Direct Observation on Reaction Intermediates and the Role of

Oct 23, 2017 - Cu is the only monometallic catalyst that produces a large amount of hydrocarbon fuels during the CO2 electrochemical reduction reactio...
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Direct Observation on Reaction Intermediates and the Role of Bicarbonate Anions in CO2 Electrochemical Reduction Reaction on Cu Surfaces Shangqian Zhu,† Bei Jiang,‡ Wen-Bin Cai,‡ and Minhua Shao*,†,§ †

Department of Chemical and Biological Engineering and §Energy Institute, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China ‡ Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, Fudan University, Shanghai 200433, China S Supporting Information *

understanding of the role of bicarbonate during CO2RR, especially on other metal surfaces that produce different products, in situ monitoring of the evolution of reaction intermediates along with potential change is of great interest. Herein, we combined the attenuated total reflection (ATR)SEIRAS, isotopic labeling, and electrochemical techniques to investigate the CO2RR mechanisms on a Cu thin film electrode, which is the only monometallic catalyst having a high selectivity of hydrocarbon fuels (CH4 and C2H4) in the products.4b ATR-SEIRAS is a powerful tool for determining adsorbed intermediates.7 Previous studies on CO2RR on Cu surfaces were mainly focused on monitoring the dynamic evolution of adsorbed CO,8 which was a crucial intermediate toward diverse products and can be easily detected due to its strong IR absorption features originating from both the chemical and the electromagnetic enhancement.9 Herein, the Cu thin film was prepared following a two-step wet chemical strategy.10 A Au layer (∼60 nm) was chemically deposited on the semispherical Si prism and electrochemically cleaned (Figure S1). A Cu overlayer was then electrochemically deposited on the Au substrate. The desired thickness (∼30 nm) could be achieved by controlling the total charge passed (Figure S2). The apparent enhancement factor based on CO adsorption was ∼28.11 It should be noted that the enhancement on other adsorbed species could be lower.7b The real-time Fourier transform infrared spectra were collected during the cathodic scan of the Cu thin film in a CO2saturated 0.1 M KHCO3 solution from +0.3 to −1.3 V and are shown in Figure 1a. The linear scanning voltammogram in Figure 1b (dotted line) showed no obvious reduction current until the potential reached about −0.3 V. No absorption band was observed between +0.3 and −0.3 V in Figure 1a, except for one moving negatively between 1544 and 1517 cm−1, which was assigned to the adsorbed carbonate anions with two oxygen atoms coordinated on the bridge site.8b,12 The desorption of carbonate anions was caused by stronger electric repulsions as potential became more negative. Various vibration features were observed between 2400 and 1300 cm−1 when potentials were lower than −0.4 V. The band assignments are summarized in Table S1. In the high wavenumber region, the CO band started to appear at −0.5 V along with the gradual depletion of CO2 in the

ABSTRACT: Cu is the only monometallic catalyst that produces a large amount of hydrocarbon fuels during the CO2 electrochemical reduction reaction (CO2RR). However, the CO2RR mechanism and the impact of electrolyte are unclear. In this communication, two important issues regarding the CO2RR on Cu surfaces are studied: (1) the direct observation on reaction intermediates and (2) the role of the electrolyte (KHCO3) in the reaction. Surfaceenhanced infrared absorption spectroscopy allows direct observation of several reaction intermediates that have never been detected before, except for the commonly detected CO. Another important finding is that CO2 molecules are mediated to the Cu surface via their equilibrium with bicarbonate anions instead of direct adsorption from the solution. These results shed light on the full understanding of the CO2RR on Cu surfaces and developing more advanced catalysts.

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he ever-increasing level of atmospheric carbon dioxide and limited fossil fuel reserves have driven intensive studies on electrochemical conversion of CO2 into value-added chemicals and fuels.1 However, several bottlenecks have hindered the wide adoption of this technology, especially the unsatisfying performance of catalysts.2 In addition to the intrinsic activities of catalysts, the electrode/electrolyte interface (e.g., electrolyte types used, specific adsorption of reaction intermediates, active site availability, etc.) also plays a critical role in electrocatalysis.3 It is interesting to note that as one of the most popular electrolytes used in the aqueous CO2 electrochemical reduction reaction (CO2RR), bicarbonate-based solutions have been regarded only as a pH buffer and proton donor for a long period.4 The exact role of bicarbonate in CO2RR has not been systematically studied regardless of its substantial impacts on the reaction rate and product selectivity.5 Recently, the role of interfacial bicarbonate anions on CO2RR on a Au thin film electrode was studied by combining the potential step and surface-enhanced infrared absorption spectroscopy (SEIRAS) techniques.6 It was found that bicarbonate could enhance the reaction rate of CO2RR on Au by increasing the CO2 concentration through a rapid equilibrium exchange between the two species, which served as the CO2 source in the reaction.6 In order to have a full © 2017 American Chemical Society

Received: September 30, 2017 Published: October 23, 2017 15664

DOI: 10.1021/jacs.7b10462 J. Am. Chem. Soc. 2017, 139, 15664−15667

Communication

Journal of the American Chemical Society

band was observed between 1420 and 1330 cm−1 (from −0.4 to −0.9 V). Fitting the spectra obtained at −0.7 and −0.8 V revealed two individual bands centered near 1370 and 1400 cm−1 (Figure S3), respectively, which can be assigned to a carboxyl intermediate (*COOH) and a bidentate COO− species with two oxygens coordinated on Cu atoms. 17 These two intermediates possibly separated in the subsequent reaction pathways to produce *CO and HCOOH, respectively.18 A sharp rising band at ∼1394 cm−1 at potentials lower than −0.9 V was assigned to carbonate anions in the solution, which was also consistent with the ATR-IR spectra of K2CO3 solutions recorded on a bare ZnSe prism surface (Figure S4). Hence, the high intensity of this band indicates the accumulation of carbonate anions near the Cu surface. In the high current density region, protons were quickly consumed, resulting in an increase of local pH, which could shift the equilibrium between bicarbonate and carbonate anions to the latter one. In other words, the band intensity of solution carbonate anions can serve as an indicator on the local pH. Most of these vibration features were also observed in the similar potential regions in the subsequent anodic scan (Figure S5). Similar experiments were also conducted in an Ar-saturated KHCO3 solution (Figure 2a). Beginning at −0.7 V in the

Figure 1. (a) Real-time ATR-SEIRAS spectra and (b) cyclic voltammogram and integrated band intensities recorded during the cathodic scan of the Cu thin film electrode in a CO2-saturated 0.1 M KHCO3 solution. Reference spectrum was taken at 0.3 V vs RHE. Figure 2. Real-time ATR-SEIRAS spectra recorded during the cyclic scan of the Cu thin film electrode in an Ar-saturated 0.1 M (a) KH12CO3 and (b) KH13CO3 solutions. Reference spectrum was taken at 0.3 V vs RHE.

solution. Its broad tailing and asymmetric shape was possibly related with the complex distribution of local CO binding environments,8c the Cu surface reconstruction and inhomogeneity,8a and Fano resonance.13 Its intensity reached the maximum at −0.8 V and then gradually decreased (red dot in Figure 1b), which was possibly caused by the CO2 mass transport and the formation of hydrocarbons,8c in which CO is an intermediate. The CO band position initially blue-shifted and then red-shifted as the potential swept negatively, which was caused by both the change of Fano line shape13 (coverage dependent) and Stark tuning effect.14 The band position shifted to lower wavenumbers from −1.0 to −1.2 V with a rate of 35 cm−1 V−1, which was larger than the pure Stark tuning rate of 20 cm−1 V−1 reported on a smooth Cu electrode.14 Interestingly, we also detected several bands that have never been detected during CO2RR on Cu-based catalysts. A weak band near 1720 cm−1 was observed between −0.6 and −1.2 V. Its position was very close to the simulated vibration frequency of adsorbed formyl (*CHO, 1741 cm−1),8c which is also a key intermediate after the subsequent protonation of surface-bonded CO. It could also be assigned to multiply adsorbed CO.15 Revealing its nature was difficult due to an extremely complicated composition of products of CO2RR on Cu surfaces.16 Its similar band intensity evolution and slightly delayed maximum peak position as compared with COL indicated that its formation may rely on the latter one (blue triangle in Figure 1b). A convoluted

cathodic scan, a broad peak located near 2020 cm−1 started to appear. The band intensity gradually increased, and the position moved to higher wavenumbers. The other band at 2070 cm−1 that was much sharper appeared at a lower potential (∼−0.9 V), whose intensity increased dramatically at −1.0 V. The intensities of both peaks started to decrease at −0.6 V in the subsequent anodic scan. The sharp band was assigned to COL on Cu surfaces.8c The broad band may originate from COL or COB.19 More studies are needed to confirm its nature. Isotopic labeling (KH13CO3 solution) was also employed to exclude the contribution of the Cu−H bond, which was observed in the same region.8b,17a The two pairs of IR absorption bands (Figure 2b) were also detected in the similar potential region. The positions of the bands, however, were ∼50 cm−1 red-shifted as compared to those recorded in the KH12CO3 solution. The red shifts are consistent with the theoretical prediction on CO using the harmonic oscillator model,8c confirming that these two bands originally come from surface-bonded CO rather than Cu−H. The possibility that CO was adsorbed on the unexpected exposure of a Au underlayer during reaction was also excluded by repeating the same experiment on the bare Au thin film, which shows no band between 2100 and 1900 cm−1 in the same 15665

DOI: 10.1021/jacs.7b10462 J. Am. Chem. Soc. 2017, 139, 15664−15667

Communication

Journal of the American Chemical Society

mainly originates from CO2 in equilibrium with bicarbonate anions instead of free molecules in the solution. In addition, the consumption of CO2 in equilibrium with bicarbonate anions and protons in water can increase the local pH near the electrode surface. Consequently, free CO2 in the solution can be absorbed and become bicarbonate anions. As a result, the IR band from 12CO was expected to appear as the reaction continued. As expected, another peak located near 2045 cm−1 that can be assigned to 12CO was observed at ∼18 s in Figure 3b. Its intensity gradually increased and became comparable to that of the 13CO band at 60 s, due to the gradual conversion of H13CO32− into H12CO32− near the electrode surface. It should be mentioned that the relative peak intensities were not necessarily proportional to the ratio of their surface coverages, as it is unclear whether there is intermolecular coupling between adsorbed 13CO and 12CO.20,21 According to the in situ spectroscopic results, it is clear that the CO2 source is mainly from bicarbonate anions instead of the free CO2 molecules in the solution during the CO2RR on Cu surfaces. Understanding the possible reasons for this phenomenon is of great interest. It should be mentioned that the potentials where CO2RR occurs are much more negative than the potential of zero charge on Cu (∼−0.7 V vs SHE).22 Hence, hydrated cations would densely accumulate in the outer Helmholtz plane,23 which may hinder the interaction between free CO2 molecules in the solution and electrode surfaces. The negatively charged bicarbonate anions would possibly approach the electrode surfaces more easily due to their electric attraction interaction with cations. Herein, we proposed that these bicarbonate anions near the electrode surface can release CO2 molecules for CO2RR via the fast equilibrium shown in the following equation:

potential region in either Ar- or CO2-saturated bicarbonate solution (Figure S6). The absence of a CO adsorption signal was possibly caused by the rather weak binding strength of CO and its fast desorption from Au surfaces, which echoed well with the previous study by Dunwell et al.6 The successful observation of adsorbed CO bands on Cu surfaces in our in situ experiments indicates that the concentration of CO2 in equilibrium with solution bicarbonate anions is sufficient to drive the CO2RR, although the reaction rate might be extremely low according to the rather weak band intensity. However, the exact concentration of CO2 in the Ar-saturated solution cannot be measured accurately. To the best of our knowledge, it is the first time confirming that CO could be formed in Ar-saturated KHCO3 solutions on Cu surfaces. Our results indicate that bicarbonate anions may play a crucial role in CO2RR on Cu surfaces instead of being only a pH buffer and proton source. To further reveal the role of bicarbonate anions in CO2RR, fast real-time IR spectra were collected, while stepping the potential from 0.2 to −0.6 V in 12CO2-saturated KH12CO3 and KH13CO3 solutions. As shown in Figure 3a, a broad peak was observed at

HCO−3 + H 2O ⇌ H 2CO3 + OH− ⇌ CO2 + H 2O + OH−

The continuous consumption and regeneration (or migration) mode of bicarbonate anions possibly mediated the CO2 to the Cu surfaces for reaction, instead of the direct adsorption of solutionphase CO2. The above proposed mechanism is illustrated in Figure 4. In summary, we investigated the CO2RR process on a Cu thin film supported on a Au substrate by the powerful ATR-SEIRAS technique. In addition to surface-bonded CO, a frequently reported and well-studied reaction intermediate/product, the evolution of several other intermediates was observed for the first time. A new band near 1720 cm−1 was detected between −0.6

Figure 3. Real-time ATR-SEIRAS spectra recorded after stepping the Cu thin film electrode to −0.6 V in a CO2 prepurged 0.1 M (a) KH12CO3 and (b) KH13CO3 solutions. Reference spectrum was taken at 0.2 V vs RHE; six interferograms were co-added for each spectrum.

∼2040 cm−1 in the first 3 s in the 12CO2-purged KH12CO3. Blue shift and increase in band intensity, along with the corresponding depletion of solution 12CO2, were simultaneously detected. Interestingly, the broad band observed in the first 3 s in 12CO2purged KH13CO3 was shifted to ∼1990 cm−1 from 2040 cm−1, and the corresponding depletion of solution CO2 was 13CO2 (2277 cm−1) instead of the unlabeled one (Figure 3b). This result gave a direct evidence that the CO2 source for CO2RR

Figure 4. Schematic illustration on proposed role of bicarbonate anions in CO2RR. 15666

DOI: 10.1021/jacs.7b10462 J. Am. Chem. Soc. 2017, 139, 15664−15667

Communication

Journal of the American Chemical Society and −1.2 V, which can be possibly assigned to multiple CO intermediate or *CHO, a key intermediate after the subsequent protonation of adsorbed CO. Two other weak bands observed near 1380 (*COOH) and 1400 (bidentate COO−) cm−1 from −0.4 to −0.9 V implied the coexistence of these two intermediates after the initial electron transfer and protonation on CO2 molecules. More importantly, surface-bonded CO was also detected in Ar-saturated KHCO3 solutions between −0.7 and −1.0 V, indicating that bicarbonate anions may play a critical role in the reaction instead of a simple pH buffer or proton source. By combining fast ATR-SEIRAS analysis, isotopic labeling, and potential stepping techniques, the isotopic nature of initial surface-generated CO was found to be identical with that of bicarbonate anions instead of the free CO2 molecules in the solution. This result indicates the CO2 reactant in CO2RR originates from the CO2 molecules in equilibrium with bicarbonate anions rather than those in the solution. Hence, we propose that CO2 is mediated to the Cu surfaces via its equilibrium with bicarbonate anions rather than direct adsorption from the solution. Our study adds significant new insights on the reaction mechanisms and role of bicarbonate anions in the CO2RR on Cu surfaces.

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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/jacs.7b10462. ATR-SEIRAS setup, Cu thin film preparation procedure, additional electrochemical measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Shangqian Zhu: 0000-0002-5813-9588 Wen-Bin Cai: 0000-0003-0500-4791 Minhua Shao: 0000-0003-4496-0057 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.Z. and M.S. acknowledge the support from Research Grant Council (26206115), and a startup fund from the Hong Kong University of Science and Technology. B.J. and W.B.C. acknowledge the financial support from the 973 Program of the Chinese Ministry of Science and Technology (2015CB932303), and Natural Science Foundation of China (21473039 and 21733004). S.Z. thanks the HKJEBN Group for providing the Ph.D. scholarship, and especially Dr. Han Wang for help in the ATR-SEIRAS construction.

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REFERENCES

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DOI: 10.1021/jacs.7b10462 J. Am. Chem. Soc. 2017, 139, 15664−15667