A nitrogen fixation catalyst based on purple phosphorus double active sites and a preparation method and application thereof

Abstract

This invention relates to the field of photocatalytic nitrogen fixation technology, and particularly to a nitrogen fixation catalyst based on purple phosphorus dual active sites, its preparation method, and its application. The nitrogen fixation catalyst has a Z-type heterojunction structure, comprising: phosphorus-doped crystalline carbon-nitrogen nanorods, wherein phosphorus atoms replace carbon atoms located at the end of the crystalline carbon-nitrogen nanorod framework and bonded to amino groups; and purple phosphorus quantum dots in situ bonded to the crystalline carbon-nitrogen nanorods; wherein the doped phosphorus atoms in the crystalline carbon-nitrogen nanorods and the unsaturated phosphorus atoms in the purple phosphorus quantum dots constitute spatially separated dual active sites. This invention constructs a Z-type heterojunction with spatially separated dual active sites, utilizing purple phosphorus to dominate the construction of doped P atoms and VPQDs dual active sites on a CCNR, achieving a side-on adsorption mode for N2 molecules and significantly reducing the activation barrier for N≡N triple bond dissociation.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic nitrogen fixation technology, and in particular to a nitrogen fixation catalyst based on purple phosphorus dual active sites, its preparation method, and its application. Background Technology

[0002] Photocatalytic nitrogen fixation for ammonia synthesis utilizes solar photocatalysis technology to react N2 and water (H2O) at room temperature to produce NH3 and oxygen (O2), and is considered one of the ideal pathways for future renewable energy. Compared with the traditional Haber-Bosch process, photocatalytic nitrogen fixation has advantages such as a simple reaction system, environmental friendliness, low cost, and safety.

[0003] The catalytic reduction of N2 to ammonia mainly involves three basic steps: (i) adsorption of N2 at the active site, (ii) dissociation of the N≡N triple bond and hydrogenation reaction, and (iii) desorption of the formed NH3 from the catalyst surface. Among these, based on the different sequences of N≡N triple bond dissociation and hydrogenation, the currently recognized nitrogen fixation mechanisms are mainly divided into two types: (I) dissociation mechanism and (II) association mechanism.

[0004] Generally, the adsorption modes of N2 on semiconductors are classified into two types: "end-on" and "side-on". In the "end-on" adsorption mode, the N atom at one end of the N2 molecule interacts with the active site, and its thermodynamic onset overpotential is approximately 0.4-0.5 V. In the "side-on" adsorption mode, under conditions of biological nitrogenase or some catalysts with strong interactions with N2, the N atoms at both ends of the N2 molecule simultaneously bind to the catalyst surface to form strong chemisorption, and its thermodynamic onset overpotential is only 0.19 V.

[0005] However, in the currently reported photocatalytic nitrogen fixation reactions, the adsorption of N2 on the active sites of photocatalysts is mostly in an "end-on" mode, which often leads to problems such as weak chemisorption between N2 and the active sites and difficulty in effectively dissociating and activating the N≡N triple bond.

[0006] Therefore, the existing technology still needs further development and improvement. Summary of the Invention

[0007] To address the aforementioned technical problems, this invention proposes a nitrogen-fixing catalyst based on dual active sites of purple phosphorus, its preparation method, and its application, aiming to achieve a "side-on" adsorption mode for N2 and effectively reduce the activation barrier for N≡N triple bond dissociation. Specifically: In a first aspect, a nitrogen-fixing catalyst based on dual active sites of purple phosphorus, wherein the nitrogen-fixing catalyst has a Z-type heterojunction structure, comprising: Phosphorus-doped crystalline carbon-nitrogen nanorods, wherein phosphorus atoms replace carbon atoms bonded to amino groups at the ends of the crystalline carbon-nitrogen nanorod framework. Purple phosphorus quantum dots in situ bonded to the crystalline carbon-nitrogen nanorods; In this context, the doped phosphorus atoms in the crystalline carbon-nitrogen nanorods and the unsaturated phosphorus atoms in the purple phosphorus quantum dots constitute spatially separated dual active sites.

[0008] This invention achieves a "side-on" adsorption mode for N2 molecules by constructing a Z-shaped heterojunction structure and forming spatially separated dual active sites composed of doped phosphorus atoms and unsaturated phosphorus atoms in purple phosphorus quantum dots on crystalline carbon-nitrogen nanorods. This significantly enhances the chemisorption between N2 and the catalyst active sites, increasing the N2 adsorption energy from -0.42 eV to -1.38 eV and the N-N bond length from 1.10 Å to 1.46 Å. This effectively lowers the activation energy barrier for N≡N triple bond dissociation, suppresses the competitive hydrogen evolution reaction, and greatly improves the conversion efficiency of photocatalytic nitrogen fixation to ammonia synthesis.

[0009] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0010] As a preferred technical solution, the nitrogen-fixing catalyst, wherein the purple phosphorus quantum dots include two particle size distributions, wherein the first particle size distribution ranges from 1 to 5 nm and the second particle size distribution ranges from 8 to 15 nm.

[0011] A dual-size distribution of purple phosphorus quantum dots (VPQDs) is employed. On the surface of small-sized VPQDs (1-5 nm), phosphorus atoms preferentially replace carbon atoms at the terminal -NH2 junctions in the CCNR, achieving effective phosphorus doping. On larger-sized VPQDs (8-15 nm), in-situ bonding occurs at the phosphorus doping sites, constructing a stable, spatially separated dual-active-site structure. This hierarchical size design optimizes the charge transfer efficiency at the heterojunction interface, significantly improving the NH3 generation rate.

[0012] Secondly, a method for preparing the nitrogen-fixing catalyst described above, comprising the following steps: Melamine was thermally condensed at 450-600℃ to obtain a preheated product. The preheated product was mixed and ground with an alkali metal salt, then calcined at 500-650°C in an inert atmosphere, and then washed and dried to obtain the crystalline carbon-nitrogen nanorods. A purple phosphorus quantum dot dispersion is provided. The crystalline carbon-nitrogen nanorods are mixed with the purple phosphorus quantum dot dispersion and subjected to a solvothermal reaction at 150-200°C to obtain the nitrogen-fixing catalyst.

[0013] This invention presents a three-step method for preparing nitrogen-fixing catalysts. First, highly crystalline CCNRs are obtained through thermal condensation and molten salt methods. Then, in-situ bonding of VPQDs with CCNRs and P-atom doping are achieved through a solvothermal method, thus completing the construction of dual active sites in one step. This preparation method is simple, has controllable conditions, and is easy to scale up for industrial production, laying the foundation for industrial applications.

[0014] As a preferred technical solution, in the preparation method, the alkali metal salt is selected from one or more of potassium chloride, lithium chloride, sodium chloride, and calcium chloride.

[0015] This invention employs an alkali metal salt-assisted molten salt method to prepare CCNR. The alkali metal salt forms a eutectic mixture at high temperature, which effectively reduces the reaction temperature, promotes the formation of crystalline carbon-nitrogen nanorods, and improves the crystallinity and morphological uniformity of the product. This is beneficial for subsequent composite with VPQDs and the construction of dual active sites.

[0016] As a preferred technical solution, in the preparation method, the alkali metal salt is a mixture of lithium chloride and potassium chloride, and the molar ratio of lithium chloride to potassium chloride is (55-65):(45-35).

[0017] This invention utilizes a LiCl / KCl mixed salt system and optimizes the molar ratio to achieve the desired effect. + and K + The synergistic effect of these components allows for the formation of a eutectic mixture at around 550℃, effectively regulating the growth kinetics of CCNR and obtaining uniform nanorod morphologies with diameters of 50-100 nm and lengths of 1-3 μm. Furthermore, these nanorods exhibit the highest crystallinity, providing an ideal carrier structure for the subsequent construction of dual active sites.

[0018] As a preferred technical solution, the preparation method wherein the purple phosphorus quantum dot dispersion is prepared by the following method: The purple phosphorus was ground, and the ground purple phosphorus was mixed with N-methylpyrrolidone to obtain a purple phosphorus mixture. The purple phosphorus mixture was subjected to ultrasonic treatment at a temperature below 5°C to obtain a purple phosphorus quantum dot dispersion.

[0019] This invention effectively exfoliates blocky purple phosphorus to obtain quantum dots by combining grinding-assisted liquid phase exfoliation with low-temperature ultrasonic treatment. The low-temperature conditions (<5℃) inhibit the oxidation and aggregation of VPQDs, maintaining the high crystallinity and surface activity of VPQDs, and providing high-quality VPQDs raw materials for subsequent composite with CCNR and construction of dual active sites.

[0020] As a preferred technical solution, the preparation method includes separating the purple phosphorus quantum dot dispersion by centrifugation speed gradient separation after ultrasonic treatment; wherein the centrifugation speed gradient separation includes: first separating purple phosphorus quantum dots of the first size distribution by centrifugation at 5000-8000 rpm, and then separating purple phosphorus quantum dots of the second size distribution by centrifugation at 12000-18000 rpm.

[0021] This invention achieves precise size fractionation of VPQDs through centrifugal velocity gradient separation technology, obtaining VPQDs with two size distributions: 1-5 nm and 8-15 nm. This separation method is simple to operate, has controllable conditions, and good repeatability, providing technical support for the precise feeding and fractionation reaction of dual-size VPQDs.

[0022] As a preferred technical solution, in the preparation method, the purple phosphorus quantum dots are added in two batches. The first batch of purple phosphorus quantum dots with the second size distribution is added for pre-reaction, and after reacting for 0.5-2 hours, purple phosphorus quantum dots with the first size distribution are added.

[0023] This invention employs a batch addition strategy. Large-size VPQDs (8-15 nm) undergo pre-reaction to form stable bonding sites on the CCNR surface. Subsequently, small-size VPQDs (1-5 nm) are added, where highly active P atoms preferentially replace C atoms on the CCNR side-linked -NH2, achieving effective P doping. This hierarchical reaction strategy ensures the orderly construction of the two active sites, optimizing the spatial distribution and synergistic effect of the dual active sites.

[0024] As a preferred technical solution, in the preparation method, the mass ratio of the first size-distributed purple phosphorus quantum dots to the second size-distributed purple phosphorus quantum dots is 1:2-5.

[0025] This invention optimizes the mass ratio of two VPQDs of different sizes, ensuring that the smaller VPQDs provide sufficient P atoms for doping, while the larger VPQDs provide ample surface sites for constructing heterojunctions. This ratio yields the best synergistic effect of the two active sites, resulting in optimal photocatalytic nitrogen fixation performance of the catalyst.

[0026] Thirdly, the application of the aforementioned nitrogen-fixing catalyst in photocatalytic nitrogen fixation for ammonia synthesis.

[0027] Applying the nitrogen-fixing catalyst of this invention to the photocatalytic nitrogen fixation and ammonia synthesis reaction can achieve efficient nitrogen fixation under normal temperature and pressure conditions and without sacrificial agents, which has good prospects for industrial application.

[0028] Beneficial effects: Compared with existing technologies, this invention constructs a Z-shaped VPQDs / P-CCNR heterojunction with spatially separated dual active sites. Utilizing phosphorus (VP) to dominate the construction of doped P atoms and VPQDs dual active sites on the CCNR, it achieves a "side-on" adsorption mode for N2 molecules, significantly reducing the activation energy barrier for N≡N triple bond dissociation. N2 is mainly adsorbed on P-CCNR / VPQDs in a "side-on" mode, with its adsorption energy significantly increased to -1.38 eV and the N / N bond length increased to 1.46 Å, effectively improving photocatalytic nitrogen fixation performance. By constructing spatially separated dual active sites to trigger a tandem photocatalytic nitrogen fixation reaction, doped P atoms serve as N2 adsorption sites, and surface-enriched hole-rich VPQDs serve as H2O adsorption sites, effectively converting the activated H2O into N2 adsorption sites. + By incorporating surface overflow into the enzymatic alternating hydrogenation pathway for ammonia synthesis, the competitive hydrogen evolution reaction of water molecules at the reduction site is effectively avoided, significantly improving the efficiency of photocatalytic nitrogen fixation. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the drawings used in the description of the embodiments or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0030] Figure 1 This is a schematic diagram of the preparation of CCNR by the molten salt method provided in an embodiment of the present invention.

[0031] Figure 2 This is a schematic diagram of the preparation of VPQDs using the grinding-assisted liquid phase stripping method and the interval centrifugation method provided in the embodiments of the present invention.

[0032] Figure 3 This is a structure-property relationship diagram of the size and thickness of VPQDs and centrifugation speed provided in the embodiments of the present invention.

[0033] Figure 4 This is a schematic diagram of the one-step solvothermal method for preparing VPQDs / P-CCNR heterojunction photocatalysts provided in this embodiment of the invention.

[0034] Figure 5 (A) is a top view of the CCNR structure; (B) is a model diagram of P atoms replacing the C sites (P1) on the side of the CCNR; and (C) is a diagram of P atoms replacing the C sites (P2) on the side of the CCNR.

[0035] Figure 6 Solid-state CCNR and VPQDs / P-CCNR provided in embodiments of the present invention 13C nuclear magnetic resonance spectrum.

[0036] Figure 7 Solid-state VPQDs / P-CCNR provided in embodiments of the present invention 31 P nuclear magnetic resonance spectrum.

[0037] Figure 8 In the diagram, (F) represents the adsorption model and adsorption energy of N2 on the BCN surface, (G) represents the adsorption model and adsorption energy of N2 on the CCNR, and (H) represents the "side-on" adsorption model and adsorption energy of N2 on the VPQDs / P-CCNR side.

[0038] Figure 9 A schematic diagram of the Z-type VPQDs / P-CCNR heterojunction and its photocatalytic nitrogen fixation mechanism provided in the embodiments of the present invention.

[0039] Figure 10 In the table, (A) represents N2 on CCNR, (B) represents N2 on P-CCNR, (C) represents N2 on CCNR / VPQDs, and (D) represents the adsorption mode and adsorption energy of N2 on P-CCNR / VPQDs.

[0040] Figure 11 In the table, (A) represents the photocatalytic NH3 generation rate of different samples, (B) represents the photocatalytic nitrogen fixation stability of P-CCNR / VPQD during cycling, and (C) represents the AQY value of P-CCNR / VPQD photocatalytic nitrogen fixation at different wavelengths. Detailed Implementation

[0041] The embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and should not be construed as limiting the scope of the invention.

[0042] In the description of the embodiments of the present invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing the embodiments of the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the embodiments of the present invention. In addition, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0043] In the description of the embodiments of the present invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "connected" and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of the present invention based on the specific circumstances.

[0044] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0045] The inventors discovered that the current photocatalytic nitrogen fixation to ammonia synthesis has a low conversion efficiency, mainly due to the weak chemisorption between N2 and the active sites of the photocatalyst, as well as the difficulty in dissociating and activating the N≡N triple bond. Furthermore, the competitive adsorption of H2O at the active sites further reduces the conversion efficiency of NH3.

[0046] To address these issues, this invention proposes a novel photocatalytic nitrogen fixation reaction scheme that utilizes VP-dominated construction of doped P atoms on CCNRs and VPQDs as dual active sites to trigger a tandem photocatalytic process. On one hand, the doped P atoms serve as N2 adsorption sites, and the synergistic effect with unsaturated P atoms on neighboring VPQDs enables a "side-on" adsorption mode for N2 molecules, significantly reducing the N≡N triple bond dissociation activation barrier. On the other hand, the hole-rich VPQDs on their surface serve as H2O adsorption sites, allowing the activated H2O to be adsorbed onto the CCNR. + By incorporating surface overflow into the N≡N bond dissociation and enzymatic alternating hydrogenation pathway for ammonia synthesis, a highly efficient tandem photocatalytic nitrogen fixation reaction is achieved while effectively suppressing the competitive hydrogen evolution reaction. Furthermore, by employing a non-metal to replace the metal in existing technologies as the dual active site, the cost of the catalyst is reduced.

[0047] In one implementation of this invention, phosphorus atoms on crystalline carbon-nitrogen nanorods are doped with two sizes, and VPQDs with dual-size distribution are prepared by a grinding-assisted liquid-phase exfoliation method and a zone centrifugation technique. During the solvothermal process, the P atoms on the surface of the smaller VPQDs preferentially replace the C atoms at the terminal -NH2 in the CCNR, forming P-CCNR. Subsequently, the larger VPQDs undergo in-situ bonding at the P-doped sites, thereby constructing a Z-type VPQDs / P-CCNR heterojunction with spatially separated dual active sites.

[0048] Combination Figures 5 to 8 As shown, this invention uses nuclear magnetic resonance spectroscopy and density functional theory to determine the sites where P atoms replace the C atoms of the terminal -NH2 atoms on the side of the CCNR (P1 in P1 and P2 sites). Based on the adsorption energies of VPQDs bonded in situ at different sites on P-CCNR, the crystal structure model of VPQDs / P-CCNR is optimized. This reveals the influencing factors on the formation of dual active sites by VP-dominated P atom doping on the CCNR and in-situ bonding of VPQDs. Furthermore, by comparing the adsorption energies of N2 molecules on BCN, CCNR, and VPQDs / P-CCNR, it is determined that the redistribution of charge between the doped P atom as an N2 adsorption site and the unsaturated P atoms on adjacent VPQDs enables "side-on" adsorption of N2 molecules. Comparing the bond length changes of N2 molecules under different adsorption modes further clarifies that the "side-on" adsorption state of N2 molecules on VPQDs / P-CCNR can significantly reduce the activation energy barrier for N≡N triple bond dissociation, effectively improving photocatalytic nitrogen fixation performance. On the other hand, theoretical calculations show that VPQDs rich in holes on the surface can be used as H2O adsorption sites. The H+ generated by activation can participate in the enzymatic association hydrogenation synthesis ammonia pathway through surface overflow, realizing a tandem photocatalytic nitrogen fixation reaction and effectively inhibiting the competitive hydrogen evolution reaction.

[0049] This invention utilizes electrochemical methods to characterize the nitrogen fixation and hydrogen evolution reactions occurring on VPQDs / P-CCNR heterojunctions under different atmospheres (mainly N2 and Ar). In Ar-saturated solution, the water reduction and hydrogen evolution reaction primarily occur, while in N2-saturated solution, nitrogen fixation and the competitive hydrogen evolution reaction occur simultaneously. Therefore, by comparing the current densities in N2 and Ar-saturated solutions, the dominant reaction in the reaction system can be determined. When a certain bias voltage is applied, a higher current density is observed in the N2-saturated solution, indicating that photocatalytic nitrogen fixation is the primary reaction on the VPQDs / P-CCNR heterojunction, and the current response is mainly related to the N2 reduction reaction. The slope of the Tafel curve is used to further characterize the amount of apparent electron transfer in the photocatalytic nitrogen fixation reaction, revealing the kinetics of photocatalytic ammonia synthesis. Furthermore, using... 14 N2 and15 The N2 isotope labeling method verified that the product NH3 originated from the reduction of N2. Based on this, the feasibility of triggering a tandem photocatalytic all-nitrogen fixation reaction under sacrificial agent-free conditions with Z-type VPQDs / P-CCNR heterojunctions was explored, and the oxidation-end products were analyzed. Finally, the cycle stability of the optimal photocatalyst and its apparent quantum efficiency under different monochromatic lights were tested. Figure 9 (As shown). This invention utilizes in-situ XPS technology to analyze the valence state changes of phosphorus in the VPQDs / P-CCNR heterojunction photocatalyst under nitrogen fixation reaction conditions, further confirming the active sites of N2 and the "side-on" adsorption mode in the nitrogen fixation reaction. Furthermore, time-resolved in-situ FT-IR technology is used to analyze the changes of various species with reaction time during nitrogen fixation and ammonia synthesis. The characteristic peak at 1616 cm⁻¹ can be attributed to N2 adsorption, and the peak at 1366 cm⁻¹... -1 and 1201 cm -1 This belongs to NH 4+ The characteristic peak of the species, 1130 cm -1 The characteristic peak at that location belongs to the product H2N, which is produced by the "side-on" adsorption mode of N2 molecules. The stretching vibrations of NH2 species were investigated. By calculating the Gibbs free energies of various reaction intermediates in the distal association pathway, alternating association pathway, and enzymatic pathway of the nitrogen fixation reaction, the advantages of the enzymatic pathway induced by the N2 "side-on" adsorption mode in lowering the activation energy barrier of N≡N triple bond dissociation and thus improving photocatalytic nitrogen fixation performance were highlighted. Simultaneously, the Gibbs free energy of H2O molecule adsorption and dissociation on the VPQDs surface was calculated, revealing the important role of VPQDs in improving H2O molecule activation and surface H+ overflow triggering of the tandem photocatalytic nitrogen fixation reaction while effectively inhibiting the competitive hydrogen evolution reaction. Finally, the mechanism by which the N2 molecule "side-on" adsorption mode and H2O molecule activation overflow synergistically enhance and trigger the tandem photocatalytic nitrogen fixation reaction on the VPQDs / P-CCNR heterojunction was derived.

[0050] The following specific preparation examples will further explain the above-mentioned technical solutions provided by the present invention.

[0051] Example 1 CCNR is synthesized via an alkali metal salt-assisted molten salt method, such as... Figure 1As shown, the specific preparation process is as follows: 3 g of melamine was weighed and placed on a corundum ceramic boat in a muffle furnace, heated to 500°C at a rate of 5°C / min and held for 4 h. After natural cooling, a LiCl / KCl mixture (molar ratio 59:41) was added to a mortar and the preheated sample was further ground. Then, the mixture was placed in a tube furnace under a N2 atmosphere and heated to 550°C at a rate of 2.5°C / min and held for 4 h. After natural cooling to room temperature, the sample was washed with boiling deionized water to remove residual impurities, and dried at 60°C for 8 h to obtain the CCNR sample, as shown in (A) of Figure 5.

[0052] Dual-size distributed VPQDs were prepared by combining grinding-assisted liquid phase exfoliation with interval centrifugation. Figure 2 The main preparation process is as follows: 30 mg of blocky VP sample is weighed and placed in a mortar, a small amount (1-2 mL) of N-methylpyrrolidone (NMP) is added and mixed, and then ground in the same direction for 30 min to ensure complete exfoliation. This is then transferred to an ultrasonic bottle containing 50 mL of NMP and treated with ultrasound (50 W output power) in a constant temperature water bath below 5℃ for 6 h to obtain a mixture containing VPQDs. Then, the VPQDs dispersion is centrifuged at 6000-9000 rpm for 15 min using a zone centrifugation method to obtain large-sized VPQDs (8-15 nm), followed by centrifugation of the filtrate at 9000-15000 rpm for 15 min to obtain small-sized VPQDs (1-5 nm). The number and size of the VPQDs can be controlled by changing the centrifugation zone. Figure 3 As shown, the structure-property relationship between the size and thickness of the VPQD and the centrifugation velocity is as follows: Figure 2 As shown, as the centrifugation range decreased from 9-11K rpm to 3-5K rpm, the lateral dimension of the VPQD increased from 6.2±0.6 nm to 112.4±21.6 nm, and the corresponding VPQD thickness increased from 1.12±0.18 nm to 12.21±3.86 nm. Therefore, by changing the centrifugation speed of the VPQD dispersion, VPQDs with controllable size and thickness were obtained.

[0053] VPQDs / P-CCNR heterojunction photocatalysts were prepared by a one-step solvothermal method. Figure 4The main preparation process is as follows: 20 mg of CCNR sample was weighed and added to 40 mL of NMP dispersion containing 0.03 mg / mL VPQDs. The mixture was stirred thoroughly for 2 h to ensure complete homogeneity. Then, the mixture was transferred to a 50 mL polytetrafluoroethylene reactor and heated to 180 °C at a rate of 5 °C / min, maintaining the temperature for 10 h. The solvothermal sample was then subjected to centrifugation, washing, and dispersion three times, and finally vacuum dried at 60 °C for 8 h to obtain the VPQDs / P-CCNR sample.

[0054] Example 2 CCNR was prepared according to the method in Example 1, except that the molar ratio of LiCl / KCl was changed to 55:45, 57:43, 59:41, 61:39, 63:37 and 65:35.

[0055] Example 3 VPQDs / P-CCNR heterojunctions were prepared according to the method in Example 1, the difference being the use of different centrifugation intervals to separate the VPQDs. The results showed that the nitrogen fixation performance of the catalyst was optimal when using VPQDs with a dual-size distribution (small size 1-5 nm and large size 8-15 nm). This is because the P atoms on the surface of the small-sized VPQDs preferentially replace the C atoms at the terminal -NH2 junctions in the CCNR, forming P-CCNR; subsequently, the large-sized VPQDs undergo in-situ bonding at the P-atom doping sites, thereby constructing a Z-type VPQDs / P-CCNR heterojunction with spatially separated dual active sites.

[0056] Example 4 The VPQDs / P-CCNR heterojunction photocatalyst prepared in Example 1 was used for photocatalytic nitrogen fixation and ammonia synthesis. The reaction conditions were: catalyst dosage 20 mg, gas-liquid-solid three-phase system, N2 flow rate 30 mL / min, H2O as the reaction solvent, and a 300 W xenon lamp (λ>420 nm) as the light source. After 3 h of reaction, the NH3 concentration was determined by Nessler's reagent spectrophotometry.

[0057] Combination Figure 10 As shown, the test results indicate that the optimized P-CCNR / VPQDs (6% VPQDs) has an NH3 generation rate of 1586.25 μmol h⁻¹. -1 g -1 The values ​​are the original BCN (98.01 μmol h⁻¹). -1 g -1 ) and CCNR (158.62 μmol h -1 g -1The apparent quantum yield (AQE) was approximately 16.2 times and 10.1 times higher than that of other previously reported nitrogen fixation reactions. After four consecutive photocatalytic nitrogen fixation reactions, the nitrogen fixation activity of P-CCNR / VPQD did not decrease significantly, and it maintained good activity even after being stored for one month, indicating that P-CCNR / VPQD has good stability and recyclability. Furthermore, the apparent quantum yield (AQE) at 420 nm reached 1.58%, which is superior to other reported nitrogen fixation reaction performances.

[0058] Example 5 The VPQDs / P-CCNR crystal structure model prepared in Example 1 was optimized by density functional theory (DFT) calculations to verify the feasibility of constructing a tandem photocatalytic nitrogen fixation reaction platform by using spatially separated doped P atoms and VPQDs as adsorption sites for N2 and H2O, respectively.

[0059] Combination Figure 11 As shown, the calculation results indicate that the adsorption energy of N2 on CCNR and P-CCNR is very weak (its adsorption energies Eads are -0.38 and -0.42 eV, respectively), and the N-N bond length shows no significant change. On CCNR / VPQDs, N2 is mainly adsorbed in an "end-on" mode, and its Eads... ads The α-ads value is -0.78 eV, and the NN bond length is 1.16 Å. For the phosphorus-dominated dual active site, N2 is mainly adsorbed on P-CCNR / VPQDs in a "side-on" mode, with its α-ads value significantly increasing to -1.38 eV and the NN bond length increasing to 1.46 Å, indicating that the N≡N triple bond is effectively activated.

[0060] In summary, this invention provides a nitrogen-fixing catalyst based on purple phosphorus dual active sites, its preparation method, and its application. The nitrogen-fixing catalyst has a Z-type heterojunction structure, comprising: phosphorus-doped crystalline carbon-nitrogen nanorods, wherein phosphorus atoms replace carbon atoms located at the lateral ends of the crystalline carbon-nitrogen nanorod framework and bonded to amino groups; and purple phosphorus quantum dots in situ bonded to the crystalline carbon-nitrogen nanorods; wherein the doped phosphorus atoms in the crystalline carbon-nitrogen nanorods and the unsaturated phosphorus atoms in the purple phosphorus quantum dots constitute spatially separated dual active sites.

[0061] This invention employs a novel tandem photocatalytic nitrogen fixation reaction novelty scheme that uses VP-dominated construction of CCNR with doped P atoms and VPQDs as dual active sites to trigger the reaction. On one hand, the doped P atoms serve as N2 adsorption sites, and the synergistic effect with the unsaturated P atoms on the neighboring VPQDs enables the "side-on" adsorption mode of N2 molecules, significantly reducing the activation energy barrier for N≡N triple bond dissociation. On the other hand, the VPQDs enriched with holes on their surface serve as H2O adsorption sites, activating the H2O generated during activation. +By participating in the N≡N bond dissociation and enzymatic alternating hydrogenation pathway through surface overflow, a highly efficient tandem photocatalytic nitrogen fixation reaction is achieved while effectively suppressing the competitive hydrogen evolution reaction.

[0062] Finally, it should be noted that the above embodiments are only for illustrating the present invention and not for limiting the present invention. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that various combinations, modifications, or equivalent substitutions of the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention and should be covered within the scope of the claims of the present invention.

Claims

1. A nitrogen fixation catalyst based on dual active sites of purple phosphorus, characterized in that, The nitrogen-fixing catalyst has a Z-type heterojunction structure, comprising: Phosphorus-doped crystalline carbon-nitrogen nanorods, wherein phosphorus atoms replace carbon atoms bonded to amino groups at the ends of the crystalline carbon-nitrogen nanorod framework; and Purple phosphorus quantum dots in situ bonded to the crystalline carbon-nitrogen nanorods; In this context, the doped phosphorus atoms in the crystalline carbon-nitrogen nanorods and the unsaturated phosphorus atoms in the purple phosphorus quantum dots constitute spatially separated dual active sites.

2. The nitrogen-fixing catalyst according to claim 1, characterized in that, The purple phosphorus quantum dots include two particle size distributions, with the first particle size distribution ranging from 1 to 5 nm and the second particle size distribution ranging from 8 to 15 nm.

3. A method for preparing the nitrogen-fixing catalyst according to any one of claims 1-2, characterized in that, Includes the following steps: Melamine was thermally condensed at 450-600℃ to obtain a preheated product. The preheated product was mixed and ground with an alkali metal salt, then calcined at 500-650°C in an inert gas atmosphere, and after washing and drying, the crystalline carbon-nitrogen nanorods were obtained. A purple phosphorus quantum dot dispersion is provided. The crystalline carbon-nitrogen nanorods are mixed with the purple phosphorus quantum dot dispersion and subjected to a solvothermal reaction at 150-200°C to obtain the nitrogen-fixing catalyst.

4. The preparation method according to claim 3, characterized in that, The alkali metal salt is selected from one or more of potassium chloride, lithium chloride, sodium chloride, and calcium chloride.

5. The preparation method according to claim 4, characterized in that, The alkali metal salt is a mixture of lithium chloride and potassium chloride, wherein the molar ratio of lithium chloride to potassium chloride is (55-65):(45-35).

6. The preparation method according to claim 3, characterized in that, The purple phosphorus quantum dot dispersion was prepared by the following method: The purple phosphorus was ground, and the ground purple phosphorus was mixed with N-methylpyrrolidone to obtain a purple phosphorus mixture. The purple phosphorus mixture was subjected to ultrasonic treatment at a temperature below 5°C to obtain a purple phosphorus quantum dot dispersion.

7. The preparation method according to claim 6, characterized in that, After ultrasonic treatment, the purple phosphorus quantum dot dispersion is subjected to centrifugal speed gradient separation; wherein, the centrifugal speed gradient separation includes: firstly centrifuging at 5000-8000 rpm to separate purple phosphorus quantum dots of a first size distribution, and then centrifuging at 12000-18000 rpm to separate purple phosphorus quantum dots of a second size distribution.

8. The preparation method according to claim 7, characterized in that, The purple phosphorus quantum dots were added in two batches. The first batch of purple phosphorus quantum dots with the second size distribution were added for pre-reaction, and after reacting for 0.5-2 hours, purple phosphorus quantum dots with the first size distribution were added.

9. The preparation method according to claim 8, characterized in that, The mass ratio of the first size distribution of purple phosphorus quantum dots to the second size distribution of purple phosphorus quantum dots is 1:2-5.

10. The application of any one of the nitrogen-fixing catalysts described in claims 1-2 in photocatalytic nitrogen fixation for ammonia synthesis.

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