Carbon-Nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution

doi:10.21203/rs.3.rs-841663/v1

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Carbon-Nitrogen bond formation on Cu electrodes during CO₂reduction in NO₃^- solution

https://doi.org/10.21203/rs.3.rs-841663/v1

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In performing electrochemical reduction of CO₂ over Cu electrodes, the anions present in solution typically do not participate in the formation of reaction products. NO₃^-is an exception, and previous reports indicate the formation of urea in certain process conditions. Here we demonstrate by use of Surface Enhanced Raman Spectroscopy and Electrochemical Mass spectrometry that simultaneous reduction of NO₃^-and CO₂ on Cu surfaces forms carbon-nitrogen bonds in the form of cyanide. The Raman peak position of C≡N is dependent on the oxidation state of the Cu surface, and Cu-C≡N can be oxidized by anodic polarization yielding NO. More importantly, Cyanide likely forms soluble Cu-C≡N complexes, which cause catalyst surface instability. The implications of this observation for practical application of a process for electrochemical formation of urea, are discussed.

Electrochemistry

Catalysis

Materials Chemistry

CO2 reduction

Cu electrodes

carbon-nitrogen bonds

Urea is one of the most important nitrogen-based fertilizers playing a very important role in securing the worldwide food supply¹. The chemical is industrially produced from CO₂ and NH₃ in an energy intensive process consuming 173 kWh per ton of urea², with an annual production capacity of 155 Mt³. Although ammonia needed for urea production is mainly produced by the Haber-Bosch process, non-conventional renewable technologies for ammonia synthesis, which can allow for small scale, green ammonia production, are gaining significant attention^4,5. Unfortunately, while economically attractive, electrochemical N₂ reduction to NH₃ suffers from very low activity⁶. Alternatively, NO₃^- can be considered as feedstock, which provides facile reaction kinetics and requires a significantly lower overpotential^7–9. Green ammonia from NO₃^- could be coupled with urea production in a consecutive reactor, located where waste NO₃^- streams are available¹⁰.

Rather than application of an electrochemical- and catalytic reactor in series, proof of principle for simultaneous reduction of CO₂ and NO₃^- in a single electrochemical reactor, using gas diffusion electrodes, was established,^11–14 including the use of different nitrogen sources such as NO₃^- ^15–17, NO₂^- ^18–20 and N₂ ^21–23. Since copper is a good catalyst for CO₂ activation²⁴ as well as for NH₃ formation from NO₃^‑ ²⁵, copper was mostly investigated for electrochemical urea synthesis. Usually anions present in solution do not participate in electrochemical transformation of CO₂ over Cu electrodes, but apparently NO₃^-is an exception. However, little is known about the C-N coupling mechanism on Cu surfaces, and the hypothesis in the literature of CO and NH_x surface intermediates is rather speculative. In the present study, we discuss the use of Surface Enhanced Raman Spectroscopy (SERS) to identify adsorbed species on a polycrystalline, rough Cu electrodes during simultaneous reduction of CO₂ and NO₃^-. SERS is a suitable technique to detect reaction intermediates and it was already used to monitor electrochemical reduction of CO₂ on Cu surfaces^26,27. Moreover, Electrochemical Mass Spectrometry (EC-MS) was used to study potential dependent desorption of products from the electrode surface, which in combination with SERS resulted in detailed understanding of the surface processes occurring during electrochemical reduction of CO₂ in the presence of NO₃^-. Formation of carbon-nitrogen bonds in the form of cyanide was revealed, which was never previously observed at room temperature in aqueous electrolyte.²⁸ We thus contribute to understanding of electrochemical C-N coupling reactions recently studied in the literature^29–32. More importantly, we demonstrate that cyanide likely leads to formation of soluble Cu-C≡N complexes, which suggests a stable process for urea formation based on Cu electrodes might be difficult to achieve.

In situ SERS of Cu electrode in the presence of CO₂ and NO₃⁻

SERS of the Cu surface in CO₂ saturated 0.1 M KHCO₃ with 50 mM KNO₃ during cyclic voltammetry was performed, and is shown separately for reductive and oxidative scans (Fig. 1b and 1c, respectively). The expected characteristic signals for Cu-oxides were observed in voltametry. In the reductive scan at 0.5 V reduction of CuO to Cu₂O is observed, followed by reduction of Cu₂O to metallic Cu (Fig. 1a). Those surface chemical changes are in agreement with alterations in intensity of the characteristic peaks at 610 and 520 cm^− 1 in the SER spectra, which have been assigned to Cu₂O³³ (Fig. 1b). At more negative potentials those signals disappear, only to re-appear again in an oxidative scan at 0.5 V (Fig. 1c) and higher, in agreement with the oxidative current in Fig. 1a, which can thus be assigned to oxidation of Cu to Cu₂O. Cyclic voltammograms of blank measurements can be found in the Supplementary Figure S1, and Raman spectra in reference conditions in Supplementary Figures S2 (CO₂ reduction in bicarbonate solution) and S3 (nitrate reduction in the absence of CO₂), confirming that the Raman peaks at 1048 and 1072 cm^− 1 in Figs. 1b and 1c can be assigned to nitrate in solution, and surface adsorbed carbonate, respectively.

In a reductive scan (Fig. 1b) up to 0.3 V, little changes are observed in the Raman spectra. When reduction of Cu-oxides is completed, a signal at ~ 2080 cm^− 1 appears and grows in intensity at more negative potential. Although the band position is similar to that assigned to adsorbed C ≡ O, the potential at which this band is observed (0.3 V vs RHE) is quite different from previous observations in the absence of nitrate. Moreover, the standard reduction potential of CO₂ to CO is -0.106 V vs RHE³⁴, and since the first CV cycle is shown excluding residual CO from previous scans, CO cannot be present at 0.3 V vs RHE. At more negative potentials (-0.7 V vs RHE), the signal at 360 cm^− 1 from Cu-CO should appear in case CO is actually formed (Supplementary Figures S2 and S4). However, such signal is not observed at any moment during a CV in the presence of NO₃⁻. In addition, the CO signal at 2080 cm^− 1 observed in the absence of nitrate has relatively low intensity compared to the peak observed here (Magnification in Fig. 1b). Surprisingly, although the intensity of adsorbed CO₃²⁻ at 1070 cm^− 1 decreases at more negative potential, it doesn’t disappear completely, and appears more stable in the presence, than in the absence of nitrate (compare the measurement in the absence of nitrate in Supplementary Figure S4). This could suggest that the presence of NO₃⁻ in the system prevents conversion of surface adsorbed CO₂ to CO to some extent, which is in agreement with the absence of signals of CO in the Raman spectra. The presence of small amounts of NO_x in CO₂ has been previously found to significantly reduce selectivity of CO₂RR (favouring nitrate reduction products), because of the lower onset potential for reduction of NO_x compared to CO₂^34,35.

During the oxidative scan (Fig. 1c), the 2080 cm^− 1 peak does not disappear at -0.4 V vs RHE like in the case of CO formed by reduction of CO₂. This is additional proof that the observed signal originates from a product formed only in the presence of NO₃⁻ and CO₃²⁻. Finally, when Cu₂O is formed (0.5 V vs RHE), the ~ 2080 cm^− 1 peak disappears while a new signal at 2150 cm^− 1 appears. The Stark effect (Supplementary Figure S5), proving that the signals originate from surface-absorbed species, is observed for both the ~ 2080 cm^− 1 and 2150 cm^− 1 bands, respectively. Besides CO, carbon-nitrogen triple bonds (C ≡ N) show significant intensity in the wavenumber range of interest, out of which cyanide is the most simple compound^43,44. Although it was also reported that Cu-H (adsorbed hydrogen) can appear at the same position in SEIRAS⁴⁵, in a control experiment in Ar saturated 0.1 M KClO₄ signals were absent (Supplementary Figure S6).

In order to prove that the reaction is electrochemically driven, SERS of a Cu surface in CO₂ saturated 0.1 M KHCO₃ with 50 mM KNO₃ was recorded at OCV (Fig. 2a). Clearly bands in the 2000–2500 cm^− 1 remain absent as a function of time. At -0.3 V vs RHE, a broad peak at 2080 cm^− 1 appears and remains visible throughout the measurement. Subsequently, at OCV when Cu₂O signals re-appear due to surface oxidation of metallic Cu, the band at 2150 cm^− 1 appears and slightly decreases in intensity over time while shifting position from 2154 cm^− 1 to 2149 cm^− 1. This suggests that the product formed under reductive potential, changes frequency when oxidation of Cu occurs to form a Cu₂O containing surface. Moreover, in order to exclude any homogeneous reactions induced by possible Cu ions present in the solution, cyclic voltammetry in blank electrolytes was performed and SERS at OCV was measured while adding KHCO₃ or KNO₃ (Fig. 2b). The signal at 2150 cm^− 1 was not observed in any experiment, proving that the reaction is driven electrochemically and can be assigned to CuC ≡ N.

A summary of the observed SERS signals can be found in Table 1.

Table 1

**Summary of SERS signals observed in this work**. *Signals related to cyanide observed in this work.
Position [cm^− 1]	Assignment	Ref.
275	Cu-CO	^27,36
360	Cu-CO	^27,36
350	CO₃²⁻ (C-O)_asym	²⁷
1540	CO₃²⁻ (C-O)_asym	²⁷
520	Cu₂O	³³
610	Cu₂O	³³
1048	NO₃⁻	^37,38
1072	CO₃²⁻ (C-O)_sym	^27,39
1360	bicarbonate	²⁷
1640	H₂O	²⁷
2070	C ≡ O	^27,39
~ 2080^*	Cu(C ≡ N)_n⁽ⁿ⁻¹⁾	^40,41
~ 2150^*	CuC ≡ N	⁴⁰
300^*	Cu-CN	⁴²

We will now discuss if C ≡ N is a possible product of reduction of urea, or formed in a route via intermediates.

Urea decomposition – Does it form Cu-CN?

Surprisingly, the bands observed in the Raman spectra of Fig. 2, do not provide much information regarding (intermediates in) the formation of urea. To evaluate urea chemistry over Cu surfaces, potential dependent SERS analysis of 50 mM urea, in CO₂ saturated 0.1 M KHCO₃, is shown in Supplementary Figure S7. Several Raman signals can be assigned to adsorbed urea on Cu surfaces, while a strong vibration at 713 cm^− 1 is apparent after reduction of Cu₂O at 0 V vs RHE (Figure S7a), shifting to lower frequency as a function of increasingly negative potential (Figures S7b and S7c). We speculate this band is due to the C-N vibration (strong at ~ 1005 cm^− 1 in solution) of urea, shifted to lower wavenumber by strong adsorption/interaction of urea with the Cu/Cu⁺ surface. Interestingly, when urea is exposed to Cu at reductive potentials, a Cu-C ≡ N related signal appears in the Raman spectra, as well as the Cu-CO band at ~ 360 cm^− 1 at the most negative potentials (< -0.9 V) applied, the result of conversion of CO₂ to CO and coinciding with the disappearance of the peak at ~ 710 cm^− 1. Apparently urea does not prevent reduction of CO₂, contrary to nitrate.

Urea in solution can undergo multiple reactions, but formation of C ≡ N has not been previously reported in reductive conditions^46,47. It can hydrolyse to cyanate and ammonium, which is a reversible reaction⁴⁸. In neutral solution, cyanate may decompose further to produce ammonium ions and carbonate ⁴⁹. Is should be noted that urea was not formed by reaction of ammonium carbonate solution, suggesting that consecutive cyanate decomposition renders urea decomposition irreversible⁵⁰. Reduction of cyanate to C ≡ N appears feasible, but was not experimentally proven.

Cu-CN: intermediate to Urea?

Also the role of C ≡ N in the electrochemical path towards urea, if any is formed from CO₂ and NO₃⁻, remains difficult to assess on the basis of the Raman experiments. Since the characteristic Raman feature at ~ 709 cm^− 1 related to urea adsorption is completely absent in Figs. 1b and 1c, C ≡ N is most likely formed by conversion of a short lived (common) intermediate, such as a C-N species.

In order to prove that the observed signals belong to cyanide, and evaluate potential consecutive reactions of C ≡ N, SERS of the Cu surface in CO₂ saturated KHCO₃ in the presence of KCN was measured as shown in Fig. 3.

In the presence of 10 ppb of KCN, a signal at 2147 cm^− 1 appears (Fig. 3a) which matches the peak observed in our experiments. It was proven that SERS can be successfully used for detection of trace amounts of cyanides as demonstrated for Au in the gas phase⁴² as well as in solution on Ag⁵¹ and Cu surfaces, where Cu(I) surfaces can be very effective in detection by forming CuCN⁵². The position of the peak is similar to the C ≡ N stretching position (frequency) of solid CuCN⁴⁰ and surprisingly, it changes depending on CN⁻ concentration (Fig. 3b). In excess of CN⁻ in aqueous solution, CuCN can form higher, soluble complexes, generalized as Cu(CN)_n⁽ⁿ⁻¹⁾ (n = 2, 3, 4), which structure depends on the CN⁻/Cu ratio⁵³. At higher CN⁻ concentration, the Raman signals of Cu₂O decrease in intensity (Supplementary Figure S8), which proves chemical reaction of CN⁻ to be likely with (surface) copper oxide, forming higher CuCN-complexes⁵⁴.

In order to verify the behaviour of the C ≡ N signal during a potential sweep, cyclic voltammetry in 0.1 M KHCO₃ with addition of 0.1 ppm of KCN was performed (Fig. 4). The obtained results, i.e. strong signals at ~ 2080 cm^− 1, are in excellent agreement with the experimental results obtained in CO₂ and NO₃⁻ containing electrolyte. Note that NO₃⁻ was not added in this measurement, thus those signals must originate from C ≡ N. Cu(CN)_n⁽ⁿ⁻¹⁾ can give different Raman signals depending on the type of complex and usually more than one is present in the system, which can depend on the concentration of CN⁻ as well as the pH of the solution^40,41. In our system, considering continuous production of CN⁻ at negative potentials in a CV cycle as well as a pH change near the electrode surface it is reasonable to assign the set of ~ 2080 cm^− 1 peaks to Cu(CN)_n⁽ⁿ⁻¹⁾. Formation of HCN is also possible considering the bulk pH of the electrolyte being 8.6, although this is much more favourable at lower pH⁴¹.

If higher KCN concentrations were used (Supplementary Figure S9), Cu₂O signals were hardly recorded during a CV, which again confirms chemical attack of CN⁻ on Cu(I)-oxide forming Cu-CN complexes. This likely explains the broad peak observed even at positive potentials, and the very small signal at 2170 cm^− 1 attributed to CuCN.

Finally, in order to definitively prove formation of cyanide, isotopic labelling experiments with KH¹³CO₃ and K¹⁵NO₃ were performed (Supplementary Figure S10). The results show peak shifts with both, ¹³C and ¹⁵N, proving involvement of carbon and nitrogen in the species responsible for the observed vibrations at ~ 2080 and ~ 2150 cm^− 1.

Consequences of Cu-C ≡ N formation

The electrode surface was analysed after chronoamperometry at -0.3 V vs RHE. The electrochemical surface area was significantly increased indicating surface roughening, and the XRD pattern shows a specific increase of the Cu (220) orientation (Supplementary Figure S11). Moreover, on SEM images surface changes can be observed (Supplementary Figure S12). These results all indicate surface reorganization occurs by C ≡ N formation, and thus surface instability during electrolysis in the presence of CO₂ and NO₃⁻ is likely. This might have significant consequences for a practical process for formation of urea, but this requires further study.

Reactions of formate and NH₄⁺

In order to reveal what intermediate species might be responsible for reaction towards cyanide, cyclic voltammetry with formate instead of bicarbonate and ammonium salt instead of nitrate were performed (Supplementary Figure S13 and S14). Since at 0.3 V no CO₂ reduction products are expected (see standard reduction potentials of CO₂ in Supplementary Table S1), formate was chosen as one of the most simple products from CO₂ reduction in a 2e⁻ transfer⁵⁵. On the other hand, NH₄⁺ was chosen as N-source, since this is the most likely product of NO₃⁻ reduction on Cu surfaces⁵⁶. In both cases, no signals related to cyanide were observed.

Since cyanide formation requires removal of oxygen from NO₃⁻, it is very likely that NH_x intermediates on the electrode surface are involved in the reaction (it was reported that stable NH_x adsorbed on cathode surface can exist⁵⁷). Furthermore, adsorbed carbonate is present in the potential range where adsorbed NH_x species exist. Therefore, we conclude that adsorbed NH_x on Cu surface reacts with adsorbed CO₃²⁻ forming cyanide and water, as shown in the schematic overview in Fig. 6.

In situ EC-MS analysis

In order to complement the results obtained by SERS, electrochemical mass spectrometry was used which allows to track gaseous products desorbed from the electrode surface with high sensitivity⁵⁸. Mass spectra of selected signals recorded during CV scans are shown in Supplementary Figure S15, Figure S16 and Figure S17. Comparison of the m/z: 30 signal, in different electrolytes is shown in Fig. 5a. Besides the obvious difference in m/z: 30 signal, assigned to NO, no other differences were observed. Formation of nitric oxide was confirmed by isotopically labelling experiments, where the peak of m/z: 31 is clearly visible when K¹⁵NO₃ was used (Supplementary Figure S18).

In CO₂ saturated KHCO₃ with 50 mM KNO₃ as well as in Ar saturated 0.1 M KNO₃, NO is observed in the reductive sweep as product of NO₃⁻ electroreduction (NO was not found in CO₂ saturated KHCO₃). The difference in onset potential for NO formation can be explained by different concentrations of NO₃⁻ or pH changes in the electrolyte. Although the starting pH is the same (6.8), in the case of CO₂ saturated KHCO₃ the buffering capacity of the electrolyte renders large pH changes less likely as compared to electrolytes containing KNO₃ only. It could also very well be that residual C ≡ N species and adsorbed carbonate inhibit the reduction of NO₃⁻ somewhat, further explaining the different trend in NO production as a function of potential. More importantly, nitric oxide is also observed in an oxidative scan if CO₂ and NO₃⁻ are present. It starts to appear at ~ 0.4 V which matches the onset potential of the formation of the peak at 2150 cm^− 1 in SERS experiments. In addition, nitric oxide formation was monitored during chronoamperometry at -0.3 V vs RHE where no signal was detected as shown in Fig. 5b. However, when approaching OCV conditions, the NO signal appears and slowly decreases over time again, which is in agreement with the SERS spectra recorded at OCV (see Fig. 2; blank measurement with 0.1 M KNO₃ can be found in Supplementary Figure S19).

The formation of NO during an oxidative scan suggests oxidation of a (surface adsorbed) product, formed in the reductive scan, which based on SERS data is cyanide. Formed cyanide in solution can thus be decomposed by direct electrooxidation⁵⁹. Cu is effective in this reaction by forming complexes which can oxidize at less positive potentials than free cyanide. It has been previously shown that those complexes can be oxidized at potentials slightly more positive than 0 V vs RHE⁶⁰. Although the most desired products of oxidation of CN are N₂ and CO₂; OCN⁻, CO₃²⁻, NO₃⁻ and NH₃ were also observed⁴⁶. Moreover, oxidation to NO₃⁻ was found to be catalysed by copper oxide⁶¹ which very likely involves NO as intermediate. In our analysis system, NO starts to show up at 0.4 V where oxidation of cyanide complexes is possible on Cu electrodes, and the peak of this signal is observed at potentials where copper is oxidized, which is in agreement with literature.

The overall mechanism deduced from SERS and EC-MS experiments is schematically shown in Fig. 6, representing one cyclic voltammetry cycle. This scheme will be discussed in the following summary.

Simultaneous electroreduction of CO₂ and NO₃⁻ on Cu electrodes was investigated using Surface Enhanced Raman Spectroscopy and Mass Spectrometry. Initially reduction of nitrate results in the formation of NO. At less positive potentials very likely surface bound NH_x are formed, which react with adsorbed carbonate to Cu containing C ≡ N complexes. Evidence for formation of urea was not found. Reversing the scan to more positive potentials, likely results in deposition of solid CuCN and at the same time nitric oxide formation from oxidation of cyanide complexes was detected. These results reveal a novel reaction scheme in the simultaneous reduction of CO₂ and NO₃⁻. Furthermore the results show the formation of other carbon-nitrogen species should be taken into account when working on electrochemical urea synthesis. Similarly, Cu instability due to chemical attack of CN⁻ as well as formation of toxic cyanides should be considered as potential difficulties in bulk electrolysis experiments with Cu. In addition we here provide important insights into research on C-N coupling reactions. Finally, the in situ formed cyanide might be coupled to organic molecules, and be a novel route towards industrially relevant cyanides or nitriles.

Materials

Cu foil (Alpha Aesar, 99.99%), or Cu disc (Pine research) and H₃PO₄ (Sigma Aldrich, 85 wt. % in H₂O,) were used for electrode preparation. Electrolyte solutions were prepared using MiliQ water and KClO₄ (Sigma Aldrich, 99.99 %), KHCO₃ (Sigma Aldrich, 99.7 %), KNO₃ (Sigma Aldrich, ≥ 99.0 %), (NH₄)₂CO₃ (Sigma Aldrich, ≥ 99.5 % ), HCOOK (Sigma Aldrich, ≥ 99.0 %), KCN (Sigma Aldrich, ≥ 98.0 %), K¹⁵NO₃ (Sigma Aldrich, 98 atom % ¹⁵N), KH¹³CO₃ (Sigma Aldrich, 99 atom% ¹³C), ¹³CO₂ (Campro Scientific, min. 99.9 atom % ¹³C).

Electrode preparation

For SERS experiments, polycrystalline rough Cu electrodes were prepared according to the procedure described elsewhere²⁷. In brief, Cu discs were mechanically polished followed by anodic treatment in 85% phosphoric acid (3 V for 2.5 min), sonication in MiliQ water and drying in an Ar stream. A photograph of the prepared electrode is shown in Supplementary Figure S20.

For EC-MS experiments a Cu disc (⌀ 5 mm) was polished following the routine of cleaning provided by Pine Research.

Characterization

Electrodes were characterized using X-ray diffraction (XRD, Bruker Phaser D2) and Scanning Electron Microscopy (SEM) (JSM-6010LA, JEOL system).

Electrochemical measurements

Raman spectra were recorded using an Avantes AvaRaman spectrometer equipped with an Intertec λ = 785 nm laser. The distance between the Raman probe with a focal length of 10mm and the Cu surface was adjusted as to obtain the largest intensity of Raman signals. Electrolyte flow was adjusted to 1.8 ml/min. Ag/AgCl (3 M NaCl, BASi) and platinized Ti mesh were used as reference and counter electrode respectively. The schematic representation of the cell design is shown in Supplementary Figure S21. Each electrode was electrochemically activated in situ in 0.1 M KClO₄ by 5 oxidation-reduction cycles in the range between 2 and − 1.4 V vs RHE (SEM of Cu electrode before and after activation is shown in Supplementary Figure S22). Next, the electrolyte was changed for compositions reflecting the process conditions of urea formation or respective blanks. The electrolyte was continuously purged with CO₂ or Ar. Raman spectra were recorded during cyclic voltammetry (unless indicated otherwise) between + 0.9 and − 1.1 V vs RHE starting at the oxidative potential with a scan speed of 10 mV/s and spectral integration time of 5 s. The potential interval between spectra in the presented graphs is 0.2 V, unless otherwise indicated.

Chronoamperometry measurements were performed in a one compartment cell, with Ag/AgCl (3 M NaCl, BASi) and platinized Ti mesh used as reference and counter electrode respectively. The electrolyte was continuously purged with CO₂ or Ar. A potential of -0.3 V vs RHE was applied for 30 min. The Cu electrode was removed from the electrolyte immediately after the open-circuit potential was applied. Then it was rinsed with MiliQ water and dried in an Ar stream. The electrochemical surface area (ECSA) was measured before and after chronoamperometry by the capacitance method. Cyclic voltammograms in the potential region where no faradaic current is observed, were measured at variable scan rates from 20 to 100 mV/s in Ar saturated 0.1 M KClO₄.

EC-MS measurements were performed using a SpectroInlets (Copenhagen, Denmark) system, with He or CO₂ as carrier gas at 1 ml/min flow rate. Different m/z signals were recorded during cyclic voltammetry (unless otherwise indicated). Ag/AgCl (sat. KCl, CH Instrument) and Pt mesh were used as reference and counter electrode respectively. The schematic representation of the cell design is shown in Supplementary Figure S21.

All electrochemical measurements were carried out at room temperature using a BioLogic VSP potentiostat. Recorded potentials were recalculated to Reversible Hydrogen Electrode scale according to following equation:

$${E}{RHE}={E}{Ag/AgCl}+0.059 pH+{E}_{Ag/AgCl}^{0}$$

Acknowledgements

This work took place within the framework of the Institute of Sustainable Process Technology, co-funded with subsidy from the Topsector Energy by the Ministry of Economic Affairs and Climate Policy, The Netherlands.

Author contributions

P. M. K. performed experiments, analysed the data and wrote the manuscript. A. P. R. performed EC-MS measurements. B. T. M., N. E. B. and G. M. supervised the work. All authors discussed the results and assisted during manuscript preparation.

Competing interests

The authors declare no competing interests.

Additional information

Supplementary information is available for this paper at LINK.

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files.

Correspondence and requests for materials should be addressed to G.M.

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Carbon-nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution
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Carbon-Nitrogen bond formation on Cu electrodes during CO₂reduction in NO₃^- solution

Status:

Version 1

Abstract

Figures

Main Text

Results And Discussion

Urea decomposition – Does it form Cu-CN?

Cu-CN: intermediate to Urea?

Consequences of Cu-C ≡ N formation

Reactions of formate and NH₄⁺

Summary

Methods

Materials

Electrode preparation

Characterization

Electrochemical measurements

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Carbon-Nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution

Status:

Version 1

Abstract

Figures

Main Text

Results And Discussion

Urea decomposition – Does it form Cu-CN?

Cu-CN: intermediate to Urea?

Consequences of Cu-C ≡ N formation

Reactions of formate and NH4+

Summary

Methods

Materials

Electrode preparation

Characterization

Electrochemical measurements

Declarations

References

Additional Declarations

Supplementary Files

Status:

Version 1

Carbon-Nitrogen bond formation on Cu electrodes during CO₂reduction in NO₃^- solution

Reactions of formate and NH₄⁺