A heterojunction catalyst, its preparation method, and its application

Abstract

This invention belongs to the field of ammonia synthesis catalyst technology, specifically relating to a heterojunction catalyst, its preparation method, and its application. The method includes the following steps: providing an ARuX precursor, where A is a rare earth element or an alkaline earth metal element, and X is Si or Ge; acid etching the ARuX precursor to obtain a Ru-X heterojunction; and surface activation treatment of the Ru-X heterojunction using a fluoride salt solution to obtain the heterojunction catalyst. This invention utilizes strong acid etching of the precursor to specifically remove A, resulting in a Ru-X heterojunction with a large specific surface area (a hundredfold increase compared to the precursor). Subsequently, surface activation treatment of the obtained Ru-X heterojunction using a fluoride salt solution removes anions from the Ru-X heterojunction surface, significantly improving the catalytic performance of the heterojunction catalyst.

Description

Technical Field

[0001] This invention specifically relates to the field of ammonia synthesis catalyst technology, and more particularly to a heterojunction catalyst, its preparation method, and its application. Background Technology

[0002] Electrode catalysts (ETCs) are materials whose electrons are charge-balancing anions. They are a special class of ammonia synthesis catalysts. Their low work function allows electrons to be easily transferred to catalytic sites, significantly reducing the N2 dissociation adsorption energy in the rate-determining step of the ammonia synthesis reaction and thus achieving high catalytic activity. Therefore, ETCs are considered a new generation of low-energy-consumption ammonia synthesis catalysts under mild conditions and have attracted much attention. In 2003, Professor H. Hosono's team reported the first room-temperature stable inorganic ETC, C12A7:e-, in Science, and achieved high-performance ammonia synthesis under mild conditions (≤400℃, 0.1-1MPa) by supporting Ru. In addition, the team also developed intermetallic ETC catalysts (RTX) with excellent water stability and ammonia synthesis performance. These studies have promoted the ammonia synthesis process under mild conditions.

[0003] Among them, the rare earth and Ru-containing compound ARuX exhibits the highest ammonia synthesis activity of 1.8 mmol / g / h under conditions of 400℃, 0.1 MPa, and 60 ml / min gas flow rate (N2:H2 = 15:45) (taking LaRuSi as an example), but it has low porosity and a small specific surface area (<2m²). 2 Problems such as / g) limit the exposure of catalytic active sites and the improvement of performance. Researchers such as Wu et al. improved the specific surface area and ammonia synthesis catalytic performance of ARuX to some extent through etching with acid solutions, alkaline solutions, or complexing agents. Among these methods, etching with hydrochloric acid yielded the material with the largest specific surface area. However, due to the poisoning effect of chloride ions on the ammonia synthesis reaction, although the specific surface area of ​​ARuX increased by a hundredfold under hydrochloric acid etching, from 1–2 m², it still fell short of the target area. 2 / g increased to 274m 2 / g, but the catalyst is almost deactivated.

[0004] Therefore, existing technologies still need to be optimized and improved. Summary of the Invention

[0005] One objective of this invention is to provide a method for preparing a heterojunction catalyst. This method involves acid etching of an ARuX precursor to selectively remove the atom, thereby obtaining a Ru-X heterojunction material with a high specific surface area. The Ru-X heterojunction material is then activated with a fluoride salt solution to eliminate the surface poisoning problem, resulting in a highly active ammonia synthesis catalyst with a high specific surface area.

[0006] Specifically, a method for preparing a heterojunction catalyst includes the following steps:

[0007] Provide ARuX precursors, where A is a rare earth element or an alkaline earth metal element, and X is Si or Ge;

[0008] The ARuX precursor was subjected to acid etching to obtain a Ru-X heterojunction;

[0009] The Ru-X heterojunction was surface activated using a fluoride salt solution to obtain the heterojunction catalyst.

[0010] In this invention, the Ru-X precursor is used, where A is a rare earth element such as La, Ce, or Pr, or an alkaline earth metal element such as Ca or Sr. Strong acids, such as sulfuric acid or hydrochloric acid, are used to etch the precursor, specifically removing A. The resulting Ru-X heterojunction has a large specific surface area (a hundred times larger than that of the precursor). Subsequently, a fluoride salt solution is used to perform surface activation treatment on the obtained Ru-X heterojunction, removing anions from the surface of the Ru-X heterojunction, thus significantly improving the catalytic performance of the heterojunction catalyst.

[0011] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The purpose and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0012] As a preferred technical solution, in the method for preparing the heterojunction catalyst, the acid used in the acid etching treatment is an inorganic acid, and the concentration of the inorganic acid is 0.01M-6M. The inorganic acid is a strong acid, such as sulfuric acid or hydrochloric acid, and its concentration is 0.01M to 0.05M, 0.05M to 0.1M, 0.1M to 0.5M, 0.5M to 1M, 1M to 1.5M, 1.5M to 2M, 2M to 2.5M, 2.5M to 3M, 3M to 3.5M, 3.5M to 4M, 4M to 4.5M, 4.5M to 5M, 5M to 5.5M, or 5.5M to 6M. By employing strong acid etching, the a metal is specifically removed, thereby increasing the specific surface area of ​​the Ru-X heterojunction.

[0013] As a preferred technical solution, the method for preparing the heterojunction catalyst, wherein the surface activation treatment of the Ru-X heterojunction with a fluoride salt solution to obtain the heterojunction catalyst specifically includes:

[0014] The Ru-X heterojunction was immersed in a fluoride solution and then removed.

[0015] The Ru-X heterojunction after soaking is washed and dried to obtain the heterojunction catalyst.

[0016] In this invention, the Ru-X heterojunction after acid etching is soaked in a fluoride salt solution to remove chloride ions, sulfate ions, and other anions from the surface of the Ru-X heterojunction. This avoids poisoning of the heterojunction catalyst by anions and inhibits the migration of internal poisoned ions, thereby effectively solving the surface poisoning problem of the catalyst material. After soaking in the fluoride salt solution, the solid material can be collected by filtration or centrifugation, washed 3-5 times with deionized water or hot deionized water, washed 1-5 times with ethanol, and then dried to obtain the heterojunction catalyst.

[0017] As a preferred technical solution, in the method for preparing the heterojunction catalyst, the fluoride salt solution is selected from one of CsF, KF, NaF and LiF.

[0018] As a preferred technical solution, in the method for preparing the heterojunction catalyst, the concentration of the fluoride salt solution is 0.1M-1M. Within this concentration range, the surface of the Ru-X heterojunction can be better activated.

[0019] As a preferred technical solution, the method for preparing the heterojunction catalyst involves immersing the Ru-X heterojunction in a fluoride salt solution for 5-24 hours, such as 5 to 8 hours, 8 to 10 hours, 10 to 12 hours, 12 to 14 hours, 14 to 16 hours, 16 to 18 hours, 18 to 20 hours, 20 to 22 hours, or 22 to 24 hours.

[0020] As a preferred technical solution, the method for preparing the heterojunction catalyst, wherein the method for preparing the ARuX precursor includes:

[0021] Metal particles A, Ru, and X were weighed according to stoichiometric ratios and mixed to obtain a mixture.

[0022] The mixture is smelted to obtain ARuX blocks;

[0023] The ARuX bulk material is ground into powder in an inert atmosphere to obtain the ARuX precursor.

[0024] As a preferred technical solution, the method for preparing the heterojunction catalyst further includes:

[0025] In an inert atmosphere, the ARuX block material is ground into powder, and the powder is pressed into a block.

[0026] The block is wrapped with a metal sheet and then placed in a vacuum-sealed container. The metal sheet is a molybdenum sheet, a stainless steel sheet, or a tantalum sheet.

[0027] The sealed container was annealed to obtain the purified ARuX precursor.

[0028] In this invention, ARuX precursors can be prepared using arc-melting, induction furnace melting, or solid-state methods, with arc-melting being the preferred method. The pure phase is then obtained through high-temperature annealing for 5-30 days.

[0029] Secondly, a heterojunction catalyst, wherein the heterojunction catalyst is prepared by the preparation method described above.

[0030] Thirdly, the application of the heterojunction catalyst described in the second aspect in the catalytic synthesis of ammonia. Surface-activated catalyst materials can be used for ammonia synthesis, as demonstrated in the experimentally constructed fixed-bed ammonia synthesis reaction system.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] This invention utilizes acid etching of the precursor ARuX to remove A, resulting in a Ru-X heterojunction with a large specific surface area. Due to its large specific surface area, the Ru-X heterojunction exhibits potentially strong catalytic activity when used as a catalyst. Furthermore, surface activation treatment with fluoride removes anions from the Ru-X heterojunction surface, significantly enhancing the catalytic performance of the heterojunction catalyst. This preparation method is simple to operate, and the resulting heterojunction catalyst possesses high activity, making it suitable for ammonia synthesis catalysis. Attached Figure Description

[0033] Figure 1 The X-ray diffraction pattern of the surface-activated Ru-Si heterojunction provided by the present invention;

[0034] Figure 2 Scanning electron microscope (SEM) image of the surface-activated Ru-Si heterojunction provided by this invention;

[0035] Figure 3 The SEM image of the Ru-Si heterojunction provided by this invention;

[0036] Figure 4 The nitrogen adsorption-desorption curve of the Ru-Si heterojunction provided by this invention;

[0037] Figure 5 A comparison diagram of the ammonia synthesis performance of the Ru-Si heterojunction provided by this invention;

[0038] Figure 6 The activation energy results for the Ru-Si heterojunction provided by this invention are shown. Detailed Implementation

[0039] This invention provides a heterojunction catalyst and its preparation method. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

[0040] Furthermore, the features, operations, or characteristics described in the specification can be combined in any suitable manner to form various embodiments. Simultaneously, the steps or actions in the method description can be rearranged or adjusted in a manner readily apparent to those skilled in the art. Therefore, the various orders in the specification and drawings are merely for the clear description of a particular embodiment and do not imply a necessary order, unless otherwise stated that a particular order must be followed. The component designations used herein, such as "first," "second," etc., are merely for distinguishing the described objects and do not have any sequential or technical meaning. All instruments and reagents used are commercially available products.

[0041] Unless otherwise defined, all terms used herein (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. It will also be understood that, unless expressly defined herein, terms such as those defined in a general dictionary shall be interpreted as having the meaning consistent with their meaning in the relevant field context, and not as having an idealized or overly formal meaning.

[0042] The preparation method of the heterojunction catalyst provided by the present invention will be further explained and illustrated below through specific embodiments.

[0043] Example 1

[0044] Metal particles A, Ru, and X were weighed according to stoichiometric ratio. Taking LaRuSi as an example, 1.4g of La, 1.01g of ruthenium, and 0.28g of silicon were weighed, mixed evenly, and placed in a metal melting furnace. The mixture was repeatedly melted under an Ar atmosphere until a uniform ARuX elliptical block was formed. After cooling, the block was removed and ground into a uniform powder (particle size <10µm) in an Ar gas glove box. An appropriate amount of powder was pressed into a tablet in the glove box. The re-pressed sample was wrapped with a clean metal Mo sheet and placed in a quartz tube that was sealed at one end and open at the other. The quartz tube was evacuated and sealed with a high-temperature oxyhydrogen flame. The sample was then annealed in a box furnace at 1000°C for 10 days to obtain a purified sample. After annealing, the sample was ground into a uniform powder again in the glove box.

[0045] The ARuX precursor was etched in 0.5 M hydrochloric acid solution at a solid-liquid ratio of 10:1 (g / L) with stirring for 3 hours. After etching, the solid was collected by centrifugation or filtration and washed twice with deionized water to obtain the heterojunction material. The obtained material underwent surface detoxification and activation. First, a 0.5 M KF solution was prepared to ionize F particles, and the resulting heterojunction material was soaked in it for 12 hours. After soaking, the solid material was collected by filtration and washed with deionized water or hot deionized water. Finally, it was washed once with ethanol and dried. The synthesis results and properties are as follows: Figures 1 to 4 As shown. Once surface detoxification and activation are completed, the material can be used for ammonia synthesis.

[0046] Example 2

[0047] Metal particles A, Ru, and X were weighed according to stoichiometric ratio. Taking CeRuSi as an example, 0.59g of Ce, 1.01g of Ruthenium, and 0.28g of Silicon were weighed, mixed evenly, and placed in a metal melting furnace. The mixture was repeatedly melted under an Ar atmosphere until a uniform ARuX elliptical block was formed. After cooling, the block was removed and ground into a uniform powder (particle size <10µm) in an Ar glove box. An appropriate amount of powder was pressed into a tablet in the glove box. The re-pressed sample was wrapped with a clean Mo sheet and placed in a quartz tube that was sealed at one end and open at the other. The quartz tube was evacuated and sealed with a high-temperature oxyhydrogen flame. The sample was then annealed in a box furnace at 1200°C for 5 days to obtain a purified sample. After annealing, the sample was ground into a uniform powder again in the glove box.

[0048] The ARuX precursor was etched in 0.1 M hydrochloric acid solution at a solid-liquid ratio of 2:1 (g / L) with stirring for 5 hours. After etching, the solid was collected by centrifugation and washed twice with deionized water to obtain the heterojunction material. The obtained material underwent surface detoxification and activation. First, a 0.1 M CsF solution was prepared to ionize F particles, and the resulting heterojunction material was soaked in the solution for 6 hours. After soaking, the solid material was collected by vacuum filtration and washed with deionized water or hot deionized water. Finally, it was washed once with ethanol and dried to obtain the heterojunction catalyst. Once surface detoxification and activation are complete, the material can be used for ammonia synthesis.

[0049] Example 3

[0050] Metal particles A, Ru, and X were weighed according to stoichiometric ratio. Taking PrRuGe as an example, 1.41g of Pr, 1.01g of ruthenium, and 0.73g of germanium were weighed, mixed evenly, and placed in a metal melting furnace. The mixture was repeatedly melted under an Ar atmosphere until a uniform ARuX elliptical block was formed. After cooling, the block was removed and ground into a uniform powder (particle size <10µm) in an Ar glove box. An appropriate amount of powder was pressed into a tablet in the glove box. The re-pressed sample was wrapped with a clean metal Mo sheet and placed in a quartz tube that was sealed at one end and open at the other. The quartz tube was evacuated and sealed with a high-temperature oxyhydrogen flame. The sample was then annealed in a box furnace at 1200°C for 10 days to obtain a purified sample. After annealing, the sample was ground into a uniform powder again in a glove box.

[0051] The ARuX precursor was etched in 1M sulfuric acid solution at a solid-liquid ratio of 15:1 (g / L) with stirring for 5 hours. After etching, the solid was collected by filtration and washed twice with deionized water to obtain the heterojunction material. The obtained material underwent surface detoxification and activation. First, a 1M NaF solution was prepared to ionize F particles, and the resulting heterojunction material was soaked in the solution for 10 hours. After soaking, the solid material was collected by filtration and washed with deionized water or hot deionized water. Finally, it was washed once with ethanol and dried to obtain the heterojunction catalyst. Once surface detoxification and activation are complete, the material can be used for ammonia synthesis.

[0052] Example 4

[0053] Metal particles A, Ru, and X were weighed according to stoichiometric ratio. Taking PrRuGe as an example, 1.41g of Pr, 1.01g of ruthenium, and 0.73g of germanium were weighed, mixed evenly, and placed in a metal melting furnace. The mixture was repeatedly melted under an Ar atmosphere until a uniform ARuX elliptical block was formed. After cooling, the block was removed and ground into a uniform powder (particle size <10µm) in an Ar glove box. An appropriate amount of powder was pressed into a tablet in the glove box. The re-pressed sample was wrapped with a clean metal Mo sheet and placed in a quartz tube that was sealed at one end and open at the other. The quartz tube was evacuated and sealed with a high-temperature oxyhydrogen flame. The sample was then annealed in a box furnace at 1200°C for 10 days to obtain a purified sample. After annealing, the sample was ground into a uniform powder again in a glove box.

[0054] The ARuX precursor was etched in 5M sulfuric acid solution at a solid-liquid ratio of 20:1 (g / L) with stirring for 5 hours. After etching, the solid was collected by filtration and washed twice with deionized water to obtain a heterojunction material. The obtained material underwent surface detoxification and activation. First, a 0.5M LiF solution was prepared to ionize F particles, and the resulting heterojunction material was soaked in the solution for 18 hours. After soaking, the solid material was collected by filtration and washed with deionized water or hot deionized water. Finally, it was washed once with ethanol and dried to obtain the heterojunction catalyst. Once surface detoxification and activation are complete, the material can be used for ammonia synthesis.

[0055] Example 5

[0056] Metal particles A, Ru, and X were weighed according to stoichiometric ratio. Taking PrRuGe as an example, 1.41g of Pr, 1.01g of ruthenium, and 0.73g of germanium were weighed, mixed evenly, and placed in a metal melting furnace. The mixture was repeatedly melted under an Ar atmosphere until a uniform ARuX elliptical block was formed. After cooling, the block was removed and ground into a uniform powder (particle size <10µm) in an Ar glove box. An appropriate amount of powder was pressed into a tablet in the glove box. The re-pressed sample was wrapped with a clean metal Mo sheet and placed in a quartz tube that was sealed at one end and open at the other. The quartz tube was evacuated and sealed with a high-temperature oxyhydrogen flame. The sample was then annealed in a box furnace at 1200°C for 10 days to obtain a purified sample. After annealing, the sample was ground into a uniform powder again in a glove box.

[0057] The ARuX precursor was etched in a 6M sulfuric acid solution at a solid-liquid ratio of 20:1 (g / L) with stirring for 5 hours. After etching, the solid was collected by filtration and washed twice with deionized water to obtain the heterojunction material. The obtained material underwent surface detoxification and activation. First, a 1M NaF solution was prepared to ionize F particles, and the resulting heterojunction material was soaked in the solution for 24 hours. After soaking, the solid material was collected by filtration and washed with deionized water or hot deionized water. Finally, it was washed once with ethanol and dried to obtain the heterojunction catalyst. Once surface detoxification and activation are complete, the material can be used for ammonia synthesis.

[0058] Example 6

[0059] Weigh 0.1 g of the catalyst material prepared in Example 1, encapsulate it in a quartz tube, fix the quartz tube in the ammonia synthesis system, and set the catalytic conditions to 400 °C, 0.1 MPa, and N2:H2 = 15 mL / min. -1 45mL min -1 The reaction tail gas was collected using a 5mM sulfuric acid solution. Then, the ammonia content was tested using ion chromatography, and ammonia synthesis performance data were collected at different time points. The reaction temperature was adjusted to 300-400℃, and ammonia data were collected to calculate the activation energy Ea of the reaction. Taking LaRuSi as an example... Figures 5 to 6 As shown, after hydrochloric acid etching and chlorine detoxification, at 400℃ and 0.1MPa, the ammonia synthesis activity of the material is 6.2mmol / g / h, and the performance can be maintained at 6.1mmol / g / h within 48h, showing good stability. The ammonia synthesis activation energy of the material was tested to be 66.2kJ / mol.

[0060] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for preparing a heterojunction catalyst, characterized in that, Includes the following steps: Provide ARuX precursors, where A is a rare earth element or an alkaline earth metal element, and X is Si or Ge; The ARuX precursor was subjected to acid etching to obtain a Ru-X heterojunction; The Ru-X heterojunction was surface activated using a fluoride salt solution to obtain the heterojunction catalyst. In this process, a fluoride salt solution is used to perform surface activation treatment on the Ru-X heterojunction to remove anions from the surface of the Ru-X heterojunction.

2. The method for preparing the heterojunction catalyst according to claim 1, characterized in that, The acid used in the acid etching process is an inorganic acid, and the concentration of the inorganic acid is 0.01M-6M.

3. The method for preparing the heterojunction catalyst according to claim 1, characterized in that, The surface activation treatment of the Ru-X heterojunction with a fluoride salt solution to obtain the heterojunction catalyst specifically includes: The Ru-X heterojunction was immersed in a fluoride solution and then removed. The Ru-X heterojunction after soaking is washed and dried to obtain the heterojunction catalyst.

4. The method for preparing the heterojunction catalyst according to claim 1, characterized in that, The fluoride salt solution is selected from one of CsF, KF, NaF and LiF.

5. The method for preparing the heterojunction catalyst according to claim 3, characterized in that, The concentration of the fluoride salt solution is 0.1M-1M.

6. The method for preparing the heterojunction catalyst according to claim 3, characterized in that, The Ru-X heterojunction is immersed in a fluoride salt solution for 6-24 hours.

7. The method for preparing the heterojunction catalyst according to claim 1, characterized in that, The method for preparing the ARuX precursor includes: Metal particles A, Ru, and X were weighed according to stoichiometric ratios and mixed to obtain a mixture. The mixture is smelted to obtain ARuX blocks; The ARuX bulk material is ground into powder in an inert atmosphere to obtain the ARuX precursor.

8. The method for preparing the heterojunction catalyst according to claim 7, characterized in that, The method for preparing the ARuX precursor further includes: In an inert atmosphere, the ARuX block material is ground into powder, and the powder is pressed into a block. The block is wrapped with a metal sheet and then placed in a vacuum-sealed container. The metal sheet is a molybdenum sheet, a stainless steel sheet, or a tantalum sheet. The sealed container was annealed to obtain the purified ARuX precursor.

9. A heterojunction catalyst, characterized in that, The heterojunction catalyst is prepared using the preparation method described in any one of claims 1-8.

10. The application of the heterojunction catalyst according to claim 9 in the catalytic synthesis of ammonia.

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