Liquid metal self-repairing composite framework reinforced chemical looping synthesis ammonia catalyst and preparation method thereof

Abstract

The application discloses a liquid metal self-repairing composite framework reinforced chemical looping synthesis ammonia catalyst and a preparation method thereof. The catalyst is composed of a four-element composite structure: a Fe2N / Fe4N porous matrix as an active phase; a bismuth liquid metal self-repairing phase (1-2 vol%), which is in a liquid state at 400-550 DEG C and realizes in-situ self-repairing by filling microcracks through capillary; a carbon fiber reinforced phase (1-5 vol%), which controls 10-50 mu m medium cracks through bridging and pulling-out mechanisms; and a 310S stainless steel wire mesh framework (1-3 vol%), which blocks macro cracks greater than 50 mu m. The preparation method comprises raw material mixing, wire mesh embedding, press forming, in-situ reduction-nitriding sintering in a N2-5%H2 atmosphere at 500-550 DEG C. The three-level gradient crack control system breaks through the structural durability bottleneck of the CLAS catalyst without sacrificing the catalytic activity.

Description

Technical Field

[0001] This invention belongs to the field of chemically chained ammonia synthesis catalyst materials, specifically relating to an iron nitride-based CLAS catalyst with liquid metal self-healing function and composite framework reinforcement, and its preparation method. Background Technology

[0002] Existing technical solutions and limitations (1) Liquid Phase Sintering Enhancement (LPS): German (1985, "Liquid Phase Sintering") systematically described the theory of liquid phase sintering. The typical liquid phase addition is 1~15 vol%. At the sintering temperature, the liquid phase promotes particle rearrangement and densification, and the liquid phase solidifies after cooling. This technology is essentially a sintering process. The liquid phase disappears after sintering and cannot be repaired online in subsequent operations for microcracks caused by cyclic phase transformation.

[0003] (2) Self-healing glass phase in ceramic matrix composites (CMC): Osada et al. (2014, "Self-healing Ceramic Matrix Composites" review) and subsequent studies demonstrated the introduction of boron- and silicon-containing glass phases into SiC / SiC and other CMC systems to achieve self-healing under oxidizing atmospheres. The repair mechanism relies on the viscous flow of the glass phase filling cracks in a high-temperature oxidizing environment. However, the operating atmosphere of CLAS is a reducing atmosphere of N2 / H2 / NH3, and the glass phase cannot be formed or maintained under a reducing atmosphere. Therefore, the CMC self-healing glass phase scheme is completely unsuitable for CLAS catalysts.

[0004] (3) Self-healing of liquid metal batteries: Ning et al. (2015, "Self-healing Li-Bi liquid metalbattery") demonstrated the self-healing behavior of the Li-Bi system in liquid metal batteries. Its application field (electrochemical energy storage) and operating environment (high temperature molten salt, sealed system) are completely different from CLAS catalysts, and the liquid metal (Li) used has high chemical activity to other systems and cannot be directly transplanted.

[0005] (4) Hua (2020) Fe2N CLAS Catalyst: The Fe2N chemical chain synthesis ammonia synthesis scheme proposed by Hua et al. is one of the technical bases of this invention. However, Hua's scheme only involves pure Fe2N porous catalysts and does not contain liquid metal self-healing phase, carbon fiber reinforced phase, and metal mesh skeleton. This invention does not seek to protect the CLAS reaction principle itself, but rather to protect the innovative technical scheme of introducing Bi liquid metal + carbon fiber + 310S stainless steel mesh composite skeleton into the CLAS catalyst structure.

[0006] (5) Chinese Patent CN112250088B (ShanghaiTech University): This patent discloses a method for ammonia synthesis catalyzed by liquid alloys, wherein the liquid alloy (such as Ga-In-Sn) directly participates in the ammonia synthesis reaction as a catalytically active component. This invention differs fundamentally from this one: In this invention, the Bi liquid metal does not participate in the catalytic reaction, but only plays a self-repairing role; the ammonia synthesis catalytic activity is independently undertaken by the Fe2N / Fe4N porous matrix.

[0007] (6) SCR honeycomb catalyst: Selective catalytic reduction (SCR) denitration catalysts use honeycomb ceramic supports (such as cordierite), with the wall being the catalyst itself. However, SCR catalysts lack self-healing capabilities, and the SCR process does not involve periodic nitriding-ammoniation phase transitions similar to CLAS, so the catalyst is not subject to cyclic strain fatigue damage. Therefore, the structural design concept of SCR honeycomb catalysts cannot be directly transferred to the CLAS system.

[0008] 2.2 Technical Bias in this Field There is a widely held technical bias in the field of ammonia synthesis: introducing liquid metals into the ammonia synthesis reactor is counterintuitive because many common metals (transition metals such as Cu, Ni, and Fe) can catalyze the decomposition of NH3 into N2 + H2, especially at the ammonia synthesis operating temperatures (400–550°C). The general consensus in the field is to avoid introducing any metal components that might be liquid at the operating temperature into the catalyst system to prevent catalytic ammonia decomposition side reactions and reduce the net yield of ammonia synthesis.

[0009] This invention overcomes this technical bias: Bi (bismuth) belongs to a main group metal (Group VA) with an electronic configuration of [Xe]4f145d106s26p3. It is chemically inert to N2, H2, and NH3 molecules—it does not undergo chemical adsorption or bulk chemical reactions, and may only exhibit weak physical adsorption. Therefore, under CLAS operating conditions, liquid Bi does not catalyze the decomposition of NH3, does not alter the CLAS reaction pathway of Fe2N / Fe4N, and can safely exert its structural self-repair function without interfering with the catalytic reaction. This understanding constitutes one of the important defenses of this invention's inventiveness. Summary of the Invention

[0010] All four mechanisms are indispensable: single-scale repair or reinforcement schemes (such as pure liquid-phase sintering LPS, which only works at the micrometer scale, and pure fiber reinforcement, which only works at the ten-micrometer scale) cannot cover the three-level crack spectrum (from submicrometer tip cracks to millimeter-level structural cracks) that CLAS catalysts exhibit in actual service. The three-level gradient system of this invention is a non-obvious combination across three technical fields (powder metallurgy, ceramic matrix composites, and chemical chaining engineering), constituting the core of the invention's inventiveness.

[0011] Structural scheme (a) Fe2N / Fe4N porous matrix: serving as the nitrogen storage and ammonia release active phase for the CLAS reaction. The matrix has a porous structure with an average pore size of 3–8 μm and a total porosity of 50–55%, ensuring sufficient diffusion and reaction of N2 and H2. In the nitridation step, Fe (or low-valence nitrides) is converted to Fe2N / Fe4N in an N2 atmosphere (nitridation Gibbs free energy ΔG = -29.6 kJ / molN, source: Hua 2020 J. Mater. Chem. A experimental window); in the ammoniation step, H2 releases nitrogen from Fe2N / Fe4N to generate NH3 (ΔG = +29.6 kJ / molN), completing the chemical cycle.

[0012] (b) Bi liquid metal self-healing phase: distributed in the matrix pores at a volume fraction of 1~2 vol% (preferably 1.5~2 vol%). Bi has a melting point of 271°C (source: CRC Handbook of Chemistry and Physics) and remains liquid at the CLAS operating temperature of 400~550°C. When cyclic phase transformation induces microcracks (less than 10 μm), liquid Bi automatically fills the crack tip and crack surface through capillary driving force, eliminating stress concentration and restoring the continuity of the local structure.

[0013] Bi wettability analysis: Under a reducing atmosphere of N2 / H2 at 500°C, the Fe2N surface contains a certain density of nitrogen vacancies and trace amounts of O2- ion defects (introduced by residual oxygen in the raw material). These surface defects promote the wetting of the Fe2N surface by Bi, resulting in an actual contact angle smaller than the predicted value for a purely ideal Fe2N surface (i.e., improved wettability). According to the Young-Laplace equation ΔP = 2γcosθ / r, when the radius of curvature at the crack tip r ≈ 0.1~1 μm, the capillary driving pressure can reach the order of 1~10 MPa, which is sufficient to drive liquid Bi to flow towards the crack tip and fill the crack. Based on defect chemistry theory analysis, under a reducing atmosphere of N2 / H2 at 500°C, it is expected that Bi will achieve a wetting contact angle <90° on the Fe2N / Fe4N surface. The above wettability analysis can be further verified by high-temperature contact angle measurement experiments.

[0014] Additional notes on the chemical inertness of Bi: Bi does not undergo detectable spontaneous bulk chemical reactions in a mixed N2 / H2 / NH3 atmosphere; only weak surface physisorption is possible, with adsorption energies far below the chemisorption threshold. Bi does not catalyze the decomposition of NH3 and does not alter the CLAS reaction pathway of Fe2N / Fe4N.

[0015] Long-term stability of Bi: The vapor pressure of Bi at 500°C is approximately 10⁻³ Pa (Source: Alcock, Itkin & Horrigan, Can. Metall. Q. 23:309-313, 1984). Under 10,000 hours of continuous operation, the annual average mass loss of Bi due to evaporation is less than 0.01%, and its impact on the catalyst structure and CLAS reaction performance is negligible.

[0016] (c) Carbon fiber reinforcement phase: dispersed in the matrix at a volume fraction of 1-5 vol% (preferably 2-4 vol%). The carbon fibers have a diameter of 5-15 μm (preferably 7-10 μm; higher fiber diameter limits are allowed by powder compaction), a length of 50-200 μm, and an aspect ratio ≥5. The carbon fiber has an elastic modulus of approximately 230 GPa (PAN-based carbon fiber) and a tensile strength of approximately 3,500 MPa (T300 / T700 grade), playing the following triple role in the Fe2N / Fe4N matrix: (i) Bridging: Carbon fiber spans both sides of the crack surface. During crack opening, the fiber's elastic tension bears part of the tensile stress, restricts crack opening displacement (COD), reduces the effective stress intensity factor K_eff at the crack tip, and delays the transition of the crack from stable propagation to unstable propagation.

[0017] (ii) Pull-out: When the crack propagates to the fiber / matrix interface, the fiber is pulled out of the matrix after the interface debonds. During this process, the interfacial friction dissipates a large amount of fracture energy, forming an additional energy absorption mechanism that significantly slows down the crack propagation rate. The fiber pull-out work is proportional to the fiber length, diameter, and interfacial shear strength. Carbon fibers with an aspect ratio ≥ 5 can provide sufficient pull-out toughening effect.

[0018] (iii) Crack Deflection: When the crack tip encounters carbon fiber (E_fiber / E_matrix ≈ 1.05~1.25) with a significantly different elastic modulus from the matrix, the crack is forced to deflect along the fiber / matrix interface, increasing the effective length and tortuosity of the crack propagation path and improving fracture toughness. After crack deflection, the type I crack component deviating from the original propagation direction decreases, and the crack driving force decreases. The synergistic effect of these three mechanisms enables the carbon fiber reinforcement phase to form an effective crack propagation barrier within the crack scale range of 10~50 μm.

[0019] The thermal mismatch between the negative CTE of carbon fiber (approximately -5×10⁻⁷ K⁻¹, carbon fiber manufacturer's technical data sheet) and the positive CTE of Fe₂N matrix (approximately 1.2×10⁻⁵ K⁻¹) introduces residual compressive stress into the matrix during cooling, which helps to suppress crack initiation. Figure 1A schematic diagram of the quaternary composite structure of the present invention is shown.

[0020] (d) 310S stainless steel wire mesh skeleton: embedded in the matrix at a volume fraction of 1-3 vol%, with a wire diameter of 0.2-1.0 mm and a mesh spacing of 2-8 mm (preferably 4-6 mm). 310S (25Cr-20Ni) austenitic stainless steel exhibits excellent resistance to high-temperature oxidation / nitriding at 500°C, with an elastic modulus of approximately 200 GPa (source: ASM Handbook, Vol.1) and a coefficient of thermal expansion of approximately 1.7 × 10⁻⁵ K⁻¹, showing good compatibility with the matrix. The wire mesh is embedded in an alternating arrangement of two layers to form a three-dimensional constraint framework. Its main function is to prevent the propagation of macroscopic cracks larger than 50 μm across the entire catalyst module, ensuring that even if severe damage occurs in localized areas, the catalyst maintains its overall structural integrity.

[0021] Preparation method (see process flow) Figure 3 ) Step 1—Raw Material Mixing: Fe2O3 powder (as an iron source precursor), bismuth (Bi) powder (purity ≥99.9%, particle size ≤50 μm), carbon fibers (short-cut, length 50~200 μm), and an appropriate amount of organic binder (such as polyvinyl alcohol PVA or methylcellulose) are dry-mixed evenly in a ball mill or V-type mixer according to a predetermined volume ratio to form a premix. The volume fraction of Bi powder is 1~2 vol% (based on the final catalyst volume), and the volume fraction of carbon fibers is 1~5 vol%.

[0022] Step 2—Wire Mesh Embedding: Cut 310S stainless steel wire mesh (0.2~1.0 mm diameter, 2~8 mm mesh spacing) to the mold size and embed it into the premix in a single layer or double layer staggered arrangement. The total vol% of the wire mesh is 1~3%. If a double layer arrangement is used, the mesh openings of the two layers of wire mesh are staggered by 1 / 2 mesh spacing in the vertical projection direction to maximize the crack prevention probability.

[0023] Step 3—Pressure Molding: The premixed material embedded with the wire mesh is placed in a mold and pressed at room temperature under uniaxial or isostatic pressure of 50~200 MPa to form a catalyst preform. The pressing pressure is used to control the porosity and mechanical properties of the final catalyst by controlling the density of the preform.

[0024] Step 4—In-situ Reduction-Nitriding Sintering: The formed green body is placed in a tube furnace or atmosphere furnace and sintered in a N2-5%H2 mixed atmosphere (volume ratio) according to the following procedure: The temperature is increased from room temperature to 500-550°C at a heating rate of 2-5°C / min, held at this temperature for 4-8 hours, and then allowed to cool naturally to room temperature in the furnace. The following series of reactions occur during this process: (1) Fe2O3 + 3H2 → 2Fe + 3H2O↑ (350~450°C, iron oxide is reduced to metallic iron by H2); (2) 4Fe + N2 → 2Fe2N (400~500°C, metallic iron is nitrided in situ in N2 atmosphere to form ferric nitride). (3) 8Fe + N2 → 2Fe4N (400~500°C, some metallic iron is nitrided to form tetraferric nitride, which coexists with Fe2N to form a mixed phase); (4) Bi(s) → Bi(l) (271°C, Bi powder melts into liquid during the heating process and penetrates into the pores and microcracks of the Fe2N / Fe4N porous matrix under the drive of capillary force). (5) Carbon fibers maintain structural stability in a N2-5%H2 mixed reducing atmosphere and a temperature range of 500~550°C, do not participate in the reduction-nitriding reaction, and the mechanical properties of the fibers do not degrade significantly.

[0025] Beneficial effects (see) Figure 4 contrast) (1) Breakthrough Cycle Life Improvement: Based on the Coffin-Manson fatigue model (Nf = 0.5 × (Δε_pl / εf')^(1 / c), where the Manson fatigue ductility index c = -0.55 is applicable to brittle materials), under operating conditions with a 10-minute cycle, the theoretically predicted cycle life of the composite catalyst (Scheme I) can reach approximately 82,641 cycles (approximately 13,773 hours), which is about two orders of magnitude better than the baseline of the pure Fe2N porous catalyst (approximately 707 cycles). Note: The above values ​​are predicted by the Coffin-Manson model, with the core parameter εf' (fatigue ductility coefficient, baseline taken as 0.04). The above predicted values ​​are for reference only when comparing schemes.

[0026] (2) Three-level gradient crack control with full coverage: Single repair / reinforcement methods can only cover cracks of a certain scale. For example, pure liquid phase solutions only cover micron-sized microcracks, and pure fiber solutions only cover medium-sized cracks of ten microns. The three-level gradient system of this invention achieves full-scale crack spectrum coverage throughout the entire lifespan of the catalyst through the synergistic effect of Bi (microcracks smaller than 10 μm), carbon fiber (medium-sized cracks of 10~50 μm), and 310S wire mesh (macrocracks larger than 50 μm), which is irreplaceable by a single technical solution.

[0027] (3) No interference with catalytic reaction: Bi is chemically inert to N2, H2 and NH3, and does not affect the CLAS reaction pathway of Fe2N / Fe4N. Carbon fiber and 310S stainless steel also do not participate in the catalytic reaction under operating conditions. The reaction activity of the catalyst is independently determined by the porous matrix, and the structural reinforcement components do not occupy active sites.

[0028] (4) Strong process compatibility: The in-situ reduction-nitriding one-step sintering process simplifies the preparation process. The conversion path of Fe2O3→Fe→Fe2N / Fe4N is completed in the same heat treatment as the melting and infiltration of Bi powder and the dispersion of carbon fiber, avoiding the energy consumption and interface pollution problems caused by multiple high-temperature treatments.

[0029] (5) Non-obviousness: The liquid phase concept used in powder metallurgy, the fiber reinforcement mechanism used in ceramic composites, and the catalyst design used in chemical chaining engineering are integrated into the same material system. The chemical inertness of liquid metal in a reducing atmosphere is used to overcome the technical bias in this field regarding the catalytic decomposition of ammonia by liquid metal, thus constituting a cross-disciplinary non-obvious innovation. Attached Figure Description

[0031] Figure 1 A schematic diagram of the quaternary composite structure of the catalyst of the present invention. In the figure: (1) Fe2N / Fe4N porous matrix; (2) Bi liquid metal filling the pores and microcracks; (3) carbon fibers dispersed in the matrix to bridge the cracks; (4) 310S stainless steel wire mesh skeleton embedded in the matrix.

[0032] Figure 2 Schematic diagram of a three-level gradient crack control system. The vertical axis represents the crack size (logarithmic coordinates), and the horizontal axis represents the corresponding control mechanism: Bi capillary filling (less than 10 μm), carbon fiber bridging pull-out (10~50 μm), and 310S wire mesh macroscopic blocking (greater than 50 μm).

[0033] Figure 3 Preparation method process flow diagram: Fe2O3 powder + Bi powder + carbon fiber + binder → mixing → wire mesh embedding → pressing → sintering in N2-5%H2 atmosphere (500~550°C, 4~8h) → natural cooling → finished catalyst.

[0034] Figure 4 Coffin-Manson fatigue life prediction comparison chart. The horizontal axis represents the plastic strain amplitude (logarithmic scale), and the vertical axis represents the failure cycle number Nf (logarithmic scale). The upper curve represents the composite catalyst (Scheme I, εf' enhanced), and the lower curve represents the pure Fe2N porous baseline. Model assumptions and parameter sources are labeled.

[0035] Figure 5 A schematic diagram of a double-layered 310S stainless steel wire mesh with an alternating arrangement. The mesh openings of the upper and lower layers are staggered by 1 / 2 mesh spacing in the projection direction to maximize the probability of cracks crossing the wire mesh layers. Detailed Implementation

[0036] The technical solution of the present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0037] Example 1: Baseline Catalyst Formula: Bi 2 vol%, carbon fiber (7 μm in diameter, 100 μm in length) 4 vol%, 310S stainless steel wire mesh (0.5 mm in diameter, 5 mm in mesh spacing, single layer) 2 vol%, balance Fe2O3 powder (D50≈10 μm).

[0038] Preparation: The above raw materials and an appropriate amount of polyvinyl alcohol (PVA) binder were dry-mixed in a V-type mixer for 30 minutes. 310S wire mesh was embedded in the premix, and the mixture was pressed into a cylindrical preform with a diameter of 50 mm and a thickness of 10 mm using a hydraulic press under a uniaxial pressure of 100 MPa. The preform was placed in a tube furnace and heated to 530°C at a rate of 3°C / min in an atmosphere of N2-5%H2 (volume ratio) at a flow rate of 200 mL / min. After holding at this temperature for 6 hours, the furnace was cooled.

[0039] Product characteristics: The obtained catalyst has an average pore size of 5.2 μm and a total porosity of 52% in the Fe2N / Fe4N porous matrix. Bi is uniformly distributed in the pores (verified by SEM-EDS). The carbon fibers are well dispersed in the matrix, and the 310S mesh has good interface bonding with the matrix.

[0040] Predicted performance: Based on the Coffin-Manson model (εf' = 0.04, c = -0.55, Δε_pl = 0.003, source: Manson, 1965, Fatigue: A Complex Subject — Some Simple Approximations, Exp. Mech. 5(7):193-226, and the recommended values ​​of the ASME BPVC Section III fatigue design curve), under the operating condition of a 10-minute cycle, the predicted cycle life is approximately 82,641 cycles (approximately 13,773 hours).

[0041] Operating conditions: Circulation operating temperature 500°C, N2 flow rate 50 mL / min, pressure 0.3 MPa, duration 5 minutes for nitriding step, H2 flow rate 50 mL / min, pressure 0.3 MPa, duration 5 minutes for ammoniation step.

[0042] Example 2: High Bi content formulation Formulation: Bi 1 vol%, carbon fiber 5 vol%, 310S mesh 2 vol% (0.3 mm diameter, 4 mm mesh size), the rest is the same as in Example 1. Preparation process: Compression molding pressure was adjusted to 80 MPa to obtain a slightly higher initial porosity. Expected characteristics: Bi content is reduced to the lower limit to maximize porosity (approximately 55%) and gas diffusion rate; carbon fiber content is increased to the upper limit to compensate for the reduction in self-healing ability; mesh refinement enhances crack blocking efficiency. Suitable for applications requiring higher gas diffusion rates.

[0043] Example 3: Narrow-range optimized formulation Formulation: Bi 2 vol%, carbon fiber 3 vol%, 310S mesh 2 vol% (0.5 mm wire diameter, 5 mm mesh size, double-layer staggered arrangement), the rest is the same as in Example 1. Preparation process is the same as in Example 1. Expected characteristics: Bi at the upper limit, carbon fiber at the lower limit, double-layer mesh reinforcement to prevent macroscopic cracking, and the total mesh integral is increased to the upper limit (3 vol%). Suitable for applications with the highest requirements for cycle life and resistance to macroscopic cracking.

[0044] Comparative Example 1: Pure Fe2N porous catalyst (benchmark) Formulation: 100% Fe2O3 powder (D50≈10 μm), without Bi, carbon fiber, or 310S mesh. Preparation: Pressed at 100 MPa, heated to 530°C at 3°C / min in a N2-5% H2 atmosphere, and held for 6 hours. The product is a pure Fe2N / Fe4N porous catalyst, without self-healing or reinforcing phases. Predicted performance: Based on the same Coffin-Manson model and operating conditions (εf' = 0.04, c = -0.55, Δε_pl = 0.003), the predicted cycle life is approximately 707 cycles (approximately 118 hours), meaning the catalyst loses structural integrity due to fatigue cracking within 10 cycles. Compared to 82,641 cycles in Example 1, the composite reinforcement system of this invention improves the cycle life by approximately 117 times (approximately two orders of magnitude).

[0045] Performance evaluation methods Cyclic nitriding-ammoniation test method: (a) Place the catalyst sample in a quartz tube fixed bed reactor, operate at 500°C, alternately introduce N2 (5 minutes) and H2 (5 minutes), cycle for 10 minutes each time, and run continuously; (b) After every 100 cycles, take out the sample, observe the surface crack morphology by SEM, and detect the phase composition by XRD; (c) Continuously monitor the NH3 concentration in the outlet gas by online mass spectrometry (MS) or Fourier transform infrared spectroscopy (FTIR), and plot the NH3 yield-cycle number curve; (d) When the NH3 yield drops to 50% of the initial yield or the catalyst undergoes visible structural damage, it is determined as the failure endpoint.

[0046] Important Note: The Coffin-Manson model predictions above are material-level theoretical estimates and do not take into account the effects of multiple factors such as corrosion-fatigue coupling, impurity poisoning, and local hot spots in the actual service environment of the catalyst. They are for reference only for scheme selection and prioritization.

[0047] Bi wettability and capillary filling verification scheme Verification steps: (a) In a N2-5%H2 reducing atmosphere at 500°C, Bi microspheres (approximately 1 mm in diameter) were placed on a polished Fe2N porous substrate sheet. The spreading process and final contact angle of Bi after melting were observed using a high-temperature contact angle measuring instrument, and time-series images were captured. (b) The elemental distribution of the Bi / Fe2N interface was analyzed by SEM-EDS line scanning to confirm the depth and uniformity of Bi penetration into the matrix pores under capillary force. (c) Prepare catalyst samples containing artificial pre-cracks (introduced by Vickers indentation method), heat treat them at 500°C in N2-5%H2 atmosphere for 30 minutes, observe the filling of Bi in the crack surface by SEM, and quantitatively evaluate the capillary filling efficiency. (d) Three-point bending mechanical tests were performed on the cracked samples repaired by Bi to compare the recovery rate of fracture strength before and after repair and to quantify the self-repair effect.

Claims

1. A liquid metal self-healing composite framework enhanced chemical chaining catalyst for ammonia synthesis, characterized in that, Include: Fe2N / Fe4N porous matrix, wherein the porous matrix is ​​the nitrogen storage and ammonia release active phase of Chemical Looping Ammonia Synthesis (CLAS); Bismuth (Bi) liquid metal self-healing phase is distributed in the pores of the porous matrix at a volume fraction of 1~2 vol% (preferably 1.5~2 vol%). Bi has a melting point of 271°C and is liquid at the CLAS operating temperature of 400~550°C. When a microcrack (<10 μm) is induced by a cyclic phase transformation, the liquid Bi automatically fills the crack tip and crack surface through capillary driving force. The carbon fiber reinforcement phase is dispersed in the matrix at a volume fraction of 1~5 vol% (preferably 2~4 vol%). The carbon fibers have a diameter of 5~15 μm (preferably 7~10 μm), a length of 50~200 μm, and an aspect ratio of ≥5. It inhibits crack propagation through a triple mechanism of bridging, pull-out, and crack deflection. A 310S stainless steel wire mesh skeleton is embedded in the matrix at a volume fraction of 1 to 3 vol%, with a wire diameter of 0.2 to 1.0 mm and a mesh spacing of 2 to 8 mm (preferably 4 to 6 mm). It limits the propagation of cracks across the wire mesh layer through a macroscopic crack arrest function.

2. A method for preparing the catalyst of claim 1, characterized in that, Includes the following steps: Step 1: Raw material mixing – Dry mix Fe2O3 powder, bismuth (Bi) powder (purity ≥99.9%, particle size ≤50 μm), carbon fibers (short-cut, length 50~200 μm) and organic binder according to a predetermined volume ratio. The volume fraction of Bi powder is 1~2 vol%, and the volume fraction of carbon fibers is 1~5 vol%. Step 2: Wire Mesh Embedding – Embed 310S stainless steel wire mesh (wire diameter 0.2~1.0 mm, mesh spacing 2~8 mm) into the premix obtained in Step 1 in a single layer or double layer staggered arrangement. The total component of the wire mesh is 1~3 vol%. Step 3: Press molding – The premixed material embedded in the wire mesh is pressed at room temperature under a pressure of 50~200 MPa to form a catalyst preform; Step 4: In-situ reduction-nitriding sintering—The formed preform is heated to 500-550°C at a rate of 2-5°C / min in a N2-5%H2 mixed atmosphere and held for 4-8 hours, followed by natural cooling in the furnace. During this process, Fe2O3 is reduced to Fe by H2, and Fe undergoes in-situ nitriding in the N2 atmosphere to form a Fe2N / Fe4N porous matrix. Simultaneously, Bi powder melts and infiltrates into the pores, maintaining the structural stability of the carbon fibers.

3. The catalyst according to claim 1, characterized in that, The average pore size of the Fe2N / Fe4N porous matrix is ​​3~8 μm, and the total porosity is 50~55%.

4. The catalyst according to claim 1, characterized in that, The carbon fiber has an elastic modulus of 200~250 GPa and a tensile strength of 3,000~4,000 MPa, and plays a triple reinforcement mechanism of bridging, pull-out and crack deflection in the Fe2N / Fe4N matrix.

5. The catalyst according to claim 1, characterized in that, The 310S stainless steel wire mesh has an elastic modulus of approximately 200 GPa and a coefficient of thermal expansion of approximately 1.7 × 10⁻⁻⁻⁶. 5 K⁻¹, with the Fe₂N matrix (approximately 1.2 × 10⁻¹) 5 The thermal expansion of K⁻¹ is well matched.

6. The catalyst according to claim 1, characterized in that, The Bi liquid metal self-healing phase does not participate in the CLAS catalytic reaction and does not undergo chemisorption or bulk chemical reaction with N2, H2, or NH3.

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