HIGHLY EFFICIENT COPPER NITRIDE BIMETALLIC CATALYSTS FOR ELECTROCHEMICAL REDUCTION OF ONE OR MORE CARBON OXIDES TO N-PROPANOL AND/OR ETHANOL

Description

TECHNOLOGICAL FIELD

The present disclosure relates to catalysts and catalytic methods for electrochemical reduction of one or more carbon oxides (such as carbon monoxide and/or carbon dioxide) into alcohols such as n-propanol and/or ethanol. In particular, the present disclosure is about a gas diffusion electrode (GDE) suitable for carbon oxide electrolysis, a gas-fed flow cell comprising such GDE as well as a method for producing such GDE and a process using such GDE.

TECHNOLOGICAL BACKGROUND

Electrochemical reduction of CO provides an opportunity to produce fuels and chemicals in a carbon-neutral manner, assuming that CO is produced from CO₂ which can be captured from the atmosphere or at least can be captured from the industrial emission before being released into the atmosphere. To do so requires efficient, selective, and stable catalysts. Copper-based materials are promising electrocatalysts for CO reduction.

Zhi-Qin Liang et al. in “Copper-on-nitride enhances the stable electrosynthesis of multi-carbon products from CO₂” Nature Communications (2018) 9:3828, reports a copper on copper (I) composite that stabilizes copper (I) during CO₂ reduction through the use of copper nitride as an underlying copper (I) species. It was a synthesized copper-on-nitride catalyst that exhibits a Faradaic efficiency of 64±2% for C₂+ products. A 40-fold enhancement was achieved in the ratio of C₂+ to the competing CH₄ compared to the case of pure copper. It was shown that the copper-on-nitride catalyst performs stable CO₂ reduction over 30 h. Mechanistic studies suggest that the use of copper nitride contributes to reducing the CO dimerization energy barrier—a rate-limiting step in CO₂ reduction to multi-carbon products.

Among the chemicals that can be produced this way, N-propanol and ethanol are of high-value interest and have not yet been produced efficiently via electrochemical CO reduction.

Mohamed Ebaid et al. in “Production of C₂/C₃ Oxygenates from Planar Copper Nitride-Derived Mesoporous Copper via Electrochemical Reduction of CO₂” Chem. Mater. 2020, 32, 3304-331, reports a highly mesoporous metallic Cu catalyst prepared by electrochemical reduction of thermally nitrided Cu foil. Under aqueous saturated CO₂ reduction conditions, the Cu₃N-derived Cu electrocatalyst produces virtually no CH₄, very little CO, and exhibits a faradaic efficiency of 68% in C₂₊ products (C₂H₄, C₂H₅OH, and C₃H₇OH) at a current density of ˜18.5 mA cm⁻² and a cathode potential of −1.0 V versus the reversible hydrogen electrode (RHE). Under these conditions, the catalyst produces more oxygenated products than hydrocarbons. It was shown that surface roughness is a good descriptor of catalytic performance. The roughest surface reached 98% CO utilization efficiency for C₂₊ product formation from CO₂ reduction and the ratio of oxygenated to hydrocarbon products correlates with the degree of surface roughness. These effects of surface roughness are attributed to the high population of undercoordinated sites as well as a high pH environment within the mesopores and adjacent to the surface of the catalyst.

WO2019098474 relates to an electrochemical cell to be used when carbon dioxide is decomposed and reduced to a useful conversion product; and a catalyst to be used in an electrode for an anode within the cell. A C₂₊ compound can be selectively generated and a C₂₊ hydrocarbon can be stably generated by providing, as a catalyst for a reduction reaction of a material, a catalyst comprising a metal compound containing a metal ion having a standard reduction potential lower than the standard reduction potential when a monovalent or divalent copper cation within copper oxide is reduced to copper or a monovalent copper cation, and thus a catalyst having durability can be provided.

Fan Lei et al., Sci. Adv. 2020; 6: eaay3111 21 Feb. 2020 discloses, in light of environmental concerns and energy transition, electrochemical CO2 reduction (ECR) to value-added multicarbon (C2+) fuels and chemicals, using renewable electricity, presents an elegant long-term solution to close the carbon cycle with added economic benefits as well. However, electrocatalytic C—C coupling in aqueous electrolytes is still an open challenge due to low selectivity, activity, and stability. This document summarizes recent progress in how to achieve efficient C—C coupling via ECR, with emphasis on strategies in electrocatalysts and electrocatalytic electrode/reactor design, and their corresponding mechanisms. In addition, current bottlenecks and future opportunities for C2+ product generation are discussed.

Li Honglin et al., Applied Catalysis B: Environmental Volume 320, January 2023, 121948 reads that copper (Cu) has been proved as an efficient catalyst in carbon dioxide electrochemical reduction reaction (CO2RR) towards hydrocarbons, but still suffers from low selectivity and poor stability. Herein, Cu-based/CxNy catalysts were fabricated by facile pyrolysis of CuNCN in sealed quartz tubes. It is found that CuNCN pyrolyzed at 300° C. (CuNCN-300) exhibits a high C2H4 Faradaic efficiency of 48.5% at 500 mA cm−2. However, increasing the pyrolysis temperature above 400° C. gives rise to CH4 being the predominant product and CuNCN-500 achieves CH4 Faradaic efficiencies of 66.3% at 300 mA cm−2. Combining experimental and DFT calculation results, Cu3N plays a crucial role in the formation of C2H4, while tri-s-triazine units in CuNCN-500 reduce the barrier of *CO hydrogenation to *CHO and retard C—C coupling on Cu surface. These findings mark the significance of precise tailoring of the synergistic effect between g-C3N4 and different Cu species for achieving the desired selectivity during CO2RR.

There is a still need for an improvement of catalyst materials for efficient electrochemical reduction reactions of one or more carbon oxides; in particular, there is still a need for a catalyst that shows improved selectivity for alcohols such as n-propanol and/or ethanol; for a system comprising such a catalyst and for process using such catalysts.

SUMMARY OF THE DISCLOSURE

One or more of the above needs can be fulfilled by the multi-metallic catalyst according to the present disclosure comprising copper nitride-metal.

According to a first aspect, the disclosure provides a catalyst suitable for electrochemical reduction reaction of one or more carbon oxides (such as carbon monoxide reduction (COR) reactions and/or carbon dioxide (CO₂R) reduction), remarkable in that it comprises a nitride-doped multi-metallic material comprising a primary metal being copper and one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, iridium, osmium, and any mixture thereof; and in that the nitride-doped multi-metallic material comprises, as determined by XRD, copper, copper nitride and copper-Me alloy wherein Me is one of the secondary metals.

A new class of catalysts has been found. The catalysts, being nitride-doped multi-metallic material comprising copper as the primary metal and at least one secondary metal, allow good selectivity into alcohol production such as n-propanol and/or ethanol.

With preference, the one or more secondary metals are selected from silver, gold, and any mixture thereof.

In an embodiment, the nitride-doped multi-metallic material comprises from 0.5 to 10.0 mol % of the one or more secondary metals based on the total molar content of copper and one or more secondary metals as determined by XPS; preferably from 0.6 to 9.0 mol. % or from 1.0 to 10.0 mol %.

In an embodiment, the nitride-doped multi-metallic material comprises only one secondary metal. For example, the nitride-doped multi-metallic material is a nitride-doped bimetallic material and comprises from 2.0 to 8.0 mol % of the secondary metal based on the total molar content of copper and the metal as determined by XPS; preferably from 3.0 to 7.0 mol. %.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, and comprises from 0.5 to 8.0 mol % of the secondary metals based on the total molar content of copper and the two or more secondary metals; with preference 1.0 to 6.0 mol %.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least one of the secondary metals is silver or gold; preferably wherein at least two secondary metals are silver and gold.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least two secondary metals Me1 and Me2 are present in a ratio Me1/Me2 ranging from 1 to 15; preferably from 1 to 10; more preferably, from 1 to 4.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least two secondary metals Me1 and Me2 differ by their reduction potential wherein Me1 has a reduction potential greater than Me2, and Me1 and Me2 are present in a ratio Me1/Me2 greater than 1; preferably greater than 2; more preferably greater than 3.

For example, the nitride-doped multi-metallic material shows Cu (111) and Cu (200) facets as determined by XRD.

For example, the nitride-doped multi-metallic material is in the form of rode-shape particles and/or the particles have an average diameter ranging from 20 nm to 500 nm as measured by transmission electron microscopy; preferably, ranging from 50 nm to 400 nm or from 80 to 350 nm.

According to a second aspect, the disclosure provides a method to produce a catalyst according to the first aspect. The method comprising

• • a) providing copper particles;
  • b) adding of one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, and any mixture thereof, wherein said step of addition comprises a step (b1) of galvanic exchange reaction or a step (b2) of electrodeposition or both steps (b1) of galvanic exchange reaction and step (b2) of electrodeposition, to obtain a multi-metallic material comprising copper as primary metal and one or more secondary metals;
  • c) nitridation of the multi-metallic material obtained in step b) to obtain a nitride-doped multi-metallic material.

For example, the step b) is a step (b1) of galvanic exchange reaction and is performed by stirring the copper particles in an aqueous solution comprising one or more selected from chloroplatinic acid hexahydrate, ruthenium acetylacetonate, iridium acetylacetonate, osmium acetylacetonate, rhodium acetylacetonate, palladium acetylacetonate, palladium nitrate, palladium chloride, gold chloride trihydrate, silver nitrate, and any mixture thereof. In a preferred embodiment, the aqueous solution comprises gold chloride trihydrate, silver nitrate, and any mixture thereof. For example, the nitride-doped multi-metallic material comprises two or more secondary metals, and the step b) is a step (b1) of galvanic exchange reaction comprising successive sub-steps of galvanic exchange reaction; with preference, the two or more secondary metals differ by their reduction potential and the successive sub-steps of galvanic exchange reaction are performed starting by the secondary metal having the highest reduction potential.

For example, the nitridation step comprises a calcination sub-step followed by a copper nitride synthesis sub-step; preferably the copper nitride synthesis sub-step is performed at a temperature ranging from 150 to 190° C. and for a time ranging from 5 to 30 hours.

For example, the copper particles have an average diameter ranging from 5 nm to 200 nm as measured by transmission electron microscopy; preferably, the copper particles are nanoparticles and have an average diameter ranging from 5 nm to 100 nm as measured by transmission electron microscopy.

According to a third aspect, the disclosure provides a gas diffusion electrode suitable for the electrochemical reduction of one or more carbon oxides, said gas diffusion electrode having a gas diffusion layer, the gas diffusion electrode further comprising an ink deposited on the gas diffusion layer; wherein the ink comprises an ion-conducting polymer, said gas diffusion electrode is remarkable in that the ink further comprises the catalyst as defined according to the first aspect or produced according to the second aspect.

One or more of the following features advantageously define the gas diffusion layer of the gas diffusion electrode of the disclosure:

• • The gas diffusion layer is a hydrophobic porous support. With preference, said hydrophobic porous support shows a pore size ranging from 400 nm to 500 nm as determined by scanning electron microscopy preferably, from 420 nm to 580 nm or from 440 nm to 560 nm.
  • The gas diffusion layer is a hydrophobic, porous and chemically inert support; with preference, the gas diffusion layer is not soluble in KOH.
  • The gas diffusion layer is or comprises polytetrafluoroethylene.
  • The gas diffusion layer has a thickness ranging from 50 μm to 150 μm as measured by scanning electron microscopy, preferably from 60 μm to 120 μm more preferably from 70 μm to 100 μm.

One or more of the following features advantageously define the ion-conducting polymer of the gas diffusion electrode of the disclosure:

• • The ion-conducting polymer comprises an ionomer.
  • The ion-conducting polymer is or comprises an ionomer with a tetrafluoroethylene backbone group (—CF₂—CF₂—).
  • The ion-conducting polymer is or comprises a perfluorinated sulfonic acid, such as Nafion®.
  • The ion-conducting polymer is or comprises tetrafluoroethylene-perfluoro(3-hydrophobioxa-4-pentenesulfonic acid) copolymer, such as Aquivion®.

In an embodiment, the ink layer has a thickness ranging from 2 μm to 20 μm as measured by scanning electron microscopy, preferably from 5 μm to 15 μm.

According to a third aspect, the disclosure provides a method for producing the gas diffusion electrode suitable for the electrochemical reduction of one or more carbon oxides as defined according to the second aspect, said method is remarkable in that it comprises the following steps:

• • providing nitride-doped multi-metallic particles;
  • adding an ion-conducting polymer and obtaining a dispersion by dispersing the nitride-doped multi-metallic particles into a solvent selected from water and/or an organic solvent; to obtain an ink
  • providing a gas-diffusion layer and depositing said ink onto said gas-diffusion layer to obtain the gas diffusion electrode.

According to a fourth aspect, the disclosure provides a gas diffusion electrode obtained by the method according to the third aspect.

According to a fifth aspect, the disclosure provides a gas-fed flow cell suitable for the electrochemical reduction of one or more carbon oxides, said gas-fed flow cell comprising a gas chamber, a catholyte chamber and an anolyte chamber, wherein said gas chamber is separated from the catholyte chamber by a gas diffusion electrode, said gas diffusion electrode having a gas diffusion layer being comprised within said gas chamber, wherein said catholyte chamber and said anolyte chamber are separated by an anion exchange membrane, and wherein said catholyte chamber and said anolyte chamber comprise respectively a cathode and an anode, said gas-fed flow cell is remarkable in that the gas diffusion electrode is as defined according to the second aspect and/or with the fourth aspect.

According to a sixth aspect, the disclosure provides a process for electrochemically reducing one or more carbon oxides into one or more hydrocarbons and alcohols characterized in that it comprises the following steps:

• • providing an electrolyser comprising a gas diffusion cathode, an anode and an ion- exchange membrane in between said gas diffusion cathode and said anode, wherein the electrolyser is selected from a zero-gap electrolyser, a two-gaps electrolyser, a catholyte-free one-gap electrolyser and a catholyte-containing one-gap electrolyser, and wherein the ion-exchange membrane is an anion-exchange membrane or a bipolar membrane;
  • providing an input flow at the gas diffusion cathode, the input flow comprising one or more carbon oxides selected from carbon monoxide, carbon dioxide, and a mixture of carbon monoxide and carbon dioxide;
  • providing an anolyte solution at the anode and a catholyte solution at the gas diffusion cathode;
  • applying an electric current between the gas diffusion cathode and the anode; and
  • recovering, an output flow of catholyte comprising n-propanol and/or ethanol;
    and in that the electrolyser further comprises a first interface in between the gas diffusion cathode and the ion-exchange membrane, wherein the first interface comprises a cathode comprising the catalyst as defined according to the first aspect or prepared according to the method according to the second aspect;

For example, the mixture of carbon monoxide and carbon dioxide comprises at least 20 mol. % based on the total molar content of the mixture; preferably at least 40 mol. %; more preferably at least 60 mol. %, even more preferably at least 80 mol. % and most preferably at least 90 mol. % or at least 95 mol. %. According to a seventh aspect, the disclosure provides for the use of a catalyst for electrochemical carbon monoxide reduction (COR) reactions to produce n-propanol and/or ethanol the use being remarkable in that the catalyst is according to the first aspect.

According to a eighth aspect, the disclosure provides for the use of a catalyst for electrochemical carbon dioxide reduction (CO2R) reactions to produce n-propanol and/or ethanol the use being remarkable in that the catalyst is according to the first aspect.

The particular features, structures, characteristics or embodiments may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments.

DESCRIPTION OF THE FIGURES

FIG. 1: Scheme of flow electrochemical cell for CO reduction. Anode (Ni foam), catholyte KOH 1 M (40 ml), anolyte KOH 1 M (60ml), CO flow (10 ml/min).

FIG. 2: XRD patterns of Cu₃N/Ag(5%), Cu₃N and Cu/Ag(5%) with the references. The black circles indicate Cu₂O.

FIG. 3: SEM images of Cu₃N/Ag(5%), Cu₃N, Cu and Cu/Ag(5%).

FIG. 4: XRD patterns of Cu₃N/Ag(5%) samples synthesized with different nitridation times (10-60 h). Black circles: Cu, white circles: Cu₃N and black squares: Ag.

FIG. 5: XRD patterns of Cu₃N/Ag samples synthesized with 20 h of nitridation time and different contents of Ag (2-8%). Black circles: Cu, white circles: Cu₃N and black squares: Ag.

FIG. 6: Activity for electrochemical CO reduction of Cu, Cu₃N, Cu₃N/Ag(5%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-200 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min−1. The scale bar of FEn-propanol of Cu₃N/Ag at 150 mA.cm⁻² was taken from the results of three independent experiments.

FIG. 7: Activity for electrochemical CO reduction of Cu₃N/Ag(5%) catalysts synthesized with different nitridation times of 10 to 60 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-200 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹. The scale bar of FEn-propanol of Cu₃N/Ag(5%) with 20 h of nitridation, at 150 mA.cm⁻² was taken from the results of three independent experiments.

FIG. 8: Activity for electrochemical CO reduction of Cu₃N/Ag(5%), Cu₃N/Au(5%) and Cu₃N/Pd(5%) catalysts synthesized with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-200 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 9: Activity for electrochemical CO reduction of Cu₃N/Ag catalysts with different contents of Ag (2-8%), synthesized with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 75-200 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 10: Activity for electrochemical CO reduction of Cu₃N/Ag(5%) with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied currents: 100-200 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 11: Activity for electrochemical CO reduction of Cu₃N/Ag(5%) with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 0.1-3 M, 60 mL. Catholyte: KOH 0.1-3 M, 40 mL. Applied currents: 100-200 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 12: XRD patterns of Cu₃N/Ag(5%) and Cu₃N synthesized with nitridation time of 20 h, after 30 minutes of CORR electrolysis.

FIG. 13: XRD patterns of Cu₃N/Ag(5%) synthesized with nitridation time of 20 h, after 30 minutes of CORR electrolysis and three scans of LSV.

FIG. 14: Activity for electrochemical CO₂ reduction of Cu₃N/Ag(5%) with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied currents: 100-200 mA.cm⁻² for 0.5 h. CO2 flow: 10 ml.min⁻¹.

FIG. 15: XPS survey spectrum and high resolution measurement of Ni1s, Cu2p and Ag3d in Cu₃N/Ag(5%) sample.

FIG. 16: Activity for electrochemical CO reduction of Cu₃N/Au(2%)Ag(0.1%), Cu₃N/Au(2%)Ag(0.3%), Cu₃N/Au(2%)Ag(0.5%) and Cu₃N/Au(2%)Ag(1%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 17: Activity for electrochemical CO reduction of Cu₃N/Au(0.3%)Ag(0.3%), Cu₃N/Au(1%)Ag(0.3%), Cu₃N/Au(2%)Ag(0.3%) and Cu₃N/Au(3%)Ag(0.3%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 18: Activity for electrochemical CO reduction of Cu₃N/Au(1%)Ag(0.3%) catalysts at KOH concentrations of 0.1, 0.5, 1, 1.5 and 3 M with applied current of 100, 125 and 150 mA.cm⁻², respectively, for 0.5 h. Nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 60 mL. Catholyte: KOH 40 mL. CO flow: 10 ml.min⁻¹. Error bars were made for ethanol and propanol, C2+ liquids alcohols is shown as the highest value obtained.

FIG. 19: Activity for electrochemical CO reduction of Cu₃N/Au(1%)Ag(0.3%), Cu₃N/Au(5%) and Cu₃N/Ag(5%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 20: the ratio between multicarbon alcohol products and ethylene from electrochemical CO reduction of Cu₃N/Au(0.3%)Ag(0.3%), Cu₃N/Au(1%)Ag(0.3%), Cu₃N/Au(2%)Ag(0.3%), Cu₃N/Au(3%)Ag(0.3%), Cu₃N/Au(5%), and Cu₃N/Ag(5%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 21: Activity for electrochemical CO reduction of Cu₃N/Au(0.3%), Cu₃N/Au(1%), Cu₃N/Au(2%), and Cu₃N/Au(3%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 22: Activity for electrochemical CO reduction of Cu₃N/Ag(0.3%), Cu₃N/Ag(2%), Cu₃N/Ag(5%), and Cu₃N/Ag(8%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 23: Activity for electrochemical CO reduction of Cu₃N/Au(1%)Ag(0.3%), and Cu₃N/Ag(0.3%) catalysts with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 0.5 h. CO flow: 10 ml.min⁻¹.

FIG. 24: Activity for electrochemical CO reduction of Cu₃N/Au(1%)Ag(0.3%) catalyst with nitridation time of 20 h. Anode: Ni foam. Anolyte: KOH 1 M, 60 mL. Catholyte: KOH 1 M, 40 mL. Applied current: 100-150 mA.cm⁻² for 30 to 800 minutes. CO flow: 10 ml.min⁻¹.

FIG. 25: XRD patterns of Cu₃N/Au(1%)Ag(0.3%) synthesized with nitridation time of 20 h, before and during 30 minutes of CORR electrolysis.

DETAILED DESCRIPTION

For the disclosure, the following definitions are given:

The terms “comprising”, “comprises” and “comprised of” as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. The terms “comprising”, “comprises” and “comprised of” also include the term “consisting of”.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4, 5 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75 and 3.80, when referring to, for example, measurements). The recitation of endpoints also includes the recited endpoint values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

FIG. 1 illustrates the gas-fed flow cell 1 of the present disclosure. Said gas-fed flow cell comprises a gas diffusion electrode suitable for electrochemical carbon oxide reduction reactions, such as carbon monoxide reduction reactions and/or for carbon dioxide reduction reactions. The following description first describes the catalyst used in the gas diffusion electrode.

The Catalyst and the Method of Preparation of the Catalyst

The disclosure provides for a catalyst suitable for electrochemical carbon oxide reduction reactions (such as carbon monoxide reduction reactions and/or for carbon dioxide reduction reactions), remarkable in that it comprises a nitride-doped multi-metallic material comprising a primary metal being copper and one or more secondary metals selected from noble metals wherein the noble metals include silver; and in that the nitride-doped multi-metallic material comprises, as determined by XRD, copper, copper nitride and copper-Me alloy wherein Me is one of the secondary metals.

For example, the one or more secondary metals are selected from silver, gold, platinum, palladium, ruthenium, and any mixture thereof; preferably, the one or more secondary metals are selected from silver, gold, palladium, and any mixture thereof; more preferably, the one or more secondary metals are selected from silver, gold, and any mixture thereof.

It is preferred that the catalyst be prepared by nitridation of a multi-metallic material. So, the first step is to prepare such multi-metallic material. This can be done by the addition of one or more secondary metals to copper particles. The copper particles have an average diameter ranging from 5 nm to 200 nm as measured by transmission electron microscopy; preferably, from 10 nm to 180 nm; more preferably, from 15 nm to 150 nm; even more preferably from 20 nm to 120 nm. In an embodiment the copper particles are nanoparticles and have an average diameter ranging from 5 nm to 100 nm as measured by transmission electron microscopy; preferably, from 10 nm to 80 nm; more preferably, from 15 nm to 60 nm; even more preferably from 20 nm to 50 nm or from 5 nm to 40 nm.

The step of addition comprises a step (b1) of galvanic exchange reaction or a step (b2) of electrodeposition or both steps (b1) of galvanic exchange reaction and step (b2) of electrodeposition; preferably the step of addition comprises a step (b1) of galvanic exchange reaction or both steps (b1) of galvanic exchange reaction and step (b2) of electrodeposition; even more preferably, the step of addition comprises a step (b1) of galvanic exchange reaction.

In an embodiment, the step (b1) using a galvanic exchange reaction, the copper particles are doped with one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, iridium, osmium, and any mixture thereof; preferably, the one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, and any mixture thereof; for example, the one or more secondary metals are selected from silver and/or gold.

For example, the step (b1) is performed by stirring the copper particles in an aqueous solution comprising one or more noble metal ions precursors. For example, the step (b1) is performed by stirring the copper particles in an aqueous solution comprising one or more selected from chloroplatinic acid hexahydrate (H₂PtCl₆·6H₂O); ruthenium acetylacetonate (Ru(acac)₃); iridium acetylacetonate (Ir(acac)₃), osmium acetylacetonate (Os(acac)₃), rhodium acetylacetonate (Rh(acac)₃), palladium acetylacetonate (Pd(acac)₃), palladium nitrate (Pd(NO₃)₂), palladium chloride (H₂PdCl₄), gold chloride trihydrate (HAuCl₄·3H₂O), silver nitrate (AgNO₃), and any mixture thereof. In a preferred embodiment, the aqueous solution comprises gold chloride trihydrate (HAuCl₄·3H₂O), silver nitrate (AgNO₃), and any mixture thereof.

For example, the one or more secondary metal concentration in the step (b1) was performed with a content of from 0.5 to 10.0 mol % of the one or more secondary metals based on the molar content of copper; preferably from 0.6 to 9.0 mol %; more preferably from 0.8 to 8.0 mol %; even more preferably from 1.0 to 7.5 mol %; most preferably from 1.1 to 7.0 mol %; even most preferably from 1.2 to 6.5 mol % or from 1.3 to 6.0 mol %.

For example, the one or more secondary metal concentration in the step (b1) was performed with a content of at least 0.5 mol % of the one or more secondary metals based on the molar content of copper; preferably at least 0.6 mol %; more preferably at least 0.8 mol %; even more preferably at least 1.0 mol %; most preferably at least 1.1 mol %; even most preferably at least 1.2 mol % or at least 1.3 mol %.

For example, the one or more secondary metal concentration in the step (b1) was performed with a content of at most 10.0 mol % of the one or more secondary metals based on the molar content of copper; preferably at most 9.0 mol %; more preferably at most 8.0 mol %; even more preferably at most 7.5 mol %; most preferably at most 7.0 mol %; even most preferably at most 6.5 mol % or at most 6.0 mol %.

For example, the nitride-doped multi-metallic material is a nitride-doped bimetallic material and only one secondary metal is used as dopant. In such a case, the secondary metal concentration in the step (b1) was performed with a content of from 1.0 to 10.0 mol % of the secondary metal based on the molar content of copper; preferably from 1.5 to 9.0 mol %; more preferably from 2.0 to 8.0 mol %; even more preferably from 2.5 to 7.5 mol %; most preferably from 3.0 to 7.0 mol %; even most preferably from 3.5 to 6.5 mol % or from 4.0 to 6.0 mol %.

With preference, the nitride-doped multi-metallic material comprises two or more secondary metals, and the step b) is a step (b1) of galvanic exchange reaction comprising successive sub-steps of galvanic exchange reaction. Thus, with preference, the secondary metals are added successively, one after the other, in separate sub-steps of galvanic exchange reaction.

Advantageously, the two or more secondary metals differ by their reduction potential, and the successive sub-steps of galvanic exchange reaction are performed starting with the secondary metal having the highest reduction potential. Such galvanic exchange reaction sub-steps are separated or followed by an equal number of washing sub-step with H₂O and ethanol.

In such a case, the nitride-doped multi-metallic material comprises two or more secondary metals, and each of the two or more secondary metal concentrations in the sub-step (b1) was performed with a content of 0.5 to 10.0 mol % or 0.5 to 8.0 mol % of the one or more secondary metals based on the molar content of copper; preferably from 0.6 to 9.0 mol %; more preferably from 0.8 to 8.0 mol %; even more preferably from 1.0 to 7.5 mol %; most preferably from 1.1 to 7.0 mol %; even most preferably from 1.2 to 6.5 mol % or from 1.3 to 6.0 mol % or from 1.0 to 6.0 mol %.

For example, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least one of the secondary metals is silver or gold; preferably wherein at least two secondary metals are silver and gold. Indeed, as demonstrated by the examples, surprising results are obtained with multi-metallic material comprising copper, silver and gold (i.e., with nitride-doped tri-metallic material).

For example, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least two secondary metals Me1 and Me2 are present in a ratio Me1/Me2 ranging from 1 to 15; preferably from 1 to 12; more preferably from 1 to 10; even more preferably, from 1 to 8; most preferably from 1 to 6; and even most preferably, from 1 to 4.

For example, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least two secondary metals Me1 and Me2 differ by their reduction potential wherein Me1 has a reduction potential greater than Me2, and Me1 and Me2 are present in a ratio Me1/Me2 greater than 1; preferably greater than 2; more preferably greater than 3.

For example, the step (b1) (or each sub-step of b1) was performed for a time ranging from 10 min to 5 hours; preferably from 15 min to 2.5 hours; more preferably from 20 min to 1.5 hours, or from 10 min to 1 hour.

For example, the step (b1) (or each sub-step of b1) was performed at a temperature ranging from 40 to 250° C.

In another embodiment, the method further comprises a step (b2) of electrodeposition on the copper particles of one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, iridium, osmium, and any mixture thereof; preferably, the one or more secondary metals are selected from silver, gold, platinum, palladium, ruthenium, and any mixture thereof; more preferably, the one or more secondary metals are selected from silver and/or gold.

The step of addition of one or more secondary metals is followed by a step of nitridation of the multi-metallic material obtained in step b) to obtain a nitride-doped multi-metallic material.

It is preferred that the step of nitridation comprises a calcination sub-step followed by a copper nitride synthesis sub-step. For example, a calcination sub-step is performed at a temperature ranging from 350 to 800° C. for a time ranging from 30 min to 2 hours. The calcination sub-step is performed in condition to transform at least a part and preferably all Cu to CuO as evidenced by XRD.

The calcination sub-step is followed by a sub-step of copper nitride (Cu₃N) synthesis that is preferably performed at a low temperature by mixing the recovered calcined multi-metallic particles obtained from the calcination sub-step with sodium amide (NaNH₂) at a temperature ranging from 150 to 190° C. for a given time. Copper nitride synthesis at low temperature is known to the person skilled in the art and described in A. Miura, et al. J. Asian Ceram. Soc., 2014, 2, 326-328, which is incorporated herein by reference.

For example, the copper nitride synthesis sub-step is performed at a temperature ranging from 120 to 200° C.; preferably from 150 to 190° C.; more preferably from 160 to 180° C.

For example, the copper nitride synthesis sub-step is performed for a time ranging from 5 to 60 hours, preferably from 5 to 30 hours, and preferably from 10 to 25 hours. It was found that, at a given temperature, the nitridation time affects the activity of the catalyst, a too-long nitridation time results in a reduction of the catalyst activity.

It is preferred that the copper nitride synthesis sub-step is performed to be incomplete (i.e. that some Cu remains in the catalyst) this can be achieved by a temperature below 150° C. or a reduced nitridation time. In a preferred embodiment, the copper nitride synthesis sub-step is performed at a temperature ranging from 150 to 190° C. and for a time ranging from 5 to 30 hours.

Copper nitride synthesis is evidenced by XRD. In a preferred embodiment, the nitride-doped multi-metallic material shows an XRD pattern with a copper (Cu°) peak and copper nitride (Cu₃N) peak. So that the catalyst comprises a mixture of copper and copper nitride.

Also, SEM images of calcined multi-metallic powder before and after the nitridation step showed that round particles of a few microns in size are changed into submicron-sized particles due to the nitridation. The nitridation step increased the catalyst surface area.

In an embodiment, the nitride-doped multi-metallic material shows Cu (111) and Cu (200) facets as determined by XRD.

So, the method to produce a catalyst may comprise:

• • a) providing copper particles;
  • b) addition of one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, iridium, osmium, and any mixture thereof, wherein said step of addition comprises a step (b1) of galvanic exchange reaction or a step (b2) of electrodeposition or both steps (b1) of galvanic exchange reaction and step (b2) of electrodeposition, to obtain a multi-metallic material comprising copper as primary metal and one or more secondary metals;
  • c) nitridation of the multi-metallic material obtained in step b) to obtain a nitride-doped multi-metallic material.

With preference, the method to produce a catalyst may comprise:

• • a) providing copper particles;
  • b) addition of one or more secondary metals selected from silver, gold, platinum, palladium, ruthenium, iridium, osmium, and any mixture thereof, wherein said step of addition comprises a step (b1) of galvanic exchange reaction;
  • c) nitridation of the multi-metallic material obtained in step b) to obtain a nitride-doped multi-metallic material.

In an embodiment, the nitride-doped multi-metallic material comprises from 0.5 to 10.0 mol % as determined by XPS of the one or more secondary metals based on the total molar content of copper and one or more secondary metals; preferably from 0.6 to 9.0 mol % more preferably from 0.8 to 8.0 mol %; even more preferably from 1.0 to 7.5 mol %; most preferably from 1.1 to 7.0 mol %; even most preferably from 1.2 to 6.5 mol % or from 1.3 to 6.0 mol %.

For example, the nitride-doped multi-metallic material comprises at least 0.5 mol % of the one or more secondary metals based on the molar content of copper and one or more secondary metals r; preferably at least 0.6 mol %; more preferably at least 0.8 mol %; even more preferably at least 1.0 mol %; most preferably at least 1.1 mol %; even most preferably at least 1.2 mol % or at least 1.3 mol %.

For example, the nitride-doped multi-metallic material comprises at most 10.0 mol % of the one or more secondary metals based on the molar content of copper and one or more secondary metals; preferably at most 9.0 mol %; more preferably at most 8.0 mol %; even more preferably at most 7.5 mol %; most preferably at most 7.0 mol %; even most preferably at most 6.5 mol % or at most 6.0 mol %.

For example, the nitride-doped multi-metallic material is a nitride-doped bimetallic material and comprises from 1.0 to 10.0 mol % of the secondary metal based on the molar content of copper and the secondary metal; preferably from 1.5 to 9.0 mol %; more preferably, from 2.0 to 8.0 mol %; even more preferably from 2.5 to 7.5 mol %; most preferably from 3.0 to 7.0 mol %; even most preferably from 3.5 to 6.5 mol % or from 4.0 to 6.0 mol %.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, and comprises from 0.5 to 10.0 mol % or 0.5 to 8.0 mol % of the one or more secondary metals based on the molar content of copper two or more secondary metals; preferably from 0.6 to 9.0 mol %; more preferably from 0.8 to 8.0 mol %; even more preferably from 1.0 to 7.5 mol %; most preferably from 1.1 to 7.0 mol %; even most preferably from 1.2 to 6.5 mol % or from 1.3 to 6.0 mol % or from 1.0 to 6.0 mol %.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least one of the secondary metals is silver or gold; preferably wherein at least two secondary metals are silver and gold.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least two secondary metals Me1 and Me2 are present in a ratio Me1/Me2 ranging from 1 to 15; preferably from 1 to 12; more preferably from 1 to 10; even more preferably, from 1 to 8; most preferably from 1 to 6; and even most preferably, from 1 to 4.

In an embodiment, the nitride-doped multi-metallic material comprises two or more secondary metals, wherein at least two secondary metals Me1 and Me2 differ by their reduction potential wherein Me1 has a reduction potential greater than Me2, and in that Me1 and Me2 are present in a ratio Me1/Me2 greater than 1; preferably greater than 2; more preferably greater than 3.

In an embodiment, the nitride-doped multi-metallic material is in the form of rode-shape particles.

In an embodiment, at least 50% of the nitride-doped multi-metallic material are in the form of particles having an average diameter ranging from 20 nm to 500 nm as measured by transmission electron microscopy; preferably, ranging from 30 nm to 450 nm; more preferably from 50 nm to 400 nm; even more preferably from or from 80 to 350 nm and most preferably from 100 nm to 300 nm.

For example, at least 60% of the nitride-doped multi-metallic material are in the form of particles having an average diameter ranging from 20 nm to 500 nm as measured by transmission electron microscopy; preferably at least 70%; more preferably, at least 80% and most preferably at least 90%.

For example, at least 80% of the nitride-doped multi-metallic material are in the form of particles having an average diameter ranging from 20 nm to 500 nm as measured by transmission electron microscopy; preferably, ranging from 30 nm to 450 nm; more preferably from 50 nm to 400 nm; even more preferably from or from 80 to 350 nm and most preferably from 100 nm to 300 nm.

The Gas Diffusion Electrode

The gas diffusion electrode has a gas diffusion layer and an ink deposited on the gas diffusion layer; wherein the ink comprises an ion-conducting polymer, said gas diffusion electrode is remarkable in that the ink further comprises the above-described catalyst comprising nitride-doped multi-metallic material, preferably in the form of particles.

The gas diffusion layer allows for the diffusion of carbon oxide (such as CO and/or CO₂) as the main reactant of the electrochemical reduction reaction (i.e. the electrolysis reaction) into the electrochemical cell and is preferably a hydrophobic porous support. In a gas-fed flow cell, the gas diffusion layer is comprised within the gas chamber of said gas-fed flow cell. With preference, said support shows a pore size ranging from 400 nm to 500 nm as determined by scanning electron microscopy, preferably from 420 nm to 580 nm or from 440 nm to 560 nm. gas diffusion layer.

It is preferred that the gas diffusion layer is a hydrophobic, porous and chemically inert support; with preference, the gas diffusion layer is not soluble in KOH. For example, the gas diffusion layer is or comprises polytetrafluoroethylene (PTFE). Examples of suitable membranes are commercially available from Dioxide Materials.

With preference, the gas diffusion layer has a circular shape and/or has a surface area of at least 1 cm², or of at least 1.5 cm². For example, the gas diffusion layer has a thickness ranging from 2 μm to 50 μm measured by scanning electron microscopy, preferably from 5 μm to 40 μm, more preferably from 8 μm to 30 μm.

The ink is deposited on the gas diffusion layer and comprises an ion-conducting polymer. With preference, the ion-conducting polymer is or comprises an ionomer. For example, the ion-conducting polymer is or comprises an ionomer with a tetrafluoroethylene backbone group (—CF2—CF2—). Such ionomers are capable of creating strongly hydrophobic nanoporous networks. For example, said ion-conducting polymer is or comprises a perfluorinated sulfonic acid, such as Nafion® (tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer); and/or the ion-conducting polymer is or comprises tetrafluoroethylene-perfluoro(3-hydrophobioxa-4-pentenesulfonic acid) copolymer, such as Aquivion®. It can form a layer on the gas diffusion layer, said layer having a thickness ranging from 2 nm and 50 nm measured by transmission electron microscopy, preferably from 5 nm and 40 nm, more preferably from 10 nm and 30 nm. For example, ink has a ratio of the nitride-doped multi-metallic particles over the ion-conducting polymer.

For example, the gas diffusion electrode has a mass loading of the ink onto said gas diffusion layer ranging from 0.50 mg/cm² to 2.00 mg/cm², preferably from 1.00 mg/cm² to 1.50 mg/cm². The mass loading can be determined by weighing before and after deposition and drying.

The Gas-Fed Flow Cell

The gas-fed flow cell suitable for the electrochemical reduction of one or more carbon oxides (such as the electrochemical reduction of carbon monoxide and/or the electrochemical reduction of carbon dioxide) of the present disclosure will then be described.

The gas-fed flow cell comprises a gas chamber, a catholyte chamber and an anolyte chamber. For example, the gas chamber has a gas channel, through which a flow of CO is circulating. The gas chamber is separated from the catholyte chamber by a gas diffusion electrode. The catholyte chamber and the anolyte chamber are separated by an anion exchange membrane (AEM). The catholyte chamber and the anolyte chamber comprise respectively a cathode and an anode, for example, a Ni foam anode. However, any oxygen evolution reaction (OER) catalyst and anode compartment design can be used. The gas-fed flow cell of the present disclosure is remarkable in that the gas diffusion electrode is as defined above and in that the gas diffusion layer of said gas diffusion electrode is comprised within the gas chamber. The ink comprising the ion-conducting polymer and the nitride-doped multi-metallic particles as described above is comprised within the catholyte chamber.

With preference, the gas-fed flow cell comprise a reference electrode. It is preferred that said reference electrode is an Ag/AgCl electrode filled with KCl at a concentration ranging from 3.0 to 3.8 M; preferably from 3.2 to 3.6 M; even more preferably with 3.4 M of KCl. In other implementations, the reference electrode could also be a reversible hydrogen electrode (RHE).

Method for Producing the Gas Diffusion Electrode

The preparation of the catalyst; i.e. of the nitride-doped multi-metallic particles, is as described above

For example, the nitride-doped multi-metallic particles described above are washed and dried after step (c) and/or before the steps required to prepare the gas diffusion electrode. With preference, the step of washing is performed with an organic solvent and the step of drying lasts at least 12 hours, preferably at least 24 hours and/or lasts no more than 48 hours, preferably no more than 36 hours. With preference, the organic solvent is selected from dichloromethane, ethyl acetate, acetone, dimethylformamide, acetonitrile, n-butanol, n-propanol, methanol, ethanol and any mixture thereof; with preference, the organic solvent is or comprises ethanol and/or methanol.

Preparation of the Gas Diffusion Electrode as Such

The nitride-doped multi-metallic particles are, in step (d), dispersed into ethanol to obtain a second dispersion. It is preferred that the second dispersion is sonicated. With preference, said sonicating is achieved at room temperature, for example at a temperature ranging from 20° C. and 30° C. With preference yet, said sonicating is achieved for at least 10 minutes, preferably for at least 15 minutes and/or for no more than 60 minutes, preferably no more than 45 minutes.

Then, in step (e), an ion-conducting polymer, such as Nafion®, is added to obtain the ink.

The ink is then deposited in step (f) on a gas-diffusion layer, for example, a gas-diffusion layer that is or comprises polytetrafluoroethylene. In a preferred embodiment, the ink is drop-casted on the gas-diffusion layer.

Electroreduction of Carbon Oxide and Production of n-Propanol and/or Ethanol

Finally, the present disclosure is about a process for electrochemically reducing carbon oxides (such as carbon monoxide and/or carbon dioxide), said process comprising the following steps:

• • i. providing a gas-fed flow cell;
  • ii. providing at least one electrolyte flow into said gas-fed flow cell wherein the at least one electrolyte flow is a catholyte flow and an anolyte flow;
  • iii. activating said gas-fed flow cell;
  • iv. providing an input flow comprising one or more carbon oxides selected from carbon monoxide, carbon dioxide, and a mixture of carbon monoxide and carbon dioxide to produce an output flow of catholyte comprising at least n-propano and/or ethanol;
  • v. recovering said output flow comprising at least n-propanol and/or ethanol;
    said process is remarkable in that the gas-fed flow cell provided at step (i) is as defined above.

The process needs to have both a flowing catholyte and a flowing anolyte.

For example, the current density is at least 100 mA cm−² with preference at least 150 mA cm−².

The “electrolyte flow” hereafter refers to the catholyte flow but also applies to the anolyte flow in the case of a flowing anolyte.

For example, the electrolyte flow provided in step (ii) has a flow rate that is ranging from 2.5 mL min−1 to 8.5 mL min−1, preferably from 3.0 mL min−1 to 8.0 mL min−1, more preferably from 3.5 mL min−1 to 7.5 mL min−1.

For example, the electrolyte flow provided in step (ii) is a flow of an aqueous solution of one or more inorganic bases. With preference, the one or more inorganic bases are alkali selected from NaOH, KOH, Ca(OH)₂, LiOH, Mg(OH)₂, RbOH, CsOH and any mixture thereof. With preference, the one or more inorganic bases are or comprise KOH and/or NaOH.

With preference, the aqueous solution of one or more inorganic bases has a pH ranging from 7 to 15.

With preference, the aqueous solution of one or more inorganic bases has a concentration of at least 0.05 M, or at least 0.1 M, or at least 1 M, or at least 2 M, or at least 3 M, or at least 4 M. With preference, the aqueous solution of one or more inorganic bases has a concentration of at most 10 M, of at most 8 M, or at most 7 M, or at most 6 M, or at most 5 M. For example, the concentration of the aqueous solution of one or more inorganic bases is ranging from 0.1 M to 10 M; preferably from 3 M to 7 M, or preferably from 0.1 M to 5 M.

With preference, the aqueous solution of one or more inorganic bases is an aqueous solution of KOH at a concentration of at least 0.05 M, or at least 0.1 M, or at least 1 M, or at least 2 M, or at least 3 M, or at least 4 M. With preference, said at least one alkaline compound is an aqueous solution of KOH at a concentration of at most 10 M, of at most 8 M, or at most 7 M, or at most 6 M or at most 5 M. For example, the concentration of KOH in a solution of water ranges from 0.1 to 10 M; preferably from 3 M to 7 M, more preferably from 0.1 M to 5 M.

Advantageously, said step (iii) is performed by injecting carbon oxide (such as carbon monoxide and/or carbon dioxide) at a potential gradient starting at −0.3 V versus a reference electrode and ending at −2.0 V versus said reference electrode at a sweep rate ranging from 15 mV s-1 to 100 mV s-1 or from 30 mV s-1 to 60 mV s-1. With preference, the potential gradient starts at −0.5 V versus a reference electrode and ends at −1.8 V versus said reference electrode. For example, the reference electrode is an Ag/AgCl electrode filled with 3.4 M of KCl.

The output flow of catholyte, in addition to comprising n-propanol and/or ethanol, can also comprise one or more selected from acetate.

With preference, said step (iv) lasts at least 1 hour, more preferably at least 2 hours, even more preferably at least 3 hours, most preferably at least 4 hours, even most preferably at least 5 hours or at least 6 hours.

For example, the input flow comprising one or more carbon oxides provided in step (iv) has a flow rate that ranges from 5 mL min⁻¹ to 150 mL min⁻¹, preferably from 8 mL min⁻¹ to 100 mL min⁻¹, more preferably from 10 mL min⁻¹ to 80 mL min⁻¹, even more preferably from 15 mL min −1 to 50 mL min⁻¹.

Advantageously, said step (iv) is performed at room temperature, for example at a temperature ranging from 20° C. to 30° C.

Advantageously, said step (iv) is performed at atmospheric pressure, for example at a pressure ranging from 0.09 MPa to 0.11 MPa.

TEST METHODS

Mass loading of the ink onto the gas diffusion layer: The membrane was weighed using an analytical balance before deposition and after drying overnight in a vacuum desiccator.

Transmission Electron Microscopy

TEM analysis was conducted using a Jeol 2100F microscope equipped with Schottky Field Emission electron gun and an ultra-high resolution polar piece.

Scanning Electron Microscopy

SEM images were obtained using a.SEM ZEISS Ultra 55.

X-Ray Diffraction: XRD data were obtained from a D8 ADVANCE diffractometer (Bruker) using a Cu Kα X-ray source (1.5406Å). Peaks were attributed using the PDF-2/release 2013 RDB database.

XPS measurement: Surface elemental composition was determined by X-ray photoelectron spectroccopy (XPS; Thermo Electron Escalab 250) using a monochromated Al Ka radiation (1486.6 eV) and photoelectron take-off angle of 90°. Survey and high resolution spectra energy was 100 eV and 20 eV, respectively. Thermo Electron software Avantage was used for curve fitting of the XPS spectra.

EXAMPLES

Example 1: The Bimetallic Catalysts

Preparation of Cu₃N/Ag, Pd or Au Catalysts

Cu powder with a size of 25 nm purchased from Sigma Aldrich was used as the substrate. Copper-silver (Cu/Ag), copper-gold (Cu/Au), and copper-palladium (Cu/Pd) bimetallic compounds were prepared by a galvanic exchange method, in which the Cu powder was stirred in an aqueous solution of AgNO₃, HAuCl₄ or Pd(NO₃)₂ with from 2 to 8 mol % as the second metal to Cu, at 60° C. for 30 minutes.

The thus obtained powder was collected and washed with water and ethanol by centrifugation.

To prepare copper nitride (Cu₃N) and Cu₃N combined with a second metal, a low-temperature nitridation method was applied. Such a method is known to the person skilled in the art and is described in A. Miura, et al. J. Asian Ceram. Soc., 2014, 2, 326-328, which is incorporated herein by reference.

Firstly, Cu, Cu/Ag, Cu/Pd or Cu/Au powder was calcined in air at 500° C. for 1 h to transform the Cu species to CuO. Then, the powder (10 mg) was mixed with NaNH₂ 98% purchased from Sigma Aldrich (100 mg) by mortar and pestle, then put into a Teflon-lined autoclave in a glovebox filled with Ar. The autoclave was heated at 170° C. for 20 h. After cooling down, ethanol was added to the mixture and the catalyst was collected after washing and centrifuging with water and ethanol.

Characterizations of Cu₃N/M (M=Ag, Au, Pd) Catalysts:

As shown in FIG. 2, the Cu₃N/Ag(5%) was composed of Cu₃N, CuAg and Cu.

The Cu₃N was mainly comprised of Cu and Cu₃N. While Cu/Ag(5%) contained Cu, CuAg and a small relative amount of Cu₂O which could be due to oxidation in air.

Further characterization such as XPS, and STEM were done to determine the surface structure and composition of this sample.

As shown in FIG. 3, Cu substrate was composed of nanoparticles with sizes from 25 to 40 nm. Cu/Ag(5%) sample contained particles of similar size and its surface was decorated with lots of smaller particles of a size of several nanometers. This result indicated that a galvanic exchange reaction between AgNO₃ and Cu substrate occurred.

After nitridation, as shown in the images of Cu₃N/Ag(5%) and Cu₃N samples, the morphology of the particles changed, in which rode-shape particles with sizes ranging from 100 to 300 nm appeared together with some 10 nm undetermined shape particles.

As shown in FIGS. 4 and 5, Cu₃N was completely formed with 25 h of nitridation. The amount of Cu₃N or Cu(0) was dependent on the nitridation time. Varying the content of Ag affected the relative amount of Cu in the sample, the Cu₃N/Ag(5%) showed both Cu(111) and Cu(200) phases, while Cu₃N/Ag(2 or 8%) showed only Cu(111) phase. Besides, the peak intensity of Ag increased as the content of Ag was increased.

Example 2—Gas Diffusion Electrodes Preparation with Bimetallic Catalysts

To prepare the cathode for the electrochemical test, the catalyst was mixed with Nafion® (tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer) and ethanol with a ratio of 1.5 mg: 5 μl: 100 μl. Then, the suspension was drop cast onto a gas diffusion layer (PTFE of 30%) purchased from dioxide materials with a loading of 1.5 mg.cm⁻². The thus obtained catalyst-loaded gas diffusion electrode was used as the cathode.

Example 3—Electrochemical Experiments with Bimetallic Catalysts

The GDEs of example 2 were electrically connected in a gas-fed flow cell for electrochemical testing.

A flow cell system was used (FIG. 1). Applied current density: 100-200 mA.cm⁻².

Activity for Electrochemical CO Reduction of Cu₃N/M Catalysts:

The activity was investigated for CO reduction of the copper nitride, copper nitride-silver and silver-copper catalysts, taking the Cu sigma powder as the reference.

As shown in FIG. 6, at −100 mA.cm⁻² of applied current density, Cu sigma showed FE for n-propanol of 12%, and those for ethanol and acetic acid were around 10%.

Cu₃N sample showed FE for n-propanol of 17-18% at −100 and −150 mA.cm⁻², while FE for ethanol was almost the same and acetate was slightly lower compared to that of Cu sigma. This result indicates that nitridation was effective to enhance the selectivity for n-propanol. Regarding the Cu/Ag sample, FE for propanol was around 10%, whereas FE for ethanol was around 20% at −100 and −150 mA.cm⁻². Interestingly, Cu₃N/Ag sample showed a significant improvement of activity for n-propanol with a high FE of 23% and 39% at −100 and −150 mA.cm⁻², respectively. This high efficiency for propanol is comparable to the highest value reported so far (by Sargent and colleagues, Nat Energy, 2022, 7, 170-176). It is noteworthy that the total FE for alcohols (propanol and ethanol) is about 62%.

The nitridation time for the sample was also optimized. The activity of the samples prepared with different nitridation times (10-60 h) is presented in FIG. 5.

As shown in FIG. 7, nitridation time had a significant effect on the activity of Cu₃N/Ag(5%) for CO reduction to propanol, the sample synthesized with 20 h of nitridation showed the highest FE for n-propanol of 39% at −150 mA.cm⁻² of current density. The nitridation time of 10 h showed the second highest FE for n-propanol of 25% at 100 and 150 mA.cm⁻², respectively. Meanwhile, a significant increase in nitridation time to 40 or 60 h gave a lower FE for n-propanol of 15%.

After optimizing the nitridation time with Cu₃N/Ag samples, the effect of two other metals including Au and Pd as the secondary metal was tested.

As shown in FIG. 8, Cu₃N/Au(5%) showed comparable activity for propanol compared to Cu₃N/Ag(5%) at −150 mA.cm⁻². Interestingly, at 100 mA.cm⁻², Cu₃N/Au(5%) showed almost 30% of FE for propanol.

The activity for CO reduction was also investigated with different content of Ag.

As shown in FIG. 9, the sample with 5% of Ag showed the highest FE for propanol at 150 mA.cm⁻². On the other hand, at 100 mA.cm⁻², the sample with 2% of Ag showed the highest FE for propanol of around 35%. Thus varying the content of secondary metal and the current density allows to further enhance the selectivity to a given alcohol.

The activity for CO reduction was also investigated in the highly active Cu₃N/Au(5%) and Cu₃N/Ag(5%) catalysts at different current densities.

As shown in FIG. 10, in the range of applied current densities of 100-200 mA.cm⁻² with 25 mA of difference, it was obtained a volcano shape tendency, in which the highest FE for propanol was achieved at −150 mA.cm⁻² using Cu₃N/Au(5%) catalyst.

The activity for CO reduction of Cu₃N/Ag(5%) catalyst at different KOH concentrations was invistigated.

As shown in FIG. 11, at very low KOH concentration (0.1 M), the selectivity of Cu₃N/Ag(5%) switched to ethanol, in which FE for ethanol reached around 53% at 100 mA.cm⁻² of applied current densities Whereas, the efficiency for ethanol was around 40% at higher currents of 150 and 200 mA.cm⁻². As the KOH concentration increased up to 3 M, FE_ethanol decreased to around 20% and FE_propanol increased to reach the highest value of 39% at KOH 1 M. These results suggest that low concentrations of hydroxyl group could enhance the formation of ethanol.

Characterizations for Cu₃N/Ag(5%) and Cu₃N samples after 30 minutes of electrolysis (i.e. of electrochemical reduction).

XPS spectra showed that after 30 minutes of electrolysis, Cu₃N was not present anymore and a NH₃⁺ species appeared, suggesting that Cu3N was not the active site of this Cu₃N/Ag(5%) catalyst. The presence of Cu(II) species could be due to the unavoidable oxidation of Cu species in air.

As shown in FIG. 12, no Cu₃N remained after the electrolysis, which is consistent with the XPS results (see FIG. 15). The presence of Cu₂O could be due to oxidation in air. Nevertheless, there is still possibility that Cu₂O could play a role in the catalytic reaction of Cu₃N/Ag(5%). Therefore, longer time of electrolysis could be done in the future.

As shown in FIG. 13, after 3 scans of linear sweep voltametry (LSV), Cu₃N disappeared. The existence of Cu₂O could be due to the oxidation in air.

Activity for CO₂RR of Cu₃N/Ag(5%)

As shown in FIG. 14, Cu₃N/Ag(5%) was very selective for CO production with FE of nearly 50% at three different applied current densities. Regarding liquid products, the electrocatalyst exhibited very low FE for ethanol, n-propanol and acetic acid, which was less than 10%. Whereas, formic acid was the main product with FE of 13-18%.

Example 4: The Tri-Metallic Catalysts

Preparation of Cu₃N/Au/Ag Catalysts

Cu powder with a size of 25 nm purchased from Sigma Aldrich was used as the substrate. Copper-gold (Cu/Au), bimetallic compounds were prepared by a galvanic exchange method, in which the Cu powder was stirred in an aqueous solution of HAuCl₄ with from 0.3 to 5 mol % as the second metal to Cu, at 60° C. for 30 minutes.

The thus obtained powder was collected and washed with water and ethanol by centrifugation.

In order to obtain the tri-metallic compounds, a second sub-step of galvanic exchange reaction was performed, in which the CuAu powder precedently obtained powder was stirred in an aqueous solution of AgNO₃ with from 0.1 to 5 wt % as the third metal to Cu/Au, at 60° C. for 30 minutes. The thus obtained powder was collected and washed with water and ethanol by centrifugation. To prepare copper nitride (Cu₃N) and Cu₃N combined with Au and Ag metals, a low-temperature nitridation method was applied. Such a method is known to the person skilled in the art and is described in A. Miura, et al. J. Asian Ceram. Soc., 2014, 2, 326-328, which is incorporated herein by reference.

Firstly, Cu, or Cu/Au/Ag powder was calcined in air at 500° C. for 1 h to transform the Cu species to CuO. Then, the powder (10 mg) was mixed with NaNH₂ 98% purchased from Sigma Aldrich (100 mg) by mortar and pestle, then put into a Teflon-lined autoclave in a glovebox filled with Ar. The autoclave was heated at 170° C. for 20 h. After cooling down, ethanol was added to the mixture and the catalyst was collected after washing and centrifuging with water and ethanol.

Example 5—Gas Diffusion Electrodes Preparation with Tri-Metallic Catalysts

To prepare the cathode for the electrochemical test, the catalyst of example 4 was mixed with Nafion® (tetrafluoroethylene-perfluoro-3,6-dioxa-4-methyl-7-octenesulfonic acid copolymer) and ethanol with a ratio of 1.5 mg: 5 μl: 100 μl. Then, the suspension was drop cast onto a gas diffusion layer (PTFE of 30%) purchased from dioxide materials with a loading of 1.5 mg.cm⁻². The thus obtained catalyst-loaded gas diffusion electrode was used as the cathode.

Example 6: Electrochemical Experiments with Tri-Metallic Catalysts

The GDEs of example 5 were electrically connected in a gas-fed flow cell for electrochemical testing.

A flow cell system was used (FIG. 1). Applied current density: 100-200 mA.cm⁻².

The activity of nitride-doped tri-metallic catalysts was investigated. Bi-metallic catalysts used for comparison experiments were produced as in example 1.

The activity was investigated for CO reduction of the trimetallic catalysts.

As shown in FIG. 16, when varying Ag percentage with a fixed value of 2% Au, Cu₃N/Au(2%)Ag(0.3%) was found to be the most selective for C₂₊ liquids and C₂₊ alcohols. 0.3% Ag was then selected for further experiments.

As shown in FIG. 17, when varying Au percentage with a fixed value of 0.3% Ag Cu₃N/Au(1%)Ag(0.3%) was found to be the most selective for C₂₊ liquids and C₂₊ alcohols and was thus selected as reference for further comparison.

As shown in FIG. 18, when varying KOH concentration in electrolytes, 1.0 M KOH showed the highest selectivity for C₂₊ alcohols production, and was thus selected for further experiments.

As shown in FIG. 19, comparing CORR by nitride-derived bimetallic and trimetallic catalysts showed a significant improvement with Cu₃N/Au(1%)Ag(0.3%) for C₂₊ liquids. Similarly, 77% FE of C₂₊ alcohol was obtained; this comes from a decrease in the selectivity of ethylene, comparing to nitrite-derived bimetallic catalysts.

As shown in FIG. 20, the highest ratio FE_alcohols/FE_ethylene of 5.4 was achieved with Cu₃N/Au(1%)Ag(0.3%). Au and Ag co-presence gave much higher FE_alcohols/FE_ethylene ratio than Au or Ag alone.

As shown in FIG. 21, using bi-metallic Cu₃N/Au with Au percentage ranging from 0.3 to 3%, ethylene is always a major product, with FE of C₂₊ alcohols being below 60%.

As shown in FIG. 22, using bi-metallic Cu₃N/Ag with Ag percentage ranging from 0.3 to 8%, ethylene is always a major product, with FE of C₂₊ alcohols being below 60%.

As shown in FIG. 23, a comparison between Cu₃N/Au(1%)Ag(0.3%) and Cu₃N/Ag(0.3%) in CORR confirms the function of Au in tuning the selectivity of CORR to favor C₂₊ alcohols production.

As shown in FIG. 24, long-term CORR electrolysis by Cu₃N/Au(1%)Ag(0.3%) indicated a relatively stable system after nearly 14 h of electrolysis. A slight decrease in C₂₊ alcohols and an increase in H2 production was observed.

TABLE 1
	FEC2+ alcohols	FEC2+ alcohols/	FEC2+ alcohols/
Catalysts	(%)	FEethylene	FEhydrogene	Reference

CuAuAgN	76	5.4	17.9	This work
CuAgN	59	3.1	8.2	This work
Cu-butyl-X	68	2.9	11.3	Nat. Commun., 2023, 14, 501
Cu(OD)0.8Ag0.2	54	4.5	4.5	Nat. Commun., 2023, 14, 698
Ag—Ru—Cu	49	1.8	4.5	Nat Energy, 2022, 7, 170
Ag—Au—Cu	37	1.3	3.1	Nat Energy, 2022, 7, 170
Cu adparticles	40	1.1	2.7	Nat. Commun, 2018, 9, 4614
Cavity Cu	35	1.7	2.2	Nat. Catal., 2018, 1, 946