S3). Table 2 Thermodynamic stability of CHO* versus COH* (eV) co-adsorbed with different coverages of CO* on Cu(100) determined as the free energy of CHO* minus the free energy of COH*. More negative values indicates that CHO* is more stable. CO* coverage (ML) No solvent water With 1 solvent water

1/16 -0.40 -0.24

1/8 -0.37 -0.20

1/4 -0.30 0.09

1/2 -0.22 0.28

3/4 0.11 0.34

Table 2 suggests that at low CO* coverage, CHO* is thermodynamically more stable, however, as CO* coverage increases COH* becomes more stable relative to CHO*. This trend is promoted and the transition occurs at a lower CO* coverage if water is included due to the increased stability of COH* in the presence of water. This difference in coverage effect for COH* versus CHO* is related to the adsorption structure. As shown in Figure S9, COH* has an upright adsorption mode. Its terminal O-H* group can form hydrogen bonds with either solvent water or a neighboring CO* molecule, with small repulsive interaction with CO*. Contrarily, adsorbed CHO* takes much larger surface area than COH*, and its C-O* group and C-H* group cause large repulsive forces with neighboring CO* molecules.

3.4.2 High potential methane and ethylene production path 25 ACS Paragon Plus Environment

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The high CO* coverage and associated switch in CHO*/COH* competition predicted at high potentials explains the high potential methane and ethylene production activity observed in experiments.25,48 At potentials higher than -0.6 V-RHE, COH* formation becomes more favorable both thermodynamically and kinetically due to the coverage effects of CO*. As a consequence, methane and ethylene may be produced through a reaction path similar to that reported on the Cu(111) facet.18 We have examined these paths on Cu(100) facets and they are shown in Figure 1. Energetics of these paths at 0 and -0.9 V-RHE are shown in Figure 5.

Figure 5. High potential methane and ethylene path via COH* on the Cu(100) facet at 0 V-RHE and at -0.9 V-RHE. Energy values are referenced to CO2(g), H+(aq) + e- pair, and clean Cu(100) surface. The high potential path is consistent with the experimentally observed CH4/C2H4 production at potentials higher than -0.8 V-RHE.9,25 The coverage effect can also explain why methanol is not produced. As discussed in Section 3.3.2, a -0.55 eV potential is required to enable the 26 ACS Paragon Plus Environment

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conversion from CHO* into CH2O*, however at such potentials the CHO* itself may not be produced due to the favorability of COH* under high CO* coverages.

3.5 Insights on the facet effects of Cu on CO2 electroreduction 3.5.1 Dominant pathway of C2 production on Cu(100) In this paper we have discussed the possible mechanisms of ethylene and ethanol production on Cu(100), which can presumably be opened up under low potentials. Other than the previously proposed CO dimerization path26,27, we suggest that CO* hydrogenation into CHO* and CHO* dimerization can be equally kinetically favorable, and, as argued below, a more reasonable mechanism on Cu(100). Because we found that the relative stability of CHO* and COH* is reversed when the potential gets more negative, which causes the flip from low potential ethylene/ethanol to high potential methane/ethylene production behavior observed on Cu(100).9,25 If CO dimerization was the major route of C2 production and there were, as suggested in previous works,

26,27

no rate-limiting steps after the CO dimer is formed, it is not

clear why the behavior of Cu(100) is different at low and high potentials. In this study, however, we found the C-C bond coupling reactions were not the rate-limiting steps. Instead, steps that are related to the C-O bond breaking are much more difficult than C-C bond coupling and likely dictate the potential requirements. In fact, our calculated kinetic barriers of C2 production after the C-C bond is formed are quite high no matter the CO dimerization (previous pathway in literature) or CO hydrogenation (current pathway proposed in this work) mechanism is considered. These high barriers are not consistent with experimental observations that C2


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production can occur on Cu(100) under very low potentials.9,25 In the next section we will propose a possible explanation to this discrepancy between experiments and DFT calculations.

3.5.2 Activity of CO2 electroreduction on different Cu facets In the Cu(100) electrokinetic experiments by Schouten et al.9,25, production of ethylene was observed at potentials as low as -0.4 V-RHE. But based on our calculations, the rate-limiting step of ethylene production should be the hydrogenation of OHC-CHO* into OHC-CHOH*, which has barriers of 0.97 and 0.78 eV at 0 and -0.4 V-RHE, respectively. This calculated high barrier would not allow the ethylene production path to be opened up at room temperature, suggesting that the Cu(100) electrode used in the electrokinetic experiments would be more active than that estimated by DFT. Based on two experimental observations, we propose a possible explanation to this underestimated activity of Cu(100) electrode by our DFT calculations. The first observation is that the Cu(100) facet is subject to surface reconstruction under hydrogen evolution reaction electrochemical conditions around a potential of -0.5 to -0.7 V-SCE.53,54 The reconstruction process was observed to be fast and reversible54, and therefore would not cause permanent change of the Cu electrode surface morphology after the potential is removed. The reconstruction coincides with high H* coverage including potentially sub-surface hydrogen. Since high H* coverage occurs also during CO2 electroreduction we speculate that a reconstruction of the Cu(100) may occur in the low potential range where Koper and co-workers have reported the ethylene formation. A recent electrochemical scanning tunneling microscopy study of a polycrystalline Cu electrode reports changes to the Cu facets55 but because it was not a single 28 ACS Paragon Plus Environment

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Cu(100) crystal electrode and involved a constant fixed potential it is difficult to make conclusive statements about the nature of the reconstruction. Although the exact structure of the reconstructed Cu surface and its dependence on reaction conditions, such as pH, concentration of H, applied potential, and CO* coverage, are still not fully clear, it was suggested that under HER conditions the reconstruction may convert a flat Cu(100) surface into non-flat structures which contain, for example, stripes and even complex Moiré superstructures.53,54 As the second observation, many recent studies have found that non-flat Cu surfaces are much more active towards C2 production and have lower overpotentials compared with flat surfaces. Examples include Cu2O derived Cu surfaces that contain Cu nanoparticles16, grain boundaries17, or cracks56, Cu mesocrystals bearing step and edge sites57, and Cu nanowires.58-60 More insightful comparisons between flat and non-flat Cu surfaces were provided by the earlier experiments of Hori and co-workers.13,14 In their experiments, Cu(100) was not the facet showing the highest C2 production selectivity. Instead, surfaces that are primarily (100) terrace but also contain a small amount of (111) step sites, such as the Cu(711) surface, showed higher C2 selectivity and lower overpotential compared with the flat (100) facet.13,14 To test the idea that (100) reconstructions offer C2 forming steps, we have calculated the potential dependent kinetic barrier of the aforementioned rate-limiting step, CHO-CHO* hydrogenation, on the Cu(711) surface. We found that with the presence of the (111) step site, Cu(711) lowers the barrier to 0.83 and 0.64 eV at 0 and -0.4 V-RHE, which is a 0.23 eV decrease compared with the Cu(100) facet (if both calculations use the 1 H2O solvation model). Pictures showing the structures of the Cu(711) surface and the transition states are provided in the Supporting Information (Figure S10). Considering the further stabilization of the transition states by additional solvent H2O


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molecules, the results suggested that Cu(711) may allow the C2 production path to open at room temperature at -0.4 V-RHE. Therefore, we suggest that the Cu(100) electrode used in electrokinetic experiments may at least undergo partial reconstruction under reaction conditions, and the reconstructed surface is no longer completely flat Cu(100) facet, but may contain steps and edges, which showed higher activity than the flat Cu(100) facet. Although we used Cu(711) as an example to demonstrate the possible effect of step edges, the exact nature of a reconstructed Cu(100) surface, especially under CO2 reduction conditions, is unclear and may be much more complex. Further experimental and theoretical studies are necessary to accurately evaluate the activity of the possibly reconstructed Cu electrode under real reaction conditions. It is also important to note that we have not considered other effects that could impact the selectivity of the CO2 reduction pathways, including the role of adsorbed cations and electron-transfer steps (instead of coupled proton-electron transfer steps). The earlier CO dimerization mechanism is an example of an electron-transfer step, but our results show that this is not the rate-limiting step. Therefore, if an electron-transfer step is in fact a rate-limiting step, it must occur after C2 intermediates are formed on the Cu(100) surface. To study such steps will require more extensive water solvation and larger supercells along with candidate rate limiting steps.

4. Conclusions DFT calculations on potential-dependent reaction energies and kinetic barriers demonstrate that CO2 reduction reaction paths, potential requirements, and product selectivity, depend on the Cu electrode facet. Cu(111) facilitates the formation of a COH* intermediate, through which 30 ACS Paragon Plus Environment

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methane and ethylene are produced via a common CH2 species. At low potentials, Cu(100) favors CHO* intermediate formation, from which ethylene is produced via C-C coupling of two CHO* and then a series of reduction steps of the C2 species. This selective preference for the key hydrogenation intermediates between CHO* and COH* accounts for the distinct features of CO2 reduction on different Cu facets. The COH*/CHO* competition is affected by the relative surface coverages of CO*, which is dictated by the applied potential. As a consequence, at high potentials and high CO* coverages, COH* is favored over CHO* on Cu(100) and methane and ethylene can be produced via a similar path as on Cu(111). The C-OH bond dissociation steps, which have substantial kinetic barriers on both (100) and (111) facets, determine the reaction rates and potential requirement of CO2 reduction. The remaining gap between DFT and electrokinetic experiments may be explained by the partial reconstruction of the Cu(100) surface, as recent experiments showed that non-flat Cu surfaces containing steps and edges can show higher C2 production activity.

ASSOCIATED CONTENT Supporting Information Supporting Information associated with this article can be found, in the online version, at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors 31 ACS Paragon Plus Environment

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*E-mail: [email protected]; *E-mail: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We acknowledge the Ohio Supercomputing Center for providing the computational resources. AA and WL were supported by the US Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-FG02-07ER15896. XN was supported by the National Natural Science Foundation of China, Grant No. 21503027 and the “Talents introduction of scientific research project” DUT15RC(3)027. MJJ was supported by the National Science Foundation, Grant No. CBET-1264104. REFERENCES (1)

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