Tuning the Selectivity of Carbon Dioxide Electroreduction toward


Nov 9, 2016 - The electrochemical reduction of carbon dioxide (CO2) to ethanol (C2H5OH) and ethylene (C2H4) using renewable electricity is a viable me...
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Research Article pubs.acs.org/acscatalysis

Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived CuxZn Catalysts Dan Ren,†,‡ Bridget Su-Hui Ang,† and Boon Siang Yeo*,†,‡ †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 The Singapore-Berkeley Research Initiative for Sustainable Energy, CREATE Tower 1, Create Way #11-00, Singapore 138602



S Supporting Information *

ABSTRACT: The electrochemical reduction of carbon dioxide (CO2) to ethanol (C2H5OH) and ethylene (C2H4) using renewable electricity is a viable method for the production of these commercially vital chemicals. Copper (Cu) and its oxides are by far the most effective electrocatalysts for this purpose. However, the formation of ethanol using these catalysts is generally less favored in comparison to that of ethylene. In this work, we demonstrate that the selectivity of CO2 reduction toward ethanol could be tuned by introducing a cocatalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amounts of Zn dopants (Cu, Cu10Zn, Cu4Zn, and Cu2Zn) were prepared and used as catalysts under ambient pressure in aqueous 0.1 M KHCO3 electrolyte. By varying the amount of Zn in the bimetallic catalysts, we found that the selectivity of ethanol versus ethylene production, defined by the ratio of their Faradaic efficiencies (FEethanol/FEethylene), could be tuned by a factor of up to ∼12.5. Ethanol formation was maximized on Cu4Zn at −1.05 V vs RHE, with a remarkable Faradaic efficiency and current density of 29.1% and −8.2 mA/cm2, respectively. The Cu4Zn catalyst was also catalytically stable for the production of ethanol for at least 5 h. The importance of Zn as a CO-producing site was demonstrated by performing CO2 reduction on Cu−Ni and Cu−Ag bimetallic catalysts. Operando Raman spectroscopy revealed that the as-deposited Cu-based oxide films were reduced to the metallic state during CO2 reduction, after which only signals belonging to CO adsorbed on Cu sites were recorded. This showed that the reduction of CO2 probably occurred on metallic sites rather than on metal oxides. A two-site mechanism to rationalize the selective reduction of CO2 to ethanol is proposed and discussed. KEYWORDS: CO2 reduction, copper−zinc, ethanol, ethylene, operando Raman spectroscopy

1. INTRODUCTION The electrochemical reduction of carbon dioxide (CO2) using solar electricity has the potential of generating useful chemicals and liquid fuels sustainably while maintaining carbon neutrality.1,2 Among all the carbonaceous products formed, ethanol (C2H5OH) and ethylene (C2H4) are two C2 molecules of great interest. Ethanol can be either used directly as a fuel or blended with gasoline to give an overall cleaner-burning fuel.3 Ethylene is a key starting material for the manufacture of polymers such as polyethylene.4 Among all the metal electrodes investigated, copper is by far the most promising electrocatalyst for reducing CO2 to ethanol and ethylene:5,6

C2 molecules, principally by modifying the chemical compositions and physical structures of the Cu catalysts.9−23 From this endeavor, it was discovered that Cu surfaces with high degrees of roughness were efficacious in giving high yields of C2 products (Table 1). For example, CO2 could be reduced on Cu2O-derived Cu films to ethanol and ethylene with Faradaic efficiencies (FE) up to 16% and 40% and partial current densities up to −5.7 and −12.0 mA/cm2, respectively.14 Interestingly, we observe here that while these two C2 compounds are usually formed in tandem across a wide range of potentials, ethanol is generally less favored. As summarized in Table 1, the FEethanol/FEethylene ratios (a measure of reaction selectivity) range from 0.02 to 0.86 for different types of Cu-based catalysts.7−9,14,18−20,22 This selectivity has been explained using density functional theory calculations.24 After C−C coupling of two adsorbed C1 intermediates such as *CHxO (x = 0−2) on the Cu surfaces, the resultant C2 species could be reduced to either ethylene or ethanol.24,25 CH2CHOads, a key intermediate proposed in the process, was found to be reduced to ethanol on a Cu(100) surface with a 0.2 eV

2CO2 + 9H 2O + 12e− → C2H5OH + 12OH− E° = 0.09 V vs RHE 2CO2 + 8H 2O + 12e− → C2H4 + 12OH− E° = 0.08 V vs RHE

However, copper is generally unselective and other less valuable products such as methane (CH4) are also simultaneously produced with these C2 compounds.7,8 Considerable effort has therefore been invested in optimizing the conversion of CO2 to © XXXX American Chemical Society

Received: July 30, 2016 Revised: October 3, 2016

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Table 1. Overview of the Performances of Cu-Based Catalysts for the Electroreduction of CO2 to Ethanol and Ethylenea Faradaic efficiency (%)

a

catalyst

conditions

ethanol

ethylene

FEethanol/FEethylene

jethanol (mA/cm2)

polycrystalline Cu7 polycrystalline Cu7 polycrystalline Cu8 Cu (100)9 Cu (110)9 Cu (111)9 Cu (310)9 CuBr10 3.6 μm Cu2O on Cu14 Cu nanoparticles (on GDEb)18 4 mM KF cycled Cu foil19 44 nm Cu nanocubes20 Cu2O-derived particles22

0.1 M KHCO3, −5 mA/cm2 0.1 M KCl, −5 mA/cm2 0.1 M KHCO3, −1.05 V vs RHE 0.1 M KHCO3, −5 mA/cm2 0.1 M KHCO3, −5 mA/cm2 0.1 M KHCO3, −5 mA/cm2 0.1 M KHCO3, −5 mA/cm2 3 M KBr, −2.4 V vs Ag/AgCl 0.1 M KHCO3, −0.99 V vs RHE 1 M KOH, −0.80 V vs RHE 0.1 M KHCO3, −1 V vs RHE 0.1 M KHCO3, −1.1 V vs RHE 0.1 M KHCO3, −31.2 mA/cm2

6.9 21.9 9.75 9.7 10.5 2.6 29.9 1.6 16.37 17 7.9 3.7 11.8

30.1 47.8 25.98 40.4 13.5 8.3 34.6 79.5 34.26 45 16.3 41.1 42.6

0.23 0.46 0.38 0.24 0.78 0.31 0.86 0.02 0.48 0.38 0.48 0.09 0.28

−0.3 −1.1 −0.6 −0.5 −0.5 −0.1 −1.5 −0.7 −5.7 −52 −0.5 −0.1 −3.7

Only catalysts with complete FE and current density data for both ethanol and ethylene are given here. bGDE: gas diffusion electrode.

The oxide films Cu, CuxZn, CuxAg, and CuxNi were galvanostatically deposited onto polished Cu disks (exposed geometric surface area 0.865 cm2, 99.99%, Goodfellow). Zinc oxide films were prepared similarly, except that they were deposited onto Zn disks (99.99%, Goodfellow). The electrodepositions were performed using a potentiostat (Autolab PGSTAT30) with applied currents of −0.92 mA/cm2 for 600 s. The electrolytes were kept at 60 °C and stirred at 300 rpm during the deposition process. 2.2. Characterization of Catalysts. The surface morphology of the catalysts was characterized by scanning electron microscopy (SEM, JEOL JSM-6710F, 5 kV, 10 μA). The catalyst films after reduction were also removed from the substrate and suspended in ethanol (99.97%, VWR). These suspensions were sonicated for 2 min and drop-coated onto nickel grids coated with lacey carbon (LC325Ni, 300 mesh, Electron Microscopy Sciences). The particles therein were characterized by transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), and selected area electron diffraction (SAED, JEOL 3010, 300 kV, 112 μA). The crystalline structures of the catalyst films were measured by X-ray diffraction (XRD, Bruker D8 Advance). X-ray photoelectron spectroscopy (XPS) was performed with a Kratos AXIS UltraDLD instrument (Kratos Analytical Ltd.) using Al Kα radiation (5 mA, 15 kV) with the chamber pressure at 5 × 10−9 Torr. Ex situ and operando Raman spectroscopy was employed to characterize the catalysts.29 2.3. Electrochemical Reduction of CO2. Aqueous 0.1 M KHCO3 (99.7%, Merck) was used as the electrolyte. The cathodic and anodic compartments of the cell were separated by an anion exchange membrane (Selemion AMV, AGC Asahi Glass). Both compartments were infused with a continuous flow of CO2 (99.999% Linde Gas) at 20 cm3/min. An Ag/AgCl electrode (saturated KCl, Pine) and Pt wire were used respectively as the reference and counter electrodes. Chronoamperometry and chronopotentiometry were controlled by a potentiostat (Gamry Reference 600). The iR drop was compensated via the current interrupt mode. All potentials cited in this work were referenced to the RHE, unless stated otherwise. The current densities reported were normalized against the exposed geometric surface area (0.865 cm3) of the working electrode. The gas and liquid products were analyzed respectively by gas chromatography (Agilent 7890A) and 1H nuclear magnetic resonance spectroscopy (Avance Bruker 500).30

higher energy barrier in comparison to that of ethylene.24 In order to significantly increase the yield of ethanol relative to that of ethylene, we believe that a new strategy to tune the selectivity of CO2 electroreduction is required. While CO2 could not be electroreduced to ethanol selectively on Cu catalysts, CO reacted quite differently. Li et al. reported that carbon monoxide (CO) could be reduced to ethanol with a FE of up to 42.9% on Cu nanoparticles in aqueous 0.1 M KOH.26 Grain boundaries within the catalysts were thought to promote the process by stabilizing the reaction intermediates.27 While the high FE of ethanol is meritorious, its maximum partial current density was only −0.39 mA/cm2. This is likely due to mass transport limitations imposed by the poor solubility of CO in the aqueous electrolyte (∼1 mM).26 Here, it is also noteworthy that the mechanistic role of CO in the selective formation of ethanol has not been fully elucidated. On the basis of the above discussion, a viable strategy to make up for the shortfall in CO solubility is to introduce a cocatalyst to catalyze CO formation in situ during CO2 reduction. Previous studies have shown that nanostructured zinc particles were selective for CO formation.28 With the objective of enhancing the selectivity and efficiency of CO2 reduction toward ethanol, we have prepared oxide-derived Cu-based catalysts with different quantities of Zn dopants (Cu10Zn, Cu4Zn, and Cu2Zn). The physical, chemical, surface, and electrocatalytic properties of these catalysts were characterized by a combination of transmission and scanning electron microscopy (TEM and SEM), energy dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), operando Raman spectroscopy, chronoamperometry, online gas chromatography (GC), and 1H nuclear magnetic resonance spectroscopy (NMR). We shall demonstrate that the selective reduction of CO2 to ethanol can be tuned by varying the concentration of Zn in the CuxZn catalysts. Supported also by control experiments using Cu−Ni and Cu−Ag catalysts, we propose a two-site mechanism to rationalize the efficient reduction of CO2 to ethanol on the CuxZn catalysts.

2. EXPERIMENTAL SECTION Detailed experimental procedures are presented in the Supporting Information. 2.1. Preparation of Catalysts. Ultrapure water (18.2 MΩ cm, Type 1, Barnstead) was used for washing and preparing solutions. 8240

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Figure 1. Electron microscopic characterization of the catalysts (A, F, K) Cu, (B, G, L) Zn, (C, H, M) Cu10Zn, (D, I, N) Cu4Zn, and (E, J, O) Cu2Zn: (A−E) SEM images of the catalysts before reduction; (F−J) SEM images of the catalysts after reduction at −1.05 V; (K−O) SAED patterns (measured during TEM) of representative particles after reduction at −1.05 V. Scale bars: 1 μm for A−J and 5 nm−1 for K−O.

3. RESULTS AND DISCUSSION 3.1. Characterization of the Catalysts. Three bimetallic CuxZn oxides with various quantities of Zn dopant, as well as monometallic Cu oxide and Zn oxide, were prepared by electrodeposition (Table S1 in the Supporting Information). The bulk elemental compositions of CuxZn films were analyzed by EDX (during TEM) and had Cu−Zn compositions of Cu10Zn, Cu4Zn, and Cu2Zn (Figure S1 and Table S2 in the Supporting Information). The morphologies and crystalline properties of these catalysts were also characterized using SEM and SAED (Figure 1). The as-deposited Cu oxides were in the form of smooth polyhedron particles of sizes 100 nm to 1 μm (Figure 1A),12,14 while the Zn oxide films were interconnected platelets with lengths of hundreds of nanometers (Figure 1B).31 The CuxZn oxide catalysts showed spherical particles that were hundreds of nanometers in sizes (Figure 1C−E). The appearance of all the films changed after they were used as catalysts for 60 min CO2 reduction in 0.1 M KHCO3 electrolyte (at a representative potential of −1.05 V; Figure 1F−J). The polyhedral Cu particles became rougher with the appearance of 10−50 nm nanoparticles decorated on their surfaces (Figure 1F), while the Zn catalyst transformed to agglomerates of particles with sizes from 50 to 500 nm (Figure 1G). The CuxZn films also exhibited rougher morphologies after reduction (Figure 1H−J). These morphological evolutions could be attributed to the dissolution/deposition of the metal ions22,32 and/or relief of structural strains33 during the reduction process. The SAED patterns (during TEM) of the Cu, Zn, and mixed CuxZn catalysts (postreduction) showed respectively Cu0, Zn0, and Cu0 + Zn0 crystallites (Figure 1K−O). The presence of distinct Cu0 and Zn0

crystallites in the CuxZn catalysts indicated the phase segregation of Cu and Zn, rather than the formation of alloys. This observation is also in line with the X-ray diffractograms of the catalysts (postreduction), which did not exhibit any signal that could be assigned to the Cu−Zn alloy (Figure S2 in the Supporting Information). The surface Cu to Zn elemental ratios of the Cu10Zn, Cu4Zn, and Cu2Zn catalysts (postreduction) were measured by XPS to be 15.9, 2.5, and 1.9 (Figure S3 in the Supporting Information). The electrochemically active surface areas of all the catalysts (postreduction) were also estimated from their double-layer capacitances. The CuxZn films had approximately 2−6 times larger electrochemical surface areas in comparison to the Cu or Zn films (Table S3 in the Supporting Information). 3.2. Operando Raman Spectroscopy of the Catalysts during CO2 Reduction. The chemical identities of the asdeposited films and their transformations during CO2 electroreduction were elucidated by Raman spectroscopy (Figure 2). The assignments of the observed signals together with the relevant reference values are summarized in Table S4 in the Supporting Information. Freshly deposited Cu and Zn catalysts showed Raman peaks belonging to Cu2O (142, 216, 525, 630 cm−1)34 and ZnO (430, 560 cm−1),35 respectively, while the three CuxZn films showed both Cu2O and ZnO peaks (Figure 2A). Operando Raman spectroscopy was then performed on the oxide catalysts in CO2-saturated 0.1 M KHCO3 electrolyte (with continuous infusion of CO2), while they were being subjected to a constant potential of −0.85 V. The Raman peaks of the Cu2O film disappeared after 50 s, which indicated its reduction (Figure 2B). This was followed by the appearance of peaks at 8241

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Figure 2. (A) Raman spectroscopy of the as-deposited catalysts. (B−D) Operando Raman spectra of Cu, Zn, and Cu4Zn catalysts, respectively, during electrochemical reduction of CO2 at −0.85 V in 0.1 M KHCO3 (inserts show the simultaneously acquired chronoamperograms).

280, 365, and 2060 cm−1 from 150 s onward. On the basis of previous operando Raman spectroscopy studies of CO2 electroreduction on Cu surfaces and gas-phase CO adsorption on Cu, the signals at 280, 365, and 2060 cm−1 could be respectively assigned to the restricted rotation of adsorbed *CO, Cu−CO stretching, and CO stretching modes.36,37 CO is a known intermediate of CO2 reduction on Cu electrodes.38,39 The presence of other reaction intermediates such as *CH2 or *CHO is putative, although we could not observe them.25,40 This is presumably due to the insufficient dynamic range of our Raman spectrometer and/or the short lifetime of these intermediates. The ZnO behaved analogously to the Cu2O film and showed a featureless spectrum after 30 s reduction (Figure 2C). This could be interpreted as the reduction of the Zn oxides to the Zn0 state. Interestingly, no CO-related vibrations were observed, even though Zn is an excellent catalyst for reducing CO2 to CO.5,28 This could be attributed to the weak adsorption of CO on Zn surfaces.41 The reductions of both Cu and Zn oxide films to their respective Cu0 and Zn0 states at −0.85 V were in accordance with their EH−pH diagrams.42,43 The operando Raman spectra of a Cu4Zn oxide film during its reduction at −0.85 V is presented in Figure 2D. Within 20 s from the application of the reduction potential, the Cu2O signals were attenuated, while ZnO signals were still present. From 300 s onward, all signals belonging to Cu and Zn oxides disappeared, which indicated that the catalyst was in the metallic state. COrelated peaks at 287, 365, and 2010 cm−1 were then observed. The shifts in the peak positions (7−50 cm−1) of CO bonded on Cu4Zn, versus Cu, could be attributed to the differences in its surface coverage or adsorption on different catalytic sites.44 We also verified that these three Raman peaks (287, 365, and 2010 cm−1) were related to electrochemically generated CO intermediates, as they disappeared within 200 s after the flow of CO2 was removed from the electrolyte (Figure S4 in the Supporting Information). The rapid reductions of all the deposited metal oxides to their metallic states are consistent with the observed chronoamperograms

(inserts of Figure 2B−D), which showed cathodic peaks at the initial stage of the reduction. The spikes in the chronoamperograms were due to the natural desorption or manual removal of gas bubbles formed on the surface of the catalyst. It is also noteworthy that the CO-related vibrations were not observed on the Cu or CuxZn catalysts until all the signals belonging to the oxides had disappeared (Figures 2B and 2D). This indicates that the reduction of CO2 to CO and hydrocarbons is likely to occur more efficiently on metallic sites rather than on oxides. This finding is in good agreement with the simultaneous linear sweep voltammetry and online mass spectrometry study by Kas et al., which showed that no CO2 reduction products were formed while the catalyst was still Cu2O.12 3.3. Electrochemistry of Cu, Cu10Zn, Cu4Zn, Cu2Zn, and Zn Catalysts. The electrochemistry of Cu, Zn, Cu10Zn, Cu4Zn, and Cu2Zn catalysts toward CO2 reduction in aqueous 0.1 M KHCO3 electrolyte were assessed using 60 min chronoamperometry at potentials between −0.65 and −1.15 V (Figure S5 in the Supporting Information). The average current densities (jtotal) exhibited by the five catalysts at different applied potentials are summarized in Figure 3 (representative chronoamperograms at −1.05 V are presented in Figure S6 in the Supporting Information). In comparison to Zn, the Cu-based catalysts showed 5−30 times higher current densities. This could be explained by differences in their surface areas (Table S3 in the Supporting Information) as well as the lower reactivity of monometallic Zn toward CO2 reduction. Previous studies have shown that polycrystalline Zn required 100−300 mV higher overpotential in comparison to polycrystalline Cu to drive −5 mA/cm2 current during CO2 reduction.5,41 3.4. Distribution of CO2 Reduction Products: Ethanol and Ethylene Formation. The gas and liquid products formed during CO2 reduction on Cu, Zn, Cu10Zn, Cu4Zn, and Cu2Zn catalysts were quantified using GC and 1H NMR (Figures S7 and S8 in the Supporting Information). Thirteen products, including H2 were detected, and their FEs added up to 91.1−109.5% 8242

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the three CuxZn catalysts (FEethanol/FEethylene = 1.38−6) compare favorably to values (0.02−0.86) previously measured on other Cu electrodes (Table 1).7−9,14,18−20,22 These trends are summarized in Figure 4F. The aforementioned trends of ethanol and ethylene formation on CuxZn indicate that Zn and Cu had acted synergistically to promote the formation of ethanol. This synergy could be demonstrated by analyzing the FE of CO on these catalysts. If Cu and Zn had functioned independently, we would expect to see increasing FEs of CO evolution on CuxZn as larger overpotentials were applied (due to the formation of CO from Zn; Figure 4B). However, the formation of CO actually decreased at more negative potentials (Figure 4C−E). For example, on the Cu4Zn and Cu2Zn catalysts, the FE of CO started to decrease at potentials negative to −1.00 V. Significantly, at these potentials, the formation of ethanol was enhanced. We thus postulate that gaseous CO formed from the Zn sites may have further reacted to give ethanol. This postulation is corroborated by the observation that the potential where the FE of ethanol had peaked shifted 100−150 mV more negative on CuxZn (x = 4, 2) in comparison to Cu catalyst. This is because the CO needed for enhanced ethanol formation could only be generated in greater quantities on the Zn sites at more cathodic potentials (Figure 4B). It is interesting to note that, although Cu and Zn produced formate with maximum FEs of 30.6% and 6.1%, respectively, the FE of formate on the three CuxZn catalysts was 65%) (Table S7 in the Supporting Information). This observation is consistent with the findings of Hori et al., which showed that the selectivity for

Figure 3. Total current density as a function of applied potential for CO2 reduction on Cu, Zn, Cu10Zn, Cu4Zn, and Cu2Zn catalysts, respectively.

(Tables S5−S9 in the Supporting Information). H2 was formed because of the competing hydrogen evolution reaction (HER). On Cu, the FEs of ethylene were generally >2× greater than those of ethanol across most potentials studied (Figure 4A). This observation agreed well with previous measurements on Cu surfaces (Table 1).7,8,14,16 Zn catalysts did not reduce CO2 to ethylene or ethanol (Figure 4B). Instead, only CO, H2, and formate were detected, with the FE of CO increasing continuously to a maximum of ∼75% at −1.10 and −1.15 V. Interestingly, the CuxZn catalysts showed different selectivities toward ethylene and ethanol formation (Figure 4C−E). As the amount of Zn in CuxZn increased, the maximum FEs of ethanol increased from 11.3% (on Cu) to 29.1% (on Cu4Zn) and then decreased to 18.1% (on Cu2Zn). On the other hand, the maximum FEs of ethylene decreased monotonically from 26.5% (on Cu) to 4.1% (on Cu2Zn). The selectivities of ethanol versus ethylene production (FEethanol/FEethylene) on the different Cu-based catalysts could also be tuned by a factor of up to ∼12.5 (from 0.48 on Cu to 6 on Cu2Zn). It is significant that the selectivities observed on

Figure 4. Electrochemical reduction of CO2 on the catalysts: (A−E) Faradaic efficiencies of ethanol, ethylene, carbon monoxide, and formate as a function of potential on Cu, Zn, Cu10Zn, Cu4Zn, and Cu2Zn catalysts, respectively; (F) maximum Faradaic efficiencies of ethanol and ethylene and the average FEethanol/FEethylene ratio (calculated on the basis of the ratios measured at different potentials) on CuxZn. 8243

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ACS Catalysis hydrocarbon formation on a Cu electrode could be easily changed to H2 evolution when there are small amounts of Zn contaminant on the electrode.46 The partial current density of ethanol (jethanol) was used to assess the efficacy of the Cu and CuxZn catalysts for producing this valuable liquid fuel (Figure 5). The trend observed here is

Figure 6. Faradaic efficiency of ethanol and ethylene during electrochemical reduction of CO2 on Cu, Cu3.5Ni, Cu3Ag, CuAg, and CuAg6 catalysts at −30 mA/cm2 in 0.1 M KHCO3.

attributed to the smaller amount of Ag atoms on the catalyst surface (Table S12 in the Supporting Information) and/or different CO binding strengths on Ag and Zn.41 Nonetheless, our results clearly demonstrated that the addition of catalytic sites favorable toward CO formation (Zn or Ag) into Cu would enhance the reduction of CO2 toward ethanol and increase the selectivity of ethanol versus ethylene (FEethanol/FEethylene). A scrutiny of previously reported data of Cu−Au bimetallic catalysts also revealed that FEethanol/FEethylene increased as the amount of Au (a CO-producing catalyst) was increased.45 In comparison with monometallic Cu particles which produced less ethanol in relation to ethylene (FEethanol = 3.7%, FEethylene = 4.6% at −1.04 V), Cu3Au produced ∼3 times more ethanol than ethylene (FEethanol = 1.6%, FEethylene = 0.6% at −1.09 V). Our results also show that the selective formation of ethanol on CuxZn (or CuxAg) catalysts cannot be simply attributed to the better “dispersion” of the Cu particles among the second metal dopant. The preceding results indicate that an in situ source of mobile CO reactant is important for promoting the catalytic reduction of CO2 to ethanol. To further demonstrate this, we performed the direct electroreduction of CO on Cu and Cu4Zn catalysts (Table S13 in the Supporting Information). On the Cu catalyst, the FEethanol/FEethylene ratio ranged from 1 to 16, which is higher than those obtained through CO2 reduction on the same catalyst (Figure 4A). Similarly, the FEethanol/FEethylene ratio is 2−20 on Cu4Zn catalyst. However, we note that the absolute Faradaic efficiency of ethanol was rather low on both catalysts (≤3.2%) and the main product is H2. This can be attributed to the low solubility of CO in the aqueous KHCO3 electrolyte, which limits the amount of CO available for the reduction. 3.6. Proposed Mechanisms for the Formation of Ethanol. On the basis of our results, we propose a two-site mechanism to rationalize how Cu and Zn atoms in the CuxZn catalysts have acted synergistically to enhance the electroreduction of CO2 to ethanol (Figure 7). Incoming CO2 molecules could first bind to either Cu or Zn sites and be reduced to CO (1 → 2).5 On the Cu sites, CO could be reduced further to CHO or CHx (x = 1−3) intermediates.25 On the other hand, CO adsorbs weakly on Zn sites and is likely to desorb.41 This proposition is in agreement with our operando Raman spectroscopy data that showed the poorer adsorption of CO on Zn in comparison to Cu (Figure 2B,C). The desorbed CO could diffuse and spill over onto the Cu sites (2 → 3). To the best of our knowledge, the spillover phenomenon is more commonly reported for H atoms

Figure 5. Partial current density of ethanol formation on Cu, Cu10Zn, Cu4Zn, and Cu2Zn catalysts at different potentials.

similar to what has been described for the FE of ethanol on the same catalysts (Figure 4F): namely, an optimum amount of Zn is required for maximizing the formation of this product. It is noteworthy that, at −1.05 V, Cu4Zn catalyzed −8.2 mA/cm2 of ethanol formation. This highly practical figure of merit compares very favorably to those of Cu and the other CuxZn catalysts studied in this work (jethanol = −2.3 to −4.0 mA/cm2 at −1.05 V) and to many previously measured values (jethanol = −0.1 to −5.7 mA/cm2) on copper electrodes (Table 1).7−9,14,19,20,22 The durability of the Cu4Zn catalyst for CO2 reduction was remarkable. Ethanol could be continuously produced over 5 h with a FEethanol value of ∼29% (Table S10 in the Supporting Information). The FE of ethanol only decreased to 25% for 10 h electrolysis. The current density and the FE of other CO2 reduction products such as ethylene remained stable during the electrolysis (Figure S9 in the Supporting Information). We attribute the stability of our catalyst to its negligible catalysis toward the formation of methane (FEmethane = 0.4%), which is known to form together with graphitic carbon poisons.30,47 The extraction of ethanol from the aqueous electrolyte must be made before it can be used as a fuel. Using a salting-out liquid−liquid extraction (SALLE) procedure with hexane as the extractant, we facilely removed ∼93.5% ethanol from the KHCO3 electrolyte (Figure S10 in the Supporting Information). 3.5. Role of CO in the Selective Formation of Ethanol. To demonstrate the importance of CO-producing sites for selective ethanol formation, we performed CO2 reduction on CuxNi and CuxAg catalysts (Figure 6). Ag and Ni were chosen as cocatalysts with Cu, because Ag is an excellent catalyst for reducing CO2 to CO, while Ni is inactive toward CO2 reduction (Tables S1 and S11 in the Supporting Information).5,48 In comparison to Cu, Cu3.5Ni only catalyzed FE = 13.6% of C2 compounds with FEethanol/FEethylene = 0.2. In contrast, the three Cu− Ag catalysts showed selectivity toward ethanol. The FE of ethanol increased on Cu−Ag catalysts in comparison to Cu catalyst, and FEethanol/FEethylene values were 0.67 on Cu6Ag, 0.73 on CuAg, and 1.01 on CuAg6. In terms of increasing the selectivity of ethanol formation, Ag was a less efficient dopant in comparison to Zn. This might be 8244

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Figure 7. Proposed mechanism for the electroreduction of CO2 to ethanol on CuxZn catalysts: stages 1 → 2, four protons and four electrons reduce two CO2 molecules to CO on Cu and Zn, respectively; stages 2 → 3, four protons and four electrons reduce CO molecule to *CH2 on Cu, while CO produced by Zn desorbs and migrates near the *CH2; stages 3 → 4, CO inserts into the bond between Cu and *CH2 to form *COCH2; stages 4 → 5, two protons and two electrons reduce *COCH2 to CH3CHO (acetaldehyde); stages 5 → 6, two protons and two electrons reduce CH3CHO to CH3CH2OH (ethanol). The protons transferred are presumably drawn from water molecules.

in electrochemical systems.49 The spillover of CO from one metal site (which binds to CO weakly) to another (which binds to CO strongly) has only been reported in gas-phase studies.50 The spilled-over CO may then insert itself into the bond between the Cu surface and *CH2, to form *COCH2 (3 → 4).18,51 Further reduction of *COCH2 will produce acetaldehyde (4 → 5) and finally ethanol (5 → 6).51,52 In this mechanism, both Zn and Cu are crucial for the production of ethanol. The role of Zn is to increase the population of free CO, which is an essential reactant for the insertion process to occur efficiently. This explains why the maximum FE of ethanol increased as the amount of Zn increased from Cu to Cu4Zn (Figure 4F). However, an excess of Zn will result in correspondingly fewer Cu surface sites to form C1 intermediates. This will thus lead to an overall lowering of the FE of ethanol (exhibited by the Cu2Zn catalyst). It is noteworthy that our proposed insertion mechanism has been shown to favor alcohol formation in the gas phase on Fischer−Tropsch catalysts such as Fe oxides or mixtures of Cu and Zn oxides.53 Similar mechanisms have also been proposed by Hori’s group and Kenis’s group for the electroreduction of CO2 to C2 compounds.18,51 Our proposed mechanism gives an insight into why CO2 could not be easily reduced to ethanol on roughened Cu surfaces (such as on Cu2O-derived Cu), even when the pressure of the CO2 feed is at 9 atm (which should generate a higher surface population of *CO).14,18,47 This is because a mobile CO is required for the insertion into the bond between Cu and *CH2. Such a CO would be easily generated on the Zn sites of the CuxZn catalyst. In contrast, CO produced from the reduction of CO2 on monometallic Cu catalyst would be more strongly adsorbed.41 The formation of ethanol could also proceed via another pathway: here, CO produced on Zn could readsorb on the Cu sites and increase the surface coverage of C1 species. Two neighboring C1 species can then undergo a C−C bond formation step to form either ethylene or ethanol (the latter via an acetaldehyde intermediate). 25,52 On the basis of DFT simulations on Cu single-crystal surfaces, Koper, Calle-Vallejo, and co-workers have suggested that the energy barrier of ethanol formation from acetaldehyde is lower on highly undercoordinated catalytic sites.24,54 This could be the case for the rough oxide-derived CuxZn catalysts used in this work. We further note that Hori et al. had investigated the electroreduction of CO on electropolished polycrystalline Cu surfaces in 0.1 M KHCO3 electrolyte at −2.5 mA/cm2 and did not observe any enhanced ethanol formation (FEethylene = 21.2%, FEethanol = 10.9%).55

These results indicated the importance of atomically roughened Cu surfaces for the preferential formation of ethanol. Due to its ease of transportation and storage, ethanol is a highly desirable liquid fuel. However, its formation from CO2 is usually poor on Cu surfaces. Li et al. had circumvented this difficulty by demonstrating the reduction of CO to ethanol with a FE of 42.9% on oxide-derived Cu nanoparticles.26 However, the solubility of CO in the aqueous 0.1 M KOH electrolyte constrained the current density for ethanol to −0.39 mA/cm2.26 Herein, we promote the direct reduction of CO2 to ethanol by using oxide-derived CuxZn bimetallic catalysts. A highly practical jethanol value of −8.2 mA/cm2 could be achieved using Cu4Zn. We propose that the role of Zn during the electrolysis is to continuously produce an in situ source of mobile CO, which can diffuse to neighboring Cu sites to react with another C1 intermediate (*CO or *CH2). The suppression of CH4 formation using the oxide-derived catalysts also mitigated the formation of graphitic carbon. This helped to confer stability on the catalysts for their prolonged electroreduction of CO2 to ethanol. The high selectivity, efficiency, and stability of ethanol formation on Cu4Zn catalyst have not been achieved on Cu surfaces (Table 1) or on other Cu-based bimetallics such Cu−Au.45 The ease of preparation of our catalyst and high abundance of Cu and Zn render it a suitable candidate for facilitating the scale-up and sustainable conversion of CO2 to ethanol.

4. CONCLUSIONS In this work, we prepared a series of oxide-derived CuxZn catalysts for the enhanced electroreduction of CO2 to ethanol. Operando Raman spectroscopy demonstrated that CO2 was reduced more preferentially on metallic rather than oxide surfaces. We discovered that the selectivity of ethanol production vs ethylene, defined by the ratio of their Faradaic efficiencies (FEethanol/FEethylene), could be tuned by a factor of ∼12.5 by varying the amount of Zn in the CuxZn catalysts. Ethanol formation was maximized on Cu4Zn at −1.05 V, with a Faradaic efficiency of 29.1% and a partial current density of −8.2 mA/cm2. Cu4Zn was also catalytically stable for the production of ethanol for at least 5 h. The importance of CO-producing sites for selective ethanol formation was demonstrated by performing CO2 reduction on Cu−Ni and Cu−Ag bimetallic catalysts. A two-site mechanism was also proposed for the formation of ethanol. Our work highlights a new methodology for the design of more efficient bimetallic catalysts and narrows the gap 8245

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ACS Catalysis

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between the reduction of CO2 to ethanol in the laboratory and the application of this process in industry.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.6b02162. Experimental details, TEM-EDX, XRD, XPS, surface area measurements using double-layer capacitance, additional Raman spectroscopy, electrochemical cell for CO 2 reduction, chronoamperograms, GC and 1H NMR data, Faradaic efficiencies of all the products, durability of Cu4Zn, salting-out liquid−liquid extraction, atomic ratios of Cu−Ag and Cu−Ni bimetallics, and electrochemical reduction of CO (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail for B.S.Y.: [email protected] Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by a research grant, R143-000-587-112, from the National University of Singapore. D.R. acknowledges a Ph.D. research scholarship from the Ministry of Education, Singapore. We thank Ze Liang Yuan for conducting the XPS analysis.



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