Modified metal oxide catalyst and plasma coupling electro-catalysis ammonia synthesis system based on modified metal oxide catalyst

Abstract

The invention discloses a modified metal oxide catalyst and a plasma coupling electro-catalysis ammonia synthesis system based on the catalyst, cobalt oxide is modified by using radio frequency inductively coupled plasma (ICP) under a vacuum condition and an argon atmosphere, and the cobalt oxide catalyst rich in defects is obtained after discharging is finished. The ammonia synthesis system comprises a plasma activated air system and an electrocatalytic ammonia synthesis system; carrying out plasma discharge on air to generate NOX, cooling, and introducing the NOX into the absorption liquid; the absorption liquid is used as catholyte for electrocatalytic ammonia production in an H-shaped pool, and a cobalt oxide catalyst modified by plasma treatment is selected as a cathode. The cobalt oxide is treated by ICP, so that the problems of low conductivity, inert surface and the like of metal oxide are solved, and the performance of the catalyst is improved. The method solves the problems that plasma air discharge products are complex, and the ammonia yield is low due to the fact that N is equivalent to N cannot be efficiently activated through electro-catalysis.

Description

Technical Field

[0001] The present invention relates to the technical field of ammonia synthesis, and in particular to a modified metal oxide catalyst and a plasma coupled electrocatalytic ammonia synthesis system based on the catalyst. Background Art

[0002] Ammonia plays a vital role in various industries such as agriculture, textiles, pharmaceuticals, dyes and plastics. It is also increasingly recognized as a potential carbon-free energy carrier and renewable energy storage medium with high energy density (4.3kWh kg-1) and large hydrogen storage capacity (17.7%). However, the conventional Habear-Bosch ammonia synthesis process requires high temperature and pressure operating conditions (~500℃, 150-300atm), consumes 1-2% of the global energy supply and contributes up to 1% of carbon dioxide emissions. The electrochemical nitrogen (N2) reduction reaction (NRR) is an attractive alternative method that uses water as a sustainable source of protons and renewable electricity, such as solar or wind, to synthesize ammonia under mild conditions.

[0003] However, due to the limited solubility of N2 and the stability of the N≡N bond (bond energy of 946 kJ mol -1 ), the ammonia yield of NRR is low (10 -6 ~10 -4 molh -1 cm -2 In contrast, nitrates and nitrites have good solubility and significantly lower dissociation energies (N═O bond energy is 204 kJ mol -1 ). In addition to naturally abundant sources such as flue gas and polluted water, nitrate and nitrite can also be effectively obtained from the air using technologies such as plasma conversion. Therefore, it is feasible to achieve the synthesis of ammonia from air by combining plasma activation and electrocatalytic synthesis of ammonia from nitrate and nitrite.

[0004] The core of electrochemical ammonia synthesis is the design of catalysts. Metal oxides are common catalysts in electrochemical ammonia synthesis because of their low price, stable chemical properties, and ability to adapt to acidic and alkaline reaction environments. However, problems such as low conductivity and inert surface have hindered their further development. Defect modification engineering is currently an effective method to improve the activity and selectivity of catalysts. The principle is to modify them by changing the electronic state of the catalyst or generating additional active sites for reduction reactions. Among them, the modification method by generating vacancies has attracted much attention for its convenience and efficiency. In a specific external environment (such as high temperature, etc.), oxygen in the lattice of metal oxides will be detached, resulting in oxygen deficiency and the formation of oxygen vacancies. The generation of oxygen vacancies in metal oxides can effectively improve the yield of synthetic ammonia and the Faraday efficiency.

[0005] However, the existing plasma-based ammonia synthesis methods have complex plasma-air discharge products. When N2 is used as the raw material for electrocatalytic synthesis of ammonia, the ammonia yield is low due to the inability of the catalytic system to efficiently activate N≡N, and the metal oxides have low conductivity and inert surface problems. Summary of the invention

[0006] The present invention aims to address the deficiencies of the prior art and to provide a modified metal oxide catalyst and a plasma coupled electrocatalytic ammonia synthesis system based on the catalyst.

[0007] The objective of the present invention is achieved through the following technical scheme: a modified metal oxide catalyst, using cobalt oxide as the initial metal oxide catalyst, modifying the cobalt oxide by treating it with radio frequency inductively coupled plasma (ICP) under vacuum conditions and an argon atmosphere, and obtaining a defective cobalt oxide catalyst after the discharge is completed.

[0008] Furthermore, Vo3O4 was prepared by a hydrothermal method, in which cobalt nitrate hexahydrate, ammonium fluoride and urea were added to deionized water to form a solution, which was continuously stirred and carbon paper was added. The solution was then transferred to a reactor, sealed and placed in an oven for heating. After the heating was completed and the carbon paper was cooled, it was taken out and rinsed with deionized water and anhydrous ethanol to dry the surface impurities and then dried. The dried precursor was placed in a tubular furnace and calcined under an Ar atmosphere to obtain cobalt oxide attached to the carbon paper.

[0009] Furthermore, the molar ratio of cobalt nitrate hexahydrate, ammonium fluoride and urea is 1.0:1.8-2.1:4.7-5.2; the stirring time is 0.5-1h; the heating temperature in the oven is 110-130°C, and the heating time is 7-9h; the drying temperature is 70-90°C, and the drying time is 7-9h; the calcination temperature is 340-360°C, and the calcination time is 1.5-2.5h.

[0010] Furthermore, the discharge power of the radio frequency inductively coupled plasma ICP is 145-155 W, and the discharge time is 4-5 min.

[0011] On the other hand, the present invention also provides a plasma-coupled electrocatalytic ammonia synthesis system based on a modified metal oxide catalyst, the system comprising a plasma-activated air system and an electrocatalytic ammonia synthesis system;

[0012] The plasma activated air system is used to generate NO by plasma discharge in the jet gliding arc plasma reactor. X , and NO X After cooling, the mixture is introduced into the absorption liquid of the electrocatalytic ammonia synthesis system for full absorption;

[0013] The electrocatalytic ammonia synthesis system uses the absorption liquid as the cathode liquid to electrocatalytically produce ammonia in an H-type cell, and the cathode uses a cobalt oxide catalyst modified by plasma treatment.

[0014] Furthermore, the discharge voltage in the plasma activated air system is 9-10 kV.

[0015] Furthermore, the cooling method in the plasma activated air system is a serpentine condenser.

[0016] Furthermore, the gas flow rate in the plasma activated air system is 4.5-5.5 L min -1 .

[0017] Furthermore, the absorption liquid in the electrocatalytic ammonia synthesis system is an alkaline solution.

[0018] Furthermore, the potential in the electrocatalytic ammonia synthesis system is -0.88 to -0.48 V vs. RHE.

[0019] Beneficial effects of the present invention:

[0020] 1. The system for synthesizing ammonia from air based on plasma-coupled electrocatalysis provided by the present invention solves the problem of complex plasma-air discharge products and low ammonia yield caused by the inability of electrocatalysis to efficiently activate N≡N by combining the plasma's ability to efficiently activate nitrogen with the directional reaction of particles in the electrocatalysis.

[0021] 2. The present invention selects a suitable absorption liquid to absorb the NO generated by plasma activated air X Electrocatalysis is carried out after discharge, rather than directly, to ensure that the NOX generated by discharge is fully reacted and reduce losses.

[0022] 3. The present invention provides the preferred working conditions for plasma activated air and electrocatalysis. The preferred working conditions for plasma activated air are a discharge voltage of 9 to 10 kV, a serpentine condenser for cooling, a 1M KOH solution for absorption liquid, and a gas flow rate of 4.5 to 5.5 L min -1 ; The working condition of electrocatalysis is the potential of -0.88~-0.48V vs.RHE. Under this condition, NO2 in the absorption liquid - The concentration can reach 7888 μg / mL, and the Faradaic efficiency and yield of synthetic ammonia can reach >85% and >15 mg h -1 cm -2 .

[0023] 4. The present invention provides a method of treating a metal oxide catalyst with radio frequency inductively coupled plasma (ICP) to overcome the problems of low conductivity and inert surface of metal oxides and improve catalyst performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0025] Figure 1 This is a diagram of the plasma-coupled electrocatalytic ammonia synthesis system.

[0026] Figure 2 The NO2 when using different absorption liquids - Concentration diagram.

[0027] Figure 3 It is a schematic diagram of the experimental system for increasing the number of gas washing bottles.

[0028] Figure 4 It is NO2 in different scrubber bottles - Concentration diagram.

[0029] Figure 5 It is a schematic diagram of the experimental system using a serpentine condenser and two gas washing bottles for cooling.

[0030] Figure 6 It is NO2 in different scrubber bottles - Concentration diagram.

[0031] Figure 7 It is NO2 at different discharge voltages - Concentration diagram.

[0032] Figure 8 The NO2 at different gas flow rates - Concentration diagram.

[0033] Fig. 9 Schematic diagram of the catalyst position (a) and discharge condition (b) during ICP treatment.

[0034] Fig.10 These are SEM images at different resolutions, where a and b are Co3O4, and c and d are V-Co3O4.

[0035] Fig.11 These are TEM images, where a and b are Co3O4, and c and d are V-Co3O4.

[0036] Fig.12 It is the N2 adsorption-desorption curve, where a is Co3O4 and b is V-Co3O4.

[0037] Fig.13This is a high-resolution electron microscope image of V-Co3O4.

[0038] Fig.14 This is the mapping map of V-Co3O4.

[0039] Fig.15 This is the XRD pattern of V-Co3O4.

[0040] Fig.16 This is the electron paramagnetic resonance spectrum of V-Co3O4.

[0041] Fig.17 These are the LSV images of carbon paper, Co3O4 supported on carbon paper, and V-Co3O4.

[0042] Fig.18 This is the electrochemical active surface area diagram of Co3O4 and V-Co3O4, where a and c are V-Co3O4, and b and d are Co3O4.

[0043] Fig.19 This is a schematic diagram of the yield and Faraday efficiency of ammonia synthesis using Co3O4 and V-Co3O4.

[0044] Fig. 20 NO2 is a byproduct of the synthesis of Co3O4 and V-Co3O4 - Schematic diagram of yield and Faraday efficiency.

[0045] Fig.21 It images of V-Co3O4 at different potentials.

[0046] Fig. 22 This is a schematic diagram comparing the performance of V-Co3O4 and other catalysts.

[0047] Fig.23 This is a schematic diagram of the stability experiment of V-Co3O4 (potential is -0.08V vs.RHE).

[0048] Fig.24 These are the TEM and XRD images of V-Co3O4 after stability experiment.

[0049] Fig.25 It is a schematic diagram of the ammonia yield and Faraday efficiency of the coupled system.

[0050] Fig.26 This is a diagram of the radio frequency inductively coupled plasma material surface modification experimental platform.

[0051] Fig. 27 It is NH4 + Standard curve graph.

[0052] Fig.28 It is NO2- Standard curve graph. DETAILED DESCRIPTION

[0053] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described below in conjunction with the accompanying drawings and implementation examples. It should be understood that the specific implementation examples described herein are only used to explain the present invention and are not used to limit the present invention.

[0054] The present invention provides a modified metal oxide catalyst, wherein the initial metal oxide catalyst is preferably Vo3O4, comprising the following steps:

[0055] Vo3O4 is prepared by a hydrothermal method: cobalt nitrate hexahydrate, ammonium fluoride and urea are added to deionized water to form a solution, stirred continuously, and the cut carbon paper is added, then transferred to the lining of the reactor, sealed in the reactor, and finally heated in an oven. After heating and cooling, the reactor is opened to take out the carbon paper, rinsed three times with deionized water and anhydrous ethanol in sequence, and then dried after the surface impurities are washed off. The dried precursor is placed in a tubular furnace and calcined under an Ar atmosphere to obtain cobalt oxide attached to the carbon paper.

[0056] Among them, the molar ratio of cobalt nitrate hexahydrate, ammonium fluoride and urea is preferably 1.0:1.8~2.1:4.7~5.2; the stirring time is preferably 0.5~1h; the heating temperature in the oven is preferably 110~130°C, and the heating time is preferably 7~9h; the flushing method is to use deionized water and anhydrous ethanol to flush three times in sequence; the drying temperature is preferably 70~90°C, and the drying time is preferably 7~9h; the calcination temperature is preferably 340~360°C, and the calcination time is preferably 1.5~2.5h.

[0057] Vo3O4 prepared by radio frequency inductively coupled plasma (ICP) treatment: 50 sccm of argon gas is continuously introduced under vacuum conditions, and the cobalt oxide obtained above is treated by ICP in an argon atmosphere to obtain defective cobalt oxide after the discharge is completed.

[0058] Among them, the discharge power is preferably 145-155W, and the discharge time is preferably 4-5min.

[0059] The present invention also provides a system for synthesizing ammonia from air based on plasma-coupled electrocatalysis, which consists of two parts. The first part is a plasma-activated air system, and the second part is an electrocatalytic ammonia synthesis system, which uses the absorption liquid as the cathode liquid to perform electrocatalytic ammonia production in an H-type cell.

[0060] The coupling of plasma is reflected in two aspects. The first aspect is that plasma is used to activate air. The second aspect is that the catalyst used in electrocatalysis is surface modified by radio frequency inductively coupled plasma (ICP) technology. The system diagram is shown in the attached figure. Figure 1 shown.

[0061] The plasma activated air system generates NO by plasma discharge from air in a jet gliding arc plasma reactor. X , and NO X After cooling, the mixture is passed into the absorption liquid for full absorption. The discharge voltage is preferably 9-10 kV. The cooling method is preferably a serpentine condenser. The absorption liquid can be an alkaline solution, and the preferred embodiment of the present invention is a 1M KOH solution. The gas flow rate is preferably 4.5-5.5 Lmin -1 .

[0062] The electrocatalytic ammonia synthesis system uses the absorption liquid as the cathode liquid to electrocatalyze the production of ammonia in an H-type cell, and the cathode uses a metal oxide catalyst modified by plasma treatment. The anode liquid is preferably a 1M KOH solution. The potential is preferably -0.88 to -0.48 V vs. RHE.

[0063] The following is the specific process of the experiment of the present invention:

[0064] (1) Preferred absorption liquid

[0065] The air flow rate was selected as 7L / min, the discharge voltage was 10kV, and no cooling treatment was performed. The absorption liquids were selected as deionized water, 0.05M KOH, 0.1M KOH and 1M KOH solutions, respectively. The absorption liquids were continuously passed for 15 minutes, and then the concentrations of nitrate and nitrite ions in the absorption liquid were measured.

[0066] As attached Figure 2 As shown, after the plasma reacts with the absorption liquid, the main product is NO2 - , NO3 - The content of NO2 is very low and can be almost ignored, so only NO2 is measured in the subsequent examples. - concentration. At the same time, it can be found that when plasma passes through alkaline solution, the NO2 - It is much higher than deionized water, and the absorption effect is better as the alkalinity of the solution increases. In 1M KOH solution, the NO2 - The concentration is up to 2480 μg / mL. Therefore, the absorption liquid is preferably 1 M KOH solution.

[0067] (2) Optimal cooling method

[0068] Select the air flow rate as 7L / min, the discharge voltage as 10kV, and the absorption liquid as 1M KOH solution. Figure 3As shown, the temperature is lowered in sequence by using four gas washing bottles, which are respectively designated as bottles A, B, C, and D. The gas is passed through for 15 minutes, and then the nitrite ion concentrations in the absorption liquids of the four gas washing bottles are measured.

[0069] As attached Figure 4 As shown, the gas is high temperature after plasma discharge, and will gradually cool down after passing through the solution. The gas temperature is different when passing through bottles A, B, C, and D. The gas temperature is even lower when passing through the washing bottle at the back. As the gas temperature decreases, NO2 in the absorption liquid - The concentration increases first and then decreases. The NO2 in bottle B - The highest concentration is 3560μg / mL, and the NO2 in bottle D - The lowest concentration is 1224μg / mL. This trend may be due to the fact that increasing the number of gas washing bottles can achieve a certain cooling effect, but when the gas continues to enter the subsequent gas washing bottles, the cooling effect may not be ideal. However, since the NOX in the gas has been absorbed by the previous gas washing bottles, NO2 - The concentration will decrease.

[0070] Subsequently, as attached Figure 5 As shown, without changing other conditions, the cooling method was changed to a serpentine condenser and two washing bottles, marked as bottles A and B, and the nitrite ion concentration in the absorption liquid of the two washing bottles was then measured.

[0071] As attached Figure 6 As shown, it can be found that after adding the serpentine condenser, the maximum NO2 - The concentration is 5068μg / mL, which is much higher than the 2838μg / mL in bottle B. The results show that the serpentine condenser can achieve a better cooling effect than increasing the number of washing bottles. Therefore, the cooling method is preferably the serpentine condenser.

[0072] (3) Optimized discharge voltage

[0073] The air flow rate was selected as 7L / min, and a serpentine condenser was used for cooling. The absorption liquid was 1M KOH solution. The discharge voltages were selected as 6kV, 7kV, 8kV, 9kV, and 10kV, respectively. The discharge voltages were continuously passed for 15 minutes, and then the nitrite ion concentration in the absorption liquid was measured.

[0074] As attached Figure 7 As shown, in the range of 6 to 10 kV, NO2 - The concentration increases with the increase of voltage. At 10kV, NO2 - The concentration reached a maximum of 5068 μg / mL, but NO2 -The intensity of concentration change with voltage is not obvious, indicating that voltage has an effect on the reaction between gas and absorption liquid after discharge, but the influence is limited. Therefore, the discharge voltage is preferably 9-10 kV.

[0075] (4) Optimizing air flow

[0076] The absorption liquid was selected as 1M KOH solution, the discharge voltage was 10kV, and a serpentine condenser was used for cooling. The gas flow rates were selected as 3L / min, 5L / min, 7L / min, 9L / min, and 11L / min, respectively, and the gas flow was continuously passed for 15 minutes, and then the nitrite ion concentration in the absorption liquid was measured.

[0077] As attached Figure 8 As shown in the figure, as the gas flow rate increases, NO2 - The concentration increases first and then decreases. At a gas flow rate of 5 L / min, NO2 - The maximum concentration can reach 7888μg / mL. This trend of change may be due to the fact that when the gas flow rate is small, the gas can fully contact and react with the absorption liquid. As the gas flow rate gradually increases, the NOx concentration generated by air discharge increases until it reaches an instantaneous saturation state. At this time, the NO2 - The concentration will increase with the increase of gas flow; when the gas flow continues to increase, the gas that has no time to react leaves the solution under the impetus of subsequent gas molecules because the instantaneous absorption has been saturated. - The concentration is lower.

[0078] (5) Catalyst preparation

[0079] First, pour cobalt nitrate hexahydrate (3mmol), ammonium fluoride (6mmol) and urea (15mmol) into a beaker to make 80mL solution. After continuous stirring for half an hour, put the cut carbon paper into the above solution, then transfer it to the lining of the reactor, seal the reactor, and finally put it into an oven and heat it at 120℃ for 8h. After the heating is completed and cooled, open the reactor and take out the carbon paper, rinse it three times with deionized water and anhydrous ethanol in turn, rinse the surface impurities, and dry it at 80℃ for 8h. The dried precursor is placed in a tubular furnace and calcined at 350℃ for 2h under Ar atmosphere to obtain cobalt oxide attached to the carbon paper.

[0080] As attached Fig. 9 As shown, 50 sccm argon gas is continuously introduced under vacuum conditions, and the cobalt oxide obtained above is treated by ICP in an argon atmosphere (discharge power is 150 W, discharge time is 4 min), and defective cobalt oxide (V-Co3O4) is obtained after the discharge.

[0081] (6) Catalyst characterization

[0082] Co3O4 and V-Co3O4 were observed by scanning electron microscopy (SEM). Fig.10 As shown in the figure, at a resolution of 5μm, the structures of the two catalysts are rod-like structures, which are composed of countless synapses, and the width of each nanorod is about 15 to 20nm. At a higher resolution of 100nm, enlarged protruding structures can be observed, which are intertwined with each other. This structure undoubtedly increases the catalyst area of ​​the electrocatalytic reaction. By comparing the SEM images, it is not difficult to find that plasma modification of cobalt oxide does not significantly change the morphology of the material.

[0083] The two catalysts were analyzed by transmission electron microscopy (TEM). Fig.11 As shown, it can be seen that the gauze-like, nearly single-layer, and nanorod-like structures of Co3O4 and V-Co3O4 are clearer, confirming the results of the SEM analysis. At a resolution of 20nm, it was observed that plasma modification can make Co3O4 particles smaller (20nm in Figure b is about the diameter of one particle, and about two particles in Figure d are about 20nm), which is consistent with the characteristics of plasma-modified catalyst materials. It may be because plasma modification is carried out in a high energy density area, which causes the unstable part of the edge of the cobalt oxide microparticles to erode, making the particles more dispersed, thereby generating more active sites, which is more conducive to the subsequent electrocatalytic synthesis of ammonia.

[0084] BET analysis of the two catalysts is shown in the attached Fig.12 The results show that the surface area of ​​cobalt oxide increased from 32.35 m 2 / g becomes 34.82m 2 / g, this tiny change is enough to show that the reason for the huge improvement in the performance of cobalt oxide is not the change in the catalyst surface area.

[0085] Attached Fig.13 This is a high-resolution transmission electron microscope image of V-Co3O4. The lattice fringes are clear, indicating the single crystal characteristics of the catalyst nanorods. Using Digital Micrograph software to analyze it, the spacing between adjacent crystal planes is 0.254nm. Combined with the subsequent XRD analysis results, it is the (311) crystal plane of Vo3O4 crystal.

[0086] Element mapping analysis of V-Co3O4 is performed, as shown in the attached Fig.14 As shown, the distribution of elements in a certain area is shown. It can be observed that Co and O elements exist in the nanorods, the distribution of Co and O elements is uniform, and the content of Co element is relatively high.

[0087] XRD analysis of V-Co3O4 is shown in the attached Fig.15As shown in the figure, V-Co3O4 has characteristic peaks at 19.0°, 31.3°, 36.9°, 44.8°, 59.4°, and 65.2°. These peaks are consistent with the peak positions of Co3O4. Referring to the PDF#43-1003 card of the cobalt oxide crystal phase structure, these characteristic peaks correspond to the (111), (220), (311), (400), (511), and (440) crystal planes of cobalt oxide, respectively. This further proves the successful synthesis of cobalt oxide materials. By comparison, it can also be found from the figure that the crystal phase structure characteristics of V-Co3O4 are significantly weaker than those of Co3O4, which usually means that V-Co3O4 has a smaller particle size or more lattice defects. In addition, we also calculated the half-peak width at the (311) crystal plane. The half-peak width of V-Co3O4 is 2.46, while the half-peak width of Co3O4 is 1.62. The half-width characterizes the order of atomic arrangement in the crystal. After plasma modification, the half-width of cobalt oxide increases, indicating that plasma treatment weakens the order of crystal phase arrangement of the catalyst material and increases crystal defects.

[0088] Attached Fig.16 The electron paramagnetic spectrum image is used to detect specific defects in the catalyst material. The g value of the material with oxygen vacancies is about 2.00, which is the basis for judging the existence of oxygen vacancies in the material. According to the results shown in the image, the g values ​​of Co3O4 and V-Co3O4 are both 2.003; and the signal intensity can characterize the relative size of the oxygen vacancy concentration. It can be observed that the oxygen vacancy concentration of V-Co3O4 is much higher than that of Co3O4. Combined with XRD analysis, it can be inferred that high-energy-density plasma treatment produces more oxygen vacancies in cobalt oxide and increases crystal defects. Specifically, the content of oxygen vacancies in Co3O4 is about 31.9%, and the content of oxygen vacancies in V-Co3O4 is about 46.9%. It can be seen that plasma treatment significantly increases the oxygen vacancy content.

[0089] In addition, the contact angle / surface tension measuring instrument was used to measure the carbon paper, Co3O4 loaded on carbon paper, and V-Co3O4 to explore the changes in the hydrophilicity and hydrophobicity of the materials before and after plasma treatment. The contact angles of carbon paper, Co3O4 loaded on carbon paper, and V-Co3O4 to water were 112.8°, 55.6°, and 12.5°, respectively, indicating that after plasma treatment, the hydrophilicity of the material was enhanced, which was more conducive to the subsequent catalytic reaction.

[0090] (7) Catalyst electrochemical performance test

[0091] An experiment on synthesizing ammonia by nitrate reduction was carried out. The reaction solution was a mixed solution of 1M KOH and 0.1M KNO3. First, cyclic voltammetry, linear sweep voltammetry, and electrochemically active surface area (ECSA) analysis were performed on the two catalysts. Then, the synthetic ammonia yield and Faraday efficiency at different potentials (-0.08, -0.18, -0.28, -0.38, -0.48, -0.58, -0.68, -0.78 V vs. RHE) were investigated.

[0092] Attached Fig.17 The trend of current change of carbon paper, Co3O4 loaded on carbon paper and V-Co3O4 at different potentials is shown. By comparison, the carbon paper catalyst exhibits the lowest current density, which is almost negligible, reflecting its characteristics as a catalyst carrier. The catalytic current density after plasma modification is higher than that before treatment at the experimental potential, and the current density can reach 360mA cm at -0.78V vs.RHE. -2 .

[0093] Attached Fig.18 It is the result of electrochemically active surface area (ECSA) analysis, which reflects the active sites of the catalyst and is characterized by double layer capacitance. The capacitance of V-Co3O4 is calculated to be 2.04 mF cm -2 , higher than the capacitance of Co3O4 1.85mF cm -2 , demonstrating the superiority of the catalyst after plasma treatment in electrochemical response and kinetics.

[0094] Attached Fig.19 The ammonia yield and Faraday efficiency of the two catalysts in the reduction of nitrate to synthesize ammonia are shown. It can be observed that the ammonia yield of V-Co3O4 and Co3O4 increases with the increase of potential in the range of -0.08V to -0.78V vs.RHE. At the lowest potential of -0.08V vs.RHE, the ammonia yield of V-Co3O4 is 7.2mg h -1 cm -2 At the highest potential of -0.78 V vs. RHE, the ammonia yield reached 27.5 mg h -1 cm -2 , which are all higher than the ammonia yield of Co3O4. It is worth noting that at -0.48V vs.RHE, the ammonia yield of V-Co3O4 is 67.9% higher than that of Co3O4, which means that plasma modification has greatly increased the ammonia yield of Co3O4, providing new ideas for the subsequent defect modification of catalysts. Similarly, after plasma treatment, the Faraday efficiency of Co3O4 for ammonia synthesis has also been improved to a certain extent, with the highest Faraday efficiency being 98.9% at -0.08V vs.RHE.

[0095] Attached Fig. 20 Demonstrates the byproduct NO2 during the reduction of nitrate to synthesize ammonia - The yield and Faraday efficiency show that NO2 - The yields of NH4Cl2 were all at a low level under the experimental potentials, with the highest yield being 0.7 mg h at -0.18 V vs. RHE. -1 cm -2 , the Faraday efficiency is also at a very low level, less than 2%. In addition, it can be found that after plasma treatment, the byproduct NO2 - And both the yield and the Faradaic efficiency will be reduced.

[0096] Attached Fig.21 The It time curves of V-Co3O4 at different potentials are shown. It is found that with the increase of potential, the current density of the catalyst also increases. The current density at the highest potential of -0.78 V vs. RHE is about 360 mA cm -2 , which is consistent with the LSV results and explains the phenomenon that the ammonia yield increases with increasing potential.

[0097] Attached Fig. 22 The catalyst used in this experiment was compared with other catalysts, including metal oxides, metal oxides with different defect types, and other cobalt oxide catalysts. The two indicators of ammonia yield and Faraday efficiency were compared. The ammonia synthesis performance of V-Co3O4 is ahead of most of the current nitrate reduction ammonia synthesis catalysts.

[0098] (8) Catalyst stability test

[0099] A stability test was conducted on V-Co3O4, i.e., the experiment was carried out at -0.08V vs. RHE where its Faradaic efficiency is maximum. Each run was 30 minutes and repeated 24 times. Its stability characteristics were evaluated by comparing the changes in ammonia yield and Faradaic efficiency of the catalyst in repeated experiments.

[0100] Attached Fig.23 The results of 24 repeated experiments were presented, and the ammonia yield of V-Co3O4 was found to be between 6.7 and 7.5 mg h -1 cm -2 , the fluctuation is not big, it is relatively stable, and the Faraday efficiency is also maintained at a stable level, maintained at about 98%. Through 24 repeated experiments, it is preliminarily proved that V-Co3O4 has good stability in synthesizing ammonia.

[0101] The catalysts after the experiment were analyzed by TEM and XRD. Fig.24 As shown, the morphology and structure of the catalyst did not change significantly compared with before the experiment.

[0102] Overall, the properties of V-Co3O4 did not change significantly, which demonstrated the high activity, high yield and excellent stability of V-Co3O4 rich in oxygen vacancy defects in the NO3RR process.

[0103] (9) Coupling system for ammonia synthesis

[0104] In exploring the production of NO2 - After the reaction conditions with the best concentration were obtained, ammonia synthesis experiments were carried out in the coupled system. Under the best conditions, the absorption liquid was 1M KOH, cooled by a serpentine condenser, the discharge voltage was 10kV, the air flow rate was 5L / min, and after 15 minutes of plasma discharge, the absorption liquid was moved into the electrochemical cell for electrochemical reduction. The experiments were carried out at -0.08, -0.28, -0.48, -0.68, and -0.88V vs. RHE potentials, and then the ammonia yield and Faraday efficiency were measured, as shown in the attached figure. Fig.25 As shown in Figure 2, with the increase of potential, the ammonia yield gradually increases and the Faradaic efficiency gradually decreases. At -0.88 V vs. RHE, the ammonia yield is as high as 45.0 mg h -1 cm -2 , the Faradaic efficiency was 85.2%; at -0.08 V vs. RHE, the ammonia yield was 12.3 mg h -1 cm -2 , while the Faradaic efficiency is 98.6%.

[0105] The experimental instruments and devices used in the present invention are as follows:

[0106] (1) Radio frequency inductively coupled plasma material surface modification experimental platform

[0107] As attached Fig.26 As shown in the figure, the RF inductively coupled plasma material surface modification experimental system consists of a vacuum system, a flow control system, an ICP power supply and a controller. Among them, the vacuum system consists of a vacuum pump and a vacuum gauge, which can promote the discharge of RF inductively coupled plasma under low pressure conditions; the flow control system consists of a high-precision mass flow meter and a gas supply system, which can achieve precise regulation of the flow rate of 20 to 100 mL / min; the power control system consists of a plasma reactor, a power supply and an impedance matcher. The plasma reactor contains a quartz tube with a length of 75 cm and a diameter of 4.2 cm. The surface of the quartz tube is surrounded by a water-cooled copper spiral coil. The power supply can control the power between 0 and 500 W, and the impedance matcher can ensure high-frequency alternating current output.

[0108] (2) H-type pool

[0109] The reaction cell used in the electrocatalytic experiment is an H-type cell, using a proton exchange membrane Nafion 117. The cobalt oxide catalyst rich in oxygen vacancies used in the experiment is used as the working electrode (cathode), Ag / AgCl is used as the reference electrode, and a Pt sheet is used as the counter electrode (anode) to form a three-electrode device. The potential is converted into a reversible hydrogen potential (E RHE =E Ag / AgCl +0.059×PH+0.197V). The electrolyte is a KOH solution containing KNO3, 100mL in total, on both sides of the H-type reaction cell. To simulate the plasma air discharge gas NO x Electrocatalytic scenario of dissolving in water. The three electrodes are connected to the electrochemical workstation. After running a reaction cycle (30 / 60min) under the set parameters, the post-reaction solution is taken for subsequent quantitative detection.

[0110] (3) Jet Gliding Arc Plasma Reactor

[0111] The jet plasma reactor consists of a cylindrical inner electrode and an outer electrode, with a minimum spacing of 1 mm between the electrodes. Inner electrodes of different lengths were used in the experiment. The reactor uses a double inlet, and the gas forms a swirl when it enters the reactor tangentially from the inlet. After the discharge, the arc excited by the discharge will slide under the push of the airflow and stretch along the electrode axis to form a three-dimensional plasma region.

[0112] (4) Scanning electron microscopy (SEM)

[0113] A high-resolution morphology and topography image of the catalyst material can be obtained by a scanning electron microscope (SEM). A scanning electron microscope is usually measured together with an energy spectrometer or a spectrometer, and can be used to identify and measure the element abundance in the sample and draw an element distribution map. The present invention obtains the morphological changes of the material and the element hot spot distribution map of Co and O by scanning the cobalt oxide and the cobalt oxide after plasma treatment, and analyzes the element types and distribution content of the selected Qu Yuan.

[0114] (5) Transmission electron microscopy (TEM)

[0115] The catalyst material is characterized by a transmission electron microscope (TEM) to obtain the microscopic structure inside the material. The present invention analyzes the morphology and crystal structure of the catalyst by characterizing different catalysts by a transmission electron microscope.

[0116] (6) X-ray diffractometer (XRD)

[0117] The crystal structure of the material can be analyzed by X-ray diffraction (XRD). The present invention explores the changes in the crystal structure of the material before and after plasma treatment by performing XRD characterization on two cobalt oxide catalysts.

[0118] (7) Paramagnetic resonance spectrometer

[0119] The vacancies, free radicals, crystal defects, etc. of the material are characterized by an electron paramagnetic resonance (EPR) spectrometer. The present invention explores the changes of vacancies before and after plasma treatment by performing EPR characterization on two cobalt oxide materials.

[0120] (8) Ammonia detection method

[0121] The present invention adopts indigo blue spectrophotometry to quantitatively detect the concentration of product ammonia, which has the advantages of rapidity and low interference. The principle of the detection method is that hypochlorite can react with ammonia in a weakly alkaline environment to generate chloramine, and chloramine reacts with hypochlorite to generate a complex indigo blue, and its absorbance is measured in an ultraviolet spectrophotometer. By comparing with the ammonia concentration standard curve, the ammonia content in the solution is obtained. The ammonia standard curve equation is obtained according to the following process:

[0122] (a) Prepare ammonia standard solution: weigh 0.3142 g of ammonium chloride (NH4Cl), dissolve it in deionized water, transfer it into a volumetric flask and dilute to 100 mL. At this point, the ammonia concentration in the volumetric flask is 1 mg / mL;

[0123] (b) Prepare 1, 2, 3, 4, and 5 μg / mL ammonia solutions in 100 mL volumetric flasks, and add 4 mL of each solution dropwise into a colorimetric tube;

[0124] (c) Add 0.32 mL of NaOH (1 M, containing 5 wt % sodium salicylate), 2.4 mL of NaClO (0.05 M), and 0.8 mL of sodium nitrosoferrocyanide (1.0 wt %) to each colorimetric tube, mix thoroughly, and let stand at room temperature in a light-free environment for 2 h. Transfer the solution to be tested to a cuvette, add deionized water as a reference, and measure the absorbance of each solution at a wavelength of 655 nm;

[0125] (d) Plot these data into a standard curve with ammonia concentration as the horizontal axis and absorbance as the vertical axis. Fit the slope and intercept of the regression curve to obtain the standard curve equation, as shown in the attached figure. Fig. 27 shown.

[0126] Since the concentration of ammonia generated in this experiment is relatively high, direct measurement of the solution after electrolysis will far exceed the range of the UV spectrometer. First, it is necessary to dilute it proportionally and then use the indophenol blue colorimetric method for detection. Take 4mL of the solution after the reaction and repeat the operation (c) in the process of drawing the standard equation of ammonia to obtain the absorbance of the solution after the reaction. According to the standard curve equation and the dilution multiple, the concentration of NH3 is calculated.

[0127] (9) Detection method of nitrite

[0128] In NO X Among the products after reaction with water, NO2 - The concentration of NO2 is the highest, so the NO2 - The concentration of nitrite ions plays a vital role in improving the efficiency of the plasma electrocatalytic coupling system. The concentration of nitrite ions is also detected by spectrophotometry, which is simple and has high selectivity and sensitivity to nitrite ions.

[0129] First, prepare the Grisee colorimetric reagent: 0.1g N-(1-naphthyl)ethylenediamine dihydrochloride, 1g sulfonamide, and 2.94ml H3PO4 to make 50ml of deionized water solution. Then prepare NH4 + Similar to the standard solution, 1, 2, 3, 4, 5 μg / mL of NO2 were prepared. - 1 mL of each solution was taken into a colorimetric tube, and then 2 mL of deionized water and 1 mL of Grisee solution were added in sequence, shaken to mix, and allowed to stand for 15 min. The absorbance at 540 nm was measured using an ultraviolet spectrophotometer and plotted as NO2 - The standard curve is as follows Fig.28 shown.

[0130] (10) Calculation of Faraday efficiency and yield

[0131] In the field of electrocatalytic ammonia synthesis, ammonia yield and Faraday efficiency are two key performance indicators that characterize electrocatalytic ammonia synthesis:

[0132] Ammonia yield: the rate of electrocatalytic ammonia synthesis, which is the ammonia production per unit catalyst per unit time calculated based on the ammonia concentration;

[0133] Faraday efficiency: reflects the selectivity of electrocatalytic synthesis of NH3.

[0134] The calculation formula of ammonia yield is:

[0135] r(NH3)=(n×V) / (t×A)

[0136] Where n is the NH3 concentration, V is the volume of the reaction solution, t is the reaction time, and A is the geometric area of ​​the catalyst.

[0137] Faraday efficiency calculation formula:

[0138] FE(%)=(n×F×c×V) / (M×Q)

[0139] Among them, F is the Faraday constant (96500C / mol), n is the number of electrons gained and lost in the redox reaction (n is taken as 8 in the reaction of nitrate reduction to ammonia), c is the NH3 concentration, V is the volume of the reaction solution, M is the relative molecular mass of NH3, and Q is the total charge.

[0140] The above embodiments are used to illustrate the present invention rather than to limit the present invention. Any modification and change made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A modified metal oxide catalyst, characterized in that: Cobalt oxide is used as an initial metal oxide catalyst, and is modified by treating the cobalt oxide with radio frequency inductively coupled plasma (ICP) under vacuum conditions and an argon atmosphere, and a defective cobalt oxide catalyst is obtained after the discharge is completed.

2. A modified metal oxide catalyst according to claim 1, characterized in that: Vo3O4 was prepared by a hydrothermal method. Cobalt nitrate hexahydrate, ammonium fluoride and urea were added to deionized water to form a solution. The solution was stirred continuously and carbon paper was added. The solution was then transferred to a reactor, sealed and placed in an oven for heating. After the heating was completed and the carbon paper was cooled, it was taken out and rinsed with deionized water and anhydrous ethanol to dry the surface impurities and then dried. The dried precursor was placed in a tubular furnace and calcined under an Ar atmosphere to obtain cobalt oxide attached to the carbon paper.

3. A modified metal oxide catalyst according to claim 1, characterized in that: The molar ratio of cobalt nitrate hexahydrate, ammonium fluoride and urea is 1.0:1.8-2.1:4.7-5.2; the heating temperature in the oven is 120°C, and the heating time is 8 hours; the drying temperature is 80°C, and the drying time is 8 hours; the calcination temperature is 350°C, and the calcination time is 2 hours.

4. A modified metal oxide catalyst according to claim 1, characterized in that: The discharge power of the radio frequency inductively coupled plasma ICP is 150 W and the discharge time is 4 min.

5. A plasma-coupled electrocatalytic ammonia synthesis system based on the modified metal oxide catalyst according to any one of claims 1 to 4, characterized in that: The system consists of two parts: a plasma activated air system and an electrocatalytic ammonia synthesis system; The plasma activated air system is used to generate NO by plasma discharge in the jet gliding arc plasma reactor. X , and NO X After cooling, the mixture is introduced into the absorption liquid of the electrocatalytic ammonia synthesis system for full absorption; The electrocatalytic ammonia synthesis system uses the absorption liquid as the cathode liquid to electrocatalytically produce ammonia in an H-type cell, and the cathode uses a cobalt oxide catalyst modified by plasma treatment.

6. The plasma-coupled electrocatalytic ammonia synthesis system according to claim 5, characterized in that: The discharge voltage in the plasma activated air system is 9 to 10 kV.

7. The plasma-coupled electrocatalytic ammonia synthesis system according to claim 5, characterized in that: The cooling method in the plasma activated air system is a serpentine condenser.

8. The plasma-coupled electrocatalytic ammonia synthesis system according to claim 5, characterized in that: The air flow rate in the plasma activated air system is 4.5-5.5 L min -1 .

9. The plasma-coupled electrocatalytic ammonia synthesis system according to claim 5, characterized in that: The absorption liquid in the electrocatalytic ammonia synthesis system is an alkaline solution.

10. The plasma-coupled electrocatalytic ammonia synthesis system according to claim 5, characterized in that: The potential in the electrocatalytic ammonia synthesis system is -0.88 to -0.48 V vs. RHE.

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