Self-growing multi-dimensional cathode, preparation method thereof and application of self-growing multi-dimensional cathode in electrocatalytic nitrate reduction synthesis of ammonia

Abstract

The invention provides a self-growing multi-dimensional cathode, a preparation method thereof and application of the self-growing multi-dimensional cathode in electrocatalytic nitrate reduction synthesis of ammonia. The preparation method comprises the following steps: providing a substrate material; the method comprises the following steps: placing a substrate material in a urea-containing transition metal salt mixed solution, carrying out an in-situ hydrothermal growth reaction, and carrying out in-situ growth of transition metal on the surface of the substrate material; and carrying out heat treatment on the substrate material of which the surface is subjected to in-situ growth of the transition metal to obtain the self-growing multi-dimensional cathode. The prepared self-growing multi-dimensional cathode is of a nano array structure, so that the self-growing multi-dimensional cathode has multiple catalytic active sites and has the advantages of high nitrate electrocatalysis reduction activity, high ammonia conversion rate and the like, an intermediate product nitrite is not accumulated in the electrocatalysis process, and ammonia is recovered in an ammonium salt form. The invention provides a novel material and method for efficiently removing nitrate in sewage and recycling ammonia resources.

Description

Technical Field

[0001] The present invention relates to the technical field of nitrate wastewater treatment and ammonia resource recovery in environmental engineering, and in particular to a self-growing multidimensional cathode and a preparation method thereof and application thereof in electrocatalytic nitrate reduction to synthesize ammonia. Background Art

[0002] Nitrate (NO 3 - ) will cause eutrophication of water bodies and cause human diseases such as methemoglobinemia and cancer, posing a serious threat to ecological security and human health. Traditional methods for removing nitrates from sewage, such as ion exchange and reverse osmosis, have disadvantages such as high cost and the need for post-treatment. In the early days, since most research was based on the removal of nitrates from the environment, the main product of electrocatalytic nitrate reduction was nitrogen gas, which eventually escaped into the atmosphere. In contrast, ammonia, as the basic raw material for the production of nitrogen fertilizers, is crucial to national food security. In addition, ammonia can also be used as a carrier for green energy storage / conversion. Therefore, electrocatalytic nitrate reduction to ammonia is a sustainable method with the advantages of pollutant removal, resource recovery, carbon emission reduction, and energy saving.

[0003] However, the reduction of nitrate to ammonia is a rather complex process involving the transfer of eight electrons and nine protons, resulting in slow reaction kinetics. In order to achieve controllable electrocatalytic synthesis of ammonia, the development of highly selective and active electrocatalysts has become a technical key. Compared with precious metals such as Pt, Rh, and Ru, which are restricted by high prices, current research tends to use low-cost transition metals such as Cu, Fe, Ni, and Zn. At the same time, single metal electrocatalysts focus on the rate-determining step of nitrate reduction to nitrite in the deoxygenation step, but their electrocatalytic activity and ammonia selectivity are insufficient. Therefore, current research focuses on bimetallic catalysts composed of a starting metal (enhancing the adsorption of nitrate and promoting the reduction of nitrate) and a second metal (enhancing the proton adsorption capacity and promoting the formation of product ammonia), using the synergistic effect between the bimetallics to further improve the performance of the electrocatalyst.

[0004] However, bimetallic catalysts still have some disadvantages: 1) The substrate used in the bimetallic preparation process is usually a uniform porous material, such as mesoporous carbon, and the selection of the substrate has certain limitations; 2) Existing studies mostly use electrodeposition technology or bonding technology to load bimetallic catalysts onto electrode substrates, but the catalytic electrodes prepared by electrodeposition are unstable and easy to fall off; bonding technology mostly uses Nafion solution as a binder, which reduces the conductivity of the catalytic electrode, and the catalyst is easy to fall off after long-term operation. Summary of the invention

[0005] In order to solve the above technical problems, the present invention provides a self-growing multidimensional cathode and a preparation method thereof and its application in electrocatalytic nitrate reduction to synthesize ammonia. The self-growing multidimensional cathode prepared by the present invention presents a nano-array structure and thus has multiple catalytic active sites, has the advantages of high electrocatalytic nitrate reduction activity and high ammonia conversion rate, and has no accumulation of intermediate product nitrite in the electrocatalytic process, and recovers ammonia in the form of ammonium salt. The present invention provides new materials and methods for efficient removal of nitrate from sewage and recovery of ammonia resources.

[0006] The first object of the present invention is to provide a method for preparing a self-grown multidimensional cathode, comprising the following steps:

[0007] providing a base material;

[0008] Placing the substrate material in a mixed solution of a transition metal salt containing urea to perform an in-situ hydrothermal growth reaction to in-situ grow a transition metal on the surface of the substrate material;

[0009] The substrate material with transition metal in-situ grown on the surface is subjected to heat treatment to obtain the self-grown multi-dimensional cathode.

[0010] In some embodiments of the present invention, the substrate material is selected from one or more of carbon paper, carbon nanotubes, carbon cloth, carbon felt, graphene, foam carbon, foam copper, foam nickel, foam iron, and stainless steel mesh.

[0011] In some embodiments of the present invention, the base material is ultrasonically cleaned, and the cleaning solution includes acetone, anhydrous ethanol, hydrochloric acid, and deionized water; the cleaned base material is placed in a vacuum drying oven at about 40-70° C. and dried for 12.0-48.0 hours.

[0012] In some embodiments of the present invention, the metal in the transition metal salt is selected from one or more of iron, cobalt, nickel, copper, and zinc; the salt of the transition metal salt is selected from one or more of chlorides, sulfates, nitrates, and acetates.

[0013] In some embodiments of the present invention, the molar ratio of urea to transition metal salt in the mixed solution is 20:(1-12).

[0014] The solvent of the mixed solution includes anhydrous ethanol and deionized water, wherein the molar ratio of anhydrous ethanol to deionized water is 1:(0.5-2.0).

[0015] In some embodiments of the present invention, the conditions of the in-situ hydrothermal growth reaction are: reaction temperature of 50 to 300° C., and reaction time of 1.0 to 24.0 h.

[0016] In some embodiments of the present invention, the heat treatment conditions are: a heating rate of 2 to 10°C / min, a temperature of 200 to 800°C, a time of 1.0 to 6.0h, and also include natural cooling to room temperature; it also includes natural cooling to room temperature, taking out and washing with deionized water and anhydrous ethanol three times respectively, placing it in a 40 to 90°C oven to dry for 1.5 to 3.0h and then cooling to room temperature for use.

[0017] The second object of the present invention is to provide a self-grown multidimensional cathode obtained by the preparation method described above. The cathode has a nanoarray structure and grows directly on a conductive substrate without the need for a binder. The strong contact interface between the nanoarray material and the substrate material can efficiently transmit electrons and reduce the reaction energy barrier.

[0018] The third object of the present invention is to provide an electrocatalytic reaction system using the self-grown multidimensional cathode as the cathode.

[0019] In some embodiments of the present invention, the electrocatalytic reaction system comprises a cathode chamber, an anode chamber, a desalination chamber, a secondary cathode chamber, and a secondary anode chamber, and can be amplified by stacking amplification;

[0020] The anode chamber and the cathode chamber are separated by a gas-permeable hydrophobic membrane, the anode chamber and the brine chamber are separated by an anion exchange membrane, the cathode chamber and the brine chamber are separated by a cation exchange membrane, the brine chamber and the secondary cathode chamber are separated by a cation exchange membrane, and the brine chamber and the secondary anode chamber are separated by an anion exchange membrane.

[0021] The self-grown multi-dimensional cathode is the cathode, the (auxiliary) anode is an iridium tantalum titanium anode, and the auxiliary cathode is a stainless steel mesh.

[0022] The nitrate wastewater enters the cathode chamber, the anode chamber contains a low-concentration ammonium salt electrolyte, the sub-anode chamber and the sub-cathode chamber contain a low-concentration brine electrolyte, and the brine chamber contains a high-concentration brine electrolyte.

[0023] The fourth object of the present invention is to provide an application of the electrocatalytic reaction system in electrocatalytic reduction of nitrates in wastewater to synthesize ammonia.

[0024] In some embodiments of the present invention, the electrocatalysis is operated in a constant current density or constant voltage mode, and the constant current is 0-100 mA / cm 2 , the constant voltage is 0~25V, and none of them is 0;

[0025] The nitrate concentration in sewage nitrate is not less than 1mM;

[0026] Nitrates come from municipal sewage, industrial sewage (in the fields of petrochemical industry, metallurgy, explosives, fertilizer, pectin production and nuclear industry), and agricultural sewage.

[0027] The above technical solution of the present invention has the following advantages compared with the prior art:

[0028] The present invention provides a method for preparing a self-growing multidimensional cathode for electrocatalytic nitrate reduction to synthesize ammonia and its application, using at least one of carbon paper, carbon nanotubes, carbon cloth, carbon felt, graphene, foam carbon, foam copper, foam nickel, foam iron, and stainless steel mesh as a substrate material, and modifying the self-growing multidimensional cathode by a physical and chemical method, wherein one or more transition metal catalytic active sites on the self-growing multidimensional cathode are focused on *NO 3 Restore to *NO 2 The rate-determining step of the reduction of *NO to *N and the selectivity-determining step of the hydrogenation of *N to *NH 3 The active hydrogen determines the step, which can reduce the nitrate pollutants in the sewage to ammonia, thereby achieving the removal of nitrate nitrogen pollutants and the recovery of ammonia resources. The new electrode has a nitrate removal efficiency of about 100% and a selectivity of more than 90% for the conversion of nitrate to ammonia, solving the problems of low catalytic electrode activity and low ammonia selectivity. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific embodiments of the present invention in conjunction with the accompanying drawings, wherein:

[0030] Figure 1 This is a preparation flow chart of the self-grown multidimensional cathode obtained in Example 1 of the present invention.

[0031] Figure 2 Figure a is a SEM image of the self-grown multidimensional cathode loaded with copper and zinc metals obtained in Example 1 of the present invention. Figure 2 (b) is the SEM image of the untreated stainless steel cathode.

[0032] Figure 3 Schematic diagram of the electrocatalytic device used in the present invention.

[0033] Figure 4 It is a diagram showing the nitrate removal efficiency of treating nitrate wastewater in Test Example 1 of the present invention.

[0034] Figure 5 It is a diagram showing the ammonia conversion efficiency of treating nitrate wastewater in Test Example 1 of the present invention.

[0035] Figure 6 It is a diagram showing the ammonia recovery efficiency of treating nitrate wastewater in Test Example 1 of the present invention. DETAILED DESCRIPTION

[0036] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments so that those skilled in the art can better understand the present invention and implement it, but the embodiments are not intended to limit the present invention.

[0037] Embodiment 1:

[0038] This embodiment provides a method for preparing a self-growing multi-dimensional cathode for electrocatalytic nitrate reduction to synthesize ammonia and its application, as follows (see the specific preparation process for details). Figure 1 ):

[0039] 1. Provide a base material using a stainless steel mesh (200 mesh, 0.1 mm thick, 4.0 cm×4.0 cm, model 304), and use 1M HCl solution, acetone, ethanol and deionized water to assist ultrasonic cleaning of the stainless steel mesh for 0.5 h in turn to obtain a pretreated stainless steel mesh.

[0040] 2. Add 20mmol urea and 6mmol CuCl 2 ·2H 2 O, 6mmol ZnCl 2 Dissolve in 60 mL of mixed solution (30 mL of anhydrous ethanol + 30 mL of deionized water) under magnetic stirring to obtain a transparent and uniform hydrothermal solution.

[0041] 3. Transfer the pretreated stainless steel mesh obtained in step 1 and the hydrothermal solution obtained in step 2 to a 100 mL reactor lined with polytetrafluoroethylene, and place the reactor in a 90°C oven for hydrothermal reaction for 4.0 hours. After the reaction is completed and cooled naturally, a stainless steel mesh loaded with Cu and Zn bimetallic catalysts is obtained. Take it out and wash it with deionized water and ethanol three times respectively.

[0042] 4. Place the stainless steel mesh loaded with Cu and Zn bimetallic catalysts obtained in step 3 in an oven and dry at 60°C for 2.0 h; then place the dried stainless steel mesh loaded with Cu and Zn bimetallic catalysts in a muffle furnace and heat at 10°C min. -1 The temperature was raised to 300°C at a heating rate of 1.5°C and maintained at 300°C for 2.0 h; the stainless steel mesh loaded with Cu and Zn bimetallic catalysts after calcination in the muffle furnace, i.e., the self-grown multidimensional cathode, was taken out and naturally cooled to room temperature for use, thereby obtaining Cu 6 Zn 6 @Stainless steel cathode. The obtained self-grown multidimensional cathode was structurally characterized ( Figure 2 a), and compared with the untreated stainless steel cathode morphology ( Figure 2 In b), it can be seen that the stainless steel mesh structure is completely covered by the three-dimensional nanostructured nanosheets.

[0043] Embodiment 2:

[0044] This embodiment provides a method for preparing a self-growing multi-dimensional cathode for electrocatalytic nitrate reduction to synthesize ammonia and its application, as follows:

[0045] 1. Provide a base material using a stainless steel mesh (200 mesh, 0.1 mm thick, 4.0 cm×4.0 cm, model 304), and use 1M HCl solution, acetone, ethanol and deionized water to assist ultrasonic cleaning of the stainless steel mesh for 0.5 h in turn to obtain a pretreated stainless steel mesh.

[0046] 2. Add 20mmol urea and 6mmol CuCl 2 ·2H 2 O was dissolved in 60 mL of a mixed solution (30 mL of anhydrous ethanol + 30 mL of deionized water) under magnetic stirring to obtain a transparent and uniform hydrothermal solution.

[0047] 3. The pretreated stainless steel mesh obtained in step 1 and the hydrothermal solution obtained in step 2 were transferred to a 100 mL reactor lined with polytetrafluoroethylene, and the reactor was placed in a 90°C oven for hydrothermal reaction for 4.0 h. After the reaction was completed and cooled naturally, a stainless steel mesh loaded with Cu metal catalyst was obtained. It was taken out and washed 3 times with deionized water and ethanol respectively.

[0048] 4. Place the stainless steel mesh loaded with Cu monometallic catalyst obtained in step 3 into an oven and dry it at 60°C for 2.0h; then place the dried stainless steel mesh loaded with Cu monometallic catalyst into a muffle furnace and heat it at 10°C·min -1 The temperature was raised to 300°C at a heating rate of 1.5°C and maintained at 300°C for 2.0 h; the stainless steel mesh loaded with Cu single metal catalyst after calcination in the muffle furnace, i.e., the self-grown multidimensional cathode, was taken out and naturally cooled to room temperature for use, thereby obtaining Cu 6 @Stainless steel cathode.

[0049] Performance test example:

[0050] This embodiment provides a stacked electrocatalytic nitrate reduction coupled ammonia recovery system, the structure of which is as follows: Figure 3As shown, it comprises a cathode chamber, an anode chamber, two desalination chambers, a secondary cathode chamber, and a secondary anode chamber. The cathode chamber is injected with simulated nitrate wastewater (15 mL) prepared with deionized water, and the simulated nitrate wastewater contains 100 mM sodium nitrate. The anode chamber is injected with 6 mM ammonium sulfate solution (15 mL). The two desalination chambers are each injected with 150 mM sodium sulfate solution (15 mL). The secondary cathode chamber and the secondary anode chamber are each injected with 500 mM sodium sulfate solution (15 mL). The (secondary) anode uses an iridium tantalum titanium anode (4.0 cm × 4.0 cm), the cathode uses the self-grown multidimensional cathode (4.0 cm × 4.0 cm) obtained in Example 1 and Example 2, respectively, and the secondary cathode uses a stainless steel mesh (200 mesh, 0.1 mm thick, 4.0 cm × 4.0 cm, model 304). The cation exchange membrane (4.0 cm × 4.0 cm) used in the system was CMI-7000, Membrane International, the anion exchange membrane (4.0 cm × 4.0 cm) was AMI-7001, Membrane International, and the gas permeable hydrophobic membrane (4.0 cm × 4.0 cm) was made of polytetrafluoroethylene with a polypropylene support layer. An electrochemical workstation (VoltLab 40PGZ301) was used to apply 6.25 mA / cm 2 Constant current, under this constant current condition, the experiment of electrocatalytic nitrate wastewater coupled ammonia recovery was carried out. The experimental results are shown in Figure 4-6 .

[0051] Nitrate removal effect Figure 4 As shown, after 12.0 h of electrolysis reaction, the stainless steel cathode (Cu 6 Zn 6 @ stainless steel cathode) and the stainless steel cathode loaded with Cu metal catalyst prepared in Example 2 (Cu 6 @Stainless steel cathode) has a nitrate degradation efficiency of 99.9%. From the whole reaction process in the figure, the difference between the two in the effect of nitrate reduction is relatively small.

[0052] The conversion efficiency of nitrate reduction to ammonia is Figure 5 As shown, the stainless steel cathode (Cu 6 Zn 6 @ stainless steel cathode) and the stainless steel cathode loaded with Cu metal catalyst prepared in Example 2 (Cu 6 @ stainless steel cathode) for nitrate to ammonia conversion efficiency of 89.4% and 69.4% respectively. Therefore, compared with only Cu metal loading, the stainless steel cathode loaded with Cu and Zn bimetallic can better promote the directional conversion of nitrate to ammonia.

[0053] Ammonia recovery efficiency Figure 6 As shown, the stainless steel cathode (Cu 6 Zn 6 @ stainless steel cathode) and the stainless steel cathode loaded with Cu metal catalyst prepared in Example 2 (Cu 6 @ stainless steel cathode) for ammonia product recovery efficiency was 100.0% and 99.6% respectively. During the whole reaction process, compared with the simultaneous loading of Cu and Zn bimetallic, the ammonia recovery efficiency of the stainless steel cathode loaded with Cu metal was slightly lower, but as the reaction time continued to extend, the effects of the two tended to be similar.

[0054] It can be seen that the self-growing multidimensional cathode preparation method provided by the present invention can simultaneously achieve the dual goals of nitrate wastewater treatment and ammonia resource recovery.

[0055] Obviously, the above embodiments are merely examples for clear explanation and are not intended to limit the implementation methods. For those skilled in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all the implementation methods here. The obvious changes or modifications derived from these are still within the protection scope of the invention.

Claims

1. A method for preparing a self-grown multidimensional cathode, characterized in that: The following steps are involved: providing a base material; Placing the substrate material in a mixed solution of a transition metal salt containing urea to perform an in-situ hydrothermal growth reaction to in-situ grow a transition metal on the surface of the substrate material; The substrate material with transition metal in-situ grown on the surface is subjected to heat treatment to obtain the self-grown multi-dimensional cathode.

2. The preparation method according to claim 1, characterized in that: The substrate material is selected from one or more of carbon paper, carbon nanotubes, carbon cloth, carbon felt, graphene, foam carbon, foam copper, foam nickel, foam iron, and stainless steel mesh; The base material is ultrasonically cleaned, and the cleaning solution includes acetone, anhydrous ethanol, hydrochloric acid and deionized water.

3. The preparation method according to claim 1, characterized in that: The metal in the transition metal salt is selected from one or more of iron, cobalt, nickel, copper and zinc; the salt of the transition metal salt is selected from one or more of chloride, sulfate, nitrate and acetate.

4. The preparation method according to claim 1, characterized in that: The molar ratio of urea to transition metal salt in the mixed solution is 20:(1-12).

5. The preparation method according to claim 1, characterized in that: The conditions of the in-situ hydrothermal growth reaction are as follows: the reaction temperature is 50 to 300° C., and the reaction time is 1.0 to 24.0 h.

6. The preparation method according to claim 1, characterized in that: The heat treatment conditions are as follows: a heating rate of 2 to 10°C / min, a temperature of 200 to 800°C, and a time of 1.0 to 6.0h.

7. A self-grown multidimensional cathode, characterized in that: Obtained by the preparation method according to any one of claims 1 to 6, the self-grown multi-dimensional cathode has a nano-array structure.

8. An electrocatalytic reaction system, characterized in that: The self-grown multi-dimensional cathode described in claim 8 is used as the cathode.

9. The application of the electrocatalytic reaction system described in claim 8 in the electrocatalytic reduction of nitrates from wastewater to synthesize ammonia.

10. The use according to claim 9, characterized in that Electrocatalysis is operated at a constant current density or constant voltage, with a constant current of 0 to 100 mA / cm 2 , the constant voltage is 0~25V, and none of them is 0; The nitrate concentration in sewage nitrate is not less than 1mM.

Images