CZ20033504A3 - Process and apparatus for electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen - Google Patents

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

Process and apparatus for the electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen

Technical field

The present invention relates to methods and apparatus for the electrochemical reduction of nitrogen oxides in a mixture of nitrogen oxides and oxygen.

Preferably, the invention relates to a working electrode for an electrochemical reactor, an electrochemical reactor comprising such a working electrode, methods for reducing nitrogen oxides in a mixture of oxides of nitrogen and oxygen using a working electrode comprising an electrically conductive lanthanum manganese ceramic with an oxygen-depleting agent.

In a further advantage, the invention relates to an electrochemical reactor comprising a nitrogen oxide absorber adapted for electrochemical regeneration and methods of electrochemical reduction of nitrogen oxides in a mixture of oxides of nitrogen and oxygen using an electrochemical reactor.

BACKGROUND OF THE INVENTION

In this context, the term "nitrogen oxides", often referred to as NO _X , refers to one or more oxygen and nitrogen compounds, such as NO and NO ₂ , etc. Furthermore, "nitrogen oxide absorber" refers to an absorber for nitrogen oxides, e.g. in the form of a compound or a mixture of compounds.

The reduction of NO _X in the presence of oxygen is known.

In a method adapted to combustion processes, excess fuel is added for a short period of time to provide a reducing agent, i.e. the addition of CH, by which NO _{X is} reduced according to concurrent reactions:

• · · (1) 2 ΝΟχ + x CH + x / 2 0 ₂ N ₂ + x CO ₂ + x / 2 H ₂ O (2) 11/2 O ₂ + CH -> CO2 +> / ₂ H ₂ O

However, the addition of CH will affect combustion processes and thus the heat produced by the engine.

In the electrochemical reduction of NO _X in the presence of O ₂ , simultaneous electrode processes occur between electrons and NO _X and O ₂ on the working electrode, as can be expressed by electrode processes at the cathode:

(3) 2NO _x + 4xe N ₂ + 2xO ² '(4) O ₂ + 4 e'-> 2O ² '

For a given voltage and current density, the available electrons react with any of the reactants

NO _X or O ₂ .

The method of increasing selectivity of NO _x reduction relative to O ₂ reduction involves increasing the amount of NO _X relative to O ₂ prior to electrochemical reduction. Alkaline earth metals such as MgO or CaO have been used to absorb NO _X. Subsequently, NO _{X is} released by heat recovery before the electrochemical reduction of NO _X.

Another method of increasing the selectivity of NO _x reduction relative to O ₂ reduction involves increasing NO _X access to the working electrode of the working electrode compared to the O ₂ approach, or similarly increasing the working electron access of the working electrode to NO _X compared to the electron approach to O ₂ .

Patents of the invention:

U.S. Patent No. 5,022,975 relates to a device for electrochemical control of particulate contamination for adjusting the composition of a gas stream including removal of SO _X and NO _X ; in an embodiment, the apparatus comprises gadolinium salts stabilized with cerium salts as electrolytes.

• · · · · · ·

U.S. Pat. No. 5,401,372 relates to an electrochemical catalytic reduction unit for NO _x reduction in exhaust gases containing oxygen using catalysts diffusing gas such as modified vanadium oxides with a layer of collecting electrons, such as conductive oxides of the type perovskytu, e.g. LSM.

US Patent No. 5,456,807 relates to a method and apparatus for selectively removing nitrogen oxides from gas mixtures, comprising absorbing NO _X with NO _X absorbents, removing absorbed NO _{X by} heat, and electrochemically reducing NO _X to N ₂ and O ₂ in an electrochemical unit with solid oxide.

WO 97/44126 relates to an electrochemical reactor comprising a mixed ion selective electrolyte and electrode material heat-treated gadolinium oxide with 20% CeO and containing about 6 vol% lanthanum oxide with 20% strontium oxide to reduce carbon in nitrogen containing 20% of oxygen. Nothing is said or suggested to reduce NO _X to

N ₂ and O ₂ .

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved method and apparatus for the selective electrochemical reduction of nitrogen oxides in the presence of oxygen.

It is an object of the present invention to provide an improved method and apparatus for the selective electrochemical reduction of nitrogen oxides in the presence of oxygen in gaseous combustion mixtures.

Other objects of the invention will be apparent from the description elsewhere.

According to the present invention, the object is preferably a working electrode for an electrochemical reactor, an electrochemical reactor comprising a working electrode, a paired electrode and an ion-selective electrolyte;

wherein the working electrode comprises an electrically conductive ceramic oxide material having the general formula:

A _X A '(1- _X ) ByB' (ly) O (3-S) wherein

A and A ´ denote the first interchangeable metals of similar sizes, said first interchangeable metals having a high efficiency for reducing the lack of oxygen ions, 0 <x <1;

B and B ´ denote second interchangeable metals of similar sizes, wherein said second interchangeable metals are smaller in size than said first interchangeable metals and have a high transition efficiency between oxidation states, 0 < y <1;

O is oxygen;

and δ denotes a small number, positive or negative, which makes it possible to compensate for differences in the valencies of said metals.

Surprisingly, it has been shown that when selecting a working electrode that includes an electrically conductive ceramic oxide material of the formula ABO3 by definition, the number of vacant sites in the oxygen ion lattice can be reduced by reducing the oxygen intuition conductivity in the ceramic oxide material.

Consequently, a working electrode can be provided which has high selectivity for NO _x reduction and at the same time low O ₂ reduction activity.

Components A, A ', B, and B' of the ΑΑΈΒΌ3 material can be selected in a wide range.

Preferably, the working electrode 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; Group 3a metals: Al, Ga and In; and groups 3b: Sc, Y, La of the periodic table, preferably La, Gd, In and Y;

• · ·

and A se is selected from the group consisting of alkaline earth metals: Mg, Ca, Sr and Ba; and Eu, preferably Ca, Sr, Ba and Eu, wherein the electrical and chemical / catalytic properties of the working electrode can be varied over a wide range.

With another advantage, B and B 'are selected from the group consisting of transition metals: Group 1b: Cu and Ag; Group 2b: Zn; Group 3a: Ga, In and TI; 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;

furthermore, it is achieved that the electrical and chemical / catalytic properties of the working electrode can be varied over a wide range.

Actual elements and stoichiometric coefficients can be selected based on experiments.

For y = 0, preferably the preferred working electrode comprises LSM.

The ceramic oxide preferably comprises lanthanum manganite with the addition of strontium oxide,

La _x Sri. _x MnO3, the stoichiometric coefficient 1-x is in the range from 0.05 to 0.20, preferably from 0.10 to 0.18, most preferably about 0.15, and good selectivity can be achieved for the reduction of nitrogen oxides compared to the reduction of oxygen.

The invention also relates to an electrochemical reactor comprising a working electrode, a paired electrode and an ion-selective electrolyte, the working electrode according to the invention. Such a reactor may be useful for the reduction of nitrogen oxides in the exhaust gases of diesel or light combustion engines where high oxygen content makes it impossible to use standard techniques, such as chemical reduction in a three-way catalyst, to reduce the nitrogen oxide content.

The invention also relates to a method for reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, and the method comprises:

providing an electrochemical reactor comprising a working electrode, a paired electrode, and an ion selective electrolyte; said working electrode adapted to reduce nitrogen oxides to form nitrogen and oxygen, and said working electrode adapted to suppress the reduction of oxygen to oxygen ions; said processes on the working electrode are according to processes on the cathode electrode:

(a) 2 NO _X + 2x e -> N ₂ + 2x O ² '(b) O ₂ + 4 e 2 O ² ' and anode electrode processes:

(c) 2 O ² '-> O ₂ + 4e said processes at cathode electrode (a) and (b) are performed at a voltage selected from the range of -1500 mV up to +1500 mV between said working electrode and said paired electrode, and at a temperature in the range of from 200 to 500 ° C;

and said working electrode, which comprises an electrically conductive lanthanum manganite ceramic; said lanthanum manganite is admixed with an oxygen deficiency replacing agent for replacing the oxygen deficiency; said ion-depleting agent is in an effective amount to suppress said reduction of oxygen to oxygen ions per working • 9444 94 9949 94 9 • 99 9 9 · · 9 «

9 ··· 99 9,9999 • 9 <9,999,999999

9,999,999

999999 99 94 99 4 so that the rate of nitrogen oxide reduction is higher than the rate of oxygen reduction at the selected voltage and temperature.

Preferably, said voltage is selected from the range of -200 mV to 800 mV, said voltage being measured against a hydrogen electrode by 8% H ₂ O and 3% H ₂ in Ar, thereby achieving selectivity further increasing and overall the energy requirement is reduced by reducing the voltage as much as possible without achieving a situation where the reduction rate becomes too low.

Even more preferably, said oxygen ion deficiency replacing agent is selected from the group consisting of Sr, Ca, Ba and Eu.

Preferably, said method uses an electrochemical reactor comprising a working electrode according to the invention, thereby achieving a particularly improved selectivity of nitrogen oxide reduction.

In some applications, the concentration of nitrogen oxides is low. Consequently, it may be desirable to first increase the concentration of nitrogen oxides.

The invention also relates to an electrochemical reactor for reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, the electrochemical reactor comprising a working electrode, a paired electrode, an ion selective electrolyte, and a nitrogen scavenger for absorbing nitrogen oxides; said nitrogen absorber being adapted for its electrochemical regeneration; as a result, the nitrogen oxides can be adapted directly, i.e. also from gas mixtures with a low concentration of nitrogen oxides. Said electrochemical reactor can then readily regenerate the NO _X scavenger by electrochemical reduction without the addition of external heating or the addition of a chemical reducing agent.

Preferably, said nitrogen sink and said working electrode are mixed to achieve close contact between the adsorbed sample containing NO _X and the working electrode. This ensures a rapid, selective and efficient reduction of the NO _X- containing substance.

• ···· 9 9 · 9 9 · · · 9

9 9 9 9 9 9

9999 98 9 9999

9 9 9 9 9 9 9 9 9 9 9 g ··· ··· ·· ·. * Í

Alternatively, said nitrogen scavenger forms a porous layer on said working electrode, thereby obtaining a separate scavenger which may be advantageous for some applications with respect to ease of assembly and maintenance.

Preferably, the nitrogen scavenger comprises a material or combination of materials selected from the group consisting of Na ₂ O, K ₂ O, MgO, CaO, SrO, and BaO, preferably BaO, whereby nitrates and nitrites are readily formed in the presence of nitrogen oxides. Further, these nitrates and nitrites can be easily converted back to oxides under reducing conditions at elevated temperature.

Preferably, in this electrochemical reactor, the working electrode is a working electrode according to the invention

The invention also relates to a method for the electrochemical reduction of nitrogen oxides in a mixture of oxides of nitrogen and oxygen, the method comprising:

providing an electrochemical reactor comprising a working electrode, a paired electrode, an ion selective electrolyte, and a nitrogen oxide scavenger for absorbing nitrogen oxides;

absorbing nitrogen oxides from the mixture of nitrogen oxides and oxygen into said nitrogen oxide scavenger;

electrochemically regenerating said nitrogen oxide scavenger comprising said absorbed nitrogen oxides with an electrochemically reducing agent; wherein said agent is produced during said absorption.

Preferably, the nitrogen oxides are absorbed in said nitrogen oxide scavenger without supplying any electrical voltage between the working electrode and the paired electrode, whereby the adsorption process becomes more efficient because the reactor is not polarized within a relatively short regeneration time.

More preferably, said nitrogen oxide scavenger is regenerated using an electrical voltage between said nitrogen oxide scavenger and said steam electrode in the range of 0 to 1.5 V, preferably of 0.2 to 1.0 V, most preferably of 0.4 to 0.7 V, · · ·

keeping the voltage as low as possible to save energy. In the case of energy disadvantageous conditions for reduction, the choice of a higher voltage can revive the process.

The invention also relates to a regeneration which is carried out at an electric current density which permits a regeneration of said nitrogen oxide scavenger of more than 80% after a regeneration time in the range from 5 to 40 s, preferably from 5 to 30 s, most preferably from 5 to 40 s. 15 sec, wherein the absorber is inactive during the regeneration process. Therefore, minimizing the regeneration time and keeping it for short periods of time compared with the time of adsorption can be optimized rate reduction of _NOx content in the exhaust gases.

By varying the length of the adsorption period and the regeneration period in relation to each other, the process can be adjusted to cope with a strongly varying content of nitrogen oxides in the exhaust gases.

The invention also relates to an electric current density which allows the regeneration of said nitrogen oxide scavenger by more than 90% after said regeneration time.

The invention also relates to the fact that said nitrogen oxide absorber absorbs more than 60%, preferably from 60 to 80%, of nitrogen oxides from a mixture of nitrogen oxides and oxygen.

The invention also relates to the absorption of nitrogen oxides until saturation of said nitrogen oxide scavenger.

The invention also relates to said nitrogen sink and said working electrode being mixed.

The invention also relates to said working electrode being a working electrode according to the invention.

Definitions of meanings:

The meaning of electric current density means electric current to the electrode surface, said electrode surface preferably being a geometric electrode surface. For • · · · · · · · · · · · ·

determining the electrode surface can be made to take into account the microstructure and porosity of the electrode material.

Short description of drawings:

In the following, a detailed description of the invention is set forth by way of example. Reference is made to the drawings in which:

Figure 1 represents an example of a cyclic voltmetric measurement of a working electrode consisting of Lao 8 S 2 O 14 Feo.1Mno 9 O 3 either in the presence of nitrous oxide or oxygen;

Figure 2 shows an example of a cyclic voltmetric measurement of a working electrode comparison consisting of CO 3 O 4 either in the presence of nitrous oxide, curve 1, or in the presence of oxygen, curve 2;

Figure 3 shows an example of a cyclic voltmetric measurement of a working electrode, which consists of Lao.ssSro.isMnOa either in the presence of nitrous oxide, curve B or in the presence of oxygen, curve A;

Figure 4 shows an example of cyclic voltmetric measurements of a series of working electrodes in the presence of nitrous oxide, wherein said working electrodes comprise an LSM material having a different degree of strontium addition as cathode;

Figure 5 shows five examples of cyclic voltmetric measurements of a series of working electrodes in the presence of oxygen, said electrodes comprising an LSM material similar to that used for the measurements presented in Figure 4;

Figure 6 is a cross-sectional drawing of an electrochemical reactor according to the invention;

Figure 7 is a cross-sectional drawing of an electrochemical unit for an electrochemical reactor according to the invention;

Figure 8 is a cross-sectional drawing of another electrochemical unit for the electrochemical reactor of the invention; and ·· ····

9

9 9 ·· 9 9 «« • ··· 9 9 9 9 * 9 9 • · · · 99999 9 * 99

- · 99999999] i ······ ·· ·· ··.

Figure 9 is a cross-sectional drawing of an electrochemical experiment for voltmetric measurements.

Detailed description:

Figure 1 shows an example of a cyclic voltmetric measurement of a working electrode, which consists of Lao.82Sro.14Feo.1Mno.

The y-axis indicates the current density in Ι / μΑ on the working electrode having an electrode surface and with 0.01 cm ² .

The x-axis indicates the voltage at the working electrode in V therefore a standard hydrogen electrode containing 2.9% H ₂ and 3.1% H ₂ O in argon in equilibrium with the platinum electrode.

The electrochemical unit contains a working electrode, which consists of Lao.82Sro.14Feo.1Mno.9O3 and is prepared according to the procedure in Example 1 (see below).

Obviously, as the voltage decreases from about 0.5 V to about -0.1 V, the reaction rate of the O2 reduction increases at a steady rate, while the NO reduction rate approaches zero. At an even lower voltage, the NO reduction rate increases very strongly. These conditions are advantageous for NO _x reduction.

Figure 2 shows an example of a cyclic voltmetric measurement of a working electrode comparison consisting of CO 3 O 4 either in the presence of nitrous oxide, curve 1, or in the presence of oxygen, curve 2.

The y-axis indicates the current density in Ι / μΑ on the working electrode having an electrode surface of about 0.01 cm ² .

The x-axis indicates the voltage at the working electrode E in V versus a standard hydrogen electrode containing 3% H ₂ and 8% H ₂ O in argon in equilibrium with the platinum electrode.

Figure 3 represents an example of a cyclic voltmetric measurement of a working electrode consisting of Lao.ssSro.isMnOs either in the presence of nitrous oxide, curve B, or in the presence of oxygen, curve A.

4 · 4 4 4 4 4 4 4

4,444,444

4 4 4 4 4 5

44 44 4

The NO reduction rate (curve B) increases numerically as the voltage decreases from about 1 V to about 0 V. It should be noted that the polarization of the working electrode is negative. The O ₂ reduction rate is very low (near zero electric current density), from about 0 V to about 0.5 V. At low voltages, the O ₂ reduction rate increases uniformly.

In the range of about 0.9 V to about 0.5 V, the NO reduction rate is more than 2 orders of magnitude higher than the O ₂ reduction rate. As a result, the LSM material, here preferably LaOssSro.isMnOj, is well suited as an electrode material for selectively reducing nitrogen oxides in the presence of oxygen.

Figure 4 shows an example of cyclic voltmetric measurements of a series of working electrodes in the presence of nitrous oxide, LSM05, LSM15, LSM25, LSM35 and LSM50, wherein said working electrodes comprise LSM materials having different degrees of strontium addition as cathode.

Determination of LSMy curves defines the use of LSM materials of formula La (i. _X ) Sr _x MnO 3 in which y is 100 * x, eg LSM15 determines the LSM material as Lao.ssSro.isMnOs.

The NO reduction rate is significantly higher for the LSM15 as cathode material than for any other LSM materials in the range of 0.2 to 0.8 V.

Figure 5 shows five examples of cyclic voltmetric measurements of a series of working electrodes in the presence of oxygen, LSM05, LSM15, LSM25, LSM35, and LSM50, said electrodes comprising LSM materials having different degrees of strontium addition as cathode similar to the LSM materials used for measurements in the picture

4.

Obviously, the rate of O ₂ reduction increases significantly with increasing x.

Figure 6 is a cross-sectional drawing of an electrochemical reactor according to the invention.

The electrochemical unit comprises an electrolyte 1 which conducts an oxygen ion, here CGO, a selective cathode 2, here an LSM 15 material, and an anode 3, here platinum. The electrochemical unit is

0 · 0 · ·· ···· 0 0

0 0 0 000 • 000 0 · 0 0 0 0 0 0 0 * 00 0000 · 0 ·

0 000 «··· * · * * .........

located in the gas supply means 21,22 through which the exhaust gases flow from the engine, here the inlet tube 21 and the outlet tube 22. The raw gas stream 11, which contains NO _X , enters the cathode surface 2 of the electrochemical unit where NO _{X is} reduced to N ₂ and O ₂ . The treated gas 12 leaves the cathode surface. The unit is polarized by an external power supply 5 with controlled voltage through line 4.

Figure 7 is a cross-sectional drawing of an electrochemical unit for an electrochemical reactor according to the invention.

The electrochemical unit comprises an oxygen ion-carrying electrolyte 1, a cathode 2 made from a mixture of cathode catalyst particles 7, here LSM15, and particles 8 that adsorb NO _X , here BaO particles and anode 3, here a platinum electrode.

For purposes of illustration, the particle size is strongly exaggerated. In the actual unit, the particle size was in the range of about 0.1 to 10 μπι.

Figure 8 is a cross-sectional drawing of another electrochemical unit for the electrochemical reactor of the invention.

The electrochemical unit comprises an electrolyte 1, which conducts an oxygen ion, cathode 2, here made of a cathode catalyst material layer 7, here LSM15, and a porous layer of material 8 that adsorbs NO _X , here fused BaO particles, and anode 3, here a platinum electrode.

Figure 9 is a cross-sectional drawing of an electrochemical experiment for voltmetric measurements.

The electrochemical unit comprises an oxygen ion-conducting electrolyte 1, a working electrode 2, eg made of a layer of cathode catalyst material such as LSM15, and formed into a cone shape with a narrow tip to improve placement on the electrode, said cathode electrode further comprising e.g. a layer of material 8 that adsorbs NO _X , the fused BaO particles here, and an anode 3, e.g. a platinum electrode.

999 999

9999 ·· 9 • 9 9 9 9 • 9 9 9 9 9 9 9

9 9 9 9 9

99 99 9

The arrangement further comprises a potentiometric power supply 51, such as a potentiometric power supply supplied by the University of Southern Denmark, Odense, which supplies electrical current via conductors 41, 42.

DETAILED DESCRIPTION OF THE INVENTION

Example 1: “Lai working electrode series. _x Sr _x MnC> 3 "

A series of 5 electrochemical units, each containing an ion-selective electrolyte made by pressing a 1 mm thick sheet of CGO (cerium oxide with addition of 10 atomic gadolinium oxide, Ceo.9Gdo.1O1.95, supplied by Rhodia Electronics) was produced. and Catalysts), whereupon the CGO sheets were placed in an electric furnace to bake the sheets at a temperature ranging from 1400 to 1500 ° C for 2-4 hours.

The LSM type working electrodes were coated with Lai _x Sr _x MnO3 on the exposed upper side of the CGO sintered sheets.

LSM materials were prepared by evaporating a solution of the corresponding metal nitrates, e.g. La (NO3) 3, Sr (NO3) ₂ and Mn (NO3) ₂ , stabilized by the addition of citric acid.

The resulting powder was calcined at a temperature ranging from 900 to 1100 ° C over 1 to 3 hours.

An LSM fine powder dispersion in water was prepared and then an organic binder such as methylcellulose and other additives such as dispersing agents were added to stabilize the dispersion.

The dispersion was then applied to one side of the sintered CGO sheets using a platinum paste containing platinum powder and an organic binder supplied by Engeldhard.

Then, the CGO sheets were sintered at 800 ° C for 1 hour.

The production of electrochemical units was repeated with different LSM materials having x values of 0.05, 0.15, 0.25, 0.35 and 0.50 in the general formula.

• 4 · 4

444

4444

4

4 4

444

4 4

44

4 • 4 4

4 4

4 4

4 “Cyclic voltmetric measurements”

Cyclic voltmetric measurements were performed on the produced electrochemical units in an N ₂ atmosphere containing 2 vol% NO and in an N ₂ atmosphere containing 10 vol% O ₂ . The N ₂ gas mixture was supplied by Hede Nielsen / Air Liquide, Denmark.

The measurements were performed at temperatures between 300-500 ° C. The results are shown in Figures 5 and 6 for measurements at 500 ° C.

With increasing x, the reaction rate of O ₂ reduction at the cathode increased. At x values less than 0.25, the reaction rate of O _{2 is} significant at voltages below 0.5 V.

It appears that the reduction of NO at the cathode reaches a maximum at x = 0.15.

Experiments suggest that good selectivity in Lao.ssSro.isMnOs between electrochemical reduction of NO and O ₂ can be achieved.

Further experiments have shown that similar good selectivity can be achieved for x values ranging from 0.12 to 0.18. Outside this range, selectivity becomes worse on account of the relatively higher O 2 reduction rate and / or the lower NO reduction rate.

Example 2: "Working electrode based on Lao.ssSro.isMnOa"

Electrochemical unit comprising a working electrode Lao.85Sr .i5Mn0 _{0 3} was prepared as described in Example 1. The unit was tested at 300 ° C in N ₂ gas stream, which contained 1000 ppm NO and 10 vol.% O _2. The unit was polarized 0.5 volts.

Since some NO ₂ is formed in a mixture of NO and O ₂ , the NO and NO ₂ content was measured in the exhaust gas downstream of the electrochemical unit by mass spectrometry analysis using a Varian mass spectrometer. The NO reduction rate was measured in the range of 40 to 80%, depending on the gas flow rate through the unit, said reduction rate being related to the combined NO and NO ₂ content being measured.

99 9

9999

9,999,999 • ··· · · 9,999,999 • 9,999,999,999 • 9,999,999

...... .......

Example 3: "Lao-based working electrode ^ Sro.isMnOs and BaO"

Electrochemical unit with working electrode containing 50 wt. %

Lao.85Sro.i5Mn03 and 50 wt. % BaO, supplied by Merck, was prepared as described in Example 1. During the preparation, the working electrodes were activated by the addition of platinum.

Platinum was added by impregnating a solution of PtCU in 0.1 N hydrochloric acid into the working electrode material. Then the working electrodes were dried and heated to 600 ° C. BaO acts as a nitrogen oxide scavenger. Pt acts as a co-catalyst for NO _x adsorption reactions. Depending on the actual composition of the flue gases, several reactions may occur, eg:

NO + 1.5 O ₂ + BaO -> (Pt) Ba (NO ₃ ) ₂

Electrochemical units were tested at 300 ° C in N ₂ gas containing 1000 ppm NO and 10% O ₂ . The units were operated for 2 min without working electrode polarization, thus allowing the start of NO absorption into the working electrode BaO material. Then the working electrode was polarized at 0.5 volts for 20 seconds. The NO and NO ₂ content was measured in the exhaust gas downstream of the electrochemical unit. The NO reduction rate was between 60 and 90% depending on the gas flow rate through the unit, said reduction rate being dependent on the measured content of both NO and NO ₂ gases.

Example 4: "Energy consumption - calculation"

In a typical automobile with a diesel engine in a 2 1, fitted with a turbocharger, which travels a constant speed of 120 km / h, the content of NO _X in the exhaust gas is typically 750 ppm.

• 0000 ····

0 0 0 0 · 0 0 0 0000 0000 0000 000 0000 000 0000 000

Under these conditions, the engine will deliver about 20 to 25 kW. The exhaust gases will flow at a rate of about 80 1 / s. The temperature will be about 300 ° C.

For the sake of simplicity, NO _{X is} calculated as NO because the diesel engine has a NO content of more than about 90% by volume of the total NO _X content. The NO content is 80 * 750 ppm = 0.06 l / s. The number of moles of NO is n = 0.06 / 0.082 / 575 = 0.00127 mol / s.

Multiplying by Faraday's constant and multiplying by two due to the number of electrons in the reaction, provides the current requirement:

I - 0.00127 * 96500 * 2 = 246A

Due to the current efficiency of 60% and at a voltage of 0.5 volts we get the power requirement:

P = 216 * 0.5 / 80 * 100 = 205 watts

This corresponds to 0.8% of the engine power.

Claims (2)

What is claimed is: 1. A method for reducing nitrogen oxides in a mixture of oxides of nitrogen and oxygen, the method comprising the use of an electrochemical reactor comprising a working electrode for reducing nitrogen oxides to nitrogen and oxygen, a vapor electrode and an ion selective electrolyte; processes that take place on electrodes are basically the following processes: at the cathode (a) 2NO _x + 2xe AN ₂ + 2x O ² '(b) O ₂ + 4e -> 2 O ² ' at the anode (c) 2 O ² '-> O ₂ + 4é wherein said processes at the cathode electrode (a) and (b) occur at a voltage of between -1500 mV and + 1500 mV between said working electrode and said counter electrode and at temperatures ranging from 200 to 500 ° C; and wherein said working electrode comprises an electrically conductive ceramic material of lanthanum manganite (LaMnCh), lanthanum chromite (LaCrC> 3), lanthanum ferrite (LaFeO ₃ ), lanthanum cobaltite (LaCoO ₃ ), or nickel-lanthanum oxide (LaNiCh); wherein one or more metals of the group comprising Sr, Ca, Ba, Eu, Fe, Co and Ni are added to said material in an effective amount to achieve a nitrogen oxide reduction rate higher than the oxygen reduction rate at a given voltage and temperature. }Ό} · ··· · · · · · ······ 262. The method of claim 1, wherein the voltage at the working electrode is between -200 mV and 800 mV, measured against a hydrogen electrode saturated with 8% H ₂ O and 3% H ₂ in Ar. Method according to claims 1 and 2, characterized in that the working electrode is Lai _x Sr _x MnO 3, where x is a number in the range of 0.12 to 0.18. The process according to any one of claims 1 to 3, wherein the mixture of nitrogen oxides and oxygen is concentrated by means of a scavenger capable of absorbing nitrogen oxides and selected from the group consisting of Na ₂ O, K ₂ O, MgO, CaO, SrO and BaO and that nitrogen oxides absorbed in the absorber can react with the working electrode. 5. The method of claim 4, wherein the absorber comprises a working electrode and a paired electrode so that the absorbed nitrogen oxides react with the working electrode of the electrochemical reactor by establishing an electrical voltage between the electrodes. The method of claim 5, wherein the nitrogen oxides are absorbed in the absence of any electrical voltage between the working electrode of the absorbed material and the paired electrode of the absorbed material. The method of any one of claims 4 to 6, wherein the absorber working electrode comprises an electrically conductive ceramic of lanthanum manganite (LaMnO ₃ ), lanthanum chromite (LaCrO ₃ ), lanthanum ferrite (LaFeO). ₃ ) lanthanum cobaltite (LaCoO ₃ ) or nickel-lanthanum oxide (LaNiO ₃ ); wherein one or more metals from the group comprising Sr, Ca, Ba, Eu, Fe are added to said material; 2X • ·· Co and Ni in an effective amount to achieve a higher nitrogen oxide reduction rate than the oxygen reduction rate at a given voltage and temperature. The method of any one of claims 4 to 7, wherein the nitrogen oxides are reduced during the same time that the absorber is regenerated. Method according to any one of claims 4 to 8, characterized in that the absorber e is regenerated by applying an electrical voltage between the absorber working electrode and the absorber pair electrode in the range from 0 to 1.5 V, preferably from 0.2 to 1.0 V, most preferably from 0.4 to 0.7 V. Method according to claims 8 and 9, characterized in that said regeneration is carried out at an electric current density which enables regeneration of said absorber by more than 80% during a regeneration period in the range from 5 to 40 s, preferably from 5 to 30 s , most preferably from 5 to 15 s. 11. The method of claim 10, wherein said electric current density allows said nitrogen oxide scavenger to be recovered by more than 90% during said regeneration time. Method according to any one of claims 4 to 11, characterized in that said absorber absorbs more than 60%, preferably in the range from 60 to 80%, of nitrogen oxides from a mixture of nitrogen oxides and oxygen. • 4 • 4 4 2 / • 44 4 ·· 4 The method of any one of claims 4 to 12, wherein said absorption of nitrogen oxides continues until saturation of the absorber. The method of any one of claims 4 to 13, wherein said absorber and said electrode are mixed with each other. 15. An electrochemical reactor for reducing nitrogen oxides in a mixture of oxides of nitrogen and oxygen, comprising a working electrode for reducing nitrogen oxides to nitrogen and oxygen, a steam electrode, an ion selective electrolyte, said working electrode comprising an electrically conductive manganite ceramic material lanthanum (LaMnO ₃ ), lanthanum chromite (LaCrO ₃ ), lanthanum ferrite (LaFeO ₃ ), lanthanum cobaltite (LaCoO ₃ ), or nickel-lanthanum oxide (LaNiO ₃ ); wherein one or more metals of Sr, Ca, Ba, Eu, Fe, Co, and Ni are added to said material in an effective amount to achieve a nitrogen oxide reduction rate higher than the oxygen reduction rate, and wherein the reactor further comprises means which: maintaining a voltage between -1500 mV and +1500 mV between said working electrode and said paired electrode, and means to maintain a temperature in the range of 200 to 500 ° C. 16. The reactor of claim 15, further comprising a nitrogen oxide scavenger. 17. The reactor according to claims 15 and 16, characterized in that said nitrogen oxide absorber comprises a material or combination of materials selected from the group consisting of Na ₂ O, K ₂ O, MgO, CaO, SrO and BaO.