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

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: A 2 A′ (1−x) B y B′ (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.

1 - 23 . (Cancelled)  
     
     
         24 . A method of reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, by use of an electrochemical reactor comprising a working electrode for reducing nitrogen oxides to nitrogen and oxygen, a counter electrode, and an ion-selective electrolyte; the processes taking place at the electrodes being substantially as follows:  
       at the cathode  
         2NO x +2 x e − ->N 2 +2 x O 2   (a)  O 2 +4e − ->2O 2−   (b)  
       at the anode  
         2O 2− ->O 2 +4e −   (c)  said cathode electrode processes (a) and (b) being carried out at a potential between −1500 my and +1500 mV 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 material of lanthanum manganite (LaMnO 3 ), lanthanum chromite (LaCrO 3 ), lanthanum ferrite (LaFeO 3 ), lanthanum cobaltite (LaCoO 3 ) or lanthanum nickel oxide (LaNiO 3 ); said material being doped with one or more of metals selected from the group consisting of Sr, Ca, Ba, Eu, Fe, Co and Ni in an effective amount to achieve a faster rate of reduction of nitrogen oxides than the rate of reduction of oxygen at the selected potential and temperature.    
     
     
         25 . A method according to  claim 24 , wherein the potential of the working electrode is between −200 mV and 800 mV, measured versus a hydrogen electrode of 8% H 2 O and 3% H 2  in Ar.  
     
     
         26 . A method according to  claim 24  or  25 , wherein the working electrode is La 1−x Sr x MnO 3  with x being in the range from 0.12 to 0.18.  
     
     
         27 . A method according to  claim 24 , wherein the mixture of nitrogen oxides and oxygen is concentrated with an absorber 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 the nitrogen oxides absorbed in the absorber are caused to react with the working electrode.  
     
     
         28 . A method according to  claim 27 , wherein the absorber comprises a working electrode and a counter electrode for causing the absorbed nitrogen oxides to react with the working electrode of the electrochemical reactor by establishing an electric potential between said electrodes.  
     
     
         29 . A method according to  claim 28 , wherein the nitrogen oxides are absorbed without applying any electrical potential between the working electrode of the absorbing material and the counter electrode of the absorbing material.  
     
     
         30 . A method according to  claim 27 , wherein nitrogen oxides are reduced at the same time as the absorber is regenerated.  
     
     
         31 . A method according to  claim 28 , wherein said absorber is regenerated by applying an electrical potential between the working electrode of the absorber and the counter electrode of the absorber in the range of from 0 to 1.5V.  
     
     
         32 . A method according to  claim 30  or  31 , wherein said regeneration is carried out at an electrical current density causing more than 80% regeneration of said absorber after a regeneration time in the range from 5 to 40 s.  
     
     
         33 . A method according to  claim 32 , wherein said electrical current density causes more than 90% regeneration of said nitrogen oxide absorber after said regeneration time.  
     
     
         34 . A method according to  claim 27 , wherein said absorber absorbs more than 60%, preferably in the range 60-80% of the nitrogen oxides of the mixture of nitrogen oxides and oxygen.  
     
     
         35 . A method according to  claim 27 , wherein said absorption of nitrogen oxides is continued until the absorber is saturated.  
     
     
         36 . A method according to  claim 27 , wherein said absorber and said working electrode are intermixed.  
     
     
         37 . An electrochemical reactor for reducing nitrogen oxides in a mixture of nitrogen oxides and oxygen, comprising a working electrode for reducing nitrogen oxides to nitrogen and oxygen, a counter electrode, an ion-selective electrolyte where said working electrode comprises an electric conductive ceramic material of lanthanum manganite (LaMnO 3 ), lanthanum chromite (LaCrO 3 ), lanthanum ferrite (LaFeO 3 ), lanthanum cobaltite (LaCoO 3 ) or lanthanum nickel oxide (LaNiO 3 ); said material being doped with one or more of metals selected from the group consisting of Sr, Ca, Ba, Eu, Fe, Co and Ni in an effective amount to achieve a faster rate of reduction of nitrogen oxides than the rate of reduction of oxygen, wherein the reactor further comprises means for maintaining a potential between −1500 mV and +1500 mV between said working electrode and said counter electrode and means for maintaining a temperature within a range of 200 to 500° C.  
     
     
         38 . A reactor according to  claim 37  which further comprises a nitrogen oxide absorber.  
     
     
         39 . A reactor according to  claim 37  or  38 , wherein said nitrogen oxide absorber comprises a material or a combination of materials selected from the group consisting of Na 2 O, K 2 O, MgO, CaO, SrO and BaO.  
     
     
         40 . A method according to  claim 28 , wherein the working electrode of the absorber comprises an electric conductive ceramic material of lanthanum manganite (LaMnO 3 ), lanthanum chromite (LaCrO 3 ), lanthanum ferrite (LaFeO 3 ), lanthanum cobaltite (LaCoO 3 ) or lanthanum nickel oxide (LaNiO 3 ); said material being doped with one or more of metals selected from the group consisting of Sr, Ca, Ba, Eu, Fe, Co and Ni in an effective amount to achieve a faster rate of reduction of nitrogen oxides than the rate of reduction of oxygen at the selected potential and temperature.