阅读说明：本技术 电催化还原氮气的合成氨方法及所用催化剂 (Ammonia synthesis method by electrocatalytic reduction of nitrogen and used catalyst ) 是由 钱超 王舒月 周少东 阮建成 陈新志 于 2020-05-25 设计创作，主要内容包括：本发明公开了一种用于电催化合成氨的过渡金属单原子催化剂,采用光沉积法,按照如下步骤制备而得：利用碳粉、硝酸钠、浓硫酸、高锰酸钾、去离子水等,制备作为载体的碳材料；在水与有机溶剂组成的混合溶剂中分散TiO<Sub>2</Sub>和碳材料,再加入过渡金属盐均匀混合,接着置于光照下室温反应,反应所得物冷冻固化后,再真空冷冻干燥,得过渡金属单原子催化剂。本发明还同时提供了利用过渡金属单原子催化剂进行的电催化还原氮气的合成氨方法。采用本发明方法合成氨具有工艺简单经济、环境友好、收率高等特点。(The invention discloses a transition metal monatomic catalyst for electrocatalytic synthesis of ammonia, which is prepared by adopting a photo-deposition method according to the following steps: preparing a carbon material serving as a carrier by using carbon powder, sodium nitrate, concentrated sulfuric acid, potassium permanganate, deionized water and the like; dispersing TiO in mixed solvent of water and organic solvent 2 And adding a transition metal salt into the carbon material, uniformly mixing, then placing the mixture under illumination to react at room temperature, freezing and solidifying a reaction product, and then carrying out vacuum freeze drying to obtain the transition metal monatomic catalyst. The invention also provides a method for utilizing transition metal monogenThe sub-catalyst is used for electrocatalytic reduction of nitrogen to synthesize ammonia. The method for synthesizing ammonia has the characteristics of simple and economical process, environmental friendliness, high yield and the like.)

1. Transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia, characterized in that: the preparation method comprises the following steps of:

1) preparing a carbon material as a carrier:

uniformly stirring 1g of carbon powder, (1 +/-0.1) g of sodium nitrate and (46 +/-2) mL of concentrated sulfuric acid in an ice bath, adding (6 +/-0.6) g of potassium permanganate, and reacting at 30-40 ℃ for (1 +/-0.2) h; then adding 40 +/-10 mL of deionized water, and heating and reacting at 90 +/-10 ℃ for 30-50 min;

stopping the reaction after the reaction time is up; carrying out post-treatment on the reaction product to obtain a carbon material;

2) and preparing a transition metal monoatomic catalyst:

firstly, mixing water and an organic solvent to form a mixed solvent;

according to TiO₂: carbon material 1: (1. + -. 0.1) by weight, dispersing TiO in the mixed solvent₂Mixing with carbon material, adding transition metal salt, mixing, and standing under illumination at room temperatureReacting for 1-6 h, freezing and solidifying the reaction product, and then carrying out vacuum freeze drying to obtain a transition metal monoatomic catalyst;

in the transition metal single-atom catalyst, the load capacity of the transition metal element is 0.1-10%.

2. The transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia according to claim 1, characterized in that:

in the step 2): the transition metal is vanadium (V), niobium (Nb) or tantalum (Ta).

3. The transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia according to claim 2, characterized in that:

the transition metal salt is sodium metavanadate, sodium pyrovanadate, sodium orthovanadate, niobium oxalate, potassium niobate, tantalum pentachloride, potassium fluotantalate or potassium metatantalate.

4. A transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia according to claim 3, characterized in that:

in the step 2): the organic solvent is methanol, ethanol, isopropanol, butanol, dioxane, dioxolane, diethylene glycol dimethyl ether and ethylene glycol dimethyl ether;

the volume ratio of the water to the organic solvent is 1: 1-10: 1.

5. A transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia according to any one of claims 1 to 4, characterized in that:

in the step 2): freezing and solidifying the reaction product at-15-25 ℃ for 1.5-2.5 h, and then freezing and drying in vacuum to obtain the transition metal monatomic catalyst.

6. A transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia according to any one of claims 1 to 4, characterized in that:

in the step 1), after the reaction time is up, adding (100 +/-10) mL of deionized water to stop the reaction, and then adding (6 +/-1) mL of hydrogen peroxide solution; repeatedly cleaning with 3% hydrochloric acid and deionized water until the pH of the cleaning solution is neutral; and (4) carrying out ultrasonic dispersion and vacuum freeze drying on the cleaned product to obtain the carbon material.

7. A method for synthesizing ammonia by carrying out electrocatalytic reduction on nitrogen by using the transition metal monatomic catalyst according to any one of claims 1 to 6, wherein an electrocatalysis device of a three-electrode system is used, a reference electrode RE and a working electrode WE are arranged in a cathode electrolytic cell, a counter electrode CE is arranged in an anode electrolytic cell, and electrolytes are filled in the cathode electrolytic cell and the anode electrolytic cell; the method is characterized in that:

and arranging a transition metal monatomic catalyst coating on the surface of the working electrode WE, introducing nitrogen into the electrolyte in the cathode electrolytic cell until the electrolyte is saturated, and applying voltage to carry out electrolysis.

Technical Field

The invention relates to an electrochemical synthesis method of ammonia.

Background

Ammonia is an important inorganic chemical product and is a main raw material in the fertilizer industry and basic organic chemical industry. In addition, ammonia is also a carbon-free energy carrier, the combustion products of the ammonia are nitrogen and water, the mass produced ammonia can replace most of the liquid fuel consumption at present, the ammonia is considered to be one of green sustainable fuel substances with a promising future, and the application of the ammonia in the fields of heavy transportation, power generation, distributed energy storage and the like is actively developed all over the world. The amount of ammonia is huge, and the demand of ammonia is continuously increased along with the development of industry and agriculture. At present, most of the sources of ammonia in the world are synthetic ammonia except a small amount of byproducts recovered from coke oven gas.

The traditional industrial method for synthesizing ammonia is a Haber-Bosch method, which requires harsh conditions of high temperature and high pressure (about 300-500 ℃, 20.26-30.40 MPa), and the energy consumption caused by the method accounts for about 1.4% of the total energy consumption of the whole world every year. The raw material hydrogen is prepared mainly by decomposing fossil energy, the consumed natural gas in the process accounts for 3-5% of the total natural gas consumption in the world, and a large amount of greenhouse gas is generated.

Up to now, the synthesis of ammonia has undergone three generations of technological changes. The first generation of ammonia synthesis technology involved the use of carbon sequestration or compensation to reduce the net carbon impact of ammonia production to zero. The carbon sequestration in ammonia production adds cost and plant complexity based on existing H-B technology, represents only a transitional solution, contributing to the establishment of ammonia markets beyond the fertilizer and chemical industries.

The second generation of ammonia synthesis technology still adopts the H-B method, but the hydrogen used as the raw material is the hydrogen produced by electrolyzing water. Siemens technicians produce hydrogen by using completely renewable electric energy generated by a 20kW wind turbine through a Proton Exchange Membrane (PEM) electrolytic cell, and about 30 kilograms of ammonia are formed every day (Physical Chemistry Chemical Physics,2012,14(3):1235-1245.), so that the problem that excessive natural gas is consumed in the original process for preparing hydrogen is effectively solved, but the defect of overlarge input energy of high temperature and high pressure in the synthesis process still exists.

The third generation of ammonia synthesis technology electrically reduces nitrogen gas to ammonia directly or indirectly, and the technology completely breaks away from the H-B process. The synthesis reaction is driven by electrochemical reduction, and the hydrogen source is from water. The reaction conditions (normal temperature and normal pressure) in the process are mild, the raw material sources are rich, and the electric energy can be from sustainable energy sources such as solar energy, wind energy and the like, so that the method has an important development prospect. Haiyuan Zou reports that an ultrathin chlorine-doped graphene catalyst is used for electrocatalytic reduction of nitrogen, and the ammonia yield is 10.7 mu g.h under the potential of-0.45V (vs RHE)^-1·cm^-2mg^-1 _Cat.The Faraday efficiency is 8.7% (ACCCATALYSIS, 2019,9(12): 10649-. Hongjie Yu reports a film material mAu₃Pd/NF is used for electrocatalytic nitrogen reduction, and the ammonia yield is 24.02 mug.h^-1·cm^-2mg^-1 _Cat.Faraday efficiency of 18.16% (ACS Applied Materials)&Interfaces,2020,12(1): 436-. Wenjie Zang reports that a nitrogen-doped carbon-supported copper monatomic catalyst is used for electrocatalytic nitrogen reduction, and the obtained ammonia yield is 49.3 mug.h in HCl electrolyte^-1·cm^-2mg^-1 _Cat.The Faraday efficiency is 11.7% (ACCCATALYSIS, 2019,9(11): 10166-10173), but the method has the problem that the test result is inaccurate due to the introduction of nitrogen impurities, and the source of nitrogen elements in the product is verified.

In summary, the early H-B method for synthesizing ammonia has the problems of harsh conditions, large energy consumption and the like, and the recent electrochemical synthesis of ammonia has the problems of low yield, low faraday efficiency, expensive catalyst materials, non-uniform test standards and the like, and the efficient green production of ammonia is to be realized, which not only relates to the design of an electrochemical system, but also relates to the development of efficient and economic catalysts.

Disclosure of Invention

The invention solves the technical problem of providing a mild, efficient and green ammonia synthesis method.

In order to solve the technical problems, the invention provides a transition metal monatomic catalyst for synthesizing ammonia by electro-catalysis, which is prepared by adopting a photo-deposition method according to the following steps:

1) preparation of a carbon material (carbon material support) as a support:

uniformly stirring 1g of carbon powder (high-purity carbon powder with the purity of more than or equal to 95%), 1 +/-0.1 g of sodium nitrate and 46 +/-2 mL of concentrated sulfuric acid (sulfuric acid solution with the mass concentration of 95-98%) in an ice bath, adding 6 +/-0.6 g of potassium permanganate, and reacting at 30-40 ℃ for 1 +/-0.2 h; then adding 40 +/-10 mL of deionized water, and heating and reacting at 90 +/-10 ℃ for 30-50 min;

stopping the reaction after the reaction time is up; carrying out post-treatment on the reaction product to obtain a carbon material;

description of the drawings: the carbon material is sealed and then stored at 16-25 ℃;

2) and preparing a transition metal monoatomic catalyst:

firstly, mixing water and an organic solvent to form a mixed solvent;

according to TiO₂: carbon material 1: (1. + -. 0.1) by weight, dispersing TiO in the mixed solvent₂Adding a transition metal salt into the carbon material, uniformly mixing, then placing the mixture under illumination for reacting at room temperature for 1-6 h, freezing and solidifying a reaction product, and then carrying out vacuum freeze drying to obtain a transition metal monatomic catalyst;

in the transition metal monatomic catalyst, the loading amount of the transition metal element (as an active center) is 0.1 to 10%, preferably 1 to 10%, more preferably 1 to 5%, and most preferably 1%.

In general: 50mg TiO/min₂Mixing 20-40 ml of mixed solvent;

as an improvement of the transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia of the present invention, in the step 2): the transition metal is vanadium (V), niobium (Nb) or tantalum (Ta). The transition metal salt is sodium metavanadate, sodium pyrovanadate, sodium orthovanadate, niobium oxalate, potassium niobate, tantalum pentachloride, potassium fluotantalate or potassium metatantalate.

As a further improvement of the transition metal monatomic catalyst for electrocatalytic synthesis of ammonia of the present invention, in said step 2): the organic solvent is methanol, ethanol, isopropanol, butanol, dioxane, dioxolane, diethylene glycol dimethyl ether and ethylene glycol dimethyl ether;

the volume ratio of the water to the organic solvent is 1: 1-10: 1.

As a further improvement of the transition metal monatomic catalyst for electrocatalytic synthesis of ammonia of the present invention, in said step 2): freezing and solidifying the reaction product at-15-25 ℃ for 1.5-2.5 h, and then freezing and drying in vacuum to obtain the transition metal monatomic catalyst.

As a further improvement of the transition metal monatomic catalyst for the electrocatalytic synthesis of ammonia of the present invention, in the step 1), after the reaction time is reached, (100 + -10) mL of deionized water is added to stop the reaction, and then, (6 + -1) mL of hydrogen peroxide solution is added; then repeatedly cleaning with 3% (mass%) hydrochloric acid and deionized water until the pH of the cleaning solution is neutral; and ultrasonically dispersing the cleaned product (the dispersion time is 1-3 h), and carrying out vacuum freeze drying to obtain the carbon material.

Description of the drawings: the hydrogen peroxide acts to reduce the residual oxidizing agent, potassium permanganate.

The invention also provides a method for synthesizing ammonia by carrying out electrocatalysis reduction on nitrogen by using the transition metal monatomic catalyst, which comprises the steps of using an electrocatalysis device of a three-electrode system, arranging a reference electrode RE and a working electrode WE in a cathode electrolytic cell, arranging a counter electrode CE in an anode electrolytic cell, and filling electrolyte in the cathode electrolytic cell and the anode electrolytic cell; and arranging a transition metal monatomic catalyst coating on the surface of the working electrode WE, introducing nitrogen into the electrolyte in the cathode electrolytic cell until the electrolyte is saturated, and applying voltage to carry out electrolysis.

The voltage is, for example, -0.4V (vs RHE), and a chronoamperometric test (CA) is carried out for 2h after electrolysisAnd obtaining the ammonia-containing electrolyte. The ammonia yield can reach as high as 136.4 mug.h^-1·cm^-2mg^-1 _Cat.The Faraday efficiency was 53.5%.

According to the preparation method, Graphene Oxide (GO) is prepared as a carbon material carrier for loading the catalyst, and then the types of organic solvents are screened in the process of preparing the monatomic catalyst, so that the proportion of water and the organic solvents in a mixed solvent is optimized, the electronic structure of the catalyst is favorably adjusted, and the catalytic activity of the catalyst is enhanced. The invention also screens the transition metal as the active center, optimizes the loading amount of the active center of the catalyst and is beneficial to improving the activity of a single site in the catalyst.

A schematic diagram of an electrochemical device of the present invention is shown in fig. 1.

The invention relates to an electro-catalysis nitrogen ammonia synthesis method, which develops a novel high-efficiency monatomic catalyst with transition metal as an active center, and electro-catalysis nitrogen is reduced to form ammonia; has the following technical advantages:

1. the prepared monoatomic transition metal catalyst has high activity, the preparation method is relatively simple, and the yield of the catalytic synthesis ammonia is high;

2. in the electrochemical process, nitrogen and water (mainly water in electrolyte) are used as raw materials, so that the raw materials are wide in source, no pollution gas is generated, and the environment friendliness in the production process is ensured;

3. the method for synthesizing ammonia has the characteristics of simple and economical process, environmental friendliness, high yield and the like.

In conclusion, the invention establishes a technical development route for synthesizing ammonia by directly utilizing transition metal monatomic catalyst through comparing the reaction characteristics of different routes and comprehensively considering the difficulty of industrialization of the reaction process, wherein nitrogen and water are used as raw materials. The key technical difficulty is the development of high-efficiency monatomic catalyst.

Drawings

The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.

FIG. 1 is a schematic view of an electrochemical device;

RE represents a reference electrode, WE represents a working electrode, and CE represents a counter electrode.

Detailed Description

The invention will be further described with reference to specific examples, but the scope of the invention is not limited thereto:

hereinafter, the reaction is carried out under conventional stirring conditions; the conditions of vacuum freeze-drying are as follows: vacuum degree of 0.001MPa and-50 ℃.

Example one, preparation of a carbon material (carbon material support) as a support:

uniformly mixing and stirring 1g of high-purity carbon powder (the purity is more than or equal to 95 percent), 1g of sodium nitrate and 46mL of concentrated sulfuric acid (sulfuric acid solution with the mass concentration of 95-98 percent) in an ice bath (about 0 ℃), adding 6g of potassium permanganate, then reacting for 1h at 30-40 ℃, continuously adding 40mL of deionized water, putting the mixture into a water bath with the temperature of 90 ℃ for heating for 30-50 min, taking out the mixture, adding 100mL of deionized water to stop the reaction, and adding 6mL of hydrogen peroxide solution (hydrogen peroxide solution with the mass concentration of 30 percent, wherein the hydrogen peroxide solution has the effect of reducing residual potassium permanganate serving as an oxidant); finally, the washing was repeated with 3% (mass%) hydrochloric acid and deionized water until the pH of the washing solution was close to neutral. Adding about 20ml of water into the cleaned product, performing ultrasonic dispersion for 2h (ultrasonic dispersion is 25 ℃, power is 400W, frequency is 20kHz), and performing vacuum freeze drying for 20 h to obtain a carbon material, wherein the carbon material is sealed and stored at 16-25 ℃.

The following examples all employ this carbon material.