Ammonia oxidation catalyst taking Mo2C as carrier and preparation method thereof

Abstract

The invention discloses preparation and application of an ammoxidation catalyst with Mo2C as a carrier, and belongs to the technical field of catalysts and ammonia fuel cells. According to the catalyst, Mo2C is used as a carrier, and an active metal component is highly loaded on the surface of the Mo2C carrier through a sodium borohydride reduction method, so that a unique active center structure is formed. In the preparation process, firstly, a metal organic carbon thermal reduction method is adopted to accurately control the carburizing temperature to prepare the Mo2C carrier, then sodium borohydride is used as a reducing agent, metal Pt and Ir are reduced from chloroplatinic acid and chloroiridic acid and loaded on the Mo2C carrier, and drying is performed to obtain the target catalyst. The prepared catalyst changes the electronic property and surface active site distribution and improves the catalytic performance. Further, a catalyst with strong metal-metal carbide interaction is constructed, catalytic performance improvement is promoted, product selective modulation is realized, excellent catalytic performance is shown in ammoxidation reaction, and the catalyst is expected to be applied to low-temperature ammonia fuel cells.

Description

Technical Field

[0001] The present invention belongs to the technical fields of catalysts and ammonia fuel cells, and particularly relates to an ammonia oxidation catalyst supported on Mo2C, a preparation method thereof, and an application thereof. Background Art

[0002] With the increasing depletion of traditional fossil fuels (such as coal, petroleum, and natural gas), finding clean and sustainable energy has become an important task globally. Ammonia is an important chemical raw material with large production, easy storage, and transportation. As a potential energy carrier, it has received increasing attention. Ammonia is a gas at normal temperature and pressure and can be liquefied under appropriate pressure (generally around 10 - 12 atmospheres). The energy density of liquid ammonia is relatively high, and compared with hydrogen, its storage and transportation are safer and more convenient. The volumetric energy density of liquid ammonia is approximately 1.5 times that of liquid hydrogen. Ammonia can also be produced in various ways, and with the development of hydrogen production technologies using renewable energy (such as wind energy and solar energy), it has also become possible to produce green ammonia by reacting hydrogen generated from renewable energy with nitrogen, which provides a sustainable approach for the fuel supply of ammonia fuel cells.

[0003] Currently, the low-temperature direct ammonia fuel cell technology is in a rapid development stage after early exploration and has received increasing attention in the energy field. However, it still faces some challenges, such as the slow kinetics of the anodic ammonia oxidation reaction, which depends on a high overpotential (>0.4 V), resulting in limited battery performance and efficiency; at the same time, there are also the poisoning effects of oxygen-containing nitrogen species on the catalyst, as well as the relatively high cost of the catalyst and the need to improve its stability. These problems have affected the performance and commercialization prospects of ammonia fuel cells. Therefore, high-performance and high-stability anodic catalysts are the key.

[0004] At present, AOR catalysts are mainly divided into noble metal catalysts and non-noble metal catalysts. Among them, the strong metal–support interaction to improve catalyst activity is widely used. By occurring phenomena such as interfacial electron transfer and surface structure changes of nanoparticles, and ultimately affecting the catalytic reaction performance. Chen et al. loaded a single layer of Pt on the surface of WC and found excellent HER reaction activity. (Angew. Chem. Int. Ed., 2010, 49, 9859-9862) Gong et al. obtained that W2C / CNT showed similar electrocatalytic hydrogen evolution performance to Pt through crystal phase regulation. (Nat. Commun, 2016, 7, 13216) Li et al. loaded Ru clusters on precisely prepared cubic and hexagonal molybdenum carbide supports and found that Ru / β-Mo2C used as a cathode catalyst showed excellent performance in PEMWE. (Nano Lett, 2024, 24: 5705–5713) Bayati et al. synthesized Pt / MoC for AOR, which was beneficial to the dispersion of Pt nanoparticles, and the bonding interaction between Pt and Mo metals improved the AOR performance. (ACS Appl. Nano Mater. 2020, 3, 843–851.) CN114214657B discloses a method for improving the performance of molybdenum-based nitride / carbide electrocatalysts by transition metal doping. Using Mo-MOF as a template, transition metal-doped porous molybdenum nitride / molybdenum carbide nanorod catalysts were prepared by solution co-deposition method and high-temperature pyrolysis-nitridation / carbonization, showing excellent OER activity. However, none of these studies have applied the catalysts with carbides as carriers to ammonia electrocatalytic oxidation. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technologies, platinum-based catalysts are expensive and have low activity. The purpose of the present invention is to provide a preparation method and application of a composite carrier composed of XC72 and Mo2C, loaded with a Pt and Ir bimetallic catalyst, which has rich molybdenum carbide content, low price, simple preparation method, and the prepared metal catalyst has good electrocatalytic activity and stability.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions: A preparation method of an ammonia oxidation catalyst using Mo2C as a carrier, comprising the following steps: Step 1: Weigh a certain amount of Vulcan XC72 carbon carrier, add 100-150 mL of deionized water, and ultrasonically disperse for 20-40 min; Step 2: Add resorcinol and a certain amount of ammonium molybdate to dissolve in the above solution, and stir vigorously for 30-60 min. During the stirring process, dropwise add a commercially available aqueous formaldehyde solution; Step 3: Transfer the mixed solution to a round-bottom flask and reflux at 85 °C for 7-24 h.

[0007] Step 4: Centrifuge and wash the solution after reflux several times, and dry it in an oven to obtain a black precursor, which is dried for later use.

[0008] Step 5: Weigh a certain amount of the product dried in Step 4 and place it in a tube furnace. Carry out carbothermal reduction in 50% H2 / Ar or pure hydrogen at a flow rate of 15 mL / min, and increase the temperature at a heating rate of 2 - 3 °C per minute to obtain a Mo2C-XC72 support.

[0009] Step 6: When the tube furnace cools down to room temperature, the prepared sample needs to be passivated with 1% O2 / 99% N2 before being taken out.

[0010] Step 7: At room temperature, mix a series of composite supports, isopropanol, and deionized water and disperse them evenly by ultrasonic treatment; add chloroplatinic acid and chloroiridic acid solutions during the ultrasonic treatment; while vigorously stirring the above solution, dropwise add a mixed solution of sodium borohydride and sodium hydroxide, stir for 6 - 12 h, then filter, wash, and dry under vacuum to obtain an ammonia oxidation catalyst based on molybdenum carbide with different crystal phases as the support, namely the PtIr / Mo2C-XC72 catalyst.

[0011] Further, the amount of resorcinol in Step 2 is 1 - 3 g, and the ammonium molybdate is ammonium molybdate tetrahydrate, and the content of molybdenum element in the ammonium molybdate is 10 - 20%.

[0012] Further, the vacuum drying in Step 2 is carried out at 60 - 80 °C for 12 to 24 h under vacuum.

[0013] Further, the carbonization temperature in Step 5 is 800 - 1000 °C, and the carbonization time is 2 - 6 h to obtain a Mo2C-XC72 support.

[0014] Further, for the mixed solution of sodium borohydride and sodium hydroxide prepared in Step 7, add 20 - 80 mg (more than 60% in excess) of sodium borohydride to 0.1 mol of NaOH solution prepared by mixing 10 mL of deionized water and 40 mg of NaOH.

[0015] Further, the total PtIr metal loading in Step 7 is 10 - 20%.

[0016] Among them, ethanol and isopropanol are used as media, ammonium molybdate is used as the molybdenum source, and XC72 is used as the support and carbon source.

[0017] A supported low-temperature ammonia oxidation reaction catalyst with Mo2C as the support is applied to an ammonia fuel cell.

[0018] The beneficial effects of the present invention are as follows: The preparation method of the composite support is simple and convenient to operate, and no harmful substances are produced. The strong metal–support interaction of the catalyst can improve its catalytic performance by changing its electronic properties and the distribution of surface active sites. The use of a composite support to load metals by the sodium borohydride reduction method is convenient to operate and does not pollute the environment. The prepared molybdenum carbide supported Pt and Ir ammonia oxidation catalyst has a lower ammonia oxidation onset potential and better performance than PtIr / XC72 and PtIr / Mo2C catalysts when applied to electrocatalytic ammonia oxidation, and has broad prospects in ammonia fuel cells, especially low-temperature ammonia fuel cells. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 XRD patterns of the prepared XC72, commercial Mo2C, and Mo2C-XC72; Figure 2 CV curve of the PtIr / XC72 catalyst prepared in Example 1 in 1M KOH + 0.1 M NH3 solution; Figure 3 CV curve of the PtIr / Mo2C catalyst prepared in Example 2 in 1M KOH + 0.1 M NH3 solution; Figure 4 CV curve of the PtIr / Mo2C-XC72 catalyst prepared in Example 3 in 1M KOH + 0.1 M NH3 solution. DETAILED DESCRIPTION OF THE INVENTION

[0020] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the embodiments of the present invention belong to the scope of protection of the present invention.

[0021] The preparation method of the ammonia oxidation catalyst based on Mo2C as the support described in the present invention includes the following steps: Step 1: Add the carbon support to 100-150 ml of deionized water and perform ultrasonic dispersion to obtain a mixed solution; Step 2: Add 1-3 g of resorcinol and ammonium molybdate to the mixed solution prepared in Step 1 and continue ultrasonic dispersion; continue stirring for more than 20 min after dispersion; Step 3: Dropwise add 1-3 ml of commercially available formaldehyde aqueous solution during the stirring in Step 2; continue stirring for 20 minutes and then transfer to an oil bath, condense and reflux at 80 °C for 6-12 hours and then dry to obtain the precursor powder; Step 4: Carbon thermal reduction of the dried precursor powder is carried out in 50% H2 / Ar or pure hydrogen at a flow rate of 15 mL / min, with a heating rate of 2 - 3 °C per minute, to obtain a Mo2C-XC72 support; Step 5: First, the Mo2C-XC72 support is mixed with isopropanol and deionized water and ultrasonically dispersed. Then, chloroplatinic acid and chloroiridic acid solutions are added and ultrasonically dispersed continuously. Then, the solution is stirred and a mixed solution containing sodium borohydride and sodium hydroxide is added dropwise. After stirring for 4 - 5 h, it is filtered, washed, and finally vacuum dried to obtain a supported low-temperature ammonia oxidation reaction catalyst.

[0022] In Step 1, the carbon support is Vulcan XC72; the Vulcan XC72 support is placed in a beaker, and a certain volume of deionized water is added for ultrasonic dispersion to obtain a mixed solution; in Step 2, ammonium molybdate, such as ammonium molybdate tetrahydrate, is added to the mixed solution prepared in Step 1 and ultrasonic dispersion is continued. After dispersion, it is vigorously stirred for more than 20 minutes; in Step 3, 1 - 3 mL of commercially available formaldehyde aqueous solution is slowly added dropwise during vigorous stirring; after dropwise addition, stirring is continued for 20 minutes, and the stirred solution is transferred to a round-bottom flask and then to an oil bath; after refluxing and condensing at 80 °C for 6 hours, it is suction filtered, washed, and dried to obtain a precursor powder; in Step 5, first, the molybdenum-based metal support is placed in a beaker, then isopropanol and deionized water are added for mixing, and after mixing, ultrasonic dispersion is carried out. Then, chloroplatinic acid and chloroiridic acid solutions are added to the above mixed solution and ultrasonic dispersion is continued; at the same time, a sodium hydroxide solution with a certain molar concentration is prepared, sodium borohydride is mixed with the prepared sodium hydroxide to obtain a mixed solution, and then a mixed solution containing 20 - 40 mg of sodium borohydride and the prepared sodium hydroxide is obtained; among them, sodium borohydride can reduce the platinum and iridium metals in the added chloroplatinic acid and chloroiridic acid solutions, which is beneficial to the loading of platinum and iridium metals on the subsequent support; under vigorous stirring of the catalyst precursor solution, the mixed solution containing sodium borohydride and sodium hydroxide is slowly added dropwise. After all the dropwise addition is completed, it is sealed with plastic wrap and stirring is continued for 4 - 5 h until the reaction is complete, then it is filtered, washed, and vacuum dried to obtain a supported low-temperature ammonia oxidation reaction catalyst.

[0023] Example 1 Step 1: 80 mg of Vulcan XC72 support is placed in a 250 mL beaker, 42 mL of deionized water and 43 mL of isopropanol are added; ultrasonic dispersion is carried out for 28 minutes; a mixed solution is obtained; Step 2: Add 0.53 mL of chloroplatinic acid solution with 10% platinum metal loading and 1.2 mL of chloroiridic acid solution with 10% iridium metal loading into the mixed solution prepared in Step 1 and continue ultrasonic dispersion; continuously stir for 20 min after dispersion; meanwhile, prepare 50 mL of 0.1 mol NaOH solution, and then dissolve 40 mg of NaBH4 in the NaOH solution; Step 3: Stir the solution and dropwise add the mixed solution containing sodium borohydride and sodium hydroxide. After stirring for 7 h, filter and wash, and finally perform vacuum drying at 60 °C for 8 h to obtain the supported low-temperature ammonia oxidation reaction catalyst PtIr / XC72.

[0024] Example 2 Step 1: Place 80 mg of commercial Mo2C support in a 300 mL beaker, add 47 mL of deionized water and 45 mL of isopropanol; perform ultrasonic dispersion for 30 minutes; obtain a mixed solution; Step 2: Add 1.07 mL of chloroplatinic acid solution with 10% platinum metal loading and 5.555 mL of chloroiridic acid solution with 10% iridium metal loading into the mixed solution prepared in Step 1 and continue ultrasonic dispersion; continuously stir for 25 min after dispersion; meanwhile, prepare 50 mL of 0.1 mol NaOH solution, and then dissolve 80 mg of NaBH4 in the NaOH solution; Step 3: Stir the solution and dropwise add the mixed solution containing sodium borohydride and sodium hydroxide. After all the addition is completed, seal it with plastic wrap, stir for 4 h, filter and wash, and finally perform vacuum drying at 80 °C for 7 h to obtain the supported low-temperature ammonia oxidation reaction catalyst PtIr / Mo2C.

[0025] Example 3 Preparation of precursor: Step 1: Place 450 mg of Vulcan XC72 support in a 250 mL beaker, add 120 mL of deionized water; perform ultrasonic dispersion for 30 minutes; obtain a mixed solution; Step 2: Add 20 mg of molybdic acid (with 10% molybdenum element content) into the mixed solution prepared in Step 1 and continue ultrasonic dispersion; continuously stir for 20 min after dispersion; Step 3: During the stirring in Step 2, dropwise add 1.5 mL of deionized water; continue stirring for 20 minutes, then transfer it to an oil bath, perform condensation reflux at 80 °C for 4 h, and then perform suction filtration and drying; obtain the precursor powder; Step 4: Heat the dried precursor powder to 900 °C at a heating rate of 3 °C per minute in a 50% H2 / Ar atmosphere, and then calcine it at 790 °C for 2 h to obtain the Mo2C-XC72 support; Preparation of catalyst: Step 1: Place 80 mg of the Mo2C-XC72 support in a 300 mL beaker, add 40 mL of deionized water and 40 mL of isopropanol; ultrasonically disperse for 30 minutes; obtain a mixed solution; Step 2: Add 1.06 mL of chloroplatinic acid solution with a platinum metal loading of 10% and 5.557 mL of chloroiridic acid solution with an iridium metal loading of 10% to the mixed solution prepared in Step 1 and continue ultrasonic dispersion; continuously stir for 20 min after dispersion; meanwhile, prepare 50 mL of 0.1 mol of NaOH solution, and then weigh 85 mg of NaBH4 and dissolve it in the NaOH solution; Step 3: Stir the solution and dropwise add the mixed solution containing sodium borohydride and sodium hydroxide. After all the addition is completed, seal it with plastic wrap, stir for 5 h, filter and wash, and finally perform vacuum drying at 60 °C for 12 hours to obtain the supported low-temperature ammonia oxidation reaction catalyst PtIr / Mo2C-XC72.

[0026] The metal catalyst of the supported low-temperature ammonia oxidation catalyst prepared through the above examples, the construction of the metal-transition metal surface interface active sites, includes active components and a composite support. The composite support is a mixture of Mo2C and a carbon support, such as Mo2C-XC72; the active components are metal platinum and metal iridium; by mass fraction, the total content of metals Pt and Ir is 10-30 wt.%.

[0027] The application of the catalyst of the supported ammonia oxidation catalyst prepared through the above examples, such as the application in ammonia electrocatalytic oxidation. More specifically, the metal catalyst of the supported low-temperature ammonia oxidation catalyst is applicable to low-temperature ammonia fuel cells.

[0028] The electrochemical test method of the catalyst prepared in the examples in an alkaline solution is as follows: Weigh 1-5 mg of the catalyst prepared in the example, and add isopropanol and Nafion solution in a ratio of 49:1 to a 5 mL vial, ultrasonically obtain a slurry, transfer a certain amount of the slurry and slowly drop it onto a glassy carbon electrode. The metal mass on the glassy carbon electrode is strictly controlled at 4 µg, and perform electrochemical tests at 25 °C; the electrochemical test uses a five-neck electrolytic cell, pour about 50-70 mL of electrolyte (at the edge of the five-neck electrolytic cell). Place a glassy carbon working electrode, a counter electrode, a reference electrode, and a liquid sealing device in the electrolytic cell. Among them, the glassy carbon electrode is the working electrode, the carbon rod is the counter electrode, and the Hg / HgO electrode is the reference electrode; before the test, pass an inert gas (Ar, N2, etc.) through the upper mouth of the inlet pipe to remove the oxygen in the solution. Subsequently, during the test, replace the inlet pipe with the liquid sealing device (maintaining an inert gas atmosphere), and place the working electrode below the lower end of the alkaline solution.

[0029] The catalyst was activated by scanning 15 - 20 cycles at a rate of 100 mV / s in an Ar-saturated 1 M KOH solution. A certain voltage range, scan rate, and number of scan cycles were set to clean and activate the electrode surface. Then, the test was switched to an ammonia solution of Ar-saturated 1 M KOH + 0.1 M NH3. The catalyst was restored at the reduction potential (-0.85 V vs. Hg / HgO) for about 100 - 150 s. Then, the CV curve was tested at 5 mV / s for 3 - 4 cycles as the basis for testing the catalyst performance.

[0030] The physical structure characterization test method of the catalyst prepared in the examples is as follows: The powder X-ray diffractometer (XRD) used the X-ray diffractometer X'Pert3 Powder of PANalytical Company in the Netherlands, with a Cu target Kα radiation source having a laser incident wavelength of 0.15406 nm, a working voltage of 45 kV, and a current of 40 mA. The results were analyzed by HighScore Plus to obtain the composition and crystal structure information of the sample, and the test range was: 2θ = 20 - 90°.

[0031] The XRD pattern of the carrier used in Example 1 is as Figure 1 , and XC-72 is mainly composed of a peak envelope at about 24.94°.

[0032] The XRD pattern of the carrier used in Example 2 is as Figure 1 , a peak envelope at about 24.94° is the diffraction peak of XC-72, and the diffraction peaks at 34.47°, 37.93°, 39.49°, 52.23°, 61.66°, and 69.64° correspond to the diffraction peaks of Mo2C (100), (002), (101), (102), (110), (103). Compared with commercial molybdenum carbide, the diffraction peaks coincide, indicating that the Mo2C carrier was successfully prepared.

[0033] The catalyst activity is as Figures 2-4 shown. The activities of PtIr / Mo2C-XC72, PtIr / Mo2C, and PtIr / XC72 are 64.1, 48.8, and 23.2 A g -1 Pt, and the initial potentials are 0.33, 0.38, and 0.39 V respectively. By comparison, it can be found that PtIr / Mo2C-XC72 has the best peak current density, the lowest ammonia oxidation initial potential, and the highest performance. Followed by PtIr / Mo2C, indicating that using Mo2C as a carrier can effectively improve the performance.

[0034] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. An ammonia oxidation catalyst with Mo2C as the carrier, characterized in that: It includes an active component and a carrier. The active component is Pt and Ir, and the carrier is a composite carrier composed of Mo2C and carbon black XC72; calculated by mass percentage, the contents of Pt and Ir in the catalyst are 10-20%, and the content of Mo is 10-20%.

2. A method for preparing an ammoxidation catalyst with Mo2C as a carrier as described in claim 1, characterized in that: It includes the following steps: (1) Add Vulcan XC72 carbon carrier into deionized water, and ultrasonically disperse for 20-40 min to obtain a dispersion liquid; (2) Add resorcinol and ammonium molybdate into the dispersion liquid, stir vigorously for 30-60 min, and dropwise add formaldehyde solution during the stirring process to obtain a mixed solution; (3) Reflux the mixed solution at 85 °C for 7-24 h, centrifuge and wash, and dry at 60-100 °C to obtain a precursor; (4) Carbonize the precursor in a 50vol% H2 / 50vol%Ar mixed gas or pure hydrogen at a flow rate of 15 mL / min and a heating rate of 2-3 °C / min, cool to room temperature, and passivate in a 1vol% O2 / 99vol% N2 mixed gas for 1-3 h to obtain a composite carrier; (5) At room temperature, mix the composite carrier, isopropanol and deionized water, add chloroplatinic acid and chloroiridic acid solutions while ultrasonically dispersing, and then dropwise add a mixed solution of sodium borohydride and sodium hydroxide while vigorously stirring, mix and stir for 6-12 h, filter, wash, and vacuum dry to obtain the ammonia oxidation catalyst PtIr / Mo2C-XC72 with Mo2C as the carrier as described above.

3. The method according to claim 2, characterized in that: In step (4), the carbonization temperature is 800-900 °C and the time is 1-3 h.

4. The method according to claim 2, wherein: In step (5), the preparation of the mixed solution of sodium borohydride and sodium hydroxide: Add 20-80 mg of sodium borohydride into a NaOH solution prepared with 10 mL of deionized water and 40 mg of NaOH.

5. The method according to claim 2, characterized in that: In step (5), the vacuum drying temperature is 60-80 °C and the time is 12-24 h.

6. Application of an ammonia oxidation catalyst with Mo2C as the carrier as described in claim 1 or an ammonia oxidation catalyst with Mo2C as the carrier prepared by the method as described in any one of claims 2-5 in low-temperature ammonia electrocatalytic oxidation.

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