Preparation method and application of molybdenum and molybdenum trioxide composite photocatalytic material with core-shell structure

Abstract

The invention discloses a preparation method of a molybdenum and molybdenum trioxide composite photocatalytic material with a core-shell structure, and belongs to the technical field of nitrogen reduction photocatalytic materials.Molybdenum powder is dispersed in an ethanol solvent, a hydrogen peroxide solution is added, and the molybdenum and molybdenum trioxide composite photocatalytic material with the core-shell structure is obtained after a hydrothermal reaction and vacuum drying. The preparation method of the molybdenum and molybdenum trioxide composite material with the core-shell structure is prepared by a hydrothermal method, and molybdenum powder is partially oxidized to obtain molybdenum and molybdenum trioxide; the layered structure of molybdenum is oxidized, resulting in the formation of molybdenum trioxide on the surface. The design of the in-situ Schottky junction provides more reaction sites for electron reduction of nitrogen on the oxygen-vacancy-rich molybdenum and molybdenum trioxide photocatalyst. The space separation of electrons and holes in the molybdenum-molybdenum trioxide photocatalytic material improves the separation efficiency of carriers, so that the recombination of the carriers is effectively inhibited, and the overall utilization efficiency of the carriers is greatly improved.

Description

Technical Field

[0001] The invention relates to the technical field of nitrogen reduction photocatalytic material preparation, and in particular to a preparation method and application of a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material. Background Art

[0002] In recent years, research on nitrogen fixation reactions that convert atmospheric nitrogen into ammonia has become a hot topic. However, the current industrial nitrogen fixation Haber-Bosch reaction has harsh conditions, consumes a lot of fossil fuels and emits a lot of carbon dioxide. Therefore, there is an urgent need to explore more environmentally friendly, cost-effective and sustainable nitrogen fixation strategies under mild reaction conditions. Photocatalytic nitrogen reduction has become a potential solution.

[0003] Molybdenum trioxide is a common transition metal oxide. It has poor light absorption ability and is an n-type semiconductor. It has been widely used in research. The presence of valence electrons in the outer layer of molybdenum trioxide leads to adjustable oxygen vacancy density. When oxygen vacancies are introduced, it becomes one of the candidate materials for constructing plasma-enhanced photocatalysts within the light absorption range. In terms of defect engineering, the introduction of anion vacancies not only promotes the nitrogen fixation reaction by enhancing the ability to capture electrons, but also realizes the donor-acceptor mechanism by injecting additional electrons into the bond orbitals of nitrogen molecules. The Schottky junction interface interaction provides diverse and rich active sites for the nitrogen fixation reaction and effectively inhibits the occurrence of unnecessary side reactions.

[0004] Therefore, providing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material with excellent catalytic activity and cyclic stability is a technical problem that technical personnel in this field urgently need to solve. Summary of the invention

[0005] In view of this, the present invention provides a method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material, which has excellent catalytic activity and cycle stability in photocatalytic nitrogen reduction.

[0006] To achieve the above object, the present invention adopts the following technical solution:

[0007] A method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material, characterized in that it comprises the following steps:

[0008] (1) dispersing molybdenum powder in an ethanol solvent, and stirring to obtain a molybdenum powder dispersion;

[0009] (2) adding hydrogen peroxide solution to the molybdenum powder dispersion, and stirring again to obtain a suspension;

[0010] (3) The suspension was transferred to a sealed polytetrafluoroethylene-lined hydrothermal autoclave for a hydrothermal reaction. After the reaction was completed, the suspension was naturally cooled to room temperature, washed with deionized water and anhydrous ethanol, and then dried in a vacuum drying oven overnight to obtain a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material.

[0011] In the present invention, after the molybdenum powder is oxidized, the interlayer spacing increases and the interlayer force strength decreases, resulting in the stratification of the molybdenum powder. Since the oxidation process is a process from the surface to the inside, the molybdenum powder is wrapped in molybdenum trioxide. This is an oxidation peeling process. Specifically, the layered structure of molybdenum is oxidized, resulting in the formation of molybdenum trioxide on the surface. In the scheme of the present invention, due to the presence of the synergistic effect between the Schottky junction and the oxygen vacancies, the molybdenum@molybdenum trioxide has good nitrogen reduction activity, and exhibits excellent structural stability and a long service life at room temperature. The synergistic effect between the Schottky junction and the oxygen vacancies is used to rapidly promote the development of the photocatalytic effect and open up new prospects.

[0012] Furthermore, in step (1), the ratio of the mass of the molybdenum powder to the volume of the ethanol solvent is 4.8-8 mg / mL, preferably 4.8-6 mg / mL.

[0013] Furthermore, the stirring rate is 400-600 r / min, and the stirring time is 0.5-1 h.

[0014] Furthermore, the volume fraction of the hydrogen peroxide solution in step (2) is 30-40%, preferably 30%-35%;

[0015] The ratio of the mass of molybdenum powder to the volume of hydrogen peroxide solution in the molybdenum powder dispersion is 21-64 mg / mL, preferably 32-64 mg / mL.

[0016] In the above scheme of the present invention, if the concentration and added volume of the hydrogen peroxide solution are too large, the molybdenum powder will be completely oxidized into molybdenum trioxide, and a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material cannot be formed. Therefore, the amount of hydrogen peroxide added needs to be strictly controlled.

[0017] Furthermore, the stirring rate is 400-600 r / min, and the stirring time is 1-2 h.

[0018] Furthermore, in step (3), the volume of the polytetrafluoroethylene-lined hydrothermal autoclave is 30-100 mL, preferably 30-50 mL.

[0019] In step (3), the hydrothermal reaction temperature is 150-170° C., and the hydrothermal reaction time is 10-14 h; preferably, the hydrothermal reaction temperature is 160° C., and the hydrothermal reaction time is 12 h.

[0020] Furthermore, the washing with deionized water and anhydrous ethanol in step (3) is repeated 3-7 times, preferably 3-5 times.

[0021] Furthermore, in step (3), the drying temperature is 60-100°C, preferably 65-90°C, more preferably 70°C, and the drying time is 10 hours.

[0022] The present invention also provides the use of the core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material prepared by the above method in photocatalytic nitrogen reduction.

[0023] The advantages and beneficial effects of the present invention are:

[0024] The preparation method of the core-shell structured molybdenum@molybdenum trioxide composite material prepared by the present invention is prepared by a hydrothermal method, and molybdenum powder is partially oxidized to obtain molybdenum@molybdenum trioxide; the layered structure of molybdenum is oxidized, resulting in the formation of molybdenum trioxide on the surface. The molybdenum trioxide nanosheets are then separated from the surface of the monolithic molybdenum, exposing a new surface that undergoes the same oxidation peeling process. Ultimately, this will lead to the production of molybdenum trioxide nanosheets, and the fewer layers and shorter diffusion lengths in the thin sheets are conducive to the rapid transfer of photogenerated carriers in the photocatalytic reaction. The design of the in-situ Schottky junction provides more reaction sites for the electronic reduction of nitrogen on the oxygen-rich vacancy molybdenum@molybdenum trioxide photocatalyst. The spatial separation of electrons and holes in the molybdenum@molybdenum trioxide photocatalytic material improves the separation efficiency between carriers, thereby effectively suppressing the recombination of carriers and greatly improving the overall utilization efficiency of carriers. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is the X-ray diffraction (XRD) diagram of the core-shell structure molybdenum@molybdenum trioxide photocatalytic material of Example 1;

[0026] Figure 2 are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the core-shell structure molybdenum@molybdenum trioxide photocatalytic material of Example 1; wherein, Figure 2 (a) is the SEM image of molybdenum@molybdenum trioxide photocatalytic material. Figure 2 (b) and Figure 2 (c) TEM and HRTEM images of Mo@MoO3 photocatalytic materials, respectively;

[0027] Figure 3 is a comparison chart of the photocatalytic nitrogen reduction performance of the photocatalytic materials obtained in Examples 1-3;

[0028] Figure 4 This is a test diagram of the cyclic stability of photocatalytic nitrogen reduction of the core-shell structure molybdenum@molybdenum trioxide photocatalytic material in Example 1;

[0029] Figure 5This is the XRD spectrum of the core-shell structure molybdenum@molybdenum trioxide photocatalytic material in Example 1 before and after the photocatalytic reaction. DETAILED DESCRIPTION

[0030] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] Example 1

[0032] (1) 0.192 g of molybdenum powder was added to 24 mL of ethanol and stirred at a speed of 500 r / min for 0.5 h to obtain a dispersion;

[0033] (2) Add 3 mL of 30% hydrogen peroxide solution to the dispersion and stir at 500 r / min for 1 h to obtain a uniform suspension;

[0034] (3) The suspension was transferred to a sealed 50 mL polytetrafluoroethylene-lined hydrothermal autoclave for hydrothermal reaction at 160 ° C for 12 h. After the reaction was completed, it was naturally cooled to room temperature; it was washed with deionized water and anhydrous ethanol for 4 times respectively, and dried in a vacuum drying oven at 70 ° C for 10 h to obtain a molybdenum@molybdenum trioxide composite material (denoted as MoO3-3).

[0035] Example 2

[0036] (1) 0.192 g of molybdenum powder was added to 24 mL of ethanol and stirred at a speed of 500 r / min for 0.5 h to obtain a dispersion;

[0037] (2) Add 6 mL of 30% hydrogen peroxide solution to the dispersion and stir at 500 r / min for 1 h to obtain a uniform suspension;

[0038] (3) The suspension was transferred to a sealed 50 mL polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction at 160°C for 12 h. After the reaction was completed, it was naturally cooled to room temperature. It was washed with deionized water and anhydrous ethanol for 4 times respectively, and then dried in a vacuum drying oven at 70°C for 10 h to obtain a molybdenum@molybdenum trioxide composite material (denoted as MoO3-6).

[0039] Example 3

[0040] (1) 0.192 g of molybdenum powder was added to 24 mL of ethanol and stirred at a speed of 500 r / min for 0.5 h to obtain a dispersion;

[0041] (2) Add 9 mL of 30% hydrogen peroxide solution to the dispersion and stir at 500 r / min for 1 h to obtain a uniform suspension;

[0042] (3) The suspension was transferred to a sealed 50 mL polytetrafluoroethylene-lined hydrothermal reactor for hydrothermal reaction at 160°C for 12 h. After the reaction was completed, it was naturally cooled to room temperature. It was washed with deionized water and anhydrous ethanol for 4 times respectively, and then dried in a vacuum drying oven at 70°C for 10 h to obtain a molybdenum@molybdenum trioxide composite material (denoted as MoO3-9).

[0043] Comparative Example 1

[0044] The same mass of molybdenum powder and commercial molybdenum trioxide were weighed and repeatedly ground in a mortar until they were completely mixed, and finally a sample was obtained, which was labeled as molybdenum@molybdenum trioxide composite material (denoted as MoO3-M).

[0045] Figure 1 This is the XRD spectrum of the molybdenum@molybdenum trioxide photocatalytic material prepared in Example 2. Analysis shows that after adding 6mL of hydrogen peroxide, most of the molybdenum is oxidized. The MoO3-6 photocatalytic material shows obvious diffraction peaks at 12.7°, 23.3°, 25.7°, 27.3°, 33.7°, 38.9° and 49.2°, which correspond to the (020), (110), (040), (021), (111), (060) and (002) crystal planes in the molybdenum trioxide crystals. The XRD results show that molybdenum and molybdenum trioxide phases exist simultaneously in the MoO3-6 photocatalytic material. This indicates that the molybdenum@molybdenum trioxide photocatalytic material was successfully prepared by a one-step hydrothermal method.

[0046] Figure 2 Figure 2 is a scanning electron microscope (SEM) and transmission electron microscope (TEM) image of the core-shell structure molybdenum@molybdenum trioxide composite material of Example 2. Figure 2 (a) It can be seen that the Mo@MoO3 composite material synthesized by the hydrothermal method is composed of thin and uniformly shaped nanosheets with a thickness of about 4-5 nm. Figure 2 (b) High-resolution transmission electron microscopy images show a nanosheet-like structure similar to that observed with scanning electron microscopy. Figure 2 (c) shows that MoO3-6 is also exposed on the (111) crystal plane, and at the same time shows the Mo (110) crystal plane with a crystal spacing of 0.22nm, providing reliable evidence for the existence of Schottky junction. The production of MoO3 nanosheets, the fewer layers and shorter diffusion length in the flakes are conducive to the rapid transfer of photogenerated carriers in the photocatalytic reaction, which is beneficial to increase the specific surface area of ​​the composite material and expose more catalytic active sites, thereby improving its photocatalytic activity.

[0047] Figure 3The photocatalytic nitrogen reduction performance comparison chart of the catalysts obtained in Examples 1-3. The overall photocatalytic activity showed a volcanic trend. The photocatalytic nitrogen fixation activity of MoO3-M (27.78 μmol·g -1 ·h -1 ) is significantly lower than MoO3-6 (50.78 μmol·g -1 ·h -1 ), this difference is mainly attributed to the fact that the MoO3-6 photocatalyst is composed of MoO3 nanosheets containing oxygen-rich vacancies and elemental Mo. During the photocatalytic nitrogen reduction reaction, the construction of the Schottky junction generates a built-in electric field to form a charge transfer channel, which can not only regulate the charge transfer behavior, but also produce efficient active adsorption sites. It is worth noting that MoO3-9 with a higher oxygen vacancy concentration (31.82 μmol·g -1 ·h -1 ) and MoO3-3 with lower oxygen vacancy concentration (36.48 μmol·g -1 ·h -1 )The photocatalytic nitrogen fixation performance showed a significant decline. This may be because the high oxygen vacancy concentration makes the oxygen vacancies become the recombination centers of electrons and holes, inhibiting the yield and rate of the ammonia synthesis reaction, while the low oxygen vacancy concentration cannot construct enough active sites to promote the nitrogen fixation reaction. In addition, MoO3-6 with low oxygen vacancy concentration is more favorable for photocatalytic nitrogen fixation than MoO3-9 with high oxygen vacancy concentration, mainly due to the presence of Schottky structure and interface interaction, which provide abundant reaction sites for nitrogen fixation reaction. Therefore, only the synergistic effect of oxygen vacancy density and Schottky structure at appropriate concentrations can significantly promote the nitrogen fixation reaction.

[0048] Figure 4 The cyclic stability of the photocatalytic nitrogen reduction of the core-shell structured molybdenum@molybdenum trioxide composite material in Example 2. It can be observed from the stability test of the ammonia synthesis sample that during the ammonia detection process, the photocatalytic nitrogen fixation performance may be slightly reduced or improved due to physical factors such as catalyst loss. However, this fluctuation is small, and after 5 cycle tests, the overall nitrogen fixation performance stability is maintained well.

[0049] Figure 5 The XRD spectra of the core-shell structured molybdenum@molybdenum trioxide composite material before and after the photocatalytic reaction of Example 2. The XRD analysis conducted before and after the photocatalytic performance test showed no obvious changes, and no new phases and impurities were generated, further confirming that the MoO3-6 photocatalytic material has excellent stability.

[0050] Although the embodiments of the present invention have been shown and described above, it is to be understood that the above embodiments are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and vary the above embodiments within the scope of the present invention.

Claims

1. A method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material, characterized in that: The following steps are involved: (1) dispersing molybdenum powder in an ethanol solvent, and stirring to obtain a molybdenum powder dispersion; (2) adding hydrogen peroxide solution to the molybdenum powder dispersion, and stirring again to obtain a suspension; (3) The suspension is transferred to a closed polytetrafluoroethylene-lined hydrothermal autoclave for a hydrothermal reaction. After the reaction is completed, the suspension is naturally cooled to room temperature, washed with deionized water and anhydrous ethanol in sequence, and then dried in a vacuum drying oven to obtain a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material.

2. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 1, characterized in that: The ratio of the mass of the molybdenum powder to the volume of the ethanol solvent in step (1) is 4.8-8 mg / mL.

3. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 2, characterized in that: The stirring rate is 400-600r / min and the stirring time is 0.5-1h.

4. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 2, characterized in that: The volume fraction of the hydrogen peroxide solution in step (2) is 30-40%, and the ratio of the mass of molybdenum powder to the volume of the hydrogen peroxide solution in the molybdenum powder dispersion is 21-64 mg / mL.

5. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 4, characterized in that: The stirring rate is 400-600 r / min, and the stirring time is 1-2 h.

6. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 1, characterized in that: In step (3), the hydrothermal reaction temperature is 150-170° C., and the hydrothermal reaction time is 10-14 h.

7. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 1, characterized in that: In step (3), washing with deionized water and anhydrous ethanol is repeated 3-7 times respectively.

8. The method for preparing a core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 1, characterized in that: In step (3), the drying temperature is 60-100° C. and the drying time is 10 h.

9. A core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material, characterized in that: Prepared by the method according to any one of claims 1 to 8.

10. Use of the core-shell structured molybdenum@molybdenum trioxide composite photocatalytic material according to claim 9 in photocatalytic nitrogen reduction.

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