Mechanistic Understanding of CO2 Electroreduction on Cu2O - The

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Mechanistic Understanding of CO Electroreduction on CuO Liujian Qi, Shanping Liu, Wang Gao, and Qing Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11842 • Publication Date (Web): 15 Feb 2018 Downloaded from http://pubs.acs.org on February 16, 2018

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The Journal of Physical Chemistry

Mechanistic Understanding of CO2 Electroreduction on Cu2O Liujian Qi, Shanping Liu, Wang Gao*, and Qing Jiang. School of Materials Science and Engineering, Jilin University 130022, Changchun (P.R. China). Corresponding Author * E-mail: [email protected].

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Abstract: Cu2O demonstrates the unique selectivity and efficiency to methanol in CO2 electroreduction which is a potential strategy to convert CO2 to important fuels and chemicals, however, its reaction mechanism is still controversial. To address this issue, we have built the model of partially reduced Cu2O(100) with the consideration of solid-liquid interface by using density functional theory (DFT) methods. These allow us to uncover inherent mechanism of CO2 electroreduction to methanol on Cu2O(100) and find the key intermediate CH3OH*-OH*, which can explain the experimental results well. Our results reveal that the synergy of surface morphology and solvation is essential to the selectivity and efficiency of Cu2O(100) in reducing CO2 to methanol. More importantly, we find that the variation trend of charge distribution on catalyst surface accounts for the minimum energy pathway of CO2 electroreduction, which could act as a descriptor for understanding the mechanism of CO2 electroreduction and designing advanced catalysts.


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1. Introduction The electroreduction of CO2 has attracted tremendous interests in recent years, primarily because it is a promising technique to reduce CO2 to valuable fuels and raw materials for chemical industry with the help of renewable energy.1-3 Once applied on a large scale, this technique will have a great potential which not only solves climate and environment problems caused by global warming, but also provides an effective strategy to mitigate energy crisis as a consequence of the shortage of fossil energy.4-7 Since the pioneering work made by Hori, large amount of work have been devoted to studying the catalytic performance of copper that catalyzes CO2 to mainly hydrocarbons, however, its poor selectivity and efficiency have become the main obstacles for the further commercialization.8-20 Therefore, understanding the inherent mechanism of CO2 electroreduction is crucial for the development of designing catalysts with high performance. In recent years, copper oxide has also been extensively investigated as one of the potential catalysts for CO2 electroreduction, and it has shown excellent performance with high selectivity and efficiency.21-26 In addition, it is suggested that Cu2O have been directly involved in the (photo)electrochemical reduction of CO2, which is emphasized to be essential for the selectivity and efficiency to methanol.27,28 For instance, Frese et al. firstly carried out the experiment of CO2 electroreduction on copper oxide electrodes in KHCO3 electrolyte and found that the main product was methanol.29 Subsequently, Chang and Le et al. reported CO2 electroreduction on the prepared Cu2O catalysts in NaOH and KHCO3 electrolytes, respectively, and methanol was further confirmed as the predominant product.30,31 On the other hand, the oxide-derived copper which is from the fully reduction of copper oxide has also been evaluated as promising strategy to promote the development of CO2 electroreduction, e.g. forming C2 products at low 3 ACS Paragon Plus Environment


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overpotential.32,33 Although Cu2O exhibits the unique selectivity and efficiency to methanol in CO2 electroreduction, the underlying mechanism is still unclear. To understand the origin of the improved selectivity to methanol, DFT has been employed to investigate CO2 electroreduction on copper oxide. Mikosch et al. studied CO2 electroreduction to methanol on Cu2O nanolayers and nanoclusters. By analyzing the electronic structures of Cu2O with various intermediates, possible reaction mechanism of CO2 electroreduction to CH3OH was proposed.34 They demonstrated that the adsorption of CO2 on copper oxide was enhanced by the active sites on Cu2O surface, and the first two hydrogenations of CO2 resulted in the formation of HCOOH*, which was thought to be the key intermediate during CO2 electroreduction to methanol. While it provided the atomic level insight into the mechanism of CO2 electroreduction on Cu2O, the effect of solvation that was essential in CO2 electroreduction was not considered. In this work, we are devoted to studying the mechanism of CO2 electroreduction on Cu2O(100)/Cu(111) surfaces to probe how Cu2O can exhibit high selectivity and efficiency to methanol. We firstly build the model of partially reduced Cu2O(100) surface which is proved stable by the ab initio molecular dynamics simulation. To illustrate the role of solvation, we model the liquid-solid interface by introducing water molecules. Through analyzing various possible intermediates on Cu2O(100)/Cu(111) surfaces, we have identified the minimum energy pathways of CO2 electroreduction on Cu2O(100)/Cu(111) surfaces. Our results indicate that partially reduced Cu2O(100) surface and the solvation play the synergic role in determining the unique selectivity and efficiency of Cu2O to methanol. More importantly, we find that the variation trend of the charge distribution on Cu2O(100) and Cu(111) surfaces account for the minimum energy pathway of CO2 electroreduction, which could act as a descriptor for understanding the mechanism of CO2 electroreduction. We have provided deep insight into CO2 4 ACS Paragon Plus Environment

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electroreduction on Cu2O, which could afford useful guidelines for designing catalysts with high performance for CO2 electroreduction.

2. Calculation Methods CASTEP code was employed to perform all the electronic structure calculations with the generalized gradient approximation (GGA) exchange-correlation functional proposed by the Perdew, Burke, and Ernzerhof (PBE) and the core electrons were treated with ultrasoft pseudopotentials.35-37 The Cu2O(100) surface was cut from the cuprous oxide bulk crystal with the experimental lattice constant of 4.27 Å.38 The copper terminated of Cu2O(100) surface was modeled with seven-layered slabs, in which the upper three layers and adsorbates were allowed to relax and the bottom four layers were fixed. We adopted (3×2) unit cells for Cu2O(100) with the adjacent slabs separated with 20 Å vacuum. The extensive tests allow us to adopt the energy cutoff of 450 eV for the plane wave basis set and the 2×3×1 Monkhorst-Pack k-points for the first Brillouin zone. The Cu(111) surface was built with (3×3) unit cells with the top two layers relaxed and the bottom three layers fasten in the geometry optimizations. The 24 Å vacuum was used to separate the adjacent slabs. Similarly, a k-point sampling of (4×4×1) and an energy cutoff of 450eV were employed for Cu(111) in our calculations. All the geometric configurations were optimized until the maximum force in all directions was less than 0.01eV/Å, and the convergence of total energy was also carefully considered. In CO2 electroreduction, the calculated barriers for each reaction step with the transfer of proton-electron pairs simultaneously are acquired by employing the approximate method proposed by Jacob et al.39 In this method, the effect of applied overpotential on the reaction barrier is treated by shifting Fermi energies on the basis that the Fermi energy of charged 5 ACS Paragon Plus Environment


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intermediates and the applied overpotential are linearly dependent. The potential dependent barrier of a proton transfer is calculated by approximately transferring proton through electrochemical double layer to solution. Although it is incapable for the atomic-scale models to simulate this complicated process, we can effectively calculate the first transfer step of proton from electrode to electrochemical double layer and the last transfer step of proton from electrochemical double layer to solution as a proton transferring process from one water to another, and the corresponding transfer barrier of proton is calculated to be 0.30 eV.39 Moreover, for the hydrogenation process of intermediate that is an exothermic step, the barrier is Ea=0.3eV, while the barrier is Ea=Er+0.30 eV for the endothermic hydrogenation step. The method applied here has been successfully applied in the previous studies in field of oxygen reduction reaction and oxidation of formic acid on Pt(111) in the electrochemical environment.16,17,40-42 To identify the stability of the structure of reduced Cu2O(100), the ab initio molecular dynamic calculation in CASTEP was performed with the ensemble of NPT at the room temperature of 300K using the thermostat of Nose. The total time of simulation is 10 ps with a time step of 1 fs.

3. Results and discussion 3.1. Structure of partially reduced Cu2O(100). It is known that the activity and selectivity of catalyst is closely related to surface morphology of catalyst. As described above, Cu2O catalyst is not stable thermodynamically in the electroreduction condition. During CO2 electroreduction, Cu2O could be reduced through the combination of oxygen on Cu2O surface with the hydrogen in solvation, forming water and active coordinatively unsaturated surface copper sites on Cu2O 6 ACS Paragon Plus Environment

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surface. In the light of original Cu2O(100), we build the model of partially reduced Cu2O(100) surface with the assumption that oxygen atoms in the subsurface of Cu2O(100) are reduced to water. As shown in Figure 1a and 1b, the isolated Cu dimers on surface transform into the Cu chains after the reduction of oxygen in the subsurface of Cu2O(100). By performing the ab initio molecular dynamic calculations, we have confirmed that the partially reduced Cu2O(100) is stable at 300 K and pressure of 100 kPa, in which the structure of the partially reduced Cu2O(100) is almost unchanged (Figure S1). In spite of the instability of Cu2O in the electroreduction condition, the reduction process of Cu2O to Cu is sluggish from the viewpoint of dynamics at low overpotentials. This suggests that our proposed partially reduced Cu2O model is sufficient for describing the cathode at low overpotential ~ -0.23 V and PH ~9, where CO2 is reduced to CH3OH. Meanwhile, we have also analyzed formation energy of the partially reduced Cu2O(100) surface and the pristine Cu2O(100) surface, the calculation results show that formation energy of the partially reduced Cu2O(100) is 0.51 eV, which is slightly smaller than 0.54 eV of the pristine Cu2O(100) surface. Additionally, we find that the redution of pristine Cu2O(100) surface to the partially reduced Cu2O(100) surface is an exothremic process thermodynamically (0.54eV/reduced oxygen atom). In general, the partially reduced Cu2O(100) model in this work is stable in both thermodynamics and dynamics, and our model is representative in studying the CO2 electroreduction to CH3OH on Cu2O. For the detailed dynamical process of CO2 electroreduction on the partially redeced Cu2O, more complex and influential model should be considered in future work. 3.2. Solid-liquid interface on partially reduced Cu2O(100) and Cu(111). To evaluate the role of solvation in CO2 electroreduction, we first consider the atomic structures of water on both Cu2O(100) and Cu(111). The solid-liquid models on Cu2O(100) and Cu(111) are built by 7 ACS Paragon Plus Environment


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incorporating two water molecules on both surfaces (Figure 1c and Figure 1d). The models used in our work are similar with that in the previous study by Nie et al. where the role of kinetics on selectivity of CO2 electroreduction and mechanism of CO2 electroreduction on Cu(111) were effectively investigated.43,44 3.3. Mechanism of methanol formation on Cu2O(100) in the gas-phase model. It has been now widely recognized that CO* (* means an adsorption state) is the key intermediate in CO2 electroreduction which can be further reduced to various hydrocarbon products. In addition, the elcetroreduction of CO has also shown the analogous distribution of hydrocarbon products to the electroreduction of CO2,9,45,46 therefore, we adopt CO* instead of CO2 to study the mechanism of CO2 electroreduction on both Cu2O(100) and Cu(111). Moreover, we have investigated the process of CO2 electroreduction without/with water to explore the role of solvation. A series of possible reaction pathways to CH3OH and the corresponding energetics have been calculated to understand the selectivity and efficiency of CO2 electroreduction to methanol on Cu2O(100). The minimum energy pathway of CO2 electroreduction to CH3OH at 0 V vs. RHE on Cu2O(100) in the gas-phase model is shown in Figure 2a. CO* preferentially adsorbs on the top site of Cu atom. The initial hydrogenation step is to add one proton-electron pair to CO*, leading to the formation of CHO*, which adsorbs on surface via carbon binding to the bridge site and oxygen binding to the adjacent Cu atom. The reaction energy (Er) and formation barrier (Ea) of this step are Er=0.06 eV and Ea=0.36 eV, respectively. The competitive reaction of this protonelectron transfer process is the formation of COH* with Er=0.90 eV and Ea=1.20 eV, highenergy barrier indicates that the protonation of oxygen in CO* is hindered, therefore it is more favorable to form CHO* both thermodynamically and kinetically. The next hydrogenation process of CHO* results in the formation of CH2O* with Er=-0.29 eV and Ea=0.30 eV. The 8 ACS Paragon Plus Environment

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alternative formation of CHOH* in this protonation process is endothermic with 0.24 eV, suggesting that the reduction of CHO* to CH2O* is more favorable thermodynamically. CH2O* can be further hydrogenated to CH3O* with Er=-1.21 eV and Ea=0.30 eV, in which CH3O* adsorbs via oxygen binding to the bridge site and methyl group orienting away from surface. While the alternative hydrogenation to form CH2OH* is also exothermic by -0.35 eV, the reaction energy of CH2O* to CH3O* is much more exothermic than that of CH2O* to CH2OH*. Accordingly, the protonation step of CH2O* to CH3O* is more favorable thermodynamically. Further hydrogenation to CH3O* is a key point which determines the formation of CH3OH by adding the hydrogen to oxygen, or the formation of CH4 by breaking the carbon-oxygen bond (followed by a hydrogenation step). Our calculations in the gas-phase model demonstrate that formation of CH4 is more thermodynamically favorable by 0.21 eV than that of CH3OH on Cu2O(100) surface, which conflicts with the available experimental results,30,31 suggesting the important role of solid-liquid model in CO2 electroreduction. (see Figure 2a and Table S1). To deeply study the mechanism of CO2 electroreduction on Cu2O(100) surface, we calculate the charge distribution of Cu2O (100) surface and the corresponding adsorption energies of various intermediates (see Figure 3 and Table S6). For instance, in the protonation process of CO* to COH*, the charge on Cu2O(100) surface changes from +1.45e in CO* system to +1.67e in COH* system. Comparably, in another protonation process of CO* to CHO*, the surface charge of Cu2O(100) changes from +1.45e to +1.88e, indicating that Cu2O(100) surface donates more charge in this pathway than that in the hydrogenation of CO* to COH*. Accordingly, CHO* is more favorable to form than COH* in the protonation of CO*, implying that the more charge the intermediate can accept, the higher selectivity of this intermediate is. For the second hydrogenation step, the charge on surface with CH2O* is +2.01e, larger than that on surface with 9 ACS Paragon Plus Environment


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CHOH* +1.71e, which also correspondes to the fact that CH2O* is more favorable to form than CHOH* in the protonation of CHO*. In addition, the adsorption energy of CHO* (-0.86 eV) is higher than that of COH* (-0.02 eV), similarly, the adsorption energy of CH2O* is -1.14 eV, also higher than that of CHOH* with -0.62 eV (see Table S6). This conclusion also holds in the following protonation steps. It is known that the chemical adsorption of intermediate depends on the electron transfer between substrate and intermediate, which corresponds to the variation of charge on the substrate. The more electron transfer during the process of chemical adsorption, the stronger chemical bonds between intermediate and substrate will be, suggesting that the intermediate is more stable on the substrate. Since the minimum energy pathway of CO2 electroreduction to CH3OH discussed above is based on the thermodynamic results that corresponds to the stability of intermediates. Therefore, the variation trend of charge on Cu2O(100) surface primely accounts for the minimum energy pathway of CO2 electroreduction, which could act as a descriptor for understanding the mechanism of CO2 electroreduction. 3.4. Mechanism of methanol formation on Cu2O(100) in the solvation model. Figure 2b shows the minimum energy pathway to CH3OH at 0 V vs. RHE on Cu2O(100) in the solvation model. The electroreduction of CO2 exhibits a different reaction pathway to CH3OH in the solvation condition. For the first protonation of CO*, comparing to the high energetics in the reduction of CO* to COH* (Er=1.13 eV and Ea=1.43 eV), the formation of CHO* is much more favorable with Er=-0.26 eV and Ea=0.30 eV, in which the carbon bonds to the bridge site with oxygen pointing to the hydrogen of H2O. Obviously, the effect of solvation changes the adsorption properties of intermediates and accelerates the formation of CHO* from CO* thermodynamically (Er=0.06 eV and Ea=0.36 eV in gas-phase model). The second protonation step leads to the formation of CH2OH*-OH* (Er=-0.45 eV and Ea=0.30 eV), in which one 10 ACS Paragon Plus Environment

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hydrogen in H2O is captured to form the hydroxyl of CH2OH*, leaving the residual OH* forming hydrogen bond with CH2OH*. It demonstrates clearly that H2O molecules not only change the adsorption properties of intermediates, but also take part in the process of CO2 electroreduction. The alternative protonation process of CHO* to CHOH* is also significantly refrained in the solvation condition (Er=1.42 eV and Ea=1.72 eV). Further protonation of carbon in CH2OH*OH* leads to formation of CH3OH*-OH* with Er=-1.09 eV and Ea=0.30 eV. One of the competitive reaction, the protonation of OH*, is found to be less favorable thermodynamically (Er=-0.29 eV and Ea=0.30 eV). In addition, another alternative formation of CH2* with hydroxyl in CH2OH* protonated to H2O is also excluded with Er=0.21 eV and Ea=0.51 eV. Then the final protonation of OH* results in the formation of H2O, which is the rate-limiting step in the formation of CH3OH. In general, the solvation modifies the protonation of intermediates on Cu2O(100) surface and leads to the formation of CH3OH without CH4, which further implies that the effect of solvation is crucial for CO2 electroreduction. Our results are in good agreement with the previous experimental observations that CO2 electroreduction mainly produces methanol instead of methane on Cu2O surface.30,31 We also analyze the charge distribution of Cu2O(100) surface and the corresponding adsorption energies of various intermediates in the solvation model, while the distribution of charge is different from the case in the gas-phase model, the variation trend of charge still reflects the minimum energy pathway to CH3OH (see Figure 3 and Table S6). For example, in the protonation of CO*, the resulting surface with CHO* has a more positive charge of +1.97e than the surface with COH* +1.58e, and CHO* is more readily formed than COH*. To verify the availability of two-water solvation model in describing the mechanism of methanol formation on Cu2O(100), we build the water bilayer to simulate solvation model, the 11 ACS Paragon Plus Environment


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minimum energy pathway of CO2 electroreduction and charge distribution on Cu2O(100) are shown in Figure 4 and Figure S2, respectively. Generally, CO2 electroreduction on Cu2O(100) with the water-bilayer solvation model generates the same minimum energy pathway to methanol as the case in two-water solvation model. For example, the hydrogenation of CO* still leads to the formation of CHO*, in spite of slightly higher reaction energy than that in two-water solvation model. In addition, the hydrogenation of CHO* still leads to the formation of CH2OH*-OH*, during which the hydrogen in hydroxyl of CH2OH* is obtained from the water bilayer (see Figure 4). It is consistent with the result in the two-water model that one hydrogen in H2O is captured to form the hydroxyl of CH2OH*. Moreover, as can be seen in Figure S2, the variation trend of charge distribution on Cu2O(100) with the water-bilayer solvation model still reflects the minimum energy pathway CO2 electroreduction to CH3OH. For instance, the Cu2O(100) surface is charged +1.34e with CHO* compared to +0.96e with COH*, while CHO* is more favorable to form than COH* in the water-bilayer solvation model, which is in accordance with the result in two-water solvation model. In general, our results has shown that the two-water solvation model is sufficient to describe the mechanism of methanol formation on Cu2O(100). 3.5. Mechanism of methane formation on Cu(111) without/with H2O. To better comprehend the unique ability of Cu2O in catalyzing CO2 to CH3OH, we study the mechanism of CO2 electroreduction and the corresponding adsorption energies of various intermediates on Cu(111) and the charge distribution on Cu(111) surface at 0 V vs. RHE (see Figure S3, Figure S4 and Table S7). In the gas-phase model, CO2 electroreduction tracks the same minimum energy pathway to CH4 as that on Cu2O(100): CO*→CHO*→CH2O*→CH3O*→CH4, however, with a higher barrier. In the first protonation process of CO*, the formation of CHO* is more


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favorable than the formation of COH*, meanwhile charge of Cu(111) surface with CHO* is 0.60e, which is more positive than that of COH* -0.68e. In addition, the adsorption energy -0.26 eV is higher than that 0.08 eV of COH* on Cu(111), suggesting the more adsorption preference of CHO*. This trend also holds in the following steps: the more charge donating from Cu(111) surface to intermediates, the more stabel and active intermediates are. These results demonstrate that the rule of surface charge distribution in describing CO2 electroreduction on Cu2O(100) surface is also appropriate for the pathways on Cu (111) without H2O (more details in Figure S4). In the solvation condition, CO2 electroreduction on Cu(111) also exhibits the same minimum energy pathway as the gas-phase model by forming CH4, which is different from the formation of CH3OH on Cu2O(100) in the solvation condition, corresponding to the unique catalytic ability of Cu2O(100). Although the charge distribution on Cu(111) is influenced by the solvation, it is still capable of accounting for the role in describing the adsorption preference of intermediates and the competitive pathways of CO2 electroreduction on Cu(111) surface (see more details in Figure S4 and Table S7). Our calculations demonstrate that CO2 electroreduction on Cu(111) leads to the formation of methane, which is consistent with the experimental result.11 As discussed above, CO2 electroreduction to methane on Cu(111) always favors the CHO* pathway in both gas-phase and two-water solvation models, which are in accordance with the previous results of CO2 electroreduction on Cu(211) in gas-phase model.13 However, for CO2 electroreduction in the water-bilayer solvation model on Cu(111), the minimum energy pathway to methane has been changed and the COH* pathway is more favored,47,48 as we found in ref 16. These results indicate that the two-water solvation model is closer to the gas-phase model on Cu(111), which is different from that on Cu2O(100). Although the different gas-phase and solvation models on Cu(111) predict the different active intermediates for CO2 electroreduction, 13 ACS Paragon Plus Environment


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methane is always predicted as the more favorable product compared to methanol. Therefore, our models are reasonable to capture the difference of CO2 electroreduction on Cu(111) and Cu2O(100). In addition, we expect that the variation trend of charge in the water-bilayer model on Cu(111) should still be applicable to account for the minimum energy pathway of CO2 electroreduction, if the predicted mechanism of methane formation is mainly based on the thermodynamic results. 3.6. Potential dependence of CO2 electroreduction on Cu2O(100). We now analyze the effect of applied overpotential on the mechanism of CO2 electroreduction on Cu2O(100). It has been experimentally observed by Le et al. that the Faradaic efficiency of CO2 electroreduction to CH3OH depends strongly on the applied overpotential.31 When the overpotential is at 0.0 V vs. RHE, the rate-limiting step of CO2 electroreduction is the hydrogenation of CH3OH*-OH* to CH3OH* and H2O*, with Er=0.23 eV and Ea=0.53 eV. This energetics indicates that it is unfavorable to form methanol, which is in agreement with the experimental result that no methanol is detected at 0.0 V vs. RHE. Once the overpotential is raised to -0.23 V vs. RHE, the rate-limiting step of CH3OH*-OH* to CH3OH* and water on Cu2O(100) becomes exothermic, suggesting that the formation of methanol is feasible. It is in accordance with experimental result that the Faradaic efficiency of CH3OH formation is observed in CO2 electroreduction on Cu2O at the relative low overpotential.29,31 Since the following decreased Faradaic efficiency of CH3OH formation is observed at the more negative overpotential in experiments, we attribute this decay to that most of Cu2O is finally reduced to metallic copper, on which no methanol is detected experimentally.8,9,45,46 Nevertheless, by consulting the results shown in the Pourbaix diagrams of Cu-H2O system, we find that Cu2O tends to be stable under the applied overpotential smaller than -0.23 eV and PH > 9,49 this indicates that our proposed partially reduced Cu2O(100) surface 14 ACS Paragon Plus Environment

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model is validity enough for describing the CO2 electroreduction to CH3OH on Cu2O at low overpotential. In contrast, the rate-limiting step of the hydrogenation of CO* to CHO* (Ea=0.80 eV) on Cu(111) is still endothermic at -0.23 V vs. RHE (Figure S3b), where the formation of CHO* is still impeded, suggesting that Cu2O(100) shows the higher activity to promote the hydrogenation of CO* to CHO*. 3.7. Mechanism difference of CO2 electroreduction between Cu2O(100) and Cu(111) in the gas-phsae/solvation model. We now focus on comparing the differences of the mechanism of CO2 electroreduction between Cu2O(100) and Cu(111). As shown in Figure 2 and Figure S3, the electroreduction of CO2 goes through the same minimum energy pathway to CH4 on both Cu2O(100) and Cu(111) in the gas-phase model. However, in the solvation model, CO2 electroreduction on Cu2O(100) and Cu(111) lead to the different minimum energy pathways to CH3OH and CH4, respectively. It indicates that the synergic effect of the atomic structure of catalysts and the solvation play the crucial role in determining the selectivity of product on Cu2O(100) and Cu(111). Cu(111) surface consists of the close-packed coordinatively unsaturated copper atoms, whereas the partially reduced Cu2O(100) surface is composed of the loose copper chains (see Figure 1). During the CO2 electroreduction in the gas-phase model, CO* intermediate prefers to adsorb on the top site of copper on Cu2O(100), whereas the most stable adsorption structure of CO* on Cu(111) is on the hollow site of copper. After the hydrogenation of CO*, CHO* prefers the similar adsorption structure on both Cu2O(100) and Cu(111) surfaces with carbon atom on the bridge site and oxygen bonding to the adjacent copper (see Figure 2a and Figure S3a). It is most likely the reason why the reaction energy of the hydrogenation of CO* to CHO* on Cu2O(100) is much smaller than that on Cu(111) (see Table S1 and Table S4). CH2O* prefers to 15 ACS Paragon Plus Environment


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adsorb on the Cu2O(100) with both carbon and oxygen atoms on bridge site, which is similar with the adsorption structure of CH2O* on Cu(111). Likewise, CH3O* prefers to adsorb on the bridge site of Cu2O(100), but on the hollow site of Cu(111), and O* prefers the hollow on both surfaces, which is most likely to result in different energetics that the reaction energy from CH3O* to CH4* on Cu2O(100) is larger than that on Cu(111). Therefore, it is different atomic configuration of Cu2O(100) and Cu(111) that makes different adsorption configuration of intermediates, resulting in the different reaction energy and reaction barrier. On the other hand, during the CO2 electroreduction in the solvation model, the additional effect of solvation not only further influences adsorption configurations of intermediates but also changes the adsorption preference of the possible intermediates via hydrogen bonds participating in CO2 electroreduction, which leads to the formation of different intermediates on both Cu2O(100) and Cu(111) surfaces and thereby makes CO2 electroreduction go through different pathways to methanol and methane, respectively, on Cu2O(100) and Cu(111). For instance, in the solvation condition, CO* prefers the bridge site with oxygen atom forming hydrogen bond with H2O molecule on Cu2O(100), which is different from the top site of CO* on Cu2O(100) in the gas-phase model, and the hydrogenation of CO* to CHO* on Cu2O(100) also becomes more favorable compared with that in the gas-phase model. As can be seen in Table S6, the adsorption strength of CHO* in the solvation model becomes stronger than that in the gas-phase model, however, the adsorption strength of COH* becomes weaker than that in the gas-phase model, suggesting that the effect of solvation adjusts the adsorption preference of intermediates during the CO2 electroreduction compared to the gas-phase model. More importantly, the hydrogenation of CHO* leads to the formation of CH2OH*-OH*, and the hydrogen in the CH2OH* is derived from the hydrogen in solvation, indicating that the solvation has also participated in the CO2 16 ACS Paragon Plus Environment

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electroreduction, which is absence in the gas-phase model, and the following hydrogenation steps of CH2OH*-OH* lead to the final product of CH3OH. As can be seen in Figure S3, for the CO2 electroreduction on Cu(111) in the solvation condition, the preferred adsorption sites of intermediates on Cu(111) are almost unchanged, which is most likely the reason why CO2 electroreduction on Cu(111) processes the same minimum energy pathway to CH4 as the case in the gas-phase model, suggesting the different effect of solvation on Cu(111) (compared to Cu2O(100)) in CO2 electroreduction. However, the adsorption strength of various intermediates has also been improved by the effect of solvation. For instance, in the hydrogenation of CO* to CHO* and COH*, the adsorption energy of CHO* is improved by -0.11 eV compared to the gasphase model, similarly, the adsorption strength of other intermediates in the following hydrogenation step has also been improved in the solvation model (more details in Table S7). To further elucidate the selectivity and efficiency to methanol in CO2 electroreduction on Cu2O(100), the charge distribution on catalyst surfaces in both gas-phase and solvation models are investigated in details. For the pristine catalyst surfaces without adsorbed intermediates, our calculations reveal that the type of charge on Cu2O(100) is positive, on the contrary, Cu(111) is negatively charged, which is originated from the different atomic structures of Cu2O(100) and Cu(111) surfaces (see Figure 3 and Figure S4). Once the solid-liquid model is taken into account, charge of both Cu2O(100) and Cu(111) surfaces is changed due to the effect of solvation, however, the intrinsic sort of charge on both surfaces remains unchanged. More importantly, as discussed above, the variation trend of charge distribution can be successfully applied as a descriptor to describe the minimum energy pathway of CO2 electroreduction on both Cu2O(100) and Cu(111) surfaces. In addition, we find that Cu2O(100) has a stronger ability to donate charge than Cu(111) in the hydrogenation of CO* to the key intermediate CHO*. In the gas-phase



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model, the changes of charge are 0.43e in the hydrogenation of CO* to CHO* on Cu2O(100) surface, which is larger than that of 0.14e on Cu(111) surface, the same as the case in the liquidphase model. This stronger ability of Cu2O(100) to donate charge can also be certified by the fact that CO* has a stronger adsorption strength on Cu2O(100) than that on Cu(111) in both gasphase and solvation models (see Table S6 and Table S7). We also calculate the work functions of clean Cu2O(100) and Cu(111) surfaces to explore the reason why Cu2O(100) has the greater charge donation ability than Cu(111) (see Table 1). In both gas-phase and liquid-phase models, the work functions of Cu2O(100) are always smaller than that of Cu(111). Since the work function is the minimal thermodynamic energy the electron needs to leave from the catalyst surface, it indicates that charge transfer from Cu2O(100) to the intermediate is more readily than that on Cu(111).

4. Conclusions In conclusion, our DFT calculations provide interesting insights into the mechanism of CO2 electroreduction on partially reduced Cu2O(100) and Cu(111) surfaces. We find that the selectivity and efficiency of CO2 electroreduction to CH3OH on Cu2O(100) is determined by the synergy of the structure of partially reduced Cu2O surface and solvation. In addition, water is found to participate in the CO2 electroreduction on Cu2O(100) directly except providing the solvation effect. More importantly, our results show that the variation trend of charge distribution on both Cu2O(100) and Cu(111) surfaces account for the minimum energy pathway of the CO2 electroreduction, which could act as a descriptor for understanding the mechanism of CO2 electroreduction and providing useful guild lines for the design of high performance catalysts. 18 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information. Ab initio molecular dynamic simulation for the partially reduced Cu2O(100); Charge distribution of Cu2O(100) surface; minimum energy pathway and charge distribution of Cu(111) surface without/with H2O; reaction energies and formation barriers of the possible reactions for the formation of CH3OH and CH4 on the reduced Cu2O(100) and Cu(111) without/with H2O, respectively. AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors acknowledge the Program of Thousand Young Talents Plan, the National Natural Science Foundation of China (No. 21673095, 51631004), the Program of Innovative Research Team (in Science and Technology) in University of Jilin Province, and the computing resources of High Performance Computing Center of Jilin University, and National Supercomputing Center in Jinan and in Tianjin China for support.



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Table 1. The calculated work functions (Φ) of clean Cu2O(100) and Cu(111) in the gas-phase model and liquid-phase model. Φ (eV)

gas

liquid

Cu2O(100)

4.18

3.83

Cu(111)

4.64

4.27


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Figure 1. Structures of Cu2O(100) (a) before and (b) after the first layer of Cu2O is reduced. Configurations of solid-liquid interfaces on (c) Cu(111) and (d) the partially reduced Cu2O(100). The solid white lines represent (a) Cu dimers, (b) zigzag arrangement of Cu chains.



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Figure 2. Intermediate configurations for the minimum energy pathway to CH3OH and the corresponding energy diagram on the reduced Cu2O(100) (a) without H2O and (b) with H2O.


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Figure 3. Charge distribution of the reduced Cu2O(100) without/with H2O when adsorbing different intermediates.



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Figure 4. Intermediate configurations for the minimum energy pathway to CH3OH and the corresponding energy diagram on Cu2O(100) with H2O bilayer.


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