CN106955699B - A kind of high-efficiency solar energy nitrogen fixation photocatalytic material and preparation method thereof - Google Patents

Abstract

The invention relates to a high-efficiency solar nitrogen-fixing photocatalytic material and a preparation method thereof. The chemical composition of the photocatalytic material is Bi ₂ O _3‑x / _{nBia MO b ,} wherein x=0～1, n=0～1 , a=0～2, b=0～6, M is at least one of V, Mo, and W. The photocatalytic material of the present invention includes bismuth-based oxide Bi ₂ O _3-x (x=0～1) or its composite material with Bi _a MO _b (M=V, Mo, W) photocatalytic material. Photocatalytic materials not only have extremely high photocatalytic nitrogen fixation activity, but also have extremely high stability. Bi ₂ O _3-x (x=0～1) in the material can adsorb and activate nitrogen in the air, promote the cleavage of nitrogen-nitrogen triple bonds, and generate ammonia in water.

Description

A kind of high-efficiency solar energy nitrogen fixation photocatalytic material and preparation method thereof

technical field

The invention relates to a high-efficiency solar energy nitrogen-fixing photocatalytic material and a preparation method thereof, and belongs to the technical field of photocatalytic materials.

Background technique

Ammonia provides nitrogen for plants, especially food crops, and is widely used in industry. It is the largest chemical product in the world except sulfuric acid, and the largest amount is currently in chemical fertilizers. The traditional nitrogen fixation methods mainly include biological nitrogen fixation and chemical nitrogen fixation. Biological nitrogen fixation can be carried out under relatively mild conditions, but it can only occur in a few plants and microorganisms, making it difficult for large-scale production applications. Chemical nitrogen fixation effectively solves the shortage of biological nitrogen fixation, but the whole process consumes a lot of energy, and a large amount of use not only causes soil compaction, but also nitrate leakage with water, causing serious environmental pollution. At present, the whole world is facing energy, food and environmental problems, and exploring new methods of nitrogen fixation and ammonia production has become a hot issue in the scientific community.

The key to the ammonia synthesis reaction is the activation of _N2 . If light energy is introduced into this reaction system, it is possible to realize this reaction. Photocatalytic nitrogen fixation is a new technology that utilizes the response of semiconductor photocatalyst to light by converting nitrogen into ammonia and light energy into chemical energy. It has become an important science and technology in the field of photocatalysis because of its energy saving and green environmental protection. question.

The core problem of photocatalytic nitrogen fixation technology is the design, development and development of suitable photocatalysts. Current research mainly focuses on Fe-doped _TiO2 -based series of photocatalysts. But for _TiO2 , there are several key technical difficulties, such as the band gap of _TiO2 is 3.2eV, which can only be excited by ultraviolet light (only 3.8% of the solar energy), in addition, the recombination rate of photogenerated carriers of _TiO2 High, low quantum efficiency (less than 4%), low utilization of solar energy, almost no light response in the visible light range; when used for photocatalytic nitrogen fixation, only 1-10 μmol/L of ammonia nitrogen can be produced.

The development of new and efficient photocatalytic materials is the key to using solar energy to achieve photocatalytic nitrogen fixation, and it is also an inevitable trend and development direction for photocatalysis to be further practical. The key problem for photocatalytic nitrogen fixation is the activation of N≡N triple bonds. The vast majority of photocatalytic materials themselves do not have the ability to activate _N2 , therefore, to fundamentally realize the solar nitrogen fixation of photocatalytic materials, it must be combined with other materials that can activate _N2 . This material can be a noble metal or an electron-rich metal oxide. Considering the cost of noble metals, electron-rich metal oxides are the first choice for cocatalysts. When the Bi element forms metal oxides, the lone pair electrons contained in the Bi atoms often do not participate in the formation, making it an electron-rich compound. Therefore, it can be considered to introduce Bi ions during the preparation of photocatalytic materials, so that a part of Bi metal oxides are simultaneously generated and supported on the surface of photocatalytic materials to activate _N2 . The preparation of this composite of Bi-based oxides and photocatalytic materials can greatly improve the nitrogen fixation efficiency of photocatalytic materials under the action of solar energy, which has high scientific value and practical significance for alleviating the energy crisis and preventing environmental pollution. .

SUMMARY OF THE INVENTION

In view of the above problems existing in the existing photocatalytic nitrogen fixation technology, the purpose of the present invention is to provide a high-efficiency solar nitrogen-fixing photocatalytic material, and at the same time provide a simple, fast and highly controllable method for preparing the photocatalytic material.

Herein, the present invention provides a high-efficiency solar nitrogen-fixing photocatalytic material, the chemical composition of the photocatalytic material is Bi ₂ O _3-x / _{nBia MO b ,} wherein x=0～1, n=0～1, a=0 to 2, b=0 to 6, and M is at least one of V, Mo, and W. Preferably x, n≠0.

The photocatalytic material of the present invention includes bismuth-based oxide Bi ₂ O _3-x (x=0～1) or its composite material with Bi _a MO _b (M=V, Mo, W) photocatalytic material. Photocatalytic materials not only have extremely high photocatalytic nitrogen fixation activity, but also have extremely high stability. Bi ₂ O _3-x (x=0-1) in the material can adsorb and activate nitrogen in the air, promote the cleavage of nitrogen-nitrogen triple bonds, and generate ammonia in water. The material has significant photocatalytic activity for solar photocatalytic nitrogen fixation. Under 24 hours of simulated sunlight, 1-10 mg/L of ammonia nitrogen can be generated in pure water. The highest efficiency achieved for nitrogen fixation. Fe-doped TiO ₂ can only produce 0.03 mg/L of ammonia nitrogen under the same conditions. Bi ₂ O _3-x (x=0～1) composite photocatalytic material shows the advantages of efficient nitrogen fixation under natural conditions and has application prospects.

In the present invention, the diameter of Bi ₂ O _3-x particles in the photocatalytic material is 1-1000 nm, preferably 1-100 nm, and the diameter of Bi _a MO _b particles is 1-1000 nm, preferably 1-100 nm. After the particle size increased to 100 nm, the catalytic activity decreased significantly with the decrease of the specific surface area of the particles.

The present invention also provides a preparation method of the high-efficiency solar nitrogen-fixing photocatalytic material, the preparation method comprising: dissolving bismuth salt and sodium oleate in water, stirring to prepare an emulsion-like precursor solution A; dispersing the M salt In water, ultrasonically obtain solution B; mix an appropriate amount of solution A and solution B, synthesize at 80-180° C. for more than 2 hours, wash and dry to obtain the high-efficiency solar nitrogen-fixing photocatalytic material. "Mix an appropriate amount of solution A and solution B", the "appropriate amount" refers to n=0-1 in the photocatalytic material Bi ₂ O _3-x / _{nBia MO b prepared} from the mixed solution.

The present invention uses sodium oleate as surfactant, disperses bismuth salt in deionized water to obtain emulsion precursor solution A, and disperses M salt (M=V, Mo, W) in deionized water to obtain solution B. When only the precursor solution A was synthesized at 80～180℃ for more than 2 hours, and after washing and drying, Bi ₂ O _3-x (x=0～1) photocatalytic material could be obtained. At this time, the high-efficiency solar nitrogen fixation photocatalytic material was prepared. The value of n in the material Bi ₂ O _3-x / _{nBia MO b is} 0. When an appropriate amount of solution A is mixed with an appropriate amount of solution B, the photocatalytic material Bi ₂ O _3-x / _nBia MO with n=0 to 1 can be prepared after synthesizing at 80-180 ℃ for more than 2 hours, washing and drying. _b . The preparation method of the invention does not need special equipment and harsh conditions, has simple process, strong controllability, is easy to realize large-scale production, and has practicability.

Preferably, the bismuth salt is at least one of bismuth salts such as bismuth nitrate, bismuth chloride, and bismuth ion complexes.

Preferably, the molar concentration of Bi in solution A is 0.001-0.2 mol/L, and if the concentration is too high, the particles of the reaction product will be too large.

Preferably, the molar concentration of sodium oleate in solution A is 0.001-1 mol/L, and the oleic acid concentration is too low to fully complex bismuth ions, and too high will increase the alkalinity and reducibility of the reaction liquid, which is not conducive to product formation.

Preferably, the stirring time for preparing the emulsion-like precursor solution A is 1-3 hours.

Preferably, the M salt is at least one of ammonium metavanadate, ammonium vanadate, vanadium pentoxide, sodium tungstate, sodium molybdate, tungsten chloride, phosphotungstic acid, phosphomolybdic acid and the like.

Preferably, the molar concentration of M salt in solution B is 0.001-0.1 mol/L.

Preferably, the synthesis time is 2 to 48 hours.

Description of drawings

₁ is the XRD diffraction pattern of the BiO/BiVO composite photocatalytic material obtained by taking BiO and BiVO as an example in Example ₁ ;

Fig. 2 is the transmission electron microscope photograph of BiO/BiVO ₄ composite photocatalytic material obtained by taking BiO and BiVO ₄ as examples in Example 1;

Figure 3 is a comparison diagram of the photocatalytic nitrogen fixation efficiency of materials such as BiO/BiVO ₄ obtained in Example 1, BiO obtained in Example 2 and BiVO ₄ obtained in Example 4 and Fe-doped TiO ₂ under simulated sunlight;

Fig. 4 obtains the XRD pattern of BiO in embodiment 2;

FIG. 5 is the XRD pattern of BiVO ₄ obtained in Example 4. FIG.

Detailed ways

The present invention will be further described below with reference to the accompanying drawings and the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention, but not to limit the present invention.

The purpose of the present invention is to provide a high-efficiency solar nitrogen-fixing photocatalytic material and a method for preparing the photocatalytic material. The chemical composition of the photocatalytic material of the present invention is Bi ₂ O _3-x /nBia MO _b , including the photocatalytic material with Bi ₂ O _{3 -x} (x=0-1) as the main catalytic active species and its combination with Bi _a Photocatalytic materials of MO _b composites. When n=0, the photocatalytic material is Bi-based oxide Bi ₂ O _3-x (x=0～1), and when n≠0, the photocatalytic material is Bi ₂ O _3-x and Bi _a MO _b (M=V, Mo, W) composite of photocatalytic materials.

The present invention adopts the liquid phase method, uses sodium oleate as surfactant, disperses bismuth salt in deionized water to obtain emulsion precursor solution A, and dissolves M salt (M=V, Mo, W) in water to obtain precursor In solution B, the precursor solution or the mixed solution thereof with M salt is heated to prepare the high-efficiency solar nitrogen-fixing photocatalytic material of the present invention. Regarding the preparation of solution A, it is preferable to dissolve sodium oleate as a surfactant in deionized water first, so that the oleic acid molecules can be fully dissolved to form micelles, and then bismuth salt is added to make it evenly dispersed. When only the precursor solution A is heated, Bi ₂ O _3-x (x=0~1) photocatalytic material can be obtained, and the high-efficiency solar nitrogen-fixing photocatalytic material Bi ₂ O _3-x / _{nBia MO b is} prepared at this time. The value of is 0. When an appropriate amount of solution A and an appropriate amount of solution B are mixed and heated, the photocatalytic material Bi ₂ O _3-x / _{nBia MO b with} n=0～1 and n is not 0 can be prepared; the “appropriate amount” is Refers to the photocatalytic material Bi ₂ O _3-x / _{nBia MO b prepared} from the mixed solution, where n=0～1 and n is not 0; The molar ratio of salt to M salt (M=V, Mo, W) is preferably 1:1 to 1:2; when the amount of bismuth salt is too large, Bi ₂ O ₃ without nitrogen fixation catalytic activity will be generated, and too little will produce Phase separation of M salts.

In the present invention, the preparation of the emulsion precursor solution includes the steps of dissolving bismuth salt and sodium oleate in water and stirring to prepare the emulsion precursor solution. The bismuth salts include, but are not limited to, bismuth nitrate, bismuth chloride, bismuth complexes, and the like. The molar concentration of Bi in the emulsion precursor solution is preferably 0.001 to 0.2 mol/L, and the molar concentration of sodium oleate is preferably 0.001 to 1 mol/L. If the concentration exceeds the concentration range, the reactant concentration will be too large, and the product particles will be too large, and the catalytic effect will be too large. performance degradation, etc. Excessive concentration of sodium oleate will lead to changes in the acidity, alkalinity and reducibility of the reaction solution, which is not conducive to the formation of products. Specifically, as an example, for example, bismuth nitrate or bismuth chloride and sodium oleate are dissolved in deionized water in a certain proportion, and stirred for 1 to 3 hours to form an emulsion-like precursor solution. Regarding the mixing ratio of bismuth salt and sodium oleate, the molar ratio of the two can be between 1:3 and 1:6, which ensures that Bi ions are fully complexed and the concentration of sodium oleate is not too high.

In the present invention, M salt (M=V, Mo, W) includes but is not limited to ammonium metavanadate, ammonium vanadate, vanadium pentoxide, sodium tungstate, sodium molybdate, tungsten chloride, phosphotungstic acid, Phosphomolybdic acid, etc. After dissolving the M salt in water, stir for 0.5 to 2 hours, and ultrasonically disperse it uniformly. The molar concentration of the M salt dissolved in water is preferably 0.001 to 0.1 mol/L.

The photocatalytic material synthesized by the invention includes the photocatalytic material with Bi ₂ O _3-x (x=0-1) as the main catalytic active species and the photocatalytic material compounded with Bi _a MO _b . As an example, the synthesis steps may include:

When the emulsion precursor solution is synthesized in an oil bath or hydrothermally at 80～180°C for more than 2 hours, washed with an organic solvent, and dried to obtain Bi ₂ O _3-x (x=0～1) photocatalytic material;

When the emulsion precursor solution is mixed with the M salt solution, it is synthesized in oil bath or hydrothermally at 80～180℃ for more than 2 hours, washed with organic solvent, and dried to obtain Bi ₂ O _3-x (x=0～1) loading composite photocatalytic materials;

The organic solvent is n-hexane, cyclohexane, acetone, alcohol and the like.

Compared with the prior art, the present invention has the following beneficial effects:

1) The Bi ₂ O _3-x / _{nBia MO b photocatalytic} material obtained by the method of the present invention not only has extremely high photocatalytic nitrogen fixation activity, but also has extremely high stability. The material can generate 1-10 mg/L of ammonia nitrogen in pure water under 24 hours of simulated sunlight illumination, which is the highest efficiency of photocatalytic nitrogen fixation in pure water under solar energy conditions reported at home and abroad. Fe-doped TiO ₂ can only produce 0.03 mg/L of ammonia nitrogen under the same conditions. Bi ₂ O _3-x (x=0～1) composite photocatalytic material shows the advantages of efficient nitrogen fixation under natural conditions, and has application prospects;

2 ₎ The preparation method of the Bi ₂ O _3-x /nBia MO _b photocatalytic material does not require special equipment and harsh conditions, the process is simple, the controllability is strong, it is easy to realize large-scale production, and has practicability.

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

Dissolve 1.2 mmol of sodium oleate in 20 mL of deionized water, add 0.194 g Bi(NO ₃ ) ₃ ·5H ₂ O after dissolving, and stir magnetically for 1.5 hours to form an emulsion-like precursor solution;

Dissolve 0.4 mmol of ammonium metavanadate in 20 mL of deionized water, stir for 1 hour and sonicate to make the dispersion uniform;

The two groups of solutions were mixed and stirred for 1 hour and then transferred to a 50mL hydrothermal kettle, hydrothermally reacted at 105°C for 16 hours, and naturally cooled to room temperature;

The solid sample in the hydrothermal kettle was washed with n-hexane, ethanol, etc., and then the powder was obtained by vacuum freeze-drying technology, which was the BiO/BiVO ₄ composite material.

Figure 1 is the XRD diffraction pattern of the BiO/BiVO ₄ composite material obtained in this example. The XRD component analysis shows that the obtained composite photocatalytic material is monoclinic BiVO ₄ and a small amount of hexagonal BiO.

Fig. 2 is a transmission electron microscope photograph of the BiO/BiVO ₄ composite material obtained in this example. It can be seen from Fig. 2 that: in the obtained composite material, BiVO ₄ is a structure composed of nanorods with a diameter of about 40-50 nm, and BiO is a diameter of about 40-50 nm. are nanorods around 3-4 nm, and the BiO nanorods are tightly grown on BiVO ₄ .

In order to study the photocatalytic nitrogen fixation performance of the prepared samples, an experiment was designed to simulate the reduction of _N2 in air to _NH4 ⁺ by the material in pure water system under sunlight. The "Fe-doped TiO ₂ " described in the examples was prepared with reference to literature (Appl. Catal. B: Environ. 2014, 144, 468-477).

The NH ₄ ⁺ concentration was measured by salicylic acid spectrophotometry (HJ 536-2009) in the experiment.

_The same amount of BiO/BiVO composite and 0.5wt% Fe-doped _TiO nanomaterials prepared by hydrothermal method were added into 200 mL of pure water, respectively, and the pH was adjusted between 3-4 with dilute hydrochloric acid, and then placed in a 500W xenon lamp. Under irradiation for 24 hours, the NH ₄ ⁺ concentration in the solution was measured and the results were recorded.

The detection results showed that the color development was carried out under light for 24 hours, and the ammonia nitrogen concentration in the suspension of BiO/BiVO ₄ composite material was found to reach 3.9 mg/L, indicating that the obtained BiO/BiVO ₄ composite material had a high photocatalytic nitrogen fixation ability.

Figure 3 shows the comparison of the photocatalytic nitrogen fixation (reduction of N ₂ in air under a xenon lamp) efficiency of the BiO/BiVO ₄ composite obtained in this example and Fe-doped TiO ₂ nanomaterials under simulated sunlight. It can be seen from Figure 3 that the obtained BiO/BiVO ₄ composites have a significantly higher nitrogen fixation efficiency than Fe-doped TiO ₂ under the same conditions.

Example 2

Dissolve 2.2 mmol of sodium oleate in 20 mL of deionized water, add 0.194 g Bi(NO ₃ ) ₃ ·5H ₂ O after dissolving, and stir magnetically for 1.5 hours to form an emulsion-like precursor solution. Add the precursor solution to 20 mL of deionized water. Transfer to a 50mL hydrothermal kettle, hydrothermally react at 105°C for 20 hours, and naturally cool to room temperature;

The solid sample is washed with ethanol, etc., and then the powder is obtained by vacuum freeze-drying technology, which is the BiO material.

According to XRD detection and analysis (see FIG. 4 ), the material obtained in this example is a BiO material (quantum sized BiO).

The BiO material obtained in this example is slightly less efficient than the composite material obtained in Example 1 in reducing nitrogen in air to generate NH ₄ ⁺ (see FIG. 3 ).

Example 3

Dissolve 1.2 mmol of sodium oleate in 20 mL of deionized water, add 0.194 g Bi(NO ₃ ) ₃ ·5H ₂ O after dissolving, and stir magnetically for 1.5 hours to form an emulsion-like precursor solution;

Dissolve 0.1 mmol of sodium tungstate in 20 mL of deionized water, stir for 1 hour and sonicate to make the dispersion uniform;

The two groups of solutions were mixed and stirred for 1 hour and then transferred to a 50mL hydrothermal kettle, hydrothermally reacted at 105°C for 16 hours, and naturally cooled to room temperature;

The solid sample in the hydrothermal kettle is washed with n-hexane, ethanol, etc., and then vacuum freeze-drying technology is used to obtain powder, which is BiO, Bi ₂ O ₃ , Bi ₂ O _2.33 /Bi ₂ WO ₆ composite material.

The Bi ₂ O _3-x composite Bi ₂ WO ₆ photocatalytic material obtained in this example is 85% of the photocatalytic nitrogen fixation efficiency of the BiO/BiVO ₄ material obtained under the same conditions as in Example 1.

Example 4

The difference between this example and Example 1 is only: sodium oleate is not added in the preparation process;

The rest are exactly the same as described in Example 1.

The product of this example was determined to be pure BiVO ₄ phase by XRD component analysis (see Figure 5), indicating that the existence of oleate ions is a necessary condition for the formation of BiO phase.

Compared with the BiO/BiVO ₄ obtained in Example 1, the nitrogen fixation efficiency of the product BiVO ₄ is only 2% of that of the composite material under the same conditions (see Figure 3).

Claims (11)

1. a kind of high-efficiency solar fixed nitrogen catalysis material, which is characterized in that the chemical composition of the catalysis material is BiO/ nBi_aMO_b, wherein n=0~1 and n are not 0, a=0~2 and a is not 0, b=0~6 and b is not 0, M V, at least one in Mo, W Kind, the diameter of BiO is 3-4 nanometers in the catalysis material.

2. catalysis material according to claim 1, which is characterized in that Bi in the catalysis material_aMO_bParticle it is straight Diameter is 1~1000 nanometer.

3. catalysis material according to claim 2, which is characterized in that Bi in the catalysis material_aMO_bParticle it is straight Diameter is 1~100 nanometer.

4. the preparation method of high-efficiency solar fixed nitrogen catalysis material, feature described in a kind of any one of claims 1 to 3 Be, the preparation method include: bismuth salt is soluble in water with enuatrol, stirring, emulsion form precursor solution A is made；By M salt It is dispersed in water, ultrasound obtains solution B；Suitable solution A is mixed with solution B, synthesized 2 hours in 80~180 DEG C or more, it washes Wash, dry after obtain the high-efficiency solar fixed nitrogen catalysis material.

5. the preparation method according to claim 4, which is characterized in that the bismuth salt is bismuth nitrate, bismuth chloride, bismuth ion network Close at least one of object.

6. the preparation method according to claim 4, which is characterized in that in solution A the molar concentration of Bi be 0.001~ 0.2mol/L。

7. the preparation method according to claim 4, which is characterized in that in solution A the molar concentration of enuatrol be 0.001~ 1mol/L。

8. the preparation method according to claim 4, which is characterized in that the mixing time of emulsion form precursor solution A is made It is 1~3 hour.

9. the preparation method according to claim 4, which is characterized in that the M salt is ammonium metavanadate, ammonium vanadate, five oxidations At least one of two vanadium, sodium tungstate, sodium molybdate, tungsten chloride, phosphotungstic acid, phosphomolybdic acid.

10. the preparation method according to claim 4, which is characterized in that in solution B the molar concentration of M salt be 0.001~ 0.1 mol/L。

11. the preparation method according to any one of claim 4 to 10, which is characterized in that generated time is 2~48 small When.