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Yulong Zhou, Baopeng Yang, Zhencong Huang, Gen Chen, Jianguo Tang, Min Liu, Xiaohe Liu, [Renzhi Ma](https://orcid.org/0000-0001-7126-2006), Zongwei Mei, Ning Zhang

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[Cu-Ni alloy nanocrystals with heterogenous active sites for efficient urea synthesis](https://mdr.nims.go.jp/datasets/e6494192-b6af-4490-81d1-286502d3278f)

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1 Cu-Ni Alloy Nanocrystals with Heterogenous Active Sites for Efficient Urea Synthesis Yulong Zhou,a Baopeng Yang,c Zhencong Huang,a Gen Chen,a Jianguo Tang,a Min Liu,c Xiaohe Liu,d Renzhi Ma,e Zongwei Mei,b,* and Ning Zhang,a,* a School of Materials Science and Engineering, Central South University, Changsha 410083, China b Yangtze Delta Region Institute (Huzhou) & School of Physics, University of Electronic Science and Technology of China, Huzhou 313001, China c School of Physics and Electronics, Central South University, Changsha 410083, China  d School of Chemical Engineering, Zhengzhou University, Zhengzhou 450001, China  e Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan *Corresponding authors: Ning Zhang (nzhang@csu.edu.cn); Zongwei Mei (meizw@csj.uestc.edu.cn).    2 Abstract  Electrocatalytic urea synthesis from CO2 and NO3- are environmental green and energy savable. Although Cu are effective for CO2 and NO3- reduction reactions, achieving high efficiency of urea production remains challenging. Herein, Cu-Ni nanocrystals are formed to provide heterogenous active sites for promoting hydrogenation and C-N coupling reactions in urea synthesis. The carbon supported Cu-Ni nanocrystals are characterized by extended X-ray absorption fine structure and X-ray photoelectron spectroscopy spectra. The Cu-Ni nanocrystals achieves the highest urea Faradaic efficiency of 25.1% and urea yield of 22.01 μmol h-1 cm-1, which is much higher than that of single Cu or Ni metal. The reaction mechanism is probed by in-situ Fourier transform infrared spectroscopy, revealing a C-N coupling reaction pathway between *CO and *NH2. Theoretical calculations suggest that hydrogenation of *NOx and coupling *CO with *NH2 are more easily occurred over co-existed Cu and Ni sites rather than homogeneous Cu or Ni site. Keywords: C-N coupling; Urea synthesis; Bimetallic catalyst; Active site; Copper alloy    3 1. Introduction Urea is an indispensable fertilizer and broadly utilized chemical, which plays an important role in human development[1, 2]. In conventional industry, urea is synthesized by the Haber-Bosch reaction to produce ammonia (NH3) and then the coupling reactions of NH3 with CO2[3], which requires continuous high-energy input under harsh conditions[4-6]. The electrocatalytic synthesis of urea is a promising alternative to conventional urea synthesis due to its mild conditions and low energy consumption[6-8]. Particularly, electrocatalytic C-N coupling of greenhouse gases CO2 and industrial effluent nitrate (NO3-) to produce urea not only contributes to the target of carbon neutrality but also maximizes the utilization of waste resources[9-11]. Generally, the formation urea includes the electrochemical reductions of CO2 and NO3- (e.g. CO2 to CO and NO3- to *NOx), hydrogenation process (e.g. *NOx to *NHx, or *CONO to *CONHx), and the C-N process (e.g. *NO2 with *CO, or *NH2 with *CO)[12, 13]. To achieve high yield of urea, all these reactions should be high efficiency[14-16]. Up to now, plenty of materials such as Pb, Sn, Ag, Cu based metal and compounds are developed as electrocatalysts for urea synthesis[17, 18]. Among them, the Cu has been recognized as a cheap and efficient transition metals for CO2 and NO3- reduction reactions, which is promising for urea synthesis[18]. For instance, Cu metal atoms are suitable for adsorbing CO2 to produce CO and hydrocarbons[19, 20]. The Cu materials are also active for adsorbing NO3- and further hydrogenating to form NH3[21]. More interestingly, it has been reported that the Cu electrocatalysts can achieve the C–N coupling to produce urea in presence of NO3- and CO2[22]. However, achieving high-performance electrocatalysts for urea synthesis remains challenging for Cu materials, in significant part because the Cu active sites of materials are not satisfying the processes of C-N coupling reactions processes such as the *NO2 with *CO or *NH2 with *CO as well as the hydrogenation reactions on *NOx or *CONO  4 intermediates[13, 14, 23]. Properly tailoring active sites of Cu based materials to satisfy required C–N coupling and hydrogenation reactions is highly desirable for urea synthesis. Recently, constructing heterogenous active sites are realizing effective strategy to minimize the reaction barriers, modify electronic state, and activate particular reactions. For instance, Zhang et al. found that Fe@C and Fe3O4 formed on CNT provided dual active sites for adsorption and activation of NO3- and CO2, resulting in the lower energy barriers of formed *NH2 and *CO intermediates as well as the C-N coupling reactions[24]. Wang et al. found that bonded Fe-Ni pairs are effective sites for the synergistic adsorption and activation of multiple reactants, which thermodynamically and kinetically enhance the critical C-N coupling[14]. Although these significate work has been achieved, constructing heterogenous active sites in Cu metals to promote the C–N coupling and hydrogenation reactions for efficient urea synthesis are still seldom concerned. Herein, proper Ni content is incorporated into Cu to form Cu-Ni alloy, which provides heterogenous active sites to promote the hydrogenation of NOx species and C-N coupling reaction for urea synthesis. In detail, a carbon supported Cu-Ni catalysts are synthesized by pyrolysis of Cu-Ni metal-organic frameworks (MOF). The X-ray absorption spectroscopy (XAS) and X-ray photoelectron spectroscopy (XPS) show that Cu and Ni are mainly metallic states. The performance of urea synthesis for Cu-Ni catalyst is higher than those of individual metal of Cu and Ni electrocatalysts, which achieves Faradaic efficiency of producing urea as high as 25.1% at -0.5 V (vs RHE) and yield of 37.53 μmolh-1cm-2 at -0.8 V (vs RHE). Such a performance is much superior to mostly reported catalysts. The mechanism of urea formation is analyzed by in-situ Fourier transform infrared spectroscopy (FT-IR), revealing a possible C-N coupling reaction pathway between *NH2 and *CO. The theoretical calculations suggest that hydrogenation of *NOx  5 and coupling of *CO and *NH2 are more easily occurred over the heterogenous Cu and Ni atoms rather than on homogeneous Cu and Ni atoms. The designed heterogeneous active sites in this work provide a facile and highly efficient strategy for urea synthesis from NO3- and CO2. 2. Experimental section  2.1 Preparation of CuxNiy BTC.  Preparation of monometallic and bimetallic MOFs as precursors by hydrothermal synthesis route[25]. For the synthesis of Cu BTC, Cu(NO3)2·3H2O (4 mmol, 0.966 g) and BTC (4.5 mmol, 0.946 g) were dissolved in 30 mL of mixed aqueous solution, where water: DMF: ethanol was 1:1:1, and the obtained solution was sonicated for half an hour to obtain a clear blue solution. The blue solution was then transferred to a 50 mL PTFE-lined stainless steel hydrothermal autoclave and kept at 120 °C for 12 h. The blue Cu BTC precursor obtained by centrifugation was further washed three times with water and ethanol alternately and dried in an oven at 60 °C for 8 h. Ni BTC was prepared from Ni(NO3)2·6H2O and BTC by the same method as Cu BTC preparation. The co-exitance of Cu and Ni BTC was synthesized using a similar hydrothermal route by adding proper amount of Cu(NO3)2·3H2O and Ni(NO3)2·6H2O, which named as CuxNiy BTC (x + y = 10, the x and y stand for the mole ratio of added Cu(NO3)2·3H2O and Ni(NO3)2·6H2O during the synthesis). The ratio of Cu and Ni in final products may have deviations in compared with the set ratio of Cu and Ni nitrates during synthesis.  2.2 Preparation of CuxNiy/C catalysts.  The above obtained BTC precursors were pyrolyzed under Ar/5% H2 atmosphere to obtain the corresponding carbon-loaded mono- and bimetallic catalysts. The specific operation is to place the synthesized precursors inside the porcelain boat, and then transfer them to the tube furnace, where the reaction is carried out at 500 °C for 3h with a heating rate of 5 °C min-1, the powder  6 obtained is the target catalyst, which named as CuxNiy/C. The actually ratio of Cu and Ni in CuxNiy/C may have deviations in compared with the set ratio of Cu and Ni nitrates during synthesis of precursor. 2.3 Electrochemical measurements.  In this work, the performance of electrocatalytic NO3- and CO2 synthesis of urea was carried out by using CHI 760E as an electrochemical workstation. Hg/HgO (1M KOH-filled) was used as the reference electrode, and a 1  1 cm2 platinum sheet was used for the counter electrode. The area of working electrode is fixed at 0.25 cm2. The working electrode was a dry carbon fiber paper (CFP) modified with catalysts. The catalyst (5 mg) was dispersed in 1 mL of a 1:1 mixture of water and ethanol, and then 20 μL of Nafion (5 wt% aqueous solution) was added and sonicated for 30 min to form a homogeneous ink. The CFP was coated with 50 μL and the catalyst loading was 1 mg cm-2. The obtained working electrodes were vacuum dried at 60℃ for further use. Before the electrochemical test, the cathode chamber of the H-type cell was pre-saturated with the corresponding gases (Ar > 99.999%, CO2 > 99.999%). During the catalytic process, the gas flow rate was maintained at 30 mL min-1. All the potentials were measured against a Hg/HgO (1M KOH solution filling) reference electrode and converted to RHE as follows: ERHE = EHg/HgO + 0.098 +0.059 × pH. The pH is 6.8 or 8.3 for the measurement with CO2 or without CO2 saturation, respectively. The scan rate for linear sweep voltammetry (LSV) tests was 10 mV s-1. The cyclic voltammetry curves in electrochemical double-layer capacitance (Cdl) determination were measured in a potential window where no Faraday process occurred in an electrolyte of 0.1M KHCO3 and 0.1M KNO3 at different scanning rates of 20, 40, 60, 80, 100, 120, 140, 160 mV s-1.  7 DFT computational details, materials, characterizations, urea quantification, NH3 quantification, NO2- quantification, FE and the yield rate of NH3 calculations, and the In-situ FTIR measurements are shown in Supporting Information. 3. Results and Discussion A carbon-loaded bimetallic catalyst (CuxNiy/C) was obtained by high-temperature pyrolysis under Ar/H2 atmosphere using CuxNiy benzene-1, 3, 5-tricarboxylic acid (BTC) as the precursor (Figure S1)[25]. The prepared materials with different mole ratio of Cu and Ni were named as Cu/C, Cu9Ni/C, Cu8Ni2/C, Cu5Ni5/C, and Ni/C. As shown in Figure S2-3, the SEM images of the MOF precursors show a regular octahedral morphology, which is gradually distorted with the increasing doping of Ni. Subsequently, the carbon-loaded Cu–Ni alloys were obtained after pyrolysis under Ar/H2 atmosphere. As illustrated in Figure 1a, the Cu and Ni correspond to PDF#70-7038 and PDF#65-2865, respectively. The XRD patterns show that Cu/C, Ni/C, and CuxNiy/C are pure metallic phases with face-centered cubic structure. As expected, the Cu (111) plane diffraction peak shifts to higher angle owing to the contraction of its lattice spacing after Ni incorporation. The scanning electron microscope (SEM) image shows the same octahedral morphology as the MOF precursor (Figure 1b and Figure S4). The magnified SEM (Figure 1c and Figure S4) shows that the octahedral structure of the catalyst is consisted of stacked nanoparticles with size about 20-50 nm. The TEM image in Figure 1d and Figure S5 confirms that the aggerated nanoparticles are formed after pyrolysis. As illustrated in Figure 1e, the high-resolution TEM (HR-TEM) image illustrates that Cu-Ni nanoparticle presents clear lattice fringes with a plane distance of 0.21 nm, which corresponds well to the (111) plane of cubic Cu (Figure S6). The fast Fourier transform (FFT) pattern exhibits the cubic structure of Cu metal with (111) plane exposed in Cu8Ni2/C[12, 26]. The transmission electron microscopy (TEM) and the corresponding EDX  8 mapping images shows that Cu and Ni elements are well distributed in CuxNiy/C (Figure 1f, Figure S7).  Figure 1. Morphology and structure of catalysts. (a) XRD pattern of Cu/C, Cu9Ni/C, Cu8Ni2/C, Cu5Ni5/C and Ni/C catalysts with enlarged (111) crystal plane. (b) SEM image, (c) local enlargement image, (d) TEM image, (e) High-resolution TEM image of Cu8Ni2/C. (f) TEM image and corresponding elemental mappings of Cu8Ni2/C, where Cu and Ni atoms are represented by blue and yellow pixels, respectively  The element states of formed Cu8Ni2/C catalysts were further investigated by X-ray absorption spectroscopy (XAS). Figure 2a presents the normalized Cu K-edge X-ray near-edge absorption structure (XANES) spectra of Cu8Ni2/C, Cu foil, CuO, and Cu2O. The absorption edge of Cu in Cu8Ni2/C is negatively shifted by ~0.3 eV compared with that of Cu foil. However, the normalized K-edge XANES of Cu8Ni2/C shows that the corresponding absorption edge of Ni is positively shifted compared to the Ni foil (Figure 2b). In addition, the first-order derivative of  9 XANES shows that the valence states of Cu and Ni elements in Cu8Ni2/C are the same as those of Cu and Ni foils (0 valence state), which are lower than those of copper oxides (Cu2O and CuO) and nickel oxide (NiO) as shown in Figure 2c,d[27]. These results indicate that both Cu and Ni are metallic states and a redistribution of electrons from Ni to Cu in Cu8Ni2/C. Moreover, Fourier transform of expanded X-ray absorption fine structure (FT-EXAFS) characterizations show a bond length (2.24 Å) of Cu8Ni2/C between the Cu-Cu (2.27 Å) in Cu foil and Ni-Ni (2.18 Å) in Ni foil, further confirming the formation of CuNi solid solution (Figure 2e). Additionally, the metallic bonds of the synthesized Cu-Ni alloys are quite different from the Cu-Cu bonds in copper oxides (Cu2O and CuO) and Ni-Ni bonds in nickel oxide (NiO).[28, 29]. Moreover, Wavelet transform (WT)-EXAFS was conducted to identify the metal-metal and metal-O bonds in Cu8Ni2/C and related references. The WT-FXAFS analysis clearly illustrates that Cu8Ni2/C is conformationally consistent with Cu or Ni foil and distinguished from the Cu and Ni oxides (Figure 2f and S8). The above results further confirm the formation of Cu-Ni alloy in Cu8Ni2/C. The electronic structure of the Cu8Ni2/C catalyst was further evaluated by X-ray photoelectron spectroscopy (XPS). For energy calibration, the position of C1s photoelectron peak was applied (binding energy 284.8 eV, Figure S9). Notably, the 2p peak of Ni0 moves toward higher binding energy with increasing Cu content, whereas the 2p peak of Cu0 moves toward lower binding energy (Figure 2g and S10), implying that Cu and Ni in Cu8Ni2/C lead to electron redistribution because of the alloying effect[30-32]. The result is also consistent with XAS, where electrons aggregate from Ni to Cu.  10  Figure 2 Electron transfer and valence state of catalysts. (a) Normalized and (c) deriv-normalized intensity Cu K-edge X-ray absorption near-edge structure (XANES) spectra for Cu2O, CuO, Cu foil and Cu8Ni2/C. (b) Normalized and (d) deriv-normalized intensity of Ni K-edge XANES spectra for NiO, Ni foil and Cu8Ni2/C. (e) Fourier transform extended X-ray absorption fine structure (FT-EXAFS) spectra of Cu8Ni2/C and other references. (f) Wavelet-transform plots for Cu foil, Cu8Ni2/C, and Ni foil. (g) XPS peaks spectra of Ni 2p of Cu/C, Cu9Ni/C, Cu8Ni2/C, Cu5Ni5/C. The electrochemical measurements were carried out in the classical three-electrode system (Figure S11), where the counter electrode was 1  1 cm2 platinum sheet and reference electrode was Hg/HgO. The concentrations of urea, nitrite, and ammonium were calibrated by diacetyl  11 monoxime[6, 33], N-(1-naphthyl)-ethylenediamine dihydrochloride[34], and indophenol blue methods[35], respectively (Figure S12). Notably, when using the diacetyl monoxime method to calibrate urea, high nitrite concentrations may influence urea calibration. It is necessary to ensure that the nitrite concentration is less than 100 μmolL-1 in the measurement system for diacetyl monoxime method[36]. Meanwhile 1H NMR chemical shift in the range of 5.1~5.2 and control tests without NO3-/CO2 was used to further confirm the urea production (Figure S13)[12]. Linear sweep voltammetry (LSV) tests were performed to evaluate the electrochemical behaviours of Cu, Ni, and Cu-Ni bimetallic catalysts in CO2-saturated electrolyte with 0.1 M KHCO3 + 0.1 M KNO3 at a scan rate of 10 mV s-1. As shown in Figure 3a, the current density is decreased in the order of Cu5Ni5/C, Cu8Ni2/C, Cu, Cu9Ni/C, and Ni. Note that the current density shows in Figure 3a is contributed by the various electrocatalytic reactions, such as urea synthesis, hydrogen evolution reaction (HER), CO2 reduction reactions (CO2RR), and NO3- reduction reactions (NO3RR). As shown in Figure 3b, the Tafel slopes are 162.12, 219.78, 276.87, 286.62, and 465.45 mV dec-1 for Cu8Ni2/C, Cu5Ni5/C, Cu, Cu9Ni/C, and Ni. The Tafel slope represents the total kinetic process of these reactions including HER, CO2RR, and NO3RR, and the side reactions play an important role in providing key intermediates in the urea synthesis process. Thus, the Tafel slope results indicate that the Cu-Ni alloys shows relatively higher overall electrocatalytic activity than the single Cu or Ni. Taking Cu8Ni2/C material as an example, then control experiments were carried out to check the effect of reactants on current density as shown in Figure 3c. A relatively low current density is produced in Ar saturated electrolyte without NO3-, which is mainly attributed to the HER and slightly lower than the CO2RR performance (CO2 without KNO3). A partial enlargement of Figure 3c is shown in Figure S14. It is noting that the current density of HER improves with the increase of Ni component in Cu-Ni alloy (Figure S15), which indicates that the  12 doping Ni is favorable to the HER process from water splitting. There is no obviously current improvement when the solution is the saturated CO2 without NO3-. The reactions are mainly HER and CO2RR in such a condition. In presence of NO3- and Ar, the current is improved greatly, which is mainly ascribed to the NO3RR and HER. When both NO3- and CO2 are co-existent, the current is decreased in compared with the existence of NO3- and Ar, which may be caused by the sluggish of urea formation.  Figure 3 Electrocatalytic performances of urea synthesis. (a) LSV curves over Cu/C, Cu9Ni/C, Cu8Ni2/C, Cu5Ni5/C, and Ni/C catalysts (catalysts loading was 1mg cm-2) measured in 0.1M KHCO3 + 0.1M KNO3 electrolyte at a scan rate of 10mV s-1 (b) Tafel slopes of Cu/C, Cu9Ni/C, Cu8Ni2/C, Cu5Ni5/C, and Ni/C in the electrolyte of 0.1M KHCO3 + 0.1M KNO3. (c) LSV curves over Cu8Ni2/C (catalysts loading was 1mg cm-2) in different variables (the solution concentrations  13 were 0.1 M) at a scan rate of 10mV s-1. (d) Urea Faraday efficiencies and corresponding yield rate on Cu/C, Cu9Ni/C, Cu8Ni2/C, Cu5Ni5/C, and Ni/C at various applied potentials (vs RHE) for 30min of electrocatalysis. (e) FE(Urea) and yield rate on Cu8Ni2/C under the applied potential of −0.5 V (vs RHE) during 10 periods of 1 h electrocatalytic. (f) Comparison of the performance of Cu8Ni2/C catalyst with other extensively reported electrocatalysts for electrocatalytic nitrate and CO2 synthesis of urea. The produced urea is examined over various potentials and materials. At potential from -0.4 to -0.8 V (vs RHE), there is a volcano distribution among Cu-Ni alloys and the highest activity is located at the Cu8Ni2/C. The Faraday efficiency (FE) and yield rate reaches as high as 25.1% with a urea yield rate of 22.01 μmol h-1 cm-1 at -0.5 V (vs RHE) on Cu8Ni2/C (Figure 3d). Furthermore, the urea yield on Cu8Ni2/C catalyst increases with more negative potential, reaching 37.53 μmol h-1 cm-1 at -0.8 V (vs RHE) as shown in Figure 3d. The FE (urea) and urea yield rate are relatively stable at -0.5 V (vs RHE) during the test of 10 cycles as shown in Figure 3e. The corresponding I-t curves are shown in Figure S16. These results indicate that Cu8Ni2/C exhibits good electrocatalytic stability for urea synthesis from CO2 and NO3-. In comparison with other reported efficient catalysts (half of which are copper materials), Cu8Ni2/C has the superior urea yield rate of (37.53 μmol h-1cm-1) at (-0.8V) with FE in the middle range (Figure 3f). The concrete measurement parameters and performance are shown in Supplementary Table S1.  As illustrated in Figure S17, the Faraday efficiency for nitrite (FE(NO2-)) on Cu/C at -0.4 V reaches 79.36%, which implies that the Cu active site is weak for *NO2 adsorption or difficult to continue the conversion of *NO2, resulting in a high selectivity for nitrite on Cu/C catalyst.[11]. As the content of Ni increased in the CuxNiy/C bimetallic catalyst, the selectivity for nitrite decreased. And the ratio of FE(NO2-) to the sum of FE(NH3) and FE (urea) decreased from 4.5 to  14 0.86 (Figure S18). This observation suggests that Ni favors the adsorption of *NO2 and the subsequent hydrogenation process for the conversion to *NH2. In addition, the electrochemical impedance spectroscopy (EIS) was tested at -0.4 V (vs RHE), as shown in Figure S19. The charge transfer resistance is decreases with the increase of Ni content in Cu-Ni materials, which will accelerate the electrocatalytic reactions. The double-layer capacitance (Cdl) was used to calculate the electrochemically active surface area (ECSA) of electrocatalysts. For urea synthesis, the Cu8Ni2/C sample exhibits the highest Cdl (506.5 μF cm-2). Generally, a larger Cdl represents higher catalytic area (ECSA), which provide more abundant active sites for electrochemical reaction (Figure S20). Subsequently, the effect of nitrate concentration on the selectivity was further explored. The experiments were carried out with different NO3- concentration (100 mM, 20 mM and 4 mM) while keeping the KHCO3 concentration at 100 mM. The experimental results showed that the current density and Faraday efficiency of urea showed an increasing trend with increasing NO3- concentration (Figure S21a, b). However, the highest selectivity for NH3 production is found at the lowest NO3- concentrations (Figure S21c). Probably, *NH2 intermediates were inhibited to achieve C-N coupling at its lower concentration, which then tended to be converted to NH3. It is worth noting that the concentration of nitrate also affects the NO2- selectivity, and there is anoptimal NO3- concentration in the interval of 4~100 mM for the highest NO2- selectivity. To explore the realistic surface states, in situ Raman spectroscopy was applied to analyze the surface of Cu-Ni catalyst during the reaction (Figure S22). Raman spectra were tested at 0.2 V intervals from the open-circuit potential, and the results showed that the catalyst surface state was stable under electrochemical action[37, 38]. Furthermore, we employed ITO conductive glass as the substrate for catalyst loading, and the XRD pattern of the catalyst before and after  15 electrochemical testing is presented in Figure S23. These results indicate that there is no shift of peak positions and new peaks after the reaction, except the decreased intensity of Cu8Ni2 diffaction signal resulting from the possible drop of catalysts from ITO during the electrochemical test. A comparison of the morphology before and after the electrochemical reaction is shown in Figure S24, where the catalyst consists of spherical particles stacked in a loose framework before the electrochemical reaction. After the electrochemical reaction, the particles were partially aggregated. In other words, the Cu8Ni2/C catalyst showed stable crystal phase with morphology reconstructure during the electrochemical reactions. In situ FTIR measurements were carried out on Cu8Ni2/C, Cu/C, and Ni/C catalysts to investigate the evolution of intermediates during the C-N coupling process. The IR signal is collected every 0.1 V interval from the open circuit potential to -0.8 V (vs RHE) during the electrochemical test. Figure 4a-c show the in-situ IR test results of Cu8Ni2/C, Cu/C, and Ni/C catalysts in CO2-saturated electrolyte with 0.1 M KNO3 and 0.1 M KHCO3. The control experiment was also carried out by the in-situ IR tests for Cu8Ni2/C in 0.1 M KNO3 + 0.1 M KHCO3 electrolyte without CO2 (Figure S25). In the range of 1000-1300 cm-1, all spectra showed distinct infrared bond around 1225 cm-1 ascribed to the anti-symmetric stretching vibration of N-O in nitrite during the reduction of NO3- to *NO2 (Figure 4a-c and Figure S25). This result indicates that *NO2 is an essential intermediate for the reduction of NO3- and urea synthesis[39]. In Figure 4b, the IR spectra of Cu/C appears as a downward peak (formation of *NO2) from 0.4V to -0.4V and shift to a upwards peak (consumption of *NO2) as the potential decreases to lower than -0.7 V, indicating that the accelerated transition of *NO2 to *NH (*NH2) at the overpotential lower than -0.7 V. Notably, the upward peak (consumption of *NO2) starts at 0 V on the Ni/C (Figure 4c) and becomes stronger with decreasing potential, which corresponds to the accelerated  16 consumption of *NO2. However, the wave number belonging to *NO2 consumption gradually moves to larger ones (bule-shift) as increasing the negative potential from -0.6 V for the synthesis of urea on Cu8Ni2/C, which indicates the weaker absorption ability of *NO2 on the catalyst surface[40]. This is consistent with its decreased FE (urea) and increased FE(NH3) at more negative potential (Figure S17).  Figure 4 In situ electrochemical spectroscopy measurements. Infrared signal in the range of 1000-3600cm-1(disconnected at 1800-2800 cm-1) under various potentials for (a) Cu8Ni2/C, (b) Cu/C and (c) Ni/C during the electrocoupling of nitrate and CO2. (d) Proposed reaction pathway for urea formation on Cu8Ni2/C. Based on the electrocatalytic performance, the *NO2 is consumed to produce the *NH and *NH2 intermediates for urea and NH3 synthesis. According to consumption signal of *NO2, it can  17 be concluded that Ni promotes the conversion of *NO2 to *NH (*NH2) at lower overpotential than Cu (Figure 4a-c). Furthermore, in Figure 4a, the infrared signals are observed around 3180 and 3350 cm-1 for Cu8Ni2/C (with CO2), which are ascribed to the bending mode of NH2 (δs NH2) and the NH2 stretching mode (νs NH2), respectively[6, 41]. Remarkably, these downward peaks were not observed in Cu8Ni2/C (without CO2, Figure S25), which can be judged that the peaks at 3180 and 3350 cm-1 are the N-H signals from urea molecule rather than the intermediates for urea or ammonia synthesis [1, 14]. For Cu/C, the same νs NH2 and δs NH2 are correspondingly detected at 3368 and 3187 cm-1, which show lower signal intensity and consequently less urea product than Cu8Ni2/C (Figure 4a, b and Figure 3d). However, the infrared spectrum at 3440 cm-1 for Ni/C belongs to the NH2 asymmetric stretching mode (νas NH2) acting as the intermediate for urea synthesis[6], which is not observed on Cu/C and Cu8Ni2/C (Figure 4a-c). It reveals that Ni is beneficial to the formation of *NH2 intermediate. Besides, the downward peak at 3300 cm-1 originates from the stretching vibration of the O-H of the associated state[42, 43], which may be caused by the hydrolysis of Ni atoms to produce *H and O-H. The *H gets transferred and the associative state O-H accumulates on the catalyst surface. In sum, the Ni active sites in Cu8Ni2/C promote the formation of *NH2 and accordingly enhance the electrocatalytic activity for urea synthesis. Subsequently, the infrared band of Cu/C at 1605 cm-1 is assigned to C=O stretching vibration in *CO intermediate[44], which is not detected on Cu8Ni2/C and Ni/C (Figure 4a-c). It suggests that the moderate adsorption of *CO on Cu atoms. In addition, Cu8Ni2/C, Ni/C, and Cu/C show downward peaks indicating the formation of C-N with asymmetric stretching vibration at 1420 cm-1, 1410 cm-1, and 1420 cm-1, respectively. It reveals that the C-N coupling is successfully achieved[6, 45]. Note that the signal intensity of C-N on Cu8Ni2/C is stronger than those on Cu/C  18 and Ni/C, which may be resulted from the synergistic effect of Cu and Ni for the correspondingly optimal formation of *CO and *NH2 intermediates. At the end, the obvious infrared bond located at 1650 cm-1 on Cu8Ni2/C is attributed to the C=O stretching vibration of the amide I band (*CONH2)[46, 47], which confirms the formation of *CONH2 intermediate (Figure 4a). The *CONH2 intermediate may be a product of the first C-N coupling of *CO and *NH2. Based on the intermediates detected in the IR spectra, we summarize a reaction path for the synthesis of urea, as illustrated in Figure 4d. In such a pathway, the CO2 is firstly reduced to form *CO through CO2 →*CO2 →*COOH →*CO, and NO3- is reduced to *NH2 through NO3- → *NO3- → *NO2 →*NO →*N →*NH →*NH2. Then the *CO and *NH2 are directly coupled to form *CONH2. At last, the urea (CO(NH2)2) is synthesized by further *NH2 coupling on *CONH2 intermediate. We verify the feasibility of this reaction pathway, and analyze the reaction mechanism through subsequent DFT calculations. Density functional theory (DFT) calculations were carried out to probe the reaction mechanism of C-N coupling for urea synthesis from NO3- and CO2. The Cu64, Ni64, and Cu48Ni16 slabs are built as models to study the properties of Cu, Ni, and Cu-Ni alloys, respectively. We established calculation models based on the (111) surface of Cu, Ni, and Cu-Ni alloy (Figure S26), which is a highly active face[48]. Firstly, the Table S2 shows the detailed adsorption models of the intermediates for the reduction of nitrate to *NH2, which present a process NO3- →*NO3 →*NO2 →*NO →*N →*NH →*NH2. The key adsorption models are presented in Figure 5a. In Cu and Ni metals, the *NOx and *NH2 homogenously bond with the Cu or Ni atoms. As for Cu-Ni alloy, the *NOx and *NH2 prefer to heterogeneously bond with both Cu and Ni atoms in DFT optimization process. The Gibbs free energy for each step is presented in Figure 5b. For Cu, the reduction of *NO2 to *NO is the rate-determining step (RDS) with a high Gibbs free energy change  19 (ΔG) of 1.95 eV for the formation of *NH2. The high energy barrier for the conversion of *NO2 to *NH2 on Cu electrocatalyst, resulting in the desorption tendency for *NO2 from Cu surface to produce NO2-. Such a result is consistent with the results of the performance tests of the Cu catalyst (FE(NO2-) = 79.36%) (Figure S17). On the contrary, the ΔG are -0.32 eV and -0.12 eV on Ni and Cu-Ni alloy for the formation of *NO from *NO2, respectively. The result shows that Ni atoms are important in promoting the conversion of *NO2 to *NH2, which has been confirmed by in-situ IR analysis as Figure 4c presented. The formation of *H is shown in Figure 5e, it is difficult to produce protons (*H) on Cu atoms with Gibbs free energy change (ΔG) of 0.60 eV. The doping of Ni facilities such a process and makes hydrogenation a spontaneous process, which greatly promotes the conversion of *NO2 to *NH2.  Figure 5 DFT calculations. (a) Schematic model of the adsorption of intermediate and (b) free  20 energy profile of NO3- reduction to *NH2. (c) Schematic model of the intermediate adsorption and (d) free energy profile of CO2 reduction to *CO. (e) Free energy profile of H+ reduction to *H. (f) Schematic model of the intermediate and (g) free energy profile of C-N coupling on Cu, Ni, and Cu-Ni alloy surfaces. (h) Electronic partial density of states in 3d orbitals and d-band center positions of Cu, Ni, and Cu-Ni alloy. “*” represents the adsorbed state.   The schematic model of the intermediate for the reduction of CO2 to *CO (Figure 5c and Table S3) shows that the C atoms in *CO adsorbed by Cu, Ni and Cu-Ni alloy are preferred to bond with Cu atoms. The Gibbs free energy of the reaction are presented in Figure 5d, and the Gibbs free energy of *COOH formation (ΔG = 0.88 eV) becomes the potential limiting step on Cu. The doping of Ni may lead to the change of electronic states of Cu atoms (will discussed later), resulting in enhanced adsorption of Cu to *COOH and *CO[49, 50]. Figure 5f present the C-N couplings between *CO and *NH2. In comparison with the Cu or Ni metals, the *CO and *NH2 prefer to heterogeneously bond with both Cu and Ni atoms in DFT optimization process. In such an adsorption model, the *CO is adsorbed on Ni site and the *NH2 is adsorbed on Ni site. As illustrated in Figure 5g, C-N coupling is an important rate-determining step. The ΔG of *CO and * NH2 for Ni and Cu is 1.42 and 1.28 eV, respectively. As for Cu-Ni alloy, as the *CO adsorbs on Cu site and *NH2 adsorbs on Cu and Ni site, the ΔG of *CO and *NH2 coupling decreased to is only 0.33 eV, which greatly promote the urea formation. The second step of C-N coupling between *NH2CO and *NH2 is a thermodynamically spontaneous process for Cu and Cu-Ni and need relatively large energy input for Ni. Therefore, Ni atoms act as the active site of *NH2 and Cu atoms act as the active site of *CO to synergistically achieve C-N coupling reactions. Furthermore, after incorporation of Ni, the d-band center of Cu-Ni alloy upshifts toward the Fermi level in  21 compared with the Cu, as the Figure 5h exhibited. The changes of d-band center can also influence the adsorption and desorption ability of intermediate such as *CO, *CO2, *NO2, and *NH2[26, 49]. 4. Conclusion In conclusion, the Ni is incorporated into Cu to form Cu-Ni alloys nanocrystals, acting as heterogenous active sites for enhancing the hydrogenation of NOx species and C-N coupling reactions to produce urea. Experimentally, the C supported Cu-Ni catalysts are synthesized by pyrolysis of Cu-Ni MOF. The analysis of EXAFS and XPS show that Cu and Ni are mainly metallic state alloy. The performance of urea synthesis for Cu-Ni catalyst is higher than those of elemental metal Cu and Ni, which achieves urea Faradaic efficiency to 25.1% at -0.5 V (vs RHE) and yield of 37.53 μmolh-1cm-2 at -0.8 V (vs RHE). The performance of Ni-Cu alloy is much superior to mostly reported catalysts. The pathway of urea formation is analysed by in-situ FT-IR, suggesting a possible C-N coupling reaction pathway between *NH2 and *CO. The theoretical calculations reveal that hydrogenation of *NOx and coupling of *CO and *NH2 are more easily occurred over the heterogenous Cu and Ni atoms rather than on homogeneous Cu and Ni atoms. The constructed heterogeneous active sites reported in this works give an efficient strategy to promote electrochemical urea synthesis from NO3- and CO2. Declaration of Competing Interest  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ACKNOWLEDGMENT We appreciated the financial supports from the following fundings: National Natural Science Foundation of China (22072183); The Natural Science Foundation of Hunan Province, China  22 (2022JJ30690); Foundation of Yangtze Delta Region Institute (Huzhou) of UESTC, China (Nos. U03210060); High Performance Computing Center of Central South University. Appendix A. Supporting information.  Supplementary data associated with this article can be found in the online version. References [1] S.-K. Geng, Y. Zheng, S.-Q. Li, H. Su, X. Zhao, J. Hu, H.-B. Shu, M. Jaroniec, P. Chen, Q.-H. Liu, S.-Z. Qiao, Nickel ferrocyanide as a high-performance urea oxidation electrocatalyst, Nature Energy, 6 (2021) 904-912. [2] R. Lan, S.W. Tao, J.T.S. 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