KR20230068756A - Single-atom nickel catalyst for electrochemical conversion of carbon dioxide with high activity and selectivity and electrode for conversion of carbon dioxide including the same - Google Patents

Abstract

The present invention relates to a single-atom nickel catalyst for electrochemically converting carbon dioxide with high activity and selectivity, which comprises an organometallic compound in which at least one nitrogen (N) atom bonded to a central nickel (Ni) ion in a nickel porphyrin-based compound is replaced with a heteroatom, and a carbon dioxide conversion electrode including the same. According to the present invention, the catalyst for electrochemically converting carbon dioxide has twisted symmetry at the atomic level around a nickel active point through controlling the coordination environment of a monatomic nickel catalyst to efficiently convert carbon dioxide to carbon monoxide with high selectivity and the carbon monoxide produced through this can be useful in the production of various petrochemical-based compounds.

Description

Monoatomic nickel catalyst for electrochemical carbon dioxide conversion with high activity and high selectivity and carbon dioxide conversion electrode containing the same SAME}

The present invention relates to a novel monoatomic nickel catalyst for carbon dioxide conversion that can be used as a catalyst for electrochemical conversion of carbon dioxide (CO ₂ ) into carbon monoxide (CO) and an electrode for carbon dioxide conversion including the same.

As global warming caused by greenhouse gases causes widespread and persistent problems such as climate change, ecosystem disturbance, and food shortage, technology for processing emitted carbon dioxide (CO ₂ ), which is considered the main cause of global warming, is attracting attention.

Carbon dioxide processing technology is divided into carbon capture & storage technology and utilization technology as a resource with useful value. Capture and storage are representative of underground storage methods such as storing carbon dioxide in deep water in supercritical form or storage in conjunction with oil extraction technology (EOR), and mineral carbonation method that stores carbon dioxide in the form of carbonate through reaction with alkali metals. . However, these methods have disadvantages of securing a storage space, limiting the amount of storage, and destroying the environment. Accordingly, a technology that has recently emerged is a technology that converts carbon dioxide into a useful resource with high added value.

Such carbon dioxide conversion technologies include biochemical, photochemical, and electrochemical methods. Most of them are technologies that produce hydrocarbon compounds through the reduction reaction of carbon dioxide, which is very stable in nature, and consume carbon dioxide rather than simply storing it. Therefore, it can be said that it is a reduction effort in a more active sense than the existing capture and storage technology.

In particular, the electrochemical carbon dioxide reduction reaction (CO ₂ RR) technology can control the selectivity of hydrocarbon compounds produced according to the reaction conditions (reduction potential, temperature, time), and has an effect on the region and environment It is receiving a lot of attention compared to other conversion technologies in that it does not receive In addition, electrical energy used for carbon dioxide reduction can be supplied from renewable energy such as wind power, solar power, and geothermal energy, and the carbon cycle stores electrical energy primarily produced through the process of converting carbon dioxide and using the generated compound as fuel. It has the advantage of being able to build an energy flow with

Among the compounds that can be produced from this electrochemical carbon dioxide conversion, carbon monoxide (CO), together with by-product hydrogen generated in the conversion process, becomes a raw material for syngas, and through continuous gas-liquid conversion reaction (Fischer-Tropsch synthesis), various types of low-carbon It can be converted and used as fuel, so its added value is very high.

Meanwhile, catalysts used in electrochemical carbon dioxide conversion technology are mainly based on expensive precious metals such as gold or silver. These noble metals have optimal adsorption energy between the reaction intermediate (*COOH) and the product (*CO) when converting carbon dioxide, so they have good activity in the conversion of carbon dioxide to carbon monoxide, but problems such as price and stability have not been solved. .

Therefore, in order to put the electrochemical conversion of carbon dioxide to practical use, it is necessary to develop a catalyst material with high performance while lowering the cost of the catalyst.

Republic of Korea Patent Publication No. 10-2013-0085635 (published date: 2013. 7. 30) International Patent Publication WO 2015/169763 (published date: 2015. 11. 12) International Patent Publication WO 2013/042695 (published date: 2013. 3. 28)

The present invention, as a catalyst for the electrochemical carbon dioxide conversion reaction, is based on a single-atomic nickel catalyst in which nickel (Ni), a transition metal rather than a noble metal, is stabilized with an organic ligand, but by controlling the coordination environment of the central nickel ion, electrochemical carbon dioxide Its object is to provide a novel monoatomic nickel catalyst showing high activity for conversion and an electrode for carbon dioxide conversion comprising the same.

In order to achieve the above object, the present invention is an organometallic compound in which one or more nitrogen (N) atoms bonded to a central nickel (Ni) ion are substituted with heteroatoms in a nickel porphyrin-based compound. A catalyst for electrochemical carbon dioxide conversion is proposed.

For example, the catalyst for electrochemical carbon dioxide conversion according to the present invention is an organometallic compound in which one nitrogen atom is substituted with an oxygen (O) atom in a nickel porphyrin-based compound, and may be represented by Formula 1 below.

<Formula 1>

(In Formula 1 above,

R are independently of each other hydrogen or a hydrocarbon group having 1 to 12 carbon atoms).

When R is a hydrocarbon group having 1 to 12 carbon atoms, the hydrocarbon group may be either saturated or unsaturated, and may be linear, branched or cyclic. For example, the hydrocarbon group is a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, an n-pentyl group, an n-hexyl group, an octyl group, a cyclohexyl group, a cyclooctyl group, a cyclohexylmethyl group, It may be a phenyl group, a naphthyl group, a benzyl group, or the like, and may further have a substituent such as a hydroxyl group, an amino group, a carboxyl group, a carbonyl group, a sulfhydryl group, a sulfonyl group, or a halo group. In addition, adjacent R's may be linked by a hydrocarbon chain to form a cyclic structure, wherein the hydrocarbon chain may be saturated or unsaturated.

Furthermore, the catalyst for electrochemical carbon dioxide conversion according to the present invention may be composed of a nickel porphyrin-based compound represented by Formula 2 below.

<Formula 2>

And, in another aspect of the invention, the present invention proposes an electrode for electrochemical carbon dioxide conversion comprising the catalyst for electrochemical carbon dioxide conversion.

For example, the electrode for electrochemical conversion of carbon dioxide according to the present invention may have a structure including a coating layer of a conductive composition including the catalyst for conversion of electrochemical carbon dioxide on a surface of a current collector.

In this case, the composition for forming the coating layer on the surface of the current collector may include the catalyst for electrochemical carbon dioxide conversion according to the present invention and a conductive material, and the conductive material may be activated carbon (acetylene black, Ketjen Black, etc.) , It is preferably a carbon material such as carbon nanotube, graphene or fullerene. In addition, metal materials may be used as the conductive material, and for example, Au, Ag, Cu, Al, Pt, Ni, Zn, Sn, Bi, Pd, etc., and alloys thereof may be used. Furthermore, the composition for forming the coating layer may include a transparent conductive metal oxide such as ITO, ZnO, FTO, AZO, or ATO as a conductive material, and in addition, a composite of the conductive carbon material, a conductive resin, a conductive ion exchange resin, etc. You may use resin materials, such as an ionomer.

Meanwhile, the aforementioned various conductive materials may be included in the composition for forming the coating layer by combining two or more types.

The catalyst for electrochemical carbon dioxide conversion according to the present invention has twisted symmetry at the atomic level around the nickel active site through the coordination environment control of the monoatomic nickel catalyst, thereby converting carbon dioxide into carbon monoxide with high selectivity, thereby generating Carbon monoxide can be used very usefully in the production of various petrochemical-based compounds.

Figure 1(a) is a diagram showing the process of synthesizing Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP, and Figure 1(b) shows single crystal X-ray diffraction (SC-XRD) 1(c) shows the 3D molecular structure of Ni(-Cl)-N ₃ O-TPP determined by Ni-N _4- TPP, Ni(-Cl)-N ₃ O-TPP and Ni This is a K-edge X-ray absorption spectroscopy (XANES) spectrum.
2a is ^{a 1} H NMR spectrum of 2,5-bis-(phenylhydroxymethyl)furan prepared in Example herein.
2b is a ¹³ C NMR spectrum of 2,5-bis-(phenylhydroxymethyl)furan prepared in Example herein.
Figure 3a is ^{a 1} H NMR spectrum for N ₃ O-TPP prepared in the present example.
Figure 3b is a ¹³ C NMR spectrum of N ₃ O-TPP prepared in the present example.
Figure 4a is ^{a 1} H NMR spectrum of Ni(-Cl)-N ₃ O-TPP prepared in the present example.
Figure 4b is a ¹³ C NMR spectrum of Ni(-Cl)-N ₃ O-TPP prepared in the present example.
5 is ^{a 1} H NMR spectrum of Ni-N _4- TPP prepared in the present example.
Figure 6(a) shows the polarization of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP measured at a potential scan rate of 5 mV s ^-1 in 0.5 M KHCO ₃ electrolyte saturated with argon/carbon dioxide. curves, and Fig. 6(b) and Fig. 6(c) show differential electrochemical masses of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP combined with a real-time scanning flow cell (SFC) system, respectively. Differential electrochemical mass spectroscopy (DEMS) results (CO ₂ ( m / z = 44), CO ( m / z = 28) and H ₂ ( m / z = 2) signals are 0.5 M KHCO ₃ saturated with carbon dioxide was collected during two cycles of cyclic voltammetry from 0 to -0.9 V _RHE , and the potential profile during DEMS measurement is indicated by a dotted line), Figure 6(d) shows the potential range of carbon dioxide saturated electrolyte, -0.5 to 0.4 V _RHE Cyclic voltammetry results of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP measured at a potential scan rate of 50 mV s ^-1 at (oxidation-reduction pair A is indicated by an arrow) , FIG. 6(e) is a reaction real-time XANES spectrum of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP measured at open circuit potential (OCP) and -0.65 V _RHE ( The difference between the spectra obtained at OCP and -0.65 V _RHE is indicated by a dotted line), Fig. 6(f) is a linear plot of the Ni K-edge XANES spectrum of Ni(-Cl)-N ₃ O-TPP collected at -0.65 V _RHE . This is the linear combination fitting result.
7(a) is a schematic diagram showing the catalytic cycles of Ni-N _4- TPP (left) and Ni(-Cl)-N ₃ O-TPP (right), and FIG. 7(b) shows a potential of -0.6 V _RHE is the corresponding density functional theory calculated free energy profile when is applied.
8(a) and 8(b) are schematic diagrams showing optimization structures of the density functional function theory calculation of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP ^l- , respectively (for nickel The binding energy of COOH is displayed, and a larger negative (-) value means a stronger bond), and FIG. 8(c) is a schematic molecular orbital (MO) diagram (to form a strong Ni-COOH bond). , the electron density of electron-rich nickel needs to be redistributed to porphyrin ligands (orange arrows), increasing the ligand-field splitting (Δ) increases the energy cost of this electron redistribution).
FIG. 9(a) shows the total Faraday efficiency (FE _Total ) and carbon monoxide production Faraday efficiency (FE _CO ) obtained for the carbon dioxide reduction reaction at various potentials for 1 hour in a double compartment H-cell, and FIG. 9(b) shows the double The total current density ( j _Total ) and the partial current density ( j _CO ) obtained for the carbon dioxide reduction reaction at various potentials in the cell H-cell for 1 hour. Oxidation peaks for Ni(-Cl)-N ₃ O-TPP measured by cyclic voltammetry (polarization curve is at the lower potential limit at a potential scan rate of 50 mV s ^-1 in 0.5 M KHCO ₃ electrolyte saturated with carbon dioxide) (Lower potential limit; LPL) was measured by sequentially changing from _-0.5 to -0.8 V _RHE , peaks (A and B) are indicated by arrows), FIG. Raman spectra of Ni(-Cl)-N ₃ O-TPP before and after cyclic voltammetry measurement.

In describing the present invention, if it is determined that a detailed description of a related known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description will be omitted.

Embodiments according to the concept of the present invention can be applied with various changes and can have various forms, so specific embodiments are illustrated in the drawings and described in detail in this specification or application. However, this is not intended to limit the embodiments according to the concept of the present invention to a specific disclosed form, and should be understood to include all changes, equivalents, or substitutes included in the spirit and technical scope of the present invention.

Terms used in this specification are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as "comprise" or "having" are intended to designate that the described feature, number, step, operation, component, part, or combination thereof exists, but one or more other features or numbers However, it should be understood that it does not preclude the presence or addition of steps, operations, components, parts, or combinations thereof.

Hereinafter, the present specification will be described in more detail with reference to examples. However, embodiments according to the present specification may be modified in many different forms, and the scope of the present specification is not construed as being limited to the embodiments described below. The embodiments herein are provided to more completely explain the present specification to those skilled in the art.

<Example>

In this embodiment, in order to derive the active site of a monatomic nickel catalyst having high activity for electrochemical carbon dioxide conversion, two organometallic complexes having precisely controlled structures, that is, a structure containing monoatomic nickel surrounded by 4 nitrogen atoms ( Ni-N _4- TPP) and a structure with distorted symmetry (Ni-N ₃ O-TPP) were synthesized by replacing one nitrogen with oxygen as a second molecule, and through various electrochemical and spectroscopic analyses, It was confirmed that the structure with distorted symmetry showed high carbon dioxide conversion performance. Through computational chemistry, it was revealed that the distorted symmetry of the active site effectively lowers the energy required to generate the reaction intermediate, resulting in high conversion performance.

1. Synthesis of Ni-Porphyrin Compounds

N _4- TPP and N ₃ O-TPP were synthesized through a condensation reaction using pyrrole and aldehyde, and in the case of N ₃ O-TPP, 2,5-bis-(phenylhydroxymethyl)furan (2,5-bis- (phenyl hydroxymethyl)furan) was used as a precursor. Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP were synthesized through recrystallization after metallization of N _4- TPP and N ₃ O-TPP.

(1) Synthesis of 2,5-Bis(phenylhydroxymethyl)furan

In a 250 mL round bottom flask, distilled n-hexane (50 mL), tetramethylethylenediamine (4.3 g, 37.5 mmol, 5.6 mL), n-butyllithium (2.4 g, 37.5 mmol, 15.0 mL of 2.5 M hexane solution ) was added and stirred for 10 minutes in a nitrogen atmosphere. Then, distilled furan (1.0 g, 15.0 mmol, 1.1 mL) was added and stirred at room temperature for 1 hour. The stirred solution is cooled on 0 degree ice, and 0 degree benzaldehyde (4.0 g, 37.5 mmol, 33.8 mL of 1.1 M tetrahydrofuran solution) is slowly added. The mixture was stirred for 15 minutes while slowly warming to room temperature, and then the reaction was terminated by adding cold ammonium chloride aqueous solution (70 mL, 1.0 M aqueous solution). The organic layer was washed with water (3 X 70 mL) and brine (1 X 70 mL), dried over magnesium sulfate, and the solvent was removed under reduced pressure. Purification using a silica gel column containing 30% (v/v) ethyl acetate/n-hexane is carried out to obtain the desired product. At this time, the yield is 35% (1.46 g).

(2) N ₃ Synthesis of O-TPP

2,5-bis-(phenylhydroxymethyl)furan (0.56 g, 2.0 mmol) and chloroform (200 mL) were added to a 500 mL round bottom flask and stirred for 15 minutes in a nitrogen atmosphere. Benzaldehyde (0.39 g, 4.0 mmol, 0.41 mL), pyrrole (0.43 g, 6.0 mmol, 0.42 mL), and BF ₃ OEt ₂ (0.22 g, 2.0 mmol, 0.25 mL) were added to the solution, and the mixture was warmed to room temperature. stirred for 2 hours. Then 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (1.36 g, 6.0 mmol) is added and the mixture is stirred for 1 hour. After removing the solvent under reduced pressure, purification using a two-stage column is performed. First, it is primarily purified using a basic alumina column containing 10% (v/v) acetone/dichloromethane. Subsequently, a silica gel column containing the same solvent is used to finally obtain the desired product. At this time, the yield is 15% (0.19 g).

(3) Ni(-Cl)-N ₃ Synthesis of O-TPP

N ₃ O-TPP (0.080 g, 0.13 mmol) was added to a 100 mL round bottom flask and dissolved in chloroform (70 mL). A solution of nickel chloride (0.037 g, 1.56 mmol) in ethanol (7 mL) was added to a round bottom flask to initiate the reaction and refluxed for 2 hours. The solvent is removed under reduced pressure, and the solid is dissolved in chloroform (5 mL). Purification using a silica gel column with 5% (v/v) acetone/chloroform gives a purple product. Thereafter, recrystallization from dichloromethane/n-hexane yields desired purple crystals. At this time, the final yield is 64% (0.059 g).

(4) Ni-N _4- Synthesis of TPP

Tetraphenylporphyrin (0.10 g, 0.16 mmol) was added to a 250 mL round bottom flask, and dimethylformamide (7 mL) was added to dissolve it. The reaction was initiated by adding nickel(II)acetic acid (0.81 g, 3.25 mmol) to the solution and refluxed overnight. Then, the solvent is removed under reduced pressure. Purification using a silica gel column with 30% (v/v) dichloromethane/n-hexane gives the desired product. At this time, the yield is 43% (0.047 g).

(5) Analysis of physical and chemical properties of the synthesized nickel porphyrin compound

From the peak at about 856.0 eV in the X-ray photoelectron spectroscopy (XPS) spectrum and the Ni K-edge position at 8345 eV in the X-ray absorption spectroscopy (XANES) spectrum (Fig. 1(c)), the oxidation state of nickel is It was found to be +2 for both nickel porphyrin compounds. The structure of Ni(-Cl)-N ₃ O-TPP was determined by single-crystal X-ray diffraction (SC-XRD; Fig. 1(b)), and nickel atoms and O(1)N(2)-N(3)N (4) shows a five-coordinate nickel complex with an interplanar vertical distance of 0.303 Å and a distance of 2.279 Å from the axial chloride ion. The η ¹ -furan ligand is coordinated to the nickel atom with θ = 18.2° (θ is the angle between the furan plane and the Ni-O bond). The 1s → 4p _z transition at about 8339 eV in the XANES spectrum is characteristic of the square planar structure clearly visible in the spectrum of Ni-N _4- TPP (Fig. 1(c)). On the other hand, a pre-edge peak is observed at about 8335 eV for Ni(-Cl)-N ₃ O-TPP due to the increased dipole-allowing 1s → 4p transition, which is a twisted D leading to hybridization of 3d and 4p orbitals. It is a proof of _4h symmetry. Thus, a Ni ^II complex with a 4-coordinate dianionic ligand (square planar Ni-N _4- TPP) and a monoanionic ligand with a condensed chloride ion (Ni(-Cl)-N ₃ O-TPP) this has been confirmed

2. Real-time reaction characterization of porphyrin compounds

The electrochemical carbon dioxide reduction reactivity on Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP was measured in 0.5 M KHCO ₃ electrolyte using a rotating disk electrode. The electrode was prepared by mixing porphyrin and activated carbon to provide sufficient electrical conductivity. Linear scanning potential measurement results show a similar current profile for Ni-N _4- TPP in argon and carbon dioxide saturated electrolytes (pH about 8.9 and 7.2, respectively) (Fig. 6(a)). On the other hand, the reduction current density of Ni(-Cl)-N ₃ O-TPP is greatly increased in a carbon dioxide saturated electrolyte, and is twice as high as that in the absence of carbon dioxide, especially at -0.8 V _RHE . Reduction current is almost absent for glassy carbon and activated carbon without nickel porphyrin compounds, and it can be confirmed that the improved reduction current in carbon dioxide saturated electrolyte is due to the electrocatalytic properties of nickel porphyrin compounds. Therefore, the difference in the polarization curves clearly observed for the Ni(-Cl)-N ₃ O-TPP electrode but not the Ni-N _4- TPP electrode is greater for Ni(-Cl)-N than Ni(-Cl)-N _4- TPP. This means that the electrochemical carbon dioxide reduction reactivity of ₃ O-TPP is higher. In order to confirm the above hypothesis, two main reactions (reduction reaction from carbon dioxide to carbon monoxide and competitive hydrogen evolution reaction) on a monatomic nickel catalyst under the same reaction conditions were analyzed using differential electricity coupled with a scanning flow cell (SFC) system. It was observed by differential electrochemical mass spectroscopy (DEMS). As a result of DEMS analysis during the linear scanning potential method, carbon dioxide ( m / z = 44) in the electrolyte is not consumed in the low overvoltage range, but about Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP, respectively. It starts to be consumed at potentials below -0.8 and -0.6 V _RHE (Fig. 6(b) and Fig. 6(c)). At the same time, carbon monoxide ( m / z = 28) increases as the overpotential increases. This change is not observed in the absence of carbon dioxide, and only hydrogen is observed in that condition. The same onset potential for carbon dioxide consumption and carbon monoxide generation means that carbon monoxide is produced by electrochemical reduction of dissolved carbon dioxide. In addition to higher onset potentials, larger signal changes for carbon dioxide consumption and carbon monoxide production were recorded in the Ni(-Cl)-N ₃ O-TPP than in the Ni-N _4- TPP. Therefore, these results show that electrochemical carbon dioxide reduction is better achieved by Ni(-Cl)-N ₃ O-TPP than by Ni—N _4- TPP.

To elucidate the source of the conflicting carbon dioxide reduction activities among nickel porphyrins, the oxidation state of nickel, which is claimed to be one of the key variables determining the reactivity, was observed under carbon dioxide reduction conditions. Cyclic voltammetry measurement results for Ni-N _4- TPP, carbon dioxide It was confirmed that there was no oxidation-reduction of nickel in the saturated electrolyte, and the initial oxidation state (+2 valency) was stabilized in the range from 0.4 to -0.8 V _RHE . However, as a result of cyclic voltammetry measurement of Ni(-Cl)-N ₃ O-TPP, a clear oxidation-reduction pair is observed at about -0.22 V _RHE (peak A). The number of electrons transferred for one Ni(-Cl)-N ₃ O-TPP in the oxidation-reduction reaction, given the amount of Ni(-Cl)-N ₃ O-TPP in the electrode and the charge of electrons transferred during the reaction. can be estimated to be about 1.9. Although oxidation-reduction pairs are observed in argon-saturated electrolytes, the oxidation-reduction potential ( E ⁰ ) of Ni(-Cl)-N ₃ O-TPP is about 110 on the pH-corrected reversible hydrogen electrode (RHE) scale. Move to positive (+) by mV. However, on the standard hydrogen electrode (SHE) scale without pH correction, the oxidation-reduction pair A is located at the same E ⁰ of -0.62 V _RHE regardless of the pH of the electrolyte. The corresponding polarization curve results indicate that the oxidation-reduction pair involves two-electron transfer without proton transfer.

During the carbon dioxide reduction reaction of the nickel porphyrin catalyst, the oxidation number change of nickel was further analyzed using XANES. XANES spectra were recorded in CO2-saturated electrolyte at OCP and -0.65 V _RHE , respectively (Fig. 6(e)). The latter potential is lower than the oxidation-reduction potential of Ni(-Cl)-N ₃ O-TPP (ie, -0.22 V _RHE ), and the carbon dioxide reduction reaction is maximized at that potential. In the case of Ni-N _4- TPP, the reaction real-time XANES spectrum is almost similar regardless of the electrode potential. However, the Ni K-edge position of Ni(-Cl)-N ₃ O-TPP changes by -2 eV when polarized to a reduction potential (-0.65 V _RHE ). The corresponding Ni K-edge position (8343 eV) means an average oxidation number of about +1.6, indicating that some of the nickel is reduced to +0 or +1. This is because the electronic state of the central nickel can play a decisive role in the electrochemical conversion of carbon dioxide, and Ni ⁰ or Ni ^I It can be assumed that it has higher activity than Ni ^II . However, when performing a linear combination fitting of the XANES spectrum of Ni(-Cl)-N ₃ O-TPP at -0.65 V _RHE with nickel species of Ni ⁰ and Ni ^II , an inaccurate fitting value (R -factor = 0.075) (Fig. 6(f)). This failure of fitting supports that the oxidation-reduction pair A at -0.22 V _RHE observed in the Ni(-Cl)-N ₃ O-TPP polarization curve results is not the oxidation-reduction pair of Ni ^II and Ni ⁰ . Thus, the single-electron reduction ^{of Ni II to Ni I involves} another single-electron reduction of the N ₃ O-TPP ligand since the oxidation-reduction pair A involves two electron transfers as confirmed by cyclic voltammetry. can be considered (Fig. 6(d)).

3. Electronic structure of active nickel species during carbon dioxide reduction reaction

Determination of nickel porphyrin compounds using density functional theory calculations found to predict reliable E ⁰ for various transition metal porphyrin systems (Me-N ₄ -TPP; Me = Fe, Co, Ni, Cu and Zn). We looked at E ⁰ theoretically. Density functional theory calculations predict the single-electron reduction potential of Ni-N _4- TPP as -1.04 V _SHE and the addition of electrons to the π* orbitals of N _4- TPP, which is consistent with the existing experimental and theoretical results. . Therefore, the density functional theory calculation results support that nickel of the +2 oxidation number of Ni-N _4- TPP is not reduced in the potential range of the carbon dioxide reduction reaction. The two-electron oxidation-reduction of Ni(-Cl)-N ₃ O-TPP is predicted to occur via the following reaction.

Density functional theory calculation predicts E ⁰ in Equation (1) as -0.54 V _SHE . This corresponds to the experimental observation of oxidation-reduction pair A located near -0.62 V _SHE . Density functional theory calculations suggest that one electron reduces Ni ^II to Ni ^I during two-electron reduction, as supported by the spin density plot of the Ni-N ₃ O-TPP ^ㅣ0 species and the +0.98 Mulliken spin population of nickel. also show The other electron occupies the π* orbital of _NO -TPP and slightly changes the Mulliken spin population of nickel from +1, but greatly increases the spin density of the porphyrin π-system.

As a result, Ni ^II of Ni-N _4- TPP and Ni ^I of Ni-N ₃ O-TPP ^l- are chemical species actually responsible for the high activity. Therefore, the carbon dioxide reduction reactivity of nickel active sites with these two different oxidation numbers is of interest. To isolate the effect of changing ligand field and oxidation state, hypothetical analysis of Ni-N ₃ O-TPP ^l+ , which can be formed by removing axial chloride without changing the oxidation number of nickel in Ni(-Cl)-N ₃ O-TPP. The model was further explored using density functional theory calculations. Unlike Ni(-Cl)-N ₃ O-TPP, there is no fifth coordination of Ni-N ₃ O-TPP ^l+ and a low-spin Ni ^II state (S = 0) with virtually no structural distortion stabilized Therefore, the metal oxidation state and coordination structure of Ni-N ₃ O-TPP ^ㅣ+ and Ni-N _4- TPP are almost identical, but D is selective only for Ni-N ₃ O-TPP ^ㅣ+ due to substitution of N and O. _{The 4h} ligand field collapses.

When a potential of -0.6 V _RHE , which is the carbon dioxide reduction reaction potential, is applied, Ni-N _4- TPP (structure having a D _4h ligand field) and Ni-N ₃ O-TPP ^l- (structure with a modified D _4h ligand field) The free energy of the carbon dioxide reduction reaction for is shown in FIG. 7 . In the *COOH phase, which exhibits the most unfavorable energy, Ni ^II of Ni-N _4- TPP shows a low stabilization ability of *COOH, so a large overpotential is predicted. On the other hand, the active species of Ni-N ₃ O-TPP, Ni ^I of Ni-N ₃ O-TPP ^l-, substantially stabilizes *COOH under real-time reaction conditions and has a much reduced overpotential. Interestingly, the hypothetical active site of Ni ^II of Ni-N ₃ O-TPP ^ㅣ+ predicts a reduction reactivity equivalent to that of Ni ^I of Ni-N ₃ O-TPP ^ㅣ .

Therefore, it can be concluded that it is important to break the symmetry of the in-plane ligand field for highly active carbon dioxide reduction. Further performed cyclic voltammetry experiments and density functional theory calculations showed that under the reaction conditions of another high-performance single-atomic nickel catalyst, nickel phthalocyanine (NiPc), a distorted D _4h ligand field was induced. Therefore, the above conclusion can support that the high-performance carbon dioxide reduction reactivity induced by the distorted ligand field is not a characteristic of Ni(-Cl)-N ₃ O-TPP, but a phenomenon that can be generally applied to monatomic nickel catalysts.

As shown in FIG. 8(a) and FIG. 8(b), Ni ^I- N ₃ O-TPP ^l- can form a stronger Ni-C bond than Ni ^II- N _4- TPP, which means that Ni ^{I -} N ₃ O-TPP ^l- is presumed to be the cause of the high activity. To explain this, a schematic molecular orbital (MO) diagram is shown in FIG. 8(c). The d orbitals of nickel species with low oxidation numbers, such as +2 and +1, are mostly filled, so the nickel ions have low Lewis acidity and prevent the formation of strong Ni-C bonds with COOH. Thus, forming strong Ni-C bonds with COOH requires a redistribution of nickel electron density. As in the case of the N ₄ -TPP ligand, when a strong ligand field causes a large ligand field splitting (Δ), the energy cost required for charge redistribution increases significantly (see Fig. 8(c)), which reduces the Ni-C bond strength. substantially weaken. Thus, in the case of twisting of the ligand field, as in the case of the N ₃ O-TPP ligand, nickel can form stronger Ni-C bonds to stabilize *COOH, which provides a favorable energy condition for the carbon dioxide reduction reaction at the nickel active site. is the key to achieving However, it also promotes the production of Ni ^I under polarization, accompanied by a significant positive (+) shift of the nickel reduction potential as the energy level of the unoccupied d orbital of nickel is lowered.

4. Electrochemical conversion of carbon dioxide to carbon monoxide

Response real-time gas chromatography (GC) in a double-compartment H-cell to determine the Faradaic efficiency (FE) and partial current density for the CO reduction reaction, along with basic understanding based on reaction-time spectroscopic analysis and computational studies The gas product was analyzed.

9(a) shows Faraday efficiencies of carbon monoxide and hydrogen production under potentiostatic polarization conditions for the carbon dioxide reduction reaction for 1 hour. In both nickel porphyrin electrodes, the sum of FE _CO and FE _H2 reached almost 100%, so the formation of other products was almost negligible. The absence (or undetectable content) of a liquid product (eg, formate or methanol) in the electrolyte after the reaction was confirmed through the ¹ H NMR spectrum. carbon dioxide During the reduction reaction, Ni-N _4- TPP is dominated by hydrogen production over the entire potential range, and carbon monoxide shows only about 2% Faraday efficiency in the high overpotential region below -0.75 V _RHE (Fig. 9(a) and Fig. 9(b)). On the other hand, Ni(-Cl)-N ₃ O-TPP shows an initiation potential for carbon monoxide generation at about -0.55 V _RHE and the maximum Faraday efficiency reaches about 80% at -0.65 V _RHE . However, the Faraday efficiency of Ni(-Cl)-N ₃ O-TPP and the partial current density of carbon monoxide production deteriorate in the high overvoltage region, whereas hydrogen production is greatly improved (Fig. 9(a)). This contradicts the results of the reaction real-time SFC-DEMS measurement showing enhanced carbon monoxide generation as the overpotential increases. Further cyclic voltammetry analysis of Ni(-Cl)-N ₃ O-TPP provides clues to understanding the discrepancy between the two analysis results (Fig. 9(c)). When the lower potential limit of cyclic voltammetry decreases below -0.7 V _RHE , the oxidation-reduction pair A of Ni(-Cl)-N ₃ O-TPP at about -0.22 V _RHE in CO2-saturated electrolyte disappears. On the other hand, a broad oxidation-reduction pair newly grows at about 0.07 V _RHE (Fig. 9(c); peak B), which is more evident as two different oxidation-reduction pairs at about 0.13 and 0.07 V _RHE in argon-saturated electrolyte. (B1 and B2 peaks, respectively). Peak B at 0.07 V _RHE in carbon dioxide saturated electrolyte (or peak B1 at about 0.13 V _RHE in argon saturated electrolyte) is responsible for the oxidation-reduction pair of nickel-free N ₃ O-TPP, which is below <-0.6 V _RHE . indicates structural instability (e.g., demetallization) of nickel at a potential of Considering the Pourbaix diagram of nickel, the precipitation of dissolved nickel ions into metallic nickel or oxidized NiO and Ni(OH) ₂ is a spontaneous process of energy release under pH conditions (i.e. pH = 7.2–8.9). and the norm E ⁰ is about 0.13 ( E ⁰ _Ni/NiO ) and 0.11 ( E ⁰ _{Ni/Ni(OH) 2} ) V _RHE , respectively. The formation of solid NiO species was clearly confirmed by the Raman spectrum of the Ni(-Cl)-N ₃ O-TPP electrode after cyclic voltammetry measurements in the range of -0.8 to 0.4 V _RHE (Fig. 9(d) ). It is noteworthy that these solid-phase nickel species are highly active in the H ₂ production reaction. When measuring the carbon dioxide reduction reaction in a double-compartment H-cell, the electrode at each potential was replaced with a new one, but the 1-hour operating time was less than several minutes (only 80 seconds at potentials below -0.7 V _RHE ) compared to the SFC-DEMS analysis. much longer, resulting in much more severe degradation of the nickel porphyrin when analyzed in a double-compartment H-cell. Despite the rapid measurement of SFC-DEMS, the instability of Ni(-Cl)-N ₃ O-TPP can be supported by the significant decrease in carbon dioxide consumption and carbon monoxide production at polarization during the second cyclic voltammetry measurement (Fig. 6( c)).

As described above, the present invention has been described based on preferred embodiments and comparative examples, but the technical spirit of the present invention is not limited thereto, and variations or modifications are possible within the scope described in the claims. It will be obvious to those skilled in the art, and such variations or modifications will fall within the scope of the appended claims.

Claims (5)

A catalyst for electrochemical carbon dioxide conversion composed of an organometallic compound in which one or more nitrogen (N) atoms bonded to a central nickel (Ni) ion in a nickel porphyrin-based compound are replaced with heteroatoms. According to claim 1,
The heteroatom is an electrochemical carbon dioxide conversion catalyst, characterized in that the oxygen (O) atom. According to claim 1,
A catalyst for electrochemical carbon dioxide conversion, characterized in that represented by the following formula (1):
<Formula 1>

(In Formula 1 above,
R are independently hydrogen or a hydrocarbon group having 1 to 12 carbon atoms). According to claim 3,
A catalyst for electrochemical carbon dioxide conversion, characterized in that represented by the following formula (2):
<Formula 2>

. An electrode for electrochemical carbon dioxide conversion comprising the catalyst according to any one of claims 1 to 4.

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