A highly efficient bimetallic catalyst and its application in hydrogen production from ammonia borane alcoholysis

Abstract

The invention discloses a high-efficiency bimetallic catalyst, comprising a substrate carbide C3N4 and a carbonization catalyst CoCu / CoCuN dispersed in the substrate carbide. x The method for preparing a highly efficient bimetallic catalyst comprises the following steps: (1) uniformly distributing an appropriate amount of dicyandiamide in four porcelain crucibles, heating to 350°C in a muffle furnace for 2 hours to obtain a product DCD-350; (2) mixing the substance DCD-350 obtained in step (1), Co(acac)2 and Cu(acac)2 in a ball mill for 1.5 hours in an alternating motion to prepare CoCu / CoCuN x Compared with the prior art, the advantages of the present invention are: it is easy to increase the reaction rate, maintain a high turnover rate, save costs, is economically feasible, and is more stable.

Description

Technical Field

[0001] The invention relates to the technical field of hydrogen production, and in particular to a high-efficiency bimetallic catalyst and application thereof in hydrogen production by alcoholysis of ammonia borane. Background Art

[0002] Ammonia borane AB is a chemically stable compound in which 3 equivalents of H2 are stored; hydrogen can be released during pyrolysis, hydrolysis or solvent decomposition; the pyrolysis of ammonia borane AB requires high temperature and power consumption, and the hydrolysis of ammonia borane produces NH4B(OH)4, which is in equilibrium with NH3 and B(OH)3, and therefore may damage the Pt-based anode material of the nephew exchange membrane fuel cell PEMFC, as shown in eq1.

[0003]

[0004] This problem can be solved if methanol is used instead of water. Ammonia borane AB reacts with methanol to form a stable borate in an unionized form that is soluble in methanol, preventing the formation of free ammonia molecules, as shown in eq2. Therefore, among these dehydrogenation routes, the solvent decomposition process is more suitable for practical application needs. However, the dehydrogenation of ammonia borane AB in solvent separation can only be achieved in ambient atmosphere and room temperature in the presence of a suitable catalyst.

[0005] NH3BH3+4CH3OH→HN4B(OCH3)4+3H2 (2)

[0006] Most H2 release reactions of hydrogen storage materials in aqueous media have a common feature, that is, adsorption of substrates (such as H2O molecules) followed by OH bond cleavage. Taking AB hydrolysis as an example, mechanism studies have shown that it follows the bimolecular Langmuir-Hinshelwood kinetic model. In the AB hydrolysis process, AB and H2O molecules are generally co-adsorbed on the catalyst surface to form H3N-BH2* and H + The activation of the OH bonds of H2O*, that is, borane AB is adsorbed on Co NPs and CoN4 sites, H2O is adsorbed on CoN4 sites, and then the OH bonds of H2O molecules are broken to form OH- and H + , which is the rate-determining step, then -OH attacks the center of the B atom to form H3N-BH2(OH)*, which is then reduced to eliminate two H from AB and H2O respectively. + , H2 is absorbed and desorbed from the catalyst surface; H2O* further reacts with H3N-BH2(OH)* to form [H4N] + and [BO2]-, releasing two more H2. However, for methanol decomposition, the adsorption kinetics and reactivity are usually relatively slow due to the large molecular weight of the CH3-OH molecule.

[0007] Precious metal nanoparticle catalysts NPs generally produce better results for the methanol decomposition of ammonia borane AB than non-precious metal nanoparticle catalysts. However, they still have a long way to go from commercialization due to technical challenges such as high cost and low stability, as shown in Table 1. Non-precious metal nanoparticle catalysts such as Co, Fe, Ni and Cu are much cheaper and more economical, but they generally exhibit low durability and slow reaction rates. Therefore, how to improve the reactivity and stability of non-precious metal nanoparticle catalysts remains a major challenge in the production of hydrogen through hydrogen storage materials in this application.

[0008] catalyst <![CDATA[TOF(mol H2·mol cat -1 ·min -1 )]]> <![CDATA[Ea(kJ·mol -1 )]]> Ru@PVP NPS 47.7 58.0 <![CDATA[Ni 20 Ru 80 @HAP NPS]]> 58.9 65.9 Pd@PVP NPS 22.3 35.0 Pd / C 1.9 / <![CDATA[Co 35 Pd 65 / C]]> 27.7 25.5 <![CDATA[Co-Co2B]]> 7.5 / <![CDATA[Ni-Ni3B]]> 5.0 / Ni@PVP NPS 12.1 62.0 <![CDATA[CoCu / CoCuN6-CNT]]> 13.3 38.0

[0009] Table 1. Comparison of performance of precious metal and non-precious metal catalysts for ammonia borane methanol decomposition Summary of the invention

[0010] The technical problem to be solved by the present invention is to overcome the above technical defects and provide a high-efficiency bimetallic catalyst which is convenient for improving the reaction rate, maintaining a high turnover rate, saving costs, being economically feasible and more stable, and its application in hydrogen production by alcoholysis of ammonia borane.

[0011] In order to solve the above technical problems, the technical solution provided by the present invention is a highly efficient bimetallic catalyst, comprising a substrate carbide C3N4 and a carbonization catalyst CoCu / CoCuN dispersed in the substrate carbide. x -CNT.

[0012] Furthermore, the method for preparing a highly efficient bimetallic catalyst comprises the following steps:

[0013] (1) Distribute an appropriate amount of dicyandiamide evenly in four porcelain crucibles, heat to 350° C. in a muffle furnace for 2 hours to obtain the product DCD-350;

[0014] (2) mixing the DCD-350 obtained in step (1), Co(acac)2 and Cu(acac)2 in an alternating motion on a ball mill for 1.5 hours to prepare a mixture of Co(acac)2 / Cu(acac)2 / DCD-350;

[0015] (3) Add citric acid solution (6 mL) to the mixture of Co(acac)2 / Cu(acac)2 / DCD-350 and dry at 90°C for 2-4 hours until the mass is constant. The presence of citric acid solution will help the metal cobalt and copper to disperse into the carbide C3N4 and increase the carbonization catalyst CoCu / CoCuN x -Specific surface area of ​​CNTs;

[0016] (4) The dried mixture obtained in step (3) was returned to the ball mill and subjected to an alternating motion cycle for another 1.5 hours. The mixture was then transferred to a ceramic crucible and heated at 2.6 °C min-1 under a steady flow of Ar. -1 The samples were heated to 800 °C at a heating rate of 100 °C for 2 h, an Ar atmosphere was generated in a tube furnace, and then cooled to room temperature by natural cooling to obtain CoCu / CoCuN x -CNT--800T.

[0017] Furthermore, the citric acid solution is prepared by dissolving citric acid (10 g) in 0.5 L of deionized water and then diluting to 1 L with 95% ethanol;

[0018] Furthermore, the alternating motion requires the ball mill to change its rotation direction every 5 minutes;

[0019] Furthermore, the catalyst is used to catalyze the hydrogen production reaction of ammonia borane AB;

[0020] Furthermore, the reaction medium of the hydrogen production reaction is anhydrous methanol;

[0021] The advantages of the present invention compared with the prior art are: firstly, the alcoholysis hydrogen production of the present application has obvious advantages over the existing high-energy consumption pyrolysis and hydrolysis hydrogen production, and can produce high-purity hydrogen on demand, avoiding the damage of the trace nitrogen generated during hydrolysis to the Pt-based anode material of the proton exchange membrane fuel cell PEMFC, and has higher safety. The byproducts after alcoholysis hydrogen production can be recovered and the original state of the hydrogen storage material can be restored in one step; secondly, the catalyst is prepared by non-precious metals, which is low in cost and economically feasible, and then the active sites of the metal atoms are effectively utilized by reducing the size of the non-precious metal bimetallic catalyst to increase the effective surface area of ​​the reaction, so that the catalyst can efficiently and completely release the stored H2 from the hydrogen storage material in the shortest possible time and with the least possible amount of catalyst, and has a high conversion rate TOF; and it can catalyze the hydrogen release reaction of ammonia borane at a relatively low temperature, and has the characteristics of low activation energy and long life. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Comparison of Co SACs and Cu SACs with CoCu diatomic catalysts: a) reaction kinetics of AB alcoholysis; b) TOF and specific generation rate. DETAILED DESCRIPTION

[0023] The following is combined with Figure 1 The present invention is described in further detail.

[0024] A high-efficiency bimetallic catalyst, characterized by comprising a substrate carbide C3N4 and a carbonization catalyst CoCu / CoCuN dispersed in the substrate carbidex -CNT.

[0025] A method for preparing a high-efficiency bimetallic catalyst comprises the following steps:

[0026] (1) 80 g (0.95 mol) of dicyandiamide was evenly distributed in four porcelain crucibles and heated to 350° C. in a muffle furnace MTI OTF-1200X for 2 hours to obtain product DCD-350;

[0027] (2) 10 g of the substance DCD-350 obtained in step (1), 0.1488 g (0.5787 mmol) of Co(acac)2, and 0.1019 g (0.2893 mmol) of Cu(acac)2 were mixed in a ball mill for 1.5 hours, with the rotation direction changed every 5 minutes, to prepare a mixture of Co(acac)2 / Cu(acac)2 / DCD-350; 6 mL of 10 g·L -1 The citric acid solution is added to the mixture Co(acac)2 / Cu(acac)2 / DCD-350 and dried at 90°C for 2-4 hours until the mass is constant; wherein the citric acid solution is prepared by dissolving 10g of citric acid in 0.5L of deionized water and then diluting it to 1L with 95% ethanol;

[0028] (3) The dried mixture obtained in step (2) was returned to the ball mill and subjected to an alternating motion cycle for another 1.5 hours, then transferred to a ceramic crucible and heated at a steady temperature of 150 mL min -1 Under Ar flow, the temperature was 2.6℃·min -1 The carbonization temperature was 800 °C and the carbonization temperature was 800 °C to obtain CoCu / CoCuNx-CNT-800T.

[0029] The application of the high-efficiency bimetallic catalyst of the present application is to use the catalyst to catalyze the hydrogen production reaction of ammonia borane AB; the reaction medium of the hydrogen production reaction is anhydrous methanol.

[0030] Among the dehydrogenation routes of pyrolysis, hydrolysis and solvent decomposition, the solvent decomposition process is more suitable for practical application needs, but the dehydrogenation of ammonia borane AB in solvent separation can only be achieved in ambient atmosphere and room temperature in the presence of a suitable catalyst; more importantly, the methanol decomposition product of NH4B(OCH3)4 can be easily converted into ammonia borane by a one-step reaction with LiAlH4 in the presence of NH4Cl, as shown in eq3.

[0031]

[0032] Compared to noble metal nanoparticle catalysts NPs, single-atom catalysts SACs or atomically dispersed metal atoms, in which the nanoparticles are shrunk down to the atomic level, exhibit outstanding performance in many catalytic reactions. These single-atom nanoparticle catalysts SACs show better reactivity and selectivity than their noble metal nanoparticle catalyst NPs analogs in many catalytic reactions. Until now, little is known about the use of non-noble metal SACs for hydrogen release from ammonia borane. We synthesized a bifunctional Co catalyst with high metal loading via a facile one-pot pyrolysis method to obtain Co and Cu NPs partially supported on CNTs with atomically dispersed CoN4 as well as CuN4. During the alcoholysis of ammonia borane AB, not only the catalytic activity was enhanced by 2 times, but also the stability of the catalyst was improved due to the anti-agglomeration type of the catalyst (strong Co as well as Cu anchoring) on ​​the N atoms. Encouragingly, the catalytic alcohol release hydrogen release reaction can be run 40 times without obvious decay, which is characterized by a long service life.

[0033] To further improve the reactivity of cobalt-copper-based catalysts, we studied the d-band model in atomic systems: In theory, the pairing and coupling of non-noble metals with another metal forms a single energy state, which becomes the d-band center. When there is electron density transfer, an upward shift of the d-band center relative to the Fermi energy will occur. The stronger the upward shift of the d-band center, the more empty antibonding states are formed, and the stronger the binding energy of the substrate; it is recorded that the reactivity TOF of CoCu NPs is about 3 times that of pure Co analogs, so we synthesized CoCu / CoCuN6-CNT-800T diatomic catalysts, which made the TOF of AB alcoholysis reach 13.3 mol at a lower temperature of 25 °C. CoCu -1 min -1 ,Preliminary results show that CoCu / CoCuN6-CNT is more effective than CoCu NPs. Compared with Co / CoN4-CNT and Cu / CuN4-CNT at 25℃, the rate of CoCu / CoCuN6-CNT is more than 6 times, such as Figure 1 shown.

[0034] The present invention and its embodiments are described above, and such description is not restrictive. The drawings show only one embodiment of the present invention, and the actual structure is not limited thereto. In short, if ordinary technicians in the field are inspired by it, without departing from the purpose of the invention, they can design a structure and embodiment similar to the technical solution without creativity, which should belong to the protection scope of the present invention.

Claims

1. A method for preparing a highly efficient bimetallic catalyst, characterized in that: The following steps are involved: (1) Distribute an appropriate amount of dicyandiamide evenly in four porcelain crucibles, heat to 350° C. in a muffle furnace for 2 hours to obtain the product DCD-350; (2) mixing the DCD-350 obtained in step (1), Co(acac)2 and Cu(acac)2 in an alternating motion on a ball mill for 1.5 hours to prepare a mixture of Co(acac)2 / Cu(acac)2 / DCD-350; (3) 6 mL of citric acid solution was added to the mixture of Co(acac)2 / Cu(acac)2 / DCD-350 and dried at 90 °C for 2-4 hours until the mass was constant; (4) The dried mixture obtained in step (3) was returned to the ball mill and subjected to an alternating motion cycle for another 1.5 hours. The mixture was then transferred to a ceramic crucible and heated at 2.6 °C min-1 under a steady flow of Ar. -1 The samples were heated to 800 °C at a heating rate of 100 °C for 2 h, an Ar atmosphere was generated in a tube furnace, and then cooled to room temperature by natural cooling to obtain CoCu / CoCuN x -CNT--800T.

2. The method for preparing a high-efficiency bimetallic catalyst according to claim 1, characterized in that: The citric acid solution was prepared by dissolving 10 g of citric acid in 0.5 L of deionized water and then diluting the solution to 1 L with 95% ethanol.

3. The method for preparing a high-efficiency bimetallic catalyst according to claim 1, characterized in that: The alternating motion requires the ball mill to change its direction of rotation every 5 minutes.

4. A high-efficiency bimetallic catalyst prepared by the preparation method according to any one of claims 1 to 3.

5. The use of the high-efficiency bimetallic catalyst according to claim 4, characterized in that: The high-efficiency bimetallic catalyst is used to catalyze the hydrogen production reaction of ammonia borane AB.

6. The use of the high-efficiency bimetallic catalyst according to claim 5, characterized in that: The reaction medium of the hydrogen production reaction is anhydrous methanol.

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