CN112295563A

CN112295563A - Co-based catalyst for breaking limitation relation of synthetic ammonia reaction and preparation method and application thereof

Info

Publication number: CN112295563A
Application number: CN202011243696.2A
Authority: CN
Inventors: 王秀云; 李玲玲; 江莉龙
Original assignee: Fuzhou University
Current assignee: Fuzhou University
Priority date: 2020-11-10
Filing date: 2020-11-10
Publication date: 2021-02-02
Anticipated expiration: 2040-11-10
Also published as: CN112295563B

Abstract

The invention discloses a Co-based catalyst for breaking the reaction restriction relation of synthetic ammonia, and a preparation method and application thereof, wherein Co with a 3d electronic structure and nonmetal elements (X = B, N, F and the like) with a 2p electronic structure are adopted to form a binary intermetallic CoX compound. Through the hybridization of a d-p orbit, the position and the width of the center of a Co 3d band are effectively adjusted, the bonding strength of N-containing intermediate species on active metal is effectively reduced, the desorption of the N-containing intermediate species at low temperature is promoted, and the N existing in the traditional ammonia synthesis catalyst is broken through₂The dissociation adsorption energy and the desorption energy of the intermediate species containing N are in a restrictive relationship, and ammonia synthesis under mild conditions is realized. The ammonia synthesis rate was 10.8 mmol g at 400 ℃ and 3 MPa^‑1h^‑1The preparation method of the catalyst is simple and easy to operate, and the catalyst shows good performanceAnd (4) industrial application prospect.

Description

Co-based catalyst for breaking limitation relation of synthetic ammonia reaction and preparation method and application thereof

Technical Field

The invention belongs to the field of catalyst material preparation, and particularly relates to a Co-based catalyst for breaking the limitation relation of synthetic ammonia reaction, and a preparation method and application thereof.

Background

Ammonia is an important component of food and fertilizers, is also a direct or indirect component of many medicines, and is a basic chemical raw material for maintaining human life. In addition, ammonia is a hydrogen-rich substance (17.6 wt%), which is an ideal hydrogen energy carrier, and is also strongly concerned. In view of the many important roles of ammonia in human production and life, researchers have focused on developing a highly efficient ammonia synthesis catalyst under mild conditions that is inexpensive and readily available. But due to N₂The N in N is very stable, so that the nitrogen hydrogenation reaction is very difficult in dynamics, and N is very stable₂Becomes also the rate-determining step in the ammonia synthesis reaction. The researchers found N₂Activation generally requires electron transfer of the active metal to N₂On the opposite bond trajectory to activate the N ≡ N bond. This motivates us to consider the catalytic synthesis of ammonia by the formation of binary intermetallics of non-metallic elements with the transition metal Co. The intermetallic compound has the advantages that: (1) charge state of transition metal Co vs N₂Has strong modification effect on the activation; (2) active centers are introduced into the lattice structure, so that the aggregation of the active centers can be prevented; (3) the structure is relatively simple. Specifically, occupied and unoccupied d orbitals of Co are hybridized with non-metallic p orbitals, the position and the width of the center of a d-band are influenced, the electron cloud density of Co metal is increased, more electrons can be provided for N [ identical to ] N, N [ identical to ] N bonds are weakened, and N [ identical to ] N bonds are reduced₂Activation of the energy barrier of the reaction to promote N₂Adsorption of (3); simultaneously, the binding energy of the N-containing intermediate species on the active metal is reduced, the desorption of the N-containing intermediate species at low temperature is promoted, and N is broken₂The reaction performance is improved by a restrictive relationship between the dissociative adsorption energy of (a) and the desorption energy of the N-containing intermediate species. And secondly, the active metal Co in the binary intermetallic compound is regularly and periodically arranged in crystal lattices, and Co atoms and nonmetal atoms interact with each other to hinder the migration, agglomeration and growth of the active metal Co and prolong the service life of the catalyst. Therefore, it is of great significance to introduce the binary intermetallic compound into the ammonia synthesis reaction.However, the use of these intermetallic compounds as catalysts for ammonia synthesis under mild conditions has not been reported.

Disclosure of Invention

The invention aims to provide a Co-based catalyst for breaking the reaction limitation relation of synthetic ammonia and a preparation method and application thereof. Co in the catalyst reacts with nonmetal elements to form a binary intermetallic compound, wherein a 3d orbit of the Co is hybridized with a nonmetal 2p orbit to influence a d-band center of active metal Co, so that N in the reaction process is influenced₂The adsorption activation and the desorption of the intermediate species containing N break the restrictive relation in the reaction of synthesizing ammonia and improve the reaction efficiency of synthesizing ammonia.

In order to achieve the purpose, the invention adopts the following technical scheme:

the catalyst is a binary intermetallic CoX compound formed by Co with a 3d electronic structure and a nonmetal element with a 2p electronic structure, wherein the nonmetal element X is at least one of boron, nitrogen, carbon, fluorine and silicon; boron, silicon, fluorine are preferred.

[1](1) When the non-metallic element X is boron, the preparation method of the Co-based catalyst specifically comprises the following steps: dissolving a cobalt precursor in a deionized water solution mixed with 50% methanol, dropwise adding a reducing agent solution into the mixture under a nitrogen atmosphere, and continuing to react for a period of time after the dropwise addition is finished, wherein the excessive reducing agent solution and the reaction time are used for completely reducing metal ions. The black product formed subsequently was washed with deionized water and anhydrous methanol. To obtain Co₂A B-based catalyst.

(2) When the non-metallic element X is fluorine, the preparation method of the Co-based catalyst specifically comprises the following steps: dissolving a cobalt precursor, ammonium fluoride and urea in ethylene glycol, and forming a uniform solution under vigorous stirring. Then, the obtained solution is transferred to a polytetrafluoroethylene hydrothermal kettle for solvothermal reaction. After completion of the reaction, the pink product was collected and washed repeatedly with water and ethanol. Obtaining CoF₂A base catalyst.

(3) When the non-metallic element X is silicon, the preparation method of the Co-based catalyst specifically comprises the following steps: will be provided withThe cobalt precursor, the silicon powder and the magnesium powder are mixed and put into a stainless steel high-pressure reaction kettle for high-temperature calcination. After naturally cooling to room temperature, washing with ethanol, distilled water and hydrochloric acid solution to remove impurities. Finally drying in a vacuum drying oven to obtain Co₂A Si-based catalyst.

Further, in (1), (2), and (3), the cobalt precursor is any one of cobalt acetate, cobalt nitrate, cobalt carbonate, cobalt chloride, and cobaltosic oxide.

Further, (1) the reduction reaction time is 0-2 h, the washing frequency is 1-4 times, preferably 1 h and 3 times, and the reducing agent is any one of sodium borohydride or potassium borohydride, preferably sodium borohydride.

Further, (2) the solvothermal reaction temperature is 120-200 ℃, and the solvothermal time is 10-24 h. Preferably 200 ℃ for 24 h. The number of product washes is 1-4, preferably 3.

Further, the temperature rise rate of the calcination process in the step (3) is 5-20 ℃ min^-1The temperature is 700 ℃ and 800 ℃, and the temperature is kept for 5-20 h. The drying temperature is 50-80 deg.C, and the drying time is 8-12 h. The temperature rise rate of the calcination process is preferably 10 ℃ min^-1Keeping the temperature at 750 ℃ for 10 h; the drying temperature is 50 ℃ and the drying time is 12 h.

The Co-based catalyst for breaking the limitation relation of the synthetic ammonia reaction can be used for catalyzing the ammonia synthetic reaction under mild conditions.

The invention has the following remarkable advantages:

1. the invention provides a preparation method and application of a Co-based catalyst for breaking the limitation relation of ammonia synthesis reaction, which is applied to ammonia synthesis reaction under mild condition for the first time.

2. According to the invention, a binary intermetallic compound is formed by Co and nonmetal elements, the position and width of the center of a 3d zone of Co are adjusted by d-p hybridization, the electron density of Co metal is improved, the catalytic reaction energy barrier is reduced, the ammonia synthesis reaction rate is improved, and the service life of the catalyst is prolonged.

3. The ammonia synthesis rate of the d-p hybridized Co-based ammonia synthesis catalyst provided by the invention is superior to that of the traditional Co-based ammonia synthesis catalyst, and the catalyst has the advantages of simple preparation method, low cost and good thermal stability, and is very beneficial to industrial production.

Drawings

FIG. 1 is an XRD pattern of catalysts prepared in examples 1-3 and comparative example 1;

FIG. 2 is a graph showing the reaction rates of ammonia synthesis in the presence of catalysts prepared in examples 1 to 3 and comparative example 1 at 3 MPa and different temperatures;

FIG. 3 is a graph showing the reaction rates of ammonia synthesis in the catalysts obtained in examples 1 to 3 and comparative example 1 at 350 ℃ and different pressures.

Detailed Description

In order to make the content of the present invention more comprehensible, the technical solutions of the present invention are further described below with reference to specific embodiments, and the exemplary embodiments and descriptions thereof are only used for explaining the present invention and are not meant to limit the present invention.

Example 1

Co₂Preparation of B

Cobalt acetate (0.498 g, 0.1M) was dissolved in a solution of deionized water mixed with 50% methanol, then sodium borohydride solution (0.228 g, 1M) was added dropwise to the above solution under a stream of nitrogen, and the reaction was continued for 1 h after the addition was complete. The black product formed was then washed thoroughly three times with deionized water and three times with anhydrous methanol. To obtain Co₂A B-based catalyst.

Example 2

CoF₂Preparation of

Cobalt nitrate (0.29 g), ammonium fluoride (0.12 g) and urea (0.3 g) were dissolved in 50 mL of ethylene glycol to form a homogeneous solution under vigorous stirring. The resulting solution was then transferred to a teflon hydrothermal kettle and placed in an oven at 200 ℃ for 20 h. After cooling to room temperature, the pink product was collected and washed three times with water and ethanol, respectively. Obtaining CoF₂A base catalyst.

Example 3

Co₂Preparation of Si

Silicon powder (0.29 g), cobaltosic oxide powder (0.80 g) and magnesium powder (2.40 g) were mixed and placed in a 20 mL stainless steel high pressure reactor. Then the autoclave is sealed at 10 ℃ for min^-1Is heated from room temperature to 750 ℃ and maintained for 10 h. After the autoclave was naturally cooled to room temperature, the obtained precipitate was washed with ethanol, distilled water, and a hydrochloric acid solution (0.50M) for 3 times. Finally dried in vacuo at 50 ℃ overnight to give Co₂A Si-based catalyst.

Comparative example 1

Preparation of Co powder

The Co catalyst is prepared by dispersing cobaltosic oxide powder (1 g) in an organic solvent ethylene glycol (20 mL), transferring the mixture into a high-pressure reaction kettle, carrying out solvent heat treatment at 200 ℃ for 10 hours in a hydrogen atmosphere (1MPa), and then separating, washing and drying the powder.

Evaluation of catalyst Performance

FIG. 1 is an X-ray powder diffraction pattern of various cobalt-based ammonia synthesis catalysts, and it can be seen from FIG. 1 that XRD patterns correspond to the crystalline phases of cobalt powder, cobalt boride, cobalt silicide and cobalt fluoride, respectively. The following materials were used in the mass ratios of 1: 1 mixed catalyst 0.3 g each, mass space velocity 60,000 mL g^-1h^-1) Measuring the ammonia synthesis rate in a continuous flow miniature fixed bed reactor, and measuring NH in tail gas₃The change in concentration was determined by ion chromatography (Thermo Scientific, DIONEX, ICS-600) with a reaction gas composition of: 75% H₂+25%N₂And (4) mixing the gases. The ammonia synthesis reaction rate of the catalyst was measured at 3 MPa and different temperatures, and the test results are shown in FIG. 2. From FIG. 2, the active Co of the catalyst can be seen₂B> CoF₂>Co≥CoSi₂. The ammonia synthesis reaction rate of the catalyst was measured at 350 c under different pressures and the results are shown in fig. 3. As can be seen from FIG. 3, the Co-X catalyst exhibited higher ammonia synthesis performance over a wider pressure range (0.2-5 MPa), and the ammonia gas generation rate increased with increasing pressure.

The above description is only a preferred embodiment of the present invention, and all equivalent changes and modifications made in accordance with the claims of the present invention should be covered by the present invention.

Claims

1. A Co-based catalyst for breaking the reaction restriction relationship in ammonia synthesis, characterized in that: the catalyst is a binary intermetallic CoX compound formed by Co with a 3d electronic structure and a nonmetal element with a 2p electronic structure, wherein the nonmetal element X is at least one of boron, nitrogen, carbon, fluorine and silicon.

2. Co-based catalyst for breaking the reaction restriction in ammonia synthesis according to claim 1, characterized in that: when the non-metal element X is boron, the preparation method specifically comprises the following steps: dissolving a cobalt precursor in 50wt% methanol water solution, then dropwise adding a reducing agent solution into the mixed solution under the nitrogen atmosphere, and cleaning a black product formed after reaction by using deionized water and anhydrous methanol to obtain Co₂A B-based catalyst.

3. Co-based catalyst for breaking the reaction restriction in ammonia synthesis according to claim 2, characterized in that: the reducing agent is sodium borohydride or potassium borohydride.

4. Co-based catalyst for breaking the reaction restriction in ammonia synthesis according to claim 1, characterized in that: when the non-metal element X is fluorine, the preparation method specifically comprises the following steps: dissolving a cobalt precursor, ammonium fluoride and urea in ethylene glycol, forming a uniform solution under vigorous stirring, transferring the obtained solution into a polytetrafluoroethylene hydrothermal kettle, carrying out solvothermal reaction, collecting a pink product after the reaction is finished, and repeatedly washing the pink product with water and ethanol to obtain CoF₂A base catalyst.

5. Co-based catalyst for breaking the reaction restriction in the synthesis of ammonia according to claim 4, characterized in that: the solvothermal reaction temperature is 120-200 ℃, and the solvothermal reaction time is 10-24 h.

6. Co-based catalyst for breaking the reaction restriction in ammonia synthesis according to claim 1, characterized in that: when the non-metal element X is silicon, the preparation method specifically comprises the following steps: mixing the cobalt precursor, the silicon powder and the magnesium powder, putting the mixture into a high-pressure reaction kettle for high-temperature calcination, naturally cooling the mixture to room temperature, washing the mixture with ethanol, distilled water and hydrochloric acid solution to remove impurities, and drying the mixture to obtain Co₂A Si-based catalyst.

7. Co-based catalyst for breaking the reaction restriction in the synthesis of ammonia according to claim 6, characterized in that: the heating rate of the high-temperature calcination is 5-20 ℃ min^-1The high-temperature calcination temperature is 700-800 ℃, and the high-temperature calcination time is 5-20 h.

8. Use of a Co-based catalyst according to any one of claims 1 to 7 for breaking the reaction restriction in ammonia synthesis.