An alkali metal-modified molybdenum nitride-based catalyst and its preparation method and application

Abstract

The present invention belongs to the technical field of catalytic materials, and in particular to a kind of alkali metal-modified molybdenum nitride-based catalyst and its preparation method and application. The specific preparation method comprises the following steps: graphene oxide and a surfactant are stirred and dispersed in a solvent, and a monolayer or few-layer graphene oxide nanosheet is formed to obtain a carrier solution; under acidic conditions, an aluminum source, a molybdenum source, an alkali metal salt and a carrier solution are stirred and mixed to form a Mo-Al precursor; at the same time, under the action of a surfactant, the metal ions in the alkali metal salt are combined with the Mo-Al precursor and dispersed on the surface of the graphene oxide nanosheet to obtain a precursor sol; the precursor sol is calcined in an air atmosphere, and then reduced in an ammonia atmosphere to obtain an alkali metal-modified molybdenum nitride-based catalyst. The alkali metal-modified molybdenum nitride-based catalyst prepared by the present invention shows excellent ammonia decomposition catalytic performance.

Description

Technical Field

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

[0002] Hydrogen, as a low-carbon, efficient new energy source, is hailed as an ideal green and sustainable energy source. However, technical challenges in its storage and transportation have severely hampered its large-scale application and development. In the research of hydrogen storage materials, ammonia has emerged as a promising hydrogen storage medium due to its low-risk storage and transportation characteristics, as well as its catalytic decomposition, which produces only hydrogen and nitrogen with no other byproducts. Producing hydrogen through the catalytic decomposition of ammonia not only avoids the risks of direct hydrogen storage and transportation, but also enables the convenient utilization of hydrogen energy.

[0003] Given the safety of ammonia and the cleanliness of its decomposition products, catalytic ammonia decomposition is considered an efficient and green hydrogen production technology. The core of this technology lies in developing highly efficient catalysts to reduce the activation energy of the ammonia decomposition reaction, thereby increasing the reaction rate and hydrogen yield. Therefore, the search for highly active and stable catalysts is a key research direction in catalytic ammonia decomposition for hydrogen production.

[0004] Molybdenum nitride catalysts, commonly used in ammonia decomposition, have attracted considerable attention due to their unique electronic structure and catalytic performance. Molybdenum nitride catalysts also possess electronic properties similar to those of precious metals, exhibiting excellent activity in ammonia decomposition reactions. However, the lack of effective control in the preparation of molybdenum nitride-based catalysts using existing technologies generally results in large molybdenum nitride particles, resulting in poor dispersion of active species and a small number of active sites. Furthermore, these particles are prone to agglomeration under high-temperature ammonia decomposition reaction conditions, significantly reducing the catalyst's catalytic activity and long-term stability. Summary of the Invention

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

[0006] The present invention introduces an alkali metal additive to lattice-dope or surface-modify molybdenum nitride, effectively regulating the electronic structure of the modified molybdenum nitride active component and optimizing its surface adsorption performance. By increasing the number of surface alkaline sites, the adsorption and deprotonation of NH3 are promoted, accelerating the ammonia decomposition reaction. Simultaneously, the modified molybdenum nitride is in situ dispersed in the pores of mesoporous alumina and supported on a graphene oxide carrier. The confinement effect of the mesoporous alumina improves the dispersibility of the active component and inhibits its deactivation due to sintering during the high-temperature ammonia decomposition reaction. At the same time, graphene is used to form a two-dimensional network structure, increasing the contact area between the catalyst and the reactant molecules, reducing the activation energy, and significantly improving the catalytic activity and durability of the catalyst, thereby resolving 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 method for preparing an alkali metal-modified molybdenum nitride-based catalyst, comprising the following steps:

[0008] Graphene oxide and a surfactant are stirred and dispersed in a solvent to form single-layer or few-layer graphene oxide nanosheets to obtain a carrier solution; an aluminum source, a molybdenum source, an alkali metal salt and the carrier solution are stirred and mixed under acidic conditions to form a Mo-Al precursor; at the same time, under the action of the surfactant, the metal ions in the alkali metal salt combine with the Mo-Al precursor and are dispersed on the surface of the graphene oxide nanosheets to obtain a precursor sol; the precursor sol is calcined in an air atmosphere to thermally decompose the Mo-Al precursor to form a composite oxide structure in which MoO3 nanoparticles are embedded in a mesoporous alumina skeleton; and then the MoO3 nanoparticles are reduced and nitrided to Mo2N nanoparticles and the graphene oxide nanosheets are reduced to conductive reduced graphene oxide nanosheets to obtain an alkali metal-modified molybdenum nitride-based catalyst.

[0009] The present invention uses graphene oxide as a carrier and a surfactant as a pore-forming agent. The graphene oxide and surfactant are dispersed in a solvent and stirred to a certain extent, disrupting the van der Waals forces between the graphene oxide sheets, causing them to disperse and exfoliate into a single or few-layer structure. Simultaneously, the surfactant adsorbs on the graphene oxide surface through hydrophobic-hydrophilic interactions, reducing interfacial tension and preventing reagglomeration of the graphene oxide. Furthermore, the surfactant serves as a pore-forming agent for the subsequent formation of a mesoporous structure.

[0010] Preferably, the surfactant is a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer.

[0011] Preferably, the graphene oxide and the surfactant are stirred and dispersed in the solvent for 6 hours to 12 hours at a temperature of 25° C. to 35° C.

[0012] The present invention generates Al(OH)3 sol by acid catalyzing hydrolysis of aluminum source and reacts with MoO4 in molybdenum source. 2- Combined to form Mo-Al precursor. By adjusting the pH value of the precursor sol to 1~3, the premature precipitation of the molybdenum source is prevented and the MoO4 2- Electrostatic bonding with Al(OH)3. At the same time, the surfactant forms micelles in the precursor sol, guiding the Mo-Al precursor to be evenly distributed on the surface of graphene oxide and forming a mesoporous structure.

[0013] The present invention introduces alkali metal salts to perform lattice doping or surface modification on Mo, which can effectively regulate the electronic structure of the Mo active component, optimize its surface adsorption performance, make the adsorption strength of the intermediate moderate, promote the desorption of the product molecules, and thus significantly improve the reaction activity.

[0014] The obtained precursor sol is dried at 60°C to prevent pore collapse caused by violent volatilization and maintain the integrity of the mesoporous structure. Furthermore, the Mo-Al precursor in the precursor sol cross-links with the graphene through hydrogen bonding and electrostatic interactions, forming a two-dimensional network structure that ensures high dispersion of the active components.

[0015] Preferably, the molar ratio of the aluminum source, the molybdenum source and the alkali metal salt is 5:3:0.9; and the molar ratio of the alkali metal salt and graphene oxide is 9:0.3.

[0016] Preferably, the aluminum source, molybdenum source, alkali metal salt and carrier solution are stirred and mixed for 8 hours to 12 hours at a temperature of 25° C. to 35° C.

[0017] Preferably, the acid used in the acidic conditions is concentrated nitric acid.

[0018] Preferably, the aluminum source is aluminum isopropoxide; the molybdenum source is ammonium molybdate; and the alkali metal salt is lithium nitrate, sodium nitrate, potassium nitrate or cesium nitrate.

[0019] It should be noted that the present invention uses a two-stage calcination process. First, the precursor sol is calcined in an air atmosphere to remove the pore-forming agent and form a mesoporous alumina skeleton. Simultaneously, the molybdenum source is pyrolyzed into MoO3 nanoparticles, which are embedded in the mesoporous alumina pores. Reduction is then carried out in an ammonia atmosphere, using ammonia to undergo a nitridation reaction, reducing and nitriding MoO3 to highly active Mo2N. Simultaneously, graphene oxide is reduced to conductive reduced graphene oxide, forming a two-dimensional network structure that is beneficial to mass transfer efficiency. Furthermore, alkali metals, as electron donors, can transfer electrons to the Mo metal center, weakening the Mo-N and NH bonds, lowering the d-band center position, and adjusting the surface electronic structure to moderate the intermediate adsorption strength, avoid excessive surface coverage, promote the desorption of N2 and H2, and further enhance the reaction activity.

[0020] Preferably, the calcination temperature is 350°C to 420°C, which 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 that the surfactant is completely removed to form a mesoporous structure.

[0021] Preferably, the reduction temperature is 700° C. to ensure that MoO 3 is fully nitrided to Mo 2 N, and the nitridation reaction kinetic threshold is 600° C.; at the same time, excessive graphitization of the reduced graphene oxide is avoided, resulting in loss of active sites.

[0022] The second object of the present invention is to provide an alkali metal-modified molybdenum nitride-based catalyst prepared by the above preparation method.

[0023] The third object of the present invention is to provide the use of the above-mentioned alkali metal-modified molybdenum nitride-based catalyst in catalytic decomposition of ammonia to produce hydrogen.

[0024] The present invention introduces alkali metal additives, which can not only adjust the electronic structure of the catalyst and increase the electron density of the metal active site, thereby significantly improving the activity of the catalyst. In addition, alkali metals, as electron donors, can transfer electrons to the metal center, weaken the Mo-N bond, lower its d-band center position, adjust the surface electronic structure, and make NH x The moderate adsorption strength of the intermediates prevents excessive surface coverage, promotes the desorption of nitrogen molecules, and further enhances reaction activity. The introduction of alkali metals also directly affects the surface alkalinity of the catalyst, making it easier for NH3 to adsorb and activate the alkali-metal-modified molybdenum nitride-based catalyst during the decomposition reaction. Therefore, the increase in surface basic sites promotes the adsorption and deprotonation of NH3, accelerating the ammonia decomposition reaction. In addition, alkali metals may promote the migration and binding of nitrogen atoms by forming surface intermediates, accelerating the generation and desorption of nitrogen.

[0025] Preferably, the particle size of the alkali metal-modified molybdenum nitride-based catalyst is 30 mesh to 60 mesh.

[0026] Preferably, the specific application method is as follows:

[0027] An alkali metal-modified molybdenum nitride-based catalyst is placed in a reactor, and ammonia is controlled to enter the reactor and react with the catalyst to catalytically decompose ammonia to produce hydrogen.

[0028] Preferably, the reaction temperature is 350°C to 650°C.

[0029] Compared with the prior art, the present invention has the following technical effects:

[0030] 1. The present invention introduces an alkali metal additive to perform lattice doping or surface modification on molybdenum nitride, and in situ disperses the modified molybdenum nitride in the pores of mesoporous alumina to form an alkali metal-modified molybdenum nitride and mesoporous alumina composite structure, which is then loaded on the surface of a graphene oxide carrier to obtain an alkali metal-modified molybdenum nitride-based catalyst. The present invention adjusts the electronic structure of molybdenum nitride by an alkali metal additive, increases the electron density of the metal active site, and thus significantly improves the intrinsic activity of the catalyst; at the same time, the confinement effect of mesoporous alumina is used to improve the dispersibility of the active component and inhibit its deactivation due to sintering during the high-temperature ammonia decomposition reaction. At the same time, graphene is used to form a two-dimensional network structure, which increases the contact area between the catalyst and the reactant molecules, reduces the activation energy, and greatly improves the catalytic activity and durability of the catalyst, solving the problem of insufficient activity and stability of existing molybdenum nitride-based catalysts.

[0031] 2. The catalyst prepared by the present invention has a large specific surface area, high porosity, highly dispersed active components, abundant active sites and efficient mass transfer characteristics.

[0032] 3. The preparation method of the present invention is simple to operate, operates under mild reaction conditions, and is applicable to the synthesis of a variety of mesoporous nitride-based catalysts. Furthermore, this method enables the simultaneous doping of other heteroatoms during the preparation of nitrides, demonstrating good universality and broad application potential. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 These are scanning electron microscope images of the catalysts prepared in Examples 1 to 4 and Comparative Example 1; wherein, a is a scanning electron microscope image of the molybdenum nitride / aluminum oxide / graphene composite catalyst prepared in Comparative Example 1, b is a scanning electron microscope image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1, c is a scanning electron microscope image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2, d is a scanning electron microscope image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3, and e is a scanning electron microscope image of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4.

[0034] Figure 2 These are the transmission electron microscopy images and high-resolution transmission electron microscopy images of the molybdenum nitride / aluminum oxide / graphene composite catalyst prepared in Comparative Example 1; wherein, a is a transmission electron microscopy image at a 50 nm scale, b is a high-resolution transmission electron microscopy image at a 20 nm scale, c is a high-resolution transmission electron microscopy image of frame 1 in b, and d is a high-resolution transmission electron microscopy image of frame 2 in b.

[0035] Figure 3The scanning transmission electron microscopy image and element surface scanning distribution map of the molybdenum nitride / aluminum oxide / graphene composite catalyst prepared in Comparative Example 1 under high-angle annular dark field are shown; wherein, a is a scanning transmission electron microscopy image under high-angle annular dark field, b is the Mo element distribution map, c is the N element distribution map, d is the O element distribution map, e is the C element distribution map, and f is the Al element distribution map.

[0036] Figure 4 These are the transmission electron microscopy images and high-resolution transmission electron microscopy images of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1; wherein, a is a transmission electron microscopy image, b is a high-resolution transmission electron microscopy image, c is a high-resolution transmission electron microscopy image of frame 1 in b, and d is a high-resolution transmission electron microscopy image of frame 2 in b.

[0037] Figure 5 The scanning transmission electron microscopy image and element surface scanning distribution map of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 1 under high-angle annular dark field; wherein, a is a scanning transmission electron microscopy image under high-angle annular dark field, b is the Mo element distribution map, c is the N element distribution map, d is the O element distribution map, e is the C element distribution map, and f is the Al element distribution map.

[0038] Figure 6 These are the transmission electron microscope images, high-resolution transmission electron microscope images, and scanning transmission electron microscope images under high-angle annular dark field of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2; wherein, a is a transmission electron microscope image, b is a high-resolution transmission electron microscope image, c is a scanning transmission electron microscope image under high-angle annular dark field, and d is a high-resolution transmission electron microscope image of frame 1 in b.

[0039] Figure 7 This is the elemental surface scan distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 2; wherein, 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.

[0040] Figure 8 These are the transmission electron microscope images, high-resolution transmission electron microscope images, and scanning transmission electron microscope images under high-angle annular dark field of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3; wherein, a is a transmission electron microscope image, b is a high-resolution transmission electron microscope image, c is a scanning transmission electron microscope image under high-angle annular dark field, and d is a high-resolution transmission electron microscope image of frame 1 in b.

[0041] Figure 9 This is the elemental surface scan distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 3; wherein, 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.

[0042] Figure 10Transmission electron microscopy (TEM) images, high-resolution TEM images, and scanning TEM images under high-angle annular dark field of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4; wherein, a is a TEM image, b is a high-resolution TEM image, c is a scanning TEM image under high-angle annular dark field, and d is a high-resolution TEM image of frame 1 in b.

[0043] Figure 11 This is the elemental surface scanning distribution diagram of the alkali metal-modified molybdenum nitride-based catalyst prepared in Example 4; wherein, 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.

[0044] Figure 12 The figures are particle size distribution diagrams of the catalysts prepared in Examples 1 to 4 and Comparative Example 1; wherein, 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.

[0045] Figure 13 The figure is a curve diagram showing the change of ammonia conversion rate of the catalysts prepared in Examples 1 to 4 and Comparative Example 1 with temperature.

[0046] Figure 14 This is a graph showing changes in ammonia conversion rate over time for the catalysts prepared in Examples 1 to 4 and Comparative Example 1 at a temperature of 600°C. DETAILED DESCRIPTION

[0047] In order to enable those skilled in the art to better understand and implement the technical solution of the present invention, the present invention is further described below with reference to specific embodiments and drawings.

[0048] In the description of the present invention, unless otherwise specified, all reagents used are commercially available and all methods used are conventional techniques in the art.

[0049] It should be noted that the abbreviation of the polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer is P123.

[0050] Example 1

[0051] A method for preparing an alkali metal-modified molybdenum nitride-based catalyst comprises the following steps:

[0052] 0.06 g of graphene oxide and 0.5 g of P123 were ultrasonically dispersed in 5 mL of anhydrous ethanol in sequence and stirred at a constant temperature of 30 °C for 10 h to obtain a carrier solution.

[0053] 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 added to the carrier solution in sequence, the pH value of the solution was controlled to 2, and the solution was stirred at a constant temperature of 30°C for 10 hours to form a homogeneous precursor sol.

[0054] The precursor sol was dried at 60°C for 16 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 3 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, which was recorded as Li-Mo2N / mAl2O3 / rGO.

[0055] Example 2

[0056] A method for preparing an alkali metal-modified molybdenum nitride-based catalyst comprises the following steps:

[0057] 0.06 g of graphene oxide and 0.5 g of P123 were ultrasonically dispersed in 5 mL of anhydrous ethanol in sequence and stirred at a constant temperature of 25° C. for 8 h to obtain a carrier solution.

[0058] 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 added to the carrier solution in sequence, the pH value of the solution was controlled to 2, and the mixture was stirred at a constant temperature of 25°C for 8 hours to form a homogeneous precursor sol.

[0059] The precursor sol was dried at 60°C for 12 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 2 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, which was recorded as Na-Mo2N / mAl2O3 / rGO.

[0060] Example 3

[0061] A method for preparing an alkali metal-modified molybdenum nitride-based catalyst comprises the following steps:

[0062] 0.06 g of graphene oxide and 0.5 g of P123 were ultrasonically dispersed in 5 mL of anhydrous ethanol, and stirred at a constant temperature of 35° C. for 12 h to obtain a carrier solution.

[0063] 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 added to the carrier solution in sequence, the pH value of the solution was controlled to 2, and the solution was stirred at a constant temperature of 35°C for 12 hours to form a homogeneous precursor sol.

[0064] The precursor sol was dried at 60°C for 24 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 4 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, which was recorded as K-Mo2N / mAl2O3 / rGO.

[0065] Example 4

[0066] A method for preparing an alkali metal-modified molybdenum nitride-based catalyst comprises the following steps:

[0067] 0.06 g of graphene oxide and 0.5 g of P123 were ultrasonically dispersed in 5 mL of anhydrous ethanol in sequence and stirred at a constant temperature of 30° C. for 10 h to obtain a carrier solution.

[0068] 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 added to the carrier solution in sequence, the pH value of the solution was controlled to 2, and the mixture was stirred at a constant temperature of 30°C for 10 hours to form a homogeneous precursor sol.

[0069] The precursor sol was dried at 60°C for 16 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 3 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and an alkali metal-modified molybdenum nitride-based catalyst was obtained, which was recorded as Cs-Mo2N / mAl2O3 / rGO.

[0070] Comparative Example 1

[0071] A method for preparing a molybdenum nitride / aluminum oxide / graphene composite catalyst comprises the following steps:

[0072] 0.06 g of graphene oxide and 0.5 g of P123 were ultrasonically dispersed in 5 mL of anhydrous ethanol in sequence and stirred at a constant temperature of 30° C. for 10 h to obtain a carrier solution.

[0073] Under stirring conditions, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide, and 3 mmol of ammonium molybdate were added to the carrier solution in sequence, and the mixture was stirred at a constant temperature of 30° C. for 10 h to form a homogeneous sol.

[0074] The sol was dried at 60°C for 16 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 3 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and a molybdenum nitride / aluminum oxide / graphene composite catalyst was obtained, which was recorded as Mo2N / mAl2O3 / rGO.

[0075] Comparative Example 2

[0076] A method for preparing a molybdenum nitride / aluminum oxide composite catalyst comprises the following steps:

[0077] 0.5 g of P123 was ultrasonically dispersed in 5 mL of anhydrous ethanol. After uniform dispersion, 0.8 mL of concentrated nitric acid, 5 mmol of aluminum isopropoxide and 3 mmol of ammonium molybdate were added in sequence. The mixture was stirred at a constant temperature of 30°C for 10 h to form a homogeneous sol.

[0078] The sol was dried at 60°C for 16 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 3 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and a molybdenum nitride / aluminum oxide composite catalyst was obtained, which was recorded as Mo2N / mAl2O3.

[0079] Comparative Example 3

[0080] A method for preparing a molybdenum nitride / graphene composite catalyst comprises the following steps:

[0081] 0.06 g of graphene oxide was ultrasonically dispersed in 5 mL of anhydrous ethanol. After uniform dispersion, 3 mmol of ammonium molybdate was added and stirred at a constant temperature of 30 °C for 10 h to form a homogeneous sol.

[0082] The sol was dried at 60°C for 16 hours and then ground. The ground sample was calcined at 400°C in an air atmosphere for 3 hours and then calcined at 700°C in an ammonia atmosphere for 2 hours; the heating rate was 2°C / min, and a molybdenum nitride / graphene composite catalyst was obtained, which was recorded as Mo2N / rGO.

[0083] Experimental test.

[0084] 1. Surface morphology characterization.

[0085] like Figure 1 As 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 relatively thin nanosheet structures, indicating the successful incorporation of graphene. Furthermore, the alkali metal modification did not alter the surface morphology of the catalysts.

[0086] The microstructure of the catalyst was further observed by transmission electron microscopy. Figure 2 As shown in a and b, the catalyst prepared in Comparative Example 1 exhibits typical mesoporous structural characteristics, with Mo2N nanoparticles with a particle size of about 1 nm to 3 nm uniformly dispersed in the mesoporous channels; Figure 2 As shown in Figures c and d, the Mo2N interplanar spacing is 0.238~0.240nm, which belongs to the Mo2N (111) crystal plane.

[0087] like Figure 3 As shown, the element surface scan shows that each element is evenly distributed throughout the composite catalyst.

[0088] like Figure 4 As 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 without obvious particle aggregation, indicating that the mesoporous alumina support can effectively prevent the sintering and agglomeration of Mo2N particles. Figure 4 As shown in Figures c and d, the high-resolution transmission electron microscopy images reveal that the Mo2N interplanar spacing is 0.241nm~0.243nm, which belongs to the Mo2N(111) crystal plane.

[0089] like Figure 5 As shown, the element surface scan shows that each element is evenly distributed throughout the composite catalyst.

[0090] like Figures 6 to 11 As shown, the alkali metal-modified molybdenum nitride-based catalysts prepared in Examples 2 to 4 all exhibit irregular mesoporous structures and are assembled onto the graphene surface. The interplanar spacings are 0.241 nm, 0.242 nm, and 0.243 nm, respectively, attributed to the Mo2N (111) crystal plane. The elemental scanning image shows that the elements are evenly distributed throughout the alkali metal-modified molybdenum nitride-based catalyst.

[0091] like Figure 12 As shown, the particle size of the catalysts prepared in Comparative Example 1 and Examples 1 to 4 is 1.6 nm to 1.7 nm.

[0092] 2. Catalytic test.

[0093] The present invention utilizes an ammonia decomposition reaction evaluation device comprising a fixed-bed quartz tube reactor with an inner diameter of 3 mm, a reaction heating furnace, a mass flow meter for controlling the ammonia flow rate, and an online gas chromatograph. The gas chromatograph utilizes a 13X packed column and a thermal conductivity detector, with hydrogen as the carrier gas.

[0094] The present invention conducts specific catalytic tests on the catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3. The specific test methods are as follows:

[0095] The catalyst prepared in the embodiment or comparative example is placed in the constant temperature zone of a quartz tube, and the quartz tube is inserted into a heating furnace. High-purity ammonia is precisely controlled by a mass flow meter to enter the reactor at a fixed flow rate, and then reacts through the catalyst layer; wherein the particle size of the catalyst is 30 mesh to 60 mesh, and the heating conditions are: heating from 350°C to 650°C at a heating rate of 10°C / min, and maintaining each temperature point for 60 minutes. The gas after the reaction is detected by an online gas chromatograph, and a spectrum is collected every 10 minutes. The conversion rate of ammonia is calculated according to the conversion rate formula based on the concentration values ​​of nitrogen and ammonia:

[0096] ;

[0097] in, is the conversion rate of ammonia, is the concentration of ammonia introduced, is the outlet ammonia concentration.

[0098] Table 1 Hydrogen production rates of catalysts prepared in Examples 1 to 4 and Comparative Examples 1 to 3

[0099]

[0100] As shown in Table 1, compared with the Mo2N / mAl2O3 / rGO catalyst prepared in Comparative Example 1, after introducing different types of alkali metals to modify Mo2N, the hydrogen production rate of the alkali metal-modified molybdenum nitride-based catalyst was improved. The main reasons are as follows: the alkali metal additive can not only adjust the electronic structure of the catalyst and increase the electron density of the metal active site, thereby 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, reduce its d-band center position, adjust the surface electronic structure, make the intermediate adsorption strength moderate, avoid excessive surface coverage, promote the desorption of nitrogen molecules, and thus enhance the reaction activity. The introduction of alkali metals also directly affects the surface alkalinity of the catalyst. In the ammonia decomposition reaction, NH3, as an alkaline molecule, is more easily adsorbed and activated on the alkaline surface. Therefore, the increase in surface alkaline sites promotes the adsorption and deprotonation process of NH3, accelerating the ammonia decomposition reaction. In addition, alkali metals may promote the migration and binding of N atoms by forming surface intermediates, thereby accelerating the generation and desorption of nitrogen.

[0101] The activity of the Mo2N / mAl2O3 / rGO catalyst was significantly improved compared to the two-component Mo2N / mAl2O3 catalyst prepared in Comparative Example 2 and the two-component Mo2N / rGO catalyst prepared in Comparative Example 3. On the one hand, the presence of mA12O3 imparts a mesoporous structure to the catalyst, resulting in a large specific surface area, high porosity, highly dispersed active components, abundant active sites, and efficient mass transfer properties. On the other hand, the introduction of graphene forms a two-dimensional network structure in the catalyst, which helps increase the contact area between the catalyst and reactant molecules, accelerates the desorption of product molecules, and thus improves reaction activity.

[0102] from Figure 13 As can be seen, the ammonia conversion rates of all catalysts increased rapidly with increasing reaction temperature, indicating that the reaction rate accelerated at high temperatures. The K-Mo2N-mAl2O3 / rGO catalyst exhibited the highest catalytic activity over the entire temperature range of 400°C to 650°C. Compared to the alkali-metal-unmodified Mo2N-mAl2O3 / rGO catalyst prepared in Comparative Example 1, the ammonia conversion rate was 73.0% at 550°C, significantly lower than that of the alkali-metal-modified molybdenum nitride-based catalysts under the same conditions. The ammonia conversion rates of the alkali-metal-modified molybdenum nitride-based catalysts prepared in Examples 1 to 4 were 92.4%, 78.0%, 94.2%, and 91.0%, respectively. This result indicates that the alkali-metal-modified catalysts possess higher ammonia decomposition activity, primarily due to the fact that the introduction of the alkali metal element alters the electronic structure of the catalyst surface, increasing the electron density at the active sites and facilitating the desorption of surface nitrogen atoms, thereby enhancing the catalyst's ammonia decomposition activity.

[0103] Stability is another important indicator for evaluating ammonia decomposition catalysts. Figure 14 It can be seen that the catalysts prepared in Comparative Example 1 and Examples 1 to 4 did not show an obvious activity decline trend within at least 24 hours, showing good catalytic stability.

[0104] It should be noted that when numerical ranges are mentioned in the present invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints may be selected. Since the steps and methods used are the same as those in the embodiments, the present invention describes preferred embodiments to avoid redundancy. Although preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they understand the basic inventive concepts, and such changes and modifications fall within the scope of the present invention.

[0105] 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 is also intended to include these modifications and variations.

Claims

1. A method for preparing an alkali metal-modified molybdenum nitride-based catalyst, characterized in that: The following steps are involved: stirring and dispersing graphene oxide and a surfactant in a solvent to simultaneously form graphene oxide nanosheets with a single-layer or few-layer structure, thereby obtaining a carrier solution; Under acidic conditions, an aluminum source, a molybdenum source, an alkali metal salt, and a carrier solution are stirred and mixed to form a Mo-Al precursor. Simultaneously, under the action of a surfactant, metal ions in the alkali metal salt combine with the Mo-Al precursor and are dispersed on the surface of graphene oxide nanosheets to obtain a precursor sol. The precursor sol is calcined in an air atmosphere to thermally decompose the Mo-Al precursor to form a composite oxide structure in which MoO3 nanoparticles are embedded in a mesoporous alumina framework; then, the MoO3 nanoparticles are reduced and nitrided to Mo2N nanoparticles in an ammonia atmosphere, and the graphene oxide nanosheets are reduced to conductive reduced graphene oxide nanosheets, thereby obtaining an alkali metal-modified molybdenum nitride-based catalyst; The molar ratio of aluminum source, molybdenum source and alkali metal salt is 5:3:0.9; The molar ratio of alkali metal salt to graphene oxide is 9:0.3; The calcination temperature is 350℃~420℃.

2. The method for preparing an alkali metal-modified molybdenum nitride-based catalyst according to claim 1, wherein: The reduction temperature is 700°C.

3. The method for preparing an alkali metal-modified molybdenum nitride-based catalyst according to claim 1, wherein: The surfactant is a polyethylene oxide-polypropylene oxide-polyethylene oxide triblock copolymer.

4. The method for preparing an alkali metal-modified molybdenum nitride-based catalyst according to claim 1, wherein: The pH value of acidic conditions is 1~3.

5. The method for preparing an alkali metal-modified molybdenum nitride-based catalyst according to claim 1, wherein: 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.

6. 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 5.

7. The alkali metal-modified molybdenum nitride-based catalyst according to claim 6, characterized in that The particle size of the alkali metal-modified molybdenum nitride-based catalyst is 30 mesh to 60 mesh.

8. Application of an alkali metal-modified molybdenum nitride-based catalyst in catalytic ammonia decomposition to produce hydrogen, characterized in that: The alkali metal-modified molybdenum nitride-based catalyst is the alkali metal-modified molybdenum nitride-based catalyst according to claim 6.

9. Use of the alkali metal-modified molybdenum nitride-based catalyst according to claim 8 in catalytic decomposition of ammonia to produce hydrogen, characterized in that: The specific application methods are as follows: An alkali metal-modified molybdenum nitride-based catalyst is placed in a reactor, and ammonia is controlled to enter the reactor and react with the catalyst to catalytically decompose ammonia to produce hydrogen.

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