WO2022171663A1

WO2022171663A1 - Electrochemical device for converting nitrogen oxides nox into ammonia and/or hydrogen

Info

Publication number: WO2022171663A1
Application number: PCT/EP2022/053115
Authority: WO
Inventors: Julian Dailly; Aayan BANERJEE; Olaf Deutschmann
Original assignee: Electricite De France; Karlsruher Institut für Technologie
Priority date: 2021-02-10
Filing date: 2022-02-09
Publication date: 2022-08-18
Also published as: FR3119553A1; FR3119553B1

Abstract

The present invention relates to an electrochemical device for converting nitrogen oxides NOx into ammonia and/or dihydrogen. The device comprises an air electrode, a fuel electrode and a solid electrolyte which is arranged between the air electrode and the fuel electrode. The fuel electrode comprises particles of a nitride-based material on its surface in contact with the electrolyte. The invention also relates to a method for producing ammonia and dihydrogen from nitrogen oxides by means of the electrochemical device.

Description

Title: Electrochemical device for the conversion of nitrogen oxides NOx into ammonia and/or hydrogen

Technical field [0001] The present invention relates to the field of electrochemical devices for the conversion of nitrogen oxides into ammonia and dihydrogen.

Prior technique

Nitrogen oxides (NOx), including nitric oxide NO, nitrogen dioxide N0 ₂ and nitrous oxide N ₂ 0, are well-known air pollutants. The presence of nitrogen oxides in the atmosphere contributes to the formation of smog (thick haze limiting visibility), fine particles and acid rain, as well as to the destruction of the ozone layer and leads to risks health issues such as, for example, respiratory diseases. In particular, in countries such as China and India where the economy is growing rapidly, excessive industrial activities lead to the emission of a large amount of nitrogen oxides, affecting both the country of origin of these emissions than neighboring countries. Nitrogen oxides are generally produced by the reaction between nitrogen and oxygen at high temperatures during the combustion of fuels such as diesel, hydrogen or natural gas. Reducing the quantity of nitrogen oxides present in the atmosphere is a major challenge for industrialized countries to enable technological advances in the field of combustion and for all technologies involving the formation of nitrogen oxides to be pursued. .

[0003] The decomposition of nitrogen oxides by selective catalytic reduction (SCR) uses gaseous ammonia and urea to convert nitrogen oxides into nitrogen and water. SCR is the most used solution in thermal power plants burning biomass, waste or coal, in petrochemical industries, in iron and steel industries but also in transport such as boats, trucks and cars diesels due to reduction in reactor size and lower catalyst prices. However, the use of urea as a reagent results in the formation of carbon dioxide, while the use of ammonia results in the indirect formation of greenhouse gases via the Haber-Bosch process for synthesizing ammonia . In both cases, SCR products are not strategic components that can be recovered and converted into an energy source.

[0004] In addition to SCR, there are other techniques allowing the elimination of nitrogen oxides. [0005] NOx absorbers, also called NOx traps (LNT), are composed of catalysts based on platinum or ruthenium and an absorbent such as barium oxide. The catalyst allows the oxidation of NOx to nitrogen dioxides which are then fixed by the barium to form Ba(N0 ₃ ) ₂ . A purge phase then takes place under a reduced atmosphere during which the NOx is reduced to nitrogen. LNTs are cyclic processes consisting of a trap phase followed by a purge phase, and are mainly used in transports.

[0006] Exhaust gas recirculation (EGR) is a technique which consists of returning the exhaust gases to the engine cylinders; it is mainly used in transport.

[0007] Flameless burners (FLOX® burners) prevent the formation of nitrogen oxides. They differ from conventional burners in the ability to recirculate the exhaust gases into the combustion chamber and mix them with the combustion air, thus preventing the formation of a flame front. Thanks to a high temperature (at least 800°C) as well as a homogeneous distribution of the temperature distribution, the formation of nitrogen oxides, which generally occurs at the tip of the combustion flame due to its temperature peak, can be limited.

[0008] For power plants using heavy fuel oil, flameless burners alone do not make it possible to achieve the reductions in pollutant emissions required. Indeed, heavy fuel oil, during its combustion, is not completely burned. The use of a water-in-fuel emulsion thus makes it possible to completely burn heavy fuel oil. The water droplets explode as soon as the fuel oil begins to burn causing the fuel oil to break down into micro-droplets that can be properly burned. This complete combustion reduces pollutant emissions, including nitrogen oxides.

[0009] Nitrogen monoxide (NO) represents about 95% of nitrogen oxides and can be oxidized or reduced by removing electrons from it or giving it electrons. Nitric oxide can therefore be converted into an environmentally friendly nitrogen compound. When it is oxidized by receiving electrons, it can be converted into nitrite ion (N0 ₂ ) and nitrate ion (N0 ₃ ). When reduced, it can be converted into dinitrogen (N ₂ ), hydroxylamine (NH ₂ OH), hydrazine (N ₂ H ₄ ) and ammonia (NH ₃ ). Nitrogen monoxide can be selectively converted into a specific nitrogenous compound, for example ammonia, provided that the operating parameters used are precisely controlled. [0010] American patent applications No. 2020/0002180 and Korean patent application No. 20200090668 describe, for example, an electrochemical system making it possible to produce ammonia from nitrogen oxides. More precisely, these documents propose an electrochemical cell allowing the reduction of nitrogen oxides at the cathode, while water is oxidized at the anode. An electrolyte allows the conduction of protons. The electrochemical cell operates at ambient temperature under atmospheric pressure, and allows the formation of ammonia and dihydrogen. To promote ammonia selectivity, the invention presented in these documents proposes complexing the nitrogen oxides with metal complexes present in the electrolyte, the nitrogen oxide complexes thus formed will then be reduced. The use of metal oxide complexes makes it possible to promote the conversion of nitrogen oxides into ammonia rather than into dihydrogen at low temperature and to use inexpensive reagents such as water and air. Indeed, the reaction leading to the production of dihydrogen consumes a large part of the energy supplied to the system and reduces the quantity of nitrogen oxides converted into ammonia. In order to reduce as much as possible the quantity of nitrogen oxides converted into dihydrogen, the electrochemical cell described in these applications requires the use of noble materials (such as silver or platinum) for the electrodes as well as catalysts which are very selective.

[0011] There therefore remains a need for a process allowing the conversion of nitrogen oxides into recoverable compounds such as ammonia and dihydrogen, which is economical, in particular which does not require the use of expensive materials. such as noble metals for the preparation of the electrodes, nor to operate with highly selective catalysts, while guaranteeing sufficiently high levels of conversion. Summary

The present invention therefore provides, according to a first aspect, an electrochemical device for the conversion of nitrogen oxides NOx into ammonia and / or hydrogen comprising an air electrode, a fuel electrode and a solid electrolyte disposed between the air electrode and the fuel electrode characterized in that the fuel electrode comprises particles of a nitride-based material.

[0013] The inventors have indeed demonstrated that the presence of nitrides in the fuel electrode made it possible to promote the adsorption of nitrogen oxides on this electrode, thus promoting their conversion. Such a process makes it possible to obtain excellent levels of conversion of nitrogen oxides into ammonia and/or hydrogen, by operating with electrodes made of inexpensive materials such as ceramics.

According to a second aspect, the subject of the invention is an electrochemical process for the production of ammonia and dihydrogen from nitrogen oxides comprising:

- the oxidation, at the level of the air electrode, of a proton donor compound to form protons,

- the circulation of the protons formed at the air electrode through the solid electrolyte towards the fuel electrode and

- the reduction of nitrogen oxides, at the level of the fuel electrode, to form ammonia and/or dihydrogen as well as water and dinitrogen.

The reactions implemented in the process of the invention can thus be carried out at high temperature (between 200 and 700° C.), thus significantly accelerating the electrochemical and electro-catalytic kinetics, and allowing the use of catalysts well less selective. The method according to the invention also guarantees an excellent level of safety for its large-scale implementation due to the use of a solid electrolyte.

The device and the method according to the invention allow the production of ammonia and dihydrogen with great selectivity by the reduction of nitrogen oxides from the exhaust gases, without producing organic pollutants or toxic waste, using just water and electricity. Nitrogen oxides are thus converted into highly recoverable components, harmless to the environment.

Brief description of the drawings

Other characteristics, details and advantages of the present invention will appear on reading the detailed description below, and on analyzing the appended drawings, in which:

[0018] [Fig. 1] is a schematic representation of a fuel cell according to one embodiment of the invention.

Description of embodiments

The present invention provides an electrochemical device for the conversion of nitrogen oxides NOx into ammonia and/or dihydrogen. The device includes an air electrode, a fuel electrode, and a solid electrolyte disposed between the air electrode and the fuel electrode. The fuel electrode includes particles of a nitride material. [0020] As illustrated in [Fig. 1], the electrochemical device comprises an air electrode forming an anode. The air electrode preferably has a porous structure.

[0021] Preferably, the air electrode allows the formation of protons during the oxidation of the proton donor compound. The air electrode can thus be made of any material capable of allowing proton formation by oxidation of a proton donor compound. Preferably, the air electrode is made of a ceramic material.

The air electrode may in particular be composed of a material of the perovskite type, for example (Pr-)BaSrCoFe0 ₃ -d or LaSrCoFe0 ₃ -d, as well as its derivatives. It can also be composed of materials of the Ruddlesden-Popper type, for example (AB) ₂ -xNi0 +d (where A may be praseodymium, niobium or lanthanum and B barium or strontium) or La ₄ Ni ₃ Oi ₀ , as well as its derivatives. It may also be composed of oxides such as Pr ₆ On or of double perovskite materials such as BaGdLaCoOe, Ba ₂ FeMo0 ₆ as well as its derivatives. [0023] The electrochemical device of [Fig. 1] further comprises a fuel electrode comprising particles of a nitride material.

The fuel electrode forms a cathode and preferably also has a porous structure. The fuel electrode must allow ionic conduction, preferably proton, as well as electronic conduction. It can therefore be made of the same material allowing both ionic conduction, preferably proton, and electronic conduction.

The materials allowing ionic conduction, preferably protonic, can for example be a ceramic material.

The proton-conducting ceramics belong in particular to the family of perovskites of formula AB0 ₃ in which A is an element chosen from group II of the periodic table, B is an element chosen from cerium or group IVB of the periodic table. More particularly, perovskites in which A is chosen from barium Ba or strontium Sr, B is chosen from zirconium Zr or cerium Ce. These ceramics can be doped with yttrium (Y) or ytterbium (Yb). Preferably, the material of perovskite type is chosen from BaCeZrY(Yb)0 ₃ or Sr(Ce,Zr)X0 ₃ where X is a metal with or without doping, as well as its derivatives.

Alternatively, the material allowing ionic conduction, preferably proton, can be an ABBΌ3 type material, in which A is an element chosen from group II of the periodic table, B is an element chosen from cerium or the group IVB of the periodic table, B' is an element chosen from among the lanthanides or the group VIIIB of the periodic table. More particularly, materials of the ABBΌ3 type, in which A is chosen from barium Ba or strontium Sr, B is chosen from zirconium Zr or cerium Ce, and B' is chosen from praseodymium Pr, vanadium V, Cobalt Co or neodymium Nd. These ceramics can be doped with yttrium (Y) or ytterbium (Yb).

Finally, the material allowing ionic conduction, preferably protonic, can also be composed of materials based on Nd ₂ Ce ₂ 0 ₇ with or without doping, as well as its derivatives.

According to a preferred embodiment, the material allowing ionic conduction, preferably protonic, of the fuel electrode is the same as that constituting the electrolyte, which facilitates chemical compatibility. Alternatively, the material allowing the ionic conduction, preferably protonic, of the fuel electrode can be different from that constituting the electrolyte.

[0030] The materials allowing the electronic conduction of the fuel electrode can themselves be metallic or ceramic materials.

Thus, the fuel electrode may consist of a single ceramic material, simultaneously allowing ionic and electronic conduction, or a mixture of at least one ionic conductive material, preferably proton conductor and at least one electronic conductive material. When the fuel electrode consists of a single ceramic material, simultaneously allowing ionic and electronic conduction, this material will preferably be different from that of the air electrode.

[0032] According to a particular embodiment, the fuel electrode may be composed of a material of the ceramic-metal composite type (cermet) comprising for example nickel and an ionic material.

[0033] According to a more particular embodiment, when the fuel electrode consists of a single ceramic material, simultaneously allowing ionic and electronic conduction, it can also be composed of a material from the perovskite family of formula ABX ₃ , where A is a rare earth and BX a metal, as well as its derivatives.

[0034] The fuel electrode can be manufactured in different ways.

If the fuel electrode also serves as a mechanical support for the electrochemical device, it can be manufactured by "tape casting", which consists of mixing electrode powders and/or organic compounds which are then cast under film form and then sintered. It can also be manufactured by "pressing", which consists of compacting the electrode powders and then sintering them.

If the fuel electrode is a thin layer, it can be manufactured by wet chemical means, that is to say by screen printing, by thermal spraying, by centrifugal coating or by induction by soaking a mixture electrode powders and/or organic compounds on the electrolyte layer. It can also be fabricated by physical routes, i.e. by physical vapor deposition or by chemical vapor deposition of precursors of the electrode material on the surface of the electrolyte. [0037] The fuel electrode comprises particles of a nitride-based material. The nitride-based material may be a metal nitride-based material preferably obtained from non-precious metals such as rare earths. Non-exhaustively, metals with oxidation number +III can be zirconium, titanium, tantalum, niobium, scandium, chromium, vanadium, manganese, copper, yttrium, molybdenum, silver, hafnium, gold, iron, cobalt, nickel, ruthenium, cesium, rhodium, palladium, platinum, osmium and iridium. The choice of metal can be made according to their catalytic activity as well as the desired reaction product. Thus, scandium and cesium are, for example, catalysts with high activity and selectivity towards ammonia under certain operating conditions.

The materials based on nitrides incorporated into the structure of the fuel electrode, are in the form of particles, preferably nanoparticles, that is to say particles whose size is of the order of the nanometer. However, their size and shape can vary, as long as the particles remain homogeneously present in the structure of the fuel electrode. However, it should be borne in mind that the larger the surface area of these nanoparticles (and therefore the smaller and smaller the particle size), the faster the kinetics of conversion of nitrogen oxides into ammonia and/or dihydrogen.

The materials based on nitrides can be prepared by heat treatment of nitride precursors, for example metal salts, under an atmosphere of ammonia or any other molecule containing nitrogen. Their size can be controlled by an appropriate adjustment of the heat treatment parameters (heating rate, temperature), or by the nature of the metallic precursors. Other parameters can also impact the particle size of materials based on nitride, such as for example the nature of the atmosphere during the heat treatment, the quantity of precursor, the microstructure of the electrode to be infiltrated.

The nitrides or nitride precursors thus prepared can then be incorporated directly into the mixture making it possible to manufacture the fuel electrode, or alternatively infiltrated into the porous electrode.

The nitride-based material is preferably distributed homogeneously in the structure of the fuel electrode. This distribution makes it possible to increase the number of contact points allowing the reaction and therefore to provide a maximum reaction surface making it possible to increase the reaction kinetics of conversion of nitrogen oxides into ammonia and/or dihydrogen.

In addition to the air electrode and the fuel electrode, the device of the invention uses a solid electrolyte placed between the air electrode and the fuel electrode, and in contact with them. The solid electrolyte allows the diffusion of protons from the air electrode to the fuel electrode and the separation between the anode and cathode compartments. To effectively separate the gaseous phases of the two compartments, the electrolyte must be sufficiently dense and therefore weakly porous, or even non-porous.

The solid electrolyte may consist of a ceramic material, different from the material constituting the air electrode.

The proton-conducting ceramics can, as indicated above, be the same as those used for the fuel electrode. These proton-conducting ceramics belong in particular to the family of perovskites of formula AB0 ₃ in which A is an element chosen from group II of the periodic table, B is an element chosen from cerium or group IVB of the periodic table. More particularly, perovskites in which A is chosen from barium Ba or strontium Sr, B is chosen from zirconium Zr or cerium Ce. These ceramics can be doped with yttrium (Y) or ytterbium (Yb). Preferably, the material of perovskite type is chosen from BaCeZrY(Yb)0 ₃ or Sr(Ce,Zr)X0 ₃ where X is a metal with or without doping, as well as its derivatives.

Alternatively, it is also possible to produce the electrolyte in a material of ABBΌ3 type, in which A is an element chosen from group II of the periodic table, B is an element chosen from cerium or group IVB from the table periodic table, B' is an element chosen from the lanthanides or group VIIIB of the periodic table. More particularly, materials of the ABBΌ3 type, in which A is chosen from barium Ba or strontium Sr, B is chosen from zirconium Zr or cerium Ce, and B' is chosen from praseodymium Pr, vanadium V, cobalt Co or neodymium Nd. These ceramics can be doped with yttrium (Y) or ytterbium (Yb).

Finally, the electrolyte can also be composed of materials based on Nd ₂ Ce ₂ 0 ₇ with or without doping, as well as its derivatives. These categories of materials presented above for the manufacture of the electrolyte have the advantage of being suitable for the operating temperatures typically encountered in PCFC proton-conducting ceramic fuel cells, between 300° C. and 700° C. C, under reducing atmosphere and steam. They also possess a suitable crystallographic structure making it possible to obtain a barrier effect to contaminants, while possessing desired properties of preferentially exclusive conduction of protons. Finally, these compounds thus offer great mechanical and physico-chemical stability to the electrochemical device with a proton conductor which is equipped with them.

The electrochemical device according to the invention can be used as a fuel cell. More particularly, it may be an electrolyser or a protonic ceramic cell (PCC for “Protonic Ceramic Cell”), in particular a protonic conductive fuel cell (PCFC, protonic conductive fuel cell) or a protonic conductive ceramic electrolytic cell (PCEC for “Protonic Ceramic Electrolysis Cell”), the latter allowing the production of pure dihydrogen from water.

According to a second aspect, the invention proposes an electrochemical process for the production of ammonia and dihydrogen from nitrogen oxides comprising:

- the oxidation, at the level of the air electrode, of a proton donor compound to form protons, - the circulation of the protons formed at the level of the air electrode through the solid electrolyte towards the fuel electrode and

The electrochemical process is preferably operated exclusively in the gas phase, that is to say that all the reactants and products are in the gas phase.

The nitrogen oxides can be composed of a mixture of nitric oxide of formula NO, of nitrogen dioxide of formula N0 ₂ and/or of nitrous oxide of formula N ₂ 0. They can be generalized under the formula N _y O _x.

The proton donor compound used in the electrochemical process of the invention can be any compound capable of releasing a proton during its oxidation at the air electrode. The proton donor compound can, for example, be water, dihydrogen, methane, etc. Preferably, the proton donor compound is water, more preferably in the form of water vapour. The oxidation of the water vapor at the level of the air electrode further leads to the formation of a water vapor enriched in dioxygen, which can also be recoverable.

[0053] The [Fig. 1] schematically illustrates the supply of water vapor at the level of the air electrode. Water vapor is oxidized to dioxygen according to the following half-equation of [Reaction Formula 1]:

[0054] [Reaction formula 1]

2y + 2x 4y + 4x , y + x 4y + 4x — - H ₂ 0 = — - H ⁺ + - - 0 ₂ + — - e ^~ yyyy

The molecules of the proton donor, and in particular water, are oxidized to produce protons, dioxygen and electrons. Depending on the nature of the nitrogen oxide treated, the stoichiometry of the products formed may vary.

[0056] The protons produced by [Reaction Formula 1] pass through the electrolyte in order to reach the fuel electrode.

At the level of the fuel electrode, the protons having passed through the electrolyte react with the nitrogen atoms of the particles of the nitride-based material arranged in the structure of the fuel electrode in order to form ammonia. The ammonia thus formed is desorbed leaving behind a nitrogen atom vacancy on the fuel electrode. The nitrogen oxides of formula N _y O _x in the gas phase brought to the level of the fuel electrode are adsorbed via their nitrogen on the gap created in the fuel electrode. The oxygen atoms of the adsorbed N _y O _x also react with the protons, thus leading to the dissociation of the N _y O _x and producing water vapour. The adsorbed nitrogen atom can in turn react with the protons coming from the electrolyte in order to produce ammonia making it possible to form a catalytic cycle. Finally, the protons dissociated from the particles of the nitride-based material can recombine and desorb in the gas phase in the form of dihydrogen. These different reactions are summarized in the following half-reaction [Reaction Formula 2]:

[0058] [Reaction formula 2] 1 1 2 ₂ 0 + - ₂ H ₂ + - 2 N ₂

The overall reaction of the electrochemical process obtained by combining [Reaction Formula 1] and [Reaction Formula 2] is shown in the following [Reaction Formula 3]:

[0060] [Reaction formula

2 -N _y 0 _x

The electrochemical process for converting nitrogen oxides into ammonia and/or dihydrogen can be carried out at high temperature, in particular at a temperature between 200° C. and 700° C., preferably between 250° C. and 650° C. °C and even more preferably between 300°C and 600°C. This temperature range makes it possible to maximize the proton conduction of the solid electrolyte as well as the electrochemical and electrocatalytic kinetics.

Claims

[Claim 1] Electrochemical device for the conversion of nitrogen oxides NOx into ammonia and/or hydrogen, comprising:

- an air electrode; - a fuel electrode, and

- a solid electrolyte disposed between the air electrode and the fuel electrode, said solid electrolyte consisting of a ceramic material, characterized in that the fuel electrode comprises particles of a nitride-based material.

[Claim 2] Electrochemical device according to claim 1, in which the material based on nitrides is a material based on metal nitrides, preferably obtained from non-precious metals such as rare earths.

[Claim 3] An electrochemical device according to any preceding claim, wherein the particles of nitride material are nanoparticles.

[Claim 4] An electrochemical device according to any preceding claim, wherein the nitride material is homogeneously distributed in the structure of the fuel electrode.

[Claim 5] An electrochemical device according to any preceding claim, wherein the air electrode is made of a ceramic material.

[Claim 6] Electrochemical device according to any one of the preceding claims, in which the fuel electrode is made of a material of the ceramic-metal composite type (cermet) comprising for example nickel and an ionic material.

[Claim 7] Electrochemical device according to any one of the preceding claims, characterized in that the said device is an electrolyser or a proton-ceramic cell, in particular a proton-conducting ceramic fuel cell or a proton-conducting ceramic electrolytic cell.

[Claim 8] An electrochemical process for the production of ammonia and dihydrogen from nitrogen oxides by means of the electrochemical device according to any one of the preceding claims, comprising:

- the circulation of said protons formed at the level of the air electrode, through the electrolyte solid, to the fuel electrode, and

- the reduction of nitrogen oxides at the fuel electrode, to form ammonia and/or dihydrogen as well as water and dinitrogen.

[Claim 9] Electrochemical process according to Claim 8, characterized in that it is carried out at a temperature of between 200°C and 700°C, preferably between 250°C and 650°C and even more preferably between 300°C and 600°C.

[Claim 10] Electrochemical process according to claim 8 or 9, characterized in that it is carried out in the gas phase, that is to say that all the reactants and products are in the gas phase.

[Claim 11] Electrochemical process according to any one of Claims 8 to 10, characterized in that the proton donor compound is water and in which the oxidation at the level of the air electrode causes the formation of a flow of oxygen-enriched water vapor.

[Claim 12] Electrochemical process according to any one of Claims 8 to 11, characterized in that the nitrogen oxides are nitrogen monoxide, nitrogen dioxide and/or nitrous oxide.