Xiao-Xue Fu‡
ac,
Hui Guo‡ac,
Duan-Hui Si‡a,
Hong-Jing Zhu‡ac,
Yi-Ying Lana,
Yuan-Biao Huangabc and
Rong Cao
*abc
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, China. E-mail: rcao@fjirsm.ac.cn
bFujian Science & Technology Innovation Laboratory for Optoelectronic Information of China, Fuzhou, 350108, China
cUniversity of Chinese Academy of Sciences, Beijing, 100049, China
First published on 24th June 2025
Electrocatalytic reduction of the pollutant nitrate to ammonia (NO3RR) using clean energy is being considered as a viable alternative to the Haber–Bosch process for producing industrially valuable ammonia. However, the multi-electron–proton transfer process of the NO3RR to ammonia usually leads to poor selectivity and low current density, which still cannot meet the industrial requirements. Stabilizing the key intermediates during the reaction is particularly important for achieving high selectivity in the NO3RR towards the production of NH3. Herein, we develop a hydrogen bonding strategy to stabilize the key intermediates of the NO3RR, which involves the design and synthesis of trinuclear copper(I) cluster-based metal–organic frameworks (MOFs). The methyl groups in the copper-based MOFs (DiMe-Cu3-MOF) can regulate the electron density around the Cu3 site and stabilize the key intermediates, *NO2, through hydrogen bonding interaction with methyl groups. Thus, the DiMe-Cu3-MOF electrocatalyst delivers a high NH3 faradaic efficiency (95%) for the NO3RR with a high ammonia production of 401 μg cm−2 h−1, and the partial current density of ammonia reaches an industrial level value of −950.6 mA cm−2. Control experiments and theoretical studies demonstrated that the introduction of methyl groups into the DiMe-Cu3-MOF can facilitate atypical hydrogen bonding with the intermediates of the NO3RR and thus enhance the adsorption of intermediates and reduce the energy barrier of the conversion of NO3− to NH3. This work highlights the vital importance of adjusting the microenvironment through hydrogen bonding for enhancing the NO3RR performance.
The competing hydrogen evolution reaction (HER) usually occurs during the NO3RR in aqueous electrolytes.18 Moreover, despite the lower activation energy barrier of N–O, the complex reaction pathway of the NO3RR (NO3− + 9H+ + 8e− → NH3 + 3H2O) involves multiple electron and proton transfers and production of several intermediates, which frequently results in low activity and selectivity of the NO3RR, thereby posing a significant challenge for practical applications.19,20 Several electrocatalysts including metal-based electrocatalysts have been applied in the NO3RR; however, their activity and selectivity still need to be improved to meet the industrial requirements for ammonia production.21,22 Compared to other electrocatalysts, copper-based materials have been widely used in electrochemical reactions such as CO2 reduction and the NO3RR due to their suitable adsorption energy for intermediates.22–24 However, the various intermediates are usually detached from the copper active sites prior to completing the eight-electron transfer process required for ammonia generation.8,25 This is due to potential-dependent phase transitions, resulting in deactivation caused by changes in the oxidation state, morphology, crystal facets, and other factors. In addition, the process of reducing nitrate to ammonia involves eight electron transfer steps, and there may be multiple potential intermediates generated in the process, such as NO2−, NO, N2 and NH2OH.26,27 Therefore, stabilizing these intermediates in the reaction process to prevent them from dissociating from the active site will be beneficial for the formation of the final product.
As is well known, regulating the microenvironments and introducing suitable groups into electrode materials could reduce the energy barrier of the electrochemical reaction and thus enhance the faradaic efficiency (FE) and selectivity.28 Interestingly, the catalytic mechanism of one of the enzymes, copper nitrite reductase (CuNiR), involved in the biological reduction of NOx to ammonia, involves the binding of NO2− to the catalytic center T2Cu, replacing a soluble molecule.29 Simultaneously, hydrogen bonding forms between the residue coordinating with T2Cu and NO2−, leading to the formation of the subsequent intermediate in the reaction process.30,31 Inspired by this, introducing molecular interactions involving hydrogen bonding into the electrocatalytic NO3RR can effectively stabilize specific intermediates and direct the reaction along the expected pathway.32 Currently, there is limited research on the precise control of electrocatalytic processes through weak interactions, such as hydrogen bonding. Therefore, it is necessary to develop new catalysts to enhance the NO3RR.
Metal–organic frameworks (MOFs) have recently emerged as a promising class of crystalline porous polymers due to their large surface area, stability, and potential in various applications such as gas adsorption, heterogeneous catalysis, optoelectronic properties, and energy storage.33–36 The unique feature of flexible and designable structure endows MOFs be easily modified the microenvironment of the metal active sites through introducing functional groups. Thus, the hydrogen bonding interaction between the intermediates of the NO3RR process with the introduced functional groups could be formed. Consequently, the selectivity of the NO3RR towards the target products could be improved by reducing the reaction energy barrier. However, to the best of our knowledge, there is no report of using the hydrogen bonding strategy to enhance the NO3RR performance.
Based on this, three examples of copper(I) cyclic trinuclear unit (Cu-CTU)-based two-dimensional (2D) MOFs were designed and synthesized to investigate how the structure of the catalyst affects the microenvironment of the copper active center during the nitrate reduction process.37 As shown in Scheme 1, the DiMe-Cu3-MOF was synthesized by the reaction of the copper(I) cyclic trinuclear cluster Cu3(Me-PyCA)3 (Me-PyCA = 3,5-dimethyl-1H-pyrazole-4-carbaldehyde), 3,5-dimethyl-1H-pyrazol-4-amine (DiMe-PyA)35,38 and Cu2O. The copper center in DiMe-Cu3-MOF is surrounded by two methyl (–CH3) groups. Atypical hydrogen bonds C–H⋯Y (where Y is usually N and O) between the –CH3 groups and the nitrogen–oxygen intermediates adsorbed on the copper center during the NO3RR process may be formed. Thus, the intermediates could be stabilized and the energy barrier for ammonia generation would be lowered. For comparison, UniMe-Cu3-MOF was prepared by the reaction of the methyl-free copper(I) cyclic trinuclear cluster Cu3(PyCA)3,39 DiMe-PyA and Cu2O. The NonMe-Cu3-MOF containing no methyl group was also synthesized by imine condensation between Cu3(PyCA)3 and an amino copper(I) cyclic trinuclear complex, in situ generated from Cu2O and 1H-pyrazol-4-amine (PyA). The activity of DiMe-Cu3-MOF with atypical hydrogen bonds was higher than that of UniMe-Cu3-MOF and NonMe-Cu3-MOF, with the trend of DiMe-Cu3-MOF > UniMe-Cu3-MOF > NonMe-Cu3-MOF. It is worth noting that DiMe-Cu3-MOF exhibited a high faradaic efficiency (FENH3) of 95% and achieved a high NH3 partial current density (jNH3) of −950.6 mA cm−2.
The successful synthesis of DiMe-Cu3-MOF, UniMe-Cu3-MOF, and NonMe-Cu3-MOF was further confirmed through Fourier transform infrared spectroscopy (FT-IR) and 13C solid-state cross-polarization magic angle-spinning nuclear magnetic resonance (CP-MAS NMR) spectroscopy. As shown in Fig. 1d and S1,† all three MOFs exhibited a newly formed CN imine signal at approximately 1610 cm−1, while the –CHO peak (1668 cm−1) and –NH2 signals (3201 cm−1 and 3325 cm−1) in the reactant monomers disappeared.40 Furthermore, due to the increased conjugation (including planar conjugation and π–π stacking) of DiMe-Cu3-MOF, a strong inductive effect is generated on the –CH3 groups, leading to an increase in their bond strength. Consequently, the peak located at 2903 cm−1 for the –CH3 group in DiMe-Cu3-MOF shows a blue shift in comparison with the peak at 2842 cm−1 in Cu3(Me-PyCA)3. These results were supported by the 13C solid-state NMR spectrum (Fig. 1f), which showed a strong C
N imine carbon resonance signal at ∼149 ppm and a prominent –CH3 signal at 19 ppm in DiMe-Cu3-MOF. Notably, the absence of methyl peaks was observed in NonMe-Cu3-MOF. Inductively coupled plasma (ICP) spectroscopy confirmed that DiMe-Cu3-MOF, UniMe-Cu3-MOF, and NonMe-Cu3-MOF had large Cu contents of 28.7 wt%, 32.7 wt%, and 41.1 wt%, respectively, indicating the integration of a large number of Cu catalytic sites into the MOF framework. N2 adsorption experiments were conducted to investigate the porosity of DiMe-Cu3-MOF, UniMe-Cu3-MOF, and NonMe-Cu3-MOF. The N2 isotherms of DiMe-Cu3-MOF at 77 K displayed type IV adsorption curves with hysteresis loops, indicating the presence of micropores. Further analysis of the pore size distribution revealed that DiMe-Cu3-MOF had micropores with an average diameter of approximately 1.85 nm (Fig. 1e and S2†), which closely corresponded to the theoretical value of 1.91 nm. The obtained DiMe-Cu3-MOF exhibited the highest Brunauer–Emmett–Teller (BET) surface area of 772 m2 g−1 among the three MOFs. The UniMe-Cu3-MOF had a BET surface area of 410 m2 g−1, sharing the same pore diameter of 1.85 nm, while the NonMe-Cu3-MOF still had a moderate BET surface area of 318 m2 g−1, along with a slightly smaller pore diameter of 1.83 nm. Such high porosity of the three materials is favorable for the accessibility and transformation of NO3−.
The scanning electron microscopy (SEM) images showed that all three MOFs have a dendritic crystal morphology (Fig. S3†). High-resolution TEM (HR-TEM) images (Fig. 2a–c) show that the ordered micropore lattice spacing of NonMe-Cu3-MOF, UniMe-Cu3-MOF, and DiMe-Cu3-MOF is approximately 1.88 nm, 1.87 nm, and 1.85 nm, respectively, which is consistent with the pore size of the simulated structural model and N2 adsorption results (Fig. 1e). Furthermore, the absence of a Cu2O lattice within the three MOFs provides additional confirmation of the successful removal of Cu2O from the initial materials. Energy-dispersive X-ray spectroscopy (EDS) element mapping images show that Cu, C and N elements are uniformly distributed across the structures of NonMe-Cu3-MOF, UniMe-Cu3-MOF, and DiMe-Cu3-MOF (Fig. 2d–f).
X-ray absorption spectroscopy (XAS) was conducted to determine the oxidation state and coordination environment of copper species in the catalysts. In the Cu K-edge of X-ray absorption near edge structure (XANES) spectra for DiMe-Cu3-MOF, the absorption edge corresponding to the 1s → 4p transition is positioned between Cu foil and Cu2O, closer to Cu2O. At the same time, the absence of the 1s → 3d pre-edge due to the dipole-forbidden indicated the absence of Cu2+ in DiMe-Cu3-MOF (Fig. 3a). It suggests that the oxidation state of copper in the material is +1. Additionally, in the Cu K-edge extended X-ray absorption fine structure (EXAFS) of DiMe-Cu3-MOF (Fig. 3b), a clear signal corresponding to the Cu–N coordination path at 1.4 Å was observed, which differs from that of Cu2O. Furthermore, no signal corresponding to Cu–Cu at 2.23 Å was observed, indicating there is no Cu or CuOx in DiMe-Cu3-MOF. Through comprehensive resolution in both K and R space, the wavelet transform (WT) analysis of Cu K-edge EXAFS was performed to further explore the coordination structure of DiMe-Cu3-MOF. Fig. 3d illustrates the distinctive features of DiMe-Cu3-MOF compared to Cu2O, providing further evidence of the absence of Cu2O particles and a non-Cu–O coordination mode in DiMe-Cu3-MOF. The results indicated the dominance of Cu–N species and the absence of Cu and Cu2O particles in DiMe-Cu3-MOF. The EXAFS fitting spectrum (Fig. 3c and Table S1†) demonstrated a coordination number of 2.0 for the copper center in DiMe-Cu3-MOF, which is consistent with the simulated structure. In addition, the Cu K-edge XANES and EXAFS spectra of UniMe-Cu3-MOF and NonMe-Cu3-MOF are also characterized in Fig. S4 and S5 and Tables S2 and S3.† According to the analysis results based on the Cu K-edge XANES and EXAFS curves in Fig. 3a and b, the Cu species in UniMe-Cu3-MOF and NonMe-Cu3-MOF exhibited similar states and coordination environments to the Cu in DiMe-Cu3-MOF, which was coordinated by two N atoms (Fig. S4 and S5 and Tables S2 and S3†). High-resolution X-ray photoelectron spectroscopy (XPS) of Cu 2p reveals two peaks at 952.8 eV and 932.9 eV in DiMe-Cu3-MOF, corresponding to Cu 2p1/2 and Cu 2p3/2, respectively (Fig. 3e). Cu LMM Auger spectroscopy provided further elucidation of the valence state of the copper center, revealing a Cu+ peak with a kinetic energy of 914.5 eV in DiMe-Cu3-MOF (Fig. 3f). The XPS data for UniMe-Cu3-MOF and NonMe-Cu3-MOF showed similar results (Fig. S4 (e and f) and S5 (d and e)†) to those of DiMe-Cu3-MOF, confirming the previous conclusion from X-ray absorption near-edge structure (XANES) that copper in the three MOFs exhibits a Cu+ state.
In order to evaluate the influence of hydrogen bonding on the intermediates of the NO3RR, the activity of DiMe-Cu3-MOF, UniMe-Cu3-MOF, and NonMe-Cu3-MOF in the NO3RR was studied in a standard three-electrode H-cell. All the potentials mentioned in this work were referenced to Ag/AgCl. The ammonia generated after electrolysis was analyzed using ultraviolet-visible (UV-vis) spectrophotometry (Fig. S6†), and the accuracy of this method was verified by ion chromatography (IC) (Fig. S7†). There were significant differences in the electrochemical NO3RR between DiMe-Cu3-MOF and its corresponding control samples. As illustrated in Fig. 4a and S8,† the DiMe-Cu3-MOF, which features –CH3 groups in both Cu-CTUs, demonstrated a more positive onset potential, larger ammonia faradaic efficiencies (FENH3), and higher partial current densities when compared to the NonMe-Cu3-MOF (lacking –CH3 groups) and UniMe-Cu3-MOF (with only half of Cu-CTUs containing –CH3 groups). Especially, the current density for NonMe-Cu3-MOF was much lower than those of the other two catalysts, indicating its poor activity for the NO3RR. The ammonia yield and FENH3 of the NO3RR for the three catalysts are shown in Fig. 4b and c, respectively. DiMe-Cu3-MOF achieved an FENH3 of 95% at −1.55 V, with an ammonia yield of 401 μg h−1 cm−2 and a current density of −64 mA cm−2 (Fig. 4d). The ammonia yield of the NO3RR for the three materials increased in the order of NonMe-Cu3-MOF < UniMe-Cu3-MOF < DiMe-Cu3-MOF. The FENH3 of NonMe-Cu3-MOF was significantly lower than that of UniMe-Cu3-MOF and DiMe-Cu3-MOF within the potential range of −1.45 to −1.85 V. In contrast, the production of NO3− was determined by a typical colorimetric method to evaluate the corresponding electrocatalytic performance (Fig. S9†).41 The results show that NO2− content generated by NonMe-Cu3-MOF is higher than those of the other two catalysts, indicating that the NO2− intermediate escaped from the Cu active sites, which was not further transformed to NH3. It may be ascribed to the NO2− intermediate that cannot be stabilized by the Cu sites of NonMe-Cu3-MOF. This highlights the importance of introducing methyl groups into DiMe-Cu3-MOF to stabilize intermediates via hydrogen bonding. To further verify the industrial applicability of DiMe-Cu3-MOF, NO3RR tests were conducted under constant current electrolysis conditions. The results demonstrated that within the current density range of −400 to −1200 mA cm−2, DiMe-Cu3-MOF consistently maintains an FENH3 above 80%, with the partial current density for ammonia reaching −950.6 mA cm−2 (Fig. 4e). In addition, DiMe-Cu3-MOF has high stability, in which the current density remained consistent throughout a 12 hour testing period, as well as without any notable decline in Faraday efficiency (Fig. 4g). PXRD patterns, TEM images and Cu K-edge EXAFS and EXAFS showed no significant changes in DiMe-Cu3-MOF after 2 h of electrolysis compared to the corresponding freshly prepared material (Fig. S10–S12†).
To confirm the source of the NH3 product for DiMe-Cu3-MOF, a series of control experiments and 15N isotope-labeled experiments were conducted.42,43 When nitrate was not added to the electrolyte or applied open circuit potential, no ammonia was produced (Fig. S13†). Additionally, by using 15NO3− as the nitrogen source, the origin of ammonia was traced to the nitrate in the electrolyte rather than environmental contamination. The typical double-peak 15NH3 signal effectively ruled out ammonia contamination and confirmed that ammonia was indeed produced through the electrocatalytic reduction of NO3− (Fig. 4f).
In order to determine whether the high activity of DiMe-Cu3-MOF originates from the effect of atypical hydrogen bonding in the NO3RR process, the activities are normalized by the number of redox-active copper sites, BET surface areas and the loading amount of catalysts on the electrode. As shown in Fig. S14 and S15 and Table S4,† DiMe-Cu3-MOF showed moderate intrinsic activity for the NO3RR towards NH3 from the ECSA-normalized I–V curves, indicating that the high performance of DiMe-Cu3-MOF is not from its high ECSA. Additionally, when the activity was normalized by the BET surface area, the DiMe-Cu3-MOF also showed moderate intrinsic activity for the NO3RR towards NH3 at moderate current density, which suggested that the high activity of DiMe-Cu3-MOF is not contributed by its high surface area (Fig. S16†). Meanwhile, different loading amounts on the electrode showed similar normalized current density and ammonia Faraday efficiency, indicating that the amount of the catalyst loading on the electrode had little effect on the catalytic activity (Fig. S17†). These results demonstrated that the high activity of DiMe-Cu3-MOF toward NH3 can be attributed to the effect of atypical hydrogen bonding. In order to elucidate the role of hydrogen bonds in the stabilization of intermediates in the NO3RR process, in situ Fourier transform infrared spectroscopy (in situ FTIR) measurements were carried out (Fig. S18†). As shown in Fig. 5a, b and S19,† the downward absorption band at 1380 cm−1 corresponds to the consumption peak of NO3−. With increasing potential, two distinct peaks corresponding to the vertical and bending adsorption of NO appear at ∼1730 cm−1 and 1649 cm−1, respectively, in DiMe-Cu3-MOF (Fig. 5a). As the applied potential increases, the intensity of these peaks decreases more noticeably. Additionally, as shown in Fig. 5b, with the increase of potential, the vertical and bending adsorption peaks of NO appear at ∼1778 cm−1 and 1643 cm−1 and their intensities increase significantly in NonMe-Cu3-MOF. However, after reaching the optimal point, the absorption peak decreases slightly. This indicates that NonMe-Cu3-MOF has a weaker ability to facilitate the conversion of NO2 into NO intermediates, which is different from DiMe-Cu3-MOF as shown in Fig. 5a, further illustrating the role of DiMe-Cu3-MOF in stabilizing NO2 intermediates. The trend of increasing absorption peaks with potential increase for NO2 (1550 cm−1) intermediates on NonMe-Cu3-MOF indicates the gradual accumulation of NO2 (Fig. 5b).44,45
In situ Raman spectra were utilized further to investigate the catalyst surface during the reaction. The Raman peaks at 986 and ∼1046 cm−1 correspond to the characteristic vibration modes of SO24− and NO3− in the electrolyte, respectively. The DiMe-Cu3-MOF shows a Raman peak of the Cu–N bond at 454 cm−1 (Fig. 5c and S20†). The intensity of this peak initially increases and then decreases with the rising potential, suggesting an accumulation of the Cu–N bond at low potential, followed by the consumption of the intermediate of the NO3RR at high potential. The Raman peaks at 850, 1160, 1221 and 1534 cm−1 are related to the symmetric and antisymmetric stretching vibrations of chelated NO2−, *NO and *NHO intermediates, respectively.46,47 Therefore, the combination of catalytic performance, in situ FTIR and Raman spectroscopy results demonstrates that DiMe-Cu3-MOF exhibits stronger intermediate (*NO2) adsorption compared to other materials, leading to excellent NO3RR performance.
To study the effects of hydrogen bonding on the NO3RR and HER, DFT calculations were performed for the DiMe-Cu3-MOF and NonMe-Cu3-MOF. Two typical NO3RR pathways were considered: the NOH pathway (*NO3− → *NO2 → *NO2H → *NO → *NOH → *N → *NH → *NH2 → *NH3) and the NHO pathway (*NO3− → *NO2 → *NO2H → *NO → *NHO → *NH2O → *NH2OH → *NH2 → *NH3), respectively. According to in situ Raman results, the presence of the *NHO intermediate instead of the *NOH (1540 cm−1) intermediate was detected on both DiMe-Cu3-MOF and NonMe-Cu3-MOF, indicating that the nitrate was reduced to ammonia via the NHO pathway (Fig. 5c, d and S21†).48 As illustrated in Fig. 6a and c, the conversion from *NO to *NHO is identified as the rate-determining step (RDS). However, the difference in the Gibbs free energy (ΔG) for DiMe-Cu3-MOF (−0.14 eV) and NonMe-Cu3-MOF (−0.18 eV) for the conversion from *NO to *NHO is minimal. In the case where the difference in ΔG for the RDS is not significant, DiMe-Cu3-MOF and NonMe-Cu3-MOF exhibit pronounced performance distinctions, primarily attributed to the hydrogen bonding interaction between DiMe-Cu3-MOF and *NO2 during the NO3RR process. DFT calculations were employed to further elucidate how the hydrogen bonding interaction facilitates the performance of the NO3RR to ammonia. As shown in Fig. 6a, the ΔG of the conversion from the *NO2 intermediate to *NO2H over DiMe-Cu3-MOF (−0.98 eV) is lower than that of NonMe-Cu3-MOF (−0.66 eV, Fig. 6c), indicating thermodynamically favorable ammonia generation. This is primarily attributed to the hydrogen bonding interaction stabilizing the *NO2 intermediate in the reaction process of DiMe-Cu3-MOF, which has been experimentally confirmed by in situ FTIR spectroscopy. In addition, the free energy of the HER of DiMe-Cu3-MOF is higher than that of NonMe-Cu3-MOF, indicating that the introduction of the methyl group could impede the HER (Fig. 6d), which is consistent with the results of electrochemical experiments. Additionally, the noncovalent interaction (NCI) was further applied to illustrate the hydrogen bond.49 As shown in Fig. S22,† an obvious van der Waals interaction was observed between the methyl groups and intermediates, indicating that the existence of a hydrogen bond could stabilize the intermediates of the NO3RR. Based on the analysis of the NO3RR and HER, we propose that the formation of hydrogen bonding by adjusting the microenvironment of the active center can promote the NO3RR by stabilizing the *NO2 intermediate and reducing the energy barrier in the rate-limiting *NO2H generation step, thus promoting the NO3RR.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5sc02208h |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry 2025 |