Fe-N-C catalyst, preparation method and application thereof

Abstract

The invention relates to a monatomic iron catalyst for electrochemical reduction of carbon dioxide, and a preparation method and application thereof. In particular, the present invention relates to a process for the preparation of Fe (II) doped Zn-ZIF precursor materials and to the use thereof in the preparation of catalysts comprising Fe monatomic atoms, obtained on an N-doped carbon matrix from the pyrolysis of such Fe (II) doped Zn-ZIF precursor materials.

Description

[0001] This application is a divisional application of the Chinese invention patent application with application number 201980013316.5 and invention name “Fe-NC catalyst, preparation method and use thereof” filed on February 12, 2019. Field of the Invention

[0002] The present invention generally relates to the field of electrochemical reduction of carbon dioxide, and in particular to catalysts for CO2 electroreduction. Background Art

[0003] Rising levels of atmospheric carbon dioxide (CO2) have become a global concern, with a range of environmental consequences foreseen, including devastating climate change, that could affect modern civilization. Due to the considerable amounts of CO2 produced over the last fifty years, a variety of approaches are needed to reduce its amount. Among them, methods have been developed for the electrochemical reduction of CO2 to useful chemicals, including carbon monoxide (CO), formic acid, methane, and ethylene (Jhong et al., 2013, Current Perspectives in Chemical Engineering, 2(2), 191-199).

[0004] Therefore, electrolytic reduction of CO2 in aqueous electrolytes to CO is considered a promising technology for converting CO2 and electricity into CO and O2, which fixes CO2 while producing valuable chemicals, and can recycle waste CO2 into a "carbon neutral" cycle. Electrochemical reduction of CO2 to form carbon-based fuels and chemicals has been widely proposed for storing and utilizing intermittent renewable energy sources such as solar and wind energy (Zhu et al., 2016, Adv Mater., 28(18), 3423-52). However, there are two major drawbacks that prevent CO2 electrolytic reduction from developing into a viable technology: low energy efficiency due to large overpotentials, and separation problems due to poor selectivity. If CO is the only product of CO2 reduction, the product can be directly used for the synthesis of hydrocarbons (Fischer-Tropsch synthesis), methanol and aldehydes by adding H2 in an appropriate CO:H2 ratio.

[0005] Over the past three decades, work has focused on screening different metal catalysts and the variety of products that can be formed using these metals. Unfortunately, at overpotentials below 400 mV, conversions are generally low (<20 mA / cm 2 ), and the selectivity of the desired product is also very low.

[0006] To date, the most active catalysts for the electroreduction of CO to produce CO are gold and silver-based nanomaterials (Chen et al., 2012, J. Am. Chem. Soc., 134, 19969-19972; Zhu et al., 2014, J. Am. Chem. Soc., 136, 16132-16135; Lu et al., 2014, Nature Communications, 5, 3242), and to date, the two most active catalysts for the electroreduction of CO to produce CO are (1) oxide-derived gold (Chen et al., 2012, supra): the CO production rate is 8 mA cm at −0.4 V relative to the reversible hydrogen electrode (RHE). -2 ; (2) Gold needle electrode (Liu et al., 2016, Nature, 537, 328): At -0.35 V relative to RHE, the current density of CO formation is about 14 mA cm -2 . However, the high cost of this catalyst limits its large-scale use. Most non-precious metal catalysts (such as copper and zinc) perform much worse (Li et al., 2012, Journal of the American Chemical Society, 134, 7231-7234; Li et al., 2017, Journal of the American Chemical Society, 139, 4290-4293; Won et al., 2016, Angewandte Chemie, 55, 9297-9300). Catalysts containing earth-abundant metals such as iron, cobalt, and nickel have been developed, but their partial current densities are still lower than those based on gold and silver. In particular, using commercially available zeolitic imidazole framework (ZIF), ZIF-8, iron (II) acetate (Fe(Ac)2) and o-phenanthroline (phen) as precursors, they were mixed by ball milling before pyrolysis, producing catalytic materials with various performances, but all achieved CO generation rates far lower than that of gold catalysts, i.e., about 2 mA·cm at -0.4 V. -2 (Huan et al., 2017, ACS Catal., 7, 1520). Iron, cobalt and nitrogen co-doped carbon catalysts were prepared by high temperature pyrolysis of Fe(II)-doped Co-ZIF precursors, wherein the Fe(II)-doped Co-ZIF precursors were prepared from 2-methylimidazole cobalt (ZIF-67) (CN107086313). The resulting catalysts are said to be useful in fuel cells and metal-air battery fields and are superior to commercial catalysts Pt / C.

[0007] Ye Yifan et al. describe a catalyst for oxygen reduction reaction obtained from a precursor consisting of ZIF-8 functionalized with ammonium ferric salt (AFC) Then a two-step pyrolysis was performed (Ye Yifan et al., 2017, Journal of Energy Chemistry, 26(6), 1174-1180). The second pyrolysis step is said to improve the mesoporous area and the formation of undesirable iron nanoparticles. It was prepared by AFC without adding reducing agent, so the precursor was Fe(III)-doped Zn-ZIF.

[0008] Therefore, CO2 conversion via catalytic processes in a selective and efficient manner is of interest, and there is a high demand for high-performance catalysts that can achieve cost-effective and selective CO2 conversion. Summary of the invention

[0009] The object of the present invention is to provide a catalytic material which can be used for the electroreduction of CO2 to CO in an aqueous electrolyte.

[0010] It would be advantageous to provide a catalytic material having high catalytic activity that is capable of increasing the rate of electroreduction of CO2 while maintaining high selectivity for CO formation.

[0011] It would be advantageous to provide a catalytic material having a production cost that permits large-scale use.

[0012] It would be advantageous to provide a catalytic material that is stable during use, thereby limiting the amount of catalytic material used per reduction process and avoiding the need for expensive catalyst regeneration, which would reduce the overall production cost of the CO2 electro-reduction product.

[0013] It would be advantageous to provide a catalyst made from abundant metals that would allow for large-scale use.

[0014] It would be advantageous to provide a cost-effective method for preparing efficient catalytic materials for the electroreduction of CO2 to CO in aqueous electrolytes.

[0015] It would be advantageous to provide a cost-effective method for preparing an effective catalytic material that exhibits a discrete distribution of Fe and Zn atoms within the structure of the catalyst.

[0016] Advantageously, the catalysts are prepared from readily available chemical reagents.

[0017] An object of the present invention is to provide a selective and cost-effective process for the electroreduction of CO2 to CO in an aqueous electrolyte.

[0018] The objects of the invention are achieved by providing a use according to claim 1 , a method according to claim 2 and a catalytic material according to claim 13 .

[0019] According to a first aspect of the present invention, disclosed herein is the use of a catalytic material prepared from a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) for electroreduction of CO2 to CO, the catalytic material comprising single Fe atoms on a N-doped carbon matrix.

[0020] According to a second aspect of the present invention, a method for preparing a catalytic material is disclosed, comprising the following steps:

[0021] a) providing a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) in dry form under an inert atmosphere;

[0022] b) pyrolyzing the Fe(II)-doped Zn-ZIF under an inert atmosphere for about 2 to about 24 hours, typically 3 hours;

[0023] c) collecting the obtained pyrolysis products as catalytic materials.

[0024] The method may advantageously comprise providing a Fe(II)-doped Zn ZIF obtained by a method comprising the steps of:

[0025] (i) providing an imidazole or an imidazole derivative precursor;

[0026] (ii) In preventing Fe 2+ Dissolving the imidazole or imidazole derivative precursor in a degassed polar solvent (e.g., methanol or ethanol) in a reducing environment of cationic oxidation, for example, in the presence of an iodide derivative (e.g., potassium iodide KI, sodium iodide NaI, or ammonium iodide NH4I), to obtain a solution A;

[0027] (iii) dissolving an iron (II) precursor such as FeCl2, FeCl2·4H2O, Fe(CH3COO)2, and a zinc salt such as Zn(NO3)2 or ZnCl2 in a degassed polar solvent such as methanol or ethanol to obtain a solution B;

[0028] (iv) adding solution B to solution A;

[0029] (v) collecting and washing the precipitate;

[0030] (vi) drying the washed precipitate at room temperature to obtain a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) in the form of dry powder, wherein steps (ii) to (vi) are performed under stirring under an inert atmosphere;

[0031] (vii) The Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) was collected in the form of dry powder under an inert atmosphere.

[0032] According to a third aspect of the present invention, a catalytic material is disclosed herein, comprising single iron atoms on N-doped matrix carbon, obtained by pyrolysis of Fe(II)-doped Zn-ZIF.

[0033] According to another aspect of the present invention, disclosed herein is a use comprising a catalytic material for electroreduction of CO2 to CO, wherein the catalytic material comprises single iron atoms on N-doped matrix carbon, obtained by pyrolysis of Fe(II)-doped Zn-ZIF.

[0034] According to another aspect of the present invention, a method for electrochemically reducing carbon dioxide to CO is disclosed herein, the method comprising the following steps:

[0035] - Providing an electroreduction system for electrochemical reduction of CO2;

[0036] - providing a catalytic material according to the invention comprising Fe single atoms on N-doped matrix carbon;

[0037] - bringing the catalytic material into contact with a working electrode of an electroreduction system;

[0038] -Carry out CO2 electroreduction reaction to generate CO;

[0039] - Collect the released gas, which contains carbon monoxide, small amounts of H2 and unreacted CO2.

[0040] According to another aspect of the present invention, disclosed herein is an electroreduction system or a portion of an electroreduction system (eg, a supporting electrode) for electrochemical reduction of CO2, comprising the catalytic material of the present invention.

[0041] According to another aspect of the present invention, disclosed herein is a kit for electroreduction of CO2, comprising the catalytic material of the present invention.

[0042] According to another aspect of the present invention, disclosed herein is a kit for preparing the catalytic material of the present invention, the kit comprising a container containing a Fe(II)-Zn doped zeolitic imidazole framework (ZIF) in dry form and under an inert atmosphere.

[0043] According to another aspect of the present invention, disclosed herein is a kit for preparing a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) suitable for preparing the catalytic material of the present invention, the kit comprising in a separate compartment or container:

[0044] a) imidazole or an imidazole derivative precursor;

[0045] b) iodide derivatives, such as potassium iodide, sodium iodide or ammonium iodide (KI or NaI or NH4I);

[0046] c) an iron(II) precursor, such as FeCl2 or Fe(CH3COO)2; and

[0047] d) Zinc salts, for example Zn(NO3)2 or ZnCl2.

[0048] Other features and advantages of the invention will be apparent from the claims, the detailed description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 shows a method (A) for obtaining a catalytic material comprising single Fe atoms on an N-doped carbon substrate according to the present invention (Fe-NC(II)), and a method (B) for obtaining a Fe(II)-doped Zn-ZIF-8 precursor according to the present invention for such a catalytic material.

[0051] Figure 2 X-ray diffraction patterns of the Fe(II)-doped Zn-ZIF-8 precursor (top) and the undoped Zn-ZIF-8 precursor (bottom) before pyrolysis (a) and the corresponding pyrolysis products (b) are provided, as well as a graphical comparison of the comparative Fe(II)-doped Co-ZIF-8 precursor (top) and the Fe(II)-doped Zn-ZIF-8 precursor of the present invention (bottom) before pyrolysis (c) and after pyrolysis (d), as described in Example 3.

[0052] Figure 3 Electron microscopic characterization of the catalytic materials of the present invention by (a) transmission electron microscopy (TEM) and (b) high resolution transmission electron microscopy (HRTEM) imaging and (c) energy dispersive spectroscopy (EDS) imaging as described in Example 3 is provided. The distribution of Fe, N and Zn is shown in c2, c3 and c4, respectively.

[0053] Figure 4 Electron microscopic characterization of the catalytic materials of the present invention by scanning electron microscopy (SEM) imaging as described in Example 3 is provided. a and b: Fe(II)-doped Zn-ZIF-8 precursor before pyrolysis; c, d and e: pyrolysis products obtained from Fe(II)-doped ZIF-8 precursor; f: particle size distribution diagram of Fe(II)-doped Zn-ZIF-8 (light color) and pyrolysis products (dark color) before pyrolysis.

[0054] Figure 5 Electron microscopic characterization of a comparative material (Fe-NC(III)) prepared by pyrolysis of Fe(III)-doped ZIF-8 by scanning electron microscopy (SEM) imaging as described in Example 3 is provided. a and b: comparative Fe(III)-doped ZIF-8 precursor before pyrolysis; c and d: pyrolysis products of Fe(III)-doped ZIF-8 precursor; e: particle size distribution of Fe(III)-doped ZIF-8 (light color) and pyrolysis products (dark color) before pyrolysis.

[0055] FIG6 shows an electrochemical characterization of the performance of the catalytic material of the present invention obtained as described in Example 2 compared with other materials in an electrolysis test using 0.5M KHCO3 as an electrolyte as described in Example 4. a: Schematic diagram of the electrolysis test performed in an electrochemical reduction system for electrochemical reduction of CO2, including a working electrode (WE, carbon cloth, on which the catalytic material is deposited by drop casting, as described in Example 4), a counter electrode (RE, platinum wire) and a reference electrode (RE, Ag / AgCl, saturated KCl solution electrode); b: Stable cyclic voltammetry (CV) curves obtained with the pyrolysis products of the Fe-NC(II) material of the present invention and the undoped ZIF-8 material in CO2 (solid line) and N2 (dashed line) saturated electrolytes; c: Chronoamperometry curve of the catalytic material "Fe-NC(II)" of the present invention at -0.37V relative to RHE in a CO2 saturated electrolyte. The dots show the Faraday efficiency of CO. d: Faraday efficiency of the Fe-NC(II) material for CO (bottom) and H2 (top) at different applied potentials. Comparison of the Faradaic efficiency (e) and partial current density (f) of CO on Fe-NC(II) material (solid line) and other catalysts (dashed lines) at different applied potentials. (1): 0.6 mg / cm 2 Fe-NC(II) material of the present invention: 3 mg of the catalyst was dispersed in 1 mL of ethanol, and 200 μL of the dispersion was dropped on a 1 cm×1 cm carbon cloth electrode. (2): 1 mg / cm 2 Comparative materials Fe-NC material (Fe 0.5d) (from Huan et al., 2017, supra): 1 mg of the catalyst was dispersed into 200 μL and then dropped on a 1 cm 2 On the carbon paper electrode; (3): 0.4 mg / cm 2 Comparative material CoPc / CNT (from Zhang et al., 2017, Nature Communications, 8, 14675): CoPc / CNT was prepared by adsorbing Co phthalocyanine molecules on multi-walled carbon nanotubes in DMF. 2 mg of the catalyst was dispersed in 1 mL, and 100 μL of the dispersion was dropped into a 0.5 cm 2on a carbon fiber paper electrode; (4): porous Ag (from Lu et al., 2014, supra): a porous Ag electrode was prepared by acid etching of an α-Al(Ag) alloy synthesized at 546 °C; and (5): oxide-derived (OD) Au (from Chen et al., 2012, supra): an OD Au electrode was prepared by applying symmetrical 1 kHz square wave pulses between 2.70 V and 0.70 V on Au foil in 0.5 M H2SO4 electrolyte for 60 min; Faradaic efficiency (g) of CO (solid line) and H2 (dashed line) and partial current density (h) of CO on Fe-NC(II) materials (squares) and other comparative Fe-doped ZIF materials (dots: Fe(II)-doped Co ZIF from CN 107086313; triangles: Fe(III)-doped ZIF from Ye Yifan et al., 2017, supra) at different applied potentials.

[0056] Figure 7 Characterization of the Fe-NC(II) catalyst of the present invention as described in Example 3 is provided, (a) by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and (b) by energy dispersive X-ray spectroscopy (EDS) in the area indicated by the white square; (c) by chronoamperometry at -0.37 V relative to RHE in different electrolytes, by measuring the Faraday efficiency of the total current density (curve) and CO (points) of the Fe-NC(II) of the present invention: (1) ultrapure K2CO3 (99.999%) and deionized water (18.2 MΩ·cm), (2) KHCO3 (99.7%) and deionized water with analytical reagent purity, and (3) KHCO3 and tap water with analytical reagent purity.

[0057] Figure 8 Electron microscopic characterization of the material of the present invention after electrolysis by (a) transmission electron microscopy (TEM) and (b) high resolution transmission electron microscopy (HRTEM) imaging and (c) EDS imaging as described in Example 5 is provided. The distribution of Fe, N and Zn is shown in c2, c3 and c4, respectively, and (d) EDS spectra of Fe-NC(II) material before (top) and after (bottom) electrolysis.

[0058] Fig. 9Characterization of the precursor Fe(II)-doped Zn-ZIF material (ac) and the catalytic material Fe-NC(II) (d) obtained by pyrolysis of the Fe(II)-doped Zn-ZIF by X-ray absorption spectroscopy (XAS) as described in Example 3 is provided. a: Fe K-edge XANES spectrum of the Fe(II)-doped Zn-ZIF of the present invention (black solid line) compared with Fe foil (black dashed line), Fe(II)(phen)3SO4 (black dots) and Fe2O3 (grey solid line); b: Fe K-edge EXAFS spectrum of the Fe(II)-doped Zn-ZIF precursor of the present invention. Black curve: fitted crystal structure of ZIF-8, in which the position of the Zn(II) ion is occupied by the Fe(II) ion; c: schematic diagram of the structure used as the EXAFS fitting model, in particular the region around the metal of the ZIF-8 unit cell; d: Fe K-edge EXAFS spectrum of the catalytic material Fe-NC(II) of the present invention obtained by pyrolysis of the Fe(II)-doped Zn-ZIF precursor. DETAILED DESCRIPTION OF THE INVENTION

[0060] Zeolitic imidazolate frameworks (ZIFs) are a subfamily of metal-organic frameworks (MOFs) that can be synthesized via solvothermal / hydrothermal and microwave-assisted reactions in the temperature range of 298–423 K. ZIFs adopt a porous crystalline structure consisting of metal ions and organic linkers arranged in a manner similar to that of silicon and oxygen in zeolites. The tetrahedral metal center [e.g., M = Zn(II)] is coordinated only by nitrogen atoms in the 1,3-positions of the imidazolate bridging ligands to provide an overall neutral framework. The imidazolate ligands are typically obtained via phenyl (benzimidazolyl (PhIM)) or methyl (2-methylimidazolate (MeIM)) groups (Park et al., 2006, Proceedings of the National Academy of Sciences of the United States of America (PNAS), 103(7), 10186–10191). ZIF-8 can be easily synthesized at room temperature and is stable in water. It is a zeolitic structure with tetrahedrally coordinated zinc metal and 2-methylimidazole rings (Zn(MeIM)2) with a sodalite topology (SOD) that exhibits a 3D structure with a cavity of 11.6 Å in diameter accessible through a window of 3.4 Å. The zinc sites in ZIF can be substituted by other divalent metal cations with tetragonal coordination modes (e.g., Fe 2+ and Co 2+ ) are replaced, the internal space of ZIF can accommodate other metal cations (such as Fe 3+ 、Ni 2+ and Cu 2+ In the framework of the present invention, it was unexpectedly found that by using Fe 2+Doped zeolitic imidazole framework (ZIF), especially ZIF-8, as a precursor material for pyrolysis according to the process of the present invention, can obtain a catalytic material containing Fe single atoms on a N-doped carbon matrix, which has CO2 electroreduction performance similar to that of oxide-derived Au catalysts.

[0061] The performance of the catalyst can be measured by the Faradaic efficiency (FE), the partial current density of CO formation and the stability (Jones et al., 2014, Isr. J. Chem., 54, 1451–1466). Typically, for an applied potential greater than -0.6 V relative to RHE, the Faradaic efficiency is greater than 80% and the partial current density of CO formation is greater than 10 mA cm -2 (This indicates that the CO production rate is higher than 3.3 mL·h -1 cm -2 ) catalyst is considered to be a very effective catalyst.

[0062] The expression "imidazole or an imidazole derivative precursor" refers to a precursor of an imidazole precursor that is capable of forming an imidazole bridge with a tetrahedral metal ion within the zeolitic imidazole ester framework structure. Examples of imidazole precursors are published in Han et al., 2009, Chemical Research Reports, 43(1), 58-67 and Park et al., 2006, supra. In particular, imidazole or an imidazole derivative precursor includes 2-ethylimidazole, 2-methylimidazole, 4-nitroimidazole, 4,5-dichloronitroimidazole, imidazole ester-2-carboxaldehyde, 4-cyanoimidazole, benzimidazole, methylbenzimidazole, 4-azabenzimidazole, 5-azabenzimidazole and purine.

[0063] The expression "iodide derivative" includes potassium iodide, sodium iodide, ammonium iodide, tetramethylammonium iodide, tetraethylammonium iodide and tetrabutylammonium iodide.

[0064] The expression "iron (II) precursor" refers to any iron (II)-containing precursor suitable for doping ZIF. Examples of iron (II) precursors according to the present invention include dehydrates or hydrates of FeCl2, FeSO4, (NH4)2Fe(SO4)2·6H2O, Fe(CH3COO)2 and iron (II) acetylacetonate.

[0065] The expression "zinc salt" refers to any zinc-containing salt suitable for preparing the zeolitic imidazolate framework structure. Examples of zinc salts according to the present invention include Zn(NO3)2, ZnCl2, ZnSO4, Zn(ClO4)2, ZnBr2, Zn(CH3COO)2 and the dehydrate or hydrate of zinc acetylacetonate.

[0066] The statement “prevent Fe 2+ The "reducing environment for cationic oxidation" includes the introduction of reducing agents, including Na2SO3, K2SO3, ascorbic acid, and formaldehyde.

[0067] With reference to the accompanying drawings, in particular first with reference to Figure 1A , which provides a schematic representation of a method for preparing a catalytic material comprising single atoms of Fe on an N-doped substrate, the method comprising the following steps:

[0068] a) providing a Fe(II)-doped zinc-zeolitic imidazole framework (ZIF), such as Fe(II)-doped ZIF-8 (7) in a dried form under an inert atmosphere;

[0069] b) pyrolyzing the Fe(II)-doped Zn-ZIF-8 under an inert atmosphere for about 2 to about 24 hours;

[0070] c) collecting the obtained pyrolysis products as catalytic materials.

[0071] In one embodiment, the Fe(II) doped Zn-zeolitic imidazole framework (ZIF) is provided in dry form in a N2 atmosphere or immersed in hexane. Generally, the Fe(II) doped Zn-zeolitic imidazole framework (ZIF) according to the present invention is stable under an inert atmosphere.

[0072] According to another further embodiment, the Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) is placed on a support under an inert atmosphere, and the pyrolysis in step b) is carried out directly on the support.

[0073] In one embodiment, the pyrolysis of step b) is performed for about 3 hours.

[0074] In one embodiment, the pyrolysis of step b) is carried out at a temperature between about 800 and 950°C, typically 900°C.

[0075] In another embodiment, the pyrolysis of step b) is carried out by heating the mixture at a temperature of about 2 to 10 °C.min -1 The temperature is raised at a rate of, for example, 5°C.min -1 .

[0076] In another embodiment, the inert atmosphere is provided by a flow of N2, for example, at a rate of 50 to 150 mL·min -1 Flow rate.

[0077] In another embodiment, the Fe(II)-doped Zn-ZIF material according to the present invention adopts the crystal structure of Zn ZIF-8 with some Zn(II) ions replaced by Fe(II) ions.

[0078] In another embodiment, the method for preparing the catalytic material according to the present invention may advantageously comprise providing a Fe(II)-doped ZIF material obtained by the method according to the present invention.

[0079] Reference Figure 1B , provides a diagram of a specific embodiment of the steps of a method for preparing a Fe(II)-doped Zn-ZIF, which is obtained by a method comprising the following steps:

[0080] (i) providing an imidazole precursor, in particular a 2-methylimidazole precursor (1);

[0081] (ii) dissolving the imidazole precursor in a degassed polar solvent such as methanol (2) in the presence of an iodide derivative such as potassium iodide (KI) (3) to obtain a solution A;

[0082] (iii) an iron (II) precursor such as FeCl2 (4) and a zinc salt such as Zn(NO3)2·6H2O (5)

[0083] Dissolve in a degassed polar solvent such as methanol (6) to obtain solution B;

[0084] (iv) adding solution B to solution A;

[0085] (v) collecting and washing the precipitate;

[0086] (vi) drying the washed precipitate at room temperature to obtain Fe(II) in the form of dry powder

[0087] doped ZIF-8 (8), wherein steps (ii) to (vi) are performed with stirring under an inert atmosphere;

[0088] (vii) The Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) was collected in the form of dry powder under an inert atmosphere.

[0089] In another embodiment, the method for preparing Fe(II)-doped Zn-ZIF comprises dissolving an iron(II) precursor and a zinc salt in step (iii) to obtain a solution B such that the molar ratio of Fe to Zn is about 1:20 to about 1:4, preferably, not higher than 1:9.

[0090] In another embodiment, a method of preparing Fe(II)-doped Zn-ZIF comprises adding solution B dropwise into solution A under stirring for about 1 hour.

[0091] In another embodiment, the method for preparing Fe(II)-doped Zn-ZIF comprises maintaining the reaction mixture obtained in step (iv) under inert atmosphere with stirring for about one day after the addition of solution B is completed.

[0092] In another embodiment, the method of preparing Fe(II)-doped Zn-ZIF comprises collecting the precipitate in step (v) by centrifugation under an inert atmosphere or by filtration under ambient pressure.

[0093] In another embodiment, the method for preparing Fe(II)-doped Zn-ZIF comprises washing the precipitate in step (v) by washing the collected precipitate in a solvent sequence. According to another specific embodiment, the washing is performed in a solvent sequence comprising washing with N,N-dimethylformamide (DMF) and washing twice with methanol.

[0094] In another embodiment, the inert atmosphere is provided by a closed reaction space under an inert atmosphere (eg, a glove box filled with N 2 ).

[0095] According to another embodiment, the dried form of the Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) of the present invention collected in step (vii) can be stored in an air protective atmosphere (such as under a N2 atmosphere or immersed in hexane) for about 1 week before being pyrolyzed.

[0096] In another embodiment, the method for preparing Fe(II)-doped Zn-ZIF further comprises step (viii), dispersing the dried precipitate obtained in step (vii) in hexane before pyrolyzing it so as to store it in an air-protected atmosphere.

[0097] In a particular embodiment, the Fe(II)-doped Zn-ZIF according to the present invention is directly obtained or deposited on a support, for example on a support electrode (eg, carbon cloth or carbon fiber paper).

[0098] According to a particular aspect, the method of the present invention for preparing a Fe(II)-doped Zn-ZIF precursor advantageously allows for doping of Fe(II) ions during the formation of the ZIF structure (in step (iv)), which results in a catalyst obtained after pyrolysis, such as the catalyst obtained from a precursor obtained by mixing only ZIF-8 and Fe(II) as described in Huan et al., 2017, supra, having better CO2 reduction performance. In a particular embodiment, a catalytic material containing single Fe atoms on a N-doped carbon matrix prepared from a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) according to the present invention can be used in the process of electroreduction of CO2 to CO, and can be useful as part of an electroreduction system for electrochemical reduction of CO2 and / or a kit for electroreduction of CO2.

[0099] According to a particular embodiment, the catalytic material according to the invention can be deposited on the surface of a working electrode of an electroreduction system.

[0100] According to another specific embodiment, the catalytic material of the present invention can be deposited on the surface by spin coating, drop casting or by dip coating, spraying. According to a specific embodiment, the catalytic material according to the present invention can be mixed with an adhesive such as a resin or a polymer to adhere to the use surface. Carbon nanotubes or carbon fibers can be added to the mixture to increase the intensity of the deposit. For example, deposition can be achieved by drying a solution of a catalyst, an adhesive and an optional carbon nanotube or carbon fiber onto the desired surface.

[0101] According to another specific embodiment, the catalytic material according to the present invention can be coated or deposited on the surface used, especially on the surface of the electrode or the electroreduction system for the electroreduction system. More particularly, the mixture of the catalytic material and the binder can be pressed into the membrane electrode for the electroreduction system.

[0102] Alternatively, according to another further embodiment, the catalytic material of the present invention can be prepared by pyrolysis of Fe(II)-doped Zn-ZIF-8 directly grown or deposited on a support, for example on a support electrode (such as carbon cloth or carbon fiber paper).

[0103] According to another aspect of the present invention, a kit for electroreduction of CO2 is provided, comprising the catalytic material of the present invention. For example, the kit comprises the catalyst in a dry form in a container (e.g., vial, cartridge, etc.), or deposited on a surface, such as a supporting electrode.

[0104] According to a specific embodiment, an electroreduction system or a portion of an electroreduction system (e.g., a supporting electrode) for electrochemical reduction of CO2 is provided, which comprises the catalytic material of the present invention, for example, the catalytic material is present in a dry form in a container (e.g., a vial, a cartridge, etc.), or is deposited on a surface, such as a supporting electrode of the electroreduction system.

[0105] According to a particular embodiment, the electroreduction system according to the invention is a three-electrode system (working electrode, counter electrode and reference electrode) using as electrolyte an aqueous solution of KHCO 3 or NaHCO 3 saturated with 1 atm of CO 2 under stirring.

[0106] According to a specific embodiment, the working electrode of the electro-reduction system is a carbon cloth.

[0107] Having described the invention, the following examples are presented by way of illustration only and not limitation. Example

[0108] Example 1: Preparation of Fe(II)-doped Zn-ZIF precursor

[0109] The method according to the invention for preparing the Fe(II)-doped Zn-zeolitic imidazole frameworks (ZIFs) according to the invention is as follows Figure 1B As shown, for Fe(II)-doped zinc imidazole (ZIF-8). In order to prevent oxidation of Fe(II) species during the reaction, the reaction medium, methanol (MeOH) or ethanol, was degassed by bubbling N2 before use, and the synthesis of Fe(II)-doped Zn-ZIF-8 material was performed in a glove box as follows:

[0110] Provide the imidazolate precursor to prepare solution A:

[0111] First, 1.314 g of 2-methylimidazole (2-mIm) was provided as an imidazole ester precursor, and 0.076 g of potassium iodide (KI) was dissolved in 15 mL of MeOH to form a solution A.

[0112] Provide iron (II) chloride to prepare solution B:

[0113] 0.080 g of FeCl2·4H2O or 0.063 g of FeCl2 as Fe(II) precursor and 1.190 g of Zn(NO3)2·6H2O were dissolved in 25 mL of MeOH to form solution B.

[0114] Add solution B to solution A with stirring under inert atmosphere:

[0115] Then, solution B was added dropwise to solution A under stirring at room temperature over 1 hour, and the solution became turbid and gradually turned pale yellow. After adding solution B, the mixture was kept under N2 atmosphere with stirring for 1 day.

[0116] Wash and collect the precipitate:

[0117] The precipitate was separated by filtration at ambient pressure in a glove box and washed once with DMF and twice with MeOH. The washed precipitate was then dried under vacuum at room temperature for 30 minutes, and the Fe(II)-doped Zn-ZIF-8 white powder was collected and placed in a corundum crucible and immersed in hexane, and the precursor was separated from the air during the transfer of the precursor from the glove box to a tube furnace for pyrolysis. The Fe(II)-doped Zn-ZIF-8 can be stored in a sealed state under a N2 atmosphere or immersed in hexane. The dried form can be stored for about a week. The yield obtained based on Zn is about 90%, and about 50% of the loaded Fe is doped into the ZIF.

[0118] Example 2: Preparation of the catalytic material of the present invention from Fe(II)-doped Zn-ZIF

[0119] exist Figure 1AThe method of the present invention for preparing a catalyst comprising single Fe atoms on an N-doped carbon matrix obtained from an Fe(II)-doped Zn-ZIF according to the present invention is described in the following, using the Fe(II)-doped zinc imidazole (ZIF-8) obtained in Example 1 as a pyrolysis precursor. During the transfer from the glove box to the tube furnace, the Fe(II)-doped zinc imidazole (ZIF-8) precursor obtained in Example 1 was immersed in hexane to prevent possible oxidation caused by exposure to air. Then, the catalyst was heated in a tube furnace at 900°C at 100 mL min -1 The pyrolysis was carried out for 3 h at a N2 flow rate of 5 °C min -1 The catalytic material obtained by pyrolysis is a black powder. The yield obtained based on Fe is about 90%.

[0120] Example 3: Characterization of Fe(II)-doped zinc imidazole (ZIF-8) precursor and its pyrolysis product

[0121] like Figure 2 As reported above, the X-ray diffraction (XRD) patterns of undoped ZIF-8 and Fe(II)-doped zinc imidazole (ZIF-8) precursor of the present invention prepared by the same method as in Example 1 without the addition of FeCl2 or FeCl2·4H2O are compared. Figure 2 As reported on b, XRD patterns of the materials obtained after pyrolysis of undoped ZIF-8 and Fe(II)-doped ZIF-8 are compared.

[0122] To prepare the samples for XRD characterization, the material in solid form was first dispersed in a small amount of ethanol, dropped on a glass slide, and then dried naturally. XRD measurements were performed on an XPert with monochromatic Cu Kα radiation and a fast Si-PIN multiband detector. TM Performed on a Philips diffractometer.

[0123] like Figure 2 As shown in a, undoped ZIF-8 (top) and Fe(II)-doped ZIF-8 (bottom) have similar X-ray diffraction (XRD) patterns, indicating that the crystal structure of ZIF-8 remains unchanged after doping with Fe(II). Figure 2b compares the XRD patterns of the pyrolysis product (top) obtained from the Fe(II)-doped ZIF-8 precursor of the present invention obtained in Example 2 and the pyrolysis product (bottom) obtained from the undoped ZIF-8 under the same conditions, wherein a wide band around 25° originating from the carbon matrix is ​​observed, and no diffraction peak corresponding to any crystalline Fe space (such as metal, carbide or nitride) is shown, indicating that the Fe element exists in the pyrolysis product of the Fe(II)-doped Zn-ZIF-8 precursor of the present invention in the form of a single atom. Inductively coupled plasma optical emission spectrometry (ICP-OES) analysis confirmed the presence of Fe and Zn in the pyrolysis product of the Fe(II)-doped Zn-ZIF-8 precursor of the present invention, wherein the weight fractions of Fe and Zn are 2.2% and 4.3%, respectively. The comparative Fe(II)-doped Co-ZIF precursor was prepared as described in CN 107086313, wherein, specifically, the molar ratio of iron / cobalt in ferrous sulfate and cobalt nitrate was 0.1; the molar ratio of total metal ions to 2-methylimidazole was 1 / 16, the solvent was nitrogen-saturated methanol, the stirring time was 5 min, the temperature was 30°C; and the standing time was 20 h. For pyrolysis, the heating rate was 5°C / min, the temperature was 900°C, and the standing time was 3 h. Under the same conditions, the comparative precursor (top) was prepared before pyrolysis ( Figure 2 c) and after pyrolysis ( Figure 2 d) is compared with the XRD pattern of the Fe(II)-doped Zn-ZIF-8 precursor of the present invention (bottom). It can be clearly seen that the comparative Fe(II)-doped Co-ZIF and the Fe(II)-doped Zn-ZIF of the present invention show very similar XRD patterns before pyrolysis, indicating that the crystal structures of the two ZIFs are similar ( Figure 2 c). However, after pyrolysis, the diffraction peaks of face-centered cubic Co (triangles) were observed in the XRD patterns of the comparative Fe(II)-doped Co-ZIF, indicating the presence of a large number of Co crystals in the sample ( Figure 2 d), which points to the formation of metal nanoparticles and the inhomogeneous doping of Fe in the material. These Co crystals may act as catalysts for hydrogen release during the electroreduction of CO2 to CO, and the Faradaic efficiency of CO formation is then very low. Figure 6g As shown, for all catalysts, the sum of the Faradaic efficiencies of CO and H2 is close to 100%. For the comparative catalyst obtained from Fe(II)-doped Co-ZIF (dots), the Faradaic efficiency of H2 is higher than 80%, indicating that the release of hydrogen during the electrolysis of this sample is the main process rather than the formation of CO, in contrast to the catalyst of the present invention (squares), where the Faradaic efficiency of CO is higher than 80%.

[0124] Figure 33a and 3b show TEM and HRTEM characterizations of the pyrolysis product of the Fe(II)-doped Zn-ZIF-8 precursor of the present invention obtained in Example 2. The material exhibits high porosity and no crystalline nanoparticles are observed. Figure 3 c shows the elemental distribution of Fe, N, and Zn obtained by EDS imaging, which are uniform in the material.

[0125] The Fe(II)-doped Zn-ZIF-8 precursor obtained according to Example 1 and its pyrolysis product obtained according to Example 2 were analyzed by SEM ( Figure 4 ) were further characterized and compared with the comparative Fe(II)-doped ZIF-8 and its pyrolysis product Fe-NC(III) ( Figure 5 ). The comparative Fe(III)-doped ZIF-8 precursor was obtained according to the reported method (Chen et al., 2017, Angewandte Chemie, 56, 6937-6941): First, 1.314 g of 2-mIm was dissolved in 15 mL of MeOH to form solution A. 0.141 g of iron (III) acetylacetonate (Fe(acac)3) and 1.190 g of Zn(NO3)2·6H2O were dissolved in 30 mL of MeOH to form solution B. Then, solution B was added dropwise to solution A under stirring at room temperature for 1 hour. The solution was then transferred to a 100 mL Teflon-lined stainless steel autoclave and heated at 120°C for 4 hours. The product was centrifuged and then washed three times with DMF, twice with methanol, and finally dried in vacuo at 70°C overnight. The product was prepared by evaporating the product in a tube furnace at 100 mL·min -1 The pyrolysis was carried out at 900 °C for 3 h with a N2 gas flow rate of 5 °C min -1 , the pyrolysis product of the comparative Fe(III)-doped ZIF-8 precursor was obtained as a black powder.

[0126] As in Figure 4 As shown, before pyrolysis (a and b), the Fe(II)-doped Zn-ZIF-8 precursor material is in the form of polyhedrons with an average size of 218 ± 35 nm and a smooth surface. After pyrolysis (c to e), the particle size is reduced to 134 ± 21 nm, and its surface becomes very rough, and some nanotubes are generated. Figure 4 f shows the Figure 4 a Measured size distribution of Fe(II)-doped Zn-ZIF-8, from Figure 4 c Measured size distribution of the corresponding pyrolyzed material. Figure 5 The SEM images of the comparative Fe(III)-doped ZIF-8 precursor (a and b) and its pyrolysis product (c and d) are clearly shown. Figure 5As shown in e, during the pyrolysis process, the average particle size decreased from 205 ± 34 nm to 112 ± 22 nm. Figure 5 a Size distribution of Fe(III)-doped ZIF-8 was measured. Figure 5 c The corresponding pyrolyzed materials were measured. After pyrolysis, the surface of the Fe-NC(III) material was smoother than that of the Fe-NC(II) material, and no nanotubes were formed.

[0127] The Fe(II)-doped Zn-ZIF-8 catalytic material of the present invention was further characterized by high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM), as shown below:

[0128] FEI Titan Themis corrected for spherical aberration in HAADF-STEM mode at 200 kV TM Atomic resolution images of the catalyst of the present invention were obtained by pyrolysis of Fe(II)-doped Zn ZIF prepared as described in Example 2. Figure 7 As shown in a, the bright spots are discrete metal atoms. The spectrum of the white square area of ​​this image obtained by energy dispersive X-ray spectroscopy (EDS) supports the discrete distribution of Fe and Zn single atomic positions in the pyrolyzed sample ( Figure 7 b).

[0129] The 12-hour chronoamperometry test (catalyst loading: 0.6 mg / cm 2 , in a CO2-saturated 0.5 M KHCO3 electrolyte, and kept for 12 h. Characterization of the catalyst Fe-NC(II) performed: the total current density and Faradaic efficiency obtained for the electrolyte made of ultrapure K2CO3 (99.999%) and deionized water (18.2 MΩ·cm), the electrolyte made of KHCO3 with analytical reagent purity (99.7%) and deionized water, and the electrolyte made of KHCO3 with analytical reagent purity and tap water did not change significantly with increasing impurity concentrations in the electrolyte, and the performance remained stable over a 12 h period ( Figure 7 c) Ultrapure electrolytes are essential for most electrocatalysts used for CO2 electroreduction, and the fact that the catalysts of the present invention can work even in electrolytes made from tap water is a great advantage for large-scale applications.

[0130] The Fe(II)-doped Zn-ZIF precursor was further characterized by X-ray absorption spectroscopy (XAS), as briefly described below:

[0131] XAS, including X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) at the Fe K-edge, were collected in full fluorescence-yield mode at ambient conditions at the BM31 beamline of the European Synchrotron Radiation Facility (ESRF). The scan range was kept in the energy range of 7000–7700 eV for the Fe K-edge. The spectra were obtained by subtracting the baseline of the leading edge and normalizing the baseline of the trailing edge. The k 3 Weighted EXAFS oscillations EXAFS analysis was performed to assess the contribution of each bond pair to the Fourier transformed peak. The collected data were normalized to the incident energy and processed using the Athena software in the IFEFFIT software package. The E0 value of 7112.0 eV was used to calibrate all data with respect to the first inflection point of the iron foil absorption K-edge. EXAFS curve fitting was performed using Artemis and IFEFFIT software using ab initio calculated phases and amplitudes from the program FEFF 8.2. The experimental data were fitted using the EXAFS equation, using CN (coordination number), R (distance between the absorber and backscattering atoms) and σ 2 (EXAFS Debye-Waller factor considering thermal and structural disorder) as a variable parameter. 2 The value (amplitude reduction factor due to shaking / shaking-off processes at the central atom) was determined to be 0.89. Fig. 9 a shows the Fe K-edge X-ray absorption near-edge structure (XANES) of the Fe(II)-doped Zn-ZIF precursor obtained in Example 1, compared with those of Fe foil, Fe(II)-sulfate o-phenanthroline (Fe(phen)3SO4) and Fe2O3. These data show that the energy of the Fe K-edge of the Fe(II)-doped Zn-ZIF of the present invention is close to that of Fe(phen)3SO4, indicating that the Fe here shows a +2 valence. Fig. 9 b shows the Fe K-edge extended X-ray absorption fine structure (EXAFS) of the Fe(II)-doped Zn-ZIF precursor obtained according to Example 1. The fitting is based on the crystal structure of ZIF-8, in which the Zn(II) ions are replaced by Fe(II) ions ( Fig. 9 c), the bond length and coordination number are free. The Fe-N path in the first shell and the Fe-C paths in the second and third shells are fitted. The lengths of the Fe-N and two Fe-C paths obtained from the fitting are and Nearly undoped Zn-ZIF-8 crystals with Zn-N Zn-C and Zn-C The coordination numbers (CN) of these three paths from the fitting are 4.4 ± 0.4, 6.5 ± 0.6, and 3.3 ± 0.7, respectively, which are also comparable to the ideal values ​​of ideal ZIF-8 crystals (4, 8, and 4, respectively).

[0132] These results indicate the formation of Fe-doped Zn-ZIF-8, in which part of the Zn(II) ions were replaced by Fe(II) ions.

[0133] Fig. 9 d shows the Fe K-edge EXAFS spectrum of the catalyst after pyrolysis of Fe(II)-doped Zn-ZIF. This figure only shows the Fe-N and Fe-C paths in the first shell coordination, and their lengths are and CN were 3.7±0.4 and 0.5±0.1, respectively.

[0134] The Fe-N and Fe-C path lengths and coordination numbers from EXAFS fitting are close to the theoretical values ​​for ZIF-8 crystals, which is consistent with the similar XRD patterns observed ( Figure 2 a) Together, it is demonstrated that the Fe(II)-doped ZIF-8 of the present invention has a crystal structure similar to that of ZIF-8, and part of the Zn(II) ions are replaced by Fe(II) ions.

[0135] These results indicate that Fe appears in the catalyst as discrete single atoms coordinated to about four X (X = N or C) atoms.

[0136] Example 4: Electrochemical Characterization of Catalytic Materials of the Invention

[0137] The catalytic performance of the pyrolyzed material prepared from the Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) of the present invention for CO2 reduction has been tested in an electroreduction system: To prepare the catalyst ink, 3 mg of the material and 40 μL of The perfluorinated resin solution (5 wt.%, Sigma) was dispersed in 1 mL of ethanol by ultrasonic treatment for 30 min. A ribbon carbon cloth (Fuel Cell Store) was used as the working electrode. The area exposed to the electrolyte was fixed to 1 cm by shielding the carbon cloth with a sealing film. 2 200 μL of catalyst ink was loaded onto the carbon cloth by dripping 4 times. Therefore, the catalyst loading on the carbon cloth was 0.6 mg cm -2 . The electrochemical characterization was performed in an electroreduction system in the form of a Gamry Reference 3000 electrochemical instrument using a gas-tight two-chamber cell such as Figure 6aAs shown. The working electrode and the reference electrode are fixed in one chamber, and the counter electrode is fixed in another chamber. The two chambers are separated by an anion exchange membrane ( FAA-3-PK-130). Ag / AgCl electrode and Pt wire with saturated KCl filling solution were used as reference electrode and counter electrode, respectively. 0.5M KHCO3 aqueous solution was used as electrolyte in the working electrode chamber and was stirred vigorously during the experiment. Bubble CO2 or N2 to one side of the working electrode for at least 30 minutes to saturate the electrolyte. The inlet gas was pre-wetted with 0.5M KHCO3 solution to minimize evaporation of the electrolyte.

[0138] Figure 6b The cyclic voltammetry (CV) curves of the pyrolysis products of Fe(II)-doped Zn-ZIF-8 and undoped ZIF-8 (obtained by pyrolysis according to the procedure described in Example 2) in N2 and CO2 saturated electrolytes were compared. Between -0.25 V and -0.5 V relative to RHE, the current density of the catalytic material of the present invention in CO2 saturated electrolyte increases significantly and is much higher than that of the pyrolysis product of undoped ZIF-8, which means that the Fe single atom site may exhibit CO2 reduction ability in this potential range. Figure 6c The chronoamperometric curve of the material of the present invention in a CO2-saturated electrolyte at -0.36 V relative to RHE is shown. The current density is always maintained at about 7 mA cm -2 CO and a very small amount of H2 were the only products detected by gas chromatography (GC), while H 1 -NMR (H 1 -NMR) did not detect solution-phase products, supporting the high specificity of the reaction. During the electrolysis of this day, the Faradaic efficiency of CO remained above 80%. Figure 6d The Faradaic efficiency of CO and H2 of the Fe-NC(II) material of the present invention at different applied potentials is shown. It can be seen that the Faradaic efficiency of CO is higher than 80% between -0.21 V and -0.46 V relative to RHE. Figure 6e and 6fThe Faradaic efficiency and partial current density of CO formation of the Fe-NC(II) material of the present invention were compared with some recently reported catalysts with the highest CO formation activity. Surprisingly, the Fe-NC(II) material showed a partial current density of CO formation comparable to that of oxide-derived Au (OD Au, one of the most active CO formation catalysts mentioned in the literature (Chen et al., 2012, supra)) at a certain applied potential, and significantly higher than other earth-abundant metal-based catalysts such as the best Fe-NC(III) material (Fe 0.5d) (Huan et al., 2017, supra), CoPc / CNT material (Zhang et al., 2017, supra), and porous Ag material (Lu et al., 2014, supra). Similarly, the Faradaic efficiencies of the comparative Fe(II)-doped Co ZIF and the comparative Fe(III)-doped ZIF are each very low and significantly lower than the Faradaic efficiency of the Fe(II)-doped Zn-ZIF of the present invention, and the partial current densities of the two comparative materials are also significantly lower than those of the Fe(II)-doped Zn-ZIF ( Figure 6g and h).

[0139] Example 5: Characterization of the pyrolysis products of Fe(II)-doped zinc imidazole (ZIF-8) precursor after electrolysis

[0140] The stability of the catalytic material of the present invention was evaluated by electron microscopy as described in Example 3 after the electrolysis test as described in Example 4 ( Figure 8 After electrolysis, the carbon cloth electrode was rinsed with deionized water and immersed in 0.5 mL of ethanol. After about 1 hour of ultrasonic treatment, a dispersion of the electrolytic catalyst was obtained, which was then used for electron microscopy characterization. As shown by TEM and HRTEM images ( Figure 8 As shown in Figs. 8a and 8b), it can be seen that the separation of Fe single atoms to form nanoparticles is not observed in the catalytic material of the present invention. EDS imaging results ( Figure 8 c) Still showing high dispersion of Fe(c2), Zn(c3) and N(c4) on the carbon matrix and EDS spectra of the catalytic material of the present invention ("Fe-NC(II)") before (top) and after (bottom) electrolysis showing similar Fe:Zn ratios (39:61 and 43:57, respectively) ( Figure 4 d), indicating that the catalytic material of the present invention has high stability during the CO2 electrolytic reduction process.

[0141] In summary, the catalytic material comprising single-atom Fe dispersed on a N-doped carbon matrix obtained by pyrolysis of Fe(II)-doped Zn-ZIF-8 shows high selectivity and activity for CO production in the CO2 electroreduction reaction in aqueous electrolyte. In the low overpotential region, the obtained Faradaic efficiency for CO is higher than 80%, and the partial current density for CO is comparable to that of OD Au, which is one of the most efficient catalysts ever reported for CO reduction (Chen et al., 2012, supra). Due to its high activity for CO production and high Fe content, this material is a promising catalyst for the practical conversion of CO2 to CO and other chemicals.

Claims

1. Use of a catalytic material containing single iron atoms on an N-doped carbon matrix obtained by pyrolysis of an Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) under an inert atmosphere for the electroreduction of CO2 to CO.

2. A method for preparing a catalytic material, comprising the following steps: a) providing a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) in dry form under an inert atmosphere; b) pyrolyzing the Fe(II)-doped Zn-ZIF under an inert atmosphere for about 2 to about 24 hours, typically 3 hours; c) collecting the obtained pyrolysis products as catalytic materials.

3. The method according to claim 2, characterized in that The pyrolysis in step b) is carried out at a temperature between about 800 and 950°C.

4. The method according to claim 2 or 3, characterized in that The pyrolysis in step b) is carried out by heating the mixture at a temperature of about 3 to 10°C.min -1 The temperature is increased at a rate of 5. A method for preparing Fe(II)-doped Zn-ZIF material, comprising the following steps: (i) providing an imidazole or an imidazole derivative precursor; (ii) In preventing Fe 2+ In a reducing environment of cationic oxidation, dissolving the imidazole or imidazole derivative precursor in a degassed non-aqueous polar solvent, wherein the degassed non-aqueous polar solvent is selected from degassed methanol and degassed ethanol to obtain a solution A; (iii) dissolving an iron (II) precursor and a zinc salt in a degassed non-aqueous polar solvent to obtain a solution B; (iv) adding solution B to solution A; (v) collecting and washing the precipitate; (vi) drying the washed precipitate at room temperature to obtain a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) in the form of dry powder, wherein steps (ii) to (vi) are carried out under stirring under an inert atmosphere, wherein the reducing environment is achieved by introducing a reducing agent that is at least one iodide derivative.

6. The method according to claim 5, characterized in that In step (iv), solution B is added dropwise to solution A with stirring for about 1 hour.

7. The method according to any one of claims 5 to 6, characterized in that After the addition of solution B was complete, the reaction mixture from step (iv) was kept under stirring under an inert atmosphere for about 1 day.

8. The method according to any one of claims 5 to 7, characterized in that The imidazole precursor is 2-methylimidazole.

9. The method according to any one of claims 5 to 8, characterized in that The iron (II) precursor is FeCl2.

10. The method according to any one of claims 5 to 9, characterized in that The iodide derivative is selected from potassium iodide, sodium iodide or ammonium iodide.

11. The method according to any one of claims 2 to 10, characterized in that The inert atmosphere was provided by a flow of N2.

12. The method according to any one of claims 5 to 11, characterized in that In step (v) the precipitate is collected by centrifugation under an inert atmosphere or by filtration at ambient pressure.

13. The method according to any one of claims 2 to 4, characterized in that The provided Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) is obtained according to the method according to any one of claims 5 to 12.

14. A catalyst comprising single Fe atoms on an N-doped carbon matrix, obtained by the process according to any one of claims 2 to 4 or 13.

15. The use according to claim 1, characterized in that The catalytic material is obtained by pyrolysis of Fe(II)-doped Zn-ZIF-8.

16. The use according to claim 1 or 15, characterized in that The catalytic material is the material according to claim 14.

17. The use according to claim 1, characterized in that The Fe(II)-doped Zn-ZIF has the crystal structure of Zn-ZIF-8, in which some Zn(II) ions are replaced by Fe(II) ions.

18. The method according to any one of claims 2 to 4 or 11, characterized in that: The Fe(II)-doped Zn-ZIF has the crystal structure of Zn-ZIF-8, in which some Zn(II) ions are replaced by Fe(II) ions.

19. An electroreduction system for electrochemical reduction of CO2, comprising a catalytic material prepared from a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) according to the method of claim 5.

20. A kit for the preparation of a catalytic material according to the present invention, the kit comprising at least one container containing a Fe(II)-doped Zn-zeolitic imidazole framework (ZIF) in dry form and under an inert atmosphere.

21. The kit according to claim 20, characterized in that The Fe(II)-doped Zn-ZIF has the crystal structure of Zn-ZIF-8, in which some Zn(II) ions are replaced by Fe(II) ions.

22. A kit for the preparation of Fe(II)-doped Zn-zeolitic imidazole framework (ZIF), the kit comprising in separate compartments or containers: a) imidazole or an imidazole derivative precursor; b) an iron (II) precursor, such as FeCl2 or Fe(CH3COO)2; c) zinc salts, such as Zn(NO3)2 or ZnCl2; and d) Optionally, a reducing agent to prevent Fe 2+ Oxidation of ions, such as iodide derivatives, such as potassium iodide, sodium iodide or ammonium iodide.

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