阅读说明：本技术 一种高效可见光合成氨的氮化碳-二氧化钛异质结材料的制备方法及其应用 (Preparation method and application of carbon nitride-titanium dioxide heterojunction material for efficiently synthesizing ammonia by visible light ) 是由 张金龙 吴仕群 王灵芝 岳文晖 何承萱 陈子钰 潘礼汉 于 2021-06-16 设计创作，主要内容包括：本发明提供了一种具有可见光合成氨性能复合材料的制备方法,该材料在可见光光催化合成氨体系中可以得到很好的应用。本发明以尿素作为前驱体合成氮化碳(CN)材料,以四氯化钛和钛酸异丙酯作为钛源,通过将氮化碳纳米片浸泡于钛源中,随后进行水解、老化、煅烧等步骤即可得到氮化碳-二氧化钛(CN-TiO-(2))异质结材料。本发明所述方法可以简单通过控制对所加氮化碳的量来控制异质结材料中两种组分的比例,且可以通过在惰性气氛下煅烧增加异质结中氧空位的浓度。制备的富含氧空位的氮化碳-二氧化钛异质结(CN-OvTiO-(2))材料具有较高可见光吸收效率、氮气捕获能力及电荷分离效率,在可见光纯水体系中,表现出优异的光催化合成氨活性及稳定性。(The invention provides a preparation method of a composite material with visible light ammonia synthesis performance, and the composite material can be well applied to a visible light ammonia synthesis system. The invention takes urea as a precursor to synthesize a Carbon Nitride (CN) material, takes titanium tetrachloride and isopropyl titanate as titanium sources, and obtains the carbon nitride-titanium dioxide (CN-TiO) by soaking carbon nitride nanosheets in the titanium sources, and then carrying out the steps of hydrolysis, aging, calcination and the like 2 ) A heterojunction material. The method of the invention can control the proportion of two components in the heterojunction material simply by controlling the amount of the added carbon nitride, and can increase the concentration of oxygen vacancies in the heterojunction by calcining in an inert atmosphere. Prepared carbon nitride-titanium dioxide heterojunction (CN-OvTiO) rich in oxygen vacancy 2 ) The material has high visible light absorption efficiency, nitrogen capture capacity and charge separation efficiencyThe photocatalyst shows excellent photocatalytic synthetic ammonia activity and stability in a visible light pure water system.)

1. A preparation method of a carbon nitride-titanium dioxide heterojunction material for synthesizing ammonia by visible light is characterized in that carbon nitride nanosheets are synthesized by a thermal polymerization method, then titanium dioxide nanoparticles grow on the nanosheets, and the oxygen vacancy concentration in the heterojunction is increased by calcination under an inert atmosphere, so that the prepared heterojunction material rich in oxygen vacancies has excellent visible light capturing capability, nitrogen capturing capability and charge separation capability and shows good visible light ammonia synthesis performance, and specifically comprises the following steps: the first step is as follows: 2 g of urea is uniformly placed in a porcelain square boat, the porcelain square boat is placed in a muffle furnace to be calcined for 4 hours under the air atmosphere, and the calcined product is ground to obtain Carbon Nitride (CN) powder.

2. The second step is that: adding a certain amount of TiCl₄Slowly dripping 7.5 mL of ethanol containing different amounts of carbon nitride, stirring for 10 minutes, then dripping 0.8 mL of isopropyl titanate, and continuously stirring for 2 hours to obtain a uniform solution.

3. Placing the solution in a constant temperature and humidity box for 2 days, then placing the solution in a 70-DEG C oven for aging for 1 day, finally placing the solution in a muffle furnace for calcining for 8 hours at 400 ℃, wherein the heating rate is 2 ℃/min, and the obtained sample is the carbon nitride-titanium dioxide heterojunction (CN-TiO)₂）。

4. The third step: and (3) placing the prepared carbon nitride-titanium dioxide heterojunction in a tubular furnace, and calcining for 4 hours in an argon atmosphere.

5. The obtained material is the carbon nitride-titanium dioxide heterojunction (CN-OvTiO) rich in oxygen vacancy₂）。

6. The method of preparing an oxygen vacancy rich carbon nitride-titanium dioxide heterojunction material as claimed in claim 1, wherein: in the first step, the calcination temperature was 530 ℃ and the rate of temperature rise was 2.3 ℃ per minute.

7. The method of preparing an oxygen vacancy rich carbon nitride-titanium dioxide heterojunction material as claimed in claim 1, wherein: in the second step, TiCl₄The amount of (2) added was 0.13 mL.

8. The method of preparing an oxygen vacancy rich carbon nitride-titanium dioxide heterojunction material as claimed in claim 1, wherein: in the second step, the ratio of the two components in the heterojunction formed can be controlled by controlling the amount of carbon nitride added.

9. The addition amount of carbon nitride was 100 mg, 300 mg, 500 mg.

10. The method of preparing an oxygen vacancy rich carbon nitride-titanium dioxide heterojunction material as claimed in claim 1, wherein: the conditions of the constant temperature and humidity box are as follows: the temperature was 40 ℃ and the humidity was 55%.

11. The method of preparing an oxygen vacancy rich carbon nitride-titanium dioxide heterojunction material as claimed in claim 1, wherein: in the third step, the calcination temperature under argon is 300 ℃ and the heating rate is 1 ℃/min.

Technical Field

Relates to a carbon nitride-titanium dioxide heterojunction material for synthesizing ammonia by visible light, belonging to the field of nano materials and the technical field of photocatalysis.

Background

The development of solar energy synthetic ammonia photocatalysts has attracted extensive attention of chemists in recent years. Compared with industrial synthetic ammonia, the photocatalytic nitrogen fixation synthetic ammonia is a green, environment-friendly, non-toxic and low-energy-consumption synthetic ammonia mode. However, the efficiency of photocatalytic synthesis of ammonia is still far lower than the requirement of practical application, so that the development of high-efficiency, stable and cheap ammonia synthesis photocatalyst is needed. Among them, most of the solar spectrum is visible light, and therefore, it is an object of the present invention to develop a nitrogen-fixing photocatalyst having visible light response. However, most semiconductors have a wide band gap and low utilization of visible light. Although many methods can regulate the band gap width of the catalyst and broaden the photoresponse range of the catalyst, the utilization rate of the modified catalyst to visible light is still limited, and the catalyst is unstable. Carbon Nitride (CN) has been favored by many photocatalytic researchers in recent years as a catalyst with visible light response. The biggest problem of carbon nitride as a photocatalyst is that the recombination of photo-generated electrons and holes is serious, and the utilization rate of photo-generated carriers is low. Secondly, for the photocatalytic synthesis of ammonia, the effective adsorption of the catalytic material on nitrogen is one of the key factors for improving the reaction rate, and the vacancies can effectively promote the chemical adsorption of nitrogen due to the unique spatial structure and electronic structure of the vacancies.

Therefore, based on the above research background, in consideration of the three aspects of absorption of visible light, effective separation of photogenerated charges and effective adsorption of nitrogen, the invention prepares the carbon nitride-titanium dioxide heterojunction rich in oxygen vacancy, and simultaneously systematically compares the material with the carbon nitride-titanium dioxide heterojunction without oxygen vacancy and the activity of the titanium dioxide rich in oxygen vacancy. On one hand, carbon nitride in the heterojunction can be used as a visible light absorption component to promote the absorption of more visible light; on the other hand, carbon nitride and titanium dioxide can form a heterojunction to promote charge separation, thereby improving photocatalytic activity. In addition, the chemisorption of nitrogen is promoted by the production of oxygen vacancies on the titanium dioxide. The carbon nitride-titanium dioxide heterojunction material rich in oxygen vacancies prepared by the invention realizes high-efficiency and high-stability ammonia synthesis under visible light.

Disclosure of Invention

The method comprises the steps of constructing a heterojunction by titanium dioxide and carbon nitride by a chemical deposition method, preparing a carbon nitride nano material by a thermal polymerization method, soaking the carbon nitride nano material in a titanium-containing precursor solution, and then performing hydrolysis, aging, crystallization and the like of a titanium source to obtain the carbon nitride-titanium dioxide heterojunction. And finally, calcining the heterojunction material in an inert atmosphere to obtain the carbon nitride-titanium dioxide heterojunction material rich in oxygen vacancies.

The method for preparing carbon nitride by thermal polymerization comprises the following steps: 2 g of urea is uniformly placed in a porcelain square boat, the porcelain square boat is placed in a muffle furnace to be calcined for 4 hours in the air atmosphere, the calcination temperature is 530 ℃, the heating rate is 2.3 ℃ per minute, and the calcined product is ground into powder to obtain the sample.

The invention relates to a method for constructing a carbon nitride-titanium dioxide heterojunction, which comprises the following steps: 0.13 mL of TiCl₄7.5 mL of ethanol containing different amounts (100 mg, 300 mg, 500 mg) of CN was slowly added dropwise thereto, followed by stirring for 10 minutes, then 0.8 mL of isopropyl titanate was added dropwise thereto, and stirring was continued for 2 hours to obtain a uniform solution. Placing the solution in a constant temperature and humidity box for 2 days at 40 ℃ and 55% humidity, then placing the solution in a 70 ℃ oven for aging for 1 day, and finally placing the solution in a muffle furnace for calcining for 8 hours at 400 ℃ with the heating rate of 2 ℃/min.

The method for preparing oxygen vacancy related to the invention comprises the following steps: and placing the prepared carbon nitride-titanium dioxide heterojunction into a tubular furnace, and calcining for 4 hours in an argon atmosphere at the calcining temperature of 300 ℃ and the heating rate of 1 ℃/min.

The invention has the advantages that

1. The carbon nitride is used as a visible light capturing component, so that the utilization rate of visible light is improved, and the problem of low sunlight utilization rate of the traditional catalyst is solved.

2. The carbon nitride and the titanium dioxide are used for constructing a heterojunction, so that the migration of photo-generated charges is promoted, and the defect of low charge utilization rate of a photocatalyst is overcome; the raw materials are cheap and easy to obtain, and the cost is reduced.

3. By preparing oxygen vacancies on the surface of the catalyst, the chemical adsorption of nitrogen is promoted, and the problem of poor adsorption capacity of the catalyst on nitrogen in the photocatalytic synthesis of ammonia is solved.

Drawings

FIG. 1a is a TEM image of bulk carbon nitride, and FIGS. 1b-c are CN-TiO in example 2₂The TEM and HRTEM images of (A) can be clearly seen from FIG. 1b with TiO on CN₂The small area in the figure is enlarged to form figure 1 c. The lattice stripe region was measured and the interplanar spacing was found to be 0.35nm, corresponding to anatase TiO₂The amorphous area beside the 101 crystal face is CN, which indicates TiO₂Successfully grown on CN. FIG. 1d shows oxygen vacancy-enriched CN-TiO calcined under argon in example 3₂And in the heterojunction, the CN region is still in a nano-sheet shape, which indicates that the morphology of the CN region is not obviously damaged by the calcination treatment. FIG. 1e is CN-OvTiO₂The corresponding element mapping chart proves the existence of C, N, Ti and O elements, and the mapping distribution of the C and N elements and the Ti and O elements is almost the same, and also shows that the formed CN-OvTiO is₂The heterojunction of (2).

FIG. 2 shows TiO in examples 1 to 3 and comparative examples 1 to 2₂、CN、OvTiO₂、CN-TiO₂、CN-OvTiO₂XRD, UV-DRS, FTIR and N of₂And (4) removing the attached drawing by adsorption. As shown in FIG. 2a, TiO₂With OvTiO₂And CN-TiO₂With CN-OvTiO₂The peak positions and the intensities of the two are almost consistent, and the preparation of the oxygen vacancy is proved to have no obvious damage to the crystal structure of the catalyst. There was a slight enhancement of the absorption of light by the catalyst after the oxygen vacancies were created, as shown in figure 2 b. Figure 2c illustrates that there is no significant change in the infrared absorption properties of the catalyst after the oxygen vacancies have been created. The specific surface area of each sample was determined and, as shown in FIG. 2d, for TiO after oxygen vacancies were created₂The specific surface area of (a) does not change significantly. CN and TiO₂After the heterojunction is formed, the specific surface area is between the two, and the shown adsorption and desorption curvesThe linear property also combines the characteristics of the two, in P/P₀=0.4-0.8 and P/P₀>The region of 0.8 presents a weaker hysteresis loop. CN-OvTiO₂Specific surface area and adsorption-desorption curve property of (A) and CN-TiO₂Similarly, it was demonstrated that the preparation of oxygen vacancies had no effect on the specific surface area and pore properties of the catalyst. The above characterization proves the success of the construction of CN-TiO₂A heterojunction.

FIG. 3(a) is a graph of nitrogen fixation activity under visible light for materials with different carbon nitride to titanium dioxide ratios; FIGS. 3(b) and (c) are TiO samples of examples and comparative examples, respectively₂、CN、OvTiO₂、CN-TiO₂With CN-OvTiO₂The nitrogen fixation activity diagram under visible light and full spectrum, (d) is CN-OvTiO₂Cyclic activity profile under visible light. CN is only 4 mu mol g under visible light^-1The nitrogen fixation activity of the heterojunction is obviously improved. OvTiO₂Has weak activity under visible light, TiO₂No activity under visible light (fig. 2 b). CN-OvTiO₂The activity in a visible light region is the highest and reaches 34 mu mol g^-1Is CN-TiO₂2.4 times of the amount of the precursor, which shows that the preparation of oxygen vacancy is beneficial to promoting the photocatalysis nitrogen fixation. Secondly, the activity rule of each sample under the irradiation of light in the full spectrum range is similar to that under visible light, and the heterojunction is proved to be capable of promoting photocatalysis nitrogen fixation. To investigate the stability of the catalyst, 5 cycles of the reaction were carried out (FIG. 2d), and CN-OvTiO was found₂Still maintain good activity, prove the activity of catalyst does not cut down obviously with reaction time.

FIG. 4 shows CN-TiO in examples 2 and 3₂And CN-OvTiO₂The diagram of Ti 2p (a), O1 s (b) and the diagram of nitrogen temperature programmed desorption (c), and the diagram of (d) is the diagram of TiO 2₂、CN、OvTiO₂、CN-TiO₂And CN-OvTiO₂EPR map of. The XPS spectrum can know CN-OvTiO₂The oxygen vacancy concentration increased significantly and the EPR plot also confirmed the difference in oxygen vacancy concentration. The nitrogen temperature programming map shows that the chemical adsorption of nitrogen is obviously enhanced along with the increase of the concentration of oxygen vacancies, and the success of the strategy is proved.

FIG. 5 shows an embodimentTiO in examples and comparative examples₂、CN、OvTiO₂、CN-TiO₂And CN-OvTiO₂The visible light transient photocurrent (a), the room temperature fluorescence spectrum (b) and the time resolution fluorescence spectrum (c) are graphs, and the graphs (d) and (e) are TiO₂And (f) is CN-OvTiO₂Energy band structure schematic diagram. Photocurrent graphs illustrate that both the heterojunction structure and the preparation of oxygen vacancies can promote photo-generated charge separation efficiency; lower fluorescence intensity, meaning better electron-hole separation efficiency; the longer fluorescence lifetime indicates that the photo-generated carrier of the catalyst has longer lifetime; therefore, the performance of photocatalytic synthesis of ammonia is improved. The Mott Schottky spectrogram and the corresponding energy band structure chart prove that the energy bands of the carbon nitride and the titanium dioxide are matched, so that the heterojunction is favorably formed.

FIG. 6 shows TiO in example 1, example 2 and comparative example 1₂、CN、CN-TiO₂The DMPO-. OH capture experiments (a) and (b) under illumination are Pt-CN-OvTiO₂HRTEM image of (a), (b), (c) and (d) are TiO₂And CN-TiO₂The band structure diagram and two charge transfer paths. It is known from radical trapping experiments that photogenerated holes in the heterojunction tend to transfer to carbon nitride, and selective deposition experiments of Pt prove that photogenerated electrons tend to gather on titanium dioxide, and the formed heterojunction is a traditional Type II heterojunction.

The experimental results prove that the carbon nitride-titanium dioxide heterojunction rich in oxygen vacancies synthesized by the method has excellent visible light synthetic ammonia performance and stability.

While the present invention has been described in detail with reference to the preferred embodiments, it should be understood that the above description should not be taken as limiting the invention.

Detailed description of the preferred embodiments

The present invention will be described in more detail below with reference to specific examples, but the scope of the present invention is not limited to these examples.

Example 1

Synthesis of carbon nitride nanosheet

And uniformly placing 2 g of urea in a porcelain square boat, placing the porcelain square boat in a muffle furnace to calcine for 4 hours in the air atmosphere, wherein the calcining temperature is 530 ℃, the heating rate is 2.3 ℃ per minute, and grinding the calcined product into powder to obtain the carbon nitride nanosheet (CN).

Example 2

Synthesis of carbon nitride-titanium dioxide heterojunction

0.13 mL of TiCl₄7.5 mL of ethanol containing different amounts (100 mg, 300 mg, 500 mg) of CN was slowly added dropwise thereto, followed by stirring for 10 minutes, then 0.8 mL of isopropyl titanate was added dropwise thereto, and stirring was continued for 2 hours to obtain a uniform solution. Placing the solution in a constant temperature and humidity cabinet for 2 days at 40 deg.C and 55% humidity, aging in a 70 deg.C oven for 1 day, calcining in a muffle furnace at 400 deg.C for 8 hr at a heating rate of 2 deg.C/min to obtain a sample of carbon nitride-titanium dioxide heterojunction (CN-TiO)₂）。

Example 3

Synthesis of oxygen vacancy rich carbon nitride-titanium dioxide heterojunction

Placing the prepared carbon nitride-titanium dioxide heterojunction into a tube furnace, calcining for 4 hours in an argon atmosphere at the calcining temperature of 300 ℃ and the heating rate of 1 ℃/min, wherein the obtained sample is the carbon nitride-titanium dioxide heterojunction (CN-OvTiO) rich in oxygen vacancy₂）。

Comparative example 1

Preparation of titanium dioxide nanoparticles

0.13 mL of TiCl₄Slowly dropping the mixture into 7.5 mL of ethanol solution, stirring the mixture for 10 minutes, then dropping 0.8 mL of isopropyl titanate, and continuing stirring the mixture for 2 hours to obtain a uniform solution. Placing the solution in a constant temperature and humidity cabinet for 2 days at 40 deg.C and humidity of 55%, aging in a 70 deg.C oven for 1 day, calcining in a muffle furnace at 400 deg.C for 8 hr at a heating rate of 2 deg.C/min to obtain titanium dioxide nanoparticles (TiO)₂）。

Comparative example 2

Synthesis of oxygen vacancy rich titanium dioxide nanoparticles

Placing the prepared titanium dioxide nano particles in a tubular furnace,calcining for 4 hours in argon atmosphere at the calcining temperature of 300 ℃ and the heating rate of 1 ℃/min to obtain the titanium dioxide nano-particle (OvTiO) rich in oxygen vacancy₂）。

Experiment and data

The activity investigation method for synthesizing ammonia by photocatalysis provided by the invention comprises the following steps:

50 mg of catalyst is weighed and ultrasonically dispersed in 100 mL of water, the reaction solution is placed in a purchased photocatalytic reactor to be stirred, and high-purity N is introduced into the reaction solution₂Bubbling for 30 minutes to remove dissolved O from the solution₂The gas flow rate was 200 mL/min. And then, turning on a xenon lamp to irradiate the reaction solution, placing a corresponding optical filter on a lamp cap as required to remove light of certain wave bands, taking out about 6 milliliters of liquid from the solution every 30 minutes for centrifugation, taking out 5 milliliters of supernatant, adding a Nashin reagent for 10 minutes, and then testing the light absorption of the mixed solution on an ultraviolet-visible spectrophotometer. The concentration of the synthetic ammonia was judged from the absorbance of the absorption curve at 420 nm.