Method for preparing nitric acid and nitrous acid under catalyst-free condition

Abstract

The invention relates to the technical field of chemical synthesis, and particularly discloses a method for preparing nitric acid and nitrous acid under a catalyst-free condition. The method comprises the following steps: atomizing deionized water to form micro-droplets, mixing the micro-droplets with high-purity nitrogen to construct a nitrogen-water micro-droplet mixed system, applying a radio frequency electric field to the system at normal temperature and normal pressure to perform micro-plasma discharge treatment, enabling nitrogen and water to react under a catalyst-free condition to generate nitric acid and nitrous acid, and collecting products through an absorption device. By adopting the technical scheme, green, low-carbon and high-selectivity synchronous synthesis of nitric acid and nitrous acid can be realized, the energy consumption and carbon emission are remarkably reduced, the process flow is simple, the raw materials are cheap, the operation is safe, the method can be coupled with a renewable energy power system, and the method is suitable for the fields of chemical fertilizers, medicines, electronic chemicals and the like.

Description

Technical Field

[0001] This invention belongs to the field of chemical synthesis technology, specifically a method for preparing nitric acid and nitrous acid under catalyst-free conditions. Background Technology

[0002] Nitric acid and nitrous acid, as important nitrogen-containing chemicals, have wide applications in industrial production and daily life. However, current mainstream preparation methods have significant limitations: nitric acid is mainly produced through ammonia oxidation, which consumes large amounts of fossil fuels and emits greenhouse gases; nitrous acid mostly relies on nitric acid reduction or amine oxidation, which not only has high raw material costs but also easily produces toxic byproducts, and involves complex processes and significant catalyst loss. Therefore, developing a green, efficient, and low-cost method for preparing nitric acid and nitrous acid has become a research hotspot and urgent need in the field of chemical synthesis.

[0003] A search revealed that invention patent CN110013873B discloses a catalyst for the catalytic decomposition of nitrates, nitrites, or dilute nitric acid, as well as its preparation and application. This patent provides a catalyst based on a composite of HZSM-5 molecular sieve and activated carbon powder, used in a fixed-bed reactor to catalytically decompose nitrates, nitrites, or dilute nitric acid aqueous solutions to generate the target product. However, this technical solution relies on the use of a specific catalyst (such as HZSM-5 molecular sieve), and the catalyst preparation process is relatively complex, requiring periodic replacement or regeneration, increasing process costs and operational difficulty. Furthermore, this method has high requirements for reaction conditions (such as temperature and atmosphere), which may limit its application in large-scale industrial production.

[0004] A search revealed that invention patent CN107228923B discloses a method and system for preparing standard gaseous nitrous acid. This patent is based on the reaction of gaseous hydrochloric acid with solid nitrite under specific temperature and humidity conditions, which can accurately and easily prepare gaseous nitrous acid of a specific concentration. However, the core of this technical solution lies in using nitrite as a reaction raw material, while nitrite itself needs to be synthesized by other methods, resulting in a long overall process chain and high raw material costs. In addition, this method has strict requirements for controlling the reaction environment (such as temperature and humidity), which may increase equipment investment and operating costs. At the same time, the use of gaseous hydrochloric acid also brings corrosiveness and safety risks.

[0005] The aforementioned problems indicate that existing methods for preparing nitric acid and nitrous acid still have certain shortcomings in terms of catalyst dependence, raw material costs, process complexity, and environmental friendliness. Therefore, this invention provides a method for achieving green nitrogen fixation and efficient preparation of nitric acid and nitrous acid through an electrically excited nitrogen-water droplet mixing system under catalyst-free conditions. This method aims to avoid the high carbon emissions and catalyst loss problems of traditional processes, significantly reduce resource dependence, and, driven by clean electricity, cut off carbon emissions at the source, meeting the needs of green chemical engineering and sustainable agriculture. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a method for preparing nitric acid and nitrous acid under catalyst-free conditions, aiming to solve the problems of traditional nitric acid and nitrous acid preparation processes relying on high temperature and high pressure, fossil energy and complex catalyst systems.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: A method for preparing nitric acid and nitrous acid under catalyst-free conditions includes the following specific steps: S1. Water droplet preparation: Deionized water is atomized through an atomizer to form water droplets.

[0008] Preferably, in step S1, the nebulizer is a compression medical nebulizer, which breaks down deionized water by means of the shear force of the airflow and the synergistic effect of the atomization structure. The average diameter of the water droplets ranges from 1 to 5 μm. The operating pressure range of the compression medical nebulizer is 60-130 kPa, its power is 130 VA, its average atomization volume is ≥0.2 mL / min, and its gas flow rate is ≥6 L / min.

[0009] S2. Preparation of the mixed system: Nitrogen gas and the water droplets are introduced into the reaction vessel to form a nitrogen-water droplet mixed system.

[0010] Preferably, in step S2, the purity of the nitrogen gas is ≥99.99%, and the content of O2 and CO2 impurities is ≤5 ppm. The flow rate of the nitrogen gas is controlled at 40-50 mL / min, the flow rate of the carrier gas carrying water droplets is controlled at 6 L / min, and the mixing system is continuously introduced into the reaction vessel for 1-2 minutes.

[0011] S3. Micro-plasma discharge treatment: Under normal temperature and pressure, a radio frequency electric field is applied to the nitrogen-water microdroplet mixture system in the reaction vessel to perform micro-plasma discharge treatment, thereby generating nitric acid and nitrous acid.

[0012] Preferably, in step S3, the reaction vessel is a quartz tube, with copper wire wound around its outer ring. The radio frequency electric field is applied by connecting positive and negative voltages to both ends of the copper wire. The left end of the copper wire in the quartz tube is connected to the negative electrode, and the right end is connected to the positive electrode. The electrode spacing of the radio frequency electric field is 10 cm, and the operating frequency of the radio frequency electric field is 13-14 MHz. In step S3, the voltage range of the radio frequency electric field is 80-110 V, and the discharge treatment lasts for 40 minutes. During the micro-plasma discharge, high-energy electrons are generated. These high-energy electrons collide with nitrogen molecules, causing their nitrogen-nitrogen triple bonds to break, generating nitrogen free radicals and high-energy nitrogen molecules. Simultaneously, water droplets decompose under the discharge to generate hydrogen free radicals, oxygen free radicals, and hydroxyl free radicals. The active nitrogen species generated in step S3 combine with oxygen free radicals to generate nitric oxide and nitrogen dioxide. Nitric oxide and nitrogen dioxide further react with hydroxyl free radicals to generate nitrous acid and nitric acid. The entire reaction is completed at room temperature and pressure without the introduction of any catalyst, and the total content of byproducts is less than or equal to 5%.

[0013] S4. Product collection: The product generated in step S3 is passed into an absorption device containing a deionized aqueous solution for product absorption and collection.

[0014] Preferably, in step S4, the absorption device is a brown volumetric flask containing 200 mL of deionized water at an initial temperature of 20 ± 2 °C. Magnetic stirring is used to promote product dissolution, with the stirring speed ranging from 300 to 500 rpm. Following step S4, the product solution is further purified by ion exchange chromatography to obtain a high-purity nitric acid or nitrous acid solution. The separation process uses a strongly acidic cation exchange resin, and the eluent is a 0.1 mol / L hydrochloric acid solution.

[0015] Compared with the prior art, the present invention has the following beneficial effects: This invention achieves a catalyst-free, green nitrogen fixation process by constructing a nitrogen-water microdroplet hybrid system and applying a radio frequency electric field for micro-plasma discharge treatment at ambient temperature and pressure. This directly converts inexhaustible nitrogen and deionized water from the air into high-value-added nitric acid and nitrous acid. This method completely avoids the dependence of traditional ammonia oxidation methods on high temperature and pressure, fossil fuels, and complex catalyst systems, significantly reducing energy consumption and carbon emissions. Simultaneously, the introduction of water droplets greatly increases the gas-liquid reaction interface, improving mass transfer efficiency and free radical yield, enabling efficient synergistic activation and oxidation reactions of nitrogen. By precisely controlling key parameters such as water droplet size, nitrogen purity, gas flow rate, radio frequency voltage, and reaction time, this invention achieves highly selective simultaneous synthesis of nitric acid and nitrous acid without any catalytic components, with a significantly higher overall product yield than existing catalytic or non-catalytic systems. Furthermore, this process is simple, uses inexpensive and readily available raw materials, is safe and controllable in operation, and can be seamlessly integrated with renewable energy power systems. It provides a new technological path for the green, low-carbon, and sustainable production of nitric acid and nitrite, and has broad application prospects in fertilizers, pharmaceuticals, electronic chemicals, and fine chemicals. Attached Figure Description

[0016] Figure 1 This is a flowchart of the method for preparing nitric acid and nitrous acid under catalyst-free conditions according to the present invention.

[0017] Figure 2 The graph shows the changes in pH and the yields of nitric acid and nitrous acid during the micro-plasma discharge treatment process.

[0018] Figure 3 The yield graphs of nitric acid and nitrous acid under different voltage radio frequency electric fields are shown. Detailed Implementation

[0019] Figure 1 shows a flowchart of a method for preparing nitric acid and nitrous acid under catalyst-free conditions. The technical solution of the present invention will be described in detail below with reference to specific embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0020] Example 1 A method for preparing nitric acid and nitrous acid under catalyst-free conditions specifically includes the following steps: S1, deionized water is atomized using an atomizer to form water droplets. Specifically, the atomizer used in step S1 is a compressor-type medical atomizer (product model: YJY-II, working pressure: 60-130 kPa, power: 130 VA, average atomization volume: ≥0.2 mL / min, gas flow rate: ≥6 L / min). This atomizer utilizes the shear force generated by high-speed airflow and the synergistic effect of its internal precision atomization structure to efficiently break down the deionized water. The working pressure range of the compressor-type medical atomizer is set to 60-130 kPa, its rated power is 130 VA, its average atomization volume is not less than 0.2 mL / min, and its matching gas flow rate is not less than 6 L / min. Under these operating parameters, the water droplets formed by the breakdown of deionized water have a highly uniform particle size distribution, with their average diameter strictly controlled within the range of 1-5 μm. The selection of this particle size range is based on engineering considerations balancing the gas-liquid interface area and droplet stability: too small a particle size can lead to rapid evaporation of the droplets during transport, reducing the effective concentration of the liquid phase in the reaction system; too large a particle size will significantly reduce the gas-liquid contact interface per unit volume, weakening the free radical generation efficiency. To ensure the stability and repeatability of the atomization process, the internal flow channel of the atomizer is made of medical-grade stainless steel with an ultra-precision polished surface, achieving a roughness Ra of less than 0.2 μm to avoid impurity introduction and droplet aggregation. The water droplets generated by atomization are output in the form of an aerosol, and their concentration is dynamically controlled by adjusting the working gas pressure and liquid supply rate to ensure a uniformly dispersed two-phase flow when subsequently mixed with nitrogen.

[0021] In step S2, nitrogen gas and the water droplets are introduced into the reaction vessel to form a nitrogen-water droplet mixture. Specifically, the nitrogen gas used in step S2 has a purity of not less than 99.99%, and the content of oxygen and carbon dioxide impurities does not exceed 5 ppm. The selection of high-purity nitrogen gas aims to minimize the occurrence of side reactions, especially to avoid premature reaction between oxygen and active nitrogen species to form non-target nitrogen oxides, or carbon dioxide dissolving in water droplets to form carbonic acid, interfering with the acid-base balance of the product system. Nitrogen gas is precisely metered using a high-precision mass flow controller, and its gas flow rate is strictly controlled within the range of 40 mL / min. At the same time, a carrier gas carrying water droplets (usually an inert gas) is introduced synchronously at a flow rate of 6 L / min. The two are fully turbulently mixed in the mixing chamber to form a stable nitrogen-water droplet mixture. The gas flow rate of the atomizer in step S1 is the same as the flow rate of the carrier gas carrying water droplets in step S2. The mixing system is continuously introduced into the reaction vessel for 1-2 minutes. This time window is determined based on comprehensive simulation calculations of the gas replacement efficiency and reactant concentration gradient within the reaction vessel, ensuring that the original gas in the vessel is completely replaced and the reactant concentration reaches a steady-state level before the reaction begins. The mixing chamber is designed with a Venturi tube structure, utilizing the Bernoulli effect to generate local negative pressure at the throat, actively drawing in water droplet aerosols to achieve efficient and uniform mixing of nitrogen and water droplets, with a standard deviation of mixing uniformity of less than 3%.

[0022] In step S3, a radio frequency electric field is applied to the nitrogen-water droplet mixture within the reaction vessel under normal temperature and pressure conditions to perform micro-plasma discharge treatment, thereby generating nitric acid and nitrous acid. Specifically, the reaction vessel used in step S3 is a high-purity fused silica tube, which possesses excellent dielectric properties, chemical inertness, and light transmittance, and can withstand the localized high temperature and strong oxidizing environment generated by the micro-plasma discharge. A 1 mm diameter oxygen-free copper wire is tightly wound around the outer ring of the quartz tube. The left end of the copper wire is connected to the negative terminal of the radio frequency power supply, and the right end is connected to the positive terminal, forming an effective discharge area of ​​10 cm in length. The radio frequency electric field is established by applying a 100 V AC voltage to both ends of the copper wire, with an operating frequency of 13.56 MHz. This frequency is the standard frequency of the Industrial, Scientific, and Medical (ISM) band, effectively avoiding electromagnetic interference to communication equipment. The discharge treatment lasts for 40 minutes, during which the interior of the reaction vessel is maintained at normal temperature (25 ± 2 °C) and normal pressure (101.3 kPa).

[0023] During micro-plasma discharge, a high-intensity alternating electromagnetic field is induced inside the quartz tube by a radio frequency electric field, repeatedly accelerating free electrons in the mixed system to gain sufficient energy. The chemical formula for the reaction is: These high-energy electrons undergo inelastic collisions with nitrogen molecules, causing the nitrogen-nitrogen triple bonds in the nitrogen molecules to break, generating highly reactive nitrogen radicals (N·) and excited-state nitrogen molecules (N2*). Simultaneously, water droplets interact with high-energy electrons (e·) in a strong electric field. - Under the combined action of nitrogen and oxygen radicals, dielectric breakdown and coulombic explosion occur, resulting in the generation of hydrogen radicals (H·), oxygen radicals (O·), and hydroxyl radicals (·OH). These radicals undergo a series of complex chain reactions at the gas-liquid interface: First, nitrogen radicals rapidly combine with oxygen radicals to generate nitric oxide (NO); nitric oxide further reacts with oxygen radicals or hydroxyl radicals to generate nitrogen dioxide (NO2); subsequently, nitric oxide and nitrogen dioxide undergo disproportionation reactions with hydroxyl radicals on the surface of water droplets, generating nitrous acid (HNO2) and nitric acid (HNO3), respectively. The entire reaction pathway can be summarized as follows: The above reaction proceeds spontaneously at room temperature and pressure without the introduction of any catalyst, and the total content of byproducts does not exceed 5%. To ensure the uniformity and stability of the discharge, the copper wire wound around the outer wall of the quartz tube is a single-layer tightly wound method with a turn spacing error of less than 0.1 mm, and is insulated and fixed with epoxy resin to prevent short circuits between turns. The RF power supply is equipped with a real-time impedance matching network, which can automatically adjust the output parameters according to the dynamic changes of the discharge load to maintain a constant discharge power of 100 W.

[0024] In step S4, the product generated in step S3 is passed into an absorption device containing a deionized aqueous solution for absorption and collection. Specifically, the absorption device used in step S4 is a 250 mL brown volumetric flask containing 200 mL of deionized aqueous solution. The brown volumetric flask is chosen to shield from ambient light and prevent photolysis of nitrous acid under light. The initial temperature of the deionized water is strictly controlled at 20±2 ℃ to optimize the solubility of the gas in water and suppress side reactions. A magnetic stirrer is used to enhance mass transfer during the absorption process, with the stirring speed set in the range of 300-500 rpm. This speed is sufficient to form a stable vortex within the flask, ensuring that the gaseous products (mainly NO, NO2, and trace amounts of N2O3 and N2O4) are in full contact with the absorption liquid and dissolve rapidly, while avoiding excessive splashing or foaming due to excessive speed. The absorption process continues until the discharge ends, ensuring that all gaseous products are completely captured. After absorption, a mixed aqueous solution containing nitric acid and nitrous acid is obtained.

[0025] Following step S4, the product solution is further purified by ion exchange chromatography to obtain a high-purity nitric acid solution or nitrite solution. The separation process uses a strongly acidic cation exchange resin as the stationary phase, and the eluent is a 0.1 mol / L hydrochloric acid solution. Through a gradient elution program, effective separation is achieved based on the difference in affinity between nitrate and nitrite ions on the resin, ultimately yielding nitric acid and nitrite solutions with purities exceeding 99%. Figure 2 The graph shows the changes in pH and the yields of nitric acid and nitrous acid during the micro-plasma discharge treatment process. Figure 3 The graph shows the yields of nitric acid and nitrous acid under different voltage radio frequency electric fields. The yield of nitric acid in this example was verified to be 27.16 μmol·L⁻¹ at room temperature and pressure. -1 ·min -1 The yield of nitrite was 16.25 μmol·L⁻¹. -1 ·min -1 .

[0026] Example 2 Unlike Example 1, in step S2 of this example, nitrogen gas and water droplets are continuously passed through a quartz tube wound with copper wire at a gas flow rate of 50 mL / min and a carrier gas flow rate of 6 L / min, respectively, while the other steps remain unchanged.

[0027] Example 3 Unlike Example 1, in step S3 of this example, the radio frequency electric field is established by applying an 80V AC voltage to both ends of the copper wire, while the other steps remain unchanged.

[0028] Comparative Example 1 A method for preparing nitric acid and nitrous acid based on HZSM-5 molecular sieve catalyst includes the following steps: (Refer to Example 1 of CN110013873B): Step 1: 80g of waste ZSM-5 molecular sieve was calcined at 550℃ for 5h and ground to 200 mesh to obtain HZSM-5 molecular sieve powder.

[0029] Step 2: Mix HZSM-5 powder, coal-based activated carbon (200 mesh, 15% ash), boehmite, methylcellulose, deionized water and 63% concentrated nitric acid in a mass ratio of 2.0:0.5:0.14:0.2:1:0.22. Knead the mixture into a plastic body, extrude it into strips, dry it at 120℃ for 8 hours, calcine it at 500℃ for 6 hours, and crush it into 20-40 mesh particles.

[0030] Step 3: Load 5g of the above catalyst into the fixed-bed reactor and activate it at 350℃ for 2h under a nitrogen atmosphere.

[0031] Step 4: Introduce 0.5 mol / L dilute nitric acid aqueous solution, control the reaction temperature at 350℃, pressure at 0.3 MPa, liquid hourly space velocity at 0.75 h⁻¹, and nitrogen carrier gas flow rate at 30 mL / min. Collect the product through an absorption device.

[0032] Comparative Example 2 Unlike Example 1, in step S3 of this example, the radio frequency electric field is established by applying an AC voltage of 120V to both ends of the copper wire, while the other steps remain unchanged.

[0033] Based on the above embodiments, the key parameters for the preparation of nitric acid and nitrous acid by micro-plasma discharge in a nitrogen-water microdroplet system under catalyst-free conditions were optimized. The experimental results are shown in the table below: Table 1. Yields of nitric acid and nitrous acid prepared by different embodiments. Based on Example 1 and Comparative Example 1, comparative experiments were conducted on the preparation of nitric acid and nitrous acid under catalyst-free and catalyst-containing conditions (referring to the preparation method in existing patent CN110013873B). The experimental results are shown in the table below: Table 2 Comparison of Catalyst-Based and Non-Catalyst-Based Methods The above results indicate that: (1) When the nitrogen flow rate increased from 40 mL / min to 50 mL / min, the yields of nitric acid and nitrous acid decreased slightly. This may be because excessive nitrogen dilutes the concentration of active free radicals in the reaction system, resulting in a decrease in nitrogen activation efficiency.

[0034] (2) When the voltage is reduced to 80 V (Example 3): the yield decreases significantly due to the reduced number of high-energy electrons and the reduced efficiency of nitrogen triple bond breaking; when the voltage is increased to 120 V (Comparative Example 2): the yield increases, but the by-product content exceeds 5%, possibly due to the generation of by-products such as N2O caused by over-discharge. Therefore, the optimal voltage is 100 V (Example 1).

[0035] (3) The non-catalytic method has a higher yield and no catalyst loss; the non-catalytic method has a lower content of by-products because it avoids non-selective reactions on the catalyst surface.

[0036] Conclusion: By optimizing the nitrogen flow rate (40 mL / min) and radio frequency voltage (100 V), this invention enables the efficient synthesis of nitric acid and nitrous acid under ambient temperature and pressure without a catalyst. The overall performance is superior to that of traditional catalytic methods, providing a feasible route for green nitrogen fixation.

[0037] Performance comparison and advantages of this invention compared with traditional nitrogen fixation technology: 1. Energy Consumption Comparison: Taking the ammonia oxidation method for nitric acid synthesis as an example, the traditional preparation of nitric acid requires the Haber-Bosch process to synthesize ammonia. This process must be carried out at a high temperature of 400-500℃ and a high pressure of 15-30MPa, relying on fossil fuels to provide heat and hydrogen as the source of hydrogen. The energy consumption per ton of ammonia is approximately 30-40 GJ, and the entire process emits a large amount of CO2, causing environmental pollution. This technology, under normal temperature and pressure, uses electrical energy to drive micro-plasma discharge, achieving "zero carbon nitrogen fixation" without the consumption of fossil fuels. The energy consumption per unit product is significantly lower than that of traditional methods.

[0038] 2. Yield Comparison: Traditional nitrogen fixation technology using the Haber-Bosch process achieves a conversion efficiency of approximately 15%-20% for ammonia synthesis, with the subsequent ammonia oxidation to nitric acid yielding approximately 85%-90%. However, this process involves numerous steps and is lengthy. This technology, under the influence of micro-plasma, directly converts nitrogen gas and water droplets into nitric acid and nitrous acid, completing nitrogen fixation in one step. Furthermore, the product yield can be optimized by adjusting reaction parameters, resulting in higher atom economy.

[0039] 3. Cost Comparison: Traditional technologies use expensive catalysts such as iron-based catalysts, which are prone to deactivation and require regular replacement. The high purity of hydrogen and complex processes also contribute to the high cost. This technology eliminates the need for catalysts, avoiding the costs, regeneration, deactivation, and loss associated with them. The raw materials are nitrogen and deionized water, which are inexpensive and widely available, and the process is direct and simple.

[0040] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. A method for preparing nitric acid and nitrous acid under catalyst-free conditions, characterized in that, Includes the following steps: S1. Water droplet preparation: Deionized water is atomized through an atomizer to form water droplets; S2. Preparation of the mixed system: Nitrogen gas and the water droplets are introduced into the reaction vessel to form a nitrogen-water droplet mixed system; S3. Micro-plasma discharge treatment: Under normal temperature and pressure, a radio frequency electric field is applied to the nitrogen-water microdroplet mixture system in the reaction vessel to perform micro-plasma discharge treatment, thereby generating nitric acid and nitrous acid; S4. Product collection: The product generated in step S3 is passed into an absorption device containing a deionized aqueous solution for product absorption and collection.

2. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 1, characterized in that, In step S1, the nebulizer is a compression medical nebulizer, which breaks down deionized water by means of the shear force of the airflow and the synergistic effect of the atomization structure.

3. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 2, characterized in that, The average diameter of the water droplets ranges from 1 to 5 μm.

4. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 2, characterized in that, The working pressure range of the compressed medical nebulizer is 60-130 kPa, its power is 130 VA, its average nebulization volume is ≥0.2 mL / min, and its gas flow rate is ≥6 L / min.

5. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 1, characterized in that, In step S2, the purity of the nitrogen gas is ≥99.99%, and the content of O2 and CO2 impurities is ≤5ppm.

6. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 1, characterized in that, In step S2, the flow rate of nitrogen is controlled at 40-50 mL / min, the flow rate of the carrier gas carrying water droplets is controlled at 6 L / min, and the mixing system is continuously introduced into the reaction vessel for 1-2 minutes.

7. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 1, characterized in that, In step S3, the reaction vessel is a quartz tube, the outer ring of which is wound with copper wire, and the radio frequency electric field is applied by connecting positive and negative voltages to both ends of the copper wire.

8. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 7, characterized in that, The copper wire of the quartz tube is connected to the negative electrode on the left and the positive electrode on the right, and the electrode spacing of the radio frequency electric field is 10cm, and the operating frequency of the radio frequency electric field is 13-14MHz.

9. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 1, characterized in that, In step S3, the voltage range of the radio frequency electric field is 80-110V, and the discharge treatment lasts for 40 minutes.

10. The method for preparing nitric acid and nitrous acid under catalyst-free conditions according to claim 1, characterized in that, In step S4, the absorption device is a brown volumetric flask containing 200 mL of deionized water solution, the initial temperature of the deionized water is 20 ± 2 °C, and the rotation speed range of the magnetic stirrer is 300-500 rpm.

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