Preparation method of nickel-based catalyst based on expanded graphite dispersion and application thereof in hydrogen production by ammonia cracking

Abstract

The application belongs to the technical field of hydrogen production, and discloses a preparation method of a nickel-based catalyst based on expanded graphite dispersion and application of the catalyst in hydrogen production by ammonia cracking, which comprises the following steps: (1) uniformly dispersing expanded graphite in deionized water; (2) adding nickel chloride hexahydrate and cerium chloride heptahydrate to uniformly disperse on the surface of the expanded graphite; (3) adding sodium borohydride to the mixture to perform reduction, centrifugation and washing; (4) calcining the obtained solid by centrifugation, and oxidizing and crystallizing Ce(III) into CeO2 under a high-temperature environment; and (5) reducing to obtain a Ni / CeO2-expanded graphite catalyst under pure hydrogen condition. The three-dimensional precursor is formed by the reduction method of nickel chloride, cerium chloride and expanded graphite, Ni and CeO2 are dispersed and fixed on the expanded graphite, and through the confinement effect and electronic transfer synergistic effect of the expanded graphite, the catalyst exhibits excellent catalytic activity in the ammonia decomposition reaction.

Description

Technical Field

[0001] This invention belongs to the field of hydrogen production technology, specifically relating to an ammonia cracking hydrogen production catalyst. Background Technology

[0002] With the global energy transition towards cleaner and lower-carbon energy sources, hydrogen energy, as a highly efficient and clean secondary energy source, has attracted significant attention. However, the low volumetric energy density of hydrogen leads to high storage and transportation costs and poses safety risks, becoming a bottleneck restricting the large-scale development of hydrogen energy. Ammonia (NH3), as a carbon-free hydrogen storage carrier, has significant advantages such as high hydrogen content, ease of liquefaction, and well-developed storage and transportation infrastructure. In-situ production of high-purity hydrogen through ammonia catalytic cracking is an effective way to solve the challenges of hydrogen energy storage and transportation.

[0003] Ammonia cracking is a strongly endothermic reaction, and efficient catalysts are crucial for lowering the activation energy and increasing the conversion rate. Currently, ruthenium (Ru)-based catalysts exhibit the highest catalytic activity under mild conditions, but their scarcity and high cost hinder large-scale industrial application. Nickel (Ni)-based catalysts, due to their similar d-electron orbital configuration to noble metals, high intrinsic activity, and low cost, have become the most promising alternative materials. However, existing nickel-based catalysts still face two major technical bottlenecks in industrial applications:

[0004] First, poor thermal stability. To achieve high ammonia conversion rates, the reaction typically needs to be carried out at high temperatures (>600℃). At these temperatures, nickel nanoparticles are prone to thermal sintering and agglomeration, leading to a sharp decrease in the active surface area and rapid catalyst deactivation. Second, limited mass and heat transfer. Ammonia cracking is a strongly endothermic process. Traditional oxide supports (such as Al2O3 and SiO2) have poor thermal conductivity, resulting in a "cold spot" effect where localized temperature drops are likely to occur within the catalytic bed, severely limiting the reaction rate.

[0005] To address heat transfer issues, high thermal conductivity carbon materials have been introduced as catalyst supports. Among them, expanded graphite (EG) not only possesses the excellent thermal conductivity of natural graphite, enabling rapid homogenization of the catalytic bed temperature and elimination of cold spots, but also boasts a rich porous worm-like structure and a large specific surface area, which is beneficial for the loading and dispersion of active metals.

[0006] To address the issues of nickel's sintering susceptibility and the need for further enhancement of its catalytic activity, modification with additives is a key approach. Cerium dioxide (CeO2) possesses unique oxygen storage and release capabilities and abundant oxygen vacancies, and can generate strong metal-support interactions (SMSI) with transition metals. Existing literature and theoretical studies indicate that introducing CeO2 as an additive into nickel-based catalysts can, on the one hand, adjust the local electronic structure of Ni through electron transfer effects, reducing the activation energy of the ammonia cracking resolution step (i.e., the recombination and desorption of nitrogen atoms); on the other hand, the physical anchoring and chemical repulsion effects of CeO2 can effectively limit the migration of Ni particles at high temperatures, significantly inhibiting sintering.

[0007] Chinese patent application number 2024100699927, entitled "A Catalyst for Hydrogen Production by Ammonia Decomposition and its Preparation Method and a Reactor for Hydrogen Production by Ammonia Decomposition," discloses a catalyst for hydrogen production by ammonia decomposition. This catalyst includes a support, an active component, and an additive. The support is a graphitized mesoporous carbon material, and the active component is at least one of noble metal nanoparticles, noble metal alloy nanoparticles, and noble metal-transition metal alloy nanoparticles. However, noble metals are extremely expensive, limiting their large-scale application and confining them mainly to laboratories or specific high-performance applications. Nickel, on the other hand, is abundant, inexpensive, and exhibits excellent and stable catalytic performance.

[0008] Chinese patent application number 2014102676714, entitled "A Ni-CeO2@graphene Composite Nanocatalyst for Hydrogen Production from Ammonia Borane Hydrolysis and Its Preparation Method," discloses a Ni-CeO2@graphene composite nanocatalyst for hydrogen production from ammonia borane hydrolysis. This catalyst uses graphene as a support and sodium borohydride (NaBH4) and ammonia borane (AB) as reducing agents, obtained by reducing the precursors nickel salt and cerium nitrate. Expanded graphite, compared to graphene, has a three-dimensional, loose, porous, worm-like structure; it also has a larger surface area, which is beneficial for dispersing catalyst particles; furthermore, expanded graphite is relatively inexpensive.

[0009] Based on the above analysis, how to apply expanded graphite to the preparation process of ammonia cracking hydrogen production catalysts and utilize the structural characteristics of expanded graphite to solve the problems of poor thermal stability and limited heat and mass transfer of existing ammonia cracking hydrogen production catalysts is a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0010] The purpose of this invention is to provide a method for preparing a nickel-based catalyst based on expanded graphite dispersion and its application in ammonia cracking for hydrogen production, which solves the problems of poor conductivity and mass transfer efficiency of conventional ammonia cracking catalysts.

[0011] To achieve the above objectives, the present invention provides the following technical solution:

[0012] The preparation method includes the following steps: (1) Dispersing expanded graphite uniformly in deionized water; each gram of expanded graphite is dispersed in 260-450 mL of deionized water.

[0013] Expanded graphite can be uniformly dispersed in deionized water using ultrasonic dispersion.

[0014] (2) Add nickel chloride hexahydrate and cerium chloride heptahydrate to disperse them evenly on the surface of expanded graphite; the mass ratio of expanded graphite to nickel chloride hexahydrate is 1:3-6; the mass ratio of expanded graphite to cerium chloride heptahydrate is 1:1-5.

[0015] After adding nickel chloride hexahydrate and cerium chloride heptahydrate, ultrasonic and magnetic stirring can be used to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0016] (3) Add sodium borohydride to the mixture of expanded graphite, deionized water, nickel chloride hexahydrate and cerium chloride heptahydrate for reduction, centrifugation and washing;

[0017] Sodium borohydride was added to the above mixture to remove Ce. 3+ with Ni 2+ Alkaline coprecipitation, while simultaneously rapid in-situ reduction of Ni 2+ For Ni 0 This yielded Ni active sites with small particle size, high dispersion, and uniform distribution.

[0018] The mass ratio of expanded graphite to sodium borohydride is 1:2-5.

[0019] First, add sodium borohydride to deionized water and ultrasonically disperse it into a sodium borohydride solution. Then, add the sodium borohydride solution to the above mixture and stir magnetically. Add 1g of sodium borohydride to every 100mL of deionized water in the sodium borohydride solution.

[0020] (4) The solid obtained by centrifugation is calcined and Ce(Ⅲ) is oxidized and crystallized into CeO2 under high temperature environment.

[0021] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction under pure hydrogen conditions; the hydrogen reduction temperature was 400℃. Because NiO species are generated during calcination or storage, this part of NiO species was reduced under pure hydrogen conditions before the catalytic reaction to obtain Ni / CeO2-expanded graphite catalyst.

[0022] The present invention also provides a Ni / CeO2-expanded graphite catalyst prepared by the above preparation method.

[0023] This invention also provides the application of Ni / CeO2-expanded graphite catalyst in ammonia cracking for hydrogen production.

[0024] Sodium borohydride, as a strong reducing agent, can rapidly and efficiently remove Ni from the precursor.2+ In-situ reduction to highly dispersed Ni 0 Because sodium borohydride reduction has a fast reaction rate and produces numerous nucleation sites, it can yield small-sized, highly dispersed, and uniformly distributed Ni active sites, achieving Ni pre-dispersion from the source. The pre-dispersed Ni is then used in the subsequent high-temperature calcination process. 0 It will be oxidized to NiO, but because it has been pre-dispersed in the early stage, NiO is still uniformly anchored on the CeO2 surface in a highly dispersed state, and the dispersibility is effectively maintained.

[0025] Before catalytic testing, reduction with H2 at 400℃ can directionally reduce NiO species generated during calcination or storage to active Ni. 0 This can restore the catalytic active sites and meet the requirements of the catalytic reaction for the valence state of the active components.

[0026] The beneficial effects of adopting the above technical solution are:

[0027] This invention relates to a three-dimensional precursor formed by the reduction of nickel chloride, cerium chloride, and expanded graphite. Ni and CeO2 are dispersed and fixed on expanded graphite. Through the confinement effect and synergistic effect of electron transfer of expanded graphite, the catalyst exhibits excellent catalytic activity in the ammonia decomposition reaction.

[0028] The multi-level porous structure of expanded graphite effectively binds the migration of nickel nanoparticles, inhibits the agglomeration and sintering of active components under high-temperature reaction conditions, and the catalyst can maintain high activity stability and significantly extend its lifespan during long-term operation.

[0029] 1. Cerium oxide nanoparticles are grown in situ and anchored in the sheets and channels of expanded graphite to form a stable three-dimensional conductive network.

[0030] 2. The synergistic effect of nickel, cerium oxide, and expanded graphite: cerium oxide promotes the dispersion and redox ability of nickel, while expanded graphite provides a high specific surface area and electron conduction channels, which together enhance catalytic activity and stability.

[0031] 3. Expanded graphite provides a huge specific surface area and abundant mesopores / macropores, providing an ideal space for the high dispersion of active components (such as cerium oxide and nickel nanoparticles), effectively preventing the agglomeration and sintering of active sites.

[0032] 4. Expanded graphite retains the layered structure of graphite, but the layers are greatly expanded. These interlayer regions can act as a unique "nanoreactor," confining the active components. This confinement effect can alter the diffusion path and residence time of reactants, while also allowing more cerium oxide and nickel to adhere to it.

[0033] 5. Excellent thermal conductivity of expanded graphite: Expanded graphite supports can quickly remove the heat of reaction from the active sites or uniformly transfer heat to the entire catalyst, thereby avoiding localized overheating and the formation of "hot spots." Benefits: Improved catalyst stability, extended service life, and prevention of sintering and deactivation of active components due to localized high temperatures.

[0034] 6. The excellent electrical conductivity of expanded graphite may promote electron transfer at the metal-carrier interface and enhance the synergistic effect.

[0035] 7. Expanded graphite has good chemical stability in non-oxidizing atmospheres and various reaction media, and is not prone to side reactions with reactants / products.

[0036] 8. Its three-dimensional network structure also gives it a certain mechanical strength, which is superior to pure carbon nanotubes or graphene aerogels and other nano-carbon materials, making it more suitable for the filling requirements of industrial reactors.

[0037] 9. Synergy between cerium oxide and expanded graphite: Cerium oxide is responsible for chemical activation and protection, while expanded graphite is responsible for physical dispersion, electrical conductivity and thermal conductivity. Together, they create an ideal microenvironment for the nickel active center.

[0038] 10. The three structures work together to form a highly exposed and stable "Ni-CeO2-EG" tertiary structure with a "Ni-CeO2 interface". This structure can maintain long-term stability under harsh reaction conditions (such as high temperature and high flow rate).

[0039] 11. Synergistic stability among the three: The self-cleaning ability of the catalyst is greatly enhanced, and its lifespan is significantly extended. Nickel particles maintain a small size, and their activity does not decline.

[0040] 12. The porous structure of expanded graphite provides unobstructed channels for the diffusion of reactants and products, reducing mass transfer resistance.

[0041] 13. The high thermal conductivity of expanded graphite is like embedding a "heat sink" in the reactor, which makes the catalyst bed temperature uniform, avoids deactivation and side reactions caused by local overheating, and also makes the reactor easier to control. Attached Figure Description

[0042] Figure 1 The image shows the XRD pattern of Ni / CeO2-expanded graphite prepared in Example 7.

[0043] Figure 2 SEM image of Ni / CeO2-expanded graphite prepared in Example 7.

[0044] Figure 3 TEM image of Ni / CeO2-expanded graphite prepared in Example 7.

[0045] Figure 4 EDX image of Ni / CeO2-expanded graphite prepared in Example 7.

[0046] Figure 5 This is a comparison chart of the ammonia decomposition efficiency of the catalyst with expanded graphite added and the catalyst without expanded graphite added, prepared in Example 7.

[0047] Figure 6 The ammonia decomposition efficiency of the catalysts prepared in Examples 1-4 was obtained by changing the mass ratio of nickel and cerium oxide while keeping the amount of expanded graphite constant.

[0048] Orange represents Example 1, green represents Example 2, purple represents Example 3, and blue represents Example 4.

[0049] Figure 7 Ammonia decomposition efficiency graphs for catalysts prepared in Examples 5-8, with varying the mass of expanded graphite while keeping the mass ratio of nickel and cerium oxide constant.

[0050] Purple is Example 5, gray is Example 6, burgundy is Example 7, and light blue is Example 8.

[0051] Figure 8 The stability test diagram of the Ni / CeO2-expanded graphite catalyst prepared in Example 7 at 580 °C is shown. Detailed Implementation

[0052] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0053] Example 1

[0054] (1) Take 0.0350g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0055] (2) Add 0.1400g NiCl2·6H2O and 0.1500g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0056] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0057] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0058] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0059] Example 2

[0060] (1) Take 0.0350g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0061] (2) Add 0.1665g NiCl2·6H2O and 0.1120g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0062] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0063] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0064] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0065] Example 3

[0066] (1) Take 0.0350g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0067] (2) Add 0.1900g NiCl2·6H2O and 0.0745g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0068] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0069] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0070] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0071] Example 4

[0072] (1) Take 0.0350g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0073] (2) Add 0.2100g NiCl2·6H2O and 0.0400g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0074] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0075] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0076] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0077] Example 5

[0078] (1) Take 0.0350g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0079] (2) Add 0.1900g NiCl2·6H2O and 0.0745g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0080] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0081] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0082] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0083] Example 6

[0084] (1) Take 0.0420g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0085] (2) Add 0.1900g NiCl2·6H2O and 0.0745g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0086] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0087] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0088] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0089] Example 7

[0090] (1) Take 0.0490g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0091] (2) Add 0.1900g NiCl2·6H2O and 0.0745g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0092] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0093] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0094] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0095] Phase analysis of the prepared Ni / CeO2-expanded graphite was performed using X-ray diffraction. (See attached image.) Figure 1 As shown in the image, the corresponding peaks for Ni, cerium oxide, and expanded graphite can be observed, proving that the desired catalyst was successfully prepared. The SEM images are attached. Figure 2 As shown.

[0096] Using a gas phase analyzer, the test temperature and the flow rate of ammonia gas were determined. Ammonia gas was passed through a catalyst for decomposition, and the results were analyzed using the gas phase analyzer. A 100-hour stability test was conducted at a temperature of 580℃ and a gas flow rate of 30 mL / min. Data was taken every 4 hours and plotted to obtain the catalyst stability. Figure 8 As shown, the ammonia decomposition rate of the catalyst remains basically stable, without significant fluctuations or obvious attenuation. Therefore, the prepared Ni / CeO2-EG catalyst possesses long-term stable performance. The ammonia decomposition efficiency under the same conditions with and without expanded graphite is shown in the figure below. Figure 5 As shown.

[0097] Example 8

[0098] (1) Take 0.0560g of expanded graphite, immerse it in 15mL of deionized water, and ultrasonically disperse it for 30min to ensure that the expanded graphite is evenly dispersed in the deionized water.

[0099] (2) Add 0.1900g NiCl2·6H2O and 0.0745g CeCl3·7H2O, sonicate for 30min, and perform magnetic stirring for 12h to ensure that nickel and cerium are evenly dispersed on the surface of expanded graphite.

[0100] (3) Take 0.1500 g of sodium borohydride, add it to 15 mL of deionized water and ultrasonically disperse it for 30 min. Then add the sodium borohydride solution to the mixture in step (2) and stir magnetically for 30 min. Centrifuge and wash the resulting solution to remove impurities.

[0101] (4) Place the centrifuged solid into a muffle furnace and calcine it at 450°C for 3 hours.

[0102] (5) Ni / CeO2-expanded graphite catalyst was obtained by reduction for 1 h under pure H2 at 400℃.

[0103] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. The application of Ni / CeO2-expanded graphite catalyst in ammonia cracking for hydrogen production, characterized by: The preparation method of the catalyst includes the following steps: (1) uniformly dispersing expanded graphite in deionized water; (2) adding nickel chloride hexahydrate and cerium chloride heptahydrate to uniformly disperse them on the surface of expanded graphite; (3) adding sodium borohydride to the mixture of expanded graphite, deionized water, nickel chloride hexahydrate and cerium chloride heptahydrate for reduction, centrifugation and washing; (4) calcining the solid obtained by centrifugation, oxidizing Ce(III) and crystallizing it into CeO2 under high temperature environment; (5) reducing it under pure hydrogen conditions to obtain Ni / CeO2-expanded graphite catalyst.

2. The application according to claim 1, characterized in that: In step (1), each gram of expanded graphite is dispersed in 260-450 mL of deionized water.

3. The application according to claim 1, characterized in that: The mass ratio of expanded graphite to nickel chloride hexahydrate is 1:3-6.

4. The application according to claim 1, characterized in that: The mass ratio of expanded graphite to cerium chloride heptahydrate is 1:1-5.

5. The application according to claim 1, characterized in that: The mass ratio of expanded graphite to sodium borohydride is 1:2-5.

6. The application of the catalyst according to claim 1, characterized in that: In step (4), the furnace is fully calcined at 450°C.

7. The application of the catalyst according to claim 1, characterized in that: In step (5), the hydrogen reduction temperature is 400℃.

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