Alkali metal modified molybdenum nitride-based catalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of catalytic materials, and particularly relates to an alkali metal modified molybdenum nitride-based catalyst and a preparation method and application thereof. The specific preparation method comprises the following steps: stirring and dispersing graphene oxide and a surfactant in a solvent, and meanwhile, forming a graphene oxide nanosheet with a single-layer or few-layer structure, so as to obtain a carrier solution; stirring and mixing an aluminum source, a molybdenum source, alkali metal salt and the carrier solution under an acidic condition to form a Mo-Al precursor; meanwhile, under the action of a surfactant, metal ions in alkali metal salt are combined with the Mo-Al precursor and dispersed on the surface of the graphene oxide nanosheet, and precursor sol is obtained; calcining the precursor sol in an air atmosphere, and then reducing in an ammonia atmosphere to obtain the alkali metal modified molybdenum nitride-based catalyst. The prepared alkali metal modified molybdenum nitride-based catalyst shows excellent ammonia decomposition catalytic performance.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalytic materials, and particularly relates to an alkali metal-modified molybdenum nitride-based catalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Hydrogen energy, as a low-carbon and efficient new energy, is regarded as an ideal green and sustainable energy. However, the technical problems of its storage and transportation severely restrict its large-scale application and development. In the research of hydrogen storage materials, ammonia has become a potential hydrogen storage medium due to its low-risk characteristics of storage and transportation, and the advantage that only hydrogen and nitrogen are produced after catalytic decomposition without other by-products. By catalytically decomposing ammonia to produce hydrogen, not only can the risks of direct hydrogen storage and transportation be avoided, but also the convenient utilization of hydrogen energy can be realized.

[0003] Based on the safety of ammonia and the cleanliness of the decomposition products, catalytic ammonia decomposition to produce hydrogen is regarded as an efficient and green hydrogen production technology route. The core of this technology is to develop efficient catalysts to reduce the activation energy of the ammonia decomposition reaction, improve the reaction rate and hydrogen production rate. Therefore, exploring catalysts with high activity and stability is an important direction in the current research field of catalytic ammonia decomposition to produce hydrogen.

[0004] As a common catalyst for ammonia decomposition, molybdenum nitride catalyst has attracted much attention due to its unique electronic structure and catalytic performance. And molybdenum nitride catalyst has electronic characteristics similar to noble metals, and can exhibit excellent activity in the catalytic reaction of ammonia decomposition. However, in the process of preparing molybdenum nitride-based catalysts in the prior art, due to the lack of effective control, the obtained molybdenum nitride particles generally have large sizes, resulting in poor dispersion of active species and few active sites, and are prone to agglomeration under the conditions of high-temperature ammonia decomposition reaction, thus significantly reducing the catalytic activity and long-term stability of the catalyst. Summary of the Invention

[0005] In order to solve the above technical problems in the prior preparation of molybdenum nitride-based catalysts, due to the lack of effective control, there are poor dispersion of active species and few active sites, and it is prone to agglomeration in the high-temperature ammonia decomposition reaction, resulting in insufficient activity and stability of the catalyst, the present invention provides an alkali metal-modified molybdenum nitride-based catalyst, a preparation method thereof, and an application thereof.

[0006] The present invention introduces alkali metal promoters to perform lattice doping or surface modification on molybdenum nitride, which can effectively regulate the electronic structure of the active components of the modified molybdenum nitride and optimize its surface adsorption performance; by increasing the surface basic sites, it promotes the adsorption and deprotonation process of NH3 and accelerates the ammonia decomposition reaction. At the same time, the modified molybdenum nitride is in-situ dispersed in the pores of mesoporous alumina and loaded on the graphene oxide support; through the confinement effect of mesoporous alumina, the dispersion of the active components is improved and their deactivation due to sintering during the high-temperature ammonia decomposition reaction is inhibited. At the same time, a two-dimensional network structure is formed by using graphene, increasing the contact area between the catalyst and reactant molecules, reducing the activation energy, and greatly enhancing the catalytic activity and durability of the catalyst, thus solving the problem of insufficient activity and stability of existing molybdenum nitride-based catalysts.

[0007] The first object of the present invention is to provide a preparation method of an alkali metal-modified molybdenum nitride-based catalyst, comprising the following steps: Stir and disperse graphene oxide and a surfactant in a solvent to simultaneously form graphene oxide nanosheets with a single-layer or few-layer structure, obtaining a carrier solution; under acidic conditions, stir and mix an aluminum source, a molybdenum source, an alkali metal salt, and the carrier solution to form a Mo-Al precursor; meanwhile, under the action of the surfactant, metal ions in the alkali metal salt combine with the Mo-Al precursor and are dispersed on the surface of the graphene oxide nanosheets, obtaining a precursor sol; calcine the precursor sol in an air atmosphere to pyrolyze the Mo-Al precursor to form a composite oxide structure with MoO3 nanoparticles embedded in the mesoporous alumina framework; then reduce it in an ammonia atmosphere to nitride the MoO3 nanoparticles to Mo2N nanoparticles and reduce the graphene oxide nanosheets to conductive reduced graphene oxide nanosheets, obtaining an alkali metal-modified molybdenum nitride-based catalyst.

[0008] The present invention uses graphene oxide as a carrier and a surfactant as a pore-forming agent. Graphene oxide and the surfactant are dispersed in a solvent, and through stirring treatment, the van der Waals force between the graphene oxide layers is partially destroyed to disperse and exfoliate it into a single-layer or few-layer structure. At the same time, the surfactant adsorbs on the surface of graphene oxide through hydrophobic-hydrophilic interactions, reducing the interfacial tension and preventing the re-aggregation of graphene oxide; moreover, the surfactant can serve as a pore-forming agent for the subsequent formation of mesoporous structures.

[0009] Preferably, the surfactant is a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer.

[0010] Preferably, the time for stirring and dispersing graphene oxide and the surfactant in the solvent is 6 h to 12 h, and the temperature is 25 °C to 35 °C.

[0011] In the present invention, an aluminum source is acid-catalyzed to hydrolyze to generate an Al(OH)3 sol, which combines with MoO4 in the molybdenum source to form a Mo-Al precursor. By adjusting the pH value of the precursor sol to 1-3, premature precipitation of the molybdenum source is prevented, and the electrostatic combination of MoO4 and Al(OH)3 is promoted. At the same time, a surfactant forms micelles in the precursor sol, guiding the Mo-Al precursor to be uniformly distributed on the surface of graphene oxide and forming a mesoporous structure. 2- Combined, 2- with the electrostatic combination of MoO4 and Al(OH)3.

[0012] In the present invention, by introducing an alkali metal salt to perform lattice doping or surface modification on Mo, the electronic structure of the Mo active component can be effectively regulated, its surface adsorption performance can be optimized, the adsorption strength of the intermediate is moderate, the desorption of the product molecules is promoted, and thus the reaction activity is significantly improved.

[0013] In the present invention, the obtained precursor sol is dried at 60 °C to avoid pore collapse caused by violent volatilization and maintain the integrity of the mesoporous structure. In addition, the Mo-Al precursor in the precursor sol crosslinks with graphene through hydrogen bonds and electrostatic interactions to form a two-dimensional network structure, ensuring the highly dispersed active components.

[0014] Preferably, the molar ratio of the aluminum source, the molybdenum source, and the alkali metal salt is 5:3:0.9; the molar ratio of the alkali metal salt to graphene oxide is 9:0.3.

[0015] Preferably, the time for stirring and mixing the aluminum source, the molybdenum source, the alkali metal salt, and the carrier solution is 8 h to 12 h, and the temperature is 25 °C to 35 °C.

[0016] Preferably, the acid used in the acidic condition is concentrated nitric acid.

[0017] Preferably, the aluminum source is aluminum isopropoxide; the molybdenum source is ammonium molybdate; the alkali metal salt is lithium nitrate, sodium nitrate, potassium nitrate, or cesium nitrate.

[0018] It should be noted that in the present invention, through two-stage calcination, the precursor sol is first calcined in an air atmosphere to remove the pore-forming agent and form a mesoporous alumina skeleton; at the same time, the molybdenum source is pyrolyzed into MoO3 nanoparticles and embedded in the mesoporous alumina pores. Then, it is reduced in an ammonia atmosphere, and a nitridation reaction occurs with ammonia to reduce and nitride MoO3 into highly active Mo2N; at the same time, graphene oxide is reduced to conductive reduced graphene oxide to form a two-dimensional network structure, which is beneficial to the mass transfer efficiency. In addition, the alkali metal, as an electron donor, can transfer electrons to the Mo metal center, weaken the Mo-N and N-H bonds, lower the position of its d-band center, regulate the surface electronic structure, make the adsorption strength of the intermediate moderate, avoid too high surface coverage, promote the desorption of N2 and H2, and further improve the reaction activity.

[0019] Preferably, the calcination temperature is 350°C to 420°C. The calcination temperature is lower than the decomposition temperature of graphene to maintain the formation of a two-dimensional nanostructure; at the same time, the calcination temperature is higher than the decomposition temperature of the surfactant to ensure complete removal of the surfactant and form a mesoporous structure.

[0020] Preferably, the reduction temperature is 700°C to ensure the full nitridation of MoO3 to Mo2N. The kinetic threshold of the nitridation reaction is 600°C; at the same time, it avoids the excessive graphitization of reduced graphene oxide resulting in the loss of active sites.

[0021] The second object of the present invention is to provide an alkali metal-modified molybdenum nitride-based catalyst prepared by the above preparation method.

[0022] The third object of the present invention is to provide the application of the above alkali metal-modified molybdenum nitride-based catalyst in the catalytic decomposition of ammonia to produce hydrogen.

[0023] The present invention introduces an alkali metal promoter, which can not only adjust the electronic structure of the catalyst, increase the electron density of metal active sites, and thus significantly improve the activity of the catalyst. Moreover, as an electron donor, the alkali metal can transfer electrons to the metal center, weaken the Mo-N bond, lower the position of its d-band center, regulate the surface electronic structure, and make the adsorption strength of the NH x intermediate moderate, avoid excessive surface coverage, promote the desorption of nitrogen molecules, and further enhance the reaction activity. The introduction of the alkali metal also directly affects the surface basicity of the catalyst, making NH3 more likely to adsorb and activate on the alkali metal-modified molybdenum nitride-based catalyst during the decomposition reaction. Therefore, the increase in surface basic sites promotes the adsorption and deprotonation process of NH3 and accelerates the ammonia decomposition reaction. In addition, the alkali metal may promote the migration and combination of N atoms through the formation of surface intermediates, accelerating the generation and desorption of nitrogen.

[0024] Preferably, the particle size of the alkali metal-modified molybdenum nitride-based catalyst is 30 mesh to 60 mesh.

[0025] Preferably, the specific application method is as follows: Place the alkali metal-modified molybdenum nitride-based catalyst in a reactor, control the entry of ammonia into the reactor and react with the catalyst to catalytically decompose ammonia to produce hydrogen.

[0026] Preferably, the reaction temperature is 350°C to 650°C.

[0027] Compared with the prior art, the present invention has the following technical effects: 1. The present invention introduces an alkali metal promoter to conduct lattice doping or surface modification on molybdenum nitride, and in-situ disperses the modified molybdenum nitride in the pores of mesoporous alumina to form a composite structure of alkali metal-modified molybdenum nitride and mesoporous alumina, and then loads it on the surface of a graphene oxide support to obtain an alkali metal-modified molybdenum nitride-based catalyst. The present invention adjusts the electronic structure of molybdenum nitride through the alkali metal promoter, increases the electron density of metal active sites, thereby significantly improving the intrinsic activity of the catalyst; at the same time, the confinement effect of mesoporous alumina is used to improve the dispersion of active components and inhibit their deactivation due to sintering during the high-temperature ammonia decomposition reaction. At the same time, a two-dimensional network structure is formed by using graphene, increasing the contact area between the catalyst and reactant molecules, reducing the activation energy, and greatly enhancing the catalytic activity and durability of the catalyst, solving the problem of insufficient activity and stability of existing molybdenum nitride-based catalysts.

[0028] 2. The catalyst prepared by the present invention has a large specific surface area, high porosity, highly dispersed active components, rich active sites, and efficient mass transfer characteristics.

[0029] 3. The preparation method of the present invention is simple in operation and mild in reaction conditions, and is applicable to the synthesis of a variety of nitride-based catalysts with mesoporous structures. In addition, this method can simultaneously achieve the doping of other heteroatoms during the preparation of nitrides, showing good universality and broad application potential. Description of the Drawings

[0030] Figure 1 SEM images of the catalysts prepared in Examples 1 to 4 and Comparative Example 1; among them, a is the SEM image of the molybdenum nitride / alumina / graphene composite catalyst prepared in Comparative Example 1, b is the SEM image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1, c is the SEM image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2, d is the SEM image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3, and e is the SEM image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4.

[0031] Figure 2 TEM images and high-resolution TEM images of the molybdenum nitride / alumina / graphene composite catalyst prepared in Comparative Example 1; among them, a is the TEM image at a scale of 50 nm, b is the high-resolution TEM image at a scale of 20 nm, c is the high-resolution TEM image of frame 1 in b, and d is the high-resolution TEM image of frame 2 in b.

[0032] Figure 3Scanning transmission electron microscopy image and elemental mapping distribution diagram of the molybdenum nitride / aluminum oxide / graphene composite catalyst prepared in Comparative Example 1 under high-angle annular dark field; where a is the scanning transmission electron microscopy image under high-angle annular dark field, b is the Mo element distribution diagram, c is the N element distribution diagram, d is the O element distribution diagram, e is the C element distribution diagram, and f is the Al element distribution diagram.

[0033] Figure 4 Transmission electron microscopy image and high-resolution transmission electron microscopy image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1; where a is the transmission electron microscopy image, b is the high-resolution transmission electron microscopy image, c is the high-resolution transmission electron microscopy image of frame 1 in b, and d is the high-resolution transmission electron microscopy image of frame 2 in b.

[0034] Figure 5 Scanning transmission electron microscopy image and elemental mapping distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1 under high-angle annular dark field; where a is the scanning transmission electron microscopy image under high-angle annular dark field, b is the Mo element distribution diagram, c is the N element distribution diagram, d is the O element distribution diagram, e is the C element distribution diagram, and f is the Al element distribution diagram.

[0035] Figure 6 Transmission electron microscopy image, high-resolution transmission electron microscopy image, and scanning transmission electron microscopy image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2 under high-angle annular dark field; where a is the transmission electron microscopy image, b is the high-resolution transmission electron microscopy image, c is the scanning transmission electron microscopy image under high-angle annular dark field, and d is the high-resolution transmission electron microscopy image of frame 1 in b.

[0036] Figure 7 Elemental mapping distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2; where a is the Mo element distribution diagram, b is the N element distribution diagram, c is the Na element distribution diagram, d is the O element distribution diagram, e is the C element distribution diagram, and f is the Al element distribution diagram.

[0037] Figure 8 Transmission electron microscopy image, high-resolution transmission electron microscopy image, and scanning transmission electron microscopy image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3 under high-angle annular dark field; where a is the transmission electron microscopy image, b is the high-resolution transmission electron microscopy image, c is the scanning transmission electron microscopy image under high-angle annular dark field, and d is the high-resolution transmission electron microscopy image of frame 1 in b.

[0038] Figure 9 Elemental mapping distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3; where a is the Mo element distribution diagram, b is the N element distribution diagram, c is the K element distribution diagram, d is the O element distribution diagram, e is the C element distribution diagram, and f is the Al element distribution diagram.

[0039] Figure 10Transmission electron microscopy (TEM) image, high-resolution transmission electron microscopy (HRTEM) image, and scanning transmission electron microscopy (STEM) image under high-angle annular dark field (HAADF) of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4; where a is the TEM image, b is the HRTEM image, c is the STEM image under HAADF, and d is the HRTEM image of the frame 1 in b.

[0040] Figure 11 Elemental mapping distribution diagrams of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4; where a is the Mo element distribution diagram, b is the N element distribution diagram, c is the Cs element distribution diagram, d is the O element distribution diagram, e is the C element distribution diagram, and f is the Al element distribution diagram.

[0041] Figure 12 Particle size distribution diagrams of the catalysts prepared in Examples 1-4 and Comparative Example 1; where a is the particle size distribution diagram of the molybdenum nitride / aluminum oxide / graphene composite catalyst prepared in Comparative Example 1, b is the particle size distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1, c is the particle size distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2, d is the particle size distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3, and e is the particle size distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4.

[0042] Figure 13 Graph of the change in ammonia conversion rate with temperature for the catalysts prepared in Examples 1-4 and Comparative Example 1.

[0043] Figure 14 Graph of the change in ammonia conversion rate with time at a temperature of 600 °C for the catalysts prepared in Examples 1-4 and Comparative Example 1. Detailed Description of the Invention

[0044] To enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific examples and drawings.

[0045] In the description of the present invention, unless otherwise specified, the reagents used are commercially available, and the methods used are conventional techniques in the art.

[0046] It should be noted that the abbreviation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer is P123.

[0047] Example 1 A method for preparing an alkali metal-modified molybdenum nitride-based catalyst, comprising the following steps: Disperse 0.06 g of graphene oxide and 0.5 g of P123 in 5 mL of absolute ethanol by ultrasonic dispersion in turn, and stir at a constant temperature of 30 °C for 10 h to obtain a carrier solution.

[0048] Under stirring conditions, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, 3 mmol of ammonium molybdate, and 1.0 mg of lithium nitrate were successively added to the carrier solution, and the pH value of the solution was controlled to be 2. Stir at a constant temperature of 30 °C for 10 h to form a homogeneous precursor sol.

[0049] The precursor sol was dried at 60 °C for 16 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 3 h and then calcined in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, denoted as Li-Mo2N / mAl2O3 / rGO.

[0050] Example 2 A preparation method of an alkali metal-modified molybdenum nitride-based catalyst includes the following steps: 0.06 g of graphene oxide and 0.5 g of P123 were successively ultrasonically dispersed in 5 mL of absolute ethanol and stirred at a constant temperature of 25 °C for 8 h to obtain a carrier solution.

[0051] Under stirring conditions, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, 3 mmol of ammonium molybdate, and 7.9 mg of sodium nitrate were successively added to the carrier solution, and the pH value of the solution was controlled to be 2. Stir at a constant temperature of 25 °C for 8 h to form a homogeneous precursor sol.

[0052] The precursor sol was dried at 60 °C for 12 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 2 h and then calcined in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, denoted as Na-Mo2N / mAl2O3 / rGO.

[0053] Example 3 A preparation method of an alkali metal-modified molybdenum nitride-based catalyst includes the following steps: 0.06 g of graphene oxide and 0.5 g of P123 were respectively ultrasonically dispersed in 5 mL of absolute ethanol and stirred at a constant temperature of 35 °C for 12 h to obtain a carrier solution.

[0054] Under stirring conditions, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, 3 mmol of ammonium molybdate, and 9.4 mg of potassium nitrate were successively added to the carrier solution, and the pH value of the solution was controlled to be 2. Stir at a constant temperature of 35 °C for 12 h to form a homogeneous precursor sol.

[0055] The precursor sol was dried at 60 °C for 24 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 4 h and then in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, denoted as K-Mo2N / mAl2O3 / rGO.

[0056] Example 4 A preparation method of an alkali metal-modified molybdenum nitride-based catalyst, comprising the following steps: 0.06 g of graphene oxide and 0.5 g of P123 were successively ultrasonically dispersed in 5 mL of absolute ethanol and stirred at a constant temperature of 30 °C for 10 h to obtain a carrier solution.

[0057] Under stirring conditions, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, 3 mmol of ammonium molybdate, and 18.1 mg of cesium nitrate were successively added to the carrier solution, and the pH value of the solution was controlled to be 2, and stirred at a constant temperature of 30 °C for 10 h to form a homogeneous precursor sol.

[0058] The precursor sol was dried at 60 °C for 16 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 3 h and then in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, denoted as Cs-Mo2N / mAl2O3 / rGO.

[0059] Comparative Example 1 A preparation method of a molybdenum nitride / aluminum oxide / graphene composite catalyst, comprising the following steps: 0.06 g of graphene oxide and 0.5 g of P123 were successively ultrasonically dispersed in 5 mL of absolute ethanol and stirred at a constant temperature of 30 °C for 10 h to obtain a carrier solution.

[0060] Under stirring conditions, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, and 3 mmol of ammonium molybdate were successively added to the carrier solution, and stirred at a constant temperature of 30 °C for 10 h to form a homogeneous sol.

[0061] The sol was dried at 60 °C for 16 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 3 h and then in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and a molybdenum nitride / aluminum oxide / graphene composite catalyst was obtained, denoted as Mo2N / mAl2O3 / rGO.

[0062] Comparative Example 2 A preparation method of a molybdenum nitride / aluminum oxide composite catalyst, comprising the following steps: 0.5 g of P123 was ultrasonically dispersed in 5 mL of absolute ethanol. After uniform dispersion, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, and 3 mmol of ammonium molybdate were successively added, and the mixture was stirred at 30 °C for 10 h to form a homogeneous sol.

[0063] The sol was dried at 60 °C for 16 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 3 h and then in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and a molybdenum nitride / aluminum oxide composite catalyst, denoted as Mo2N / mAl2O3, was obtained.

[0064] Comparative Example 3 A preparation method of a molybdenum nitride / graphene composite catalyst includes the following steps: 0.06 g of graphene oxide was ultrasonically dispersed in 5 mL of absolute ethanol. After uniform dispersion, 3 mmol of ammonium molybdate was added, and the mixture was stirred at 30 °C for 10 h to form a homogeneous sol.

[0065] The sol was dried at 60 °C for 16 h and then ground. The ground sample was calcined in an air atmosphere at 400 °C for 3 h and then in an ammonia atmosphere at 700 °C for 2 h; among them, the heating rate was 2 °C / min, and a molybdenum nitride / graphene composite catalyst, denoted as Mo2N / rGO, was obtained.

[0066] Experimental tests.

[0067] 1. Surface morphology characterization.

[0068] As Figure 1 shown, the alkali metal-modified molybdenum nitride-based catalysts prepared in Examples 1 to 4 and the molybdenum nitride / aluminum oxide / graphene composite catalyst prepared in Comparative Example 1 all exhibited a relatively thin nanosheet structure, indicating the successful introduction of graphene. And the modification of the alkali metal did not change the surface morphology of the catalyst.

[0069] The microstructure of the catalyst was further observed by transmission electron microscopy. As Figure 2 shown in a and b of Figure 2 , the catalyst prepared in Comparative Example 1 exhibited typical mesoporous structure characteristics, and Mo2N nanoparticles with a particle size of about 1 nm to 3 nm were uniformly dispersed in the mesoporous channels; as

[0070] shown in c and d of Figure 3 , the Mo2N crystal plane spacing was 0.238 to 0.240 nm, belonging to the Mo2N(111) crystal plane.

[0071] As Figure 4As shown in a and b, the Na-Mo2N-mAl2O3 prepared in Example 1 has been successfully assembled onto the graphene surface, and the catalyst exhibits an irregular mesoporous structure. Mo2N is evenly dispersed on the surface of the mesoporous alumina support, and no obvious particle aggregation phenomenon appears, indicating that the mesoporous alumina support can effectively prevent the sintering and agglomeration of Mo2N particles. As Figure 4 As shown in c and d, the high-resolution transmission electron microscopy image reveals that the crystal plane spacing of Mo2N is 0.241 nm - 0.243 nm, which belongs to the Mo2N(111) crystal plane.

[0072] As Figure 5 shown, the elemental mapping shows that each element is evenly distributed throughout the composite catalyst.

[0073] As Figures 6 - 11 shown, the alkali metal-modified molybdenum nitride-based catalysts prepared in Example 2 - Example 4 all exhibit an irregular mesoporous structure and are assembled onto the graphene surface. And the crystal plane spacings are 0.241 nm, 0.242 nm, and 0.243 nm respectively, which belong to the Mo2N (111) crystal plane. The elemental mapping shows that each element is evenly distributed on the entire alkali metal-modified molybdenum nitride-based catalyst.

[0074] As Figure 12 shown, the particle sizes of the catalysts prepared in Comparative Example 1 and Examples 1 - 4 are 1.6 nm - 1.7 nm.

[0075] 2. Catalytic test.

[0076] The present invention utilizes an ammonia decomposition reaction evaluation device, which includes a fixed-bed quartz tube reactor with an inner diameter of 3 mm, a reaction heating furnace, a mass flowmeter for controlling the ammonia gas flow rate, and an on-line gas chromatograph. The gas chromatograph uses a 13X packed column and a thermal conductivity detector, and the carrier gas is hydrogen.

[0077] The present invention conducts specific catalytic tests on the catalysts prepared in Examples 1 - 4 and Comparative Examples 1 - 3. The specific test method is as follows: Place the catalyst prepared in the example or comparative example in the constant temperature zone of the quartz tube, insert the quartz tube into the heating furnace, precisely control the high-purity ammonia gas to enter the reactor at a fixed flow rate through the mass flowmeter, and then carry out the reaction through the catalyst layer; wherein the particle size of the catalyst is 30 mesh - 60 mesh, and the heating conditions are: heating from 350 °C to 650 °C at a heating rate of 10 °C / min and maintaining for 60 min at each temperature point. Use the on-line gas chromatograph to detect the gas after the reaction, collect the spectra every 10 minutes, and calculate the conversion rate of ammonia according to the formula of the conversion rate based on the concentration values of nitrogen and ammonia: ; wherein, is the conversion rate of ammonia, is the concentration of ammonia introduced, is the concentration of ammonia at the outlet.

[0078] Table 1 Hydrogen production rates of the catalysts prepared in Examples 1 - 4 and Comparative Examples 1 - 3

[0079] As shown in Table 1, compared with the Mo2N / mAl2O3 / rGO catalyst prepared in Comparative Example 1, after modifying Mo2N with different kinds of alkali metals, the hydrogen production rates of the alkali - metal - modified molybdenum nitride - based catalysts are all increased. The main reasons are as follows: Alkali - metal promoters can not only adjust the electronic structure of the catalyst, increase the electron density of metal active sites, thus significantly improving the activity of the catalyst. Moreover, as an electron donor, the alkali metal can transfer electrons to the metal center, weaken the Mo - N bond, lower the position of its d - band center, regulate the surface electronic structure, make the adsorption strength of intermediates moderate, avoid too high surface coverage, promote the desorption of nitrogen molecules, and thus enhance the reaction activity. The introduction of alkali metals also directly affects the surface basicity of the catalyst. In the ammonia decomposition reaction, NH3, as a basic molecule, is more likely to be adsorbed and activated on the basic surface. Therefore, the increase in surface basic sites promotes the adsorption and deprotonation process of NH3, accelerating the ammonia decomposition reaction. In addition, alkali metals may promote the migration and combination of N atoms by forming surface intermediates, accelerating the generation and desorption of nitrogen.

[0080] Compared with the binary Mo2N / mAl2O3 catalyst prepared in Comparative Example 2 and the binary Mo2N / rGO catalyst prepared in Comparative Example 3, the activity of the Mo2N / mAl2O3 / rGO catalyst is also significantly improved. On the one hand, the presence of mAl2O3 endows the catalyst with a mesoporous structure, making it have a large specific surface area, high porosity, highly dispersed active components, rich active sites and efficient mass - transfer characteristics. On the other hand, the introduction of graphene enables the catalyst to form a two - dimensional network structure, which helps to increase the contact area between the catalyst and reactant molecules, accelerate the desorption of product molecules, and thus improve the reaction activity.

[0081] From Figure 13It can be seen that the ammonia conversion rates of all catalysts increase rapidly with the increase of reaction temperature, indicating that the reaction rate accelerates at high temperatures. In the entire temperature range of 400°C to 650°C, the K-Mo2N-mAl2O3 / rGO catalyst exhibits the highest catalytic activity. Compared with the unmodified alkali metal Mo2N-mAl2O3 / rGO catalyst prepared in Comparative Example 1, at a temperature of 550°C, the ammonia conversion rate is 73.0%, which is significantly lower than that of the alkali metal-modified molybdenum nitride-based catalyst under the same conditions. Among them, the ammonia conversion rates of the alkali metal-modified molybdenum nitride-based catalysts prepared in Examples 1 to 4 are 92.4%, 78.0%, 94.2% and 91.0% respectively. This result shows that the alkali metal-modified catalyst has higher ammonia decomposition activity, which is mainly attributed to the fact that the introduction of alkali metal elements will change the electronic structure of the catalyst surface, and increasing the electron density of the active sites helps the desorption of surface N atoms, thus improving the ammonia decomposition activity of the catalyst.

[0082] Stability is another important index for evaluating ammonia decomposition catalysts. As can be seen from Figure 14 it, the catalysts prepared in Comparative Example 1 and Examples 1 to 4 did not show an obvious trend of activity decline within at least 24 hours, showing good catalytic stability.

[0083] It should be noted that when the present invention involves numerical ranges, it should be understood that any value between the two endpoints of each numerical range and the two endpoints can be selected. Since the adopted step methods are the same as those in the examples, in order to prevent repetition, the present invention describes the preferred embodiments. Although the preferred embodiments of the present invention have been described, those skilled in the art can make additional changes and modifications to these embodiments once they know the basic creative concept, and these changes and modifications all fall within the scope of the present invention.

[0084] Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. If these modifications and variations of the present invention fall within the scope of the equivalent technology of the present invention, the present invention also intends to include these changes and modifications.

Claims

1. A preparation method of an alkali metal-modified molybdenum nitride-based catalyst, characterized in that, It includes the following steps: Stir and disperse graphene oxide and a surfactant in a solvent to simultaneously form graphene oxide nanosheets with a single-layer or few-layer structure, obtaining a carrier solution; Under acidic conditions, stir and mix an aluminum source, a molybdenum source, an alkali metal salt, and the carrier solution to form a Mo-Al precursor; meanwhile, under the action of the surfactant, metal ions in the alkali metal salt combine with the Mo-Al precursor and are dispersed on the surface of the graphene oxide nanosheets, obtaining a precursor sol; Calcine the precursor sol in an air atmosphere to pyrolyze the Mo-Al precursor to form a composite oxide structure with MoO3 nanoparticles embedded in a mesoporous alumina framework; then reduce it in an ammonia atmosphere to reduce and nitride the MoO3 nanoparticles into Mo2N nanoparticles, and reduce the graphene oxide nanosheets into conductive reduced graphene oxide nanosheets, obtaining an alkali metal-modified molybdenum nitride-based catalyst.

2. The preparation method of the alkali metal-modified molybdenum nitride-based catalyst according to claim 1, wherein, The molar ratio of the aluminum source, the molybdenum source, and the alkali metal salt is 5:3:0.9; The molar ratio of the alkali metal salt to graphene oxide is 9:0.

3.

3. The preparation method of the alkali metal-modified molybdenum nitride-based catalyst according to claim 1, characterized in that, The calcination temperature is 350°C to 420°C; The reduction temperature is 700°C.

4. The preparation method of the alkali metal-modified molybdenum nitride-based catalyst according to claim 1, characterized in that, The surfactant is a poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer.

5. The preparation method of the alkali metal-modified molybdenum nitride-based catalyst according to claim 1, characterized in that, The pH value of the acidic condition is 1 to 3.

6. The preparation method of the alkali metal-modified molybdenum nitride-based catalyst according to claim 1, characterized in that, The aluminum source is aluminum isopropoxide; The molybdenum source is ammonium molybdate; The alkali metal salt is lithium nitrate, sodium nitrate, potassium nitrate, or cesium nitrate.

7. An alkali metal-modified molybdenum nitride-based catalyst, characterized in that, The alkali metal-modified molybdenum nitride-based catalyst is prepared by the preparation method of the alkali metal-modified molybdenum nitride-based catalyst according to any one of claims 1 to 6.

8. The alkali metal-modified molybdenum nitride-based catalyst according to claim 7, wherein, The particle size of the alkali metal-modified molybdenum nitride-based catalyst is 30 mesh to 60 mesh.

9. Application of an alkali metal-modified molybdenum nitride-based catalyst in catalytic ammonia decomposition for hydrogen production, characterized in that, The alkali metal-modified molybdenum nitride-based catalyst is the alkali metal-modified molybdenum nitride-based catalyst according to claim 7.

10. Use of the alkali metal-modified molybdenum nitride-based catalyst according to claim 9 in catalytic ammonia decomposition for hydrogen production, characterized in that, The specific application method is as follows: Place the alkali metal-modified molybdenum nitride-based catalyst in a reactor, control the ammonia to enter the reactor and react with the catalyst, and catalytically decompose ammonia to produce hydrogen.

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