EP2766512A1 - Method for generating hydrogen and oxygen by steam electrolysis - Google Patents

Abstract

The present invention relates to a method for generating hydrogen and oxygen adsorbates by steam electrolysis at 200 to 800°C using an electrolysis cell (30) comprising a solid electrolyte (31) which is made of a proton-conducting ceramic and which is arranged between an anode (32) and a cathode (33), each of which comprises a proton-conducting ceramic, and the ratio of the electroactive surface to the geometric surface of each of which is equal to at least 10, said method comprising the following steps: circulating a current between the anode (32) and the cathode (33), wherein the density of the current is no less than 500 mA/cm²; inserting water in the form of steam, which is fed under pressure to the anode (32); oxidizing said water in the form of steam at the anode (32), and generating highly reactive oxygen at the anode (32) after said oxidation; generating protonated species in the electrolyte (31) after said oxidation and migrating said protonated species in the electrolyte (31); and reducing said protonated species at the surface of the cathode (33) in the form of reactive hydrogen atoms.

Description

 Process for generating hydrogen and oxygen by electrolysis of water vapor

The present invention relates to a method of generating highly reactive hydrogen and oxygen by electrolysis of water vapor using a proton conduction membrane.

Conductive ceramic membranes are now the subject of much research to increase their performance; in particular, these membranes find particularly interesting applications in fields such as the electrolysis of water at high temperature for the production of hydrogen or the treatment of carbonaceous gases (CO ₂ , CO) by electrochemical hydrogenation. Patent applications WO2008152317 and WO2009150352 describe examples of such methods.

Hydrogen (H ₂ ) appears today as a very interesting energy vector, which will become increasingly important for processing petroleum products, among other things, and which could, in the long term, be a good substitute for oil. and fossil fuels, whose reserves will decline sharply in the coming decades. In this perspective, however, it is necessary to develop efficient processes for producing hydrogen.

 Many hydrogen production processes have been described from different sources, but many of these processes are unsuitable for massive industrial hydrogen production.

In this context, it is possible, for example, to synthesize hydrogen from the steam reforming of hydrocarbons. One of the major problems of this synthetic route is that it generates, as by-products, significant amounts of C0 ₂ type greenhouse gases. In fact, 8 to 10 tonnes of C0 ₂ are released to produce 1 tonne of hydrogen.

 There are two challenges for future years: to find a new usable energy carrier that is safe for our environment, such as hydrogen, and to reduce the amount of carbon dioxide.

Techno-economic estimates of industrial processes now take this latter data into account. However, this is essentially sequestration, especially underground sequestration in crevices that do not necessarily correspond to old oil deposits, which eventually may not be safe.

 A promising route for the industrial production of hydrogen is the technique known as electrolysis of water vapor, for example at high temperature (EHT), at an average temperature, typically above 200 ° C., or at intermediate temperature. between 200 ° C and 1000 ° C.

 At present, two methods of producing electrolysis of water vapor are known:

According to a first method illustrated in FIG. 1, an electrolyte capable of conducting the O ^{2 -} ions and operating at temperatures generally of between 750 ° C. and 1000 ° C. is used.

More precisely, FIG. 1 schematically represents an electrolyser 1 comprising a ceramic membrane 2, conducting O ^{2 "} ions, providing the electrolyte function separating an anode 3 and a cathode 4.

The application of a potential difference between the anode 3 and the cathode 4 causes a reduction in the water vapor H ₂ 0 on the side of the cathode 4. This reduction forms hydrogen H ₂ and O ^{2 -} ions (O in the Kroger-Vink notation) on the surface of the cathode 4 according to the reaction:

2e + V ₀ + H ₂ 0 → 0 ₀ ^X + H ₂ The O ^{2 -} ions, more precisely the oxygen vacancies (Vj), migrate through the electrolyte 2 to form oxygen O ₂ on the surface of the anode 3, e electrons being released following the oxidation reaction:

0 ₀ ^X → ^ 0 ₂ + V ₀ + 2e Thus, this first method makes it possible to generate at the outlet of the electrolyser 1 oxygen - anode compartment - and hydrogen mixed with water vapor - cathode compartment.

 According to a second method illustrated in FIG. 2, an electrolyte capable of driving the protons and operating at temperatures lower than those required by the first method described above, generally between 200 ° C. and 800 ° C., is used.

 More precisely, this FIG. 2 schematically represents an electrolyzer 10 comprising a proton-conducting ceramic membrane 1 1 providing the electrolyte function separating an anode 12 and a cathode 13.

The application of a potential difference between the anode 12 and the cathode 13 causes an oxidation of the water vapor H ₂ 0 on the side of the anode 12. The water vapor introduced into the anode 12 is thus oxidized to form O ₂ oxygen and H ⁺ ions (or OH _G in the Kröger-Vink notation), this reaction releasing electrons e ^" according to the equation:

H ₂ 0 + 20 → 20H ₀ + ^ 0 ₂ + 2e '

The H ⁺ ions (or OH ₀ in the Kröger-Vink notation) migrate through the electrolyte 11, to form hydrogen H ₂ on the surface of the cathode 13 according to the equation:

2nd + 20H ₀ → 20o + H ₂

Thus, this process provides at the outlet of the electrolyzer 10 pure hydrogen - cathode compartment - and oxygen mixed with water vapor - anodic compartment.

More specifically, the formation of H ₂ passes through the formation of intermediate compounds which are hydrogen atoms adsorbed on the surface of the cathode with varying energies and degrees of interaction and / or radical hydrogen atoms ^. (or H _ode in the notation of Kröger-Vink). These species being highly reactive, they usually recombine to form hydrogen H ₂ according to the equation:

"""Electrode ^{T AA} 2

Patent application WO2008152317 has shown that the insertion of pressurized water vapor makes it possible to remain at moderate operating temperatures (of the order of 500 to 600 ° C.) while obtaining conductivity values ensured by the displacement. relatively high H ⁺ protons.

However, this type of proton conduction electrolysis is mainly studied and developed at the laboratory level with low current levels. Some fear, as in the case of O ^{2 "} conduction electrolysis, electrode delamination phenomena that can induce a disintegration between this electrode and the electrolyte when used with higher current densities.

 Indeed, unlike the field of anionic conduction electrolyses, the charge carriers (protons) are not intrinsic to the structure of the membrane and are therefore more limited in the structure than the charge carriers of anionic conduction which are formed by the gaps in the structure.

 Therefore, it is known to use in the field of anionic conduction electrolysis current densities at the terminals of the electrodes greater than the current densities used in the field of proton conduction electrolysis.

 However, the application of such current densities across the electrodes of a proton conductive electrolyzer in the state of the art electrolyzers would cause localized overvoltages which would lead to delamination phenomena of the electrodes.

In this context, the present invention aims to provide a method of generating highly reactive hydrogen and oxygen adsorbates by electrolysis of water vapor by means of an electrolysis cell comprising both a solid electrolyte proton conduction, said process being industrializable by limiting the risk of delamination of the electrodes.

 To this end, the invention proposes a process for generating adsorbates of hydrogen and oxygen by electrolysis of water vapor between 200 ° C. and 800 ° C. by means of an electrolysis cell comprising a solid electrolyte. made in a proton-conductive ceramic, said electrolyte being disposed between an anode and a cathode, said anode and cathode each comprising a proton-conduction ceramic and each having a ratio between their electroactive surface and their geometric surface at least equal to 10, said process comprising the following steps:

circulation of a current between the anode and the cathode whose density is greater than or equal to 500 mA / cm ² ;

 - insertion of water in the form of steam introduced under pressure at the anode;

 oxidizing said water in vapor form at the level of the anode;

- Generation of highly reactive oxygen at the anode following said oxidation;

 generation of protonated species in the electrolyte following said oxidation;

 migration of said protonated species into the electrolyte;

 - Reducing said protonated species on the surface of the cathode in the form of reactive hydrogen atoms.

Note that the current can be continuous or pulsed; in the case of a pulsed current, the term current density means the current density corresponding to the maximum value of the current intensity reached during the tap.

 The generation of the current can be obtained by various means:

a generator imposing a voltage across the terminals of the assembly can be used (ie a potential difference between the electrodes); a source of current imposing a current between the electrodes can be used; it is also possible to use operation in potentiostatic mode; in other words, in addition to the two cathode and anode electrodes, at least one third so-called reference electrode is used. The working electrode (preferably the cathode) will then be brought to a given potential with respect to the reference electrode (in which it is avoided to pass too much current so as not to modify its potential which serves as a reference) . The generator for automatically maintaining the potential of the working electrode, even under current, is called potentiostat.

As explained above, the term "reactive hydrogen atoms" means hydrogen atoms adsorbed on the surface of the cathode and / or radical hydrogen atoms H ^' (or H _electrode in the Kröger-Vink notation).

By geometric surface of an electrode is meant its plane outer surface and electroactive surface, the surface formed by the inner surface of the pores of the electrode in which the electrochemical reaction occurs; in other words, it is the internal surface on which the reaction occurs: 2e + 20H _Q - 20 + H ₂ . The electrodes according to the invention therefore have a large number of triple points, namely points or contact surfaces between an ion conductor, an electronic conductor and a gas phase.

The invention results from the observation made by the applicant that the use of electrodes (cathode and anode) comprising a proton-conductive ceramic (typically electrodes formed by a cermet including a mixture of said perovskite type ceramic and a metal alloy and / or perovskite doped with a lanthanide at one or more degrees of oxidation) surrounding a proton-conductive electrolyte and having a ratio electroactive surface / geometric surface sufficiently high allows to work at much higher current densities than those provided in the state of the art without risk of delamination of said electrodes. Indeed, the consequent increase in the ratio between the electrostatic surface and the geometrical surface of the electrodes compared to the ratio of the electrodes of the state of the art makes it possible to reduce the local overvoltages which are responsible for the delamination phenomena of the electrodes. .

The process according to the invention generates highly reactive hydrogen at the cathode of the electrolyser (in particular hydrogen atoms adsorbed at the surface of the electrode and / or radical).

These highly reactive hydrogen atoms H * _electrode are formed on the cathode surface according to the reaction: e ^' + OH ₀ → 0 ₀ ^x + H _E ^{x the} _electrode

These highly reactive hydrogen atoms can be used as such for the production of hydrogen or for other applications which we will detail later.

 The method according to the invention may also have one or more of the following characteristics, considered individually or in any technically possible combination:

 in a particularly advantageous manner, said ratio between the electroactive surface and the geometrical surface of said cathode and anode is greater than or equal to 100; such a ratio makes it possible to further improve the resistance of the electrodes at high current densities without the risk of delamination;

said current density is greater than or equal to 1 A / cm ² ;

 the partial and relative pressure of water vapor is advantageously greater than or equal to 1 bar and preferably greater than or equal to 10 bar;

 - The flow of current is between an anode and a cathode each made in a cermet consisting of a mixture of a proton conductive ceramic and a conductive material;

said conductive material is a passivable material with a high melting point which may comprise at least 40% of chromium; the flow of current is between an anode and a cathode each comprising a proton-conductive ceramic formed by a perovskite doped with a lanthanide at one or more oxidation states, said ceramic being doped by a complementary doping element taken from the following group : niobium, tantalum, vanadium, phosphorus, arsenic, antimony, bismuth;

the method according to the invention comprises the following steps:

o introduction of carbon dioxide C0 ₂ and / or carbon monoxide CO at the cathode of the electrolysis cell;

o reduction of CO ₂ and / or CO introduced at the cathode from said generated atoms of reactive hydrogen;

o formation of compounds of the type C _x H _y O _z, with x> 1, 0 <y ((2x + 2) and 0 <z 2 2 x following the reduction of CO ₂ and / or CO;

the method according to the invention comprises the following steps:

 o introduction of nitrogen compounds to the cathode of the electrolysis cell;

 o reducing said nitrogen compounds introduced to the cathode from said generated atoms of reactive hydrogen;

said nitrogenous compounds are compounds of the type NO _x with x> 1, said process comprising a step of forming compounds of the NtOyHz type, with t greater than or equal to 1, y greater than or equal to 0 and z greater than or equal to zero, following the reduction of NO _x ;

said nitrogenous compounds are N ₂ compounds, said process comprising a step of forming N _x H _y compounds with x> 1 and y> 0 to result in the formation of NH ₃ following reduction of N ₂ ; said reactive hydrogen atoms are used to carry out a hydrocracking step at the cathode;

said reactive hydrogen atoms are used to convert aromatic compounds to the cathode, for example saturated alkanes (paraffins) or cycloalkanes (naphthenes);

the method according to the invention comprises a step of reacting said highly reactive oxygen with a compound introduced to the anode so that the latter undergoes oxygenation. The present invention also relates to an electrolysis cell for implementing the method according to the invention comprising:

 a solid electrolyte produced in a proton conducting ceramic;

 an anode comprising a proton-conduction ceramic, each of said anodes and cathode having a ratio of its electroactive surface to its geometrical surface of at least 10;

 a cathode comprising a proton-conduction ceramic, said electrolyte being disposed between said anode and said cathode;

 means for inserting water in the form of steam introduced under pressure to the anode;

means for inducing a current flowing between the anode and the cathode whose density is greater than or equal to 500 mA / cm ² .

 Said means for inducing a current flowing between the anode and the cathode may be a voltage, current or potentios generator (in this case, the cell will also include at least one cathodic or anodic reference electrode).

 Depending on the applications, the cell may also comprise means for introducing and evacuating pressurized gas into the cathode compartment and / or means for introducing and evacuating pressurized gas into the anode compartment.

 Other features and advantages of the invention will emerge clearly from the description which is given below, as an indication and in no way limiting, with reference to the appended figures, among which:

 FIGS. 1 and 2, already described, are simplified schematic representations of steam electrolyzers,

 FIG. 3 is a general simplified schematic representation of an electrolysis cell for implementing the method according to the invention;

FIGS. 4 to 6 are illustrations of applications using the cell of FIG. FIG. 3 generally shows, schematically and simplified, an electrolysis cell 30, also called an elementary assembly, implementing the electrolysis method according to the invention.

 This electrolysis cell 30 has a structure similar to that of the device 20 of FIG. 2. Thus, the cell 30 comprises:

 an anode 32,

 a cathode 33,

 an electrolyte 31 formed by an electrolytic membrane with proton conduction,

means 34 for inducing a current flowing between the anode 32 and the cathode 33 whose density is greater than or equal to 500 mA / cm ² ,

- the means 35 for inserting water vapor pressurized pH ₂ 0 in the membrane 31 via the anode 32 (and relative partial pressure of steam is greater than or equal to 1 bar and preferentially greater than or equal to 10 bar).

 It will be noted here that the term "partial and relative pressure" refers to the insertion pressure relative to the atmospheric pressure.

 It should be noted that it is possible to use either a gaseous stream containing only steam or a gaseous stream partially containing water vapor. Thus, depending on the case, the term "partial pressure" denotes either the total pressure of the gas stream in the case where the latter consists solely of water vapor or the partial pressure of water vapor in the case where the gas stream includes other gases than water vapor.

 According to a first embodiment, the anode 32 and the cathode 33 are preferably formed by a cermet constituted by the mixture of a proton-conductive ceramic and an electrically conductive passivable alloy which is capable of forming a passive protection layer in order to protect it in an oxidizing environment (ie at the anode of an electrolyser). This passivable alloy is preferably a metal alloy

The passivable alloy comprises, for example, chromium (and preferably at least 40% of chromium) so as to have a cermet present both the special feature of not oxidizing temperature. The chromium content of the alloy is determined so that the melting point of the alloy is greater than the sintering temperature of the ceramic. By sintering temperature is meant the sintering temperature necessary to sinter the electrolyte membrane so as to make it gas tight.

 The chromium alloy may also include a transition metal so as to maintain an electronic conductive character of the passive layer. Thus, the chromium alloy is an alloy of chromium and one of the following transition metals: cobalt, nickel, iron, titanium, niobium, molybdenum, tantalum, tungsten, etc.

 The ceramic of the anode and cathode electrodes 32 and 33 is advantageously the same ceramic as that used for producing the electrolyte membrane of the electrolyte 31.

 According to an advantageous embodiment of the invention, the proton-conducting ceramic used for producing the cermet of the electrodes 32 and 33 and of the electrolyte 31 is a zirconate perovskite of formula of general formula AZrO 3 which can advantageously be doped with an element A selected from lanthanides.

 The use of this type of ceramic for the production of the membrane therefore requires the use of a high sintering temperature in order to obtain a densification sufficient to be gastight. The sintering temperature of the electrolyte 31 is more particularly defined according to the nature of the ceramic but also according to the desired porosity level. Conventionally, it is estimated that to be gas-tight, the electrolyte 31 must have a porosity of less than 6% (or a density greater than 94%).

Advantageously, the sintering of the ceramic is carried out under a reducing atmosphere so as to avoid the oxidation of the metal at high temperature, that is to say under an atmosphere of hydrogen (H ₂ ) and argon (Ar) or even carbon monoxide (CO) if there is no risk of carburation. The electrodes 32 and 33 of the cell 30 are also sintered at a temperature above 1500 ° C (according to the example of sintering a zirconate type ceramic).

According to a second embodiment, the anode 32 and the cathode 33 may also be formed by a ceramic material which is a perovskite doped with a lanthanide. Perovskite can be a zirconate of formula AZr0 ₃ . The zirconate is doped with a lanthanide which is, for example, erbium. In addition, the lanthanide-doped perovskite is doped with a doping element taken from the following group: niobium, tantalum, vanadium, phosphorus, arsenic, antimony, bismuth. These doping elements are chosen to dope the ceramic since they can go from an oxidation degree of 5 to an oxidation degree of 3, which allows to release oxygen during sintering. More specifically, the doping element is preferably niobium or tantalum. Each electrode may also comprise a metal mixed with the ceramic so as to form a cermet. The ceramic comprises for example between 0.1% and 0.5% by weight of niobium, between 4 and 4.5% by weight of erbium and the remainder of zirconate. Boosting the ceramic with niobium, tantalum, vanadium, phosphorus, arsenic, antimony or bismuth makes the ceramic conductive electrons. The ceramic is then a mixed conduction ceramic; in other words, it is conductive to both electrons and protons, whereas in the absence of these doping elements, the perovskite doped with a lanthanide with a single oxidation state is not electron conducting. . Such a configuration makes it possible to have electrodes made of a material of the same nature as the solid electrolyte which has good conductivity of both protons and electrons, even when the ceramic is not mixed. to a metal (as is the case of the first embodiment).

 According to the invention, the electrodes 32 and 33 of the cell 30 are designed to have a ratio between their electroactive surface and their geometric surface at least equal to 10 and preferably greater than or equal to 100.

By geometric surface is meant the plane outer surface of the electrode, that is to say the surface receiving the flow of electrons. By specific (or developed) surface is meant the surface accessible to a gas within the electrode: it is therefore essentially constituted by the inner surface of the pores.

 By electroactive surface is meant that part of the specific surface on which the electrochemical reaction occurs; in other words, it is the internal surface on which the reaction occurs:

H ₂ 0 + 20o → 20H ₀ + θ ₂ + 2e '2e + 20H ₀ → 20o + H ₂ .

According to the invention, the means 34 make it possible to inject a current flowing between the anode 32 and the cathode 33 whose density is greater than or equal to 500 mA / cm ² and preferably greater than or equal to 2 A / cm ² without risk. of current drop or delamination of the electrodes

 The applicant has advantageously found that the fact of using electrodes made of a proton-conduction material and having a sufficient electroactive surface (advantageously greater than or equal to 100) makes it possible to increase significantly the usable current density without the risk of delamination of the electrodes. .

 The determination of the ratio between the electroactive surface and the geometrical surface is carried out for example by means of a method for characterizing the porous surface of a cermet electrode detailed in the publication "Characterization of porous texture of cermet electrode for steam electrolysis". At Intermediate Temperature, C. Deslouis, M. Keddam, K. Rahmouni, H. Takenouti, F. Grasset, O. Lacroix, B. Sala, Electrochimica Acta 56 (201 1) 7890-7898.

 The general operation of the cell is described below.

The circulation of the current between the anode 32 and the cathode 33 causes an oxidation of the water vapor H ₂ O on the side of the anode 32. The water vapor introduced under pressure into the anode 32 is thus oxidized to to form O ₂ oxygen and H ⁺ ions (or OH ₀ in the Kröger-Vink notation), this reaction releasing electrons e ^" according to the equation: H ₂ 0 + 20 → 20H ₀ + ^ 0 ₂ + 2e '

The H ⁺ ions (or OH ₀ in the Kröger-Vink notation) migrate through the electrolyte 31, to form hydrogen H ₂ on the surface of the cathode 33 according to the equation:

2nd + 20H ₀ → 20o + H ₂

Thus, this process provides at the outlet of the cell 30 pure hydrogen - cathode compartment - and oxygen mixed with water vapor - anodic compartment.

More specifically, the formation of H ₂ passes through the formation of intermediate compounds which are hydrogen atoms adsorbed on the surface of the cathode 33 and / or radical hydrogen atoms H ^' (or H _{j Lctrode} in the notation of Kröger-Vink). These species being highly re-active,

or they recombine to form hydrogen H ₂ according to the equation: 2H _electrode → H ₂ (see FIG. 3);

 or they react with other compounds injected on the cathode side 33 (as will be seen with reference to FIGS. 4 and 4).

It should be noted that the oxygen atoms adsorbed on the surface of the anode 32 can advantageously be used to produce the oxygen adsorbate C> E _{electrode that} can be used in an anode oxygenation reaction, by for example by injecting SO ₂ sulfur dioxide or SOx into the anode which reacts with oxygen to form sulfuric acid H ₂ SO ₄ or to make oxygen for oxyfuel combustion. We have for example the following equations:

H ₂ 0 + 20 ₀ ^x → 20H ₀ + + 2e '

H ₂ 0 + SO _x + (3 - x) 0 ^ _trode → H ₂ SQ ₄ O Electrode

Regarding the T1 operating temperature of the device 30, the latter depends on the type of material used for the membrane 31; in any case, this temperature is greater than 200 ° C and generally less than 800 ° C, or even lower than 600 ° C. This operating temperature corresponds to a conduction provided by H ⁺ protons.

 Figures 4 and following each illustrate a particular use of the cell of Figure 3 wherein the highly reactive hydrogen is used to recombine with other compounds at the cathode

33.

FIG. 4 illustrates a first example in which the electrolysis cell 30 is used to form compounds of the CxH _y Oz type, (with x≥1, 0 <y≤ (2x + 2) and 0 <z≤2x). to the reduction of C0 ₂ and / or CO.

The cell 30 of FIG. 3 further comprises means 36 for inserting gas (pCO ₂ or / and CO) into the cathode compartment 33 under pressure.

At the anode 32, the water is oxidized by releasing electrons while H ⁺ ions (in OH _G form) are generated.

These H ⁺ ions migrate through the electrolyte 31 and are therefore capable of reacting with various compounds that would be injected at the cathode 33, the carbon compounds of the CO ₂ and / or CO type reacting at the cathode 33 with these H ⁺ ions for form compounds of type C _x H _y O _z (with x> 1, 0 <y≤ (2x + 2) and 0 <z≤2x) and water at the cathode.

 The chemical equations of the different reactions can be written in particular:

(6n + 2) H X

Electrode + nC0 ₂ → C _n H _{2n + 2} + 2nH ₂ 0

 6nH X

Electrode + nC0 ₂ → C _n H _2n + 2nH ₂ 0

 6nH X

Electrode + nC0 ₂ → C _n H _{2n + 2} 0+ ( _2n- 1) H ₂ 0

 (6n- 2) H X

Electrode + nC0 ₂ → C _n H _2n O + (2n-1) H ₂ 0 The nature of the compound formed depending on the operating conditions, the global formation reaction of CxH _y O _z can therefore be written:

(4x-2z + y) H _ode + xC0 ₂ → C _x H _y O _z + (2x-z) H ₂ 0

The nature of the compounds C _x H _y O _z synthesized at the cathode depends on many operating parameters such as, for example, the pressure of the cathode compartment, the partial pressure of the gases, the operating temperature T1, the potential / current torque applied to the cathode, the residence time of the gas and the nature of the electrodes.

As regards the gas pressure, the relative pressure of CO ₂ and / or CO is greater than or equal to 1 bar and less than or equal to the rupture pressure of the assembly.

 It should be noted that the total pressure imposed in one compartment - catalytic or anodic - can be compensated in the other compartment so as to have a pressure difference between the two compartments to prevent the rupture of the membrane assembly, the support electrode if the one - Ci at a breaking strength too low.

The temperature T1 of operation of the device 30 also depends, in the range between 200 and 800 ° C, of the nature of the carbon compounds CxH _y Oz that it is desired to generate.

5 illustrates a second example in which the electrolytic cell 30 is used for reducing compounds of the type NO _x (x <2) to form compounds of the type N _t O _y H _z, (with t> 1, y > 0 and z> 0).

The cell 30 of FIG. 3 further comprises means 36 for inserting under pressure NO _x (x <2) type compounds into the cathode compartment 33.

The problem consists in allowing the reduction by electro-catalytic hydrogenation of the NO _x content of the effluents produced for example during the combustion of hydrocarbons or gases. The production of these molecules is made up to 60% by urban transport and 40% by boilers and thermal power plants. These molecules easily penetrate the bronchioles and affect breathing, causing hyperreactivity bronchial tubes in asthmatics; and increased susceptibility of bronchial tubes to microbes, at least in children. As a result, current regulations require industries to limit their releases to ΝΟχ.

It is known to one skilled in the art to reduce NO _x by two types of process: selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). Whatever solution is used (ie with or without a catalyst), the latter is based on the use of ammonia to reduce NO _x to N ₂ . These solutions have the disadvantage of using ammonia as a hydrogen carrier whereas it would be more interesting to directly treat NO _x with hydrogen. The production of ammonia assumes the use of a process for steam reforming hydrocarbons generating CO ₂ . This process also involves the use of a second reactor for the production of ammonia.

The method of using the cell 30 according to FIG. 5 is based on the following principle: pressurized water vapor is introduced at the level of the anode compartment 32 and the NO _x is introduced under pressure at the cathode compartment 33. The incorporation under pressure of water vapor will cause an oxidation of this water in the form of steam on the surface of the anode so as to generate protonated species in the membrane which, after migration within the membrane, are reduced on the surface of the cathode very reactive hydrogen capable of reducing by hydrogenation NO _x introduced into the cathode compartment such that the NO _x are reduced to NO _y (with _{y x x} ) less oxidized then nitrogen and ammonia.

 So, in the proposed solution, we associate in a single reactor

(ie cell 30) proton generation and electro-catalytic hydrogenation of NO _x .

Adsorbates monatomic hydrogen are formed at the cathode surface 33 according to the reaction: e ^'+ OH → 0 _{Q 0} ^x + H _electrode.

In fact, in the presence of NO _x on the cathode side 33, the highly reactive _electrolyte adsorbates H react with the nitrogen compounds at the cathode 33 to give reduced compounds of nitrogen oxides of the type N _t O _y H _z.

with x> 1 and y> 0 and z> 0 depending on the reaction:

t O _x + (2tx - 2y + z) II, L, → (* -) H ₂ O + N _t O _y H _z .

By way of example, these compounds are either NO _and less oxidized than the NO _x compounds introduced under pressure, N ₂ nitrogen, or NH ₃ ammonia.

 The global reactions to the electrodes are written:

H ₂ 0+ 20o → 20H ₀ + -0 ₂ + 2e '(anode 32)

tNO _x + (ls - 2y + z) e + (ls - 2y + z) OH _G → (ls - 2y + Z) O _Q + (is - y) H ₂ O + N _t O _y H _z (cathode 33).

 The solution according to the invention makes it possible to reduce the number of reactors required for the reduction of NOx to a single and only reactor seat of the electro-hydrogenation.

Figure 6 illustrates a third example in which the electrolysis cell 30 is used to produce ammonia by electro-catalytic hydrogenation of N ₂ . It should be noted that, according to this embodiment, it is also possible to produce other N _x H _y compounds with x> 1 and y> 0 before leading to the formation of NH ₃ .

The cell 30 of FIG. 3 further comprises means 36 for inserting pressurized nitrogen N ₂ under pressure into the cathode compartment 33.

The problem here is to produce in massive quantity, at low cost and without emission of CO ₂ of the ammonia, by electro-catalytic hydrogenation of N ₂ .

Currently ammonia is produced by catalytic reaction of hydrogenation of N ₂ during the steam reforming of hydrocarbons. The synthesis of this product is therefore indirectly emitting CO ₂ . Moreover, the method of synthesis induces a very high volatility of the production price of

NH ₃ . Indeed 80% of the price of NH ₃ is directly dependent on the price of the gas from which is produced the hydrogen necessary for the synthesis. Thus, the volatility of the price of ammonia is very large and dependent on that of gas. In addition, according to the known techniques, even when the hydrogen is produced by electrolysis, it is necessary to use two reactors, one for the production of hydrogen and the other for the catalytic reaction.

 The solution implemented in cell 30 of Figure 6 is to produce ammonia using a single reactor.

As before, the monoatomic hydrogenated compounds are formed on the cathode surface according to the reaction: e + OH ₀ → 0 ₀ ^x + ¾ _atod ,

Thereby, in the presence of N ₂ on the cathode side 33, hydrogen ° ^{tr "Sc} reagent reacts with the hydrogenated compounds on the electrode 33 to give NH ₃ according to the reaction: N ₂ + 6H _{lK ^; trode} → 2NH ₃ .

 The global reactions to the electrodes are written:

H ₂ 0 + 20o → 20H ₀ + -0 ₂ + 2e

N ₂ + 6e + 60H ₀ → 60 + 2NH ₃

The solution of FIG. 6 makes it possible to reduce the number of reactors necessary for the production of NH ₃ (which serves as an H ₂ vector) for a single electro-hydrogenation seat reactor.

In the proposed solution, the hydrogen needed to reduce nitrogen is no longer produced from fossil fuels; the process according to the invention is "cleaner" insofar as it does not generate C0 ₂ .

 In addition, such a method makes it possible to dispense with the use of catalyst, which it is necessary to change and recycle because of its deactivation by the water produced during the catalytic reduction reaction.

Finally, the proposed solution avoids storage of H ₂ since the reactive hydrogen production and hydrogenation reduction reactions take place in the same reactor.

As we have seen in reference to FIGS. 4 to 6, the highly reactive hydrogen produced by the cell 30 of FIG. 3 can be used industrially for very different applications. Well understood, the invention is not limited to the embodiments which have just been described. Thus, hydrogenation by highly reactive hydrogen atoms can also be used in the petrochemical industry, for example to convert aromatic compounds to saturated alkanes (paraffins) and cycloalkanes (naphthenes). The process according to the invention can also be used for hydrocracking for converting heavy petroleum products into light products under hydrogen pressure and at a sufficiently high temperature: typically, hydrocracking makes it possible to obtain products such as diesel or kerosene from heavy residues.

Claims

Process for the generation of hydrogen and oxygen adsorbates by electrolysis of steam between 200 ° C. and 800 ° C. by means of an electrolysis cell (30) comprising a solid electrolyte (31) produced in a protonically conductive ceramic, said electrolyte (31) being disposed between an anode (32) and a cathode (33), said anode and cathode each having a proton-conductive ceramic and each having a ratio between their electrically active surface and their surface at least 10, said method comprising the following steps: - circulation of a current between the anode (32) and the cathode (33) whose density is greater than or equal to 500 mA / cm ² ; - insertion of water in the form of steam introduced under pressure to the anode (32);  oxidizing said water in vapor form at the level of the anode (32);  - Generation of highly reactive oxygen at the anode (32) following said oxidation;  generation of protonated species in the electrolyte (31) following said oxidation;  migration of said protonated species into the electrolyte (31); - Reducing said protonated species on the surface of the cathode (33) in the form of reactive hydrogen atoms. Method according to the preceding claim characterized in that said ratio between the electroactive surface and the geometric surface of said cathode and anode is greater than or equal to 100. 3. Method according to one of the preceding claims characterized in that said current density is greater than or equal to 1 A / cm ² . Process according to one of the preceding claims, characterized in that the partial and relative pressure of water vapor is advantageously greater than or equal to 1 bar and preferably greater than or equal to 10 bar. Method according to one of the preceding claims, characterized in that the flow of current is between an anode and a cathode each made in a cermet consisting of a mixture of a proton conductive ceramic and a conductive material. Method according to one of the preceding claims characterized in that said conductive material is a passivable material with high melting point may contain at least 40% chromium. Method according to one of the preceding claims, characterized in that the flow of current is between an anode and a cathode each comprising a proton-conductive ceramic formed by a perovskite doped with a lanthanide at one or more oxidation levels. 8. Method according to one of the preceding claims characterized in that it comprises the following steps: introduction of carbon dioxide C0 ₂ and / or carbon monoxide CO to the cathode of the electrolysis cell; - reduction of CO ₂ and / or CO introduced at the cathode from said generated atoms of reactive hydrogen; - formation of compounds of the type C _x H _y O _z, with x> 1, 0 <y ((2x + 2) and 0 <z 2 2 x following the reduction of CO ₂ and / or CO. 9. Method according to one of claims 1 to 7 characterized in that it comprises the following steps: introduction of nitrogen compounds to the cathode of the electrolysis cell;  - Reducing said nitrogenous compounds introduced to the cathode from said generated atoms of reactive hydrogen. 10. Process according to the preceding claim, characterized in that the said nitrogen compounds are compounds of the type NO _x with x> 1, the said process comprising a step of forming compounds of the type N _t O _y H _z , with t higher or equal to 1, y greater than or equal to 0 and z greater than or equal to zero, following the reduction of NO _x . 1 1. Process according to Claim 9, characterized in that the said nitrogenous compounds are N ₂ compounds, the said process comprising a step of forming N _x H _y compounds with x> 1 and y> 0 to result in the formation of NH _3. to the reduction of N ₂ . 12. Method according to one of claims 1 to 7 characterized in that said reactive hydrogen atoms are used to perform a hydrocracking step at the cathode. 13. Method according to one of claims 1 to 7 characterized in that said reactive hydrogen atoms are used to convert aromatic compounds to the cathode. 14. Method according to one of the preceding claims characterized in that it comprises a step of reacting said highly reactive oxygen with a compound introduced to the anode so that the latter undergoes oxygenation. 15. Electrolysis cell for implementing the method according to one of the preceding claims, comprising: a solid electrolyte produced in a proton conducting ceramic;  an anode comprising a proton-conduction ceramic, each of said anodes and cathode having a ratio of its electroactive surface to its geometrical surface of at least 10;  a cathode comprising a proton-conduction ceramic, said electrolyte being disposed between said anode and said cathode;  means for inserting water in the form of steam introduced under pressure to the anode; means for inducing a current flowing between the anode and the cathode whose density is greater than or equal to 500 mA / cm ² .