U.S. patent application number 12/992740 was filed with the patent office on 2011-06-09 for process for producing compounds of the cxhyoz type by reduction of carbon dioxide (co2) and/or carbon monoxide (co).
Invention is credited to Olivier Lacroix, Beatrice Sala.
Application Number | 20110132770 12/992740 |
Document ID | / |
Family ID | 40122470 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110132770 |
Kind Code |
A1 |
Sala; Beatrice ; et
al. |
June 9, 2011 |
PROCESS FOR PRODUCING COMPOUNDS OF THE CXHYOZ TYPE BY REDUCTION OF
CARBON DIOXIDE (CO2) AND/OR CARBON MONOXIDE (CO)
Abstract
The present invention relates to a process for electrolysing
steam introduced under pressure into an anode in compartment (32)
of an electrolyser (30) provided with a proton-conducting membrane
(31) made of a material that allows protonated species to be
incorporated into this membrane under steam, water injected in
steam form being oxidized at the anode (32) so as to generate
protonated species in the membrane that migrate within this same
membrane and are reduced at the surface of the cathode (33) in the
form of reactive hydrogen atoms capable of reducing carbon dioxide
and/or carbon monoxide, said process comprising the following
steps: injection of CO.sub.2 and/or CO under pressure into the
cathode compartment (33) of the electrolyser (30); -reduction of
the CO.sub.2 and/or CO injected into the cathode compartment (33)
by said reactive hydrogen atoms generated, in such a way that the
CO.sub.2 and/or the CO form compounds of the C.sub.xH.sub.yO.sub.z
type where x>1, y is between 0 and 2x+2 and z is between 0 and
2x.
Inventors: |
Sala; Beatrice; (St. Gely du
Fesc, FR) ; Lacroix; Olivier; (Montpellier,
FR) |
Family ID: |
40122470 |
Appl. No.: |
12/992740 |
Filed: |
May 15, 2009 |
PCT Filed: |
May 15, 2009 |
PCT NO: |
PCT/FR2009/050909 |
371 Date: |
February 4, 2011 |
Current U.S.
Class: |
205/338 |
Current CPC
Class: |
C25B 3/25 20210101 |
Class at
Publication: |
205/338 |
International
Class: |
C25B 3/04 20060101
C25B003/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 15, 2008 |
FR |
0853161 |
Claims
1. A process for electrolyzing steam injected under pressure into
an anode compartment of an electrolyzer provided with a
proton-conducting membrane made of a material enabling protonated
species to be incorporated into this membrane under steam,
oxidation of the water injected in steam form taking place at the
anode so as to generate protonated species in the membrane that
migrate within this same membrane and are reduced at the surface of
the cathode in the form of reactive hydrogen atoms capable of
reducing carbon dioxide CO.sub.2 and/or carbon monoxide CO, said
process comprising the following steps: the introduction of
CO.sub.2 and/or CO under pressure into the cathode compartment of
the electrolyzer, the reduction of CO.sub.2 and/or CO introduced
into the cathode compartment from said reactive hydrogen atoms
generated such that the CO.sub.2 and/or CO form
C.sub.xH.sub.yO.sub.x type compounds, with x.gtoreq.1; y is between
0 and 2x+2 and z is between 0 and 2x.
2. The electrolysis process according to claim 1, wherein the
process comprises a step of controlling the nature of the
C.sub.xH.sub.yO.sub.z type compounds formed according to the
voltage-current pair applied to the cathode.
3. The electrolysis process according to claim 1 wherein the
process comprises a step of the utilization of a proton-conducting
membrane that is impermeable to the diffusion of oxygen O.sub.2 and
H.sub.2 and enables the incorporation of protonated species into
this membrane under steam pressure.
4. The electrolysis process according to claim 3, wherein the
process comprises a step of utilizing a proton-conducting membrane
of the type: perovskite vacancies, non-stoichiometric perovskites
and/or perovskites doped with general formula ABO.sub.3, of
fluorite, pyrochlore A.sub.2B.sub.2X.sub.7, apatite
Me.sub.10(XO.sub.4).sub.6Y.sub.2, oxyapatite
Me.sub.10(XO.sub.4).sub.6O.sub.2 structure, of hydroxylapatite
Me.sub.10(XO.sub.4).sub.6(OH).sub.2 structure, of silicate
structure, aluminosilicates, phyllosilicates, zeolite, silicates
grafted with oxyacids or silicates grafted with phosphates.
5. The electrolysis process according to claim 4, wherein the
process comprises a step of utilizing, as a proton-conducting
membrane, an electrolyte supported by the cathode or by the anode
so as to reduce its thickness in order to increase its mechanical
strength.
6. The electrolysis process according to claim 1, wherein the
process comprises a step of the utilization of a relative partial
pressure of steam greater than or equal to 1 bar and less than or
equal to a burst pressure of the assembly, the latter being greater
than or equal to at least 100 bars.
7. The electrolysis process according to claim 1, wherein the
relative partial pressure of steam is advantageously greater than
or equal to 50 bars.
8. The electrolysis process according to claim 1, wherein the
relative pressure of CO.sub.2 and/or CO is greater than or equal to
1 bar and less than or equal to the burst pressure of the assembly,
the latter being greater than or equal to at least 100 bars.
9. The electrolysis process according to claim 1, wherein the
electrolysis temperature is greater than or equal to 200.degree. C.
and less than or equal to 800.degree. C., advantageously between
350.degree. C. and 650.degree. C.
10. The electrolysis process according to claim 1, wherein the
electrodes, of porous structure, are either ceramic-metal
materials, or "ceramic" electrodes with mixed electronic and ionic
conduction.
11. The electrolysis process according to claim 10, wherein the
ceramic-metal materials are, for the cathode, ceramic-metal
materials in which the ceramic is compatible with the electrolyte
forming the membrane and in which the nature of the metal dispersed
is advantageously a metal and/or a metal alloy among which metals
such as cobalt, copper, molybdenum, silver, iron, zinc, noble
metals (gold, platinum, palladium) and/or transition elements may
be cited.
12. The electrolysis process according to claim 10, wherein the
ceramic-metal materials are, for the anode, ceramic-metal materials
in which the ceramics are compatible with the electrolyte forming
the membrane and in which the nature of the metal dispersed is
advantageously a metal alloy or a passivable metal.
13. A steam electrolysis device for electrolyzing steam introduced
under pressure into an anode compartment of an electrolyzer
provided with a proton-conducting membrane, made of a material
enabling the injection of protonated species into this membrane
under steam after oxidation, comprising: an electrolyte in the form
of an ion conducting membrane made in said material enabling the
injection of protonated species under the effect of the water
pressure in said membrane, an anode, a cathode, a generator
enabling current to be generated and to apply a potential
difference between said anode and said cathode, means for the
insertion of steam under pressure in said electrolyte via said
anode, means to inject CO.sub.2 and/or CO under pressure into the
cathode compartment of the electrolyzer, and means to reduce the
CO.sub.2 and/or the CO introduced into the cathode compartment
according to a process in conformance with one of the previous
embodiments.
14. The device according to claim 13, wherein the material enabling
the injection of protonated species is impermeable to O.sub.2 and
H.sub.2 gases.
15. The device according to claim 13, wherein the material enabling
the injection of protonated species has a densification level of
over 88%, preferably equal at least to 94%.
16. The device according to claim 13, wherein the material enabling
the injection of protonated species is an oxygen atom-defective
oxide such as an oxygen-defective perovskite acting as a proton
conductor.
17. The device according to claim 16, wherein the oxygen
atom-defective oxide presents stoichiometric intervals and/or is
doped.
Description
[0001] The present invention relates to a process for producing
compounds of the C.sub.xH.sub.yO.sub.z type, particularly with
x>1; y is between 0 and 2x+2 and z is between 0 and 2.times., by
reduction of carbon dioxide (CO.sub.2) and/or carbon monoxide (CO),
particularly from very reactive hydrogen species generated by water
electrolysis.
[0002] Today ceramic conducting membranes are the subject of much
research to increase their performance; these membranes find
particularly interesting applications, among others, in the fields
of: [0003] water electrolysis at high temperature for producing
hydrogen, [0004] in the treatment of carbon-containing gas
(CO.sub.2, CO) by electrochemical hydrogenation to obtain compounds
of the C.sub.xH.sub.yO.sub.z type (x>1; y is between 0 and 2x+2
and z is between 0 and 2x).
[0005] Today hydrogen (H.sub.2) appears to be a very interesting
energy carrier, that is called upon to take on more and more
importance to treat, among other substances, petroleum, oils and
lubricants, and that may, in the end, be advantageously substituted
for petroleum and fossil energies, whose reserves are going to
sharply decrease in the decades to come. However, in this
perspective, it is necessary to develop effective hydrogen
production processes.
[0006] Admittedly, many hydrogen production processes have been
described from various sources, but a number of these processes
have proved to be unsuitable for the massive industrial production
of hydrogen.
[0007] In this context, one may, for example, cite the synthesis of
hydrogen from steam reforming hydrocarbons. One of the main
problems with this synthetic pathway is that it produces, as
by-products, significant quantities of CO.sub.2-type greenhouse
gases. In fact, 8 to 10 tons of CO.sub.2 are released to produce 1
ton of hydrogen.
[0008] Therefore two challenges are presented for future years: To
search for a new energy carrier that is usable without danger to
our environment such as hydrogen, and to reduce the quantity of
carbon dioxide.
[0009] The techno-economic valuations of industrial processes now
take the latter data into account. However, it is essentially
sequestration, in particular underground sequestration in crevices
that do not necessarily correspond to old oil deposits, which in
the end cannot be without danger.
[0010] It would seem wise to recycle this carbon dioxide in the
form of compounds usable in the chemistry field or in the energy
production field. The energy necessary for this transformation
would be electricity, for example of nuclear origin, and in
particular that from reactors such as HTR "High Temperature
Reactor" type nuclear reactors or EPR (registered trademark)
European pressurized water reactors.
[0011] A promising path for industrial hydrogen production is the
technique known as steam electrolysis, for example at high
temperature (EHT), at medium temperature, typically over
200.degree. C., or even at an intermediate temperature of between
200.degree. C. and 1000.degree. C.
[0012] Two steam electrolysis production processes are presently
known:
[0013] According to a first process illustrated in FIG. 1, an
electrolyte capable of carrying O.sup.2- ions and operating at
temperatures that are generally between 750.degree. C. and
1000.degree. C. is utilized.
[0014] More precisely, FIG. 1 schematically represents an
electrolyser 1 comprising a ceramic membrane 2, conductor of
O.sup.2- ions, ensuring the electrolyte function separating an
anode 3 and a cathode 4.
[0015] The application of a potential difference between anode 3
and cathode 4 leads to a reduction in H.sub.2O steam from the
cathode 4 side. This reduction forms hydrogen H.sub.2 and ions
O.sup.2- (O.sub.o.sup.x in Kroger-Vink notation) at the surface of
the cathode 4 according to the reaction:
2e'+V.sub.o.sup...+H.sub.2O.fwdarw.O.sub.o.sup.x+H.sub.2
[0016] The O.sup.2- ions, more precisely the oxygen vacancies
(V.sub.o.sup...), migrate through electrolyte 2 to form oxygen
O.sub.2 at the surface of anode 3, electrons e being released
according to the oxidation reaction:
O o x .fwdarw. 1 2 O 2 + V o .. + 2 e ' ##EQU00001##
[0017] Thus, the first process enables, from the output of the
electrolyzer 1, oxygen--anode compartment--and hydrogen mixed with
steam--cathode compartment--to be generated.
[0018] According to a second process illustrated in FIG. 2, an
electrolyte capable of carrying the protons and operating at lower
temperatures than those required for the first process described
above, generally between 200.degree. C. and 800.degree. C., is
utilized.
[0019] More precisely, this FIG. 2 schematically represents an
electrolyzer 10 comprising a proton-conducting ceramic membrane 11
ensuring the electrolyte function separating an anode 12 and a
cathode 13.
[0020] The application of a potential difference between anode 12
and cathode 13 leads to oxidation of the H.sub.2O steam from the
anode side 12. The steam injected in anode 12 is thus oxidized to
form oxygen O.sub.2 and H.sup.+ ions (or OH.sub.o.sup..in
Kroger-Vink notation), this reaction releasing electrons
e--according to the equation:
H 2 O + 2 O o x .fwdarw. 2 OH o . + 1 2 O 2 + 2 e '
##EQU00002##
[0021] The H.sup.+ ions (or OH.sub.o.sup..Kroger-Vink notation)
migrate through the electrolyte 11 to form hydrogen H.sub.2 at the
surface of cathode 13 according to the equation:
2e'+2OH.sub.o.sup...fwdarw.2O.sub.o.sup.x+H.sub.2
[0022] Thus, this process provides, from the output of electrolyzer
10, pure hydrogen--cathode compartment--and oxygen mixed with
steam--anode compartment.
[0023] More precisely, the formation of H.sub.2 passes by the
formation of intermediate compounds that are hydrogen atoms
adsorbed at the surface of the cathode with variable energies and
degrees of interaction and/or radical hydrogen atoms H.sup..cndot.
(or H.sub.Electrode.sup.x in Kroger-Vink notation). As these
species are highly reactive, they usually recombine to form
hydrogen H.sub.2 according to the equation:
2H.sub.Electrode.sup.x.fwdarw.H.sub.2
[0024] The present invention aims to reduce the quantity of
existing carbon dioxide, for example by recycling this carbon
dioxide in the form of compounds usable in the chemistry field or
in the energy production field.
[0025] For this purpose, the invention proposes a process for
electrolyzing water steam injected under pressure into an anode
compartment of an electrolyzer provided with a proton-conducting
membrane made of a material enabling protonated species to be
incorporated into this membrane under steam, oxidation of the water
injected in steam form taking place at the anode so as to generate
protonated species in the membrane that migrate within this same
membrane and are reduced at the surface of the cathode in the form
of reactive hydrogen atoms capable of reducing carbon dioxide
CO.sub.2 and/or carbon monoxide CO, said process comprising the
following steps: [0026] injection of CO.sub.2 and/or CO under
pressure into the cathode compartment of the electrolyzer, [0027]
the reduction of the CO.sub.2 and/or CO injected into the cathode
compartment by said reactive hydrogen atoms generated such that the
CO.sub.2 and/or the CO form compounds of the C.sub.xH.sub.yO.sub.z,
type, with x>1; y is between 0 and 2x+2 and z is between 0 and
2x.
[0028] As explained above, reactive hydrogen atoms is understood to
refer to hydrogen atoms adsorbed at the surface of the cathode with
variable energies and degrees of interaction and/or radical
hydrogen atoms H.sup..cndot. (or H.sub.Electrode.sup.x in
Kroger-Vink notation).
[0029] The invention results from the observation that the second
process described above generates highly reactive hydrogen at the
electrolyzer cathode (particularly hydrogen atoms adsorbed at the
surface of the electrode and/or radical hydrogen atoms).
[0030] These highly reactive hydrogen atoms H.sub.Electrode.sup.x
are formed at the surface of the cathode according to the
reaction:
e'+OH.sub.o.sup...fwdarw.O.sub.o.sup.x-H.sub.Electrode.sup.x
[0031] In fact, in the presence of CO.sub.2 and/or CO on the
cathode side, the highly reactive hydrogen H.sub.Electrode.sup.x
reacts with the carbon compounds on the electrode to give reduced
carbon dioxide and/or carbon monoxide compounds of the
C.sub.xH.sub.yO.sub.z type with x>1; y is between 0 and 2x+2 and
z is between 0 and 2x.
[0032] By way of example, these compounds are paraffins
C.sub.nH.sub.2n.sub.+2, olefins C.sub.2nH.sub.2n, alcohols
C.sub.nH.sub.2n+2OH or C.sub.nH.sub.2n-1OH, aldehydes and ketones
C.sub.nH.sub.2nO, acids C.sub.n-1H.sub.2n+1 with n>1.
[0033] The invention thus enables steam to be electrolyzed with the
joint carbon dioxide and/or carbon monoxide electroreduction as
described subsequently.
[0034] The process according to the invention may also present one
or more of the characteristics below, considered individually or
according to all technically possible combinations: [0035] the
process comprises a step of controlling the nature of the
C.sub.xH.sub.yO.sub.z, type compounds, formed according to the
voltage-current pair applied to the cathode; [0036] the process
comprises a step of utilizing a proton-conducting membrane that is
impermeable to the diffusion of oxygen O.sub.2 and H2 allowing the
injection of protonated species into the membrane under steam
pressure; [0037] the process comprises a step of utilizing a
proton-conducting membrane of the type: perovskite vacancies,
non-stoichiometric perovskites and/or perovskites doped with
general formula ABO.sub.3, of fluorite, pyrochlore
A.sub.2B.sub.2X.sub.7, apatite Me.sub.10(XO.sub.4).sub.6Y.sub.2,
oxyapatite Me.sub.10(XO.sub.4).sub.6O.sub.2 structure, of
hydroxylapatite Me.sub.10(XO.sub.4).sub.6(OH).sub.2 structure, of
silicate, aluminosilicate (phyllosilicate or zeolite) structure,
silicates grafted with oxyacids or silicates grafted with
phosphates; [0038] the process comprises a step of utilizing an
electrolyte supported by the cathode or by the anode so as to
reduce its thickness in order to increase its mechanical strength;
[0039] the process comprises a step of utilizing relative partial
steam pressure greater than or equal to 1 bar and less than or
equal to a burst pressure of the assembly, the latter being greater
than or equal to at least 100 bars; [0040] the relative partial
steam pressure is advantageously greater than or equal to 50 bars;
[0041] the relative pressure of CO.sub.2 and/or CO is greater than
or equal to 1 bar and less than or equal to a burst pressure of the
assembly, the latter being greater than or equal to at least 100
bars; [0042] the electrolysis temperature is greater than or equal
to 200.degree. C. and less than or equal to 800.degree. C.,
advantageously between 350.degree. C. and 650.degree. C.; [0043]
the electrodes, of porous structure, are either of ceramic-metal
materials or "ceramic" electrodes of mixed electronic and ionic
conduction; [0044] the ceramic-metal materials are, for the
cathodes, ceramics compatible with the electrolyte in which the
nature of the metal dispersed is advantageously a metal and or a
metal alloy, among which one may cite metals such as cobalt,
copper, molybdenum, silver, iron, zinc, noble metals (gold,
platinum, palladium) and/or transition elements; [0045] the
ceramic-metal materials are, for the anodes, ceramics compatibles
with the electrolyte in which the nature of the metal dispersed is
advantageously a metal alloy or a passivable metal.
[0046] The invention also relates to a steam electrolysis device
for electrolyzing water steam introduced under pressure into an
anode compartment of an electrolyzer provided with a
proton-conducting membrane, made of a material enabling the
injection of protonated species into this membrane under steam
after oxidation, comprising: [0047] an electrolyte in the form of
an ion conducting membrane made in said material enabling the
injection of protonated species under the effect of the water
pressure in said membrane, [0048] an anode, [0049] a cathode,
[0050] a generator enabling current to be generated and to apply a
potential difference between said anode and said cathode,
characterized in that the generator comprises: [0051] means for the
insertion of steam under pressure in said electrolyte via said
anode; [0052] means to inject CO.sub.2 and/or CO under pressure
into the cathode compartment of the electrolyzer, and [0053] means
to reduce the CO.sub.2 and/or the CO introduced into the cathode
compartment according to a process in conformance with one of the
previous embodiments.
[0054] The device according to the invention may also present one
or more of the characteristics below, considered individually or
according to all technically possible combinations: [0055] the
material enabling the injection of protonated species is
impermeable to O.sub.2 and H.sub.2 gases; [0056] the material
enabling the injection of protonated species has a densification
level of over 88%, preferably equal to at least 94%; [0057] the
material enabling the injection of protonated species is an oxygen
atom-defective oxide such as an oxygen-defective perovskite acting
as a proton conductor; In this case, the oxygen atom-defective
oxide may present stoichiometric intervals and/or may be doped.
[0058] Other characteristics and advantages of the invention will
clearly emerge from the description given below, for indicative and
in no way limiting purposes, with reference to the attached
figures, among which: [0059] FIGS. 1 and 2, already described, are
simplified schematic representations of steam, and [0060] FIG. 3 is
a simplified schematic representation of a steam electrolyser
performing joint CO.sub.2 and/or CO electroreduction.
[0061] FIG. 3 schematically and in a simplified manner represents
an embodiment of an electrolysis device for the production of
hydrogen implementing the joint CO.sub.2 and/or CO electroreduction
process according to the invention.
[0062] This electrolysis device has a structure similar to that of
the device from FIG. 2. Thus, it comprises: [0063] an anode 32,
[0064] a cathode 33, [0065] an electrolyte 31, [0066] a generator
34 ensuring a potential difference between the anode 32 and the
cathode 33, [0067] means 35 enabling the insertion under pressure
of steam pH.sub.2O into membrane 31 via cathode 33 (the relative
partial steam pressure is greater than or equal to 1 bar and less
than or equal to a burst pressure of the assembly, this latter
being greater than or equal to at least 100 bars).
[0068] In conformance with the invention, the device also comprises
means 36 enabling the insertion under pressure of gas (pCO.sub.2
and/or CO) into the cathode compartment 33.
[0069] The injection of steam is done via means 35 at the level of
anode 32 while the injection of gas CO.sub.2 and/or CO is done via
means 36 at the level of cathode 33.
[0070] At anode 32, the water is oxidized by releasing electrons
while H.sup.+ ions (in OH.sub.o.sup.. form) are generated according
to a process that is similar to the process described with the help
of FIG. 2.
[0071] These H.sup.+ ions migrate through the electrolyte 31,
carbon compounds of the CO.sub.2 and/or CO type react at cathode 33
with these H.sup.+ ions to form compounds of the
C.sub.xH.sub.yO.sub.z type (with x>1; y is between 0 and 2x+2
and z is between 0 and 2x) and water at the cathode.
[0072] In particular, the chemical equations of the various
reactions may be written as:
(6n+2)H.sub.Electrode.sup.x+nCO.sub.2.fwdarw.C.sub.nC.sub.2n+22nH.sub.2O
6nH.sub.Electrode.sup.xnCO.sub.2.fwdarw.C.sub.nH.sub.2n+2nH.sub.2O
6nH.sub.Electrode.sup.x+nCO.sub.2.fwdarw.C.sub.nH.sub.2n+2O+(2n-1)H.sub.-
2O
(6n-2)H.sub.Electrode.sup.x+nCO.sub.2H.sub.2nO+(2n-1)H.sub.2O
[0073] As the nature of the compound formed depends on the process
conditions, the overall C.sub.xH.sub.yO.sub.z formation reaction
may thus be written as:
(4x-2z+y)H.sub.Electrode.sup.++xCO.sub.2+C.sub.xH.sub.yO.sub.z+(2x-z)H.s-
ub.2O
[0074] The nature of the C.sub.xH.sub.yO.sub.z compounds
synthesized at the cathode depends on many process parameters such
as, for example, gas pressure, operating temperature T1 and the
voltage-current pair applied to the cathode as described below:
[0075] Concerning the gas pressure, the relative pressure of
CO.sub.2 and/or CO is greater than or equal to 1 bar and less than
or equal to a burst pressure of the assembly, the latter being
greater than or equal to at least 100 bars.
[0076] It will be noted that here the term relative pressure
designates the pressure of insertion with relation to atmospheric
pressure.
[0077] It will be noted that it is possible to use either a gas
stream containing only steam or a gas stream partially containing
steam. Thus, depending on the case, the term "partial pressure"
will designate either the total pressure of the gas stream in the
case where the latter is only constituted of steam or the partial
pressure of the steam in the case where the gas stream comprises
gases other than steam.
[0078] We add that the total pressure imposed in a cathode or anode
compartment may be compensated for in the other compartment so as
to have a pressure difference between the two compartments to
prevent rupture of the membrane, electrode support assembly if its
rupture resistance is too low.
[0079] Concerning the operating temperature T1 of the device, the
latter depends on the type of material utilized for membrane 31; in
any case, this temperature is over 200.degree. C. and is generally
under 800.degree. C., or even under 600.degree. C. This operating
temperature corresponds to a conduction ensured by W protons.
[0080] The operating temperature T1 of the device is also
dependent, within the 200 to 800.degree. C. range, according to the
nature of the C.sub.xH.sub.yO.sub.z carbon compounds that one
wishes to generate.
[0081] In fact, a large variety of compounds may be obtained, such
as methane, methanol, formaldehyde, carboxylic acids (formic acid,
etc.) and other compounds with longer chains, which may go as far
as the formation of synthetic fuel.
[0082] For example, one may have the following reactions at the
cathode:
8H.sub.Electrode.sup.x+CO.sub.2.fwdarw.CH.sub.4+2H.sub.2O
6H.sub.Electrode.sup.x+CO.sub.2.fwdarw.CH.sub.2+2H.sub.2O
6H.sub.Electrode.sup.x+CO.sub.2.fwdarw.CH.sub.3OH+H.sub.2O
4H.sub.Electrode.sup.x+CO.sub.2.fwdarw.CH.sub.2O+H.sub.2O
H.sub.Electrode.sup.x+CO.sub.2.fwdarw.COOH
[0083] Concerning the voltage-current pair applied to the cathode,
it should be noted that the nature of the carbon compounds formed
also depends on this voltage. In fact, the more reductive the
cathode medium (low oxidoreduction potential E), the more the
carbon compounds generated are hydrogenated, as diagrammed in the
diagram below (R being, for example, an alkyl group).
##STR00001##
[0084] For the advantageous embodiment of these reactions, it is
necessary to have electrodes presenting a large number of triple
contact points, i.e., points or contact surfaces between an ionic
conductor, an electronic conductor and a gas phase.
[0085] For example, the electrodes considered are preferentially
ceramic-metal materials formed by a mixture of ion-conducting
ceramic and an electron-conducting metal.
[0086] However, the utilization of "all-ceramic"
electron-conducting electrodes may also be considered instead of a
ceramic-metal material.
[0087] It should be noted that a given electrolyte may be an
O.sup.2- proton or ion conductor according to the temperature and
the pressure of the applied steam.
[0088] But the utilization of proton-conducting membranes generates
hydrogen (in hydrogen atom form more or less adsorbed at the
surface of the cathode) that is much more reactive than H.sub.2
hydrogen (or dihydrogen), thus enabling better hydrogenation of the
CO.sub.2 and CO compared to a conventional hydrogenation process
(in the presence of H.sub.2).
[0089] Moreover, the utilization of H.sup.+ ion conducting
membranes operating at moderate temperature enables the synthesis
of C.sub.xHyO.sub.z type complex compounds (with x, y and z greater
than 1) while the utilization of O.sup.2- conducting membranes,
operating at a much higher temperature, preferentially generates
CO, a product that is stable at high temperature.
[0090] The objective of studies implemented is to obtain the
maximum yield for the production of hydrogen and/or the
hydrogenation of CO.sub.2 and/or CO. To do this, most of the
current utilized must intervene in the faraday process, i.e., be
utilized for the reduction of water and consequently the production
of highly reactive hydrogen.
[0091] Thus the voltage utilized for polarization must be affected
at least by [0092] overvoltage at the electrodes [0093] contact
resistance at the electrode/electrolyte interfaces [0094] ohmic
drop within materials and particularly within the electrolyte
[0095] the standard thermodynamic reaction voltage at the
electrodes.
[0096] In this context, the present invention proposes the
utilization of proton-conducting electrolyte under steam pressure
for the electrolysis of water at high temperature for hydrogen
production as well as for electroreduction of CO.sub.2 and/or CO at
the cathode.
[0097] Thus, the process comprises the following steps: [0098] the
insertion of protonated species under the effect of the pressure of
a gas stream containing steam, into said membrane, [0099]
electrolysis of the steam and reduction of the gas (CO.sub.2 and/or
CO) in the cathode compartment.
[0100] Thanks to the gas stream containing steam, protonation of
the membrane is promoted by the steam under pressure and this
pressure is advantageously utilized to obtain the desired
conductivity at a given temperature. Such a process is described,
for example, in the French patent application filed under No.
07/55418 on Jun. 1, 2007.
[0101] As indicated in this application, the applicant observed
that the increase in the relative partial pressure of steam leads
to an increase in the ionic conductivity of the membrane.
[0102] This correlation between the increase in relative partial
pressure with the increase in conductivity enables suitable
materials to be worked at lower temperatures. In other words, the
lowering in conductivity driven by operating at lower temperatures
is compensated for by the increase in relative partial pressure of
steam.
[0103] In conformance with the invention, "highly reactive"
hydrogen is produced at the cathode that may generate hydrogen
(H.sub.2), in the absence of a reducible compound, or
C.sub.xH.sub.yO.sub.z type compounds, in the presence of CO.sub.2
and/or CO with x>1; y is between 0 and 2x+2 and z is between 0
and 2x.
[0104] The proton-conducting membrane is made from a material
promoting the insertion of water such as a doped perovskite
material of general formula AB.sub.1-xD.sub.2O.sub.3-x/2. The
materials utilized for the anode and the cathode are preferentially
ceramic-metal materials (mixture of metal with the perovskite
material utilized for the electrolyte). The membrane is preferably
impermeable to O.sub.2 and H.sub.2 gases.
[0105] In general, the membrane may be of the type: perovskite
vacancies, non-stoichiometric perovskites and/or perovskites doped
with general formula ABO.sub.3, of fluorite, pyrochlore
A.sub.2B.sub.2X.sub.7, apatite Me.sub.10(XO.sub.4).sub.6Y.sub.2,
oxyapatite Me.sub.10(XO.sub.4).sub.6O.sub.2 structure, of
hydroxylapatite Me.sub.10(XO.sub.4).sub.6(OH).sub.2 structure,
silicate structures, aluminosilicates (phyllosilicate or zeolite),
silicates grafted with oxyacids or silicates grafted with
phosphates.
[0106] More generally, the electrolytes may advantageously be all
of the compounds utilized as high temperature or intermediate
temperature proton conductors either by virtue of their tunnel or
sheet structure and/or by the presence of vacancies capable of
inserting protonated species whose molecular size is small.
[0107] The present invention is capable of many variations. In
particular, the material enabling the incorporation of protonated
species may be impermeable to O.sub.2 and H.sub.2 gases and/or may
enable the incorporation of protonated species at a densification
level of over 88%, preferably equal at least to 94%.
[0108] In fact, a good compromise between the densification level
that must be the highest possible (particularly for the mechanical
strength of electrolytes and gas permeation) and the capacity of
the material to enable incorporation of protonated species must be
found. Increasing the partial steam pressure that forces the
incorporation of protonated species into the membrane compensates
for the densification level increase.
[0109] According to a variation, the material enabling the
injection of water is an oxygen atom-defective oxide such as an
oxygen-defective perovskite acting as a proton conductor. In
addition, the oxygen atom-defective oxide may present
stoichiometric intervals and/or may be doped.
[0110] In fact, non-stoichiometry and/or doping allow the creation
of oxygen atom vacancies. Thus, in the case of proton conduction,
the exposure under pressure of a perovskite presenting
stoichiometric intervals and/or being doped (and therefore being
deficient in oxygen) to steam induces the incorporation of
protonated species into the structure. The molecules of water fill
the oxygen vacancies and dissociate into 2 hydroxyl groups (or H+
proton on an oxide site) according to the reaction:
O.sub.o.sup.x+V.sub.o.sup...+H.sub.2O 2OH.sub.o.sup..
[0111] It will be noted that materials other than
non-stoichiometric and/or doped perovskites may be utilized as
materials promoting the incorporation of water and its dissociation
in the form of protonated species and/or hydroxides.
[0112] For example, crystallographic structures, such as fluorite
structures, pyrochlore A.sub.2B.sub.2X.sub.7 structures, apatite
Me.sub.10(XO.sub.4).sub.6Y.sub.2 structures, oxyapatite Me.sub.10
(XO.sub.4).sub.6O.sub.2 structures, hydroxylapatite
Me.sub.10(XO.sub.4).sub.6 (OH).sub.2 structures, silicates,
aluminosilicates, phyllosilicates, or phosphates may be cited.
[0113] These structures may possibly be grafted by oxyacid
groups.
[0114] In fact, all structures having a high affinity with water
and/or protons may be contemplated.
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