A nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst, a preparation method and application thereof

Abstract

The application discloses a nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst and a preparation method and application thereof, and belongs to the field of catalytic materials and hydrogen energy technologies. The application takes ZIF-67 as a precursor, and prepares a nitrogen-doped carbon-coated cobalt nanoparticle catalyst (Co@CN) through pyrolysis under an inert atmosphere. When the catalyst is used in an ammonia decomposition reaction, the ammonia conversion rate can reach more than 68%, and is close to 100% at 550 DEG C, and the catalyst does not deactivate after continuous operation for 40 hours. The application can further introduce Ni or Ru to form a bimetallic catalyst, and the catalytic activity is significantly improved. The method is simple to operate, does not need a pre-reduction treatment, and has low cost, and has an excellent industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of catalytic materials and hydrogen energy technology, specifically relating to a nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst, its preparation method, and its application. Background Technology

[0002] Hydrogen energy, as a clean and efficient secondary energy source, is an important carrier for achieving the "dual carbon" goal. However, the storage and transportation of hydrogen has long faced technical bottlenecks such as low volumetric energy density, extremely low liquefaction temperature, and high leakage risk, which severely restricts its large-scale application. Ammonia, with its high hydrogen storage capacity of 17.7 wt%, easy liquefaction at room temperature, combustion products consisting only of nitrogen and water, and no carbon emissions, is considered an ideal chemical hydrogen storage medium. Hydrogen can be produced online through ammonia decomposition reactions and directly supplied to fuel cells or internal combustion engines, thereby realizing an "ammonia-hydrogen" energy cycle.

[0003] Ammonia decomposition is a strongly endothermic process, with theoretical conversion rates exceeding 99% at 400°C and atmospheric pressure. However, due to slow reaction kinetics, actual yields typically require temperatures above 500°C. Developing efficient catalysts to lower reaction temperatures and increase reaction rates is a core research focus. While ruthenium (Ru)-based catalysts exhibit the highest ammonia decomposition activity, Ru resources are scarce and expensive, hindering large-scale industrial applications. Therefore, developing low-cost, highly active non-noble metal catalysts has become a research hotspot.

[0004] Among non-precious metals, cobalt (Co) has attracted attention due to its moderate nitrogen binding energy and relatively low price. However, traditional supported cobalt catalysts generally suffer from the following problems: (1) cobalt nanoparticles are prone to sintering and agglomeration in high-temperature reactions, leading to a rapid decrease in activity; (2) the metal-support interaction is weak, and the electronic structure is difficult to control; (3) the preparation process usually requires hydrogen pre-reduction, which increases the complexity of operation and safety risks.

[0005] Therefore, developing a method for preparing nitrogen-doped carbon-coated cobalt-based catalysts via pyrolysis using ZIF-67 as a precursor, and systematically optimizing the pyrolysis temperature and exploring the synergistic catalytic mechanisms of bimetallic and ternary metals, is of great scientific significance and application value for achieving efficient, low-cost, and pre-reduction-free ammonia decomposition for hydrogen production. Summary of the Invention

[0006] The purpose of this invention is to provide a nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst, its preparation method, and its applications. This method is simple to operate, low in cost, requires no pre-reduction treatment, and the resulting catalyst exhibits high activity, high stability, and excellent ammonia decomposition performance.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst, comprising the following steps: S1. Cobalt salt and 2-methylimidazole were mixed in methanol, stirred, aged, centrifuged and dried to obtain ZIF-67 precursor; S2. The ZIF-67 precursor is pyrolyzed under an inert atmosphere to obtain a nitrogen-doped carbon-coated cobalt nanoparticle catalyst.

[0008] Preferably, the cobalt salt in S1 is cobalt nitrate hexahydrate, and the molar ratio of cobalt salt to 2-methylimidazole is 1:4. The amount of methanol used is 5 to 15 mL per millimole of cobalt salt, preferably 10 mL.

[0009] Preferably, the stirring time is 30 minutes, the aging time is 24 hours, and the drying temperature is 60°C.

[0010] Preferably, the pyrolysis temperature in S2 is 550–800°C, the time is 2 hours, the heating rate is 5–10°C / min, preferably 10°C / min; the inert atmosphere is argon or nitrogen.

[0011] Furthermore, the cobalt nanoparticles in the obtained Co@CN catalyst have a particle size of 5–10 nm and are uniformly dispersed in a nitrogen-doped porous carbon matrix; the BET specific surface area is 300–400 m². 2 / g, with an average pore size of 6–11 nm.

[0012] Furthermore, after S1 and before S2, the process includes immersing ZIF-67 in a methanol solution containing nickel or ruthenium salts, stirring, and drying to obtain a ZIF-67 precursor loaded with a second metal; after pyrolysis, a bimetallic catalyst Ni-Co@CN or Ru-Co@CN is obtained.

[0013] Preferably, the nickel salt is nickel nitrate hexahydrate, and the ruthenium salt is ruthenium trichloride; the mass ratio of nickel to cobalt is (0.01-0.78):1, and the mass ratio of ruthenium to cobalt is (0.01-0.05):1.

[0014] More preferably, the mass ratio of Ni to Co in Ni-Co@CN is from 0.02:38 to 0.78:39.23, with the optimum being 0.03:37.43; In Ru-Co@CN, the mass ratio of Ru to Co ranges from 0.17:36.71 to 3.69:44.64, with the optimum being 0.77:41.33.

[0015] The present invention also provides a nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst prepared by the above preparation method.

[0016] This invention further provides the application of the above-mentioned catalyst in the ammonia decomposition reaction to produce hydrogen. The application conditions are: fixed-bed reactor, catalyst loading of 50–200 mg, reaction temperature of 350–600°C, and space velocity of 6000–30000 mL·g. -1 ·H -1 The pressure is at atmospheric pressure. No hydrogen pre-reduction is required before the reaction; ammonia gas can be directly introduced and the temperature raised.

[0017] Furthermore, the present invention can also support Ni and Ru together in the ZIF-67 precursor to form a Ni-Ru-Co@CN ternary catalyst.

[0018] It contains at least the following beneficial technical effects: This invention uses ZIF-67 as a precursor to prepare a nitrogen-doped carbon-coated cobalt nanoparticle catalyst (Co@CN) via a one-step pyrolysis method. This method is simple to operate, requires no hydrogen pre-reduction treatment, and is low in cost. The cobalt nanoparticles (5-10 nm in diameter) are uniformly dispersed in a porous nitrogen-doped carbon matrix, effectively suppressing high-temperature sintering. By optimizing the pyrolysis temperature to 600°C, the pyridine nitrogen content in the catalyst reaches as high as 80.33%, forming a strong synergistic electronic effect with cobalt, which significantly reduces the apparent activation energy of the ammonia decomposition reaction. The ammonia conversion rate reaches over 68%, and is close to 100% at 550°C. Moreover, it does not deactivate after 40 hours of continuous operation. Its activity and stability are superior to most reported non-precious metal catalysts. Further introduction of nickel or ruthenium to prepare bimetallic catalysts (Ni-Co@CN, Ru-Co@CN) and utilizing the intermetallic synergistic effect to increase the ammonia decomposition conversion rate to 90% and 84% respectively. DFT calculations confirmed that the electronic structure optimization of Ni / Co and Ru / Co reduced the NH bond cleavage energy barrier. Furthermore, this invention expands upon the ternary catalyst Ni-Ru-Co@CN, achieving a 79% conversion rate at 500℃ and maintaining stability for 60 hours. Therefore, this invention provides a highly active, highly stable, low-cost cobalt-based ammonia decomposition catalyst and its preparation method that do not require pre-reduction, showing significant industrial application prospects in the field of ammonia-hydrogen energy conversion. Attached Figure Description

[0019] Figure 1 The XRD patterns of ZIF-67 and Co@CN-600 prepared in Example 1.

[0020] Figure 2 SEM images of ZIF-67 and Co@CN-600 prepared in Example 1 before and after calcination; (ab) SEM images of ZIF-67 and (cd) Co@CN-600 and (e) elemental mapping of Co@CN-600.

[0021] Figure 3 The morphology of Co@CN was characterized using TEM and HRTEM. (ab) TEM, (c) HRTEM and (d) particle size distribution diagrams of Co@CN-600 are shown.

[0022] Figure 4 The images show the N2 adsorption-desorption isotherms and corresponding pore size distribution curves of the Co@CN catalysts prepared at different pyrolysis temperatures in Example 1.

[0023] Figure 5 The performance of Co@CN catalysts prepared at different pyrolysis temperatures in NH3 decomposition in Example 1.

[0024] Figure 6 Example 1 illustrates the effect of different space velocities and catalyst loading on the catalytic performance of Co@CN-600.

[0025] Figure 7 Performance testing of Ru-Co@CN bimetallic catalysts with different Ru and Ni loadings in Example 2; (a) NH3 conversion of Ru-Co@CN-X with different solvent amounts, (c) Ru-Co@CN-X with different metal loadings, and (e) Ni-Co@CN-X with different metal loadings; (b), (d), and (f) H2 generation rates (space velocity = 12000 mL·gcat) corresponding to different catalysts. -1 ·H -1 ).

[0026] Figure 8 Performance of NiRu-Co@CN catalyst in NH3 decomposition Figure 9 Stability tests were conducted on the NiRu-Co@CN catalyst at 500 °C. Detailed Implementation

[0027] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0028] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0029] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0030] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be obvious to those skilled in the art. This application specification and embodiments are merely exemplary.

[0031] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0032] Unless otherwise specified, "room temperature" and "normal temperature" in this invention refer to 25±2℃.

[0033] Unless otherwise specified, all raw materials or instruments used in the following embodiments of the present invention are commercially available.

[0034] Example 1 Synthesis of ZIF-67 precursor: 20 mmol Co(NO3)2·6H2O was dissolved in 200 mL methanol (solution A), and 80 mmol 2-methylimidazole was dissolved in 200 mL methanol (solution B). Solution A was quickly poured into solution B, stirred for 30 minutes, and aged at room temperature for 24 hours. Purple crystals were collected by centrifugation, washed five times with methanol, and dried at 60°C for 16 hours to obtain the ZIF-67 precursor.

[0035] Preparation of Co@CN catalysts at different pyrolysis temperatures: Take 1 g of ZIF-67 precursor and place it in a tube furnace. Under an argon atmosphere, heat the furnace to 550°C, 575°C, 600°C, 700°C and 800°C respectively at a rate of 10°C / min. Hold the temperature for 2 hours for pyrolysis and allow to cool naturally to obtain black powder, which is then labeled as Co@CN-550, Co@CN-575, Co@CN-600, Co@CN-700 and Co@CN-800 respectively.

[0036] 1. Structural characterization: Figure 1(a) shows the XRD pattern of the synthesized precursor ZIF-67. The diffraction peaks perfectly match the standard simulated XRD pattern of ZIF-67. The absence of other impurity peaks indicates that the synthesized ZIF-67 has high crystallinity. The XRD pattern of the product formed by subsequent calcination under a nitrogen atmosphere is shown below. Figure 1 In Figure (b), characteristic diffraction peaks belonging to the (111), (200), and (220) crystal planes of Co (PDF No. 15-0806) are observed at 44.2°, 51.5°, and 75.8°. The broad peak at 25° is attributed to amorphous carbon, indicating that after calcination, the ZIF-67 organic framework has been transformed into Co nanoparticles embedded in a nitrogen-doped carbon framework. Furthermore, the average particle size of the Co nanoparticles is calculated to be 7.5 nm according to the Scherrer formula. This indicates that metallic cobalt exhibits high dispersibility under calcination conditions. The nitrogen-doped carbon framework not only stabilizes these cobalt nanoparticles, preventing sintering and agglomeration, but may also provide electronic effects and synergistic effects for subsequent catalytic reactions, contributing to improved catalytic performance.

[0037] Figure 2 SEM images of ZIF-67 before and after calcination with optimized solvent content. The morphology of ZIF-67 is as follows. Figure 2 As shown in (ab). ZIF-67 has a uniform dodecahedral structure with a size of approximately 400-500 nm and exhibits good dispersibility. Co@CN-600, formed by calcination in argon at 600 °C, is shown in... Figure 2 As shown in (cd), the sample surface is no longer smooth, collapsing and folding inwards, but still retaining a dodecahedral structure. This wrinkled surface has a larger specific surface area, which can effectively increase the contact area with NH3 and create more active sites for the catalyst. Mapping of the Co@CN-600 catalyst. Figure 2 (e) indicates that C, N and Co elements are uniformly distributed in Co@CN-600.

[0038] Figure 3 shows the morphology of Co@CN characterized by TEM and HRTEM. As shown in Figure 3(ab), the Co nanoparticles of the specified size are uniformly dispersed and embedded in the CN support to form a coating structure. The CN obtained by pyrolysis can effectively protect the aggregation of Co nanoparticles. In Figure 3(c), the lattice fringes of Co can be clearly observed with a spacing of 0.194 nm, corresponding to the cobalt (111) plane, which is consistent with the XRD results. From the particle size distribution histogram 3(d), it can be seen that the particle size of Co@CN-600 is in the range of 5-10 nm. This indicates that the Co nanoparticles are almost completely encapsulated in the carbon framework.

[0039] According to IUPAC classification, Figure 4 The N2 adsorption-desorption isotherm of Co@CN (a) belongs to type IV and exhibits a characteristic type H1 hysteresis loop in the relative pressure (P / P0) range of 0.42-1.0, indicating the presence of its mesoporous structure. Furthermore, the pore size distribution curve... Figure 4 In (b), the average pore size of the catalyst is observed to be concentrated between 3 and 4 nm. The surface area, pore volume, and pore size of different catalysts show that the CN layer formed by calcination of ZIF-67 still possesses mesoporous characteristics. The nanopores and defects of CN provide a certain surface area, which is beneficial to the mass transfer process of ammonia decomposition. With increasing pyrolysis temperature, the average pore size of the catalyst first increases and then decreases. This may be due to the deterioration of mesoporosity during pyrolysis. The average pore size of Co@CN-600 is 10.02 nm. Co nanoparticles can be confined on the support, thus providing more catalytic active sites for ammonia decomposition.

[0040] 2. Ammonia decomposition performance test: Figure 5 Performance of Co@CN-X catalyst in NH3 decomposition: (a) NH3 conversion rate of different catalysts at different reaction temperatures; (b) H2 generation rate of different catalysts at different reaction temperatures (space velocity = 9000 mL·gcat). -1 ·H -1 ) As shown in Figure 5(a) of the performance of the Co@CN-X catalyst in NH3 decomposition, Co@CN-600 exhibits the highest NH3 conversion rate among all catalysts, especially above 500 °C. Co@CN-800 and Co@CN-550 show poor NH3 decomposition performance. This indicates that the Co@CN structure prepared at different calcination temperatures has a significant impact on the NH3 decomposition reaction. At a calcination temperature of 600 °C, the ZIF-67 precursor undergoes more complete decomposition, resulting in highly dispersed metallic Co NPs and an appropriate amount of active sites distributed on the carbon support surface. H2-TPR analysis shows that, compared with other catalysts, the active metallic Co in Co@CN-600 exhibits superior reducibility and stronger interaction with the support, thus significantly enhancing catalytic activity. Furthermore, the calcination temperature of 600 °C optimizes the pore structure of the carbon support, retaining more pyridine nitrogen groups. N2 physisorption and XRD analysis showed that Co@CN-600 possesses a high specific surface area and well-developed pore structure, which facilitates the diffusion of NH3 molecules to active sites, thereby enhancing catalytic performance. The increased specific surface area and optimized pore structure improved the adsorption and decomposition efficiency of NH3 molecules. In contrast, at a calcination temperature of 550 °C, the ZIF-67 precursor underwent incomplete decomposition, leading to insufficient Co reduction and fewer active sites, thus resulting in poorer catalytic performance of Co@CN-550. Furthermore, the lower calcination temperature hindered the graphitization of the carbon support, as confirmed by Raman spectroscopy, which reduced electronic conductivity, thereby weakening catalytic performance. XPS results indicated that a calcination temperature of 800 °C might lead to the decomposition or migration of nitrogen species. Although Co... 0 While the content is high, its performance is degraded due to the low pyridine nitrogen content. BET specific surface area and pore size distribution analysis showed that excessive graphitization and pore structure collapse significantly reduced the specific surface area and the exposure of active sites, further weakening the catalytic performance.

[0041] As shown in Figure 5(b), the H2 generation rate of different catalysts at different reaction temperatures indicates that the H2 generation rate and the NH3 decomposition conversion rate of the catalysts show the same trend. Specifically, the Co@CN-600 catalyst achieved a 69% NH3 conversion rate at 500 °C, with an H2 generation rate of 7.02 mmol H2·gcat. -1 ·min -1 (GHSV is 9000mL·gcat) -1 ·H -1 The results showed that the cobalt-based catalyst synthesized in this study outperformed catalysts from other studies; furthermore, Co@CN-600 showed better performance at 12000 mL·gcat.-1 ·H -1 Even at a space velocity of [value missing], it can still achieve a 68% NH3 conversion rate, compared to 9000 mL·gcat [value missing]. -1 ·H -1 Since the timing was similar, subsequent studies all used 12000 mL·gcat. -1 ·H -1 Under the conditions.

[0042] 3. Effects of different space velocities and catalyst loading on the catalytic performance of Co@CN-600 Figure 6 shows the ammonia conversion rate of Co@CN-600 under different process conditions. As can be seen from the figure, despite different process conditions (different space velocities and catalyst dosages), the reactivity of ammonia decomposition gradually increases with increasing temperature. Especially in the higher temperature range (e.g., above 500 °C), the increasing trend of ammonia decomposition activity is more pronounced. This phenomenon is consistent with the characteristics of an endothermic reaction, further verifying that ammonia decomposition is an endothermic reaction. Figure 6(a) shows that space velocity has a significant impact on the ammonia decomposition conversion rate. At lower space velocities (6000-12000 mL·g⁻¹), the conversion rate increases with increasing space velocity. -1 ·H -1 At the same level, the conversion rate of ammonia remained almost constant. However, with further increases in space velocity (>15000 mL·g), the conversion rate of ammonia decreased. -1 ·H -1 (especially up to 30000 mL·g) -1 ·H -1 At low temperatures (6000-12000 mL·g⁻¹), the conversion rate decreases significantly, especially in the low-temperature region below 500 °C. -1 ·H -1 Under these conditions, the gas flow rate is relatively slow, resulting in a longer contact time with the catalyst. The reaction is primarily controlled by the intrinsic activity of the catalyst, leading to a higher conversion rate and minimal differences between the curves, exhibiting kinetic control characteristics. However, at high space velocities (15000-30000 mL·g⁻¹), the reaction is much more kinetically controlled. -1 ·H -1Under certain conditions, increased gas flow rate leads to a shorter gas-solid contact time, and the reaction becomes more dependent on the diffusion rate of ammonia molecules. This is especially true at low temperatures (below 450 °C), where mass transfer effects significantly impact the conversion rate, resulting in a substantial decrease in conversion. Figure 6(b) shows that different catalyst loading amounts (50-200 mg) have almost no effect on the conversion rate of the ammonia decomposition reaction; the curves essentially overlap, indicating that under experimental conditions (moderate space velocity and suitable temperature range), the contribution of loading amount to reaction performance has reached saturation. This is because 50 mg of catalyst provides sufficient active sites to meet the requirements of the ammonia decomposition reaction. Within the kinetic control range, the reaction rate is determined by the intrinsic activity of the catalyst; further increasing the loading amount does not significantly improve the utilization rate of active sites. Additional catalyst only increases redundant active sites that do not participate in the reaction, without further improving reaction performance. This result is significant for practical industrial applications, indicating that only an appropriate amount of catalyst is needed to meet reaction requirements in industrial applications, eliminating the need for excessive use, thereby reducing costs and improving efficiency.

[0043] Example 2 Ni-Co@CN bimetallic catalysts with different Ni loadings 0.5 g of ZIF-67 (synthesized according to Example 1) was dispersed in 400 mL of methanol and sonicated for 10 minutes. Then, 0.05 g, 0.1 g, 0.25 g, and 0.5 g of Ni(NO3)2·6H2O were added, respectively. The mixture was stirred for 6 hours, allowed to stand for 24 hours, washed by centrifugation with methanol, and dried at 60°C for 16 hours to obtain the Ni-ZIF-67 precursor. This precursor was then pyrolyzed at 600°C for 2 hours under argon atmosphere. The products were sequentially labeled as Ni-Co@CN-1, Ni-Co@CN-2, Ni-Co@CN-3, and Ni-Co@CN-4.

[0044] Ru-Co@CN bimetallic catalysts with different Ru loadings 0.5 g of ZIF-67 was dispersed in 400 mL of methanol, and 0.01 g, 0.025 g, and 0.05 g of RuCl3 were added respectively. The mixture was stirred for 6 hours, allowed to stand for 24 hours, washed and dried, and then pyrolyzed at 600°C for 2 hours. The resulting products were labeled as Ru-Co@CN-1, Ru-Co@CN-2, and Ru-Co@CN-3.

[0045] 1. Catalyst performance testing Figure 7(a) shows the ammonia decomposition performance of Ru-Co@CN-X prepared with different solvent amounts. Combining FESEM results and ammonia decomposition catalytic performance analysis, in the Ru-Co@CN-25 to Ru-Co@CN-400 systems, the morphology and dispersibility of the catalyst are gradually optimized with increasing methanol volume. Ru-Co@CN-400 has the best particle uniformity and pore distribution, thus exhibiting the highest catalytic activity at 500℃. Furthermore, Figure 7(c) shows the ammonia decomposition performance of Ru-Co@CN with different metal loadings. It can be seen from the figure that the catalytic performance gradually improves with increasing Ru loading (Ru-Co@CN-1 to Ru-Co@CN-3). Among them, Ru-Co@CN-3 exhibits the best performance. In contrast, Figure 7(e) shows the ammonia decomposition performance of Ni-Co@CN with different metal loadings. In the Ni-Co@CN-1 to Ni-Co@CN-4 systems, the catalytic performance exhibits a negative correlation with the loading. Excess Ni intensifies particle aggregation and alters the electronic structure of Co, leading to the same trend in H2 generation rate and NH3 decomposition conversion rate, as shown in Figures 7(b), (d), and (f). Specifically, the Ni-Co@CN-400 catalyst achieved a 90% NH3 conversion rate at 500 °C, with an H2 generation rate of 12.23 mmol H2·gcat. -1 ·min -1 The Ru-Co@CN-400 catalyst achieved an NH3 conversion rate of 84% at 500 °C, with an H2 formation rate of 11.31 mmol H2·gcat. -1 ·min -1 (GHSV is 12,000 mL·gcat) -1 ·H -1 ).

[0046] Example 3: Ni-Ru-Co@CN ternary catalyst 0.5 g of ZIF-67 was dispersed in 400 mL of methanol, and Ni(NO3)2·6H2O and RuCl3 were added simultaneously. The rest of the process was the same as in Example 2, to obtain NiRu-Co@CN. The specific ratios are shown in Table 1.

[0047] Table 1 1. Catalyst performance testing The performance of NiRu-Co@CN catalyst in NH3 decomposition is as follows: Figure 8As shown in Figure (a), the ammonia decomposition conversion rates of catalysts with different Ni and Ru contents vary significantly at different temperatures. Overall, all catalysts show a trend of gradually increasing ammonia decomposition conversion rate with increasing temperature, with Ni3Ru1-Co@CN exhibiting the best catalytic activity above 500°C. Ni4Ru2-Co@CN and Ni3Ru2-Co@CN show slightly lower activity compared to Ni3Ru1-Co@CN, indicating that further increasing the Ru content did not improve catalytic performance. This may be due to reduced dispersibility caused by excessive Ru or the influence of intermetallic interactions on catalytic activity. Ni1Ru1-Co@CN and Ni1Ru2-Co@CN show significantly lower ammonia decomposition conversion rates at lower temperatures (≤450°C), indicating that catalytic activity is limited when Ni content is low. Ni2Ru1-Co@CN and Ni2Ru2-Co@CN show moderate conversion rates in the 450~550°C range, indicating that the Ni to Ru ratio has a significant impact on catalytic performance. Appropriately increasing the Ni and Ru content can help improve catalytic performance through synergistic effects, but excessive amounts may lead to agglomeration, reducing the specific surface area and thus affecting catalytic activity. Therefore, Ni3Ru1-Co@CN exhibits the best catalytic activity. Furthermore, temperature has a significant impact on catalytic performance; all catalysts achieve high ammonia decomposition conversion rates at higher temperatures (>500°C), further demonstrating the crucial role of temperature in promoting the ammonia decomposition reaction.

[0048] Figure 8 Figure (b) shows the hydrogen generation rate of the NiRu-Co@CN catalyst. The H2 generation rate and the NH3 decomposition conversion rate of the catalyst show the same trend. Specifically, the Ni3Ru1-Co@CN catalyst achieved a 79% NH3 conversion rate at 500 °C, with an H2 generation rate of 10.59 mmol H2·gcat. -1 ·min -1 (GHSV is 12000 mL·gcat) -1 ·H -1 ).

[0049] Figure 9 shows the stability test results for Ni3Ru1-Co@CN. It can be seen that at 500 ℃ and 12000 mL·gcat... -1 ·H -1 Under the given conditions, the stability curves of the two catalysts showed that the conversion rate of ammonia and the rate of hydrogen production hardly decreased over a continuous reaction period of 60 hours.

[0050] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst, characterized in that, Includes the following steps: S1. Cobalt salt and 2-methylimidazole were mixed in methanol, and after stirring, aging, centrifugation and drying, ZIF-67 precursor was obtained; S2. The ZIF-67 precursor is pyrolyzed under an inert atmosphere to obtain a nitrogen-doped carbon-coated cobalt nanoparticle catalyst.

2. The preparation method according to claim 1, characterized in that, The cobalt salt in S1 is cobalt nitrate hexahydrate, and the molar ratio of cobalt salt to 2-methylimidazole is 1:

4.

3. The preparation method according to claim 1, characterized in that, The amount of methanol used in S1 is 5-15 mL per millimole of cobalt salt.

4. The preparation method according to claim 1, characterized in that, The pyrolysis temperature in S2 is 550~800℃, the time is 2 hours, and the heating rate is 5~10℃ / min.

5. The preparation method according to claim 1, characterized in that, The inert atmosphere in S2 is argon or nitrogen.

6. The preparation method according to claim 1, characterized in that, The cobalt nanoparticles in the catalyst obtained after the pyrolysis of S2 have a particle size of 5-10 nm and a specific surface area of ​​300-400 m². 2 / g, with an average pore size of 6~11 nm.

7. The preparation method according to claim 1, characterized in that, After S1 and before S2, the process further includes immersing ZIF-67 in a methanol solution containing nickel salt or ruthenium salt, stirring, and drying to obtain a ZIF-67 precursor loaded with a second metal. The S2 pyrolysis yields a bimetallic catalyst Ni-Co@CN or Ru-Co@CN.

8. The preparation method according to claim 7, characterized in that, The nickel salt is nickel nitrate hexahydrate, and the mass ratio of nickel to cobalt is (0.01~0.78):1; The ruthenium salt is ruthenium trichloride, and the mass ratio of ruthenium to cobalt is (0.01~0.05):

1.

9. A nitrogen-doped carbon-coated cobalt-based ammonia decomposition catalyst prepared by the preparation method according to any one of claims 1 to 8.

10. The application of the catalyst according to claim 9 in the ammonia decomposition reaction to produce hydrogen, characterized in that, The reaction temperature was 350–600 °C, and the space velocity was 6000–30000 mL·g. -1 ·H -1 The pressure is normal.

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