Article pubs.acs.org/JPCC
Electrochemical Reduction of Carbon Dioxide to Ethane Using Nanostructured Cu2O‑Derived Copper Catalyst and Palladium(II) Chloride Chung Shou Chen, Jane Hui Wan, and Boon Siang Yeo* Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543 S Supporting Information *
ABSTRACT: A method to facilitate the electrochemical reduction of carbon dioxide (CO2) to ethane (C2H6) was developed. The electrolyte used was aqueous 0.1 M KHCO3. Chronoamperometry, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, online gas chromatography, and nuclear magnetic resonance spectroscopy were used to characterize the electrochemical system and products formed. Carbon dioxide reduction using a Cu2Oderived copper working electrode gave ethylene (C2H4) and ethanol as main C2 products, with optimized faradic efficiencies (FE) of 32.1 and 16.4% at −1.0 V vs RHE. The active catalysts were ∼500 nm-sized crystalline Cu0 particles, which were formed via the reduction of the Cu2O precursor during the initial phase of the CO2 reduction reaction. When palladium(II) chloride was added to the electrolyte, C2H6 formation could be achieved with a significant FE of 30.1% at the said potential. The production of C2H4 was, on the other hand, suppressed to a FE of 3.4%. The alternate use of Pd0, PdO, or Pd−Al2O3 dopants did not afford the same conversion efficiency. Extensive mechanistic studies demonstrate that C2H4 was first produced from CO2 reduction at the Cu0 sites, followed by hydrogenation to C2H6 with the assistance of adsorbed PdClx. Interestingly, we discover that both Cu and PdClx sites are necessary for the efficient reduction of C2H4 to C2H6. The PdCl2 was “consumed” during the reaction, and a hypothesis for how it contributes to the reduction of CO2 to ethane is proposed. C2H4/CH4 ratio) can be enhanced up to ∼100× with the use of stepped-Cu(100) surfaces, CuCl-confined Cu-mesh, foamlike Cu structures, thick deposited Cu2O films, Cu mesocrystals, Cu nanocubes, or Cu nanoparticles.3,4,8,13,16−18 Undercoordinated Cu atoms and/or (100) facets on these surfaces are thought to aid in the chemisorption and dimerization of the pertinent C1 intermediates (CO, CHO, or CH2O) to C2H4. An increase in local pH at the electrode surface is also believed to contribute to the enhanced C2H4 formation.2,3,6,19 Interestingly, on some thick Cu films, ethane (C2H6) was formed with faradic efficiencies (FE) up to ∼10%.5,6,20,21 This observation is intriguing as single- or polycrystalline Cu metal surfaces do not electrocatalyze the reduction of CO2 to C2H6.8,9 As a fuel, C2H6 has a higher gross energy of 65.8 MJ/m3 as compared to that of CH4 at 35.6 MJ/m3.22 It is also being investigated as a feedstock to produce commodity chemicals such as acetic acid and vinyl chloride.23 The development of an electrochemical system to facilely synthesize C2H6 from CO2 thus not only will help to establish a green closed-loop fuel production cycle but also will open up valuable carbon valorization pathways. CO2 has been proposed to electroreduce to C2H6 via the hydrogenation of an ethylene intermediate.6,20 This mechanism
1. INTRODUCTION The electrochemical reduction of carbon dioxide (CO2) into target molecules has the potential of becoming an environmentally friendly and sustainable way of converting waste carbon to industrially valuable chemical feedstocks and fuels.1 This is especially so if the process uses electricity generated from renewable sources such as wind and solar. An ideal electrocatalyst for CO2 reduction should be highly active and selective toward the desired molecules at low overpotentials.2 To date, copper and its oxides are the most promising catalysts for this purpose, albeit highly unselective.3−8 This is exemplified by the formation of at least 16 different products when CO2 reduction is performed using a polycrystalline copper electrode.9 The poor selectivity could be attributed to the variety of catalytic sites present, which impacts the energetics and kinetics of the elementary steps of the reaction.10 The electrolytes employed and potentials applied also have strong influences on the product selectivity.2,9,11 As such, considerable effort has been dedicated to discover the type of atomic scale features on Cu catalysts and electrochemical parameters that are optimum for the selective reduction of CO2 to molecules such as ethylene, ethanol, carbon monoxide, formate, methane, etc.3−7,10,12−15 Single crystal Cu(111) and Cu(100) surfaces can selectively catalyze the reduction of CO2 to methane and ethylene, respectively.8 The selectivity toward C2H4 (as gauged by the © XXXX American Chemical Society
Received: September 19, 2015 Revised: October 23, 2015
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DOI: 10.1021/acs.jpcc.5b09144 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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°C, with a fixed current of −0.7 mA for 1200 s. The plating electrolyte contained 0.3 M CuSO4 (GCE, 99%), 3.2 M NaOH (Chemicob, 99%) and 2.3 M lactic acid (VWR, >90%). Excess electrolyte was removed from the disc by sonication in water. Metallic Pd0 electrodes were prepared by galvanostatically reducing Pd0 films onto polished graphite discs (Ted Pella) at −0.519 mA/cm2 for 10 min. The plating solution consisted of 0.005 M PdCl2 dissolved in aqueous 0.1 M HCl. PdCl2 (99.999%, Aldrich), PdO (99.97%, Aldrich), Pd (99.999%, Aldrich), and Pd−Al2O3 (5 wt % Pd loaded onto Al2O3 support, Aldrich) particles were used as received. Only ultrapure type 1 water (18.2 MΩ, Barnstead, Thermo Scientific) was used for preparing the sample solutions, electrolytes, and for washing. 2.2. Characterization of Catalysts. The Cu catalysts and Pd particles were analyzed by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM and EDX, JEOL JSM-6710F) operated in the secondary electron mode (5 keV, 10 mA probe current). X-ray photoelectron spectroscopy (XPS) was performed using a Kratos AXIS Ultra (Al Kα emission source). The signals were calibrated using the C1s peak at 284.5 eV. X-ray diffraction (XRD) of the films was performed using a Bruker D8 (Cu Kα, 40 kV, 40 mA). Standard XRD patterns were used for identifying the peaks (PDF 00-003-0892 for Cu2O, PDF 00-001-1242 for Cu, PDF 00-001-0228 for PdCl2, PDF 01-075-0200 for PdO, and PDF 01-087-0643 for Pd). The added PdCl2 catalysts were also analyzed after CO2 reduction. The particles were collected by vacuum filtration, washed with water, and dried in low vacuum prior to analysis. The electrochemical surface areas of the electrodes were determined by double layer capacitance measurements.3 2.3. Electrochemical Reduction of Carbon Dioxide. The electrochemical reduction of CO2 was performed in a gastight Teflon electrochemical cell, which was modeled after the design of Kuhl et al. (S1).9 A three-electrode configuration was used. The anodic and cathodic compartments were separated by an anion-exchange membrane (Selemion AMV, AGC Asahi Glass). The counter and reference electrodes were a Pt wire and a Ag/AgCl electrode (Saturated KCl, Pine Research Instrumentation), respectively. Electrochemical measurements were made using a potentiostat (Gamry Instruments Reference 600). CO2-saturated aqueous 0.1 M KHCO3 (pH = 6.8) was used as an electrolyte. The volume of the catholyte and anolyte were 12 and 8 mL, respectively. CO2 was bubbled into the electrolyte at 20 sccm during the experiment. The catholyte was stirred at 500 rpm with a Teflon-coated stirring bar to minimize CO2 mass transport limitations. A typical CO2 reduction experiment lasts for 4210 s (or 1 h, 10 min). Each prepared catalyst was only used once for CO2 reduction at a chosen potential. Each data point is an average of measurements collected from at least three separate NMR or GC experiments. All the potentials cited in this work are referenced to the reversible hydrogen electrode (RHE) and are iR-corrected using the current interrupt mode. The current density values are normalized to the exposed geometric surface areas of the working electrodes (0.385 cm2). 2.4. Quantification of Liquid and Gas Products. The gas and liquid products were quantified by online gas chromatography (GC) and nuclear magnetic resonance spectroscopy, respectively (S1). Their yields are expressed as faradic efficiencies.3,4
is supported by the reduced production of ethylene whenever ethane is formed. Active hydrogen is crucial for the hydrogenation of ethylene.20,24 Defective Cu surfaces are believed to support a higher population of adsorbed H, as compared to smooth Cu surfaces.24 This explains the general observation that thick Cu films with high surface roughness can reduce CO2 to C2H6, albeit rather inefficiently.5,6,20 For example, a thin reduced-Cu2O film converts CO2 to C2H4 with a FE of ∼32%. When the thickness of the film is increased by several-fold, the respective FEs of C2H4 and C2H6 only change to ∼12 and ∼10%.6 It is also difficult to quantify the amount of active hydrogen present in these Cu films. On the basis of the above considerations, we envisage that a viable method to promote CO2 reduction to C2H6 is to first increase the surface population of adsorbed C2H4 intermediates. This could be actualized by using reduced-Cu2O films as electrocatalysts. This class of catalysts has been shown highly efficacious for catalyzing the production of C2H4 with FEs up to ∼40%.3,6 Hydrogenation of the C2H4 intermediates to C2H6 can then be subsequently enhanced by increasing the quantity of active H present in the electrocatalytic system. A possible way to do so is to add palladium dopants, which are known carriers for active hydrogen in electrochemical, gas-, and liquidphase hydrogenation reactions.2,25−27 They also facilitate in the process by binding and activating the olefinic CC functional group. In view of this strategy, we note that CO2 electroreduction to ethane has been demonstrated previously by the addition of Pd-alumina to a CO2 reduction cell consisting of a Cu working electrode.28 However, little information about the durability and mechanistic aspects of the system was shown. We also note here that alumina could reduce to Al0 films under the very cathodic potentials required for CO2 reduction.29 Al0 is inert toward CO2 reduction, and will instead catalyze the hydrogen evolution reaction (HER).30 Here in this work, we develop an electrochemical system, consisting of Cu2O-derived Cu working electrodes and PdCl2 dopants, for the targeted reduction of CO2 to C2H6, via C2H4 intermediates. PdCl2 was chosen as it is a known support for active hydrogen and it also binds to olefins efficiently.31 Aqueous 0.1 M KHCO3 was used as electrolyte. Chronoamperometry, scanning electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, online gas chromatography, and nuclear magnetic resonance spectroscopy were used to characterize the catalytic system and products formed. Indepth mechanistic studies revealed, for the first time, how CO2 electroreduces to ethane.
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Cu discs (Goodfellow, Ø = 10 mm, 99.99%) were used as the substrate for all catalysts. These were successively polished with SiC polishing paper (Grit 1200 Struers) and diamond slurries (9 and 3 μm, Struers). The polished Cu discs were then sonicated in water for 5 min to remove residual diamond polish. The following catalysts were prepared with these mechanically polished Cu discs. Electropolished copper surfaces: the Cu discs were electropolished in phosphoric acid (85%, RCI Labscan) at a 259.7 mA cm−2 anodic current for 60 s.16 The disc was then rinsed with water. Cu2O nanoparticles: Cu2O was electrochemically deposited onto a polished Cu electrode using a two-electrode cell with a platinum counter electrode.32 Deposition was performed at 60 B
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3. RESULTS AND DISCUSSION 3.1. Morphology and Chemical Composition of CuBased Catalysts. The morphology and chemical compositions of the catalysts were characterized by scanning electron microscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (Figure 1, and S2). The surface of an electropolished Cu electrode was smooth and featureless (Figure 1A). Little morphological changes occurred after it was used as the working electrode for 4210 s of CO2 reduction in a 0.1 M KHCO3 electrolyte (at a representative potential of −1.0 V, Figure 1A insert). Cu2O electrodeposited on a planar Cu disc was in the form of ∼500 nm-sized
polyhedrons (Figure 1B). These polyhedrons remained approximately the same size after CO2 reduction, albeit with their surfaces highly corrugated (Figure 1C).3 XRD analyses revealed that the Cu2O particles had reduced to crystalline Cu0 during the CO2 reduction (Figure 1, panels E and F). This catalyst will be termed as Cu2O-derived Cu. 100 mg PdCl2 was also added to the catholyte when a Cu2O working electrode was used. PdCl2 is highly insoluble in aqueous KHCO3 and appears as a black suspension. After 4210 s of CO2 reduction at −1.0 V, numerous 20−100 nm-sized nanoparticles were found attached to the surface of the Cu2Oderived Cu working electrode (Figure 1D). These were shown by EDX and XPS to be Pd0, suggesting that PdCl2 had reduced to Pd0 concurrently during the CO2 reduction process (Figure 1, panels G and H) (S2).33,34 X-ray diffraction also demonstrates that the deposited Cu2O layer had reduced to metallic Cu0 (Figure 1F). 3.2. Electrochemical Reduction of CO2 to C2H6. 3.2.1. Chronoamperometry Data for Cu-Based Catalysts. Chronoamperograms recorded during CO2 reduction at a representative potential of −1.0 V are presented in Figure 2A.
Figure 2. (A) Chronoamperograms collected during CO2 reduction at −1.0 V using electropolished Cu electrode, the Cu2O-derived Cu electrode, and the Cu2O-derived Cu electrode with 100 mg PdCl2 added to the electrolyte. (B) Measured steady state currents of these electrodes as a function of potential. Figure 1. SEM images of an electropolished Cu electrode (A) before and (inset) after CO2 reduction; Cu2O electrode (B) before and (C) after CO2 reduction; (D) Cu2O electrode after CO2 reduction with 100 mg PdCl2 added to the electrolyte. X-ray diffractograms of an electropolished Cu, Cu2O electrode, and Cu2O electrode with PdCl2 added to the electrolyte (E) before and (F) after CO2 reduction. The Cu reflexions for the Cu2O sample prior to CO2 reduction are from the underlying Cu substrate. (G) Pd3d and (H) Cl2p XPS spectra of asreceived PdCl2 and Cu2O-derived Cu/PdCl2 after CO2 reduction. CO2 reduction was performed at −1.0 V.
Cathodic peaks were observed in the chronoamperograms of the Cu2O-derived Cu catalysts during the first 200 s. This feature, in agreement with the XRD results presented in Section 3.1 and predictions from the Pourbaix diagram of the copperwater system, can be ascribed to the reduction of Cu2O to metallic Cu0.3,35 In the case where PdCl2 was added, its reduction to Pd0 would also contribute to the said cathodic peak. The cathodic currents subsequently stabilized. The average current densities exhibited by the Cu catalysts at C
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Figure 3. Faradic efficiencies of products from CO2 electroreduction as a function of potential using (A and B) electropolished Cu, (C and D) Cu2O-derived Cu, and (E and F) Cu2O-derived Cu with PdCl2 added to the electrolyte.
different potentials are presented in Figure 2B. The currents increased as more negative potentials were applied for all three catalysts. The Cu2O-derived Cu catalyst displayed a generally higher steady state current compared with electropolished Cu. This can be attributed to the former’s larger electrochemical surface area. We found, using the double layer capacitance method, that the Cu2O-derived Cu catalyst has a roughness factor of 6−7 times larger than that of the electropolished Cu substrate. 3.2.2. CO2 Reduction Products Formed Using Cu-Based Catalysts. The faradic efficiencies of carbonaceous products formed during CO2 reduction at different electrochemical potentials are presented in Figure 3 and Table S3 in S1. H2 gas was also produced from the competitive hydrogen evolution reaction. The trends for the products formed on an electropolished Cu electrode are consistent with our previous work and Jaramillo’s et al. (Figure 3, panels A and B).4,9 Formate (FEmax = 26.7%) and CO (FEmax = 15.1%) are formed at relatively positive potentials from −0.8 to −1.0 V. C−C bond coupling then prevailed from −1.0 to −1.1 V, which explained the increased production of C2H4 (FEmax = 18.5%) and other C2 and C3 products such as ethanol, propanol, etc. At more negative potentials, proton and electron transfers dominate, which thus resulted in enhanced CH4 (FEmax = 40.1%) and H2 formation.9
When Cu2O-derived Cu working electrodes were used as CO2 reduction catalysts, large quantities of C2H4 and ethanol were produced at −1.0 V with respective FEs of 32.1 and 16.4% (Figure 3, panels C and D). The FE for CH4 was suppressed to ∼1% at all potentials. These findings agree with the results of our previous studies on the same type of catalyst.3 The more selective formation of C2H4 can be attributed to morphological factors and to a rise in local pH at the surface of the electrode.2,3,6,19 Regardless of the applied potential (−0.7 to −1.2 V), C2H6 was formed with FEs of ≤1%. Attempts to substantially boost its production by varying the thickness of the Cu2O-derived Cu films were not successful. To increase the quantity of active hydrogen in the electrocatalytic system, 100 mg PdCl2 was added to the KHCO3 electrolyte before CO2 reduction using a Cu2O catalyst. The most significant result with this inclusion is that C2H6 is now formed at remarkably larger faradic efficiencies and which maximized at 30.1% at −1.0 V (Figure 3, panels E and F). C2H4 formation, in contrast, was greatly suppressed to 3.4% at the same potential. The FE of C2H6 formed here was notably higher as compared to the use of only roughened Cu films, where the FEs of C2H6 was up to ∼10%.5,6,20,21 We assume here when performing the FE calculations that C2H6 was formed electrochemically; more will be seen in the next section that this is the case. Interestingly, while ethanol and D
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The Journal of Physical Chemistry C ethylene have been shown by quantum chemical calculations to originate from common C2 intermediates such as CH2CHO(ad), the FE of ethanol was not greatly affected.12 The FEs of the rest of the carbonaceous products also remained relatively unchanged. These observations suggest that ethane must have been formed from ethylene. To assess the role of PdCl2, CO2 electroreduction was performed at −1.0 V using a Cu2O-derived Cu working electrode, but adding instead PdO or metallic Pd0 particles to the catholyte. These particles were chosen as they absorb/ adsorb H2 gas with similar efficiency as PdCl2 (S3). A reduced formation of C2H6 with respectively FEs of 9.4 (with PdO) and 2.8% (with Pd) was observed (Table S4 in S1). SEM analyses revealed poor adsorption of these palladium particles to the Cu substrate, which would lead to a decrease in surface population of active hydrogen or binding sites for hydrogenating ethylene (S4). We also tested the effects of Pd−Al2O3 dopants (S5). At −1.0 V, ethane formed with a maximum FE of 20.8%. However, the entire catalytic system deactivated from 20 min onward as shown by the increase in H2 production via HER. Al2O3 particles coated onto the working electrode is the likely cause of the deactivation as aluminum does not catalyze CO2 reduction.30 These findings demonstrate that the type of palladium added is critical to the optimum reduction of CO2 to C2H6. Whenever PdCl2 (or PdO/Pd) is added to the electrolyte, the total FE for the products is lower at 48−87% (Figure 3). This apparent discrepancy can be ascribed to charges being used for the reduction of PdCl2 throughout the experiment (S6). The adsorption/absorption of H2 gas (produced from HER) into the PdCl2 particles could also reduce the total FE (S3). Similar observations have been made by Hori et al. for the electrochemical reduction of CO2 on bulk Pd electrodes in aqueous KHCO3 electrolyte (total FE of products = 60.2%).36 3.2.3. Elucidating the Formation of C2H6. To understand how ethane is formed, CO2 reduction was performed using a graphite working electrode and with 100 mg PdCl2 added to the KHCO3 electrolyte. Graphite is not known to catalyze CO2 reduction.30 Any CO2 reduction products formed must thus be attributed to the mediation of the added PdCl2. A potential of −1.0 V was applied. Formate, CO, and CH4 were formed with respective FEs of 4.7, 8.1, and 0.2% (Table S4 in S1). No C2H6 could be detected. The use of a Pd0 working electrode yielded similar findings, which are consistent with Hori’s work that CO2 could not be reduced to C2H6 on solely palladium (Table S4 in S1, and S7).36 On the basis of the above observations, we hypothesize that CO2 must reduce first to C2H4 on the Cu sites. The C2H4 then hydrogenate to C2H6 through the mediation of the PdCl2 particles. It is also of interest to determine if the reduction of C2H4 to C2H6 takes place electrochemically. To test these hypotheses, a 200 mol ppm of C2H4 (N2 balance) gas mixture was passed through the electrochemical cell at parameters stated in Figure 4. The gas exhaust was analyzed by GC over 4210 s. The findings are (i) C2H4 cannot be reduced to C2H6 in the absence of applied potentials or catalysts (experiments A− D); (ii) 2−4% of the incoming C2H4 was electrochemically reduced to C2H6 when either a Cu2O-derived Cu or a Pd0 working electrode was used (experiments E and F); (iii) 16% of the incoming C2H4 was reduced to C2H6 when a graphite working electrode was used and PdCl2 added to the electrolyte (experiment G); and (iv) 45% of the C2H4 were reduced to C2H6 when a Cu2O-derived Cu working electrode was used
Figure 4. Analysis of gas exhaust after 200 mol ppm of C2H4 was flowed into an electrochemical cell under parameters described in Experiments A to H. Flow rate of gas = 20 sccm.
and PdCl2 added to the electrolyte (experiment H). These experiments demonstrate directly that C2H6 was formed electrochemically from the hydrogenation of a C 2 H 4 intermediate. More significantly, of all the combinations of working electrodes and dopants studied, we found that the use of Cu2O-derived Cu electrode and PdCl2 (experiment H) offers the most effective environment for the hydrogenation of ethylene to ethane. This observation strongly suggests that ethylene must have chemisorbed first on the copper sites, before being hydrogenated on the adsorbed palladium sites to ethane (S8). Since PdCl2 reduced to Pd0 during CO2 reduction (Figure 1, panels G and H), the production of ethane is expected to decrease with time. A time-resolved CO2 reduction experiment (using a Cu2O-derived Cu working electrode and 100 mg PdCl2 added to the electrolyte) at −1.0 V confirmed that only ∼3% FE C2H6 was formed after 8 h (Figure 5A). C2H4 production, on the other hand, resumed with a higher FE. This change in activity has to be attributed to the weaker activity of the adsorbed Pd0 for hydrogenating C2H4 to C2H6. Interestingly, the buildup of a thick reduced Pd0 overlayer on the working electrode was not observed even after 12 h of experiment, as evidenced by the clear visibility of the Cu2Oderived Cu particles (Figure 5B). SEM of the added PdCl2 particles which were originally 500−1500 nm in size, revealed that these had reduced to 20−50 nm after CO2 reduction (Figure 5, panels C and D). XRD of these particles (the suspension in the electrolyte) showed that they are completely Pd0 (Figure 5E). This analysis indicates that although PdCl2 reduce to Pd0 adsorbates during the experiment, the latter only reside temporarily on the Cu surface before desorbing back into the electrolyte. 3.3. Mechanism for CO2 Reduction to C2H6. On the basis of the experimental results shown in Sections 3.1 and 3.2, a mechanism for the reduction of CO2 to C2H6 is proposed (Figure 6). An electrodeposited Cu2O layer is reduced to E
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Figure 5. (A) Composite plot of current density vs time at −1.0 V and faradic efficiencies of CH4, C2H4, C2H6, CO, and H2 over 12 h of CO2 reduction using a Cu2O-derived Cu electrode with PdCl2 added to the electrolyte. SEM images of (B) the working electrode after 12 h of CO2 reduction at −1.0 V, (C) as-received PdCl2, and (D) Pd particles collected from the catholyte after 12 h of CO2 reduction. The inset in (D) is a zoomed-in of the same particles. (E) X-ray diffractograms of the particles shown in (C and D).
the PdClx islands (compared to Pd0 islands), which thus makes them more receptive toward H transfer from the PdClx. In support of this hypothesis, we note that the salts and complexes of platinum group metals (for example, PdII) are known to coordinate to olefins and facilitate their hydrogenation.39 Hydrogen embrittlement is a possible explanation for the desorption of the Pd0 adsorbates from the surface of the Cu2Oderived Cu catalyst.40,41 Hydrogen that is formed through the HER, could diffuse into the Pd metal to form α- and β- phase PdHx, which have lattice constants of 3.893 and 4.04 Å respectively. β-PdHx is thermodynamically unstable and spontaneously decomposed to give H2 and α-PdHx. This caused lattice contraction and mechanical stress to the Pd islands, which is relieved by cracking/fragmentation. The adhesion of the deposited Pd to the working electrode is thus weakened, which enables it to desorb from the Cu surface (assisted by gas evolution on the electrode). This work provides first compelling evidence that during CO2 reduction to C2H6, the latter is formed from the hydrogenation of a C2H4 intermediate, rather than from the dimerization of two CH3 adsorbates.6 The commonly observed selectivity toward ethylene (rather than ethane) during CO2 reduction on single- or polycrystalline Cu surfaces can thus be understood by the lack of hydrogenation sites on these Cu electrocatalysts.3,4,24 Roughened Cu surfaces which contain more active hydrogen can aid in the conversion of CO2 to C2H6, although rather inefficiently.5,6,20 Here we show that efficacious C2H6 formation could be enabled by using PdCl2 dopants as adsorption sites for the C2H4 and carrier for active hydrogen. We are aware that palladium is a precious metal. However, it should be noted that the less-active form of Pd0 can be regenerated back to PdCl2 by using CuCl2 as the oxidizing agent (similar to the catalyst regeneration process in the Wacker-Smidt reaction).42,43 We predict that the use of high pressure electrochemistry to increase the formation of C2H4 from CO2 reduction would lead to a correspondingly higher yield of C2H6.19 The use of electrolytes with larger cations, for example, cesium+, could also be explored as a means to decrease HER and to improve the selectivity toward C2H6.2
Figure 6. Scheme illustrating the electrochemical reduction of CO2 to C2H6 via an adsorbed C2H4 intermediate.
crystalline Cu0 particles within the first few minutes of applying a negative potential.3,35 Incoming CO2 is then reduced to ethylene on these Cu2O-derived Cu active sites, most likely via the C−C bond coupling of two adjacent CHxO (x = 0, 1, or 2) adsorbates. Quantum chemical simulations using density functional theory has previously shown that this process is more kinetically favorable when the reactants are CHO or CH2O, rather than CO.37 PdCl2 added to the 0.1 M KHCO3 electrolyte is concurrently reduced on the surface of the working electrode to PdClx particles. These particles adsorb hydrogen via 2H+ + 2e → 2H(ad) or from H2 gas produced during HER (S3).38 Ethylene intermediates then migrate from the Cu sites to the PdClx islands, where they are hydrogenated to ethane. The deposited PdClx eventually reduces and desorbs as Pd0 particles, freeing the surface for new PdCl2 to be absorbed. The process continues until all the added PdCl2 has been reduced to Pd0. Since Pd0 is less active for hydrogenation, C2H4 production increases. The chemical stoichiometry of the adsorbed PdClx could not be determined from ex situ XPS as it has a short lifetime (due to its rapid reduction to Pd0 during the CO2 reduction process). It is interesting to understand the reasons behind the enhanced hydrogenation of C2H4 using PdCl2. We posit that a critical step for C2H4 hydrogenation is its adsorption on the palladium site.31 C2H4 could have adsorbed more favorably on F
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The Journal of Physical Chemistry C
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4. CONCLUSION The production and underlying mechanism in which C2H6 is electrochemically produced from CO2 is described in this work. A Cu2O-derived Cu working electrode was used, and PdCl2 was added to the 0.1 M KHCO3 catholyte. Significant ethane formation of up to 30.1% faradic efficiency could be achieved at an applied potential of −1.0 V. Detailed mechanistic studies demonstrate unambiguously that C2H4 was first produced from CO2 reduction at the Cu sites. It then underwent hydrogenation with the assistance of adsorbed PdClx to give C2H6. Both copper and PdClx sites are necessary for the efficient reduction of C2H4 to C2H6. The addition of other palladiumbased particles such as Pd0, PdO, or Pd−Al2O3 to the electrolyte did not afford the same conversion efficiency. The use of PdCl2 dopants alongside a copper working electrode is a new methodology to expand on the types of molecules that could be formed from CO2 building blocks. We thus expect that the principles presented here will open up new pathways toward the valorization of carbon dioxide to more complex molecules.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b09144. Experimental procedures, NMR peak assignments, electrochemical half-reactions, and faradic efficiencies of CO2 reduction products on various catalysts, EDX and SEM images of catalysts, and determination of H2 dissolution capabilities of catalysts (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +65 6779 1691. Tel: +65 6516 2836. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a start-up grant (WBS: R143-000515-133) from the National University of Singapore. We thank Mr. Henche Kuan (Department of Materials Science and Engineering, National University of Singapore) for performing the X-ray photoelectron spectroscopy measurements.
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DOI: 10.1021/acs.jpcc.5b09144 J. Phys. Chem. C XXXX, XXX, XXX−XXX
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