Fe-Mo-V ternary sulfide electrocatalytic synthesis ammonia catalyst

Abstract

The invention relates to a Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst, in particular to the technical field of electrochemical green hydrogen production of green ammonia. The catalyst is composed of iron, molybdenum, vanadium and sulfur elements, has a chemical general formula of FeaMo [beta] V [gamma] S, and has a porous nanosheet structure with a specific surface area. The preparation method comprises the following steps: dissolving an iron source, a molybdenum source and a vanadium source in water in proportion, adding a sulfur source, carrying out a hydrothermal reaction, carrying out centrifugal washing, and annealing in a hydrogen-containing inert atmosphere. According to the catalyst, an iron-molybdenum electronic structure is reconstructed through vanadium doping, Fe-V double active centers are formed to synergistically promote nitrogen adsorption and N = N bond breakage, and meanwhile, a proton transfer path is perfected through molybdenum and sulfur sites. The invention is applied to proton exchange membrane electrolytic cell cathodes. The porous nanosheet structure accelerates nitrogen mass transfer, V-S bonds strengthen lattice stability, and the problems of weak nitrogen activation, serious hydrogen evolution competition and sulfur loss of a traditional catalyst are solved. The process is suitable for large-scale production, the energy consumption of single electrolysis is reduced to 28.6 kWh / kgNH3, and a high-efficiency and low-cost catalysis scheme is provided for green ammonia industrialization.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of electrochemical green hydrogen to green ammonia, and particularly relates to a Fe-Mo-V ternary sulfide electrocatalytic synthesis ammonia catalyst. More particularly, it relates to a transition metal sulfide catalyst for electrochemical nitrogen reduction to ammonia (eNRR), which is particularly suitable for a proton exchange membrane electrolysis cell system BACKGROUND

[0002] Green ammonia (NH3) as a zero-carbon energy carrier and hydrogen energy storage medium has important strategic significance in large-scale consumption of renewable energy and realization of carbon neutralization goal. The traditional Haber-Bosch method for synthesizing ammonia relies on hydrogen production from fossil energy, and has high carbon emission intensity. However, the green hydrogen to green ammonia technology can realize zero carbon emission in the whole process through coupling of hydrogen production by water electrolysis and nitrogen reduction reaction. However, the electrochemical synthesis of ammonia technology still faces key challenges such as low catalytic efficiency, poor selectivity, and high energy consumption, which restrict its industrial application.

[0003] 1. Status of electrochemical ammonia synthesis technology The electrochemical nitrogen reduction reaction (NRR) can be carried out at room temperature and pressure, and is theoretically more energy-saving than the Haber-Bosch method. However, the high stability of nitrogen molecules (N2) makes it difficult to activate, and the dissociation energy of N≡N triple bond is as high as 941 kJ / mol, making the NRR kinetics extremely slow. In addition, the existence of competitive hydrogen evolution reaction (HER) in aqueous solution further reduces the faradic efficiency of ammonia synthesis. At present, the yield and selectivity of electrochemical ammonia synthesis are still far lower than the industrial demand, which is mainly limited by the insufficient activity of the catalyst.

[0004] The existing NRR catalysts are mainly divided into two categories: noble metal-based and non-noble metal-based. Although noble metals (such as Pt, Ru, Au) have strong nitrogen adsorption capacity, they are high in cost and have serious HER competition, resulting in ammonia selectivity generally lower than 20%. Non-noble metal catalysts (such as Fe, Mo, Co-based materials) are low in cost, but have weak nitrogen activation capacity and are prone to deactivation due to surface passivation. Transition metal sulfides (such as FeS2, MoS2) have attracted attention due to their unique electronic structure and low cost, but binary sulfides have single active sites, making it difficult to simultaneously accelerate the nitrogen adsorption and proton transfer processes.

[0005] 2. Research progress of transition metal sulfides Transition metal sulfides are considered as potential NRR catalysts due to their adjustable electronic structure and rich defect sites. FeS2 and MoS2 materials have been proven to be able to catalyze nitrogen reduction in experiments, but still have the following problems: (1) Insufficient nitrogen adsorption capacity Nitrogen adsorption on most sulfide surfaces is weak, making it difficult to effectively activate the N≡N bond. For example, the nitrogen adsorption energy of pure FeS2 is only -0.3 eV, much lower than the ideal value (-0.8 eV or higher), making the initial hydrogenation step (N2→ N2H) the rate-determining step.

[0006] (2) Severe competition for hydrogen evolution reaction Sulfide catalysts are prone to HER at a reduction potential, especially in acidic or neutral electrolytes. H + Reduction dominates. For example, the HER current density of MoS2 can reach 10 mA / cm 2 at -0.4 V (vs. RHE), while the NRR current density is less than 0.1 mA / cm 2 , resulting in a faradic efficiency of less than 10%.

[0007] (3) Poor catalyst stability The surface sulfur atoms of sulfides are prone to dissolution or oxidation during electrocatalysis, resulting in loss of active sites. For example, the surface sulfur loss rate of FeS2 can reach 30% after 24 hours of continuous electrolysis, resulting in a decrease in catalytic activity.

[0008] 3. Modification strategies for existing catalysts and their limitations To improve NRR performance, researchers have tried various modification methods, but have not yet broken through the efficiency bottleneck: (1) Morphology control Nanosheets, porous structures, and other structures can increase the exposure of active sites, but the improvement of specific surface area has limited effect on the activation of nitrogen. For example, the ammonia yield of porous MoS2 is twice that of bulk MoS2, but the faradic efficiency is still less than 15%.

[0009] (2) Defect engineering Sulfur vacancies or metal vacancies can enhance nitrogen adsorption, but excessive defects can exacerbate HER competition. For example, FeS2 rich in sulfur vacancies has a NRR selectivity of 25% at -0.3 V, but when the potential is shifted to -0.5 V, the HER proportion rapidly rises to 90%.

[0010] (3) Heteroatom doping Non-metallic doping such as nitrogen and phosphorus can adjust the electronic structure, but the doping sites are usually randomly distributed, making it difficult to precisely control the active centers. For example, N-doped MoS2 has a 50% increase in ammonia yield, but its stability has not improved significantly.

[0011] 4. Summary of technical bottlenecks Currently, the development of electrochemical ammonia synthesis catalysts still faces the following core challenges: Insufficient nitrogen activation: existing catalysts are difficult to effectively weaken the N≡N bond, resulting in a high reaction energy barrier; Low selectivity: HER competition is serious, especially in the low overpotential region, and the NRR faradic efficiency is generally lower than 30%; Poor stability: the catalyst is prone to structural degradation or active site poisoning during long-term operation.

[0012] In summary, developing an electrochemical ammonia synthesis catalyst with high activity, high selectivity and long service life is still a key problem for the industrialization of green ammonia technology. The existing research has not solved the problem of simultaneous improvement of nitrogen activation and proton transfer, and it is urgent to break through the performance bottleneck through new catalyst design. SUMMARY

[0013] In order to make up for the deficiencies of the prior art in the field of electrocatalytic synthesis of ammonia, the present application is committed to providing a Fe-Mo-V ternary sulfide electrocatalytic synthesis of ammonia catalyst with excellent performance, aiming to break through the many bottlenecks faced by traditional catalysts and promote the development of electrochemical green hydrogen to green ammonia technology.

[0014] I. Fe-Mo-V ternary sulfide electrocatalytic synthesis of ammonia catalyst The core product of the present application is a Fe-Mo-V ternary sulfide electrocatalytic synthesis of ammonia catalyst, which includes Fe, Mo, V and S in its composition, and its chemical general formula is Fe α Mo β V γ S. In this chemical formula, the atomic ratio of each element satisfies the following specific range: 0.5≤a≤3, 0.1≤β≤1, 0.01≤γ≤0.5, and a+β+γ=n, where n is a positive real number satisfying 1.5≤n≤4.5. The catalyst exhibits a unique porous nanosheet morphology, with a specific surface area in the range of 80-150 m 2 / g.

[0015] By adjusting the Fe-Mo-V atomic ratio, i.e. limiting it within the range of 0.5≤a≤3, 0.1≤β≤1, 0.01≤γ≤0.5, heteroactive sites are formed. Among them, the doping of V element plays a key role, and the d electrons of V 3+ feedback to the 3d orbit of Fe, enhancing the electron injection ability of Fe sites to the π antibonding orbital of N2 molecules. At the same time, the Mo sites stabilize the N2H intermediate through the S-Mo-S bridging structure, thereby reducing the initial hydrogenation energy barrier. This electronic structure mechanism is an important basis for the performance improvement of the catalyst of the present application.

[0016] The porous nanosheet structure possessed by the catalyst has a thickness range of 5-20 nm, and this structure provides the catalyst with a specific surface area of 80-150 m 2 / g. The high specific surface area property promotes the mass transfer process, enabling the reactants to reach the active sites more quickly, while increasing the degree of exposure of the active sites. The advantages brought by this structure can be seen through specific experimental data: ammonia yield is greatly improved compared to traditional Fe2MoS catalysts, from 12.1 μg•h -1 •mgcat -1 to 28.5 μg•h -1 •mgcat -1 ; N2 adsorption energy is also enhanced from -0.3 eV to -0.75 eV (calculated by DFT); in the flow cell test, the mass transfer efficiency is improved by 3.2 times, proving the positive impact of this structure on catalyst performance.

[0017] Further specification of the atomic ratio range, when n is between 2.5 and 4.0, and the thickness of the porous nanosheet is controlled between 5 and 20 nm, a synergistic effect is produced. Specifically, the high-density active sites corresponding to n = 2.5-4.0, combined with the ultra-thin nanosheet with a thickness of 5-20 nm, shortens the charge transport path. At the same time, the mesoporous structure distributed on the surface of the catalyst, with a pore size range of 2-10 nm, accelerates the process of N2 diffusion to the active site. At a potential of -0.4 V, 42.5% is achieved (compared with 30% for traditional non-noble metal catalysts); at a high potential of -0.6 V, the current density stability is good, and the faradic efficiency can be maintained above 35%, while the Fe2MoS catalyst is only 22% at this potential.

[0018] Further, when the atomic ratio is set as a = 1.8-2.2, β = 0.9-1.1, γ = 0.05-0.15, and n = 3.0. Under this specific atomic ratio condition, Fe 2+ / Fe 3+ redox pairs and V 3+ synergistic effect, further enhancing the adsorption capacity of N2. At the same time, Mo 4+ sites maintain the state of reaction intermediates through S bridging structure. Through theoretical calculations, it is known that the initial hydrogenation energy barrier is greatly reduced from 1.58 eV to 0.92 eV (transition state calculation results), improving the catalytic efficiency. In the long-term stability test, after 100 hours of continuous operation, the attenuation rate of the catalyst is less than 8%, while the attenuation rate of the Fe2MoS catalyst is 35%.

[0019] The specific surface area of the catalyst should be between 100 and 130 m 2When the range of 3~10 is reached, mesopores (2-10 nm) act as material transport channels, delivering N2 to active sites and accelerating the reaction, while micropores (<2 nm) can enrich proton concentration and accelerate the conversion process of N2H to N2H2. Through testing methods such as BET (specific surface area test) combined with molecular simulation, it is found that the N2 diffusion coefficient is increased by 2.8 times, which proves the role of the porous structure in improving the material transport efficiency. In a low-temperature environment, such as 25℃, the ammonia yield can still reach 24.3μg•h -1 •mgcat -1 Although it is slightly lower than 35.1μg•h -1 •mgcat -1 at 80℃, it still shows good low-temperature activity, which makes the catalyst maintain certain catalytic performance at different temperature conditions.

[0020] II. Preparation method of Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst The catalyst preparation method of the present application comprises the following key steps: 1. Raw material dissolution step: First, dissolve the iron source, molybdenum source and vanadium source in water according to the atomic ratio of Fe:Mo:V a:β:γ. This step requires control of the purity and ratio of raw materials to ensure the accuracy and consistency of subsequent reactions. For example, in the specific implementation process, high-purity iron source, molybdenum source and vanadium source are selected, weighed and added to a certain amount of deionized water, and fully dissolved under constant temperature stirring conditions to form a uniform mixed solution.

[0021] 2. Hydrothermal reaction step: Then, add the sulfur source to control the total molar ratio of sulfur source to iron source, molybdenum source and vanadium source in the range of (3~10):1. Then, the mixed solution is placed under specific temperature conditions for hydrothermal reaction, and the reaction temperature is set to 180~220℃ and the reaction time is 12~24 hours. Under the hydrothermal conditions of 200℃, the molecules of each raw material move and interact in the solution, gradually forming precursor nanosheets with specific structures, and this process is controlled by temperature, time and raw material ratio and other factors.

[0022] 3. Annealing step: After the hydrothermal reaction, the product is centrifuged and washed to remove impurities that may have been generated during the reaction. Then, the washed product is subjected to annealing treatment in an inert atmosphere containing 5-10% H2, with the annealing temperature controlled at 300-400°C and the time being 1-3 hours. During the annealing process, the H2 / Ar mixed gas environment accelerates the crystallization process of the Fe-Mo-V-S tetrahedral framework, enhancing the lattice stability. Through this hydrothermal-annealing process, the prepared catalyst has strong structural stability, for example, during 100 hours of electrolysis, the sulfur loss rate is 7%, while the traditional Fe2MoS catalyst has a sulfur loss rate of 28%. At the same time, through TEM statistical analysis, it is found that the coefficient of variation (CV) of the thickness of the nanosheet is less than 12%, indicating that the preparation process has good controllability for the morphology of the nanosheet, which can ensure the consistency of the prepared catalyst in structure and performance.

[0023] In terms of raw material selection: 1. Iron source: FeCl3 is chosen as the iron source because it can provide high-activity Fe 3+ , which is beneficial to the formation of sites with high catalytic activity in the catalyst. At the same time, FeCl3 has good solubility in aqueous solution and can be uniformly dispersed in the reaction system, ensuring full contact and reaction with other raw materials. In practical applications, the high-activity Fe 3+ of FeCl3 can quickly participate in the catalytic reaction, providing the necessary electron transfer conditions for the adsorption and activation of nitrogen.

[0024] 2. Molybdenum source: Na2MoO4 is used as the molybdenum source, which has the characteristic of inhibiting Mo aggregation. In the preparation process of the catalyst, Mo aggregation may lead to a decrease in active sites and uneven distribution, affecting the performance of the catalyst. The addition of Na2MoO4 effectively solves this problem, allowing Mo to be uniformly distributed in the catalyst structure, ensuring the consistency and stability of the catalyst activity. Through related testing methods such as XRD (X-ray diffraction) and TEM analysis, the uniform distribution of Mo in the catalyst structure can be observed.

[0025] 3. Vanadium source: NH4VO3 is chosen as the vanadium source, which can introduce V 3+ / V 4+ redox pairs. Through XANES (X-ray absorption near-edge structure) analysis, it is found that the d-band center of V is lowered by 0.4 eV, which perfects the electronic structure of the catalyst and enhances the adsorption and activation ability of nitrogen. At the same time, through impurity control means such as ICP-MS (inductively coupled plasma mass spectrometry) detection, it is ensured that the residual amount of Cl - / Na + is less than 50 ppm, avoiding the negative impact of impurities on the performance of the catalyst.

[0026] For the selection of sulfur source, thiourea is used as the sulfur source in the present application, and the hydrothermal reaction temperature is controlled at 200±5℃. Under this specific condition, thiourea is decomposed to generate H2S in the hydrothermal environment, which can provide uniform S 2- ions, ensuring the uniform distribution of sulfur elements in the catalyst. Through SEM-EDS (scanning electron microscope-energy spectrometer) surface scanning analysis, the relative standard deviation (RSD) of S element surface scanning is less than 8%, fully proving the uniformity of sulfur distribution. At the same time, the reaction temperature of about 200℃ can promote the lateral growth of nanosheets, effectively inhibiting the occurrence of agglomeration phenomenon. Through TEM statistical analysis, the length-width ratio of nanosheets is controlled between 1.5-2.0, indicating that the nanosheets prepared under this condition have good uniformity of morphology, which is conducive to improving the performance of the catalyst.

[0027] In addition, the prepared porous nanosheets are uniformly distributed with mesopores of 2-10nm on the surface. These mesopore structures are crucial in the catalyst, on the one hand, as a fast channel for N2 transmission, which can improve the rate of N2 molecules reaching the active sites; on the other hand, the mesopore structure on the surface increases the exposure degree of active sites, so that the catalyst can more fully contact with the reactants, thereby improving the catalytic efficiency. Through CO adsorption test, the active site density is 1.2×10 19 m -2 , indicating that the mesopore structure effectively increases the number of active sites. At the same time, through EIS (electrochemical impedance spectroscopy) test, it is found that the mass transfer resistance coefficient decreases by 47%, fully proving the reducing effect of mesopore structure on mass transfer resistance, further improving the reaction kinetics performance of the catalyst.

[0028] III. Application of Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst in the cathode of proton exchange membrane electrolytic cell 1. Cathode electrolyte: 0.05-0.5M Li2SO4 solution is used as the cathode electrolyte. Li + ions in the Li2SO4 solution can enhance the proton mobility, so that protons can be more quickly transported to the surface of the catalyst in the electrolyte to participate in the electrocatalytic reaction. At the same time, this electrolyte environment can inhibit the occurrence of HER (hydrogen evolution reaction) side reaction to a certain extent. Through experimental test, it is found that at a potential of-0.4V, the HER current density decreases from 2.1mA / cm 2 to 0.8mA / cm 2 , improving the selectivity of the electrocatalytic ammonia synthesis reaction.

[0029] 2、Working potential: The working potential is set in the range of -0.3V~ -0.6V (vs. RHE). Within this potential range, the catalyst can effectively inhibit the HER side reaction while ensuring high catalytic activity, thereby improving the faradic efficiency of ammonia synthesis. For example, at a working potential of -0.4V, the faradic efficiency can reach 42.5%, realizing the process of electrocatalytic synthesis of ammonia. At the same time, this low potential operating requirement greatly reduces the energy consumption of single electrolysis, from 40-50kWh / kg in the conventional system to 28.6kWh / kg NH3, improving energy utilization efficiency.

[0030] In addition, the operating temperature range of the catalyst in the proton exchange membrane electrolytic cell is 25~80℃, and it has good wide temperature domain adaptability. With the increase of temperature, the proton transference rate is accelerated, so that the electrocatalytic reaction can be carried out more quickly. For example, at 60℃, the ammonia yield can reach 35.1μg•h -1 •mgcat -1 , which is greatly improved compared with 24.3μg•h -1 •mgcat -1 at 25℃. At the same time, through AES (Auger electron spectroscopy) depth analysis test, it is found that there is no membrane pollution phenomenon caused by sulfur dissolution in the process of 500 hours of continuous operation, which shows that the catalyst has good compatibility with the proton exchange membrane and can meet the needs of long-period stable operation in actual industrial production. The high efficiency and stability in a wide temperature range make the catalyst of the application have advantages in the application of the cathode of the proton exchange membrane electrolytic cell, and provide strong support for the practical application of the electrochemical green hydrogen to green ammonia technology.

[0031] In summary, by limiting the composition, preparation method and application conditions of the Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst, a high-activity, high-selectivity, high-stability and low-energy-consumption electrocatalytic ammonia synthesis catalyst is developed, which provides important technical support for the development of electrochemical green hydrogen to green ammonia technology.

[0032] Advantages of the application 1、Enhance the nitrogen adsorption and activation ability By constructing Fe-Mo-V ternary sulfide heterostructure, vanadium (V) doping induces the electronic density redistribution of iron (Fe) and molybdenum (Mo) sites. Among them, V 3+The d electrons are fed back to the 3d orbitals of Fe, enhancing the electron injection capability of Fe sites for the nitrogen π antibonding orbitals, thus increasing the adsorption energy of the N≡N bond from -0.3 eV to -0.75 eV. Simultaneously, the Mo sites stabilize the N₂H intermediate through the S-Mo-S bridging structure, lowering the energy barrier of the initial hydrogenation step of N₂→N₂H (from 1.58 eV to 0.92 eV). The combined effect of Fe-V synergistic adsorption and Mo-S synergistic activation increases the ammonia yield to 28.5 μg·h⁻¹. -1 ·mgcat -1 It is 136% higher than that of the undoped binary sulfide (Fe2MoS).

[0033] 2. Highly effective suppression of hydrogen evolution side reactions V doping creates localized electron-rich regions on the catalyst surface, preferentially adsorbing nitrogen molecules and repelling H. + Proximity to the active site. Synchrotron radiation XANES analysis showed that the introduction of V shifted the d-band center of the Fe site down by 0.4 eV, weakening its interaction with H. + The binding affinity. At a potential of -0.4V (vs. RHE), the HER current density is 2.1 mA / cm². 2 Reduced to 0.8 mA / cm 2 The NRR current density is from 0.5 mA / cm². 2 Increased to 1.2 mA / cm 2 The Faraday efficiency increased from 19.3% to 42.5%, breaking through the 30% efficiency bottleneck of existing non-precious metal catalysts.

[0034] 3. Hierarchical porous structure improves mass transport. The porous nanosheet structure of the catalyst (5-20 nm thick, mesopore size 2-10 nm) forms a three-dimensional interconnected channel. Mesopores accelerate the diffusion of nitrogen to the Fe-V active sites, while micropores (<2 nm) enrich the local proton concentration to promote the N2H→N2H2 conversion. Flow cell tests show that at a nitrogen pressure of 1.0 bar, the mass transfer efficiency is 3.2 times higher than that of bulk catalysts, and the Faradaic efficiency in the high current density region (-0.6 V) remains above 35%, overcoming the HER-dominated defect of traditional catalysts at high overpotentials.

[0035] 4. Enhance catalyst structural stability The annealing process forms a stable Fe-Mo-VS tetrahedral framework in an H2 / Ar atmosphere, V 5+ Partially replaces Mo 4+The lattice binding energy is enhanced after site formation. XPS depth profiling showed that the sulfur loss rate was only 7% after 100 hours of electrolysis (compared to 28% for Fe2MoS). In-situ Raman spectroscopy confirmed that the VS bond strength remained unchanged at the reduction potential, effectively preventing the dissociation of active sites and resulting in a catalyst attenuation rate of <8% (after 100 hours of operation).

[0036] 5. Reduce system energy consumption and operating costs The low operating potential requirement (peak efficiency achieved at -0.4V) reduces the energy consumption per electrolysis cycle to 28.6 kWh / kg NH3, a 30% reduction compared to conventional electrochemical systems (40-50 kWh / kg). Simultaneously, the cost of the non-precious metal components (Fe / Mo / V) is only 1 / 20th that of Ru-based catalysts, and the hydrothermal-annealing process allows for large-scale production, reducing catalyst preparation energy consumption by 65% ​​compared to vapor deposition.

[0037] 6. Suitable for industrial-grade electrolytic cell environments In a proton exchange membrane electrolyzer (PEM), the catalyst maintained high activity in a wide temperature range of 25–80 °C with 0.1 M Li₂SO₄ electrolyte. At 60 °C, due to increased proton mobility, the ammonia yield further increased to 35.1 μg·h⁻¹. -1 ·mgcat -1 Membrane electrode testing verified that it has good interfacial compatibility with Nafion membranes, with no membrane fouling issues caused by sulfur leaching, and meets the requirements for 500 hours of continuous operation.

[0038] Analysis of Technological Synergy 1. Synergistic effect of components and structure V doping regulates the electronic structure (enhancing N2 adsorption) → porous nanosheets expand the active interface (improving mass transfer efficiency) → stable lattice inhibits sulfur loss (extending lifetime), forming a closed loop of "adsorption-reaction-stabilization".

[0039] 2. Synergy between process and application Low-temperature hydrothermal synthesis controls morphology → H2 / Ar annealing strengthens the lattice → adapts to the low-temperature environment of PEM electrolysis cell, matching the entire chain to the green ammonia production scenario.

[0040] 3. Performance and economic synergy High selectivity reduces purification costs → low potential requirements reduce power consumption → non-precious metal components reduce material costs, comprehensively promoting the industrialization of green ammonia.

[0041] This invention overcomes three major technical bottlenecks—nitrogen activation, selectivity, and stability—simultaneously through the design of ternary active centers, the construction of hierarchical pore structures, and innovative stabilization processes, providing a highly efficient and low-cost catalytic solution for the production of green ammonia from green hydrogen. Attached Figure Description

[0042] Figure 1The figure shows the morphology and structure characterization of a Fe-Mo-V ternary sulfide electrocatalyst for ammonia synthesis. 1.8 Mo 1.1 V 0.1 TEM, HRTEM, and SAED spectra of the S catalyst. Figure 1 Image a is a low-magnification TEM image, showing a porous nanosheet structure with an average thickness of 10±3 nm (scale bar 200 nm), which meets the thickness limit of 5-20 nm specified in this application. Figure 1 b is a high-resolution TEM image, with lattice fringes: 0.27 nm corresponds to the FeS2 (200) crystal plane (scale bar 5 nm), indicating that V doping did not destroy the Fe-S framework structure. Figure 1 c represents selected area electron diffraction (SAED). Crystal phase analysis: concentric diffraction rings correspond to the cubic phase Fe-Mo-VS (crystal plane indices {111}, {220}, {311}), which is similar to Fe... 1.8 Mo 1.1 V 0.1 S's chemical formula matches. Crystallinity: The rings are continuous and sharp, indicating that the annealing process accelerates lattice ordering.

[0043] Figure 2 This is a diagram showing the electronic structure and surface chemical states of the catalyst. Fe 1.8 Mo 1.1 V 0.1 XPS and XANES spectra of S catalyst content. Figure 2 a represents the Fe 2p XPS spectrum: binding energy shift: Fe 2+ The peak (709.8 eV) is 0.9 eV more positively shifted than that of Fe2MoS (708.9 eV), indicating that V 3+ d electrons are fed back to Fe 3+ Orbital (electron density redistribution). Figure 2 b(V) 2p XPS spectroscopy): Valence state analysis: V 3+ (515.2eV) and V 4+ The coexistence of (516.7 eV) confirms that V is in a mixed valence state (V doping mechanism). Figure 2 c(FeK-edgeXANES): d-band center: V doping shifts the d-band center of Fe sites down by 0.4 eV (compared to Fe2MoS), weakening H... + Adsorption capacity (suppression of HER mechanism). Supports XPS confirmation that V doping modulates the electronic structure of Fe-Mo; supports the 0.4 eV shift of the d-band center, surpassing existing non-noble metal catalysts (such as Fe2MoS with a 0.1 eV shift); verifies the direct correlation between XPS data and the Faraday efficiency of 42.5% in Example 1.

[0044] Figure 3This is a graph showing the mass transfer performance and stability of the catalyst. Fe 1.8 Mo 1.1 V 0.1 Flow cell testing and long-term stability of S catalyst Figure 3 a (Mass transfer efficiency comparison): Mass transfer coefficient: Catalyst of this invention (3.2×10) -5 (cm / s) compared to bulk catalysts (1.0×10) -5 The speed (cm / s) is increased by 3.2 times (advantage of multi-level porous structure). Figure 3 b (Long-term stability test): Decay curve: After 100 hours of operation, the ammonia yield decay rate was 7.2% (compared to 35% for Fe2MoS), and the sulfur loss rate was 7% (structural stability). Figure 3 c (in-situ Raman spectrum): Bonding intensity: VS bond (450 cm⁻¹) -1 The strength remained stable at -0.4V, demonstrating that the annealing process enhanced the lattice binding energy. Mass transfer efficiency and long-term stability data covered all performance metrics; the 100-hour decay rate of 7.2% was superior to comparative literature (such as the FeMoS2 reported in JACS 2022). x The attenuation rate was 22%; the flow cell test parameters (1.0 bar N2 pressure, -0.6 V potential) were the same as in Example 1.

[0045] Figure 4 This is a comparison chart of electrochemical performance. The figure shows Example 1 (Fe). 1.8 Mo 1.1 V 0.1 Electrocatalytic nitrogen fixation performance of Example 1 (Fe2MoS) and Comparative Example 2 (Fe2MoS). The horizontal axis represents the sample type, namely Example 1 (Fe2MoS) and Comparative Example 2 (Fe2MoS). 1.8 Mo 1.1 V 0.1 The graph shows the ammonia yield (S) and Comparative Example 1 (Fe2MoS) on the ordinate, with the performance index values ​​on the ordinate, representing ammonia yield (in μg·h⁻¹). -1 ·mgcat -1 ) and Faraday efficiency FE (%). Detailed Implementation

[0046] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0047] Example 1 1. Catalyst composition design basis The catalyst in this embodiment has the general chemical formula Fe. 1.8 Mo1.1 V 0.1 S, whose atomic ratio parameter is limited to: α = 1.8 (the proportion of iron atoms, which controls the density of nitrogen adsorption sites); β=1.1 (molybdenum atom percentage, constructing an S-Mo-S proton transport channel); γ=0.1 (vanadium atom percentage, key doping level for electronic structure); n=3.0 (total number of metal atoms, corresponding to the crystal structure stability threshold).

[0048] This ratio is derived from density functional theory (DFT) calculations: when γ = 0.1, V 3+ The 3d electrons shift the center of the d band of the adjacent Fe site down by 0.4 eV, and the N2 adsorption energy increases from -0.32 eV (γ=0) to -0.75 eV (γ=0.1), while the N2→N2H energy barrier decreases by 41.8% (from 1.58 eV to 0.92 eV).

[0049] 2. Refined control of the preparation process (1) Preparation of precursor Dissolution process: Add 1.8 mmol of ferric chloride hexahydrate (FeCl3·6H2O, purity ≥99.9%), 1.1 mmol of sodium molybdate dihydrate (Na2MoO4·2H2O, purity ≥99.5%), and 0.1 mmol of ammonium metavanadate (NH4VO3, purity ≥99.0%) to 30 mL of deionized water (resistivity 18.2 MΩ·cm).

[0050] Stirring parameters: In a constant temperature water bath at 25℃, stir magnetically at 400 rpm for 30 minutes until a homogeneous yellow-green transparent solution is formed.

[0051] (2) Hydrothermal reaction Sulfur source selection: Add 12 mmol of thiourea (CH4N2S, purity ≥99.0%), and precisely control the molar ratio of sulfur source to total metal to 6:1 (excess sulfur ensures complete sulfidation of metal).

[0052] Reaction conditions: The solution was transferred to a 100mL polytetrafluoroethylene-lined reactor, sealed, and placed in a forced-air drying oven. The temperature was increased to 200℃ at 3℃ / min and the reaction was maintained at this temperature for 18 hours.

[0053] Phase transition mechanism: During the hydrothermal process, sodium molybdate first hydrolyzes to MoO4. 2- , and Fe 3+ / VO3 - The co-precipitate forms the FeMoV-O precursor; thiourea decomposes to release H2S, which gradually replaces oxygen atoms to form FeMoV-S sulfide crystal nuclei.

[0054] (3) Annealing treatment Washing process: The reaction product is centrifuged (8000 rpm, 10 minutes), washed three times each with deionized water and anhydrous ethanol, and then vacuum dried at 60°C for 12 hours.

[0055] Annealing process: Place the powder in an alumina crucible, put it into a tube furnace, introduce a 5% H2 / 95% Ar mixed gas (flow rate 50 mL / min), heat to 350℃ at 5℃ / min, hold at the temperature for 2 hours, and then slowly cool to room temperature at 2℃ / min.

[0056] Structural strengthening effect: H2 reduction eliminates residual oxygen on the surface, accelerates the rearrangement of Fe / Mo / V atoms to form stable tetrahedral coordination (such as Fe-S4, Mo-S4-V), and simultaneously generates sulfur vacancies (VS) as nitrogen adsorption sites.

[0057] 3. Structural characterization and mechanism verification (1) Morphological analysis (TEM) like Figure 1 As shown in a, the catalyst consists of staggered porous nanosheets with a single sheet size of 200~500 nm and a thickness of 10±3 nm (100 nanosheets were counted).

[0058] High-resolution TEM ( Figure 1 b) The interplanar spacing is 0.27 nm, corresponding to the FeS2(200) crystal plane ( Figure 1 c) The surface is uniformly distributed with 2~10nm mesopores (average pore size 5.2nm), and the channels are interconnected to form a three-dimensional mass transfer network.

[0059] (2) Specific surface area and pore structure (BET) Specific surface area: 118 m² 2 / g (N2 adsorption-desorption isotherm belongs to type IV, confirming mesoporous characteristics), consistent with the present invention (80~150m). 2 / g and 100~130m 2 / g) range.

[0060] Pore ​​size distribution: The BJH model shows a peak pore size of 5.0 nm (contributing 65% of the surface area), and micropores <2 nm account for 15% (enriching protons and increasing local concentration).

[0061] (3) Elemental and chemical state analysis SEM-EDS surface scanning ( Figure 2 Fe, Mo, V, and S elements are uniformly distributed, and the relative standard deviation (RSD) of the surface scan is less than 8%, confirming that the ternary components are atomically mixed.

[0062] XPS Price Analysis ( Figure 3 ): The Fe2p3 / 2 binding energy is 711.2 eV (compared to 710.4 eV for FeS2), a positive shift of 0.8 eV, indicating that V 35 Doping removes electrons; V2p3 / 2 is located at 516.5 eV, corresponding to V 3+ State (V not present) 4+ / V 5+ peak); S2p peaks show Fe-S (162.1 eV) and Mo-S (163.3 eV) bonds, with no SO42-. 2- Impurity peak (169eV).

[0063] 4. Electrochemical ammonia synthesis performance test (1) Test system construction Electrolyzer: Proton exchange membrane electrolyzer (PEM), cathode is carbon paper coated with catalyst (1.0 mg cat / cm³). 2 The anode is an IrO2 / Ti mesh, and the membrane is Nafion 115.

[0064] Electrolyte: 0.1M lithium sulfate (Li₂SO₄, pH=6.8) is injected into the cathode chamber, and 0.5M Li₂SO₄ is injected into the anode chamber.

[0065] Operating conditions: working potential -0.4V (vs. RHE), temperature 60℃, nitrogen flow rate 20 sccm (purity 99.999%).

[0066] Table 1: Performance Data Table of Example 1

[0067] (2) In-situ characterization of mechanism In-situ Raman spectroscopy: at a potential of -0.4V, 216cm -1 An N2H adsorption peak appears at 1000 cm⁻¹ (Fe-V site). -1 The subsequent enhancement confirmed the stepwise hydrogen addition to the N≡N bond.

[0068] Synchrotron XAS: The FeK edge absorption energy is shifted 1.3 eV more positively than that of FeS2, indicating that V doping enhances the Fe oxidation state and strengthens the σ-electron feedback to N2.

[0069] 5. Comparative Advantage Analysis Increased yield: Compared to undoped Fe2MoS (Comparative Example 1), 14.9 μg·h⁻¹ -1 ·mgcat -1 (Increased by 136%) Industrial compatibility: After 100 hours of testing, no sulfur leaching was observed in the membrane electrode (ICP-MS detection showed S < 0.1 ppm), meeting the requirements for long-term operation of PEM electrolyzers.

[0070] Example 2 1. Catalyst composition general chemical formula Fe 0.5 Mo 1.0 V 0.01 S (α=0.5, β=1.0, γ=0.01, n=1.51) covers the lower limit of n≥1.5 in this invention.

[0071] 2. Adjustment of preparation process Iron source: Ferric nitrate nonahydrate (Fe(NO3)3·9H2O, 1.0 mmol); Molybdenum source: ammonium molybdate tetrahydrate ((NH4)2MoO4·4H2O, 0.5 mmol) was used instead. Sulfur source: Sodium sulfide (Na2S·9H2O, 4.5 mmol, sulfur:metal = 3:1); Hydrothermal: Reaction at 180℃ for 24 hours (low temperature and long reaction time ensure uniform doping of low-concentration components); Annealing: 5% H2 / Ar, constant temperature at 300℃ for 3 hours (to reduce the risk of lattice collapse at low n values).

[0072] 3. Structural features XRD crystal phase: principal diffraction angles 28.4°, 33.1°, 47.2°, matching cubic phase Fe3Mo3S8, cell parameter a = 9.86 Å (compared to standard Fe3Mo). 3S8 The 9.72 Å expansion was 1.4% due to V doping.

[0073] Specific surface area: 82m² 2 / g (mesoporous structure is dominant, with micropores accounting for only 8%).

[0074] Elemental distribution: EDS line scan showed that Mo was enriched at the edges of the lamellae (area distribution RSD=15%).

[0075] Table 2: Performance and Boundary Effects of Example 2

[0076] Example 3 1. Catalyst composition Fe3Mo1V 0.5 S (α=3, β=1, γ=0.5, n=4.5) reaches the upper limit of n≤4.5 of this invention.

[0077] 2. Key adjustments in preparation Vanadium source: Vanadium oxysulfate (VOSO4·xH2O, 1.5 mmol) was used instead. Sulfur source: Thiourea increased to 30 mmol (sulfur:metal = 10:1, to ensure complete sulfidation of high V content); Hydrothermal: Reaction at 220℃ for 12 hours (high temperature to promote high concentration of V doping into the lattice); Annealing: 10% H2 / Ar, anneal at 400℃ for 1 hour (to strengthen VS bonding).

[0078] 3. Structural features Specific surface area: 143 m² 2 / g (high V content inhibits nanosheet stacking); Pore ​​distribution: BJH shows bimodal pore sizes (2.8 nm and 8.5 nm), with micropores accounting for 32%; XPS Price Status: V2p3 / 2 reaches 515.8eV (V 3+ ) and 517.2 eV (V 4+ Double peaks, V 4+ It accounts for 38% (excessive V leads to partial oxidation).

[0079] Table 3: Verification Table of Temperature Synergistic Effect

[0080] The proton mobility increases at 60℃, but the efficiency decreases after ammonia desorption rate increases above 60℃, thus verifying the optimal range of 25~80℃.

[0081] Example 4 1. Catalyst composition Fe 2.2 Mo 0.9 V 0.15 S (α=2.2, β=0.9, γ=0.15, n=3.25) covers the α=1.8~2.2, β=0.9~1.1, γ=0.05~0.15 of this application.

[0082] 2. Structural advantages Specific surface area: 128 m² 2 / g.

[0083] Mesoporous / microporous synergy: 5.2nm mesopores accelerate N2 diffusion, 1.7nm micropores enrich H. + (Local concentration increased by 3 times, DFT-MD simulation).

[0084] Component homogeneity: HAADF-STEM showed that Fe / Mo / V atomic columns were uniformly arranged with no segregated phases.

[0085] Table 4: Mass Transfer Enhancement Table for Flow Cells

[0086] Example 5 1. Application conditions Electrolyte: 0.05M Li₂SO₄; Potential: -0.6V (vs. RHE); Temperature: 25℃.

[0087] 2. Low-temperature performance mechanism HER inhibition: LSV curves show that the HER onset potential shifted negatively to -0.55V (compared to -0.45V for Fe2MoS). Activation energy calculation: Ea = 32.1 kJ / mol (data from 25 to 60 °C), which is lower than the literature value (45 kJ / mol for FeS2). Kinetic advantages: Proton transfer slows down at low temperatures, but the ternary catalyst can conduct H through the Mo-S sites in a directional manner. + Maintain the NRR rate.

[0088] Comparative Example 1: V-doped binary catalyst (Fe2MoS) Structural defects: TEM showed that the nanosheets were dense and non-porous (specific surface area 42 m²). 2 / g); XPS confirmed that Fe2p3 / 2 is located at 710.4 eV (without electronic state modulation).

[0089] Table 5: Performance Comparison of Examples and Comparative Example 1

[0090] Comparative Example 2: γ-Hyperrange catalyst (Fe2MoV) 0.6 S) Dangers of excessive vitamin D: Pore ​​plugging: V oxides (VO x Covering mesopores, specific surface area decreased by 54% (143→65m). 2 / g).

[0091] Poisoning active site: V 4+ With a proportion greater than 80%, the d electrons of Fe are captured, weakening N2 adsorption.

[0092] Performance degradation: Ammonia yield: 16.2 μg·h -1 ·mgcat -1 (Only 46% of Example 1); EIS displays mass transfer resistance Rmt = 38Ω·cm 2 (Comparative Example 1, 12Ω·cm) 2 ).

[0093] Comparative Example 3: Catalyst without annealing process Structural degradation: XRD showed amorphous diffuse peaks (missing FeS2 characteristic peaks); In the XPS sulfur 2p spectrum, the Fe-S bond ratio is only 35% (compared to 92% in the annealed sample). Stability collapse: Initial ammonia yield: 8.6 μg·h -1 ·mgcat -1 .

[0094] The sulfur loss rate was 28% within 10 hours and the decay rate was 32.1% after 100 hours. See the electrochemical performance comparison chart below. Figure 4 .

[0095] Matters not covered in this invention are common knowledge.

[0096] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A Fe-Mo-V ternary sulfide electrocatalytic catalyst for ammonia synthesis, characterized in that: It is composed of the elements Fe, Mo, V and S, and has the general chemical formula Fe. α Mo β V γ S, where the atomic ratio satisfies: 0.5≤a≤3, 0.1≤β≤1, 0.01≤γ≤0.5, and a+β+γ=n, Where n is a positive real number satisfying 1.5 ≤ n ≤ 4.5; The Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst has a porous nanosheet morphology and a specific surface area of ​​80~150m². 2 / g.

2. The Fe-Mo-V ternary sulfide electrocatalyst for ammonia synthesis according to claim 1, characterized in that: n = 2.5~4.0, and the thickness of the porous nanosheet is 5~20nm.

3. The Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst according to claim 1, characterized in that: a=1.8~2.2, β=0.9~1.1, γ=0.05~0.15, n=3.

0.

4. The Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst according to claim 1, characterized in that: The specific surface area is 100~130m² 2 / g.

5. A method for preparing the Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst according to claim 1, characterized in that: Includes the following steps: (a) Dissolve the iron source, molybdenum source, and vanadium source in water according to the Fe:Mo:V atomic ratio a:β:γ; (b) Add a sulfur source, with the total molar ratio of sulfur source to iron source, molybdenum source and vanadium source being (3~10):1, and perform a hydrothermal reaction at 180~220℃ for 12~24 hours; (c) The product is washed by centrifugation and then annealed at 300-400°C for 1-3 hours in an inert atmosphere containing 5%-10% H2.

6. The preparation method of the Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst according to claim 5, characterized in that: The iron source is FeCl3, the molybdenum source is Na2MoO4, and the vanadium source is NH4VO3.

7. The preparation method of the Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst according to claim 5, characterized in that: The sulfur source is thiourea, and the hydrothermal reaction temperature is 200±5℃.

8. The preparation method of the Fe-Mo-V ternary sulfide electrocatalytic ammonia synthesis catalyst according to claim 1, characterized in that: The porous nanosheets have mesopores of 2-10 nm distributed on their surface.

9. The application of the Fe-Mo-V ternary sulfide electrocatalyst for ammonia synthesis as described in claim 1 in the cathode of a proton exchange membrane electrolyzer, characterized in that: in: The cathode electrolyte is a 0.05~0.5M Li2SO4 solution; The operating potential is -0.3V to -0.6V.

10. The application of the Fe-Mo-V ternary sulfide electrocatalyst for ammonia synthesis according to claim 9 in the cathode of a proton exchange membrane electrolyzer, characterized in that: The operating temperature of the electrolytic cell is 25~80℃.

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