A method for preparing a metal monatomic catalyst

Abstract

This invention discloses a method for preparing a metal single-atom catalyst, comprising the following steps: (1) prepolymerizing a PCN precursor to obtain a prepolymerized PCN intermediate; (2) uniformly mixing the PCN intermediate, the metal precursor, and a pore-forming agent, followed by calcination to obtain an SA-M / PCN catalyst. The metal single-atom catalyst of this invention comprises a metal single atom SA-M and a support PCN, wherein the type M of the metal single atom 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 60-120 m². 2 / g. This invention employs a stepwise pyrolysis strategy, controlling the timing of metal precursor introduction during pyrolysis to increase the specific surface area of ​​the catalyst and achieve high exposure of metal sites in single-atom metal catalysts with high metal loading.

Description

Technical Field

[0001] This invention belongs to the field of catalyst preparation technology, and specifically relates to a method for preparing a polymer carbon nitride (PCN) supported metal single-atom catalyst. Background Technology

[0002] In recent years, single-atom catalysts have been widely used in various catalytic reactions due to their high atomic utilization, uniform active sites, and advantages such as high activity, high stability, and recyclability. In single-atom catalysts, the metal single atoms are mainly stabilized through interactions with the support. Therefore, the strength of the support-metal interaction and the number of anchoring sites provided by the support directly affect the dispersion and stability of the metal component. Polymer carbon nitride (PCN), as a highly stable two-dimensional material, offers sufficient anchoring sites to stabilize metal single atoms due to its high nitrogen content, heptaazine ring pores, and surface and edge defects, making it an ideal single-atom catalyst support. Furthermore, PCN is simple to prepare, has high physicochemical stability, absorbs visible light (band gap of approximately 2.72 eV), and its band edge positions meet the requirements for H2O redox, making it an important member of photocatalysts. Therefore, polymer carbon nitride (PCN) supported metal single-atom (SA-M / PCN) catalysts can be widely used in various photocatalytic reactions (photodegradation of organic pollutants, photocatalytic carbon dioxide reduction, and photocatalytic water splitting for hydrogen production, etc.).

[0003] SA-M / PCN catalysts are generally prepared using post-treatment and direct pyrolysis methods, such as those described by Xie et al. ( Adv. Mater., 2016, 28 A single-atom Pt / PCN photocatalyst was synthesized using a post-treatment method (2427-2431). This method requires first synthesizing PCN, then using a solution method followed by low-temperature annealing to synthesize the single-atom Pt / PCN photocatalyst. Furthermore, the solution adsorption of Pt species requires multiple washings until the solution is free of Pt precursors. Therefore, this method can only synthesize single-atom Pt / PCN photocatalysts with very low Pt loadings. When the Pt loading is 0.075, 0.11, and 0.16 wt%, Pt is dispersed as single atoms; when the Pt loading is 0.38 wt%, Pt clusters are obtained; and increasing the Pt loading to 3.2 wt% only yields Pt nanoparticles. Direct pyrolysis can synthesize SA-M / PCN catalysts with higher metal loadings, for example, as described by Park et al. (2427-2431). Carbon, 2017, 124A high-load (10 wt%) Ni / PCN single-atom catalyst was synthesized by direct pyrolysis of a mixture of PCN and metal precursors (pp. 180-187). However, due to the low specific surface area of ​​PCN and its tendency to form bulk structures, the synthesized SA-M / PCN catalyst also had a low specific surface area. Moreover, because the metal precursor intervened too early in the polymerization process of the PCN precursor, most of the metal active sites were encapsulated inside the PCN. Therefore, even if the metal species achieved single-atom-level dispersion, only a small portion of the active sites were exposed, limiting the catalytic performance. Therefore, improving the metal loading of the SA-M / PCN catalyst and achieving a high degree of metal site exposure has become a major challenge in the field of SA-M / PCN catalyst preparation technology. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a method for preparing a metal single-atom catalyst. This invention employs a stepwise pyrolysis strategy, controls the timing of the introduction of the metal precursor during the pyrolysis process, increases the specific surface area of ​​the catalyst, and achieves high exposure of metal sites in the metal single-atom (SA-M / PCN) catalyst under a high metal loading.

[0005] The metal single-atom (SA-M / PCN) catalyst of the present invention comprises a metal single atom SA-M and a support PCN, wherein the type M of the metal single atom 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 60-120 m². 2 / g.

[0006] The preparation method of the metal single atom (SA-M / PCN) catalyst of the present invention includes the following: (1) prepolymerizing the PCN precursor to obtain a prepolymerized PCN intermediate; (2) mixing the PCN intermediate, the metal precursor and the pore-forming agent evenly and calcining them under an argon atmosphere to obtain the 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℃, and the prepolymerization treatment time is 1~5 hours.

[0009] In the method of the present invention, the metal precursors in step (2) are: nitrates, chlorides or sulfates of Fe, Co, Ni, Cu, Mn, Zn, and In, and chlorates of Pt and Pd.

[0010] In the method of the present invention, the mass ratio of PCN intermediate to 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 roasting temperature in step (2) is 480~650℃ and the roasting time is 1~5 hours.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] A stepwise pyrolysis strategy was employed to prepolymerize the PCN precursor, avoiding the premature involvement of the metal precursor in the PCN precursor polymerization process, which would otherwise result in the encapsulation of most of the metal active sites. The synthesized SA-M / PCN catalyst exhibits a single-atom-level dispersion of metal species, along with high metal loading and specific surface area. Therefore, more active sites are exposed on the catalyst surface. When applied to catalytic reactions, these exposed active sites can better contact the reaction substrate, improving catalytic performance. Attached Figure Description

[0016] Figure 1 HAADF-STEM electron microscope image of SA-Fe / PCN-1 catalyst A prepared in Example 1 of this invention.

[0017] Figure 2 This is a SEM image of the SA-Fe / PCN-1 catalyst A prepared in Example 1 of the present invention.

[0018] Figure 3 This is a SEM image of the SA-Fe / PCN-4 catalyst E prepared in Comparative Example 1 of this invention. Detailed Implementation

[0019] The technical solution of the present invention will be described in more detail below through specific embodiments, but the present invention is not limited to these embodiments.

[0020] Catalyst characterization experiments:

[0021] 1) N2 physical adsorption-desorption analysis (N2-BET)

[0022] Nitrogen physisorption-desorption data of the samples were obtained using a Quadrasorb SI gas adsorption instrument from Quantachrome Instruments. After pretreating and degassing 0.1 g of the sample at 250 °C for 8 hours, physisorption-desorption experimental data of the sample were obtained at liquid nitrogen temperature (-196 °C). The BET specific surface area of ​​the sample was calculated using the multi-point BET method.

[0023] 2) Aberration-corrected transmission electron microscopy (TEM) analysis

[0024] High-angle annular dark-field scanning electron microscope (HAADF-STEM) images were obtained using a JEM-ARM200F spherical aberration electron microscope from NEC. The experimental procedure was as follows: the catalyst sample was first finely ground in an agate mortar, a small amount of powder was placed in anhydrous ethanol for ultrasonic dispersion, and then the suspension was dropped onto a copper grid supporting a carbon film. After natural drying, the sample was deposited on the copper grid, and finally the sample was observed using an electron microscope. Example 1

[0025] The SA-Fe / PCN-1 catalyst A was prepared using the stepwise pyrolysis strategy of the present invention: 10g of PCN precursor (melamine) was placed in a muffle furnace and pre-calcined at 350℃ for 3h. After natural cooling, a PCN intermediate was obtained. 2g of PCN intermediate, 0.3g of ammonium nitrate and 0.5g of FeCl3·6H2O were weighed and mixed evenly in an agate mortar. Then, the mixture was placed in a tube furnace and calcined at 600℃ for 5h under an argon atmosphere (heating rate 5℃ / min). After natural cooling, catalyst A was obtained. Example 2

[0026] SA-Fe / PCN-2 catalyst B was prepared using the stepwise pyrolysis strategy of the present invention: The preparation method of catalyst B is the same as in Example 1, except that the pre-calcination at 350°C for 3 hours is changed to pre-calcination at 400°C for 3 hours to obtain catalyst B. Example 3

[0027] The SA-Fe / PCN-3 catalyst C was prepared using the stepwise pyrolysis strategy of the present invention: The preparation method of catalyst C is the same as in Example 1, except that the PCN precursor (melamine) is replaced with the PCN precursor (urea), and the calcination at 600°C for 5 hours is replaced with calcination at 550°C for 5 hours to obtain catalyst C. Example 4

[0028] The SA-Ni / PCN catalyst D was prepared using the stepwise pyrolysis strategy of the present invention: The preparation method of catalyst D is the same as in Example 1, except that FeCl3·6H2O is replaced with NiCl2·6H2O to obtain catalyst D. Comparative Example 1

[0029] Reference (Chin. J. Catal., 2020, 41, 1198-1207) describes the preparation of SA-Fe / PCN catalyst E using a traditional direct pyrolysis method: Fe(NO3)3·9H2O is mixed evenly with an appropriate amount of melamine (the molar ratio of melamine to iron atoms is 10) in an agate mortar, placed in a tube furnace, and calcined at 600℃ for 5h under N2 atmosphere (heating rate 5℃ / min), followed by natural cooling to obtain catalyst E.

[0030] Table 1 Catalyst Parameters

[0031] 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

[0032] As attached Figure 1 As shown, the iron species in the SA-Fe / PCN-1 catalyst A prepared in Example 1 are dispersed as single atoms, indicating that the stepwise pyrolysis strategy of the present invention can successfully prepare single-atom catalysts.

[0033] As shown in Table 1, compared with the SA-Fe / PCN-4 catalyst E prepared in Comparative Example 1, the specific surface areas of SA-M / PCN catalysts A, B, C, and D prepared in Examples 1, 2, 3, and 4 of this invention are significantly increased, while their metal loadings are relatively similar. Therefore, SA-M / PCN catalysts A, B, C, and D can expose more metal active sites. Further analysis using SEM images shows that the SA-M / PCN-1 catalyst A prepared in Example 1 has a porous structure, corresponding to its high specific surface area, which is beneficial for the exposure of metal sites. In contrast, the SA-Fe / PCN-4 catalyst E prepared in Comparative Example 1 exhibits a bulk shape, which is not conducive to the exposure of metal sites. It is evident that the stepwise pyrolysis strategy of this 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 support (PCN), wherein the metal single atom SA-M / PCN catalyst has a porous structure; the type of metal single atom 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; and the specific surface area of ​​the SA-M / PCN catalyst is 60-120 m². 2 / g; The preparation method of the metal single-atom SA-M / PCN catalyst includes the following: (1) prepolymerizing the PCN precursor to obtain a prepolymerized PCN intermediate; (2) mixing the PCN intermediate, the metal precursor and the pore-forming agent evenly and calcining them under an argon atmosphere to obtain the SA-M / PCN catalyst; the metal precursor in step (2) is: nitrate, chloride or sulfate of Fe, Co, Ni, Cu, Mn, Zn and In, and chlorate of Pt and Pd.

2. A method for preparing the metal single-atom SA-M / PCN catalyst according to claim 1, characterized in that... The process includes the following: (1) Prepolymerizing the PCN precursor to obtain a prepolymerized PCN intermediate; (2) Mixing the PCN intermediate, the metal precursor and the pore-forming agent evenly and calcining them under an argon atmosphere to obtain an SA-M / PCN catalyst.

3. The method according to claim 2, characterized in that: The PCN precursor mentioned 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℃, and the prepolymerization treatment time is 1~5 hours.

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

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

1.

8. The method according to claim 2, characterized in that: The pore-forming agent mentioned 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.

9. 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.

10. The method according to claim 2, characterized in that: The roasting temperature in step (2) is 480~650℃ and the roasting time is 1~5 hours.

11. The application of the metal single-atom SA-M / PCN catalyst according to claim 1 in photocatalytic reactions.

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