WO2022089658A1 - 一种常压下催化合成氨的方法 - Google Patents

一种常压下催化合成氨的方法 Download PDF

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WO2022089658A1
WO2022089658A1 PCT/CN2021/131847 CN2021131847W WO2022089658A1 WO 2022089658 A1 WO2022089658 A1 WO 2022089658A1 CN 2021131847 W CN2021131847 W CN 2021131847W WO 2022089658 A1 WO2022089658 A1 WO 2022089658A1
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metal
liquid alloy
ammonia
molten salt
reaction
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PCT/CN2021/131847
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French (fr)
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管晓飞
汤祖建
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上海科技大学
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    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01C—AMMONIA; CYANOGEN; COMPOUNDS THEREOF
    • C01C1/00—Ammonia; Compounds thereof
    • C01C1/02—Preparation, purification or separation of ammonia
    • C01C1/04—Preparation of ammonia by synthesis in the gas phase
    • C01C1/0405—Preparation of ammonia by synthesis in the gas phase from N2 and H2 in presence of a catalyst
    • C01C1/0411—Preparation of ammonia by synthesis in the gas phase from N2 and H2 in presence of a catalyst characterised by the catalyst
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01C—AMMONIA; CYANOGEN; COMPOUNDS THEREOF
    • C01C1/00—Ammonia; Compounds thereof
    • C01C1/02—Preparation, purification or separation of ammonia
    • C01C1/04—Preparation of ammonia by synthesis in the gas phase
    • C01C1/0405—Preparation of ammonia by synthesis in the gas phase from N2 and H2 in presence of a catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/02—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of the alkali- or alkaline earth metals or beryllium
    • B01J23/04—Alkali metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/14—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of germanium, tin or lead
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/20—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state
    • B01J35/27—Catalysts, in general, characterised by their form or physical properties characterised by their non-solid state in a liquid or molten state
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/60—Catalysts, in general, characterised by their form or physical properties characterised by their surface properties or porosity
    • B01J35/61—Surface area
    • B01J35/618—Surface area more than 1000 m2/g
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01C—AMMONIA; CYANOGEN; COMPOUNDS THEREOF
    • C01C1/00—Ammonia; Compounds thereof
    • C01C1/02—Preparation, purification or separation of ammonia
    • C01C1/04—Preparation of ammonia by synthesis in the gas phase
    • C01C1/0405—Preparation of ammonia by synthesis in the gas phase from N2 and H2 in presence of a catalyst
    • C01C1/0482—Process control; Start-up or cooling-down procedures

Definitions

  • the invention relates to a method for synthesizing ammonia, in particular to a method for catalyzing ammonia synthesis under normal pressure.
  • Ammonia (NH 3 ) is an important chemical raw material and occupies an important position in the national economy. About 80% of ammonia is used to produce chemical fertilizers, and 20% is used as raw material for other chemical products. Ammonia is mainly used to prepare nitrogen fertilizer, ammonium salt, nitric acid, cyanide, etc. In addition, ammonia is also an important hydrogen storage material. The mass fraction of the hydrogen content of ammonia is not only as high as 18%, but also at room temperature (300K, 27°C), the saturated vapor pressure of ammonia is low, only about 10 atmospheres, so ammonia is easier to liquefy storage and transportation than hydrogen.
  • ammonia has the potential to become an important material for energy storage and application, playing an important role in the future global sustainable energy layout (MacFarlane, DR, et al., A Roadmap to the Ammonia Economy, Joule, volume 4, pages 1186-1205, 2020).
  • the Haber process for ammonia synthesis is based on hydrogen and nitrogen as raw materials. Under the conditions of high temperature (400 ⁇ 500°C) and high pressure (100 ⁇ 500atm), nitrogen and hydrogen are reacted under the action of iron catalyst to prepare ammonia.
  • high temperature 400 ⁇ 500°C
  • high pressure 100 ⁇ 500atm
  • iron catalyst reacted under the action of iron catalyst to prepare ammonia.
  • the overall energy consumption of the synthetic ammonia industry is huge, and it also brings serious environmental problems (International Energy Agency, World Energy Outlook, 2007).
  • Patent CN109803923 provides a method for synthesizing ammonia by using alkali metal as a catalyst, but the content of synthetic ammonia is very low and the synthesis rate is also very low according to the patented method, so it is not easy to realize large-scale ammonia synthesis.
  • the purpose of the present invention is to provide a method for catalyzing ammonia synthesis, which can catalyze the synthesis of ammonia from hydrogen and nitrogen using a liquid alloy as a catalyst under normal pressure (one atmospheric pressure).
  • the object of the present invention is to provide a method for catalyzing ammonia synthesis under normal pressure.
  • hydrogen and nitrogen are reacted with liquid alloy as catalyst to synthesize ammonia under normal pressure, and molten salt is used in the reaction, and the density of the molten salt is lower than that of the liquid alloy.
  • the density of the molten salt is used to provide the reaction interface and to isolate the liquid alloy from introducing impurities into the reaction environment.
  • the molten salt floating above the liquid alloy plays three roles: 1) The metal nitride formed by the reaction between the alloy and nitrogen has a certain solubility in the molten salt, wherein nitrogen in the metal nitride is usually represented by nitrogen ions It is dissolved in molten salt in the form of molten salt, so molten salt can provide a new reaction interface for the reaction between metal nitride and hydrogen; 2) There may be oxygen and water vapor from the outside atmosphere in the reactor, and molten salt can avoid the liquid alloy and the outside atmosphere. 3) The raw gas nitrogen and hydrogen may contain a small amount of impurity gases (such as water vapor and oxygen), which can interact with the active substances in the liquid alloy. Metals react to form metal oxides. Metal oxides are less dense than liquid alloys and have low solubility in liquid alloys. Under the action of bubbles, the metal oxide will float to the molten salt for dissolution, which can remove impurities in hydrogen and nitrogen, and make the reaction proceed efficiently.
  • the molten salt is selected from one or more of LiCl, NaCl, KCl, MgCl 2 , CaCl 2 , SrCl 2 , BaCl 2 , ZnCl 2 , AlCl 3 , LiBr, NaBr, KBr, LiI, NaI, KI Melt of salt.
  • the molten salt is a molten salt of LiCl and KCl.
  • the molar ratio of LiCl and KCl is (55-60): (35-45). More specifically, the molar ratio of LiCl to KCl can be (55-57): (35-42), or (55-57): (38-45), or (56-60): (35 to 42), or (56 to 60): (38 to 45).
  • the molten salt one or more molten salts are put into plastic bottles, and mixed on a mixer; after the mixing is completed, put into an alumina crucible, and under argon protection and 400 ° C, a mechanical vacuum pump is used. Vacuum and dry; after cooling, the molten salt can be obtained.
  • the mass ratio of the molten salt to the liquid alloy is (0.02-0.9):(0.1-0.98).
  • the mass ratio of the molten salt to the liquid alloy may be (0.02-0.3):(0.14-0.32), (0.25-0.6):(0.23-0.61), or (0.52-0.9):(0.50 ⁇ 0.98).
  • the liquid alloy includes a first metal, and the melting point of the first metal is 27-180.54°C.
  • the first metal is an active metal, which can react with nitrogen to form metal nitride.
  • the first metal is an alkali metal, and the alkali metal is selected from one or more of Li, Na, K, Rb, Cs, and Fr. Further preferably, the first metal is Li.
  • the liquid alloy further includes a second metal, and the melting point of the second metal is 29.77-630°C.
  • the second metal can reduce the activity of the first metal, on the one hand, it can prevent the first metal from corroding the reactor, and on the other hand, it can drive the decomposition of the first metal hydride formed during the reaction.
  • the second metal is an amphoteric metal selected from one or more of Zn, Sn, Bi, Ga, In, Pb, and Sb.
  • the second metal is Sn.
  • the molar ratio of the first metal and the second metal is (0.2-0.7):(0.3-0.8).
  • the molar ratio of the first metal and the second metal is (0.4-0.6):(0.4-0.7).
  • the alloy of the present invention is preferably a liquid alloy of the first metal and the second metal.
  • the method for catalyzing synthetic ammonia under normal pressure in the present invention, described synthetic ammonia comprises two steps:
  • the second step is the synthesis of ammonia.
  • Hydrogen reacts with metal nitride to generate ammonia and metal.
  • the product ammonia ( NH3 ) leaves the reactor as a gas and is then separated and collected.
  • the reaction between hydrogen and metal nitrides will also generate ammonia and metal hydrides.
  • the second metal reduces the activity of the first metal, it will promote the decomposition of the metal hydride.
  • the released first metal can continue to participate in the reaction with nitrogen to generate metal nitride, and then the metal nitride reacts with hydrogen to generate ammonia.
  • the released first metal can continue to participate in the reaction with nitrogen to generate metal nitride, and then the metal nitride reacts with hydrogen to generate ammonia.
  • the method for forming the liquid alloy in the present invention is the prior art, which can be directly heating and melting the first metal or/or the mixture of the first metal and the second metal in the reactor to obtain the liquid alloy, or the first metal Or the first metal and the second metal are melted and charged into the reactor as a liquid alloy.
  • In-situ electrolysis can also be used to introduce the first metal into the second metal, specifically selecting the liquid second metal as the cathode, applying a voltage to electrolyze the oxide or salt of the first metal, and generating the second metal in situ on the surface of the second metal cathode. a metal, and by diffusion forms a liquid alloy with a second metal.
  • molten salt not only molten salt is used, but also a liquid alloy formed by the first metal and the second metal is used. Since the first metal is simply used, the density of the first metal is low and the first metal will As bubbles are generated, they will float, resulting in low ammonia synthesis rate and production, shortening the service life of the first metal, and even causing the reactor to rupture.
  • the formation of a liquid alloy between the second metal and the first metal can effectively increase the density. In addition, it can not only prevent the first metal from corroding the ceramic reactor, but also drive the decomposition of the first metal hydride formed during the reaction.
  • the reaction temperature in the ammonia synthesis process is 300-600°C. More preferably, the temperature is 400-500°C. According to thermodynamic analysis, too high temperature is not conducive to the synthesis and collection of ammonia. In addition, according to kinetic analysis, too low temperature reduces the reaction rate of ammonia synthesis, so an appropriate reaction temperature range should be selected.
  • a third metal is also used in the catalytic reaction, the melting point of the third metal is higher than that of the first metal, and exists in the liquid alloy in a solid form for activating the reaction gas; the third metal for transition metals.
  • the third metal has a relatively high specific surface area (6500-8000 cm 2 /g), the third metal can promote the activity of nitrogen molecules and hydrogen molecules, and has catalytic activity for the synthesis of ammonia.
  • the third metal can be nickel foam, which is fixed on the gas outlet of nitrogen and hydrogen; it can also be placed in the liquid alloy in the form of a wire mesh; or the third metal can be directly mixed with the first metal and the second metal .
  • the third metal is selected from one or more of Ni, Fe, Mo, Co, Ru, Re and Pt.
  • the mass ratio of the third metal and the liquid alloy is (0.1-10):(90-99.9). Further preferably, the mass ratio of the third metal and the liquid alloy is (0.3-7): (94-96).
  • the flow rate ratio of the nitrogen and the hydrogen added is (1-99):(1-99).
  • the flow rate ratio of the nitrogen gas to the hydrogen gas is 1:(2 ⁇ 50).
  • the nitrogen gas and the hydrogen gas are respectively introduced into the liquid alloy through different ventilation pipes, the nitrogen gas is introduced into the bottom of the liquid alloy, and the hydrogen gas is introduced into the upper part of the liquid alloy or the molten salt.
  • the flow rate of the nitrogen gas is 5-100 cm 3 /min
  • the flow rate of the hydrogen gas is 5-100 cm 3 /min.
  • the reaction is carried out under a protective gas for preventing the decomposition of ammonia.
  • the protective gas is selected from one of nitrogen, hydrogen and inert gas.
  • the flow rate of the gas is 60-100 cm 3 /min.
  • the invention can make the generated ammonia gas quickly leave the high temperature area of the reactor through the purging of the protective gas, and can effectively prevent the decomposition of the ammonia generated by the catalytic reaction.
  • the invention provides a reactor for a method for catalyzing ammonia synthesis under normal pressure, comprising a reactor, a heating mechanism for providing a heat source, a hydrogen source and a nitrogen source.
  • the reactor includes a reactor body, a nitrogen gas vent pipe and a hydrogen gas vent pipe, and the nitrogen gas vent pipe and the hydrogen gas vent pipe penetrate into the reactor body and extend into the reactor body.
  • the upper part of the reactor body is provided with a product gas outlet.
  • the product gas outlet is communicated with an ammonia gas collection mechanism, which is used to avoid environmental pollution and unsafe factors caused by ammonia gas discharge.
  • the ammonia gas collection mechanism is a collection tank containing an acid aqueous solution.
  • the reactor is further provided with a protective gas vent pipe, the inlet pipe of the protective gas vent pipe is connected with the protective gas source, and the gas outlet end of the protective gas vent pipe penetrates the reactor body and enters the reactor within the body.
  • the reactor in order to ensure that the reaction temperature is 300-600° C., the reactor can be transferred to a heating mechanism such as a tube furnace for heating.
  • the present invention has the following beneficial effects:
  • the catalyst of the present invention uses alkali metals with high activity, which can easily react with nitrogen to form alkali metal nitrides, and can synthesize ammonia under normal pressure. Compared with noble metal catalysts, alkali metals usually have wider sources and lower costs.
  • the second metal in the present invention can reduce the activity of the first metal, on the one hand, it can prevent the first metal from corroding the reactor, and on the other hand, it can drive the decomposition of the first metal hydride formed in the reaction process.
  • the molten salt can provide a new reaction interface for the reaction of metal nitrides and hydrogen to synthesize ammonia, so that the metal nitrides continuously generate ammonia, effectively improving the rate of reaction and the synthetic amount of ammonia. Avoid contact of the liquid alloy with oxygen and water vapor in the outside atmosphere, so as to prevent the liquid alloy from being oxidized, thereby prolonging the service life of the liquid alloy.
  • the density of the first metal nitride generated by the nitrogen fixation reaction is small and the solubility is low, so the first metal nitride will float up with the bubbles, and the generated first metal nitride will react with hydrogen to synthesize ammonia, and the first metal nitride will react with hydrogen to synthesize ammonia.
  • a metal hydride is decomposed into the first metal and hydrogen under the promotion of the second metal, realizing a continuous closed chemical chain, which can effectively synthesize ammonia continuously under normal pressure.
  • FIG. 1 is a schematic diagram showing the structure of the reactor of the present invention.
  • FIG. 1 is a reactor
  • 2 is a nitrogen gas vent pipe
  • 3 is a hydrogen gas vent pipe
  • 4 is a product gas outlet
  • 5 is a molten salt
  • 6 is a liquid alloy.
  • the alloy is a prefabricated Li-Sn alloy, specifically: 1) in an argon glove box, mix 6.87g Li and 78.32g Sn in a crucible; 2) mix 1.52g LiCl and 1.86g KCl Mix and cover the surface to isolate the metal from the atmosphere; 3) Transfer the crucible as a whole to a tube furnace, heat it to 500°C under an argon protective gas, keep it for 12 hours, cool it to room temperature, and remove LiCl and KCl , thereby obtaining a prefabricated Li-Sn alloy in which the molar ratio of Li and Sn is 60:40.
  • the molten salt is a prefabricated LiCl-KCl molten salt: 100 g of anhydrous LiCl and 122 g of anhydrous KCl are weighed into a plastic bottle, and mixed on a mixer for 24 hours; The salt was put into an alumina crucible, evacuated with a mechanical vacuum pump under argon protection and 400 ° C, and dried for 24 hours; the cooled sample was transferred into a glove box, and the mixture of LiCl and KCl in the LiCl-KCl molten salt was obtained. The molar ratio was 59:41.
  • This embodiment discloses a specific device for catalyzing ammonia synthesis under normal pressure, which includes a reactor, a heating mechanism for providing a heat source, a hydrogen source and a nitrogen source.
  • the reactor includes a reactor body 1, a nitrogen gas vent pipe 2, and a hydrogen gas vent pipe 3; the upper part of the reactor body 1 is provided with a product gas outlet 4, and the The nitrogen gas vent pipe 2 and the hydrogen gas vent pipe 3 penetrate into the reactor body 1 and extend into the reactor body 1 .
  • the product gas outlet 4 is connected with an ammonia gas collection mechanism, such as a collection tank containing an acid aqueous solution.
  • the reactors are all made of alumina.
  • the reactor can also It includes a protective gas vent pipe, the inlet pipe of the protective gas vent pipe is connected with the protective gas source, and the gas outlet end of the protective gas vent pipe penetrates the reactor body 1 and enters the reactor body 1 .
  • the protective gas vent can be argon, nitrogen, hydrogen or inert gas.
  • the material of the protective gas vent pipe is alumina material.
  • the solid alloy as a catalyst is put into the reactor body 1, and the molten salt 5 is covered on the surface of the alloy, and then heated under the protection of argon to make the solid alloy form a liquid alloy, and the inlet end of the nitrogen gas vent pipe 2 is communicated with the nitrogen source, The outlet end of the nitrogen breather pipe 2 is located in the liquid alloy 6, the inlet end of the hydrogen breather pipe 3 is communicated with the hydrogen source, and the gas outlet end of the hydrogen breather pipe 3 is located in the liquid alloy 6 or can also be located in the molten salt 5. Ammonia gas is collected through the product gas outlet 4 .
  • the reactor can be transferred to a heating mechanism such as a tube furnace for heating.
  • Examples 2 to 4 and Comparative Examples 1 to 2 all use reactor 1 for catalytic reaction, but it should be noted that, in order to achieve the purpose of the invention in this application, it is not limited to the reactors listed in the present invention, as long as Any reactor or device or equipment that can meet the requirement of introducing nitrogen and hydrogen under normal pressure and synthesizing ammonia under the catalysis of liquid alloy can be used.
  • Li-Sn liquid alloy is used as a catalyst, and molten salt of LiCl-KCl is added to synthesize ammonia at normal pressure.
  • the entire reactor was transferred to a tube furnace.
  • the reactor was filled with argon gas, the flow rate of argon gas was 5cm 3 /min, and the temperature was raised to 500°C at a rate of 4°C/min, so that the prefabricated Li-Sn alloy became a Li-Sn liquid alloy, and then the flow of argon gas was stopped.
  • the reaction was carried out at 500°C and normal pressure for 24 hours.
  • the product ammonia gas is passed into the dilute sulfuric acid aqueous solution through the product gas outlet, and the ammonia is collected.
  • concentration of the dilute sulfuric acid aqueous solution is 0.1 mol/L, and the volume is 20 ml.
  • the content of ammonium ions in the dilute sulfuric acid aqueous solution was determined by an ion chromatograph to be 182.22 mg/l.
  • the synthesis rate of ammonia can be calculated to be 143.41 ⁇ g/h, which indicates that with the addition of molten salt, the liquid alloy can be used as a catalyst to efficiently catalyze the synthesis of ammonia under normal pressure.
  • Li-Sn liquid alloy is used as a catalyst, and molten salt of LiCl-KCl is added to synthesize ammonia at normal pressure.
  • the product ammonia gas is passed into the dilute sulfuric acid aqueous solution through the product gas outlet, and the ammonia is collected.
  • concentration of the dilute sulfuric acid aqueous solution is 0.1 mol/L, and the volume is 20 ml.
  • the content of ammonium ions in the dilute sulfuric acid aqueous solution was determined by an ion chromatograph to be 3.9 mg/l.
  • the synthesis rate of ammonia can be calculated to be 2.66 ⁇ g/h, which indicates that with the addition of molten salt, the liquid alloy can be used as a catalyst to efficiently catalyze the synthesis of ammonia under normal pressure. This result is less than the ammonia synthesis rate of Example 2, mainly due to the relatively low temperature of the reaction, which makes the reaction rate lower.
  • low addition of molten salt and low lithium content in the liquid alloy also reduce the rate of catalytic ammonia synthesis and the ammonia content.
  • the steps and conditions of the catalytic reaction in this example are the same as those in Example 2. However, 0.126g of nickel foam was fixed at the outlet ends of the hydrogen breather pipe and the nitrogen breather pipe with nickel wires respectively. As the third metal, nickel foam is used to activate the reactive gas.
  • the concentration of the dilute sulfuric acid aqueous solution is 0.1 mol/L, and the volume is 20 ml.
  • Lithium nitride has a certain solubility in molten salt, wherein nitrogen in lithium nitride is usually dissolved in molten salt in the form of nitrogen ions, so molten salt can be The reaction between lithium nitride and hydrogen provides a new reaction interface; 2) There may be oxygen and water vapor from the outside atmosphere in the reactor, and the molten salt can prevent the liquid alloy from contacting the oxygen and water vapor in the outside atmosphere, thereby preventing the liquid alloy from being oxidized , prolong the service life of the liquid alloy; 3)
  • the raw gas nitrogen and hydrogen may contain a small amount of impurity gases (such as water vapor and oxygen), and these impurity gases can react with the active metals in the liquid alloy to form metal oxides. Metal oxides are less dense than liquid alloys and have low solubility in liquid alloys. Under the action of bubbles, the metal oxide will float to the molten salt for dissolution, which can remove im
  • Example 2 Compared with Comparative Example 1 without the addition of molten salt, the content of ammonia in Example 2 was increased by 43 times, and the synthesis rate of ammonia was increased by 43 times.
  • Comparative Example 2 Sn liquid alloy was used as catalyst, LiCl-KCl molten salt was added, and ammonia was synthesized under normal pressure.

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Abstract

本发明涉及一种常压下催化合成氨的方法。本发明氢气和氮气在反应器中 以液态合金为催化剂在常压下合成氨，反应中采用熔融盐，所述熔融盐的密度小于液态合金的密度，所述熔融盐用于提供反应界面并用于隔离液态合金避免引入反应环境中杂质。本发明中，第一金属与氮气反应生产金属氮化物，熔融盐能为金属氮化物与氢气反应合成氨提供新的反应界面，使金属氮化物不断生成氨，并有效催化了反应的速率和氨的合成量，此外，熔融盐可以避免液态合金与外界大气的氧气和水蒸汽接触，从而避免液态合金被氧化，从而延长液态合金的使用寿命。

Description

一种常压下催化合成氨的方法 技术领域
本发明涉及一种合成氨的方法，具体涉及一种常压下催化合成氨的方法。
背景技术
氨(NH 3)是一种重要的化工原料，在国民经济中占有重要地位，其中约有80％的氨用来生产化学肥料，20％为其它化工产品的原料。氨主要用于制备氮肥、铵盐、硝酸、氰化物等。另外，氨也是一种重要的储氢材料。氨的含氢量的质量分数不仅高达18％，而且在室温条件(300K,27℃)下，氨的饱和蒸汽压低，仅为10个大气压左右，所以，氨比氢更易于液化存储和运输。因此，氨有潜力成为一种重要的能源存储和应用的物质，在未来全球可持续能源的布局中发挥重要作用(MacFarlane,D.R.,et al.,A Roadmap to the Ammonia Economy,Joule,volume 4,pages 1186-1205,2020)。
目前，工业上使用哈伯法大规模合成氨。哈伯法合成氨是以氢气和氮气为原料，在高温(400～500℃)和高压(100～500atm)条件下，使氮气和氢气在铁催化剂作用下反应制备氨。然而，由于哈伯法所需的高温高压条件，合成氨工业总体消耗的能量巨大，同时也带来了严重的环境问题(International Energy Agency,World Energy Outlook,2007)。
专利CN109803923提供了一种以碱金属为催化剂合成氨的方法，但按照专利方法操作合成氨的含量非常低，而且合成速率也非常低，不容易实现大规模化合成氨。
发明内容
鉴于以上所述现有技术的缺点，本发明的目的在于提供一种催化合成氨的方法，其能够在常压下(一个大气压)以液态合金为催化剂催化氢气和氮气合成氨。
为了实现上述目的及其他相关目的，本发明是通过包括采用如下技术方案获得。
本发明的目的在于提供一种常压下催化合成氨的方法，在反应器中，氢气和氮气以液态合金为催化剂常压下反应合成氨，反应中采用熔融盐，所述熔融盐的密度小于液态合金的密度，所述熔融盐用于提供反应界面并用于隔离液态合金避免引入反应环境中杂质。
本发明中，浮在液态合金上方的熔融盐起到了三方面的作用，1)合金和氮气反应生成的金属氮化物在熔融盐中有一定的溶解度，其中金属氮化物中的氮通常以氮离子的形式溶于熔融盐中，因此熔融盐可以为金属氮化物与氢气反应提供新的反应界面；2)反应器内可能有来自外界大气的氧气和水蒸汽，熔融盐可以避免液态合金与外界大气的氧气和水蒸汽接触，从而避免液态合金被氧化，延长液态合金的使用寿命；3)原料气体氮气和氢气可能含有少量杂 质气体(如水蒸汽和氧气)，这些杂质气体可与液态合金中的活泼金属反应生成金属氧化物。金属氧化物比液态合金的密度小，而且在液态合金的溶解度低。在气泡的作用下，金属氧化物会上浮到熔融盐中进行溶解，可去除氢气和氮气中的杂质，使反应高效进行。
优选地，所述熔融盐选自LiCl、NaCl、KCl、MgCl 2、CaCl 2、SrCl 2、BaCl 2、ZnCl 2、AlCl 3、LiBr、NaBr、KBr、LiI、NaI、KI的一种或多种盐的熔融体。
更优选地，所述熔融盐为LiCl和KCl的熔融盐。
进一步优选地，所述LiCl和KCl的摩尔比为(55～60)：(35～45)。更具体地，所述LiCl和KCl的摩尔比可以是(55～57)：(35～42)，也可以是(55～57)：(38～45)，也可以是(56～60)：(35～42)，也可以是(56～60)：(38～45)。所述熔融盐是将一种及以上熔融盐装入塑料瓶子中，在混料机上进行混料；混料完成后，装入氧化铝坩埚中，在氩气保护以及400℃下，用机械真空泵抽真空，干燥；冷却后即可得到熔融盐。
优选地，所述熔融盐与液态合金的质量比为(0.02～0.9):(0.1～0.98)。所述熔融盐与液态合金的质量比可以是(0.02～0.3):(0.14～0.32)，也可以是(0.25～0.6):(0.23～0.61)，也可以是(0.52～0.9):(0.50～0.98)。
优选地，所述液态合金包括第一金属，所述第一金属的熔点为27～180.54℃。本发明中第一金属为活性金属，可以和氮气反应生成金属氮化物。
更优选地，所述第一金属为碱金属，所述碱金属选自Li、Na、K、Rb、Cs、Fr的一种或多种。进一步优选地，所述第一金属为Li。
优选地，所述液态合金还包括第二金属，所述第二金属的熔点为29.77～630℃。本发明中第二金属可以降低第一金属的活度，一方面可以避免第一金属腐蚀反应器，另一方面可以驱动反应过程中所形成的第一金属氢化物的分解。
更优选地，所述第二金属为两性金属，所述两性金属选自Zn、Sn、Bi、Ga、In、Pb、Sb的一种或多种。
进一步优选地，所述第二金属为Sn。
更优选地，所述第一金属和第二金属的摩尔比为(0.2～0.7):(0.3～0.8)。
进一步优选地，所述第一金属和第二金属的摩尔比为(0.4～0.6):(0.4～0.7)。本发明的合金优选第一金属和第二金属形成的液态合金。
本发明中常压下催化合成氨的方法，所述合成氨包括两步：
1)第一步是氮的固定。氮气与第一金属反应生成金属氮化物，以Li为例，反应为：6Li+N 2＝2Li 3N。金属氮化物通常比液态合金的密度小，而且在液态合金中的溶解度低，所以在气 体气泡的作用下，金属氮化物通常会上浮。
2)第二步是氨的合成。氢气与金属氮化物反应生成氨和金属，以Li为例，反应为2Li 3N+3H 2＝2NH 3+6Li。产物氨(NH 3)以气体形式离开反应器，然后被分离和收集。此外，氢气与金属氮化物反应也会生成氨和金属氢化物，以Li为例，反应为Li 3N+6H 2＝NH 3+3LiH。但由于第二金属降低了第一金属的活度，因此将促进金属氢化物分解，以Li为例，反应为2LiH＝2Li+H 2。释放出的第一金属可以继续参与和氮气的反应生成金属氮化物，然后金属氮化物再和氢气反应生成氨。通过上述反应过程，这样就建立了一个持续进行的闭合化学链，在常压下，不间断地有效合成氨。
本发明中形成液态合金的方法是现有技术，可以是将第一金属或/或第一金属与第二金属的混合物在反应器中直接加热熔融制得液态合金，也可以是将第一金属或第一金属与第二金属熔融后呈液态合金装入反应器中。也可以采取原位电解的方法把第一金属引入第二金属，具体为选用液态的第二金属为阴极，施加电压电解第一金属的氧化物或盐，在第二金属阴极表面原位生成第一金属，并通过扩散与第二金属形成液态合金。
在本发明一个更优选的技术方案中，不仅采用熔融盐，还采用了由第一金属和第二金属形成的液态合金，因为单纯使用第一金属，第一金属的密度低，第一金属会随着气泡的产生会上浮，导致氨的合成速率低和产量低，缩短第一金属的使用寿命，甚至会导致反应器破裂。而第二金属与第一金属形成液态合金可以有效地提高密度，此外，不仅可以避免第一金属腐蚀陶瓷反应器，还可以驱动反应过程中所形成的第一金属氢化物的分解。
优选地，所述合成氨过程中反应的温度为300～600℃。更优选地，所述温度为400～500℃。根据热力学分析，过高的温度不利于氨的合成和收集。此外，根据动力学分析，过低的温度又使合成氨反应的速率降低，因此应选择合适的反应温度范围。
优选地，所述催化反应中还采用有第三金属，所述第三金属熔点比第一金属的熔点高，并且以固态形式存在于液态合金中，用于活化反应气体；所述第三金属为过渡金属。本发明中第三金属具有较高的比表面积(6500～8000cm 2/g)，第三金属可以促进氮气分子和氢气分子的活性，对氨的合成具有催化活性。具体地，所述第三金属可以采用泡沫镍，固定在氮气和氢气的出气部；也可以以铁丝网的形式放置在液体合金中；也可以是第三金属与第一金属和第二金属直接混合。
更优选地，所述第三金属选自Ni、Fe、Mo、Co、Ru、Re和Pt中的一种或多种。
更优选地，所述第三金属和液态合金的质量比为(0.1～10)：(90～99.9)。进一步优选地，所述第三金属和液态合金的质量比为(0.3～7)：(94～96)。
优选地，所述氮气和所述氢气加入的流速比例为(1～99):(1～99)。
更优选地，所述氮气和氢气的流速比1:(2～50)。
优选地，所述氮气和氢气通入液态合金的方式至少有两种：
一种是，所述氮气和氢气分别经不同的通气管通入液态合金中，所述氮气通入液态合金的底部，所述氢气通入液态合金或熔融盐的上部。
另外一种是，所述氮气和氢气先混合，然后经通气管通入液态合金中。
优选地，所述氮气的流速为5～100cm 3/min，氢气的流速为5～100cm 3/min。
优选地，所述反应在保护气体下进行，所述保护气体用于防止氨的分解。所述保护气体选自氮气、氢气和惰性气体中的一种。所述气体的流速为60～100cm 3/min。本发明可以通过保护气体的吹扫，使已生成的氨气快速离开反应器的高温区域，可以有效防止催化反应生成氨的分解。
本发明提供了一种常压下催化合成氨的方法的反应器，包括反应器、用于提供热源的加热机构，氢气源和氮气源。
优选地，所述反应器包括反应器本体、氮气通气管和氢气通气管，所述氮气通气管和所述氢气通气管贯穿进入所述的反应器本体且延伸至所述的反应器本体内。
更优选地，所述反应器本体的上部设有产物气体出口。进一步优选地，所述产物气体出口连通有氨气收集机构，用于避免氨气排出对环境的污染以及引发的不安全因素。更具体地，氨气收集机构为盛装有酸水溶液的收集槽。
优选地，所述反应器还设有保护气体通气管，所述保护气体通气管的进气管与保护气体源连通，所述保护气体通气管的出气端贯穿所述反应器本体进入所述反应器本体内。
本发明的反应过程中为了确保反应温度为300～600℃，可以将反应器转移至加热机构如管式炉中进行加热。
与现有技术相比，本发明具有以下有益效果：
1)本发明催化剂使用活性高的碱金属，与氮气较易反应生成碱金属氮化物，并能在常压下合成氨，而且相比于贵金属催化剂，碱金属通常来源更广泛，成本更低。
2)本发明中的第二金属能降低第一金属的活度，一方面可以避免第一金属腐蚀反应器，另一方面可以驱动反应过程中所形成的第一金属氢化物的分解。
3)本发明催化反应中，熔融盐能为金属氮化物与氢气反应合成氨提供新的反应界面，使金属氮化物不断生成氨，有效提高了反应的速率和氨的合成量，此外，熔融盐可以避免液态合金与外界大气的氧气和水蒸汽接触，从而避免液态合金被氧化，从而延长液态合金的使 用寿命。
4)本发明催化反应中，固氮反应生成的第一金属氮化物密度较小而且溶解度低，所以第一金属氮化物将随着气泡上浮，生成的第一金属氮化物再与氢气反应合成氨，第一金属氢化物在第二金属的促进下分解为第一金属和氢气，实现了持续进行的闭合化学链，可在常压下不间断地有效合成氨。
5)本发明合成氨反应的工艺和设计简单，反应条件相对温和，易于实现大规模化合成氨。
附图说明
图1显示为本发明的反应器的结构示意图。
图1中附图标记如下：1为反应器，2为氮气通气管，3为氢气通气管，4为产物气体出口，5为熔融盐，6为液态合金。
具体实施方式
以下由特定的具体实施例说明本发明的实施方式，熟悉此技术的人士可由本说明书所揭露的内容轻易地了解本发明的其他优点及功效。
在进一步描述本发明具体实施方式之前，应理解，本发明的保护范围不局限于下述特定的具体实施方案；还应当理解，本发明实施例中使用的术语是为了描述特定的具体实施方案，而不是为了限制本发明的保护范围。下列实施例中未注明具体条件的试验方法，通常按照常规条件，或者按照各制造商所建议的条件。
当实施例给出数值范围时，应理解，除非本发明另有说明，每个数值范围的两个端点以及两个端点之间任何一个数值均可选用。除非另外定义，本发明中使用的所有技术和科学术语与本技术领域技术人员通常理解的意义相同。除实施例中使用的具体方法、设备、材料外，根据本技术领域的技术人员对现有技术的掌握及本发明的记载，还可以使用与本发明实施例中所述的方法、设备、材料相似或等同的现有技术的任何方法、设备和材料来实现本发明。
本申请实施例中，所述合金为预制Li-Sn合金，具体为：1)在氩气手套箱中，将6.87gLi和78.32gSn混合放置在坩埚中；2)把1.52g LiCl和1.86g KCl混合覆盖在表面，用于使金属与气氛隔离；3)将坩埚整体转移至管式炉中，在氩气保护气下，加热至500℃，保温12h后，冷却至室温，将LiCl和KCl去除，从而获得预制Li-Sn合金，其中Li和Sn的摩尔比为60：40。
本申请实施例中，所述熔融盐为预制LiCl-KCl熔融盐：称取100g无水LiCl和122g无水KCl装入塑料瓶子中，在混料机上进行混料24小时；混料完成后，将盐装入氧化铝坩埚 中，在氩气保护以及400℃下，用机械真空泵抽真空，干燥24h；冷却后的样品转移进入手套箱中即得，其中LiCl-KCl熔融盐中LiCl和KCl的摩尔比为59：41。但是，从实现本申请中发明目的来说，只要熔融盐的密度小于液态合金的密度，能够起到提供反应界面并隔离液态合金从而避免引入反应环境中杂质的效果均可以达到采用本申请实施例中这种具体的LiCl-KCl熔融盐的技术效果。
实施例1
本实施例中公开了一种具体的常压下催化合成氨的装置，其包括反应器、用于提供热源的加热机构，氢气源和氮气源。
图1为本发明的反应器的结构示意图，所述反应器包括反应器本体1、氮气通气管2、氢气通气管3；所述的反应本体器1的上部设有产物气体出口4，所述氮气通气管2和所述氢气通气管3贯穿进入所述的反应器本体1且延伸至所述的反应器本体1内。
为了避免氨气排出对环境的污染以及引发的不安全因素，所述产物气体出口4连通有氨气收集机构，如盛装有酸水溶液的收集槽。
为了使得这一反应器能够用于本申请中合成过程，所述反应器均采用氧化铝制成。
为了使得用于作为催化剂的合金在反应器本体1内形成加热熔融成液态合金，并排除这一过程中反应器本体1内的空气对液态合金的氧化作用等的影响，所述反应器还可以包括保护气体通气管，所述保护气体通气管的进气管与保护气体源连通，所述保护气体通气管的出气端贯穿所述反应器本体1进入所述反应器本体1内。所述保护气体通气管可以为氩气、氮气、氢气或惰性气体。同样地，所述保护气体通气管的材质为氧化铝材质。
在一个具体在使用上述装置进行常压下合成氨时，
将作为催化剂的固态合金放入反应器本体1中，并在合金表面覆盖熔融盐5，然后在氩气保护下加热使得固态合金形成液态合金，氮气通气管2的进气端与氮气源连通，氮气通气管2的出气端位于液态合金6中，氢气通气管3的进气端与氢气源连通，氢气通气管3的出气端位于液态合金6中或者也可以位于熔融盐5中，反应产生的氨气通过产物气体出口4进行收集。
在反应过程中为了确保反应温度为300～600℃，可以将反应器转移至加热机构如管式炉中进行加热。
本申请中实施例2～4以及对比例1～2均采用反应器1进行催化反应，但是注意的是，从实现本申请中发明目的来说，不限于本发明所举列的反应器，只要能满足在常压下氮气和氢气通入，在液态合金的催化下合成氨的反应器或装置或设备都可以。
实施例2
本实施例中，以Li-Sn液态合金为催化剂，添加LiCl-KCl的熔融盐，常压合成氨。
本实施例中常压下催化合成氨的方法，催化反应步骤如下：
1)在氩气手套箱中，称取21.3g预制Li-Sn合金和8.46g预制LiCl-KCl熔融盐，并先后装入反应器中。
2)将反应器整体转移至管式炉中。反应器通入氩气，氩气的流速为5cm 3/min，并以4℃/min的速率升温至500℃，使预制Li-Sn合金成为Li-Sn液态合金，然后停止通氩气。将氮气经过氮气通气管通入Li-Sn液态合金的底部，氮气的流速为5cm 3/min；将氢气经过氢气通气管通入Li-Sn液态合金的上部，氢气的流速为15cm 3/min。反应在500℃和常压条件下进行24小时。将产物氨气经产物气体出口通入稀硫酸水溶液，对氨进行收集。稀硫酸水溶液的浓度为0.1mol/L，体积为20ml。
实施结果：用离子色谱仪测定稀硫酸水溶液中氨根离子的含量为182.22mg/l。氨的合成速率可以计算为143.41μg/h，这说明了在添加熔融盐下，以液态合金为催化剂，可在常压下高效催化合成氨。
相比于专利CN109803923提供的仅使用液态纯钠在500℃和常压下合成氨的方法，本实施例的氨的合成速率提高了5倍。
实施例3
本实施例中，以Li-Sn液态合金为催化剂，添加LiCl-KCl的熔融盐，常压合成氨。
本实施例中常压下催化合成氨的方法，催化反应步骤如下：
1)在氩气手套箱中，称取11.76g预制Li-Sn合金，再称取26.97g Sn混合，使Li和Sn的摩尔比为30:70，再称取3.38g预制LiCl-KCl熔融盐，并先后装入反应器中。
2)将反应器整体转移至管式炉中。反应器通入氩气，氩气的流速为5cm 3/min，并以4℃/min的速率升温至400℃，使预制Li-Sn合金成为Li-Sn液态合金，然后停止通氩气。将氮气经过氮气通气管通入Li-Sn液态合金的底部，氮气的流速为5cm 3/min；将氢气经过氢气通气管通入熔融盐中，氢气的流速为15cm 3/min。反应在400℃和常压条件下进行24小时。将产物氨气经产物气体出口通入稀硫酸水溶液，对氨进行收集。稀硫酸水溶液的浓度为0.1mol/L，体积为20ml。
实施结果：用离子色谱仪测定稀硫酸水溶液中氨根离子的含量为3.9mg/l。氨的合成速率可以计算为2.66μg/h，这说明在添加熔融盐下，以液态合金为催化剂，可在常压下高效催化合成氨。这个结果小于实施例2的氨合成速率，主要归因于反应的温度相对较低，使得反应速率较低。此外，熔融盐添加少以及液态合金中锂含量少也会降低催化合成氨的速率和氨的 含量。
实施例4
本实施例中催化反应步骤和条件同实施例2。但在氢气通气管和氮气通气管的出气端分别用镍丝固定了0.126g的泡沫镍。作为第三金属，泡沫镍用于活化反应气体。
实施结果：用离子色谱仪测定稀硫酸水溶液中氨根离子的含量为189.47mg/l。氨的合成速率可以计算为149.12μg/h。与未引入第三金属的实施例2相比，氨的含量和氨的合成速率都有提高。这说明在反应中引入第三金属泡沫镍进一步促进合成氨反应。
对比例1
对比例1中，以Li-Sn液态合金为催化剂，常压下合成氨。
合成氨的催化反应步骤如下：
1)在氩气手套箱中，将21.3g预制Li-Sn合金装入反应器中。
2)将反应器整体转移至管式炉中。反应器通氩气，氩气的流速为5cm 3/min，并以4℃/min的速率升温至500℃，使预制Li-Sn合金成为Li-Sn液态合金，然后停止通氩气。将氮气经过氮气通气管通入Li-Sn液态合金的底部，氮气的流速为5cm 3/min；将氢气经过氢气通气管通入Li-Sn液态合金中，氢气的流速为15cm 3/min。反应在500℃和常压条件下进行24小时，将产物氨气经产物气体出口通入稀硫酸水溶液，对氨进行收集。稀硫酸水溶液的浓度为0.1mol/L，体积为20ml。
实验结果：用离子色谱仪测定稀硫酸水溶液中氨根离子的含量为4.22mg/l。氨的合成速率可以计算为3.32μg/h。这个结果远远小于实施例2的氨合成速率，说明了熔融盐对于在常压下催化合成氨反应起到重要的作用。熔融盐在氨合成中的作用有三方面的，1)氮化锂在熔融盐中有一定的溶解度，其中氮化锂中的氮通常以氮离子的形式溶于熔融盐中，因此熔融盐可以为氮化锂与氢气反应提供新的反应界面；2)反应器内可能有来自外界大气的氧气和水蒸汽，熔融盐可以避免液态合金与外界大气的氧气和水蒸汽接触，从而避免液态合金被氧化，延长液态合金的使用寿命；3)原料气体氮气和氢气可能含有少量杂质气体(如水蒸汽和氧气)，这些杂质气体可与液态合金中的活泼金属反应生成金属氧化物。金属氧化物比液态合金的密度小，而且在液态合金的溶解度低。在气泡的作用下，金属氧化物会上浮到熔融盐中进行溶解，可去除氢气和氮气中的杂质，使反应高效进行。
与没有添加熔融盐的对比例1相比，实施例2中氨的含量提高了43倍，氨的合成速率提高了43倍。
对比例2
对比例2中，以Sn液态合金为催化剂，添加LiCl-KCl熔融盐，常压下合成氨。
合成氨的催化反应步骤如下：
1)在氩气手套箱中，称取45.87g Sn和3.38g预制LiCl-KCl熔融盐，并先后装入反应器中。
2)将反应器整体转移至管式炉中。反应器通氩气，氩气的流速为5cm 3/min，并以4℃/min的速率升温至400℃，然后停止通氩气。将氮气经过氮气通气管通入Sn液态合金的底部，氮气的流速为5cm 3/min；将氢气经过氢气通气管通入熔融盐中，氢气的流速为15cm 3/min。反应在400℃和常压条件下进行24小时。将产物氨气经产物气体出口通入稀硫酸水溶液，对氨进行收集。稀硫酸水溶液的浓度为0.1mol/L，体积为20ml。
实验结果：用离子色谱仪测定稀硫酸水溶液中氨根离子的含量为1.77mg/l。氨的合成速率计算为1.40μg/h。这个结果小于实施例2的氨合成速率，说明了第一金属锂和熔融盐在催化过程中相互协作，对于在常压下催化合成氨反应起到重要的作用。
上述实施例仅例示性说明本发明的原理及其功效，而非用于限制本发明。任何熟悉此技术的人士皆可在不违背本发明的精神及范畴下，对上述实施例进行修饰或改变。因此，举凡所属技术领域中具有通常知识者在未脱离本发明所揭示的精神与技术思想下所完成的一切等效修饰或改变，仍应由本发明的权利要求所涵盖。

Claims (10)

  1. 一种常压下催化合成氨的方法，其特征在于，氢气和氮气在反应器中以液态合金为催化剂在常压下合成氨，反应中采用熔融盐，所述熔融盐的密度小于液态合金的密度，所述熔融盐用于提供反应界面并用于隔离液态合金避免引入反应环境中杂质。
  2. 根据权利要求1所述的常压下催化合成氨的方法，其特征在于，所述熔融盐为选自LiCl、NaCl、KCl、MgCl 2、CaCl 2、SrCl 2、BaCl 2、ZnCl 2、AlCl 3、LiBr、NaBr、KBr、LiI、NaI、KI的一种或多种盐的熔融体。
  3. 根据权利要求1所述的常压下催化合成氨的方法，其特征在于，所述熔融盐与液态合金的质量比为(0.02～0.9):(0.1～0.98)。
  4. 根据权利要求1所述的常压下催化合成氨的方法，其特征在于，
    所述液态合金包括第一金属，所述第一金属的熔点为27～180.54℃；
    和/或，所述液态合金还包括第二金属，所述第二金属的熔点为29.77～630℃。
  5. 根据权利要求4所述的常压下催化合成氨的方法，其特征在于，
    所述第一金属为碱金属，所述碱金属选自Li、Na、K、Rb、Cs、Fr的一种或多种；
    和/或，所述第二金属为两性金属，所述两性金属选自Zn、Sn、Bi、Ga、In、Pb、Sb的一种或多种。
  6. 根据权利要求4所述的常压下催化合成氨的方法，其特征在于，所述第一金属和第二金属的摩尔比为(0.2～0.7):(0.3～0.8)。
  7. 根据权利要求1所述的常压下催化合成氨的方法，其特征在于，所述合成氨过程中反应的温度为300～600℃。
  8. 根据权利要求1所述的常压下催化合成氨的方法，其特征在于，在反应器中，所述熔融盐覆盖在液态合金的表面，所述氮气经氮气通气管通入液态合金中，所述氢气经氢气通气管通入液态合金中或通入熔融盐中。
  9. 根据权利要求1所述的常压下催化合成氨的方法，其特征在于，所述反应中还采用有第三金属，所述第三金属熔点比第一金属的熔点高，并且以固态形式存在于液态合金中，用于活化反应气体；所述第三金属为过渡金属。
  10. 根据权利要求9所述的常压下催化合成氨的方法，其特征在于，所述第三金属的比表面积为6500～8000cm 2/g。
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