Preparation method of polymer carbon nitride-based metal monatomic catalyst

Abstract

The invention discloses a preparation method of a polymer carbon nitride-based metal monatomic catalyst, which comprises the following steps: (1) firstly carrying out prepolymerization treatment on a PCN precursor to obtain a prepolymerized PCN intermediate; (2) fully and uniformly mixing the prepolymerized PCN intermediate, a metal precursor and metal salt according to a certain proportion, and roasting; and (3) removing metal salt from solid powder obtained by roasting to obtain the SA-M / PCN catalyst. According to the method, a step-by-step pyrolysis strategy is adopted, a main polymerization process and a metal anchoring process of the PCN are separately carried out, and a certain amount of metal salt is added, so that the crystallinity and the metal loading capacity of the PCN in the SA-M / PCN catalyst are improved, and the photocatalytic activity of the catalyst is favorably improved.

Description

Technical Field

[0001] The invention belongs to the technical field of metal single-atom catalyst preparation, and specifically relates to a method for preparing a highly crystalline polymer carbon nitride-based single-atom catalyst. Background Art

[0002] In recent years, single-atom catalysts have been widely used in various catalytic reactions due to their high atomic utilization, uniform active sites, high activity, high stability, and recyclability. Metal atoms in single-atom catalysts are mainly stabilized by the interaction with the support. Therefore, the strength of the interaction between the support and the metal and the number of anchoring sites that the support can provide directly affect the dispersion and stability of the metal component. Polymeric carbon nitride (PCN) is a highly stable two-dimensional material. Its high-density N content, heptazine ring holes, and surface and edge defects can provide sufficient anchoring sites to stabilize metal atoms, making it an ideal single-atom catalyst support. Moreover, PCN has the characteristics of simple preparation, high physicochemical stability, visible light absorption (band gap of about 2.72 eV), and band edge positions that meet the requirements of H2O redox, making it an important member of photocatalysts. Therefore, metal single-atom catalysts (SA-M / PCN) supported by polymeric carbon nitride (PCN) can be widely used in various photocatalytic reactions (photodegradation of organic pollutants, photocatalytic carbon dioxide reduction, and photohydrogen production from water).

[0003] The preparation of SA-M / PCN catalyst is generally carried out by direct pyrolysis and post-treatment. ACS Nano 2018 , 12, 9441-9450) synthesized a high-load (Fe loading> 4 wt%) Fe / PCN single-atom catalyst by directly pyrolyzing a mixture of PCN and metal precursors. This method can synthesize SA-M / PCN catalysts with high metal loadings. However, since the metal precursor participates in the thermal decomposition and polymerization process of the PCN precursor too early, the organic matter and metal species released by its decomposition will introduce impurities, produce defects, and destroy the structural integrity of PCN, resulting in poor crystallinity of PCN, which is not conducive to further improvement of photocatalytic performance. The structural integrity of PCN can be ensured by using a post-treatment loading method. This method generally requires the synthesis of PCN first and then the loading of metal Pt. For example, Xie et al. ( Adv. Mater., 2016, 28, 2427-2431) synthesized a single-atom Pt / PCN photocatalyst by post-treatment method. First, PCN was synthesized, and then Pt was loaded on PCN by solution method plus low-temperature annealing. Although this method can ensure that PCN has high crystallinity, it can only obtain single-atom Pt / PCN catalysts with very low Pt loading (0.075, 0.11, 0.16wt%), which is not conducive to the improvement of photocatalytic performance. Therefore, how to prepare SA-M / PCN catalysts with high metal loading while ensuring high crystallinity of PCN has become a major challenge in the field of PCN-based single-atom catalyst preparation technology. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for preparing a polymer carbon nitride-based metal single atom (SA-M / PCN) catalyst. A step-by-step pyrolysis strategy is adopted to separate the main polymerization process of PCN from the metal anchoring process, and a certain amount of metal salt is added to improve the crystallinity and metal loading of PCN in the SA-M / PCN catalyst, which helps to improve the photocatalytic activity of the catalyst.

[0005] The preparation method of the polymer carbon nitride-based metal single atom catalyst of the present invention comprises the following contents: (1) firstly prepolymerizing a PCN precursor to obtain a prepolymerized PCN intermediate; (2) fully and evenly mixing the prepolymerized PCN intermediate, a metal precursor and a metal salt in a certain proportion, and calcining the mixture under the protection of an argon atmosphere; (3) removing the metal salt from the solid powder obtained by calcining to obtain a SA-M / PCN catalyst.

[0006] In the method of the present invention, the PCN precursor in step (1) is selected from one or more of melamine, dicyandiamide, monocyanamide, urea, thiourea or ammonium bisulfate.

[0007] In the method of the present invention, the prepolymerization treatment temperature in step (1) is 300-500° C., the prepolymerization treatment time is 1-5 hours, and the prepolymerization treatment is generally carried out in an air atmosphere.

[0008] In the method of the present invention, the metal precursor described in step (2) is one or more of nitrates of Fe, Co, Ni, Cu, Mn, Zn, In and chlorates of Pt and Pd.

[0009] In the method of the present invention, the metal salt in step (2) is selected from one or more of NaCl, LiCl, KCl and CsCl.

[0010] In the method of the present invention, the mass ratio of the PCN intermediate to the metal precursor in step (2) is 2-20:1.

[0011] In the method of the present invention, the mass ratio of the PCN intermediate to the metal salt in step (2) is 1:0.2-10.

[0012] In the method of the present invention, the mixing method in step (2) can be manual grinding or mechanical ball milling, and the mixing time is generally 1 to 5 hours.

[0013] In the method of the present invention, the roasting temperature in step (2) is 480-600° C., and the roasting time is 1-5 hours.

[0014] In the method of the present invention, the method for removing the metal salt in step (3) is generally to use water washing, and the metal salt is dissolved and removed by solvent washing. The solvent can be water or an alcohol solvent. For example, water washing can be performed followed by centrifugation and then drying to obtain a SA-M / PCN catalyst.

[0015] The drying temperature is 60-120° C., and the drying time is 4-12 hours.

[0016] The polymer carbon nitride-based metal single atom catalyst of the present invention comprises a metal single atom SA-M and a carrier PCN, wherein the metal single atom type M is selected from one or more of Fe, Co, Ni, Cu, Mn, Zn, In, Pt, and Pd, and the metal single atom SA-M accounts for 2wt% to 6wt% of the total mass of the catalyst.

[0017] The present invention constructs a step-by-step pyrolysis strategy, in which a PCN precursor is prepolymerized at a certain temperature to obtain a prepolymerized PCN intermediate, the prepolymerized PCN intermediate and a metal precursor are fully mixed, and a certain amount of metal salt is added, and the uniformly mixed mixture is then placed in a tubular furnace for calcination, so that the polymer intermediate is fully polymerized and the metal species are dispersed with the assistance of the metal salt to obtain a solid powder, and then the metal salt is removed by washing with water to obtain a SA-M / PCN catalyst.

[0018] The beneficial effects of the present invention are as follows: the step-by-step pyrolysis strategy includes two processes: PCN precursor prepolymerization and high-temperature pyrolysis. The main polymerization process of PCN can be separated from the decomposition process of the metal precursor, thereby avoiding the introduction of impurities, defects, and destruction of the structural integrity and crystallinity of PCN due to the premature involvement of the metal precursor in the main polymerization process of the PCN precursor. Moreover, the addition of metal salts in the high-temperature pyrolysis stage can assist the PCN intermediate to further fully polymerize to form highly crystalline PCN. Therefore, the PCN in the synthesized SA-M / PCN catalyst has high crystallinity and high metal loading, and is expected to be applied to a variety of efficient photocatalytic reactions. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1XRD patterns of SA-Fe / PCN-1 catalyst A prepared in Example 1 of the present invention and SA-Fe / PCN catalyst D prepared in Comparative Example 1.

[0020] Figure 2 The photo-Fenton degradation performance diagram of Rhodamine B of SA-Fe / PCN-1 catalyst A prepared in Example 1 of the present invention and SA-Fe / PCN catalyst D prepared in Comparative Example 1. DETAILED DESCRIPTION

[0021] The technical solution of the present invention is described in more detail below through specific embodiments, but the present invention is not limited to these embodiments.

[0022] Catalyst characterization experiments: 1) X-ray powder diffraction (XRD) X-ray diffraction (Powder X-ray Diffraction) was used to analyze the phase structure of the catalyst. The instrument model was a SmartLab X-ray diffractometer from Rigaku, Japan. The instrument was equipped with a Cu Kα X-ray source. The operating voltage and current when testing the sample were 40 KV and 100 mA, respectively. The scanning range was 2θ = 5-80 °, and the scanning speed was 8 ° / min. Example 1

[0023] SA-Fe / PCN-1 catalyst A was prepared by adopting the step-by-step pyrolysis strategy of the present invention: 10 g of PCN precursor (melamine) was put into a muffle furnace, pre-calcined at 300° C. for 3 h, and naturally cooled to obtain a PCN intermediate; 2 g of PCN intermediate, 0.22 g of Fe(NO3)3·9H2O, and 1 g of NaCl were weighed and mixed evenly in an agate mortar, then put into a tubular furnace, and calcined at 550° C. for 5 h (heating rate of 5° C. / min) under an argon atmosphere, and naturally cooled to obtain a solid powder; the solid powder was ultrasonically washed with water for 2 hours, and centrifugally washed with water 4 times to obtain catalyst A. Example 2

[0024] The SA-Fe / PCN-2 catalyst B was prepared by the step-by-step pyrolysis strategy of the present invention: the preparation method of catalyst B was the same as that of Example 1, except that 0.22 g Fe(NO3)3·9H2O was replaced with 0.16 g Fe(NO3)3·9H2O to obtain catalyst B. Example 3

[0025] The SA-Fe / PCN-3 catalyst C was prepared by adopting the step-by-step pyrolysis strategy of the present invention: the preparation method of catalyst C was the same as that of Example 1, except that 1 g of NaCl was replaced with 0 g of NaCl to obtain catalyst C. Example 4

[0026] The photo-Fenton reaction performance of catalyst A, catalyst B, catalyst C and catalyst D was tested. The photo-Fenton reaction conditions were as follows: the photo-Fenton reaction was carried out in a customized 50 mL glass reactor. During the entire reaction process, circulating water was passed to maintain a constant temperature of 308 K. The catalyst dosage was 20 mg. The model organic pollutant selected was rhodamine B, and the reaction solution was 50 mL with a concentration of 50 mg / L. Before starting the reaction, it was stirred for 30 min under dark conditions to allow the catalyst to adsorb and desorb the model pollutant to reach equilibrium. Then 100 mL of hydrogen peroxide was added and the xenon light source was turned on immediately. Samples were taken every 20 min. During sampling, 0.4 mL of the reaction solution was taken to filter out the catalyst with a filter membrane and tested on a UV spectrophotometer. The residual concentration of rhodamine B was analyzed, and the degree of pollutant removal at each time point was calculated to evaluate the photocatalytic performance of the catalyst. The evaluation performance and catalyst parameters are shown in Table 1. Comparative Example 1

[0027] References ACS Nano 2018 , 12, 9441-9450), SA-Fe / PCN catalyst D was prepared by conventional direct pyrolysis method: 2 g PCN precursor (melamine) and 0.2 g Fe(NO3)3·9H2O were weighed and mixed evenly in an agate mortar, then placed in a tubular furnace, calcined at 600 °C for 5 h (heating rate 5 °C / min) under N2 atmosphere, and cooled naturally to obtain catalyst D.

[0028] Table 1 Catalyst parameters and photo-Fenton reaction performance catalyst Metal loading (wt%) Removal rate of Rhodamine B after 2 hours of reaction Catalyst ASA-Fe / PCN-1 6.5 99.0% Catalyst BSA-Fe / PCN-2 5.1 93.6% Catalyst CSA-Fe / PCN-3 6.2 87.7% Catalyst DSA-Fe / PCN 6.3 85.5% As attached Figure 1 As shown in the figure, the characteristic diffraction peak (27.8°) of PCN in catalyst A prepared in Example 1 of the present invention is significantly stronger than that in catalyst D prepared in comparative example 1, indicating that PCN in catalyst A has better structural integrity and crystallinity than catalyst D. Figure 2 As shown, the catalysts are applied to the photo-Fenton degradation of Rhodamine B reaction, and catalyst A exhibits better photocatalytic degradation of Rhodamine B than catalyst D. This is due to the fact that the preparation method of the present invention enables the PCN in the synthesized catalyst A to have better crystallinity, thereby exhibiting better photocatalytic performance.

Claims

1. A method for preparing a polymer carbon nitride-based metal single atom catalyst, characterized in that The method comprises the following contents: (1) firstly subjecting a PCN precursor to a prepolymerization treatment to obtain a prepolymerized PCN intermediate; (2) fully and evenly mixing the prepolymerized PCN intermediate, a metal precursor and a metal salt in a certain proportion, and calcining the mixture under the protection of an argon atmosphere; (3) removing the metal salt from the solid powder obtained by calcination to obtain a SA-M / PCN catalyst.

2. The method according to claim 1, characterized in that: The PCN precursor in step (1) is selected from one or more of melamine, dicyandiamide, cyanamide, urea, thiourea or ammonium bisulfate.

3. The method according to claim 1, characterized in that: The prepolymerization treatment temperature in step (1) is 300-500° C., the prepolymerization treatment time is 1-5 hours, and the prepolymerization treatment is generally carried out in an air atmosphere.

4. The method according to claim 1, characterized in that: The metal precursor described in step (2) is one or more of the nitrates of Fe, Co, Ni, Cu, Mn, Zn, In and the chlorates of Pt and Pd.

5. The method according to claim 1, characterized in that: The metal salt in step (2) is selected from one or more of NaCl, LiCl, KCl and CsCl.

6. The method according to claim 1, characterized in that: The mass ratio of the PCN intermediate to the metal precursor in step (2) is 2-20:

1.

7. The method according to claim 1, characterized in that: The mass ratio of the PCN intermediate to the metal salt in step (2) is 1:0.2-10.

8. The method according to claim 1, characterized in that: The mixing method in step (2) can be manual grinding or mechanical ball milling, and the mixing time is generally 1 to 5 hours.

9. The method according to claim 1, characterized in that: The calcination temperature in step (2) is 480-600° C., and the calcination time is 1-5 hours.

10. The method according to claim 1, characterized in that: The method for removing the metal salt in step (3) is to dissolve and remove the metal salt by washing with a solvent, wherein the solvent is water or an alcohol solvent.

11. A polymer carbon nitride-based metal single atom catalyst prepared by the method described in any one of claims 1 to 10, characterized in that: It includes metal single atoms SA-M and carrier PCN, wherein the metal single atom type M is selected from one or more of Fe, Co, Ni, Cu, Mn, Zn, In, Pt, and Pd, and the metal single atom SA-M accounts for 2wt% to 6wt% of the total mass of the catalyst.

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