CN108262034B

CN108262034B - Catalyst, preparation method thereof and application thereof in ammonia synthesis under normal pressure and low temperature

Info

Publication number: CN108262034B
Application number: CN201710001631.9A
Authority: CN
Inventors: 包信和; 崔亭亭; 潘秀莲
Original assignee: Dalian Institute of Chemical Physics of CAS
Current assignee: Dalian Institute of Chemical Physics of CAS
Priority date: 2017-01-03
Filing date: 2017-01-03
Publication date: 2021-03-30
Anticipated expiration: 2037-01-03
Also published as: CN108262034A

Abstract

The invention discloses a synthetic ammonia catalyst used under normal pressure and low temperature conditions and a preparation method thereof, wherein the catalyst comprises a transition metal nanocluster and a main group metal electronic assistant which are dispersed in a carbon nanotube cavity with the inner diameter of about 1-4 nm. Due to the unique limited-area environment formed by the one-dimensional nanometer straight-through pore canal of the carbon nano tube, the transition metal nanocluster catalyst wrapped by the carbon nano tube has high synthetic ammonia activity and stability under the conditions of normal pressure and low temperature, and lays a foundation for further developing the synthetic ammonia catalyst under the mild condition.

Description

Catalyst, preparation method thereof and application thereof in ammonia synthesis under normal pressure and low temperature

Technical Field

The invention relates to a catalyst technology, and provides a preparation method of a catalyst for normal-pressure low-temperature 25-350 ℃ synthetic ammonia reaction and application of the catalyst in normal-pressure low-temperature synthetic ammonia reaction.

Background

Ammonia is an important inorganic chemical product, with an annual production capacity of about 0.16 million tons in industry, higher than that of any other chemical (M.Kitano, ethyl., Ammonia synthesis using a stable electrolyte an electron donor and reversible hydrogen store, Nat.chem.2012,4(11):934-]80% of the fertilizer is used for producing chemical fertilizers, and 20% of the fertilizer is used for producing raw materials of other chemicals. N is a radical of₂Is one of the most stable simple substances in nature, the N ≡ N bond energy is up to 945kJ/mol, and the bond needs extremely high energy for breaking, so that N is bonded with N₂The direct conversion of molecules to ammonia is kinetically extremely hindered. Currently, the Harber-Bosch process is mainly used for industrial ammonia synthesis, i.e. N is performed under the conditions of high temperature (300-₂And H₂Ammonia is generated on the surface of the Fe-based catalyst, the extremely harsh reaction conditions cause the process to consume huge energy, and the energy consumed every year is the total energy consumption of the world1.4% of the amount [ Cornelis J.M., et al, Challenges in reduction of dinotgrogen by proton and electron transfer, chem. Soc. Rev.2014,43(15),5183-]. Therefore, the development of a novel high-efficiency ammonia synthesis catalyst under mild conditions is of great significance and is always a great challenge in the chemical industry.

In recent years, some progress has been made with respect to ammonia synthesis catalysts under mild conditions. In nature, nitrogen-fixing microorganisms can directly reduce nitrogen in the air into ammonia under normal temperature and pressure by using nitrogen-fixing enzymes in the bodies. Biological nitrogen fixation greatly exceeds chemical nitrogen fixation in both required conditions and production capacity, so that researchers at home and abroad carry out a series of deep researches on chemical simulation biological nitrogen fixation from the sixties of the last century. The most successful Catalytic system is the Molybdenum-based Dinitrogen complex system, Schrock, 2003 reports a mononuclear Molybdenum-based Dinitrogen complex catalyst with triamino amine as a ligand, and the direct Catalytic Reduction of nitrogen under normal temperature and pressure conditions is realized for the first time with the aid of a suitable proton source and a reducing agent [ D.V. Yandulov, et al, Catalytic Reduction of Dinitrogen to Ammonia at a Single Molybdenum catalyst Center, science.2003,301(5629),76-78]. Using a similar approach, Nishibayashi in 2011 reported a double molybdenum-double nitrogen complex catalyst with a PNP pincer ligand that successfully increased the number of nitrogen conversions on the center of the single molybdenum from 4 in the Schrock system to 12[ K.Arashiba, et al, A molybdenum complex ligands to the catalytic reduction of dinitrogen intra-ammonia, Nat.chem.2011,3(2),120-]. Although these homogeneous systems achieve direct catalytic conversion of nitrogen at ambient temperature and pressure, the conversion is stoichiometric with respect to the metal center. Further, the calculated energy required for reducing a unit mole of nitrogen to two moles of ammonia was 580kJ mol each depending on the proton source and the reducing agent used therefor^-1(Schrock System) and 700kJ mol^-1(Nishibayashi system) [ F.Neese, et al., The Yangulov/Schrock Cycle and The nitrogene Reaction: Pathways of nitrogene fire cooled by Density function Theory, Angew.chem., int.Ed.,2006,45(2),196-]Are all higher than Harber-Bosch process (485kJ mol)^-1). Besides homogeneous catalysis systems, heterogeneous catalysis ammonia synthesis under mild conditions has also made some research progress. Kitano et al have recently reported that a series of electronic compound-supported Ru catalysts show excellent catalytic activity in the reaction of synthesizing Ammonia at normal pressure [ Ammonia synthesis a stable electronic as an electron donor and reversible hydrogen store, nat. chem.2012,4(11): 934-; essential role of hydride in ruthenium-based ammonia synthesis catalysts, chem.Sci.2016,7,4036-4043]. Ammonia is synthesized at normal pressure and the reaction temperature is as low as 200 deg.c, and the specific activity of the catalyst is 0.71mol NH₃mol Ru^-1h^-1Is obviously superior to other Ru-based catalysts reported in the past. However, the relatively high price of ruthenium is a great obstacle to its widespread use.

Patent application CN106064097A discloses a normal temperature ammonia synthesis catalyst, which relates to the use of bismuth subcarbonate BSC load gold Au catalyst, but the content of noble metal gold in the catalyst is up to 20% -80%, and the catalyst preparation method is tedious, involves five steps, and the production cost is too high, therefore, the application in industry has great limitation.

Patent application CN103977828A discloses a synthetic ammonia catalyst containing nitrogen compounds of main group elements and related carriers and additives, but the reaction temperature of the involved synthetic ammonia is very high (400 ℃), and the energy consumption is high.

In summary, the research on the ammonia synthesis catalyst under mild conditions has made some progress, but the industrialization is still far from being realized. How to develop and design a synthetic ammonia catalyst with more excellent performance by taking the existing basic theory and technology as reference and reduce the cost of the catalyst still needs further research.

Carbon nanotubes are of great interest to researchers in the field of catalysis due to their unique one-dimensional nanoscale through-channels, highly graphitized tube walls, high specific surface area, and good electrical and thermal conductors. The carbon nanotube is different from other carbon materials in the quasi-one-dimensional nano-scale tubular cavity structure, and the nano-reactor made of the carbon material can be obtained by assembling the metal catalyst into the nano-pore channel. The multi-walled carbon nanotube-wrapped metal catalysts reported in the literature in recent years all exhibit superior catalytic performance in a series of catalytic Reactions compared to the catalyst outside the tubes, showing a synergistic confinement effect [ x.l.pan, x.h.bao, Reactions over catalysts confined in carbon nanotubes, chem.commu.s. 2008,6271-6281; X.L.Pan, X.H.Bao, The Effects of The cosmetic carbonate on catalysts, Acc.chem.Res.,2011, 553-. The synergistic confinement effect is caused by the unique structure of the carbon nano tube and comprises several aspects, firstly, the space restriction effect of the carbon nano tube ensures that the metal nano particles wrapped by the carbon nano tube are not easy to grow in the reaction process, and the stability of the particles is kept at higher temperature; in addition, due to the curvature of the graphene surface, pi electrons migrate from inside to outside of the tube, a unique electronic confinement environment is formed in the tube, and the structure and Properties of a series of metals or metal Oxides can be modified through interaction [ W Chen, X.L.Pan, X.H.Bao, Tuning of Redox Properties of Iron and Iron Oxides via Encapsulation with Carbon Nanotubes, J.Am.chem.Soc.2007,129, 7421-7426; J.P.Xiao, X.L.Pan, X.H.Bao, heated Fundamentals of defined Catalysis in Carbon Nanotubes, J.Am.chem.Soc.2015,137,477-482 ]; more interesting is the ability of the reactant molecules to be selectively enriched in the space of the Carbon nanotube domains [ J, Guan, X.L.Pan, Syngas differentiation Induced by design in Carbon Nanotubes: A Combined First-Principles and Monte Carlo Study, J.Phys.chem.C.2009,113(52), 21687-; zhang, X.L.Pan, X.H.Bao, enhanced chemical reactions in a defined hydrophosphonic environment an NMR study of benzene hydroxylation in carbon nanotubes, chem.Sci.,2012,4(3), 1075) 1078, thereby reducing the apparent pressure of the reaction. Small-diameter carbon nanotubes have smaller diameters, larger curvatures, and exhibit stronger confinement effects in catalytic reactions than multi-walled tubes [ j.p.xiao, x.l.pan, x.h.bao, Size-dependence of carbon nanotube confinement in catalysis, chem.sci.2016 ]. However, the small-diameter carbon tube confinement catalyst is limited by the preparation technology and has been reported recently and is a bottleneck.

Disclosure of Invention

One of the technical problems to be solved by the present invention is to provide a catalystThe catalyst is used in the synthetic ammonia reaction under the conditions of normal pressure and low temperature, shows high activity and high stability, and has the specific activity as high as 1279-47631mmol NH under the conditions of normal pressure and 25-350 DEG C₃mol metal^-1h^-1And the activity of the catalyst is kept stable for more than 140 hours on-line monitoring.

The second technical problem to be solved by the invention is to provide an efficient preparation method for preparing the transition metal nanocluster catalyst doped with main group metal and limited by the small-caliber carbon nanotubes.

In order to solve one of the above technical problems, the technical scheme adopted by the invention is as follows:

a catalyst, comprising:

the carbon nano tube is used as a carrier; (carbon nanotubes as the remainder)

The transition metal nanoclusters dispersed in the carbon nanotube cavity are catalytic active metals, and the mass content of the transition metal nanoclusters is 0.1-10%; the transition metal refers to one or more of iron, ruthenium, rhenium and platinum;

the main group metal simple substance or the main group metal alloy is taken as an auxiliary agent, and the mass content is 5-50%.

The mass content of the catalytic active metal is preferably 0.1-2%; the mass content of the auxiliary agent is preferably 20-30%.

The carbon nano tube is a thin-wall carbon nano tube with the inner diameter of a tube cavity of 1-4nm, and preferably 1-2 nm.

The auxiliary agent is IA, IIA and IIIA group elements, and comprises one or more than two of Li, Na, K, Rb, Cs, Mg, Ca, Ba and Al; for transition metal iron, the preferred auxiliaries are Na, K; for rhenium and platinum, the preferred auxiliaries are Na, K, Rb and Cs; for ruthenium, preferred promoters are Ca, Ba, Cs.

A method of preparing a catalyst comprising the steps of:

a) dispersing the original carbon nano tube in a mixed solution of concentrated sulfuric acid and concentrated nitric acid, performing ultrasonic treatment for 4-7h, performing purification and truncation, and performing vacuum freeze drying for 70-100h to obtain an opened and truncated carbon nano tube which is marked as s-TWCNT.

b) The s-TWCNT is treated for 3-5H at 700-1000 ℃ in H2 or Ar to remove the oxygen-containing functional groups generated on the surface during the acid treatment.

c) Loading of transition metal: sealing the carrier treated in the step b) and a certain amount of organic metal compound or other metal salt precursors in a quartz filling device, and filling gas phase in a steam form, wherein the mass ratio of the metal precursor to the carbon tube is 0.5-20 to obtain the carbon tube compound filled with the metal precursor in the tube, leaching the compound with methanol, ethanol or toluene solvent to primarily remove the metal precursor outside the tube, and then thermally decomposing the compound in an oxidation, reduction or inert atmosphere, wherein the oxidation atmosphere required by thermal decomposition is the inert atmosphere containing 20-100% (volume percentage content) of O2; the reducing atmosphere is an inert atmosphere containing 10-100 percent (volume percentage) of H2; the inert atmosphere is one or more of N2, Ar and He; the thermal decomposition pressure is normal pressure-8 MPa; the thermal decomposition temperature is 100-400 ℃.

d) Reducing the decomposed catalyst for 1-5h by using a certain amount of hydrogen at the temperature of 200-500 ℃ to obtain the reduced catalyst. The hydrogen flow is 20-50 mL/min.

e) Doping of the promoter: sealing and placing the reduced catalyst in the step d) in an argon glove box, grinding and mixing the catalyst with the main group metal simple substance or the alloy uniformly according to a certain proportion, and stirring and activating the mixture for 10 to 16 hours at the temperature of 100-200 ℃ in an inert atmosphere to obtain the activated catalyst.

The metal precursor is one or more than two of volatile organic metal compound or metal salt.

The volatile organic metal compound used for preparing the iron-based catalyst is one or more than two of ferrocene, tert-butyl ferrocene, cyclooctatetraene iron tricarbonyl, cyclohexadiene iron tricarbonyl and carbonyl iron; the iron-based catalyst is prepared (the adopted volatile metal salt refers to one or more of ferric chloride and ferric nitrate; the adopted volatile organic metal complex for preparing the rhenium-based catalyst is one or more of methyl rhenium trioxide, rhenium decacarbonyl and cyclopentadiene rhenium tricarbonyl compounds; the adopted volatile metal salt for preparing the rhenium-based catalyst refers to one or more of rhenium (III) chloride and rhenium (V) chloride; and the adopted volatile organic metal complex for preparing the platinum-based catalyst is one or more of platinum acetylacetonate (II),1,1,1,5,5, 5-hexafluoro-platinum acetylacetonate (II), pi-cyclopentadiene (trimethyl) platinum (IV) and trimethyl methyl cyclopentadiene (IV).

The application of the catalyst in the reaction for synthesizing ammonia at normal pressure and low temperature comprises the following steps: reducing the catalyst in situ for 30-120min in a hydrogen atmosphere at the temperature of 300-400 ℃ and normal pressure, reducing the temperature to a certain temperature in the hydrogen atmosphere, raising the temperature to the reaction temperature, and switching to a normal-pressure nitrogen-hydrogen mixed gas, wherein the volume ratio of the nitrogen-hydrogen mixed gas to the N is₂：H₂1:3-3:1, the flow rate of reaction gas is 1-50mL/min, the reaction temperature is 25-350 ℃, and the ammonia product is obtained.

Compared with the prior art, the invention has the advantages that:

(1) the existing industrial synthesis ammonia mainly uses molten iron and a carbon-supported noble metal ruthenium catalyst, the related reaction conditions are very harsh, high temperature (400-. In contrast, the carbon nanotube-confined transition metal nanocluster catalyst prepared by using a high specific surface area carbon nanotube with an inner diameter of less than 4nm as a carrier, filling highly dispersed transition metal nanoclusters in a tube cavity of the carbon nanotube-confined transition metal nanocluster, and doping a main group metal auxiliary agent has very high reaction activity under mild reaction conditions of normal pressure and low temperature. Compared with other traditional ammonia synthesis catalysts, the transition metal nanocluster catalyst with the unique structure wrapped by the carbon nano tubes has the advantages that due to the selective enrichment effect of the carbon tube cavities on gas molecules, nitrogen molecules can be effectively and preferentially adsorbed in the tubes, ammonia molecules are mainly distributed outside the tubes, and the electronic modulation effect of the carbon nano tubes on the transition metal nanocluster catalyst can effectively accelerate the reaction. Under the reaction condition of normal pressure of 25-350 ℃, the obtained activity is 4-8 times of that of the noble metal ruthenium-based catalyst with the optimal activity reported at present, the reaction efficiency of ammonia synthesis is obviously improved, and compared with the patent application CN103977828A, the catalyst has the advantages of obviously reducing the reaction temperature and improving the catalyst activity.

(2) The transition metal nanocluster catalyst wrapped by the carbon nanotube is a nanocatalyst with a novel structure, and a reaction path can be modulated by utilizing the synergistic interaction among the carbon nanotube tube cavity, the transition metal nanocluster and reactant molecules in the reaction process, so that the reaction rate is changed, and the stability of the catalyst is improved.

(3) The catalytic confinement effect of the transition metal nanocluster catalyst wrapped by the carbon nanotube on catalytic reaction is enhanced along with the reduction of the diameter of a tube cavity, so that the thin-wall carbon nanotube with the tube cavity diameter smaller than 4nm is more beneficial to improving the performance of the catalyst compared with a multi-wall carbon nanotube with the tube cavity diameter larger than 4-10 nm.

(4) The preparation method of the catalyst has the advantages that (a) the pretreatment method of the carbon nano tube has high efficiency, and the yield of the opened and truncated carbon nano tube obtained after the mixed acid treatment is up to more than 90 percent; (b) the transition metal nanocluster loading method can selectively load transition metal nanoclusters in the hollow carbon nanotube cavity, and more than 85% of the transition metal nanoclusters in the product obtained after oxidation reduction are confined in the carbon nanotube cavity; (c) the preparation method is simple and efficient, and gram-level catalyst for catalytic reaction evaluation can be prepared in batches.

Drawings

FIG. 1(A) is a high angle annular dark field image (HAADF-STEM) of a Fe @ TWCNT catalyst; (B) high Resolution Transmission Electron Microscopy (HRTEM) images of Ru @ TWCNT; (C) high Resolution Transmission Electron Microscopy (HRTEM) images of Re @ TWCNT;

FIG. 2 shows K-Fe @ TWCNT, K-PtRe @ TWCNT, K-Fe/TWCNT, K-Fe @ CB, K-Fe @ MWCNT, K-Fe/MWCNT, and Ru/Ca₂N:e^-Testing the catalytic synthesis ammonia reaction activity;

FIG. 3 is a stability test of K-Fe @ TWCNT at 200 ℃ under normal pressure.

Detailed Description

To further illustrate the present invention, the following specific examples are set forth, but the scope of the claims of the present invention is not limited by these examples. Meanwhile, the embodiment only gives some conditions for achieving the purpose, but does not mean that the conditions must be met for achieving the purpose.

Example 1

Putting 4 parts of 60mg few-walled carbon nanotubes (TWCNTs) into four 100-milliliter round-bottom flasks, adding 60mL of mixed acid (concentrated sulfuric acid: concentrated nitric acid: 3:1) into the round-bottom flasks, putting the round-bottom flasks into an ultrasonic water bath, carrying out ultrasonic treatment for 5.5 hours, keeping the temperature of the water bath at 40-50 ℃, taking 45mL of supernatant after ultrasonic treatment, rinsing the supernatant to be neutral, and carrying out vacuum freeze drying for 90 hours. And (3) treating the dried sample in argon at 1000 ℃ for 4h, wherein the flow of the argon is 50mL/min, cooling to room temperature, and taking out for later use.

The 220mg of the few-wall carbon nano-tubes (s-TWCNTs) are obtained after truncation and purification, the yield is up to 96 percent, the lengths of the truncated carbon tubes are uniformly distributed at 0.4-1.0 mu m, the tube cavities are hollow, and the purity is higher than 95 percent.

Example 2 preparation of K-Fe @ TWCNT catalyst

(1) 100mg of truncated carbon nanotubes (s-TWCNTs) were placed in a vacuum filling apparatus, and 38mg of cyclooctatetraene tricarbonyl was placed in the other end of the vacuum apparatus. At 1X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. And after cooling, mixing the carbon nano tube and the precursor in vacuum, and then treating for 72 hours in an oven at 80 ℃.

(2) And fully leaching the filled sample with toluene, removing residual Fe precursors outside the tube, filtering, drying at 60 ℃ overnight, oxidizing for 24 hours with high-pressure oxygen at 100 ℃, stirring with 2M nitric acid for 1 hour, filtering, drying at 60 ℃ overnight, and reducing for 5 hours with hydrogen at 450 ℃ to obtain the reduced 1 wt.% Fe @ TWCNTs catalyst.

(3) And (3) sealing and placing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and mixing uniformly, and stirring and activating for 12 hours at 150 ℃ in an inert atmosphere to obtain the activated catalyst K-Fe @ TWCNT.

The morphology of the catalyst and the morphology of the medium Fe are shown as A in figure 1, and the electron microscope photo shows that the iron nano particles are efficiently filled in the carbon nano tube with the tube diameter being less than the wall, and the filling rate is as high as more than 85%. The reaction activity test result of the synthetic ammonia is shown in figure 2, the activity is up to 1279-47631mmol NH at the normal pressure of 160-320 DEG C₃mol metal^-1h^-15-15 times of transition metal nanocluster catalyst K-Fe/TWCNT loaded outside the carbon nanotube. The stability test result is shown in fig. 3, and the catalyst activity is kept stable for more than 140 hours under the condition of online monitoring at the normal pressure of 200 ℃.

Example 3 preparation of Cs-Ru @ TWCNT catalyst

(1) 100mg of truncated carbon nanotubes (s-TWCNTs) was placed in a vacuum filling apparatus, and 2.5mg of bis (2, 4-dimethylpentadiene) ruthenium was placed in the other end of the vacuum apparatus. At 1.5X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. And after cooling, mixing the carbon nano tube and the precursor in vacuum, and then treating for 48 hours in an oven at 120 ℃.

(2) And fully leaching the filled sample with toluene, removing the residual Ru precursor outside the tube, filtering, drying at 60 ℃ overnight, and reducing with hydrogen at 450 ℃ for 5 hours to obtain the reduced 1 wt.% Ru @ TWCNT catalyst.

(3) And (3) sealing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of Cs metal simple substance, grinding and mixing uniformly, and stirring and activating for 12h at 100 ℃ in an inert atmosphere to obtain the activated catalyst Cs-Ru @ TWCNT.

The morphology of the catalyst and the morphology of the medium Ru are shown as B in figure 1, and the electron microscope photo shows that the ruthenium nano particles are efficiently filled and uniformly dispersed in the carbon nano tube with less walls, and the filling rate is as high as more than 85%.

Example 4 preparation of K-Re @ TWCNT catalyst

(1) 100mg of truncated carbon nanotubes (s-TWCNTs) were placed in a vacuum filling apparatus, and 3mg of methyltrioxorhenium was placed at the other end of the vacuum apparatus. At 1.5X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. And after cooling, mixing the carbon nano tube and the precursor in vacuum, and then treating for 48 hours in a 90 ℃ oven.

(2) And fully leaching the filled sample with toluene, removing residual Re precursor outside the tube, filtering, drying at 60 ℃ overnight, and reducing with hydrogen at 450 ℃ for 5 hours to obtain the reduced 2 wt.% Re @ TWCNT catalyst.

(3) And (3) sealing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and mixing uniformly, and stirring and activating for 12h at 100 ℃ in an inert atmosphere to obtain the activated catalyst K-Re @ TWCNT.

The morphology of the catalyst and the morphology of the medium Re are shown as C in figure 1, and the electron microscope photo shows that the rhenium nano particles are efficiently filled and uniformly dispersed in the carbon nanotube with few walls, and the filling rate is as high as more than 85%.

Example 5 preparation of K-Pt @ TWCNT catalyst

(1) 100mg of truncated carbon nanotubes (s-TWCNTs) was placed in a vacuum filling apparatus, and 2mg of cyclopentadienylplatinum (trimethyl) was placed at the other end of the vacuum apparatus. At 1.5X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. And after cooling, mixing the carbon nano tube and the precursor in vacuum, and then treating for 48 hours in an oven at 80 ℃.

(2) And fully leaching the filled sample with toluene, removing residual Pt precursor outside the tube, filtering, drying at 60 ℃ overnight, and reducing with hydrogen at 450 ℃ for 5 hours to obtain the reduced 1.5 wt.% Pt @ TWCNT catalyst.

(3) And (3) sealing and placing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and mixing uniformly, and stirring and activating for 12h at 100 ℃ in an inert atmosphere to obtain the activated catalyst K-Pt @ TWCNT.

The platinum nano particles are efficiently filled and uniformly dispersed in the carbon nano tube with less wall, and the filling rate is as high as more than 85%.

Example 6K-Re₂Ru₁Preparation of @ TWCNT catalyst

(1) 100mg of truncated carbon nanotubes (s-TWCNTs) were placed in a vacuum filling apparatus, and 3mg of methyltrioxorhenium and 2.5mg of bis (2, 4-dimethylpentadiene) ruthenium were placed at the other end of the vacuum apparatus. At 1.5X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. Cooling, mixing the carbon nanotube and the precursor in vacuum, and coolingOven-treating at 120 deg.C for 48 h.

(2) Fully leaching the filled sample with toluene, removing residual Re and Ru precursors outside the tube, filtering, drying at 60 ℃ overnight, and reducing with hydrogen at 450 ℃ for 5 hours to obtain reduced Re₂Ru₁@ TWCNT catalyst.

(3) Placing the reduced catalyst in an argon glove box in a sealed manner, taking 50mg, adding 26 mu L KNa alloy, grinding and mixing uniformly, stirring and activating for 12h at 100 ℃ in an inert atmosphere to obtain the activated catalyst K-Re₂Ru₁@TWCNT。

The ruthenium-rhenium alloy nano particles are efficiently filled and uniformly dispersed in the carbon nanotube with less wall, the filling rate is up to more than 85 percent, and the molar ratio Re of two metals is measured by EDX (enhanced dispersive X) energy spectrum: ru is 2: 1.

example 7K-Pt₁Re₁Preparation of @ TWCNT catalyst

(1) 100mg of truncated carbon nanotubes (s-TWCNTs) were placed in a vacuum filling apparatus, and 1.5mg of methyltrioxorhenium and 1.5mg of cyclopentadienylplatinum (trimethyl) were placed at the other end of the vacuum apparatus. At 1.5X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. And after cooling, mixing the carbon nano tube and the precursor in vacuum, and then treating for 48 hours in a 90 ℃ oven.

(2) Fully leaching the filled sample with toluene, removing residual Re and Pt precursors outside the tube, filtering, drying at 60 ℃ overnight, and reducing with hydrogen at 450 ℃ for 5 hours to obtain reduced Pt₁Re₁@ TWCNT catalyst.

(3) Sealing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L KNa alloy, grinding and mixing uniformly, stirring and activating for 12h at 100 ℃ in an inert atmosphere to obtain the activated catalyst K-Pt₁Re₁@TWCNT。

The platinum-rhenium alloy nano particles are efficiently filled and uniformly dispersed in the carbon nanotube with less wall, the filling rate is up to more than 85 percent, and the molar ratio Pt of two metals is measured by EDX (enhanced dispersive X-ray spectroscopy): re 1:1, the test result of the reaction activity of the synthetic ammonia is shown in figure 2, and the catalytic synthesis is carried out at the normal pressure of 160-320 DEG CThe reaction activity of ammonia is 40-7010mmol NH₃mol metal^-1h^-1。

Example 8 synthetic Ammonia reaction Performance test

Evaluation of the activity of the ammonia synthesis reaction K-Fe @ TWCNT catalyst was carried out in a continuous fixed bed. 10 mg of Fe catalyst (example 1) was added to a quartz reaction tube, and after reduction at 400 ℃ for 80 minutes under normal pressure with hydrogen, the temperature was lowered to 160 ℃ and the reaction tube was switched to a normal pressure mixed gas of nitrogen and hydrogen (volume ratio N)₂：H₂1:3), the flow rate of reaction gas is 5mL/min, the generation amount of ammonia gas is continuously detected on line by using a flight time mass spectrum, after the reaction is carried out at 160 ℃ for 9 hours, the reaction is balanced, the temperature is increased to 200 ℃, the activity is stabilized for 6 hours, the temperature is increased to 240 ℃, the activity is stabilized for 2 hours, the temperature is increased to 280 ℃, the activity is stabilized after 1 hour, the temperature is increased to 320 ℃, and the activity is stabilized after 6 hours. Using NH formed per unit time per unit mass of metal₃The number of moles of (a) reflects the activity of the catalyst at different temperatures (as shown in figure 2).

Comparative example 1 preparation of K-Fe/TWCNT catalyst

(1) Adding 100mg of truncated carbon nanotubes (s-TWCNTs) into a beaker, dropwise adding a toluene solution containing tricarbonyl cyclooctatetraene iron (the mass percentage of Fe is 0.1-2% relative to the TWCNT) while stirring, stirring to dry, drying overnight at 60 ℃, and reducing for 5 hours by hydrogen at 450 ℃ to obtain the reduced Fe/TWCNTs catalyst.

(2) And (3) sealing and placing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and uniformly mixing, and stirring and activating for 12 hours at 150 ℃ in an inert atmosphere to obtain the activated catalyst K-Fe/TWCNT.

The K-Fe/TWCNT catalyst is a comparative catalyst with iron nanoclusters loaded outside the carbon nanotube with few walls, the test result of the activity of the ammonia synthesis reaction is shown in figure 2, and the activity of the ammonia synthesis reaction is 27-9203mmol NH at the temperature of 160-320 ℃ under normal pressure₃mol metal^-1h^-1The catalyst is only 2-18% of the transition metal nanocluster catalyst loaded in the carbon nanotube, and the efficiency is extremely low.

Comparative example 2 preparation of K-Fe @ CB catalyst

(1) 100mg of Carbon Black (Carbon Black) was charged into the vacuum filling apparatus, and 4.5mg of iron tricarbonyl cyclooctatetraene was placed at the other end of the vacuum apparatus. At 1X 10^-4The temperature of the device with the carbon black is programmed to 450 ℃ under the Pa vacuum condition, the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. After cooling, the carbon black and the precursor are mixed under vacuum and then are treated in an oven at 80 ℃ for 72 hours. Reducing with hydrogen at 450 ℃ for 5h to obtain a reduced 1 wt.% Fe @ CB catalyst.

(2) And (3) sealing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and mixing uniformly, and stirring and activating for 12 hours at 150 ℃ in an inert atmosphere to obtain the activated catalyst K-Fe @ CB.

The activity of the K-Fe @ CB for catalyzing and synthesizing ammonia at the temperature of 160-280 ℃ under normal pressure is 17-1370mmol of NH₃mol metal^- ¹h^-1The catalyst is only 1-5% of the transition metal nanocluster catalyst loaded in the carbon nanotube, and the efficiency is extremely low.

Comparative example 3 preparation of K-Fe @ MWCNT catalyst

(1) Respectively putting 4 parts of 60mg multi-walled carbon nanotubes (MWCNTs) into four 100-milliliter round-bottom flasks, respectively adding 60 milliliters of mixed acid (concentrated sulfuric acid: concentrated nitric acid: 3:1), simultaneously putting into an ultrasonic water bath, carrying out ultrasonic treatment for 5.5 hours, keeping the temperature of the water bath at 40-50 ℃, taking 45 milliliters of supernatant after ultrasonic treatment, leaching to be neutral, and carrying out vacuum freeze drying for 90 hours. The dried sample was treated at 1000 ℃ for 4h under argon with an argon flow of 50 mL/min.

(2) 100mg of truncated carbon nanotubes (s-MWCNTs) was placed in a vacuum filling apparatus, and 38mg of cyclooctatetraene iron tricarbonyl was placed in the other end of the vacuum apparatus. At 1X 10^-4And (3) programming the device with the carbon nano tube to 450 ℃ under the Pa vacuum condition, wherein the heating rate is 5 ℃/min, and the temperature is kept at 450 ℃ for 16 hours. And after cooling, mixing the carbon nano tube and the precursor in vacuum, and then treating for 72 hours in an oven at 80 ℃.

(3) And fully leaching the filled sample with toluene, removing residual Fe precursors outside the tube, filtering, drying at 60 ℃ overnight, oxidizing for 24 hours with high-pressure oxygen at 100 ℃, stirring with 2M nitric acid for 1 hour, filtering, drying at 60 ℃ overnight, and reducing for 5 hours with hydrogen at 450 ℃ to obtain the reduced 1 wt.% Fe @ MWCNTs catalyst.

(4) And (3) sealing and placing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and mixing uniformly, and stirring and activating for 12 hours at 150 ℃ in an inert atmosphere to obtain the activated catalyst K-Fe @ MWCNT.

The K-Fe @ MWCNT catalyst is a comparative catalyst for loading iron nanoclusters in a multi-walled carbon nanotube, and the test result of the synthetic ammonia reaction activity is shown in figure 2, wherein the catalytic synthetic ammonia reaction activity is 0-1396mmol NH at the temperature of 160-₃mol metal^-1h^-1The catalyst is only 0-2% of the transition metal iron nanocluster catalyst loaded in the carbon nanotube, and the efficiency is extremely low.

Comparative example 4 preparation of K-Fe/MWCNT catalyst

(2) Adding 100mg of truncated carbon nanotubes (s-MWCNTs) into a beaker, dropwise adding a toluene solution containing tricarbonyl cyclooctatetraene iron (the mass percentage of Fe is 0.1-2 percent relative to the MWCNTs) while stirring, stirring to dry, drying overnight at 60 ℃, and reducing for 5 hours by hydrogen at 450 ℃ to obtain the reduced Fe/MWCNTs catalyst.

(3) And (3) sealing and placing the reduced catalyst in an argon glove box, taking 50mg, adding 26 mu L of KNa alloy, grinding and mixing uniformly, and stirring and activating for 12 hours at 150 ℃ in an inert atmosphere to obtain the activated catalyst K-Fe/MWCNT.

The K-Fe/MWCNT catalyst is a comparative catalyst for loading iron nanoclusters outside a multi-walled carbon nanotube, and the test result of the synthetic ammonia reaction activity is shown in figure 2, wherein the catalytic synthetic ammonia reaction activity is 0-956mmol NH at the temperature of 160-320 ℃ under normal pressure₃mol metal^-1h^-1Is onlyThe transition metal iron nanocluster catalyst loaded in the carbon nanotube tube accounts for 0-2%, and the efficiency is extremely low.

Comparative example 5 for comparison, the test conditions for the performance of the reactions for the catalytic synthesis of ammonia using K-Fe/TWCNT, K-Fe @ CB, K-Fe @ MWCNT, K-Fe/MWCNT were exactly the same as in example 8 and were carried out in a continuous fixed bed. Adding corresponding catalyst 10 mg into a miniature quartz reaction tube, reducing at 400 ℃ for 80 minutes by using normal pressure hydrogen, cooling to 160 ℃, and switching to normal pressure nitrogen-hydrogen mixed gas (volume ratio N)₂：H₂1:3), the flow rate of reaction gas is 5mL/min, the generation amount of ammonia gas is continuously detected on line by using a flight time mass spectrum, after the reaction is carried out at 160 ℃ for 9 hours, the reaction is balanced, the temperature is increased to 200 ℃, the activity is stabilized for 6 hours, the temperature is increased to 240 ℃, the activity is stabilized for 2 hours, the temperature is increased to 280 ℃, the activity is stabilized after 1 hour, the temperature is increased to 320 ℃, and the activity is stabilized after 6 hours. Using NH formed per unit time per unit mass of metal₃The number of moles of (a) reflects the activity of the catalyst at different temperatures (as shown in figure 2).

As can be seen from the above example 8 and comparative example 5, the activity of the transition metal nanocluster catalyst dispersed in the carbon nanotube lumen with an inner diameter of about 1-4nm under the reaction condition of normal pressure of 160-.

The above examples are provided only for the purpose of describing the present invention, and are not intended to limit the scope of the present invention. The scope of the invention is defined by the appended claims. Various equivalent substitutions and modifications can be made without departing from the spirit and principles of the invention, and are intended to be within the scope of the invention.

Claims

1. A method for preparing a catalyst, comprising:

the carbon nano tube is used as a carrier;

the transition metal nanoclusters dispersed in the carbon nanotube cavity are catalytic active metals, the mass percentage content is 0.1-10%, and the filling rate in the carbon nanotube cavity is more than 85%; the transition metal refers to one or more of iron, ruthenium, rhenium and platinum;

the main group metal simple substance or the main group metal alloy is taken as an auxiliary agent, the mass percentage content is 5-50%, and the carbon nano tube is a thin-wall carbon nano tube with the inner diameter of a tube cavity of 1-4 nm;

the method comprises the following steps: (1) pretreatment of the carbon nanotubes: dispersing original carbon nano tubes in a mixed solution of concentrated sulfuric acid and concentrated nitric acid, performing ultrasonic treatment, filtering, leaching to be neutral, performing vacuum freeze sublimation drying, and performing high-temperature treatment in hydrogen or argon for 3-5 hours to obtain truncated carbon nano tubes with open tube orifices; the carbon nano tube is a thin-wall carbon nano tube with the inner diameter of a tube cavity of 1-2 nm;

(2) the transition metal nanocluster loading method comprises the following steps: packaging the spare carbon tube in a container, evacuating the container to dewater to a vacuum degree of 10^-2Gasifying a transition metal precursor below Pa, introducing the gasified transition metal precursor into the carbon tube, maintaining the gasified transition metal precursor at the temperature of between 70 and 100 ℃ for 24 to 50 hours, heating and decomposing the obtained product in an oxidizing atmosphere, a reducing atmosphere or an inert atmosphere, and reducing the product by hydrogen to prepare a reduced catalyst;

(3) doping of main group metal simple substance or alloy auxiliary agent: placing the reduced catalyst in an inert atmosphere, grinding and uniformly mixing the reduced catalyst with a main group metal simple substance or alloy according to a certain proportion, and stirring and activating the mixture for 10 to 16 hours at the temperature of 100-200 ℃ in the inert atmosphere to obtain an activated catalyst;

the auxiliary agent is one or more than two of Li, Na, K, Rb, Cs, Mg, Ca, Ba and Al.

2. The method for preparing a catalyst according to claim 1, characterized in that: the mass content of the catalytic active metal is 0.1-2%; the mass content of the auxiliary agent is 20-30%.

3. The method for preparing a catalyst according to claim 1 or 2, characterized in that: for transition metal iron, the auxiliary agents are Na and K; for rhenium and platinum, the auxiliary agents are Na, K, Rb and Cs; for ruthenium, the auxiliaries are Ca, Ba, Cs.

4. The method for preparing a catalyst according to claim 1, characterized in that: the inert atmosphere is one of nitrogen, argon and helium.

5. The method for preparing a catalyst according to claim 1, characterized in that: the volume ratio of concentrated sulfuric acid to concentrated nitric acid in the mixed solution in the step (1) is 1:10-10: 1.

6. The method for preparing a catalyst according to claim 1, characterized in that: the metal precursor is one or more of volatile organic metal compound or volatile metal salt.

7. The method for preparing a catalyst according to claim 6, characterized in that: the volatile organic metal compound adopted by the iron precursor is one or more than two of ferrocene, tert-butyl ferrocene, cyclooctatetraene iron tricarbonyl, cyclohexadiene iron tricarbonyl and iron carbonyl; the volatile metal salt adopted by the iron precursor refers to one or more than two of ferric chloride and ferric nitrate; the volatile organic metal compound adopted by the rhenium precursor is one or more than two of methyl rhenium trioxide, rhenium decacarbonyl and cyclopentadiene rhenium tricarbonyl compounds; the volatile metal salt adopted by the rhenium precursor refers to one or two of rhenium (III) chloride and rhenium (V) chloride; the volatile organic metal compound adopted by the platinum precursor is one or more than two of acetylacetone platinum (II),1,1,1,5,5, 5-hexafluoroacetylacetone platinum (II), pi-cyclopentadiene (trimethyl) platinum (IV) and trimethyl methyl cyclopentadiene platinum (IV).

8. The method for preparing a catalyst according to claim 1, characterized in that: in the step (2), O is in an oxidizing atmosphere₂20-100% by volume, and the balance inert gas; h in the reducing atmosphere₂The volume percentage of the catalyst is 10-100 percent, and the rest is inert gas; the inert atmosphere isN₂One or more of Ar and He.

9. The method for preparing a catalyst according to claim 1, characterized in that: in the step (2), the heating decomposition pressure is normal pressure-8 MPa; the thermal decomposition temperature is 100-400 ℃.

10. Use of a catalyst prepared by the preparation method according to any one of claims 1 to 4, wherein: the method is used for the normal-pressure low-temperature ammonia synthesis reaction, and the reaction temperature is 25-350 ℃.

11. Use according to claim 10, characterized in that: the reaction method for synthesizing ammonia at normal pressure and low temperature comprises the following steps: reducing the catalyst in situ for 30-120min in a hydrogen atmosphere at the temperature of 300-400 ℃ and normal pressure, reducing the temperature to a certain temperature in the hydrogen atmosphere, raising the temperature to the reaction temperature, and switching to a normal-pressure nitrogen-hydrogen mixed gas, wherein the volume ratio of the nitrogen-hydrogen mixed gas to the N is₂：H₂And (3) =1:3-3:1, wherein the flow rate of reaction gas is 1-50mL/min, the reaction temperature is 25-350 ℃, and an ammonia gas product is obtained.