Metal sub-nanocluster catalyst, preparation method thereof and application of metal sub-nanocluster catalyst in catalytic conversion of methane into C2 oxygen-containing product

Abstract

The invention provides a metal sub-nanocluster catalyst, a preparation method thereof and application of the metal sub-nanocluster catalyst in catalytic conversion of methane into C2 oxygen-containing products, and particularly relates to the technical field of catalysts. The catalyst provided by the invention comprises a carrier and a metal sub-nanocluster loaded on the carrier, the metal in the metal sub-nano cluster comprises Fe, Cu, Ru, Pd, Mo or Ni; the particle size of the metal sub-nano cluster is 0.1 to 2 nm. According to the metal sub-nanocluster catalyst provided by the invention, due to the quantum effect and nuclear number effect of the sub-nanocluster, the metal sub-nanocluster catalyst shows a unique energy band and electronic structure, the catalytic activity and selectivity of the catalyst are remarkably improved, a plurality of active sites are provided for the catalyst, at least two molecules of methane and intermediate species thereof can be activated at the same time, and the catalytic activity of the catalyst is improved. The C-C coupling reaction is promoted, and the yield and the selectivity of catalyzing methane to be converted into C2 oxygen-containing products are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a metal sub-nanocluster catalyst, a preparation method thereof, and an application thereof in catalyzing the conversion of methane into C2 oxygen-containing products. Background Art

[0002] Methane is a resource with rich reserves and low cost, and is the main component of natural gas, shale gas, and coalbed methane, accounting for 21% of the total global energy. Using methane instead of crude oil for synthesizing high-value chemicals is a very attractive strategy. However, the high symmetry and stability of methane molecules result in low activity of methane conversion reactions. Traditional methane conversion processes have problems such as harsh reaction conditions and high energy consumption. Moreover, the products of methane conversion are often more reactive than methane itself and are more easily activated, which makes the products prone to over-oxidation and poor selectivity.

[0003] C2 oxygen-containing products such as ethanol, acetaldehyde, and acetic acid are important clean energy sources and chemicals with huge demand. Therefore, the low-temperature conversion of methane to C2 oxygen-containing chemicals has important practical significance and application prospects. Currently, in the common processes for catalytically converting methane into C2 oxygen-containing products, using O2, H2O2, etc. as oxidants and transition metal nanoparticle catalysts as the main catalysts, there are problems of low methane conversion rate and low product selectivity. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a metal sub-nanocluster catalyst, a preparation method thereof, and an application thereof in catalyzing the conversion of methane into C2 oxygen-containing products. The metal sub-nanocluster catalyst provided by the present invention has high yields and selectivities of C2 oxygen-containing products in the reaction of converting methane into C2 oxygen-containing products.

[0005] To solve the above technical problems, the present invention provides the following technical solutions:

[0006] The present invention provides a metal sub-nanocluster catalyst, including a carrier and metal sub-nanoclusters loaded on the carrier; the metal in the metal sub-nanoclusters includes Fe, Cu, Ru, Pd, Mo, or Ni;

[0007] The particle size of the metal sub-nanoclusters is 0.1 - 2 nm.

[0008] Preferably, the loading amount of the metal sub-nanoclusters in the metal sub-nanocluster catalyst is 0.5 - 10 wt%.

[0009] Preferably, the carrier includes a heteroatom-doped carbon carrier, a molecular sieve, or a metal oxide.

[0010] Preferably, when the carrier is a heteroatom-doped carbon carrier, the heteroatoms include one or more of N atoms, O atoms, S atoms, and P atoms, and the mass fraction of the heteroatoms in the carrier is 1-20%.

[0011] Preferably, when the carrier is a molecular sieve, the molecular sieve includes ZSM-5 molecular sieve; when the carrier is a metal oxide, the metal oxide includes MnO2, CeO2, TiO2, or ZnO.

[0012] The present invention also provides a method for preparing the metal sub-nanocluster catalyst described in the above technical solution, including the following steps:

[0013] Coat the carrier dispersion on the surface of the substrate, and after removing the dispersant, obtain the dispersed carrier;

[0014] Deposit metal sub-nanoclusters on the dispersed carrier by atomic layer deposition to obtain a metal sub-nanocluster catalyst.

[0015] Preferably, the conditions of the atomic layer deposition include: the temperature is 150-300 °C, the pressure is 10-200 Pa, and the number of deposition cycles is 1-30.

[0016] Preferably, the carrier gas for the atomic layer deposition is an inert gas, and the flow rate of the carrier gas is 15-300 mL / min.

[0017] The present invention also provides the application of the metal sub-nanocluster catalyst prepared by the preparation method described in the above technical solution in catalyzing the conversion of methane into C2 oxygen-containing products.

[0018] Preferably, the catalysis of methane conversion into C2 oxygen-containing products includes the following steps: Mix the metal sub-nanocluster catalyst, oxidant, and water described in the above technical solution, introduce methane, and carry out an oxidation reaction to obtain C2 oxygen-containing products;

[0019] The oxidant includes hydrogen peroxide or oxygen, and the pressure of introducing methane is 0.1-3 MPa;

[0020] The temperature of the oxidation reaction is 25-150 °C.

[0021] The present invention provides a metal sub-nanocluster catalyst, comprising a support and metal sub-nanoclusters supported on the support; the metal in the metal sub-nanoclusters includes Fe, Cu, Ru, Pd, Mo or Ni; the particle size of the metal sub-nanoclusters is 0.1 - 2 nm. Due to the quantum effect and nuclear number effect of the sub-nanoclusters, the metal sub-nanocluster catalyst provided by the present invention exhibits unique energy bands and electronic structures, significantly improving the catalytic activity and selectivity of the catalyst. Each sub-nanocluster contains multiple metal atoms, providing multiple active sites for the catalyst, having more surface coordination unsaturated sites, higher metal dispersion, capable of simultaneously activating at least two molecules of methane and its intermediate species, promoting the C-C coupling reaction, and improving the yield and selectivity of catalytic methane conversion to C2 oxygen-containing products; there are significant metal-metal bonds in the sub-nanocluster catalyst while having high metal utilization rate, which provides more possibilities for the bonding of reactants and reaction paths; at the same time, the sub-nanoclusters have unique electronic structures with discrete energy levels, optimizing the adsorption and desorption ability of reaction intermediates in the process of methane conversion to C2 oxygen-containing products, and improving the selectivity of methane conversion to C2 oxygen-containing products.

[0022] In addition, the presence of heteroatoms in the support coordinates with the metal sub-nanoclusters, further regulating the electronic structure of the metal sub-nanoclusters, making them exhibit appropriate spin states and oxygen species binding strengths, facilitating the realization of C-C coupling, and further improving the selectivity and yield of methane conversion to C2 oxygen-containing products. It can be seen from the example data that when the metal sub-nanocluster catalyst prepared by the present invention is used to catalyze methane conversion to C2 oxygen-containing products, under the conditions of 25 - 150 °C, the selectivity of C2 oxygen-containing products is 50 - 95%.

[0023] The present invention also provides a preparation method of the metal sub-nanocluster catalyst. By controlling the deposition cycle parameters of atomic layer deposition (ALD), the nuclear number and size of the sub-nanoclusters are precisely controlled, thereby improving the catalytic activity and selectivity of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0025] Figure 1 SEM image of the metal sub-nanocluster catalyst prepared in Example 1;

[0026] Figure 2SEM image of the single-atom metal catalyst prepared in Comparative Example 1. Detailed implementation manners

[0027] The present invention provides a metal sub-nanocluster catalyst, including a support and metal sub-nanoclusters loaded on the support; the metal in the metal sub-nanoclusters includes Fe, Cu, Ru, Pd, Mo or Ni;

[0028] The particle size of the metal sub-nanoclusters is 0.1 - 2 nm.

[0029] In the present invention, the support preferably includes a heteroatom-doped carbon support, a molecular sieve or a metal oxide. In the present invention, when the support is preferably a heteroatom-doped carbon support, the heteroatom preferably includes one or several of N atom, O atom, S atom and P atom. In the detailed implementation manners, the heteroatom-doped carbon support can be an N-doped carbon support, an O-doped carbon support, an S-doped carbon support, a P-doped carbon support, an N,O co-doped carbon support, an N,S co-doped carbon support, an N,P co-doped carbon support, an O,S co-doped carbon support, an O,P co-doped carbon support, an S,P co-doped carbon support, an N,O,S co-doped carbon support, an N,O,P co-doped carbon support, an O,S,P co-doped carbon support or an N,P,S co-doped carbon support. In the present invention, the mass fraction of the heteroatom in the support is preferably 1 - 20%, and in the detailed implementation manners, the mass fraction of the heteroatom in the support can be 1%, 2%, 5%, 8%, 10%, 12%, 15%, 18% or 20%.

[0030] In the present invention, when the support is preferably a molecular sieve, the molecular sieve preferably includes ZSM-5 molecular sieve.

[0031] In the present invention, when the support is preferably a metal oxide, the metal oxide preferably includes MnO2, CeO2, TiO2 or ZnO.

[0032] In the present invention, the preparation method of the support is not specifically limited, and the well-known operations in the art can be adopted, such as hydrothermal method, template method, organic precursor pyrolysis method, plasma method or atmosphere treatment method.

[0033] In the present invention, the metal in the metal sub-nanoclusters includes Fe, Cu, Ru, Pd, Mo or Ni, and the particle size of the metal sub-nanoclusters is 0.1 - 2 nm. In the detailed implementation manners, the particle size of the metal sub-nanoclusters can be 0.1 nm, 0.15 nm, 0.18 nm, 0.2 nm, 0.5 nm, 0.8 nm, 1.0 nm, 1.25 nm, 1.3 nm, 1.5 nm, 1.8 nm or 2 nm.

[0034] In the present invention, the loading amount of the metal sub-nanocluster in the metal sub-nanocluster catalyst is preferably 0.5 to 10.0 wt%. In specific embodiments, the loading amount of the metal sub-nanocluster in the metal sub-nanocluster catalyst can be 0.5 wt%, 1.0 wt%, 1.5 wt%, 2.0 wt%, 2.5 wt%, 3.0 wt%, 3.5 wt%, 4.0 wt%, 4.5 wt%, 5.0 wt%, 5.5 wt%, 6.0 wt%, 6.5 wt%, 7.0 wt%, 7.5 wt%, 8.0 wt%, 8.5 wt%, 9.0 wt%, 9.5 wt%, 10.0 wt%.

[0035] For the metal sub-nanocluster catalyst of the present invention, due to the scale of the sub-nanoclusters, it exhibits a unique energy band structure, improving the selectivity of the catalyst. Each sub-nanocluster contains multiple metal atoms, providing multiple active sites for the catalyst, having more surface coordination unsaturated sites, and a higher metal dispersion degree. It can simultaneously activate at least two methane molecules, improving the yield of catalytic conversion of methane to C2 oxygen-containing products; while having a high metal utilization rate, the sub-nanocluster catalyst has significant metal-metal bonds, which provide more possibilities for the bonding of reactants and reaction paths; at the same time, the sub-nanoclusters have a unique electronic structure with discrete energy levels, optimizing the adsorption and desorption ability of reaction intermediates during the conversion of methane to C2 oxygen-containing products, and improving the selectivity of methane conversion to C2 oxygen-containing products. In addition, the presence of heteroatoms in the support coordinates with the metal sub-nanoclusters, further regulating the electronic structure of the metal sub-nanoclusters, making them exhibit an appropriate spin state and oxygen species binding strength, realizing C-C coupling, and further improving the selectivity and yield of methane conversion to C2 oxygen-containing products.

[0036] The present invention also provides a preparation method of the metal sub-nanocluster catalyst described in the above technical solution, including the following steps:

[0037] Coat the carrier dispersion liquid on the surface of the substrate, and after removing the dispersant, obtain the dispersed carrier;

[0038] Deposit metal sub-nanoclusters on the dispersed carrier by atomic layer deposition to obtain the metal sub-nanocluster catalyst.

[0039] In the present invention, without special instructions, the raw materials and equipment used are all well-known commercially available products in the art.

[0040] In the present invention, the carrier dispersion liquid is coated on the surface of the substrate, and after removing the dispersant, the dispersed carrier is obtained.

[0041] In the present invention, the carrier dispersion preferably comprises a carrier and a dispersant. In a specific embodiment, it is preferred to mix the carrier and the dispersant. The present invention has no special requirements for the manner and method of the mixing, and well-known technical means in the art can be adopted.

[0042] In the present invention, the dispersant is preferably anhydrous ethanol.

[0043] In the present invention, the mass concentration of the carrier in the carrier dispersion is preferably 0.8 - 1 mg / mL. In a specific embodiment, the mass concentration of the carrier in the carrier dispersion can be 0.8 mg / mL, 0.9 mg / mL, or 1 mg / mL.

[0044] The present invention has no special requirements for the manner and method of removing the dispersant, and well-known technical means in the art can be adopted, such as natural air drying, drying, etc.

[0045] In the present invention, there are no special requirements for the substrate, and materials with high temperature resistance and a flat surface can be used. In the embodiments of the present invention, a quartz wafer is specifically preferred. In the present invention, the specifications of the quartz wafer are preferably 80 mm × 80 mm × 2 mm.

[0046] To obtain the dispersed carrier, the present invention uses atomic layer deposition to deposit metal sub-nanoclusters on the dispersed carrier to obtain a metal sub-nanocluster catalyst.

[0047] In the present invention, the atomic layer deposition is preferably carried out in an atomic layer deposition vacuum reaction chamber. In the present invention, the temperature of the atomic layer deposition is preferably 150 - 300 °C. In a specific embodiment, the temperature of the atomic layer deposition can be 150 °C, 160 °C, 170 °C, 180 °C, 190 °C, 200 °C, 210 °C, 220 °C, 230 °C, 240 °C, 250 °C, 260 °C, 270 °C, 280 °C, 290 °C, or 300 °C; the pressure is preferably 10 - 200 Pa; the number of deposition cycles is preferably 1 - 30 times. In a specific embodiment, the number of deposition cycles is preferably 1 time, 2 times, 5 times, 8 times, 10 times, 15 times, 20 times, 25 times, or 30 times.

[0048] In the present invention, the carrier gas for the atomic layer deposition is preferably an inert gas. The inert gas is preferably nitrogen or argon, and the flow rate of the carrier gas is preferably 15 - 300 mL / min.

[0049] In the present invention, when depositing metal sub-nanoclusters Fe, the atomic layer deposition preferably uses ferrocene and ozone as deposition precursors; when depositing metal sub-nanoclusters Cu, the atomic layer deposition preferably uses copper isopropoxide and ozone as deposition precursors; when depositing metal sub-nanoclusters Ni, the atomic layer deposition preferably uses nickelocene and ozone as deposition precursors; when depositing metal sub-nanoclusters Ru, the atomic layer deposition preferably uses (2’3-dimethyl-1,3-butadiene) tricarbonyl ruthenium and ozone as deposition precursors; when depositing metal sub-nanoclusters Pd, the atomic layer deposition preferably uses palladium hexafluoroacetylacetonate and HCHO as deposition precursors; when depositing metal sub-nanoclusters Mo, the atomic layer deposition preferably uses molybdenum hexacarbonyl and ozone as deposition precursors.

[0050] In the present invention, due to the different carriers and metal species, the interaction between the metal and the carrier is inconsistent, resulting in different locations of the metal sub-nanoclusters, different atomic layer (ALD) deposition parameters, and ultimately different metal sizes. Moreover, minor changes in the number of metal catalysts and the coordination environment (such as adding or removing a metal or coordination atom) will cause huge differences in electronic properties and geometric structures, thereby significantly affecting the catalytic performance.

[0051] The present invention also provides the application of the metal sub-nanocluster catalyst prepared by the preparation method described in the above technical solution in the catalytic conversion of methane to C2 oxygen-containing products.

[0052] In the present invention, the catalytic conversion of methane to C2 oxygen-containing products preferably includes the following steps: mixing the metal sub-nanocluster catalyst, an oxidant, and water, introducing methane, and performing an oxidation reaction to obtain C2 oxygen-containing products.

[0053] In the present invention, the oxidant preferably includes hydrogen peroxide or oxygen. In a specific embodiment, the oxidant is preferably hydrogen peroxide, and the concentration of the hydrogen peroxide is preferably 30 wt%.

[0054] In the present invention, the mass ratio of the metal sub-nanocluster catalyst to the volume of the oxidant is preferably 5 mg: 3 mL.

[0055] In the present invention, the pressure of introducing methane is preferably 0.1 to 3 MPa. In a specific embodiment, the pressure of introducing methane can be 0.1 MPa, 0.5 MPa, 1 MPa, 1.5 MPa, 2 MPa, or 3 MPa.

[0056] In the present invention, the temperature of the oxidation reaction is preferably 25 to 150 °C. In specific embodiments, the temperature of the oxidation reaction can be 25 °C, 50 °C, 70 °C, 100 °C or 150 °C; the time of the oxidation reaction is preferably 1.5 to 3 h. In specific embodiments, the time of the oxidation reaction can be 1.5 h, 2 h, 2.5 h or 3 h.

[0057] In the present invention, the C2 oxygen-containing products are preferably acetic acid, acetaldehyde and ethanol.

[0058] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Any modification, equivalent replacement, improvement, etc. made to the embodiments of the present invention without creative work based on the technical essence and general principles of the present invention shall fall within the protection scope of the present invention.

[0059] Preparation of MnO2

[0060] Mix MnSO4·H2O, (NH4)2S2O8, (NH4)2SO4 and water, carry out hydrothermal reaction at 140 °C for 12 h, filter and dry to obtain MnO2.

[0061] Preparation of N,S co-doped carbon carrier

[0062] Using CdS nanowires as a template agent, coat dopamine hydrochloride on the outside, stir for 12 h, filter and dry, place it in a tubular furnace and calcine at 1000 °C for 3 h in an argon atmosphere to obtain an N,S-doped carbon carrier.

[0063] Preparation of N,O co-doped carbon carrier

[0064] Polyaniline nanorods are obtained by calcining in a tubular furnace at 800 °C for 1 h in an argon atmosphere.

[0065] Preparation of N,P co-doped carbon carrier

[0066] After phosphorylating polyaniline rods, place them in a tubular furnace and calcine at 800 °C for 1 h in an argon atmosphere.

[0067] Preparation of O,S co-doped carbon carrier

[0068] Add thiourea to carbon nanotubes, place them in a tubular furnace and calcine at 800 °C for 1 h in an argon atmosphere.

[0069] Preparation of O,P co-doped carbon carrier

[0070] Add phosphoric acid to carbon nanotubes, place them in a tubular furnace and calcine at 800 °C for 1 h in an argon atmosphere.

[0071] Preparation of P, S Co-doped Carbon Support

[0072] Add phosphoric acid and thiourea into carbon nanotubes, place them in a tube furnace, and calcine at 800 °C for 1 h in an argon atmosphere.

[0073] Example 1

[0074] Take 10 mg of N, S co-doped carbon support (N 2.8 wt%, S 3.5 wt%) and dry it, then mix it with 10 mL of absolute ethanol to obtain a dispersion of heteroatom-doped carbon support; evenly coat the obtained carbon support dispersion on the surface of a quartz sheet with a size of 80 mm × 80 mm × 2 mm, and after air drying, obtain a dispersed carbon support.

[0075] Place the dispersed carbon support into an atomic layer deposition vacuum reaction chamber, use ozone and ferrocene as precursors, and deposit 1 cycle on the surface of the carbon support by atomic layer deposition (ALD). The temperature of the chamber during deposition is 160 °C, the pressure of the chamber is 10 - 200 Pa, and the flow rate of the carrier gas during deposition is 50 mL / min (the carrier gas is nitrogen) to obtain an N, S co-coordinated Fe sub-nanocluster catalyst, labeled as Fe-NSC. Among them, the loading amount of Fe is 1.17 wt%, Fe exists in the form of sub-nanoclusters, and the cluster size is 1.0 nm.

[0076] Perform scanning electron microscopy test on the sub-nanocluster catalyst obtained in Example 1 to obtain Figure 1 . From Figure 1 it can be seen that Fe particles exist in the form of sub-nanoclusters.

[0077] Example 2

[0078] Use ozone and copper isopropoxide as precursors to deposit 3 cycles on the surface of N, S co-doped carbon support (N 2.8 wt%, S 3.5 wt%). The temperature of the chamber during deposition is 180 °C, and the flow rate of the carrier gas during deposition is 70 mL / min. Other conditions are the same as those in Example 1 to obtain an N, S co-coordinated Cu sub-nanocluster catalyst, labeled as Cu-NSC. Among them, the loading amount of Cu is 1.12 wt%, Cu exists in the form of sub-nanoclusters, and the cluster size is 1.1 nm.

[0079] Example 3

[0080] Using ozone and nickelocene as precursors, one cycle of deposition was carried out on the surface of an N,S co-doped carbon support (N 2.8 wt%, S 3.5 wt%). The temperature of the chamber during deposition was 150 °C, and the carrier gas flow rate during deposition was 75 mL / min. Other conditions were the same as in Example 1, obtaining an N,S co-coordinated Ni sub-nanocluster catalyst, labeled Ni-NSC. Among them, the loading of Ni was 1.18 wt%, Ni existed in the form of sub-nanoclusters, and the cluster size was 1.0 nm.

[0081] Example 4

[0082] Using ozone and (2’3-dimethyl-1,3-butadiene) tricarbonyl ruthenium as precursors, one cycle of deposition was carried out on the surface of an N,S co-doped carbon support (N 2.8 wt%, S 3.5 wt%). The temperature of the chamber during deposition was 200 °C, and the carrier gas flow rate during deposition was 75 mL / min. Other conditions were the same as in Example 1, obtaining an N,S co-coordinated Ru sub-nanocluster catalyst, labeled Ru-NSC. Among them, the loading of Ru was 1.14 wt%, Ru existed in the form of sub-nanoclusters, and the cluster size was 1.2 nm.

[0083] Example 5

[0084] Using HCHO and palladium hexafluoroacetylacetonate as precursors, three cycles of deposition were carried out on the surface of an N,S co-doped carbon support (N 2.8 wt%, S 3.5 wt%). The temperature of the chamber during deposition was 240 °C, and the carrier gas flow rate during deposition was 50 mL / min. Other conditions were the same as in Example 1, obtaining an N,S co-coordinated Pd sub-nanocluster catalyst, labeled Pd-NSC. Among them, the loading of Pd was 1.08 wt%, Pd existed in the form of sub-nanoclusters, and the cluster size was 1.4 nm.

[0085] Example 6

[0086] Using ozone and molybdenum hexacarbonyl as precursors, three cycles of deposition were carried out on the surface of an N,S co-doped carbon support (N 2.8 wt%, S 3.5 wt%). The temperature of the chamber during deposition was 170 °C, and the carrier gas flow rate during deposition was 80 mL / min. Other conditions were the same as in Example 1, obtaining an N,S co-coordinated Mo sub-nanocluster catalyst, labeled Mo-NSC. Among them, the loading of Mo was 1.27 wt%, Mo existed in the form of sub-nanoclusters, and the cluster size was 1.3 nm.

[0087] Example 7

[0088] Using ozone and ferrocene as precursors, 20 cycles of deposition were carried out on the surface of an N,O co-doped carbon support (N 8.6 wt%, O 9.7 wt%). Other conditions were the same as in Example 1, obtaining an Fe sub-nanocluster catalyst with N,O co-coordination, labeled as Fe-NOC. Among them, the loading of Fe was 2.14 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 0.8 nm.

[0089] Example 8

[0090] Using ozone and ferrocene as precursors, 15 cycles of deposition were carried out on the surface of an N,P co-doped carbon support (N 7.2 wt%, P 2.4 wt%). Other conditions were the same as in Example 1, obtaining an Fe sub-nanocluster catalyst with N,P co-coordination, labeled as Fe-NPC. Among them, the loading of Fe was 1.97 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 0.8 nm.

[0091] Example 9

[0092] Using ozone and ferrocene as precursors, 15 cycles of deposition were carried out on the surface of an O,S co-doped carbon support (O 6.2 wt%, S 3.1 wt%). Other conditions were the same as in Example 1, obtaining an Fe sub-nanocluster catalyst with O,S co-coordination, labeled as Fe-OSC. Among them, the loading of Fe was 1.62 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 0.9 nm.

[0093] Example 10

[0094] Using ozone and ferrocene as precursors, 15 cycles of deposition were carried out on the surface of an O,P co-doped carbon support (O 5.6 wt%, P 1.9 wt%). Other conditions were the same as in Example 1, obtaining an Fe sub-nanocluster catalyst with O,P co-coordination, labeled as Fe-OPC. Among them, the loading of Fe was 1.74 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 1.0 nm.

[0095] Example 11

[0096] Using ozone and ferrocene as precursors, 15 cycles of deposition were carried out on the surface of a P,S co-doped carbon support (P 2.1 wt%, S 3.3 wt%). Other conditions were the same as in Example 1, obtaining an Fe sub-nanocluster catalyst with P,S co-coordination, labeled as Fe-PSC. Among them, the loading of Fe was 1.81 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 0.9 nm.

[0097] Example 12

[0098] Using ozone and ferrocene as precursors, 15 cycles of deposition were carried out on the surface of commercial ZSM-5 support. The temperature of the cavity during deposition was 200 °C, and the carrier gas flow rate during deposition was 75 L / min. Other conditions were the same as in Example 1, obtaining a Fe sub-nanocluster catalyst co-coordinated with commercial ZSM-5, labeled as Fe-ZSM-5. Among them, the loading of Fe was 1.71 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 0.8 nm.

[0099] Example 13

[0100] Using ozone and ferrocene as precursors, 20 cycles of deposition were carried out on the surface of MnO2 support. The temperature of the cavity during deposition was 180 °C, and the carrier gas flow rate during deposition was 75 mL / min. Other conditions were the same as in Example 1, obtaining a Fe sub-nanocluster catalyst co-coordinated with MnO2, labeled as Fe-MnO2. Among them, the loading of Fe was 2.01 wt%, and Fe existed in the form of sub-nanoclusters with a cluster size of 1.1 nm.

[0101] The catalysts prepared in Examples 1 to 13 were subjected to catalyst activity evaluation. The experimental method was as follows: Add 5 mg of the catalysts prepared in Examples 1 to 13, 7 mL of deionized water, 3 mL of hydrogen peroxide (30 wt%), and 0.5 MPa of CH4 into a 20 mL polytetrafluoroethylene inner liner. Stir and react at 50 °C for 2 h to obtain C2 oxygen-containing products (including acetic acid, ethanol, acetaldehyde, etc.). The yields and selectivities of the C2 oxygen-containing products were tested. The test results are shown in Table 1.

[0102] Table 1 Yields and selectivities of the catalysts prepared in Examples 1 to 13 for the low-temperature conversion of methane to C2 oxygen-containing products

[0103]

[0104]

[0105] Comparative Example 1

[0106] Using ozone and ferrocene as precursors, 1 cycle of deposition was carried out on the surface of N,O co-doped carbon support (N 8.6 wt%, O 9.7 wt%). The temperature of the cavity during deposition was 160 °C, and the carrier gas flow rate during deposition was 50 mL / min. Other conditions were the same as in Example 1, obtaining a Fe single-atom catalyst co-coordinated with N,O, labeled as Fe1-NOC. Among them, the loading of Fe was 0.32 wt%, and Fe existed in the form of single atoms.

[0107] The single-atom catalyst obtained in Comparative Example 1 was subjected to scanning electron microscopy test to obtain Figure 2 . From Figure 2 it can be seen that Fe particles exist in the form of atoms.

[0108] Comparative Example 2

[0109] Using ozone and copper isopropoxide as precursors, two cycles of deposition were carried out on the surface of an N,O co-doped carbon support (N 8.6 wt%, O 9.7 wt%). The temperature of the deposition chamber was 180 °C, and the carrier gas flow rate during the deposition process was 70 mL / min. Other conditions were the same as in Example 1, obtaining a Cu single-atom catalyst with N,O co-coordination, labeled as Cu1-NOC. Among them, the Cu loading was 0.31 wt%, and Cu existed in the form of single atoms.

[0110] Comparative Example 3

[0111] Using ozone and nickelocene as precursors, one cycle of deposition was carried out on the surface of an N,O co-doped carbon support (N 8.6 wt%, O 9.7 wt%). The temperature of the deposition chamber was 150 °C, and the carrier gas flow rate during the deposition process was 75 mL / min. Other conditions were the same as in Example 1, obtaining an Ni single-atom catalyst with N,O co-coordination, labeled as Ni1-NOC. Among them, the Ni loading was 0.28 wt%, and Ni existed in the form of single atoms.

[0112] Comparative Example 4

[0113] Using ozone and ferrocene as precursors, three cycles of deposition were carried out on the surface of the MnO2 support. The temperature of the deposition chamber was 160 °C, and the carrier gas flow rate during the deposition process was 75 mL / min. Other conditions were the same as in Example 1, obtaining an Fe single-atom catalyst with MnO2 co-coordination, labeled as Fe1-MnO2. Among them, the Fe loading was 0.52 wt%, and Fe existed in the form of single atoms.

[0114] The catalysts prepared in Comparative Examples 1-4 were evaluated for catalytic activity. The experimental method was as follows: 5 mg of the catalysts prepared in Examples 14-17, 7 mL of deionized water, 3 mL of hydrogen peroxide (30 wt%), and 0.5 MPa of CH4 were added to a 20 mL polytetrafluoroethylene inner liner, and the mixture was stirred and reacted at 50 °C for 2 h to obtain a liquid-phase product. There was almost no C2 oxygen-containing product in the liquid-phase product. The yield and selectivity of the liquid-phase product were tested. The test results are shown in Table 2.

[0115] Table 2 Yield and selectivity of liquid-phase products of the catalysts prepared in Comparative Examples 1-4 in the low-temperature conversion reaction of methane

[0116]

[0117] From the data in Table 1 and Table 2, it can be seen that the catalysts prepared in Examples 1-13 can catalyze the conversion of methane to C2 oxygen-containing products at low temperature, while the catalysts prepared in Comparative Examples 1-4 catalyze the conversion of methane to C1 oxygen-containing products under the same conditions. It shows that due to different deposition parameters, the two catalysts obtained by ALD deposition have different numbers and sizes of metal nuclei on the same support. At the sub-nanometer scale, small changes in the number of metal nuclei and coordination environment in the metal catalyst (such as adding or subtracting a metal or coordination atom) cause huge differences in electronic properties and geometric structures, thereby significantly changing the catalytic performance. And compared with single-atom catalysts, sub-nanocluster catalysts have significant metal-metal bonds while having high metal utilization, which provides more possibilities for the bonding and reaction paths of reactants. At the same time, compared with nanoparticle catalysts, sub-nanocluster catalysts have more surface coordination unsaturated sites and higher metal dispersion; and sub-nanocluster catalysts have a unique electronic structure with discrete energy levels, and this unique quantum electronic structure provides sufficient space for regulating the adsorption and activation of reactants, thus showing excellent catalytic performance.

[0118] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A metal sub-nano cluster catalyst, characterized in that: It includes a carrier and a metal sub-nano cluster loaded on the carrier; the metal in the metal sub-nano cluster includes Fe, Cu, Ru, Pd, Mo or Ni; The particle size of the metal sub-nano cluster is 0.1-2 nm.

2. The metal sub-nano cluster catalyst according to claim 1, characterized in that: The loading amount of the metal sub-nano clusters in the metal sub-nano cluster catalyst is 0.5-10 wt %.

3. The metal sub-nano cluster catalyst according to claim 1, characterized in that: The carrier includes a heteroatom-doped carbon carrier, a molecular sieve or a metal oxide.

4. The metal sub-nano cluster catalyst according to claim 3, characterized in that: When the carrier is a carbon carrier doped with heteroatoms, the heteroatoms include one or more of N atoms, O atoms, S atoms and P atoms, and the mass fraction of the heteroatoms in the carrier is 1-20%.

5. The metal sub-nano cluster catalyst according to claim 3, characterized in that: When the carrier is a molecular sieve, the molecular sieve includes ZSM-5 molecular sieve; when the carrier is a metal oxide, the metal oxide includes MnO2, CeO2, TiO2 or ZnO.

6. The method for preparing the metal sub-nano cluster catalyst according to any one of claims 1 to 5, characterized in that: The following steps are involved: Applying the carrier dispersion on the surface of the substrate and removing the dispersant to obtain a dispersed carrier; Atomic layer deposition is used to deposit metal sub-nano clusters on the dispersed carrier to obtain a metal sub-nano cluster catalyst.

7. The preparation method according to claim 6, characterized in that: The conditions of the atomic layer deposition include: a temperature of 160 to 300° C., a pressure of 10 to 200 Pa, and a deposition cycle number of 1 to 30.

8. The preparation method according to claim 6 or 7, characterized in that: The carrier gas for the atomic layer deposition is an inert gas, and the flow rate of the carrier gas is 15 to 300 mL / min.

9. Use of the metal sub-nano cluster catalyst according to any one of claims 1 to 5 or the metal sub-nano cluster catalyst prepared by the preparation method according to any one of claims 6 to 8 in catalyzing the conversion of methane into C2 oxygen-containing products.

10. The use according to claim 9, characterized in that: The catalytic conversion of methane into C2 oxygen-containing products comprises the following steps: mixing the metal sub-nano cluster catalyst according to any one of claims 1 to 5, an oxidant and water, introducing methane, and performing an oxidation reaction to obtain C2 oxygen-containing products; The oxidant includes hydrogen peroxide or oxygen, and the pressure of the methane introduced is 0.1-3 MPa; The temperature of the oxidation reaction is 25-150°C.

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