Chloride-Derived Bimetallic Cu-Fe Nanoparticles for High-Selective Nitrate-to-Ammonia Electrochemical Catalysis

: Cu-based bimetallic materials have been widely reported as efﬁcient catalysts for electrocatalytic nitrate reduction. However, the faradaic efﬁciency and selectivity are still far from satisfactory. Herein, Cu-Fe bimetallic nanoalloys with adjustable Cu/Fe ratios are successfully prepared through a reactive mechanical milling approach with CuCl 2 , FeCl 3 and Na as the starting materials. The optimized Cu 3 Fe exhibits excellent nitrate conversion efﬁciency of 81.1% and 70.3% ammonia selectivity at − 0.7 V vs. RHE within 6 h under 0.1 M Na 2 SO 4 and 100 ppm NO 3 − . The Fe-introduction-induced upshift of the d -band center is identiﬁed to be beneﬁcial for promoting nitrate adsorption on Cu 3 Fe. Moreover, favorable H generation under the assistance of Fe could effectively accelerate the stepwise hydrogenation during electrocatalytic nitrate reduction, resulting in signiﬁcantly improved NH 4+ selectivity. This work supplies valuable insights for the rational design of transition-metal-based bimetallic catalysts for electrocatalytic nitrate reduction.


Introduction
Intensified industrialization breaks the global nitrogen cycle, resulting in concentrated nitrate in groundwater and other water bodies [1]. Excessive nitrate discharge always leads to severe environmental contamination and impending threat to public health [2,3]. To address the nitrate contamination and restore the nitrogen cycle in nature, several physicochemical and biological approaches have been commercially applied [4,5]. Although nitrate enrichment could be successfully achieved based on physicochemical strategies, subsequent hazard-free treatment is still mandatorily required, which always involves complex procedures with high costs. The microbial approach always suffers from long process flow and poor resistance to environmental disturbance. Therefore, developing highly efficient, stable and cost-effective alternative strategies to achieve nitrate removal is of great significance for water restoration. In this context, electrocatalytic nitrate reduction is attracting intensive attention due to its ability to apply external circuit current rather than chemical agents as an electron donor to reduce nitrate. The booming green energy systems enable the electrochemical reduction of nitrate to be more eco-friendly and cost-effective. In addition, electrocatalytic denitrification exhibits a strong anti-interference capability to adapt to environmental fluctuation. Thus, compared with physicochemical and biological approaches, the electrochemical strategy is more efficient, stable and flexible for nitrate removal.
Recently, intensive studies have focused on electrocatalytic reduction of nitrate into NH 4 + , which exhibits the potential to replace or at last supplement the energy-intensive Haber-Bosch process for NH 3 production. However, achieving high selectivity toward After mechanical milling, 50 mL absolute ethanol was added into the milling vessel and stirred to remove unreacted metal sodium. Centrifuges were used to separate the remaining particles. In this step, no deionized water was added to avoid the reaction between Na and H 2 O to form NaOH and further react with Fe in the bimetallic nanoparticle. Subsequently, 50 mL deionized water was added into the centrifugal tube, stirred and centrifuged to remove NaCl wrapped on catalyst particles. To obtain pure alloy nanoparticle sediment, repeated washing operations were necessary. Finally, the centrifugal tube with catalyst was freeze-dried for 10 h to avoid intense nanoparticle agglomeration during the evaporation process. The Cu catalyst was synthesized according to the same steps described above without adding FeCl 3 .

Instrumentation
X-ray diffraction (XRD) patterns were taken on a TD-3500 diffractometer (Dandong Tongda, Dandong, China) using Cu Kα radiation (λ = 1.54056 Å) at 40 kV and 40 mA and collected in the 2θ range 3-80 • . Energy-dispersive X-ray spectroscopy (EDS) analysis coupled to Scanning Electron Microscopy (SEM) was conducted on a MIRA3 instrument (TESCAN, Prague, Czech Republic). Transmission Electron Microscope (TEM) images were taken on a JEM-2100-F instrument (JEOL, Tokyo, Japan). X-ray photoelectron spectra (XPS) and ultraviolet photo-electron spectroscopy (UPS) were measured by an ESCALAB 250Xi Multi Technique System (Thermo Fisher, Waltham, MA, USA) equipped with an Al Kα X-ray exciting source (1486.6 eV). The binding energies were calibrated by using the C 1 s peak at 284.8 eV as an internal standard. UV-Vis spectra were obtained from a Unico 2800 spectrophotometer (Thermo Fisher, Waltham, MA, USA) with 1 cm quartz cells.

Concentration Detection for NO 3 − , NO 2 − and NH 4 + by UV-Vis Spectrometry
A CuFe nanoparticle was deposited in Ni foam supports using Nafion as a binder to investigate product distribution in a catalysis cycle. NO 3 − , NO 2 − , and NH 4 + concentration was recorded as a function of Cu/Fe ratio and reaction time. The characteristic absorption peak of NO 3 − , NO 2 − and NH 4 + was identified and the corresponding standard curve was prepared ( Figure S1) to detect the concentration. The concentration of NH 4 + in the solution was detected by salicylic acid spectrophotometry, and deionized water was used as the background. The concentration of NO 2 − was detected by diazotization coupling spectrophotometry, and deionized water was used as the background. NO 3 − was simply detected by ultraviolet spectrophotometry, and for the NO 3 − characteristic absorption peak that could be detected at 220 nm without adding any chromogenic agent, 0.5 mL 1 M HCl was added to shield the interference from a characteristic absorption peak of CO 2 dissolved in the solution; 0.5 mL 1 M HCl was also added to the deionized water as the background. All the ion concentrations we detected were diluted to the corresponding linear area to fit their standard curve. Before electrolysis, argon was introduced into the electrolytic cell for 20 min to replace dissolved O 2 . After that, the initial electrolyte sample was detected, and the electrolyte sample was detected every 2 h to track the product concentration variation.

Electrocatalytic Measurement of Nitrate Reduction
The cyclic voltammetry (CV) test of Cu-Fe nanoparticles with different Cu-Fe ratios was carried out to observe the electrocatalytic reaction process at different voltages. CV test was carried out in a three-electrode system: platinum wire electrode as a counter electrode; Ag/AgCl electrode as a reference electrode; a glassy carbon electrode coated with catalyst as a working electrode; 0.007 M NaNO 3 and 0.1 M Na 2 SO 4 solution as electrolyte. All the potentials recorded were converted into the potential versus the reversible hydrogen electrode (RHE) during the CV test, and the scan rate of each test as set as 10 mV s −1 . Before the CV test, the electrode material was subjected to 3 cycles of CV scanning within the working voltage range to activate the electrode material, and then the CV test was carried out and recorded. Koutecký-Levich (K-L) plots were obtained by linear sweep voltammetry (LSV) carried out on rotating disk electrode (RDE) at different rotation speeds. We used the K-L equation and K-L plots to calculate the number (n) of transferred electrons in the NRA reaction process.
K-L analyses were performed according to the following K-L equation: where A is the electrode area; i m is the measured current; i K is the kinetic current from NO 3 − electroreduction; n is the number of electron transfers in the reaction; F is Faraday's constant (96,485 C mol −1 ); D represents the effective diffusion coefficient of nitrate, which is 1.4 × 10 −5 cm 2 s −1 for 0.1 M nitrate at 25 • C; ν is the kinematic viscosity of water at 25 • C (1 × 10 −6 m 2 s −1 ); C is the nitrate concentration, and ω represents the electrode's rate of rotation (rpm).

Results and Discussion
As shown in Scheme 1, the Cu-Fe nanoalloys with different Cu/Fe ratios of 3:1, 1:1 and 1:3, donated as Cu 3 Fe, CuFe and CuFe 3 , respectively, are successfully achieved through changing the CuCl 2 /FeCl 3 ratio during reactive mechanical milling. The phase structure of the as-prepared Cu-Fe nanoalloys is characterized by powder X-ray diffraction (XRD). As shown in Figure 1d With the content increase of FeCl 3 during mechanical milling, a sharp peak intensity decrease of cubic Cu takes place, combined with a rapid peak increase of Fe (101). Noteworthily, there is no intermetallic compound between Cu and Fe according to the corresponding phase diagram; thus, the as-prepared Cu-Fe nanoalloys were speculated mainly to exist in the form of a solid solution state. Morphology of the samples was investigated via scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure S2, all the samples appear in the form of slightly aggregated particles of about 100 nm. Magnified TEM image further reveals that the aggregated particles are composed of nanoparticles 10 nm in diameter (Figure 1a,b). Furthermore, the distinct clear lattice spacing of 0.209 nm for the Cu (111) plane of Cu 3 Fe is well resolved [14] (Figure 1b), which is 1% larger than that of pure Cu, indicating lattice stretching of Cu (111) by Fe doping. Also, the distinct clear lattice spacing of 0.204 nm for Fe (101) plane of Cu 3 Fe is well resolved, which is 2% larger than that of pure Fe, indicating lattice stretching of Fe (101) Meanwhile, energy dispersive X-ray (EDS) elemental mapping analysis demonstrates the uniform distribution of Cu and Fe in the Cu 3 Fe nanoalloy ( Figure 1c). The percentages of each element is presented by an EDS mapping sum spectrum, and the detected Cu/Fe ratio of Cu 3 Fe is close to 3/1, which is the initial starting material proportion, indicating that the mechanical milling method can retain an atomic ratio consistency ( Figure S3).  XPS analysis is further performed to study the surface composition and chem state of the sample. The survey spectra of the as-prepared samples confirm the prese of Cu, Fe and O in all the samples ( Figure S4). Noteworthily, the strong O signal sugg that all the Cu-Fe nanoalloys involve surface oxidation. As shown in Figure 1e, two typ peaks located at 933.8 and 953.7 eV are resolved, corresponding to Cu 2+ of Cu 2p3/2 and Scheme 1. A schematic illustration explaining why Bulk Na cannot be pulverized by mechanical milling (a) while the mixture of Na, CuCl 2 and FeCl 3 can be converted to Cu-Fe cluster by mechanical milling (b).  XPS analysis is further performed to study the surface composition and chemical state of the sample. The survey spectra of the as-prepared samples confirm the presence of Cu, Fe and O in all the samples ( Figure S4). Noteworthily, the strong O signal suggests that all the Cu-Fe nanoalloys involve surface oxidation. As shown in Figure 1e, two typical peaks located at 933.8 and 953.7 eV are resolved, corresponding to Cu 2+ of Cu 2p3/2 and Cu The plain Cu prepared through the same condition shows a characteristic peak at 932.7 eV for Cu 0 and a characteristic peak at 934.0 eV for Cu 2+ ( Figure S5). The result shows a 0.2 eV negative shift on Cu 0 species characteristic peaks and a positive shift on Cu 2+ species characteristic peaks, indicating the electron transfer takes place at CuO/Cu interface [15]. In addition, a broad Fe 2p peak corresponding to the oxidized state is observed in Figure 1f. With the increase of Fe content, a negative shift of the Cu 2p peaks takes place, and the ratio of Cu 2+ /Cu 0 decreases, indicating that introduction of Fe could effectively reduce Cu 2+ and further regulate the electron structure between Cu 0 and Cu 2+ , while strong Fe 2p peaks at 709.4 eV and 711.9 eV are resolved in CuFe and CuFe 3 , suggesting the intensified oxidation and the electronic density decrease of Fe [16,17] (Figure 1f). The above XPS results demonstrate that the addition of Fe could effectively regulate the distribution of Cu 3d band, leading to the high electron density of Cu 0 , which induces the decrease of reaction barrier and suppresses the competitive H 2 production, resulting in a high conversion rate, Faradaic efficiency (FE) and NH 3 selectivity for NRA [18][19][20][21][22].
The electrochemical denitrification performances of the as-prepared Cu-Fe nanoalloy samples are conducted based on a standard three-electrode system. All the reported potentials are converted to a reversible hydrogen electrode (RHE) scale. The nitrate removal rate is compared via electrolysis test at 0.7 V vs. RHE, Cu 5 Fe, Cu 3 Fe, CuFe and CuFe 3 shows a higher nitrate conversion than Fe and Cu prepared under the same condition ( Figure 2a). It is worth noting that Cu 3 Fe, CuFe and CuFe 3 show more effective electroreduction of nitrate than Cu 5 Fe, thus pointing out the direction for the experiments subsequently. Cyclic voltammetry (CV) tests are carried out on Cu 3 Fe, CuFe and CuFe 3 to compare their NRA activity in the electrolyte of 0.1 M Na 2 SO 4 and 100 ppm NO 3 − -N ( Figure 2b). Compared with CuFe and CuFe 3 , Cu 3 Fe exhibits a much larger peak current density, indicating its highest intrinsic activity for catalyzing nitrate reduction. Cu 3 Fe shows a greater current density in the electrolyte that contains nitrite than in the electrolyte containing nitrate, with 0.1 M Na 2 SO 4 as the supporting electrolyte ( Figure S6). Potentiostatic electrolysis test is conducted at −0.7 V vs. RHE. The reactant and reductive products, including nitrite and ammonia, are detected according to UV-Vis spectrophotometry results [23] (Figures 2c and S1). Continuous nitrate decrease takes place after electrolysis at different times on Cu 3 Fe, CuFe and CuFe 3 . Compared with CuFe and CuFe 3 , Cu 3 Fe shows the highest ammonium selectivity and nitrate conversion efficiency. The conversion of nitrate and the selectivity for ammonium on Cu 3 Fe reaches 81.1% and 70.3% after 6 h (Figure 2c), respectively, which is comparable or even superior to most reported Cu-based nonnoble bimetallic catalysts [24,25]. The catalytic activity of Cu 3 Fe for NRA is investigated under 0.1 M Na 2 SO 4 with various concentrations of nitrate (Figure 2d). The significant current increase takes place with the rise of the NO 3 − concentration, showing the stepwise activation of Cu 3 Fe.
Electrochemical transformation of nitrate into NH 4 + involves complex proton-coupled electron transfer (PCET) reactions, which always compete with hydrogen evolution reaction (HER) [26]. Thus, controllable generation of highly reactive H plays a vital role in boosting NRA. According to previous research results, Fe-based materials are identified to be an efficient catalyst for the generation of H [7]. To verify the effect of Fe doping on boosting PCET, linear sweep voltammetry (LSV) characterization is performed on Cu-Fe nanoalloys with different Fe contents under 0.1 M KOH (Figure 2e). Obviously, the reduction current density grows with the content increase of Fe. The corresponding Tafel slope plots are depicted in Figure 2f; CuFe 3 exhibits the smallest Tafel slope of 0.29 V dec −1 , suggesting that the introduction of Fe is beneficial for boosting the apparent catalytic reaction kinetics. However, excess Fe introduction is detrimental to the nitrate conversion efficiency and NH 4 + selectivity (Figure 2c). Therefore, it is reasonable to deduce that the greatly enhanced reduction current density and apparent reaction kinetics on Fe-rich nanoalloys are mainly triggered by the boosted HER. Generally, Cu plays a vital role in nitrate adsorption and subsequent reduction into nitrite, but it is insufficient for catalyzing following stepwise reduction reactions during NRA [27][28][29]. Proper introduction of Fe into Cu is beneficial for promoting stepwise hydrogenation reactions of nitrite, while superfluous Fe will suppress NRA by covering the active sites of Cu and boosting the side HER reactions [30]. Electrochemical transformation of nitrate into NH4 + involves complex proton-coupled electron transfer (PCET) reactions, which always compete with hydrogen evolution reaction (HER) [26]. Thus, controllable generation of highly reactive H plays a vital role in boosting NRA. According to previous research results, Fe-based materials are identified to be an efficient catalyst for the generation of H [7]. To verify the effect of Fe doping on boosting PCET, linear sweep voltammetry (LSV) characterization is performed on Cu-Fe nanoalloys with different Fe contents under 0.1 M KOH (Figure 2e). Obviously, the reduction current density grows with the content increase of Fe. The corresponding Tafel slope plots are depicted in Figure 2f; CuFe3 exhibits the smallest Tafel slope of 0.29 V dec −1 , suggesting that the introduction of Fe is beneficial for boosting the apparent catalytic reaction kinetics. However, excess Fe introduction is detrimental to the nitrate conversion efficiency and NH4 + selectivity (Figure 2c). Therefore, it is reasonable to deduce that the greatly enhanced reduction current density and apparent reaction kinetics on Fe-rich nanoalloys are mainly triggered by the boosted HER. Generally, Cu plays a vital role in nitrate adsorption and subsequent reduction into nitrite, but it is insufficient for catalyzing following stepwise reduction reactions during NRA [27][28][29]. Proper introduction of Fe into Cu is beneficial for promoting stepwise hydrogenation reactions of nitrite, while superfluous Fe will suppress NRA by covering the active sites of Cu and boosting the side HER reactions [30].
To investigate electrocatalytic behaviors of the samples towards NRA, they are deposited on rotating disk electrodes (RDEs) and tested in a neutral electrolyte containing 0.1 M Na2SO4 and 0.01 M NaNO3. A three-electrode configuration is applied with RDEs as the working electrode, and Pt wire and an Ag/AgCl electrode as counter electrode and reference electrode, respectively. Electrocatalytic behaviors of the Cu-Fe catalysts with different Fe contents were investigated. The half-wave potentials (E1/2) of the catalysts towards electrocatalytic nitrate reduction were obtained from the LSV curves tested on RDEs (Figure 3a). It was observed that the Cu3Fe exhibited the highest E1/2 of −0.33 V vs. RHE, while the Cu possessed E1/2 values of −0.37 V under the same testing conditions. The results indicated that Fe doping greatly enhanced NRA activity of the Cu catalysts [31]. In To investigate electrocatalytic behaviors of the samples towards NRA, they are deposited on rotating disk electrodes (RDEs) and tested in a neutral electrolyte containing 0.1 M Na 2 SO 4 and 0.01 M NaNO 3 . A three-electrode configuration is applied with RDEs as the working electrode, and Pt wire and an Ag/AgCl electrode as counter electrode and reference electrode, respectively. Electrocatalytic behaviors of the Cu-Fe catalysts with different Fe contents were investigated. The half-wave potentials (E 1/2 ) of the catalysts towards electrocatalytic nitrate reduction were obtained from the LSV curves tested on RDEs (Figure 3a). It was observed that the Cu 3 Fe exhibited the highest E 1/2 of −0.33 V vs. RHE, while the Cu possessed E 1/2 values of −0.37 V under the same testing conditions. The results indicated that Fe doping greatly enhanced NRA activity of the Cu catalysts [31]. In addition, an electrochemically active surface area (ECSA) test was conducted on Ni foam coated with Cu 3 Fe and Cu, respectively. The result indicated that Cu 3 Fe has greater intrinsic activity than Cu toward NRA, because Cu 3 Fe presents a larger ECSA-normalized current density than Cu ( Figure S7).
To determine the electron transfer number (n) of the Cu 3 Fe and Cu in NRA, LSV curves of the RDEs at various rotating speeds were recorded (Figure 3b,c). The plain Cu catalyst was used as the control sample. The current density increased with increased rotation rate, owing to the shortened diffusion distance at high rotation speeds [32,33]. The corresponding Koutecky-Levich (K-L) plots at −0.7 V vs. RHE for the Cu 3 Fe and plain Cu are shown in Figure 3d, which displays good linearity and near coincidence. The analysis of K-L equation revealed a 6-electron-transfer process for nitrate reduction on Cu 3 Fe and an 8-electron-transfer process for nitrate reduction on plain Cu. Therefore, it is speculated that the reaction at −0.7 V vs. RHE on Cu 3 Fe is the reduction of nitrite to ammonia, as the valence of N-containing species is reduced from +3 to -3 [5,34]. The nitrate-to-nitrite reduction potential was shifted positively because of the upshift of the d-band center (E d ). The current density of Cu 3 Fe was calculated based on the intercept of the K-L plot [35]. At an applied potential of −0.7 V vs. RHE, the kinetic current density of the Cu 3 Fe is 46.3 mA cm −2 , which is 1.8 times that of the Cu catalyst (25.2 mA cm −2 ), indicating the remarkably enhanced NRA activity of the Cu 3 Fe (Table S1).
Processes 2022, 10, x FOR PEER REVIEW 8 of addition, an electrochemically active surface area (ECSA) test was conducted on Ni foam coated with Cu3Fe and Cu, respectively. The result indicated that Cu3Fe has greater intrin sic activity than Cu toward NRA, because Cu3Fe presents a larger ECSA-normalized cu rent density than Cu ( Figure S7). To determine the electron transfer number (n) of the Cu3Fe and Cu in NRA, LS curves of the RDEs at various rotating speeds were recorded (Figure 3b,c). The plain C catalyst was used as the control sample. The current density increased with increased ro tation rate, owing to the shortened diffusion distance at high rotation speeds [32,33]. Th corresponding Koutecky-Levich (K-L) plots at −0.7 V vs. RHE for the Cu3Fe and plain C are shown in Figure 3d, which displays good linearity and near coincidence. The analys of K-L equation revealed a 6-electron-transfer process for nitrate reduction on Cu3Fe an an 8-electron-transfer process for nitrate reduction on plain Cu. Therefore, it is speculate that the reaction at −0.7 V vs. RHE on Cu3Fe is the reduction of nitrite to ammonia, as th valence of N-containing species is reduced from +3 to -3 [5,34]. The nitrate-to-nitrite r duction potential was shifted positively because of the upshift of the d-band center (Ed The current density of Cu3Fe was calculated based on the intercept of the K-L plot [35]. A an applied potential of −0.7 V vs. RHE, the kinetic current density of the Cu3Fe is 46.3 m cm −2 , which is 1.8 times that of the Cu catalyst (25.2 mA cm −2 ), indicating the remarkabl enhanced NRA activity of the Cu3Fe (Table S1).
Electrolysis tests were conducted on Cu3Fe to determine the most efficient potenti which has the highest Faradaic efficiency (FE) for NRA (Figure 4a). The conversion o nitrate increased gradually as the potential rose from −0.6 V to −0.9 V vs. RHE, while th Electrolysis tests were conducted on Cu 3 Fe to determine the most efficient potential which has the highest Faradaic efficiency (FE) for NRA (Figure 4a). The conversion of nitrate increased gradually as the potential rose from −0.6 V to −0.9 V vs. RHE, while the Faraday efficiency plot showed a volcanic curve with a maximum value of 74.2% at −0.7 V vs. RHE. Therefore, −0.7 V vs. RHE was selected as the test potential. With the extension of reaction time, the concentration of nitrate decreased while the concentration of ammonia increased, indicating the appearance of nitrate-to-ammonia electrochemical conversion (Figure 4d). After 5 cycles, the ammonia selectivity remained at a relatively high level, while the nitrate removal rate slightly decreased (Figure 4b). The degradation of nitrate removal rate may attribute to the excess loading of Cu-Fe nanoparticle on Ni foam for the weight of working electrode gradually decreased after 5 cycles of an electrocatalysis test. The effect of initial nitrate concentration on selectivity was investigated ( Figure 4c). As the concentration of NO 3 − -N increased from 100 ppm to 2000 ppm, the selectivity of ammonia remained almost unchanged, showing a wide range of applications. After 6 h of the electrocatalytic process, the morphology of the catalyst transferred from granular to flake ( Figure S8). removal rate may attribute to the excess loading of Cu-Fe nanoparticle on Ni foam for the weight of working electrode gradually decreased after 5 cycles of an electrocatalysis test. The effect of initial nitrate concentration on selectivity was investigated ( Figure 4c). As the concentration of NO3 − -N increased from 100 ppm to 2000 ppm, the selectivity of ammonia remained almost unchanged, showing a wide range of applications. After 6 h of the electrocatalytic process, the morphology of the catalyst transferred from granular to flake (Figure S8).  According to the high-Resolution Transmission Electron Microscope (HRTEM) results (Figure 1b), the Cu (111) plane of Cu 3 Fe nanoalloy exhibited a relatively larger lattice space than that of pure Cu nanoparticles. Crystal lattice expansion of Cu resulted in contraction and upshift of the corresponding d band energy level [36]. Generally, the d-band center (E d ) was considered a descriptor for the adsorbate-metal interaction [37,38]. When the catalyst surface chemically adsorbed nitrate and the corresponding intermediates, the d-band of Cu would couple with the energy level orbital of the adsorbates to form a bonding orbital and an anti-bonding orbital. The strength of chemical adsorption depended on the filling of the anti-bonding orbital. The up-shifted E d of Cu 3 Fe signifies the electron occupancy decrease of the anti-bonding orbital, and thus the interaction between the adsorbates and the surface of Cu 3 Fe would be enhanced. In addition, the Fe catalyzed generation of reactive H was beneficial for the hydrogenation of the adsorbed oxynitrides. Hence, the adsorbed nitrite tended to be over hydrogenated to form ammonium, rather than falling off from the adsorption site to form nitrogen or other intermediate products [39].
To further explore the underlying electrocatalytic mechanism of the catalyst during RNA, ultraviolet photoelectron spectroscopy (UPS) was performed (Figure 4e). The position of the d-band center was compared according to the value band maximum values of UPS, as shown in Figure 4 The valence band maximum values were calculated to be 9.16 eV, 9.64 eV and 10.39 eV for CuFe 3 , Cu and CuFe 3 , respectively. The valence band maximum (VBM) was shifted toward the Fermi level by moderate Fe-doping, while the opposite shift was observed after the excess of Fe content on CuFe 3 . Since the valence electrons close to the Fermi level mainly contribute the d states [40,41], the shift of the valence band indicated the position of the d-band center also shifted (Figure 4f). It was found that the maximum value band of Cu 3 Fe was 0.5 eV lower than that of Cu, indicating the corresponding upshift of E d by 25% Fe-doping content, and the rearrangement of electron structure emerged in the presence of Fe-substituted.

Conclusions
In conclusion, Cu-Fe nanoparticles catalysts with flexible Cu/Fe ratios were successfully achieved simply through reactive mechanical milling strategy with metal chloride as the metal source and sodium as the reducing agent. Systematic structural and electrochemical analysis endorse that Fe introduction was beneficial for pushing up the d-band center of Cu 3 Fe, resulting in greatly enhanced nitrate adsorption. Moreover, the intrinsic catalytic activity of Fe for the generation of H enabled stepwise hydrogenation reactions during nitrate reduction to make it more feasible and kinetically favorable. Benefitting from the unique structure, the optimized Cu 3 Fe exhibited excellent electrocatalytic NRA performance under a neutral electrolyte. When the applied potential was −0.7 V vs. RHE, the kinetic current density provided by Cu 3 Fe was recorded as 46.3 mA cm −2 , which is 1.8 times beyond that of Cu. In addition, a high Faraday efficiency of 74.2% and NH 4 + selectivity of 70.3% was achieved under Cu 3 Fe with great structure robustness. This study provides fundamental understanding and rational design strategies for developing highly efficient Cu-based electrocatalytic nitrate reduction catalysts.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/pr10040751/s1, Figure S1: The calibration curves of (a) nitrate-N, (b) nitrite-N, (c) ammonia-N for NO 3 − electroreduction and (d) the UV-Vis absorption spectra corresponding to ammonia-N with deionized water as background; Figure S2: The TEM images of (a,c) CuFe and (b,d) CuFe 3 nanoparticles; Figure S3: Element percentage of Cu 3 Fe by EDS mapping sum spectrum results; Figure S4: XPS survey spectra of the (a) Cu3Fe, (b) CuFe and (c) CuFe3 nanoparticles; Figure S5: XPS Cu 2p of nano Cu catalyst prepared by ball-milling method; Figure  S6: CV plots of different electrolyte by Cu 3 Fe, 0.1 M Na 2 SO 4 is used as background electrolyte for all the CV tests; Figure S7: ECSA-normalized current densities of Cu 3 Fe and plain Cu catalysts measured by glassy carbon electrode in a 1 M NaOH electrolyte solution; Figure S8: The SEM images of Cu 3 Fe catalyst (a) before and (b) after 6 h electrocatalysis test; Table S1: K-L analysis results of