CN111701596B - A method for preparing an atomic-scale active site catalyst for ammonia synthesis under mild conditions - Google Patents

Abstract

The invention discloses a preparation method of an atomic-level active site catalyst capable of catalyzing the synthesis of ammonia from nitrogen and hydrogen under mild conditions. The catalyst uses reduced graphene oxide (RGO) and molybdenum disulfide (MoS ₂ ) as composite supports, atomic The grade-dispersed platinum (Pt) is the active component, and the synthesized Pt‑MoS ₂ /RGO catalyst can significantly increase the rate of ammonia synthesis. The composite carrier has the strong adsorption performance of MoS ₂ and the good conductivity of RGO. MoS ₂ is deposited on the surface of RGO in a multi-layer form, and the two form a continuous electron transport network. The atomic-level Pt is evenly dispersed on the composite carrier, achieving The limit of atomic dispersion significantly improves the activity and stability of the ammonia synthesis reaction, enables the ammonia synthesis reaction to be driven under mild conditions, and reduces the energy consumption of the traditional ammonia synthesis industry.

Description

A method for preparing an atomic-scale active site catalyst for ammonia synthesis under mild conditions

technical field

The invention belongs to the field of preparation methods of ammonia synthesis catalysts, and in particular relates to a preparation method of an atomic-level active site catalyst that catalyzes synthesis of ammonia from nitrogen and hydrogen under mild conditions.

Background technique

The industrial synthesis of ammonia mainly relies on the energy-intensive Haber-Bosch process, which requires high temperature (300-550°C) and high-pressure (15-25MPa) operating conditions, and is a process with high energy consumption and high carbon dioxide emissions. In order to reduce energy consumption, people began to explore synthetic ammonia under mild conditions.

–Evans–Polanyi(BEP)关系的限制。但是，由于人们对于生物固氮机理还未完全了解，目前在温和条件下通过人工模拟生物固氮机制合成氨效率仍旧很低，并不适合大规模推广应用。在传统热催化过程中，多利用过渡金属催化剂催化氮气氢气合成氨气，但N₂分子解离会受到BEP关系的限制，N₂分子活化势垒与反应中间物种吸附能间存在矛盾关系，一个理想的合成氨催化剂应该能平衡两者之间的关系，在火山型曲线的顶部附近。在热催化合成氨过程中，需要打破BEP关系的限制，才有可能实现氨的高效合成。At present, the mainstream ammonia synthesis technologies under mild conditions include photocatalytic ammonia synthesis, electrocatalytic ammonia synthesis, artificial biomimetic catalytic ammonia synthesis and thermal catalytic ammonia synthesis. The use of solar photocatalytic ammonia synthesis technology is limited by the extremely high chemical stability of nitrogen molecules, which limits the adsorption, activation and dissociation process of nitrogen molecules, which leads to a low yield of photocatalytic ammonia synthesis. In addition, the extremely low solubility of nitrogen molecules in aqueous solution and the slow kinetic characteristics of water oxidation reaction limit the supply of reactant nitrogen and protons, further affecting the efficiency of photocatalytic ammonia synthesis. In the process of photocatalytic ammonia synthesis, there is also the problem that a large amount of sunlight in the visible-infrared spectrum region is wasted, and the efficiency of the solar-chemical energy conversion process is generally low. The electrocatalytic synthesis of ammonia realizes the synthesis of ammonia under mild conditions with the help of external input electric energy, which has the advantages of reducing energy consumption, clean and pollution-free. Researchers hope to improve the performance of electrocatalytic ammonia synthesis by modifying the catalyst and adding cocatalysts. However, the electrocatalytic ammonia synthesis is limited by the competitive hydrogen evolution reaction brought about by the close proximity of the hydrogen evolution reaction (HER) potential and the nitrogen reduction potential. The ammonia rate and faradaic current efficiency are still low. In the future, it is necessary to develop more efficient non-noble metal catalysts to improve the performance of electrochemical synthesis of ammonia. Artificial biomimetic catalytic synthesis of ammonia is an artificial method to simulate the Fe-Mo synergistic catalytic mechanism of biological nitrogenase in nature. Different from the traditional industrial ammonia synthesis process, biological nitrogen fixation can be carried out under mild conditions, not limited by the high-temperature and high-pressure catalytic conditions of heterogeneous thermal catalytic ammonia synthesis, so as to preferentially combine the hydrogenation mechanism to reduce the activation energy barrier of _N2 and avoid

– Limitations of the Evans–Polanyi (BEP) relationship. However, since the mechanism of biological nitrogen fixation is not yet fully understood, the efficiency of ammonia synthesis under mild conditions is still very low by artificially simulating the mechanism of biological nitrogen fixation, which is not suitable for large-scale application. In the traditional thermocatalytic process, transition metal catalysts are often used to catalyze the synthesis of ammonia from nitrogen and hydrogen, but the dissociation of _N2 molecules will be limited by the BEP relationship, and there is a contradictory relationship between the activation barrier of _N2 molecules and the adsorption energy of reaction intermediate species. An ideal A good ammonia synthesis catalyst should balance the relationship between the two, near the top of the volcano-type curve. In the process of thermocatalytic ammonia synthesis, it is necessary to break the limitation of the BEP relationship in order to achieve efficient synthesis of ammonia.

Contents of the invention

The purpose of the present invention is to overcome the shortcomings of the prior art and provide a method for preparing an atomic-level active site catalyst that catalyzes the synthesis of ammonia from nitrogen and hydrogen under mild conditions. The synthesized Pt-MoS ₂ /RGO catalyst can significantly improve the activity and stability of the ammonia synthesis reaction, enabling the ammonia synthesis reaction to be driven under mild conditions.

In order to achieve the above object, the present invention adopts the following technical solutions:

An atomic-level active site catalyst for catalyzing the synthesis of ammonia from nitrogen and hydrogen under mild conditions of the present invention comprises the following steps:

An atomic-level active site catalyst that catalyzes the synthesis of ammonia from nitrogen and hydrogen under mild conditions is characterized in that it comprises the following steps:

Step 1, preparation of reduced graphene oxide carrier:

(1) Under the condition of ice-water bath at 0-4°C, slowly add 50-100mL of concentrated sulfuric acid with a mass fraction of 98% into a round-bottomed flask under stirring with a glass rod, and then slowly add 1.0000-2.0000g NaNO ₃ and 1.0000-2.0000 g of graphite powder to form a mixed solution; when the temperature of the mixed solution drops to 0-4°C, slowly add 6.0000-12.0000g of potassium permanganate three times, and keep the temperature in an ice-water bath at 0-4°C after adding potassium permanganate for the last time After reacting for 20-40 minutes, the graphite oxide powder forms a sulfuric acid-graphite intercalation compound. The mixed solution obtained at this time is recorded as mixed solution A; remove the ice-water bath to raise the temperature of the mixed solution A to 20-25°C and continue the reaction for 1-3 hours. Oxidized sulfuric acid-graphite intercalation compound, the mixed solution that obtains this moment is denoted as mixed solution B;

(2) Add 60-100 mL of deionized water into the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) interacts with water to gradually increase the temperature of the mixed solution B to 96-100 °C, and then React for 0.3~0.7h under stirring with a glass rod, so that the sulfuric acid-graphite intercalation compound in the mixed solution B undergoes hydrolysis reaction; add 5~25mL of H ₂ O ₂ with a mass fraction of 10~30% to the hydrolyzed mixed solution Solution, remove excess potassium permanganate, and the obtained solution is recorded as mixed solution C; then transfer the mixed solution C to a high-speed centrifuge at 8000-12000r/min for 10-40min, pour off the supernatant of the centrifuge tube, and centrifuge The sediment in the lower layer of the tube is graphene oxide;

(3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 60-100mL deionized water and 30-60mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; Put it in a vacuum drying oven at 100-140°C for 8-10 hours to obtain the RGO carrier, wash the RGO carrier with deionized water three times, remove the residual hydrazine hydrate, put it in a vacuum drying oven again, and dry it at 100-140°C 1 to 3 hours to obtain pure RGO carrier;

Step 2. Preparation of MoS ₂ /RGO composite carrier:

(1) Put 0.0024-0.0048g of pure RGO carrier in a beaker, add 60-100mL of deionized water, and use an ultrasonic cleaner to sonicate for 10-30min to completely dissolve the pure RGO to obtain a mixed solution of RGO;

(2) 0.0242g sodium molybdate is added in the RGO solution and is evenly stirred with a glass rod to obtain the RGO mixed solution that is dissolved with sodium molybdate; 0.0076～0.0380g of thiourea is dissolved in 40～60mL deionized water to obtain a thiourea solution; Slowly drop the thiourea solution into the RGO mixed solution dissolved in sodium molybdate with a dropper, stir evenly with a glass rod, and then add hydrochloric acid solution with a mass fraction of 10% to adjust the pH value to 0.1-7 to obtain a mixed solution E;

(3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 100-250°C, take it out after 18-30 hours of growth, and transfer it to a high-speed centrifuge at 8000-12000r/min Centrifuge for 10-40min, then pour off the supernatant of the centrifuge tube, the sediment in the lower layer of the centrifuge tube is MoS ₂ /RGO, wash the MoS ₂ /RGO product with deionized water three times, and place it in a vacuum oven at 100-140°C Dry for 1-3 hours to obtain MoS ₂ /RGO composite carrier;

Step 3. Preparation of Pt-MoS ₂ /RGO catalyst:

(1) Put 0.0200-0.0800g of MoS ₂ /RGO composite carrier in a beaker, add 60-100mL of deionized water, and use an ultrasonic cleaner to sonicate for 10-30 minutes to completely dissolve MoS ₂ /RGO, and obtain a mixed solution F;

(2) Add 0.0020～0.0100g H ₂ PtCl ₆ ·6H ₂ O into 60～100mL mixed solution F and stir evenly, then add 60～100mL 0.5mol/ L sodium borohydride solution, stir evenly with a glass rod while adding dropwise, transfer the reacted mixture to a high-speed centrifuge at 8000-12000r/min for 10-40min, pour off the supernatant of the centrifuge tube, and the lower layer of the centrifuge tube It is the Pt-MoS ₂ /RGO catalyst; the Pt-MoS ₂ /RGO catalyst was washed three times with deionized water and absolute ethanol with a mass fraction of 10-30%, and then dried in a vacuum oven at 160-240°C for 2 ~5h, a pure Pt-MoS ₂ /RGO catalyst was obtained.

Compared with the prior art, the present invention has the following advantages:

(1) Atomic-scale dispersion of Pt. The active sites of MoS ₂ are only located on the edge of the MoS ₂ sheet, and the inside of the sheet is chemically inert, which limits its wide application. In order to further increase the number of active sites and increase the performance of ammonia synthesis, the introduction of atomic-level dispersion Pt. When the Pt dispersion reaches the atomic size, it can cause drastic changes in properties such as surface free energy, quantum size effect, unsaturated coordination environment, and metal-support interaction, which can significantly improve the ammonia synthesis performance of the catalyst.

(2) The spatial confinement effect of the catalyst support. The presence of a large number of dangling bonds on the edge of MoS ₂ can provide more adsorption sites for catalytic reactions, and the defects on the surface of reduced graphene oxide provide sites for the growth of MoS ₂ , so that MoS ₂ grows on the surface of reduced graphene oxide. Adding carbon materials into the hydrothermal reaction system can form a steric hindrance effect in the solution, which can inhibit the accumulation of MoS ₂ nanosheets, and at the same time limit the aggregation of MoS ₂ nanosheets in space, thereby improving the specificity of the material. Larger surface area, more active sites, and efficient catalytic synthesis of ammonia under mild conditions.

Detailed ways

In order to make the cases described in the invention easier to understand, the following specific examples are listed, but the scope of the claims of the present invention is not limited by these examples. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts fall within the protection scope of the present invention.

An atomic-level active site catalyst for catalyzing the synthesis of ammonia from nitrogen and hydrogen under mild conditions of the present invention comprises the following steps:

Step 1, preparation of reduced graphene oxide carrier:

(1) Under the condition of ice-water bath at 0-4°C, slowly add 50-100mL of concentrated sulfuric acid with a mass fraction of 98% into a round-bottomed flask under stirring with a glass rod, and then slowly add 1.0000-2.0000g NaNO ₃ and 1.0000-2.0000 g of graphite powder to form a mixed solution; when the temperature of the mixed solution drops to 0-4°C, slowly add 6.0000-12.0000g of potassium permanganate three times, and keep the temperature in an ice-water bath at 0-4°C after adding potassium permanganate for the last time After reacting for 20-40 minutes, the graphite oxide powder forms a sulfuric acid-graphite intercalation compound. The mixed solution obtained at this time is recorded as mixed solution A; remove the ice-water bath to raise the temperature of the mixed solution A to 20-25°C and continue the reaction for 1-3 hours. Oxidize the sulfuric acid-graphite intercalation compound, and the mixed solution obtained at this time is denoted as mixed solution B.

(2) Add 60-100 mL of deionized water into the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) interacts with water to gradually increase the temperature of the mixed solution B to 96-100 °C, and then React for 0.3-0.7 h under stirring with a glass rod, so that the sulfuric acid-graphite intercalation compound in the mixed solution B undergoes a hydrolysis reaction. Add 5 to 25 _mL of _H2O2 solution with a mass fraction of 10 to 30% to the mixed solution after hydrolysis, remove excess potassium permanganate, and record the obtained solution as mixed solution C; then transfer mixed solution C to 8000 Centrifuge in a high-speed centrifuge at ~12000r/min for 10-40min, pour off the supernatant of the centrifuge tube, and the sediment in the lower layer of the centrifuge tube is graphene oxide.

(3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 60-100mL deionized water and 30-60mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; Put it in a vacuum drying oven at 100-140°C for 8-10 hours to obtain the RGO (reduced graphene oxide) carrier, wash the RGO carrier with deionized water three times, remove the residual hydrazine hydrate, and then put it in the vacuum drying oven again. Dry at 100-140° C. for 1-3 hours to obtain a pure RGO carrier.

Step 2. Preparation of MoS ₂ /RGO composite carrier:

(1) Put 0.0024-0.0048 g of pure RGO carrier in a beaker, add 60-100 mL of deionized water, and use an ultrasonic cleaner to sonicate for 10-30 minutes to completely dissolve the pure RGO to obtain a mixed solution of RGO.

(2) 0.0242g sodium molybdate is added in the RGO solution and is evenly stirred with a glass rod to obtain the RGO mixed solution that is dissolved with sodium molybdate; 0.0076～0.0380g of thiourea is dissolved in 40～60mL deionized water to obtain a thiourea solution; Slowly drop the thiourea solution into the RGO mixed solution dissolved in sodium molybdate with a dropper, stir evenly with a glass rod, and then add 10% hydrochloric acid solution to adjust the pH to 0.1-7 to obtain mixed solution E.

(3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 100-250°C, take it out after 18-30 hours of growth, and transfer it to a high-speed centrifuge at 8000-12000r/min Centrifuge for 10-40min, then pour off the supernatant of the centrifuge tube, the sediment in the lower layer of the centrifuge tube is MoS ₂ /RGO, wash the MoS ₂ /RGO product with deionized water three times, and place it in a vacuum oven at 100-140°C Dry for 1-3 hours to obtain the MoS ₂ /RGO composite carrier.

Step 3. Preparation of Pt-MoS ₂ /RGO catalyst:

(1) Put 0.0200-0.0800g of MoS ₂ /RGO composite carrier in a beaker, add 60-100mL of deionized water, and use an ultrasonic cleaner to sonicate for 10-30min to completely dissolve the MoS ₂ /RGO, and obtain a mixed solution F.

(2) Add 0.0020～0.0100g H ₂ PtCl ₆ ·6H ₂ O into 60～100mL mixed solution F and stir evenly, then add 60～100mL 0.5mol/ L sodium borohydride solution, stir evenly with a glass rod while adding dropwise, transfer the reacted mixture to a high-speed centrifuge at 8000-12000r/min for 10-40min, pour off the supernatant of the centrifuge tube, and the lower layer of the centrifuge tube It is the Pt-MoS ₂ /RGO catalyst; the Pt-MoS ₂ /RGO catalyst was washed three times with deionized water and absolute ethanol with a mass fraction of 10-30%, and then dried in a vacuum oven at 160-240°C for 2 ~5h, a pure Pt-MoS ₂ /RGO catalyst was obtained.

Example 1

Step 1, preparation of reduced graphene oxide carrier:

(1) Under the condition of 0 ℃ of ice-water bath, the concentrated sulfuric acid that 50mL mass fraction is 98% is slowly added in the round-bottomed flask under glass rod stirring, then slowly add 1.0000g NaNO ₃ and 1.0000g graphite powder form mixed solution; After the temperature of the mixed solution is lowered to 0°C, slowly add 6.0000g of potassium permanganate three times. After adding potassium permanganate for the last time, react at a constant temperature in an ice-water bath at 0°C for 20 minutes. The graphite oxide powder forms a sulfuric acid-graphite interlayer compound. The mixed solution obtained at this time was designated as mixed solution A; the ice-water bath was removed to raise the temperature of mixed solution A to 20°C, and then the reaction was continued for 1 h to deeply oxidize the sulfuric acid-graphite intercalation compound. The mixed solution obtained at this time was designated as mixed solution B.

(2) Add 60mL of deionized water to the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) interacts with water to gradually raise the temperature of the mixed solution B to 96°C, and then stir it with a glass rod After reacting for 0.3h, the sulfuric acid-graphite intercalation compound in the mixed solution B undergoes a hydrolysis reaction. Add 5mL of _H2O2 solution with a mass fraction of 10% to the mixed solution after hydrolysis, remove excess potassium permanganate, and record the obtained solution as mixed solution C; then transfer _the mixed solution C to a high-speed 8000r/min Centrifuge in a centrifuge for 10 minutes, pour off the supernatant of the centrifuge tube, and the sediment in the lower layer of the centrifuge tube is graphene oxide.

(3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 60mL deionized water and 30mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; place the mixed solution D at 100°C React in a vacuum drying oven for 8 hours to obtain an RGO (reduced graphene oxide) carrier, wash the RGO carrier three times with deionized water, remove the residual hydrazine hydrate, put it in a vacuum drying oven again, and dry it at 100°C for 1 hour to obtain pure The RGO vector.

Step 2. Preparation of MoS ₂ /RGO composite carrier:

(1) Put 0.0024 g of pure RGO carrier in a beaker, add 60 mL of deionized water, and use an ultrasonic cleaner to sonicate for 10 min to completely dissolve the pure RGO to obtain a mixed solution of RGO.

(2) Add 0.0242g sodium molybdate in the RGO solution and stir with a glass rod to obtain a mixed solution of RGO that is dissolved with sodium molybdate; dissolve 0.0076g of thiourea in 40mL deionized water to obtain a thiourea solution; The thiourea solution was slowly dropped into the RGO mixed solution dissolved in sodium molybdate, stirred evenly with a glass rod, and then 10% hydrochloric acid solution was added to adjust the pH value to 0.1 to obtain the mixed solution E.

(3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 100°C, take it out after 18 hours of growth, transfer it to a high-speed centrifuge at 8000r/min for 10 minutes, and then pour Remove the supernatant of the centrifuge tube, and the precipitate in the lower layer of the centrifuge tube is the MoS ₂ /RGO product. The MoS ₂ /RGO product is washed three times with deionized water, and dried in a vacuum oven at 100°C for 1 hour to obtain the MoS ₂ /RGO composite carrier.

Step 3. Preparation of Pt-MoS ₂ /RGO catalyst:

(1) Put 0.0200g of MoS ₂ /RGO composite carrier in a beaker, add 60mL of deionized water, and use an ultrasonic cleaner to sonicate for 10 minutes to completely dissolve the MoS ₂ /RGO, and obtain a mixed solution F.

(2) Add 0.0020g H ₂ PtCl ₆ ·6H ₂ O into 60mL mixed solution F and stir evenly, then add 60mL 0.5mol/L sodium borohydride solution dropwise with a dropper at 20°C, dropwise add While stirring evenly with a glass rod, the obtained reaction mixture was transferred to an 8000r/min high-speed centrifuge for 10 minutes, and the supernatant of the centrifuge tube was discarded, and the lower layer of the centrifuge tube was the Pt-MoS ₂ /RGO catalyst; the Pt -The MoS ₂ /RGO catalyst was washed three times with deionized water and 10% absolute ethanol, and then dried in a vacuum oven at 160°C for 2 hours to obtain a pure Pt-MoS ₂ /RGO catalyst.

Activity evaluation of Pt-MoS ₂ /RGO catalyst:

Catalyst activity measurement was carried out in a high temperature and high pressure activity testing device, and 2 mg of catalyst was taken and placed within the range of the isothermal zone of the reaction. The catalyst was activated at 300° C. for 2 h in the reaction apparatus, and then stabilized under the test conditions for 2 h. The measurement conditions are: temperature 250°C, pressure 5MPa, reaction gas N ₂ : H ₂ ratio 1:3 (volume ratio), space velocity 15000h ^-1 . The ammonia concentration in the outlet reaction gas was measured by chemical absorption method to be 131.4mmol g _cat ^-1 h ^-1 , and the ammonia synthesis activity of the atomic-level active site catalyst did not decrease significantly within 54 hours. It shows that the atomic-level active site ammonia synthesis catalyst prepared by the present invention has better catalytic activity and stability under mild conditions.

Example 2

Step 1, preparation of reduced graphene oxide carrier:

(1) under 2 ℃ of ice-water bath conditions, the concentrated sulfuric acid that 60mL mass fraction is 98% is slowly added in the round-bottomed flask under glass rod stirring, then slowly add 1.5000g NaNO ₃ and 1.5000g graphite powder form mixed solution; After the temperature of the mixed solution drops to 2°C, slowly add 9.0000g of potassium permanganate three times. After the last addition of potassium permanganate, react at a constant temperature in an ice-water bath at 2°C for 30 minutes. The graphite oxide powder forms a sulfuric acid-graphite interlayer compound. The mixed solution obtained at this time is designated as mixed solution A; remove the ice-water bath to raise the temperature of mixed solution A to 22°C and continue the reaction for 1.5 hours to deeply oxidize the sulfuric acid-graphite interlayer compound. The mixed solution obtained at this time is designated as mixed solution B .

(2) Add 70mL deionized water to the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) reacts with water to gradually raise the temperature of the mixed solution B to 98°C, and then stir it with a glass rod React for 0.5h to make the sulfuric acid-graphite intercalation compound in the mixed solution B undergo a hydrolysis reaction. Add 10 mL of _H2O2 solution with a mass fraction of 15% to the mixed solution after hydrolysis, remove excess potassium permanganate, and record the obtained _solution as mixed solution C; then transfer mixed solution C to a high-speed Centrifuge in a centrifuge for 20 minutes, pour off the supernatant of the centrifuge tube, and the sediment in the lower layer of the centrifuge tube is graphene oxide.

(3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 70mL deionized water and 45mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; put the mixed solution D at 120°C React in a vacuum drying oven for 9 hours to obtain an RGO (reduced graphene oxide) carrier, wash the RGO carrier three times with deionized water, remove the residual hydrazine hydrate, put it in a vacuum drying oven again, and dry it at 120°C for 2 hours to obtain pure The RGO vector.

Step 2. Preparation of MoS ₂ /RGO composite carrier:

(1) Put 0.0036 g of pure RGO carrier into a beaker, add 70 mL of deionized water, and use an ultrasonic cleaner to sonicate for 20 min to completely dissolve the pure RGO to obtain a mixed solution of RGO.

(2) Add 0.0242g sodium molybdate in the RGO solution and stir with a glass rod to obtain a RGO mixed solution that is dissolved in sodium molybdate; dissolve 0.0152g of thiourea in 50mL deionized water to obtain a thiourea solution; The thiourea solution was slowly dropped into the RGO mixed solution dissolved in sodium molybdate, stirred evenly with a glass rod, and then 10% hydrochloric acid solution was added to adjust the pH value to 3 to obtain the mixed solution E.

(3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 150°C, take it out after 24 hours of growth, transfer it to a high-speed centrifuge at 10,000 r/min for 20 minutes, and then pour Remove the supernatant from the centrifuge tube, and the precipitate in the lower layer of the centrifuge tube is MoS ₂ /RGO. Wash the MoS ₂ /RGO product three times with deionized water, and dry it in a vacuum oven at 120°C for 1.5 hours to obtain the MoS ₂ /RGO composite carrier.

Step 3. Preparation of Pt-MoS ₂ /RGO catalyst:

(1) Put 0.0400g of MoS ₂ /RGO composite carrier in a beaker, add 70mL of deionized water, and use an ultrasonic cleaner to sonicate for 20 minutes to completely dissolve the MoS ₂ /RGO, and obtain a mixed solution F.

(2) Add 0.0040g H ₂ PtCl ₆ ·6H ₂ O into 70mL mixed solution F and stir evenly, then add 70mL 0.5mol/L sodium borohydride solution dropwise with a dropper at 22°C, dropwise add While stirring evenly with a glass rod, the obtained reacted mixture was transferred to a high-speed centrifuge at 10,000r/min for 20 minutes, and the supernatant of the centrifuge tube was discarded, and the lower layer of the centrifuge tube was the Pt-MoS ₂ /RGO catalyst; the Pt -The MoS ₂ /RGO catalyst was washed three times with deionized water and 20% absolute ethanol respectively, and then dried in a vacuum oven at 180°C for 3 hours to obtain a pure Pt-MoS ₂ /RGO catalyst.

Activity evaluation of Pt-MoS ₂ /RGO catalyst:

Catalyst activity measurement was carried out in a high temperature and high pressure activity testing device, and 2 mg of catalyst was taken and placed within the range of the isothermal zone of the reaction. The catalyst was activated at 300° C. for 2 h in the reaction apparatus, and then stabilized under the test conditions for 2 h. The measurement conditions are: temperature 250°C, pressure 5MPa, reaction gas N ₂ : H ₂ ratio 1:3 (volume ratio), space velocity 15000h ^-1 . The ammonia concentration in the outlet reaction gas was measured by chemical absorption method to be 742.6mmol g _cat ^-1 h ^-1 , and the catalytic activity of the atomic-level active site catalyst for ammonia synthesis did not decrease significantly within 54 hours. It shows that the atomic-level active site ammonia synthesis catalyst prepared by the present invention has better catalytic activity and stability under mild conditions.

Example 3

Step 1, preparation of reduced graphene oxide carrier:

(1) under 3 ℃ of ice-water bath conditions, the concentrated sulfuric acid that 90mL mass fraction is 98% is slowly added in the round-bottomed flask under glass rod stirring, then slowly adds 1.8000g NaNO ₃ and 1.8000g graphite powder form mixed solution; After the temperature of the mixed solution is lowered to 3°C, slowly add 10.0000g of potassium permanganate three times. After adding potassium permanganate for the last time, react at a constant temperature in an ice-water bath at 3°C for 35 minutes. The graphite oxide powder forms a sulfuric acid-graphite interlayer compound. The mixed solution obtained at this time is designated as mixed solution A; remove the ice-water bath to raise the temperature of mixed solution A to 24°C and continue to react for 2.5 hours to deeply oxidize the sulfuric acid-graphite interlayer compound. The mixed solution obtained at this time is designated as mixed solution B .

(2) Add 90mL deionized water into the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) interacts with water to gradually raise the temperature of the mixed solution B to 99°C, and then stir with a glass rod The reaction was carried out for 0.6h, so that the sulfuric acid-graphite intercalation compound in the mixed solution B was hydrolyzed. Add 20mL of _H2O2 solution with a mass fraction of 28% to the mixed solution after hydrolysis, remove excess potassium permanganate, and record the obtained solution as _mixed solution C; then transfer mixed solution C to a high-speed 11000r/min Centrifuge in a centrifuge for 30 minutes, pour off the supernatant of the centrifuge tube, and the sediment in the lower layer of the centrifuge tube is graphene oxide.

(3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 90mL deionized water and 50mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; put the mixed solution D at 130°C React in a vacuum drying oven for 9.5 hours to obtain an RGO (reduced graphene oxide) carrier, wash the RGO carrier three times with deionized water, remove the residual hydrazine hydrate, put it in a vacuum drying oven again, and dry it at 130°C for 2.5 hours. Obtain pure RGO carrier.

Step 2. Preparation of MoS ₂ /RGO composite carrier:

(1) Put 0.0040 g of pure RGO carrier in a beaker, add 90 mL of deionized water, and use an ultrasonic cleaner to sonicate for 28 min to completely dissolve the pure RGO to obtain a mixed solution of RGO.

(2) Add 0.0242g of sodium molybdate into the RGO solution and stir with a glass rod to obtain a RGO mixed solution of sodium molybdate; dissolve 0.0290g of thiourea in 58mL of deionized water to obtain a thiourea solution; The thiourea solution was slowly dropped into the RGO mixed solution dissolved in sodium molybdate, stirred evenly with a glass rod, and then 10% hydrochloric acid solution was added to adjust the pH value to 5 to obtain the mixed solution E.

(3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 220°C, take it out after 26 hours of growth, transfer it to a high-speed centrifuge at 11,000 r/min for 35 minutes, and then pour Remove the supernatant of the centrifuge tube, and the precipitate in the lower layer of the centrifuge tube is the MoS ₂ /RGO product. The MoS ₂ /RGO product is washed three times with deionized water, and dried in a vacuum oven at 130°C for 2.5 hours to obtain MoS ₂ /RGO Composite carrier.

Step 3. Preparation of Pt-MoS ₂ /RGO catalyst:

(1) Put 0.0600g of MoS ₂ /RGO composite carrier in a beaker, add 90mL of deionized water, and use an ultrasonic cleaner to sonicate for 28 minutes to completely dissolve the MoS ₂ /RGO, and obtain a mixed solution F.

(2) Add 0.0080g H ₂ PtCl ₆ ·6H ₂ O into 90mL mixed solution F and stir evenly, then add 90mL 0.5mol/L sodium borohydride solution dropwise with a dropper at 24°C, dropwise add While stirring evenly with a glass rod, the obtained reacted mixture was transferred to a high-speed centrifuge at 11000r/min for 35 minutes, and the supernatant of the centrifuge tube was discarded, and the lower layer of the centrifuge tube was the Pt-MoS ₂ /RGO catalyst; the Pt - The MoS ₂ /RGO catalyst was washed three times with deionized water and 28% absolute ethanol, and then dried in a vacuum oven at 220°C for 4 hours to obtain a pure Pt-MoS ₂ /RGO catalyst.

Activity evaluation of Pt-MoS ₂ /RGO catalyst:

Catalyst activity measurement was carried out in a high temperature and high pressure activity testing device, and 2 mg of catalyst was taken and placed within the range of the isothermal zone of the reaction. The catalyst was activated at 300° C. for 2 h in the reaction apparatus, and then stabilized under the test conditions for 2 h. The measurement conditions are: temperature 250°C, pressure 5MPa, reaction gas N ₂ : H ₂ ratio 1:3 (volume ratio), space velocity 15000h ^-1 . The ammonia concentration in the outlet reaction gas was measured by chemical absorption method to be 631.2mmol g _cat ^-1 h ^-1 , and the catalytic activity of the atomic-level active site catalyst for ammonia synthesis did not decrease significantly within 54 hours. It shows that the atomic-level active site ammonia synthesis catalyst prepared by the present invention has better catalytic activity and stability under mild conditions.

Example 4

Step 1, preparation of reduced graphene oxide carrier:

(1) under 4 ℃ of ice-water bath conditions, the concentrated sulfuric acid that 100mL mass fraction is 98% is slowly added in the round-bottomed flask under glass rod stirring, then slowly add 2.0000g NaNO ₃ and 2.0000g graphite powder form mixed solution; After the temperature of the mixed solution is lowered to 4°C, slowly add 12.0000g of potassium permanganate three times. After adding potassium permanganate for the last time, react at a constant temperature in an ice-water bath at 4°C for 40 minutes. The graphite oxide powder forms a sulfuric acid-graphite interlayer compound. The mixed solution obtained at this time was designated as mixed solution A; the ice-water bath was removed to raise the temperature of mixed solution A to 25°C, and then the reaction was continued for 3 hours to deeply oxidize the sulfuric acid-graphite intercalation compound. The mixed solution obtained at this time was designated as mixed solution B.

(2) Add 100mL deionized water to the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) interacts with water to gradually raise the temperature of the mixed solution B to 100°C, and then stir it with a glass rod After reacting for 0.7h, the sulfuric acid-graphite intercalation compound in the mixed solution B undergoes a hydrolysis reaction. Add 25mL of _H2O2 solution with a mass fraction of 30% to the mixed solution after hydrolysis, remove excess potassium permanganate, and record the obtained solution as mixed solution C _; then transfer mixed solution C to a high-speed 12000r/min Centrifuge in a centrifuge for 40 minutes, pour off the supernatant of the centrifuge tube, and the sediment in the lower layer of the centrifuge tube is graphene oxide.

(3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 100mL deionized water and 60mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; put the mixed solution D at 140°C React in a vacuum drying oven for 10 hours to obtain an RGO (reduced graphene oxide) carrier, wash the RGO carrier three times with deionized water, remove the residual hydrazine hydrate, put it in a vacuum drying oven again, and dry it at 140°C for 3 hours to obtain pure The RGO vector.

Step 2. Preparation of MoS ₂ /RGO composite carrier:

(1) Put 0.0048 g of pure RGO carrier in a beaker, add 100 mL of deionized water, and use an ultrasonic cleaner to sonicate for 30 min to completely dissolve the pure RGO to obtain a mixed solution of RGO.

(2) Add 0.0242g of sodium molybdate into the RGO solution and stir evenly with a glass rod to obtain a mixed solution of RGO having sodium molybdate; dissolve 0.0380g of thiourea in 60mL of deionized water to obtain a thiourea solution, and use a The thiourea solution was slowly dropped into the RGO mixed solution dissolved in sodium molybdate, stirred evenly with a glass rod, and then 10% hydrochloric acid solution was added to adjust the pH value to 7 to obtain the mixed solution E.

(3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 250°C, take it out after 30 hours of growth, transfer it to a high-speed centrifuge at 12,000 r/min for 40 minutes, and then pour Remove the supernatant of the centrifuge tube, and the precipitate in the lower layer of the centrifuge tube is the MoS ₂ /RGO product. The MoS ₂ /RGO product is washed three times with deionized water, and dried in a vacuum oven at 140°C for 3 hours to obtain the MoS ₂ /RGO composite carrier.

Step 3. Preparation of Pt-MoS ₂ /RGO catalyst:

(1) Put 0.0800g of MoS ₂ /RGO composite carrier in a beaker, add 100mL of deionized water, and use an ultrasonic cleaner to sonicate for 30 minutes to completely dissolve the MoS ₂ /RGO, and obtain a mixed solution F.

(2) Add 0.0100g H ₂ PtCl ₆ ·6H ₂ O into 100mL mixed solution F and stir evenly, then add 100mL 0.5mol/L sodium borohydride solution dropwise with a dropper at 25°C, dropwise While stirring evenly with a glass rod, the obtained reaction mixture was transferred to a high-speed centrifuge at 12000r/min for 40 minutes, and the supernatant of the centrifuge tube was discarded, and the lower layer of the centrifuge tube was the Pt-MoS ₂ /RGO catalyst; the Pt -The MoS ₂ /RGO catalyst was washed three times with deionized water and 30% absolute ethanol, and then dried in a vacuum oven at 240°C for 5 hours to obtain a pure Pt-MoS ₂ /RGO catalyst.

Activity evaluation of Pt-MoS ₂ /RGO catalyst:

Catalyst activity measurement was carried out in a high temperature and high pressure activity testing device, and 2 mg of catalyst was taken and placed within the range of the isothermal zone of the reaction. The catalyst was activated at 300° C. for 2 h in the reaction apparatus, and then stabilized under the test conditions for 2 h. The measurement conditions are: temperature 250°C, pressure 5MPa, reaction gas N ₂ : H ₂ ratio 1:3 (volume ratio), space velocity 15000h ^-1 . The ammonia concentration in the outlet reaction gas was measured by chemical absorption method to be 398.1mmol g _cat ^-1 h ^-1 , and the catalytic activity of the atomic-level active site catalyst for ammonia synthesis did not decrease significantly within 54 hours. It shows that the atomic-level active site ammonia synthesis catalyst prepared by the present invention has better catalytic activity and stability under mild conditions.

From the above four examples, it can be seen that the catalyst Pt-MoS ₂ /RGO prepared by the present invention for ammonia synthesis with atomic active sites under mild conditions has excellent performance, and has high ammonia synthesis activity and stability.

The above embodiments are provided only for the purpose of describing the present invention, not to limit the scope of the present invention. Within the scope of the gist of the present invention, flexible changes to the experimental conditions and methods all belong to the protection scope of the present invention, and the scope of the present invention is defined by the appended claims.

Claims (1)

1. a catalytic nitrogen hydrogen synthesis ammonia atomic level active site catalyst under mild conditions, is characterized in that comprising the following steps: Step 1, preparation of reduced graphene oxide carrier: (1) Under the condition of ice-water bath at 0-4°C, slowly add 50-100mL of concentrated sulfuric acid with a mass fraction of 98% into a round-bottomed flask under stirring with a glass rod, and then slowly add 1.0000-2.0000g NaNO ₃ and 1.0000-2.0000 g of graphite powder to form a mixed solution; when the temperature of the mixed solution drops to 0-4°C, slowly add 6.0000-12.0000g of potassium permanganate three times, and keep the temperature in an ice-water bath at 0-4°C after adding potassium permanganate for the last time After reacting for 20-40 minutes, the graphite oxide powder forms a sulfuric acid-graphite intercalation compound. The mixed solution obtained at this time is recorded as mixed solution A; remove the ice-water bath to raise the temperature of the mixed solution A to 20-25°C and continue the reaction for 1-3 hours. Oxidized sulfuric acid-graphite intercalation compound, the mixed solution that obtains this moment is denoted as mixed solution B; (2) Add 60-100 mL of deionized water into the mixed solution B, use the heat released when the residual concentrated sulfuric acid added in step (1) interacts with water to gradually increase the temperature of the mixed solution B to 96-100 °C, and then React for 0.3~0.7h under stirring with a glass rod, so that the sulfuric acid-graphite intercalation compound in the mixed solution B undergoes hydrolysis reaction; add 5~25mL of H ₂ O ₂ with a mass fraction of 10~30% to the hydrolyzed mixed solution Solution, remove excess potassium permanganate, and the obtained solution is recorded as mixed solution C; then transfer the mixed solution C to a high-speed centrifuge at 8000-12000r/min for 10-40min, pour off the supernatant of the centrifuge tube, and centrifuge The sediment in the lower layer of the tube is graphene oxide; (3) Move the graphene oxide in the lower layer of the centrifuge tube to a beaker, add 60-100mL deionized water and 30-60mL pure hydrazine hydrate to the beaker in turn, and stir with a glass rod to obtain a mixed solution D; Put it in a vacuum drying oven at 100-140°C for 8-10 hours to obtain the RGO carrier, wash the RGO carrier with deionized water three times, remove the residual hydrazine hydrate, put it in a vacuum drying oven again, and dry it at 100-140°C 1 to 3 hours to obtain pure RGO carrier; Step 2. Preparation of MoS ₂ /RGO composite carrier: (1) Put 0.0024-0.0048g of pure RGO carrier in a beaker, add 60-100mL of deionized water, and use an ultrasonic cleaner to sonicate for 10-30min to completely dissolve the pure RGO to obtain a mixed solution of RGO; (2) 0.0242g sodium molybdate is added in the RGO solution and is evenly stirred with a glass rod to obtain the RGO mixed solution that is dissolved with sodium molybdate; 0.0076～0.0380g of thiourea is dissolved in 40～60mL deionized water to obtain a thiourea solution; Slowly drop the thiourea solution into the RGO mixed solution dissolved in sodium molybdate with a dropper, stir evenly with a glass rod, and then add hydrochloric acid solution with a mass fraction of 10% to adjust the pH value to 0.1-7 to obtain a mixed solution E; (3) Transfer the mixed solution E to a polytetrafluoroethylene-lined stainless steel reactor, grow MoS ₂ at 100-250°C, take it out after 18-30 hours of growth, and transfer it to a high-speed centrifuge at 8000-12000r/min Centrifuge for 10-40min, then pour off the supernatant of the centrifuge tube, the sediment in the lower layer of the centrifuge tube is MoS ₂ /RGO, wash the MoS ₂ /RGO product with deionized water three times, and place it in a vacuum oven at 100-140°C Dry for 1-3 hours to obtain MoS ₂ /RGO composite carrier; Step 3. Preparation of Pt-MoS ₂ /RGO catalyst: (1) Put 0.0200-0.0800g of MoS ₂ /RGO composite carrier in a beaker, add 60-100mL of deionized water, and use an ultrasonic cleaner to sonicate for 10-30 minutes to completely dissolve MoS ₂ /RGO, and obtain a mixed solution F; (2) Add 0.0020～0.0100g H ₂ PtCl ₆ ·6H ₂ O into 60～100mL mixed solution F and stir evenly, then add 60～100mL 0.5mol/ L sodium borohydride solution, stir evenly with a glass rod while adding dropwise, transfer the reacted mixture to a high-speed centrifuge at 8000-12000r/min for 10-40min, pour off the supernatant of the centrifuge tube, and the lower layer of the centrifuge tube It is the Pt-MoS ₂ /RGO catalyst; the Pt-MoS ₂ /RGO catalyst was washed three times with deionized water and absolute ethanol with a mass fraction of 10-30%, and then dried in a vacuum oven at 160-240°C for 2 ~5h to obtain a pure Pt-MoS ₂ /RGO catalyst, and then conduct an activity test on the Pt-MoS ₂ /RGO catalyst: the catalyst activity measurement is carried out in a high-temperature and high-pressure activity testing device, and 2 mg of the catalyst is placed in a reaction isothermal Within the scope of the zone, activate the catalyst in the reaction device at 300°C for 2h, and then stabilize it under the test conditions for 2h; the measurement conditions are: temperature 250°C, pressure 5MPa, reaction gas N ₂ : H ₂ volume ratio 1:3, With a space velocity of 15000h ^-1 , the ammonia synthesis activity of the Pt-MoS ₂ /RGO catalyst did not decrease significantly within 54h.