CN111408390A - A pure phase polygonal W2C nanomaterial and preparation method thereof - Google Patents

Abstract

The invention provides a pure phase polygon W₂C nanometer material and a preparation method thereof, wherein the preparation method comprises the following steps: 1) mixing and stirring a tungsten source and ethylene glycol uniformly to obtain a first solution; 2) mixing and stirring a carbon source, ethylene glycol and water uniformly to obtain a second solution; 3) mixing and stirring the first solution and the second solution uniformly to obtain a suspension, and separating out solids; 4) carrying out heat treatment on the solid obtained in the step 3) in a reducing atmosphere to obtain a pure-phase polygon W₂And C, nano-materials. According to the method, pure-phase, high-surface-area and high-activity W can be prepared under the condition of adding no toxic, harmless and organic matters₂And C, nano-materials.

Description

A pure phase polygonal W2C nanomaterial and preparation method thereof

technical field

The invention belongs to the technical field of inorganic nanomaterials, and in particular relates to an environment-friendly method for preparing _W2C nanomaterials with a pure phase polygonal structure.

Background technique

Tungsten carbide (WC and W ₂ C), due to its d-band electron density of states similar to Pt, is a powerful alternative to platinum-based catalysts and can be widely used in water electrolysis hydrogen evolution half-reaction (HER) to improve its reaction efficiency. In recent years, in particular, a series of studies on the performance of IVB-VIB transition metal carbides under acidic conditions conducted by Wirth et al. in 2012 pointed out that tungsten carbide has the best catalytic activity, even better than that which has been used in commercial all-alum oxidation The commercial Mo _x C at the hydrogen evolution end of the combined hydrogen production by reduction flow battery and electrolysis of water has important commercial value. And in a large number of subsequent studies, Lee's group obtained through theoretical calculations that W ₂ C has higher catalytic activity than WC. The results of this study were published in Nature Communication, which is the basis for the use of pure phase W ₂ C nanomaterials in commercial electrolysis. The application of hydrogen in aquatic products opens a door.

However, W ₂ C is often used as a by-product in the synthesis of WC and is thermodynamically unstable at 1250 °C, so the synthesis of pure phase W ₂ C is challenging and rarely reported. In addition, the synthesis of W ₂ C requires high temperature (>1000K), which may easily lead to excessive growth of W ₂ C crystals, resulting in a decrease in the degree of nanometerization of the active center of the material, which affects its catalytic performance. W ₂ C nanomaterials with high specific surface area can expose more catalytically active sites, which will also facilitate and promote the catalytic reaction. Therefore, a preparation method for synthesizing pure phase W ₂ C nanomaterials with high specific surface area is urgently needed in this field.

SUMMARY OF THE INVENTION

The invention aims to overcome the defects of the existing tungsten carbide materials such as small specific surface area, low activity and difficult synthesis of high _- activity W2C nanomaterials, and provides a pure-phase polygonal _W2C nanomaterial with high HER activity and a preparation method thereof.

In a first aspect, the present application provides a method for preparing a pure-phase polygonal W ₂ C nanomaterial with high HER activity, the preparation method comprising the following steps:

1) Mix the tungsten source and ethylene glycol and stir to obtain the first solution;

2) mixing the carbon source with ethylene glycol and water and stirring to obtain the second solution;

3) the first solution and the second solution are mixed and stirred to obtain a suspension, and the solid is isolated;

4) Heat treatment of the solid obtained in step 3) in a reducing atmosphere to obtain a pure-phase polygonal W ₂ C nanomaterial.

In the above preparation method, adding water to the precursor can make the precursor spontaneously protonated and electrostatically cross-linked, and the degree of spontaneous protonation and electrostatic cross-linking of the precursor can be controlled by controlling the amount of water added to the precursor, that is, in the first step. In the stirring process of one step (that is, the above-mentioned steps 1 and 2), the precursor starts the spontaneous protonation process to form a chain structure during the high-temperature stirring process, and the spontaneous protonation degree of the material is controlled by adjusting the amount of water added; in the second step (that is, the above-mentioned Step 3) During the stirring process, due to spontaneous protonation, the surface of the material is charged with heterogeneous charges, and "electrostatic action" occurs to cause cross-linking, and the material spontaneously arranges into a regular dodecahedron structure to form an "invisible template", and heat treatment is performed to obtain pure Phase W ₂ C nanomaterials. The growth of nanoparticles can be controlled by changing the heat treatment temperature, and W ₂ C nanoparticles with different sizes and morphologies can be obtained. According to the above method, the morphology and particle size of the material can be controlled only by controlling the water content without using a template (and pure-phase W ₂ C nanoparticles in regular dodecahedron have never been reported), and the particle size is small and uniform The regular dodecahedron can not only increase the specific surface area, but also fully expose some catalytically active crystal planes (such as (220) (111) isocrystal planes) of the material, which increases the number of active sites. At the same time, the intrinsic catalytic activity of the material is maximized. In addition, the method has no organic materials and no dangerous gas in the preparation process, is environmentally friendly and easy to handle, and can be used for mass production.

According to the above method, pure-phase, high-surface-area, high-activity W ₂ C nanomaterials can be prepared under the conditions of non-toxic, harmless, and no organic matter addition.

Preferably, the tungsten source is ammonium tungstate, and the carbon source is melamine.

Preferably, in step 1), the dosage ratio of the tungsten source to the ethylene glycol is: 0.1-10 L of ethylene glycol is used per mole of the tungsten source.

Preferably, in step 2), the dosage ratio of carbon source to ethylene glycol is: 0.1-10 L of ethylene glycol is used per mole of carbon source.

Preferably, in step 2), the volume of water is 0.2-0.6 times the volume of ethylene glycol.

Preferably, the molar ratio of the tungsten source to the carbon source is 1:10-1:2, preferably 5:32.

Preferably, in step 1), the stirring temperature is controlled at 50-100° C., preferably 60-80° C., and the stirring time is controlled at 1-10 h.

Preferably, in step 2), the stirring temperature is controlled at 50-100° C., preferably 60-80° C., and the stirring time is controlled at 1-10 h.

Preferably, in step 3), the stirring temperature is controlled at 20-80° C., preferably 20-40° C., and the stirring time is controlled at 1-4 h.

Preferably, in step 4), the reducing atmosphere is 95% H ₂ +5% Ar.

Preferably, in step 4), the heat treatment temperature is 800-1000° C., and the heat treatment time is 1-4 h.

In a second aspect, the present application provides a pure-phase polygonal W ₂ C nanomaterial prepared by any of the above preparation methods, wherein the particle size of the pure-phase polygonal W ₂ C nanomaterial is 100-2000 nm, preferably 100-500 nm, and the faceted structure.

Beneficial effects:

(1) The preparation method provided in this application can prepare pure-phase W ₂ C nanomaterials;

(2) The method does not use a template. By controlling the amount of water added, the degree of protonation of the precursor is controlled, thereby controlling the exposure of the W ₂ C edge; by controlling the heat treatment temperature, the growth of nanoparticles is controlled, and the pure phase W 2 C is formed in the formation of pure phase W ₂ C. At the same time, nanoparticles of different sizes and morphologies were obtained;

(3) The method is simple and easy to implement, the precursor has no organic matter, is environmentally friendly, and has mild preparation conditions, and can realize mass production.

Description of drawings

FIG. 1 shows the XRD patterns of pure phase W ₂ C nanomaterials prepared in Examples 1, 2, and 3. FIG.

FIG. 2 shows the XRD patterns of the materials prepared in Comparative Example 1 and Example 4. FIG.

FIG. 3 shows the FE-SEM photographs of the pure-phase W ₂ C nanomaterials prepared in Examples 1 and 2. FIG.

FIG. 4 shows a FE-SEM photograph of the W ₂ C nanomaterial prepared in Example 4. FIG.

FIG. 5 shows the hydrogen evolution electrocatalytic activity test results of the materials obtained in Examples and Comparative Examples.

Detailed ways

The present invention is further described below through the following embodiments, and it should be understood that the following embodiments are only used to illustrate the present invention, but not to limit the present invention.

An embodiment of the present invention provides a template-free, substrate-free, and environment-friendly method for preparing pure-phase W ₂ C nanomaterials. This method adopts the "two-step stirring method", which controls the spontaneous protonation and electrostatic crosslinking degree of the precursor by controlling the amount of water added to the precursor, that is, during the first stirring process, the precursor starts the spontaneous protonation process at high temperature A chain-like structure is formed, and the degree of spontaneous protonation of the material is controlled by adjusting the amount of water added; in the second stirring process, the surface of the material is charged with heterogeneous charges due to spontaneous protonation, and "electrostatic action" occurs to cause cross-linking, and the material spontaneously arranges A regular dodecahedron structure is formed to form a "stealth template", and heat treatment is performed in a specific atmosphere, and the growth of nanoparticles is controlled by changing the heat treatment temperature to obtain W ₂ C nanoparticles with different sizes and morphologies. The prepared pure phase W ₂ C nanomaterial has a particle size of about 100-500 nm. The method has the advantages of simple preparation process, mild conditions, no addition of any organic matter, environmental friendliness and easy operation, and can be used for commercial large-scale use. The method belongs to the field of inorganic nanomaterial synthesis. This method is described in detail below.

The tungsten source is added to a certain amount of ethylene glycol solution, and stirred at a certain temperature for a certain period of time to make it evenly dispersed to obtain a first solution.

The first solution is, for example, a white solution. The tungsten source can be selected from ammonium tungstate ((NH ₄ ) ₁₀ W ₁₂ O ₄₁ ·xH ₂ O), tungstic acid (H ₂ WO ₄ ), phosphotungstic acid (H ₃ O ₄₀ PW ₁₂ ), among which ammonium tungstate is preferred , because some ammonia gas escapes during the high-temperature stirring process, which makes the ammonium tungstate positively charged and cross-linked with the solvent ethylene glycol to form a precursor with a long-chain structure. The dosage ratio of the tungsten source to the ethylene glycol can be as follows: 0.1-10 L of ethylene glycol is used per mole of the tungsten source. At this ratio, ethylene glycol is sufficiently protonated to form a chain structure with tungstate ions. The stirring temperature may be 50 to 100°C, preferably 60 to 80°C, and stirring at a higher temperature enables sufficient protonation. The stirring time can be controlled at 1-10h, preferably 5-10h.

The carbon source is added to a mixed aqueous solution containing a certain amount of ethylene glycol and a little water (preferably deionized water), and stirred at a certain temperature for a certain period of time to uniformly disperse to obtain a second solution.

The second solution is, for example, a white solution. The carbon source can be selected from melamine (C ₃ N ₃ (NH ₂ ) ₃ ), sucrose (C ₁₂ H ₂₂ O ₁₁ ), glucose (C ₆ H ₁₂ O ₆ ), among which melamine is preferred because it has three primary amino groups , easily protonated to form positive ions. The molar ratio of the tungsten source and the carbon source can be controlled at 1:10 to 1:2, preferably 5:32, and the long chains formed at this molar ratio are connected to each other through electrostatic interactions between ions (such as NH4 ⁺ and OH ^- ) makes it spontaneously arrange to form a certain structure. The dosage ratio of carbon source to ethylene glycol can be as follows: 0.1-10 L of ethylene glycol is used per mole of carbon source. At this ratio, the carbon source can achieve sufficient protonation. The exposed edges and morphology of the final W ₂ C nanomaterials can be regulated by controlling the amount of water added. Preferably, the amount of water added to the solution is 0.2-0.6 times the volume of the added ethylene glycol. In this range, a dodecahedral structure, such as a nearly regular dodecahedral structure, can be obtained. Selecting different ratios within this range can obtain pure-phase W ₂ C nanomaterials with different particle sizes. More preferably, the amount of water added to the solution is 0.2-0.4 times the volume of the added ethylene glycol. If the amount of water is too small, the protonation degree of the carbon source is small, and the subsequent substitution/polymerization reaction is difficult to occur; if the amount of water is too much, there may be excess protonated and aprotonated groups in the substitution chain, and the subsequent substitution/polymerization reaction It occurs violently at the same time, and it is difficult to control the degree of its reaction. The stirring temperature may be 50 to 100°C, preferably 60 to 80°C, and stirring at a higher temperature enables sufficient protonation. The stirring time can be controlled at 1-10h, preferably 5-10h.

The first solution and the second solution are mixed, and stirred at a certain temperature for a certain period of time to make the mixture uniform to obtain a suspension. This suspension is, for example, white. In one example, the first solution is added dropwise to the second solution to mix the two. The stirring temperature can be controlled at 20-80°C, preferably 20-40°C, to ensure that the substitution/polymerization reaction (such as the reaction of NH ₄ ⁺ and OH ^- ) occurs mildly and orderly, thereby controlling the obtained pure phase W ₂ C nanoparticles shape and size. The stirring time can be controlled within 1-4h.

The above-mentioned reaction system uses ethylene glycol as a solvent, which can not only realize the protonation process in the first solution, but also the protonated ethylene glycol can form a long-chain structure with the tungstate chain, and can fully dissolve the ethylene glycol in the second solution. The carbon source causes it to undergo a protonation process. When the first and second solutions are fully mixed, no chemical reaction occurs because the solvent is consistent, thereby ensuring the occurrence of subsequent substitution/polymerization reactions.

The precipitate was collected from the suspension and dried in vacuo to give a solid. The precipitate is, for example, a white flocculent precipitate. The obtained solid is, for example, a white powder. The collection method can be centrifugation or the like. The drying temperature is controlled at 50-100°C.

The obtained solid is heat-treated to obtain W ₂ C nanomaterials. The heat treatment atmosphere can be a reducing atmosphere, for example, a 95% H ₂ /5% Ar atmosphere, and in this atmosphere, the generated material can be guaranteed to be pure phase W ₂ C. The particle size of the W ₂ C crystals can be controlled by controlling the heat treatment temperature. The heat treatment temperature may be 860-960°C, preferably 880-920°C. The heat treatment time can be 1-4h.

The present application provides a method for preparing pure-phase W ₂ C nanomaterials without templates and substrates. The amount of water added is controlled, and the appropriate tungsten source, carbon source and solvent are selected to make the precursor spontaneously protonate to form heterogeneous charges and then cross-link each other. The arrangement forms a "stealth template". Specifically, the first and second solutions are prepared separately, so that the solutions can be fully protonated during the high-temperature stirring process, and then they are mixed. NH4 ⁺ and OH ^- ) thus undergo a substitution/polymerization reaction, resulting in a spontaneous arrangement of the crystal structure. In the first solution, the solvent ethylene glycol is protonated (protonated by the action of ethylene glycol and ammonium tungstate); in the second solution, the carbon source is protonated (the carbon source is protonated by the action of water and ethylene glycol). Using ethylene glycol as a solvent, not only can the protonation process be realized in the first solution, but the protonated ethylene glycol can also combine with tungstate chains to form a long-chain structure, and can fully dissolve the carbon source in the second solution. It undergoes a protonation process. When the first and second solutions are fully mixed, no chemical reaction occurs due to the consistent solvent, thus ensuring the occurrence of subsequent substitution/polymerization reactions; then, a pure-phase polygon W ₂ C with a certain edge structure is synthesized by heat treatment in a reducing atmosphere Nanomaterials greatly increase the catalytic active area of the material, which has not been reported internationally. The W ₂ C nanomaterial prepared in one embodiment of the present invention is a pure W ₂ C phase with a nearly regular dodecahedron structure and a particle size of about 100-500 nm.

In one example, the preparation method of pure phase polygonal W ₂ C nanomaterials is as follows:

(1) 5mmol of ammonium tungstate was dissolved in 20mL of ethylene glycol solution, and stirred at 80°C for 8h;

(2) 32 mmol of melamine was dissolved in 20 mL of ethylene glycol solution, and deionized water with a volume of 0.2 to 0.6 times the volume of the ethylene glycol solution was added, and stirred at 80 °C for 8 h;

(3) The homogeneous solution obtained in (1) was added dropwise to the solution obtained in (2), and stirred at 25° C. for 1 h;

(4) The white flocculent precipitate obtained in (3) was collected by centrifugation, washed with ethanol for 3 to 5 times, and dried in vacuum for 12 h;

(5) Heat treatment of the white powder obtained in step (4) at 860-960° C. for 2 hours in a tube furnace in a 95% H ₂ /5% Ar atmosphere to obtain a pure-phase polygonal W ₂ C nanomaterial.

The advantages of the preparation method provided by the application are as follows:

(1) The precursors used in this method are all inorganic substances. The exposed edges of W ₂ C are controlled by controlling the amount of water added, and the particle size of W ₂ C crystals is controlled by controlling the heat treatment temperature. Control the morphology of the material while phase W ₂ C;

(2) The pure-phase W ₂ C nanoparticles prepared by this method have nearly dodecahedral morphology, and the particle size can be in the range of 100-500 nm;

(3) The method is simple and feasible, the preparation conditions are mild, and the environment is friendly, and the industrial mass production can be realized.

The following further examples are given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention, and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art according to the above content of the present invention belong to the present invention. scope of protection. The specific process parameters and the like in the following examples are only an example of a suitable range, that is, those skilled in the art can make selections within the suitable range through the description herein, and are not intended to be limited to the specific numerical values exemplified below.

Example 1

First, 5 mmol of ammonium tungstate (ie 15.2129 g) was dissolved in 20 mL of ethylene glycol solution, placed in a water bath at 80 °C and stirred for 8 h to obtain a uniform white solution. Then, 32 mmol of melamine (ie, 4.03584 g) was dissolved in another 20 mL of ethylene glycol solution, and deionized water with a volume of 0.4 times that of the ethylene glycol solution was added, and stirred in a water bath at 80 °C for 8 h to obtain a uniform white solution. The above two white solutions were mixed and stirred at 25°C for 1 h to obtain a white flocculent precipitate. After that, the white precipitate was collected by centrifugation, washed with ethanol three times, and dried in a vacuum drying oven at 70 °C for 12 h. Finally, the collected precipitates were heat-treated at 900° C. for 2 h in a 95% H ₂ /5% Ar atmosphere to obtain pure-phase W ₂ C nanomaterials, as shown by the W-2-900 line in the XRD pattern of FIG. 1 .

The particle size of the prepared material is about 200 nm (Table 1), and the morphology is nearly dodecahedron, as shown in the SEM pictures of c and d in Figure 3.

Example 2

Dissolve 32 mmol of melamine (ie, 4.03584 g) in 20 mL of ethylene glycol solution, and add deionized water whose volume is 0.2 times that of the ethylene glycol solution. Other operating conditions are the same as those in Example 1. The structure of the obtained material is pure phase W ₂ C, as shown by the W-1-900 line in the XRD pattern of Figure 1, the particle size is about 250 nm (Table 1), and the morphology is nearly dodecahedron, as shown in Figure 3 a , shown in the SEM image of b.

Example 3

Dissolve 32 mmol of melamine (ie, 4.03584 g) in 20 mL of ethylene glycol solution, and add deionized water whose volume is 0.6 times that of the ethylene glycol solution. Other operating conditions are the same as in Example 1. The structure of the obtained material is pure phase W ₂ C, as shown by the W-3-900 line in the XRD pattern of Figure 1, the particle size is about 500 nm (Table 1), and the morphology is uneven, as shown in Figure 3 e, f SEM pictures are shown.

Example 4

The collected precipitates were heat-treated at 950°C in a 95% H ₂ /5% Ar atmosphere for 2 hours. Other operations were the same as those in Example 1. The phase of the prepared material was W ₂ C, as shown in the XRD pattern of W-2- in Figure 2. As shown by the 950 spectral line, some of the grains grow significantly (particle size is 1-2 μm), and they are fully grown regular dodecahedral grains, as shown in the SEM photos of g and h in Figure 3.

Comparative Example 1

The collected precipitates were heat-treated at 850°C in 95% H ₂ /5% Ar atmosphere for 2 hours. Other operations were the same as those in Example 1. The phase of the prepared material was WO ₂ , as shown in the XRD pattern of W-2-850 in Figure 2. Spectrum is shown.

Comparative Example 2

32 mmol of melamine (ie, 4.03584 g) was dissolved in 20 mL of ethylene glycol solution, and deionized water whose volume was 0.1 times that of the ethylene glycol solution was added. Other operating conditions were the same as those in Example 1. The obtained material is an incompletely grown small nanoparticle with a particle size of 1-20 nm, as shown in the SEM photo of a in Figure 4 .

Comparative Example 3

32 mmol of melamine (ie, 4.03584 g) was dissolved in 20 mL of ethylene glycol solution, and deionized water whose volume was 0.8 times that of the ethylene glycol solution was added. Other operating conditions were the same as those in Example 1. The obtained material is a material in which large and small particles are cross-linked with each other, wherein the particle size of the small particles is 10-20 nm, and the particle size of the large particles is about 1 μm, which has a polygonal structure, as shown in the SEM picture of b in Figure 4 .

Particle size of pure phase W ₂ C nanomaterials prepared in Table 1

The materials obtained in the above examples and comparative examples were tested for hydrogen evolution electrocatalytic activity, and the test method was: three-electrode method (using Shanghai Chenhua CHI 760E electrochemical workstation). The test results are shown in Figure 5. When the water content is 0.4 times and the heat treatment temperature is 900 °C, the obtained pure phase polygonal W ₂ C has the best performance, and the overpotential in 0.5MH ₂ SO ₄ solution is about 220mV.

Claims (10)

1. a kind of preparation method of environment-friendly pure phase polygonal W ₂ C nanomaterial, is characterized in that, comprises the following steps: 1) Mix the tungsten source with ethylene glycol and stir evenly to obtain the first solution; 2) Mix the carbon source with ethylene glycol and water and stir evenly to obtain the second solution; 3) The first solution and the second solution are mixed and stirred evenly to obtain a suspension, and the solid is separated; 4) Heat treatment of the solid obtained in step 3) in a reducing atmosphere to obtain a pure-phase polygonal W ₂ C nanomaterial. 2 . The preparation method according to claim 1 , wherein the tungsten source is ammonium tungstate, and the carbon source is melamine. 3 . 3. The preparation method according to claim 1 or 2, wherein in step 1), the dosage ratio of the tungsten source to the ethylene glycol is: 0.1-10 L of ethylene glycol is used per mole of the tungsten source. 4. The preparation method according to any one of claims 1-3, characterized in that, in step 2), the consumption ratio of carbon source to ethylene glycol is: 0.1-10 L of ethylene glycol is used per mole of carbon source. 5 . The preparation method according to claim 1 , wherein in step 2), the volume of water is 0.2 to 0.6 times the volume of ethylene glycol. 6 . 6 . The preparation method according to claim 1 , wherein the molar ratio of the tungsten source to the carbon source is 1:10˜1:2. 7 . 7. The preparation method according to any one of claims 1-6, wherein in step 1), the stirring temperature is controlled at 50-100°C, preferably 60-80°C, and the stirring time is controlled at 1-10 h In step 2), the stirring temperature is controlled at 50-100 °C, preferably 60-80 °C, and the stirring time is controlled at 1-10 h; in step 3), the stirring temperature is controlled at 20-80 °C, preferably 20-40 °C ℃, and the stirring time was controlled at 1-4 h. 8 . The preparation method according to claim 1 , wherein in step 4), the reducing atmosphere is 95% H ₂ +5% Ar. 9 . 9 . The preparation method according to claim 1 , wherein in step 4), the heat treatment temperature is 860-960° C., and the heat treatment time is 1-4 hours. 10 . 10 . A pure-phase polygonal W ₂ C nanomaterial prepared by the preparation method according to any one of claims 1 to 9, wherein the particle size of the pure-phase polygonal W ₂ C nanomaterial is 100-2000 nm, It is preferably 100 to 500 nm and has a dodecahedral structure.

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