KR102618469B1 - 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 catalyst for electrochemical carbon dioxide conversion consisting of an organometallic compound in which at least one nitrogen (N) atom bonded to the central nickel (Ni) ion in a nickel porphyrin-based compound is replaced with a heteroatom, and the same. As an electrode for carbon dioxide conversion, the electrochemical catalyst for carbon dioxide conversion according to the present invention has twisted symmetry at the atomic level around the nickel active site through control of the coordination environment of the monoatomic nickel catalyst, thereby efficiently converting carbon dioxide with high selectivity. Carbon dioxide can be converted into carbon monoxide, and the carbon monoxide produced through this can be very useful in the production of various petrochemical-based compounds.

Description

Single-atomic nickel catalyst for electrochemical carbon dioxide conversion with high activity and high selectivity and electrode for carbon dioxide conversion 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 in the electrochemical conversion reaction of carbon dioxide (CO ₂ ) to carbon monoxide (CO) and an electrode for carbon dioxide conversion containing the same.

As global warming caused by greenhouse gases causes widespread and persistent problems such as climate change, ecosystem disturbance, and food shortages, 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 and storage technology and utilization technology as a resource with useful value. Representative capture and storage methods include underground storage methods such as storing carbon dioxide in deep water in a supercritical form or storage in conjunction with oil extraction technology (EOR), and mineral carbonation methods that store carbon dioxide in the form of carbonate through reaction with alkali metals. . However, these methods have disadvantages such as limited storage space and storage amount, and environmental destruction. Accordingly, a technology that has recently emerged is a technology that converts carbon dioxide into a useful resource of high added value.

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

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

Among the compounds that can be generated from this electrochemical carbon dioxide conversion, carbon monoxide (CO) becomes a raw material for synthesis gas along with by-product hydrogen generated during the conversion process, and is converted into various forms of low carbon through continuous gas-liquid conversion reaction (Fischer-Tropsch synthesis). It can be converted and utilized 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 precious metals have an optimal adsorption energy between the reaction intermediate (*COOH) and the product (*CO) when converting carbon dioxide and have good activity in converting carbon dioxide to carbon monoxide, but problems such as price and stability have not been solved. .

Therefore, in order to commercialize carbon dioxide electrochemical conversion, it is necessary to reduce the price of the catalyst and at the same time develop catalyst materials with high performance.

Republic of Korea Patent Publication No. 10-2013-0085635 (Publication date: July 30, 2013) International Publication Patent WO 2015/169763 (Publication date: November 12, 2015) International Publication Patent WO 2013/042695 (Publication Date: March 28, 2013)

The present invention is a catalyst for electrochemical carbon dioxide conversion reaction and is based on a monoatomic nickel catalyst in which nickel (Ni), a transition metal rather than a noble metal, is stabilized with an organic ligand, and the coordination environment of the central nickel ion is controlled to convert electrochemical carbon dioxide. The purpose is to provide a novel monoatomic nickel catalyst showing high activity in conversion and an electrode for carbon dioxide conversion containing the same.

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

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

<Formula 1>

(In Formula 1 above,

R is independently 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 includes methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, n-pentyl group, n-hexyl group, octyl group, cyclohexyl group, cyclooctyl group, cyclohexylmethyl group, It may be a phenyl group, naphthyl group, benzyl group, etc., and may further have a substituent such as a hydroxyl group, amino group, carboxyl group, carbonyl group, sulfhydryl group, sulfonyl group, or halo group. Additionally, adjacent R may be connected to a hydrocarbon chain to form a cyclic structure, and in this case, the hydrocarbon chain may be saturated or unsaturated.

Furthermore, the catalyst for electrochemical carbon dioxide conversion according to the present invention may be made of a nickel porphyrin-based compound represented by the following formula (2).

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

At this time, the composition for forming a coating layer on the surface of the current collector may include a catalyst for electrochemical carbon dioxide conversion according to the present invention and a conductive material, and the conductive material is activated carbon (acetylene black, Ketjen black, etc.) , it is preferable that it is a carbon material such as carbon nanotubes, graphene, or fullerene. Additionally, metal materials may be used as the conductive material, for example, Au, Ag, Cu, Al, Pt, Ni, Zn, Sn, Bi, and Pd, and alloys thereof. Furthermore, the composition for forming the coating layer may include a transparent conductive metal oxide such as ITO, ZnO, FTO, AZO, and ATO as a conductive material, and may also include a composite of the conductive carbon material, a conductive resin, a conductive ion exchange resin, etc. Resin materials such as ionomers may be used.

Meanwhile, the various conductive materials described above 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 can convert carbon dioxide into carbon monoxide with high selectivity by having twisted symmetry at the atomic level around the nickel active site through controlling the coordination environment of the monoatomic nickel catalyst, thereby generating Carbon monoxide can be very useful 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) is a diagram showing the process of synthesizing Ni-N 4- TPP and Ni(-Cl)-N 3 O-TPP, and Figure 1(b) is a diagram showing the process of synthesizing Ni-N 4- TPP and Ni(-Cl)-N 3 O-TPP. This is a diagram showing the 3D molecular structure of Ni(-Cl)-N ₃ O-TPP, and Figure 1(c) shows Ni-N _4- TPP, Ni(-Cl)-N ₃ O-TPP, and Ni of nickel. K-edge X-ray absorption spectroscopy (XANES) spectrum.
Figure 2a is ^{a 1} H NMR spectrum for 2,5-bis-(phenylhydroxymethyl)furan prepared in the examples herein.
Figure 2b is a ¹³ C NMR spectrum for 2,5-bis-(phenylhydroxymethyl)furan prepared in the examples herein.
Figure 3a is ^{a 1} H NMR spectrum for N ₃ O-TPP prepared in the examples herein.
Figure 3b is a ¹³ C NMR spectrum for N ₃ O-TPP prepared in the examples herein.
Figure 4a is ^{a 1} H NMR spectrum for Ni(-Cl)-N ₃ O-TPP prepared in the examples herein.
Figure 4b is a ¹³ C NMR spectrum for Ni(-Cl)-N ₃ O-TPP prepared in the examples herein.
Figure 5 is ^{a 1} H NMR spectrum for Ni-N _4- TPP prepared in the examples herein.
Figure 6(a) shows the polarization of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP measured at a potential scanning rate of 5 mV s ^-1 in 0.5 M KHCO ₃ electrolyte saturated with argon/carbon dioxide. curves, and Figures 6(b) and 6(c) are the differential electrochemical masses of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP, respectively, coupled to a real-time scanning flow cell (SFC) system. Differential electrochemical mass spectroscopy (DEMS) results (CO ₂ ( m / z = 44), CO ( m / z = 28) and H ₂ ( m / z = 2) signals are obtained in 0.5 M KHCO ₃ saturated with carbon dioxide. (collected during two cycles of cyclic voltammetry from 0 to -0.9 V _RHE , the potential profile during DEMS measurements is indicated by a dotted line), Figure 6(d) shows the potential range from -0.5 to 0.4 V _RHE in carbon dioxide-saturated electrolyte. Cyclic voltammetry results of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP measured at a potential scanning rate of 50 mV s ^-1 (oxidation-reduction pair A is indicated by an arrow) , Figure 6(e) is the real-time XANES spectrum of the reaction of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP measured at open circuit potential (OCP) and -0.65 V _RHE ( Figure 6(f) is _a linear plot _{of the} Ni K-edge This is the linear combination fitting result.
Figure 7(a) is a schematic diagram showing the catalytic cycle of Ni-N _4- TPP (left) and Ni(-Cl)-N ₃ O-TPP (right), and Figure 7(b) is a schematic diagram showing the potential of -0.6 V _RHE . This is the free energy profile calculated from density functional theory when is applied.
Figures 8(a) and 8(b) are schematic diagrams showing the density functional theory calculation optimization structure of Ni-N _4- TPP and Ni(-Cl)-N ₃ O-TPP ^ㅣ-, respectively (for nickel The bonding energy of COOH is displayed, with a larger negative value meaning a stronger bond), and Figure 8(c) is a schematic molecular orbital (MO) diagram (to form a strong Ni-COOH bond). , the electron density of the electron-rich nickel needs to be redistributed to the porphyrin ligand (orange arrow); as the ligand-field splitting (Δ) increases, the energetic cost of this electron redistribution becomes greater.
Figure 9(a) shows the total Faraday efficiency (FE _Total ) and carbon monoxide generation Faraday efficiency (FE _CO ) obtained for the carbon dioxide reduction reaction conducted for 1 hour at various potentials in a double-compartment H-cell, and Figure 9(b) shows the double The total current density ( j _Total ) and the partial current density of carbon monoxide production ( j _CO ) obtained for the carbon dioxide reduction reaction conducted for 1 hour at various potentials in the Khan H-cell. Figure 9(c) shows the results obtained using different potential ranges. Oxidation peak for Ni(-Cl)-N ₃ O-TPP measured by cyclic voltammetry (polarization curve shows the lower potential limit with 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 , and the peaks (A and B) are indicated by arrows), Figure 9(d) shows sequential changes from 0.4 to -0.8 V _RHE . Raman spectrum 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.

Since the embodiments according to the concept of the present invention can make various changes and have various forms, specific embodiments will be illustrated in the drawings and described in detail in the present 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, and substitutes included in the spirit and technical scope of the present invention.

The terms used herein are only used to describe specific embodiments and are not intended to limit the invention. Singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, terms such as “include” or “have” are intended to indicate the existence of a described feature, number, step, operation, component, part, or combination thereof, but are not intended to indicate the presence of one or more other features or numbers. It should be understood that this does not preclude the existence or addition of steps, operations, components, parts, or combinations thereof.

Hereinafter, the present specification will be described in more detail through examples. However, the embodiments according to the present specification may be modified into various other forms, and the scope of the present specification is not to be construed as being limited to the embodiments described in detail below. The embodiments of this specification are provided to more completely explain the present specification to those with average knowledge in the art.

<Example>

In this example, in order to derive the active site of a monoatomic nickel catalyst with high activity in electrochemical carbon dioxide conversion, two types of organometallic complexes with accurately controlled structures, that is, a structure containing monoatomic nickel surrounded by four nitrogen atoms ( Ni-N _4- TPP) and a second molecule (Ni-N ₃ O-TPP) with distorted symmetry in which one nitrogen is replaced with oxygen were synthesized, 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 twisted symmetry of the active site effectively lowers the energy required to generate reaction intermediates, resulting in high conversion performance.

1. Synthesis of Ni-Porphyrin compound

N _4- TPP and N ₃ O-TPP were synthesized through a condensation reaction using pyrrole and aldehyde, and for 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 the metallization reaction of N _4- TPP and N ₃ O-TPP.

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

A 2.5 M hexane solution of distilled n-hexane (50 mL), tetramethylethylenediamine (4.3 g, 37.5 mmol, 5.6 mL), and n-butyllithium (2.4 g, 37.5 mmol, 15.0 mL) in a 250 mL round bottom flask. ) and stirred for 10 minutes in a nitrogen atmosphere. Afterwards, 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 in ice at 0 degrees, and benzaldehyde (4.0 g, 37.5 mmol, 33.8 mL of 1.1 M tetrahydrofuran solution) at 0 degrees is slowly added. The mixture is slowly warmed to room temperature and stirred for 15 minutes, and then the reaction is terminated by adding a cold aqueous ammonium chloride solution (70 mL, 1.0 M aqueous solution). The organic layer was washed with water (3 Purification using a silica gel column containing 30% (v/v) ethyl acetate/n-hexane is performed to obtain the desired product. At this time, the yield is 35% (1.46 g).

(2)N ₃ Synthesis of O-TPP

Add 2,5-bis-(phenylhydroxymethyl)furan (0.56 g, 2.0 mmol) and chloroform (200 mL) to a 500 mL round bottom flask and stir 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 cooled to room temperature. Stir 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

Add N ₃ O-TPP (0.080 g, 0.13 mmol) to a 100 mL round bottom flask and dissolve in chloroform (70 mL). A solution of nickel chloride (0.037 g, 1.56 mmol) dissolved in ethanol (7 mL) was added to the 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 containing 5% (v/v) acetone/chloroform is performed to obtain a purple product. Afterwards, it is recrystallized from dichloromethane/n-hexane to obtain the desired purple crystal. At this time, the final yield was 64% (0.059 g).

(4)Ni-N _4- Synthesis of TPP

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

(5) Physical and chemical property analysis of synthesized nickel porphyrin compounds

From the peak at approximately 856.0 eV in the X-ray photoelectron spectroscopy (XPS) spectrum and the Ni K-edge position at 8345 eV in the It was found to be +2 for both nickel porphyrin compounds. The structure of Ni(-Cl)-N ₃ O-TPP refers to single crystal X-ray diffraction (SC-XRD; Figure 1(b)), showing nickel atoms and O(1)N(2)-N(3)N. (4) It shows a five-coordinate nickel complex with a vertical distance between planes 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 at θ = 18.2° (θ is the angle between the furan plane and the Ni-O bond). The 1s → 4p _z transition at approximately 8339 eV in the XANES spectrum is a characteristic of the square planar structure that can be clearly seen in the spectrum of Ni-N _4- TPP (Figure 1(c)). On the other hand, a pre-edge peak is observed at around 8335 eV for Ni(-Cl)-N ₃ O-TPP due to the increased dipole-tolerant 1s → 4p transition, which leads to the twisted D leading to hybridization of the 3d and 4p orbitals. This is evidence of _4h symmetry. Therefore, the Ni ^II complex with a four-coordinate dianionic ligand (square planar Ni-N _4- TPP) and a monoanionic ligand with an axial 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 of 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 scan potential measurement results show similar current reforming for Ni-N _4- TPP in argon and carbon dioxide saturated electrolytes (pH of approximately 8.9 and 7.2, respectively) (Figure 6(a)). On the other hand, the reduction current density of Ni(-Cl)-N ₃ O-TPP increases significantly in a carbon dioxide-saturated electrolyte, and is two times higher than in the absence of carbon dioxide, especially at -0.8 V _RHE . The reduction current is almost absent for glassy carbon and activated carbon without nickel porphyrin compound, and it can be confirmed that the improved reduction current in carbon dioxide-saturated electrolyte is due to the electrocatalytic properties of the nickel porphyrin compound. Therefore, the difference in polarization curves, which is clearly observed in the Ni(-Cl)-N ₃ O-TPP electrode but not in the Ni-N _4- TPP electrode, is greater for Ni(-Cl)-N than for Ni-N _4- TPP. ₃ This means that the electrochemical carbon dioxide reduction reactivity of O-TPP is higher. To confirm the above hypothesis, two major reactions (reduction of carbon dioxide to carbon monoxide and competitive hydrogen evolution) on a monoatomic nickel catalyst were performed under the same reaction conditions using differential electrophoresis coupled to a scanning flow cell (SFC) system. It was observed using differential electrochemical mass spectroscopy (DEMS). The results of DEMS analysis during the linear scanning potential method show that carbon dioxide ( m / z = 44) in the electrolyte is not consumed in the _low overpotential range, but _is consumed at approximately Consumption begins at potentials below -0.8 and -0.6 V _RHE (Figures 6(b) and 6(c)). At the same time, carbon monoxide ( m / z = 28) increases as overvoltage increases. This change is a phenomenon that is not observed in the absence of carbon dioxide, and only the production of hydrogen is observed under these conditions. 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 the higher onset potential, greater signal changes for carbon dioxide consumption and carbon monoxide production were recorded for Ni(-Cl)-N ₃ O-TPP than for Ni-N _4- TPP. Therefore, these results show that electrochemical carbon dioxide reduction is achieved better by Ni(-Cl)-N ₃ O-TPP than by Ni-N _4- TPP.

To reveal the source of the conflicting carbon dioxide reduction activity between nickel porphyrins, the oxidation state of nickel, which is claimed to be one of the key variables determining reactivity, was observed under carbon dioxide reduction conditions. Cyclic voltammetry measurement results for Ni-N _4- TPP, carbon dioxide It was confirmed that nickel oxidation-reduction was absent in the saturated electrolyte, and the initial oxidation state (+2 valence) was stabilized in the range from 0.4 to -0.8 V _RHE . However, as a result of cyclic voltammetry measurements of Ni(-Cl)-N ₃ O-TPP, a clear redox pair is observed at about -0.22 V _RHE (peak A). Considering the amount of Ni(-Cl)-N ₃ O-TPP on the electrode and the charge of electrons transferred during the reaction, the number of electrons transferred for one Ni(-Cl)-N ₃ O-TPP during the oxidation-reduction reaction can be estimated to be about 1.9. Although oxidation-reduction pairs are also observed in argon-saturated electrolytes, the oxidation-reduction potential ( E ⁰ ) of Ni(-Cl)-N ₃ O-TPP is approximately 110 on the pH-corrected reversible hydrogen electrode (RHE) scale. It moves 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 redox pair involves two electron transfer without proton transfer.

During the carbon dioxide reduction reaction of the nickel porphyrin catalyst, the change in the oxidation number of nickel was further analyzed using XANES. XANES spectra were recorded in carbon dioxide-saturated electrolyte at OCP and -0.65 V _RHE , respectively (Figure 6(e)). The latter potential is lower than the oxidation-reduction potential of Ni(-Cl)-N ₃ O-TPP (i.e., -0.22 V _RHE ), and the carbon dioxide reduction reaction is maximized at that potential. For Ni-N _4- TPP, the reaction real-time XANES spectra are almost similar regardless of electrode potential. However, the Ni K-edge position of Ni(-Cl)-N ₃ O-TPP changes by -2 eV when polarized to the reduction potential (-0.65 V _RHE ). The corresponding Ni K-edge position (8343 eV) indicates an average oxidation number of approximately +1.6, indicating that a portion of nickel has been reduced to +0 or +1. This means that the electronic state of the central nickel may play a critical role in electrochemical carbon dioxide conversion, and Ni ⁰ or Ni ^I It can be inferred that it has higher activity than Ni ^II . However, when performing ^a _linear combination fitting ^of _the -factor = 0.075) (Figure 6(f)). This failure of fitting supports that the redox pair A at -0.22 V _RHE observed in the Ni(-Cl)-N ₃ O-TPP polarization curve results is not the redox pair to Ni ^II and Ni ⁰ . Therefore, since the redox pair A involves a two-electron transfer, as confirmed by cyclic voltammetry, one-electron reduction of Ni ^II to Ni ^I followed by another one-electron reduction of the N ₃ O-TPP ligand. can be considered (Figure 6(d)).

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

of nickel porphyrin compounds using density functional theory calculations, which were found to predict E ⁰ reliably for a variety of transition metal porphyrin systems (Me-N _4- TPP; Me = Fe, Co, Ni, Cu, and Zn). E ⁰ was examined theoretically. Density functional theory calculations predict the single-electron reduction potential of Ni-N _4- TPP to be -1.04 V _SHE and that electrons are added to the π* orbital of N _4- TPP, which is consistent with existing experimental and theoretical results. . Therefore, the density functional theory calculation results support that nickel with +2 oxidation number of Ni-N _4- TPP is not reduced in the potential range of carbon dioxide reduction reaction. The two-electron oxidation-reduction of Ni(-Cl)-N ₃ O-TPP is predicted to occur through the following reaction.

Density functional theory calculations predict E ⁰ in equation (1) above to be -0.54 V _SHE . This corresponds to the experimental observation of redox pair A located near -0.62 V _SHE . Density functional theory calculations show that during two-electron reduction, one electron reduces Ni ^II to Ni ^I , 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. It also shows. The other electron occupies the π* orbital of N ₃ O-TPP and slightly changes the Mulliken spin population of nickel from +1, but significantly 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 ^ㅣ- are the chemical species actually responsible for the high activity. Therefore, the carbon dioxide reduction reactivity at these two different oxidation states of nickel active sites is of interest. To isolate the effects of changes in the ligand field and the oxidation state, we hypothesize that Ni-N ₃ O-TPP ^ㅣ+ 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 ^ㅣ+ and the low-spin Ni ^II state (S = 0) is virtually free from structural distortion. It stabilizes. Therefore, the metal oxidation state and coordination structure of Ni-N ₃ O-TPP ^ㅣ+ and Ni-N _4- TPP are almost identical, but due to the substitution of N and O, D is selective only for Ni-N ₃ O-TPP ^ㅣ+ _4h The 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 with D _4h ligand field) and Ni-N ₃ O-TPP ^ㅣ- (structure with modified D _4h ligand field) The free energy of the carbon dioxide reduction reaction for is shown in Figure 7. In the *COOH phase, which represents the most unfavorable energy, Ni ^II of Ni-N _4- TPP shows a low stabilizing ability of *COOH, and a large overpotential is predicted. On the other hand, under real-time reaction conditions, the active species of Ni-N ₃ O-TPP, Ni ^I of Ni-N ₃ O-TPP ^ㅣ-, substantially stabilizes *COOH, resulting in a much reduced overpotential. Interestingly, the hypothetical active site of Ni ^II of Ni-N ₃ O-TPP ^ㅣ+ predicts reduction reactivity equivalent to Ni ^I of Ni-N ₃ O-TPP ^ㅣ- .

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

As can be seen in Figures 8(a) and 8(b), Ni ^I- N ₃ O-TPP ^ㅣ- can form a stronger Ni-C bond than Ni ^II- N _4- TPP, which is similar to Ni ^{I -} N ₃ O-TPP ^ㅣ- is presumed to be the cause of the high activity shown. To explain this, a schematic molecular orbital (MO) diagram is shown in Figure 8(c). The d orbitals of low oxidation number nickel species 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. Therefore, forming a strong Ni-C bond with COOH requires redistribution of nickel electron density. If a strong ligand field causes large ligand field separation (Δ), as in the case of the N _4- TPP ligand, the energy cost required for charge redistribution is significantly higher (see Figure 8(c)), which increases the Ni-C bond strength. substantially weakens it. Therefore, in case of distortion of the ligand field, as in the case of N ₃ O-TPP ligand, nickel can form stronger Ni-C bonds to stabilize *COOH, which creates favorable energy conditions for the carbon dioxide reduction reaction at the nickel active site. becomes the key to achieving it. However, it is also accompanied by a significant positive shift of the nickel reduction potential as the energy level of nickel's empty d orbital is lowered, promoting the production of Ni ^I under polarization.

4. Electrochemical conversion of carbon dioxide to carbon monoxide

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

Figure 9(a) shows the Faraday efficiency of carbon monoxide and hydrogen production under constant potential polarization conditions for the carbon dioxide reduction reaction for 1 hour. For 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 liquid products (e.g., formate or methanol) in the electrolyte after reaction was confirmed through ¹ H NMR spectrum. carbon dioxide During the reduction reaction, Ni-N _4- TPP is dominated by hydrogen production over the entire potential range, while carbon monoxide shows a Faraday efficiency of only about 2% in the high overpotential region below -0.75 V _RHE (Figures 9(a) and 9(b)). On the other hand, Ni(-Cl)-N ₃ O-TPP shows the onset potential of 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 generation deteriorate in the high overpotential region, while hydrogen generation is greatly enhanced (Figure 9(a)). This is contrary to the results of real-time SFC-DEMS measurements, which showed improved carbon monoxide generation as overvoltage increased. Additional cyclic voltammetry analysis of Ni(-Cl)-N ₃ O-TPP provides clues to understanding the discrepancy between the two analysis results (Figure 9(c)). When the lower potential limit of cyclic voltammetry is reduced below -0.7 V _RHE , the redox pair A of Ni(-Cl)-N ₃ O-TPP at about -0.22 V _RHE in carbon dioxide-saturated electrolyte disappears. Meanwhile, a broad redox pair newly grows at about 0.07 V _RHE (Figure 9(c); peak B), which is more clearly seen as two different redox pairs at about 0.13 and 0.07 V _RHE in argon-saturated electrolyte. can be distinguished (B1 and B2 peaks, respectively). Peak B at 0.07 V _RHE in carbon dioxide-saturated electrolytes (or peak B1 at approximately 0.13 V _RHE in argon-saturated electrolytes) is responsible for the redox pair of nickel-free N ₃ O-TPP, which is <-0.6 V _RHE . indicates structural instability (e.g. demetalization) of nickel at the potential of . Considering the Pourbaix diagram of nickel, 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 standard E ⁰ is approximately 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 from -0.8 to 0.4 V _RHE (Figure 9(d) ). It is worth noting that these solid-phase nickel species are very active in the H ₂ production reaction. When measuring the carbon dioxide reduction reaction in a double-chamber H-cell, the electrode was replaced with a new electrode at each potential, but the operating time of 1 hour was several minutes (only 80 s at potentials below -0.7 V _RHE ) compared to SFC-DEMS analysis. It is much longer and consequently results in much more severe degradation of the nickel porphyrin when analyzed in a double-chamber H-cell. The instability of Ni(-Cl)-N ₃ O-TPP despite the fast measurements by SFC-DEMS can be supported by the significant decrease in carbon dioxide consumption and carbon monoxide production in polarization during the second cyclic voltammetry measurement (Figure 6 ( c)).

As described above, the present invention has been described based on preferred examples and comparative examples. However, the technical idea of the present invention is not limited thereto, and modifications and changes may be made within the scope set forth in the claims, as is known in the technical field to which the present invention pertains. It is obvious to a person with knowledge, and such modifications or changes fall within the scope of the appended patent claims.

Claims (5)

Catalyst for electrochemical carbon dioxide conversion, characterized in that represented by the following formula (2):
<Formula 2>
. delete delete delete An electrode for electrochemical carbon dioxide conversion comprising the catalyst according to claim 1.