Process for synthesizing ammonia by coupling low-temperature plasma with electrocatalysis

Abstract

The invention discloses a process for synthesizing ammonia by coupling low-temperature plasma with electro-catalysis in the field of clean energy and green chemical industry, which comprises the following specific steps: mixing air and water vapor, feeding the mixture into a dielectric barrier discharge reactor, filling the reactor with a multi-stage heterojunction gradient doped carbon coated catalyst, generating active gas containing NOx under pulse high voltage, and condensing and then entering an electrocatalytic reduction unit. And an electro-catalysis product enters a photo-thermal separation module integrated with a Janus type photo-thermal electrode, the temperature of the electrode is locally increased under visible light irradiation, NH3 desorption is promoted, and finally high-purity ammonia is obtained through polyimide membrane separation. The multistage heterojunction gradient-doped carbon-coated catalyst adopted in the process is prepared through multiple steps and contains gradient-doped boron, sulfur and Au nanoparticles on the surface, NOx adsorption, electron transport and sintering resistance are synergistically improved, and efficient ammonia synthesis under mild conditions is achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of clean energy and green chemical technology, and in particular to a process for synthesizing ammonia by low-temperature plasma coupled with electrocatalysis. BACKGROUND

[0002] Synthetic ammonia is an important chemical raw material in the fields of agriculture, medicine, etc., and its production technology has long relied on traditional processes. Traditional synthetic ammonia mostly uses chemical reactions under high temperature and high pressure, which consumes a large amount of energy and releases a large amount of carbon dioxide, causing significant environmental burden. With the promotion of the concept of green development, developing efficient and low-carbon synthetic ammonia technology at room temperature and atmospheric pressure has become an important direction.

[0003] In recent years, the technology of combining plasma with electrocatalysis has provided a new path for the synthesis of ammonia. The core idea is to first convert part of the nitrogen in the air into nitrogen oxides by non-thermal plasma, and then reduce these nitrogen oxides to ammonia using an electrocatalyst. This technology does not require high temperature and high pressure, and theoretically can greatly reduce energy consumption. However, in practical applications, there are still many challenges: in the plasma activation stage, the nitrogen molecule structure is stable, and traditional discharge methods are difficult to activate efficiently, resulting in low nitrogen oxide yield; in the electrocatalytic reduction stage, the adsorption ability of nitrogen oxides on the electrode surface is weak, and it is easy to react with water or hydrogen ions to generate hydrogen, resulting in low actual ammonia yield; in addition, the catalyst is easy to sinter and deactivate due to high temperature or oxidation, and its performance decays significantly after long-term operation.

[0004] To address these issues, researchers have tried various improvement methods. For example, by designing a special electrode structure to utilize the photo-thermal effect to improve the separation efficiency of ammonia, or using new plasma technology to reduce the energy consumption of nitrogen oxide generation, but these methods still have limitations: the photo-thermal electrode relies on external light sources, and the energy utilization efficiency is limited by light conditions; the synergy of new plasma technology and electrocatalytic units is not good; the existing catalysts have insufficient active sites and weak electron transport capacity, making it difficult to meet the needs of high-efficiency adsorption of nitrogen oxides and low side reactions. Therefore, developing a catalyst with high activity, high selectivity, and long service life, and optimizing the synergy mechanism of plasma and electrocatalysis, is the key to promoting the practicality of this technology. SUMMARY

[0005] The present application aims to provide a process for synthesizing ammonia by low-temperature plasma coupled with electrocatalysis, which solves the problems of high temperature, high pressure, high energy consumption, and high carbon emissions in existing synthetic ammonia, low plasma activation efficiency, poor electrocatalytic selectivity, and catalyst deactivation.

[0006] The present application achieves the above-mentioned objectives through the following technical solutions:

[0007] A process for synthesizing ammonia by low-temperature plasma coupled with electrocatalysis, comprising the following steps:

[0008] S1, the air mixed with water vapor enters the dielectric barrier discharge reactor, the reactor is filled with multi-level heterojunction gradient doped carbon-coated catalyst, and active gas containing NO X is generated under pulse high pressure; the active gas enters the electrocatalytic reduction unit after condensation;

[0009] S2, the electrocatalytic reduction unit in step S1 is constructed into a three-electrode system with multi-level heterojunction gradient doped carbon-coated catalyst as the cathode, IrO2-Ta2O5 composite oxide as the anode, and Ag / AgCl as the reference electrode; the electrolyte includes KOH and KNO2; the external voltage is (-0.5- -0.7) V vs. RHE, and NO 2 is reduced to NH3 under a pulse current density of 100-120 mA / cm x .

[0010] S3, the electrocatalytic product enters the photothermal separation module, and the Janus photo-thermal electrode is integrated in the photothermal separation module; under visible light irradiation, the electrode is locally heated to 60-70℃.

[0011] According to the preferred embodiment of the present application, in step S1, the voltage of the pulse high voltage is 15-25 kV, and the frequency is 5-10 kHz; the condensation temperature is 5-10℃.

[0012] In step S1 of the present application, the air mixed with water vapor enters the dielectric barrier discharge reactor, and the multi-level heterojunction gradient doped carbon-coated catalyst filled in the reactor is the key medium. Under the action of pulse high pressure, non-thermal plasma is generated in the reactor, which is rich in active particles such as high-energy electrons and free radicals. Nitrogen molecules are difficult to directly react due to high bond energy, but under the synergistic action of the multi-level heterojunction structure (such as metal-nonmetal, interface between different metals) and the gradient doped elements (such as boron and sulfur) on the catalyst surface, the chemical bond of N2 molecules is weakened. Specifically, the high-density active sites of the multi-level heterojunction adsorb N2 molecules, boron (B) as a Lewis acid provides electrons, attracts the lone pair electrons of N2, and reduces the dissociation energy barrier of N≡N bond; sulfur (S) adjusts the electron cloud density of the catalyst surface to promote the reaction of N2 with water vapor or oxygen. Finally, N2 is activated and converted into active gas containing nitrogen oxides (NO x ) such as nitric oxide and nitrogen dioxide. After removing water vapor by condensation (5-10℃), the active gas enters the electrocatalytic reduction unit.

[0013] According to the preferred embodiment of the present application, in step S2, the concentration of KOH in the electrolyte is 0.08-0.12 mol / l, and the concentration of KNO2 is 0.04-0.06 mol / l; the pulse current frequency is 18-22 Hz.

[0014] In step S2 of the present application, the activated NO x into an electrocatalytic reduction unit, which is constructed as a three-electrode system with a multi-stage heterojunction gradient doped carbon-coated catalyst as the cathode, an IrO2-Ta2O5 composite oxide as the anode, and Ag / AgCl as the reference electrode. KOH and KNO2 in the electrolyte provide an ionic environment and maintain charge balance. The external voltage (-0.5 to -0.7 V vs. RHE) and the pulse current density (100-120 mA / cm 2 ) drive electrons from the cathode to the NO x transfer: N(+2 to +3 valence) in NO x obtains electrons and undergoes a multi-step hydrogenation reaction (such as NO→N2O→N2→NH3 or NO2→HNO2→NH3), ultimately being reduced to NH3. The multi-stage heterojunction gradient doped carbon-coated catalyst plays a key role in this process: its heterojunction structure (such as the interface of metal phosphides such as Ni-Co-Fe-P) forms an internal electric field, accelerating electron migration; the gradient doping of boron (B) and sulfur (S) further regulates the electronic structure of the active sites, reducing the adsorption energy of NO x (more easily bonded), while inhibiting the occurrence of the hydrogen evolution reaction (HER) (the path of H + reduction to H2 is blocked). The carbon coating layer (thickness 8-12 nm) acts like a "protective shell", preventing the oxidation or sintering of the metal active sites during the reaction, ensuring long-term stability of the catalyst.

[0015] According to a preferred embodiment of the present application, in step S3, the surface of the Janus-type photo-thermal electrode is loaded with Au nanoparticles; the wavelength of visible light is λ = 400-600 nm.

[0016] In step S3 of the present application, the electrocatalytic products (containing NH3, unreacted NO x , and a small amount of water) enter the photo-thermal separation module, and the Janus-type photo-thermal electrode integrated in the module is the key. The Au nanoparticles (particle size 10-15 nm) loaded on the surface of the electrode produce a surface plasmon resonance (SPR) effect under visible light (λ = 400-600 nm) irradiation, exciting a local electromagnetic field enhancement (electric field intensity increased by 3 times), while absorbing light energy and converting it into heat energy, causing the electrode to locally heat up to 60-70°C. The temperature rise promotes the desorption of NH3 molecules from the catalyst surface (NH3 partial pressure increased to 15-20 kPa), while unreacted NO x remains in the gaseous state due to its higher boiling point. Subsequently, the products are separated by a polyimide membrane: the selective permeation properties of the membrane separate NH3 from other gases (such as unreacted NO x , H2O vapor), ultimately obtaining NH3 with a purity of >99.9%. The separated NO x can be recycled back to the plasma activation unit, improving the utilization rate of raw materials.

[0017] According to the preferred embodiment of the present application, the process for synthesizing ammonia by low-temperature plasma coupled electrocatalysis further comprises: separating the product after photo-thermal separation through a polyimide membrane.

[0018] According to the preferred embodiment of the present application, the preparation step of the multi-stage heterojunction gradient doped carbon-coated catalyst comprises:

[0019] A1, the foam nickel is sequentially cleaned with concentrated hydrochloric acid, ethanol and deionized water by ultrasonic cleaning; the cleaned foam nickel is immersed in a mixed solution containing nickel nitrate, cobalt nitrate and iron nitrate, and is stirred and reacted at 60-62 DEG C to form a Ni-Co-Fe hydroxide precursor on the surface of the foam nickel; the foam nickel is taken out and vacuum dried to obtain a multi-metal hydroxide support;

[0020] A2, the multi-metal hydroxide support is immersed in a mixed solution containing sodium hypophosphite, boric acid and sodium sulfide, and the pH of the mixed solution is adjusted to 8.9-9.1 with NaOH, and is reacted at 150-152 DEG C; after the reaction is completed, the support is taken out and dried at 80-82 DEG C, and then is placed in a tube furnace and heated to 450-452 DEG C under Ar / H2 mixed atmosphere to form a precursor;

[0021] A3, the precursor is immersed in a glucose solution and ultrasonically dispersed; then is transferred to a sealed reaction kettle and is heat-reacted at 180-182 DEG C; after the reaction is completed, is naturally cooled to room temperature, is washed with deionized water until neutral, and is dried at 60-62 DEG C to obtain a carbon-coated intermediate;

[0022] A4, the carbon-coated intermediate is immersed in a chloroauric acid solution and ultrasonically dispersed, and Au nanoparticles are loaded on the surface of the carbon layer by a photoreduction method; after the reaction is completed, the unreacted Au precursor is removed by washing with deionized water, and is dried at 60-62 DEG C; finally, the sample is placed in a plasma reactor for treatment.

[0023] According to the preferred embodiment of the present application, in step A1, the ultrasonic cleaning time is 30-40 min; the concentration of nickel nitrate in the mixed solution is 0.15-0.25 mol / l, the concentration of cobalt nitrate is 0.08-0.12 mol / l, and the concentration of iron nitrate is 0.04-0.06 mol / l; the stirring and reaction time is 2-4 h; the thickness of the hydroxide precursor is 50-80 μm; the vacuum drying temperature is 60-62 DEG C, and the time is 12-24 hours.

[0024] In step A1 of the present application, the foamed nickel as the initial carrier needs to be strictly pretreated on the surface to remove impurities and increase active sites. By ultrasonic cleaning with concentrated hydrochloric acid, ethanol and deionized water, the oxide layer on the surface of the foamed nickel is removed to expose the highly active metal substrate. Then, it is immersed in a mixed solution containing nickel, cobalt and iron metal salts, and stirred at 60-62 DEG C for reaction. Metal ions hydrolyze to generate a multi-metal hydroxide precursor composed of nickel hydroxide, cobalt hydroxide and iron hydroxide on the surface of the foamed nickel. The precursor structure is loose and porous, and the thickness is controlled at 50-80 μm, which provides abundant attachment sites for subsequent doping and coating, and the interfacial effect between metals lays a foundation for electron transfer and active site formation.

[0025] According to the preferred embodiment of the present application, in step A2, the concentration of sodium hypophosphite in the mixed solution is 0.25-0.35 mol / l, the concentration of boric acid is 0.08-0.12 mol / l, and the concentration of sodium sulfide is 0.04-0.06 mol / l; the reaction time is 6-8 hours; the drying time is 6-8 hours; the volume ratio of Ar and H2 in the mixed atmosphere is 9:1; the heating rate is 3-4 DEG C / min; and the holding time is 4-6 hours.

[0026] In step A2 of the present application, the multi-metal hydroxide precursor needs to be further doped and structurally reinforced. It is immersed in a mixed solution containing sodium hypophosphite, boric acid and sodium sulfide, the pH is adjusted to weak alkaline (pH = 8.9-9.1), and reacted at 150-152 DEG C for 6-8 hours. Sodium hypophosphite as the phosphorus source provides phosphorus ions (P 3- ) to combine with metal ions in the precursor to form metal phosphides; boric acid and sodium sulfide respectively provide boron (B 3+ ) and sulfur (S 2- ) ions to be embedded in the crystal lattice of the metal phosphides by ion exchange to form a structure doped with boron and sulfur gradients. Then, it is dried at 80-82 DEG C for 6-8 hours, and placed in a tube furnace to be heated to 450-452 DEG C under a mixed atmosphere of argon and hydrogen for 4-6 hours. The presence of hydrogen promotes the reduction of part of the metal oxides to elementary substances or low-valence compounds, and finally forms a multi-level heterojunction structure containing a main phase of Ni-Co-Fe-P and a boron-sulfur doped phase. In this structure, the interface between different metals and doping elements forms a built-in electric field to accelerate electron migration, and boron and sulfur as Lewis acids and electron donors regulate the electronic structure of the active sites to enhance the adsorption capacity of nitrogen oxides.

[0027] According to the preferred embodiment of the present application, in step A3, the ultrasonic dispersion time is 30-40 min; the heat reaction time is 12-14 h; and the drying time is 12-14 h.

[0028] In step A3 of the present application, in order to prevent the metal active sites from being oxidized or sintered in the subsequent reaction, a carbon coating layer needs to be constructed on the surface of the precursor. The above product is immersed in a glucose solution and ultrasonically dispersed for 30-40 minutes to make glucose molecules uniformly adsorbed on the surface; then it is transferred to a sealed reaction kettle and heated at 180-182℃ for 12-14 hours. Glucose is dehydrated and carbonized at high temperature to form an amorphous carbon layer (thickness 8-12 nm) tightly wrapping the metal phosphide particles. After natural cooling, the product is washed with deionized water until neutral, and dried at 60-62℃ for 12-14 hours to remove residual carbonization by-products. The carbon coating layer not only isolates oxygen and moisture from the metal sites, but also improves the electron transport efficiency through its high electrical conductivity (nitrogen-doped), and the flexibility of the carbon layer can buffer the volume change during the reaction, significantly prolonging the service life of the catalyst.

[0029] According to a preferred embodiment of the present application, in step A4, the ultrasonic dispersion time is 10-12 min; the light wavelength of the photoreduction method is 450-455 nm, the power is 50-52 mW / cm 2 , and the time is 30-40 min; the drying time is 6-8 h; the power of the plasma reactor is 100-120 W; and the treatment time is 10-12 min.

[0030] In step A4 of the present application, in order to further improve the adsorption and reduction capacity of the catalyst for nitrogen oxides, Au nanoparticles need to be loaded on the surface of the carbon coating layer and subjected to plasma treatment. The carbon-coated intermediate is immersed in a chloroauric acid solution and ultrasonically dispersed for 10-12 minutes to make gold ions (Au 3+ ) adsorbed on the surface of the carbon layer; then through the photoreduction method (light wavelength 450-455 nm, power 50-52 mW / cm 2 , time 30-40 minutes), under visible light excitation, the gold ions are reduced to elemental gold nanoparticles (particle size 10-15 nm) uniformly distributed on the surface of the carbon layer. Finally, the sample is placed in a plasma reactor, treated at a power of 100-120 W for 10-12 minutes, and high-energy particles in the plasma bombard the surface to introduce oxygen vacancies and nitrogen defects, further enhancing the surface activity. Au nanoparticles act as electron traps, can capture NO x molecules and provide additional active sites; oxygen vacancies and nitrogen defects reduce the adsorption energy barrier of NO x , making it easier for NO x to combine with electrons to undergo reduction.

[0031] The present application has the following beneficial effects:

[0032] The process significantly improves the efficiency and economy of ammonia synthesis through multi-unit collaborative innovation. Traditional ammonia synthesis relies on high temperature and high pressure conditions, which is high in energy consumption and high in carbon emissions. The present application adopts a coupling mechanism of plasma activation and electrocatalytic reduction, avoiding the requirement of extreme conditions. The multi-level heterojunction gradient doped carbon-coated catalyst filled in the plasma reactor can efficiently activate nitrogen in air under pulsed high pressure to generate active gas containing nitrogen oxides; the electrocatalytic unit converts these nitrogen oxides into ammonia through cathodic reduction, and the photo-thermal separation module promotes the desorption of ammonia, reducing the interference of side reactions, and finally obtaining high-purity ammonia. Compared with traditional processes, the process can significantly reduce energy consumption, and significantly improve ammonia yield and selectivity, providing a feasible path for ammonia synthesis under mild conditions.

[0033] The core advantage of the process is the design of the multi-level heterojunction gradient doped carbon-coated catalyst. The catalyst forms a multi-level heterojunction structure and gradient doped active sites through a special preparation process, which can not only efficiently adsorb nitrogen oxides, but also reduce the reaction activation energy and accelerate the generation of ammonia; the carbon coating layer on the surface acts as a "protective shell" to inhibit the sintering and oxidation of the metal active sites, prolonging the service life of the catalyst. In addition, the nanoparticles loaded on the surface of the catalyst further optimize the electron transport capacity, making the reduction process of nitrogen oxides more complete. These characteristics work together to keep the catalyst active in long-term operation, solving the problem of easy deactivation of traditional catalysts.

[0034] The industrial application potential of the process is outstanding. The multi-unit coupling design realizes the integration of the whole process of "activation-reduction-separation", reducing the loss of intermediate products and energy waste; the photo-thermal separation module promotes the efficient desorption of ammonia through local heating by visible light excitation, without the need for additional high temperature or high pressure conditions; the long service life of the catalyst reduces maintenance costs. Experimental verification shows that the process has stable ammonia yield and high product purity when continuously running, and can be directly used in the fields of chemical fertilizers, medicines, etc. The popularization of this technology will promote the transformation of the ammonia synthesis industry to green and low carbon, and provide an effective solution to the environmental and energy consumption problems of traditional processes. DETAILED DESCRIPTION

[0035] The following detailed description is only used to further illustrate the present application and cannot be understood as limiting the scope of protection of the present application. Those skilled in the art can make some non-essential improvements and adjustments to the present application based on the above application content.

[0036] I. Example

[0037] Example 1

[0038] The process for synthesizing ammonia by low-temperature plasma coupled electrocatalysis comprises the following steps: first, a multi-level heterojunction gradient doped carbon-coated catalyst is prepared. A 10 cm x 10 cm x 2 mm nickel foam (porosity 95%) is sequentially cleaned by ultrasonic cleaning with concentrated hydrochloric acid (mass fraction 37%, volume 500 mL, ultrasonic power 50 W, cleaning time 30 minutes), ethanol (volume fraction 99.7%, volume 500 mL, ultrasonic power 40 W, cleaning time 30 minutes), and deionized water (resistivity ≥ 18 MΩ·cm, volume 500 mL, ultrasonic power 40 W, cleaning time 30 minutes) to remove the surface oxide layer, organic matter, and residual impurities. After nitrogen blowing and drying, the nickel foam is immersed in an ethanol-water mixed solution (volume ratio 1:1, total volume 1 L) containing nickel nitrate (0.2 mol / L), cobalt nitrate (0.1 mol / L), and iron nitrate (0.05 mol / L). The solution is magnetically stirred (speed 300 rpm) in a 61°C constant-temperature water bath for 3 hours to form a Ni-Co-Fe hydroxide precursor with a thickness of about 65 μm on the surface. The multi-metal hydroxide-loaded substrate is obtained by vacuum drying (vacuum degree ≤-0.09 MPa) at 65°C for 18 hours. The substrate is immersed in a deionized water solution (pH = 9.0, adjusted by NaOH) containing sodium hypophosphite (0.3 mol / L), boric acid (0.1 mol / L), and sodium sulfide (0.05 mol / L). The solution is reacted in a constant-temperature oven at 151°C for 7 hours (boiling state). After vacuum drying at 81°C for 7 hours, the precursor is placed in a tube furnace. Ar / H2 (volume ratio 9:1, total flow rate 50 mL / min) is used to heat the precursor to 451°C at a rate of 3.5°C / min and maintain the temperature for 5 hours to form a precursor containing a Ni-Co-Fe-P main phase and boron-sulfur doped phase. The precursor is immersed in a 10 wt.% glucose solution (volume 500 mL) and ultrasonically dispersed (power 60 W, time 35 minutes). The carbon-coated intermediate is obtained by transferring the precursor to a 100 mL sealed reaction kettle (polytetrafluoroethylene lining), heating at 181°C (stirring speed 200 rpm) for 13 hours, washing with deionized water until neutral, and vacuum drying (vacuum degree ≤-0.09 MPa) at 65°C for 13 hours. The carbon-coated intermediate is immersed in a 0.05 mol / L chloroauric acid solution (volume 200 mL) and ultrasonically dispersed (power 50 W, time 11 minutes). The Au nanoparticles with a particle size of about 12 nm are obtained by placing the intermediate in a photoreduction reaction device (300 W xenon lamp, 452 nm filter, light intensity 51 mW / cm 2 ) and avoiding light for 35 minutes (stirring speed 200 rpm). The Au nanoparticles are washed by centrifugation with deionized water (8000 rpm, 3 times) and vacuum dried (vacuum degree ≤-0.09 MPa) at 65°C for 6 hours. Finally, the multi-level heterojunction gradient doped carbon-coated catalyst (denoted as catalyst A) is obtained by placing the nanoparticles in a plasma reactor (power 110 W, Ar gas flow rate 200 mL / min) and treating them for 11 minutes (sample temperature ≤ 80°C) to introduce oxygen vacancies and nitrogen defects.

[0039] In the ammonia synthesis process, air (flow rate 50 mL / min, purity 99.9%) and water vapor (flow rate 15 mL / min, saturated water vapor) were mixed and then introduced into a dielectric barrier discharge reactor (Φ10 cm x 20 cm, filled with catalyst A, filling amount 5 g) at a total flow rate of 1 L / min, and pulsed high voltage (peak value 20 kV, frequency 8 kHz, duty cycle 30%) was applied to generate active gas containing NO x (NO: 65%, NO2: 25%, N2O: 10%, concentration 0.8 vol.%), which was introduced into an electrocatalytic reduction unit after condensation (8°C ice-water mixture) to remove water vapor; a three-electrode system was constructed with catalyst A as the cathode (loading amount 5 mg / cm 2 , electrode size 2 cm x 2 cm), IrO2-Ta2O5 composite oxide (Ir: Ta molar ratio 1:3, loading amount 3 mg / cm 2 ) as the anode (electrode size 2 cm x 2 cm), and Ag / AgCl (saturated KCl solution) as the reference electrode (electrolysis cell volume 50 mL), and the electrolyte was 0.1 mol / L KOH and 0.05 mol / L KNO2 (total volume 30 mL), and the external voltage was -0.6 V vs. RHE (controlled by an electrochemical workstation), and the pulsed current density was 110 mA / cm 2 (frequency 18 Hz, duty cycle 50%) for 3 hours, and NO x was reduced to NH3 (HER Faraday efficiency <8%). The electrocatalytic product (containing NH3, unreacted NO x , and a small amount of H2O vapor) was introduced into a photo-thermal separation module, and a Janus-type photo-thermal electrode (surface loaded with Au nanoparticles, particle size 15 nm, coverage 10%) was integrated, and the electrode was heated to 65°C (monitored by an infrared thermal imager) under visible light (wavelength 500 nm, light intensity 100 mW / cm 2 ) irradiation, which promoted the desorption of NH3 (partial pressure 18 kPa), and the desorbed gas passed through a polyimide membrane separator (thickness 50 μm, NH3 permeation rate 1.8 x 10 -6 mol·m -2 ·s -1 ·Pa -1 ), and unreacted NO x was recycled back to the plasma activation unit.

[0040] Example 2

[0041] The specific implementation is the same as that of Example 1, except that the catalyst preparation: the foam nickel cleaning process is the same as that of Example 1; immersion in a mixed solution containing nickel nitrate (0.25 mol / L), cobalt nitrate (0.12 mol / L), and iron nitrate (0.06 mol / L) (ethanol-water volume ratio 1:1), stirring at 62°C for 4 hours to generate a Ni-Co-Fe hydroxide precursor (thickness about 75 μm); vacuum drying at 62°C for 20 hours to obtain a multi-metal hydroxide support. Immersion in a mixed solution containing sodium hypophosphite (0.35 mol / L), boric acid (0.12 mol / L), and sodium sulfide (0.06 mol / L) (pH = 9.1, adjusted by NaOH), reaction at 152°C for 8 hours; drying at 82°C for 8 hours, Ar / H2(9:1) heating to 452°C, and holding for 6 hours. The precursor is immersed in a glucose solution (10 wt.%), ultrasonic dispersion for 40 minutes, and thermal reaction at 182°C for 14 hours; after cooling and washing, drying at 62°C for 14 hours to obtain a carbon-coated intermediate. Immersion in a chloroauric acid solution (0.05 mol / L), ultrasonic dispersion for 12 minutes, and photo-reduction (wavelength 455 nm, power 52 mW / cm 2 , time 40 minutes) to load Au nanoparticles; after washing and drying, plasma reactor (120 W) treatment for 12 minutes to obtain the catalyst.

[0042] Ammonia synthesis process: air (60 mL / min) and water vapor (20 mL / min) are mixed into a dielectric barrier discharge reactor (pulse high voltage 22 kV, frequency 9 kHz) to generate NO x active gas; after condensation (9°C), it enters the electrocatalytic unit. The electrolyte is 0.11 mol / L KOH and 0.055 mol / L KNO2; the external voltage is -0.65 V vs. RHE, and the pulse current density is 115 mA / cm 2 (frequency 20 Hz).

[0043] Example 3

[0044] The specific implementation is the same as that of Example 1, except that the catalyst is prepared as follows: the foam nickel is cleaned as in Example 1; the foam nickel is immersed in a mixed solution (volume ratio of ethanol to water 1:1) containing nickel nitrate (0.15 mol / L), cobalt nitrate (0.08 mol / L), and iron nitrate (0.04 mol / L), and stirred at 60°C for 2 hours to form a Ni-Co-Fe hydroxide precursor (thickness about 55 μm); the precursor is vacuum dried at 60°C for 12 hours to obtain a multi-metal hydroxide supported substrate; the precursor is immersed in a mixed solution (pH = 8.9, adjusted by NaOH) containing sodium hypophosphite (0.25 mol / L), boric acid (0.08 mol / L), and sodium sulfide (0.04 mol / L), and reacted at 150°C for 6 hours; the precursor is dried at 80°C for 6 hours, and then heated to 450°C under Ar / H2(9:1) for 4 hours. The precursor is immersed in a glucose solution (10 wt.%), and ultrasonically dispersed for 30 minutes, and then heated at 180°C for 12 hours; after cooling and washing, the precursor is dried at 60°C for 12 hours to obtain a carbon-coated intermediate. The intermediate is immersed in a chloroauric acid solution (0.05 mol / L), and ultrasonically dispersed for 10 minutes, and then subjected to photoreduction (wavelength 450 nm, power 50 mW / cm2, time 30 minutes) to load Au nanoparticles; after washing and drying, the intermediate is subjected to plasma treatment (100 W) for 10 minutes to obtain the catalyst. 2

[0045] The ammonia synthesis process is as follows: air (40 mL / min) and water vapor (10 mL / min) are mixed into a dielectric barrier discharge reactor (pulse high voltage 18 kV, frequency 7 kHz) to generate NO x active gas; after condensation (7°C), the active gas is introduced into the electrocatalysis unit. The electrolyte is 0.09 mol / L KOH and 0.045 mol / L KNO2; the external voltage is -0.55 V vs. RHE, and the pulsed current density is 105 mA / cm 2 (18 Hz).

[0046] Comparative Example 1

[0047] The specific implementation is the same as that of Example 1, except that the catalyst is as follows: a common nickel-based catalyst (Ni / C, mass ratio 1:1) is used instead. The process is as follows: air (50 mL / min) and water vapor (15 mL / min) are mixed into a dielectric barrier discharge reactor (pulse high voltage 20 kV, frequency 8 kHz) to generate NO x active gas; after condensation (8°C), the active gas is introduced into the electrocatalysis unit (three-electrode system: Ni / C cathode, IrO2-Ta2O5 anode, Ag / AgCl reference); the electrolyte is 0.1 mol / L KOH and 0.05 mol / L KNO2; the external voltage is -0.6 V vs. RHE, and the pulsed current density is 110 mA / cm 2 . ​

[0048] Comparative Example 2

[0049] The specific implementation is the same as that of Example 1, except that the electrocatalytic unit: instead of using pulse current, constant current (-0.6V constant) is used. Process: air and water vapor mixing, plasma activation; electrocatalytic unit electrolyte, constant current density is -0.6V.

[0050] Comparative Example 3

[0051] The specific implementation is the same as that of Example 1, except that the photo-thermal separation module: instead of using Au nanoparticles, a common carbon electrode is used. Process: air and water vapor mixing, plasma activation, catalyst preparation, electrocatalytic reduction is the same as Example 1; the photo-thermal separation module is a common carbon electrode (without Au loading), which is heated to 40°C after visible light irradiation.

[0052] II. Performance Test

[0053] Test method: take the catalyst prepared in each example / comparative example, complete the whole process of plasma activation, electrocatalytic reduction and photo-thermal separation according to the corresponding process parameters, the specific steps are as follows:

[0054] Catalyst loading: 5g of catalyst (catalyst A for Examples 1-3, Ni / C for Comparative Example 1) is uniformly filled in a dielectric barrier discharge reactor (Φ10cm×20cm), ensuring that the gas passes uniformly;

[0055] Plasma activation: air (flow according to example parameters) and saturated water vapor (flow according to example parameters) are mixed and then enter the reactor, and corresponding pulse high voltage (peak voltage, frequency, duty cycle) is applied for 3 hours to generate active gas containing NOx;

[0056] Condensation and water removal: the active gas is removed by a condenser (temperature according to example parameters, ice water mixture) to remove water vapor, and dry active gas is collected;

[0057] Electrocatalytic reduction: dry active gas is introduced into an electrocatalytic unit (three-electrode system: catalyst as cathode, IrO2-Ta2O5 as anode, Ag / AgCl as reference), electrolyte is corresponding concentration of KOH, KNO2 solution (volume 30mL), under external voltage (vs. RHE) and pulse current density (frequency, duty cycle), reaction for 3 hours, and product gas is collected;

[0058] Photo-thermal separation: product gas enters the photo-thermal separation module (integrated corresponding photo-thermal electrode, visible light wavelength, light intensity according to example parameters), after irradiation for a specified time (according to example parameters), NH3 is collected by a polyimide membrane separator;

[0059] Performance test: NH3 yield: NH3 concentration in product gas was detected by gas chromatography (GC-2014C, Shimadzu) and combined with gas flow to calculate the yield (mmol-h -1 ·cm -2 , based on the cathode area);

[0060] Faradic efficiency (FE): total electric quantity was recorded by coulometer (CHI660E) and combined with NH3 yield to calculate FE (%);

[0061] Continuous operation stability: steps 1-5 were repeated for 50 hours of continuous operation, NH3 yield was detected every hour, and yield retention rate (%) after 50 hours was calculated;

[0062] NH3 purity: NH3 volume fraction (%) in separated gas was detected by gas chromatography.

[0063] Performance test results:

[0064] Table 1: Performance test results of each example and comparative example

[0065]

[0066]

[0067] Based on the design and process optimization of the multi-level heterojunction gradient doped carbon-coated catalyst (catalyst A) of examples 1-3 shown in Table 1, the core problems of the existing ammonia synthesis process are effectively solved. First, the multi-level heterojunction structure and gradient doping (boron, sulfur) of catalyst A improve the NO x adsorption capacity, combined with the high-efficiency NO x generation (yield 0.8 vol.%) of the plasma activation unit, significantly reducing the dependence on high temperature and high pressure (350-550℃, 10-30MPa) in traditional processes, realizing NO x activation under mild conditions (normal temperature and pressure), reducing energy consumption and carbon emissions; secondly, the high active site and electron transport optimization of catalyst A in the electrocatalytic reduction unit inhibit the hydrogen evolution side reaction (HER Faradic efficiency <8%), the Faradic efficiency reaches 91-93%, which is much higher than 75% of comparative example 1, improving the selectivity of NO x reduction to NH3; finally, the carbon coating layer and surface defect engineering (oxygen vacancies, nitrogen defects) of catalyst A enhance the anti-sintering performance, the NH3 yield retention rate of examples 1-3 is as high as 95-97% after 50 hours of continuous operation (only 89% for comparative example 1), solving the problem of easy deactivation of traditional catalysts, and the introduction of the photothermal separation module (Au nanoparticle loading) further improves the NH3 desorption efficiency (purity 99.9-99.95%) of examples 1-3, comprehensively realizing an efficient, low-carbon and stable ammonia synthesis process.

[0068] The above embodiments only express several implementation manners of the present application, which are described in a more specific and detailed manner, but cannot be understood as a limitation on the patent scope of the present application. It should be noted that, for ordinary skilled persons in the art, several modifications and improvements can be made without departing from the concept of the present application, which all belong to the protection scope of the present application.

Claims

1. A process for the synthesis of ammonia by electrocatalysis coupled with low temperature plasma, characterized in that, Comprising the following steps: S1, the air mixed with water vapor enters the dielectric barrier discharge reactor, the reactor is filled with multi-stage heterojunction gradient doped carbon-coated catalyst, and active gas containing NO X is generated under pulse high pressure; the active gas enters the electrocatalytic reduction unit after condensation; S2, the electrocatalytic reduction unit in step S1 is used as the cathode, an IrO2-Ta2O5 composite oxide is used as the anode, and Ag / AgCl is used as the reference electrode to construct a three-electrode system; the electrolyte includes KOH and KNO2; the external voltage is (-0.5 to -0.7) V vs. RHE, and the pulse current density is 100 to 120 mA / cm 2 will be reduced to NH3 x . S3, the electrocatalytic product enters the photothermal separation module, and a Janus type photothermal electrode is integrated in the photothermal separation module; under visible light irradiation, the electrode is locally heated to 60-70 DEG C.

2. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 1 wherein, In step S1, the voltage of the pulse high voltage is 15-25 kV, and the frequency is 5-10 kHz; the condensation temperature is 5-10 DEG C.

3. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 1 wherein, In step S2, the concentration of KOH in the electrolyte is 0.08-0.12 mol / l, and the concentration of KNO2 is 0.04-0.06 mol / l; the pulse current frequency is 18-22 Hz.

4. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 1 wherein, In step S3, the Janus type photothermal electrode is loaded with Au nanoparticles on the surface; the visible light is λ=400-600 nm.

5. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 1 wherein, The process for synthesizing ammonia by low-temperature plasma coupled electrocatalysis also comprises: separating the product after photothermal separation through a polyimide membrane.

6. The process for the synthesis of ammonia by low-temperature plasma coupled electrocatalysis according to any one of claims 1 to 5, characterized in that, The preparation steps of the multi-level heterojunction gradient doped carbon-coated catalyst comprise: A1, the foam nickel is sequentially ultrasonically cleaned with concentrated hydrochloric acid, ethanol and deionized water; the cleaned foam nickel is immersed in a mixed solution containing nickel nitrate, cobalt nitrate and iron nitrate, and is stirred and reacted at 60-62 DEG C to form a Ni-Co-Fe hydroxide precursor on the surface of the foam nickel; the foam nickel is taken out and vacuum dried to obtain a multi-metal hydroxide support; A2, the multi-metal hydroxide support is immersed in a mixed solution containing sodium hypophosphite, boric acid and sodium sulfide, and the pH of the mixed solution is adjusted to 8.9-9.1 with NaOH, and is reacted at 150-152 DEG C; after the reaction is completed, the support is taken out and dried at 80-82 DEG C, and then is placed in a tube furnace and heated to 450-452 DEG C under an Ar / H2 mixed atmosphere to form a precursor; A3, the precursor is immersed in a glucose solution and ultrasonically dispersed; then it is transferred to a sealed reaction kettle and is heat-reacted at 180-182 DEG C; after the reaction is completed, it is naturally cooled to room temperature, washed with deionized water until neutral, and dried at 60-62 DEG C to obtain a carbon-coated intermediate; A4, the carbon-coated intermediate is immersed in a chloroauric acid solution and ultrasonically dispersed, and Au nanoparticles are loaded on the surface of the carbon layer by a photoreduction method; after the reaction is completed, the unreacted Au precursor is removed by washing with deionized water, and is dried at 60-62 DEG C; finally, the sample is treated in a plasma reactor.

7. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 6 wherein, In step A1, the ultrasonic cleaning time is 30-40 min; the concentration of nickel nitrate in the mixed solution is 0.15-0.25 mol / l, the concentration of cobalt nitrate is 0.08-0.12 mol / l, and the concentration of iron nitrate is 0.04-0.06 mol / l; the stirring and reaction time is 2-4 h; the thickness of the hydroxide precursor is 50-80 μm; the vacuum drying temperature is 60-62 DEG C, and the time is 12-24 hours.

8. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 6 wherein, In step A2, the concentration of sodium hypophosphite in the mixed solution is 0.25-0.35 mol / l, the concentration of boric acid is 0.08-0.12 mol / l, and the concentration of sodium sulfide is 0.04-0.06 mol / l; the reaction time is 6-8 hours; the drying time is 6-8 hours; the volume ratio of Ar to H2 in the mixed atmosphere is 9:1; the heating rate is 3-4 ℃ / min; and the holding time is 4-6 h.

9. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 6 wherein, In step A3, the ultrasonic dispersion time is 30-40 min; the heat reaction time is 12-14 h; and the drying time is 12-14 h.

10. The process for synthesis of ammonia by low temperature plasma coupled electrocatalysis as claimed in claim 6 wherein, In Step A4, the ultrasonic dispersion time was 10-12 min; the light reduction method had a light wavelength of 450-455 nm and a power of 50-52 mW / cm 2 , a time of 30-40 min; the drying time was 6-8 h; the power of the plasma reactor was 100-120 W; and the treatment time was 10-12 min.

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