Mo-doped δ-MnO₂ nanoflowers enable efficient nitrogen oxidation to nitrate under mild conditions

Abstract

The development of efficient electrocatalysts for the nitrogen oxidation reaction (NOR) under mild conditions is crucial for sustainable nitrate synthesis. Mo-doped δ-MnO₂ electrocatalysts with varying Mo concentrations were successfully prepared for the NOR. Structural and electrochemical analyses revealed that Mo doping simultaneously enhanced the conductivity and electrochemically active surface area (ECSA) while promoting N₂ adsorption and activation through electronic structure modulation. The optimized 2.5% Mo-doped δ-MnO₂ (denoted as MM2.5) exhibited superior NOR performance in 0.1 M KOH, delivering a NO₃⁻ production rate of 116.75 μg h⁻¹ mg_cat⁻¹ with a faradaic efficiency (FE) of 7.04% and excellent long-term stability. In addition, a Zn–N₂ device was formed with MM2.5 as the anode and a Zn plate as the cathode, and the NO₃⁻ yield obtained in this device was even higher than 144.5 μg h⁻¹ mg_cat⁻¹. However, structural characterization revealed that excessive Mo doping disrupted the δ-MnO₂ crystal structure, reducing specific surface area and active site density. Density functional theory (DFT) calculations demonstrated that Mo doping lowered the Gibbs free energy of the rate-determining step (*N₂ → *NNOH) from 2.41 eV to 1.94 eV by facilitating electron transfer, thereby optimizing the reaction pathway. This study provides a new strategy for the design of transition metal oxide-based electrocatalysts, as well as the application in artificial nitrogen fixation.

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1. Introduction

The pursuit of sustainable nitrogen conversion technologies has gained particular urgency in the context of global carbon neutrality initiatives, as these methods offer a cleaner pathway for essential chemical production.^1,2 The electrocatalytic nitrogen oxidation reaction (NOR) has emerged as a promising green alternative to the energy-intensive Haber–Bosch process, owing to its unique ability to directly synthesize nitrate species under ambient conditions.^3–5 However, the practical implementation of the NOR faces significant thermodynamic challenges, primarily due to the exceptionally strong N≡N bond (941 kJ mol⁻¹) that leads to sluggish activation kinetics. This fundamental limitation has positioned the rational design of high-performance electrocatalysts as the pivotal research frontier in advancing nitrogen electrooxidation technologies.^6–8

Transition metals facilitate electron transfer and modulate the electronic structure, thereby enhancing catalytic performance and are extensively utilized in electrocatalytic synthesis.^9–11 In addition, transition metal oxides have garnered significant attention in nitrogen fixation catalysis owing to their versatile electronic configurations and abundant exposed active sites.^12–15 Particularly, manganese dioxide (MnO₂) demonstrates exceptional potential through its distinctive δ-phase layered architecture and dynamic Mn³⁺/Mn⁴⁺ redox couples, which create metastable surface states conducive to nitrogen activation.^16,17 The engineered oxygen vacancies in MnO₂ nanosheets can facilitate the formation of high-spin Mn³⁺–Mn³⁺ dimers.¹⁵ These coordinatively unsaturated pairs exhibit enhanced d-orbital overlap with N₂ antibonding orbitals, effectively lowering the activation barrier for N≡N bond cleavage through optimized electron transfer pathways. A manganese phthalocyanine catalyst with hollow nanotube structures for the electrocatalytic NOR was successfully prepared by Ghorai et al., who found that Mn atoms not only act as the main active sites of the reaction, but also significantly promote the generation of reactive species, which effectively enhances the NOR performance.¹⁸ Hou et al. prepared a Ru/Mn_1.04–RuO₂ catalyst for the electrocatalytic NOR, and enhanced the catalytic selectivity and activity for the NOR by constructing a Mn → Ru electron transfer pathway to modulate the electronic structure of the catalyst surface and increase the electron density of the Ru center.¹⁹

However, unmodified MnO₂ still suffers from insufficient electrical conductivity issues, including weak adsorption of intermediates and structural instability. These limitations result in a significant gap between its catalytic performance and practical application requirements.^20,21 Doping of molybdenum (Mo) has been demonstrated as an effective strategy to modulate the electronic structure of catalysts. For instance, the doping of high electronegativity Mo in TiO₂ induces electron transfer from adjacent Ti atoms upon substituting Ti sites, significantly enhancing the adsorption and activation of N₂.²² The Mo atoms in FeP-doped systems exhibit electron-deficient characteristics, enabling them to accept electrons from adjacent N atoms. This interaction amplifies the charge disparity between the two N atoms, thereby polarizing and activating the N≡N bond.²³ The doping of Mo into MnO₂ can enhance structural stability and induce local electronic reconstruction, thereby optimizing the reaction pathway.^17,24,25 Among several MnO₂ crystal types, δ-MnO₂ has attracted much attention due to its unique layered structure, and this open crystalline framework not only facilitates the homogeneous distribution of dopant elements, but also provides an ideal channel for reactant diffusion and electron transport.^26–28 The doping of Mo in δ-MnO₂ facilitates electron injection from electron-deficient Mo sites into the antibonding orbitals of N₂, and therefore, directly promotes the cleavage of the strong N≡N triple bond.¹³

To investigate the constitutive relationship and application potential of highly efficient electrocatalytic nitrate reduction (NOR) materials, five types of δ-MnO₂ nanoflowers with different Mo doping concentrations were synthesized by the hydrothermal method in this study. Through combined XPS and DFT analyses, it was demonstrated that Mo doping could modulate the electronic structure and active sites of δ-MnO₂, enhance the electrical conductivity, promote an increase in the proportion of low-valent Mn species, and enhance the adsorption capacity and reduce the thermodynamic barrier of the NOR through electron cloud density reconfiguration. The electron cloud density reconstruction enhances the N₂ adsorption capacity and lowers the thermodynamic barrier of the NOR, showing a NOR-specific gradient mechanism (Mo doping → increase in low-valent Mn → electron cloud reconstruction → enhanced N₂ adsorption). The Zn–N₂ battery system was further developed to achieve the synergistic coupling of the NOR and power output, and its NO₃⁻ yield is higher than that of the traditional H-type electrolytic cell, and it enables the dual functionality of “nitrate synthesis-energy storage” through the cell discharge. This study offers novel insights and technical approaches for efficient NOR material design and distributed green nitrate synthesis.

2. Experimental section

2.1 Chemicals and reagents

In this research, the chemical reagents employed were potassium permanganate (KMnO₄, AR, 99.0%), manganese sulfate monohydrate (MnSO₄·H₂O, AR, 99.0%), sodium molybdate dihydrate (Na₂MoO₄·2H₂O, AR, 99.0%), and ethanol (AR, 99.7%). These reagents were all procured from Sinopharm Chemical Reagent Co., Ltd in Shanghai, China. Nafion (5% by mass, a mixture of low-grade fatty alcohol and water) was purchased from DuPont. Nitrogen (99.999% purity) and argon (99.99% purity) were supplied by Qingdao Dehai Specialty Gases Co., while the duplicating paper was purchased from Torayca®. All reagents were utilized directly in their as-received state without any further purification prior to use. The water utilized in the experiments was deionized water (DI).

2.2 Synthesis of a series of δ-MnO₂ nanoflowers with different Mo doping contents

As illustrated in Fig. 1, Mo-doped δ-MnO₂ nanoflowers were successfully synthesized via a one-step hydrothermal approach. Specifically, Mo atoms were controllably doped into the δ-MnO₂ lattice by precisely adjusting the molar ratio of Mn/Mo in the precursor. This was carried out through a 16-hour hydrothermal reaction at 160 °C in a stainless-steel high-pressure reactor lined with polytetrafluoroethylene (PTFE). KMnO₄ and MnSO₄·H₂O served as the Mn sources, while Na₂MoO₄·2H₂O functioned as the Mo source. A series of Mo-doped δ-MnO₂ catalysts were finally prepared by maintaining specific additive ratios of elemental manganese and molybdenum compounds with doping levels of 0%, 1%, 2.5%, 5% and 10% and named MM0, MM1, MM2.5, MM5 and MM10, respectively (specific experimental details are given in the SI).

Fig. 1 Schematic representation of the preparation of Mo-doped δ-MnO₂.

3. Results and discussion

3.1 Morphological and structural characterization

Scanning electron microscopy (SEM) analysis, as depicted in Fig. 2a, indicated that a representative MM2.5 sample presented a three-dimensional nanoflower-like structure. This structure was formed through the self-assembly of countless ultrathin two-dimensional nanosheets. Transmission electron microscopy (TEM) further disclosed the intricate structural characteristics of the material. The MM2.5 samples showed an ultrathin translucent layered morphology, and a typical folded structure could be observed in the edge region (Fig. 2b and c). This structure was conducive to enhancing its contact area with the electrolyte. High-resolution transmission electron microscopy (HRTEM) images (Fig. 2d) displayed distinct lattice stripes with a spacing of 0.654 nm, which corresponded to the (001) crystal plane of δ-MnO₂. The energy dispersive X-ray spectroscopy (EDX) elemental distribution map (Fig. 2e) verified that the elements Mn, Mo and O exhibited a homogeneous spatial distribution within the main body of the nanoflower and it also implied the uniform doping of the Mo element into δ-MnO₂.

Fig. 2 (a) SEM images of MM2.5. (b and c) TEM images of MM2.5. (d) HRTEM images of MM2.5. (e) Lattice stripe diagram of MM2.5. (f) EDX elemental mapping images of Mn, Mo and O for MM2.5.

The X-ray diffraction (XRD) patterns, as shown in Fig. 3a, reveal that all samples display typical characteristic peaks of δ-MnO₂ at 12.55° (001), 25.25° (002), 37.31° (−111), and 65.62° (020) (JCPDS #80-1098). Notably, when the Mo doping amount was ≤5%, there were no significant shifts in the positions and intensities of the diffraction peaks, indicating that low-concentration doping did not disrupt the main crystal structure. As shown in Fig. S1, when the doping amount was raised to 10% (MM10), new diffraction peaks appeared at 26.3°, 37.2°, and 54.8°, which matched with the monoclinic phase MoO₂ (JCPDS #32-0671), confirming that excessive doping led to the formation of a molybdenum oxide heterogeneous phase.

Fig. 3 (a) XRD patterns of MM0 and MM2.5. (b and c) Nitrogen adsorption–desorption isotherms and pore size distribution curves for MM0, MM2.5 and MM10. (d–f) XPS spectra of Mn 2p, Mo 3d and O 1s.

The specific surface areas of MM0, MM2.5 and MM10 were determined by the Brunauer–Emmett–Teller (BET) method (Fig. 3b). The results show that the three samples can be categorized as those with typical II isotherms with hysteresis loops, indicating the presence of mesoporous structures.²⁹ Among them, the moderately optimized sample MM2.5 possessed a specific surface area of 76.43 m² g⁻¹, which is significantly higher than those of the undoped sample MM0 (57.46 m² g⁻¹) and the highly doped sample MM10 (21.22 m² g⁻¹). This indicates that moderate doping of Mo (MM2.5) increases the specific surface area of δ-MnO₂, while the decrease in the specific surface area of the sample with excessive addition of Mo (MM10) may be due to the generation of new phases of MoO₂, which is consistent with the XRD analysis results.^30,31 More detailed information about the pore structure of Mo-doped δ-MnO₂ was obtained using the pore size distribution curve determined by the DFT method (Fig. 3c). The pore size distribution curves show that all three samples have a mesoporous structure of 3–8 nm, with a greater proportion of MM2.5 pores located in this range. This hierarchical porous characteristic is favorable for increasing the exposure density of active sites and facilitating the electrolyte permeation and mass transfer process.³²

The high-resolution XPS characterization system revealed the modulation mechanism of doped Mo on the electronic structure of the material surface. The full-spectrum XPS scanning results (Fig. S2) clearly indicate the presence of characteristic peaks corresponding to Mn and O in all synthesized samples, suggesting the consistent retention of the base material composition. Notably, in the series of Mo-doped samples (MM1, MM2.5, MM5, and MM10), well-defined Mo 3d photoelectron peaks emerged within the binding energy range of 230–238 eV. This spectral feature aligns precisely with the anticipated values for the Mo⁶⁺ oxidation state, confirming the successful incorporation of molybdenum into the lattice structure. Conversely, the undoped Mo sample (MM0) shows no evidence of Mo 3d spectral features.

The high-resolution spectral analysis of the Mn 2p orbitals (Fig. 3d) shows that all samples exhibit the typical structure of two deconvolution peaks, Mn³⁺ (641.8 eV, 653.3 eV) and Mn⁴⁺ (643.4 eV, 654.7 eV), which correspond to the 2p_3/2 and 2p_1/2 spin orbitals, respectively.³³ Mo doping changed the valence distribution of Mn, and the atomic ratios of Mn³⁺/Mn⁴⁺ were 1.002 : 1, 1.145 : 1, 1.227 : 1, 1.425 : 1, and 1.472 : 1 for samples MM0, MM1, MM2.5, MM5, and MM10, respectively, with a gradual increase in the percentage of Mn³⁺. This suggests that the addition of Mo promotes the shift of elemental manganese to the lower valence state; this change occurs moderately and is likely to greatly improve the N₂ activation capacity of δ-MnO₂.³⁴ The Mn 2p peak positions of the MM2.5 to MM10 samples were negatively shifted by about 0.1, 0.3, 0.5, and 0.8 eV, respectively, compared with that of MM0.

As shown in Fig. 3e, the O 1s spectrum of MM0 can be decomposed into three characteristic peaks at 529.5, 531.8, and 533.2 eV, corresponding to Mn–O–Mn, Mn–O–H and H–O–H, respectively.³⁵ The peak positions of Mo-doped samples show negative shifts similar to those of the Mn 2p peaks as compared with that of MM0, which may be due to electron transfer effects induced by the high electronegativity of Mo⁶⁺, resulting in the negative shift of the Mn 2p peak position of MM10 samples. These changes may be due to the electron transfer effect caused by the high electronegativity of Mo⁶⁺, resulting in an increase in the electron cloud density at the Mn and O sites.^36,37 It is noteworthy that the peak of the Mo–O bond (530.1 eV) can also be obtained in the O 1s spectrum of MM10, which further suggests that the excess addition of Mo elements leads to the generation of MoO₂, which is consistent with the XRD results.³⁸

Fine spectral analysis of the Mo 3d orbitals (Fig. 3f) confirms that the characteristic double peaks at 231.6 eV (3d_5/2) and 234.8 eV (3d_3/2) are exclusively observed in the Mo-doped sample (MM2.5).³⁹ This finding is consistent with the typical characteristics of Mo⁶⁺ and suggests that Mo is stably present in the δ-MnO₂ lattice with substitutional doping. In contrast, these characteristic peaks were not detected in the undoped pristine MM0 sample. Notably, unlike the pronounced negative shift in the Mn 2p peaks, all Mo 3d doublet peaks show a consistent but obvious positive binding energy shift. This shift likely originates from a decrease in localized electron density around Mo, possibly due to charge transfer or changes in the coordination environment.^40,41

3.2 Electrochemical NOR properties

The influence of Mo doping on the charge transfer process was assessed using an electrochemical impedance spectroscopy (EIS) system. As shown in Fig. 4a, the Nyquist plots revealed that the diameters of the semicircular arcs for all Mo-doped samples were notably smaller than that of the pure phase δ-MnO₂ (MM0). This observation implies that the doping process effectively reduced the charge transfer resistance. Clearly, the incorporation of Mo improved the electrical conductivity of δ-MnO₂ and accelerated the electron transfer process. Among the samples, MM2.5 exhibited the lowest charge transfer resistance, indicating superior electron transfer performance. Cyclic voltammetry (CV) curves in Fig. S3 were used to determine the double-layer capacitances (C_dl) of the five samples in the non-Faraday interval (1.15–1.25 V vs. RHE), which were further used to assess the electrochemically active surface area (ECSA). As shown in Fig. 4b, the C_dl of the undoped MM0 was 0.707 mF cm⁻². The doping of Mo led to an increase in the C_dl values of δ-MnO₂ to varying extents. Specifically, the C_dl of MM2.5 reached 1.884 mF cm⁻², which was 2.66 times that of the pure phase. This enhancement in C_dl is beneficial for exposing a greater number of active sites, thereby facilitating the adsorption and activation of N₂.^42,43

Fig. 4 (a) Nyquist plots of five samples. (b) Double-layer capacitances of five samples. (c) LSV curves of the MM2.5 electrode in N₂-saturated and Ar-saturated 0.1 M KOH electrolytes. (d) i–t curves of the MM2.5 electrode at different potentials. (e) NO₃⁻ yield and FE of five samples at different potentials. (f) By-product experiments for MM0 and MM2.5. (g) Control test of NOR performance under different conditions. (h) Performance comparison of MM2.5 with reported catalysts.

The NOR activity of MM2.5 was evaluated by linear scanning voltammetry (LSV) in N₂-saturated and Ar-saturated 0.1 M KOH electrolytes, respectively. As shown in Fig. 4c, the current density of MM2.5 increased significantly in the N₂-saturated electrolyte, confirming its significant NOR activity, and the potential interval of 1.8–2.2 V (vs. RHE) was chosen as the range for further NOR tests. The chronoamperometry test (I–T) curves depicted in Fig. 4d revealed that within the potential range of 1.8–2.2 V, the current densities of MM2.5 increased substantially as the voltage rose (LSV and I–T curves for the remaining four samples are shown in Fig. S4). The quantitative analysis in Fig. 4e shows that the NO₃⁻ yield of MM2.5 is 116.75 μg h⁻¹ mg_cat⁻¹ at 2.0 V, which is about 2.4 times that of the unmodified sample (MM0), while the FE of MM2.5 is 7.04%, which is up to 2.8 times that of MM0. Upon analyzing the by-products, as shown in Fig. 4f, it was revealed that there were no detectable NO_x species within the gas phase products. The FE values of MM0 and MM2.5 for O₂ were 78.86% and 79.37%, respectively. The liquid products H₂O₂ and NO₂⁻ were determined by the standard curves as shown in Fig. S5. MM2.5 showed a decreased H₂O₂ yield of 13.47%, compared to the 17.06% for MM0, while the yield of NO₂⁻ only showed a slight decrease (0.71% → 0.63%). Therefore, it is reasonable to speculate that doped Mo can improve the selectivity of the NOR by inhibiting the pathway of H₂O₂ generation. Furthermore, the inherent characteristics of the NOR were validated through systematically conducted controlled experiments, as illustrated in Fig. 4g. The results indicated that when a voltage was applied and N₂ was passed through, the blank CP produced virtually no NO₃⁻. Conversely, the CP loaded with MM2.5 (M) generated a substantial amount of NO₃⁻ under the same conditions. At an open circuit potential (OCP), there was almost no NO₃⁻ at the MM2.5 electrode. Upon replacing N₂ with Ar, almost no NO₃⁻ was detected at a voltage of 2.0 V. These results verified the origin of N in NO₃⁻ from the feedstock N₂. When compared with the electrocatalytic NOR catalysts reported previously (Fig. 4h), MM2.5 demonstrated a higher NO₃⁻ yield and FE (detailed data comparisons along with relevant references are presented in Table S1).

The practical utility potential of MM2.5 was assessed via multidimensional stability tests. As illustrated in Fig. 5a–c, MM2.5 exhibited remarkable cycling stability, reproducibility, and long-term stability. Specifically, it endured five consecutive NOR test cycles, with each cycle lasting for 1 h, and the fluctuations in the NO₃⁻ yield and FE were less than 5%. For five batches of electrodes prepared independently, there was no noticeable decline in the NO₃⁻ yield, which verified the outstanding reproducibility of the synthesis method. When the electrodes were operated at a constant potential of 2.0 V versus RHE for 24 h, only a minimal decay in the current density was observed. This observation indicated the exceptional long-term durability of the MM2.5 electrode. Fig. 5d–f present the XPS spectra of Mn 2p, O 1s, and Mo 3d for MM2.5 before and after the NOR. It is evident that no significant shifts in peak positions were observed for any of the elements. This observation further supports the remarkable structural stability of MM2.5.

Fig. 5 (a) Repeatability cycling stability test for MM2.5. (b) Repeatability test. (c) Long-term stability test. (d–f) High resolution XPS spectrogram of Mn 2p, O 1s, and Mo 3d of MM2.5 before and after the NOR.

We developed a Zn–N₂ battery system based on a Mo-doped δ-MnO₂ (MM2.5) anode, which successfully realized the synergistic coupling of the NOR and electrical energy output (Fig. 6a and b). The device used MM2.5-modified composite carbon paper (CP) as the anode (the catalyst came in contact the with N₂-saturated 0.1 M KOH electrolyte) and a zinc plate as the cathode (0.5 M Na₂SO₄ electrolyte), and separation of the two chambers was achieved using a bipolar membrane. MM2.5 was coated on the conductive side of the CP, and N₂ was permeated from the water-resistant side. A peristaltic pump was connected to both sides of the anode chamber for better circulation of the electrolyte saturated with nitrogen to facilitate the NOR. The formation of a gas–liquid–solid three phase interface significantly improves the mass transfer efficiency of gaseous N₂. During the charging process, the NOR occurs at the anode (N₂ + 12OH⁻ → 2NO₃⁻ + 10e⁻ + 6H₂O),³⁹ accompanied by hydrogen precipitation at the cathode (2H₂O + 2e⁻ → H₂↑ + 2OH⁻). The open circuit potential (OCP) of the system was found to be stable at 1.420 V using a digital multimeter (Fig. 6c), which is in great agreement with the results of the electrochemical workstation test.⁴⁴ Electrochemical tests showed that the device voltage was quite stable over a wide current range of 0.25 to 1.25 mA cm⁻² (Fig. 6d), and the NO₃⁻ yield steadily increased with increasing current density, reaching 144.5 μg h⁻¹ mg_cat⁻¹ at 0.7 mA cm⁻², which is an enhancement of about 23.7% over the yield of the conventional H-type cell (Fig. 7d). Discharge mode was achieved by NO₃⁻ cathodic reduction (NO₃⁻ + 7H₂O + 8e⁻ → NH₄OH + 9OH⁻) with Zn anodic oxidation (Zn → Zn²⁺ + 2e⁻) to realize chemical–electrical energy conversion,⁴⁰ which can continuously light up 22 LED lamps (Fig. 6e), verifying the utility of its dual function (nitrate synthesis and energy storage). This study provides an innovative technological path for distributed nitrate green synthesis and residual power recovery.

Fig. 6 Schematic diagram of (a) the Zn–N₂ reaction device and (b) Zn–NO₃⁻ battery. (c) OCP of the MM2.5 based Zn–N₂ device. (d) Charging test and NO₃⁻ yield of the H-type cell with the Zn–N₂ reaction device at different current densities. (e) LED lamps illuminated with a Zn–NO₃⁻ battery.

Fig. 7 (a and b) Electron density difference of δ-MnO₂ and Mo-doped δ-MnO₂. (c) Three potential active sites on the Mo-doped δ-MnO₂ (001) crystal surface. (d) COHP maps on the three sites. (e) Bond length effects and adsorption energy maps of the three sites on N₂ and intermediate NNOH. (f) Projected density of states plots for δ-MnO₂ and Mo-doped δ-MnO₂. (g) Gibbs free energy plots of NOR catalysis on δ-MnO₂ (001) and Mo-doped δ-MnO₂ (001).

3.3 Density functional theory calculations

Density functional theory (DFT) calculations were systematically performed to elucidate the mechanism through which Mo doping enhanced the NOR electrocatalytic activity of δ-MnO₂. The full description of the computational parameters and methodological details are given in the SI. The electron density difference (EDD) maps, as shown in Fig. 7(a and b), reveal that substantial electron depletion takes place around the Mo atoms within the Mo-doped system. Conversely, electron-rich regions are formed between Mo and the Mn atoms that are coordinated to it (the blue regions signify electron depletion, while the red regions denote electron accumulation), which is in good agreement with the XPS results. Quantitative Mulliken charge analysis shows that each Mo atom transfers about 0.011|e| to the coordinating Mn atom (Fig. S7a and b), leading to an enhanced electron cloud density in the vicinity of the Mn site (Fig. S7c and d), which corresponds well to the negative shift of the Mn 2p binding energy observed through XPS and this electron redistribution property is conducive to the enhancement of the polarized adsorption of N₂ molecules at the active sites on the catalyst surface.¹³

Based on the differential charge density information and the data in the literature,¹³ as shown in Fig. 7c, three characteristic sites, the Mn atom active site (Mn site) on the pristine δ-MnO₂ (001) surface, the Mo atom active site (Mo site) on the Mo-doped surface, and the Mn atom active site (Mn_Mo site) coordinated with Mo on the Mo-doped surface, were selected for Crystal Orbital Hamilton Populations (COHP) analysis to analyze the strength of the interaction between the active sites and the N atoms. The anti-bonding (grey area) and bonding states (white area) correspond to the negative and positive values of –ICOHP, respectively. The metal–N bond strength was assessed from the magnitude of the integral of the –ICOHP value. Fig. 7d reveals that the chemical bonding between the Mo site and the adsorbed N₂ molecules exhibited a remarkably negative COHP integral value of −2.645 eV, which was about two times higher than that of the original Mn site (−1.189 eV). This change implies that Mo doping significantly strengthened the antibonding orbitals between the catalyst surface and the terminal nitrogen atoms of N₂.⁴⁵ Such a strong interaction is capable of effectively reducing the bonding energy of the N≡N triple bond, thereby decreasing its dissociation energy barrier. Notably, the COHP integral value of the Mn_Mo site, which was −1.410 eV, lay between the values of the Mo site and the original Mn site. This indicates that the doped Mo atoms can enhance the activation ability of the adjacent Mn atoms toward N₂via the electronic modulation effect.

In addition, we explored the crucial modulation of the adsorption energies of N₂ molecules, the lengths of the N≡N bonds and the potential determining step (PDS) intermediates, specifically the intermediate NNOH, by different adsorption sites on the catalyst surface, namely the Mn site, the Mn_Mo site, and the Mo site (Fig. 7e). The computational results demonstrate that the Mo site exhibits superior synergistic catalytic effects. Notably, the Mo site induces the most pronounced bond elongation in both N₂ (1.153 Å) and NNOH (1.154 Å) species, compared to the reference bond length of free N₂ molecules (1.102 Å). This substantial lengthening of the N≡N triple bond indicates a significant weakening of bond strength, which effectively reduces the activation energy barrier required for nitrogen bond cleavage. Simultaneously, the Mo active center demonstrates the most pronounced adsorption energies for both N₂ (−0.517 eV) and NNOH (−0.628 eV), reflecting its superior thermodynamic stabilization of these intermediates. This strong adsorption capability directly facilitates the reduction of energy barriers in the rate determining step of the reaction pathway.⁴⁶ In contrast, Mn sites exhibit weaker interactions, with adsorption energies of −0.349 eV (*N₂) and −0.485 eV (*NNOH), accompanied by less effective N≡N bond elongation (1.114 Å and 1.124 Å, respectively). Interestingly, under Mo doping modulation, the Mn sites show marginally improved performance in both bond stretching (1.122 Å and 1.126 Å for N₂ and NNOH) and adsorption energies (−0.352 eV and −0.507 eV) compared to undoped Mn. This enhancement correlates with the increased electron-deficient character of Mn atoms when coordinated to Mo, which likely promotes the critical nitrogen–oxygen bond activation step.

The density of states (DOS) analysis shows (Fig. 7f) that Mo doping introduces new electronic states near the Fermi level, effectively bridges a part of the band gap of δ-MnO₂, and enhances the electrical conductivity of the catalysts, which is in agreement with the EIS results.^25,47 Crucially, Mo doping induces a 0.097 eV upward shift in the material's d-band center, from −4.836 eV in pristine δ-MnO₂ to −4.739 eV, positioning it closer to the Fermi level. This positive d-band shift aligns with d-band center theory, which posits that such electronic modulation strengthens hybridization between catalyst surface d-orbitals and N₂ π*-antibonding orbitals, thereby enhancing nitrogen adsorption capacity.⁴⁸ The Gibbs free energy diagram of the NOR, constructed based on DFT calculations, as shown in Fig. 7g, reveals that both the δ-MnO₂ (001) and Mo-doped δ-MnO₂ (001) surfaces adhere to the sequential “+OH, −H” process:

which is in agreement with previous reports.^3,49 It is worth noting that the RDS for both surfaces is the “*N₂ → *NNOH” process. For the pristine surface, the energy barrier is 2.41 eV, whereas for the Mo-doped surface, it is significantly reduced to 1.94 eV. These results indicate that Mo doping can effectively reduce the thermodynamic barrier of δ-MnO₂ as a catalyst for catalytic NOR progress, and this effect directly contributes to NO₃⁻ synthesis efficiency.

4. Conclusion

We successfully synthesized a series of Mo-doped δ-MnO₂ electrocatalysts (0–10 wt% Mo) for the NOR under ambient temperature and pressure conditions. The optimized 2.5 wt% Mo-doped δ-MnO₂ catalyst demonstrated exceptional NOR performance in N₂-saturated 0.1 M KOH, achieving a NO₃⁻ yield rate of 116.75 μg h⁻¹ mg_cat⁻¹ with a FE of 7.04%. This performance correlates with its expanded ECSA and enhanced N₂ adsorption capacity, attributable to the introduction of high-density active sites. However, XRD analysis revealed that excessive Mo doping disrupts the δ-MnO₂ lattice structure, reducing critical parameters such as the BET surface area and ECSA, thereby diminishing NOR activity. DFT calculations show that the superior catalytic performance is mainly attributed to the modification of the electronic structure due to the moderate doping of Mo, and Mo doping was capable of significantly reducing the energy barrier of the NOR and facilitating the formation of key active species. As a consequence, it expedited the nitrogen fixation process. This study not only confirmed the good catalytic activity of Mo-doped δ-MnO₂ in oxidizing N₂ to nitrate, but also provided a promising strategy for transition metal-based catalyst design in the field of artificial nitrogen fixation.

Conflicts of interest

There are no conflicts to declare.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article and its supplementary information (SI). The additional XRD, XPS characterization results,CV, LSV and IT curves, standard curves, Mulliken charge analysis and differential charge density results are presented in the supplementary information. Supplementary information is available. See DOI: https://doi.org/10.1039/d5qi01648g.

Acknowledgements

The authors acknowledge the support from the Natural Science Foundation of Shandong Province (ZR2021MB075 and ZR2025MS209), the National Natural Science Foundation of China (51602297), and Fundamental Research Funds for the Central Universities, Ocean University of China (202461021 and 202364004).

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