Preparation method of metal monatomic catalyst

Abstract

The invention discloses a preparation method of a metal monatomic catalyst, which comprises the following steps: (1) carrying out prepolymerization treatment on a PCN precursor to obtain a prepolymerized PCN intermediate; and (2) uniformly mixing the PCN intermediate, a metal precursor and a pore-forming agent, and roasting to obtain the SA-M / PCN catalyst. The metal monatomic catalyst comprises a metal monatomic SA-M and a carrier PCN, the metal monatomic type M is selected from one or more of Fe, Co, Ni, Cu, Mn, Zn, In, Pt and Pd, and the metal monatomic SA-M accounts for 6 wt%-15 wt% of the total mass of the catalyst; the specific surface area of the SA-M / PCN catalyst is 60-120m < 2 > / g. According to the method, a step-by-step pyrolysis strategy is adopted, the introduction opportunity of the metal precursor in the pyrolysis process is regulated and controlled, the specific surface area of the catalyst is increased, and high exposure of metal sites of the metal monatomic catalyst under the high metal loading capacity is achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of catalyst preparation, and specifically relates to a method for preparing a polymer carbon nitride (PCN)-supported metal 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 (SA-M / PCN) catalysts 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] SA-M / PCN catalysts are generally prepared by post-treatment and direct pyrolysis. For example, Xie et al. ( Adv. Mater., 2016, 28 , 2427-2431) synthesized a single-atom Pt / PCN photocatalyst by a post-treatment method. This method requires the synthesis of PCN first, and then the single-atom Pt / PCN photocatalyst is synthesized by a solution method plus low-temperature annealing treatment. In addition, the adsorption of Pt species by the solution method requires multiple washings until the solution does not contain Pt precursors. Therefore, this method can only synthesize single-atom Pt / PCN photocatalysts with very low loadings. When the Pt loading is 0.075, 0.11, and 0.16 wt%, Pt is dispersed in a single atom. When the Pt loading is 0.38 wt%, Pt atomic clusters are obtained. When the Pt loading is increased to 3.2 wt%, only Pt nanoparticles can be obtained. SA-M / PCN catalysts with higher metal loadings can be synthesized by direct thermal decomposition, for example, Park et al. ( Carbon, 2017, 124, 180-187) A high-load (10wt%) Ni / PCN single-atom catalyst was synthesized by directly pyrolyzing a mixture of PCN and metal precursors. However, due to the low specific surface area of ​​PCN itself and the disadvantages of being easily bulky, the synthesized SA-M / PCN catalyst also has a low specific surface area. Moreover, since the metal precursor intervenes in the polymerization process of the PCN precursor too early, most of the metal active sites are encapsulated inside the PCN. Therefore, even if the metal species are dispersed at the single-atom level, only a small number of active sites can be exposed, limiting the catalytic performance. Therefore, how to increase the metal loading of the SA-M / PCN catalyst and achieve a high exposure of the metal sites has become a major challenge in the field of SA-M / PCN 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 metal single atom catalyst. The present invention adopts a step-by-step pyrolysis strategy to regulate the timing of introducing the metal precursor during the pyrolysis process, thereby increasing the specific surface area of ​​the catalyst and achieving high exposure of metal sites in the metal single atom (SA-M / PCN) catalyst at a higher metal loading.

[0005] The metal single atom (SA-M / PCN) 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 6wt% to 15wt% of the total mass of the catalyst; the specific surface area of ​​the SA-M / PCN catalyst is 60-120m 2 / g.

[0006] The preparation method of the metal single atom (SA-M / PCN) catalyst of the present invention comprises the following contents: (1) prepolymerizing a PCN precursor to obtain a prepolymerized PCN intermediate; (2) uniformly mixing the PCN intermediate, the metal precursor and the pore-forming agent, and calcining the mixture under an argon atmosphere to obtain a SA-M / PCN catalyst.

[0007] In the method of the present invention, the PCN precursor in step (1) is selected from one or more of melamine, dicyandiamide, monocyanamide, urea and thiourea.

[0008] In the method of the present invention, the prepolymerization treatment temperature in step (1) is 250-550° C., and the prepolymerization treatment time is 1-5 hours.

[0009] In the method of the present invention, the metal precursor described in step (2) is: nitrate, chloride or sulfate of Fe, Co, Ni, Cu, Mn, Zn, In, and chlorate of Pt and Pd.

[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, preferably 5-15.

[0011] In the method of the present invention, the pore-forming agent in step (2) is one or more of ammonium nitrate, ammonium chloride or urea, and the mass ratio of the pore-forming agent to the metal precursor is 0-4:1.

[0012] In the method of the present invention, the mixing method in step (2) is manual grinding or mechanical ball milling, and the mixing time is 0.5 to 3 hours.

[0013] In the method of the present invention, the calcination temperature in step (2) is 480-650° C., and the calcination time is 1-5 hours.

[0014] Compared with the prior art, the present invention has the following advantages: By adopting a step-by-step pyrolysis strategy and prepolymerizing the PCN precursor, most of the metal active sites are avoided from being encapsulated due to the premature involvement of the metal precursor in the polymerization process of the PCN precursor. The metal species in the synthesized SA-M / PCN catalyst are dispersed at the single-atom level, and have a high metal loading and specific surface area. Therefore, more active sites can be exposed on the catalyst surface. When used in catalytic reactions, the active sites exposed on the surface can better contact with the reaction substrate, thereby improving the catalytic reaction performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is the HAADF-STEM electron microscope image of SA-Fe / PCN-1 catalyst A prepared in Example 1 of the present invention.

[0016] Figure 2 This is a SEM image of SA-Fe / PCN-1 catalyst A prepared in Example 1 of the present invention.

[0017] Figure 3 This is a SEM image of SA-Fe / PCN-4 catalyst E prepared in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0018] 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.

[0019] Catalyst characterization experiments: 1) N2 physical adsorption and desorption analysis (N2-BET) The nitrogen physical adsorption-desorption data of the sample was obtained using the Quadrasorb SI gas adsorption instrument of Quantachrome Instruments, Inc., USA. After 0.1 g of the sample was pretreated and degassed at 250°C for 8 hours, the physical adsorption-desorption experimental data of the sample was obtained at liquid nitrogen temperature (-196°C), and the BET specific surface area of ​​the sample was calculated by the multi-point BET method.

[0020] 2) Spherical aberration corrected transmission electron microscopy (TEM) analysis The high-angle annular dark-field scanning electron microscope image (HAADF-STEM) was obtained using the JEM-ARM200F spherical aberration electron microscope produced by JEOL. The experimental steps are to first grind the catalyst sample in an agate mortar, take a small amount of powder and put it into anhydrous ethanol for ultrasonic dispersion, then add the suspension with a dropper to the copper mesh supporting the carbon film, and the sample will be deposited on the copper mesh after natural drying. Finally, the sample is observed using an electron microscope. Example 1

[0021] 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 350° C. for 3 h, and cooled naturally to obtain a PCN intermediate; 2 g of the PCN intermediate, 0.3 g of ammonium nitrate and 0.5 g of FeCl3·6H2O were weighed and mixed evenly in an agate mortar, and then put into a tubular furnace, calcined at 600° C. for 5 h (heating rate of 5° C. / min) under an argon atmosphere, and cooled naturally to obtain catalyst A. Example 2

[0022] 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 the pre-calcination at 350°C for 3h was changed to the pre-calcination at 400°C for 3h to obtain catalyst B. Example 3

[0023] 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 the PCN precursor (melamine) was changed to PCN precursor (urea), and the calcination at 600°C for 5h was changed to calcination at 550°C for 5h to obtain catalyst C. Example 4

[0024] The SA-Ni / PCN catalyst D was prepared by adopting the step-by-step pyrolysis strategy of the present invention: the preparation method of catalyst D was the same as that of Example 1, except that FeCl3·6H2O was replaced with NiCl2·6H2O to obtain catalyst D. Comparative Example 1

[0025] Reference (Chin. J. Catal., 2020, 41, 1198-1207), SA-Fe / PCN catalyst E was prepared by traditional direct pyrolysis method: Fe(NO3)3·9H2O and an appropriate amount of melamine (the molar ratio of melamine to iron atoms is 10) were mixed evenly in an agate mortar, placed in a tubular furnace, calcined at 600℃ for 5h (heating rate of 5℃ / min) under N2 atmosphere, and cooled naturally to obtain catalyst E.

[0026] Table 1 Catalyst parameters catalyst <![CDATA[Specific surface area (m 2 / g)]]> Metal loading (wt%) Catalyst ASA-Fe / PCN-1 62.7 7.5 Catalyst BSA-Fe / PCN-2 63.2 7.1 Catalyst CSA-Fe / PCN-3 108.5 9.3 Catalyst DSA-Ni / PCN 67.3 6.7 Catalyst ESA-Fe / PCN-4 13.5 6.3 As attached Figure 1 As shown, the iron species in the SA-Fe / PCN-1 catalyst A prepared in Example 1 is dispersed in single atoms, indicating that the single-atom catalyst can be successfully prepared by adopting the step-by-step pyrolysis strategy of the present invention.

[0027] As shown in Table 1, compared with the SA-Fe / PCN-4 catalyst E prepared in Comparative Example 1, the specific surface areas of the SA-M / PCN catalysts A, B, C, and D prepared in Examples 1, 2, 3, and 4 of the present invention are greatly improved, and their metal loadings are relatively close, so SA-M / PCN catalysts A, B, C, and D can expose more metal active sites. It can be further seen from the SEM image that the SA-M / PCN-1 catalyst A prepared by Example 1 has a porous structure, corresponding to its higher specific surface area, which is conducive to the exposure of metal sites. The SA-Fe / PCN-4 catalyst E prepared in Comparative Example 1 presents a block shape, which is not conducive to the exposure of metal sites. It can be seen that the step-by-step pyrolysis strategy of the present invention can successfully prepare SA-M / PCN catalysts with porous structures, high specific surface areas, and highly exposed active sites.

Claims

1. A metal single atom SA-M / PCN catalyst, characterized in that: The catalyst 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 or Pd; the metal single atom SA-M accounts for 6wt% to 15wt% of the total mass of the catalyst; the specific surface area of ​​the SA-M / PCN catalyst is 60-120m 2 / g.

2. A method for preparing the metal single atom SA-M / PCN catalyst according to claim 1, characterized in that The method comprises the following contents: (1) prepolymerizing a PCN precursor to obtain a prepolymerized PCN intermediate; (2) uniformly mixing the PCN intermediate, a metal precursor and a pore-forming agent, and calcining the mixture under an argon atmosphere to obtain a SA-M / PCN catalyst.

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

4. The method according to claim 2, characterized in that: The prepolymerization treatment temperature in step (1) is 250-550° C., and the prepolymerization treatment time is 1-5 hours.

5. The method according to claim 2, characterized in that: The metal precursors described in step (2) are: nitrates, chlorides or sulfates of Fe, Co, Ni, Cu, Mn, Zn, In, and chlorates of Pt and Pd.

6. The method according to claim 2, characterized in that: The mass ratio of the PCN intermediate to the metal precursor in step (2) is 2-20:1, preferably 5-15.

7. The method according to claim 2, characterized in that: The pore-forming agent in step (2) is one or more of ammonium nitrate, ammonium chloride or urea, and the mass ratio of the pore-forming agent to the metal precursor is 0-4:

1.

8. The method according to claim 2, characterized in that: The mixing method in step (2) is manual grinding or mechanical ball milling, and the mixing time is 0.5 to 3 hours.

9. The method according to claim 2, characterized in that: The calcination temperature in step (2) is 480-650° C., and the calcination time is 1-5 hours.

10. Use of the metal single atom SA-M / PCN catalyst according to claim 1 in a photocatalytic reaction.

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