Monatomic catalyst and preparation method thereof

Abstract

According to the present invention, there is provided a monatomic catalyst, the monatomic catalyst comprising: a nitrogen-doped carbon structure; and a monatomic metal wherein the monatomic metal forms a coordinate bond with a nitrogen atom of the nitrogen-doped carbon structure. When an electrochemical cell having an electrode pair containing the monatomic catalyst is used in an electrochemical advanced oxidation process for water treatment, organic pollutants in wastewater can be effectively decomposed under mild pH conditions.

Description

Technical Field

[0001] The invention relates to a single-atom catalyst and a preparation method thereof. Background Art

[0002] Electrochemical advanced oxidation process (EAOP) has high efficiency, the use of renewable energy, and the possibility of modularization of distributed systems. Therefore, the electrochemical advanced oxidation process has attracted increasing attention in the field of water treatment.

[0003] The electrochemical advanced oxidation process includes the generation of hydroxyl radicals (·OH) in the anode and the oxidation of refractory organic pollutants in wastewater. In the cathode, hydrogen peroxide (H2O2), a precursor of hydroxyl radicals, is generated through the selective reduction of oxygen.

[0004] In the past 10 years, electrochemical advanced oxidation processes have made significant progress, and various studies on improving electrochemical advanced oxidation processes are still ongoing. In recent years, research on the use of single atom catalysts in electrochemical advanced oxidation processes has also been conducted.

[0005] [Prior art literature]

[0006] [Patent Document]

[0007] (Patent Document 1) KR10-2342524B1 Summary of the invention

[0008] 1. Technical issues to be resolved

[0009] According to an aspect of the present disclosures, a single-atom catalyst can be provided, which can expand the optimal pH range of electrochemical advanced oxidation and prevent the adsorption of hydroxyl radicals on electrodes.

[0010] (II) Technical solution

[0011] According to the present invention, a single atom catalyst is provided, comprising: a nitrogen-doped carbon structure; and a single atom metal, wherein the single atom metal forms a coordination bond with a nitrogen atom of the nitrogen-doped carbon structure.

[0012] According to one embodiment, the carbon structure may be carbon black.

[0013] According to one embodiment, the single-atom metal may be cobalt (Co).

[0014] According to one embodiment, the content of the single-atom metal in the single-atom catalyst may be more than 0 wt % and less than 3 wt %.

[0015] According to the present invention, a method for preparing a single atom catalyst is provided, the method comprising the following steps: mixing a single atom metal precursor and a nitrogen-dopant precursor in a solvent to prepare a mixed solution; mixing a carbon structure powder in the mixed solution to prepare a suspension; oscillating and drying the suspension to prepare a single atom catalyst precursor; and calcining the single atom catalyst precursor to prepare a single atom catalyst.

[0016] According to the present invention, an electrode is provided, comprising: a substrate; and a catalyst layer, wherein the catalyst layer is coated on the substrate, wherein the catalyst layer contains the above-mentioned single-atom catalyst.

[0017] According to one embodiment, the substrate may include stainless steel.

[0018] According to one embodiment, a ratio of the thickness of the substrate to the thickness of the catalyst layer may be 10:1 to 1000:1.

[0019] According to the present invention, an electrochemical cell is provided, which comprises the above-mentioned electrodes as a positive electrode (cathode) and a negative electrode (anode).

[0020] According to the present invention, an electrochemical water treatment method is provided, wherein the electrochemical water treatment method utilizes the electrochemical cell described above.

[0021] (III) Beneficial effects

[0022] According to one embodiment of the present invention, the electrochemical advanced oxidation process can be performed under various pH conditions, and the efficiency of removing organic matter in wastewater by the electrochemical advanced oxidation process can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1a is a schematic diagram of a formation process of a chemical structure of a single atom catalyst according to one embodiment.

[0024] Figure 1b is a schematic diagram of a process for manufacturing an electrode including a single atom catalyst according to one embodiment.

[0025] Figure 2a to Figure 2f is a HAADF-STEM image of a single atom catalyst according to one embodiment.

[0026] Figure 3a and Figure 3b is an XRD diffraction pattern of a single atom catalyst according to one embodiment.

[0027] Figure 4a is a XANES spectrum of a single atom catalyst according to an embodiment, Figure 4b is the Fourier transform k of a single atom catalyst according to one embodiment 2 Weighted EXAFS spectrum.

[0028] Figure 4c and Figure 4d is an EXAFS fit of the first shell of a single atom catalyst according to one embodiment.

[0029] Figure 4e to Figure 4h is a wavelet transform (WT) analysis result of χ(k) of a single atom catalyst according to one embodiment.

[0030] Figure 5 is a soft XAS plot of a single atom catalyst according to one embodiment.

[0031] Figure 6 is an electron paramagnetic resonance (EPR) spectrum of a single atom catalyst according to an embodiment.

[0032] Figure 7a is an EPR spectrum of a DMPO adduct according to current density of an embodiment, Figure 7b : is an EPR spectrum of the DMPO adduct according to the initial pH of one embodiment.

[0033] Figure 8a and Figure 8b is an embodiment of a color change of a H2O2 test strip according to time, Figure 8c is a linear sweep voltammetry (LSV) curve of a single atom catalyst electrode according to one embodiment.

[0034] Figure 9a is a change in absorbance at a specific wavelength according to time, Figure 9b 1 is a graph showing changes in the concentration of hydrogen peroxide according to time according to an embodiment.

[0035] Fig.10 An embodiment is a color change of a H2O2 test strip according to time.

[0036] Fig.11a and Fig.11b are graphs showing the electrochemical oxidation efficiency of phenol using a single atom catalyst electrode at different current densities and different initial pH values ​​according to one embodiment, Fig.11c This is a comparison diagram of the phenol removal efficiency when both the positive electrode and the negative electrode are single-atom catalyst electrodes and when only the negative electrode is a single-atom catalyst electrode. DETAILED DESCRIPTION

[0037] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings, but this is merely exemplary and the present invention is not limited to the specific embodiments described exemplarily.

[0038] In the present invention, "single atom catalyst" refers to a catalyst in which a metal is dispersed on a carrier in the size of a single atom.

[0039] A single-atom catalyst according to one aspect of the present invention comprises: a nitrogen-doped carbon structure; and a single-atom metal, wherein the single-atom metal forms a coordination bond with a nitrogen atom of the nitrogen-doped carbon structure.

[0040] The nitrogen-doped carbon structure acts as a carrier of a metal dispersed with a single-atom catalyst. The nitrogen atoms contained in the carbon structure form coordination bonds with the metal, so that the metal is fixed in the carbon structure at a single-atom size. In addition, when a metal oxide is used as a carrier, the hydroxyl radicals (·OH) generated during the electrochemical advanced oxidation process are adsorbed on the surface of the metal oxide, which may lead to a decrease in the water treatment efficiency of the electrochemical advanced oxidation, but may have the advantage that the carbon structure does not adsorb hydroxyl radicals. The nitrogen doping method of the nitrogen-doped carbon structure and the type of the carbon structure are not particularly limited, as long as the metal can be dispersed at a single-atom size.

[0041] In one embodiment, the carbon structure may be carbon black. The carbon structure may be graphene, graphene oxide, carbon nanotube (CNT), graphite, etc., but in terms of cost and ease of mass production, the carbon structure is preferably carbon black.

[0042] The single-atom metal of the single-atom catalyst corresponds to the reaction site of the catalyst. The single-atom metal forms a coordination bond with the nitrogen atom of the nitrogen-doped carbon structure, thereby being fixed in the nitrogen-doped carbon structure in the form of a single atom. Compared with the case where metal atoms are aggregated, the advantage of the single-atom metal is that a wider reaction active site is provided. As the single-atom metal of the single-atom catalyst of the present invention, transition metals such as Co, Zn, V, Cr, Fe, Ni, Cu, Zr, Nb, and Mo can generally be used, but are not limited thereto.

[0043] In one embodiment, the single-atom metal may be cobalt (Co). Among the above transition metals, cobalt (Co) is advantageous because it has high activity for generating hydroxyl radicals through water oxidation reaction at the negative electrode and generating hydrogen peroxide through selective oxygen reduction reaction at the positive electrode when the single-atom catalyst is used for electrochemical advanced oxidation.

[0044] In one embodiment, the content of the single-atom metal in the single-atom catalyst can be more than 0% by weight and less than 3% by weight. When the content of the single-atom metal in the single-atom catalyst is more than 3% by weight, the excess single-atom metal may form a metal oxide, and the spacing between adjacent single-atom metals may be insufficient during the preparation of the single-atom catalyst such as heat treatment, so they may aggregate with each other to form clusters or nanoparticles. In one embodiment, the content of the single-atom metal in the single-atom catalyst can specifically be more than 0% by weight and less than 2% by weight, and more specifically can be more than 0.5% by weight and less than 1% by weight.

[0045] According to another aspect of the present invention, a method for preparing a single atom catalyst is provided, the method comprising the following steps: mixing a single atom metal precursor and a nitrogen-dopant precursor in a solvent to prepare a mixed solution; mixing a carbon structure powder in the mixed solution to prepare a suspension; oscillating and drying the suspension to prepare a single atom catalyst precursor; and calcining the single atom catalyst precursor to prepare a single atom catalyst.

[0046] The solvent in the step of preparing the mixed solution needs to have polarity so that the monoatomic metal precursor and the nitrogen-dopant precursor can be dissolved, and needs to be easily volatilized in the drying step. In this aspect, the solvent is preferably alcohol, and in terms of reducing the preparation cost, the solvent is preferably ethanol.

[0047] The single atomic metal precursor may be a substance containing a single atomic metal of the single atomic catalyst. For example, when the single atomic metal is cobalt (Co), the single atomic metal precursor may be cobalt chloride hydrate.

[0048] The nitrogen-dopant precursor includes a species that provides nitrogen as a dopant for the nitrogen-doped carbon structure of the single atom catalyst.

[0049] In the above solvent, the monatomic metal precursor and the nitrogen-dopant precursor are mixed at a prescribed temperature and in a prescribed molar ratio to form a mixed solution.

[0050] Then, in the step of preparing the suspension, the carbon structure powder is added to the mixed solution at a prescribed molar ratio relative to the nitrogen-dopant precursor to obtain the suspension. For example, the carbon structure powder can be added at a weight ratio of about 8:1 to 10:1 relative to the nitrogen-dopant precursor.

[0051] In the step of preparing the single atom catalyst precursor, the suspension is shaken for 8-12 hours to uniformly disperse the carbon structure powder in the suspension. Thereafter, the suspension is dried at a temperature of about 80-120° C. to volatilize the solvent, thereby obtaining the single atom catalyst precursor.

[0052] In the step of preparing a single atom catalyst, the single atom catalyst precursor is heat treated in an inert gas atmosphere for about 1-5 hours. The calcination temperature can be kept constant, and can also be increased from a low temperature at a specified heating rate. For example, the calcination temperature can be in a temperature range of about 500-700°C, and can be reached from room temperature at a heating rate of about 5°C / minute to about 10°C / minute. In one embodiment, the calcination temperature can be specifically about 550-700°C, and more specifically about 600-650°C. By calcining as described above, the nitrogen atoms of the nitrogen-dopant precursor are doped in the carbon structure. In addition, the nitrogen atoms of the nitrogen-dopant precursor form coordination bonds with the metal atoms of the single atom metal precursor.

[0053] Therefore, a single atom catalyst in which metal atoms are arranged on a carbon structure in a single atom form is obtained, and the obtained single atom catalyst may be the single atom catalyst mentioned in one aspect of the present invention.

[0054] According to another aspect of the present invention, an electrode is provided, comprising: a substrate; and a catalyst layer, wherein the catalyst layer is coated on the substrate, wherein the catalyst layer contains the above-mentioned single-atom catalyst.

[0055] The substrate acts as a support for the catalyst layer and is directly connected to a power source to transmit the current applied when the power is supplied to the catalyst layer. In order to play the role as described above, the substrate needs to have high adhesion to the catalyst layer and needs to have high electrical conductivity. In addition, the substrate needs to have oxidation stability so that electrical conductivity can be maintained during the electrochemical advanced oxidation process. There is no restriction on the material used as the substrate, as long as it is a material with the characteristics as described above.

[0056] In one embodiment, the substrate may include stainless steel, which corresponds to a material having adhesion to the catalyst layer and high electrical conductivity.

[0057] The catalyst layer includes the above-mentioned single-atom catalyst and is coated on the substrate. The method of coating the catalyst layer is not limited. The single-atom metal site of the catalyst layer acts as a reaction site, and when an electric current is applied through the substrate, an oxidation reaction or a reduction reaction is performed according to polarity.

[0058] In one embodiment, the ratio of the thickness of the substrate to the thickness of the catalyst layer may be 10:1 to 1000:1. When the thickness ratio is less than 10:1 or exceeds 1000:1, the electric energy applied to the catalyst layer through the substrate may be reduced, which may hinder the oxidation reaction or reduction reaction of the catalyst. In one embodiment, the ratio of the thickness of the substrate to the thickness of the catalyst layer may specifically be 50:1 to 800:1, more specifically 100:1 to 500:1.

[0059] According to another aspect of the present invention, an electrochemical cell is provided, the electrochemical cell comprising the above-mentioned electrodes as a positive electrode and a negative electrode. The positive electrode and the negative electrode of the electrochemical cell both contain single-atom catalysts, and therefore can have advantages in electrochemical advanced oxidation processes. Specifically, the single-atom catalyst of the negative electrode shows high activity for generating hydroxyl radicals through the oxidation reaction of water, and the single-atom catalyst of the positive electrode shows high activity for the selective oxygen reduction reaction of hydrogen peroxide that is a precursor of hydroxyl radicals. That is, since the positive electrode and the negative electrode of the electrochemical cell both contain single-atom catalysts, it can have a high hydroxyl radical generation efficiency. Hydroxyl radicals act as oxidants for organic pollutants, and therefore the electrochemical cell has excellent decomposition efficiency of organic pollutants.

[0060] According to another aspect of the present invention, an electrochemical water treatment method is provided, wherein the electrochemical water treatment method utilizes the electrochemical cell.

[0061] As described above, since both the positive electrode and the negative electrode of the electrochemical cell contain single-atom catalysts, the electrochemical cell has excellent decomposition efficiency of organic pollutants.

[0062] In addition, when using a conventional electrochemical cell including electrodes such as boron-doped diamond (BDD), lead oxide (PbO2) as the negative electrode, due to the oxidation reaction of water occurring at the negative electrode, hydroxyl radicals can be formed at the negative electrode only when the pH of the treated wastewater is about 3 to 4, so it has the disadvantage of being difficult to treat wastewater with a pH other than this. The negative electrode of the electrochemical cell according to the present invention contains a single-atom catalyst, so it can have the advantage that wastewater with a wider range of pH can also be treated. Specifically, the electrochemical water treatment using the electrochemical cell can be carried out at a pH of about 3 to 9.

[0063] Furthermore, the positive electrode of the electrochemical cell of the present invention contains a single-atom catalyst, so that the reactivity of the selective oxygen reduction reaction to generate hydrogen peroxide is higher than that of a conventional electrochemical cell, and thus there is an advantage that the generation efficiency of hydroxyl radicals can be further improved.

[0064] The embodiments of the present invention are further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only used to illustrate the present invention, not to limit the scope of rights. Various changes and modifications can be made to the embodiments within the scope of the present invention and the technical concept, which is obvious to those skilled in the art, and such variations and modifications also belong to the scope of rights.

[0065] Preparation Example 1 - Preparation of Single Atom Catalyst (Co1-NCB)

[0066] Cobalt (II) chloride hexahydrate (CoCl2·6H2O) and 1,10-phenanthroline were dissolved in pure ethanol in a molar ratio of 1:3. It was mixed with carbon black powder to obtain a suspension, and the obtained suspension was shaken overnight and then dried at a temperature of 80°C. Under Ar atmosphere, a tubular furnace (SH-FU-80STG, SH Scientific, Korea) was used to calcine the powder obtained after drying at 600°C for 2 hours, wherein the temperature of the furnace was increased at a heating rate of 10°C / min and reached 600°C. Finally, Co1-NCB powder (hereinafter referred to as 1 wt% Co1-NCB) was obtained. The process of preparing Co1-NCB powder is schematically shown in Figure 1a The resulting Co1-NCB powder was cooled and stored in an Ar atmosphere until further use. Co1-NCBs with a higher weight % of Co (hereinafter referred to as 3 wt % Co1-NCB and 5 wt % Co1-NCB) were synthesized by a similar process, except that a larger amount of cobalt (II) chloride hexahydrate and 1,10-phenanthroline were used in the same molar ratio compared to 1 wt % Co1-NCB.

[0067] Preparation Example 2 - Fabrication of Co1-NCB Electrode

[0068] The fabrication process of Co1-NCB electrode is shown in Figure 1b20 mg of Co1-NCB powder, 2 mL of isopropanol (IPA) and 80 μL of Nafion®117 were mixed (5% wt) to prepare a catalyst ink. Nafion®117 refers to tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octene sulfonic acid copolymer. Nafion 5%wt refers to a 5 wt% solution of Nafion polymer in a solvent of isopropanol and water. Nafion polymer is a trademark product purchased from CHEMOURS Chemical Company. It is a polymer with polytetrafluoroethylene as a hydrophobic skeleton and a sulfonic acid group (-SO3H) in the hydrophilic side chain. The catalyst ink was ultrasonically treated for 2 hours and then 2 The geometric area of ​​​​the electrode substrate (SUS304) was sprayed. The amount of catalyst ink sprayed on each electrode substrate was 0.5ml. The negative electrode and the positive electrode were coated in the same way.

[0069] Experimental Example 1 - Measurement of Surface Properties of Co1-NCB Catalyst

[0070] The morphology of Co1-NCB of Preparation Example 1 was analyzed using an aberration-corrected high-angle annular dark field scanning transmission electron microscope (HAADF-STEM, JEM134 ARM200CF) at 200 kV. X-ray diffraction (XRD, Rigaku) ​​analysis was performed using CuKα rays (λ=1.54Å) in the 2θ range of 10-80°. The X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectra of the Co K-edge were measured at the beamline of the 6-BM (BMM) beamline for material measurement in NSLS-II at Brookhaven National Laboratory (BNL). The measurement was performed in fluorescence mode using a four-element silicon drift detector (SDD). Co foil was measured simultaneously and used for spectrum alignment. Cobalt oxide (CoO) and cobalt phthalocyanine (Co-Pc) were used as reference substances. Pre-edge and post-edge backgrounds were subtracted, and the spectra were normalized to 1 based on the edge height. Data reduction was performed using Athena, and EXAFS fitting was performed using Artemis in the Demeter software package. Wavelet transform analysis was performed using the Larch software package with a continuous Cauchy mother wavelet.

[0071] Figure 2a is the HAADF-STEM image of the Co1-NCB, Figure 2b and Figure 2c These are the EDS element maps of carbon and cobalt respectively. Figure 2a to Figure 2c It shows that carbon and cobalt elements are relatively evenly distributed on the spherical carbon black carrier with an average diameter of about 30 nm. Figure 2d to Figure 2f HAADF-STEM images of Co single atoms on carbon black were taken at various magnifications. Figure 2d to Figure 2f It shows that Co does not aggregate into metallic clusters or particles. In contrast, Figure 2f The portion indicated by a circle, where only a few angstroms are measured, shows that Co is fixed in atomic form on the carbon black support.

[0072] Figure 3a and Figure 3b is an XRD diffraction pattern, which indicates the lack of crystalline Co species in Co1-NCB containing 1 wt% Co. The two broad diffraction peaks at about 24° and 43° correspond to the (002) plane and (101) plane of carbon black (JCPDS card No. 34-0567).

[0073] Reference Figure 3b When the Co loading exceeds 1 wt%, characteristic diffraction peaks of Co in both metal and metal oxide forms appear. In addition, at 3 wt% and 5 wt% Co loading, the decrease in XRD peak intensity and the broadening of the CB peak indicate the formation of metallic Co and metal oxide nanoparticles consistent with the transmission electron microscopy (TEM) images.

[0074] Figure 4a is the normalized Co K-edge XANES spectra of Co1-NCB, Co foil, CoO and Co-Pc, Figure 4b is the Fourier transform k of Co1-NCB, Co foil, CoO and Co-Pc 2 Weighted EXAFS spectra. Stronger evidence supporting the atomic dispersion of Co in NCB was obtained by XANES and EXAFS analysis. Figure 4aThe spectrum of Co1-NCB consists of three regions: i) weak pre-edge features associated with forbidden electronic transitions from 1s to 3d levels, ii) major absorption edges associated with electronic transitions from occupied 1s levels to unoccupied 4p bands, which provide good inferences about the formal oxidation state of Co, iii) extended EXAFS that can be used to determine the local structure around the absorbing atoms. The XANES of Co1-NCB shows an edge located between the Co foil and the CoO benchmark, which indicates formal oxidation states between 0 and +2. The 5 wt% Co1-NCB spectrum closely follows the Co foil and has a shoulder at about 7712 eV. The 3 wt% Co1-NCB has a smaller shoulder at 7712 eV. In contrast, the XANES spectrum of 1 wt% Co1-NCB shows almost no shoulder, similar to the CoO benchmark.

[0075] Reference Figure 4b , it can be confirmed that the EXAFS spectra of 3 wt% Co1-NCB and 5 wt% Co1-NCB are very consistent with the spectrum of Co foil. On the other hand, the EXAFS of 1 wt% Co1-NCB is very consistent with the spectra of CoO and Co-Pc.

[0076] Figure 4c and Figure 4d is the EXAFS fit of the first shell of 1 wt% Co1-NCB. Figure 4c and Figure 4d , 1 wt% Co1-NCB shows a Co-N / O path length of 1.98 Å in the first shell, which is longer than the Co-N path of Co-Pc. ​​The existence of the Co-O path and the coordination number of 4.96 support the XANES results of 1 wt% Co1-NCB similar to CoO.

[0077] Both 3 wt% Co1-NCB and 5 wt% Co1-NCB have Co-Co in the first shell, which indicates metal clusters. This result is consistent with previous studies on Co nanoparticles.

[0078] Figure 4e to Figure 4h The wavelet transform (WT) analysis results of χ(k) of 1 wt% Co1-NCB, Co-Pc, CoO and Co foil are shown in Figure 2. Figure 4e to Figure 4h As shown, the maximum intensity of the WT profile of 1 wt% Co1-NCB occurs at about 4.9 Å, which is very consistent with Co-Pc and CoO. -1 This indicates the presence of a first shell composed of Co-N / O. In the case of 3 wt% Co1-NCB and 5 wt% Co1-NCB, the contour maximum intensity is shown to be about 7.5 Å -1, similar to Co foil, which suggests that Co forms coordination bonds with other clusters formed by Co atoms. The EXAFS fitting parameters of Co K edge are shown in Table 1 below.

[0079] [Table 1]

[0080]

[0081] Where R is the interatomic distance (the bond length between the central atom and the surrounding coordinating atoms); σ 2 is the Debye-Waller factor; the R-factor indicates the goodness of fit. S0 2 The amplitude reduction factor is estimated by fitting the Co foil. The EXAFS fitting value of the sample is set to 0.768. The coordination number is fixed to a known crystallographic value of a reference compound.

[0082] The changes in the oxidation state of the metal sites of Co1-NCBs with different Co loadings were further investigated by soft X-ray absorption spectroscopy. Compared with the above XANES and EXAFS (hard X-ray absorption spectroscopy), soft X-ray absorption spectroscopy provides more surface-focused information through the Co L-edge spectrum (the transition from 2p electrode to unfilled d-orbital). Figure 5 The soft X-ray absorption spectrum of 1 wt% Co1-NCB shows that the oxidation state of Co in 1 wt% Co1-NCB is between 0 and +2, which supports the existence of Co in a single atomic structure. According to the above results, in Co1-NCB with high Co loading, due to the excess Co atoms and insufficient spacing between them, Co atoms show a tendency to aggregate into clusters or nanoparticles during the annealing process. Taking this into account, in the subsequent experimental examples, only Co1-NCB electrodes with a Co loading of 1.0 wt% were considered.

[0083] Experimental Example 2 - Production of Hydroxyl Radicals at 1 wt% Co1-NCB Anode

[0084] The production of reactive oxygen species (ROS) was monitored in an H-cell separated by a Nafion 117 membrane (a perfluorosulfonic acid membrane composed of a sulfonated tetrafluoroethylene copolymer). To identify ROS, electron paramagnetic resonance (EPR) spectroscopy (JES-X310, JEOL) was performed using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin trapping agent. Samples were taken at predetermined time intervals and analyzed under the following conditions: microwave frequency = 9417 MHz; microwave power = 5 mW; modulation frequency = 100 MHz; modulation amplitude = 2.0 G.

[0085] The oxidation reaction of water (H2O→·OH+H + +e - ) produces electrogenerated hydroxyl radicals. Other reactive oxygen species generated along with hydroxyl radicals at the 1 wt % Co1-NCB anode were analyzed using electron paramagnetic resonance (EPR) spectroscopy. After 10 minutes of electrolysis, a 1:2:2:1 quartet characteristic of DMPO-·OH adduct was observed at the Co1-NCB anode. Figure 6 As shown, the intensity of the adduct signal gradually increased within 30 minutes.

[0086] Figure 7a is the EPR spectrum of DMPO adduct according to current density, Figure 7b is the EPR spectrum of DMPO adduct according to the initial pH. Figure 7a , with the increase of current density, the EPR signal intensity of free radicals increases. In addition, referring to Figure 7b , unlike previous studies that suggested that more hydroxyl radicals would be formed under acidic conditions, the Co1-NCB anode showed consistent generation of hydroxyl radicals under a wide range of solution pH values ​​from 3 to 9. In addition, no DMPO-·O2 - The 1:1:1:1 quadruplet peak of the adduct indicates that there is no O2 at the Co1-NCB anode. - Competitive electrogeneration of other less oxidizing substances.

[0087] Experimental Example 3 - Production of Hydrogen Peroxide at 1 wt% Co1-NCB Cathode

[0088] Transition metal single-atom catalysts have been reported to promote the selective oxygen reduction reaction (O2+2e - +2H +→H2O2) effectively promotes the production of H2O2. Therefore, the electrocatalytic performance of the Co1-NCB cathode in the two-electron selective oxygen reduction reaction and the activation of H2O2 for the production of hydroxyl radicals were further investigated.

[0089] The concentration of H2O2 in the solution was quantified by iodine titration using a UV-vis spectrophotometer (Biotek Synergy Mx). Briefly, 1 mL of the sample was mixed with 1 L of deionized water (DI water), 0.75 mL of potassium hydrogen phthalate (0.1 M), 0.75 mL of potassium iodide (0.4 M), NaOH (0.06 M), and ammonium molybdate tetrahydrate (10 -4 M) and stirred vigorously for 2 min, and then the absorbance was measured at 348 nm.

[0090] H2O2 test strips are used to monitor the concentration of H2O2 during the electrolysis process. Figure 8a and Figure 8b It is shown that the test strip turns blue after 10 min of reaction, which indicates the generation of H2O2 through selective oxygen reduction reaction at the Co1-NCB cathode. Figure 9a and Figure 9b The graphs are respectively the change of absorbance at a specific wavelength according to time and the change of concentration of hydrogen peroxide according to time, refer to Figure 9b , the H2O2 concentration first increased, reached a maximum of about 1 mg / L after 10 minutes, and then decreased. Figure 9a The colorimetric results are consistent.

[0091] Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements of the Co1-NCB positive electrode were performed using a potentiometer (Ivium Technologies, The Netherlands). All measurements were performed in a three-electrode electrochemical cell consisting of a platinum (Pt) mesh as a counter electrode, Ag / AgCl as a reference electrode, and Co1-NCB as a working electrode.

[0092] To evaluate the oxygen reduction reaction activity of the cathode, electrochemical studies were performed in 100 mM NaClO4 electrolyte saturated with O2 and saturated with N2. CV was performed at a scan rate of 10 mV / s in an applied voltage window of +1.0 V to -1.5 V (vs. Ag / AgCl).

[0093] Interestingly, H2O2 was generated at the Co1-NCB anode of the H-cell only under O2-saturated conditions, suggesting that the oxygen reduction reaction occurs by consuming oxygen in the cathode compartment to generate H2O2. Fig.10This was verified by detecting H2O2 in a single cell, in which oxygen generated at the Co1-NCB anode moved to the Co1-NCB cathode. Figure 8c Comparative LSV curves of Co1-NCB cathode in O2 / N2 saturated 100mM NaClO4 electrolyte, the inset corresponds to the CV curve. Figure 8c In the CV analysis, a reduction current peak appeared at about -0.8 V in an O2-saturated environment, which further confirmed that H2O2 was synthesized through the oxygen reduction reaction.

[0094] Experimental Example 4 - Electrochemical Oxidation of Phenol

[0095] The hydroxyl radicals generated electrochemically and catalytically at the negative and positive electrodes of Co1-NCB can effectively decompose the organic pollutants in water, thereby evaluating the performance of Co1-NCB electrodes in electrochemical advanced oxidation processes.

[0096] Electrochemical phenol oxidation was performed using a 60 mL single-chamber cell containing 100 mM NaClO4 electrolyte (pH = 6.5, conductivity = 9.91 mS / cm). The electrochemical cell was equipped with a 1.0 cm 2 Co1-NCB electrodes of defined geometric area (separated by Teflon). The spacing between the electrodes was 1.0 cm. The experiment was performed as follows: 5-20 mA / cm was applied under magnetic stirring (500 rpm) at room temperature. 2 The measurements were performed in a constant current mode using a potentiometer (Ivium Technologies, The Netherlands).

[0097] Fig.11a and Fig.11b are graphs showing the electrochemical oxidation efficiency of phenol using a Co1-NCB electrode at different current densities and different initial pHs, respectively, Fig.11c It is a comparison diagram of phenol removal efficiency when both the positive electrode and the negative electrode are Co1-NCB electrodes and when only the negative electrode is a Co1-NCB electrode.

[0098] Reference Fig.11a As the current density increases from 5 mA / cm 2 Increase to 20mA / cm 2 , the phenol removal efficiency increased. The removal of phenol at various current densities also conformed to the rate constant of 0.398h -1 , 0.664h -1 , 0.794h -1 and 0.832h -1 (For each case, R 2>0.99). The most important side reaction that can reduce the generation of hydroxyl radicals is (i) the generation of H2 at the cathode (2H + +2e - →H2) and (ii) the generation of O2 at the negative electrode (2H2O→O2+4H + 4e - ). However, up to 20mA / cm 2 At a current density of 20 mA / cm2, no decrease in phenol removal efficiency due to the side reaction of generating hydrogen at the positive electrode was observed. This is inferred to be due to the generation of H2O2 by the oxygen reduction reaction at the positive electrode and the generation of free radicals by its activation. 2 The current density at the negative electrode is 2.34 W, and the oxygen generation reaction at the negative electrode as another side reaction does not inhibit the generation of free radicals at the Co1-NCB negative electrode.

[0099] Fig.11b The electrochemical phenol removal of two Co1-NCB electrodes at initial pH values ​​of 3.0, 6.5, and 9.0 is shown in FIG. The pseudo-first-order model (R 2 >0.99). The phenol removal efficiency was the highest at pH 6.5. However, in all cases, the phenol removal efficiency exceeded 90% within 240 min, indicating that the electrochemical advanced oxidation process based on Co1-NCB electrode has high electrochemical activity in a wide pH range, which is also consistent with the EPR results.

[0100] These results indicate that the generation of free radicals and the removal of phenol in the Co1-NCB electrode system show similar trends. Therefore, the effect of the composition of the Co1-NCB electrode on the generation of free radicals was evaluated based on the phenol removal efficiency. Fig.11c When the Co1-NCB anode was paired with the Co1-NCB cathode, significant improvements in both the phenol removal efficiency and kinetic constant were observed compared to the battery with the Co1-NCB anode paired with a stainless steel anode. This comparison indicates that the Co1-NCB cathode has a significant contribution in the electrochemical advanced oxidation process. In essence, the Co1-NCB cathode enhances the production of hydroxyl radicals through the cathode oxygen reduction reaction, thereby improving the oxidation capacity of the system.

[0101] The above description is merely an example of applying the principles of the present invention, and other configurations may be included without departing from the scope of the present invention.

Claims

1. A single atom catalyst comprising: Nitrogen-doped carbon structures; and Monatomic metals, in, The single-atom metal forms a coordination bond with a nitrogen atom of the nitrogen-doped carbon structure.

2. The single atom catalyst according to claim 1, wherein The carbon structure is carbon black.

3. The single atom catalyst according to claim 1, wherein The monoatomic metal is cobalt (Co).

4. The single atom catalyst according to claim 1, wherein The content of the single-atom metal in the single-atom catalyst is more than 0 wt % and less than 3 wt %.

5. A method for preparing a single atom catalyst, comprising the following steps: mixing a single atom metal precursor and a nitrogen-dopant precursor in a solvent to prepare a mixed solution; mixing a carbon structure powder into the mixed solution to prepare a suspension; shaking and drying the suspension to prepare a single atom catalyst precursor; The single atom catalyst precursor is calcined to prepare a single atom catalyst.

6. An electrode comprising: substrate; as well as a catalyst layer, wherein the catalyst layer is coated on the substrate, Wherein, the catalyst layer comprises the single atom catalyst according to any one of claims 1 to 4.

7. The electrode according to claim 6, wherein The substrate includes stainless steel.

8. The electrode according to claim 6, wherein The ratio of the thickness of the substrate to the thickness of the catalyst layer is 10:1 to 1000:

1.

9. An electrochemical cell comprising the electrode according to claim 6 as a positive electrode and a negative electrode.

10. An electrochemical water treatment method using the electrochemical cell according to claim 9.

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