CN114682280A

CN114682280A - Nickel monoatomic catalyst and its preparation method and use

Info

Publication number: CN114682280A
Application number: CN202210372538.XA
Authority: CN
Inventors: 乔涵; 索红日; 刘崇炫; 胡世文; 唐港
Original assignee: Southwest University of Science and Technology
Current assignee: Southwest University of Science and Technology
Priority date: 2022-04-11
Filing date: 2022-04-11
Publication date: 2022-07-01
Anticipated expiration: 2042-04-11
Also published as: CN114682280B

Abstract

The invention relates to the technical field of nano catalytic materials, in particular to a nickel monatomic catalyst and a preparation method and application thereof. According to the invention, chitosan sol is used for providing a carbon source, a sodium silicate aqueous solution is used for providing a silicon source, the mixture is mixed with a nickel salt aqueous solution in a proper pH range, and then the nickel-chitosan-sodium silicate ternary composite hydrogel with high nickel dispersion is rapidly formed through supermolecule complexation of three components, and the composite hydrogel can be subjected to synchronous drying and carbonization treatment by adopting a supercritical fluid with the temperature of more than 243.1 ℃ and the pressure of more than 6.39kPa to prepare the nickel monatomic catalyst. The preparation method is simple to operate, prevents excessive degradation of the pore network structure of the hydrogel, can avoid the problem that metal atoms are easy to agglomerate caused by high-temperature carbonization treatment in the conventional technology to the maximum extent, effectively improves the loading capacity of nickel monoatomic atoms on a carbon-based carrier, and endows the nickel monoatomic catalyst with excellent catalytic performance.

Description

Nickel monoatomic catalyst and its preparation method and use

Technical Field

The invention relates to the technical field of nano catalytic materials, in particular to a nickel monatomic catalyst and a preparation method and application thereof.

Background

Monatomic catalysts (SACs) are a new type of catalytic material that has emerged in recent years, and SACs are typically formed by uniformly distributing individual metal atoms on a support. Compared with the traditional carrier type catalyst, the SACs has the atom utilization rate close to 100 percent, high activity and good selectivity, shows excellent catalytic performance in various reactions, and greatly reduces the cost of the noble metal catalyst. At present, various SACs are applied to the catalytic fields of environment, biology and the like.

Nickel is a cheap metal and is widely applied to various novel chemical reactions, such as photocatalytic reaction and CO₂Reduction, water splitting and hydrogen evolution, and the like, and has the advantages of environmental protection, low cost and the like. The existing nickel monatomic catalyst mainly takes a metal organic framework compound, molybdenum disulfide and the like as carriers, and the carriers have the defects of sensitive synthesis conditions, time-consuming synthesis steps and low cost benefit.

SACs on carbon-based supports have the advantages of high specific surface area, developed pore structure, high electrical conductivity, excellent thermal/chemical stability, and potentially low manufacturing cost, and are considered to be the most promising advanced nanocatalysts with sustainable potential. Furthermore, the carbon-based support material often has heteroatom doping such as N, O, S, which not only can improve the dispersion of single atoms, but also can further improve the catalytic activity of the SACs. At present, there are two strategies, top-down and bottom-up, mainly for the synthesis of SACs of carbon-based carriers, which are widely reported. The "top-down" strategy typically starts with an existing carbon-based support (e.g., graphene sheets, carbon nanotubes, nanocarbon spheres, etc.) and then results in the formation of a physical confinement of the metal atoms by carbon vacancies on the carbon support. However, because the size of the carbon vacancies is not easily controlled and the number of vacancies tends to be very limited, clusters tend to form in the larger vacancies under high metal loading conditions, resulting in a generally lower atomic loading of the monatomic catalyst from a "top-down" approach. The "bottom-up" strategy is to start with metal and organic precursors, such as metal-porphyrin molecules, and then to form a carbon-based carrier embedded with metal single atoms through a high temperature carbonization process. The metal and the organic ligand in the synthesis by the bottom-up method are tightly connected in a coordination mode, the binding sites are rich, the metal types and the metal content are controllable, and the controllable preparation of the high-monatomic-supported catalyst is facilitated. However, the synthesis process of this method tends to be complex and time consuming. More importantly, the carbonization step in this process is usually carried out at high temperatures (> 700 ℃ C.). High temperature processing may result in reduced or even misplaced metal-organic/inorganic molecule coordination, such that metal ions tend to agglomerate into clusters or nanoparticles during annealing, resulting in a significant reduction in the loading of metal monoatomic atoms. Thus, the loading of metal monoatomic species in conventional carbon-based supported catalysts synthesized by the "bottom-up" strategy is typically relatively low (less than 2 wt%), which limits their practical applications.

Disclosure of Invention

Therefore, it is necessary to provide a nickel monatomic catalyst which is fast and simple to prepare and has high metal loading capacity and takes carbon as a carrier, and a preparation method and application thereof.

In one aspect of the present invention, a method for preparing a nickel monatomic catalyst is provided, which comprises the following steps:

respectively providing chitosan sol, sodium silicate aqueous solution and nickel salt aqueous solution; mixing the sodium silicate aqueous solution and the chitosan sol, adjusting the pH value to 5-7, and adding the nickel salt aqueous solution to prepare a reaction system; oscillating the reaction system at the frequency of 45 Hz-60 Hz to prepare the nickel-chitosan-sodium silicate composite hydrogel; synchronously drying and carbonizing the nickel-chitosan-sodium silicate composite hydrogel by adopting a supercritical fluid with the temperature of more than 243.1 ℃ and the pressure of more than 6.39 kPa;

the chitosan sol is prepared by dissolving chitosan in an acid aqueous solution, wherein the acid aqueous solution is one or more of an acetic acid aqueous solution, a citric acid aqueous solution and a hydrochloric acid aqueous solution.

In some embodiments, the supercritical fluid is an ethanol supercritical fluid having a temperature of 250 ℃ to 600 ℃ and a pressure of 7kPa to 15 kPa.

In some embodiments, the aqueous acid solution is 0.5-2% by weight of aqueous acetic acid.

In some embodiments, the aqueous acid solution is an aqueous acetic acid solution, and the chitosan is used in an amount of 1g to 10g per 100mL of the aqueous acetic acid solution.

In some embodiments, the chitosan has a degree of deacetylation of greater than or equal to 75%.

In some embodiments, the concentration of the aqueous sodium silicate solution is 50mg/mL to 150 mg/mL.

In some embodiments, the sodium silicate of the aqueous sodium silicate solution comprises Na₂O and SiO₂The amount ratio of the substance(s) is 1 (1 to 1.5).

In some embodiments, the aqueous nickel salt solution is prepared by dissolving a water-soluble nickel salt in water, the water-soluble nickel salt being one or more of nickel nitrate, nickel chloride, and nickel acetate.

In some embodiments, the nickel salt solution contains nickel element in an amount of 20mg/mL to 50 mg/mL.

In some embodiments, the mass ratio of the nickel element, the chitosan and the sodium silicate in the nickel-chitosan-sodium silicate composite hydrogel is 1 (29-64): (29-64).

In some embodiments, the drying and carbonizing treatment is performed for 3 to 6 hours.

In another aspect of the present invention, there is also provided a nickel monatomic catalyst produced by the production method according to any one of the preceding embodiments.

In some embodiments, the nickel monatomic catalyst has a nickel atom content of 5% to 11% by mass.

In another aspect of the invention, the application of the nickel monatomic catalyst in photocatalytic reaction is also provided.

In some embodiments, the photocatalytic reaction is a reaction that catalyzes the degradation of tetracycline by visible light.

According to the invention, chitosan sol is adopted to provide a carbon source, a sodium silicate aqueous solution is adopted to provide a silicon source, and after the chitosan sol is mixed with a nickel salt aqueous solution in a proper pH range, nickel, amino and hydroxyl of chitosan and silicon hydroxyl in hydrolyzed sodium silicate can rapidly form nickel-chitosan-sodium silicate ternary composite hydrogel with high nickel dispersion through supermolecular complexation, and the composite hydrogel can be used for preparing a nickel monatomic catalyst through synchronous drying and carbonization treatment by adopting a supercritical fluid with the temperature of more than 243.1 ℃ and the pressure of more than 6.39 kPa. The preparation method is simple to operate, bypasses a liquid-solid-gas triple point, and greatly weakens the surface tension among hydrogel molecules, so that excessive degradation of a pore network structure of the hydrogel is prevented, the problem that metal atoms are easy to agglomerate caused by high-temperature carbonization treatment in the conventional technology can be avoided to the greatest extent, and the loading rate of nickel single atoms on the carbon-based carrier is effectively improved. In addition, a SiC phase is generated during the supercritical carbonization, and the heterostructure thereof with the carbon support gives the nickel monatomic catalyst excellent catalytic performance.

The nickel monatomic catalyst prepared by the invention has high nickel monatomic loading rate, is a cheap metal monatomic catalyst with low cost and high catalytic efficiency, can be widely applied to various catalytic systems, particularly visible light catalytic reaction, can be used for visible light catalytic degradation of antibiotics such as tetracycline and the like, and provides a green and low-cost path for pollution treatment of the antibiotics.

Drawings

FIG. 1 shows 10% Ni-chitosan-sodium silicate supramolecular composite hydrogel (10% Ni-CS-Si) prepared in example 2;

FIG. 2 is a high angle annular dark field image-scanning transmission electron microscope (HADDF-STEM) image of 10.7% Ni-C-Si of the nickel monatomic catalyst prepared in example 2;

FIG. 3 is an X-ray diffraction (XRD) pattern of the catalysts prepared in examples 1 to 2 and comparative examples 1 to 2;

FIG. 4 is an extended edge X-ray fine structure (EXAFS) diagram of the catalyst prepared in examples 1-2;

FIG. 5 is a graph showing a comparison of catalytic effects of the catalysts prepared in examples 1 to 2 and comparative examples 1 to 3 and silicon carbide;

FIG. 6 is a graph showing the recycling effect of 10.7% Ni-C-Si of the Ni monatomic catalyst prepared in example 2 on the photocatalytic degradation of tetracycline.

Detailed Description

To facilitate an understanding of the invention, the invention will now be described more fully with reference to the accompanying drawings. Preferred embodiments of the present invention are shown in the drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete.

Furthermore, the terms "first", "second" and "first" are used for descriptive purposes only and are not to be construed as indicating or implying relative importance or implicitly indicating the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one such feature. In the description of the invention, "a plurality" means at least two, e.g., two, three, etc., unless explicitly specified otherwise. In the description of the present invention, "a plurality" means at least one, e.g., one, two, etc., unless explicitly specified otherwise.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the term "and/or" includes any and all combinations of one or more of the associated listed items.

In the present invention, the technical features described in the open type include a closed technical solution composed of the listed features, and also include an open technical solution including the listed features.

In the present invention, the numerical intervals are regarded as continuous, and include the minimum and maximum values of the range and each value between the minimum and maximum values, unless otherwise specified. Further, when a range refers to an integer, each integer between the minimum and maximum values of the range is included. Further, when multiple range-describing features or characteristics are provided, the ranges may be combined. In other words, unless otherwise indicated, all ranges disclosed herein are to be understood to encompass any and all subranges subsumed therein.

The percentage contents referred to in the present invention mean, unless otherwise specified, mass percentages for solid-liquid mixing and solid-solid mixing, and volume percentages for liquid-liquid mixing.

The percentage concentrations referred to in the present invention refer to the final concentrations unless otherwise specified. The final concentration refers to the ratio of the additive component in the system to which the component is added.

The temperature parameter in the present invention is not particularly limited, and may be a constant temperature treatment or a treatment within a certain temperature range. The constant temperature process allows the temperature to fluctuate within the accuracy of the instrument control.

The water-soluble nickel salt of the invention may or may not have crystal water, for example, nickel nitrate is understood to mean nickel nitrate without crystal water, or Ni (NO) with six crystal waters₃)₂·6H₂O。

In one aspect of the present invention, a method for preparing a nickel monatomic catalyst is provided, comprising the steps of:

respectively providing chitosan sol, sodium silicate aqueous solution and nickel salt aqueous solution; mixing a sodium silicate aqueous solution and the chitosan sol, adjusting the pH value to 5-7, and adding a nickel salt aqueous solution to prepare a reaction system; oscillating the reaction system at the frequency of 45-60 Hz to prepare the nickel-chitosan-sodium silicate composite hydrogel; drying and carbonizing the nickel-chitosan-sodium silicate composite hydrogel by adopting a supercritical fluid with the temperature of more than 243.1 ℃ and the pressure of more than 6.39 kPa;

the chitosan sol is prepared by dissolving chitosan in an acetic acid aqueous solution to prepare an acid aqueous solution, wherein the acid aqueous solution is one or more of an acetic acid aqueous solution, a citric acid aqueous solution and a hydrochloric acid aqueous solution.

By adopting the chitosan sol as a carbon source and the sodium silicate aqueous solution as a silicon source, and mixing the chitosan sol and the nickel salt aqueous solution within a proper pH range, the amino and hydroxyl groups of the nickel and the chitosan and the silicon hydroxyl groups in the hydrolyzed sodium silicate can quickly form the nickel-chitosan-sodium silicate composite hydrogel (the time is less than or equal to 2s) through supermolecular complexation. In the composite hydrogel, due to the complexation among three components, the nickel metal nodes and the chitosan organic ligand are highly ordered in arrangement, so that the nickel has excellent dispersibility, and the subsequent drying and carbonization treatment of the supercritical ethanol fluid can be carried out at a lower temperature (higher than 243.1 ℃, compared with the temperature of more than 700 ℃ in the traditional technology), so that liquid-solid-gas triple points are bypassed, the surface tension among hydrogel molecules is greatly reduced, the excessive degradation of the pore network structure of the hydrogel is prevented, the excessive aggregation of metal atoms in the pyrolysis process is effectively avoided, the active site density of nickel monoatomic atoms is favorably improved, the loading rate of the nickel monoatomic atoms on a carbon carrier is effectively improved, and the monoatomic catalyst has excellent catalytic performance. In addition, the synthesis method takes chitosan and industrial-grade sodium silicate as raw materials, has low cost, and is quick, simple and easy to operate in the synthesis process, thereby being a novel synthesis method of the carbon-based single-atom catalyst with expansion potential.

In some embodiments, the pH adjusted after mixing the aqueous sodium silicate solution and the chitosan sol may be, for example, 5.5, 6, or 6.5. The pH value directly influences the formation of the composite hydrogel, and the hydrogel with proper viscosity can be formed by controlling the pH value within a proper range, so that the arrangement of the nickel metal nodes and the organic/inorganic ligands is more ordered, and the density of the active sites of the nickel monoatomic atoms in the subsequently prepared catalyst is improved.

In some embodiments, the reaction system is oscillated at a frequency of, for example, 50Hz or 55 Hz.

In some embodiments, the container that is agitated with the reaction system is a vortex mixer.

In some embodiments, the temperature of the ethanol supercritical fluid may be, for example, 300 ℃, 350 ℃, 400 ℃, 450 ℃, 500 ℃, 550 ℃ or 600 ℃ in the dry carbonization treatment.

In some embodiments, the pressure of the ethanol supercritical fluid during the dry carbonization treatment may be, for example, 8kPa, 9kPa, 10kPa, 11kPa, 12kPa, 13kPa, or 14 kPa.

The temperature and pressure of the ethanol supercritical fluid have direct influence on the drying and carbonizing treatment result, and proper temperature and pressure not only can ensure thorough drying and carbonization, but also is beneficial to improving the loading capacity and stability of nickel single atoms in the catalyst.

In some embodiments, the aqueous acid solution is 0.5-2% by weight of aqueous acetic acid. Alternatively, the aqueous acetic acid solution may be, for example, 0.75%, 1%, 1.25%, 1.5%, or 1.75% by mass. The mass percentage of the acetic acid aqueous solution is related to the state of the chitosan sol, so that the state of the subsequent composite hydrogel is influenced, and the proper percentage can ensure that the nickel atoms in the formed composite hydrogel are more fully complexed with the chitosan, thereby being beneficial to improving the density of the active sites of the nickel single atoms in the catalyst finished product.

In some embodiments, the aqueous acid solution is an aqueous acetic acid solution, and the amount of chitosan used is 1g to 5g per 100mL of the aqueous acetic acid solution. Alternatively, the amount of chitosan used per 100mL of aqueous acetic acid solution may be, for example, 1.5g, 2g, 2.5g, 3g, 3.5g, 4g, or 4.5 g. The chitosan dosage corresponding to each 100mL of acetic acid aqueous solution also affects the formation of chitosan sol, the dosage is too small, the system is too dilute to form sol, and the dosage is too large, the viscosity is too high, the formation of composite hydrogel is not facilitated, and the uneven distribution of all components in the finished catalyst product is easily caused.

In some embodiments, the degree of deacetylation of chitosan is 75% or greater, and can be, for example, 80%, 85%, 90%, or 95%.

In some embodiments, the aqueous sodium silicate solution (in SiO)₂Content) was 50mg/mL or 150 mg/mL. Alternatively, the concentration of sodium silicate may be, for example, 50mg/mL, 80mg/mL, 100mg/mL, 120mg/mL, or 150 mg/mL.

In some embodiments, the sodium silicate of the aqueous sodium silicate solution comprises Na₂O and SiO₂The ratio of the amounts of the substances (1) to (1.5) is 1. Alternatively, Na₂O and SiO₂The ratio of the amounts of substances of (a) may be, for example, 1:1.2 or 1: 1.4. Na (Na)₂O and SiO₂The ratio of the amounts of the substances (a) to (b) shows the composition of the sodium silicate, which is an important parameter of sodium silicate, and the sodium silicate is polymerized to a different extent and thus the hydrolysate is also different for different ratios. Because sodium silicate is used as a silicon source of the nickel monatomic catalyst, Na is controlled₂O and SiO₂In a proper range, the ratio of the amount of the SiO component is set to be₂The content of the chitosan is more proper, and the chitosan can coordinate with nickel monoatomic atoms better in the process of forming the composite hydrogel, so that the nickel element in the final catalyst can exist in a monoatomic form more, and the preparation efficiency is improved.

In some embodiments, the nickel salt solution contains nickel in an amount of 20mg/mL to 50 mg/mL. Alternatively, the concentration of elemental nickel may be, for example, 20mg/mL, 30mg/mL, 40mg/mL, or 50 mg/mL.

In some embodiments, the mass ratio of the nickel element, the chitosan and the sodium silicate in the nickel-chitosan-sodium silicate composite hydrogel is 1 (29-64): 29-64, and the mass ratio of the nickel element, the chitosan and the sodium silicate may be 1 (30-63.33): 30-63.33, for example. The mass ratio of the three components is controlled in a proper range, so that the prepared catalyst has proper nickel monoatomic load, and has higher catalytic activity. In some embodiments, the time for the dry carbonization treatment is 3 to 6 hours. The time for the dry carbonization treatment may be, for example, 3 hours, 4 hours, 5 hours, or 6 hours. The time of drying and carbonizing treatment can ensure that the composite hydrogel can be completely dried and carbonized, and the prepared catalyst has higher catalytic activity and stability.

In another aspect of the present invention, there is also provided a nickel monatomic catalyst produced by the production method according to any one of the preceding embodiments. The nickel monatomic catalyst prepared by the invention is black powder in character, has high nickel monatomic loading rate, is a cheap metal monatomic catalyst with low cost and high catalytic efficiency, can be used for multiple times of catalysis after the catalytic reaction is finished and can be widely applied to various catalytic systems including but not limited to visible light catalytic reaction including visible light catalytic degradation of antibiotics such as tetracycline, and provides a green and low-cost path for pollution treatment of the antibiotics.

In some embodiments, the nickel monatomic catalyst has a nickel atom content of 5% to 11% by mass. Alternatively, the content of nickel element by mass may be, for example, 6%, 7%, 8%, 9%, or 10%. The proper nickel loading can ensure that the catalyst has enough catalytic activity, and meanwhile, the nickel monoatomic group is not agglomerated due to excessive loading, and the performance of the catalyst is not reduced.

In another aspect of the invention, the application of the nickel monatomic catalyst in the photocatalytic reaction is also provided.

In some embodiments, the reaction steps for visible light catalyzed tetracycline degradation are as follows:

dispersing the nickel monatomic catalyst in tetracycline aqueous solution, stirring under the condition of keeping out of the sun until adsorption-desorption equilibrium, and then continuously stirring for reaction under the irradiation of visible light.

In some embodiments, the mass ratio of the nickel monatomic catalyst to tetracycline is (15-25): 1, preferably 20: 1.

In some embodiments, the concentration of tetracycline in the aqueous tetracycline solution is from 25mg/L to 75mg/L, preferably 50 mg/L.

In some embodiments, the visible light source is a xenon lamp. Further, the power of the xenon lamp is 200W to 400W, and further, the power of the xenon lamp is 300W.

In some embodiments, after the visible light catalytic degradation tetracycline reaction is finished, the reaction system is centrifuged, the nickel monatomic catalyst is taken out, washed and dried by deionized water, and the nickel monatomic catalyst is recovered. The recovered nickel monatomic catalyst can still be continuously used for photocatalytic degradation of tetracycline in the next cycle (the repeated use times are more than or equal to 5), and the degradation efficiency of the tetracycline is almost unchanged and is maintained at more than 98.7%.

The present invention will be described in further detail with reference to specific examples and comparative examples. Experimental parameters not described in the following specific examples are preferably referred to the guidelines given in the present application, and may be referred to experimental manuals in the art or other experimental methods known in the art, or to experimental conditions recommended by the manufacturer. It is understood that the following examples are specific to the particular apparatus and materials used, and in other embodiments, are not limited thereto; the weight of the related components mentioned in the embodiments of the present specification may not only refer to the specific content of each component, but also represent the proportional relationship of the weight among the components, and therefore, the content of the related components is scaled up or down within the scope disclosed in the embodiments of the present specification according to the present specification. Specifically, the weight described in the description of the embodiment of the present invention may be a mass unit known in the chemical engineering field such as μ g, mg, g, kg, etc.

And (3) reagent sources:

and (3) chitosan: the deacetylation degree of the abamectin (C299272) is more than or equal to 75 percent.

Example 1

10mL of sodium silicate solution (Na)₂O and SiO₂The ratio of the amount of the substance is 1:1, SiO₂Concentration 100mg/mL) was added to 40mL of chitosan sol having a concentration of 50mg/mL (solvent: 1% by mass aqueous acetic acid solution), thoroughly mixed and adjusted to pH 6.5, followed by addition of3.16mL of freshly prepared nickel nitrate solution (50mg/L in Ni)²⁺Metering), immediately oscillating the reaction system by using a vortex mixer at the frequency of 50Hz, and performing ultra-fast cogelation on the ternary system within 2s to obtain homogeneous nickel-chitosan-sodium silicate composite hydrogel; the nickel-chitosan-sodium silicate composite hydrogel is synchronously dried and carbonized for 4 hours by adopting ethanol supercritical fluid with the temperature and the pressure of 300 ℃ and 10kPa respectively to obtain the nickel monatomic catalyst with the nickel monatomic load of 5.3 percent, which is recorded as 5.3 percent of Ni-C-Si.

Example 2

10mL of sodium silicate solution (Na)₂O and SiO₂The ratio of the amount of the substances is 1:1, SiO₂Concentration 100mg/mL) was added to 40mL of chitosan sol having a concentration of 50mg/L (solvent: 1% by mass aqueous acetic acid), thoroughly mixed and adjusted to pH 6.5, followed by addition of 6.67mL of freshly prepared nickel nitrate solution (50mg/L in Ni)²⁺Metering), immediately oscillating the reaction system by using a vortex mixer at the frequency of 50Hz, and ultra-rapidly cogelating the internal system in 2s to obtain homogeneous nickel-chitosan-sodium silicate composite hydrogel which is recorded as 10% Ni-CS-Si; drying and carbonizing the nickel-chitosan-sodium silicate composite hydrogel by adopting ethanol supercritical fluid with the temperature and the pressure of 300 ℃ and 10kPa respectively for 4 hours to obtain the nickel monatomic catalyst with the nickel monatomic load of 10.7%, which is recorded as 10.7% of Ni-C-Si.

Comparative example 1

10mL of sodium silicate solution (Na)₂O and SiO₂The ratio of the amount of the substances is 1:1, SiO₂Concentration 100mg/mL) was added to 40mL of chitosan sol having a concentration of 50mg/L (solvent: 1% by mass aqueous acetic acid), thoroughly mixed and adjusted to pH 6.5, followed by addition of 20mL of freshly prepared nickel nitrate solution (50mg/L in Ni)²⁺Metering), immediately shaking the reaction system by a vortex mixer at the frequency of 50Hz, and ultra-fast cogelling of the internal system for 2s to obtain the nickel-chitosan-sodium silicate composite hydrogel; drying and carbonizing the nickel-chitosan-sodium silicate composite hydrogel by adopting ethanol supercritical fluid with the temperature and the pressure of 300 ℃ and 10kPa respectively for 4 hours to obtain a catalyst with the nickel element loading of 22.3 percent, which is recorded as 22.3 percent of Ni-C-Si。

Comparative example 2

10mL of sodium silicate solution (Na)₂O and SiO₂The ratio of the amount of the substances is 1:1, SiO₂Concentration 100mg/mL) was added to 40mL of chitosan sol having a concentration of 50mg/L (solvent: 1% by mass aqueous acetic acid), thoroughly mixed and adjusted to pH 6.5, followed by addition of 40mL of freshly prepared nickel nitrate solution (50mg/L in Ni)²⁺Metering), immediately oscillating the reaction system by using a vortex mixer at the frequency of 50Hz, and performing ultra-fast cogelation on the internal system for 2s to obtain the nickel-chitosan-sodium silicate composite hydrogel; drying and carbonizing the nickel-chitosan-sodium silicate composite hydrogel by adopting ethanol supercritical fluid with the temperature and the pressure of 300 ℃ and 10kPa respectively for 4 hours to obtain a catalyst with the nickel element loading of 30.8%, which is recorded as 30.8% Ni-C-Si.

Comparative example 3

40mL of chitosan sol (solvent: 1% by mass of aqueous acetic acid solution) at a concentration of 50mg/L was prepared, pH was adjusted to 6.5, and 2.1mL of freshly prepared nickel nitrate solution (50mg/L in terms of Ni) was added²⁺Metering), immediately oscillating the reaction system by using a vortex mixer at the frequency of 50Hz, and ultra-fast cogelating the binary system within 2s to obtain the nickel-chitosan composite hydrogel; drying and carbonizing the nickel-chitosan composite hydrogel by adopting ethanol supercritical fluid with the temperature and the pressure of 300 ℃ and 10kPa respectively for 4 hours to obtain the nickel monatomic catalyst with the nickel monatomic load of 5.6 percent, which is recorded as 5.6 percent Ni-C.

Characterization test:

(1) the 10% Ni-CS-Si composite gel prepared in example 2 is a homogeneous, pale green hydrogel (FIG. 1); when the nickel monatomic catalyst prepared in example 2, 10.7% Ni-C-Si, was analyzed by a high-angle annular dark field image-scanning transmission electron microscope (HAADF-STEM), it was clearly seen that a large number of bright spots of individual nickel atoms were uniformly dispersed on the support (fig. 2); the XRD pattern of the composite material has two diffraction peaks at 34.5 degrees and 60.7 degrees, which correspond to layered SiC (111) and SiC (220), respectively, and shows that the composite hydrogel forms a C/SiC composite after supercritical carbonization (figure 3), and in addition, 30.8 percent of Ni-C-Si and 22.3 percent of Ni-C-Si are 43.1 percent of Ni-C-SiA distinct Ni (111) diffraction peak at ° which is not found in the 5.3% Ni-C-Si and 10.7% Ni-C-Si spectra for the lower nickel loadings, indicating that nickel is present in a monoatomic dispersion on the C/SiC support when the nickel loading is low; when the nickel is excessively loaded, agglomeration of nickel on the carrier may be caused, which is disadvantageous to the formation of the nickel monatomic catalyst. (2) The EXAFS spectrum of the nickel foil (FIG. 4) can be observed

The atomic-scale dispersion of nickel in the 10.7% Ni-C-Si and 5.3% Ni-C-Si composites was further confirmed by the peaks of the Ni-Ni bonds, which were not present in the 10.7% Ni-C-Si and 5.3% Ni-C-Si composites.

(3) And (3) testing the catalytic performance:

taking 50mg of the catalyst prepared in each example and comparative example, respectively dispersing the catalyst in 50mL of tetracycline aqueous solution with the concentration of 50mg/L, and stirring the solution for 1h in the dark until the adsorption-desorption balance is achieved; then continuously stirring under the irradiation of visible light (lambda is more than 420nm) of a 300W xenon lamp, sampling and filtering at different illumination time, analyzing the filtrate by using a high performance liquid chromatography equipped with an ultraviolet detector, measuring the concentration of tetracycline in the solution, recording data and finishing to obtain a catalytic effect comparison graph shown in figure 5.

As can be seen from FIG. 5, the monatomic catalysts 5.3% Ni-C-Si and 10.7% Ni-C-Si have the best catalytic effects (two lines closest to the abscissa time axis in the horizontal direction), and degradation of tetracycline is substantially completed after 20 minutes of reaction. When the loading of nickel is too high (22.3%, 30.8%), the catalytic performance is significantly reduced, probably because the nickel atoms are agglomerated and no longer supported on the carbon support in the form of a single atom; when no silicon component is introduced into the catalyst (5.6 percent of Ni-C), the performance of the catalyst is also obviously reduced, which shows that silicon element is indispensable in the catalyst, and the silicon element and chitosan jointly participate in the coordination of nickel atoms during the formation of the composite hydrogel, so that the nickel atoms can exist in a more highly dispersed form, and the catalyst loaded in a nickel monoatomic form can be smoothly formed; while the silicon carbide without nickel element is worse in catalytic performance. The results show that the carbon-based carrier catalyst doped with the single-atom nickel and the silicon at the same time in a certain proportion has better catalytic activity compared with silicon carbide and a carbon-based material loaded with nickel alone, and the silicon and the nickel in a certain proportion play a role in synergy.

In addition, the catalyst for catalytic degradation reaction prepared in example 2 was recovered by centrifugation, washed with deionized water and dried, and then subjected to cyclic photocatalytic degradation reaction (fig. 6), and the results showed that 10.7% Ni-C-Si has no significant change in the degradation efficiency of tetracycline (99.37%, 99.25%, 99.17%, 99.14% and 98.78%, respectively) in 5 consecutive catalytic runs, and catalytic degradation of tetracycline could be substantially completed within about 20 minutes of reaction, which indicated that the 5% Ni-C-Si catalyst had higher stability and durability.

The technical features of the embodiments described above may be arbitrarily combined, and for the sake of brevity, all possible combinations of the technical features in the embodiments described above are not described, but should be considered as being within the scope of the present specification as long as there is no contradiction between the combinations of the technical features.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is specific and detailed, but not to be understood as limiting the scope of the invention. It should be noted that various changes and modifications can be made by those skilled in the art without departing from the spirit of the invention, and these changes and modifications are all within the scope of the invention. Therefore, the protection scope of the patent of the present invention should be subject to the appended claims, and the description and the drawings can be used for explaining the contents of the claims.

Claims

1. A preparation method of a nickel monatomic catalyst is characterized by comprising the following steps:

respectively providing chitosan sol, sodium silicate aqueous solution and nickel salt aqueous solution; mixing the sodium silicate aqueous solution and the chitosan sol, adjusting the pH value to 5-7, and adding the nickel salt aqueous solution to prepare a reaction system; oscillating the reaction system at the frequency of 45-60 Hz to prepare the nickel-chitosan-sodium silicate ternary composite hydrogel; synchronously drying and carbonizing the nickel-chitosan-sodium silicate composite hydrogel by adopting a supercritical fluid with the temperature of more than 243.1 ℃ and the pressure of more than 6.39 kPa;

2. The production method according to claim 1, wherein the supercritical fluid is an ethanol supercritical fluid having a temperature of 250 to 600 ℃ and a pressure of 7 to 15 kPa; and/or

The acid water solution adopts 0.5 to 2 mass percent of acetic acid water solution; and/or

The acid aqueous solution is acetic acid aqueous solution, and the dosage of the chitosan corresponding to each 100mL of the acetic acid aqueous solution is 1 g-10 g; and/or

The deacetylation degree of the chitosan is more than or equal to 75 percent.

3. The preparation method according to claim 1, wherein the concentration of the sodium silicate aqueous solution is 50mg/mL to 150 mg/mL; and/or

Na in sodium silicate of the sodium silicate aqueous solution₂O and SiO₂The amount ratio of the substance(s) is 1 (1 to 1.5).

4. The preparation method according to claim 1, wherein the aqueous nickel salt solution is prepared by dissolving a water-soluble nickel salt in water, wherein the water-soluble nickel salt is one or more of nickel nitrate, nickel chloride and nickel acetate; and/or

The content of nickel element in the nickel salt water solution is 20 mg/mL-50 mg/mL.

5. The preparation method according to any one of claims 1 to 4, wherein the mass ratio of the nickel element to the chitosan to the sodium silicate in the nickel-chitosan-sodium silicate composite hydrogel is 1 (29-64) to (29-64).

6. The method according to any one of claims 1 to 4, wherein the drying and carbonizing are performed for 3 to 6 hours.

7. A nickel monatomic catalyst produced by the production method according to any one of claims 1 to 6.

8. The nickel monatomic catalyst of claim 7, wherein the nickel monatomic catalyst has a nickel atom content of 5% to 11% by mass.

9. Use of the nickel monatomic catalyst of claim 7 or 8 in a photocatalytic reaction.

10. The use according to claim 9, wherein the photocatalytic reaction is a visible light catalyzed tetracycline degradation reaction.