CN117772259B - Double-active-center ammonia synthesis catalyst and preparation method and application thereof - Google Patents
Double-active-center ammonia synthesis catalyst and preparation method and application thereof Download PDFInfo
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- CN117772259B CN117772259B CN202410206250.4A CN202410206250A CN117772259B CN 117772259 B CN117772259 B CN 117772259B CN 202410206250 A CN202410206250 A CN 202410206250A CN 117772259 B CN117772259 B CN 117772259B
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- QGZKDVFQNNGYKY-UHFFFAOYSA-N Ammonia Chemical compound N QGZKDVFQNNGYKY-UHFFFAOYSA-N 0.000 title claims abstract description 176
- 229910021529 ammonia Inorganic materials 0.000 title claims abstract description 88
- 239000003054 catalyst Substances 0.000 title claims abstract description 76
- 238000003786 synthesis reaction Methods 0.000 title claims abstract description 60
- 230000015572 biosynthetic process Effects 0.000 title claims abstract description 57
- 238000002360 preparation method Methods 0.000 title abstract description 12
- 229910052739 hydrogen Inorganic materials 0.000 claims abstract description 65
- 239000001257 hydrogen Substances 0.000 claims abstract description 59
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 claims abstract description 48
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims abstract description 48
- 229910052757 nitrogen Inorganic materials 0.000 claims abstract description 30
- 238000006243 chemical reaction Methods 0.000 claims abstract description 24
- 238000010438 heat treatment Methods 0.000 claims abstract description 22
- 230000003197 catalytic effect Effects 0.000 claims abstract description 21
- 239000000463 material Substances 0.000 claims abstract description 21
- 238000000034 method Methods 0.000 claims abstract description 19
- 229910052784 alkaline earth metal Inorganic materials 0.000 claims abstract description 14
- 230000008569 process Effects 0.000 claims abstract description 14
- 238000011065 in-situ storage Methods 0.000 claims abstract description 10
- 150000001875 compounds Chemical class 0.000 claims abstract description 6
- 125000002924 primary amino group Chemical group [H]N([H])* 0.000 claims abstract description 6
- -1 alkaline earth metal amino compound Chemical class 0.000 claims abstract description 5
- 239000000843 powder Substances 0.000 claims description 26
- 238000005984 hydrogenation reaction Methods 0.000 claims description 19
- 238000007599 discharging Methods 0.000 claims description 14
- XKRFYHLGVUSROY-UHFFFAOYSA-N Argon Chemical compound [Ar] XKRFYHLGVUSROY-UHFFFAOYSA-N 0.000 claims description 12
- 150000002431 hydrogen Chemical class 0.000 claims description 11
- 239000002245 particle Substances 0.000 claims description 9
- 238000011068 loading method Methods 0.000 claims description 7
- 239000000126 substance Substances 0.000 claims description 7
- 229910052786 argon Inorganic materials 0.000 claims description 6
- 238000003825 pressing Methods 0.000 claims description 6
- 229910000048 titanium hydride Inorganic materials 0.000 claims description 6
- 238000000227 grinding Methods 0.000 claims description 5
- 238000002156 mixing Methods 0.000 claims description 4
- 230000000630 rising effect Effects 0.000 claims description 3
- 238000001816 cooling Methods 0.000 claims description 2
- 238000010335 hydrothermal treatment Methods 0.000 claims description 2
- 230000009977 dual effect Effects 0.000 claims 2
- 230000007246 mechanism Effects 0.000 abstract description 10
- 150000001342 alkaline earth metals Chemical class 0.000 abstract description 8
- 230000002195 synergetic effect Effects 0.000 abstract description 2
- 238000010494 dissociation reaction Methods 0.000 description 9
- 230000005593 dissociations Effects 0.000 description 9
- 238000004519 manufacturing process Methods 0.000 description 9
- 238000006555 catalytic reaction Methods 0.000 description 8
- 230000000694 effects Effects 0.000 description 7
- 239000010936 titanium Substances 0.000 description 7
- 230000002194 synthesizing effect Effects 0.000 description 5
- KJTLSVCANCCWHF-UHFFFAOYSA-N Ruthenium Chemical compound [Ru] KJTLSVCANCCWHF-UHFFFAOYSA-N 0.000 description 4
- 239000011812 mixed powder Substances 0.000 description 4
- 239000004570 mortar (masonry) Substances 0.000 description 4
- 125000004433 nitrogen atom Chemical group N* 0.000 description 4
- 239000002994 raw material Substances 0.000 description 4
- 239000012495 reaction gas Substances 0.000 description 4
- 229910052707 ruthenium Inorganic materials 0.000 description 4
- 238000007873 sieving Methods 0.000 description 4
- 238000001179 sorption measurement Methods 0.000 description 4
- 229910001220 stainless steel Inorganic materials 0.000 description 4
- 239000010935 stainless steel Substances 0.000 description 4
- 238000002441 X-ray diffraction Methods 0.000 description 3
- 230000000052 comparative effect Effects 0.000 description 3
- 229910000510 noble metal Inorganic materials 0.000 description 3
- 229910006425 Li—N—H Inorganic materials 0.000 description 2
- 230000004913 activation Effects 0.000 description 2
- 230000009286 beneficial effect Effects 0.000 description 2
- 239000002131 composite material Substances 0.000 description 2
- 238000003795 desorption Methods 0.000 description 2
- 238000005265 energy consumption Methods 0.000 description 2
- 238000005516 engineering process Methods 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 238000009776 industrial production Methods 0.000 description 2
- 230000001788 irregular Effects 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 239000000376 reactant Substances 0.000 description 2
- 238000009620 Haber process Methods 0.000 description 1
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 1
- 229910052783 alkali metal Inorganic materials 0.000 description 1
- 150000001340 alkali metals Chemical class 0.000 description 1
- 239000012752 auxiliary agent Substances 0.000 description 1
- 229910052799 carbon Inorganic materials 0.000 description 1
- 239000000969 carrier Substances 0.000 description 1
- 238000007036 catalytic synthesis reaction Methods 0.000 description 1
- 239000003245 coal Substances 0.000 description 1
- 230000002860 competitive effect Effects 0.000 description 1
- 125000004122 cyclic group Chemical group 0.000 description 1
- 230000018109 developmental process Effects 0.000 description 1
- 238000002635 electroconvulsive therapy Methods 0.000 description 1
- 230000007613 environmental effect Effects 0.000 description 1
- 239000002803 fossil fuel Substances 0.000 description 1
- 239000005431 greenhouse gas Substances 0.000 description 1
- 238000007210 heterogeneous catalysis Methods 0.000 description 1
- 229910052744 lithium Inorganic materials 0.000 description 1
- AFRJJFRNGGLMDW-UHFFFAOYSA-N lithium amide Chemical compound [Li+].[NH2-] AFRJJFRNGGLMDW-UHFFFAOYSA-N 0.000 description 1
- 229910052751 metal Inorganic materials 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 229910052987 metal hydride Inorganic materials 0.000 description 1
- 150000004681 metal hydrides Chemical class 0.000 description 1
- 238000001000 micrograph Methods 0.000 description 1
- 239000000203 mixture Substances 0.000 description 1
- 229910052761 rare earth metal Inorganic materials 0.000 description 1
- 150000002910 rare earth metals Chemical class 0.000 description 1
- 238000009790 rate-determining step (RDS) Methods 0.000 description 1
- 230000036632 reaction speed Effects 0.000 description 1
- 230000008929 regeneration Effects 0.000 description 1
- 238000011069 regeneration method Methods 0.000 description 1
- 230000003716 rejuvenation Effects 0.000 description 1
- 229910052719 titanium Inorganic materials 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
Classifications
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P20/00—Technologies relating to chemical industry
- Y02P20/50—Improvements relating to the production of bulk chemicals
- Y02P20/52—Improvements relating to the production of bulk chemicals using catalysts, e.g. selective catalysts
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Abstract
The invention belongs to the field of catalyst material preparation, and particularly relates to a double-active-center ammonia synthesis catalyst, and a preparation method and application thereof. The invention utilizes an in-situ reaction mechanism activated by a high-pressure hydrogen impact heat treatment process to prepare the double-active-center ammonia synthesis catalyst taking the alkaline earth metal amino compound as a nitrogen carrier, and the in-situ reaction generates the double active centers of the alkaline earth metal amino compound nitrogen carrier, tiH 2 and LiH, which have fresh and clean surface interfaces and high ammonia synthesis catalytic activity, and the synergistic catalytic effect ensures that the catalyst has excellent normal-pressure low-temperature ammonia synthesis catalytic performance and higher thermal stability. The catalyst provided by the invention does not contain rare and noble elements, has low cost, simple preparation method and no pollution in the whole process, and has obvious industrial application prospect and application value of synthetic ammonia.
Description
Technical Field
The invention relates to a catalyst technology, and particularly provides a double-active-center ammonia synthesis catalyst under mild conditions, and a preparation method and application thereof.
Background
Synthetic ammonia is one of the largest chemical industries in the world with annual production rates of about 1.8 million tons, and is currently considered to be one of the greatest contributions of catalytic technology to humans. Ammonia synthesis reactions have been the most widely studied and in-depth reaction in the heterogeneous catalysis field for over a hundred years, and provide guidance for many other heterogeneous catalytic reactions, which are known as "holy cup" and "pilot" reactions in the catalysis community. Although developed for over a century, the present ammonia synthesis industry still takes the Haber-Bosch process as the dominant one, and high-purity N 2 and H 2 feed gases are required to be converted into NH 3 under the catalysis of Fe-based or Ru-based catalysts at high temperature and high pressure (350-550 ℃ and 10-30 MPa). Although the Haber-Bosch ammonia synthesis process is relatively efficient, the synthesis conditions are harsh, and the energy and H 2 feed gas are both from the conversion of fossil fuels (such as methane, coal, etc.), so that the process is a high-energy and high-carbon emission process, the annual average energy consumption of which is 1-2% of the total global energy supply, and the annual average CO 2 emission of which is about 1.5% of the total greenhouse gas emission. In view of the increasingly serious resource, energy and environmental problems worldwide, finding a suitable green alternative scheme to realize high-efficiency, low-energy consumption and low-emission synthesis of ammonia under mild conditions becomes a scientific and technical problem to be solved urgently.
Since the beginning of the 20 th century, scientific researchers have conducted a great deal of research on the kinetics and reaction mechanisms of the synthetic ammonia reaction, and it is generally considered that the mechanism of the synthetic ammonia catalytic reaction is mainly of two types, namely a dissociative mechanism and an associative mechanism. In the dissociation mechanism, N 2 and H 2 molecules are dissociated first on the catalyst surface to generate N and H, then N gradually combines with H to generate intermediate species (NH, NH 2 and NH 3), and finally NH 3 is generated to be desorbed from the surface of the catalyst. In the dissociation mechanism, the adsorption dissociation of N 2 is considered as a rate controlling step of the entire reaction. In the associative mechanism, N 2 remains in the form of adsorbed molecules prior to hydrogenation, with partial hydrogenation followed by dissociation of n≡n. In the associative mechanism, adsorption of N 2 and dissociation of N.ident.N bonds by partial hydrogenation are all possible steps for controlling the reaction speed. It is currently widely accepted that the dissociation activation of N 2 is the rate-determining step in the synthesis of ammonia, due to the high dissociation energy (945 kJ/mol) of the N≡N bond. Therefore, to achieve synthesis of ammonia under mild conditions, it is critical to design a new high performance catalyst to break the limiting relationship between the dissociation activation energy of the n≡n bond and the bond energy of the intermediate species. Compared with the traditional first generation of iron-melting ammonia synthesis catalyst (active components mainly comprise Fe 3O4 or Fe 1-x O)), the ruthenium-based ammonia synthesis catalyst (mainly comprising active metal ruthenium, a carrier and an auxiliary agent (alkali metal, alkaline earth metal and/or rare earth metal) is known as a second generation ammonia synthesis catalyst after the iron-melting catalyst due to the characteristics of lower ammonia synthesis temperature pressure, high activity and the like, Chinese patent CN 111097410B discloses a ruthenium-based ammonia synthesis catalyst, a preparation method and application thereof, the raw material ruthenium used by the catalyst is expensive, the preparation process is complex, the synthesis ammonia conditions of 400 ℃ and 10MPa are still more severe, the high-efficiency synthesis ammonia at normal pressure and low temperature (0.1-5.0 MPa, 150-300 ℃) cannot be realized, and the development of a third-generation high-efficiency, normal pressure and low temperature catalyst is needed.
In recent years, with the increasing demand of renewable energy source ammonia synthesis, renewable energy sources such as solar energy, wind energy and the like are coupled with a chemical chain process, so that the chemical chain is rejuvenated as a substitute ammonia synthesis mode. Compared with the synthetic ammonia catalytic process, the chemical chain process has the following characteristics: the method can be operated under low temperature and low pressure conditions, is beneficial to simplifying the process flow, and can be used for producing ammonia in a distributed and miniaturized way; the reactants, temperature, pressure and the like in the steps of nitrogen fixation, ammonia production and the like can be optimized respectively; the competitive adsorption problem of N 2 and H 2 or H 2 O can be avoided, but the chemical chain synthesis ammonia still has urgent demands for the high-efficiency synthesis ammonia catalyst at normal pressure and low temperature (0.1-5.0 MPa, 150-300 ℃).
Disclosure of Invention
The invention aims to provide a double-active-center ammonia synthesis catalyst taking alkaline earth metal amino compounds as nitrogen carriers and a preparation method thereof, wherein the prepared double-active-center catalyst has high catalytic activity for synthesizing ammonia at normal pressure and low temperature (0.1-5.0 MPa, 150-300 ℃), the preparation method is simple and pollution-free, the used raw materials do not contain rare noble metals, the cost is low, the large-scale industrial production is easy, the catalyst is suitable for chemical-chain ammonia synthesis application, and the catalyst has wide industrial application prospect and value for synthesizing ammonia.
In order to achieve the above purpose, the present invention adopts the following technical scheme:
A double active center ammonia synthesis catalyst comprises an alkaline earth metal amino compound nitrogen carrier and double active catalytic centers, wherein the alkaline earth metal amino compound nitrogen carrier and the double active catalytic centers are uniformly dispersed and distributed; the alkaline earth metal amino compound nitrogen carrier (M (NH 2)2) is any one of Mg (NH 2)2、Ca(NH2)2、Ba(NH2)2), and the double active catalytic centers are TiH 2 and LiH respectively;
in the double-active-center ammonia synthesis catalyst, M (NH 2)2、TiH2 and LiH phases separated out through in-situ reaction have fresh and clean surface interfaces and extremely high catalytic activity, tiH 2 accounts for 1.04-11.48% of the total weight of the double-active-center ammonia synthesis catalyst, and the balance of M (NH 2)2 and LiH, the molar ratio of LiH to M (NH 2)2 is 2:1-3;
In the double active center ammonia synthesis catalyst, tiH 2 is uniformly dispersed in the catalyst in the form of nano clusters, and the particle size of the clusters is 10-100 nm.
A method for preparing a double active center ammonia synthesis catalyst with an alkaline earth metal amide compound as a nitrogen carrier, the method comprising the steps of:
(1) Mixing and grinding alkaline earth metal hydride (MH 2) powder, titanium (Ti) powder and lithium amide (LiNH 2) powder for 1-3 h under the protection of nitrogen or argon, and pressing into a sheet with the thickness of 0.5-1 mm; the pressure of the tablet press ranges from 10 MPa to 20MPa, and alkaline earth metal hydride (MH 2) is any one of MgH 2、CaH2、BaH2;
(2) Loading the pressed slices into a reactor, slowly heating to 300-400 ℃, and introducing high-pressure hydrogen to perform hydrogen heat treatment; the temperature rising rate is 1-3 ℃/min.
(3) And cooling the material subjected to the hydro-thermal treatment to room temperature, and crushing the material under the protection of nitrogen or argon to obtain a target product.
In the step (1), the molar ratio of the materials is LiNH 2: MH2 =2:1-3, and the weight percentage of the Ti powder in the whole materials is 0.996-9.96 wt%.
The hydrogen heat treatment in the step (2) is a high-pressure hydrogen impact heat treatment process, wherein the high-pressure hydrogen is used for charging hydrogen to 10-20 MPa in a hydrogenation reactor, reacting for 0.5-1.0 h, then discharging hydrogen to normal pressure, and repeating the operations of charging hydrogen, reacting and discharging hydrogen for 5-10 h in total, wherein the purity of the hydrogen is more than or equal to 99.99%.
The invention has the innovation points that firstly, an alkaline earth metal amino compound is used as a double-active-center ammonia synthesis catalyst system of a nitrogen carrier, M (NH 2)2、TiH2 and LiH phases which are separated out through in-situ reaction have fresh clean surface interfaces and extremely high catalytic activity, an active site with high activity of a TiH 2 nano cluster plays a role by a MARS VAN KREVELEN heterogeneous catalytic mechanism, N 2 molecules can be rapidly dissociated into N atoms, then the activated N atoms are transferred to a LiH second active site to generate Li-N-H species, finally, the Li-N-H species are hydrogenated to generate ammonia, liH is regenerated, and in addition, tiH 2 is used as a hydrogenation catalyst to facilitate desorption of NH 3 to prepare ammonia, so that the general energy restriction relation existing between adsorption energy of reaction species on the surface of the ammonia synthesis catalyst and transition state energy is broken, and the catalyst system is suitable for synthesizing ammonia by chemical chains; the innovation point is that the components of the double-active-center ammonia synthesis catalyst are all derived from an in-situ reaction preparation process in the high-pressure hydrogen thermal shock treatment process, the alkaline earth metal amino compound nitrogen carrier, tiH 2 and LiH double-active catalytic centers which are separated out by reaction all have clean surface interfaces and high catalytic activity, and the three active matters are in the cooperation of division work and all participate in and act on the catalytic synthesis ammonia reaction, so that the synthesis of multi-component and multi-mechanism synergistic high-efficiency catalytic ammonia is realized.
Compared with the prior art, the invention has the following advantages:
1. The alkaline earth metal amino compound is firstly prepared as a double-active-center ammonia synthesis catalyst of a nitrogen carrier, is applied to the synthesis ammonia reaction under mild conditions, can obtain better synthesis ammonia performance under mild conditions (0.1-5.0 MPa, 150-300 ℃), is heated to 300 ℃ in a reactor, is introduced with reaction gas consisting of 1.0MPa, 25vol.% N 2 and 75vol.% H 2 to perform the synthesis ammonia catalytic reaction, and has the ammonia production rate as high as 13.56 mmol/(g.h), and is obviously superior to the synthesis ammonia performance of high-activity Ru-based and Fe-based synthesis ammonia catalysts reported in most documents.
2. The preparation method is simple, has no pollution in the whole process, does not contain rare noble metals, has low price and easy acquisition of raw materials, is suitable for large-scale industrial production, and has wide industrial application prospect and remarkable application value of synthetic ammonia.
Drawings
FIG. 1 is an X-ray diffraction (XRD) pattern of the catalyst prepared in example 1;
FIG. 2 is a 500-fold Scanning Electron Microscope (SEM) secondary electron image of the catalyst prepared in example 1;
FIG. 3 is a Scanning Electron Microscope (SEM) secondary electron image of the catalyst 6000 times prepared in example 1;
FIG. 4 is a Scanning Electron Microscope (SEM) back-scattered electron image of the catalyst 20000 times prepared in example 1;
FIG. 5 is a view of 100000 times Scanning Electron Microscope (SEM) back-scattered electron images of the catalyst prepared in example 1;
FIG. 6 is a graph showing the stability of ammonia production rate of the catalyst prepared in example 1 after the catalyst was subjected to ammonia synthesis catalytic reaction for 100 hours by introducing a reaction gas consisting of 25vol.% N 2 and 75vol.% H 2 at 300℃under 1.0 MPa.
Detailed Description
For a better understanding of the present invention, reference will now be made to the following description of the invention taken in conjunction with the accompanying drawings and examples, but the scope of the invention is not limited to the expression of the examples.
Example 1
Mixing LiNH 2 powder, ti powder and MgH 2 powder according to a molar ratio of 32:1:16 (i.e. 3.98wt.% of Ti powder) under the protection of nitrogen, grinding for 2h in a mortar, and then pressing the mixed powder into a sheet with the thickness of 0.5mm on a tablet press with the pressure of 15MPa; loading the pressed slices into a stainless steel hydrogenation reactor, arranging self-control valves at two ends of the hydrogenation reactor, heating to 350 ℃ at a speed of 1 ℃/min, charging hydrogen into the hydrogenation reactor to 20MPa by high-pressure hydrogen, reacting for 0.5h, discharging hydrogen to normal pressure, and repeating the operations of charging hydrogen, reacting and discharging hydrogen for 8h in total, wherein the purity of the hydrogen is 99.99%; and taking out the material cooled to room temperature after hydrogen heat treatment, crushing the material under the protection of nitrogen, and sieving the crushed material with a 200-mesh sieve to obtain the target product double-active-center ammonia synthesis catalyst (Mg (NH 2)2, liH and TiH 2）,TiH2 account for 4.14% of the total weight of the double-active-center ammonia synthesis catalyst), and the balance of Mg (NH 2)2 and LiH, liH: mg (molar ratio of NH 2)2 is 2:1).
FIG. 1 is an X-ray diffraction (XRD) pattern of the catalyst prepared in example 1, and it can be seen that the main phase compositions are Mg (NH 2)2 and LiH, since the TiH 2 content is too low, the XRD diffraction peaks are submerged in the background peak shapes of the main phases Mg (NH 2)2 and LiH), But is clearly visible in the electron microscope image. As shown in fig. 2, which shows a 500-time Scanning Electron Microscope (SEM) secondary electron image of the prepared catalyst, the particle size of the crushed material is 1-15 μm, the shape is irregular, one of the particles with the size of 10 μm is selected to be amplified to 6000 times as shown in fig. 3, and the large particles of the composite phase are observed to be formed by mutually bonding a plurality of spherical-like particles with smaller particle size, so that a multi-layer, multi-channel and abnormally rich reaction interface is formed; The back scattering electron mode images which are further amplified to 20000 times and 100000 times are shown in fig. 4 and 5, the TiH 2 nano-cluster is clearly visible, the brightness of the heavier Ti-containing element phase is obviously higher than that of the phase containing Li, mg, N, H element in the back scattering mode, and the particle size of TiH 2 particles is 10-100 nm, and the shape is irregular. Mg (NH 2)2、TiH2 and LiH phases) separated out through in-situ reaction has fresh and clean surface interface and extremely high catalytic activity, the TiH 2 nanocluster is used as a first active center of a catalyst, on one hand, N 2 molecules can be rapidly dissociated into N atoms in the synthesis reaction of ammonia, on the other hand, tiH 2 metal hydride is used as a hydrogenation catalyst, i.e. is beneficial to desorption of NH 3, is a novel high-quality synthetic ammonia catalyst, and activated N atoms are transferred to a second active site of LiH to generate Li 2 NH through reaction shown in the following formula (1), then Li 2 NH reacts with H 2 to generate NH 3 and LiH, and the cyclic regeneration of the reactant species is realized. High-activity Mg (NH 2)2 nitrogen carrier can effectively activate N 2 molecules under the condition of ammonia synthesis reaction, promote dissociation of N-N bond and generation of NH 3, and further improve the activity of the catalyst, the generation of ammonia is accelerated. Mg (NH 2)2 nitrogen carrier, tiH 2 and LiH double-activity catalytic center cooperate to show the activity of synthesizing ammonia far exceeding noble metal Ru-based catalyst.
4LiH+N2=2Li2NH+H2 （1）
5Li2NH+10H2=10LiH+5NH3 （2）
FIG. 6 is a graph showing the thermal stability of the catalytic activity of the catalyst after 100 hours of catalytic reaction of synthetic ammonia by heating the prepared catalyst product to 300℃in a reactor and introducing a reaction gas consisting of 1.0MPa, 25vol.% N 2 and 75vol.% H 2, wherein the ammonia production rate is as high as 13.56 mmol/(g.h), and the fluctuation of the ammonia production rate after 100 hours reaction is small, which indicates that the prepared catalyst has high-efficiency and stable catalytic performance for synthesizing ammonia. Example 2
LiNH 2 powder, ti powder and BaH 2 powder are mixed according to a molar ratio of 20:0.682:20 (i.e., 0.996wt.% of Ti powder) was mixed and ground in a mortar for 1 hour, followed by pressing the mixed powder into 0.7mm flakes on a tablet press at a pressure of 10MPa; loading the pressed slices into a stainless steel hydrogenation reactor, arranging self-control valves at two ends of the hydrogenation reactor, heating to 300 ℃ at the speed of 2 ℃/min, charging hydrogen into the hydrogenation reactor to 10MPa by high-pressure hydrogen, reacting for 1.0h, discharging hydrogen to normal pressure, and repeating the operations of charging hydrogen, reacting and discharging hydrogen for 5h in total, wherein the purity of the hydrogen is 99.999%; and taking out the material cooled to room temperature after hydrogen heat treatment, crushing the material under the protection of argon, and sieving the crushed material with a 200-mesh sieve to obtain target products TiH 2、Ba(NH2)2 and LiH, namely the double-active-center ammonia synthesis catalyst. TiH 2 accounts for 1.04 percent of the total weight of the double-active-center ammonia synthesis catalyst, and the balance of Ba (NH 2)2 and LiH, and the molar ratio of LiH to Ba (NH 2)2) is 1:1.
Example 3
LiNH 2 powder, ti powder and CaH 2 powder are mixed according to a molar ratio of 20:3.98:30 (i.e., 9.96wt.% of Ti powder) and grinding in a mortar for 3 hours, and then pressing the mixed powder into 1.0mm flakes on a tablet press at 20MPa; loading the pressed slices into a stainless steel hydrogenation reactor, arranging self-control valves at two ends of the hydrogenation reactor, heating to 400 ℃ at the speed of 3 ℃/min, charging hydrogen into the hydrogenation reactor to 15MPa by high-pressure hydrogen, reacting for 0.7h, discharging hydrogen to normal pressure, and repeating the operations of charging hydrogen, reacting and discharging hydrogen for 10h in total, wherein the purity of the hydrogen is 99.99%; and taking out the material cooled to room temperature after hydrogen heat treatment, crushing the material under the protection of nitrogen, and sieving the crushed material with a 200-mesh sieve to obtain the target product double-active-center ammonia synthesis catalyst. TiH 2 accounts for 11.48% of the total weight of the double active center ammonia synthesis catalyst, and the balance is Ca (NH 2)2 and LiH, liH: ca (molar ratio of NH 2)2 is 2:3).
Comparative example
Mixing LiH, tiH 2 powder and Mg (NH 2)2 powder according to a molar ratio of 32:1:16 under the protection of nitrogen, grinding for 2 hours in a mortar, pressing the mixed powder into 0.5mm slices on a tablet press, wherein the pressure of the tablet press is 15MPa, loading the pressed slices into a stainless steel hydrogenation reactor, heating to 350 ℃ at a speed of 1 ℃/min, charging hydrogen into the hydrogenation reactor to 20MPa with high-pressure hydrogen, reacting for 0.5 hours, discharging hydrogen to normal pressure, repeating the operations of charging hydrogen, reacting and discharging hydrogen for 8 hours, wherein the total time is 99.99 percent, taking out the materials cooled to room temperature after hydrogen heat treatment, crushing the materials under the protection of nitrogen, and sieving with a 200-mesh sieve to obtain the target product comparative catalyst.
50G of the catalysts of examples 1, 2,3 and comparative example are respectively taken for carrying out the synthetic ammonia reaction, the catalyst product is heated to 300 ℃ in a reactor, reaction gas with the pressure of 1.0MPa, 25vol.% N 2 and 75vol.% H 2 is introduced for carrying out the synthetic ammonia catalytic reaction, and the ammonia production rates are shown in the table 1, so that the ammonia production rates of the catalysts corresponding to examples 1, 2 and 3 separated out through the in-situ reaction are far higher than the ammonia production rates of the catalysts prepared by taking Mg (NH 2)2、TiH2 and LiH powder as raw materials, and the double active center composite catalyst separated out through the in-situ reaction has excellent normal pressure low temperature catalytic synthetic ammonia performance and stability.
Claims (5)
1. The double-active-center ammonia synthesis catalyst is characterized by comprising an alkaline earth metal amino compound carrier M (NH 2)2, double-active catalytic centers TiH 2 and LiH), wherein M (NH 2)2、TiH2 and LiH are separated out through in-situ reaction and are uniformly dispersed, tiH 2 accounts for 1.04-11.48% of the total weight of the double-active-center ammonia synthesis catalyst, and the balance M (NH 2)2 and LiH, liH: M (molar ratio of NH 2)2 is 2:1-3; tiH 2 is uniformly dispersed in the catalyst in the form of nanoclusters, and the cluster particle size is 10-100 nm);
The in-situ reaction is that under the protection of nitrogen or argon, MH 2 powder, ti powder and LiNH 2 powder are mixed and ground for 1-3 hours, and pressed into a sheet with the thickness of 0.5-1 mm by a tablet press; the pressure range of the tablet press is 10-20 MPa; loading the pressed slices into a reactor, slowly heating to 300-400 ℃, and introducing high-pressure hydrogen to perform hydrogen heat treatment; the temperature rising rate is 1-3 ℃/min; the hydrogen heat treatment is a high-pressure hydrogen impact heat treatment process, wherein the high-pressure hydrogen is used for charging hydrogen to 10-20 MPa in a hydrogenation reactor, reacting for 0.5-1.0 h, discharging hydrogen to normal pressure, and repeating the operations of charging hydrogen, reacting and discharging hydrogen for 5-10 h in total, wherein the purity of the hydrogen is more than or equal to 99.99%.
2. The catalyst of claim 1, wherein the alkaline earth amino compound nitrogen carrier M (NH 2)2 is one of Mg (NH 2)2、Ca(NH2)2、Ba(NH2)2).
3. A method for preparing the double active site ammonia synthesis catalyst according to claim 1 or 2, comprising the steps of:
(1) Under the protection of nitrogen or argon, mixing and grinding MH 2 powder, ti powder and LiNH 2 powder for 1-3 h, and pressing into a sheet with the thickness of 0.5-1 mm by a tablet press; the pressure range of the tablet press is 10-20 MPa; the MH 2 is any one of MgH 2、CaH2、BaH2;
(2) Loading the pressed slices into a reactor, slowly heating to 300-400 ℃, and introducing high-pressure hydrogen to perform hydrogen heat treatment; the temperature rising rate is 1-3 ℃/min; the hydrogen heat treatment is a high-pressure hydrogen impact heat treatment process, wherein high-pressure hydrogen is used for charging hydrogen to 10-20 MPa in a hydrogenation reactor, reacting for 0.5-1.0 h, discharging hydrogen to normal pressure, and repeating the operations of charging hydrogen, reacting and discharging hydrogen for 5-10 h in total, wherein the purity of the hydrogen is more than or equal to 99.99%;
(3) And cooling the material subjected to the hydro-thermal treatment to room temperature, and crushing the material under the protection of nitrogen or argon to prepare the target product double-active-center ammonia synthesis catalyst.
4. The method for preparing a catalyst for synthesis of ammonia with double active centers according to claim 3, wherein in the step (1), the molar ratio of materials is LiNH 2:MH2 = 2:1-3, and the weight percentage of ti powder in the total materials is 0.996-9.96 wt.%.
5. Use of a dual active center ammonia synthesis catalyst according to claim 1 or 2, wherein the dual active center ammonia synthesis catalyst is used for synthesis of ammonia under mild conditions or for chemical chain synthesis of ammonia.
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