Catalyst for nitrogen fixation in non-thermal plasma environment, catalytic material, catalytic system and low-temperature nitrogen fixation method

Abstract

The invention discloses a catalyst for nitrogen fixation in a non-thermal plasma environment, a catalytic material, a catalytic system and a low-temperature nitrogen fixation method, and the catalyst can promote nitrogen and oxygen to be activated and converted into nitrogen oxides through redox cycles and defect sites on the surface of the catalyst. The non-thermal plasma can directly realize nitrogen fixation from air at a temperature close to the ambient temperature. The dielectric barrier discharge reactor is coupled with manganese doped TiO2 (Mn-TiO2) to guide the NOx pathway to generate nitrate. The comprehensive characterization associates performance with lattice distortion, rich oxygen vacancies, and Mn < 2 + > / Mn < 3 + > / Mn < 4 + > redox cycles. Compared with pure plasma and pure TiO2, the Mn-TiO2 has the advantages that higher NO2 <-> / NO3 <-> formation rate (37.77 / 151.20 mol / h), higher yield (0.02% / 0.09%) and higher energy efficiency (50.40 mmol / kWh) are realized at the temperature of 56.9 DEG C.

Description

Technical Field

[0001] This invention relates to the fields of non-thermal plasma catalysis, nitrogen cycle conversion, and distributed chemical synthesis. Specifically, it relates to a nitrogen-fixing catalyst, catalytic material, catalytic system, and low-temperature nitrogen-fixing method that can simultaneously activate nitrogen and oxygen molecules in the air and efficiently generate nitrogen oxides under ambient to low-temperature conditions by controlling the multivalent states of transition metals, defect structures, and the synergistic effect of non-thermal plasma. This technology can directly convert air into nitrate-type liquid nitrogen fertilizer, exhibiting significant greening, distributed processing, low carbon emissions, and industrial feasibility. Background Technology

[0002] According to current technology, nitrogen fertilizer is one of the most critical production factors in modern agricultural systems, and nitrogen is generally considered a major limiting nutrient for plant growth. Currently, the vast majority of nitrogen fertilizer globally is produced using the Haber-Bosch (HB) process, which synthesizes ammonia (NH3) from atmospheric nitrogen (N2) and fossil fuel-based hydrogen (H2) under high temperature and pressure. While this process laid the foundation for modern agricultural systems, it is highly energy-intensive and heavily reliant on fossil resources. Literature reports that the HB process consumes approximately 2% of the global annual energy supply and contributes about 1.4% of global carbon dioxide emissions. Furthermore, the process is accompanied by methane leaks and greenhouse gas emissions, causing a sustained impact on the ecological environment.

[0003] Despite global ammonia production exceeding 170 million tons annually, the utilization efficiency of traditional ammonia-based nitrogen fertilizers remains low. Typically, less than 30-40% of the nitrogen is ultimately absorbed by crops, with the remainder lost through volatilization, leaching, or denitrification. After application to the soil, NH3 must first be oxidized to nitrite (NO2) by nitrifying bacteria. - It is further converted into nitrate (NO3) - Nitrification is a process that requires time and energy to be efficiently absorbed by plants. It also produces greenhouse gases such as N2O, further reducing nitrogen fertilizer efficiency and exacerbating the environmental burden.

[0004] NO3 - It is a more direct and efficient nitrogen source for plants to absorb, and compared with NH3 base fertilizer, it has significant advantages in root nutrient absorption, cation balance, and stable crop yield. Although nitrite (NO2) - Nitrates can also serve as a potential nitrogen source, but they are relatively unstable in soil and are prone to microbial reduction, leading to nitrogen loss and N2O emissions. Therefore, from the perspective of sustainable agriculture and emission reduction, directly obtaining stable nitrates would significantly improve nitrogen fertilizer conversion efficiency, environmental friendliness, and agricultural returns.

[0005] In recent years, novel nitrogen fixation technologies have gained increasing attention due to the high energy consumption and carbon emissions of traditional HB processes. In particular, non-thermal plasma technology can excite N2 and O2 at near-ambient temperatures to generate various reactive species, offering new possibilities for low-temperature nitrogen fixation. However, current plasma nitrogen fixation research still generally suffers from the following limitations: (1) Most technical routes still target NH3 as the product, which cannot avoid nitrogen loss caused by subsequent nitrification; (2) Many systems require the use of pure nitrogen or auxiliary gases, resulting in relatively high energy consumption; (3) The lack of efficient catalysts that can synergize with plasma active species results in a limited NOx generation rate; (4) The process of "air → NOx → NO3" has not yet been formed. - The process mode is a one-step, continuously operating process.

[0006] Therefore, there is an urgent need to develop a catalytic system and nitrogen fixation method that can achieve direct nitrogen fixation from air and efficiently generate nitrates under low-temperature conditions, in order to solve the key problems of high energy consumption, high emissions and low utilization rate in traditional nitrogen fertilizer production.

[0007] Among various alternatives, nonthermal plasma (NTP)-driven nitrogen fixation is considered a highly attractive candidate to replace the Haber-Bosch process. This approach relies on a thermodynamic nonequilibrium between high-energy electrons and a near-room-temperature bulk gas to generate active substances, thereby promoting chemical reactions under mild conditions. Various NTP discharge techniques, including dielectric barrier discharge (DBD), transient spark discharge, and DC glow discharge, have been explored for NOx generation and nitrogen fixation. Jogi et al. demonstrated that an NTP-assisted dielectric barrier discharge (DBD) reactor can uniformly generate reactive oxygen species (ROS), promoting nitrogen oxidation and improving nitrogen fixation yield. Tang et al. discovered that different active species in DBD, such as ROS and excited N2 (A... 3 Σᵤ + This can accelerate NOx formation and conversion, achieving maximum NOx yield at optimal energy density. Similarly, Janda et al. and Pei et al. investigated DC discharge systems, revealing not only their potential but also highlighting the limitations of relying solely on plasma parameters to improve conversion efficiency. To overcome these challenges, researchers introduced catalysts into plasma systems to lower activation energy, alter discharge characteristics, and improve performance. 13-15 Patil et al. reported that by loading Co3O4, MoO3, or V2O5 onto γ-Al2O3 and adding it to a DBD reactor, the NOx yield was increased by two times. Meanwhile, Chen et al. demonstrated that WO3 promoted the deep oxidation of NOx species. Cao et al. further showed that loading MnO... xAl₂O₃ with added Mn exhibits superior NO₂ production capacity compared to Al₂O₃ alone, revealing the crucial role of catalysts in enhancing plasma reactivity. In particular, Mn-doped metal oxides demonstrate superiority due to their abundant redox properties, rich oxygen vacancies, and excellent adsorption-desorption behavior, enabling efficient oxygen activation and electron transfer. Extensive studies have confirmed the high catalytic activity of Mn in photocatalysis, electrocatalysis, and thermocatalysis (including VOCs oxidation and NO oxidation). However, the synergistic effect of Mn catalysts with NTPs in nitrogen fixation remains unexplored, representing a promising research direction for achieving scalable and energy-efficient nitrogen fixation systems. Summary of the Invention

[0008] 1) Technical problems to be solved This invention aims to overcome the prominent problems of existing nitrogen fixation technologies, such as reliance on high-energy-consuming processes, limited product types, low nitrogen fixation efficiency, and difficulty in forming distributed applications. It proposes an overall technical solution that can achieve direct nitrogen fixation of air and efficient preparation of nitrates under ambient to low-temperature conditions through the deep synergy of non-thermal plasma and multivalent metal-doped oxide catalysts.

[0009] (ii) Technical Solution To achieve the above objectives, the present invention provides: A Mn–TiO2 catalyst with Mn multivalent states and a high oxygen vacancy structure; An air nitrogen fixation system comprising a DBD reactor, a catalyst bed, and an aqueous phase absorption unit; A nitrogen fixation method that uses this system to directly convert air into nitrate solution under low-temperature conditions.

[0010] The catalysts, systems and methods mentioned above have an inherent synergistic relationship and can work together to achieve direct nitrogen fixation from air, achieving the goals of high efficiency, low energy consumption and continuous operation.

[0011] To achieve the above objectives, the present invention provides the following technical solution: a catalyst for nitrogen fixation in a non-thermal plasma environment, wherein the catalyst can promote the activation of nitrogen and oxygen and their conversion into nitrogen oxides through redox cycles and defect sites on its surface.

[0012] Furthermore, the redox cycle includes the cycling of manganese ions between +2, +3 and +4 valence states.

[0013] Furthermore, the defect sites include oxygen vacancies, which can enrich and stabilize chemisorbed active oxides.

[0014] Furthermore, the catalyst enables nitrogen fixation from air using non-thermal plasma and at a reaction temperature below 120°C.

[0015] Furthermore, the catalyst can guide the reaction pathway to selectively generate nitrogen dioxide and nitrate and nitrite ions that are ultimately captured in the aqueous phase.

[0016] Furthermore, the catalyst is able to dynamically increase the proportion of divalent manganese and the proportion of chemically adsorbed oxygen on its surface during plasma catalytic reactions.

[0017] The present invention also provides a catalytic material for nitrogen fixation, for use in any of the catalysts for nitrogen fixation in a non-thermal plasma environment, wherein the material is capable of generating surface states that weaken the triple bond of the N2 molecule and provide activated electrons in response to non-thermal plasma excitation.

[0018] Furthermore, the generation of the surface states is related to the mixed valence states of the transition metal ions doped in the material and their electronic interactions with the host lattice.

[0019] The present invention also provides a plasma catalysis system for producing nitrogen-containing products from a nitrogen- and oxygen-containing gas stream, the system comprising: A non-thermal plasma generating device configured to generate high-energy electrons and active materials in a gas flow; The catalytic reaction zone is located inside or downstream of the plasma generating device and contains a catalyst for catalyzing the conversion of plasma-generated active substances into target nitrogen-containing products.

[0020] Furthermore, the non-thermal plasma generating device operates directly with ambient air as the feed gas, without the need to pre-separate pure nitrogen.

[0021] Furthermore, the catalytic reaction zone includes a product capture device configured to absorb gaseous nitrogen oxides into an aqueous solution to form water-soluble nitrates and / or nitrites.

[0022] Furthermore, the non-thermal plasma generating device is a dielectric barrier discharge reactor, and the catalyst is disposed in the discharge gap of the dielectric barrier discharge reactor.

[0023] This invention also provides a method for low-temperature nitrogen fixation, the method comprising the following steps: 1) In the presence of non-thermal plasma, a gas stream containing nitrogen and oxygen is brought into contact with a catalyst; 2) Utilizing the function of the catalyst, nitrogen and oxygen in the gas stream are converted into nitrogen oxides through interfacial redox cycle and defect-assisted activation.

[0024] Furthermore, steps 1) and 2) are performed at temperatures below 100°C.

[0025] Furthermore, the catalyst supplies Mn 2+ Mn 3+ Mn 4+ One or more of the redox pairs can accelerate the insertion reaction of oxygen atoms into nitric oxide.

[0026] Furthermore, the catalyst provides oxygen vacancies to stabilize the reaction intermediates and inhibit over-oxidation to nitrous oxide.

[0027] Furthermore, the nonthermal plasma provides vibrationally excited nitrogen molecules and oxygen atoms, while the catalyst provides surface sites to lower the activation energy for subsequent reactions of these species.

[0028] Furthermore, the catalyst can utilize plasma-generated actives and selectively direct the reaction towards higher-value nitrate products via surface catalysis, thereby reducing energy loss caused by non-selective gas-phase reactions.

[0029] Furthermore, when the catalyst is used in the non-thermal plasma environment, it can promote the surface reaction between adsorbed nitric oxide and adsorbed oxide through the Langmuir-Hinshelwood mechanism.

[0030] Furthermore, the catalyst can maintain a high concentration of surface chemisorbed oxygen and rapidly replenish the lattice oxygen consumed in the reaction.

[0031] Furthermore, the structure of the catalyst is capable of maintaining its redox cycling capability during use, and its catalytic activity decreases by no more than 5% after undergoing multiple plasma on / off cycles.

[0032] Furthermore, by utilizing the synergistic effect of non-thermal plasma and catalyst, a mixture of nitrates and nitrites that can be absorbed by aqueous solutions can be produced directly from air in a one-step process.

[0033] Furthermore, the synergistic effect involves the plasma being responsible for generating NO from N2 and O2, while the catalyst is responsible for the deep oxidation of NO to NO2 and higher-order nitrogen oxides, which are readily hydrolyzed to form NO3. - and NO2 - .

[0034] Furthermore, by doping with specific elements to introduce oxygen vacancies and mixed valence metal centers into the catalyst matrix, its ability to receive and transfer electrons from plasma is enhanced.

[0035] Furthermore, optical emission spectroscopy was used to monitor the reaction zone and its interaction with N2 and N2. + The intensity changes of characteristic emission lines related to NO were investigated to assess the effect of catalyst introduction on plasma active concentration.

[0036] A method for regenerating a catalyst, wherein the catalyst is a manganese-based catalyst for plasma nitrogen fixation, comprises heat-treating the catalyst in an oxygen-containing atmosphere to restore its oxidation state and surface oxygen.

[0037] Nonthermal plasma can achieve nitrogen fixation directly from air at near-ambient temperatures. Here, we couple a dielectric barrier discharge reactor with manganese-doped TiO2 (Mn-TiO2) to guide the NOx-to-nitrate conversion pathway. Comprehensive characterization integrates performance with lattice distortion, abundant oxygen vacancies, and Mn content. 2+ / Mn 3+ / Mn 4+ The redox cycle is associated. Compared with pure plasma and pure TiO2, Mn-TiO2 achieved higher NO2 at 56.9°C. - / NO3 - Formation rate (37.77 / 151.20 µmol / h), yield (0.02% / 0.09%), and energy efficiency (50.40 mmol / kWh). N2SPS and N2 were detected by in-situ OES and RGA. + The FNS, NOγ bands, and gaseous NO2 / N2O spectra confirmed the synergistic effect of plasma and catalyst in NO formation and subsequent oxidation in the aqueous phase to nitrate. The catalyst remained stable without structural degradation during a 12-hour on / off cycle. These results establish defect-rich Mn-TiO2 as an efficient, low-temperature, plasma-assisted synthesis of nitrate from air.

[0038] This invention constructs a novel nitrogen fixation catalytic system with Mn-doped TiO2 (Mn–TiO2) catalyst as the core and deeply couples it with a dielectric barrier discharge (DBD) athermal plasma reactor. This is achieved by modulating Mn… 2+ / Mn 3+ / Mn 4+ Multivalent state centers, oxygen vacancy concentration and Ti 3+ Regarding the number of defects, this invention achieves precise control over the activation pathways of nitrogen (N2) and oxygen (O2), thereby significantly promoting the stepwise generation of NO and NO2, and ultimately efficiently obtaining nitrate (NO3) through hydrolysis. - ) solution.

[0039] Compared with existing plasma nitrogen fixation methods, the present invention has the following advantages: I. A multi-valence synergistic regulation strategy for Mn–TiO2 is proposed to construct an efficient nitrogen and oxygen activation center.

[0040] This invention is the first to utilize the multivalent state electronic modulation capability of Mn-doped TiO2, through Mn2+ / Mn 3+ / Mn 4+ The redox cycle, and Ti in the TiO2 lattice 3+ The defect centers and high concentrations of oxygen vacancies (Vo) form a synergistic structure, enabling the catalyst to: Electron, excitation-state N2 (N2*), and ion-state N2 that promote plasma generation + It exhibits synergistic effects with the catalyst surface; Accelerating the partial breakage of the N2 bond makes it easier for it to combine with reactive oxygen species; Increase the breakdown of O2 into O∙ and O2. - O - Efficiency; Increase NO → NO2 → NO3 - Stepwise oxidation rate.

[0041] This synergistic structure is a previously unreported multi-defect, multi-valence metal oxide nitrogen fixation catalytic center.

[0042] II. Design of a deep coupling nitrogen fixation pathway using non-thermal plasma and solid catalysts.

[0043] This invention, through the rational design of the DBD plasma reactor, enables the concentrated generation of high-energy electrons, excited-state molecules, and reactive oxygen species in the maximum field strength region of the catalyst bed, thereby: To achieve simultaneous excitation and synergistic activation of N2 and O2; It accelerates the formation of NO in the gas phase and provides sufficient precursors; By leveraging the abundant oxygen vacancies and electron enrichment interface of the Mn–TiO2 catalyst, NO is efficiently oxidized to NO2. NO2 can be rapidly absorbed and hydrolyzed through gas-liquid contact to form stable NO3. - .

[0044] The entire reaction process can operate in a low temperature range of 20–120°C without the need for light, hydrogen source, or external oxidants such as H2O2.

[0045] III. Constructing a one-step nitrogen fixation system from air to NOx to nitrates.

[0046] This invention realizes a holistic nitrogen fixation pathway using air as the sole raw material. Non-thermal plasma provides nitrogen and oxygen activation energy, the Mn–TiO2 catalyst regulates the electronic structure and reactive oxygen species, and the aqueous phase absorption unit is responsible for downstream conversion, achieving system integration.

[0047] This complete process chain has: No air separation equipment required No hydrogen or hydrogen source compound required No need for traditional high temperature and high pressure conditions No photochemical or Fenton auxiliary systems required It has strong advantages in system simplification and energy utilization.

[0048] IV. Comprehensive Technical Effects of the Catalyst, System, and Method Provided by the Invention Through comprehensive material characterization and reaction performance testing, the technical system of this invention exhibits the following superior properties: Mn doping significantly increases oxygen vacancies, which, along with Ti, creates oxygen vacancies. 3+ Together they form an electron-rich region, enhancing the activation capacity of nitrogen and oxygen.

[0049] NO2 - and NO3 - The generation rate is much higher than that of the undoped TiO2 system, and the energy efficiency is significantly improved.

[0050] The system maintains structural and performance stability after running continuously for more than 12 hours, with no obvious catalyst passivation.

[0051] The synergistic effect between plasma and Mn–TiO2 catalyst was fully verified, including OES signal enhancement and stable NO2 gas generation.

[0052] The process is simple, the conditions are mild, and the equipment is compact, making it a promising candidate for distributed agricultural liquid fertilizer preparation. Attached image description: Figure 1 (a) XRD patterns of TiO2, fresh Mn-TiO2 and used Mn-TiO2; (b) Magnified view of the diffraction peaks of the (110) crystal plane.

[0053] Figure 2 (a) SEM image, (b) HAADF-STEM image, (c) HRTEM image, (dh) elemental distribution map, and (i) SAED image of Mn-TiO2.

[0054] Figure 3 (a) N2 adsorption-desorption isotherms and (b) pore size distribution of TiO2, fresh Mn-TiO2 and used Mn-TiO2.

[0055] Figure 4 XPS spectra of TiO2, fresh and used Mn-TiO2: (a) Ti2p, (b) Mn2p, (c) O1s.

[0056] Figure 5(a) NH3-TPD, (b) H2-TPR and (c) O2-TPD spectra of TiO2, fresh Mn-TiO2 and used Mn-TiO2.

[0057] Figure 6 (a) EPR spectrum and (b) Tauc plot of TiO2, fresh Mn-TiO2 and used Mn-TiO2 (used to calculate band gap).

[0058] Figure 7 (a,b) Effect of power on reaction performance (a) production rate and yield, (b) energy efficiency and N2 conversion (flow rate: 10 sccm); (c,d) Comparison of nitrogen fixation performance of pure plasma, plasma-TiO2 and plasma-Mn-TiO2 systems (power: 3W, flow rate: 40 sccm).

[0059] Figure 8 (a) Stability test of Mn-TiO2 catalyst, and (b) Comparison of NO oxidation performance of pure plasma, TiO2 and Mn-TiO2.

[0060] Figure 9 (a) Optical emission spectra (OES) of nitrogen fixation from air assisted by plasma. (a) Overall OES spectrum (200-800 nm), (b) Locally magnified OES spectrum of NO (200-300 nm), (c) RGA spectrum of products in the reaction, (d) Plasma reaction temperature measured using an IR thermal imager.

[0061] Figure 10 A rational plasma catalytic pathway for nitrogen fixation from air. Detailed Implementation

[0062] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] The purpose of this application is to synthesize a manganese-doped TiO2 catalyst to investigate the synergistic effect of catalyst integration with nonthermal plasma (NTP) for direct nitrogen fixation from air. The resulting Mn-TiO2 exhibited excellent catalytic activity and underwent a series of comprehensive physicochemical characterizations.

[0064] The present invention provides a catalyst for nitrogen fixation in a non-thermal plasma environment, the catalyst being able to promote the activation of nitrogen and oxygen and their conversion into nitrogen oxides through redox cycles and defect sites on its surface.

[0065] Furthermore, the redox cycle includes the cycling of manganese ions between +2, +3 and +4 valence states.

[0066] Furthermore, the defect sites include oxygen vacancies, which can enrich and stabilize chemisorbed active oxides.

[0067] Furthermore, the catalyst enables nitrogen fixation from air using non-thermal plasma and at a reaction temperature below 120°C.

[0068] Furthermore, the catalyst can guide the reaction pathway to selectively generate nitrogen dioxide and nitrate and nitrite ions that are ultimately captured in the aqueous phase.

[0069] Furthermore, the catalyst is able to dynamically increase the proportion of divalent manganese and the proportion of chemically adsorbed oxygen on its surface during plasma catalytic reactions.

[0070] The present invention also provides a catalytic material for nitrogen fixation, for use in any of the catalysts for nitrogen fixation in a non-thermal plasma environment, wherein the material is capable of generating surface states that weaken the triple bond of the N2 molecule and provide activated electrons in response to non-thermal plasma excitation.

[0071] Furthermore, the generation of the surface states is related to the mixed valence states of the transition metal ions doped in the material and their electronic interactions with the host lattice.

[0072] The present invention also provides a plasma catalysis system for producing nitrogen-containing products from a nitrogen- and oxygen-containing gas stream, the system comprising: A non-thermal plasma generating device configured to generate high-energy electrons and active materials in a gas flow; The catalytic reaction zone is located inside or downstream of the plasma generating device and contains a catalyst for catalyzing the conversion of plasma-generated active substances into target nitrogen-containing products.

[0073] Furthermore, the non-thermal plasma generating device operates directly with ambient air as the feed gas, without the need to pre-separate pure nitrogen.

[0074] Furthermore, the catalytic reaction zone includes a product capture device configured to absorb gaseous nitrogen oxides into an aqueous solution to form water-soluble nitrates and / or nitrites.

[0075] Furthermore, the non-thermal plasma generating device is a dielectric barrier discharge reactor, and the catalyst is disposed in the discharge gap of the dielectric barrier discharge reactor.

[0076] This invention also provides a method for low-temperature nitrogen fixation, the method comprising the following steps: 1) In the presence of non-thermal plasma, a gas stream containing nitrogen and oxygen is brought into contact with a catalyst; 2) Utilizing the function of the catalyst, nitrogen and oxygen in the gas stream are converted into nitrogen oxides through interfacial redox cycle and defect-assisted activation.

[0077] Furthermore, steps 1) and 2) are performed at temperatures below 100°C.

[0078] Furthermore, the catalyst supplies Mn 2+ Mn 3+ Mn 4+ One or more of the redox pairs can accelerate the insertion reaction of oxygen atoms into nitric oxide.

[0079] Furthermore, the catalyst provides oxygen vacancies to stabilize the reaction intermediates and inhibit over-oxidation to nitrous oxide.

[0080] Furthermore, the nonthermal plasma provides vibrationally excited nitrogen molecules and oxygen atoms, while the catalyst provides surface sites to lower the activation energy for subsequent reactions of these species.

[0081] Furthermore, the catalyst can utilize plasma-generated actives and selectively direct the reaction towards higher-value nitrate products via surface catalysis, thereby reducing energy loss caused by non-selective gas-phase reactions.

[0082] Furthermore, when the catalyst is used in the non-thermal plasma environment, it can promote the surface reaction between adsorbed nitric oxide and adsorbed oxide through the Langmuir-Hinshelwood mechanism.

[0083] Furthermore, the catalyst can maintain a high concentration of surface chemisorbed oxygen and rapidly replenish the lattice oxygen consumed in the reaction.

[0084] Furthermore, the structure of the catalyst is capable of maintaining its redox cycling capability during use, and its catalytic activity decreases by no more than 5% after undergoing multiple plasma on / off cycles.

[0085] Furthermore, by utilizing the synergistic effect of non-thermal plasma and catalyst, a mixture of nitrates and nitrites that can be absorbed by aqueous solutions can be produced directly from air in a one-step process.

[0086] Furthermore, the synergistic effect involves the plasma being responsible for generating NO from N2 and O2, while the catalyst is responsible for the deep oxidation of NO to NO2 and higher-order nitrogen oxides, which are readily hydrolyzed to form NO3. - and NO2 - .

[0087] Furthermore, by doping with specific elements to introduce oxygen vacancies and mixed valence metal centers into the catalyst matrix, its ability to receive and transfer electrons from plasma is enhanced.

[0088] Furthermore, optical emission spectroscopy was used to monitor the reaction zone and its interaction with N2 and N2. + The intensity changes of characteristic emission lines related to NO were investigated to assess the effect of catalyst introduction on plasma active concentration.

[0089] A method for regenerating a catalyst, wherein the catalyst is a manganese-based catalyst for plasma nitrogen fixation, comprises heat-treating the catalyst in an oxygen-containing atmosphere to restore its oxidation state and surface oxygen.

[0090] The specific implementation examples are as follows: Catalyst preparation TiO2 rutile (nano powder, <100 nm particles, from Millipore Sigma) was used as the support material. Manganese nitrate tetrahydrate (Mn(NO3)2·4H2O, 99.99%, Aldrich) was used as the manganese precursor. A Mn-TiO2 catalyst was prepared on TiO2 using an impregnation method with a loading of 10 wt%. In this process, the required amount of manganese nitrate was dissolved in 50 mL of deionized water in a 100 mL beaker containing 1.0 g of TiO2 support. The resulting mixture was allowed to stand at room temperature for 3 hours, and then dried overnight at 80°C. The dried solid was then calcined at 500°C with a heating rate of 5°C / min for 4 hours. The resulting sample is labeled Mn-TiO2 in this paper.

[0091] Catalyst characterization The phase composition of the prepared catalyst was analyzed using a Bruker D8 Advance diffractometer equipped with CuKα1 radiation (λ = 1.5406 Å). The instrument used a CuKα radiation source operating at 40 kV and 25 mA. Diffraction scans were performed at a scan rate of 2° / min over the 2θ range of 10° to 80°. N2 adsorption-desorption isotherms were performed on a Micromeritics ASAP2020 Plus system. Prior to analysis, the sample was degassed at 350°C for 4 h at a heating rate of 10°C / min under a vacuum of 10 mmHg. Adsorption analysis was performed using liquid nitrogen, generating 28 adsorption isotherms. The total specific surface area was determined using the Brunauer-Emmett-Teller (BET) method, while the total pore volume was measured at a relative pressure of 0.995. High-resolution transmission electron microscopy (HRTEM) was performed using a FEITecnai F20 system operating at a voltage range of 80 to 200 kV. The microscope is equipped with a high-angle annular dark-field (HAADF) detector for Z-contrast imaging and a SuperX-EDX detector for compositional analysis, facilitating detailed structural and elemental characterization. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo-Fisher Escalab QXi XPS spectrometer equipped with a monochromatic Al-Kα X-ray source (1486.6 eV) to analyze the chemical state of the samples. X-band (9.5 GHz) EPR spectra of fresh and used Mn-TiO2 were performed at 77 K on a Bruker EMX spectrometer. Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) was performed on a Perkin Elmer Lambda 365 spectrometer equipped with an integrating sphere, using BaSO4 as a reference, recording absorption spectra in the 200–800 nm range. X-ray fluorescence (XRF) analysis was performed using a Bruker S2 PUMA benchtop spectrometer equipped with an AgX-ray tube. H2-Temperature-Programmed Reduction (H2-TPR) experiments were performed using a Micromeritics AutoChemII 2920 chemisorption analyzer equipped with a thermal conductivity detector (TCD). H2-TPR curves were obtained using a 5 vol% H2 / Ar gas mixture. The sample was heated from 50°C to 800°C at a rate of 10°C / min and then held at 800°C for 30 minutes. Before each measurement, approximately 200–300 mg of sample was pretreated by heating from room temperature to 500°C at 10°C / min using a pure He stream (50 mL / min) and holding at that temperature for 30 minutes to remove weakly adsorbed moisture and gases from the catalyst surface. The sample was then cooled to 50°C, and the gas flow was switched to 5 vol% H2 / Ar (50 mL / min) and purged for 30 minutes until the TCD signal stabilized.O2-Programmed Temperature Desorption (O2-TPD) experiments were also performed using a Micromeritics AutoChemII 2920 chemisorption analyzer. O2-TPD curves were recorded at a He flow (50 mL / min) with the temperature increased from 100°C to 800°C at a rate of 10°C / min, and then held at 800°C for 30 minutes. Prior to each measurement, approximately 200 mg of sample was pretreated in a U-shaped quartz tube at 500°C with flowing He (50 mL / min) for 30 minutes, and then cooled to 100°C. Oxygen adsorption was then performed at 100°C for approximately 30 minutes using a 10 vol% O2 / He mixture until the O2 signal stabilized, after which O2-TPD data were collected.

[0092] Experimental setup Plasma catalysis experiments were conducted using a self-assembled apparatus, as shown in Figure S1. A DBD reactor was used for nitrogen fixation. The reactor was a quartz tube with an inner diameter of 8 mm and an outer diameter of 10 mm, containing a coaxial stainless steel rod electrode grounded via a stainless steel mesh wrapped around the tube. Plasma discharge was generated using a LEAP100© plasma generator, which provided microsecond pulsed discharges with an adjustable pulse voltage range from 0 to 80 kV (peak-to-peak), a pulse frequency between 100 and 3000 Hz, and a maximum power output of 700 W. The electrical characteristics of the system were continuously monitored using an oscilloscope (Keysight, DSOX1204GSeries). Voltage measurements were performed using a high-voltage probe (P6150A, Tektronix), while current measurements were performed using a current probe (TCP202, Tektronix).

[0093] In a typical catalytic performance evaluation experiment, 100 mg of powdered catalyst was placed in the center of the reactor. Quartz wool was packed above and below the catalyst to prevent displacement. The reactor temperature was maintained at 50°C using cooling air and monitored using a thermal imager (FLIROne®). The outlet gas was absorbed into a 0.005 mol / L H₂SO₄ aqueous solution in a reagent bottle placed in a cold trap. The collected solution was analyzed using a UV-Vis spectrophotometer to determine NO₃⁻. - and NO2 - The concentration of N2 was measured in the outlet gas after condensation of water vapor using a micro gas chromatograph (Agilent 490 micro-GC). In-situ monitoring of light emission from the reactor was performed using optical emission spectroscopy (OES). The emission spectrum of the central glow region of the reactor was captured using a single-channel UV-VIS-NIR spectrophotometer (Avantes Inc., USB2000 Series) equipped with a 400 μm optical fiber. Measurements were performed in the wavelength range of 200–800 nm, with a grating of 600 lines / mm and a spectral resolution of 0.4 nm.

[0094] To evaluate the effect of Mn-TiO2 incorporation on plasma-assisted NO oxidation, comparative experiments were conducted using an empty reactor, pure TiO2, and Mn-TiO2 as catalysts. See ESI for details.

[0095] Product Analysis NO3 - and NO2 - Concentration was determined using ultraviolet-visible spectrophotometry. For NO3... - The absorbance was measured using a UV-Vis spectrophotometer, analyzing 3 mL of the solution at λ = 220 nm and λ = 275 nm. The absorbance intensity was then analyzed using the formula... Calculations show that this value is related to NO3. - Concentration remains linear 33 Nitrite (NO2) - The concentration of nitrite was determined using UV-Vis spectrophotometry. Under acidic conditions, nitrite reacts with sulfonamide to form a diazo compound, which then reacts with N-(1-naphthyl)-ethylenediamine dihydrochloride (NEDA) to generate a red azo dye. In a typical procedure, 0.5 mL of the sample is mixed with 3 mL of colorimetric reagent (containing 25 mL of 85% H3PO4, 2.5 g of sulfonamide, and 0.125 g of NEDA, for a total volume of 250 mL). The solution is then incubated at room temperature for 20 minutes, and its absorbance at λ = 540 nm is measured using a UV-Vis spectrophotometer.

[0096] Experimental calculations Key parameters, including discharge power (P), formation rate (r), yield, and energy yield, are crucial for evaluating the catalytic performance of nonthermal plasma (NTP)-assisted nitrogen fixation. These parameters were determined using the following calculations.

[0097] Here, v(t) and I(t) represent the voltage and current measured by the oscilloscope, respectively, while f represents the pulse repetition frequency.

[0098] ; C NO3- and C NO2- (mg / L) is NO3 obtained from the absorption tank. - and NO2 - Concentration, measured using UV-Vis spectroscopy, V (mL) represents the volume of the adsorption solution used in the experiment, t (minutes) is the reaction time, and 14 corresponds to the molar mass of nitrogen.

[0099] Catalyst characterization To explore the crystallographic changes caused by Mn incorporation and exposure to catalytic reactions. Figure 1XRD patterns of pristine TiO2, fresh Mn-doped TiO2 (Mn-TiO2), and used Mn-TiO2 catalysts are shown. Figure 1 In (a), all samples exhibit characteristic peaks corresponding to the rutile phase of TiO2, matching well with standard JCPDS card No. 89-4920. The peak positions of all three samples are consistent, indicating that the rutile structure is retained after Mn doping and catalytic use. However, subtle differences in peak intensity and width can be observed. Compared to undoped TiO2, the Mn-doped samples (fresh and used) exhibit slightly broadened peaks and lower intensities, implying reduced crystallinity and / or smaller grain size, likely due to lattice distortion caused by Mn incorporation. No distinct peaks belonging to manganese oxide species (e.g., MnO2, Mn2O3) were observed, suggesting that Mn is either highly dispersed or incorporated into the TiO2 lattice rather than forming a separate crystalline phase. Figure 1 (b) shows a magnified view of the strongest diffraction peak at approximately 27.4°, corresponding to the (110) plane of rutile TiO2. Here, a significant peak shift due to Mn doping is observed. Specifically, compared to pristine TiO2, the diffraction peak of fresh Mn-TiO2 shifts slightly to a higher 2θ value, indicating a decrease in interplanar spacing according to Bragg's law. This can be attributed to the smaller Mn content. 4+ (0.53Å) ions substituted Ti 4+ (Ionic radius ~0.605 Å), leading to lattice contraction. No new peaks further confirm the hypothesis that Mn atoms are substituted into the TiO2 lattice. Interestingly, compared to fresh Mn-TiO2, the used Mn-TiO2 catalyst shows a slight peak shift to a lower 2θ value, indicating partial lattice recovery or expansion after the catalytic reaction. This may be due to the redox cycle of Mn during catalysis (e.g., Mn...). 4+ ↔Mn 2+ This is caused by Mn, which may involve 4+ Reduced to a larger Mn 3+ (Ionic radius ~0.645Å) or Mn 2+ (Ionic radius ~0.83 Å), thus increasing the average ionic radius of Ti sites and leading to slight lattice expansion. Furthermore, the thermal and chemical stresses during the catalytic reaction may contribute to structural relaxation or reconstruction, affecting lattice parameters. Additionally, the Mn content, determined by XRF, was 9.79%.

[0100] The structure and composition characteristics of Mn-TiO2 were further investigated using SEM, TEM, HAADF-STEM, and EDS elemental mapping. Figure 2 The SEM image in (a) shows aggregated nanoparticles with irregular morphology. Figure 2The HAADF-STEM image in (b) further demonstrates the presence of polyhedral crystal domains with strong atomic number contrast, indicating a uniform distribution of heavier elements such as Ti and Mn. Figure 2 (c) The rutile structure was confirmed, with lattice spacings of 0.324 nm and 0.248 nm corresponding to the (110) and (101) planes, respectively. These planes are characteristic of the rutile TiO2 phase, and the clear lattice striations demonstrate the high crystallinity of the material. Figure 2 The element mapping image in (dg) shows the spatial distribution of Ti, O, and Mn. Overlay image ( Figure 2 (d) shows the uniform distribution of all elements, and the individual element plots ( Figure 2 (e.g.) This confirms that Mn is uniformly distributed throughout the TiO2 matrix, supporting successful substitution doping rather than surface segregation or the formation of MnO. x Cluster. Figure 2 The EDS spectrum in (h) further confirms this, showing distinct peaks for Ti, O, and Mn, with no impurities detected. The simultaneous presence of Mn with Ti and O confirms its incorporation into the host lattice. Figure 2 The selected area electron diffraction (SAED) pattern in (i) shows bright and sharp diffraction rings corresponding to the (110), (101), (211) and (301) planes of rutile TiO2, which verifies the phase purity and polycrystalline properties of the sample.

[0101] Using the N2 adsorption-desorption isotherm ( Figure 3 (a) and BJH aperture distribution ( Figure 3 (b) The BET specific surface area, pore volume, and pore size distribution of fresh and used Mn-TiO2 and TiO2 were analyzed. Quantitative textural properties, including BET specific surface area and total pore volume, are summarized in Table S1. According to the IUPAC classification, all catalysts exhibited type IV isotherms and type H1 hysteresis loops, indicating a mesoporous structure. BET analysis showed that Mn doping increased the specific surface area and pore volume of TiO2 from 38.20 m² / s² to 38.20 m² / s² / s². 2 / g and 0.12cm 3 / g decreased to 28.16m 2 / g and 0.11cm 3 The concentration of Mn in the sample is approximately 26.18 m² / g, indicating that the incorporation of Mn led to partial pore blockage or framework densification. After catalytic use, the specific surface area and pore volume decreased to 26.18 m² / g. 2 / g and 0.10cm 3 / g indicates the presence of slight structural degradation or surface coating from reaction intermediates. The retained isotherm shape suggests that mesoporous properties are preserved. Figure 3The pore size distribution curves in (b) show that all samples exhibit a broad mesoporous structure, mainly in the 2–50 nm range. TiO2 exhibits a higher pore volume in the mesoporous region. In contrast, both fresh and used Mn-TiO2 samples show a reduction in total pore volume, particularly significant in pores smaller than ~20 nm, consistent with the structural modification induced by Mn doping. The used catalyst maintained its mesoporous framework, indicating good structural stability.

[0102] XPS measurements were performed to investigate the surface chemical state of the prepared catalyst. Figure 4 XPS results are presented, including full spectrum, Ti2p, Mn2p, and O1s XPS spectra. Table S2 summarizes the corresponding quantitative results, including the Mn valence state distribution and surface adsorbed oxygen (O2). a d s The relative proportion of ). Figure 4 (a) It can be observed that the Ti2p peak splits into two distinct peaks, located at approximately 464.1 eV and 458.5 eV, respectively, indicating the splitting of the Ti2p1 / 2 and Ti2p3 / 2 orbitals, which can be entirely attributed to Ti 4+ The shift of the Ti2p3 / 2 XPS peak to lower binding energies in fresh Mn-TiO2 is likely due to charge transfer from the Mn species to TiO2. After the reaction, the Ti2p3 / 2 peak undergoes a slight shift and broadening, which can be attributed to Ti... 3+ The generation of oxygen vacancies promotes charge transfer and enhances the activation of N2 molecules by increasing the electron density on the catalyst surface.

[0103] like Figure 4 As shown in (b), the Mn2p peaks are observed at approximately 653.6 eV and 641.8 eV, corresponding to the spin-orbit splitting peaks of Mn2p1 / 2 and Mn2p3 / 2, respectively. After deconvolution of these peaks, it was found that the spectra of Mn2p3 / 2 and Mn2p1 / 2 each consist of three fitted components, which are attributed to Mn2p1 / 2 and Mn2p3 / 2, respectively. 2+ Mn 3+ and Mn 4+ Species. For fresh Mn-TiO2 catalysts, Mn is distributed in Mn... 2+ (42.6%), Mn 3+ (45.5%) and Mn 4+ (11.9%). After plasma catalytic reaction, Mn 2+ The percentage of Mn increased significantly to 66.8%, while Mn... 3+ It decreased to 20.8%, while Mn 4+ It remained almost unchanged (~12%). Mn 2+ / Mn 4+The ratio correspondingly increased from 3.58 (fresh) to 5.40 (after use). These changes indicate that high-energy plasma species drive Mn 3+ / Mn 4+ To Mn 2+ The net reduction of Mn enriches electron-rich surface states. 2+ The species acts as an electron pool, feeding electrons back to the antibonding π* orbitals of N2, weakening the N≡N triple bond and thus promoting molecular activation. This observation is consistent with previous studies on manganese-based oxides, where the redox flexibility of Mn plays a crucial role in plasma-assisted catalysis.

[0104] The O1s XPS spectrum of the catalyst showed two main peaks, which could be further deconvolved into two distinct peaks. The peak located at 529.4–530.1 eV belongs to the lattice oxygen species O. 2- ( The peak at 530.5–531.5 eV corresponds to the chemisorbed oxygen species O2 associated with oxygen vacancies. - Or O - Group ( Table S2 shows the concentration of chemisorbed oxygen on the catalyst surface, with the increasing trend in the following order: TiO2 (12.81%) < fresh Mn-TiO2 (19.74%) < used Mn-TiO2 (27.42%). Surface chemisorbed oxygen plays a crucial role in the oxidation of NO to NO2 due to its high mobility, thereby improving nitrogen fixation efficiency. Quantitative analysis shows that the original TiO2 contains 12.8% chemisorbed oxygen. Mn doping increases this value to 19.7%. Following the plasma-catalyzed reaction, O... a d s The percentage further increased to 27.42%, which is attributed to plasma-induced oxygen vacancy formation and Mn redox cycle, which together promote the stabilization of reactive oxygen species on the Mn-TiO2 surface.

[0105] Electron paramagnetic resonance (EPR) spectra of fresh and used Mn-TiO2 measured at 150 K Figure 6 (a) reveals key information about the electronic and magnetic properties of the material before and after catalytic use. The fresh Mn-TiO2 sample exhibits a weak but broad EPR signal, indicating a low concentration of paramagnetic Mn species and / or strong interaction with the TiO2 lattice, possibly due to the low concentration of paramagnetic Mn species and / or their strong interaction with the TiO2 lattice. 4+ Or antiferromagnetically coupled Mn ions. In contrast, the used Mn-TiO2 samples showed a significantly stronger and sharper EPR signal, indicating an increase in unpaired electrons after catalysis and lower oxidation states (e.g., Mn). 2+ or Mn 3+The concentration of Mn species was higher. The significant changes in signal intensity and shape suggest that the catalytic process altered the oxidation state and coordination environment of Mn, likely due to redox cycles under the reaction conditions. This enhanced EPR response may be related to oxygen vacancies or surface defects generated during NTP catalysis, which are known to facilitate electron transfer processes. 53 .

[0106] The light absorption behavior and band gap characteristics of TiO2 and Mn-TiO2 samples were examined using ultraviolet-visible diffuse reflectance spectroscopy (UV-VisDRS), such as... Figure 6 As shown in (b). The bandgap energy was determined using the Tauc method proposed by Tauc, Davis, and Mott. This was achieved by plotting (αhν). 1 / n For the curve of hv, extrapolating the linear portion of the curve to zero reflectivity yields a new curve. The corresponding formula is expressed as: (7) Where h is Planck's constant, ν is the photon frequency, and E g Here, α is the band gap energy, A is a constant, and the exponent n depends on the type of electronic transition. For indirect semiconductors, such as rutile TiO2, n=2, while n=1 / 2 applies to directly allowed transitions. This can be illustrated by plotting (αhν). 1 / n The bandgap value was determined by extrapolating the linear region to the energy axis using the hν (Tauc plot).

[0107] The original TiO2 exhibited a band gap of approximately 3.08 eV, consistent with its inherent UV absorption edge. Upon Mn doping, a significant red shift in the absorption edge occurred, with fresh Mn-TiO2 samples displaying a narrow band gap of approximately 2.33 eV. After catalytic cycling, used Mn-TiO2 retained this narrow band gap but shifted further to ~2.15 eV, reflecting the continued presence and even slight increase of subbandgap electronic states. This band gap narrowing can be attributed to Mn-induced defect states and the introduction of oxygen vacancy levels within the bandgap, which extend light absorption into the visible region and promote electronic excitation at lower photon energies.

[0108] UV-Vis results and EPR analysis ( Figure 6 a) Completely consistent, the latter showing that used Mn-TiO2 exhibits a significantly stronger and sharper paramagnetic signal than fresh samples. The enhanced EPR signal confirms the generation of more unpaired electrons and defect centers during plasma-assisted catalysis, particularly in Mn. 3+ / Mn 2+Species and oxygen vacancies. These newly formed electronic states are directly correlated with the observed additional band gap narrowing in the used samples. In summary, UV-VisDRS and EPR results indicate that Mn doping not only reduces the intrinsic band gap of TiO2 but also promotes the dynamic formation of defect states under catalytic operation, thereby enhancing visible light absorption, electron availability, and charge transfer. This electronic modification is crucial for plasma-assisted nitrogen fixation, as the activation of N2 and O2 molecules on the catalyst surface requires an efficient supply of active electrons.

[0109] To comprehensively evaluate the catalyst's acidity, redox properties, and oxygen mobility, NH3-TPD, H2-TPR, and O2-TPD analyses were performed. The results are summarized in... Figure 5 middle. Figure 5 The NH3-TPD curves in (a) show that Mn doping significantly enhances surface acidity. TiO2 exhibits negligible ammonia desorption, reflecting a low density of acidic sites. In contrast, fresh Mn-TiO2 shows distinct desorption peaks centered at approximately 550°C, 625°C, and 790°C, corresponding to the presence of weak, moderate, and strong acidic sites, respectively. These acidic sites are crucial for the adsorption and activation of nitrogen molecules under plasma conditions. Mn incorporation enhances Lewis acidity by introducing surface defects and oxygen vacancies, which can serve as active centers for N2 adsorption. Notably, used Mn-TiO2 retains similar peak positions but with broader characteristics and slightly reduced intensity, indicating partial loss or transformation of surface acidic sites during catalytic cycling. However, the continued significant acidity after use implies good structural stability.

[0110] Figure 5 (b) illustrates the H2-TPR performance of the prepared catalyst. The H2-TPR of pure TiO2 shows a weak and broad reduction peak at approximately 650°C, possibly due to oxygen on the surface of rutile TiO2 or Ti... 4+ Restore to Ti 3+ Species-dependent. When Mn species are introduced into TiO2, the H2-TPR curve shows a peak between 300-400°C, with two shoulder peaks at 450 and 540°C. These two shoulder peaks can be identified as being related to reducible small clusters of surface Mn oxides. The reduction peak at the lower temperature (358°C) can be attributed to the reduction of MnO2 to Mn2O3. The two shoulder peaks are attributed to the sequential reduction of Mn2O3 to Mn3O4 and the reduction of Mn2O3 / Mn3O4 to MnO. The used Mn-TiO2 maintains similar redox characteristics, indicating that reducibility and the dynamic redox properties of the catalyst are preserved, which is crucial for continuous operation under plasma conditions. Figure 5(c) shows the O2-TPD curves illustrating the desorption behavior of oxygen species in pure TiO2, fresh, and used Mn-TiO2. The pristine TiO2 sample exhibits a weak desorption peak at approximately 300°C, indicating limited oxygen mobility and a low concentration of surface oxygen species. In contrast, Mn-TiO2 shows three distinct oxygen desorption peaks at approximately 540°C, 610°C, and 760°C. The desorption peak at 540°C may be attributed to chemisorbed oxygen species (O2). - and O - The release of oxygen and the peaks at 610°C and 760°C correspond to the desorption of lattice oxygen, indicating a strong metal-support interaction that facilitates oxygen activation. The presence of multiple desorption peaks in Mn-TiO2 compared to pure TiO2 suggests that Mn incorporation enhances oxygen adsorption and promotes lattice oxygen activation. These oxygen species play a crucial role in stabilizing nitrogen intermediates and participating in surface reactions during nitrogen fixation. Used Mn-TiO2 exhibits similar desorption behavior, characterized by broad peaks indicating slight structural rearrangement while preserving oxygen mobility. This oxygen availability promotes the formation of surface-bound -NO or -NO2 intermediates, which are essential in the plasma-driven synthesis of reactive nitrogen species, including nitrates.

[0111] Catalytic activity assessment Flow rate is a key parameter for plasma-assisted nitrogen fixation from air because it directly affects residence time, energy density, and reaction selectivity within the plasma reactor. In our experiments, the effect of flow rate in the range of 10 to 40 sccm was investigated. Figure S4 illustrates the effect of gas flow rate on plasma-driven nitrogen fixation from air. As the flow rate increases, the total nitrogen productivity (Figure S4(a)) increases, while the NOx productivity (Figure S4(a)) decreases. Conversely, Figure S4(b) shows that energy efficiency increases with increasing flow rate, while N2 conversion steadily decreases. These opposing trends reflect the interaction between gas residence time, electron-molecule interactions, and plasma energy utilization efficiency. When the flow rate increases from 10 to 40 sccm, the total nitrogen productivity increases from ~107 to 137 μmol / h. -1 Energy efficiency increased from 30 to nearly 38 mmol kWh -1 This indicates that higher flow rates promote greater nitrogen activation and more efficient utilization of input energy. However, both NOx yield and nitrogen conversion rate decrease with increasing flow rate, decreasing from 0.25% to 0.09% and from 1.38% to 1.01%, respectively. This inverse relationship can be explained by the reduced residence time at higher flow rates, which limits the gradual oxidation of plasma-activated N2 to stable NO and NO2.

[0112] Figure 7(a, b) illustrates the effect of discharge power on nitrogen fixation performance in the absence of a catalyst. At low discharge power, the overall nitrogen fixation rate is minimal, reflected in low yields of NOx species. Limited electron density and energy limit the degree of nitrogen activation, resulting in weak vibrational excitation and dissociation of N2 molecules. Therefore, the plasma is primarily dominated by basic ionization and limited free radical generation, producing only trace amounts of fixed nitrogen products. With increasing plasma power, a sharp increase in nitride formation was observed. The enhanced electron density and higher average electron energy increase the frequency of inelastic collisions between N2 and O2 molecules, generating more vibrationally excited N2*, atomic N, O radicals, and subsequently NOx intermediates. This effect manifests as a near-linear increase in product concentration with power, confirming that energy input is the driving factor for maintaining the reaction chemistry in a pure plasma nitrogen fixation system. Similar trends have been reported in previous dielectric barrier discharge (DBD) studies, where higher plasma power promotes a stronger oxidizing environment and greater NOx formation. However, despite the increase in absolute product formation, a decrease in energy efficiency accompanies higher discharge power. This is attributed to several factors. First, as more energy is deposited into the plasma, a larger portion of this energy is dissipated as heat and nonproductive electronic excitations rather than used for selective N2 dissociation. Second, the higher energy density accelerates side reactions such as radical recombination (e.g., N + N → N2, NO + O → NO2), which reduces the net fixed yield per unit power. Therefore, while higher power increases throughput, it compromises the efficiency of energy utilization.

[0113] Figure 8 (c, d) compare the nitrogen fixation performance in pure plasma (empty reactor), plasma-TiO2, and plasma-Mn-TiO2 systems, highlighting the crucial role of the catalyst in enhancing reaction performance. In the empty reactor, nitrogen fixation is limited, and relatively low concentrations of NO and NO2 were detected. - and NO3 - The products originated solely from plasma-induced electron collisional dissociation of N2 and O2, primarily forming NO, with only moderate further oxidation. The lack of a catalytic surface limited the reaction pathway, resulting in low overall productivity and efficiency. The introduction of TiO2 significantly improved performance, increasing both the overall yield and the distribution of NOx species compared to the empty reactor. This enhancement stemmed from TiO2's ability to provide adsorption sites and facilitate electron transfer, thereby stabilizing reactive nitrogen and oxygen intermediates. The presence of surface oxygen also promoted secondary oxidation, reflected in a higher proportion of NO2. - and NO3 -Above. It is noteworthy that the catalytic effect of TiO2 demonstrates the importance of surface-assisted pathways in supplementing plasma activation. The Mn-TiO2 catalyst exhibits the highest nitrogen fixation performance, producing significantly more NO2 compared to pure plasma and TiO2 systems. - and NO3 - This superior activity is attributed to the Mn species (Mn 2+ / Mn 3+ / Mn 4+ The redox flexibility of ) accelerates nitrogen activation and oxygen transfer processes, as well as surface oxygen species (O a d s The increased abundance of Mn was confirmed by XPS analysis. The ability of Mn to cycle between oxidation states allows for more efficient conversion of NO into higher nitrogen oxides. In summary, the results confirm that the plasma-catalyst synergy is crucial for improving yield and selectivity, and that Mn-TiO2 provides the most efficient pathway by coupling plasma-generated radicals with a redox-active catalytic surface.

[0114] The long-term stability of the catalyst during the reaction is a critical parameter that needs to be evaluated, as excellent stability ensures sustained catalytic performance and reduces the need for catalyst regeneration, thereby preventing high operating costs. A 12-hour continuous stability test was conducted on plasma-assisted nitrogen fixation from air using a Mn-TiO2 catalyst, with 6 hours of plasma on / off cycles to simulate actual industrial shift work. Results are as follows: Figure 8 As shown in (a), the overall yield exhibited a relatively stable value of approximately 0.1%, decreasing slightly by about 0.014% throughout the evaluation period, indicating excellent long-term stability. Furthermore, the XRD analysis and SEM images (Figure S3) of the used Mn-TiO2 catalyst remained unchanged, further demonstrating its excellent morphological and structural stability.

[0115] To illustrate the effect of Mn-TiO2 combined with plasma on the NO oxidation process, comparative experiments on NO oxidation were conducted in an empty reactor, on TiO2, and on Mn-TiO2. Figure 8 The bar graph in (b) shows that coupling NTP with Mn-TiO2 significantly enhances NO oxidation performance compared to pure plasma (empty) or plasma-TiO2. Mn-TiO2 achieved the highest NO conversion (~55%), NO2 yield (~28%), and NO2 selectivity (~55%), highlighting its superior catalytic effect. This improvement stems from the substitution introduction of Mn, which introduces oxygen vacancies and enables Mn to oxidize oxygen. 2+ / Mn 3+ / Mn 4+ The redox cycle stabilizes the chemisorbed oxygen species (O2). -O - This process facilitates electron transfer across the Mn-O-Ti aggregate. These defective redox sites promote the adsorption of NO as a nitroso / nitrite intermediate and accelerate the O insertion step (NO* + O* → NO2*), while the plasma provides a continuous supply of reactive species. Therefore, Mn-TiO2 not only improves the overall NO conversion efficiency but also directs the reaction pathway towards selective NO2 formation, a key intermediate for the sustainable nitrogen fixation of nitrates / nitrites from the air.

[0116] A rational approach to plasma-derived air nitrogen fertilizer Figure 9 (a) Optical emission spectrum of plasma-assisted nitrogen fixation from air. (a) Overall OES spectrum (200-800 nm), (b) Magnified OES spectrum of NO (200-300 nm), (c) RGA spectrum of products in the reaction, (d) Plasma reaction temperature measured using an IR thermal imager. To gain a deeper understanding of the behavior of adsorbates and the fundamental mechanisms of oxidation reactions under reaction conditions, in-situ OES experiments were conducted to monitor the dynamic evolution of intermediate species on the catalyst surface. Figure 9 It showcases optical emission spectroscopy (OES), mass spectrometry (MS) analysis, and thermal imaging, which together provide insights into plasma-catalyst interactions and nitrogen activation pathways.

[0117] The peaks at 337, 357, and 380 nm are attributed to emission bands generated by electronic transitions in molecular nitrogen, corresponding to the nitrogen second positive series (SPS) N2 (C 3 Πᵤ→B 3 Regarding intensity, the introduction of the catalyst was observed to enhance it, indicating that higher plasma discharge leads to more reactive species, particularly for Mn-TiO2. Furthermore, first negative series (FNS) N2 was also detected. + (B) 2 Σᵤ + →X 2 Σᵢ + ) and NOγ (A 2 Σ + -X 2 Peaks related to П. Ionized nitrogen (N2) + Observed at 390-430 nm, it is called N2. + (B) 2 Σᵤ + →X 2 Σᵢ + The transition from N2 under plasma discharge 63Because their emission line intensities are weaker than the strong signal of N2, no oxygen-related bands were observed. The peaks between 200-300 nm are attributed to NOγ (A 2 Σ + -X 2 The system confirmed the formation of NO fragments during the reaction. There are two possible pathways for NO fragment formation: (i) NO molecules formed during the plasma-driven chemical reaction, and (ii) the dissociation of the products NO2, N2O, or N2O5. To confirm the presence of NO2, N2O, or N2O5, the outlet gas was analyzed using a residual gas analyzer (RGA), such as... Figure 9 As shown in (c), the signal at m / z=44 corresponds to the ion ^14^N2^16^O. + This is related to nitrous oxide (N2O) + Typical ions associated with N₂O were observed, indicating the formation of N₂O in the plasma-induced reaction. A key identifier for NO₂ was observed in the mass spectrometer at m / z=46, indicating the presence of NO₂ in the process. The enhanced signal intensity in the plasma-catalyzed system suggests that the Mn-TiO₂ catalyst promoted the oxidation of plasma-activated NO to NO₂, a crucial step in nitrogen oxide conversion and environmental remediation. The most characteristic and stable fragment for identifying N₂O₅ was not observed at m / z=62, likely due to its thermal instability as it readily decomposes into NO₂ and O₂.

[0118] Plasma-derived air reactions are complex, primarily leading to NO formation, which is crucial for plasma-assisted nitrogen fixation. In nonthermal plasma, high-energy electrons collide with N2 and O2 molecules, causing ionization and dissociation, mainly through vibrational and electronic excitation. Vibrational excitation of N2 (particularly the formation of metastable N2(ν)) plays a dominant role, as it significantly lowers the energy barrier for subsequent reactions. Breaking the strong triple bond in N2 (bond energy ≈ 9.79 eV) is the most challenging step. However, vibrationally excited N2 molecules can more readily react with O atoms or excited O2 species (O2 and O*) to form NO. Oxygen, with its lower bond energy (≈ 5.15 eV), dissociates more easily, providing the reactive O atoms required for NO formation. Subsequently, the formed NO can recombine and undergo side reactions, thus sustaining the production of N2O and NO2. In this study, the temperature was recorded using an IR thermal imager and showed to be approximately 56.9°C ( Figure 9 (d) Typically, the conventional catalytic oxidation reaction between N2 and O2 requires an operating temperature of approximately 1200°C, while the catalytic oxidation of NO to NO2 usually occurs between 200 and 500°C. Compared to conventional catalytic oxidation, the temperature in our plasma-assisted reaction is significantly lower, demonstrating low energy consumption and benefits to catalyst activity.

[0119] Once Mn-TiO2 is introduced into the reaction system, these plasma-generated species (N, N2, O2*, and O*) adsorb onto the Mn-TiO2 surface, where Mn... 2+ / Mn 3+ / Mn 4+ Redox pairs and oxygen vacancy-catalyzed stepwise NO oxidation, such as Figure 10 As shown. Initially, NO is oxidized to NO2 at the Mn site, and then further oxidized by stabilizing at the oxygen vacancy site via transient intermediates (N2O, N2O5). The hydrolysis of N2O5 and NO2 leads to nitrate (NO3). - ) and nitrite (NO2) - The formation of these substances allows them to be captured in the aqueous phase. This plasma-catalyst system enables efficient nitrogen fixation and NOx conversion at low temperatures (~60°C), converting air-derived nitrogen into recyclable fertilizer precursors while reducing NOx emissions.

[0120] This work demonstrates an efficient, low-temperature pathway for direct air-to-nitrate synthesis by coupling thermal plasma (DBD) with Mn-doped TiO2. Systematic characterization correlates catalyst performance with lattice distortion, abundant oxygen vacancies, and Mn content. 2+ / Mn 3+ / Mn 4+ The redox cycles are linked, and together they enhance electron availability and direct the formation / oxidation of NO towards NO2. - / NO3 - Compared to pure plasma or pure TiO2, Mn-TiO2 provides higher nitrate / nitrite yields at 56.9°C while maintaining structure and activity during a 12-hour on / off cycle (with only ~0.014% yield loss). In-situ OES / RGA (N2SPS, N2 + The study demonstrated the synergistic effect of plasma and catalyst (FNS, NOγ, NO2 / N2O), and a two-step pathway: NOx from air during discharge, followed by secondary catalytic oxidation on Mn-TiO2 to produce stable, recyclable nitrates / nitrites. This establishes defect-rich Mn-TiO2 as a robust platform for sustainable plasma-assisted nitrogen fixation directly from air, providing a pathway for electrified, decentralized nitrate production as a green fertilizer with a significantly reduced carbon footprint.

Claims

1. A catalyst for nitrogen fixation in a non-thermal plasma environment, characterized in that, The catalyst can promote the activation of nitrogen and oxygen and their conversion into nitrogen oxides through redox cycles and defect sites on its surface.

2. The catalyst for nitrogen fixation in a non-thermal plasma environment as described in claim 1, characterized in that, The redox cycle includes the cycling of manganese ions between +2, +3 and +4 valence states.

3. The catalyst for nitrogen fixation in a non-thermal plasma environment as described in claim 1 or 2, characterized in that, The defect sites include oxygen vacancies, which can enrich and stabilize chemisorbed active oxides.

4. The catalyst for nitrogen fixation in a non-thermal plasma environment as described in claim 1, characterized in that, The catalyst enables nitrogen fixation from air using non-thermal plasma and at a reaction temperature below 120°C.

5. The catalyst for nitrogen fixation in a non-thermal plasma environment as described in claim 4, characterized in that, The catalyst guides the reaction pathway, selectively generating nitrogen dioxide and ultimately capturing nitrate and nitrite in the aqueous phase.

6. The catalyst for nitrogen fixation in a non-thermal plasma environment as described in claim 2, characterized in that, The catalyst can dynamically increase the proportion of divalent manganese and the proportion of chemically adsorbed oxygen on its surface during plasma catalytic reactions.

7. A catalytic material for nitrogen fixation, used in any one of the catalysts for nitrogen fixation in a non-thermal plasma environment as described in claims 1-6, characterized in that, The material is capable of responding to non-thermal plasma excitation to generate surface states that weaken the triple bonds of the N2 molecule and provide activated electrons.

8. The catalytic material for nitrogen fixation as described in claim 7, characterized in that, The generation of the surface states is related to the mixed valence states of the transition metal ions doped in the material and their electronic interactions with the host lattice.

9. A plasma catalytic system for producing nitrogen-containing products from a nitrogen- and oxygen-containing gas stream, characterized in that, The system includes: A non-thermal plasma generating device configured to generate high-energy electrons and active materials in a gas flow; The catalytic reaction zone is located inside or downstream of the plasma generating device and contains a catalyst for catalyzing the conversion of plasma-generated active substances into target nitrogen-containing products.

10. The plasma catalysis system as described in claim 9, characterized in that, The non-thermal plasma generating device operates directly with ambient air as the feed gas, without the need to pre-separate pure nitrogen.

11. The plasma catalysis system as described in claim 9, characterized in that, The catalytic reaction zone includes a product capture device configured to absorb gaseous nitrogen oxides into an aqueous solution to form water-soluble nitrates and / or nitrites.

12. The plasma catalysis system as described in claims 9-11, characterized in that, The non-thermal plasma generating device is a dielectric barrier discharge reactor, and the catalyst is placed in the discharge gap of the dielectric barrier discharge reactor.

13. A method for low-temperature nitrogen fixation, characterized in that, The method includes the following steps: 1) In the presence of non-thermal plasma, a gas stream containing nitrogen and oxygen is brought into contact with a catalyst; 2) Utilizing the function of the catalyst, nitrogen and oxygen in the gas stream are converted into nitrogen oxides through interfacial redox cycle and defect-assisted activation.

14. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, Steps 1) and 2) are performed at temperatures below 100°C.

15. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, The catalyst supplies Mn 2+ Mn 3+ Mn 4+ One or more of the redox pairs can accelerate the insertion reaction of oxygen atoms into nitric oxide.

16. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, The catalyst provides oxygen vacancies to stabilize the reaction intermediates and inhibits over-oxidation to nitrous oxide.

17. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, The nonthermal plasma provides vibrationally excited nitrogen molecules and oxygen atoms, while the catalyst provides surface sites to lower the activation energy of subsequent reactions of these species.

18. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, The catalyst can utilize plasma-generated actives and selectively direct the reaction towards higher-value nitrate products via surface catalysis, thereby reducing energy loss caused by non-selective gas-phase reactions.

19. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, When the catalyst is used in the non-thermal plasma environment, it can promote the surface reaction between adsorbed nitric oxide and adsorbed oxide through the Langmuir-Hinshelwood mechanism.

20. The method for low-temperature nitrogen fixation as described in claim 19, characterized in that, The catalyst can maintain a high concentration of surface chemisorbed oxygen and rapidly replenish the lattice oxygen consumed in the reaction.

21. The method for low-temperature nitrogen fixation as described in claim 19 or 20, characterized in that, The structure of the catalyst enables it to maintain its redox cycling capability during use, and its catalytic activity decreases by no more than 5% after multiple plasma on / off cycles.

22. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, By utilizing the synergistic effect of nonthermal plasma and catalyst, a mixture of nitrates and nitrites that can be absorbed by aqueous solutions can be produced directly from air in a one-step process.

23. The method for low-temperature nitrogen fixation as described in claim 22, characterized in that, The synergistic effect is that the plasma is responsible for generating NO from N2 and O2, while the catalyst is responsible for further oxidizing NO to NO2 and higher-order nitrogen oxides, which are readily hydrolyzed to form NO3. - and NO2 - .

24. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, By doping with specific elements to introduce oxygen vacancies and mixed valence metal centers into the catalyst matrix, its ability to receive and transfer electrons from plasma is enhanced.

25. The method for low-temperature nitrogen fixation as described in claim 13, characterized in that, Optical emission spectroscopy was used to monitor the reaction zone and its interaction with N2 and N2. + The intensity changes of characteristic emission lines related to NO were investigated to assess the effect of catalyst introduction on plasma active concentration.

26. A method for regenerating a catalyst, characterized in that, The catalyst is a manganese-based catalyst for plasma nitrogen fixation, which includes heat treatment of the catalyst in an oxygen-containing atmosphere to restore its oxidation state and surface oxygen.

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