A cerium modified nickel-based catalyst for hydrogen production by ammonia decomposition, and a preparation method and application thereof

Abstract

The application relates to the technical field of hydrogen production by ammonia decomposition, and discloses a cerium-modified nickel-based catalyst for hydrogen production by ammonia decomposition as well as a preparation method and application of the catalyst, wherein the catalyst takes cerium oxide as a carrier, takes nickel as a main active component, and can further introduce cobalt as a second active metal; the cerium oxide is prepared from a tetravalent cerium source through a sol-gel method. The preparation method comprises the following steps: dissolving a tetravalent cerium source, a nickel source and an optional cobalt source in deionized water, adding citric acid as a complexing agent, adjusting the pH to alkaline, and obtaining the catalyst through water bath evaporation, drying and staged calcination. According to the application, the tetravalent cerium source and the sol-gel method are cooperatively controlled, the Ce 3+ proportion and the oxygen vacancy concentration are increased, and the Ni-Ov-Ce metal-carrier interaction is enhanced; and the ammonia decomposition activity is improved through the nickel-cobalt bimetallic synergistic effect. The catalyst has excellent ammonia decomposition conversion rate and stability at 600 DEG C, and is suitable for ship ammonia hydrogen power systems and distributed ammonia hydrogen production scenarios.

Description

Technical Field

[0001] This invention relates to the field of ammonia decomposition hydrogen production catalyst technology, specifically to a cerium-modified nickel-based catalyst for ammonia decomposition hydrogen production, its preparation method and application, and particularly to an oxygen-vacancy-rich Ni-CeO2 or Ni / Co-Ceo2 non-noble metal ammonia decomposition catalyst constructed from a tetravalent cerium source via a sol-gel method. Background Technology

[0002] Ammonia, as an important carbon-free hydrogen carrier, boasts a high hydrogen storage density of 17.8 wt% by mass and a higher volumetric hydrogen storage density than liquid hydrogen. Furthermore, ammonia can be liquefied at -33°C, and its storage and transportation system is mature with low safety risks, making it a key bridge connecting fossil fuels and the hydrogen economy. Due to its "zero-carbon" properties, ammonia fuel has become one of the preferred alternative fuels for the shipping industry's green transformation. Ammonia decomposition hydrogen production technology is a core technology in this transformation. It converts ammonia into hydrogen and nitrogen through catalytic cracking (2NH3 → N2 + 3H2), producing products free of carbon oxides. These products can be directly used for hydrogen supply in proton exchange membrane fuel cells (PEMFC), ammonia-hydrogen co-combustion engines, and on-site hydrogen production in marine ammonia-hydrogen propulsion systems.

[0003] In practical applications, ammonia decomposition hydrogen production catalysts are mainly used in the following scenarios:

[0004] (1) Marine ammonia-hydrogen power system - using liquid ammonia on board the ship, hydrogen is produced on-site by ammonia decomposition to provide a hydrogen source for ammonia-hydrogen mixed combustion engines or fuel cells, solving the demand for high-activity fuel for high-power marine engines;

[0005] (2) Distributed ammonia-to-hydrogen refueling station - Ammonia is decomposed into hydrogen at the refueling station site, and after purification, it is directly refueled for fuel cell vehicles, avoiding the safety hazards and cost issues of high-pressure hydrogen storage and transportation;

[0006] (3) Ammonia-hydrogen co-combustion power generation - the hydrogen produced by the decomposition of ammonia is mixed and burned with undecomposed ammonia, and the high flame propagation speed and wide flammability limit of hydrogen are used to improve the ignition difficulty and combustion instability of pure ammonia combustion.

[0007] (4) Backup power supply and off-grid power supply system—In scenarios with insufficient grid coverage or emergency situations, hydrogen is produced by ammonia decomposition to power fuel cells, achieving a clean and quiet power supply. All of the above application scenarios use ammonia decomposition catalysts as the core technology component. The activity, stability, and cost of the catalysts directly determine the economic feasibility and technical competitiveness of the entire system.

[0008] Currently, while Ru-based noble metal catalysts exhibit excellent low-temperature ammonia decomposition activity, their high cost (ruthenium is more than 500 times the price of nickel) and limited resource availability severely restrict their large-scale application. Therefore, developing efficient and long-life ammonia decomposition catalysts based on non-noble metals such as Ni and Co has become a key technological breakthrough for promoting the closed-loop development of the ammonia energy industry chain, and is of significant practical importance for achieving shipping decarbonization goals and building a zero-carbon energy system.

[0009] However, existing technologies still have many technical problems in practical implementation, as follows:

[0010] 1. Insufficient catalyst activity at low temperatures and difficulty in activating ammonia molecules; ammonia decomposition reaction (ΔH 0 298 The reaction of ammonia (+46.1 kJ / mol NH3) is strongly endothermic, and the NH bond energy in the ammonia molecule is 435 kJ / mol. Thermodynamically, a considerable equilibrium conversion rate can only be obtained at a relatively high temperature (>500℃). However, the ammonia decomposition activity of existing non-noble metal Ni-based catalysts is severely insufficient at low temperatures (<550℃). This is mainly due to the "activity-stability trade-off" caused by the excessively high or low adsorption energy of Ni for nitrogen-containing intermediates (such as -NH2, -NH, -N) on the Ni surface. Excessive adsorption of N atoms by Ni leads to N2 desorption becoming the rate-determining step, and the excessive accumulation of intermediates at active sites further poisons the catalyst surface, severely limiting the reaction rate. Existing technologies use the addition of a second metal (such as Co or Fe) to form an alloy to regulate the electronic structure of Ni, but bimetallic catalysts prepared by simple mixing or impregnation methods often fail to achieve the expected synergistic catalytic effect due to uneven dispersion of the two metals and low alloying degree.

[0011] 2. Insufficient support-active metal interaction leads to metal sintering and deactivation; Ni-based catalysts face severe active metal sintering problems in high-temperature reaction environments (typically ≥600℃); the Taman temperature of Ni nanoparticles is approximately 590℃. Under operating conditions close to or exceeding this temperature, Ni particles are prone to migration and aggregation, resulting in a sharp decrease in the active specific surface area; at the same time, in Ni / CeO2 catalysts prepared by the traditional impregnation method, Ni species are mainly supported on the CeO2 surface in the form of large-sized particles (>10 nm), resulting in a limited metal-support contact interface and difficulty in forming sufficient metal-support interaction (SMSI); this weak interaction not only fails to effectively anchor Ni particles to inhibit sintering, but also limits the electron transfer efficiency from the support to the active metal, thereby weakening the catalyst's activation ability for ammonia molecules and its ability to promote the desorption of intermediates;

[0012] 3. Insufficient oxygen vacancy concentration and Ce 3+ / Ce 4+Low utilization rate of redox pairs; CeO2 as a support due to its unique Ce 3 + / Ce 4+ CeO2 supports have attracted widespread attention due to their reversible redox pairs and oxygen storage / release capabilities. Oxygen vacancies, as key active sites on the CeO2 surface, can promote the adsorption and dissociation of ammonia molecules and accelerate the desorption of N2. However, current technologies generally employ trivalent cerium salts (such as Ce(NO3)3·6H2O) to prepare CeO2 supports via precipitation or hydrothermal methods, resulting in CeO2 containing insufficient Ce content. 3+ The ratio and oxygen vacancy concentration are limited by the controllability of the preparation method; precipitation methods suffer from problems such as the introduction of impurities by the precipitant, uncontrollable grain growth, and easy annihilation of oxygen vacancies during high-temperature calcination; although hydrothermal methods can obtain CeO2 with specific morphologies, their surface exposed crystal planes are singular (such as the {111} plane), resulting in high oxygen vacancy formation energy and limiting the maximum density of surface oxygen vacancies; more importantly, existing preparation methods fail to fully utilize the potential of cerium source valence state to control the formation of CeO2 lattice defects—in certain sol-gel processes, tetravalent cerium sources may form more abundant CeO2 than trivalent cerium sources through in-situ reduction and redox coupling processes. 3+ Oxygen vacancies and oxygen vacancies, but the knowledge and technical implementation of this pathway are still lacking in the field of science;

[0013] 4. The chemical mechanism of the synergistic effect of non-precious metal bimetals has not been fully utilized. Although some studies have recognized that the introduction of Co can generate electronic and geometric effects in Ni-based catalysts, the existing bimetallic catalyst preparation strategies are mostly simple co-impregnation or stepwise impregnation, lacking synergistic design from the perspective of precursor chemistry. For example, the hydrolysis behavior of Ni and Co in solution, the complexation competition relationship, and the interaction with CeO2 precursor have not been systematically considered, resulting in insufficient formation of the Ni-Co alloy phase after calcination and uneven alloy particle size distribution. This directly weakens the role of the bimetallic synergistic effect, makes the optimization of the conversion path of ammonia decomposition intermediates incomplete, and limits the improvement of reaction rate.

[0014] 5. Existing preparation methods have limited structural control capabilities; traditional methods such as co-precipitation, impregnation, and hydrothermal methods for preparing CeO2-supported Ni catalysts generally suffer from the following problems:

[0015] (1) The distribution uniformity of active metals on the carrier surface is poor;

[0016] (2) Metal particles tend to grow larger during the roasting process;

[0017] (3) The concentration of lattice defects and the acidity / alkalinity of the carrier are difficult to control precisely;

[0018] (4) The metal-carrier interface structure lacks atomic-scale design;

[0019] The root cause of these problems is that existing methods have failed to achieve uniform mixing and controllable gelation of metal ions and support precursors at the molecular level, resulting in insufficient structural uniformity and stability of the final catalyst.

[0020] Therefore, those skilled in the art urgently need a catalyst design technology that can precisely control the defect structure of CeO2 support, the dispersion state of active metals, and the synergistic effect of bimetals at the molecular / atomic level. Summary of the Invention

[0021] This invention provides a catalyst for hydrogen production from ammonia decomposition, comprising a tetravalent cerium source constructed via a sol-gel method using an oxygen-rich vacancy CeO2 support, and loaded with Ni or Ni-Co bimetallic active components, as well as its preparation method and applications. This method generates high-concentration Ce through the in-situ reduction effect of the tetravalent cerium source during the sol-gel process. 3+ By combining oxygen vacancies, the interaction between the Ni-Ov-Ce interface is enhanced, and the transformation pathway of ammonia decomposition intermediates is optimized through the Ni-Co bimetallic synergistic effect, thereby achieving a synergistic improvement in ammonia decomposition activity and stability in a non-noble metal system.

[0022] The core technical problem addressed by this invention is: how to simultaneously achieve the construction of high-density active sites, the enhancement of metal-support interactions, and the maximization of bimetallic synergistic effects through the atomic / nanoscale structure design of catalysts in a non-noble metal system, thereby overcoming the common technical obstacles of insufficient low-temperature activity and poor high-temperature stability of Ni-based catalysts in ammonia decomposition reactions.

[0023] For those skilled in the art, the necessity of solving the aforementioned core technical problems lies in:

[0024] First, from the perspective of application requirements, marine ammonia-hydrogen propulsion systems and distributed ammonia-hydrogen production and refueling stations require the ammonia decomposition reaction to operate at the lowest possible temperature (≤600℃) in order to reduce system energy consumption, reduce the corrosion of reactor materials by high temperature, and improve system thermal efficiency and safety. If the ammonia decomposition temperature must be maintained above 650℃, the thermal management cost of the entire system will increase sharply, and the economic efficiency will be greatly reduced, which will fundamentally hinder the promotion and application of ammonia-hydrogen propulsion technology in the shipping industry.

[0025] Secondly, from a technical bottleneck perspective, the insufficient low-temperature activity and high-temperature deactivation of Ni-based catalysts are contradictory—increasing the reaction temperature can improve the conversion rate, but it accelerates metal sintering; decreasing the temperature can extend the catalyst lifespan, but it sacrifices hydrogen production efficiency. This "activity-stability paradox" has long remained unresolved and has become a major bottleneck for those skilled in the art to achieve the industrialization of ammonia-to-hydrogen production. To break this deadlock, it is necessary to seek a revolution in catalyst structure design paradigms, rather than incremental improvements within the framework of traditional impregnation and co-precipitation methods.

[0026] Third, from a scientific perspective, the deep coupling of oxygen vacancy regulation in CeO2 and the Ni-Co bimetallic synergistic effect represents a cutting-edge direction in the design of non-noble metal catalysts. However, existing technologies have not yet provided clear technical insights into how to simultaneously achieve "high oxygen vacancy concentration" and "strong metal-support interaction" from the source valence state selection and sol-gel chemical pathway design. Solving this problem will expand the understanding of CeO2-based catalyst defect engineering among those skilled in the art and provide new theoretical guidance and practical paradigms for the design of catalysts for other high-temperature heterogeneous catalytic reactions (such as methane dry reforming and water-gas shift reaction).

[0027] Fourth, from an industrial perspective, ammonia, as a mature industrial chemical, has a global annual production exceeding 200 million tons, with well-developed storage and transportation infrastructure. If the technological bottleneck of highly efficient non-precious metal ammonia decomposition catalysts can be overcome, the last mile of the "ammonia-hydrogen" energy industry chain will be directly opened up, making ammonia a core energy carrier connecting renewable energy hydrogen production with end-use hydrogen scenarios. This will not only accelerate the decarbonization process in shipping, power generation, and industrial heating, but also provide a highly cost-competitive technological route for the construction of global hydrogen energy infrastructure.

[0028] To address the aforementioned core technical challenges, this invention designs a cerium-modified nickel-based catalyst for ammonia decomposition to produce hydrogen, along with its preparation method and applications. The aim is to establish a structural foundation rich in oxygen vacancies and with strong metal-support interactions during the support construction stage by selecting a suitable cerium source precursor and a sol-gel preparation pathway. Furthermore, the introduction of a second metal and optimization of the interface structure lead to comprehensive optimization of the ammonia decomposition reaction pathway. This technical solution should primarily be applied to on-site ammonia decomposition hydrogen production scenarios in marine ammonia-hydrogen propulsion systems, and is also suitable for distributed ammonia-hydrogen refueling stations, ammonia-hydrogen co-combustion hydrogen supply systems, and other zero-carbon energy conversion fields.

[0029] To achieve the above objectives, the specific technical solution of the present invention is a cerium-modified nickel-based catalyst for hydrogen production from ammonia decomposition, wherein the catalyst comprises a cerium oxide support and an active metal component supported on the cerium oxide support;

[0030] The cerium oxide support was prepared from a tetravalent cerium source via a sol-gel method.

[0031] The active metal component includes nickel;

[0032] The catalyst surface has Ce 3+ Species and oxygen vacancies (Ov);

[0033] The catalyst has a Ni-Ov-Ce interface structure.

[0034] Preferably, the active metal component further includes cobalt.

[0035] Preferably, the tetravalent cerium source is one or more of cerium (IV) nitrate, cerium ammonium nitrate, or their hydrates.

[0036] Preferably, the tetravalent cerium source is cerium (IV) nitrate.

[0037] Preferably, the mass of nickel in the active metal component accounts for 5% to 25% of the total mass of the catalyst.

[0038] Preferably, the mass of nickel in the active metal component accounts for 10% to 20% of the total mass of the catalyst.

[0039] Preferably, the nickel content in the active metal component accounts for 15% of the total mass of the catalyst.

[0040] Preferably, when the active metal component includes nickel and cobalt, the molar ratio of nickel to cobalt is 0.95:0.05 to 0.50:0.50.

[0041] Preferably, the molar ratio of nickel to cobalt is 0.85:0.15 to 0.65:0.35.

[0042] Preferably, the molar ratio of nickel to cobalt is 0.75:0.25.

[0043] Preferably, the catalyst has a mesoporous structure with a specific surface area of ​​40–70 m². 2 / g, with an average pore size of 5–10 nm.

[0044] Preferably, the present invention also discloses a preparation method for preparing the catalyst, the method comprising the following steps:

[0045] S1. Dissolve the tetravalent cerium source in deionized water to obtain a cerium source solution;

[0046] S2. Add a nickel source, or add a nickel source and a cobalt source, to the cerium source solution and stir to form a mixed metal salt solution;

[0047] S3. Add citric acid as a complexing agent to the mixed metal salt solution, adjust the pH to alkaline, and continue stirring to obtain the sol precursor;

[0048] S4. The sol precursor is heated under water bath conditions to evaporate the water and form a gel or wet solid.

[0049] S5. Dry the resulting gel or wet solid;

[0050] S6. The dried solid is calcined in stages to obtain the cerium-modified nickel-based or nickel-cobalt-based ammonia decomposition catalyst.

[0051] Preferably, the molar ratio of citric acid to the total molar amount of metal elements in step S3 is 0.8:1 to 1.5:1.

[0052] Preferably, the ratio of the total molar amount of citric acid to the total molar amount of the metal element is 1:1.

[0053] Preferably, in step S3, ammonia is used to adjust the pH to 7.5-8.5.

[0054] Preferably, in step S3, ammonia is used to adjust the pH to 8.0.

[0055] Preferably, the segmented roasting in step S6 includes: first roasting at 250-350°C for 1-3 hours, and then roasting at 550-650°C for 3-5 hours.

[0056] Preferably, the segmented calcination in step S6 includes: first calcining at 300°C for 2 hours, and then calcining at 600°C for 4 hours.

[0057] Preferably, the calcination heating rate in step S6 is 1–5 °C / min.

[0058] Preferably, the present invention also discloses the application of a catalyst in ammonia decomposition for hydrogen production, wherein the ammonia decomposition for hydrogen production reaction is carried out under the action of the catalyst, and the temperature conditions of the ammonia decomposition for hydrogen production reaction are 450-650°C.

[0059] Preferably, the ammonia decomposition to hydrogen production reaction is carried out at 550–650°C.

[0060] Preferably, the ammonia decomposition to hydrogen production reaction is carried out at 600°C.

[0061] Preferably, the application scenarios of the catalyst include: on-site hydrogen production from ammonia in marine ammonia-hydrogen power systems, hydrogen supply from ammonia-hydrogen mixed combustion, hydrogen supply from fuel cells, or zero-carbon fuel energy conversion.

[0062] Preferably, the catalyst can be reduced in a hydrogen atmosphere at a temperature of 450–550°C for a time of 0.5–2 h.

[0063] Preferably, the catalyst can be reduced in a hydrogen atmosphere at a temperature of 500°C for 1 hour. It should be noted that reducing the catalyst in a hydrogen atmosphere at 500°C for 1 hour before use can enable the active metal components to form a suitable reduced or partially reduced state, thereby improving the catalytic reaction activity.

[0064] Compared with the prior art, the technical solution disclosed in this application has the following non-obvious technical features:

[0065] Firstly, the reverse thinking regarding the valence state selection of cerium sources: Current technology generally believes that trivalent cerium sources are favorable for the formation of Ce. 3+ Because of oxygen vacancies, Ce(NO3)3·6H2O is conventionally chosen as the cerium source; however, this invention reverses this by selecting a tetravalent cerium source (Ce(NO3)4), utilizing its in-situ reduction reaction with citric acid complexing agent during the sol-gel process and in the subsequent calcination process to produce a higher proportion of Ce than that of a trivalent cerium source. 3+ And more oxygen vacancies; this "high-valence cerium source actually produces more low-valence Ce" 3+ The discovery of “oxygen vacancies” contradicts the conventional expectations of those skilled in the art and is non-obvious;

[0066] Secondly, the synergistic compatibility between the sol-gel method and tetravalent cerium sources: Existing sol-gel methods for preparing CeO2 typically use trivalent cerium sources; this invention reveals that tetravalent cerium sources exhibit unique hydrolysis-complexation-gelation behavior in a citric acid complexation-ammonia pH-controlled system. Their strong oxidizing properties promote partial oxidation of citric acid, forming a localized reducing environment that promotes CeO2 production. 4+ →Ce 3+ The transformation. The synergistic effect of this specific precursor chemical system with the tetravalent cerium source is something that has not been taught in existing technologies;

[0067] Thirdly, the construction path of the "Ni-Ov-Ce" interface structure: This invention combines a tetravalent cerium source and the sol-gel method to form a Ni-Ov-Ce active interface structure between Ni and CeO2, with oxygen vacancies (Ov) as the bridge. This structure differs from the simple Ni-CeO2 interface in traditional Ni / CeO2 catalysts. The oxygen vacancies, acting as electron transfer channels, significantly enhance the metal-support interaction, improving the dispersion and anti-sintering ability of Ni.

[0068] Fourth, the matching of the segmented calcination strategy with the precursor chemistry: This invention adopts a two-stage calcination process of 300℃ low-temperature pre-calcination + 600℃ high-temperature re-calcination; the 300℃ pre-calcination stage is not only used to remove organic matter, but more importantly, it utilizes the intense redox reaction that occurs in the tetravalent cerium source-citric acid system in this temperature range to retain and stabilize oxygen vacancies to the maximum extent; the 600℃ high-temperature calcination ensures that the CeO2 lattice is fully formed but does not completely annihilate the oxygen vacancies; the selection of this temperature window is precisely matched with the thermal decomposition behavior of the tetravalent cerium source precursor, which cannot be obtained by simple conventional optimization;

[0069] Fifth, the synergistic regulation of Ni / Co molar ratio and oxygen vacancy concentration: The optimal catalytic activity is achieved when the Ni:Co molar ratio specified in this invention is 0.75:0.25 (total loading 15%). At this specific ratio, the introduction of Co not only changes the electron density of Ni, but also further stabilizes oxygen vacancies through Co-O-Ce interactions, forming a synergistic enhancement of the "Ni-Co bimetallic effect" and the "oxygen vacancy stabilization effect".

[0070] Compared with the prior art, the present invention has the following beneficial effects:

[0071] 1. High ammonia decomposition activity: Under conditions of 600℃ and space velocity, the ammonia decomposition conversion rate of the catalyst of this invention is significantly higher than that of the trivalent cerium source comparative catalyst and the hydrothermal method comparative catalyst. It exhibits considerable ammonia decomposition activity at 550℃, breaking through the technical bottleneck of insufficient low-temperature activity of non-noble metal Ni-based catalysts;

[0072] 2. Excellent anti-sintering stability: Due to the high concentration of oxygen vacancies induced by the tetravalent cerium source, the Ni-Ov-Ce metal-support interaction is enhanced, and the active metal Ni is firmly anchored on the CeO2 surface. After continuous operation at 600℃ for 50 hours, it still maintains a high ammonia decomposition conversion rate and there is no significant decrease in activity.

[0073] 3. Abundant surface oxygen vacancies: CeO2 prepared from tetravalent cerium sources via the sol-gel method has higher Ce content. 3+ The ratio, XPS and Raman characterization confirmed that its oxygen vacancy concentration was significantly higher than that of the trivalent cerium source control sample, providing sufficient active sites for the adsorption and activation of ammonia molecules and the desorption of N2.

[0074] 4. Ni-Co bimetallic synergistic effect: The introduction of Co regulates the central position of Ni's d-band through electronic effects, promotes the further conversion of nitrogen-containing intermediates (-NH2, -NH), avoids the accumulation of intermediates at active sites, and at the same time, the addition of an appropriate amount of Co does not destroy the Ni-Ov-Ce interface structure, thereby further enhancing the activity.

[0075] 5. High dispersibility of active metals: The sol-gel method achieves uniform mixing of Ni, Co and Ce at the molecular level. XRD characterization did not observe obvious large-size Ni or Co phase diffraction peaks. The active metals are loaded on the CeO2 surface in a highly dispersed state, which improves the density of active sites and atomic utilization.

[0076] 6. Significant cost advantage: The catalyst does not contain precious metals such as Ru, Pt, and Pd, and the raw material cost is only 1 / 500 to 1 / 1000 of that of precious metal catalysts. Moreover, the preparation method is simple and easy to scale up, and it has good prospects for industrialization.

[0077] 7. Universal application value: The catalyst of this invention is applicable to a variety of ammonia decomposition hydrogen production scenarios, including but not limited to marine ammonia-hydrogen power systems, distributed ammonia-hydrogen production and refueling stations, ammonia-hydrogen mixed combustion hydrogen supply, fuel cell hydrogen supply, and zero-carbon energy conversion. Attached Figure Description

[0078] Figure 1 This is a comparison chart of the ammonia decomposition rates of Ni-based catalysts with different supports described in this invention;

[0079] Figure 2 This is a comparison chart of the ammonia decomposition rates of catalysts with different active metal components described in this invention;

[0080] Figure 3 This is a comparison chart of the ammonia decomposition efficiency of the catalysts prepared by the sol-gel method and the hydrothermal method described in this invention;

[0081] Figure 4 This is a comparison chart of the ammonia decomposition efficiency of Ni-CeO2 catalysts with different cerium source valence states described in this invention;

[0082] Figure 5 These are SEM characterization images of the Ni-CeO2(Ⅲ) and Ni-CeO2(Ⅳ) catalysts described in this invention;

[0083] Figure 6 These are the Raman spectra of the Ni-CeO2(Ⅲ) and Ni-CeO2(Ⅳ) catalysts described in this invention;

[0084] Figure 7 This is a stability test diagram of the Ni-CeO2(Ⅳ) catalyst described in this invention;

[0085] Figure 8 This is a comparison chart of ammonia decomposition rates of catalysts with different Ni / Co ratios described in this invention;

[0086] Figure 9 This is a stability test diagram of the Ni / Co-CeO2 catalyst described in this invention;

[0087] Figure 10 This is a flowchart of the method described in this invention. Detailed Implementation

[0088] The main technical features and embodiments of the present invention will now be described in detail with reference to the accompanying drawings;

[0089] In the technical solution of this invention, the tetravalent cerium source is selected as a cerium precursor for the CeO2 support (specifically, cerium(IV)Ce(NO3)4 or its hydrate), which differs from the trivalent cerium source (such as Ce(NO3)3·6H2O) conventionally used in the prior art. During the sol-gel process, in an alkaline environment regulated by citric acid and ammonia, the tetravalent cerium source can generate a higher proportion of Ce through in-situ oxidation-reduction reactions during complexation-gelation-calcination. 3+ The species and abundant oxygen vacancies; this feature is the core innovation of this invention, directly determining the subsequent oxygen vacancy concentration, metal-support interaction strength and catalytic activity level, and is the most fundamental feature that distinguishes it from the prior art.

[0090] In the technical solution of this invention, the sol-gel preparation process is to prepare the catalyst using a sol-gel method. Specifically, a soluble metal salt (tetravalent cerium salt, nickel salt, and optionally cobalt salt) is used as a precursor, and citric acid is used as a complexing agent to form a uniform sol in the liquid phase. The catalyst is then obtained through gelation, drying, and calcination. The sol-gel method can achieve uniform mixing of Ni, Co, and Ce elements at the molecular / atomic level, ensuring high dispersion of the active metal on the support surface. Simultaneously, a uniform three-dimensional network structure is formed during gelation, providing a structural basis for the formation of mesoporous structures and the stabilization of oxygen vacancies. This method has a synergistic compatibility with the selection of a tetravalent cerium source, and is a key process guarantee for achieving the conversion from "tetravalent cerium source → high concentration of oxygen vacancies".

[0091] In the technical solution of this invention, oxygen vacancies (Ov) are anionic vacancy defects formed by the absence of oxygen atoms in the CeO2 lattice, which are usually associated with Ce. 4+ Restored to Ce 3+ To maintain charge balance. The catalyst surface of this invention has a higher Ce content than conventional CeO2 supports. 3+ The ratio and oxygen vacancy concentration are crucial; oxygen vacancies are key active sites for the adsorption and activation of ammonia molecules, promoting the breaking of NH bonds and the desorption of N2. High concentrations of oxygen vacancies also enhance electron transfer between Ni and CeO2, forming stronger metal-support interactions. This characteristic forms the microstructural basis for the high activity of the catalyst in this invention.

[0092] In the technical solution of this invention, the Ni-Ov-Ce interface structure refers to an interface structure in which the Ni active metal species forms a chemical bond with the CeO2 support through oxygen vacancies (Ov). The oxygen vacancies act as a bridge connecting the Ni and CeO2 lattices, allowing electrons to transfer from CeO2 to Ni, or vice versa. The Ni-Ov-Ce interface is the core active interface structure of the catalyst of this invention. This interface has the following characteristics:

[0093] (1) The high-temperature sintering of Ni particles is suppressed by the anchoring effect of oxygen vacancies;

[0094] (2) The adsorption energy of ammonia and its intermediates is optimized by regulating the electron density of Ni through interfacial electron transfer;

[0095] (3) It provides a unique reaction pathway for ammonia decomposition and lowers the reaction energy barrier. The formation of this interface structure depends on the high concentration of oxygen vacancies induced by the tetravalent cerium source and the close metal-support contact achieved by the sol-gel method.

[0096] In the technical solution of this invention, the nickel-cobalt bimetallic active component refers to a catalyst that can use Ni as the active metal alone, or simultaneously use Ni and Co as active metal components, and when the two coexist, the Ni:Co molar ratio is preferably 0.75:0.25. The introduction of Co regulates the d-band center position of Ni through electronic effects (Ni-Co alloy effect), optimizes the adsorption strength of nitrogen-containing intermediates (-NH2, -NH) at the active sites, promotes their further conversion, and avoids site poisoning caused by intermediate accumulation. At the same time, the addition of Co further stabilizes the oxygen vacancy structure through its interaction with oxygen vacancies (Co-Ov-Ce). This feature is a further performance improvement method based on the single Ni catalyst of this invention.

[0097] In the technical solution of this invention, the segmented calcination process employs a two-stage calcination process of "low-temperature pre-calcination + high-temperature re-calcination." First, calcination is carried out at 250–350°C (preferably 300°C) for 1–3 hours, followed by calcination at 550–650°C (preferably 600°C) for 3–5 hours, with a heating rate of 1–5°C / min (preferably 2°C / min). This segmented calcination precisely matches the thermal decomposition behavior of the tetravalent cerium source-citric acid precursor. The 300°C pre-calcination stage utilizes the redox reaction between the tetravalent cerium source and citric acid within this temperature range to maximize the generation and stabilization of oxygen vacancies. The 600°C high-temperature calcination ensures sufficient formation of the CeO2 fluorite lattice while avoiding excessive annihilation of oxygen vacancies. This calcination process is a key parameter for ensuring oxygen vacancy retention and active metal dispersion.

[0098] In the technical solution of this invention, the catalyst has a mesoporous structure and a specific surface area of ​​40–70 m². 2 / g, with an average pore size of 5–10 nm; the mesoporous structure originates from the uniform distribution of citric acid complexes during the sol-gel process and the controllable removal of organic matter during segmented calcination; this pore structure provides a smooth mass transfer channel for ammonia molecule diffusion and product gas escape, while the high specific surface area is beneficial for the high dispersion loading of active metals. This characteristic reflects the macroscopic structural level of the sol-gel preparation process and has a direct impact on the mass transfer efficiency of the catalytic reaction.

[0099] refer to Figures 1 to 9This paper illustrates the structural characterization and performance test results of a cerium-modified nickel-based ammonia decomposition hydrogen production catalyst according to one or more embodiments of this application. The catalyst is applied in zero-carbon energy conversion scenarios such as marine ammonia-hydrogen propulsion systems, distributed ammonia-hydrogen production and refueling stations, ammonia-hydrogen co-combustion hydrogen supply, and fuel cell hydrogen supply. It includes a cerium oxide support and an active metal component supported on the cerium oxide support. The cerium oxide support is prepared from a tetravalent cerium source via a sol-gel method. The active metal component includes nickel, or includes both nickel and cobalt. The catalyst surface has a Ce2+ content. 3+ It contains species and oxygen vacancies (Ov), and has a Ni-Ov-Ce interface structure. Figure 5 The SEM characterization images of the Ni-CeO2(Ⅲ) and Ni-CeO2(Ⅳ) catalysts are shown. Figure 6 The Raman spectra of both are shown. Figure 7 and Figure 9 The stability test results of Ni-CeO2(Ⅳ) and Ni / Co-CeO2 catalysts are shown respectively.

[0100] In one or more embodiments, the cerium oxide support has a fluorite-type cubic crystal structure, in which some oxygen atoms are missing in the crystal lattice, forming oxygen vacancy defects. This support is prepared from a tetravalent cerium source (preferably cerium(IV)Ce(NO3)4) via a sol-gel method, and has a specific surface area of ​​40–70 m² / g. 2 / g, with an average pore size of 5–10 nm, exhibiting a mesoporous structure. The support surface possesses a high proportion of Ce³⁺ species and oxygen vacancies, of which Ce... 3+ / (Ce 3+ +Ce 4+ The molar ratio, determined by XPS, was 0.25–0.45. The oxygen vacancy concentration was determined by the area ratio (I0.05) of the F2g peak to the defect peak in the Raman spectrum. D / I F The value was characterized as 0.35–0.65.

[0101] In one or more embodiments, the active metal component is supported on the surface of the cerium oxide support. When nickel is used as the active metal alone, the mass of nickel accounts for 5% to 25% of the total mass of the catalyst, preferably 10% to 20%, and more preferably 15%. The nickel species exist in a highly dispersed state, and no obvious large-size crystalline phase diffraction peaks of NiO or metallic Ni are observed in the XRD diffraction pattern, indicating that nickel mainly exists as nanoclusters (size <5nm) or highly dispersed Ni. 2+It exists in species form. When nickel and cobalt are introduced simultaneously as dual active metals, the molar ratio of nickel to cobalt is 0.95:0.05 to 0.50:0.50, preferably 0.85:0.15 to 0.65:0.35, and more preferably 0.75:0.25. After reduction treatment, nickel and cobalt form a Ni-Co alloy phase with uniform particle size, and form stable Ni-Ov-Ce and Co-Ov-Ce interface structures with the CeO2 support through oxygen vacancies.

[0102] In one or more embodiments, the Ni-Ov-Ce interface structure in the catalyst refers to the following: Ni atoms form chemical bonds with Ce atoms in the CeO2 lattice through oxygen vacancies (Ov), where the oxygen vacancies act as electron transfer channels, enabling the CeO2 support to transfer electrons to the Ni species, while simultaneously regulating the electron density of Ni. The formation of this interface structure is confirmed by the shift in the 2p binding energy of Ni in XPS (a shift of 0.3–0.8 eV towards lower binding energy compared to pure NiO) and the relative changes in the proportion of lattice oxygen (approximately 529 eV) and surface adsorbed oxygen (approximately 531 eV) in the 1s spectrum of O.

[0103] In one or more embodiments, the tetravalent cerium source is cerium ammonium nitrate ((NH4)2Ce(NO3)6) or its hydrate, replacing cerium(IV) nitrate. When cerium ammonium nitrate is used, because it contains ammonium ions, it decomposes during calcination to produce ammonia gas, which can further create a reducing micro-atmosphere and promote the reduction of Ce. 4+ →Ce 3+ Through this conversion, the oxygen vacancy concentration can be further increased by 5%–15%. The corresponding preparation process does not require changing other parameters to obtain a product with higher Ce content. 3+ The proportion of catalyst.

[0104] In one or more embodiments, the proportion of nickel in the active metal component to the total mass of the catalyst can be selected as a non-preferred value such as 8%, 12%, 18%, or 22% to achieve different active metal loading adjustments to meet the needs of different reaction space velocities and ammonia processing volumes. When the loading is less than 5%, the insufficient number of active sites leads to a decrease in ammonia decomposition conversion rate; when the loading is greater than 25%, nickel particles are prone to agglomeration, the metal-support interaction is weakened, and the stability is reduced.

[0105] In one or more embodiments, when the active metal component includes both nickel and cobalt, the molar ratio of nickel to cobalt can be selected as an intermediate ratio such as 0.90:0.10, 0.80:0.20, 0.70:0.30, or 0.60:0.40. When the Co ratio is low (Ni:Co>0.90:0.10), the bimetallic synergistic effect is not obvious; when the Co ratio is high (Ni:Co<0.50:0.50), excess Co may occupy the active sites of Ni and change the Ni-Ov-Ce interface structure, thus reducing the ammonia decomposition activity. Preferably, when Ni:Co=0.75:0.25, the electronic regulation effect of Co and the oxygen vacancy stabilization effect are optimally balanced.

[0106] In one or more embodiments, the catalyst can be used after a reduction treatment. The reduction treatment is carried out in a hydrogen atmosphere at a temperature of 450–550°C for 0.5–2 hours, preferably at 500°C for 1 hour. After reduction, the Ni species are NiO or Ni 2+ Reduced to metallic Ni 0 Co species are reduced to metallic Co. 0 The two form a Ni-Co alloy, which further enhances the catalytic activity of NH bond breaking and N2 desorption in the ammonia decomposition reaction.

[0107] In one or more embodiments, the mesoporous structure of the catalyst is characterized and determined in the following manner:

[0108] First, the adsorption-desorption isotherm of the catalyst was determined at liquid nitrogen temperature (-196℃) using the N2 physical adsorption-desorption method.

[0109] Then, the specific surface area is calculated according to the BET (Brunauer-Emmett-Teller) formula. Specifically, at least 5 pressure points are selected in the range of relative pressure p / p0 = 0.05 to 0.30 for linear fitting to obtain the specific surface area value.

[0110] Next, the pore size distribution was calculated based on the desorption branches using the BJH (Barrett-Joyner-Halenda) model, and the total pore volume was calculated using the cumulative adsorption amount.

[0111] Finally, the average pore size is determined based on the peak position of the pore size distribution curve.

[0112] Among them, the specific surface area is controlled between 40 and 70 m². 2 Within the range of / g, the average pore size is controlled within the range of 5–10 nm. When the specific surface area is less than 40 m², 2 When the specific surface area is above 70 m² / g, the dispersion of active metals decreases; when the specific surface area is above 70 m² / g, the dispersion of active metals decreases. 2 At / g, the mesoporous structure may be too small, leading to increased mass transfer resistance.

[0113] In one or more embodiments, the oxygen vacancy concentration of the catalyst was quantitatively analyzed by Raman spectroscopy. Specifically, Raman spectra were acquired under 532 nm laser excitation to obtain the F2g characteristic peak (approximately 465 cm⁻¹) and oxygen vacancy defect peak (approximately 600 cm⁻¹) of CeO₂. -1 (corresponding to the D peak); calculate the integral area ratio I between the defect peak and the F2g peak. D / I F The larger this ratio, the higher the oxygen vacancy concentration. Catalyst I of this invention D / I F The ratio ranges from 0.35 to 0.65, while the comparative catalyst I prepared using a trivalent cerium source... D / I F The ratio is only 0.15 to 0.30. Furthermore, Ce... 3+ The proportion was determined by peak fitting of XPS Ce3d spectra, where Ce 3+ Corresponding to the characteristic peaks u' (approximately 885 eV) and v' (approximately 903 eV), Ce 4+ The corresponding characteristic peaks are u (approximately 882 eV), u'' (approximately 889 eV), u''' (approximately 898 eV) and v (approximately 900 eV), v'' (approximately 907 eV), and v''' (approximately 916 eV). Ce 3+ The ratio = (area u' + area v') / total Ce area. The Ce of the catalyst of this invention... 3+ The proportion reached 25% to 45%, which was significantly higher than that of the comparative catalyst (10% to 20%).

[0114] In one or more embodiments, through the comprehensive design of the cerium oxide support, active metal component, Ni-Ov-Ce interface structure, and mesoporous structure characteristics, the catalyst can achieve a high density of active sites (oxygen vacancies and Ni). 0 / Co 0 Cooperative construction of sites. Figures 1 to 4 As shown, the ammonia decomposition conversion rates of the Ni-CeO2(Ⅳ) catalyst at 550℃, 600℃, and 650℃ reached 65%, 88%, and 96%, respectively, significantly higher than those of the Ni-CeO2(Ⅲ) catalyst (42%, 65%, and 82%, respectively) and the hydrothermal method comparative catalyst (38%, 58%, and 75%, respectively). Figure 7 and Figure 9 As shown, after continuous operation at 600℃ for 50 hours, the conversion rate of the catalyst decreased by less than 3%, demonstrating excellent anti-sintering stability and long-term operating potential. This catalyst contains no precious metals, has low raw material costs, and is suitable for large-scale industrial production and various ammonia decomposition hydrogen production applications.

[0115] refer to Figure 10The diagram illustrates a flowchart of a method for preparing a cerium-modified nickel-based ammonia decomposition hydrogen production catalyst according to one or more embodiments of this application. This method is applicable to both large-scale catalyst preparation and laboratory synthesis scenarios, and includes the following steps: S1 (cerium source dissolution), S2 (mixed metal salt preparation), S3 (citric acid complexation and pH adjustment), S4 (water bath evaporation and gelation), S5 (drying), and S6 (segmented calcination).

[0116] In one or more embodiments, step S1 involves dissolving a tetravalent cerium source in deionized water to obtain a cerium source solution. Specifically, a certain amount of tetravalent cerium source (preferably cerium(IV) nitrate Ce(NO3)4, purity ≥99.9%) is weighed, and the required mass is calculated based on the mass percentage of cerium in the final catalyst (typically 50%–85%, depending on the active metal loading). This mass is then added to 50–200 mL of deionized water and magnetically stirred at 300–600 rpm at room temperature (20–30°C) until completely dissolved, yielding a clear cerium source solution. The conductivity of the deionized water is ≤1 μS / cm to avoid the influence of impurity ions on the subsequent sol-gel process.

[0117] In one or more embodiments, step S2 involves adding a nickel source, or a nickel source and a cobalt source, to the cerium source solution and stirring to form a mixed metal salt solution. The nickel source is preferably nickel nitrate hexahydrate (Ni(NO3)2·6H2O), and the cobalt source is preferably cobalt nitrate hexahydrate (Co(NO3)2·6H2O). The amount of each metal salt is calculated based on the total loading of active metals in the target catalyst (5%–25%) and the nickel-cobalt molar ratio (1:0 when only nickel is present, and Ni:Co = 0.95:0.05–0.50:0.50 when nickel and cobalt are present). The weighed nickel salt and / or cobalt salt are added to the cerium source solution from step S1, and stirring continues at 400–800 rpm for 20–40 min at room temperature until all salts are completely dissolved, forming a homogeneous and transparent mixed metal salt solution. At this point, the solution pH is approximately 1–2 (due to the acidity caused by the hydrolysis of the metal salts).

[0118] In one or more embodiments, step S3 involves adding citric acid as a complexing agent to the mixed metal salt solution, adjusting the pH to alkaline, and continuing stirring to obtain a sol precursor. Specifically, citric acid (C6H8O7·H2O, analytical grade) is weighed to achieve a molar ratio of citric acid to the total molar amount of metal elements (the sum of Ce, Ni, and Co) of 0.8:1 to 1.5:1, preferably 1:1. Solid citric acid is added to the mixed solution from step S2, and the mixture is heated and stirred in a water bath at 40–60°C for 30–60 min to promote the complexation reaction between citric acid and metal ions, forming a metal-citric acid complex. Subsequently, ammonia water (25%–28% by mass) is added dropwise at room temperature, controlling the dropping rate at 1 mL / min to 2 mL / min, while simultaneously stirring rapidly at 500 rpm to 800 rpm to adjust the pH of the system to 7.5–8.5, preferably 8.0. Under these alkaline conditions, the metal-citric acid complex undergoes partial hydrolysis and condensation reactions, and the solution gradually changes from transparent to a light blue or light green sol. Stirring continues at room temperature for 20–40 minutes to homogenize the sol, yielding a stable, precipitate-free sol precursor.

[0119] In one or more embodiments, step S4 involves heating the sol precursor in a water bath to evaporate the water and form a gel or wet solid. The sol precursor obtained in step S3 is transferred to an evaporating dish or beaker and placed in a constant-temperature water bath at 70°C to 90°C (preferably 80°C) for evaporation in an air atmosphere. During heating, the water gradually evaporates, the sol viscosity increases, and the color changes from light to dark green or brown. When the volume decreases to 1 / 5 to 1 / 3 of the original volume, a viscous gel-like substance is formed; heating continues until the water is almost completely evaporated, yielding a wet solid precursor, which is fluffy or lumpy and contains a large amount of citric acid and metal complexes.

[0120] In one or more embodiments, step S5 involves drying the resulting gel or wet solid. The gel or wet solid obtained in step S4 is transferred to a forced-air drying oven and dried at 100°C–130°C (preferably 120°C) for 8–16 hours (overnight) to thoroughly remove residual moisture and some volatile organic compounds. The dried solid is dark brown or black, hard in texture, and easy to grind.

[0121] In one or more embodiments, step S6 involves segmented calcination of the dried solid to obtain the cerium-modified nickel-based or nickel-cobalt-based ammonia decomposition catalyst. Specifically, the solid dried in step S5 is ground into a fine powder (passing through an 80-100 mesh sieve), placed in a corundum crucible or ceramic crucible, and then placed in a muffle furnace for segmented calcination. The calcination procedure is as follows: first, the temperature is increased from room temperature to 250-350°C (preferably 300°C) at a heating rate of 1-5°C / min (preferably 2°C / min), and held at this temperature for 1-3 hours (preferably 2 hours) for low-temperature pre-calcination; then, the temperature is increased to 550-650°C (preferably 600°C) at the same heating rate, and held at this temperature for 3-5 hours (preferably 4 hours) for high-temperature re-calcination. After calcination, the catalyst is naturally cooled to room temperature to obtain the final catalyst.

[0122] In one or more embodiments, the molar ratio of citric acid to the total molar amount of metal elements in step S3 can be selected as a non-preferred value such as 0.9:1, 1.1:1, or 1.3:1. When the proportion of citric acid is too low (<0.8:1), the metal ion complexation is incomplete, resulting in an unstable sol that is prone to precipitation; when the proportion of citric acid is too high (>1.5:1), the excess citric acid will release heat violently during calcination, which may lead to local overheating and oxygen vacancy annihilation, and increase the risk of organic residue.

[0123] In one or more embodiments, the alkaline reagent used to adjust the pH in step S3 can be any one of ammonia, sodium hydroxide solution, or ammonium carbonate solution. When sodium hydroxide is used, it should be noted that sodium ions may remain, which may negatively affect the catalyst activity. Therefore, ammonia is preferred because ammonium ions can decompose into gas and escape during calcination without introducing metallic impurities.

[0124] In one or more embodiments, the water bath evaporation in step S4 can be performed in a rotary evaporator to accelerate the evaporation rate and improve uniformity. Placing the sol precursor in a pear-shaped flask of a rotary evaporator and evaporating under reduced pressure (vacuum degree -0.08MPa to -0.1MPa) at a water bath temperature of 70℃ to 90℃ and a rotation speed of 50rpm to 100rpm can shorten the evaporation time to 1h to 3h, and the resulting gel is more uniform, making it suitable for scale-up production.

[0125] In one or more embodiments, the segmented calcination in step S6 can be performed under different atmospheres. The low-temperature pre-calcination stage (300°C) can be performed in an air atmosphere to fully remove organic matter; the high-temperature calcination stage (600°C) can be performed in an inert atmosphere (such as N2 or Ar) to suppress the re-oxidation of oxygen vacancies. However, the inventors of this invention have found that performing full-segment calcination in an air atmosphere can achieve a sufficiently high oxygen vacancy concentration, and the operation is simpler and less costly; therefore, an air atmosphere is preferred.

[0126] In one or more embodiments, the calculation of the ratio of citric acid to the total molar amount of metal elements in step S3 is performed as follows:

[0127] First, based on the masses of tetravalent cerium salt, nickel salt, and cobalt salt weighed in steps S1 and S2, calculate the molar amount of each metallic element. For example, if the mass m of Ce(NO3)4 is weighed... Ce Its molar mass M Ce =332.12 g / mol, then the molar amount of Ce is n Ce =m Ce / M Ce Weigh out m masses of Ni(NO3)2·6H2O. Ni Its molar mass M Ni =290.81 g / mol, then the molar amount of Ni n Ni =m Ni / M Ni Weigh out m masses of Co(NO3)2·6H2O. Co Its molar mass M Co =291.03 g / mol, then the molar amount of Co, n Co =m Co / M Co Total molar mass of metallic elements n total =n Ce +n Ni +n Co ;

[0128] Then, based on the ratio R of the target citric acid to the total molar mass of the metal element (R = 0.8 to 1.5, preferably R = 1), the required citric acid (C6H8O7·H2O, molar mass M) is calculated. cit The mass m of (210.14 g / mol) cit =R×n total ×M cit Weigh out the required amount of citric acid and add it to the solution.

[0129] In one or more embodiments, the specific operation of pH adjustment in step S3 is as follows: The pH value of the solution is monitored in real time using a precision pH meter (accuracy ±0.01). During the addition of ammonia, stirring is stopped after every 0.5 mL of ammonia is added, and the pH value is measured until the pH value stabilizes within the target range (7.5–8.5). Since ammonia is a weak base, the pH rises rapidly in the initial stage of addition, but slows down after approaching 7.0; therefore, it needs to be added slowly to avoid over-addition. When the pH reaches 8.0, the addition is stopped, and stirring continues for 30 minutes to allow the system to fully equilibrium.

[0130] In one or more embodiments, the temperature regime and heating rate of the segmented calcination in step S6 need to be strictly controlled. The key role of the low-temperature pre-calcination stage (300°C) is that citric acid and the metal-citric acid complex undergo a violent oxidative decomposition reaction in this temperature range, releasing a large amount of heat and gases (CO2, H2O, NO). X (etc.), while tetravalent cerium (Ce) 4+ It was partially reduced to trivalent cerium (Ce). 3+ This creates oxygen vacancies. Controlling the heating rate to 2℃ / min ensures a gentle decomposition reaction, preventing sample splashing or abnormal grain growth due to concentrated exothermic reactions. The key role of the high-temperature calcination stage (600℃) is to allow CeO2 grains to fully crystallize, forming a stable fluorite structure, while simultaneously allowing some Ni... 2+ and Co 2+ It embeds itself in the CeO2 lattice or forms a strong interaction with CeO2, stabilizing the oxygen vacancy structure. A calcination time of 4 hours ensures complete phase transformation while avoiding excessive annihilation of oxygen vacancies and sintering of metal particles caused by prolonged high temperatures.

[0131] In one or more embodiments, to ensure the repeatability of the preparation process and the stability of product quality, the preparation method of the present invention also incorporates several quality control measures. After step S3, the viscosity of the sol precursor can be measured using a viscometer (viscosity range of 5–20 mPa·s at 25°C). If the viscosity is too low, it indicates incomplete complexation, and a small amount of citric acid can be added or the stirring time can be extended; if the viscosity is too high, it indicates excessive hydrolysis and polycondensation, and an appropriate amount of water can be added for dilution. After drying in step S5, the residual organic matter in the precursor can be detected by thermogravimetric analysis (TGA) to ensure that its residual amount is less than 5 wt%; otherwise, the drying time needs to be extended or the drying temperature increased.

[0132] In one or more embodiments, after the calcination of step S6 is completed, the catalyst is further subjected to grinding, sieving and homogenization to obtain catalyst particles with uniform particle size distribution (usually 40-60 mesh), which facilitates the subsequent loading of the fixed bed reactor.

[0133] In one or more embodiments, in response to an abnormal condition occurring in any preparation step (such as precipitation of sol, pH adjustment out of range, runaway calcination temperature, etc.), an error handling process is executed, wherein the error handling process includes at least one of the following operations: terminating the current batch operation, recording an error log, adjusting the corresponding parameters, and re-executing steps S1 to S6.

[0134] In one or more embodiments, the preparation method, through the synergistic operation of steps S1 to S6, enables in-situ reduction of the tetravalent cerium source and efficient generation of oxygen vacancies in the sol-gel process, while ensuring high dispersion of the Ni / Co active metal on the CeO2 support and sufficient formation of the Ni-Ov-Ce interface structure. Figure 6 As shown in the Raman spectrum, the Ni-CeO2(Ⅳ) catalyst prepared by this method exhibits a stronger defect peak signal (I... D / I F =0.52), while the trivalent cerium source catalyst has a value of only 0.25. For example Figure 3 As shown in the comparison chart of ammonia decomposition efficiency, the catalyst prepared by the sol-gel method achieved an ammonia decomposition conversion rate of 88% at 600℃, significantly higher than that of the hydrothermal method (75%). This indicates that the preparation method of the present invention has significant advantages in controlling the microstructure of the catalyst and improving its catalytic performance, and the process is simple, easy to scale up, and suitable for industrial production.

[0135] refer to Figures 1 to 4 and Figures 7 to 9 This paper illustrates the performance test results of a cerium-modified nickel-based ammonia decomposition hydrogen production catalyst according to one or more embodiments of this application in the ammonia decomposition reaction. The method is applied to marine ammonia-hydrogen propulsion systems, distributed ammonia-hydrogen refueling stations, ammonia-hydrogen mixed combustion hydrogen supply, fuel cell hydrogen supply, and zero-carbon fuel energy conversion scenarios. It includes loading the catalyst into a fixed-bed reactor or a fluidized-bed reactor, introducing ammonia gas to carry out a catalytic decomposition reaction, and generating a mixture of hydrogen and nitrogen gas.

[0136] In one or more embodiments, the application process is as follows:

[0137] First, the catalyst is loaded and pretreated for reduction. Take 100mg to 500g of the catalyst (determined according to reactor size) and load it into the isothermal zone of a stainless steel or quartz reaction tube. The catalyst bed is fixed at both ends with quartz wool or ceramic balls to ensure uniform gas flow distribution. After loading, the reaction tube is sealed and connected to a gas pipeline for a gas tightness check (introduce N2 or Ar, maintain pressure at 0.5MPa for 30 minutes; a pressure drop of <1% is acceptable).

[0138] Next, a reduction treatment is performed. In this reduction step, hydrogen gas (pure H2 or a H2 / Ar mixture, with an H2 volume fraction ≥5%) is introduced into the reaction tube at a flow rate of 50–200 mL / min. The reaction tube is then heated to 450–550 °C (preferably 500 °C) at a heating rate of 1–10 °C / min and maintained at this temperature for 0.5–2 h (preferably 1 h) to reduce NiO and CoO in the catalyst to metallic Ni. 0 and Co 0Simultaneously, Ni-Co alloy and Ni-Ov-Ce interface structures are formed. After reduction, the temperature is lowered to the reaction initiation temperature (usually 450℃) under H2 atmosphere, or the inert gas (N2 or Ar) is switched to purge for 10-30 minutes to remove residual H2, and then ammonia is switched to feed.

[0139] Subsequently, an ammonia decomposition reaction takes place. In this reaction step, pure ammonia gas (purity ≥99.99%) is introduced at a certain flow rate (mass hourly space velocity WHSV = 0.5–10 h⁻¹). -1 Preferably 1-4 hours -1 The reaction tube is vented under atmospheric pressure (0.1 MPa) or slightly positive pressure (≤0.5 MPa). The reaction temperature is set at 450–650 °C, preferably 550–650 °C, and more preferably 600 °C. After stabilizing at each temperature for 20–30 min, the composition of the reactor outlet gas is analyzed using an online gas chromatograph (equipped with a thermal conductivity detector (TCD) or a flame ionization detector (FID)). The components detected include H2, N2, and unreacted NH3. A Porapak Q or molecular sieve 5A packed column is used, with Ar or He as the carrier gas. Based on the molar fractions of N2 and NH3 in the outlet gas, the ammonia decomposition conversion rate (X) is calculated according to the following formula:

[0140]

[0141] in, For the outlet gas molar flow rate, This represents the molar flow rate of the imported ammonia gas. It can also be calculated directly from the outlet NH3 concentration.

[0142]

[0143] in, and These are the concentrations of ammonia at the outlet and inlet, respectively.

[0144] In one or more embodiments, the ammonia decomposition reaction can be carried out in a non-isothermal mode, that is, multiple temperature control zones are set up to make the catalyst bed temperature have a gradient distribution (450-500°C in the first section and 550-600°C in the second section), so as to utilize the lower temperature in the first section for preliminary activation and the higher temperature in the second section for deep conversion, thereby improving the overall energy efficiency.

[0145] In one or more embodiments, the ammonia decomposition reaction can be carried out in a hydrogen co-feed mode, i.e., a small amount of hydrogen (H2 / NH3 molar ratio 0.01-0.10) is mixed into the ammonia feed to suppress the formation of nitrides on the catalyst surface and prolong the catalyst lifetime. However, it should be noted that excessive hydrogen will dilute the partial pressure of ammonia and reduce the reaction rate.

[0146] In one or more embodiments, the mixed gas (H2 / N2 / NH3) after the ammonia decomposition reaction can be directly fed into an ammonia-hydrogen co-combustion engine for combustion and hydrogen supply, or it can be purified by a purification unit (such as palladium membrane separation, pressure swing adsorption PSA, or water washing to absorb NH3) to obtain high-purity hydrogen (purity ≥99.99%), which is then supplied to a proton exchange membrane fuel cell. When used in a marine ammonia-hydrogen propulsion system, the reactor outlet gas recovers waste heat through a heat exchanger to preheat the feed ammonia, improving the system's thermal integration.

[0147] In one or more embodiments, the catalyst's operating parameters can be flexibly adjusted under different space velocities and temperatures. For distributed ammonia-to-hydrogen refueling stations, due to the high requirements for hydrogen purity, a lower space velocity (WHSV = 0.5–1 h⁻¹) can be selected. -1 Higher temperatures (600–650°C) are required to achieve near 100% ammonia decomposition conversion; for marine propulsion systems, due to the high requirements for system compactness and response speed, higher airspeeds (WHSV = 3–5 h) can be selected. -1 The system utilizes moderate temperatures (550–600°C) to ensure sufficient hydrogen production while meeting the demands of dynamic load changes.

[0148] In one or more embodiments, the ammonia decomposition reaction rate constant and activation energy can be obtained by fitting the Arrhenius equation to variable-temperature experimental data.

[0149] First, the ammonia decomposition conversion rate was measured at five temperature points: 450℃, 500℃, 550℃, 600℃, and 650℃, and the reaction rate was calculated. (molNH3·g) -1 ·h -1 ), ,in, This refers to the catalyst mass.

[0150] Then, assuming the reaction is a first-order reaction (for pure ammonia feed, it is generally considered to follow first-order kinetics), according to ,in, Given the ammonia concentration, the reaction rate constant k at various temperatures can be calculated.

[0151] Finally, plot lnk and the slope of the linear fit is -Ea / R, thus obtaining the apparent activation energy Ea.

[0152] The apparent activation energy of the catalyst of this invention is calculated to be 75–95 kJ / mol, which is significantly lower than that of the trivalent cerium source comparative catalyst (110–130 kJ / mol) and the hydrothermal method comparative catalyst (125–145 kJ / mol), indicating that the catalyst of this invention lowers the energy barrier of the ammonia decomposition reaction.

[0153] In one or more embodiments, catalyst stability evaluation is performed under constant reaction conditions (preferably 600°C, WHSV = 2h). -1 The system was continuously operated, with samples taken every 2–4 hours for gas chromatography analysis to record the change in ammonia decomposition conversion rate over time. The rate of decrease in conversion rate was calculated based on the initial conversion rate. .when When the concentration exceeds 5%, it indicates significant catalyst deactivation. The catalyst of this invention, after operating at 600°C for 50 hours, <3%, such as Figure 7 and Figure 9 As shown, it exhibits excellent long-term stability. Furthermore, the catalyst can be regenerated after a 50-hour stability test: calcination at 500°C for 1 hour in air to remove carbon deposits, followed by reduction at 500°C for 1 hour in H2 atmosphere. After regeneration, the catalyst activity can be restored to over 95% of its initial level.

[0154] In one or more embodiments, in response to abnormal conditions (such as a sudden increase in reactor pressure, temperature runaway, ammonia leak alarm, or a sudden drop in conversion rate exceeding 10%), an error handling process is executed, the error handling process including at least one of the following operations: automatically cutting off ammonia feed, switching to inert gas purging, activating the emergency cooling system, recording a fault log and alarming the operator.

[0155] In one or more embodiments, the application process, through the synergistic operation of catalyst loading, reduction treatment, ammonia decomposition reaction, and post-treatment steps, achieves efficient, stable, and low-cost hydrogen production from ammonia decomposition, significantly improving the low-temperature activity and high-temperature stability of non-precious metal ammonia decomposition catalysts. Specifically, as... Figure 1 and Figure 2 As shown, the Ni-CeO2(Ⅳ) catalyst of this invention exhibits an ammonia decomposition conversion rate approximately 23 percentage points higher than that of Ni-CeO2(Ⅲ) at 600℃, and the Ni / Co-CeO2(Ⅳ) ratio is further increased to over 92%. Figure 3 As shown, the sol-gel catalyst achieves a 13 percentage point higher conversion rate than the hydrothermal catalyst at the same temperature. Figure 4 As shown, the tetravalent cerium source sample outperforms the trivalent cerium source sample at all temperature points. Figure 7 and Figure 9 As shown, the conversion rate fluctuated little during the 50-hour stability test, indicating that the catalyst has good potential for industrial application.

[0156] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitations, the phrase "comprising an element defined as..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0157] The above technical solutions only embody the preferred technical solutions of the present invention. Any modifications that may be made by those skilled in the art to certain parts thereof embody the principles of the present invention and fall within the protection scope of the present invention.

Claims

1. A cerium-modified nickel-based catalyst for hydrogen production from ammonia decomposition, characterized in that, The catalyst comprises a cerium oxide support and an active metal component supported on the cerium oxide support; The cerium oxide support was prepared from a tetravalent cerium source via a sol-gel method. The active metal component includes nickel; The catalyst surface has Ce 3+ Species and oxygen vacancies (Ov); The catalyst has a Ni-Ov-Ce interface structure.

2. The catalyst according to claim 1, characterized in that, The active metal component also includes: cobalt.

3. The catalyst according to claim 1, characterized in that, The tetravalent cerium source is one or more of cerium IV nitrate, cerium ammonium nitrate, or their hydrates.

4. The catalyst according to claim 1, characterized in that, The active metal component contains nickel, which accounts for 5% to 25% of the total mass of the catalyst.

5. The catalyst according to claim 2, characterized in that, When the active metal component includes nickel and cobalt, the molar ratio of nickel to cobalt is 0.95:0.05 to 0.50:0.

50.

6. The catalyst according to claim 1, characterized in that, The catalyst has a mesoporous structure and a specific surface area of ​​40–70 m². 2 / g, with an average pore size of 5–10 nm.

7. A preparation method for preparing a catalyst as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1. Dissolve the tetravalent cerium source in deionized water to obtain a cerium source solution; S2. Add a nickel source, or add a nickel source and a cobalt source, to the cerium source solution and stir to form a mixed metal salt solution; S3. Add citric acid as a complexing agent to the mixed metal salt solution, adjust the pH to alkaline, and continue stirring to obtain the sol precursor; S4. The sol precursor is heated under water bath conditions to evaporate the water and form a gel or wet solid. S5. Dry the resulting gel or wet solid; S6. The dried solid is calcined in stages to obtain the cerium-modified nickel-based or nickel-cobalt-based ammonia decomposition catalyst.

8. The cerium-modified nickel-based catalyst for ammonia decomposition to hydrogen production according to claim 7, its preparation method, and its application, characterized in that, The ratio of citric acid to the total molar amount of metal elements in step S3 is 0.8:1 to 1.5:

1.

9. The application of a catalyst in the decomposition of ammonia to produce hydrogen, wherein the catalyst is the catalyst according to any one of claims 1 to 6 or the catalyst prepared by the method according to any one of claims 7 to 8, characterized in that, The ammonia decomposition to hydrogen production reaction is carried out under the action of the catalyst, and the temperature conditions for the ammonia decomposition to hydrogen production reaction are 450-650℃.

10. The application according to claim 9, characterized in that, The catalyst can be reduced in a hydrogen atmosphere at a temperature of 450–550°C for a time of 0.5–2 hours.

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