WO2002094418A1 - Method and apparatus for electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen - Google Patents

Method and apparatus for electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen Download PDF

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Publication number
WO2002094418A1
WO2002094418A1 PCT/DK2002/000334 DK0200334W WO02094418A1 WO 2002094418 A1 WO2002094418 A1 WO 2002094418A1 DK 0200334 W DK0200334 W DK 0200334W WO 02094418 A1 WO02094418 A1 WO 02094418A1
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WIPO (PCT)
Prior art keywords
working electrode
oxygen
nitrogen
nitrogen oxides
electrode
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Application number
PCT/DK2002/000334
Other languages
French (fr)
Inventor
Henrik Christensen
Kent Kammer Hansen
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Dinex A/S
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Publication date
Application filed by Dinex A/S filed Critical Dinex A/S
Priority to US10/478,591 priority Critical patent/US20050016864A1/en
Priority to CA002448002A priority patent/CA2448002A1/en
Priority to PL02367694A priority patent/PL367694A1/en
Priority to HU0400621A priority patent/HUP0400621A2/en
Priority to EP02771626A priority patent/EP1397195A1/en
Publication of WO2002094418A1 publication Critical patent/WO2002094418A1/en
Priority to NO20035189A priority patent/NO20035189D0/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/02—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography
    • B01D53/04—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by adsorption, e.g. preparative gas chromatography with stationary adsorbents
    • B01D53/0407—Constructional details of adsorbing systems
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/32—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by electrical effects other than those provided for in group B01D61/00
    • B01D53/326—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by electrical effects other than those provided for in group B01D61/00 in electrochemical cells
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2253/00—Adsorbents used in seperation treatment of gases and vapours
    • B01D2253/10—Inorganic adsorbents
    • B01D2253/112—Metals or metal compounds not provided for in B01D2253/104 or B01D2253/106
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2257/00—Components to be removed
    • B01D2257/40—Nitrogen compounds
    • B01D2257/404—Nitrogen oxides other than dinitrogen oxide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2259/00—Type of treatment
    • B01D2259/40—Further details for adsorption processes and devices
    • B01D2259/40083—Regeneration of adsorbents in processes other than pressure or temperature swing adsorption

Definitions

  • the present invention relates to a method and apparatus for electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen.
  • the invention relates to a working electrode for an electrochemical reactor, an electro ⁇ chemical reactor comprising such a working electrode, a method of reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen using an working electrode comprising an electric conductive ceramic of lanthanum manganite doped with an oxygen ion vacancy quencher.
  • the invention in another aspect, relates to an electrochemical reactor comprising nitrogen oxide absorber adapted for electrochemical regeneration, and a method of electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen using such an electrochemical reactor.
  • nitrogen oxides which are often denoted by the term NO ⁇ , is intended to designate one or more compounds of oxygen and nitrogen, e.g. NO, and N0 2 , etc.
  • nitrogen oxide absorber is intended to designate an absorber for nitrogen oxides, e.g. in form of a compound or a composition of compounds.
  • the available electrons react with either of the reactants NO x or 0 2 .
  • a method of increasing the selectivity of NO x -reduction relative to 0 2 -reduction comprises increasing the amount of NO x relative to that of 0 2 prior to electrochemical reduction.
  • Alcaline earth metals such as MgO or CaO have been used to absorb NO x .
  • NO x is released by heat regeneration before electrochemical reduction of NO x .
  • Another method of increasing the selectivity of NO x - reduction relative to 0 2 -reduction comprises increasing the access of N0 X to reactive electrons of the working electrode compared to the access of 0 2 , or equivalent by increasing access of electrons of the working electrode to N0 X compared to access of electrons to 0 2 .
  • US patent No. 5 022 975 discloses a solid state electrochemical pollution control device for altering the composition of a gas stream including removing SO x and NO x ; in an embodiment said device comprises gadolinia stabilized ceria as electrolyte.
  • US patent No. 5 401 372 discloses an electrochemical catalytic reduction cell for reduction of N0 X in an 0 2 - containing exhaust emission using a gas-diffusion cata- lysts such as supported vanadium oxides with an electron collecting layer such as a conductive perovskite-type oxide, e.g. LSM.
  • a gas-diffusion cata- lysts such as supported vanadium oxides with an electron collecting layer
  • an electron collecting layer such as a conductive perovskite-type oxide, e.g. LSM.
  • US patent No. 5 456 807 discloses a method and apparatus for selectively removing nitrogen oxides from gaseous mixtures comprising absorption of NO x with NO x adsorbents, heating release of absorbed NO x , and electrochemical reduction of NO x to N 2 and 0 2 in solid- oxid electrochemical cells.
  • WO 97/44126 discloses an electrochemical reactor comprising a mixed ion-selective electrolyte and electrode material of heat treated gadoliniumoxide doped with 20% CeO and containing about 6 vol.-% lanthanium oxide doped with 20% strontiumoxide for reduction of carbon black in nitrogen containing 20% oxygen. None is indicated nor suggested about reducing NO x to N 2 and 0, .
  • a working electrode for an electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte;
  • the working electrode comprising an electric conductive ceramic oxide material having the general formula:
  • a and A' designate first substitution metals of similar sizes, said' first substitution metals having a high efficiency for reducing vacancies for oxygen ions, O ⁇ x ⁇ l;
  • B and B' designate second substitution metals of similar sizes, said second substitution metals being of smaller sizes than those of said first substitution metals, and having a high transition efficiency between oxidation states, O ⁇ y ⁇ l;
  • ⁇ is a small number, positive or negative, that allows for compensation of differences in valences of said metals .
  • the components A, A', B, and B' of the AA'BB'Oj material can be selected within wide ranges.
  • the working electrode A is selected from the group consisting of rare earth metals: Sc, Y, La, Ce, Pr, Nd, S , Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; metals of group 3a: Al, Ga, and In; and group 3b: Sc, Y, La of the periodic table, preferably La, Gd, In and Y;
  • A' is selected from the group consisting of alkaline earth metals: Mg, Ca, Sr, and Ba; and Eu, preferably Ca, Sr, Ba, and Eu
  • B and B' are selected from the group consisting of transition metals:
  • ggrroouupp llbb Cu and Ag; group 2b Zn; group 3a Ga, In, and Tl; group 3b Sc, and Y; group 4b Ti, Zr, Hf; ggrroouupp 5 5bb: V, Nb, Ta; group 6b Cr, Mo, W; group 7b Mn and Re; and ggrroouupp 8 8:: Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt;
  • a particularly preferred working electrode comprises a LSM material.
  • the ceramic oxide comprises lanthanum manganite doped with strontium oxide, La ⁇ r ⁇ x Mn0 3 , the stoichiometric coefficient 1-x being in the range 0.05 to 0.20, preferably 0A0 to 0.18, most preferred about 0.15 whereby a good selectivity can be obtained for reduction of nitrogen oxides compared to reduction of oxygen.
  • An aspect of the invention relates to an electrochemical reactor comprising the working electrode, the counter electrode, and the ion-selective electrolyte wherein the working electrode is according to the invention.
  • Such a reactor can be utilised for the reduction of nitrogen oxides in the exhaust gas from diesel engines or lean burn otto engines, where the high content of oxygen precludes the use of standard techniques, such as chemical reduction in a three way catalyst, for the reduction of the content of nitrogen oxides.
  • Another aspect of the invention relates to a method of reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, the method comprising:
  • an electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte; said working electrode being adapted to reduce nitrogen oxides to nitrogen and oxygen, and said working electrode being adapted to suppress reduction of oxygen to oxygen ions; said working electrode processes being substantially according to the cathode electrode processes : (a) 2N0 X + 2xe " -> N 2 + 2x0 2
  • said cathode electrode processes (a) and (b) being carried out at a potential selected within a range of -1500 mV to +1500 V between said working electrode and said counter electrode, and at a temperature within a range of 200 to 500 °C;
  • said working electrode comprising an electric conductive ceramic of lanthanum manganite; said lanthanum manganite being doped with an oxygen ion vacancy quencher for quenching vacancies for oxygen ions; said oxygen ion vacancy quencher being in an effective amount to suppress said reduction of oxygen to oxygen ions at the working electrode so that the rate of reduction of nitrogen oxides is faster than the rate of reduction of oxygen at the selected potential and temperature.
  • said selected potential is selected within the range from -200 mV to 800 mV, said potential being measured versus a hydrogen electrode of 8% H 2 0 and 3% H 2 in Ar whereby it is obtained that the selectivity is further enhanced, and the total power demand decreased by lowering the potential as much as possible, without reaching a situation where the reduction rate becomes too small.
  • said oxygen ion vacancy quencher is selected from the group consisting of Sr, Ca, Ba, and Eu.
  • said method uses electrochemical reactor comprising a working electrode according to the invention whereby particular improved selectivity of reduction of nitrogen oxides is obtained.
  • the concentration of nitrogen oxides is low. Consequently, a preconcentration of nitrogen oxides may be desired.
  • An aspect of the invention relates to an electrochemical reactor for reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen, the electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen absorber for absorbing nitrogen oxides; wherein said nitrogen absorber is adapted for electrochemical regeneration thereof; whereby it is achieved that nitrogen oxides can be adsorbed readily, even from gas mixtures with low concentrations of nitrogen oxides. Said electrochemical reactor can then easily regenerate the NOx adsorber by electrochemical reduction, without the need for addition of external heat or a chemical reducing agent.
  • said nitrogen absorber and said working electrode are intermixed whereby it is achieved that there is an intimate contact between the adsorbed NOx-containing species and the working electrode. This assures a fast, selective and efficient reduction of the NOx-containing compound.
  • said nitrogen absorber comprises a porous layer on said working electrode whereby a separate absorber is obtained which can be advantageous for some applications with respect to easy assembling and maintenance.
  • the nitrogen absorber comprises a material or a combination of materials selected from the group consisting of Na 2 0, K 2 0, MgO, CaO, SrO, and BaO, preferably BaO whereby nitrates and nitrites are easily formed in the presence of nitrogen oxides. Further, these nitrates and nitrites can easily be converted back to oxides under reducing conditions at elevated temperature.
  • said working electrode is a working electrode according to the invention.
  • the invention in another aspect relates to a method of electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen, the method comprising:
  • an electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen oxide absorber for absorbing nitrogen oxides;
  • the nitrogen oxides are absorbed in said nitrogen oxide absorber without applying any electrical potential between the working electrode and the counter electrode whereby the adsorption process is made more efficient by not polarising the reactor and furthermore power is saved by only polarising the reactor during the relatively short regeneration period.
  • said nitrogen oxide absorber is regenerated by applying an electrical potential between said nitrogen oxide absorber and said counter electrode in the range from 0 to 1.5 V, preferably from 0.2 to 1.0 V, most preferred from 0.4 to 0.7 V whereby the potential can be kept as low as possible to save power.
  • an electrical potential between said nitrogen oxide absorber and said counter electrode in the range from 0 to 1.5 V, preferably from 0.2 to 1.0 V, most preferred from 0.4 to 0.7 V whereby the potential can be kept as low as possible to save power.
  • selecting a higher potential can boost the process.
  • said regeneration is carried out at an electrical current density allowing more than 80% regeneration of said nitrogen oxide absorber after a regeneration time in the range from 5- 40 s, preferably 5-30 s, most preferred 5-15 s whereby the adsorber is inactive during the regeneration process. Therefore, by minimising the regeneration time and keeping it short compared to the adsorption time, the total reduction rate for the NOx content in the exhaust has can be optimised.
  • the process can be adapted to cope with strongly varying contents of nitrogen oxides in the exhaust gas.
  • said electrical current density allowing more than 90% regeneration of said nitrogen oxide absorber after said regeneration time.
  • said nitrogen oxide absorber absorbs more than 60%, preferably in the range 60-80% of the nitrogen oxides of the mixutre of nitrogen oxides and oxygen.
  • said absorption of nitrogen oxides is carried out to saturation of said nitrogen oxide absorber.
  • said nitrogen absorber and said working electrode are intermixed.
  • said working electrode is a working electrode according to the invention.
  • electrical current density is intended to designate electrical current per electrode area, said electrode area typically being the geometrical area of the electrode. In assessment of a measure of an electrode area, adjustment for variations of the microstructure and porosity of the electrode material can be done.
  • Fig. 1 shows an exemplary cyclic voltametric measurement of a working electrode comprising La 0#B2 Sr 0-14 Fe 0-1 Mn 0-9 O 3 in presence of nitrogen monooxide, and in presence of oxygen, respectively;
  • Fig. 2 shows an exemplary cyclic voltametric measurement of a comparison working electrode comprising Co 3 0 4 presence of nitrogen monooxide, curve 1, and in presence of oxygen, curve 2, respectively;
  • Fig. 3 shows an exempel of cyclic voltametric measurement of a working electrode comprising La 085 Sr 015 Mn0 3 presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively;
  • Fig. 4 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of nitrogen monooxide, said working electrodes comprising LSM materials having different degrees of doped strontium as cathode;
  • Fig. 5 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of oxygen, said working electrodes comprising LSM materials similar to those used for the measurements shown in Fig. 4;
  • Fig. 6 shows a cross sectional sketch of an embodiment of an electrochemical reactor according to the invention
  • Fig. 7 shows a cross sectional sketch of an embodiment of an electrochemical cell for an electrochemical reactor according to the invention
  • Fig. 8 shows a cross sectional sketch of another embodiment of an electrochemical cell for an electrochemical reactor according to the invention.
  • Fig. 9 shows a cross sectional sketch of an experimentel electrochemical set-up for voltametric measurements.
  • Fig. 1 shows an exemplary cyclic voltametric measurement of a working electrode comprising La 0,e2 Sr 0.14 Fe 0.1 Mn 09 O 3 in presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively.
  • the y-axis indicates electric current density in I/ ⁇ A of the working electrode having an electrode area of about 0.01 cm 2 .
  • the x-axis indicates the potential of the working electrode E in V versus a standard hydrogen gas electrode of 2.9% H 2 and 3.1% H 2 0 in argon in equilibrium with a platinum electrode.
  • An electrochemical cell comprising a working electrode comprising La 0/82 Sr 0.14 Fe 0 was prepared according to the procedure used in Example 1 (see below) .
  • Fig. 2 shows an exemplary cyclic voltametric measurement of a comparison working electrode comprising Co 3 0 4 in presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively.
  • the y-axis indicates electric current density in I/ ⁇ A of the working electrode having an electrode area of about
  • the x-axis indicates the potential of the working electrode E in V versus a standard hydrogen gas electrode of 3% H 2 and 8% H 2 0 in argon in equilibrium with a platinum electrode.
  • Fig. 3 shows an exe pel of cyclic voltametric measurement of a working electrode comprising La 0.B5 Sr o.15 Mn0 3 in presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively.
  • the reduction rate for NO is more than 2 orders of magnitude higher than the reduction rate for 0 2 . Consequently, the LSM material, here specifically La 0 . 85 Sr 015 MnO 3 , is very well suited as electrode material for selective reduction of nitrogen oxides in presence of oxygen.
  • Fig. 4 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of nitrogen monooxide, curves LSM05, LSM15, LSM25, LSM35 and LSM50, said working electrodes comprising LSM materials having different degrees of doped strontium as cathode.
  • the designation of the curves LSMy defines used LSM materials of the formula La (1 . ⁇ ) Sr x Mn0 3 wherein y is 100*x, e.g. LSM15 designates the LSM matial La 085 Sr 015 MnO 3 .
  • the reduction rate for NO is significantly higher for LSM15 as the cathode material than for any of the other tested LSM materials in the range 0.2 to 0.8 V.
  • Fig. 5 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of oxygen, curves LSM05, LSM15, LSM25, LSM35 and LSM50, said working electrodes comprising LSM materials having different degrees of doped strontium as cathode similar as the LSM materials used for the measurements shown in Fig. 4.
  • Fig. 6 shows a cross sectional sketch of an embodiment of an electrochemical reactor according to the invention
  • the electrochemical cell comprises an oxygen ion conducting electrolyte 1, here CGO, a selective cathode 2, here an LSM15 material, and an anode 3, here platinum.
  • the electrochemical cell is placed in an gas conduit means 21,22 for an exhaust gas stream from an engine, here a gas inlet tube 21 and a gas outlet tube 22.
  • the raw gas stream 11 containing NOx enters the cathode area 2 of the electrochemical cell, where the NOx is reduced to N 2 and 0 2.
  • the treated gas 12 leaves the cathode area.
  • the cell is polarised from an external power supply 5 with controlled potential through the leads 4.
  • Fig. 7 shows a cross sectional sketch of an embodiment of an electrochemical cell for an electrochemical reactor according to the invention.
  • the electrochemical cell comprises an oxygen ion conducting electrolyte 1, a cathode, 2, made from a mixture of cathode catalyst particles 7, here LSM15, and NOx adsorbing particles 8, here BaO particles, and an anode 3, here a platinum electrode.
  • the size of the particles is strongly exaggerated. In the real cell the particle size was in the range of about 0.1 to 10 ⁇ m.
  • Fig. 8 shows a cross sectional sketch of another embodiment of an electrochemical cell for an electrochemical reactor according to the invention.
  • the electrochemical cell comprises an oxygen ion conducting electrolyte 1, a cathode 2, here made from a layer of cathode catalyst material 7, here LSM15, and a porous layer of a NOx adsorbing material 8, here sintered BaO particles, and an anode 3, here a platinum electrode.
  • Fig. 9 shows a cross sectional sketch of an experimentel electrochemical set-up for cyclic voltametric measurements .
  • the electrochemical cell comprises an oxygen ion conducting electrolyte 1, a working cathode electrode 2, e.g. made from a layer of cathode catalyst material as LSM15 and formed in the shape of a cone with a narrow tip for improved positioning of the electrode, said working cathode electrode further comprising e.g. a porous layer of a NOx adsorbing material 8, here sintered BaO particles, and an anode 3, e.g. a platinum electrode.
  • a working cathode electrode 2 e.g. made from a layer of cathode catalyst material as LSM15 and formed in the shape of a cone with a narrow tip for improved positioning of the electrode, said working cathode electrode further comprising e.g. a porous layer of a NOx adsorbing material 8, here sintered BaO particles, and an anode 3, e.g. a platinum electrode.
  • the set-up further shows a potentimetric power supply 51, e.g. a potentiostatic power supply supplied by University of Southern Denmark, Odense, supplying electrical currenct through the leads 41,42.
  • a potentimetric power supply 51 e.g. a potentiostatic power supply supplied by University of Southern Denmark, Odense, supplying electrical currenct through the leads 41,42.
  • Preferred embodiments of the invention are further illustrated by examples of production of electrochemical cells having working electrodes based on LSM materials.
  • a series of 5 electrochemical cells were produced, each comprising an ion selective electrolyte produced by pressing 1 mm thin plates of CGO (cerium oxide doped with 10 atomic-% gadolinium oxide, i.e. Ce 0,9 Gd 0.1 O 1. g 5 , supplied Rhodia Electronics and Catalyst, and subsequently placing the CGO plates in an electrical furnace sintering the plates at a temperature in the range 1400-1550 °C for 2-4 hours .
  • CGO cerium oxide doped with 10 atomic-% gadolinium oxide, i.e. Ce 0,9 Gd 0.1 O 1.
  • Rhodia Electronics and Catalyst supplied Rhodia Electronics and Catalyst
  • Working electrodes of the LSM type were provided by depositing La 1 . x Sr x Mn0 3 onto the exposed upper side of the sintered CGO plates.
  • LSM materials were prepared by evaporating a solution of the corresponding metal nitrates, e.g. La(N0 3 ) 3( Sr(N0 3 ) 2 , and Mn(N0 3 ) 2 stabilised by addition of citric acid.
  • the residue powder was calcinated at a temperature in the range of 900-1100 °C for 1-3 hours.
  • a slurry of fine powder of LSM in water was prepared and organic binder, e.g. methylcellulose and other additves, e.g. dispersing agents were added to stabilize the slurry.
  • organic binder e.g. methylcellulose
  • other additves e.g. dispersing agents
  • the slurry was then applied to one side of the sintered CGO plates by painting or screen printing.
  • the CGO plates were then sintered further at a temperature in the range 1000-1200 °C for 2-4 hours.
  • Counter electrodes were provided on the other side of the sintered CGO plates by applying platinum paste comprising platinum powder and organic binder supplied from Engeldhard.
  • Cyclic voltametric measurements were performed on the produced electrochemical cells in a N 2 gas containing 2 vol-% NO and in an N 2 gas containing 10 vol-% 0 2 .
  • the N 2 -gas mixture was supplied by Hede Nielsen/Air Liquide, Denmark.
  • reaction rate of the cathodic reduction of 0 2 increases.
  • reaction rate of 0 2 is significant at potentials below 0.5 V.
  • An electrochemical cell with a working electrode comprising La 0 . 85 Sr 0 . 15 MnO 3 was prepared as described in Example 1.
  • the cell was tested at a temperature of 300 °C in a flowing N 2 gas containing 1000 ppm NO and 10 vol.-% 02.
  • the cell was polarised with 0.5 volts.
  • N0 2 Because some N0 2 is formed in the mixture of NO and 0 2 , the contents of NO and N0 2 were measured in the exhaust gas of the electrochemical cell by mass spectrometry analysis using a Varian mass spectrometer.
  • the reduction rate of NO was measure in the range of 40 to 80% depending on the gas flow rate through the cell, said reduction rate being based on the combined content of NO and N0 2 measured.
  • An electrochemical cell with a working electrode comprising a mixture of 50 weight-% La 0 . 85 Sr 015 Mn0 3 and 50 weight-% BaO supplied from Merck was prepared as described in Example 1.
  • the working electrodes were activated by addition of platinum. Platinum was added by impregnating a solution of PtCl 4 in 0.1 N hydrochloric acid into the working electrode material. Then the working electrode were dried and heated to a temperature of 600 °C.
  • BaO functions as an absorber of nitrogen oxides.
  • Pt functions as an auxiliary catalyst for the NOx adsorption reactions. Depending on the exact composition of the exhaust gas several possible reaction can take place, e.g.
  • the electrochemical cells were tested at a temperature of
  • NO x in exhaust gas is typically 750 ppm.
  • the engine will deliver about 20- 25 kW.
  • the exhaust gas flow will be about 80 1/s.
  • the temperature will be about 300 °C .
  • NO x is calculated as NO, since in a diesel engine, the NO content is more than about 90 vol.-% of the total NO x .

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Abstract

A working electrode for an electrochemical reactor, the electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte; the working electrode comprising an electric conductive ceramic oxide material having the general formula: AxA'(1-x)ByB'(1-y)O(3-Δ) wherein A and A' designate first substitution metals of similar sizes, said first substitution metals having a high efficiency for reducing vacancies for oxygen ions, 0≤x≤1; B and B' designate second substitution metals of similar sizes, said second substitution metals being of smaller sizes, said second substitution metals being of smaller sizes than those of said first substitution metals, and having a high transition efficiency between oxidation states, 0≤y≤1; O designates oxygen; and Δ is a small number, positive or negative, that allows for compensation of differences in valences of said metals. An electrochemical reactor comprising said working electrode. Methods and an electrochemical reactor for reduction of nitrogen oxides in a mixture og nitrogen oxides and oxygen, the electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen absorber for absorbing nitrogen oxides; wherein said nitrogen absorber is adapted for electrochemical regeneration thereof.

Description

METHOD AND APPARATUS FOR ELECTROCHEMICAL REDUCTION OF NITROGEN OXIDES IN A MIXTURE OF NITROGEN OXIDES AND OXYGEN
DESCRIPTION
1. BACKGROUND OF THE INVENTION
The present invention relates to a method and apparatus for electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen.
In an aspect, the invention relates to a working electrode for an electrochemical reactor, an electro¬ chemical reactor comprising such a working electrode, a method of reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen using an working electrode comprising an electric conductive ceramic of lanthanum manganite doped with an oxygen ion vacancy quencher.
In another aspect, the invention relates to an electrochemical reactor comprising nitrogen oxide absorber adapted for electrochemical regeneration, and a method of electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen using such an electrochemical reactor.
The Technical Field
In the present context the expression "nitrogen oxides", which are often denoted by the term NOκ, is intended to designate one or more compounds of oxygen and nitrogen, e.g. NO, and N02, etc. Further, the expression "nitrogen oxide absorber" is intended to designate an absorber for nitrogen oxides, e.g. in form of a compound or a composition of compounds.
Reduction of N0X in presence of oxygen is known.
In a method adapted to combustion processes, an excess of fuel is added for a short period of time thereby providing a reducing agent, i.e. addition of CH, whereby NOx is reduced according to the concurrent reactions:
(1) 2N0X + xCH +x/202 -> N2 + xC02 +x/2H20
(2) ll/202 + CH -> C02 + *s H20
However, the addition of CH affects the combustion processes and thereby the produced heat of the engine.
In electrochemical reduction of NOx in presence of 02, concurrent electrode processes between electrons and NOx and 02 takes place at the working electrode, e.g. as expressed by the electrode processes at the cathode:
(3) 2NOx + 4xe" -> N2 + 2x02"
(4) 02 + 4e~ -> 202"
For a given potential and current density, the available electrons react with either of the reactants NOx or 02.
A method of increasing the selectivity of NOx-reduction relative to 02-reduction comprises increasing the amount of NOx relative to that of 02 prior to electrochemical reduction. Alcaline earth metals such as MgO or CaO have been used to absorb NOx. Subsequently, NOx is released by heat regeneration before electrochemical reduction of NOx. Another method of increasing the selectivity of NOx- reduction relative to 02-reduction comprises increasing the access of N0X to reactive electrons of the working electrode compared to the access of 02, or equivalent by increasing access of electrons of the working electrode to N0X compared to access of electrons to 02.
Prior Art Disclosures
US patent No. 5 022 975 discloses a solid state electrochemical pollution control device for altering the composition of a gas stream including removing SOx and NOx; in an embodiment said device comprises gadolinia stabilized ceria as electrolyte.
US patent No. 5 401 372 discloses an electrochemical catalytic reduction cell for reduction of N0X in an 02- containing exhaust emission using a gas-diffusion cata- lysts such as supported vanadium oxides with an electron collecting layer such as a conductive perovskite-type oxide, e.g. LSM.
US patent No. 5 456 807 discloses a method and apparatus for selectively removing nitrogen oxides from gaseous mixtures comprising absorption of NOx with NOx adsorbents, heating release of absorbed NOx, and electrochemical reduction of NOx to N2 and 02 in solid- oxid electrochemical cells.
WO 97/44126 discloses an electrochemical reactor comprising a mixed ion-selective electrolyte and electrode material of heat treated gadoliniumoxide doped with 20% CeO and containing about 6 vol.-% lanthanium oxide doped with 20% strontiumoxide for reduction of carbon black in nitrogen containing 20% oxygen. Nothing is indicated nor suggested about reducing NOx to N2 and 0, .
2. DISCLOSURE OF THE INVENTION
Object of the Invention
It is an object of the present invention to seek to provide an improved method and apparatus for selective electrochemical reduction of nitrogen oxides in presence of oxygen.
It is an object of the present invention to seek to provide such an improved method and apparatus for selective electrochemical reduction of nitrogen oxides in presence of oxygen in gaseous combustion mixtures.
Further objects appear from the description elsewhere.
Solution According to the Invention
According to the present invention, these objects are fulfilled by providing a working electrode for an electrochemical reactor, the electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte;
the working electrode comprising an electric conductive ceramic oxide material having the general formula:
Figure imgf000006_0001
wherein A and A' designate first substitution metals of similar sizes, said' first substitution metals having a high efficiency for reducing vacancies for oxygen ions, O≤x≤l;
B and B' designate second substitution metals of similar sizes, said second substitution metals being of smaller sizes than those of said first substitution metals, and having a high transition efficiency between oxidation states, O≤y≤l;
0 designates oxygen;
and δ is a small number, positive or negative, that allows for compensation of differences in valences of said metals .
It has surprisingly turned out that selecting a working electrode comprising an electric conductive ceramic oxide material having the AB03 formula as defined, the number of vacances in the oxygen ion lattice can be minimized whereby oxygen ion conductivity in the ceramic oxide material can be minimized.
Consequently, a working electrode having high selectivity for reduction of NOx and at the same low activity for reduction of 02 can be provided.
The components A, A', B, and B' of the AA'BB'Oj material can be selected within wide ranges.
In a preferred embodiment, the working electrode A is selected from the group consisting of rare earth metals: Sc, Y, La, Ce, Pr, Nd, S , Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; metals of group 3a: Al, Ga, and In; and group 3b: Sc, Y, La of the periodic table, preferably La, Gd, In and Y;
and A' is selected from the group consisting of alkaline earth metals: Mg, Ca, Sr, and Ba; and Eu, preferably Ca, Sr, Ba, and Eu
whereby it is achieved that the electrical and chemical/catalytic properties of the working electrode can be tailored within wide ranges.
In another preferred imbodiment, B and B' are selected from the group consisting of transition metals:
ggrroouupp llbb: Cu and Ag; group 2b Zn; group 3a Ga, In, and Tl; group 3b Sc, and Y; group 4b Ti, Zr, Hf; ggrroouupp 5 5bb: V, Nb, Ta; group 6b Cr, Mo, W; group 7b Mn and Re; and ggrroouupp 8 8:: Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt;
preferably Cr, Mn, Fe, Co, and Ni
whereby it is further achieved that the electrical and chemical/catalytic properties of the working electrode can be tailored within wide ranges.
The actual elements and stoichiometric coefficients can be selected by experimentation. For y=0, a particularly preferred working electrode comprises a LSM material.
In a preferred embodiment, the ceramic oxide comprises lanthanum manganite doped with strontium oxide, La^r^ xMn03, the stoichiometric coefficient 1-x being in the range 0.05 to 0.20, preferably 0A0 to 0.18, most preferred about 0.15 whereby a good selectivity can be obtained for reduction of nitrogen oxides compared to reduction of oxygen.
An aspect of the invention relates to an electrochemical reactor comprising the working electrode, the counter electrode, and the ion-selective electrolyte wherein the working electrode is according to the invention. Such a reactor can be utilised for the reduction of nitrogen oxides in the exhaust gas from diesel engines or lean burn otto engines, where the high content of oxygen precludes the use of standard techniques, such as chemical reduction in a three way catalyst, for the reduction of the content of nitrogen oxides.
Another aspect of the invention relates to a method of reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, the method comprising:
providing an electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte; said working electrode being adapted to reduce nitrogen oxides to nitrogen and oxygen, and said working electrode being adapted to suppress reduction of oxygen to oxygen ions; said working electrode processes being substantially according to the cathode electrode processes : (a) 2N0X + 2xe" -> N2 + 2x02
(b) 02 + 4e~ -> 202
and the anode electrode process
(c) 202~ -> 0, + 4e"
said cathode electrode processes (a) and (b) being carried out at a potential selected within a range of -1500 mV to +1500 V between said working electrode and said counter electrode, and at a temperature within a range of 200 to 500 °C;
and said working electrode comprising an electric conductive ceramic of lanthanum manganite; said lanthanum manganite being doped with an oxygen ion vacancy quencher for quenching vacancies for oxygen ions; said oxygen ion vacancy quencher being in an effective amount to suppress said reduction of oxygen to oxygen ions at the working electrode so that the rate of reduction of nitrogen oxides is faster than the rate of reduction of oxygen at the selected potential and temperature.
In a preferred embodiment said selected potential is selected within the range from -200 mV to 800 mV, said potential being measured versus a hydrogen electrode of 8% H20 and 3% H2 in Ar whereby it is obtained that the selectivity is further enhanced, and the total power demand decreased by lowering the potential as much as possible, without reaching a situation where the reduction rate becomes too small.
In a particularly preferred embodiment said oxygen ion vacancy quencher is selected from the group consisting of Sr, Ca, Ba, and Eu. In still another embodiment said method uses electrochemical reactor comprising a working electrode according to the invention whereby particular improved selectivity of reduction of nitrogen oxides is obtained.
In some applications the concentration of nitrogen oxides is low. Consequently, a preconcentration of nitrogen oxides may be desired.
An aspect of the invention relates to an electrochemical reactor for reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen, the electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen absorber for absorbing nitrogen oxides; wherein said nitrogen absorber is adapted for electrochemical regeneration thereof; whereby it is achieved that nitrogen oxides can be adsorbed readily, even from gas mixtures with low concentrations of nitrogen oxides. Said electrochemical reactor can then easily regenerate the NOx adsorber by electrochemical reduction, without the need for addition of external heat or a chemical reducing agent.
In a preferred embodiment said nitrogen absorber and said working electrode are intermixed whereby it is achieved that there is an intimate contact between the adsorbed NOx-containing species and the working electrode. This assures a fast, selective and efficient reduction of the NOx-containing compound.
In another preferred embodiment said nitrogen absorber comprises a porous layer on said working electrode whereby a separate absorber is obtained which can be advantageous for some applications with respect to easy assembling and maintenance.
In a preferred embodiement the nitrogen absorber comprises a material or a combination of materials selected from the group consisting of Na20, K20, MgO, CaO, SrO, and BaO, preferably BaO whereby nitrates and nitrites are easily formed in the presence of nitrogen oxides. Further, these nitrates and nitrites can easily be converted back to oxides under reducing conditions at elevated temperature.
In a preferred embodiment of this electrochemical reactor said working electrode is a working electrode according to the invention.
In another aspect the invention relates to a method of electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen, the method comprising:
providing an electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen oxide absorber for absorbing nitrogen oxides;
absorbing nitrogen oxides from the mixture of nitrogen oxides and oxygen into said nitrogen oxide absorber;
electrochemically regenerating said nitrogen oxide absorber by electrochemically reducing species containing said absorbed nitrogen oxides; said species being produced during said absorption.
In a preferred embodiment the nitrogen oxides are absorbed in said nitrogen oxide absorber without applying any electrical potential between the working electrode and the counter electrode whereby the adsorption process is made more efficient by not polarising the reactor and furthermore power is saved by only polarising the reactor during the relatively short regeneration period.
In a particularly preferred embodiment said nitrogen oxide absorber is regenerated by applying an electrical potential between said nitrogen oxide absorber and said counter electrode in the range from 0 to 1.5 V, preferably from 0.2 to 1.0 V, most preferred from 0.4 to 0.7 V whereby the potential can be kept as low as possible to save power. In the case of energetically unfavourable conditions for the reduction, selecting a higher potential can boost the process.
In still preferred embodiment said regeneration is carried out at an electrical current density allowing more than 80% regeneration of said nitrogen oxide absorber after a regeneration time in the range from 5- 40 s, preferably 5-30 s, most preferred 5-15 s whereby the adsorber is inactive during the regeneration process. Therefore, by minimising the regeneration time and keeping it short compared to the adsorption time, the total reduction rate for the NOx content in the exhaust has can be optimised.
By changing the length of the adsorption period and the regeneration period relative' to each other, the process can be adapted to cope with strongly varying contents of nitrogen oxides in the exhaust gas.
In another preferred embodiment said electrical current density allowing more than 90% regeneration of said nitrogen oxide absorber after said regeneration time. In another preferred embodiment said nitrogen oxide absorber absorbs more than 60%, preferably in the range 60-80% of the nitrogen oxides of the mixutre of nitrogen oxides and oxygen.
In another preferred embodiment said absorption of nitrogen oxides is carried out to saturation of said nitrogen oxide absorber.
In another preferred embodiment said nitrogen absorber and said working electrode are intermixed.
In another preferred embodiment said working electrode is a working electrode according to the invention.
Definition of expressions
The expression electrical current density is intended to designate electrical current per electrode area, said electrode area typically being the geometrical area of the electrode. In assessment of a measure of an electrode area, adjustment for variations of the microstructure and porosity of the electrode material can be done.
3. BRIEF DESCRIPTION OF THE DRAWINGS
In the following, by way of examples only, the invention is further disclosed with detailed description of preferred embodiments. Reference is made to the drawings in which
Fig. 1 shows an exemplary cyclic voltametric measurement of a working electrode comprising La0#B2Sr0-14Fe0-1Mn0-9O3 in presence of nitrogen monooxide, and in presence of oxygen, respectively;
Fig. 2 shows an exemplary cyclic voltametric measurement of a comparison working electrode comprising Co304 presence of nitrogen monooxide, curve 1, and in presence of oxygen, curve 2, respectively;
Fig. 3 shows an exempel of cyclic voltametric measurement of a working electrode comprising La085Sr015Mn03 presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively;
Fig. 4 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of nitrogen monooxide, said working electrodes comprising LSM materials having different degrees of doped strontium as cathode;
Fig. 5 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of oxygen, said working electrodes comprising LSM materials similar to those used for the measurements shown in Fig. 4;
Fig. 6 shows a cross sectional sketch of an embodiment of an electrochemical reactor according to the invention;
Fig. 7 shows a cross sectional sketch of an embodiment of an electrochemical cell for an electrochemical reactor according to the invention; Fig. 8 shows a cross sectional sketch of another embodiment of an electrochemical cell for an electrochemical reactor according to the invention; and
Fig. 9 shows a cross sectional sketch of an experimentel electrochemical set-up for voltametric measurements.
4. DETAILED DESCRIPTION
Fig. 1 shows an exemplary cyclic voltametric measurement of a working electrode comprising La0,e2Sr0.14Fe0.1Mn09O3 in presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively.
The y-axis indicates electric current density in I/μA of the working electrode having an electrode area of about 0.01 cm2.
The x-axis indicates the potential of the working electrode E in V versus a standard hydrogen gas electrode of 2.9% H2 and 3.1% H20 in argon in equilibrium with a platinum electrode.
An electrochemical cell comprising a working electrode comprising La0/82Sr0.14Fe0
Figure imgf000016_0001
was prepared according to the procedure used in Example 1 (see below) .
It is seen that at an decreasing potential from about 0.5 V to about -0.1 V, the reaction rate of the reduction of 02 increases steadily, while the reduction rate for NO is close to zero. For even lower potentials, the reaction rate for NO increases very strongly. These conditions are not in favour for NOy reduction. Fig. 2 shows an exemplary cyclic voltametric measurement of a comparison working electrode comprising Co304 in presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively.
The y-axis indicates electric current density in I/μA of the working electrode having an electrode area of about
0.01 cm'.
The x-axis indicates the potential of the working electrode E in V versus a standard hydrogen gas electrode of 3% H2 and 8% H20 in argon in equilibrium with a platinum electrode.
Fig. 3 shows an exe pel of cyclic voltametric measurement of a working electrode comprising La0.B5Sro.15Mn03 in presence of nitrogen monooxide, curve B, and in presence of oxygen, curve A, respectively.
The reduction rate for NO (curve B) steadily increases numerically as the potential is lowered from about 1 V to about 0 V. Note the polarisation is negative for the working electrode. The reduction rate for 02 is very low (close to zero electric current density) from about 0 V to about 0.5 V. At lower potentials the reduction rate for 02 increases steadily.
In the range of about 0.9 V to about 0.5 V, the reduction rate for NO is more than 2 orders of magnitude higher than the reduction rate for 02. Consequently, the LSM material, here specifically La0.85Sr015MnO3, is very well suited as electrode material for selective reduction of nitrogen oxides in presence of oxygen. Fig. 4 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of nitrogen monooxide, curves LSM05, LSM15, LSM25, LSM35 and LSM50, said working electrodes comprising LSM materials having different degrees of doped strontium as cathode.
The designation of the curves LSMy defines used LSM materials of the formula La(1.κ)SrxMn03 wherein y is 100*x, e.g. LSM15 designates the LSM matial La085Sr015MnO3.
The reduction rate for NO is significantly higher for LSM15 as the cathode material than for any of the other tested LSM materials in the range 0.2 to 0.8 V.
Fig. 5 shows five examples of cyclic voltametric measurements of a series of working electrodes in presence of oxygen, curves LSM05, LSM15, LSM25, LSM35 and LSM50, said working electrodes comprising LSM materials having different degrees of doped strontium as cathode similar as the LSM materials used for the measurements shown in Fig. 4.
It is seen that the reduction rate for 02 increases significantly with increasing x.
Fig. 6 shows a cross sectional sketch of an embodiment of an electrochemical reactor according to the invention;
The electrochemical cell comprises an oxygen ion conducting electrolyte 1, here CGO, a selective cathode 2, here an LSM15 material, and an anode 3, here platinum. The electrochemical cell is placed in an gas conduit means 21,22 for an exhaust gas stream from an engine, here a gas inlet tube 21 and a gas outlet tube 22. The raw gas stream 11 containing NOx enters the cathode area 2 of the electrochemical cell, where the NOx is reduced to N2 and 02. The treated gas 12 leaves the cathode area. The cell is polarised from an external power supply 5 with controlled potential through the leads 4.
Fig. 7 shows a cross sectional sketch of an embodiment of an electrochemical cell for an electrochemical reactor according to the invention.
The electrochemical cell comprises an oxygen ion conducting electrolyte 1, a cathode, 2, made from a mixture of cathode catalyst particles 7, here LSM15, and NOx adsorbing particles 8, here BaO particles, and an anode 3, here a platinum electrode.
For illustrative purpose the size of the particles is strongly exaggerated. In the real cell the particle size was in the range of about 0.1 to 10 μm.
Fig. 8 shows a cross sectional sketch of another embodiment of an electrochemical cell for an electrochemical reactor according to the invention.
The electrochemical cell comprises an oxygen ion conducting electrolyte 1, a cathode 2, here made from a layer of cathode catalyst material 7, here LSM15, and a porous layer of a NOx adsorbing material 8, here sintered BaO particles, and an anode 3, here a platinum electrode. Fig. 9 shows a cross sectional sketch of an experimentel electrochemical set-up for cyclic voltametric measurements .
The electrochemical cell comprises an oxygen ion conducting electrolyte 1, a working cathode electrode 2, e.g. made from a layer of cathode catalyst material as LSM15 and formed in the shape of a cone with a narrow tip for improved positioning of the electrode, said working cathode electrode further comprising e.g. a porous layer of a NOx adsorbing material 8, here sintered BaO particles, and an anode 3, e.g. a platinum electrode.
The set-up further shows a potentimetric power supply 51, e.g. a potentiostatic power supply supplied by University of Southern Denmark, Odense, supplying electrical currenct through the leads 41,42.
EXAMPLES
Preferred embodiments of the invention are further illustrated by examples of production of electrochemical cells having working electrodes based on LSM materials.
Example 1 w Series of La1_xSrxMn03 working electrodes"
"Preparation"
A series of 5 electrochemical cells were produced, each comprising an ion selective electrolyte produced by pressing 1 mm thin plates of CGO (cerium oxide doped with 10 atomic-% gadolinium oxide, i.e. Ce0,9Gd0.1O1.g5, supplied Rhodia Electronics and Catalyst, and subsequently placing the CGO plates in an electrical furnace sintering the plates at a temperature in the range 1400-1550 °C for 2-4 hours .
Working electrodes of the LSM type were provided by depositing La1.xSrxMn03 onto the exposed upper side of the sintered CGO plates.
LSM materials were prepared by evaporating a solution of the corresponding metal nitrates, e.g. La(N03)3( Sr(N03)2, and Mn(N03)2 stabilised by addition of citric acid. The residue powder was calcinated at a temperature in the range of 900-1100 °C for 1-3 hours.
A slurry of fine powder of LSM in water was prepared and organic binder, e.g. methylcellulose and other additves, e.g. dispersing agents were added to stabilize the slurry.
The slurry was then applied to one side of the sintered CGO plates by painting or screen printing.
The CGO plates were then sintered further at a temperature in the range 1000-1200 °C for 2-4 hours.
Counter electrodes were provided on the other side of the sintered CGO plates by applying platinum paste comprising platinum powder and organic binder supplied from Engeldhard.
Then CGO plates were then sintered at 800 °C for 1 hour.
The preparation of electrochemical cells were repeated with different LSM materials having values for x in the general formula of 0.05, 0.15, 0.25, 0.35, and 0.50. "Cyclic voltametric measurements"
Cyclic voltametric measurements were performed on the produced electrochemical cells in a N2 gas containing 2 vol-% NO and in an N2 gas containing 10 vol-% 02. The N2-gas mixture was supplied by Hede Nielsen/Air Liquide, Denmark.
Measurements were performed at temperatures in the range between 300 and 500 °C . The results are shown in Fig. 5 and 6 for measurements at 500 °C.
At increasing x the reaction rate of the cathodic reduction of 02 increases. At lower x values than 0,25, the reaction rate of 02 is significant at potentials below 0.5 V.
It appears that cathodic reduction of NO reaches a maximum at x = 0,15.
The experiments show that good selectivity for La0.B5Sr015MnO3 between electrochemical reduction of NO and 02 can be obtained.
Further experiments have shown that similar good selectivity can be obtained for x values in the range of 0.12 to 0.18. Outside this range, the selectivity becomes less good either because of a relatively faster reduction rate of 02 and/or a slower reduction rate of NO.
Example 2 wLa0.85Sro.15Mn03 based working electrode''
An electrochemical cell with a working electrode comprising La0.85Sr0.15MnO3 was prepared as described in Example 1. The cell was tested at a temperature of 300 °C in a flowing N2 gas containing 1000 ppm NO and 10 vol.-% 02. The cell was polarised with 0.5 volts.
Because some N02 is formed in the mixture of NO and 02, the contents of NO and N02 were measured in the exhaust gas of the electrochemical cell by mass spectrometry analysis using a Varian mass spectrometer. The reduction rate of NO was measure in the range of 40 to 80% depending on the gas flow rate through the cell, said reduction rate being based on the combined content of NO and N02 measured.
Example 3 wLa0.85Sro.15Mn03 and BaO based working electrode''
An electrochemical cell with a working electrode comprising a mixture of 50 weight-% La0.85Sr015Mn03 and 50 weight-% BaO supplied from Merck was prepared as described in Example 1. During the preparation, the working electrodes were activated by addition of platinum. Platinum was added by impregnating a solution of PtCl4 in 0.1 N hydrochloric acid into the working electrode material. Then the working electrode were dried and heated to a temperature of 600 °C. BaO functions as an absorber of nitrogen oxides. Pt functions as an auxiliary catalyst for the NOx adsorption reactions. Depending on the exact composition of the exhaust gas several possible reaction can take place, e.g.
2 NO + 1.502 + BaO -> (Pt) Ba(N03)2
The electrochemical cells were tested at a temperature of
300 °C in a N2 gas containing 1000 ppm NO and 10% 02. The cells were run for 2 min without polarisation of the working electrode allowing NO to become absorbed into the working electrode BaO material. Then the working electrode was polarised with 0.5 volts for 20 seconds. The content of NO and N02 were measured in the exhaust gas of the electrochemical cell. The reduction rate of NO was between 60 and 90% depending on the gas flow rate through the cell, said reduction rate being based on both the measured contents of NO and N02.
Example 4 "Energy consumption - calculation"
In a typical automobile with turbo charged diesel engine of 2 1 displacement, driving at constant speed of 120 km/h, NOx in exhaust gas is typically 750 ppm.
Under these conditions, the engine will deliver about 20- 25 kW. The exhaust gas flow will be about 80 1/s. The temperature will be about 300 °C .
For simplicity NOx is calculated as NO, since in a diesel engine, the NO content is more than about 90 vol.-% of the total NOx. The NO content is 80*750 ppm = 0.06 1/s. The number of moles of NO is n = 0.06/0.082/575 = 0.00127 mol/s .
Multiplying with Faradays constant and multiplying with 2 for the number of electrons in the reaction provides the demand for current :
I = 0.00127 * 96500 * 2 = 246 A
With a current efficiency of 60% and a potential of 0,5 volts, this provides a power demand of
P= 216 * 0.5/80 *100 = 205 W This corresponds to 0.8% of the engine's power.

Claims

1. A working electrode for an electrochemical reactor, the electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte;
the working electrode comprising an electric conductive ceramic oxide material having the general formula:
AXA (1_x)ByB (1_y)0(3_δ)
wherein A and A' designate first substitution metals of similar sizes, said first substitution metals having a high efficiency for reducing vacancies for oxygen ions, O≤x≤l ;
B and B1 designate second substitution metals of similar sizes, said second substitution metals being of smaller sizes than those of said first substitution metals, and having a high transition efficiency between oxidation states, O≤y≤l ;
0 designates oxygen;
and δ is a small number, positive or negative, that allows for compensation of differences in valences of said metals .
2. The working electrode according to claim 1 wherein A is selected from the group consisting of rare earth metals: Sc, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; metals of group 3a: Al, Ga, and In; and group 3b: Sc, Y, La of the periodic table, preferably La, Gd, In and Y; and A' is selected from the group consisting of alcaline earth metals: Mg, Ca, Sr, and Ba; and Eu, preferably Ca, Sr, Ba, and Eu.
3. The working electrode according to claims 1 or 2 wherein B and B' are selected from the group consisting of transition metals:
group lb: Cu and Ag; group 2b: Zn; group 3a: Ga, In, and Tl; group 3b: Sc, and Y; group 4b: Ti, Zr, Hf; group 5b: V, Nb, Ta; group 6b: Cr, Mo, W; group 7b: Mn and Re; and group 8: Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt;
preferably Cr, Mn, Fe, Co, and Ni .
4. The working electrode according to claims 1 to 3 wherein said ceramic oxide comprises lanthanum manganite doped with strontium oxide, LaxSr1.xMn03, the stoichio- metric coefficient 1-x being in the range 0.05 to 0.20, preferably 0.10 to 0.18, most preferred about 0.15.
5. An electrochemical reactor comprising the working electrode, the counter electrode, and the ion-selective electrolyte wherein the working electrode is a working electrode as defined in claims 1-4.
6. A method of reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, the method comprising: providing an electrochemical reactor comprising a working electrode, a counter electrode, and an ion-selective electrolyte; said working electrode being adapted to reduce nitrogen oxides to nitrogen and oxygen, and said working electrode being adapted to suppress reduction of oxygen to oxygen ions; said working electrode processes being substantially according to the cathode electrode processes :
(a) 2N0X + 2xe" -> N2 + 2x02" (b) 02 + 4e~ -> 202~
and the anode electrode process:
(c) 202~ -> 02 + 4e~
said cathode electrode processes (a) and (b) being carried out at a potential selected within a range of -1500 mV to +1500 V between said working electrode and said counter electrode, and at a temperature within a range of 200 to 500 °C;
and said working electrode comprising an electric conductive ceramic of lanthanum manganite; said lanthanum manganite being doped with an oxygen ion vacancy quencher for quenching vacancies for oxygen ions; said oxygen ion vacancy quencher being in an effective amount to suppress said reduction of oxygen to oxygen ions at the working electrode so that the rate of reduction of nitrogen oxides is faster than the rate of reduction of oxygen at the selected potential and temperature.
7. The method according to claim 6 wherein said selected potential is selected within the range from -200 V to 800 mV, said potential being measured versus a hydrogen electrode of 8% H20 and 3% H2 in Ar.
8. The method according to claims 6 and 7 wherein said oxygen ion vacancy' quencher is selected from the group consisting of Sr, Ca, Ba, and Eu.
9. The method according to claims 6-8 wherein said electrochemical reactor comprises a working electrode as defined in claims 1-4.
10. An electrochemical reactor for reduction of nitrogen oxides in a mixture og nitrogen oxides and oxygen, the electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen absorber for absorbing nitrogen oxides; wherein said nitrogen absorber is adapted for electrochemical regeneration thereof.
11. The reactor according to claim 10 wherein said nitrogen absorber and said working electrode are intermixed.
12. The reactor according to claim 10 wherein said nitrogen absorber comprises a porous layer on said working electrode.
13. The reactor according to claims 10-12 wherein the nitrogen absorber comprises a material or a combination of materials selected from the group consisting of Na20, K20, MgO, CaO, SrO, and BaO, preferably BaO.
14. The reactor according to claims 10-13 wherein said working electrode is a working electrode as defined in claims 1-4.
15. A method of electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen, the method comprising:
providing an electrochemical reactor comprising a working electrode, a counter electrode, an ion-selective electrolyte, and a nitrogen oxide absorber for absorbing nitrogen oxides;
absorbing nitrogen oxides from the mixture of nitrogen oxides and oxygen into said nitrogen oxide absorber;
electrochemically regenerating said nitrogen oxide absorber by electrochemically reducing species containing said absorbed nitrogen oxides; said species being produced during said absorption.
16. The method according to claim 15 wherein the nitrogen oxides are absorbed in said nitrogen oxide absorber without applying any electrical potential between the working electrode and the counter electrode.
17. The method according to claims 15 and 16 wherein said nitrogen oxide absorber is regenerated by applying an electrical potential between said nitrogen oxide absorber and said counter electrode in the range from 0 to 1.5 V, preferably from 0.2 to 1.0 V, most preferred from 0.4 to 0.7 V.
18. The method according to claims 15-17 wherein said regeneration is carried out at an electrical current density allowing more than 80% regeneration of said nitrogen oxide absorber after a regeneration time in the range from 5-40 s, preferably 5-30 s, most preferred 5- 15 s.
19. The method according to claim 18 wherein said electrical current density allow more than 90% regeneration of said nitrogen oxide absorber after said regeneration time.
20. The method according to claims 15-19 wherein said nitrogen oxide absorber absorbs more than 60%, preferably in the range 60-80% of the nitrogen oxides of the mixutre of nitrogen oxides and oxygen.
21. The method according to claims 15-20 wherein said absorption of nitrogen oxides is carried out to saturation of said nitrogen oxide absorber.
22. The method according to claims 15-21 wherein said nitrogen absorber and said working electrode are intermixed.
23. The method according to claims 15-22 wherein said working electrode is a working electrode defined in claims 1-4.
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