Nitrided copper-iron composite oxides derived from layered double hydroxides for enhanced carbon dioxide electroreduction to methane and formic acid

: The reduction of carbon dioxide into valuable chemical products is a promising solution to address carbon balance and energy issues. Herein, amorphous nitrided copper-iron oxides are prepared by gas-phase nitriding of CuFe-layered double hydroxide precursors with urea as a nitrogen source. The obtained materials show high activity for CO 2 electroreduction to methane and formic acid, achieving a total Faraday efficiency of 74.7% at −0.7 V vs. RHE and exhibiting continuous 10 h durability in the H-cell. The uniformly distributed Cu+ sites act as active sites by losing electrons to activate CO 2 . During the CO 2 electroreduction, CO 2 is converted to *COOH via proton-electron coupling; *COOH combines directly with a proton in solution to produce the HCOOH product; and the other part of *COOH undergoes a protonated dehydration process to form the *CHO intermediate, which dehydrates again to form CH 4 . This study provides a new approach for designing CO 2 electroreduction catalysts.


Introduction
Carbon dioxide (CO2) is a key contributor to the global warming crisis in modernday society.Excessive emissions of CO2 have resulted in severe environmental issues, including the greenhouse effect, acidification of water bodies, and climate change.These challenges present a significant threat to the sustainable development of society [1].The combustion of fossil fuels from anthropogenic sources is the major source of atmospheric CO2 emissions [2].The main ways to achieve zero CO2 emissions are through the decarbonization of raw materials, carbon sequestration and capture, as well as carbon recovery and neutralization [3][4][5].The most efficient method for converting CO2 into valuable chemicals today is electrocatalytic reduction [6,7].However, the electrocatalytic reduction of CO2 faces challenges such as the hydrogen evolution reaction (HER) competing with the desired reaction [8].Additionally, the highly thermodynamic stability and kinetic inhibitory effects of CO2 molecules result in their deactivation [9,10].Therefore, the top priority in research in electrocatalytic reduction of carbon dioxide is to design catalysts with low overpotential that can suppress the HER and effectively promote the reduction of carbon dioxide.These catalysts should also possess advantages such as low cost, high selectivity, stability, and durability.
Due to limited reserves and high costs of precious metals, transition metals with significant stability and high economic feasibility in alkaline media have attracted considerable attention in the field of electrocatalysis [11].Therefore, it is necessary to search for transition metal catalysts with higher catalytic activity [12][13][14].Iron-based materials are considered to be very promising catalysts due to their low cost and environmental friendliness [15].However, iron-based materials continue to face a significant challenge in terms of low conductivity [16], limiting their electrocatalytic performance.The reduction intermediate CO in the carbon dioxide is harmful to ironbased materials because CO can damage the active sites of iron-based catalysts., further limiting their effectiveness in electrocatalysis [17].Scientists are now extensively overcoming this limitation by coupling iron-based materials with other materials such as metal oxides or organic carbon frameworks [18,19].Copper-based catalysts have garnered significant research attention due to their high efficiency in converting carbon dioxide into hydrocarbons [20,21].In recent years, considerable research has been conducted with the aim of designing efficient Cu-based heterogeneous materials as Carbon Dioxide Reduction Reaction (CO2RR) electrocatalysts.These materials include monometallic Cu, Cu-based oxides, Cubased bimetallic systems, single/dual Cu atoms, and Cu-based metal-organic frameworks (MOFs) [22].To improve the CO2 reduction activity of catalysts, structural adaptations such as modification of active sites, morphology or size [23,24], introducing other elements [25,26], as well as surface modification and electroplating [27][28][29] are being investigated.Among of these methods, nitrides of transition metals exhibit exceptional electronic [30][31][32], mechanical and optical properties, making them very suitable for reducing CO2 to C1 products [32][33][34][35].Nitrides of transition metals are usually solid compounds with strong metal-nitrogen bonds [36], which gives them a high degree of structural stability.Compared to other catalysts, transition metal nitrides cause less corrosion or structural changes during electrochemical processes.Therefore, it can participate in the reaction stably for a long time [37].Despite their high electrochemical activity, the limited number of electrochemically active sites in nitrides of transition metals may affect their catalytic efficiency in practical reactions [38].Amorphous materials are rich in active defect sites, thus favoring efficient CO2 electroreduction.Shan et al. synthesized Cu@CuEu nanoparticles with amorphous CuEu alloy shells.Defect sites in the amorphous material, which may interact effectively with CO2 molecules or their reduction intermediates, optimize the reaction pathway for CO2 reduction [39].Yang et al. prepared Bi-based catalysts with amorphous layers.The disordered arrangement of atoms in the amorphous materials provides more CO2 adsorption sites and may reduce the activation energy of CO2 molecules by changing the electronic structure of the catalysts.This facilitates the subsequent reduction steps [40].
The structural units of Layered Double Hydroxides (LDHs) primarily exhibit an octahedral geometry, where hydroxyl ions are situated at the vertices of the octahedron, and the encapsulated metal ions are positioned at the center.LDHs have a general chemical formula of x+ (A n− )x/n•yH2O, in which M 2+ , M 3+ , and A respectively represent divalent cations (e.g., Ni 2+ , Mg 2+ , Cu 2+ , Ca 2+ ), trivalent cations (e.g., Fe 3+ , Al 3+ ), and anions [41].By selecting different metals and anionic species, various types of LDHs can be synthesized.The high degree of design flexibility gives LDHs extensive potential applications in various catalytic reactions, such as photocatalysis, thermocatalysis, and electrocatalysis, including CO2RR [42][43][44].Ma et al. have demonstrated that Cd-MgAl-LDHs exhibit exceptional activity in the electrochemical reduction of CO2 to CO [45].Zhang et al. found that Cu9Zn1/Cu0.8Zn0.2Al2O4generated from CuZnAl-LDH enhanced the adsorption of CO intermediates and promoted C-C coupling, greatly improving the activity of CO2RR [46].
Our group has previously demonstrated that CuFe-LDH could serve as an outstanding precursor for electrochemical catalysts due to its unique properties [47].In this study, we successfully synthesized amorphous nitrided copper-iron oxides through the gas-phase nitridation of CuFe-LDH, exploiting the distinctive and highly dispersed metal components present in LDH.To understand the correlation between the material structure and the performance of CO2RR, the chemical composition and the existence state of surface elements are analyzed, and the formed gas and liquid products of CO2RR are detected.The reaction mechanism was studied through X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and in situ attenuated total reflection fourier transform infrared spectroscopy (ATR-FTIR).The dispersion of copper in nanoparticles results in a highly uniform distribution, thereby enhancing its activity in the electroreduction of CO2.

Fabrication of nitrided copper-iron composite oxides
The CuFe-LDH precursor was prepared using the co-precipitation process.0.0075 mol of Cu(NO3)2•3H2O and 0.0025 mol of Fe(NO3)3•9H2O were dissolved in 60 mL of deionized water.0.02 mol of NaOH and 0.005 mol of Na2CO3 were dissolved in 60 mL of deionized water.The two solutions were added dropwise into a threenecked flask under continuous magnetic stirring, maintaining the pH of mixed solution at 5 in the above process.After addition, the mixed solution was stirred for one hour and transferred into a hydrothermal reactor for 5 h reaction at 110 ℃.After the completion of hydrothermal treatment, the obtained CuFe-LDH samples were collected through centrifugation, filtration and drying.
The nitrided copper-iron composite oxides were prepared by the gas-phase nitridation of CuFe-LDH precursors with urea as nitrogen source.0.5 g CuFe-LDH sample and 5 g urea were transferred to tube furnace, then the nitridation reaction was conducted at 300 ℃ for 2 h with a heating rate of 1 ℃•min −1 in N2.After the nitridation, the obtained sample was washed with ethanol, centrifuged, and vacuum dried.

Characterization
XRD patterns were acquired by a Bruker D8A25 instrument with Cu Kα radiation and a scanning step of 10°•min −1 .Scanning electron microscope (SEM) images were captured on a Zeiss SUPRA 55 instruments operated at 10 kV.High-resolution transmission electron microscopy (HRTEM) images were taken using a JEOL JEM-F200 instrument equipped with an energy dispersive X-ray spectrometer (EDS), and the test was conducted at an accelerating voltage of 200 kV.XPS and Cu LMM Auger spectra were acquired through a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer with Al Kα X-rays at approximately 2 × 10 −9 Pa.ATR-FTIR was conducted in situ using a Tensor II (Bruker) FTIR spectrometer with an MCT detector.The measurements were performed with a VeeMAX III variable angle specular reflection accessory (Pike, USA) and a spectroelectrochemical cell, combined with a CHI660E instrument to control the reduction potential.Gas chromatography (GC, Shimadzu, GC2014) at 20 ℃ was utilized to quantify gaseous product.The concentration of Cu and Fe in the material was determined using inductively coupled plasma mass spectrometry (ICP-MS, iCAP6300 Radial, Thermo, US).

Electrochemical study of CO2 reduction and products analysis
Electrochemical measurements were performed using a CHI660E electrochemical workstation (Shanghai Chenhua Instruments Co., China) in a threeelectrode system within an H-cell.The cathode and anode were separated by a Nafion 115 membrane, which acted as a proton exchange membrane.A 0.5 M KHCO3 electrolyte was utilized for the measurements.The working, counter, and reference electrodes are nitrided copper-iron composite oxide powder, graphite rod, and Ag/AgCl (3 mol•L −1 KCl), respectively.The given potential was converted to a reversible hydrogen electrode (RHE) using the Nernst equation.The electrolyte that contained 0.5 M KHCO3 saturated with CO2 exhibited a pH of 7.12. =  / + (0.21 + 0.059 × ) During the linear sweep voltammetry (LSV) tests, CO2 (≥99.999%) or Ar (≥99.999%) with high purity were continuously injected into the H-cell.The electrolyte was saturated with CO2 at a mass flow rate of 30 mL•min −1 for over 20 min, following which the cell was hermetically sealed before executing the measurements.After each measurement that lasted for 1.5 h at various overpotentials, the formed products were analyzed.

Structure of as-prepared catalysts
The XRD patterns of CuFe-LDH and nitrided copper-iron composite oxides are shown in Figure 1a.The spectrum of the CuFe-LDH precursor can observe diffraction peaks at 12.8°, 25.8°, 33.1°, 36.6°,43.6°, 60.9°, and 62.5°, which are respectively attributed to the (003), (006), (009), (015), (018), (110), and (113) crystal faces of the LDH phase [48].This indicates the successful synthesis of the hydrotalcite precursor.The XRD pattern of nitrided copper-iron composite oxides shows that there are no additional XRD signal peaks except for the C signal peak at 18° indexed to the standard C (JCPDS No. , confirming the amorphous nature of obtained material.Amorphous materials are more likely to produce defective vacancies during the reaction process, which can serve as active sites for catalytic reactions [49].Furthermore, a detailed investigation of this composite was conducted by analyzing XPS spectra.(Figures 1b and 1c).It is visible in the spectrum that peaks of Cu 2p at 932.37 and 952.38 eV correspond to zero valent Cu (Cu 0 ); in addition to that, Cu 2p at 932.37 and 953.38 eV peaks correspond to the Cu 2p3/2 and 2p1/2 (Figure 1a), but it cannot confirm that Cu + or Cu 0 coexist.They were analyzed in conjunction with the LMM spectrum of Cu (Figure 1b).The apparent appearance of a signal for Cu + [50].So the presence of Cu + was established.Two peaks of Fe 2p at 709.20 and 714.6 eV are within the scope of the Fe 2p3/2 binding energy, corresponding to Fe 2+ and Fe 3+ , respectively (Figure A1), further confirms the existence of Fe 2+ and Fe 3+ in the composite.The overlapping of the 2P1/2 and 2P3/2 peaks of Fe could be due to the interference of the LMM spectrum of Cu.This suggests that charge rearrangement may have occurred within the material after nitriding, which may have led to increased conductivity of the material and enhanced electrochemical activity.SEM images of CuFe-LDH (Figure A2) show that CuFe-LDH has a nanosheet morphology with a smooth surface and approximately 100 nm of thickness.After the nitriding reaction, the catalyst was transformed into nanoparticle with diameters ranging from 50 to 100 nm (as shown in Figures 2a and 2b).The HRTEM image (Figure 2h) shows no lattice fringing detected, which was considered a typical characteristic of amorphous state [51].This further supports an amorphous nature of nitrided copper-iron composite oxides.In addition, EDS elemental mapping (Figures 2c-2g) confirms the homogeneous distribution of Cu, Fe, O, and N atoms on the surface of the catalyst.The EDS results indicate that the elemental contents of Cu, Fe, N, and O are 47.6%, 14.59%, 17.44%, and 20.37%, respectively (Figure A3).

CO2RR performance of as-prepared catalysts
The CO2 electroreduction performance of nitrided copper-iron composite oxide was investigated in an H-type cell containing 0.5 M KHCO3.Figures 3a and 3b show the stable LSV curves of nitrided copper-iron composite oxide and CuFe-LDH in Arsaturated (dashed line) and CO2-saturated (solid line) electrolytes, respectively.The current density of the catalyst is significantly lower than that of the precursor at the same overpotential.As shown in Figures 3a and 3b, the current density remained stable with the increase of overpotential, suggesting that the catalyst did not fall off throughout the reaction process.Moreover, there was no significant fluctuation in the current density during the test (Figure 3c), indicating the high stability of the catalyst.In the CO2 atmosphere, the catalyst exhibits a lower initial potential (−0.1 V vs. RHE).Under Ar-saturated and CO2-saturated environment, the maximum difference in total current density is 14 mA•cm −2 at −0.7 V vs. RHE.The main product of electrocatalytic CO2RR over the catalyst is the C1 product including HCOOH and CH4, the faradaic efficiency (FE) of HCOOH and CH4 is 47.55% and 27.13% respectively with the maximum FE of 96.77 % at −0.7 V vs. RHE (Figures 3d and 3e).In particular, at −0.7 V, the partial current densities of HCOOH and CH4 are 17.29 mA cm −2 and 9.8 mA cm −2 , respectively.However, only a tiny amount of HCOOH is produced due to CO2 reduction by CuFe-LDH precursor (Figure A4).Compared with the literature, it becomes evident that the FE of C1 products achieved by the catalyst in this study exceeds that of numerous copper-based catalysts (Table 1).During the electroreduction of CO2, the cationic reduction produced by the catalyst consumes part of the charge, resulting in a total FE of the obtained product of less than 100%.To confirm the electrochemical stability of nitrided copper-iron composite oxide, cycling tests were carried out for 10 h at the optimum potential of −0.7 V (Figure A5).The results show that the FE of CH4 decreased slightly, indicating that this catalyst has good electrochemical stability.As revealed in Figure A6, the XPS spectra of the catalyst after cycling tests show no obvious change in the valence state or proportion of surface elements, further confirming the excellent stability of the material.
In addition, the high surface area and fast electron transport kinetics of nitrided copper-iron composite oxide were confirmed by electrochemical active area (ECSA) and electrochemical impedance spectroscopy (EIS).As shown in Figures 3f and 3g, the ECSA value of nitrided copper-iron composite oxide (Cdl = 4.50 mF•cm −1 ) is 3 times that of CuFe-LDH (Cdl = 1.53 mF•cm −1 ) (Figure A7).The Nyquist diagram in Figure 3h shows that the semicircle diameter (charge transfer resistance, 1.8 Ω) of nitrided copper-iron composite oxide at operating voltage is also smaller than that of CuFe-LDH (2.6 Ω).The above results indicate that the kinetic performance of the obtained catalyst through nitridation surpasses that of its precursor.

Mechanisms of CH4 and HCOOH formation
The catalysts underwent chronoamperometric tests at different potentials for 1000 s with the intention of investigating the reaction mechanism and active sites of the catalyst.The SEM images of nitrided copper-iron composite oxide after CO2 reduction test at −0.10, −0.30, −0.50, −0.70 and −0.90 V are shown in Figure A8.The structure of the nitrided catalyst has shown a transition from particles to agglomerates during CO2 electroreduction.
The existence state of surface elements of the catalyst after chronoamperometric tests at potentials of −0.10, −0.30, −0.50, −0.70 and −0.90 V were analyzed by XPS.(Figures 4a and 4b).The Cu atoms in the catalyst were shown to be mainly Cu + and Cu 2+ during the reaction.In Cu 2p XPS spectra, peaks at 935.19 eV and 955.01 eV are attributed to Cu 2+ , and peaks at 932.37 eV and 953.38 eV are attributed to Cu 0 or Cu + (Figure 4a).The precise state of Cu was further identified by AES (Figure 4b), there is a peak assigned to Cu + in the Cu LMM spectrum of nitrided copper-iron composite oxide, but no peak attributed Cu + appeared [59,60].The Cu + /Cu 2+ ratio decreases and then increases with increasing potential as the potential increases from −0.1 V vs. RHE to −0.9 V vs. RHE (Table S1).This is the same trend as the total FE.This result indicates that Cu + is converted to Cu 2+ during the reaction process.Therefore, it is believed that the reaction process involves the loss of electrons from Cu + as the active site to activate CO2.Different from Cu, the existence state of Fe remained relatively unchanged, indicated by the lack of significant differences in the integrated XPS area when compared to the un-reacted state (Figure A9).The overlapping of the 2P1/2 and 2P3/2 peaks of Fe could be due to the interference of the LMM spectrum of Cu.This provides further evidence for the excellent stability of the catalyst.To investigate the reaction mechanism in detail, in situ ATR-FTIR was used to detect the intermediates formation during CO2 electroreduction (Figure 5).

Conclusion and outlook
The nitrided copper-iron composite nanocomposites were synthesized by urea nitridation of CuFe-LDH precursor at high temperature.The obtained catalyst has low production cost and exhibits good electrocatalytic CO2RR performance.The nitrided copper-iron composite FE was 47.55 % for HCOOH and 27.13 % for CH4 at −0.7 V, achieving a total FE of 96.77 %.The performance of the catalyst remained stability after 10 h of continuous testing with the FE of CH4 product remaining stable above 27 % at −0.7 V and the structure also remaining stable.In situ ATR-FTIR studies show that the existence of Cu + sites can furnish electrons to *CHO.This study demonstrates the potential of amorphous copper iron oxide nitride as a catalyst for CO2 electroreduction, and provides new ideas for the design of efficient catalysts.But amorphous materials face several major challenges as catalysts in the CO2RR.Firstly, amorphous materials may have multiple active sites, which may lead to competition for multiple reaction pathways and products, thus reducing the selectivity of the target product.Secondly, amorphous materials are often more complex to prepare than crystalline materials, and their characterization methods are more limited.This may limit an in-depth understanding of the structure and properties of the materials.Amorphous oxide electrocatalysts should be studied more thoroughly using more advanced characterization tools to understand their microstructures (e.g., defects), active sites, and electrocatalytic processes, which will help to design amorphous oxides with desirable electrocatalytic properties.

Supporting information
Additional tables and figures for characterization of the synthesized materials' morphology and composition include SEM images of CuFe-LDH, the EDS spectrum of nitrided copper-iron composite nanocomposites, CO2 electroreduction activity of CuFe-LDH, stability test results, SEM images of nitrided copper-iron composite oxide after the chronoamperometric i-t tests at different potentials, Nyquist plots of CuFe-LDH and nitrided copper-iron composite oxide, XPS spectra of Fe 2p of nitrided copper-iron composite, and ratios of Cu + , Cu 2+ , and Cu + /Cu 2+ calculated by Cu 2p spectra of nitrided copper-iron composite oxide.
Author contributions: Conceptualization, XL; methodology, DS and JH; software, validation, XZ, XL, FZ and YW; formal analysis, DS; investigation, JH; resources, XL; data curation, DS; writing-original draft preparation, DS; writing-review and editing, XL and YW; visualization, supervision, XL and YW; project administration, funding acquisition, XL.All authors have read and agreed to the published version of the manuscript.

Figure 3 .
Figure 3. (a) LSV of nitrided copper-iron composite; (b) LSV of CuFe-LDH; (c) Chronoamperometry test cyclic tests; (d-e) the products FE of nitrided copper-iron composite at different test potentials; (f) Cyclic voltammograms; (g) Linear fit of double layer capacitance current density to scan rate from CV plot; (g) The Nyquist plots.

Figure 4 .
Figure 4. (a) XPS spectra of Cu 2p; (b) Cu LMM; (c) XRD of material and substrate after the reaction.
The peak at about 2400 cm −1 belongs to the stretching vibration of Stretching Vibration of CO2.The peak of the stretching vibration of *COOH appears in the resonant band at around 1490 cm −1 .(* represents the adsorption state).The vibrational peaks of desorbed *CO, bridged *CO, and linear adsorbed *CO should appear at the resonance band of about 2100-2000 cm −1 .However, no corresponding signal was detected, possibly due to the high reactivity of *CO during the reaction process.The *CO readily reacts with protons to form *CHO for the following reaction, as indicated by the signals of the *CHO vibrational stretching peaks at 1046 cm −1 .

Figure 5 .
Figure 5. In-situ ATR-FTIR of CO2 reduction on nitrided copper-iron composite oxides during CO2 reduction LSV tests with different potentials.

Figure A6 .
Figure A6.HER polarization plots of MoSe2/NiSe2 at a scan rate of 5 mV s -1 in 1 M KOH electrolyte with and without 0.5 M urea.

Table 1 .
Comparison of the electrocatalytic activity of catalyst in this work with other catalysts for electrochemical reduction of CO2.