NiMoMn-bi melt alloy catalyst and method for hydrogen co-production by low carbon hydrocarbon cracking

Abstract

This invention relates to a NiMoMn-Bi molten alloy catalyst and a method for co-producing carbon materials through low-carbon hydrocarbon cracking for hydrogen production. The catalyst uses Bi as a low-melting-point molten support and Ni, Mo, and Mn as active components, with a preferred molar ratio of Ni:Mo:Mn:Bi of 12:4:4:80. After mechanical alloying, melt stabilization, and hydrogen reduction activation under an inert atmosphere, the catalyst can catalyze the cracking of low-carbon hydrocarbons in the temperature range of 350℃ to 1050℃. Under conditions of 800℃, a methane flow rate of 4 mL / min, and a catalyst bed height of approximately 1 cm, the methane conversion rate is 23.67%, the H2 selectivity is 99.98%, and the hydrogen production rate is 9.590 mL g. Ni ‑1 min ‑1 It can operate continuously and stably for 260 hours, thus providing a molten alloy catalytic solution for low-carbon hydrogen production and solid carbon resource utilization.

Description

Technical Field

[0001] This invention relates to the field of low-carbon hydrocarbon catalytic cracking for hydrogen production and the resource utilization of solid carbon materials, specifically to the preparation, activation, low-carbon hydrocarbon cracking reaction, and recovery and utilization of solid carbon by-products of NiMoMn-Bi molten alloy catalyst with Bi as a low-melting-point molten support and Ni, Mo, and Mn as active components. Background Technology

[0002] Hydrogen possesses both chemical feedstock and energy carrier properties, and has wide applications in ammonia synthesis, refining hydrogenation, metallurgical reduction, fuel cells, and long-cycle energy storage. Current large-scale hydrogen production still primarily relies on fossil resource conversion, a mature technology but facing significant carbon emission pressures. While water electrolysis can be coupled with renewable electricity, its equipment investment, power consumption, and key material costs limit large-scale, low-cost applications. Therefore, developing hydrogen production technologies that do not rely on oxidative reforming and can reduce subsequent carbon separation pressures is of practical significance.

[0003] Direct cracking of low-carbon hydrocarbons to produce hydrogen converts hydrogen from hydrocarbon molecules into hydrogen gas and fixes carbon into solid carbon, avoiding the direct generation of carbon dioxide in the main reaction. For natural gas or light hydrocarbon resources containing methane, ethane, and propane, this route can not only provide hydrogen but also yield byproducts such as carbon powder, flake carbon, graphitized carbon, and some tubular carbon. Traditional solid Ni-based, Fe-based, or supported catalysts are prone to degradation in this type of reaction due to carbon deposition, particle sintering, and bed blockage. In contrast, molten metal or molten alloy systems have a flowing interface, allowing solid carbon to gradually leave the catalytic phase under density differences and interfacial migration, making them more suitable for long-cycle cracking to produce hydrogen. Summary of the Invention

[0004] The purpose of this invention is to provide a NiMoMn-Bi molten alloy catalyst and its application in efficient hydrogen production and co-production of carbon materials. By controlling the electronic structure, active metal dispersion state and CH bond activation ability in the NiMo-Bi molten system through Mn composition, it can achieve efficient cracking of low-carbon hydrocarbons, highly selective hydrogen production, and recycling of solid carbon by-products.

[0005] A method for preparing a NiMoMn-Bi molten alloy catalyst involves using Ni, Mo, Mn, and Bi as raw materials, followed by mechanical alloying, melt stabilization, and reduction activation to form a quaternary molten alloy catalyst containing Ni-Mo-Mn active components and using Bi as the molten support. The catalyst exhibits a molten flow state at the reaction temperature. The molar ratio of Ni, Mo, Mn, and Bi ranges from 7.6 to 24:2.6 to 8:1 to 8:60 to 89.8. When the Bi content is 60 to 89.8%, the system can maintain molten flow at the reaction temperature. An excessively high proportion of active metal will reduce the fluidity of the system at the reaction temperature.

[0006] Preferably, the molar ratio of Ni, Mo, Mn and Bi is 12:4:4:80.

[0007] Preferably, the mechanical alloying treatment includes: first, ball milling Ni, Mo and Mn powders in an Ar atmosphere for 20-30 hours, then adding Bi powder and continuing ball milling for 3-8 hours. The ball milling process adopts an intermittent operation mode, with a 5-minute interval between every 15 minutes of ball milling to control the local temperature of the powder, thereby obtaining the alloy precursor.

[0008] Preferably, the melt stabilization treatment includes: heating to 550~650℃ in an inert atmosphere and holding at that temperature for 1~3 hours; the reduction activation treatment includes: treating at 650~750℃ in a hydrogen-containing atmosphere for 1~4 hours, with the treatment endpoint being that there is no significant change in the amount of hydrogen consumed in the gas phase.

[0009] The present invention also provides a method for preparing a NiMoMn-Bi molten alloy catalyst.

[0010] This invention also provides a method for producing hydrogen and co-producing carbon materials from low-carbon hydrocarbons through cracking using a NiMoMn-Bi molten alloy catalyst, the method comprising the following steps: Step S1: Atmosphere replacement and melt stabilization: The catalyst precursor is loaded into a reactor with a porous gas distributor at the bottom, and inert gas is introduced to replace the air in the reactor. Then, the temperature is raised to 550~650℃ under an inert atmosphere and held at the temperature to complete the melt stabilization. Step S2: Reduction and activation: Switch to a hydrogen-containing mixed atmosphere, heat to 650~750℃ to reduce and activate the molten alloy, and after activation, purge the reactor with inert gas; Step S3: Cracking reaction: Low-carbon hydrocarbon feed gas is introduced. The feed gas passes through the molten alloy catalyst bed in the form of bubbles through a porous gas distributor and undergoes a cracking reaction at 350~1050℃ to generate hydrogen and solid carbon. Step S4: Product separation and recovery: Hydrogen is obtained after cooling and purification of the gaseous product; solid carbon floats to the top of the molten alloy bed under the action of density difference, and carbon material is obtained after separation.

[0011] Preferably, the low-carbon hydrocarbon is one or more of methane, ethane, and propane, and the feed flow rate of the low-carbon hydrocarbon under standard conditions is 4~50 mL / min.

[0012] Preferably, when the low-carbon hydrocarbon is methane, the catalyst bed height is 1 cm, the methane feed flow rate is 4 mL / min, and the reaction temperature is 800 °C, the hydrogen selectivity is not less than 99%, and the methane conversion rate fluctuation does not exceed 5% within 260 h of continuous operation.

[0013] Preferably, in step S4, the solid carbon is recovered by one or more of the following methods: washing, drying, sieving, and acid washing; the recovered carbon material includes one or more of the following: carbon powder, flake carbon, graphitized carbon, and carbon nanotubes.

[0014] Preferably, in step S4, a carbon material collection zone is set at the top of the reactor, and solid carbon continuously floats into the collection zone during the reaction process to achieve continuous or semi-continuous recovery.

[0015] The beneficial effects of this invention are: Compared with the prior art, the present invention has at least the following beneficial effects: First, this invention uses Bi as a low-melting-point molten support and introduces Ni, Mo, and Mn as active components to keep the catalyst in a molten state at the methane cracking temperature. When the gas passes through the molten alloy catalyst layer in the form of bubbles, the gas-liquid interface can continuously expose active sites, and solid carbon products can be separated from the molten alloy phase, thereby reducing the risk of rapid deactivation of traditional solid catalysts due to carbon deposition.

[0016] Second, the present invention introduces Mn element into the NiMo-Bi system, which can adjust the electronic structure near Ni and Mo, promote CH bond activation and reduce the apparent activation energy of low-carbon hydrocarbon cracking, so that NiMoMn-Bi has stable hydrogen production capability over a wide temperature range.

[0017] Third, the NiMoMn-Bi catalyst of the present invention can achieve a high methane conversion rate and near-complete hydrogen selectivity under methane cracking conditions at 800℃, and can maintain a stable methane conversion and hydrogen generation rate during long-term operation, and has good resistance to carbon deposition and deactivation.

[0018] Fourth, the solid carbon generated by the pyrolysis of this invention is mainly concentrated in the upper part of the molten alloy or the upper part of the reactor, which facilitates separation and recovery from the catalyst; the obtained carbon material can be used as conductive filler, electrode material or other carbon material applications after being washed with water, dried, sieved or acid-washed. Attached Figure Description

[0019] Figure 1 A schematic diagram of a process for hydrogen production from low-carbon hydrocarbon cracking and carbon material recovery; Figure 2 For Ni 3x Mo x Mn x -Bi hydrogen generation rate graph in the range of 350℃ to 1000℃, where different bars correspond to different x values, and stars indicate preferred composition; Figure 3 For Ni 12 Mo4Mn x-Bi hydrogen generation rate graph in the range of 350℃ to 1000℃, where different bars correspond to different Mn contents, and stars indicate preferred composition; Figure 4 For catalytic reaction activity and kinetics; where: (a) Ni 12 Mo4-Bi and Ni 12 Apparent activation energy of Mo4Mn4-Bi (Arrhenius fit at 500-800℃); (b) Ni 3x Mo x CH4 conversion, H2 selectivity, and H2 yield of β-Bi alloys were compared using a binary catalyst Ni. 27 -Bi 73 Comparative data for three-way catalysts; (c) Ni 12 CH4 conversion, H2 selectivity, and H2 yield of Mo4M4-Bi alloy using a ternary catalyst Ni 7.6 Mo 2.6 -Bi represents comparative data for quaternary catalysts; (d) Ni 12 CH4 conversion, H2 selectivity, and H2 yield of Mo4Mn4-Bi as a function of temperature in the range of 350–1050 °C; (e) Ni 3x Mo x Mn x CH4 conversion, H2 selectivity, and H2 yield of the -Bi alloy; (f) Ni 12 Mo4Mn x CH4 conversion, H2 selectivity, and H2 yield of (g)Ni alloys; 12 Stability of Mo4Mn4-Bi over 260 h; Figure 5 Ni 2p XPS spectra of NiMo-Bi and NiMoMn-Bi; Figure 6 For Ni 12 Plots of methane conversion, hydrogen selectivity and hydrogen generation rate of Mo4Mn4-Bi under (a) different methane flow rates, (b) different molten catalyst heights and (c) different methane partial pressures; Figure 7 The results of molecular dynamics simulations of Mo3 in Mo-Bi, NiMo-Bi and NiMoMn-Bi molten alloys are shown, where (a) is a schematic diagram of the structure and (b) is a diagram of the average bond length change after running at 1500K for 1000fs. Figure 8 For Ni 3x Mo x Mn xScanning electron microscope images of solid carbon obtained by -Bi at 800℃, where (a) to (g) correspond to alloy compositions with different x values, respectively; Figure 9 For Ni 12 Mo4Mn x Scanning electron microscope images of solid carbon obtained by -Bi at 800℃, where (a) to (h) correspond to alloy compositions with different Mn contents. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention.

[0021] Low-melting-point Bi provides a stable molten support environment, but its activity is limited when used alone for CH bond activation. Introducing Ni and Mo into the Bi molten phase creates a metallic environment conducive to the activation of low-carbon hydrocarbons; further introduction of Mn can adjust the electronic structure and active metal dispersion near Ni-Mo, increasing the cracking reaction rate of low-carbon hydrocarbons such as methane. The reactor can employ a vertical quartz tube or a high-temperature resistant metal tube, with a porous gas distribution structure at the bottom, allowing the reactant gas to pass through the molten alloy catalyst layer in the form of bubbles. The bubble interface is not only the region for low-carbon hydrocarbon activation and hydrogen generation, but also an important interface for the formation, migration, and precipitation of carbon species.

[0022] During the reaction start-up process, the atmosphere inside the reactor needs to be replaced with an inert gas to reduce the risk of oxygen entering the high-temperature molten alloy system. Maintaining an inert atmosphere during the heating phase allows the alloy precursor to gradually melt and stabilize the bed. Subsequently, a hydrogen-containing atmosphere is introduced for reduction activation to remove residual oxygen on the surface and obtain a more stable initial catalytic state. After reduction, purging with an inert gas reduces the interference of residual hydrogen on the subsequent low-carbon hydrocarbon cracking initiation stage.

[0023] Low-carbon hydrocarbon cracking is an endothermic process, where reaction temperature, gas residence time, catalyst bed height, and hydrocarbon partial pressure all influence conversion rate, hydrogen selectivity, and carbon material morphology. For the NiMoMn-Bi system, which aims for efficient hydrogen production, it is necessary to improve CH bond activation efficiency and control the precipitation site of solid carbon in the reactor while ensuring catalyst fluidity and anti-coking stability.

[0024] The catalyst composition can be adjusted by changing the feed amounts of Ni, Mo, Mn, and Bi. For scenarios requiring lower melting temperatures, the Bi content can be increased; for scenarios requiring higher activation capacity for low-carbon hydrocarbons, the Ni, Mo, and Mn contents can be appropriately increased, but sufficient fluidity of the system should be ensured at the reaction temperature.

[0025] Furthermore, the catalyst is prepared using a mechanical alloying method. During the mechanical alloying process, the mill jar and grinding balls are preferably made of wear-resistant materials, and the ball-to-material ratio, rotation speed, and interval can be adjusted according to the equipment size.

[0026] The active metal powder is first ball-milled in an inert atmosphere to ensure sufficient contact and premixing of Ni, Mo, and Mn. Bi powder is then added and ball-milling continues, allowing Bi to act as a low-melting-point molten carrier and be uniformly dispersed in the catalyst precursor. This avoids excessive cold welding of low-melting-point Bi during prolonged high-energy ball milling, while ensuring uniform mixing of Bi and the active metal. Intermittent ball milling reduces the risk of localized overheating and agglomeration of the powder, which is beneficial for obtaining an alloy precursor with a uniform composition distribution.

[0027] Before the reaction, the alloy is melted in an inert atmosphere and reduced in a hydrogen-containing atmosphere to obtain a molten alloy system with low oxygen residue and uniform component distribution.

[0028] During the reduction activation process, the ratio of H2 to Ar can be adjusted according to the oxygen content of the precursor. If metal powder is used as the raw material, the reduction time can be appropriately shortened; if oxide or salt precursors are used, the reduction time can be extended to ensure that residual oxygen is fully removed.

[0029] In gas product analysis, the inlet low-carbon hydrocarbon flow rate is controlled by a mass flow controller, and the outlet gas is analyzed by gas chromatography for H2, CH4, and any possible C2-C4 hydrocarbon byproducts. Since methane cracking causes gas volume expansion, the total outlet molar flow rate can be corrected based on the conservation of hydrogen atoms, and then the absolute molar flow rates of each component can be calculated.

[0030] Methane conversion rate is calculated by dividing the difference between the inlet and outlet methane molar flow rates by the inlet methane molar flow rate; H2 selectivity is expressed as the proportion of hydrogen atoms generated entering H2, minus the hydrogen atoms entering by-product hydrocarbons such as ethane, ethylene, propane, propylene, and butane.

[0031] The morphology of carbon materials was characterized by SEM, TEM, and HRTEM. The degree of structural order was evaluated by the intensity ratio of the D and G peaks in the Raman spectrum. Elemental residues were analyzed by XRF or EDS. Different catalyst compositions and reaction conditions can form carbon powder, sheet-like carbon, graphitized carbon, and tubular carbon structures.

[0032] The recovery of carbon materials after the reaction can be carried out in a batch, continuous, or semi-continuous manner. The batch method is suitable for laboratory evaluation and small-scale preparation; the continuous method can set up a carbon material collection zone at the top of the reactor, so that the carbon material gradually leaves the catalyst layer and is collected and recovered.

[0033] The resulting hydrogen can be cooled, filtered, and subjected to necessary gas separation steps before entering hydrogen storage or downstream utilization units. Since the reaction system does not introduce an oxygen source, theoretically no CO2 is generated in the products, which helps reduce the burden of subsequent gas purification.

[0034] Raman spectroscopy can be performed using a 532nm laser. During testing, excessive laser power should be avoided to prevent localized heating of the carbon material. By comparing the characteristics of the D-peak, G-peak, and 2D-peak, defects, the degree of graphitization, and the layered structure of the carbon material can be determined.

[0035] like Figure 1 As shown, low-carbon hydrocarbons, after being controlled by mass flow rate, enter a bubble reactor loaded with a NiMoMn-Bi molten alloy catalyst, where they undergo cracking at the molten catalyst interface to produce hydrogen and solid carbon. The gaseous products are condensed and cooled before being analyzed by gas chromatography, while the solid carbon is washed with water, acid-washed, and dried before being characterized by SEM, XRF, and other methods.

[0036] like Figure 2 As shown, Ni 3x Mo x Mn x The hydrogen generation rate of the -Bi series catalysts increases significantly with increasing reaction temperature, with the best hydrogen production capacity observed near x=4, indicating that the ratio of Ni, Mo and Mn has a significant impact on CH bond activation.

[0037] like Figure 3 As shown, Ni 12 Mo4Mn x In the -Bi series, changes in Mn content affect the hydrogen production rate. Appropriate amounts of Mn are beneficial for improving methane cracking activity in both low and high temperature ranges, while excessively low or high Mn content is detrimental to overall hydrogen production performance.

[0038] like Figure 4 As shown in (a), Arrhenius fitting was performed on data within the range of 500-800℃, Ni 12 The apparent activation energy of Mo4Mn4-Bi is 77.51 ± 3.49 kJ / mol. -1 lower than Ni 12 Mo4-Bi 93.20±2.78kJ mol -1 ;like Figure 4 As shown in (b), Ni 3x Mo x The -Bi series all reach peak methane conversion, H2 selectivity, and hydrogen production rate at x=4; for example... Figure 4 As shown in (c), Ni 12 In the Mo4M4-Bi series, only when M is Mn does the highest methane conversion rate and hydrogen production rate are achieved simultaneously; for example... Figure 4 As shown in (d), Ni12 The methane conversion, H2 selectivity, and hydrogen generation rate of Mo4Mn4-Bi all increase with increasing reaction temperature; for example... Figure 4 As shown in (e) and (f), Ni 3x Mo x Mn x -Bi and Ni 12 Mo4Mn x The -Bi series all exhibited the best overall hydrogen production performance around x=4; such as Figure 4 As shown in (g), Ni 12 The Mo4Mn4-Bi catalyst maintained a stable methane conversion rate and hydrogen production rate during a continuous methane cracking process lasting up to 260 hours, indicating that the molten alloy catalytic system has good resistance to carbon deposition and deactivation and long-term operational stability.

[0039] like Figure 5 As shown, the Ni 2p XPS spectra of NiMo-Bi and NiMoMn-Bi show a significant shift, indicating that the electronic structure around Ni is regulated after the introduction of Mn, and there is an electronic interaction between the active metal and the Bi support.

[0040] like Figure 6 As shown, methane flow rate, molten catalyst height, and methane partial pressure all affect methane conversion, hydrogen selectivity, and hydrogen generation rate. Longer residence time and higher methane partial pressure are beneficial for increasing conversion, while higher flow rate can increase hydrogen generation per unit time.

[0041] like Figure 7 As shown in (a), molecular dynamics simulations compared the structural evolution of Mo3 in Mo-Bi, NiMo-Bi and NiMoMn-Bi molten alloys. The introduction of Mn helps to change the average bond length and dispersion state of Mo clusters, thereby affecting the formation of active centers.

[0042] like Figure 8 As shown, different Ni 3x Mo x Mn x The morphology of solid carbon obtained at 800℃ with -Bi composition showed significant differences, indicating that the proportion of active metals not only affects hydrogen production performance, but also regulates carbon deposition and the microstructure of solid carbon.

[0043] like Figure 9 As shown, Ni 12 Mo4Mn x The solid carbon obtained from the -Bi series at 800℃ exhibits different particle, flake, or agglomerated morphologies depending on the Mn content, further demonstrating that the Mn content can regulate the precipitation behavior of carbon species and the structure of carbon materials.

[0044] Example of the working process of the present invention: Example 1

[0045] Preparation of NiMoMn-Bi molten alloy catalyst. The raw materials were high-purity metal powders, wherein Bi had a purity of 99.99%, Ni 99.95%, Mo 99.95%, and Mn 99.5%, and the purity of methane, ethane, propane, hydrogen, and argon was 99.999%. Ni powder, Mo powder, Mn powder, and Bi powder were weighed according to the molar ratio n(Ni):n(Mo):n(Mn):n(Bi) = 12:4:4:80. The catalyst loading was 5.05 g, of which Ni mass was 0.198 g. Ni, Mo, and Mn powders were first ball-milled for 25 hours in an Ar atmosphere ball mill jar. Both the jar and grinding balls were made of zirconium oxide (ZrO2) to prevent the introduction of foreign metals into the milling equipment. The ball milling was conducted intermittently, with a 5-minute pause after every 15 minutes of milling to suppress localized overheating and agglomeration of the powder during mechanical alloying. Bi powder was then added and milling continued for another 5 hours, allowing the low-melting-point Bi to act as a molten carrier, uniformly mixing with the active metals and completing the initial alloying, yielding the NiMoMn-Bi alloy precursor. SEM-EDS and ICP-OES analysis showed no residual Zr in the ball-milled product. Direct alloying with high-purity metal powders eliminates the need for the calcination, drying, and reduction processes required by the oxide route, facilitating larger-scale preparation.

[0046] Furthermore, the alloy precursor was loaded into a vertical quartz tube bubble reactor with a length of 560 mm and an inner diameter of 10 mm. A SiO2 sand core gas distributor was installed at the bottom to allow the reactant gas to pass through the catalyst bed in the form of bubbles. The catalyst bed height was 1 cm at the reaction temperature. First, the reactor was purged with 20 mL / min Ar for 30 min to replace the air. Then, under an inert atmosphere, the temperature was increased to 600 °C at 10 °C / min and held for 2 h to achieve melting and stabilization. Next, a mixture of 10 mL / min H2 and 30 mL / min Ar was used for reduction activation at 700 °C for 3 h. The activation endpoint was when no significant hydrogen consumption was detected by gas chromatography. After reduction, the reactor was cooled in a 10 mL / min Ar stream for 2 h to suppress oxidation. After purging with Ar for 1 h, methane was introduced to begin the cracking reaction. The process flow is as follows: Figure 1 As shown. The ratio of H2 to Ar can be adjusted according to the oxygen content of the precursor: if metal powder is used as raw material, the reduction time can be shortened appropriately; if oxides or salts are used as precursors, the reduction time can be extended to ensure that residual oxygen is fully removed.

[0047] Further, methane was introduced at a rate of 4 mL / min, and the pyrolysis reaction was carried out at 800 °C. The outlet gas was analyzed using online gas chromatography. H2, CH4, and CO2 were detected by a thermal conductivity detector with a Hayesep Q / Porapak Q packed column, while C2-C4 hydrocarbon byproducts were detected by a flame ionization detector with an HP-PLOT Al2O3 S capillary column. Each measurement was repeated three times to assess repeatability. Due to the increase in the total number of gaseous molecules during methane pyrolysis, the total outlet molar flow rate was corrected for hydrogen atom conservation before calculating the absolute molar flow rates of each component. The methane conversion rate was calculated by dividing the difference between the inlet and outlet methane molar flow rates by the inlet methane molar flow rate. H2 selectivity was calculated based on the proportion of generated hydrogen atoms entering H2, after deducting the proportions entering C2H6, C2H4, C2H2, C3H8, C3H6, C3H4, and C4H6. 10 Hydrogen atoms in by-product hydrocarbons.

[0048] Furthermore, under the conditions of a catalyst bed height of approximately 1 cm, a methane flow rate of 4 mL / min, and a reaction temperature of 800 °C, the methane conversion rate was 23.67%, the H2 selectivity was 99.98%, and the hydrogen generation rate was 0.375 mL gcat. -1 min -1 After normalization by Ni mass, it is 9.590 mL gNi -1 min -1 Arrhenius fitting was performed using data within the 500-800℃ range (e.g., ...). Figure 4 (as shown in (a)), Ni 12 The apparent activation energy of Mo4Mn4-Bi is 77.51 ± 3.49 kJ / mol. -1 Ni without the introduction of Mn 12 The Mo4-Bi content was 93.20 ± 2.78 kJ / mol. -1 Under the same conditions, Ni 12 The Mo4-Bi exhibits a methane conversion rate of only 16%, an H2 selectivity of 97.7%, and a hydrogen generation rate of 0.289 mL gcat. -1 min -1 Therefore, the introduction of Mn lowers the apparent activation energy by approximately 15.7 kJ / mol. -1 The hydrogen generation rate is increased by approximately 29% (1.29 times), while the H2 selectivity is increased from 97.7% to 99.98%. Mn can modulate the electronic structure near Ni and Mo, improve the dispersion state of active metals in Bi-rich melts, and make the Ni-Mo active center more conducive to the activation of low-carbon hydrocarbon CH bonds.

[0049] Furthermore, in the temperature range of 350℃ to 1050℃, Ni 12All Mo4Mn4-Bi can generate hydrogen, and the hydrogen production rate increases significantly with increasing temperature (e.g., Figure 4 As shown in (e): Hydrogen generation can be detected from 350℃, and the hydrogen generation rate reaches 0.0222 mL gcat at 1050℃, catalyst bed height of 1 cm, and methane flow rate of 4 mL / min. -1 min -1 This indicates that the molten alloy system is suitable for the cracking of low-carbon hydrocarbons over a wide temperature range. For example... Figure 2 and Figure 3 As shown, in Ni 3x Mo x Mn x -Bi and Ni 12 Mo4Mn x In the -Bi series, x=4 (i.e., Ni) 12 Mo4Mn4-Bi) exhibited optimal overall hydrogen production performance across the entire test temperature range of 350℃ to 1000℃, indicating that the promoting effect of Mn depends on the balanced ratio of Ni / Mo / Mn, rather than simply increasing the active metal loading; in the high-temperature region of 1000-1050℃, Ni... 15 Mo5Mn5-Bi, Ni 18 Mo6Mn6-Bi, Ni 21 Mo7Mn7-Bi and Ni 24 The hydrogen production rate of highly loaded alloys such as Mo8Mn8-Bi can approach that of Ni. 12 Mo4Mn4-Bi, but none of them exceeded this composition under standard conditions at 800℃. Regarding Ni... 3x Mo x -Bi、Ni 12 Mo4M4-Bi, Ni 3x Mo x Mn x -Bi and Ni 12 Mo4Mn x The screening results of 29 catalysts from four Bi series (e.g.) Figure 4 (as shown in (b), (d), (f), and (g)) also indicate that the optimal overall hydrogen production performance is found near x=4.

[0050] Furthermore, the composition of Ni, Mo, Mn, and Bi in the feed was determined based on the predicted melting point. To ensure the predicted melting point of the alloy is below 800℃ (1073.15K) and thus maintain molten flow at the reaction temperature, the molar fraction of Bi should not be less than 60%. Therefore, in Ni... 3x Mo x Mn xIn the -Bi series, x is limited to no more than 8, and the molar ratio of Ni to Mo is fixed at 3:1. An excessively high proportion of active metals will increase the melting point of the system and reduce fluidity at the reaction temperature, which is detrimental to the decomposition of solid carbon from the catalytic phase. For scenarios requiring lower melting temperatures, the Bi content can be appropriately increased. For scenarios requiring higher activation capacity for low-carbon hydrocarbons, the Ni, Mo, and Mn contents can be appropriately increased, but sufficient fluidity of the system at the reaction temperature should be ensured. The actual composition of the alloy after cooling was determined using inductively coupled plasma optical emission spectrometry (ICP-OES). Ni 12 The actual molar fractions of Ni, Mo, and Mn in Mo4Mn4-Bi are 10.42%, 3.15%, and 3.16%, respectively, which are slightly lower than the theoretical values ​​of 12.00%, 4.00%, and 4.00%. The measured n(Ni):n(Mo):n(Mn) ratio is 3.31:1.00:1.00, and the ratio of Mo to Mn is still close to the designed 1:1, indicating that the mechanical alloying and melting process did not cause obvious component segregation.

[0051] Furthermore, Ni 12 Mo4Mn4-Bi is applicable not only to methane cracking but also to ethane and propane cracking. The apparent activation energy for propane cracking is 72.6 ± 1.2 kJ mol. -1 This is lower than the 77.51 ± 3.49 kJ / mol of methane cracking. -1 The hydrogen production rates of all three low-carbon hydrocarbons increased with increasing temperature, with propane exhibiting the highest rate. The results indicate that this catalyst possesses CH bond activation capability for low-carbon hydrocarbons with different carbon numbers, making it suitable for the conversion of mixed resources including natural gas and low-carbon hydrocarbons.

[0052] Furthermore, the structure of the cooled alloy was characterized. X-ray diffraction (XRD) was performed using Cu Kα radiation (λ=0.15406nm), with a 2θ scan range of 5-120° and a scan rate of 10° / min. The results showed that Ni 7.6 Mo 2.6 -Bi、Ni 12 Mo4-Bi and Ni 12 Bi3Ni (Pnma), rhombohedral Bi (R-3m), and a small amount of orthorhombic Ni3Mo phases are present in both Mo4Mn4-Bi. 12 The MnBi (P63 / mmc) phase also appears in Mo4Mn4-Bi; the low-angle diffraction peaks shift to lower angles after the introduction of Mn, indicating lattice expansion and changes in intermetallic interactions. In-situ high-temperature XRD shows that Bi melts around 250℃, and the diffraction peaks of Bi3Ni, MnBi, and Ni3Mo disappear at 350℃, indicating that a liquid alloy has formed in the system at the methane cracking reaction temperature. X-ray photoelectron spectroscopy (XPS) calibrated the binding energy with C 1s284.8 eV, and the results are as follows. Figure 5 As shown: In NiMo-Bi, the Ni 2p peak is located at 852.1 eV, which is 0.6 eV lower than the 852.7 eV of metallic Ni, indicating that electrons are transferred from Bi to Ni; after the introduction of Mn, the peak shifts to 852.5 eV, indicating that Ni tends to be metallic but is still electron-rich; the Mo 3d spectrum shows that Mo is partially electron-deficient in the alloy, and the introduction of Mn causes Mo to shift towards the metallic state; the Mn 2p binding energy is between that of metallic Mn and MnO, indicating an intermediate valence state rather than bulk metallic Mn or manganese oxide. X-ray absorption fine structure spectroscopy (XAFS) was acquired at the BL14W beamline of the Shanghai Synchrotron Radiation Facility using a Si(111) dual-crystal monochromator. K-edge spectra were acquired at room temperature in transmission or fluorescence modes. Ni K-edge XANES showed that both NiMo-Bi and NiMoMn-Bi shifted to lower energies relative to the Ni foil, with NiMoMn-Bi showing a larger shift. Ni K-edge EXAFS showed that the first shell was dominated by metallic coordination and no obvious Ni-O features were observed. The k2-weighted oscillations were in the range of 6-8 Å. -1 The amplitude decreases despite the interval being close to that of the Ni foil, indicating enhanced metallic properties and increased disorder. Wavelet transform EXAFS results show that the scattering characteristics of Ni become more localized after the introduction of Mn, shifting towards higher R values ​​and suppressing the contribution of the second shell. The side spectra of Mo and Mn also indicate that Mo and Mn are highly dispersed and have not agglomerated into Mo-Mo or Mn-Mn clusters. HAADF-STEM elemental surface scans show that Ni, Mo, Mn, and Bi are uniformly distributed. These structural changes correspond to a lower apparent activation energy and higher methane cracking activity.

[0053] Furthermore, density functional theory (DFT) and molecular dynamics simulations were used to investigate the dispersion state and CH bond activation behavior of active metals in the molten state, supplementing information about the liquid environment that is difficult to directly reflect in the characterization of cooled samples. Figure 7 As shown in (b), the average Mo-Mo distance of Mo3 clusters in Mo-Bi, NiMo-Bi, and NiMoMn-Bi melts was statistically analyzed. The results show that Mo3 has poor dispersion in Mo-Bi melt, and the addition of Ni can partially improve the dispersion of Mo. In NiMoMn-Bi, the average Mo-Mo distance remains above 5 Å after about 200 fs, and Ni3 clusters also exhibit a similar dispersion effect, indicating that the interaction between Mn-Ni and Mn-Mo helps the active metal to maintain dispersion in Bi solvent. Methane activation simulation at 1500 K shows that the first CH bond breaking occurs at about 150 fs (Ni 7.6 Mo 2.6 -Bi), 130fs (Ni 12 Mo4-Bi) and 60fs (Ni 12The order of apparent activation energy (Mo4Mn4-Bi) is consistent with that of the measured apparent activation energy. Bader charge and charge density difference analysis shows that in NiMo-Bi, electrons are mainly transferred from Ni to the adsorbed CH4, while in NiMoMn-Bi, Ni and Mn jointly participate in charge transfer, changing the charge of C atoms in the adsorbed CH4 from -0.186e to -0.261e, the charge of H atoms from +0.060e to +0.107e, and the net charge of CH4 from -0.127e to -0.154e, thereby enhancing CH bond polarization and promoting its breaking. Example 2

[0054] This embodiment examines Ni 12 Long-term stability of the Mo4Mn4-Bi molten alloy catalyst. Using the catalyst obtained in Example 1, continuous operation for 260 h was performed at 800℃, a catalyst bed height of 1 cm, and a methane flow rate of 4 mL / min. The results are as follows: Figure 4 As shown in (g): the hydrogen production rate increased from an initial 0.375 mL gcat. -1 min -1 Reduced to 0.325 mL gcat -1 min -1 The total decrease was 13%, with the majority of the decrease occurring in the first 25 hours, after which it stabilized at approximately 0.325 mL gcat. -1 min -1 During 260 hours of continuous operation, the methane conversion rate fluctuated by no more than 5%, and the rapid carbon deposition and deactivation, as well as bed blockage, which are common in traditional solid catalysts, did not occur. The initial rate decrease may be due to the partial dissolution of generated carbon in the molten alloy, which subsequently reached a dynamic equilibrium. After stability testing, the Ni, Mo, Mn, Bi, and C elements still showed a near-uniform distribution.

[0055] Furthermore, this embodiment also investigated the effect of reaction conditions on catalytic performance, and the results are as follows: Figure 6 As shown. Figure 6 As shown in (a), increasing the methane flow rate from 4 mL / min to 50 mL / min reduced the methane conversion rate to 6.18%, while increasing the hydrogen production per unit time from 0.375 mL gcat. -1 min -1 Increase to 0.485 mL gcat -1 min -1 According to the Davidson-Schuler constant-flow model, as the apparent gas velocity increased from 0.85 mm / s to 10.61 mm / s, the equivalent bubble diameter increased from 1.106 mm to 3.037 mm. The increased bubble size reduced the gas-liquid interface area per unit volume and shortened the gas-liquid contact time, thus decreasing the single-pass conversion rate. However, the increased inlet flow rate still improved the absolute hydrogen production rate. Figure 6 As shown in (b), increasing the height of the molten catalyst bed from 1 cm to 6 cm increased the methane conversion rate, hydrogen selectivity, and hydrogen generation rate, indicating that extending the gas residence time is beneficial to the conversion. Figure 6 As shown in (c), diluting methane with Ar, the hydrogen production rate increased to 0.375 mL gcat when the methane volume fraction increased from 10% to 100%. -1 min -1 The methane conversion rate decreased from 35.47% to 23.67%. A power-law fit was performed on six methane partial pressure points in the range of 0.1-1.0 atm at 800℃, yielding an apparent reaction order of 1.70 relative to the methane partial pressure, indicating that the reaction rate is highly sensitive to methane partial pressure. In summary, a longer residence time and appropriate methane partial pressure are beneficial for improving methane conversion, while increasing the feed flow rate increases the hydrogen production per unit time. Example 3

[0056] This embodiment investigates the recovery and characterization of solid carbon products obtained from methane cracking. The reaction apparatus of Example 1 and Ni were used. 12 The Mo4Mn4-Bi catalyst was continuously reacted at 800℃ and a methane flow rate of 4 mL / min for 3 h to obtain sufficient solid carbon. After the reaction, the mixture was cooled to room temperature under an Ar atmosphere. After cooling and crushing the quartz tube, it was found that the solid carbon was mainly concentrated in the upper part of the molten alloy bed or the upper part of the reactor. Due to the density difference, it was spatially separated from the catalyst, and the upper solid carbon could be directly collected. The collected product was successively washed with deionized water, acid-washed with 0.1 mol / L dilute hydrochloric acid, washed until neutral, dried, and sieved to obtain the solid carbon material separated from the catalyst.

[0057] Furthermore, the morphology of the carbon material was observed using a scanning electron microscope, and Raman spectroscopy (532nm laser, avoiding excessive laser power during testing to prevent localized heating of the carbon material) was used to measure the intensity ratio of the D peak to the G peak (IL). D / I G The degree of structural order was evaluated using X-ray fluorescence spectroscopy (XRF) or energy-dispersive X-ray spectroscopy (EDS) to analyze elemental residues. The results showed that Ni... 7.6 Mo 2.6 -Bi and Ni 12 The solid carbon obtained from Mo4Mn4-Bi is mainly composed of irregular particles and aggregates. D / I G The values ​​are 1.24 and 1.42 respectively, with the D peak intensity higher than the G peak, indicating amorphous carbon with abundant defects and a low degree of graphitization. For example... Figure 8 and Figure 9 As shown, Ni 3x Mo x Mn x -Bi and Ni12 Mo4Mn x The morphology of solid carbon obtained from the -Bi series catalysts at 800℃ varies significantly with the proportion of active metal and Mn content, indicating that the catalyst composition not only affects hydrogen production performance but also regulates the precipitation behavior of carbon species and the microstructure of solid carbon. The resulting carbon material can be used as a conductive filler, electrode material, or other carbon material applications after washing, drying, sieving, or acid washing. Since no oxygen source is introduced into the reaction system, CO2 is theoretically not generated in the gaseous products, which helps reduce the burden of subsequent gas purification.

[0058] Comparative Example 1 A NiMo-Bi molten alloy catalytic system without the introduction of Mn. Ni was prepared using the same method as in Example 1, except that Mn powder was not added. 12 The Mo4-Bi catalyst was used in the reaction under the same methane cracking conditions (800 °C, catalyst bed height 1 cm, methane flow rate 4 mL / min). The results showed that Ni… 12 The apparent activation energy of Mo4-Bi is 93.20 ± 2.78 kJ / mol. -1 The methane conversion rate was 16%, the H2 selectivity was 97.7%, and the hydrogen production rate was 0.2894 mL gcat. -1 min -1 (Number normalized to 7.3178 mL g) Ni -1 min -1 Further reduce the active metal loading of Ni 7.6 Mo 2.6 -Bi produces hydrogen at a rate of only 0.0596 mL gcat under the same conditions. -1 min -1 In contrast, Ni in Example 1 12 The hydrogen production rate of Mo4Mn4-Bi relative to Ni 7.6 Mo 2.6 -Bi increased by 6.29 times, relative to Ni 12 Mo4-Bi increased by 1.29 times, while the apparent activation energy decreased by approximately 15.7 kJ / mol. -1 The H2 selectivity was improved to 99.98%, and it could operate stably continuously for 260 hours. This comparative example demonstrates that the Mn component can adjust the electronic structure near Ni and Mo, promote CH bond activation and active metal dispersion, thereby improving the hydrogen production activity, hydrogen selectivity, and long-term stability of the molten alloy catalyst.

[0059] Comparative Example 2 Ni with other metals replacing Mn as the fourth component 12Mo4M4-Bi molten alloy catalytic system. Ni was prepared using the same method as in Example 1, except that Mn was replaced with equimolar amounts of Fe, Co, Al, Cr, Cu, and Mg. 12 The Mo4M4-Bi catalyst was used, and the reaction was carried out under the same methane cracking conditions (800 °C, catalyst bed height 1 cm, methane flow rate 4 mL / min). The results (e.g.) Figure 4 (d) The promoting effect of the fourth metal (Ni) exhibits significant element specificity. 12 Mo4Fe4-Bi, Ni 12 Mo4Co4-Bi, Ni 12 Mo4Al4-Bi, Ni 12 Mo4Cr4-Bi, Ni 12 Mo4Cu4-Bi and Ni 12 The hydrogen generation rates of Mo4Mg4-Bi were 0.0840, 0.0764, 0.2744, 0.0499, 0.1693 and 0.1772 mL gcat, respectively. -1 min -1 The apparent activation energies were 106.66, 120.41, 215.84, 174.61, 188.57 and 227.25 kJ / mol, respectively. -1 All are lower than Ni 12 0.2894 mL gcat of Mo4-Bi -1 min -1 Furthermore, the apparent activation energies of all substituted elements are higher than that of Mn (77.51 kJ / mol). -1 This comparative example demonstrates that the introduction of Mn into the NiMo-Bi molten system cannot be simply replaced by other fourth metals. Mn exhibits significantly better overall performance than other candidate metals in terms of improving hydrogen production activity, reducing apparent activation energy, and balancing low melting point and melt flowability.

[0060] This invention introduces Mn into a NiMo-Bi molten alloy system and uses Bi as a low-melting-point molten support, enabling low-carbon hydrocarbons such as methane, ethane, and propane to crack at the molten alloy interface to generate hydrogen and solid carbon. Compared to traditional solid catalysts, this molten alloy system can reduce the risk of carbon buildup and bed blockage through the flow interface. Simultaneously, the Mn component can modulate the electronic structure of the Ni-Mo active centers, promoting CH bond activation, thereby improving the efficiency of hydrogen production from low-carbon hydrocarbon cracking.

[0061] Compared with the NiMo-Bi system without Mn, the NiMoMn-Bi catalyst provided by this invention has a higher methane conversion rate and near-complete hydrogen selectivity under methane cracking conditions at 800℃, and can operate continuously and stably for 260 hours. At the same time, the catalyst is also suitable for ethane and propane cracking, which can improve the applicability and process stability of natural gas and low-carbon hydrocarbon mixed resource conversion.

[0062] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, various modifications, equivalent substitutions, or improvements can be made to the catalyst composition, activation conditions, types of low-carbon hydrocarbons, reaction temperature, feed flow rate, catalyst bed height, and carbon material recovery methods without departing from the spirit and principles of the present invention. All modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a NiMoMn-Bi molten alloy catalyst, characterized in that, Using Ni, Mo, Mn and Bi as raw materials, a quaternary molten alloy catalyst containing Ni-Mo-Mn active components and Bi as the molten support is formed through mechanical alloying, melt stabilization and reduction activation treatment. The catalyst is in a molten state at the reaction temperature. The molar ratio of Ni, Mo, Mn and Bi is in the range of 7.6~24:2.6~8:1~8:60~89.

8.

2. The method according to claim 1, characterized in that, The molar ratio of Ni, Mo, Mn and Bi is 12:4:4:

80.

3. The method according to claim 1, characterized in that, The mechanical alloying process includes: first, ball milling Ni, Mo and Mn powders in an Ar atmosphere for 20-30 hours, then adding Bi powder and continuing ball milling for 3-8 hours. The ball milling process adopts an intermittent operation mode, with a 5-minute interval between every 15 minutes of ball milling to control the local temperature of the powder, thereby obtaining the alloy precursor.

4. The method according to claim 1, characterized in that, The melt stabilization treatment includes: heating to 550~650℃ in an inert atmosphere and holding for 1~3h; the reduction activation treatment includes: treating at 650~750℃ in a hydrogen-containing atmosphere for 1~4h, with the treatment endpoint being no significant change in the amount of hydrogen consumed in the gas phase.

5. A NiMoMn-Bi molten alloy catalyst prepared by the method according to any one of claims 1-4.

6. A method for producing hydrogen and co-producing carbon materials from low-carbon hydrocarbons through cracking using the NiMoMn-Bi molten alloy catalyst described in claim 5, characterized in that, The method includes the following steps: Step S1: Atmosphere replacement and melt stabilization: The catalyst precursor is loaded into a reactor with a porous gas distributor at the bottom, and inert gas is introduced to replace the air in the reactor. Then, the temperature is raised to 550~650℃ under an inert atmosphere and held at the temperature to complete the melt stabilization. Step S2: Reduction and activation: Switch to a hydrogen-containing mixed atmosphere, heat to 650~750℃ to reduce and activate the molten alloy, and after activation, purge the reactor with inert gas; Step S3: Cracking reaction: Low-carbon hydrocarbon feed gas is introduced. The feed gas passes through the molten alloy catalyst bed in the form of bubbles through a porous gas distributor and undergoes a cracking reaction at 350~1050℃ to generate hydrogen and solid carbon. Step S4: Product separation and recovery: Hydrogen is obtained after cooling and purification of the gaseous product; solid carbon floats to the top of the molten alloy bed under the action of density difference, and carbon material is obtained after separation.

7. The method according to claim 6, characterized in that, The low-carbon hydrocarbon is one or more of methane, ethane, and propane, and the feed flow rate of the low-carbon hydrocarbon under standard conditions is 4~50 mL / min.

8. The method according to claim 7, characterized in that, When the low-carbon hydrocarbon is methane, the catalyst bed height is 1 cm, the methane feed flow rate is 4 mL / min, and the reaction temperature is 800 °C, the hydrogen selectivity is not less than 99%, and the methane conversion rate fluctuation does not exceed 5% within 260 h of continuous operation.

9. The method according to claim 6, characterized in that, In step S4, the solid carbon is recovered by one or more of the following methods: washing, drying, sieving, and acid washing; the recovered carbon materials include one or more of the following: carbon powder, flake carbon, graphitized carbon, and carbon nanotubes.

10. The method according to claim 9, characterized in that, In step S4, a carbon material collection zone is set at the top of the reactor. During the reaction, solid carbon continuously floats to the collection zone, achieving continuous or semi-continuous recovery.

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