Preparation method of vanadium nitride-supported single metal atom catalyst

Abstract

The present invention relates to a method for preparing a metal single-atom catalyst supported on vanadium nitride. In this method, an ammonium metavanadate solution is dropped into a mixed solution containing zinc nitrate and a second metal source, and a metal-doped zinc vanadate precursor is synthesized by chemical precipitation. Subsequently, in an ammonia atmosphere, the metal-doped zinc vanadate precursor is subjected to high-temperature nitridation to prepare the metal single-atom catalyst supported on vanadium nitride. The metal single-atom loading in the metal single-atom catalyst supported on vanadium nitride is 0.01 to 4.5 wt%. The synthesis method of the present invention only requires two steps and is applicable to various metals. The synthesized metal single-atom catalyst supported on vanadium nitride can be used in the catalytic field.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalyst preparation, and particularly relates to a preparation method of a metal single-atom catalyst supported on vanadium nitride. Background Art

[0002] Catalysts with high activity, high selectivity and low cost are crucial for the sustainable development of human society. Research shows that reducing the particle size of catalyst active sites can effectively improve their catalytic performance. As the particle size of the active sites decreases, the proportion of uncoordinated atoms gradually increases, endowing the catalyst with higher catalytic activity. When the particle size of the active sites is reduced to the single-atom scale, its energy level structure and electronic structure will change greatly, forming a single-atom catalyst. Single-atom catalysts have the characteristics of maximizing atomic utilization, high coordination unsaturation of metal sites, and strong interaction between metal sites and carriers, and usually exhibit ultra-high catalytic activity and selectivity.

[0003] In 2011, the research group of Tao Zhang prepared a single-atom Pt1 / FeO x catalyst (Nat. Chem. 2011, 3, 634 - 640), and first proposed the concept of single-atom catalysis. The existence of single-atom Pt was determined by aberration-corrected electron microscopy, X-ray absorption fine structure spectroscopy and infrared methods. This single-atom catalyst exhibits excellent catalytic activity, stability and selectivity in both CO oxidation reaction and CO selective oxidation reaction. Currently, single-atom catalysts have been widely applied to oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and carbon dioxide reduction reaction (CO2RR), etc.

[0004] The existing preparation methods of single-atom catalysts are mainly divided into wet chemical method, high-temperature pyrolysis method, atomic layer deposition method, electrochemical deposition method and other methods; (1) The wet chemical method includes co-precipitation method, impregnation method and ion exchange method. First, the metal precursor is anchored on the carrier, then the unnecessary ligands are removed by appropriate treatment methods, and then calcination or reduction is carried out, etc.; (2) The high-temperature pyrolysis method is to thermally decompose the corresponding precursor in a suitable temperature and atmosphere to prepare a single-atom catalyst. Metal-organic frameworks (MOFs) have atomically dispersed metal nodes and well-defined organic ligands, and are considered to be one of the ideal templates or precursors in the high-temperature pyrolysis method; (3) The electrochemical deposition method is usually carried out in a three-electrode system, and the deposition rate of metal atoms is crucial. The deposited products will be affected by parameters such as potential, current density and pH value; (4) The atomic layer deposition method realizes the growth of materials at the atomic scale by changing the deposition cycle, is applicable to a variety of metals, and is an effective method for preparing single-atom catalysts.

[0005] Although the preparation of different single-atom catalysts has been reported, the development and application of single-atom catalysts are severely restricted due to the complex synthesis method, high cost, and easy agglomeration of metal atoms in current single-atom catalysts. Therefore, it is urgent to develop a general method for the simple synthesis of stable single-atom catalysts. Summary of the Invention

[0006] In order to overcome the disadvantages and deficiencies of the existing single-atom catalyst synthesis methods, such as complex synthesis method, high cost, and easy agglomeration, the present invention proposes a method for preparing a metal single-atom catalyst supported on vanadium nitride.

[0007] To achieve the object of the present invention, the following technical solutions are adopted:

[0008] A method for preparing a metal single-atom catalyst supported on vanadium nitride, the method steps are as follows:

[0009] (1) Drop ammonium metavanadate solution into a mixed solution containing zinc nitrate and a second metal source, stir to make them react fully, and obtain a zinc vanadate precursor doped with metal through filtration and washing.

[0010] (2) In an ammonia atmosphere, perform high-temperature nitridation on the zinc vanadate precursor doped with metal to prepare a metal single-atom catalyst supported on vanadium nitride.

[0011] The metal single-atom loading amount in the metal single-atom catalyst supported on vanadium nitride is 0.01 - 4.5 wt%.

[0012] As a preferred embodiment of the method for preparing a metal single-atom catalyst supported on vanadium nitride described in this application, the second metal source includes but is not limited to any one of the following: metal nitrates, acetates, sulfates, chlorides, ethylenediaminetetraacetates, etc.

[0013] As a preferred embodiment of the method for preparing a metal single-atom catalyst supported on vanadium nitride described in this application, the metal types include but are not limited to iron, cobalt, nickel, rhodium, copper, indium, cerium, silver, manganese, etc.

[0014] As a preferred embodiment of the method for preparing a metal single-atom catalyst supported on vanadium nitride described in this application, in step (1), the stirring temperature is 10 - 60 °C, the stirring rate is 300 - 800 rpm, and in step (1), first mix zinc nitrate and the second metal source evenly, and the mixing time is more than 30 min, and the methods include stirring, ultrasonic dispersion, etc.

[0015] As a preferred embodiment of the method for preparing a metal single-atom catalyst supported on vanadium nitride according to the present application, wherein: in step 1, the concentration of ammonium metavanadate solution is 0.05 - 0.5 mol / L, the dropping rate is 0.1 - 10 mL / min, and after the dropping is completed, stirring is continued for 3 - 5 h.

[0016] As a preferred embodiment of the method for preparing a metal single-atom catalyst supported on vanadium nitride according to the present application, wherein: in the mixed solution containing zinc nitrate and the second metal source, the molar ratio of the metal source of iron / cobalt / nickel / copper / indium / cerium / silver / manganese / rhodium to zinc nitrate is 1:(200 - 650).

[0017] As a preferred embodiment of the method for preparing a metal single-atom catalyst supported on vanadium nitride according to the present application, wherein: during high-temperature nitridation, the heating rate is 1 - 10 °C / min, the nitridation temperature is 600 - 1100 °C, and the nitridation time is 2 - 8 h.

[0018] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0019] (1) The preparation method of the present invention is a general method, which can simultaneously prepare single-atom catalysts of at least 25 metal elements.

[0020] (2) The preparation method of the present invention is simple and effective, and can be used for large-scale synthesis, solving the deficiencies of complex synthesis and difficulty in large-scale preparation in other existing technologies. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a spherical aberration corrected high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image of a copper single-atom catalyst supported on vanadium nitride (SAC: Single Atom Catalyst) (Cu-SACs / VN) prepared according to Example 1 of the present invention;

[0022] Figure 2 It is an X-ray photoelectron spectroscopy (XPS) graph of Cu-SACs / VN prepared according to Example 1 of the present invention;

[0023] Figure 3 It is a comparison graph of the electrocatalytic acetylene semi-hydrogenation reaction activity between Cu-SACs / VN prepared according to Example 1 of the present invention and the carrier vanadium nitride (LSV curve of the electrocatalytic acetylene semi-hydrogenation reaction);

[0024] Figure 4 It is a Faraday efficiency graph of the electrocatalytic acetylene semi-hydrogenation of Cu-SACs / VN prepared according to Example 1 of the present invention;

[0025] Figure 5Spherical aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of the cerium single-atom catalyst supported on vanadium nitride (Ce-SACs / VN) prepared according to Example 4 of the present invention;

[0026] Figure 6 X-ray photoelectron spectroscopy (XPS) graph of Ce-SACs / VN prepared according to Example 4 of the present invention;

[0027] Figure 7 Electrocatalytic hydrogen evolution reaction activity graph (LSV curve of the electrocatalytic hydrogen evolution reaction) of the rhodium single-atom catalyst supported on vanadium nitride (Rh-SACs / VN), vanadium nitride, and the hydrogen evolution benchmark Pt / C prepared according to Example 2 of the present invention. Detailed implementation manners

[0028] To more clearly understand the purpose, technical solution, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. The raw materials can be obtained from public commercial channels unless otherwise specified.

[0029] The present invention discloses a method for preparing a metal single-atom catalyst supported on vanadium nitride. In this method, an ammonium metavanadate solution is dropped into a mixed solution containing zinc nitrate and a second metal source, and a metal-doped zinc vanadate precursor is synthesized by chemical precipitation. Subsequently, in an ammonia atmosphere, the metal-doped zinc vanadate precursor is subjected to high-temperature nitridation to prepare a metal single-atom catalyst supported on vanadium nitride. The metal single-atom loading in the metal single-atom catalyst supported on vanadium nitride is 0.01 - 4.5 wt%. The synthesis method of the present invention only requires two steps and is applicable to various metals.

[0030]

Example 1

[0031] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 9.778 mg of Cu(NO3)2·6H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Ultrasonically disperse for 30 min to obtain a uniform solution A.

[0032] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a uniform solution B.

[0033] (3) Slowly drop solution B into the beaker containing solution A, and stir at 25 °C and 800 rpm for 3 h to obtain a mixed solution.

[0034] (4) Filter the mixed solution by suction filtration, and wash the solid obtained by the suction filtration treatment three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0035] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is a Cu single-atom catalyst supported on a vanadium nitride support.

[0036]

Example 2

[0037] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 20 μL of 10 wt% Rh(NO3)3 solution and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Ultrasonically disperse for 30 min to obtain a uniform solution A.

[0038] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a uniform solution B.

[0039] (3) Slowly drop solution B into the beaker containing solution A, and stir at 25 °C and 800 rpm for 3 h to obtain a mixed solution.

[0040] (4) Filter the mixed solution by suction filtration, and wash the solid obtained by the suction filtration treatment three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0041] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is a Rh single-atom catalyst supported on a vanadium nitride support.

[0042]

Example 3

[0043] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 8.795 mg of AgNO3 and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Stir and disperse for 30 min to obtain a uniform solution A.

[0044] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a uniform solution B.

[0045] (3) Slowly add solution B dropwise to the beaker containing solution A, and stir for 3 h at 25 °C and 800 rpm to obtain a mixed solution.

[0046] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0047] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is an Ag single-atom catalyst supported on a vanadium nitride support.

[0048]

Example 4

[0049] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 20.435 mg of Ce(NO3)3·6H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Ultrasonically disperse for 30 min to obtain a homogeneous solution A.

[0050] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a homogeneous solution B.

[0051] (3) Slowly add solution B dropwise to the beaker containing solution A, and stir for 3 h at 25 °C and 800 rpm to obtain a mixed solution.

[0052] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0053] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is a Ce single-atom catalyst supported on a vanadium nitride support.

[0054]

Example 5

[0055] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 14.825 mg of In(NO3)3·4H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Stir and disperse for 30 min to obtain a homogeneous solution A.

[0056] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve it at 80 °C to obtain a homogeneous solution B.

[0057] (3) Slowly add solution B dropwise to the beaker containing solution A. Stir at 25 °C and 800 rpm for 3 h to obtain a mixed solution.

[0058] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0059] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is an In single-atom catalyst supported on a vanadium nitride support.

[0060]

Example 6

[0061] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 18.095 mg of Fe(NO3)3·9H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Ultrasonically disperse for 30 min to obtain a homogeneous solution A.

[0062] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve it at 80 °C to obtain a homogeneous solution B.

[0063] (3) Slowly add solution B dropwise to the beaker containing solution A. Stir at 25 °C and 800 rpm for 3 h to obtain a mixed solution.

[0064] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0065] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is an Fe single-atom catalyst supported on a vanadium nitride support.

[0066]

Example 7

[0067] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 19.596 mg of Co(NO3)3·6H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Ultrasonically disperse for 30 min to obtain a homogeneous solution A.

[0068] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a homogeneous solution B.

[0069] (3) Slowly add solution B dropwise to the beaker containing solution A and stir at 25 °C and 800 rpm for 4 h to obtain a mixed solution.

[0070] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0071] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is a Co single-atom catalyst supported on a vanadium nitride support.

[0072]

Example 8

[0073] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 21.585 mg of Ni(NO3)2·6H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Stir and disperse for 30 min to obtain a homogeneous solution A.

[0074] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a homogeneous solution B.

[0075] (3) Slowly add solution B dropwise to the beaker containing solution A and stir at 25 °C and 800 rpm for 5 h to obtain a mixed solution.

[0076] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After washing, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0077] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is a Ni single-atom catalyst supported on a vanadium nitride support.

[0078]

Example 9

[0079] (1) Weigh 2.963 g of Zn(NO3)2·6H2O and 18.436 mg of Mn(NO3)2·6H2O and place them in a 250 mL beaker. Add 100 mL of deionized water to the beaker. Ultrasonically disperse for 30 min to obtain a uniform solution A.

[0080] (2) Weigh 1.169 g of NH4VO3 and place it in a 200 mL beaker. Add 100 mL of deionized water to the beaker. Stir and dissolve at 80 °C to obtain a uniform solution B.

[0081] (3) Slowly drip solution B into the beaker where solution A is located, and stir at 25 °C and 800 rpm for 3 h to obtain a mixed solution.

[0082] (4) Filter the mixed solution by suction, and wash the solid obtained by suction filtration three times with deionized water. After the washing is completed, place it in a vacuum drying oven at 60 °C and dry for 12 h to obtain a powder sample.

[0083] (5) Place the powder in a quartz boat, heat it to 1000 °C at a heating rate of 5 °C / min in an ammonia atmosphere, hold for 3 h, and cool to room temperature to obtain a black powder, which is a Mn single-atom catalyst supported on a vanadium nitride support.

[0084] Figure 1 It is the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Cu-SACs / VN prepared through Example 1. Figure 1 The independent bright spots in it are individual Cu atoms.

[0085] Figure 2 It is the X-ray photoelectron spectroscopy (XPS) diagram of Cu-SACs / VN prepared through Example 1.

[0086] Figure 3 、 Figure 4 They are respectively the reaction activity diagram (LSV curve of the electrocatalytic acetylene semi-hydrogenation reaction) and the Faraday efficiency diagram of the single-atom catalyst with Cu active sites prepared through Example 1. The test conditions are as follows:

[0087] (1) Disperse 15 mg of the catalyst and 50 μL of Nafion (5 wt%) solution in 3 mL of ethanol and ultrasonically treat for 1 h to prepare a catalyst slurry. Then spray the catalyst slurry onto a carbon paper with a gas diffusion layer (spraying size is 0.5 cm × 2 cm, and the catalyst loading is 1 mg / cm 2 ) as the working electrode.

[0088] (2) Using nickel foam as the counter electrode, Hg / HgO as the reference electrode, and 1 mol / L KOH solution as the electrolyte, a three-electrode test system was assembled. The electrocatalytic performance of the catalyst was characterized by cyclic voltammetry and linear sweep voltammetry; the product concentration was measured by gas chromatography, and its Faraday efficiency was calculated.

[0089] Figure 5 It is the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Ce-SACs / VN prepared through Example 4. Figure 5 The independent bright spots in it are individual Ce atoms.

[0090] Figure 6 It is the X-ray photoelectron spectroscopy (XPS) diagram of Ce-SACs / VN prepared through Example 1.

[0091] Figure 7 It is the electrocatalytic hydrogen evolution reaction activity diagram (LSV curve of electrocatalytic hydrogen evolution reaction) of the single-atom catalyst with Rh active sites, vanadium nitride, and the hydrogen evolution benchmark Pt / C prepared through Example 2. Test conditions:

[0092] (1) 5 mg of the powder catalyst was taken and dispersed in a mixed solution of 960 μL of absolute ethanol, 960 μL of deionized water, and 80 μL of Nafion (5 wt%), and ultrasonically treated at a constant temperature for 1 h to make it evenly dispersed, which was the catalyst slurry.

[0093] (2) The slurry was evenly coated on the platinum-carbon electrode at a loading amount of 0.3 mg / cm 2 and dried at room temperature. Subsequently, its HER performance was tested in 1 M KOH, and the LSV curve scanning rate was 5 mV / s. At the same potential, the greater the current density, the better the catalytic activity. It can be seen that the activity of the single-atom catalyst of the present invention is close to that of the benchmark 20 wt.% Pt / C, proving the superiority of this catalyst.

[0094] In summary, the present invention has the following beneficial effects:

[0095] (1) The method of the present invention is a general method, which is effective for metals such as Cu, Ce, Rh, Mn, Fe, Mn, Co, Ni, and In, and solves the defect that other methods are usually only effective for one metal.

[0096] (2) The synthesis method of the present invention is simple and effective, and can be used for large-scale synthesis of single-atom catalysts, solving the deficiencies of existing other technologies that are complex in synthesis and difficult to prepare on a large scale.

[0097] (3) The catalyst prepared by the synthesis method of the present invention has excellent performance, and at the same time has high activity and selectivity.

[0098] (4) The method of the present invention is carried out at high temperature to ensure that only the most stable sites anchor metal single atoms, forming a single-atom catalyst with anti-sintering and stable structure.

Claims

1. A preparation method of a metal single-atom catalyst supported on vanadium nitride, comprising the following steps: (1) Drop an ammonium metavanadate solution into a mixed solution containing zinc nitrate and a second metal source, and after sufficient reaction, filter and wash to obtain a zinc vanadate precursor doped with the second metal; (2) Under an ammonia atmosphere, perform high-temperature nitridation on the zinc vanadate precursor doped with the second metal to obtain a second metal single-atom catalyst supported on vanadium nitride.

2. The method according to claim 1, wherein the metal in the second metal source is selected from iron, cobalt, nickel, rhodium, copper, indium, cerium, silver or manganese.

3. The method according to claim 2, wherein the molar ratio of the second metal source to zinc nitrate is 1:200 - 650.

4. The method according to claim 2, wherein the second metal source is selected from nitrates, acetates, sulfates, chlorides or ethylenediaminetetraacetates of the second metal.

5. The method according to claim 1, wherein the loading amount of the second metal single atoms in the second metal single-atom catalyst supported on vanadium nitride is 0.01 - 4.5 wt%.

6. The method according to claim 1, wherein in step (1), the concentration of the ammonium metavanadate solution is 0.05 - 0.5 mol / L, the dropping rate is 0.1 - 10 mL / min, and after dropping, stirring is continued for 3 - 5 h.

7. The method according to claim 1, wherein in step (1), the sufficient reaction is carried out by stirring during the dropping process, the stirring temperature is 10 - 60 °C, and the stirring rate is 300 - 800 rpm.

8. The method according to claim 7, further comprising mixing the zinc nitrate and the second metal source evenly by stirring or ultrasonic dispersion before dropping in step (1).

9. The method according to claim 1, wherein during high-temperature nitridation, the heating rate is 1 - 10 °C / min, the nitridation temperature is 600 - 1100 °C, and the nitridation time is 2 - 8 h.

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