Significance The understanding of a catalytic reaction starts with understanding its first elementary step. Surprisingly, despite the large number of studies, it is unclear whether one common or two different first intermediates control the selectivity of CO 2 electroreduction to formate and CO. We settle this controversy for Cu, which is best known for its unique capacity to synthesize C 1+ products but is just emerging as a superior earth-abundant catalyst for CO and formate. We provide solid experimental and theoretical support of the one common first-intermediate (Hori’s) model, the first intermediate being carboxylate. This outcome is an essential milestone toward accurate specification of the reaction descriptors in the growing effort to accelerate the discovery of a viable CO 2 electroreduction catalyst.

Abstract We resolve the long-standing controversy about the first step of the CO 2 electroreduction to fuels in aqueous electrolytes by providing direct spectroscopic evidence that the first intermediate of the CO 2 conversion to formate on copper is a carboxylate anion *CO 2 − coordinated to the surface through one of its C–O bonds. We identify this intermediate and gain insight into its formation, its chemical and electronic properties, as well as its dependence on the electrode potential by taking advantage of a cutting-edge methodology that includes operando surface-enhanced Raman scattering (SERS) empowered by isotope exchange and electrochemical Stark effects, reaction kinetics (Tafel) analysis, and density functional theory (DFT) simulations. The SERS spectra are measured on an operating Cu surface. These results advance the mechanistic understanding of CO 2 electroreduction and its selectivity to carbon monoxide and formate.

The electrocatalytic conversion of abundant CO 2 to fuels and other useful chemicals has attracted significant interest in the past decade as a promising complementary way to store excessive renewable energy (1, 2). However, commercialization of this green technology is hampered by the absence of a viable catalyst. Even though the catalyst discovery can significantly be accelerated using the emerging approach of computational catalyst design (3), its applicability to CO 2 electroreduction has been limited mostly to explaining experimental trends, rather than to predicting new materials (4⇓–6). This situation roots from a poor mechanistic understanding of this reaction, which starts with a poor understanding of its very first step—activation of CO 2 .

In fact, it is currently unclear whether CO 2 electroreduction on different materials starts with one common or several different first intermediates and what their structures are. Specifically, Hori et al. (7, 8) have postulated that, on metals such as Au, Cu, and Ag with medium to high overpotentials for hydrogen evolution reaction (HER), CO 2 is reduced through one common first intermediate, which is carboxylate *CO 2 − (* denotes the surface-coordinated state of the ligand): C O 2 + e − = * CO 2 − . [1]This first step is supported by the reaction kinetics analysis (7, 9⇓⇓⇓⇓⇓–15).

If so, it is still unclear what the structure of *CO 2 − is and how it controls the reaction pathway. As a ligand, CO 2 is very versatile in terms of its coordination to metal centers (16, 17). In the context of CO 2 electroreduction, the most often cited carboxylate structures include η2(O,O)-CO 2 − (O,O-down) (18⇓–20), η1(C)-CO 2 − (C-down) (21), and η2(C,O)-CO 2 − (C,O-down) (22⇓–24) (Fig. 1). As proposed by Hori et al. (7, 8), strongly adsorbed η1(C)-CO 2 − or η2(C,O)-CO 2 − can be converted to CO after they are protonated to carboxyl *COOH (Fig. 1): * CO 2 − + H + = * COOH , [2]which reductively dissociates to CO and H 2 O upon proton coupled electron transfer (PCET): * COOH + e − + H + = CO + H 2 O . [3]In contrast, the reaction can be directed toward formate if *CO 2 − is adsorbed through oxygen(s) or weakly through carbon. This structure allows the carbon atom to be hydrogenated by either a direct Tafel-like reaction with surface hydride *H, * CO 2 − + * H = HCOO − , [4a]or by PCET, * CO 2 − + e − + H + = HCOO − . [4b]An alternative is a more contemporary model that has largely been drawn from density functional theory (DFT) simulations (5, 6, 22, 25, 26). It postulates that CO and formate have different precursors—carboxyl *COOH and O (or O,O)-coordinated formyloxyl *OCHO, respectively (Fig. 1). However, there is no consensus about how these two species are formed. According to Nørskov and coworkers (5, 26), carboxyl and formyloxyl are formed through PCET: C O 2 + e − + H + = * COOH [5]and C O 2 + e − + H + = * OCHO , [6]respectively. In contrast, Goddard and coworkers (22, 25) adhere to the [1] and [2] pathway for carboxyl, and the hydride transfer (Eley–Rideal) mechanism for formyloxyl: C O 2 + * H = * OCHO . [7]The third possibility can be the direct two-electron reduction of adsorbed bicarbonate HCO 3 − (27, 28).

Fig. 1. Possible first intermediates of CO 2 electroreduction in aqueous electrolytes.

The aforementioned uncertainty is explained by the phenomenological complexity of CO 2 electroreduction, coupled with intrinsic limitations of the electrochemical and computational methods. Electrochemical surface-enhanced vibrational spectroscopy has not helped much in clarifying this issue either, even though this method has already established itself as a powerful tool to magnify our view of electrified interfaces (29). The only surface species unambiguously identified in aqueous electrolytes on Cu (30⇓⇓⇓⇓–35), Au (14), and Ag (36, 37) is carbon monoxide, while the structure of the other surface species remains highly controversial. This situation stems from the fact that the adsorbate structures have mostly been proposed based on the similarity of their one C–O stretching peak with a peak theoretically predicted or observed in other systems without corroborating by other spectral peaks and the 13C/12C and D 2 O/H 2 O isotope exchange effects. This approach is evidently reductionistic as it neglects the strong effects of the catalyst surface, interfacial electric field (including ionic pairing with a counterion), and hydration on the adsorbate spectrum, which can lead to great confusion (38, 39). Moreover, frequencies of the C–O stretching vibrations involve the same CO 2 moiety. Hence, their values are not characteristic of the species shown in Fig. 1. Another source of confusion can be uncontrollable organic contaminations, which can be spotted by the 13C/12C isotope exchange effect. Finally, to get surface enhancement, the earlier studies have been performed on model roughened surfaces, without validating their catalytic activity. As a result, even if the CO 2 -derived adsorbates are identified correctly, it does not warrant that the reaction intermediate is among them because the latter can be formed only on an operating catalyst.

In our study, we focus on Cu because it is one of the most effective pure metal catalysts of CO 2 electroreduction, along with Ag, Pd, and Au (18). At the same time, Cu stands out of this group due to its low cost and unique capacity to synthesize C 1+ products. The interest in Cu has additionally been elevated by recent findings of its remarkably high efficiency in the synthesis of CO and formate (12, 40, 41)—two of the most feasible targets of the CO 2 conversion technology (2).

Paradoxically, among the large number of studies of CO 2 electroreduction on Cu, only three have attempted to gain insight into the first step of this reaction (12, 22, 26). In particular, Li and Kanan (12) have concluded on the basis of the experimental Tafel slope that the CO synthesis on Cu starts with the formation of carboxylate *CO 2 − [1]. This result is supported by a quantum chemical study of Goddard and coworkers (22), which predicts that CO 2 is converted to CO on Cu(100) through steps [1]–[3]. In contrast, according to Nørskov and coworkers (26), Cu(111) and Cu(100) prefer to synthesize formate because these surfaces provide a more favorable thermodynamics for the formation of formyloxyl [6] over carboxyl [5]. Thus, the chemical identity of the first intermediate of the CO 2 electroreduction on Cu and hence the nature of the selectivity of Cu to CO and formate remains elusive.

To bridge this gap, we take advantage of SERS. In contrast to its more often used rival, surface-enhanced infrared absorption spectroscopy (SEIRAS), SERS can be measured on an actual catalyst. In addition, SERS can access a much broader spectral range, which allows for more reliable interpretation of the adsorbate structure. Moreover, SERS spectra are absolute, while SEIRAS spectra are differential and therefore are distorted by the negative peaks of the species present on the electrode surface at the reference potential.

To verify our assignment of SERS peaks, we use the D 2 O/H 2 O and 13C/12C isotope exchange, as well as electrochemical Stark effects. The latter presents the shift of the vibrational frequency of the adsorbate with electrode potential. To the first order of the theory, the frequency shift Δ ν by electric field F → can be written as Δ ν ≈ − | Δ μ → | ⋅ | Δ F → | ⋅ cos ⁡ φ , where Δ F → is the change in the local electric field, Δ μ → is the change in the vibrating dipole moment between the ground and first excited vibrational states, and φ is the angle between the vectors of the dipole and the field (42). Hence, the sign of the Stark effect characterizes the orientation of the adsorbate with respect to the applied electric field F → . A change in the Stark tuning rate ( Δ ν / Δ E , where Δ E is a change in the electrode potential) suggests that the adsorbate or its local environment is perturbed by a certain surface reaction, which can help identify peaks that characterize the same adsorbate. We further corroborate the spectroscopy-based conclusions with the Tafel analysis of the reaction kinetics and with DFT simulations.

Using this advanced approach, we validate Hori’s hypothesis that CO 2 electroreduction starts with a common first intermediate, which is carboxylate. We identify the structure of this intermediate as η2(C,O)-CO 2 −, find that it can be formed at potentials significantly anodic of the onset of CO 2 electroreduction, and conclude that the electrocatalytic activity of the metal ties with strengths of the metal–C and metal–O bonds of η2(C,O)-CO 2 −.

Conclusions By applying a comprehensive suite of experimental and theoretical methods, we establish that the CO 2 conversion to formate on Cu starts with the formation of a carboxylate intermediate with the η2(C,O)-CO 2 − structure. This result can be generalized toward at least coinage metals (Cu, Au, and Ag), given their similarity in terms of the reaction kinetics and the electronic properties. At the same time, the formation of *COOH is unlikely on Cu at circumneutral and basic pH, which implies that η2(C,O)-CO 2 − is converted to CO through direct dissociation [9] as in gas phase. η2(C,O)-CO 2 − is stabilized on a Cu surface due to the strong covalency of its two bonds with the surface, charge polarization in the system, as well as by the electrostatic interactions with a hydrated Na cation in the on-top position and the positively charged coordinating Cu atoms. The observed formation of η2(C,O)-CO 2 − at potentials much more anodic of the onset of the CO 2 electroreduction suggests the critical role of additional cooperative effects such as surface defects, residual surface oxide, and coadsorbed electron donors, which are yet to be understood. A negative potential activates the Cu–C and C–O bonds of η2(C,O)-CO 2 − en route to HCOO− (Fig. 7), which is explained by the electrostatic and chemical effects. In contrast, the Cu–O bond can be stabilized, underscoring its important role in the selectivity of η2(C,O)-CO 2 − to CO and HCOO−. These results indicate that the descriptors of the CO 2 conversion to HCOO− and CO are the chemical and structural properties of η2(C,O)-CO 2 −, raising intriguing questions about the exact relationship of these properties to the selectivity, activity, and energy efficiency of the catalyst.

Acknowledgments We thank Prof. Bob Farrauto for sharing a micro-GC instrument, Dr. Qinghe (Angela) Zheng for initial technical support of the micro-GC measurements, as well as the anonymous reviewers for their suggestions and corrections. This research was enabled in part by computational support provided by Compute Canada. I.V.C. and P.S. acknowledge funding support from the National Science Foundation under Award 1336845 as well as through the Industry/University Cooperative Research Center’s Center for Particulate and Surfactant Systems (IIP-0749461). S.P. acknowledges the support from Chemical and Petroleum Engineering, Canada First Research Excellence Fund at University of Calgary, and Natural Sciences and Engineering Research Council of Canada Discovery Grant RGPIN-2016-03851.

Footnotes Author contributions: I.V.C. designed research; I.V.C. and S.P. performed research; I.V.C. and P.S. contributed new analytic tools; I.V.C. and S.P. analyzed data; I.V.C. conceived the main ideas; P.S. discussed results; and I.V.C. and S.P. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

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