A highly efficient photocatalytic hydrogen production from water by Se-doped graphite carbon nitride with nitrogen vacancies

Abstract

A nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalyst for efficient photolysis of water to produce hydrogen belongs to the field of catalyst technology. By adopting a Se-assisted chemical vapor deposition (CVD) controllable preparation process, a nitrogen-rich vacancy Se-doped graphite phase carbon nitride (V N / Se‑CN) photocatalyst. During the preparation process, the graphite carbon nitride sealed in a quartz ampoule was subjected to high-temperature etching reaction of Se vapor, which simultaneously achieved Se doping and the formation of nitrogen vacancies. The nitrogen vacancy-rich Se-doped graphite carbon nitride photocatalyst showed excellent visible light water splitting hydrogen production performance (5418μmol h ‑1 g ‑1 ).

Description

Technical Field

[0001] The present invention provides a nitrogen-rich vacancy Se-doped graphite phase carbon nitride (V N / Se-CN) photocatalyst. The present invention firstly prepares bulk graphite phase carbon nitride by a simple thermal polycondensation method, and then adopts Se vapor-assisted chemical vapor deposition method, using Se single substance nanoparticle powder with different contents as precursors, to prepare a series of nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalysts. Among them, nitrogen-rich vacancy Se-doped graphite phase carbon nitride with a Se dosage of 2wt% exhibits the highest visible light photolysis water hydrogen production performance and stability. Background Art

[0002] As a new type of clean energy, hydrogen energy has attracted much attention due to its advantages such as high combustion calorific value, renewable nature, and environmental friendliness. Its development and utilization are of great significance to the realization of the "dual carbon" goal. Photocatalytic water splitting technology is an ideal way to prepare hydrogen energy with high efficiency, low cost, and low energy consumption. The key lies in the research and development of efficient photocatalysts. Graphitic carbon nitride (g-C3N4) is a non-metallic compound photocatalyst with simple preparation, strong light absorption ability, and adjustable band gap. It has good application prospects in the field of photocatalytic water splitting to produce hydrogen. However, at this stage, the photocatalytic water splitting hydrogen production performance of bulk graphitic carbon nitride still cannot meet the needs of industrial hydrogen production. Therefore, further modification research is needed to improve the photocatalytic activity.

[0003] Studies have shown that by doping graphite carbon nitride with elements or regulating vacancy defects, its physicochemical properties and energy level structure can be effectively optimized, thereby improving its photocatalytic activity. For example, Gou et al. prepared phosphorus-doped graphite carbon nitride. The introduction of phosphorus reduced the band gap energy, improved its conductivity, and inhibited the recombination of photogenerated electrons and holes inside it, thereby improving the visible light photocatalytic hydrogen production efficiency of the catalyst (Angew. Chem. Int. Ed., 2016, 55, 1830-1834); Zeng et al. synthesized oxygen-doped graphite carbon nitride by directly heating aqueous melamine. The incorporation of oxygen accelerated its internal charge separation and promoted its charge transfer, which greatly improved the catalytic activity of the catalyst (Appl. Catal. B, 2018,224,1-9); Sun et al. prepared defective graphite-phase carbon nitride, whose internal structural defects can promote the dissociation of excitons and the separation of photogenerated carriers, thereby enhancing its ability to produce hydrogen by photohydrolysis (Appl. Catal. B, 2020,264,118480); Zhang et al. constructed graphite-phase carbon nitride containing nitrogen vacancies. The introduction of N vacancies improved its energy level structure, alleviated the recombination of photogenerated charges and holes, and thus improved the photocatalytic activity of graphite-phase carbon nitride (Chem. Eng. J., 2022,431,134101).

[0004] Based on the above modification strategy, the present invention firstly adopts a thermal polymerization condensation method to prepare bulk graphite phase carbon nitride, and then adopts Se vapor-assisted chemical vapor deposition to modify the physicochemical properties and energy level structure of graphite phase carbon nitride, and by adjusting the relative amount of Se elemental nanoparticle powder and graphite phase carbon nitride (1wt%, 2wt%, 3wt%, 5wt%), a series of nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalysts are prepared. This invention discloses a preparation method of such photocatalyst. Among them, the catalyst with a Se dosage of 2wt% exhibits the highest visible light photocatalytic water splitting hydrogen production performance and stability. Summary of the invention

[0005] This invention uses Se vapor-assisted chemical vapor deposition to achieve the synchronous regulation of Se element doping and nitrogen vacancy defect construction in the graphite phase carbon nitride framework structure through the high-temperature etching effect of Se vapor, thereby successfully preparing a nitrogen vacancy-rich Se-doped graphite phase carbon nitride photocatalyst. The structure of the catalyst is a two-dimensional layered structure; Se doping helps to improve the visible light absorption capacity of graphite phase carbon nitride; nitrogen vacancies help to construct the defect energy level of graphite phase carbon nitride. The synergistic effect of Se doping and nitrogen vacancies greatly improves the efficiency of photocatalytic water splitting to produce hydrogen in the graphite phase under visible light drive.

[0006] The preparation method of the nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalyst comprises the following steps:

[0007] (1) Using urea as a precursor, bulk graphite phase carbon nitride is prepared, such as calcining urea as a precursor at 550°C for 4h;

[0008] (2) Bulk graphite carbon nitride and Se elemental nanoparticle powder are mixed and ground, then vacuum-sealed in a quartz ampoule and calcined at high temperature; when the temperature gradually drops to room temperature, the sample is taken out of the quartz ampoule and washed with carbon disulfide and deionized water to remove by-products; finally, after centrifugation and drying, a nitrogen vacancy-rich Se-doped graphite carbon nitride photocatalyst is obtained.

[0009] In step (2), the mass ratio of Se elemental nanoparticle powder to graphite phase carbon nitride is (1-5):100, preferably 2:100.

[0010] The high temperature calcination in step (2) refers to: heating to 450°C at a rate of 5°C / min and keeping the temperature for 4 hours.

[0011] The nitrogen vacancy-rich Se-doped graphite phase carbon nitride photocatalyst obtained by the present invention is used for photocatalytic water decomposition to produce hydrogen. The photocatalyst is further dispersed in distilled water containing triethanolamine, Pt is used as a co-catalyst, and visible light is used as a light source to perform photocatalytic water decomposition to produce hydrogen.

[0012] The specific operation steps for evaluating the photocatalytic hydrogen production performance of nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalyst prepared by the present invention are as follows: 20 mg of nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalyst is dispersed in 50 mL of distilled water containing 10 vol% triethanolamine, and 3 wt% Pt is added as a co-catalyst. The photocatalytic hydrogen evolution experiment uses a y-type reaction cell connected to a cooling system and a closed gas circulation / exhaust system, a 300W xenon lamp equipped with an L40 filter as a visible light source, and a GC7900 gas chromatograph is used to detect the hydrogen content produced by the photocatalytic reaction.

[0013] The Se element of the present invention is doped into the framework structure of graphite phase carbon nitride by replacing C atoms, and at the same time induces the generation of a large number of nitrogen vacancies. The raw materials of the present invention are cheap and easy to obtain, the preparation process is simple, the morphology and nitrogen vacancy content of the product are controllable, and it exhibits good photocatalytic water splitting hydrogen production activity under the irradiation of visible light. Among them, the nitrogen vacancy-rich Se-doped graphite phase carbon nitride (Se precursor dosage is 2wt%) has the highest hydrogen production activity, reaching 5418μmol h -1 g -1 , which is 18 times that of bulk graphite phase carbon nitride. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 , are the TEM, STEM images and EDS elemental analysis of the prepared nitrogen-rich vacancy Se-doped graphite phase carbon nitride (Se precursor content is 2wt%). Figure (a) is a TEM image, Figure (b) is a STEM image, and Figures (cf) are C, N, and Se element distribution maps, respectively.

[0015] Figure 2 , which are the EPR spectra of bulk graphite carbon nitride and nitrogen-rich vacancy Se-doped graphite carbon nitride (Se precursor dosage is 2wt%). The EPR spectrum can reflect the content of unpaired electrons in the sample. From the figure, it can be seen that the unpaired electrons in nitrogen-rich vacancy Se-doped graphite carbon nitride (Se precursor dosage is 2wt%) are significantly increased.

[0016] Figure 3 , which are UV-vis graphs of bulk graphite carbon nitride and nitrogen-rich vacancy Se-doped graphite carbon nitride (Se precursor dosage is 2wt%). Compared with bulk graphite carbon nitride, nitrogen-rich vacancy Se-doped graphite carbon nitride (Se precursor dosage is 2wt%) has improved light absorption capacity.

[0017] Figure 4 , which is the visible light photocatalytic hydrogen production activity diagram of the prepared samples. The prepared photocatalysts all showed excellent performance in photocatalytic hydrogen production from water, among which the photocatalytic hydrogen production activity of nitrogen vacancy-rich Se-doped graphite phase carbon nitride (Se precursor dosage is 2wt%) reached 5418μmol h -1 g -1 , which is 18 times that of bulk graphite phase carbon nitride. DETAILED DESCRIPTION

[0018] Specific embodiments are listed below to explain the present invention in more detail, and the prepared photocatalyst is described with reference to the accompanying drawings.

[0019] Example 1: Preparation of bulk graphite carbon nitride. Weigh 50 g of urea, put it into a 100 mL crucible and seal it. Place the crucible in a muffle furnace, heat it to 550°C at a rate of 5°C / min, and keep it for 4 hours. When the muffle furnace cools to room temperature, take out the prepared sample.

[0020] Preparation of nitrogen vacancy-rich Se-doped graphite-phase carbon nitride photocatalyst. 100 mg of the bulk graphite-phase carbon nitride prepared above was mixed with different masses of Se elemental nanoparticle powder (0, 1, 2, 3, 5 mg) and ground evenly in a mortar; the mixture was then vacuum-sealed in a quartz ampoule; the ampoule was placed in a muffle furnace, heated to 450°C at a heating rate of 5°C / min, and maintained at this temperature for 4 hours; when the muffle furnace was cooled to room temperature, the sample was taken out and washed alternately with carbon disulfide and distilled water to remove impurities.

[0021] The raw materials used in the present invention are cheap and readily available, the preparation process is simple, and the morphology and defect content of the product are controllable. The prepared nitrogen-rich vacancy Se-doped graphite phase carbon nitride (Se precursor dosage is 2wt%) has the best water hydrogen production activity under visible light.

Claims

1. A method for preparing a nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalyst, comprising the following steps: (1) Using urea as a precursor, bulk graphite carbon nitride is prepared; (2) Mixing and grinding bulk graphite carbon nitride and Se single-substance nanoparticle powder, and then vacuum-sealing them in a quartz ampoule and calcining them at high temperature; when the temperature gradually drops to room temperature, taking out the sample from the quartz ampoule, and washing it with carbon disulfide and deionized water to remove by-products; finally, centrifuging and drying, to obtain nitrogen vacancy-rich Se-doped graphite carbon nitride photocatalyst; in step (2), the mass ratio of Se single-substance nanoparticle powder to graphite carbon nitride is (1-5):100; in step (2), high-temperature calcination refers to: heating to 450°C at a rate of 5°C / min and keeping warm for 4h.

2. The method according to claim 1, characterized in that In step (2), the mass ratio of Se elemental nanoparticle powder to graphite phase carbon nitride is 2:

100.

3. The nitrogen vacancy-rich Se-doped graphite phase carbon nitride photocatalyst prepared by the method described in any one of claims 1-2, wherein the Se element is doped into the framework structure of the graphite phase carbon nitride by replacing C atoms and simultaneously inducing the generation of a large number of nitrogen vacancies.

4. Use of the nitrogen-rich vacancy Se-doped graphite phase carbon nitride photocatalyst prepared by the method according to any one of claims 1 to 2 for photocatalytic water decomposition to produce hydrogen.

5. According to the use of claim 4, the photocatalyst is dispersed in distilled water containing triethanolamine, Pt is used as a co-catalyst, and visible light is used as a light source to perform photocatalytic water decomposition to produce hydrogen.

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