CN110270362B - Manganese-nitrogen co-doped molybdenum carbide nanorod and preparation method and application thereof - Google Patents

Abstract

The invention relates to a method for preparing a manganese-nitrogen co-doped molybdenum carbide nanorod, comprising the following steps: S1, adding a molybdenum source, a manganese salt and aniline into water to ultrasonically form a mixed turbid liquid, wherein the molar ratio of manganese atoms to molybdenum atoms is is 0.001-0.5:1, and the molar ratio of aniline to molybdenum atom is 1-10:1; S2, by adding inorganic acid to adjust the pH of the mixed turbid liquid to 3-6, the reaction obtains a manganese-modified molybdenum amine precursor; S3 and calcining the manganese-modified molybdenum amine precursor in a reducing calcining atmosphere to obtain manganese-nitrogen co-doped molybdenum carbide nanorods. The present invention also provides the manganese-nitrogen co-doped molybdenum carbide nanorod obtained by the above preparation method and its application as an electrocatalyst. According to the preparation method of the manganese-nitrogen co-doped molybdenum carbide nanorods of the present invention, the process is simple, the yield is high, the cost is low, and the mass production is easy.

Description

A kind of manganese nitrogen co-doped molybdenum carbide nanorod and its preparation method and application

technical field

The invention relates to electrocatalytic hydrogen production, and more particularly to a manganese-nitrogen co-doped molybdenum carbide nanorod and a preparation method and application thereof.

Background technique

Hydrogen production by splitting water with electricity converted from booming renewable energy sources (such as solar energy, wind energy, etc.) is an important method to achieve large-scale production of high-purity hydrogen. The electrolytic water hydrogen production catalyst currently used in the market is a platinum-based precious metal catalyst, which has outstanding hydrogen production performance, but also has problems such as high price and limited reserves. Therefore, it is of great practical significance to explore inexpensive, abundant and efficient non-precious metal catalysts for hydrogen production.

Molybdenum carbide, a transition metal carbide, has attracted attention due to its platinum-like electronic structure, high chemical stability, and outstanding catalytic properties. However, in the preparation process of molybdenum carbide, there are common problems such as particle agglomeration and surface contamination carbon coverage. At the same time, due to the high density of d orbitals unoccupied by molybdenum atoms in molybdenum carbide, the catalyst-hydrogen adsorption energy (ΔG _H* ) is too large, which is not conducive to the desorption of hydrogen, and the kinetics of hydrogen evolution is slow.

SUMMARY OF THE INVENTION

In order to solve the problems of particle agglomeration, surface coverage of polluted carbon and unfavorable desorption of hydrogen in the molybdenum carbide nanorods in the prior art, the present invention provides a manganese-nitrogen co-doped molybdenum carbide nanorod and a preparation method and application thereof.

The present invention provides a method for preparing manganese-nitrogen co-doped molybdenum carbide nanorods, comprising the following steps: S1, adding molybdenum source, manganese salt and aniline into water to ultrasonically form a mixed turbid liquid, wherein the molar ratio of manganese atoms to molybdenum atoms is is 0.001-0.5: 1, and the molar ratio of aniline to molybdenum atom is 1-10: 1; S2, by adding inorganic acid, the pH of the mixed turbid liquid is regulated to 3-6, and the reaction obtains a manganese-modified molybdenum amine precursor (also It is called MoO _x - amine organic-inorganic hybrid precursor); S3, calcining manganese-modified molybdenum amine precursor in a reducing calcining atmosphere to obtain manganese-nitrogen co-doped molybdenum carbide nanorods.

The invention optimizes the electronic structure of molybdenum carbide by co-doping with manganese and nitrogen elements, reduces the catalyst-hydrogen adsorption energy, accelerates the kinetics of hydrogen evolution of molybdenum carbide, and improves the hydrogen evolution activity of electrolyzed water. Specifically, using manganese-modified molybdenum amine as a precursor, regulating the element ratio of Mn and Mo in the precursor ingredients, and calcining in a reducing atmosphere to obtain a manganese-nitrogen co-doped molybdenum carbide nanorod electrocatalyst. The manganese-nitrogen co-doped molybdenum carbide nanorods obtained by the preparation method of the present invention do not contain characteristic peaks and lattice fringes of carbon in Raman spectra and high-power transmission electron microscope photos, indicating that their surfaces do not contain polluted carbon; Coupled plasma emission spectroscopy, X-ray photoelectron spectroscopy, etc. proved the existence of Mn and N elements. In a word, the preparation method according to the present invention is simple, economical and reasonable, and is easy to realize large-scale production.

Preferably, in the step S1, the molybdenum source and the manganese salt are ultrasonically dissolved in water, and aniline is added as the carbon source.

Preferably, the molar concentration of molybdenum atoms in the mixed turbid solution is 0.1-1.0 mol/L. In a preferred embodiment, the molar concentration of molybdenum atoms in the mixed turbid solution is 0.3-1 mol/L.

Preferably, the molar ratio of the manganese atoms to the molybdenum atoms is 0.01-0.5:1.

Preferably, the molar ratio of aniline to molybdenum atoms is 2.4-10:1.

Preferably, the molybdenum source is selected from at least one of ammonium heptamolybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O), sodium molybdate and potassium molybdate.

Preferably, the manganese salt is selected from at least one of manganese acetate, manganese nitrate and manganese chloride.

Preferably, in the step S2, the pH is adjusted dropwise with an inorganic acid until a yellow precipitate appears, the reaction is carried out at 40-80° C. for 2-12 hours, and the manganese-modified molybdenum amine precursor is obtained by suction filtration, washing and drying. In a preferred embodiment, the reactants are reacted in a water bath, and the suction-filtered product is fully washed with ethanol. In a preferred embodiment, the reaction is stirred at 60°C for 4 hours.

Preferably, the inorganic acid is at least one selected from dilute hydrochloric acid, dilute sulfuric acid and dilute nitric acid. In a preferred embodiment, the inorganic acid is dilute hydrochloric acid, dilute sulfuric acid, or dilute nitric acid with a concentration of 0.1-3M. In a preferred embodiment, the inorganic acid is a 1M HCl solution.

Preferably, the pH of the mixed turbid solution is adjusted to 4-5 by adding inorganic acid.

Preferably, in the step S3, calcination is carried out in a tube furnace, and the calcined product is acid-bubble, centrifugally washed to neutrality and then dried to obtain manganese-nitrogen co-doped molybdenum carbide nanorods.

Preferably, the acid used in the acid foam is selected from at least one of hydrochloric acid and sulfuric acid. In a preferred embodiment, the acid used in the acid bubble is a 0.5M H ₂ SO ₄ solution.

Preferably, in the step S3, calcination is carried out at a calcination temperature of 600-800° C. for 2-10 hours at a heating rate of 2-10° C./min. In a preferred embodiment, the calcination temperature is 650-750°C. In a preferred embodiment, the calcination temperature is 700°C.

Preferably, the calcining atmosphere is a mixed gas of inert gas and hydrogen. In a preferred embodiment, the volume content of hydrogen in the calcining atmosphere is 0.05-0.25. In a preferred embodiment, the calcination atmosphere is a mixed atmosphere of H ₂ and Ar.

The present invention also provides the manganese-nitrogen co-doped molybdenum carbide nanorods obtained by the above preparation method.

Preferably, the manganese-nitrogen co-doped molybdenum carbide nanorods are formed by stacking molybdenum carbide nanoparticles with uniform particle size. In particular, the manganese-nitrogen co-doped molybdenum carbide nanorods have abundant pore structures, which are favorable for the infiltration of electrolyte and the escape of hydrogen.

The present invention also provides the application of the above manganese-nitrogen co-doped molybdenum carbide nanorod as an electrocatalyst.

Preferably, the manganese-nitrogen co-doped molybdenum carbide nanorods are used for electrolysis of water to produce hydrogen.

Preferably, the manganese-nitrogen co-doped molybdenum carbide nanorods are used as a catalyst for electrolysis of water for hydrogen evolution in an acidic medium. In a preferred embodiment, in 0.5MH ₂ SO ₄ solution, the overpotential at 20 mA/cm ² current density is 192-226 mV (relative to the reversible hydrogen electrode), the Tafel slope is 66-103 mV/dec, and Continuous operation for 24h still maintains good stability.

According to the preparation method of the manganese-nitrogen co-doped molybdenum carbide nanorods of the present invention, the process is simple, the yield is high, the cost is low, and the mass production is easy. Specifically, the present invention utilizes an organic-inorganic hybridization method to obtain a manganese-modified molybdenum amine precursor, and then calcinations in a reducing atmosphere to obtain manganese-nitrogen co-doped molybdenum carbide nanorods. This preparation method through the precursor can effectively Reduce agglomeration. Moreover, the manganese-nitrogen co-doped molybdenum carbide nanorods of the present invention regulate the electronic structure of molybdenum carbide by doping manganese elements with abundant valence electrons (d ⁵ ) and low electronegativity, accelerate the hydrogen evolution kinetics of molybdenum carbide, and improve the electrolysis of water. Hydrogen evolution activity. In addition, according to the fact that the typical D peaks and G peaks of carbon are not detected in the Raman spectrum, it can be seen that the manganese-nitrogen co-doped molybdenum carbide nanorods provided by the present invention do not have the problem that the surface is covered with contaminated carbon, and the catalytic sites can be fully exposed. Thereby, a highly active electrolytic water hydrogen evolution catalyst free of precious metals is obtained, which exhibits good electrolytic water hydrogen evolution activity and stability in an acidic medium.

Description of drawings

Fig. 1 is the XRD pattern of manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different manganese-molybdenum ratios;

Fig. 2 is the XRD pattern of sample A and sample B;

Fig. 3 is the SEM image (a) and high magnification TEM image (b) of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared in Example 1;

Fig. 4 is the Raman characterization of manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different manganese-molybdenum ratios;

5 is the XPS full spectrum characterization of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared in Example 1;

Fig. 6 is the electrolytic water hydrogen production catalytic performance characterization of manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different manganese-molybdenum ratios;

Fig. 7 is the stability characterization of the electrolysis of water for hydrogen production of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared in Example 1.

Detailed ways

Below in conjunction with the accompanying drawings, preferred embodiments of the present invention are given and described in detail.

Example 1

Weigh out 1.25 g of carbon source aniline, 1 g of ammonium heptamolybdate ((NH ₄ ) ₆ Mo ₇ O ₂₄ ·4H ₂ O), and 13.8 mg of manganese acetate (Mn(C ₂ H ₃ O ₂ ) ₂ ·4H ₂ O) (The molar concentration of molybdenum atoms is 0.3mol/L, the molar ratio of aniline and molybdenum atoms is 2.4:1, and the molar ratio of Mn:Mo is 0.01) in a beaker, add 20 mL of water, ultrasonically disperse, and add 1M HCl solution to adjust pH to 4 -5. After stirring in a 60°C water bath for 4 hours, the yellow product was separated by suction filtration and dried. The dried product was calcined to 700°C in a 10% H ₂ /Ar atmosphere, kept for 3 h, and then the calcined product was soaked in 0.5 M H ₂ SO ₄ solution for 24 h to remove unstable species in the catalyst, and finally centrifuged After washing with water until it becomes neutral, it can be dried, and the obtained product is recorded as Mn, N-Mo ₂ C-0.01.

The electrochemical test of manganese-nitrogen co-doped molybdenum carbide nanorods for electrolysis of water for hydrogen production is carried out according to the following steps:

First, 4 mg of the above product and 0.5 mg of carbon black were weighed and dispersed in a mixed solution of 2 ml of isopropanol and 60 uLNafion solution (5%), sonicated to obtain a uniformly dispersed suspension, and then 80 uL of the above suspension was dropped onto a wave with a diameter of 6 mm. On the carbon electrode, the working electrode can be obtained after natural drying; then the hydrogen production performance test of electrolyzed water is carried out in an electrochemical workstation (CHI760E, Shanghai Chenhua), using a three-electrode system, with 0.5MH ₂ SO ₄ solution as electrolyte, graphite The rod is the counter electrode, the Ag/AgCl electrode filled with 3M KCl solution is the reference electrode, the scan rate of the linear sweep voltammetry curve is 5mV/s, the experimental data is not IR corrected, and the electrode potential is relative to the reversible hydrogen electrode (RHE), The conversion method is as follows: E _RHE =E _Ag/AgCl +0.059*pH+0.209.

Example 2

The preparation process and steps in Example 2 are basically the same as those in Example 1 above, except that the mass of manganese acetate (Mn(C ₂ H ₃ O ₂ ) ₂ ·4H ₂ O) was weighed to be 1.4 mg (Mn: Mo The molar ratio is 0.001), and the prepared catalyst is denoted as Mn, N-Mo ₂ C-0.001. The catalytic performance of the catalyst for electrolysis of water for hydrogen production is the same as

Example 1.

Example 3

The preparation process and steps in Example 3 are basically the same as those in Example 1 above, except that the mass of manganese acetate (Mn(C ₂ H ₃ O ₂ ) ₂ ·4H ₂ O) was weighed to be 7.0 mg (Mn:Mo The molar ratio is 0.005), and the prepared catalyst is denoted as Mn, N-Mo ₂ C-0.005. The catalytic performance of the catalyst for electrolysis of water for hydrogen production is the same as

Example 1.

Example 4

The preparation process and steps in Example 4 are basically the same as those in Example 1 above, except that the mass of manganese acetate (Mn(C ₂ H ₃ O ₂ ) ₂ ·4H ₂ O) was weighed to be 69.4 mg (Mn:Mo The molar ratio is 0.05), and the prepared catalyst is denoted as Mn, N-Mo ₂ C-0.05. The characterization of the catalytic performance of the catalyst for electrolysis of water for hydrogen production is the same as that in Example 1.

Example 5

The preparation process and steps in Example 5 are basically the same as those in Example 1 above, except that manganese acetate (Mn(C ₂ H ₃ O ₂ ) ₂ ·4H ₂ O) was not added, and the prepared catalyst was denoted as Mn, N-Mo ₂ C-0. The characterization of the catalytic performance of the catalyst for electrolysis of water for hydrogen production is the same as that in Example 1.

Example 6

The preparation process and steps in Example 6 are basically the same as in Example 1 above, except that the molar ratio of aniline and molybdenum atom is 10:1, the molar concentration of molybdenum atom is 1.0mol/L, and the Mn:Mo molar ratio is 0.5. This sample is labeled Sample A.

Example 7

The preparation process and steps in Example 7 are basically the same as those in Example 1 above, except that the molar ratio of aniline and molybdenum atom is 1:1, the molar concentration of molybdenum atom is 0.3mol/L, and the Mn:Mo molar ratio is 0.05. This sample is labeled sample B.

Table 1 below is the ICP-OES result of the manganese content of manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different manganese-molybdenum ratios, indicating that manganese is successfully incorporated into the molybdenum carbide crystal.

Table 1

Figure 1 shows the XRD patterns of manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different ratios of manganese and molybdenum. It can be seen from Figure 1 that the prepared catalyst is β-Mo ₂ C, and the manganese doping in a certain content range does not will affect the crystal composition.

Figure 2 is the XRD pattern of sample A and sample B. It can be seen from Figure 2 that both sample A and sample B are β-Mo ₂ C.

Fig. 3 is the SEM image (a) and the high magnification TEM image (b) of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared in Example 1. The molybdenum carbide nanorods are formed by stacking molybdenum carbide nanoparticles with uniform particle size. It has abundant pore structure and no carbon lattice fringes under high magnification transmission electron microscope.

Figure 4 shows the Raman characterization of manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different ratios of manganese and molybdenum. In the Raman spectrum, there are only characteristic peaks of molybdenum carbide and no characteristic peaks of carbon (peak D and peak G), indicating that The prepared catalyst surface was not covered with contaminating carbon.

Figure 5 is the XPS full spectrum characterization of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared in Example 1, and the nitrogen signal in the full spectrum indicates that nitrogen is incorporated into the molybdenum carbide crystal.

Figure 6 shows the characterization of the catalytic performance of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared with different ratios of manganese and molybdenum for the electrolysis of water for hydrogen production, (a) is the linear sweep voltammetry curve, (b) is the Tafel curve. It can be seen from (a) that when the current density is 20 mA/cm ² , Mn, N-Mo ₂ C-0, Mn, N-Mo ₂ C-0.001, Mn, N-Mo ₂ C-0.005, Mn, The overpotentials required for N-Mo ₂ C-0.01, Mn, and N-Mo ₂ C-0.05 were 226mV, 208mV, 195mV, 192mV, and 218mV, respectively, which indicated that the electrolysis of Mn and N-Mo ₂ C-0.01 could produce hydrogen by electrolysis of water. the best catalytic performance. As can be seen from (b), Mn, N-Mo2C _- 0, Mn, N-Mo2C _- 0.001, Mn, N-Mo2C _- 0.005, Mn, N-Mo2C _- 0.01, Mn , the Tafel slopes of N-Mo ₂ C-0.05 are 103mV/dec, 72mV/dec, 70mV/dec, 66mV/dec, 88mV/dec, respectively, indicating that their rate limiting steps are Volmer-Heyrovsky steps, where, The Volmer-Heyrovsky step is the adsorption-desorption step of hydrogen protons in the process of electrolysis of water for hydrogen production. According to the Tafel slope, the rate-limiting step of the reaction can be obtained. The smaller the Tafel slope, the better.

Figure 7 shows the stability characterization (it curve) of the electrolysis of water for hydrogen production of the manganese-nitrogen co-doped molybdenum carbide nanorods prepared in Example 1. After running for 24 h, the current density still remains at about 7.2 mA/cm ² , indicating that its good stability.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the scope of the present invention. Various changes can also be made to the above-mentioned embodiments of the present invention. That is, all simple, equivalent changes and modifications made according to the claims and description content of the application for the present invention all fall into the scope of protection of the claims of the present invention. What is not described in detail in the present invention is conventional technical content.

Claims (10)

1. The preparation method of the manganese-nitrogen co-doped molybdenum carbide nanorod is characterized by comprising the following steps of:

s1, adding a molybdenum source, manganese salt and aniline into water, and performing ultrasonic treatment to form a mixed turbid liquid, wherein the molar ratio of manganese atoms to molybdenum atoms is 0.001-0.5:1, and the molar ratio of aniline to molybdenum atoms is 1-10: 1;

s2, regulating the pH value of the mixed turbid liquid to 3-6 by adding inorganic acid, and reacting to obtain a manganese-modified molybdenum amine precursor;

and S3, calcining the manganese modified molybdenum amine precursor in a reductive calcining atmosphere to obtain the manganese-nitrogen co-doped molybdenum carbide nanorod without carbon on the surface.

2. The method according to claim 1, wherein the molar concentration of molybdenum atoms in the mixed turbid liquid is 0.1-1.0 mol/L.

3. The method according to claim 1, wherein the molybdenum source is at least one selected from ammonium heptamolybdate, sodium molybdate, and potassium molybdate.

4. The method according to claim 1, wherein the manganese salt is at least one selected from the group consisting of manganese acetate, manganese nitrate and manganese chloride.

5. The production method according to claim 1, wherein the inorganic acid is at least one selected from the group consisting of dilute hydrochloric acid, dilute sulfuric acid, and dilute nitric acid.

6. The method as claimed in claim 1, wherein in step S3, the calcination is carried out at a calcination temperature raised to 600-800 ℃ at a temperature raising rate of 2-10 ℃/min for 2-10 hours.

7. The method according to claim 1, wherein the calcining atmosphere is a mixed gas of an inert gas and hydrogen gas.

8. The method according to claim 7, wherein the hydrogen gas is contained in the calcination atmosphere in an amount of 0.05 to 0.25 by volume.

9. Manganese-nitrogen-codoped molybdenum carbide nanorod obtained according to the preparation method of any one of claims 1-8.

10. Use of the manganese-nitrogen co-doped molybdenum carbide nanorod according to claim 9 as an electrocatalyst.

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