A phase-regulated cobalt-based catalyst for electrocatalytic reduction of nitrate to ammonia and a preparation method and application thereof

Abstract

This invention discloses a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, its preparation method, and its application, relating to the field of electrocatalytic reduction technology. The catalyst proposed in this invention uses cobalt foam as a substrate with β-phase cobalt hydroxide in situ supported on its surface. Through the reconstruction effect of the cobalt hydroxide precatalyst in the early stage of nitrate reduction, the α-phase cobalt hydroxide is reconstructed into β-phase cobalt hydroxide, thereby significantly improving the Faradaic efficiency of the catalyst. Furthermore, because metallic cobalt continuously reacts with nitrate at a low rate to generate cobalt hydroxide during nitrate reduction and is then reconstructed in situ into β-phase cobalt hydroxide, dynamic regeneration of active species is achieved, ensuring the long-term stability and Faradaic efficiency of the catalyst. In this way, using only metallic cobalt as a precursor, long-term stability and high Faradaic efficiency of nitrate electrocatalytic reduction can be achieved at high current densities.

Description

Technical Field

[0001] This invention relates to the field of electrocatalytic reduction technology, and more specifically, to a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, its preparation method, and its application. Background Technology

[0002] With the acceleration of industrialization and agricultural intensification, the excessive accumulation of nitrate pollutants in groundwater and radioactive wastewater has become an urgent environmental problem, posing significant public health risks and ecological imbalances. However, nitrates, as an indispensable nitrogen source in key industrial sectors such as ammonia synthesis and energetic material preparation, possess the dual attributes of both pollutant and resource, thus creating an urgent need for a synergistic "pollution control-resource regeneration" technology system. Among numerous treatment technologies, electrocatalytic reduction stands out as one of the most promising solutions for industrialization due to its high conversion efficiency under mild conditions, environmental compatibility, and the ability to achieve selective product conversion through precise control of electrochemical parameters.

[0003] In recent years, significant progress has been made in the research of electrocatalytic reduction of nitrate. From the development of copper-based and cobalt-based high intrinsically active catalysts to the development of iron-based economical materials, from bimetallic synergistic regulation to the engineering design of nanostructures, and even breakthroughs in the atom utilization of single-atom catalysts, all have demonstrated unique value in improving catalytic performance. However, existing technologies still face severe challenges: long-term stability under actual operating conditions is generally insufficient (currently only maintained for tens to hundreds of hours, far lower than the thousands of hours required for industrial applications), and current density is limited to hundreds of milliamperes per square centimeter, resulting in high ammonia synthesis costs. Furthermore, the synergistic optimization of product selectivity and catalytic efficiency is difficult to achieve, severely restricting the industrialization of this technology. Therefore, developing electrocatalytic systems that combine high stability, high selectivity, and low cost remains a core issue for promoting the practical application of nitrate resource utilization technology.

[0004] In view of this, the present invention is proposed. Summary of the Invention

[0005] The purpose of this invention is to provide a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, its preparation method, and its application. Through the remodeling effect of the cobalt hydroxide precatalyst in the initial stage of nitrate reduction, the α-phase cobalt hydroxide is remodeled into the β-phase cobalt hydroxide, thereby significantly improving the Faradaic efficiency of the catalyst.

[0006] This invention is implemented as follows: In a first aspect, the present invention provides a phase-controlled cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, wherein the catalyst is based on cobalt foam, and β-phase cobalt hydroxide is in situ supported on the surface of the cobalt foam.

[0007] Secondly, the present invention provides a method for preparing a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, comprising the following steps: Surface pretreatment of cobalt foam; The pretreated cobalt foam was placed in a first nitrate solution and allowed to stand to obtain a cobalt foam precatalyst loaded with cobalt hydroxide. The foamed cobalt precatalyst was prepared and placed in a second nitrate solution for cyclic voltammetric activation to convert α-phase cobalt hydroxide into β-phase cobalt hydroxide, thus obtaining the phase-controlled cobalt-based catalyst.

[0008] In an optional embodiment, the surface pretreatment includes: ultrasonically cleaning the cobalt foam in a 3-4 mol / L acid solution, followed by ultrasonic cleaning with ethanol and deionized water for 15-20 min each.

[0009] In an optional embodiment, the nitrate concentration in the first nitrate solution is >0.1 mol / L; the nitrate concentration in the second nitrate solution is 0.1-1 mol / L. The first nitrate solution and the second nitrate solution are adjusted to pH ≥ 7 using an alkaline solution.

[0010] In an optional embodiment, the nitrate solution is selected from at least one of sodium nitrate, potassium nitrate, lithium nitrate, or cesium nitrate.

[0011] In an optional embodiment, the alkaline solution is selected from at least one of sodium hydroxide, potassium hydroxide, barium hydroxide, or ammonia water.

[0012] In an optional implementation, the settling time is 3 days to 2 weeks.

[0013] In an optional implementation, the voltage range of the cyclic voltammetric scan is -0.3V RHE to -1.2VRHE, and the scan rate is 300mV / s.

[0014] Thirdly, the present invention provides a method for producing ammonia by electrocatalytic reduction of nitrate, using the above-mentioned phase-controlled cobalt-based catalyst as the working electrode and a mixed solution containing nitrate and alkaline electrolyte as the electrolyte to carry out an electrocatalytic reduction reaction to produce ammonia from nitrate.

[0015] Fourthly, the present invention provides an application of a cobalt-based catalyst in the electrocatalytic reduction of nitrate to prepare ammonia.

[0016] The present invention has the following beneficial effects: This invention utilizes a phase-controlled catalyst, cobalt, under alkaline conditions, to successfully achieve the directed conversion of the active species from α-phase cobalt hydroxide to β-phase cobalt hydroxide during the reaction process through its oxidative reconstruction by nitrate ions. Because β-phase cobalt hydroxide suppresses the hydrogen evolution side reaction, this reconstruction process significantly improves the Faraday efficiency of nitrate reduction. Furthermore, the reaction between cobalt and nitrate ions proceeds continuously at a mild rate, constantly generating and reconstructing highly active β-phase cobalt hydroxide. This allows for the simultaneous achievement of excellent stability and selectivity at high current densities using only cobalt. Ultimately, this system achieves a current density as high as 498 mg·h⁻¹. - ¹·cm - The increased ammonia yield significantly accelerates the industrialization of electrocatalytic nitrate reduction ammonia production technology, providing a new path with great economic and practical value for building a synergistic system of "pollution control-resource regeneration". Attached Figure Description

[0017] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The following are examples of the nitrate reduction performance of cobalt foam loaded with cobalt hydroxide in different concentrations of potassium nitrate and potassium hydroxide electrolytes in this invention: (a) nitrate reduction performance of cobalt foam loaded with cobalt hydroxide in 0.1 mol / L potassium nitrate and 1 mol / L potassium hydroxide electrolytes; (b) nitrate reduction performance of cobalt foam loaded with cobalt hydroxide in 0.5 mol / L potassium nitrate and 1 mol / L potassium hydroxide electrolytes. Figure 2 The current density, yield, and Faraday efficiency of nitrate reduction in 0.5 mol / L potassium sulfate and 0.1 mol / L potassium nitrate electrolytes for metallic cobalt and cobalt foam grown with cobalt hydroxide in this invention are shown. Figure 3 This is a scanning electron microscope image of metallic cobalt after standing in potassium nitrate solutions of different concentrations for 16 hours in this invention. Figure 4 These are scanning electron microscope images of metallic cobalt in potassium nitrate solutions of different concentrations, after being left to stand for different times in this invention. Figure 5 This represents the rate at which metallic cobalt reacts with nitrate ions to produce ammonia in potassium nitrate solutions of different concentrations in this invention. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0020] The following is a detailed description of a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, its preparation method, and its application.

[0021] In a first aspect, the present invention provides a phase-controlled cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, wherein the catalyst is based on cobalt foam, and β-phase cobalt hydroxide is in situ supported on the surface of the cobalt foam.

[0022] It should be noted that the in-situ loaded β-phase cobalt hydroxide includes a portion obtained from the electrochemical reconstruction of α-phase cobalt hydroxide.

[0023] Secondly, the present invention provides a method for preparing a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, comprising the following steps: S1. Surface pretreatment of cobalt foam; In an embodiment of this application, the surface pretreatment includes: ultrasonically cleaning cobalt foam in a 3-4 mol / L acid solution to remove surface oxides, followed by ultrasonic cleaning with ethanol for 15 minutes to remove surface organic matter; and finally ultrasonic cleaning with deionized water for 15 minutes to remove residual acid and ethanol from the surface.

[0024] The preferred acid is hydrochloric acid.

[0025] S2. The pretreated cobalt foam is placed in the first nitrate solution and allowed to stand to obtain a cobalt foam precatalyst loaded with cobalt hydroxide. In some preferred embodiments, the nitrate concentration in the first nitrate solution is >0.1 mol / L, and the pH of the first nitrate solution is adjusted to ≥7 by an alkaline solution.

[0026] In an optional embodiment, the settling time is 3 days to 2 weeks, and the settling process further includes a post-processing step: rinsing the sample with deionized water 1-3 times and then drying it with nitrogen.

[0027] S3. After the foamed cobalt precatalyst is prepared, it is placed in a second nitrate solution and subjected to cyclic voltammetric scanning activation treatment to convert α-phase cobalt hydroxide into β-phase cobalt hydroxide, thereby obtaining the phase-controlled cobalt-based catalyst.

[0028] In some preferred embodiments, the nitrate concentration in the second nitrate solution is 0.1-1 mol / L, and the pH of the second nitrate solution is adjusted to ≥7 by an alkaline solution.

[0029] Preferably, the first nitrate solution and the second nitrate solution are selected from at least one of sodium nitrate, potassium nitrate, lithium nitrate or cesium nitrate.

[0030] The alkaline solution mentioned above is selected from at least one of sodium hydroxide, potassium hydroxide, barium hydroxide, or ammonia water.

[0031] In an optional implementation, the voltage range of the cyclic voltammetric scan is -0.3V RHE to -1.2VRHE, and the scan rate is 300mV / s.

[0032] Thirdly, the present invention provides a method for producing ammonia by electrocatalytic reduction of nitrate, using the above-mentioned phase-controlled cobalt-based catalyst as the working electrode and a mixed solution containing nitrate and alkaline electrolyte as the electrolyte to carry out an electrocatalytic reduction reaction to produce ammonia from nitrate.

[0033] Fourthly, the present invention provides an application of a cobalt-based catalyst in the electrocatalytic reduction of nitrate to prepare ammonia.

[0034] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0035] Example 1 This embodiment provides a method for preparing a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, comprising the following steps: S1. Place the cobalt foam in 3M hydrochloric acid and ultrasonically clean it to remove the oxides on the surface. Then ultrasonically clean it with ethanol for 15 minutes to remove organic matter on the surface. Finally, ultrasonically clean it with deionized water for 15 minutes to remove the residual acid and ethanol on the surface.

[0036] S2. Place the pretreated cobalt foam in a mixed solution of 0.1 M potassium nitrate and 1 M potassium hydroxide and let it stand for three to two weeks. After standing, take out the sample, rinse it three times with deionized water and then blow it dry with nitrogen to obtain a cobalt foam sample loaded with cobalt hydroxide.

[0037] S3. Cut the prepared cobalt hydroxide-loaded foam into appropriate sizes and place it in a mixed electrolyte of 0.1 M potassium nitrate solution and 1 M potassium hydroxide solution. Set the voltage range to -0.3 V RHE to -1.2 V RHE and perform CV scan at a scan rate of 300 mV / s until the catalyst no longer produces bubbles.

[0038] The linear sweep voltammetry (LSV) curves and nitrate reduction performance under constant potential conditions were tested: The prepared phase-modulated cobalt-based catalyst was placed in 0.1 M potassium nitrate and 1 M potassium hydroxide electrolytes and a mixed electrolyte of 0.5 M potassium nitrate and 1 M potassium hydroxide, respectively, and the nitrate reduction performance was tested. The results are as follows: Figure 1 As shown.

[0039] The results show that the catalytic efficiency of the 0.5 M potassium nitrate system is significantly better in terms of ammonia yield, reaching 498 mg h⁻¹ near a potential of -1.8 V (vs. RHE). - ¹ cm - ², compared to approximately 285 mg h in the 0.1 M system. - ¹ cm - The increase of approximately 75% demonstrates that higher substrate concentrations can effectively enhance the ammonia production rate. In terms of Faradaic efficiency, in the 0.1 M potassium nitrate system, the catalyst reaches a peak efficiency of approximately 95% near -0.4 V (vs. RHE) and remains stably above 90% in the subsequent potential range. In the 0.5 M potassium nitrate system, the catalyst also achieves a Faradaic efficiency as high as 90.5%, ensuring high selectivity while achieving efficient ammonia production under high current. Overall, the electrolyte combination of 0.5 M potassium nitrate and 1 M potassium hydroxide better leverages the performance advantages of phase-controlled cobalt-based catalysts, achieving a significant increase in ammonia yield while maintaining excellent Faradaic efficiency, providing more industrially promising process conditions for nitrate reduction to ammonia.

[0040] Experimental Example 1 The performance of cobalt foam loaded with cobalt hydroxide prepared in Example 1 and pure cobalt foam in nitrate reduction was compared in an electrolyte of 0.5 M potassium sulfate and 0.1 M potassium nitrate. The comparisons were made in terms of current density, ammonia yield, and Faraday efficiency. The results are as follows: Figure 2 As shown.

[0041] The results show that, in terms of current density, within the test potential range of -0.2 V to -1.4 V (vs. RHE), the cobalt hydroxide-supported foam exhibits a higher current density at all potentials, and the difference in current density between the two gradually widens as the potential shifts negatively, indicating that the electron transport and reaction kinetics of the catalyst are significantly enhanced after phase-modified modification. Ammonia yield data show that the cobalt hydroxide-supported foam exhibits higher NH4+ yield at the same potential. +The formation rate was consistently superior to that of pure cobalt foam, with a particularly pronounced yield advantage near -1.2 V (vs. RHE), demonstrating a significant improvement in the selectivity and conversion efficiency of the modified catalyst for the reduction of nitrate to ammonia. In terms of Faraday efficiency, the cobalt foam supported on cobalt hydroxide maintained a high level across the entire potential range, generally higher than that of pure cobalt foam, indicating that it can more efficiently utilize electrons for the target reduction reaction and suppress side reactions such as hydrogen evolution.

[0042] In summary, cobalt foam loaded with cobalt hydroxide outperforms pure cobalt foam in all three core indicators: current density, ammonia yield, and Faraday efficiency. This fully verifies that introducing β-phase cobalt hydroxide through phase modulation can significantly improve the catalytic activity and selectivity of cobalt-based catalysts in the electrocatalytic reduction of nitrate to ammonia, providing efficient catalytic material support for nitrate pollution control and resource-based ammonia production.

[0043] Experimental Example 2 The pretreated cobalt foam was placed in potassium nitrate solutions of different concentrations and allowed to stand for 16 hours before being examined by scanning electron microscopy (SEM). The results are as follows: Figure 3 As shown.

[0044] Based on the above experimental results, in a 0.1 M low-concentration potassium nitrate solution, α-cobalt hydroxide mainly forms on the cobalt surface, exhibiting a fine filamentous or network morphology. When the concentration is increased to 0.2 M, the product phase transitions to be dominated by β-cobalt hydroxide, and the morphology also changes to a typical hexagonal plate-like structure. In the medium concentration range of 0.3-0.8 M, the α and β phases coexist, with the two morphological characteristics intertwined, reflecting the transitional state of the phase transition. When the concentration is further increased to 0.6 M, 0.9 M, and 1.0 M, the product reverts to a hexagonal plate-like structure dominated by β-cobalt hydroxide, and the crystallinity and regularity increase with increasing concentration. Overall, low-concentration environments tend to induce the formation of the α phase, while higher concentrations are more conducive to the formation and growth of the β phase (hexagonal plate-like).

[0045] Experimental Example 3 The pretreated cobalt foam was placed in potassium nitrate solutions of different concentrations and allowed to stand for different times before being examined by scanning electron microscopy (SEM). The results are as follows: Figure 4 As shown.

[0046] The experimental results show that in 0.1 M potassium nitrate solution, after 16 h of reaction, the product is mainly α-cobalt hydroxide, exhibiting a fine fibrous / nanowire interwoven structure. When the reaction time is extended to 2 weeks, the product completely transforms into β-cobalt hydroxide, with the morphology evolving into regular hexagonal plate-like or dish-like crystals, and the size significantly increases. In 0.5 M potassium nitrate solution, after 16 h of reaction, a mixed phase of α and β-cobalt hydroxide is present, with both residual fine fibrous structures and a large number of hexagonal plate-like crystals. As the reaction time is extended to 3 days, the product also completely transforms into β-cobalt hydroxide, with a uniform long hexagonal rod-like / plate-like crystal morphology. This indicates that regardless of the potassium nitrate concentration, with the extension of reaction time, the thermodynamically more stable β phase gradually becomes the dominant product, reflecting the phase transition process from α to β phase.

[0047] Increasing the potassium nitrate concentration accelerates the phase transition process: at a concentration of 0.5 M, a distinct β phase appears after 16 h, while at a concentration of 0.1 M, the phase transition takes much longer. Reaction time is the core factor driving the transformation of cobalt hydroxide from the metastable α phase to the stable β phase. The potassium nitrate concentration mainly plays a role in accelerating the phase transition. After a long reaction time, the products are predominantly β-cobalt hydroxide, and its morphology evolves from the initial fibrous structure to a regular hexagonal sheet-like structure.

[0048] Test Example 4 Cobalt metal was reacted in potassium nitrate solutions of different concentrations, and the rate of ammonia formation was measured. The results are as follows: Figure 5 As shown.

[0049] Based on the above experimental results, it can be seen that in potassium nitrate solutions of different concentrations, the rate of reaction between metallic cobalt and potassium nitrate solution to produce ammonia is in the range of 4-6 micrograms per hour per square centimeter, which is negligible compared with the yield of ammonium ions in the electrocatalytic reduction of nitrate ions, which is tens to hundreds of milligrams per hour per square centimeter.

[0050] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A phase-regulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia, characterized in that, The catalyst is based on cobalt foam, and β-phase cobalt hydroxide is in situ loaded on the surface of the cobalt foam.

2. A method for preparing a phase-modulated cobalt-based catalyst for the electrocatalytic reduction of nitrate to ammonia as described in claim 1, characterized in that, Includes the following steps: Surface pretreatment of cobalt foam; The pretreated cobalt foam was placed in a first nitrate solution and allowed to stand to obtain a cobalt foam precatalyst loaded with cobalt hydroxide. The foamed cobalt precatalyst was prepared and placed in a second nitrate solution for cyclic voltammetric activation to convert α-phase cobalt hydroxide into β-phase cobalt hydroxide, thus obtaining the phase-controlled cobalt-based catalyst.

3. The preparation method according to claim 2, characterized in that, The surface pretreatment includes: ultrasonically cleaning the cobalt foam in a 3-4 mol / L acid solution, followed by ultrasonic cleaning with ethanol and deionized water for 15-20 min each.

4. The preparation method according to claim 2, characterized in that, The nitrate concentration in the first nitrate solution is >0.1 mol / L; the nitrate concentration in the second nitrate solution is 0.1-1 mol / L. The first nitrate solution and the second nitrate solution are adjusted to pH ≥ 7 using an alkaline solution.

5. The preparation method according to claim 4, characterized in that, The nitrate solution is selected from at least one of sodium nitrate, potassium nitrate, lithium nitrate, or cesium nitrate.

6. The preparation method according to claim 4, characterized in that, The alkaline solution is selected from at least one of sodium hydroxide, potassium hydroxide, barium hydroxide, or ammonia water.

7. The preparation method according to claim 2, characterized in that, The settling time is 3 days to 2 weeks.

8. The preparation method according to claim 2, characterized in that, The voltage range of the cyclic voltammetric scan is -0.3V RHE to -1.2V RHE, and the scan rate is 300mV / s.

9. A method for producing ammonia by electrocatalytic reduction of nitrate, characterized in that, Using the phase-controlled cobalt-based catalyst as described in claim 1 as the working electrode, and a mixed solution containing nitrate and alkaline electrolyte as the electrolyte, an electrocatalytic reduction reaction is carried out to produce ammonia from nitrate.

10. The application of the cobalt-based catalyst as described in claim 1 in the electrocatalytic reduction of nitrate to prepare ammonia.

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