H / WO3-x material as well as preparation method and application thereof

Abstract

The invention belongs to the field of photocatalysts, and particularly relates to an H / WO3-x material and a preparation method and application thereof.The preparation method comprises the steps that ammonium paratungstate and urea are mixed and ground, the obtained mixture is heated, calcined, cooled and then ground, and WO3-x powder is obtained; dispersing the obtained WO3-x powder in deionized water, performing ultrasonic treatment, then adding a sodium borohydride solution, and performing stirring reaction; and washing a solid product obtained after the reaction with deionized water, carrying out suction filtration, and carrying out vacuum freeze drying to obtain the H / WO3-x material. The H / WO3-x material prepared by the invention shows excellent formaldehyde generation selectivity in photocatalytic methane oxidation reaction, the selectivity is close to 100%, and the generation of excessive oxidation products such as CO or CO2 can be effectively inhibited. The catalyst still keeps stable structure and high activity after being continuously recycled, and has good reusability.

Description

Technical Field

[0001] This invention belongs to the field of photocatalysts, specifically relating to an H / WO3 catalyst. 3-x Materials, their preparation methods, and applications. Background Technology

[0002] Methane (CH4), a potent greenhouse gas, has a global warming potential approximately 28 times that of carbon dioxide (CO2) on a centennial timescale. With the continued increase in fossil fuel consumption, anthropogenic methane emissions are becoming increasingly severe. Therefore, developing efficient and green methane conversion and resource utilization technologies has become a major issue in the energy and environmental fields. Among numerous conversion pathways, photocatalytic selective oxidation of methane to formaldehyde has attracted much attention due to its ability to directly convert methane into high-value-added chemicals under mild conditions, and is considered a green catalytic route with broad application prospects. However, this technology still faces a key scientific challenge in practical applications: the C–H bond energy in the formaldehyde molecule (approximately 368 kJ / mol) is much lower than that in the methane molecule (approximately 435 kJ / mol). This makes formaldehyde more susceptible to continuous dehydrogenation and deep oxidation during the reaction, leading to the generation of low-value byproducts such as carbon monoxide (CO) or carbon dioxide (CO2), severely limiting the selective yield of formaldehyde. To suppress over-oxidation, traditional methods often rely on strongly acidic reaction media or high-valence oxidants (such as hydrogen peroxide and permanganates) to block further C–H bond breaking. These strategies are not only complex and costly to operate, but can also lead to equipment corrosion and secondary pollution, hindering their economic viability and environmental sustainability for large-scale application. Therefore, developing a highly selective photocatalytic system that does not rely on strong acid environments or expensive oxidants has become a key bottleneck in advancing this technology towards practical application.

[0003] In recent years, hydrogenation modification, as a surface regulation strategy, has been proven to effectively improve the selectivity of photocatalytic reactions. However, systematic research on the application of hydrogenation strategies in photocatalytic methane conversion is still lacking, especially the regulatory mechanism of the degree of hydrogenation on the reaction pathway remains unclear. Based on this, we have developed a hydrogenated WO3 that can generate and precisely utilize active hydrogen species in situ. 3-x The catalytic material uses a "sacrificial bond"—a WH bond with a bond energy lower than the CH bond in formaldehyde—to block the over-oxidation pathway. Simultaneously, it utilizes the broken hydrogen species to reverse-reduc the over-oxidation intermediate, achieving 100% selective methane production to formaldehyde. This has become a highly promising strategy to overcome the current selective bottleneck in photocatalytic methane-to-formaldehyde conversion, providing a new approach and technological direction for achieving highly selective methane conversion without the need for external strong acids or high-valent oxidants. Summary of the Invention

[0004] This invention aims to overcome the problems of low product selectivity and poor stability in existing photocatalytic methane conversion technologies, and provides an H / WO3... 3-x Materials, their preparation methods, and applications.

[0005] This invention is achieved through the following technical solution: Firstly, a type of H / WO 3-x The method for preparing the material includes the following steps: (1) Ammonium paratungstate and urea were mixed and ground, the resulting mixture was heated and calcined, cooled and then ground to obtain WO3. 3-x powder; (2) Take the WO obtained in step (1) 3-x The powder was dispersed in deionized water, sonicated, and then sodium borohydride solution was added and the mixture was stirred to react. (3) The solid product obtained after the reaction in step (2) is washed with deionized water, filtered, and then freeze-dried under vacuum to obtain H / WO 3-x Material.

[0006] As a further improvement to the technical solution of the present invention, the molar ratio of urea to ammonium paratungstate in step (1) is 25-175:1.

[0007] As a further improvement to the technical solution of the present invention, the specific operation of heating and calcining in step (1) is as follows: heating to 400°C at a rate of 2°C / min and holding for 35 min, and then heating to 650°C at a rate of 1°C / min for calcining for 5 hours.

[0008] As a further improvement to the technical solution of the present invention, in step (2), WO 3-x The mass ratio of powder to sodium borohydride was 0.3:0.01-0.02, and the stirring reaction time was 2.5 h.

[0009] As a further improvement to the technical solution of the present invention, in step (3), the vacuum freeze-drying time is 12 hours.

[0010] Secondly, an H / WO3 prepared by the above preparation method 3-x Material.

[0011] Thirdly, H / WO prepared by the above-mentioned preparation method 3-x Application of materials as photocatalysts in the photocatalytic oxidation of methane to formaldehyde.

[0012] As a further improvement to the application technology solution of the present invention, the specific method of the application includes: H / WO 3-xThe material is added to water and placed in a high-pressure reactor. The air inside the reactor is replaced with a mixture of methane and oxygen to maintain the reaction pressure at 0.1-3 MPa. The reactor is then irradiated with a light source with a wavelength of 300-850 nm to catalyze the oxidation of methane to formaldehyde.

[0013] As a further improvement to the application technology of the present invention, the volume ratio of methane to oxygen is 1.3:0.7.

[0014] As a further improvement to the application technology of this invention, the formaldehyde concentration is detected by the acetylacetone colorimetric method. Specifically, the reaction solution and the acetylacetone solution are mixed at a volume ratio of 1:1 and reacted at 35°C for 60 min. The absorbance at a wavelength of 412 nm is measured using a UV-Vis spectrophotometer, and the formaldehyde yield is calculated based on the standard curve of formaldehyde concentration versus absorbance.

[0015] This invention provides an H / WO 3-x Compared with existing technologies, the materials, their preparation methods, and applications have the following advantages: (1) Excellent catalytic performance, high selectivity and good stability: Compared with traditional TiO2 (P25) and unhydrogenated WO4 3-x Catalyst, H / WO prepared in this invention 3-x The material exhibits excellent formaldehyde formation selectivity in the photocatalytic methane oxidation reaction, with a selectivity approaching 100%, and effectively inhibits the formation of over-oxidation products such as CO or CO2. The catalyst maintains structural stability and high activity even after continuous cycling, demonstrating good reusability.

[0016] (2) Wide light absorption range and high solar energy utilization efficiency: The catalyst has a significant visible light response capability, and the light absorption band edge can be extended to 650 nm. It has a narrow band gap (about 2.72 eV), which greatly improves the efficiency of capturing and utilizing sunlight.

[0017] (3) The preparation process is simple, safe and easy to scale up: The present invention uses a two-step method to prepare H / WO3 by urea-assisted calcination combined with wet hydrogenation of sodium borohydride solution. 3-x The process is simple, the reaction conditions are mild, the operating parameters are easy to control, and there is no need for complex equipment or high-risk high-pressure hydrogen environment. It has better safety and controllability and is suitable for large-scale preparation.

[0018] (4) Surface hydrogen bonds regulate the reaction pathway and inhibit excessive oxidation: by controlling the reaction pathway through WO3 3-xBy introducing W–H bonds on the surface, which have lower bond energies than C–H bonds in formaldehyde, they can act as “sacrificial bonds” to preferentially react with holes or strong oxide species in the reaction, effectively blocking the continuous dehydrogenation pathway of formaldehyde. At the same time, the active hydrogen species generated after the W–H bonds break can participate in the reverse reduction reaction, converting over-oxidized intermediates (such as formic acid) back into formaldehyde, thereby synergistically improving the reaction selectivity.

[0019] (4) Environmentally friendly and requires no harsh reagents: The reaction system of this invention uses water as solvent and oxygen as oxidant. It can achieve highly selective conversion of methane under normal temperature and pressure conditions, avoiding the use of strong acid media or high-valence oxidants, and has good environmental compatibility and green process. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0021] 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, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 The H / WO prepared in Example 1 3-x X-ray diffraction (XRD) pattern of photocatalyst.

[0023] Figure 2 The H / WO prepared in Example 1 3-x Scanning electron microscope (SEM) image of a photocatalyst.

[0024] Figure 3 The H / WO prepared in Example 1 3-x WO prepared in Comparative Example 1 3-x Comparison of UV-Vis DRS spectra of photocatalysts.

[0025] Figure 4 The H / WO prepared in Example 1 3-x Compared with the WO prepared in Comparative Example 1 3-x Comparison of infrared (IR) spectra of photocatalysts.

[0026] Figure 5 The H / WO prepared in Example 1 3-x WO prepared in Comparative Example 1 3-x Figure showing the test results of the hydrogen release performance of the photocatalyst.

[0027] Figure 6 The H / WO prepared in Example 1 3-x WO prepared in Comparative Example 1 3-x Comparison of product distribution in the photocatalytic reaction of methane to formaldehyde.

[0028] Figure 7 The H / WO prepared in Example 1 3-x WO prepared in Comparative Example 1 3-x A schematic diagram of the test results for the generation of ·OOH active species by the photocatalyst.

[0029] Figure 8 The H / WO prepared in Example 2 3-x Figure showing the results of the cycle stability test of the photocatalyst. Detailed Implementation

[0030] To better understand the above-mentioned objectives, features, and advantages of the present invention, the solutions of the present invention will be further described below. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other.

[0031] Many specific details are set forth in the following description in order to provide a full understanding of the invention, but the invention may also be practiced in other ways different from those described herein; obviously, the embodiments in the specification are only some embodiments of the invention, and not all embodiments.

[0032] This invention provides an H / WO 3-x A specific embodiment of the material preparation method includes the following steps: (1) Ammonium paratungstate and urea were mixed and ground, the resulting mixture was heated and calcined, cooled and then ground to obtain WO3. 3-x powder; (2) Take the WO obtained in step (1) 3-x The powder was dispersed in deionized water, sonicated, and then sodium borohydride solution was added and the mixture was stirred to react. (3) The solid product obtained after the reaction in step (2) is washed with deionized water, filtered, and then freeze-dried under vacuum to obtain H / WO 3-x Material.

[0033] In step (1), the mixing and grinding of ammonium paratungstate and urea is carried out at room temperature, and the molar ratio of urea to ammonium paratungstate is 25-175:1. In one example provided by the present invention, specific ratios of 25:1, 50:1, 75:1, 100:1, 125:1, 150:1, and 175:1 can be selected, or any molar ratio between any two of the above-mentioned specified values ​​can be used, such as 25-50:1, 100-125:1, etc.

[0034] Preferably, the specific operation of heating and calcining in step (1) is as follows: heat up to 400°C at a rate of 2°C / min and hold for 35 min, then heat up to 650°C at a rate of 1°C / min and calcine for 5 hours.

[0035] In another example provided by the present invention, in step (2), WO 3-x The mass ratio of powder to sodium borohydride is 0.3:0.01-0.02, and the reaction time is 2.5 h. The concentration of the sodium borohydride solution can be selected from 0.05 mmol / L, 0.1 mmol / L, 0.15 mmol / L, or 0.2 mmol / L. Preferably, the concentration of the sodium borohydride solution is set to 0.1 mmol / L.

[0036] In one example provided by the present invention, in step (3), the vacuum freeze-drying time is 12 hours.

[0037] The present invention also provides an H / WO prepared by the above preparation method. 3-x Material.

[0038] Furthermore, the present invention provides H / WO prepared by the above preparation method. 3-x Application of materials as photocatalysts in the photocatalytic oxidation of methane to formaldehyde.

[0039] Specifically, the specific method of the application includes: H / WO 3-x The material is added to water and placed in a high-pressure reactor. The air inside the reactor is replaced with a mixture of methane and oxygen to maintain the reaction pressure at 0.1-3 MPa. The reactor is then irradiated with a light source with a wavelength of 300-850 nm to catalyze the oxidation of methane to formaldehyde.

[0040] The volume ratio of methane to oxygen is 1.3:0.7.

[0041] Furthermore, the formaldehyde concentration was detected using the acetylacetone colorimetric method. Specifically, the reaction solution was mixed with the acetylacetone solution at a 1:1 volume ratio, and reacted at 35°C for 60 min. The absorbance at a wavelength of 412 nm was measured using a UV-Vis spectrophotometer, and the formaldehyde yield was calculated based on the standard curve of formaldehyde concentration versus absorbance.

[0042] To verify the H / WO described in this invention 3-x The material, acting as a photocatalyst, can promote the formation of •OOH under the aforementioned catalytic environment. This invention further discloses a specific method for detecting •OOH: (1) Add H / WO to methanol 3-x Photocatalyst, with the concentration of the catalyst in methanol controlled at 5~30 mg / mL; (2) Add DMPO (as a •OOH scavenger) to the mixed solution in step (1) and maintain the DMPO concentration at 3~5 mmol / L. Then purge the solution with oxygen for 10~30 min. (3) Transfer the processed solution from step (2) into a capillary tube, place the capillary tube into a special tube for EPR testing, install the special tube into the EPR detection instrument, irradiate the reaction system with a light source of wavelength 300~850 nm, and collect the EPR signal at the set illumination time to complete the detection of •OOH.

[0043] The specific embodiments of the present invention will be described in further detail below. Example 1

[0044] 1) At room temperature, urea and ammonium paratungstate were thoroughly ground at a molar ratio of 100:1 to ensure uniform mixing and obtain a mixed powder. This mixed powder was placed in a covered crucible and calcined in a muffle furnace in stages: first, it was heated to 400℃ at a heating rate of 2℃ / min and held for 35 min; then, it was heated to 650℃ at a heating rate of 1℃ / min and calcined at this temperature for 5 h. After calcination, it was allowed to cool naturally to room temperature, and the product was ground into a fine powder to obtain WO3. 3-x .

[0045] 2) Take the WO prepared above 3-x 0.3 g was dispersed in 45 mL of deionized water and sonicated for 20 min to ensure uniform dispersion of the powder. Under continuous stirring, 5 mL of 0.1 mmol / L sodium borohydride solution was added to the dispersion, and the reaction was continued for 2.5 h.

[0046] 3) The reaction system was then centrifuged to collect the solid product. The solid product was repeatedly washed with deionized water, and then filtered to remove residual impurities. Finally, the solid obtained by filtration was placed in a freeze dryer to dry, thus obtaining H / WO3. 3-x Materials (relevant characterization results such as) Figure 1 , Figure 2 , Figure 3 (As shown).

[0047] H / WO 3-x The material was used for photocatalytic methane oxidation. The specific reaction conditions were as follows: The catalyst and water were mixed at a mass-to-volume ratio of 2 g / L. The mixture was added to a micro high-pressure reactor. The air inside the reactor was repeatedly replaced with a mixture of methane and oxygen (volume ratio 13:7) until the pressure inside the reactor reached 2 MPa. Finally, the reaction system was irradiated with a light source with a wavelength of 300–850 nm. The reaction produced formaldehyde (results are shown in the figure). Figure 6 (As shown).

[0048] For H / WO 3-x The detection conditions and procedures for reactive oxygen species during photocatalysis are as follows: Under light-protected conditions, add 5 mg of H / WO3 to 200 μL of methanol. 3-x The photocatalyst was ultrasonically treated to ensure uniform dispersion, resulting in a suspension. 2 μL of DMPO (as a scavenger of •OOH) was added to this suspension, and oxygen was purged for 10–30 min. The treated solution was then loaded into a capillary tube, which was placed in a dedicated EPR testing tube. This tube was then connected to an EPR detection instrument, and the capillary tube was irradiated with a light source of 300–850 nm. The corresponding EPR signal was acquired after 3 min of illumination (specific detection results are shown in the figure). Figure 7 (As shown). Experimental Example 1

[0049] Using the preparation method of Example 1, WO3 was treated with 0 mmol / L sodium borohydride solution. 3-x (i.e., sodium borohydride is not added in step 2), and other reaction conditions and operations are consistent with those in Example 1 above, thus obtaining WO3. 3-x Materials, results obtained are as follows Figure 4 , Figure 5 As shown.

[0050] The material characterization results are as follows: Figure 1 The X-ray powder diffraction (XRD) pattern shows that the prepared sample is a monoclinic H / WO3 crystal. 3-x . Figure 2 Scanning electron microscope (SEM) images show that the synthesized H / WO 3-x The photocatalyst has a spherical structure with fluffy pores distributed on its surface. Figure 3 The UV-Vis diffuse reflectance spectrum indicates that H / WO 3-x The photocatalyst exhibits an extremely wide light absorption range, with the absorption band extending to 570 nm, a theoretical light utilization efficiency as high as 42.3%, and a band gap of 2.72 eV calculated using the Kubelka-Munk function. Figure 4 and Figure 5 It is evident that WH bonds were formed after hydrogenation, and hydrogen gas was detected during the hydrogen release experiment, confirming that the material had been successfully hydrogenated. This enables the H / WO bond... 3-x It has become a highly promising photocatalytic material.

[0051] Photocatalytic performance test results ( Figure 6 Display H / WO 3-x It exhibits no deep oxidation products (CO2) and near 100% selectivity for formaldehyde, with products including CH3OH (23.87 μmol·g⁻¹) and HCHO (1293.82 μmol·g⁻¹). In contrast, WO3... 3-xIn addition to generating CH3OH and HCHO, it also produces a large amount of worthless over-oxidation product CO2, and its formaldehyde selectivity is only 51.0%.

[0052] Results of reactive oxygen species detection ( Figure 7 The DMPO capture of OOH experiment showed that: H / WO 3-x The DMPO-・OOH signal intensity of the photocatalyst is WO 3-x This is 1.78 times that of the previous generation, indicating that it can generate ・OOH reactive oxygen species more efficiently. Example 2

[0053] To test the H / WO prepared in Example 1 3-x The stability of the material was assessed, and it was used in a photocatalytic methane cycle reaction. The specific reaction conditions were as follows: the catalyst and water were mixed at a mass-to-volume ratio of 2 g / L, and then the mixture was added to a micro high-pressure reactor. The air inside the reactor was replaced multiple times with a mixture of methane and oxygen (volume ratio of 13:7) until the pressure reached 2 MPa. Finally, the reaction system was irradiated with a light source with a wavelength of 300~850 nm to generate formaldehyde.

[0054] After the reaction was completed, the hydrogenated catalyst was restored and its activity was tested. The catalyst was then recovered, and the above reaction steps were repeated for cyclic testing. The results are as follows: Figure 8 As shown.

[0055] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the present invention. Although detailed descriptions have been provided with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments, and they should all be covered within the protection scope of the claims.

Claims

1. An H / WO 3-x The method for preparing the material is characterized by, Includes the following steps: (1) Ammonium paratungstate and urea were mixed and ground, the resulting mixture was heated and calcined, cooled and then ground to obtain WO3. 3-x powder; (2) Take the WO obtained in step (1) 3-x The powder was dispersed in deionized water, sonicated, and then sodium borohydride solution was added and the mixture was stirred to react. (3) The solid product obtained after the reaction in step (2) is washed with deionized water, filtered, and then freeze-dried under vacuum to obtain H / WO 3-x Material.

2. An H / WO according to claim 1 3-x The method for preparing the material is characterized by, The molar ratio of urea to ammonium paratungstate in step (1) is 25-175:

1.

3. An H / WO according to claim 1 3-x The method for preparing the material is characterized by, The specific operation of heating and calcining in step (1) is as follows: heat up to 400℃ at a rate of 2℃ / min and hold for 35 min, then heat up to 650℃ at a rate of 1℃ / min and calcine for 5 hours.

4. An H / WO according to claim 1 3-x The method for preparing the material is characterized by, In step (2), WO 3-x The mass ratio of powder to sodium borohydride was 0.3:0.01-0.02, and the stirring reaction time was 2.5 h.

5. An H / WO according to claim 1 3-x The method for preparing the material is characterized by, In step (3), the vacuum freeze-drying time is 12 hours.

6. An H / WO3 prepared by the preparation method according to any one of claims 1 to 5 3-x Material.

7. H / WO prepared by the preparation method according to any one of claims 1 to 5 3-x Application of materials as photocatalysts in the photocatalytic oxidation of methane to formaldehyde.

8. The application according to claim 7, characterized in that, The specific method of the application includes: H / WO 3-x The material is added to water and placed in a high-pressure reactor. The air inside the reactor is replaced with a mixture of methane and oxygen to maintain the reaction pressure at 0.1-3 MPa. The reactor is then irradiated with a light source with a wavelength of 300-850 nm to catalyze the oxidation of methane to formaldehyde.

9. The application according to claim 8, characterized in that, The volume ratio of methane to oxygen is 1.3:0.

7.

10. The application according to claim 8, characterized in that, Formaldehyde concentration was determined using the acetylacetone colorimetric method. Specifically, the reaction solution was mixed with the acetylacetone solution at a 1:1 volume ratio, and reacted at 35°C for 60 min. The absorbance at a wavelength of 412 nm was measured using a UV-Vis spectrophotometer, and the formaldehyde yield was calculated based on the standard curve of formaldehyde concentration versus absorbance.

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