A method for preparing columnar crystal tungsten dioxide by hydrogen step reduction

Abstract

The application discloses a method for preparing columnar crystal tungsten dioxide through hydrogen step-by-step reduction, and belongs to the technical field of tungsten oxide material preparation. The method comprises the following steps: placing tungsten trioxide in a reduction reactor and purging under an inert atmosphere; then performing hydrogen two-step reduction, first performing first-step hydrogen reduction, i.e. under a mixed atmosphere composed of argon and hydrogen, keeping at 550-650 DEG C for 3-5 h, so that the tungsten trioxide is converted into fibrous purple tungsten; then performing second-step hydrogen reduction, i.e. under a hydrogen atmosphere, keeping at 700-780 DEG C for 2-4 h, so that the fibrous purple tungsten is converted into columnar crystal tungsten dioxide; and cooling under a protective atmosphere after the reduction is completed to obtain the columnar crystal tungsten dioxide. Through the synergic control of the two-step hydrogen reduction process, the chemical vapor transmission effect on the morphology is effectively inhibited, directional and controllable preparation of the columnar crystal tungsten dioxide is realized, the product has high purity and no impurity phase, the process is stable and reliable, and is easy to scale up.

Description

Technical Field

[0001] This invention relates to the field of tungsten oxide material preparation technology, and in particular to a method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen. Background Technology

[0002] Tungsten dioxide (WO2), as an important transition metal oxide, possesses a unique monoclinic crystal structure, combining metallic conductivity with reversible redox activity. It exhibits excellent application potential in fields such as smart windows, lithium-ion battery anodes, hydrogenation catalysts, and gas sensing. The morphology of WO2 significantly influences its performance. Columnar / fibrous WO2, due to its larger aspect ratio, higher specific surface area, and unobstructed ion transport channels, demonstrates superior performance in electrochromic response speed, catalytic activity, and sensing sensitivity compared to flat or granular WO2.

[0003] Currently, the main methods for preparing WO2 include hydrogen reduction, hydrothermal synthesis, and vapor deposition. Among these, hydrogen reduction has become the mainstream method for industrial and laboratory preparation of WO2 due to its simple process, readily available raw materials, and high product purity. Traditional hydrogen reduction methods often use WO3 (tungsten trioxide) as a precursor for one-step direct reduction to WO2, which has significant drawbacks: First, the reaction is difficult to control precisely, and over-reduction is very likely to occur, directly generating metallic tungsten (W) or forming a mixed phase of WO2 and W, resulting in an impure product phase; second, at higher temperatures (usually >800℃), significant chemical vapor transport (CVT) occurs, causing WO2 crystals to re-nucleate and grow, ultimately forming a more thermodynamically stable flat or equiaxed granular morphology, rather than obtaining a columnar crystal morphology with a high aspect ratio that is conducive to electron / ion transport. While there have been attempts to use stepwise reduction in existing technologies, these have mostly focused on increasing the reaction rate or preparing ultrafine powders, without solving the technical challenge of directionally preparing high-purity, high aspect ratio columnar WO2 crystals while avoiding over-reduction and gas phase migration.

[0004] Therefore, developing a stable and controllable method to prepare high-purity, uniformly morphological columnar WO2 by stepwise reduction of WO3 with hydrogen has become a pressing technical problem in this field. Summary of the Invention

[0005] The present invention aims to solve the technical problems in the existing WO2 preparation technology, such as easy over-reduction and difficulty in stably generating columnar WO2 with uniform morphology.

[0006] To achieve the above objectives, this invention provides a method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen. By precisely controlling the atmosphere, temperature, heating rate, hydrogen flow rate, and holding time in both reduction steps, the first step involves suppressing chemical vapor migration in a mixed atmosphere with a low hydrogen concentration, causing WO3 to undergo a solid-state transformation to directionally generate fibrous purple tungsten WO3. 2.72 As a morphological template; the second step involves promoting a topological transformation in a pure hydrogen atmosphere by controlling the reduction temperature and heating rate, resulting in fibrous WO3. 2.72 The morphological characteristics are inherited by WO2, and columnar WO2 with high aspect ratio, high purity and uniform morphology is finally obtained.

[0007] The technical solution of the present invention is as follows: This invention provides a method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen, comprising the following steps: Tungsten trioxide is first reduced by hydrogen in an inert gas mixture containing hydrogen to convert it into fibrous purple tungsten; then, a second reduction by hydrogen is performed in a hydrogen atmosphere to convert the fibrous purple tungsten into columnar crystalline tungsten dioxide, resulting in columnar crystalline tungsten dioxide as the product; wherein the temperature of the first reduction by hydrogen is lower than the temperature of the second reduction by hydrogen.

[0008] Preferably, the method for preparing columnar tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Place tungsten trioxide in a reduction reactor and purge it under an inert atmosphere; Step 2: Perform a two-step hydrogen reduction. First, perform the first step of hydrogen reduction: in a mixed atmosphere of argon and hydrogen, keep the temperature at 550-650℃ for 3-5 hours to convert tungsten trioxide into fibrous purple tungsten. Then, perform the second step of hydrogen reduction: in a hydrogen atmosphere, keep the temperature at 700-780℃ for 2-4 hours to convert fibrous purple tungsten into columnar crystalline tungsten dioxide. Step 3: After reduction, cool under a protective atmosphere to obtain columnar tungsten dioxide.

[0009] Preferably, the volume fraction of hydrogen in the mixed atmosphere is 30-50%.

[0010] Regarding further explanation of the present invention, in the prior art, the preparation of WO2 by hydrogen reduction typically involves one or two steps using pure hydrogen or high-concentration hydrogen (hydrogen gas fraction ≥ 80%). However, under high-concentration hydrogen reduction conditions, WO3 is reduced to WO. 2.72 and WO 2.72 During the reduction to WO2, the chemical vapor transport effect is significant, easily causing tungsten to migrate in gaseous form, disrupting the original crystal morphology and structure. The resulting WO2 is flat or granular, making it difficult to maintain the WO2 composition. 2.72The fibrous / columnar crystal morphology is observed. To address this, the present invention strictly controls the hydrogen gas fraction in the first-step reducing atmosphere to 30-50%, thus providing sufficient reducing power to completely convert WO3 into fibrous WO3. 2.72 On the other hand, by reducing the hydrogen concentration, the chemical vapor transport effect is suppressed, thus reducing WO3 to WO. 2.72 The morphological inheritance of the first stage was completely preserved. Based on this, the second reduction step used a pure hydrogen atmosphere but controlled the temperature to not exceed 780℃, ensuring the preservation of WO3. 2.72 The reduction is completely reduced to WO2, while avoiding over-reduction or morphological collapse due to excessive temperature. Through the coordinated control of the above two reduction steps, WO2... 2.72 The columnar crystal morphology was inherited by the final product WO2, realizing the preparation of regular columnar WO2 with an aspect ratio ≥10 by hydrogen reduction.

[0011] Preferably, in step 1, the purging step is argon purging for 30-60 minutes, with an argon flow rate of 800-1000 mL / min.

[0012] Preferably, in step 2, the heating rate of the first reduction step is 5-10℃ / min.

[0013] Preferably, in step 2, the total flow rate of the first-step mixing atmosphere is 600-1000 mL / min.

[0014] Preferably, in step 2, the heating rate of the second reduction step is 2-5℃ / min.

[0015] Preferably, in step 2, the hydrogen flow rate for the second reduction step is 800-1000 mL / min.

[0016] Preferably, in step 3, the cooling step involves cooling to below 150°C under hydrogen protection, and then switching to argon cooling to room temperature.

[0017] Preferably, the columnar tungsten dioxide powder is a single-phase tungsten dioxide with columnar grains and an aspect ratio ≥10.

[0018] The present invention has the following beneficial effects: 1. Compared with existing technologies, this invention provides a method for preparing columnar tungsten dioxide by stepwise reduction with hydrogen, achieving directional and controllable preparation of columnar tungsten dioxide. In the first step, the invention controls the hydrogen concentration and reduction rate in a mixed atmosphere of hydrogen and inert gas to suppress chemical vapor migration, allowing tungsten trioxide to undergo a solid-phase transformation to directionally generate fibrous purple tungsten as a morphological template. In the second step, in a pure hydrogen atmosphere, the reduction temperature and heating rate are controlled to promote a topological transformation, allowing the morphological characteristics of the fibrous purple tungsten to be inherited by the tungsten dioxide, ultimately obtaining columnar tungsten dioxide with a high aspect ratio. The tungsten dioxide product prepared by the method provided by this invention is a regular columnar crystal with a smooth and dense surface, free from damage or agglomeration, solving the defects of traditional methods where the product is flat or granular, the morphology is uncontrollable, and impurities are easily introduced.

[0019] 2. Compared to existing technologies, the method provided by this invention avoids excessive reduction and the formation of impurity phases, and can prepare high-purity columnar tungsten dioxide. This invention precisely controls the temperature and atmosphere of the two-step reduction process. In the first step, tungsten trioxide is completely converted to tungsten oxide; in the second step, tungsten oxide is fully converted to tungsten dioxide. The product, after testing, is a single-phase tungsten dioxide, without any residual impurity phases such as tungsten oxide, tungsten trioxide, or metallic tungsten. The process of this invention is simple, stable, and reliable, and can be achieved using a conventional tube furnace. It does not require a fluidized bed vibrating device or carbon nanoparticle additives, is easy to operate, has good repeatability, and is easy to scale up for production. Attached Figure Description

[0020] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0021] Figure 1 The intermediate product fibrous purple tungsten (WO) obtained in Example 1 of this invention 2.72 SEM image; Figure 2 The intermediate product fibrous purple tungsten (WO) obtained in Example 1 of this invention 2.72 XRD pattern; Figure 3 This is a SEM image of the columnar WO2 powder obtained in Example 1 of the present invention; Figure 4 The image shows the XRD pattern of columnar WO2 powder obtained in Example 1 of this invention. Figure 5 This is a SEM image of the product obtained in Comparative Example 1 of the present invention. Figure 6This is a SEM image of the product obtained in Comparative Example 2 of the present invention. Figure 7 This is a SEM image of the product obtained in Comparative Example 3 of the present invention. Figure 8 The image shows the XRD pattern of the product obtained in Comparative Example 3 of this invention. Figure 9 This is a SEM image of the product obtained in Comparative Example 4 of the present invention.

[0022] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0023] The technical solutions described below in conjunction with the embodiments will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention provides a method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen, comprising the following steps: Tungsten trioxide is first reduced by hydrogen in an inert gas mixture containing hydrogen to convert it into fibrous purple tungsten; then, a second reduction by hydrogen is performed in a hydrogen atmosphere to convert the fibrous purple tungsten into columnar crystalline tungsten dioxide, resulting in columnar crystalline tungsten dioxide as the product; wherein the temperature of the first reduction by hydrogen is lower than the temperature of the second reduction by hydrogen.

[0025] Preferably, the method for preparing columnar tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Place tungsten trioxide in a reduction reactor and purge it under an inert atmosphere; Step 2: Perform a two-step hydrogen reduction. First, perform the first step of hydrogen reduction: in a mixed atmosphere of argon and hydrogen, keep the temperature at 550-650℃ for 3-5 hours to convert tungsten trioxide into fibrous purple tungsten. Then, perform the second step of hydrogen reduction: in a hydrogen atmosphere, keep the temperature at 700-780℃ for 2-4 hours to convert fibrous purple tungsten into columnar crystalline tungsten dioxide. Specifically, in the first step of hydrogen reduction, the reduction temperature can be any one of 550℃, 580℃, 600℃, 620℃, 650℃ or a range between two of these, and the holding time can be any one of 3h, 3.5h, 4h, 4.5h, 5h or a range between two of these; in the second step of hydrogen reduction, the reduction temperature can be any one of 700℃, 720℃, 750℃, 760℃, 780℃ or a range between two of these, and the holding time can be any one of 2h, 2.5h, 3h, 3.5h, 4h or a range between two of these. Step 3: After reduction, cool under a protective atmosphere to obtain columnar tungsten dioxide.

[0026] Preferably, the volume fraction of hydrogen in the mixed atmosphere is 30-50%.

[0027] Specifically, the volume fraction of hydrogen in the mixed atmosphere can be any one of 30%, 35%, 40%, 45%, 50%, or a range between two of them.

[0028] Preferably, in step 1, the purging step is argon purging for 30-60 minutes, with an argon flow rate of 800-1000 mL / min.

[0029] Specifically, the argon purging time can be any one of 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or a range between two of them, and the argon flow rate can be any one of 800 mL / min, 850 mL / min, 900 mL / min, 950 mL / min, 1000 mL / min, or a range between two of them.

[0030] Preferably, in step 2, the heating rate of the first reduction step is 5-10℃ / min.

[0031] Specifically, the heating rate of the first reduction step can be any one of 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, or a range between two of them.

[0032] Preferably, in step 2, the total flow rate of the first-step mixing atmosphere is 600-1000 mL / min.

[0033] Specifically, the total flow rate of the mixing atmosphere in the first step can be any one of 600 mL / min, 650 mL / min, 700 mL / min, 750 mL / min, 800 mL / min, 850 mL / min, 900 mL / min, 950 mL / min, 1000 mL / min, or a range between two of them.

[0034] Preferably, in step 2, the heating rate of the second reduction step is 2-5℃ / min.

[0035] Specifically, the heating rate of the second reduction step can be any one of 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, or a range between two of them.

[0036] Preferably, in step 2, the hydrogen flow rate for the second reduction step is 800-1000 mL / min.

[0037] Specifically, the hydrogen flow rate for the second step of reduction can be any one of 800 mL / min, 850 mL / min, 900 mL / min, 950 mL / min, 1000 mL / min, or a range between two of them.

[0038] Preferably, in step 3, the cooling step involves cooling to below 150°C under hydrogen protection, and then switching to argon cooling to room temperature.

[0039] Preferably, the columnar tungsten dioxide powder is a single-phase tungsten dioxide with columnar grains and an aspect ratio ≥10.

[0040] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0041] Example 1 A method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Spread WO3 evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 900 mL / min for 45 min. Step 2: After purging, the tube furnace is heated for two-step hydrogen reduction. The first reduction process is as follows: a mixed atmosphere consisting of 60% argon and 40% hydrogen is introduced, maintaining a total flow rate of 1000 mL / min. Under this mixed atmosphere, the tube furnace is heated to 600℃ at a heating rate of 8℃ / min and held at that temperature for 4 hours. This first hydrogen reduction completely converts WO3 into the intermediate product fibrous purple tungsten WO3. 2.72The second reduction process is as follows: Hydrogen gas is introduced, maintaining a flow rate of 1000 mL / min. Under a hydrogen atmosphere, the tube furnace is heated to 750°C at a rate of 3°C / min and held at that temperature for 3 hours. This second hydrogen reduction process produces the intermediate product fibrous tungsten WO3. 2.72 It transforms into columnar WO2 crystals; Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously flowing in until the furnace temperature cools naturally to no higher than 150°C. Then switch to argon gas. After the furnace temperature cools to room temperature, the product obtained is columnar crystalline WO2 powder. The columnar crystalline WO2 powder has a length of 8.3 μm, a diameter of 0.7 μm, and an aspect ratio of 11.43.

[0042] The intermediate product fibrous purple tungsten (WO) obtained in this embodiment 2.72 SEM image as follows Figure 1 As shown, the intermediate product is fibrous purple tungsten WO3. 2.72 XRD pattern as shown Figure 2 As shown; by Figure 1 It can be seen that WO 2.72 It exhibits a regular fibrous morphology, with a complete crystal structure, uniform size distribution, and no obvious morphological damage or granulation; Figure 2 It can be seen that WO3 has been fully restored to the pure form WO 2.72 The product has a single phase and high purity, which provides a good phase basis for the subsequent preparation of columnar WO2 by topological transformation; The SEM image of the columnar WO2 powder obtained in this embodiment is shown below. Figure 3 As shown, the XRD pattern of the columnar WO2 powder product is as follows. Figure 4 As shown; by Figure 3 It can be seen that WO2 exhibits a regular columnar crystal morphology, with a complete crystal structure, uniform size distribution, and no obvious morphological damage or aggregation; from Figure 4 It can be seen that columnar WO2 has no impurity characteristic peaks and has high phase purity.

[0043] Example 2 A method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Spread WO3 evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 800 mL / min for 60 min. Step 2: After purging, the tube furnace is heated for two-step hydrogen reduction. The first reduction process is as follows: a mixed atmosphere consisting of 70% argon and 30% hydrogen is introduced, maintaining a total flow rate of 600 mL / min. Under this mixed atmosphere, the tube furnace is heated to 550°C at a heating rate of 5°C / min and held at that temperature for 6 hours. This first hydrogen reduction completely converts WO3 into the intermediate product fibrous purple tungsten WO3. 2.72 The second reduction process is as follows: hydrogen gas is introduced and maintained at a flow rate of 800 mL / min. Under the hydrogen atmosphere, the tube furnace is heated to 700°C at a heating rate of 2°C / min and held at that temperature for 4 hours. The second hydrogen reduction process produces the intermediate product fibrous tungsten WO3. 2.72 It transforms into columnar WO2 crystals; Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously flowing in until the furnace temperature naturally cools down to no more than 150°C. Then switch to argon gas. After the furnace temperature cools down to room temperature, the product obtained is columnar crystalline WO2 powder. The columnar crystalline WO2 powder has a length of 6.3 μm, a diameter of 0.6 μm, and an aspect ratio of 10.50.

[0044] Example 3 A method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Spread WO3 evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 1000 mL / min for 30 min. Step 2: After purging, the tube furnace is heated for two-step hydrogen reduction. The first reduction process is as follows: a mixed atmosphere consisting of 50% argon and 50% hydrogen is introduced, maintaining a total flow rate of 1000 mL / min. Under this mixed atmosphere, the tube furnace is heated to 650°C at a heating rate of 10°C / min and held at that temperature for 3 hours. This first hydrogen reduction completely converts WO3 into the intermediate product fibrous purple tungsten WO3. 2.72 The second reduction process is as follows: Hydrogen gas is introduced, maintaining a flow rate of 1000 mL / min. Under a hydrogen atmosphere, the tube furnace is heated to 780°C at a rate of 5°C / min and held for 2 hours. This second hydrogen reduction process produces the intermediate product fibrous tungsten WO3. 2.72 It transforms into columnar WO2 crystals; Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously flowing in until the furnace temperature cools naturally to no higher than 150°C. Then switch to argon gas. After the furnace temperature cools to room temperature, the product obtained is columnar crystalline WO2 powder. The columnar crystalline WO2 powder has a length of 10.3 μm, a diameter of 0.8 μm, and an aspect ratio of 12.9.

[0045] Comparative Example 1 A method for preparing tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Spread WO3 evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 900 mL / min for 45 min. Step 2: After purging, the tube furnace is heated for two-step hydrogen reduction. The first reduction process is as follows: a mixed atmosphere consisting of 40% argon and 60% hydrogen is introduced, maintaining a total flow rate of 1000 mL / min. Under this mixed atmosphere, the tube furnace is heated to 600℃ at a heating rate of 8℃ / min and held for 4 hours. This first hydrogen reduction completely converts WO3 into the intermediate product fibrous purple tungsten WO3. 2.72 The second reduction process is as follows: Hydrogen gas is introduced, maintaining a flow rate of 1000 mL / min. Under a hydrogen atmosphere, the tube furnace is heated to 750°C at a rate of 3°C / min and held at that temperature for 3 hours. This second hydrogen reduction process produces the intermediate product fibrous tungsten WO3. 2.72 It transforms into columnar WO2 crystals; Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously introduced until the furnace temperature cools down naturally to no higher than 150°C. Then switch to argon gas and wait for the furnace temperature to cool down to room temperature to obtain WO2 powder, which is a mixture of flat and columnar crystals with columnar crystals as the main component.

[0046] The SEM image of the product obtained in this comparative example is shown below. Figure 5 As shown, by Figure 5 It can be seen that the columnar crystal morphology of some products is destroyed, and a large number of spherical granular products appear, and the morphological integrity and uniformity are significantly reduced.

[0047] Comparative Example 2 A method for preparing tungsten dioxide by stepwise reduction with hydrogen includes the following steps: Step 1: Spread WO3 evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 900 mL / min for 45 min. Step 2: After purging, the tube furnace is heated for two-step hydrogen reduction. The first reduction process is as follows: a mixed atmosphere consisting of 60% argon and 40% hydrogen is introduced, maintaining a total flow rate of 1000 mL / min. Under this mixed atmosphere, the tube furnace is heated to 600℃ at a heating rate of 8℃ / min and held at that temperature for 4 hours. This first hydrogen reduction completely converts WO3 into the intermediate product fibrous purple tungsten WO3. 2.72The second reduction process is as follows: Hydrogen gas is introduced, maintaining a flow rate of 1000 mL / min. Under a hydrogen atmosphere, the tube furnace is heated to 800°C at a rate of 3°C / min and held at that temperature for 3 hours. This second hydrogen reduction process produces the intermediate product fibrous tungsten WO3. 2.72 It transforms into columnar WO2 crystals; Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously introduced until the furnace temperature cools down naturally to no higher than 150°C. Then switch to argon gas and wait for the furnace temperature to cool down to room temperature to obtain WO2 powder, which is a mixture of flat and columnar crystals with flat crystals as the main component.

[0048] The SEM image of the product obtained in this comparative example is shown below. Figure 6 As shown, by Figure 6 It can be seen that most of the one-dimensional structure in the product has disappeared, forming a large number of irregular granular products, with only a small number of incomplete needle-like structures remaining, and the morphological integrity and uniformity are severely reduced.

[0049] Comparative Example 3 A method for preparing tungsten dioxide by hydrogen reduction includes the following steps: Step 1: Spread WO3 evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 900 mL / min for 45 min. Step 2: After purging, the tube furnace is heated for hydrogen reduction. The hydrogen reduction process is as follows: hydrogen is introduced and the hydrogen flow rate is maintained at 1000 mL / min. The tube furnace is heated to 750°C in a hydrogen atmosphere at a heating rate of 8°C / min and held at that temperature for 7 hours. Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously introduced until the furnace temperature cools down naturally to no higher than 150°C. Then switch to argon gas and wait for the furnace temperature to cool down to room temperature. The product obtained is a mixture of flat WO2 powder and tungsten powder.

[0050] The SEM image of the product obtained in this comparative example is shown below. Figure 7 As shown, the XRD pattern of the obtained product is as follows. Figure 8 As shown; by Figure 7 It can be seen that the product has completely lost its one-dimensional structural characteristics, consisting entirely of irregular granular products with no obvious columnar crystal morphology residue; from Figure 8 It can be seen that WO2 underwent excessive reduction, generating a metallic tungsten impurity phase. The product was a mixed phase of WO2 and metallic tungsten, with a significant decrease in purity, making it impossible to obtain a single-phase WO2 product.

[0051] Comparative Example 4 A method for preparing tungsten dioxide by hydrogen reduction includes the following steps: Step 1: Take fibrous purple tungsten (WO4) 2.72Spread evenly in a corundum boat dish and place it in a tube furnace; then purge with argon gas at a flow rate of 900 mL / min for 45 min; Step 2: After purging, the tube furnace is heated for hydrogen reduction. The hydrogen reduction process is as follows: hydrogen is introduced and the hydrogen flow rate is maintained at 1000 mL / min. The tube furnace is heated to 750°C in a hydrogen atmosphere at a heating rate of 8°C / min and held at that temperature for 3 hours. Step 3: After the reduction is complete, turn off the heating device and keep the hydrogen atmosphere continuously introduced until the furnace temperature cools down naturally to no higher than 150°C. Then switch to argon gas and wait for the furnace temperature to cool down to room temperature to obtain columnar crystal fragmented WO2 powder.

[0052] The SEM image of the product obtained in this comparative example is shown below. Figure 9 As shown, by Figure 9 It can be seen that the one-dimensional structure of the product has been severely damaged, and the crystal exhibits a porous and broken morphology, with an overall loose and irregular morphology.

[0053] In summary, this invention provides a method for preparing columnar tungsten dioxide by stepwise hydrogen reduction. The method uses tungsten trioxide as raw material and achieves the directional preparation of columnar tungsten dioxide through stepwise hydrogen reduction. The first step of the hydrogen reduction involves reduction in a mixed atmosphere of hydrogen and an inert gas, controlling the hydrogen concentration and reduction rate to transform tungsten trioxide into fibrous purple tungsten via a solid-phase transformation, serving as a morphological template. The second step involves reduction in a pure hydrogen atmosphere, controlling the reduction temperature and heating rate to transform the fibrous purple tungsten into columnar tungsten dioxide via a topological transformation. The resulting product inherits the morphological characteristics of the fibrous purple tungsten. Compared with existing technologies, this invention achieves directional and controllable preparation of columnar tungsten dioxide, yielding regular columnar crystals with a smooth and dense surface, free from damage or agglomeration. It also avoids over-reduction and impurity phase formation, producing a single-phase tungsten dioxide product with high purity. Furthermore, the process is stable and reliable, simple to operate, highly reproducible, and easily scalable for mass production.

[0054] The above description is merely a preferred embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing columnar crystalline tungsten dioxide by stepwise reduction with hydrogen, characterized in that, Includes the following steps: Tungsten trioxide is first reduced by hydrogen in an inert gas mixture containing hydrogen to convert it into fibrous purple tungsten; then a second reduction by hydrogen is carried out in a hydrogen atmosphere to convert the fibrous purple tungsten into columnar crystalline tungsten dioxide, and the product is columnar crystalline tungsten dioxide; wherein the temperature of the first reduction by hydrogen is lower than the temperature of the second reduction by hydrogen. Specifically, the following steps are included: Step 1: Place tungsten trioxide in a reduction reactor and purge it under an inert atmosphere; Step 2: Perform a two-step reduction with hydrogen. First, perform the first reduction: in a mixed atmosphere of argon and hydrogen, keep the temperature at 550-650℃ for 3-5 hours to convert tungsten trioxide into fibrous purple tungsten. Then, perform the second reduction: in a hydrogen atmosphere, keep the temperature at 700-780℃ for 2-4 hours to convert fibrous purple tungsten into columnar crystalline tungsten dioxide. Step 3: After reduction, cool under a protective atmosphere to obtain columnar tungsten dioxide. The volume fraction of hydrogen in the mixed atmosphere is 30-50%. In step 2, the heating rate of the first reduction step is 5-10℃ / min; In step 2, the total flow rate of the mixed atmosphere is 600-1000 mL / min; In step 2, the heating rate of the second reduction step is 2-5℃ / min; In step 2, the hydrogen flow rate for the second reduction step is 800-1000 mL / min.

2. The method for preparing columnar tungsten dioxide by stepwise reduction with hydrogen according to claim 1, characterized in that, In step 1, the purging step is argon purging for 30-60 minutes, with an argon flow rate of 800-1000 mL / min.

3. The method for preparing columnar tungsten dioxide by stepwise reduction with hydrogen according to claim 1, characterized in that, In step 3, the cooling step involves cooling to below 150°C under hydrogen protection, and then switching to argon cooling to room temperature.

4. The method for preparing columnar tungsten dioxide by stepwise reduction with hydrogen according to claim 1, characterized in that, The columnar tungsten dioxide is a single-phase tungsten dioxide with columnar grains and an aspect ratio ≥10.

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