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−CH3 Mediated Pathway for the Electroreduction of CO2 to Ethane and Ethanol on Thick Oxide-Derived Copper Catalysts at Low Overpotentials Albertus D. Handoko,†,‡ Kuang Wen Chan,† and Boon Siang Yeo*,† †

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Institute of Materials Research and Engineering, Agency of Science, Technology and Research (A*STAR), 2 Fusionopolis Way, Innovis, Singapore 138634

‡

S Supporting Information *

ABSTRACT: Oxide-derived copper is known for its unique ability to catalyze the selective electroreduction of CO2 to C2 and higher carbon compounds at low overpotentials. To understand this phenomenon, mechanistic studies typically chose ethylene (C2H4) as the model compound. The pathways to form other C2 compounds such as ethane (C2H6) and ethanol are then generally considered to be similar to that of C2H4. However, regular detection of C2H6 or ethanol on thick oxidederived Cu at low overpotentials, often with selectivities exceeding that of C2H4, raises an important question: does the formation of these two C2 molecules really share a common route with C2H4? In this work, through an investigation of CO2 electroreduction on oxide-derived Cu of different thicknesses and oxidation states, we show that the formation of C2H6 and ethanol on thick oxide-derived Cu films could proceed through routes distinct from that of C2H4 at low overpotentials. Investigations using select molecular precursors such as diacetyl [(CH3CO)2] suggest that the formation of C2H6 on thick oxide-derived Cu surfaces is likely to originate from the dimerization of −CH3 intermediates. We attribute the higher selectivity for C2H6 and ethanol to a higher population of Cu+ sites in the thick oxide-derived Cu films, which helped to stabilize the −CH3 intermediates. additional hydrogenation steps.16−19 Many derived Cu films, especially the thinner ones, displayed a particular efficacy toward the production of C2H4, ethanol, or n-propanol, while exhibiting CH4 suppression.15,20−23 On the other hand, thicker derived Cu (and those originating from higher valenced Cu2+ precursors) films appeared to have a very different product distribution than their thinner counterparts. These reports indicate that thicker derived Cu could be a better catalyst for CO and formic acid production24−26 or enhanced C2H6, acetate, or ethanol production.25,27−31 These thicker derived Cu films appeared to have one thing in common: they showed lower production of C2H4 when compared to the thinner derived Cu films. Such discord in the CO2RR product distribution, among the thin and thick (or CuI- and CuII-) derived Cu, calls into question whether the general view of a universal C2 product formation pathway applies for all derived Cu surfaces.32,33

C

arbon dioxide (CO2) capture and conversion to valueadded products is a desirable and sustainable pathway to tackle the rising level of this greenhouse gas.1,2 Over the past 4 decades, research activities on CO2 utilization, especially through the electrochemical route, has been exceptionally intense.3 A number of new materials have been identified as catalysts for the CO2 reduction reaction (CO2RR),4,5 with copper exhibiting the highest selectivity toward the formation of hydrocarbons and alcohols.6−10 Among the copper electrodes investigated, those obtained from the reduction of Cu precursors with high oxidation states (usually in the initial forms of oxides and halides) were found to be exceptional in terms of energetic efficiency and selectivity for producing multicarbon (≥C2) compounds. The underlying causes for this efficacy have been pinpointed to crystallite sizes and facets,11,12 grain boundaries,13 strains,14 and the rise in local pH at the surface of the electrode.15 Ethylene (C2H4) is frequently chosen as the model compound to represent C2 products formed during CO2RR. The formation of this molecule, as shown from theory and experiment, is generally thought to proceed through the ratedetermining C−C coupling of CO intermediates, followed by © XXXX American Chemical Society

Received: June 14, 2017 Accepted: August 18, 2017

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Information section S1). Samples A and B consisted of ∼0.5−2 μm sized Cu2O polygons (Figure 1a). These morphologies are consistent with those reported earlier by our group on similar samples.12 Only signals originating from Cu2O and the metallic Cu substrate were observed from their X-ray diffractograms (Figure 1b). Simple H2O2 treatment at 100 °C was then employed to oxidize the surface of Sample B to CuII (termed Sample C).37 This oxidative procedure allows us to probe the effects of increasing the oxidation state of Cu without introducing more chemical precursors or excessive heating that can significantly alter the thickness of the film. After H2O2 treatment, the surface of Sample B changed to that of CuO nanoflakes, as indicated from its SEM image and XRD (Sample C, Figure 1a,b). The electrochemical reduction of CO2 was performed using Samples A−C in CO2-saturated aqueous 0.1 M KHCO3 electrolyte. A wide range of potentials from −0.48 to −1.13 V vs RHE (all potentials in this work are with respect to the RHE) was used. Increasing film thickness generally resulted in larger electroactive surface areas, and thus the total steady-state geometric current density (jtot) was enhanced (Table 1, Figure 1c). We however note that the increase in jtot from Samples A to B is not directly proportional to the increase in electroactive surface areas, suggesting that the overall electrocatalytic activity may have changed with the thicker film B. The contribution from H2 evolution becomes particularly dominant at potentials more negative than −0.83 V on Sample B, adversely affecting the CO2RR activity (SI section S2). This suggests that protoncoupled electron transfer (PCET) occurs at smaller overpotentials with the thicker film, or there is a more severe mass transport limitation of CO2 to the electrode.38,39 For comparison, increased HER was observed only at potentials negative to −1.1 V on a planar Cu surface.7,21 The mild H2O2 treatment did not significantly affect the film thickness,

The issue is this: the formation of C2 products such as C2H6 and ethanol, often detected on derived Cu in considerable quantities, has not been subjected to the same rigor of investigation like C2H4.15,29 As the CO dimerization mechanism to form C2H4 on Cu surfaces (especially on Cu(100)) becomes more widely accepted,18,34,35 the mechanisms for the formation of these aforementioned products are often sidestepped, or conveniently tagged to that of C2H4, that is, through the dimerization of −CO with further hydrogenation steps.27,36 Such oversimplifications ignore the fact that C2H6 and ethanol production have sometimes surpassed C2H4 at low overpotentials, especially on thick derived Cu surfaces.27,29 We note that Ma et al. have recognized that small amounts of C2H6 and ethanol detected on their Cu(OH)2-derived Cu surface may have been formed from a separate pathway, namely, through −CH3 dimerization or from the hydrogenation of a −OCH2CH3 intermediate.30 However, no data was offered to support this hypothesis. In this work, we report the significant formation of C2H6 and ethanol via different pathways on thick oxide-derived Cu. Two types of Cu2O film precursors with different thicknesses (Sample A: 1.3 μm; Sample B: 11.2 μm) were synthesized hydrothermally on metallic Cu disks (Table 1; Supporting Table 1. Physical Properties of Oxide-Derived Cu Films sample

capacitance (mF)

ECSA (cm2)

relative roughness (normalized to Sample A)

estimated film thickness (μm)

A B C

2.44 4.93 8.77

33.59 67.71 120.55

1.0 2.0 3.6

1.3 11.2 11.5

Figure 1. (a) Scanning electron micrographs, (b) X-ray diffractograms, and (c) plot of the steady-state total current density (jtot) vs the applied potential for Samples A, B, and C. 2104

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Figure 2. FEs of CO2RR products (a) C2H4, (b) C2H6, (c) CO, and (d) ethanol for Samples A, B, and C.

Figure 3. Detected hydrocarbon products from experiments with feed molecule or gas (a) C2H4(g), (b) CH3CHO(aq), and (c) diacetyl(aq). The purging gas was N2 at 20 sccm flow rate for all of these experiments. (d) Proposed mechanism of C2H4 and C2H6 formation from the diacetyl feed molecule at low overpotentials.

significantly lower peak FEC2H4 of 14.9% at −0.83 V. The C2H4 production trend of the more oxidized Sample C is similar to that of Sample B, peaking also at −0.83 V with an FEC2H4 of 12.5%. The factors underlying the differing FEC2H4 peak potential positions on films with increasing thicknesses have been elaborated in our previous work:12 we found that thicker/ rougher derived Cu samples can achieve similar jtot (and thus local pH) at smaller overpotentials. Hence, when comparisons

although a further increase in the surface area of Sample C was observed, attributable to the overgrowth of CuO nanoflakes (Table 1). The CO2RR activities of Samples A−C are presented in Figure 2. Sample A displayed activities that are typically observed on a thin oxide-derived Cu. C2H4 was the main C2 product, with its Faradaic efficiency (FE) peaking at 26.7% at −0.98 V (Figure 2a).12,22 In contrast, Sample B displayed a 2105

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has been proposed by Ma et al.30 To assess the feasibility of this route, we conducted experiments on Sample C in N2-purged KHCO3 electrolyte with 10 mM acetaldehyde added as the reactant. Acetaldehyde is the precursor molecule to −OCH2CH3.18,45 The applied potential was kept at −0.78 V vs RHE. Ethanol was found to be the sole product from this experiment (SI section S3), while C2H6 was not detected (Figure 3b). This finding strongly indicates that the formation of C2H6 from the −OCH2CH3 intermediate can be ruled out. This is not unexpected because this route requires stabilization of the C−O bond after sequential protonation of the terminal CH2 (of the −OCHCH2), and then, the carbonyl group (otherwise C2H4(g) is yielded).18 Even if −OCHCH2 does become −OCH2CH3, the breaking of the C−O bond to give ethane is not likely, with the latter preferring instead to be reduced to ethanol.34 We further note that acetaldehyde is a well-known precursor to ethanol during CO2 or CO reduction on a Cu surface.46,47 Here, we propose that C2H6 could be formed through the dimerization of −CH3 intermediates.30,46 Recent density functional theory calculations has suggested that the formation of −CH3 proceeds from −COH, which is also an intermediate for CH4.48 Due to the greatly suppressed CH4 production on derived Cu surfaces15 and the high −CO coverage required to stabilize the −COH intermediate,33 the −CH3 dimerization pathway to C2H6 is often viewed as infeasible. However, we note that a significant barrier of 0.4 eV, surmountable only with −1.15 V (RHE) bias, is required for the downhill conversion of −COH to CH4 on Cu(111).48 Such conversion is proposed to go through M−CHx (x = 0−3) intermediates. The existence of carbenoid (CH2) intermediates on Cu catalyst has been proposed.49 It is thus not unreasonable to posit that the suppression of CH4(g) formation on the thick derived Cu catalysts (SI section S2) may actually indicate a more stable −CH3 intermediate. To investigate the possible −CH3 related mechanism for C2H6 formation, 10 mM diacetyl [(CH3CO)2] was reduced on Sample C at −0.78 V in N2-purged 0.1 M KHCO3, the prime condition for C2H6 formation (Figure 3c). Diacetyl is a good choice of precursor molecule to investigate the possible involvement of −CH3 as this molecule is expected to cleave into two −COCH3 units and finally convert to 1:1 parts of adsorbed −CO and −CH3 on the catalyst surface (Figure 3d). If C2H4 and C2H6 were formed respectively from separate pathways of −CO and −CH3 dimerization, an equimolar ratio of adsorbed −CO and −CH3 obtained from diacetyl would lead to a C2H4/C2H6 molar ratio of 1. Otherwise, a much higher C2H4/C2H6 ratio would indicate that C2H4 and C2H6 are more likely to follow a unified pathway (i.e., through the dimerization of −CO, followed by hydrogenation of the C2 intermediate). Analysis of the headspace gas by gas chromatography indicates a C2H4/C2H6 molar ratio of 1.2 when diacetyl was used as an intermediate (Figure 3c). This suggests that the C2H6 observed from our sample C was formed through a distinct pathway from that of C2H4. Neither C2H4 nor C2H6 was observed in the absence of an applied potential. We note that no CH4 was observed on Sample C during the experiment involving diacetyl (expected elution time in the chromatogram, ∼2 min; Figure 3c). This is in stark contrast with results found on bare polycrystalline Cu (SI section S4) where only CH4 (but not C2H4 or C2H6) was observed. This demonstrates that the alternative, low overpotential pathway to C2H6 is only accessible on thick oxide-derived Cu catalyst. The

were taken at the same jtot, Cu crystallite size emerges as the dominant activity descriptor governing the selective formation of C2H4. The reduction of CO2 to C2H6 was strongly enhanced on Cu catalysts that are thicker and have higher valenced Cu (Figure 2b). On the thinner Sample A, very little C2H6 was observed, with a maximum FEC2H6 of 2.5% at −0.83 V. Sample B, in contrast, displayed a much higher C2H6 production, peaking at −0.78 V with an FEC2H6 of 7.8%. Sample C showed an even higher C2H6 selectivity, with its FEC2H6 maximizing at 10.5% at the same potential as that of Sample B. Note that the peak potentials for maximum C2H6 selectivity on all samples was found at more positive potentials (−0.78 V) as compared to those for optimum FEC2H4. The selectivity toward C2H6 (FEC2H6 up to 8.2%) also exceeded that of C2H4 (FEC2H4 up to 3.9%) at potentials more positive than −0.78 V on the thicker Sample C. Furthermore, higher CO and ethanol production was observed at much more positive potentials on thicker Samples B and C, compared to thinner Sample A (Figure 2c,d). These trends were found to be stable for the entire duration of the electrolysis (Figure S2). The presence of thick porous structures in some oxidederived Cu films to trap C2H4(g), leading to its longer residence time and, ultimately, hydrogenation to C2H6, is frequently invoked to rationalize the enhanced C 2 H 6 production during CO2RR.15,27,36 The hydrogenation of simple alkenes, in this case, C2H4 to C2H6, can be proposed to follow the Horuiti−Polanyi mechanism.40 Such a reaction has been found favorable on 4d or 5d transition metals such as Ru, Pd, or Pt.41,42 In this scheme, the alkene first needs to be adsorbed on the surface. This is followed by two sequential hydrogen addition steps from a reservoir of adsorbed H, followed by desorption of the alkane. For CO2RR conducted in aqueous electrolytes, the supply of hydrogen can be from direct proton/ electron transfers or from adsorbed −H* intermediates produced during H2 evolution. To test this possibility, we use gaseous C2H4 (200 ppm, N2 balance) as the reaction gas in place of CO2 at −0.78 V vs RHE. Sample C was selected as the catalyst in this experiment because it showed the largest selectivity for C2H6 (FE 10.5%) in CO2-purged electrolyte. Using this C2H4 gas feed, only trace amounts of C2H6 ( 16.4% at −0.48 V, Figure 2c).35,52 These findings strongly show that the −CH3 intermediate may be stabilized at low overpotentials on these thick oxide-derived Cu, leading to enhanced formation of C2H6, rather than CH4 (despite a thermodynamic propensity for forming CH4).33 We have also considered if morphological differences between Samples A−C could result in the enhanced production of C2H6. However, as independently found by Bell and our group, changes in surface morphologies or crystallite sizes of oxide-derived Cu have only been shown to correlate with changes in C2H4 production. We thus rule out the effects of morphological factors as the cause for the selective reduction of CO2 to ethane (C2H6).12,22,53 Further, we believe that the enhanced −CH3 stability on thicker oxide-derived Cu may also be responsible for the heightened ethanol production at small overpotentials (Figure 2d). Ethanol formation on Cu has been proposed to share a common pathway with C2H4 through CO dimerization followed by the −OCHCH2 intermediate.32 In fact, the potential-dependent profile of FEEtOH usually tracks that of FEC2H4.7,22,53 However, it is clear that the FEEtOH profile on thicker Samples B and C tailed much further to the lower overpotential region and even surpassed the FEC2H4 at potentials positive to −0.78 V (FEEtOH/FEC2H4 = 6.6−23.7; Figure 2a,d). The observation of a higher FEEtOH compared to FEC2H4 is unusual as hydrogenation on the carbonyl (OCH) side of the −OCHCH2 intermediate has been shown to be kinetically more favorable on Cu surfaces.18 The production of C2H4(g) thus usually dominates over that of ethanol on Cu catalysts.7,54 Disproportionally smaller C 2H4 production indicates that ethanol could be formed via a separate pathway at lower overpotential. One of such pathways has been proposed to involve CO insertion to −CH3 or −CH 2 intermediate.46,55,56 Unlike the CO dimerization route, the formation of −COCH3 (or −COCH2) intermediate through CO insertion may have allowed the formation of ethanol without having to go through the −OCHCH2 intermediate.

In summary, examination of the CO2RR behavior of oxidederived Cu has shown that C2H6 and ethanol products could be formed through a separate pathway from C2H4 on thick derived Cu at significantly smaller overpotentials. A higher oxidation state of the originating Cu species appears to further improve the selectivity for C2H6 and ethanol production. Mechanistic investigations using select intermediate molecules suggest that the bulk of C2H6 is likely to be produced from the dimerization of more stable −CH3 species, rather than further hydrogenation of C2H4(g) or −OCH2CH3 related intermediates. A different CO2RR strategy using a modest voltage generated from lowgrade heat sources57 could thus be used to generate these low overpotentials to obtain a higher total energy conversion efficiency of CO2 to fuels.

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EXPERIMENTAL SECTION Details of the experimental procedures can be found in the Supporting Information section S1. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00514. Detailed experimental procedures; Faradaic efficiency data on wider potential ranges; NMR spectrum; GC data; and SIMS data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Albertus D. Handoko: 0000-0002-5157-8633 Boon Siang Yeo: 0000-0003-1609-0867 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by a research grant from the Ministry of Education, Singapore (R-143-000-683-112). The authors thank Dr. Hwee Leng Seng (IMRE Materials Processing & Characterization) for the ToF-SIMS analysis.

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REFERENCES

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DOI: 10.1021/acsenergylett.7b00514 ACS Energy Lett. 2017, 2, 2103−2109

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DOI: 10.1021/acsenergylett.7b00514 ACS Energy Lett. 2017, 2, 2103−2109