Method for synthesizing ammonia with high activity under mild condition

Abstract

The invention provides an ammonia synthesis method with high activity under mild conditions, ammonia is synthesized through photo-thermal catalysis, a photo-thermal catalyst is a transition metal composite metal hydride, the metal hydride is an alkali metal hydride or an alkaline earth metal hydride and has high catalytic activity, and a high ammonia synthesis rate can be obtained. When the transition metal composite metal hydride is loaded on the carrier, the ammonia synthesis rate can be further improved, and the catalyst has a good application prospect.

Description

Technical Field

[0001] The invention belongs to the technical field of synthetic ammonia and catalytic reaction, and is an ammonia synthesis method with high activity under mild conditions. Background Art

[0002] Ammonia's key role in industry and agriculture makes it one of the most important chemicals in the world. In addition, ammonia has the characteristics of high energy density, large hydrogen storage capacity, high H2 purity, mature synthetic ammonia technology, easy liquefaction, convenient storage and transportation, etc., and is regarded as an energy carrier with application prospects. The energy utilization system using ammonia as an energy carrier mainly includes three parts: ammonia synthesis, ammonia storage and transportation, and ammonia utilization. Ammonia synthesis is one of the key links. In order to meet the needs of ammonia, more requirements are put forward for synthetic ammonia, such as: low temperature and low pressure, miniaturization, and distribution.

[0003] There are three main types of ammonia synthesis methods, namely biological nitrogen fixation, heterogeneous chemical ammonia synthesis, and homogeneous chemical ammonia synthesis. Heterogeneous chemical ammonia synthesis includes thermal catalysis, electrocatalysis, photocatalysis, chemical chaining, plasma ammonia synthesis, etc. Thermocatalysis is widely used in industry to synthesize ammonia on a large scale, and Ru-based and Fe-based catalysts are widely used. It has the characteristics of mature synthesis technology and high yield. However, due to the large kinetic resistance in the ammonia synthesis process, harsh conditions of high temperature and high pressure are required. Usually, a higher export ammonia concentration can be achieved under the reaction conditions of 400℃-500℃ and 10MPa-20MPa. This process consumes about 2% of the world's energy and indirectly leads to about 300 million tons of carbon dioxide emissions.

[0004] Photocatalytic ammonia synthesis is the process of reducing N2 to ammonia under light irradiation using semiconductors, which can achieve ammonia synthesis at room temperature and pressure. Similar to electrocatalysis, whether photocatalytic ammonia synthesis can occur also depends largely on the reduction potential of N2, the width of the semiconductor band gap, and the position of the conduction band and valence band. During the reduction reaction, H2 is usually produced as the main by-product, reducing the selectivity and efficiency of NRR. The main challenges currently faced by photocatalytic ammonia synthesis technology can be divided into two aspects: on the one hand, the strong hydrogen evolution side reaction leads to poor selectivity; on the other hand, the low solubility of N2 molecules leads to extremely low ammonia yields, making it difficult to apply them on a large scale in industrial applications. Summary of the invention

[0005] In view of the above technical status, the present invention aims to provide a method for synthesizing ammonia with high activity under mild conditions, which can increase the ammonia synthesis rate.

[0006] In order to achieve the above technical objectives, the inventors used photothermal catalysis to synthesize ammonia, that is, photothermal synergistic catalytic reaction of nitrogen and hydrogen to synthesize ammonia, and found that the complex of transition metal and alkali metal hydride or alkaline earth metal hydride (referred to as transition metal composite metal hydride) can be used as a photothermal catalyst, and has high ammonia synthesis activity, and can obtain a high ammonia synthesis rate under mild conditions.

[0007] That is, the technical solution of the present invention is: a method for synthesizing ammonia with high activity under mild conditions, which adopts photothermal catalysis to synthesize ammonia, and the photothermal catalyst is a complex of transition metal and metal hydride, which is recorded as transition metal composite metal hydride.

[0008] In the transition metal composite metal hydride, the transition metal is not limited, and can be selected from one or more of Cr, W, Mo, Mn, Ti, Fe, Co, Rh, Ru, Pt, Pd, Au, Ag, Ni, Cu, Zn, etc.

[0009] In the transition metal composite metal hydride, the metal hydride is an alkali metal hydride or an alkaline earth metal hydride, for example, one or more of LiH, KH, NaH, MgH2, CaH2, BaH2, SrH2, LiMgH3, LiCaH3, LiBaH3, etc. can be selected.

[0010] In the transition metal composite metal hydride, the mass percentage of the transition metal is preferably 0.1%-90%.

[0011] The preparation method of the transition metal composite metal hydride is not limited. As a preparation method, it comprises the following steps:

[0012] The transition metal salt powder and the hydride powder of the metal M are mixed and reacted to obtain a complex of the transition metal element and the hydride of the metal M; the metal M is an alkali metal or an alkaline earth metal.

[0013] Preferably, the transition metal salt powder is mixed with the hydride powder of the metal M in an inert atmosphere, and the inert atmosphere is not limited and includes at least one of Ar, N2, He, Ne, etc.

[0014] Preferably, the transition metal salt powder and the metal M hydride powder are mixed by ball milling, and the ball milling speed is preferably 50 rmin -1 -400rmin -1 .

[0015] The transition metal salt is not limited, and includes at least one of nitrates, chlorides, acetates, sulfates, etc. of transition metals.

[0016] The molar ratio of the transition metal salt to the hydride of metal M is preferably 1:1-1:100.

[0017] In order to further improve the catalytic effect, the inventors have found that the transition metal composite metal hydride is loaded on a carrier to obtain a loaded transition metal composite metal hydride, which can further improve the catalytic effect in the photothermal catalytic ammonia synthesis reaction, thereby further improving the ammonia synthesis rate. The carrier is not limited, including one or more of MgO, CeO2, Co2O3, TiO2, SiO2, Al2O3, BaO, CaO, BN carbon materials, rare earth oxides, rare earth nitrides, perovskites, molecular sieves, carbon materials, and metal organic framework materials.

[0018] The preparation method of the supported transition metal composite metal hydride is not limited. As a preparation method, it comprises the following steps:

[0019] (1) dispersing a support material in a transition metal salt solution to obtain a mixed solution; calcining the mixed solution at 100° C. to 800° C., and then reducing the mixed solution at 100° C. to 800° C. in a H2 atmosphere, or reducing the mixed solution at 100° C. to 800° C. in a H2 and inert gas atmosphere to obtain a supported transition metal;

[0020] (2) Mixing the supported transition metal with a metal hydride to obtain a supported transition metal composite metal hydride catalyst.

[0021] The transition metal salt solution is a solution obtained by dissolving a transition metal salt in a solvent. The solvent is not limited and includes water, ethanol, acetone, and the like.

[0022] The transition metal salt is not limited and includes at least one of nitrates, chlorides, acetates and sulfates of transition metals.

[0023] Preferably, in the supported transition metal, the mass ratio of the metal to the carrier is in the range of 100:1-1:200.

[0024] In the step (1), the calcination is carried out under air, Ar, N2 or a mixed atmosphere.

[0025] In the step (1), the calcination time is preferably 1 h to 12 h.

[0026] In the step (2), the supported transition metal and the metal hydride are preferably ground and mixed, and more preferably ground and mixed in an inert atmosphere. Preferably, the mixture is placed in a ball mill for ball milling, and the rotation speed is preferably 50 rmin -1 -400rmin -1 .

[0027] The present invention can carry out photothermal catalytic synthesis of ammonia in a high-pressure reactor, and can also carry out photothermal catalytic synthesis of ammonia in a fixed bed reaction system.

[0028] When photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, the catalyst is placed in a high-temperature and high-pressure reactor, and reaction gases N2 and H2 are introduced to synthesize ammonia under light conditions. The reaction atmosphere pressure, that is, the pressure of N2 and H2 is preferably 0.1MPa-5MPa, wherein the gas pressure ratio of N2 to H2 is preferably 1:10-10:1, and more preferably 1:5-5:1; the reaction temperature is preferably 20°C-800°C; the light intensity is preferably 0.1mWcm -1 -10Wcm -1 ; The reaction time is preferably 1h-50h. In order to reduce energy consumption, it is preferred to carry out photothermal catalysis under mild conditions, with a reaction atmosphere pressure of 0.5MPa-2MPa, a reaction temperature of 100℃-300℃, and a light intensity of 0.5mWcm -1 -5Wcm -1 .

[0029] When photothermal catalytic ammonia synthesis is carried out in a high-pressure fixed bed, the catalyst is placed in a high-pressure fixed bed, and reaction gases N2 and H2 are introduced to synthesize ammonia under light conditions. The reaction gas space velocity is preferably 100 ml / min. -1 h -1 -1000000mlg -1 h -1 , wherein the ratio of N2 to H2 is preferably 1:10-10:1, and more preferably 1:5-5:1; the reaction atmosphere pressure, i.e., the pressure of N2 and H2 is preferably 0.1MPa-5MPa; the reaction temperature is preferably 20°C-1000°C; the light intensity is preferably 0.1mW / cm -1 -10W / cm -1 ; The reaction time is preferably 1h-50h. In order to reduce energy consumption, it is preferred to carry out photothermal catalysis under mild conditions, with a reaction atmosphere pressure of 0.5MPa-2MPa, a reaction temperature of 100℃-300℃, and a light intensity of 0.5mWcm -1 -5Wcm -1 .

[0030] In the present invention, the ammonia synthesis rate is defined as: the molar amount of ammonia synthesized per unit time and per unit catalyst mass. For example, 1 μmol g -1 h -1 It means that the number of moles of ammonia synthesized per gram of catalyst per hour is 1 μmol.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) The present invention adopts photothermal catalysis to synthesize ammonia. Photothermal catalysis is a catalytic system under the dual driving force of coupled light effect and thermal effect. In addition, the present invention selects transition metal composite metal hydride as a photothermal catalyst for photothermal catalytic synthesis of ammonia, which can achieve high catalytic activity under mild conditions, realize high ammonia synthesis rate, and reduce energy consumption;

[0033] (2) In the present invention, the photothermal catalytic synthesis of ammonia is suitable for being carried out in a high-temperature and high-pressure fixed-bed gas phase reaction system, and is also suitable for being carried out in a high-temperature and high-pressure reactor;

[0034] In a high-temperature and high-pressure fixed-bed gas-phase reaction system, when transition metal composite metal hydrides are used as catalysts, high catalytic activity can be achieved under mild conditions. For example, the reaction temperature can be reduced to below 200°C. When the reaction temperature is 150°C, the reaction gas pressure is 1Mpa, and the light intensity is 1Wcm -1 The ammonia synthesis rate can be higher than 1500 μmol g - 1 h -1 , preferably higher than 1800 μmol g -1 h -1 , even higher than 3000μmolg -1 h -1 .

[0035] In a high-temperature and high-pressure reactor, when transition metal composite metal hydrides are used as catalysts, high catalytic activity can be achieved under mild conditions. For example, the reaction temperature can be reduced to below 200°C. When the reaction temperature is 150°C, the reaction gas pressure is 1Mpa, and the light intensity is 1Wcm -1 The ammonia synthesis rate can be higher than 1400 μmol g -1 h -1 , preferably higher than 1600 μmol g -1 h -1 , even higher than 2500 μmol g -1 h -1 .

[0036] (3) The present invention preferably loads the transition metal composite metal hydride on a carrier, which can increase the dispersion of the active sites of the catalyst and the light absorption range, further improve the catalytic activity, and thus further improve the ammonia synthesis rate.

[0037] For example, in a high-temperature and high-pressure fixed-bed gas-phase reaction system, when a supported transition metal composite metal hydride is used as a catalyst, high catalytic activity can be achieved under mild conditions. For example, the reaction temperature can be reduced to below 200°C. When the reaction temperature is 150°C, the reaction gas pressure is 1 MPa, and the light intensity is 1 W cm -1The ammonia synthesis rate can be higher than 2500 μmol g -1 h -1 , even higher than 4000 μmol g -1 h -1 .

[0038] In a high-temperature and high-pressure reactor, when a supported transition metal composite metal hydride is used as a catalyst, high catalytic activity can be achieved under mild conditions. For example, the reaction temperature can be reduced to below 200°C. When the reaction temperature is 150°C, the reaction gas pressure is 1Mpa, and the light intensity is 1Wcm -1 The ammonia synthesis rate can be higher than 2000 μmol g -1 h -1 , even higher than 3000μmolg -1 h -1 .

[0039] (4) The present invention can adjust the ammonia synthesis rate by adjusting one or more of the reaction pressure, space velocity, gas pressure ratio, temperature, and light intensity.

[0040] Synthesis rate BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 It is a comparison chart of the synthesis rates of ammonia synthesized in Examples 1-4 and Comparative Examples 1-4. DETAILED DESCRIPTION

[0042] The present invention is further described in detail below in conjunction with embodiments. It should be pointed out that the embodiments described below are intended to facilitate the understanding of the present invention, and non-essential improvements and adjustments made to the present invention by those skilled in the art based on the above-mentioned contents of the present invention still fall within the scope of protection of the present invention.

[0043] The words “include”, “including” and the like used in the present invention should be interpreted as including rather than being exclusive or exhaustive; that is, the meaning is “including but not limited to”.

[0044] Embodiment 1:

[0045] (1) Weigh 500 mg of FeCl2 and 1 g of LiH2 in a ball mill, and mill them for 3 h at a speed of 200 r / min in an Ar atmosphere. Take out the mixture after milling to obtain transition metal composite metal hydride Fe / LiH2.

[0046] (2) Photothermal catalytic synthesis of ammonia is carried out in a fixed bed, and the photothermal catalyst is Fe / LiH obtained in step (1). The reaction conditions are as follows:

[0047] Light intensity is 1Wcm -1; Temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; The reaction gas space velocity is 60000 mLg -1 h -1 .

[0048] Ammonia synthesis rate: 3200 μmol g -1 h -1 .

[0049] Embodiment 2:

[0050] (1) Weigh 500 mg of MnCl2 and 1 g of LiH2 in a ball mill and mill for 3 h at a speed of 200 rmin in an Ar atmosphere. -1 After ball milling, the product is taken out to obtain transition metal composite metal hydride Mn / LiH.

[0051] (2) Photothermal catalytic synthesis of ammonia is carried out in a fixed bed, and the photothermal catalyst is Mn / LiH obtained in step (1). The reaction conditions are as follows:

[0052] Light intensity is 1Wcm -1 ; Temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; The reaction gas space velocity is 60000 mLg -1 h -1 .

[0053] Ammonia synthesis rate: 1850 μmol g -1 h -1 .

[0054] Embodiment 3:

[0055] (1) Weigh 200 mg of Fe(NO3)39H2O and dissolve it in 2 ml of water, then add 1 g of MgO and disperse it evenly. The resulting solution is soaked overnight and dried. Then, it is calcined at 500°C for 2 h in an air atmosphere in a tube furnace, and then switched to a H2 atmosphere and reduced at 500°C for 2 h to obtain transition metal Fe supported on MgO, which is recorded as Fe / MgO.

[0056] 0.5 g Fe / MgO and 0.5 g LiH were placed in a ball mill, and ball milled for 3 h at a speed of 200 r / min under an Ar atmosphere to obtain a supported transition metal composite metal hydride Fe / LiH / MgO.

[0057] (2) Photothermal catalytic synthesis of ammonia is carried out in a fixed bed, and the photothermal catalyst is Fe / LiH / MgO obtained in step (1). The reaction conditions are as follows:

[0058] Light intensity is 1Wcm -1; Temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; The reaction gas space velocity is 60000 mLg -1 h -1 .

[0059] Ammonia synthesis rate: 4570 μmol g -1 h -1 .

[0060] Embodiment 4:

[0061] (1) 180 mg of Mn(NO3)26H2O was weighed and dissolved in 2 ml of water, and then 1 g of MgO was added and dispersed evenly. The resulting solution was immersed overnight and dried; then, it was calcined at 500°C for 2 h in an air atmosphere in a tube furnace, and then switched to a H2 atmosphere and reduced at 500°C for 2 h to obtain transition metal Mn supported on MgO, which was recorded as Mn / MgO;

[0062] 0.5 g of Mn / MgO and 0.5 g of LiH were placed in a ball mill, and ball milled for 3 h at a speed of 200 r / min under an Ar atmosphere to obtain a supported transition metal composite metal hydride Mn / LiH / MgO.

[0063] (2) Photothermal catalytic synthesis of ammonia is carried out in a fixed bed, and the photothermal catalyst is Mn / LiH / MgO obtained in step (1). The reaction conditions are as follows:

[0064] Light intensity is 1Wcm -1 ; Temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; The reaction gas space velocity is 60000 mLg -1 h -1 .

[0065] Ammonia synthesis rate: 2620 μmol g -1 h -1 .

[0066] Comparative Example 1:

[0067] Thermal catalytic ammonia synthesis was carried out in a fixed bed, and the thermal catalyst was Fe / LiH obtained in Example (1). The reaction conditions were as follows:

[0068] The temperature is 150°C; the pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; the reaction gas space velocity is 60000 mLg -1 h -1 .

[0069] Ammonia synthesis rate: 500 μmol g-1 h -1 .

[0070] Comparative Example 2:

[0071] Thermal catalytic synthesis of ammonia was carried out in a fixed bed, and the thermal catalyst was Mn / LiH obtained in Example (2). The reaction conditions were as follows:

[0072] The temperature is 150°C; the pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; the reaction gas space velocity is 60000 mLg -1 h -1 .

[0073] Ammonia synthesis rate: 300 μmol g -1 h -1 .

[0074] Comparative Example 3:

[0075] Thermal catalytic synthesis of ammonia is carried out in a fixed bed, and the thermal catalyst is Fe / LiH / MgO obtained in Example (3).

[0076] The reaction conditions are as follows:

[0077] The temperature is 150°C; the pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; the reaction gas space velocity is 60000 mLg -1 h -1 .

[0078] Ammonia synthesis rate: 850 μmol g -1 h -1 .

[0079] Comparative Example 4:

[0080] Thermal catalytic ammonia synthesis was carried out in a fixed bed, and the thermal catalyst was Mn / LiH / MgO obtained in Example (4). The reaction conditions were as follows:

[0081] The temperature is 150°C; the pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3; the reaction gas space velocity is 60000 mLg -1 h -1 .

[0082] Ammonia synthesis rate: 480 μmol g -1 h -1 .

[0083] The synthesis rates of synthetic ammonia in Examples 1-4 and Comparative Examples 1-4 are as follows: Figure 1 As shown, the display is as follows:

[0084] (1) Example 1 is compared with Comparative Example 1, Example 2 is compared with Comparative Example 2, Example 3 is compared with Comparative Example 3, and Example 4 is compared with Comparative Example 4. It can be concluded that when other conditions are the same, the ammonia synthesis rate using photothermal catalysis is much higher than the ammonia synthesis rate using thermal catalysis.

[0085] (2) Compared with the transition metal composite metal hydride Fe / LiH as a photothermal catalyst in Example 1, the supported transition metal composite metal hydride Fe / LiH / MgO in Example 3 is used as a photothermal catalyst for photothermal catalytic synthesis of ammonia, and the ammonia synthesis rate is increased by 42.8%. Similarly, compared with the transition metal composite metal hydride Mn / LiH as a photothermal catalyst in Example 2, the supported transition metal composite metal hydride Mn / LiH / MgO in Example 4 is used as a photothermal catalyst for photothermal catalytic synthesis of ammonia, and the ammonia synthesis rate is increased by 41.6%.

[0086] The present inventors also tested other catalysts, such as Co / LiH, Cr / LiH, Fe / KH, Fe / KH / MgO, and Fe / LiH / graphite, and obtained the above conclusions (1) and (2).

[0087] (3) In Example 1-2, when the metal composite metal hydride is used as the catalyst, the photothermal catalytic ammonia synthesis rate can be higher than 1500 μmol g under mild conditions of a temperature of 150°C and a reaction gas pressure of 1.0 MPa. -1 h -1 , even higher than 3000 μmol g -1 h -1 In Example 3-4, when the supported metal composite metal hydride was used as the catalyst, the photothermal catalytic ammonia synthesis rate was further improved to 4000 μmol g -1 h -1 , achieving a high ammonia synthesis rate under mild conditions.

[0088] Embodiment 5:

[0089] In this embodiment, photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, and the photothermal catalyst is Fe / LiH obtained in step (1) of Example 1. The reaction conditions are as follows:

[0090] Light intensity is 1Wcm -1 ; The temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3.

[0091] The synthesis rate of ammonia is 2850 μmol g -1 h -1 .

[0092] Embodiment 6:

[0093] In this embodiment, photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, and the photothermal catalyst is Mn / LiH obtained in step (1) of Example 2. The reaction conditions are as follows:

[0094] Light intensity is 1Wcm -1 ; The temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3.

[0095] The synthesis rate of ammonia is 1440 μmol g -1 h -1 .

[0096] Embodiment 7:

[0097] In this embodiment, photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, and the photothermal catalyst is Fe / LiH / MgO obtained in step (1) of Example 3. The reaction conditions are as follows:

[0098] Light intensity is 1Wcm -1 ; The temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3.

[0099] The synthesis rate of ammonia is 3260 μmol g -1 h -1 .

[0100] Embodiment 8:

[0101] In this embodiment, photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, and the photothermal catalyst is Mn / LiH / MgO obtained in step (1) of Example 4. The reaction conditions are as follows:

[0102] Light intensity is 1Wcm -1 ; The temperature is 150°C; The pressure of the reaction gases N2 and H2 is 1.0 MPa, and the pressure ratio of N2 to H2 is N2:H2=1:3.

[0103] The synthesis rate of ammonia is 2180 μmol g -1 h -1 .

[0104] Embodiment 9-18:

[0105] Photothermal catalytic synthesis of ammonia was carried out in a fixed bed, and the photothermal catalyst was Fe / LiH obtained in step (1) of Example 1. The reaction conditions and ammonia synthesis rate are shown in Table 1 below.

[0106] Table 1: Reaction conditions and ammonia synthesis rates in Example 1 and Examples 9-18

[0107]

[0108] It can be seen from Table 1 that the ammonia synthesis rate can be adjusted by adjusting one or more of the reaction pressure, space velocity, gas pressure ratio, temperature and light intensity.

[0109] The above-described embodiments provide a detailed description of the technical solutions of the present invention. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, supplements or similar substitutions made within the scope of the principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for synthesizing ammonia with high activity under mild conditions, characterized in that: Photothermal catalysis is used to synthesize ammonia. The photothermal catalyst is a complex of a transition metal and a metal hydride, referred to as a transition metal composite metal hydride. The metal hydride is an alkali metal hydride or an alkaline earth metal hydride.

2. The method for synthesizing ammonia according to claim 1, characterized in that: The transition metal is one or more of Cr, W, Mo, Mn, Ti, Fe, Co, Rh, Ru, Pt, Pd, Au, Ag, Ni, Cu, and Zn; Preferably, the metal hydride is one or more of LiH, KH, NaH, MgH2, CaH2, BaH2, SrH2, LiMgH3, LiCaH3, and LiBaH3; Preferably, in the transition metal composite metal hydride, the mass percentage of transition metal is 0.1%-90%.

3. The method for synthesizing ammonia according to claim 1, characterized in that: The preparation method of the transition metal composite metal hydride comprises the following steps: The transition metal salt powder and the hydride powder of the metal M are mixed and reacted to obtain a complex of the transition metal element and the hydride of the metal M; The metal M is an alkali metal or an alkaline earth metal; Preferably, the transition metal salt powder and the hydride powder of the metal M are ground and mixed in an inert atmosphere; Preferably, the inert atmosphere is one or more of Ar, N2, He, and Ne gases; Preferably, the transition metal salt powder and the metal M hydride powder are ball-milled and mixed at a speed of preferably 50 rmin. -1 -400rmin -1 ; Preferably, the transition metal salt is at least one of nitrate, chloride, acetate and sulfate of a transition metal; Preferably, the molar ratio of the transition metal salt to the hydride of metal M is 1:1-1:

100.

4. The method for synthesizing ammonia according to claim 1, characterized in that: The transition metal composite metal hydride is loaded on a carrier, and is a loaded transition metal composite metal hydride; Preferably, the carrier includes one or more of MgO, CeO2, Co2O3, TiO2, SiO2, Al2O3, BaO, CaO, BN, carbon materials, rare earth oxides, rare earth nitrides, perovskites, molecular sieves, carbon materials, and metal-organic framework materials.

5. The method for synthesizing ammonia according to claim 4, characterized in that: The preparation method of the supported transition metal composite metal hydride comprises the following steps: (1) dispersing a support material in a transition metal salt solution to obtain a mixed solution; calcining the mixed solution at 100° C. to 800° C., and then reducing the mixed solution at 100° C. to 800° C. in a H2 atmosphere, or reducing the mixed solution at 100° C. to 800° C. in a H2 and inert gas atmosphere to obtain a supported transition metal material; (2) mixing the supported transition metal material with the metal hydride to obtain a supported transition metal composite metal hydride catalyst; Preferably, the solvent in the transition metal salt solution includes one or more of water, ethanol and acetone; Preferably, the transition metal salt is at least one of nitrate, chloride, acetate and sulfate of a transition metal; Preferably, in the supported transition metal material, the mass ratio of metal to carrier is 100:1-1:

200.

6. The method for synthesizing ammonia according to claim 5, characterized in that: In the step (1), the calcination is carried out under the conditions of air, Ar, N2 or a mixed gas atmosphere; Preferably, in step (1), the calcination time is 1h-12h; Preferably, in step (2), the supported transition metal material is mixed with the metal hydride, ground and mixed in an inert atmosphere, and more preferably, placed in a ball mill for ball milling, and more preferably, the rotation speed is 50 rmin -1 -400rmin -1 .

7. The method for synthesizing ammonia according to any one of claims 1 to 6, characterized in that: The photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor or a fixed bed.

8. The method for synthesizing ammonia according to claim 7, characterized in that: When photothermal catalytic ammonia synthesis is carried out in a fixed bed, the reaction temperature is lower than 200°C; Preferably, the reaction temperature is lower than 150°C; As a preferred embodiment, the reaction temperature is 150°C, the reaction gas pressure is 1 MPa, and the light intensity is 1 W cm -1 The ammonia synthesis rate was higher than 1500 μmol g -1 h -1 , further preferably higher than 1800 μmol g -1 h -1 , more preferably higher than 3000 μmol g -1 h -1 .

9. The method for synthesizing ammonia according to claim 7, characterized in that: When photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, the reaction temperature is lower than 200°C; Preferably, the reaction temperature is lower than 150°C; As a preferred embodiment, the reaction temperature is 150°C, the reaction gas pressure is 1 MPa, and the light intensity is 1 W cm -1 The ammonia synthesis rate was higher than 1400 μmol g -1 h -1 , further preferably higher than 1600 μmol g -1 h -1 , more preferably higher than 2500 μmol g -1 h -1 .

10. The method for synthesizing ammonia according to claim 7, characterized in that: Photothermal catalytic synthesis of ammonia is carried out in a fixed bed, wherein the photothermal catalyst is a supported transition metal composite metal hydride; The reaction temperature is 150°C, the reaction gas pressure is 1Mpa, and the light intensity is 1Wcm -1 The ammonia synthesis rate was higher than 2500 μmol g -1 h -1 , further preferably higher than 4000 μmol g -1 h -1 .

11. The method for synthesizing ammonia according to claim 7, characterized in that: Photothermal catalytic synthesis of ammonia is carried out in a high-pressure reactor, wherein the photothermal catalyst is a supported transition metal composite metal hydride; The reaction temperature is 150°C, the reaction gas pressure is 1Mpa, and the light intensity is 1Wcm -1 The ammonia synthesis rate was higher than 2000 μmol g -1 h -1 , further preferably higher than 3000 μmol g -1 h -1 .

Images