A molybdenum-doped iron monatomic composite material catalyst, a preparation method and application thereof

Abstract

The application relates to the technical field of composite material catalyst preparation, in particular to a molybdenum-doped iron monatomic composite material catalyst and a preparation method and application thereof. The catalyst comprises a substrate of zirconium dioxide ZrO2 and nitrogen-doped carbon and iron and molybdenum dispersed on the substrate, the iron is dispersed on the substrate in the form of monatomic and is coordinated with nitrogen atoms in the substrate. The catalyst is in a nanoparticulate structure formed by stacking of sheets and can be used for catalyzing nitrogen N2 hydrogenation to synthesize ammonia NH3, has good stability, can realize N2 hydrogenation to synthesize NH3 under the condition of a minimum of 100 DEG C and 1 MPa, and the NH3 generation rate reaches the highest under the condition of 400 DEG C and 1 MPa. The catalytic system is low in cost, high in stability and activity and mild in reaction condition, and can realize replacement of a noble metal catalytic system.

Description

Technical Field

[0001] This application relates to the field of composite catalyst preparation technology, specifically to a molybdenum-doped iron single-atom composite catalyst, its preparation method, and its application. Background Technology

[0002] NH3, as an important industrial and agricultural chemical, has significant applications in fertilizers, pharmaceuticals, polymer materials, hydrogen storage, and energy storage. However, the traditional industrial ammonia synthesis process, using the Haber-Bosch method, requires extremely high temperatures and pressures, resulting in the emission of large amounts of CO2, which is detrimental to environmental and energy sustainability. Therefore, developing catalysts for ammonia synthesis under mild conditions is of great importance for improving ammonia synthesis processes and achieving green ammonia synthesis.

[0003] Single-atom catalysts are widely used in heterogeneous catalysis due to their 100% atomic utilization and high catalytic activity. However, due to the high surface energy of single-atom systems, they are prone to migration and aggregation. Therefore, how to prepare and stabilize single-atom catalytic systems is an urgent problem to be solved. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this application is to provide a molybdenum-doped iron single-atom composite catalyst, its preparation method, and its application. The following technical solutions address the problems of high energy consumption, high emissions, and high costs mentioned in the background technology.

[0005] The primary objective of this application is to provide a molybdenum-doped iron single-atom composite catalyst, which comprises a zirconium dioxide (ZrO2) and nitrogen-doped carbon substrate and iron and molybdenum dispersed on the substrate, wherein the iron is dispersed on the substrate in single-atom form and coordinated with nitrogen atoms therein; the catalyst is a black solid with a microstructure of nanoparticles formed by stacked sheets.

[0006] The second objective of this application is to provide a method for preparing the above-mentioned molybdenum-doped iron single-atom composite catalyst, comprising the following steps: S1, preparing a catalyst precursor: mixing porphyrin iron ligand, zirconium-containing metal salt, and molybdenum-containing metal salt in a molar ratio of 1:7-8:0.5-4, adding a solvent to obtain a mixed system, and carrying out a solvothermal reaction after uniform mixing, and preparing a catalyst precursor after a series of post-treatments; S2, calcining the catalyst precursor in a tube furnace under inert gas protection to obtain a molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C.

[0007] Further, step S1 must satisfy at least one of the following: (1) the porphyrin iron ligand is a compound containing a porphyrin skeleton and a central iron atom, selected from tetracarboxyphenylporphyrin iron FeTCPPCl; (2) the zirconium-containing metal salt is selected from zirconium tetrachloride ZrCl4, zirconium dichloride ZroCl2, or zirconium dichloride hydrate; (3) the molybdenum-containing metal salt is selected from sodium molybdate Na2MoO4, sodium molybdate hydrate, ammonium molybdate (NH3)6Mo7O 24 Or ammonium molybdate hydrate.

[0008] Furthermore, the solvent in step S1 is a mixed solution of ultrapure water, formic acid (HCOOH), and N,N-dimethylformamide (DMF) in a volume ratio of 1:4-50:26-300.

[0009] Furthermore, the mixing method in step S1 is to stir at room temperature for 1-2 hours or sonicate at 25-35°C for 30-60 minutes.

[0010] Furthermore, the solvothermal reaction conditions in step S1 are as follows: the container containing the mixture is placed in a high-temperature oven, heated to 120°C at a heating rate of 2°C / min-10°C / min and maintained for 20h-24h, and then cooled to room temperature at a cooling rate of 0.2°C / min-0.5°C / min.

[0011] Furthermore, the post-processing method of step S1 is as follows: the solvothermal reaction product is washed several times with DMF and anhydrous ethanol, and the resulting solution is centrifuged to remove the upper liquid.

[0012] Furthermore, the calcination conditions in step S2 are as follows: the temperature is increased from room temperature to 800℃ at a heating rate of 1℃ / min-5℃ / min, held for 2h-3h, and then cooled to room temperature.

[0013] The third objective of this application is to provide an application of a molybdenum-doped iron single-atom composite catalyst in the catalytic hydrogenation of nitrogen to ammonia.

[0014] Furthermore, the application includes the following steps: S1, pressing and shaping the molybdenum-doped iron single-atom composite catalyst into tablets, then pulverizing and sieving it, and mixing it with quartz sand to obtain a reaction tube packing material, the volume of which is 1 / 3 of the reaction tube volume; S2, using the reaction tube packing material as a catalyst bed, and using nitrogen and hydrogen as reaction gases, carrying out a catalytic reaction under conditions of 50℃-500℃ and 0.1Mpa-5Mpa.

[0015] Compared with existing technologies, this application has the following advantages: This application relates to a molybdenum-doped iron single-atom composite catalyst, which can realize the synthesis of ammonia under mild conditions, reducing the energy consumption and emissions required for the reaction. This molybdenum-doped iron single-atom composite catalyst has high catalytic activity for the hydrogenation of N2 to NH3. Simultaneously, due to the synergistic effect of molybdenum, the single-atom iron component is less prone to migration, exhibiting good long-term catalytic stability and potential for industrial application. This application also provides a method for preparing molybdenum-doped iron single atoms. This method successfully introduces molybdenum into the iron single-atom system for the first time. The preparation process is simple, mild, highly controllable, and reproducible. It does not use precious metal components and can be industrialized. The prepared Mo / Fe-N / C composite material has high activity and high stability, and can catalyze the hydrogenation of N2 to NH3 at temperatures as low as 100℃ and 1MPa, thus replacing precious metal catalytic systems. Attached Figure Description

[0016] Other features, objects, and advantages of this application will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings.

[0017] Figure 1 The powder X-ray diffraction (PXRD) pattern of the obtained Mo / Fe-N / C composite material is shown below.

[0018] Figure 2 The image shows a scanning electron microscope (SEM) image of the obtained Mo / Fe-N / C composite material.

[0019] Figure 3 The image shows a transmission electron microscope (TEM) image of the obtained Mo / Fe-N / C composite material; where, Figure 3 a represents the morphology of the composite material at low resolution. Figure 3 b represents the single-atom structure observed under a high-resolution spherical aberration electron microscope;

[0020] Figure 4 The Mössbauer spectrum of the obtained Mo / Fe-N / C composite material is shown below.

[0021] Figure 5 The graph shows the catalytic performance test results of the obtained Mo / Fe-N / C composite material; where, Figure 5 a represents the catalytic performance results at different temperatures. Figure 5 b represents the catalytic stability test result;

[0022] Figure 6 The graph shows the catalytic performance test results of the obtained Fe-N / C composite material; where, Figure 6 a represents the catalytic performance results at different temperatures. Figure 6 b represents the catalytic stability test result. Detailed Implementation

[0023] The present application will be further described below with reference to specific embodiments. These embodiments are only used to more clearly illustrate the technical solutions of the present application and should not be construed as limiting the scope of protection of the present application. Anything not described in detail in this patent application is considered common knowledge in the art.

[0024] The term "and / or" as used in this application includes any and all combinations of one or more of the associated listed items. Unless otherwise stated, all parts, percentages, and ratios reported in the following examples are based on weight, and all reagents used in the examples are commercially available or synthesized by conventional methods and are ready for use without further processing, as are the instruments used in the examples. All technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in this application and in its specification is for the purpose of describing particular embodiments only and is not intended to be limiting of this application.

[0025] The inventors chose iron-based catalysts (one of the most mature ammonia synthesis catalytic systems). Iron single-atom catalysts play an important role in improving metal utilization efficiency and enhancing reaction activity. However, the inventors also considered the limitations of single-site catalytic activity and the limitations of single-atom systems on long-term catalytic stability, which restrict the application of single-atom catalysts in the field of ammonia synthesis. Therefore, they provided a molybdenum-doped iron single-atom composite catalyst and its preparation method, and applied it to the reaction of N2 catalytic hydrogenation to NH3.

[0026] In some specific embodiments, a molybdenum-doped iron single-atom composite catalyst is provided, comprising a zirconium dioxide (ZrO2) and nitrogen-doped carbon substrate and iron and molybdenum dispersed thereon. The iron is dispersed on the substrate in single-atom form and coordinated with nitrogen atoms therein, while the molybdenum is also dispersed on the substrate and replaces zirconium atoms in the zirconium dioxide in the substrate. The macroscopic morphology of the catalyst is a black solid, and the microscopic morphology is a nanoparticle structure formed by stacked sheets.

[0027] In some specific embodiments, the preparation method of the molybdenum-doped iron single-atom composite catalyst is as follows: S1, preparing the catalyst precursor: porphyrin iron ligands (compounds containing a porphyrin framework and a central iron atom structure, including but not limited to tetracarboxyphenylporphyrin iron FeTCPPCl), zirconium-containing metal salts (including but not limited to zirconium tetrachloride ZrCl4 or zirconium dichloride ZrCl2 and their hydrates), and molybdenum-containing metal salts (including but not limited to sodium molybdate Na2MoO4 and its hydrate or ammonium molybdate (NH3)6Mo7O) are prepared. 24The mixture (including its hydrate) was prepared in a molar ratio of 1:7-8:0.5-4, followed by the sequential addition of ultrapure water, formic acid (HCOOH), and N,N-dimethylformamide (DMF) in a volume ratio of 1:4-50:26-300 as solvents. The resulting mixture was ultrasonicated in a water bath at 25℃-35℃ for 30 min or stirred at room temperature for 1-2 h to achieve homogeneity. The mixture was then transferred to a container such as a polytetrafluoroethylene (PTFE)-lined reactor and placed in a high-temperature oven. The mixture was heated to 120℃ at a heating rate of 2℃ / min-10℃ / min and maintained for 20-24 h. Finally, the temperature was increased by 0.2... The temperature was cooled to room temperature at a rate of ℃ / min-0.5℃ / min to complete the solvothermal reaction. Finally, the product was washed several times with DMF and anhydrous ethanol, and the resulting solution was centrifuged to remove the supernatant to obtain the catalyst precursor, which was then vacuum dried for later use. S2. The obtained catalyst precursor was placed in a tube furnace and calcined under an inert gas atmosphere. The temperature was increased from room temperature to 800℃ at a rate of 1℃ / min-5℃ / min and held for 2-3 hours. Then, it was cooled to room temperature to obtain the molybdenum-doped iron single-atom composite catalyst powder Mo / Fe-N / C.

[0028] In some embodiments, the molybdenum-doped iron single-atom composite catalyst is in powder form. In other embodiments, the powdered catalyst can be shaped into tablets, sieved to obtain a granular structure, and then mixed with a certain amount of quartz sand for application.

[0029] In some specific embodiments, the molybdenum-doped iron single-atom composite catalyst is used in the catalytic hydrogenation of nitrogen to synthesize ammonia. The specific steps are as follows: S1, the molybdenum-doped iron single-atom composite catalyst is pressed into tablets, then crushed, sieved, and mixed with quartz sand to obtain a reaction tube packing material. The volume ratio of the reaction tube packing material is 1 / 3 of the reaction tube volume; S2, using the reaction tube packing material as the catalyst bed, and using nitrogen and hydrogen as the reaction gases, a catalytic reaction is carried out under the conditions of 50℃-500℃ and 0.1Mpa-5Mpa.

[0030] Example 1

[0031] This embodiment provides a molybdenum-doped iron single-atom composite catalyst. The preparation method includes: S1, preparing the catalyst precursor: weighing 0.4500 g of ZrCl4, 0.2400 g of FeTCPPCl and 0.1320 g of Na2MoO4·2H2O, then adding 1.5 mL of ultrapure water, 50.0 mL of DMF and 7.2 mL of HCOOH in sequence, ultrasonically dispersing for 30 min, transferring to a polytetrafluoroethylene (PTFE) lined reactor, and heating in a high-temperature oven at 5 °C / min. The temperature was raised to 120℃ and maintained for 24 hours. After cooling to room temperature, the product was transferred to a centrifuge tube and ultrasonically washed several times with DMF and ethanol. The resulting solution was centrifuged to remove the upper liquid layer and vacuum dried to obtain a dark green solid powder catalyst precursor. S2. The obtained catalyst precursor was placed in a tube furnace and heated from room temperature to 800℃ at a heating rate of 5℃ / min under argon protection. The temperature was maintained for 2 hours and then cooled to room temperature to obtain a black solid powder molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C.

[0032] Example 2

[0033] This embodiment provides a molybdenum-doped iron single-atom composite catalyst. The preparation method includes: S1, preparing the catalyst precursor: weighing 0.6900 g of ZrOCl2·8H2O, 0.2500 g of FeTCPPCl and 0.0660 g of Na2MoO4·2H2O, then adding 1.5 mL of ultrapure water, 45.0 mL of DMF and 6.8 mL of HCOOH in sequence, ultrasonically dispersing for 30 min, transferring to a polytetrafluoroethylene (PTFE) lined reactor, and placing it in a high-temperature oven at 10℃ / m S1. Heat to 120℃ and hold for 22h. Cool to room temperature and transfer the product to a centrifuge tube. Wash several times with DMF and ethanol using ultrasound. Centrifuge the resulting solution to remove the supernatant liquid and vacuum dry to obtain a dark green solid powder catalyst precursor. S2. Place the obtained catalyst precursor into a tube furnace and, under argon protection, heat from room temperature to 800℃ at a rate of 3℃ / min, hold for 3h, and cool to room temperature to obtain a black solid powder molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C.

[0034] Example 3

[0035] This embodiment provides a molybdenum-doped iron single-atom composite catalyst, the preparation method of which includes: S1, preparing the catalyst precursor: weighing 0.5000g of ZrCl4, 0.2400g of FeTCPPCl and 0.175g of (NH3)6Mo7O 24·4H2O, then 0.15 mL of ultrapure water, 50.0 mL of DMF and 7.0 mL of HCOOH were added sequentially, and the mixture was ultrasonically dispersed for 30 min. The mixture was then transferred to a PTFE-lined reactor and placed in a high-temperature oven to heat to 120 °C at 2 °C / min and maintained for 24 h. After cooling to room temperature, the product was transferred to a centrifuge tube and ultrasonically washed several times with DMF and ethanol. The resulting solution was centrifuged to remove the supernatant and vacuum dried to obtain a dark green solid powder catalyst precursor. S2. The obtained catalyst precursor was placed in a tube furnace and heated from room temperature to 800 °C at a rate of 5 °C / min under argon protection. The temperature was maintained for 3 h and then cooled to room temperature to obtain a black solid powder molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C.

[0036] Comparative Example 1

[0037] This embodiment provides a molybdenum-free iron single-atom composite catalyst. The preparation method includes: S1, preparing the catalyst precursor: weigh 0.4500g of ZrCl4 and 0.2400g of FeTCPPCl, then add 1.5mL of ultrapure water, 50.0mL of DMF and 7.2mL of HCOOH in sequence, ultrasonically disperse for 30min, transfer to a polytetrafluoroethylene (PTFE) lined reactor, place in a high-temperature oven and heat to 120℃, maintain for 24h, cool to room temperature, transfer the obtained product to a centrifuge tube, ultrasonically wash several times with DMF and ethanol, centrifuge the obtained solution, remove the upper liquid, vacuum dry to obtain a dark green solid powder catalyst precursor; S2, place the obtained catalyst precursor in a tube furnace, under argon protection, heat from room temperature to 800℃ at a heating rate of 5℃ / min, maintain for 2h, cool to room temperature to obtain a black solid powder iron single-atom composite catalyst Fe-N / C.

[0038] Test Example 1: Structural Inspection

[0039] To determine the macroscopic and microscopic structures of the composite materials of this application, the molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C obtained in Example 1 was subjected to X-ray diffraction analysis, scanning electron microscopy analysis, and transmission electron microscopy analysis.

[0040] The structure of the molybdenum-doped iron single-atom composite catalyst obtained in Example 1 was determined using an X-ray diffractometer (Rigaku Ultima IV), and the obtained XRD pattern is shown below. Figure 1 As shown in the figure, the molybdenum-doped iron single-atom composite catalyst only contains the diffraction peaks of the support ZrO2, and there are no diffraction peaks of Fe and Mo elements, indicating that Fe and Mo elements are successfully dispersed in the catalyst.

[0041] The morphology of the molybdenum-doped iron single-atom composite catalyst obtained in Example 1 was observed using a scanning electron microscope (Thermo Fisher Scientific Apreo S LoVac). The SEM images are shown below. Figure 2 As shown in the figure, the molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C composite material exists in the form of small particles with a length of about 30nm-50nm, specifically presenting a plate-like stacked structure, exposing more catalytic sites.

[0042] The structural characteristics of the Mo / Fe-N / C composite material obtained in Example 1 were further confirmed using a transmission electron microscope (JEM-ARM300F). Figure 3 The TEM image of a shows that the molybdenum-doped iron single-atom composite catalyst has a particulate structure. Figure 3 The bright spot circled in red in b further confirms the presence of Fe single-atom components in the composite material.

[0043] Test Example 2, Performance Testing

[0044] The Mo / Fe-N / C composite material prepared in Example 2 was subjected to Mössbauer spectroscopy (Topologic Systems MFD-500AV), and the data obtained are as follows: Figure 4 As shown, the composite material structure mainly contains high-spin trivalent Fe single atoms, with one Fe atom coordinated with four N atoms, and a small portion (about 10%) of aggregated metallic Fe particles.

[0045] Experimental Example 3: Catalytic Performance Testing

[0046] The performance of the obtained Mo / Fe-N / C composite material was tested in a fixed-bed reactor. The catalyst was activated under vacuum at 150 °C, then treated at 500 °C under an argon atmosphere until the NH4 content of the outlet absorbent was measured twice consecutively. + The concentration was set to 0 to exclude the effects of DMF decomposition and environmental N pollution during the synthesis process. The catalyst was pretreated by reduction at 500℃, 1MPa, and 25% N2-75% H2 before the formal reaction. The NH3 formation rate of Mo / Fe-N / C was measured at 100℃, 200℃, 300℃, 400℃, and 500℃. Figure 5 The results showed that ammonia could be generated at temperatures as low as 100℃, with the generation rate reaching a maximum of 2728 μmol / g at 400℃. cat h. Long-term stability tests show that the Mo / Fe-N / C composite material can stably catalyze for more than 300 hours, and after prolonged exposure to air, it can still achieve considerable catalytic performance upon reuse. Figure 5 b). Figure 6 a and 6b are performance tests conducted under the same conditions for Comparative Example 1. The results show that the catalytic performance and catalytic stability of Mo / Fe-N / C are significantly improved.

[0047] Based on the preferred embodiments of this application, and through the above description, those skilled in the art can make various changes and modifications without departing from the technical concept of this application. The technical scope is not limited to the contents of the specification.

Claims

1. A molybdenum-doped iron monatomic composite catalyst, characterized in that, The catalyst comprises a zirconium dioxide (ZrO2) substrate and nitrogen-doped carbon, and iron and molybdenum dispersed on the substrate. The iron is dispersed on the substrate in the form of single atoms and coordinated with nitrogen atoms therein. The catalyst is a black solid with a microstructure of nanoparticles formed by stacked sheets.

2. A method of preparing the molybdenum-doped iron monatomic composite catalyst of claim 1, characterized in that, Including the following steps: S1. Preparation of catalyst precursor: Porphyrin iron ligand, zirconium-containing metal salt, and molybdenum-containing metal salt are mixed in a molar ratio of 1:7-8:0.5-4, and a solvent is added to obtain a mixed system. After uniform mixing, a solvothermal reaction is carried out, and the catalyst precursor is prepared after a series of post-treatments. S2. The catalyst precursor is calcined in a tube furnace under inert gas protection to obtain a molybdenum-doped iron single-atom composite catalyst Mo / Fe-N / C.

3. The production method according to claim 2, characterized by, Step S1 must satisfy at least one of the following: (1) The porphyrin iron ligand is a compound containing a porphyrin skeleton and a central iron atom structure, and is selected from tetracarboxyphenylporphyrin iron FeTCPPCl; (2) The zirconium-containing metal salt is selected from zirconium tetrachloride ZrCl4, zirconium dichloride ZroCl2 or zirconium dichloride hydrate; (3) the molybdenum-containing metal salt is selected from sodium molybdate Na2Mo04, sodium molybdate hydrate, ammonium molybdate (NH3)6Mo070 24 or ammonium molybdate hydrate.

4. The preparation method according to claim 2, characterized in that, The solvent in step S1 is a mixed solution of ultrapure water, formic acid (HCOOH), and N,N-dimethylformamide (DMF) in a volume ratio of 1:4-50:26-300.

5. The preparation method according to claim 2, characterized in that, The mixing method in step S1 is to stir at room temperature for 1-2 hours or to sonicate at 25-35°C for 30-60 minutes.

6. The preparation method according to claim 2, characterized in that, The solvothermal reaction conditions in step S1 are as follows: the container containing the mixture is placed in a high-temperature oven and heated to 120°C at a heating rate of 2°C / min-10°C / min and maintained for 20h-24h, and then cooled to room temperature at a cooling rate of 0.2°C / min-0.5°C / min.

7. The preparation method according to claim 2, characterized in that, The post-processing method of step S1 is as follows: the solvothermal reaction product is washed several times with DMF and anhydrous ethanol, and the resulting solution is centrifuged to remove the upper liquid.

8. The preparation method according to claim 2, characterized in that, The calcination conditions in step S2 are as follows: the temperature is increased from room temperature to 800℃ at a heating rate of 1℃ / min-5℃ / min, held for 2h-3h, and then reduced to room temperature.

9. The application of a molybdenum-doped iron single-atom composite catalyst as described in any one of claims 1-8 in the catalytic hydrogenation of nitrogen to ammonia.

10. Use according to claim 9, characterized in that, Includes the following steps: S1. The molybdenum-doped iron single-atom composite catalyst is pressed into tablets, shaped, crushed and sieved, and then mixed with quartz sand to obtain a reaction tube filler, the volume of which is 1 / 3 of the reaction tube volume. S2. Using the reaction tube packing material as a catalyst bed and nitrogen and hydrogen as reaction gases, a catalytic reaction is carried out at 50℃-500℃ and 0.1Mpa-5Mpa.

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