Abstract:
A catalytic converter for the treatment of combustion exhaust gases includes a first large surface, which is exposed to the exhaust gas to be purified and which has a layer of a catalytically active material suitable for converting gas components, and a planar first electrode which is in physical contact with the layer made of a catalytically active material suitable for converting gas components, and which is able to receive an electric pumping voltage.

Description:
FIELD OF INVENTION 
       [0001]    The present invention relates to a catalytic converter for the treatment of combustion exhaust gases, a method for its production, and a method for its use. 
       BACKGROUND INFORMATION 
       [0002]    Catalytic converters are used for the aftertreatment of combustion exhaust gases, which are produced in industrial processes such as in the combustion of fossil fuels in the engine, for instance. These catalytic converters are used to eliminate gaseous components of the combustion exhaust gases. 
         [0003]    The elimination of nitrogen oxides from exhaust gases of combustion engines is becoming ever more important. Among others, nitrogen oxide adsorption catalysts are used for this purpose, which store nitrogen oxides contained in a combustion exhaust gas at least temporarily and release them again during a subsequent periodic regeneration process. Another possibility consists of the utilization of exhaust gas aftertreatment devices based on what is known as SCR technology (selective catalytic reduction), in which chemical reducing agents such as ammonia can be mixed with the exhaust gas to be purified, and the nitrogen oxides eliminated. 
         [0004]    In order to optimize the elimination rate of such a denitrification catalyst, it is known from JP 07-275714, for instance, to convert nitrogen oxides into nitrogen and oxygen catalytically, the produced oxygen at the cathode of an electrochemical cell being converted into oxygen ions and removed from the reaction region. This makes it possible to prevent a reverse reaction of nitrogen with oxygen into nitrogen oxides. This catalytic converter includes a catalytically active layer on the basis of a mixed oxide, and an electrochemical cell, which is designed on the basis of zirconium dioxide as solid electrolyte. 
       SUMMARY OF THE INVENTION 
       [0005]    It is an object of the present invention to provide a catalytic converter for the treatment of combustion exhaust gases, which is able to remove nitrogen oxides from combustion exhaust gases in an efficient manner. 
         [0006]    In an advantageous manner, this object may be achieved by a catalytic converter and by a method for its production having the characterizing features described herein. In particular, this is based on the fact that the catalytic converter includes a planar first electrode, which is positioned so as to be in physical contact with a layer that is made of a catalytically active material suitable for converting gas components of the combustion exhaust gas. An electric pumping voltage is able to be applied to this planar first electrode. 
         [0007]    In this way, the oxygen produced in the decomposition of, e.g., nitrogen oxide, in the form of  02  ions is able to be electrochemically removed from the region of the layer that is made of a catalytically active material suitable for converting gas components of the combustion exhaust gas. This achieves a low oxygen concentration in the region of this layer, and the catalytically active material is effectively protected against a deactivation caused by oxygen. 
         [0008]    Further advantageous developments of the present catalytic converter are described herein. 
         [0009]    For instance, it is advantageous if an additional electrode, which acts as counter-electrode to the first electrode, forms an electrochemical cell together with the first electrode and has an open, porous design. In this way, it is easy for the oxygen released in the decomposition of nitrogen oxides to be carried off on the anode side through the open-pored structure of the electrode. 
         [0010]    Furthermore, it is advantageous if the catalytic layer containing the catalytically active material has a wavy structure. Such a structure can be produced, for instance, by a brief lateral elongation of a composite in the form of a green ceramic body, the composite body being made up of the catalytic layer, the first and the second electrode, and a solid electrolyte. The wavy structure has the advantage of greater stability while simultaneously increasing the reactive surface, available for the conversion of exhaust-gas components, of the catalytic converter of the present invention. 
         [0011]    According to one advantageous embodiment of the present invention, following a brief lateral elongation, a composite made up of a solid electrolyte, the first and the second electrode and a catalytic converter layer, is rolled up parallel to a longitudinal axis of the green body, so that a rotationally symmetric body is formed. This makes it possible to produce the final three-dimensional form of the catalytic converter in a simple manner. 
         [0012]    According to one additional advantageous embodiment of the present invention, a plurality of first electrodes and a plurality of second electrodes are provided in the region of the catalytic layer, which jointly form an interdigital structure. This interdigital structure allows simple, joint contacting of all first electrodes and of all second electrodes in each case, via a shared electric supply line, e.g., at the end face on the produced rotationally symmetric catalytic converter body. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  shows a schematic, sectional view of a catalytic converter for the treatment of combustion exhaust gases according to a first exemplary embodiment of the present invention. 
           [0014]      FIG. 2  shows a schematic, sectional view of a catalytic converter for the treatment of combustion exhaust gases according to a second exemplary embodiment of the present invention. 
           [0015]      FIG. 3  shows a schematic, sectional view of the catalytic converter shown in  FIG. 2 , the catalytic converter including a wavy structure. 
           [0016]      FIG. 4  shows a schematic, sectional view of a catalytic converter according to one of the aforementioned exemplary embodiments, in the form of a rotationally symmetric catalyst body. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  schematically shows a catalytic converter according to the present invention for the treatment of combustion exhaust gases. It is preferably used for the conversion of gaseous or solid pollutant components of the combustion exhaust gases, in particular for the conversion of oxygen-containing, gaseous components. Catalytic converter  10  has a catalytic layer  12  of a catalytically active material and is preferably covered by a solid electrolyte layer  14 . Suitable as solid electrolyte is, for example, zirconium dioxide partially or fully stabilized by Bi 2 O 3 , or Gd-doped CeO 2 . Furthermore, one or more first electrodes  16  is/are positioned on a further large surface of catalytic layer  12 . The at least one first electrode  16  forms an electrochemical cell together with at least one second electrode  18 , which preferably is positioned on the side of solid electrolyte layer  14  facing away from layer  12 . Electrodes  16 ,  18  are produced from, for example, nickel, silver, platinum or palladium, or from alloys of these elements. 
         [0018]    Suitable as catalytically active components of catalytic layer  12  are, for instance, metallic components such as rhodium, palladium, platinum, iridium or nickel, or alloys thereof. For example, these metallic components may be present as nanoscale dispersion on the surface of zirconium-dioxide particles. Furthermore, mixed oxides, such as specifically spinels of the general formula A 2 BO 4  and perovskites of the general formula ABO 3 , are suitable as catalytically active components of catalytic layer  12 , in which A refers to, e.g., elements of the lanthanides, actinides, the alkali metals, alkaline-earth metals, or for yttrium, and B refers to, e.g., copper, iron, nickel or cobalt. The A- or B-positions of the mentioned compounds can also be occupied by several of the aforementioned elements, possibly also in a non-stoichiometric manner. Especially preferred are perovskites in which the A-position is occupied by the elements of lanthanum or strontium, and the B-positions are occupied by at least one of the elements of iron, cobalt or nickel. Exemplary representatives of these suitable perovskites are (La 0,9 Sr 0,1 ) (Fe 0,9 Ni 0,1 )O 3 , (La 0,9 Sr 0,1 )FeO 3  or La 0,5 Sr 0,5 CoO 3 ; the LaNiO 3  compound is rather unsuitable for technical reasons. 
         [0019]    To produce a catalytic converter according to the first exemplary embodiment shown in  FIG. 1 , as an initial step, a foil of a ceramic solid electrolyte material is preferably provided in the form of a green body, which is then printed over on a first large surface with at least one first electrode  16 , and on an opposite-lying additional large surface with at least one additional electrode  18 . Then, catalytic layer  12  is deposited on the large surface of solid electrolyte layer  14  provided with the at least one first electrode  16 . 
         [0020]    It is preferred if electrodes  16 ,  18  are applied in the form of an interdigital structure. To this end, electrodes  16 ,  18  are designed in the form of, e.g., a plurality of metallic, in particular parallel strips, for instance, the first electrodes  16  being guided right into the immediate vicinity of a first end face of the produced layer composite; first electrodes  16  have a minimum distance of I to  4  mm from the opposite lying front end of the layer composite. The further electrodes  18  have an oppositely directed orientation; they maintain a minimum distance of  1  to  4  mm from the first end face of the layer composite and essentially abut directly against the opposite-lying end face of the layer composite. This measure allows an especially simple contacting of the first and additional electrodes  16 ,  18 , in that an exclusive contacting of first electrode  16  takes place at the first end face of the layer composite, and an exclusive contacting of additional electrodes  18  takes place at the end face lying opposite. The layer composite produced in this manner is subjected to a heat treatment, specifically a sintering treatment, during which the catalytic converter structure shown in  FIG. 1  is formed. 
         [0021]    However, prior to the heat treatment, what is known as a drawing process can optionally be implemented, during which the composite body made up of catalytic layer  12 , solid electrolyte layer  14  and electrodes  16 ,  18  is subjected to, for example, a brief lateral elongation, so that a wavy structure of the composite body is obtained. This structure is retained in a subsequent heat treatment and/or sintering process, thereby resulting in an enlarged catalytically active surface of the obtained catalytic converter. 
         [0022]    Another exemplary embodiment of a catalytic converter according to the present invention is shown in  FIG. 2 . Matching reference numerals indicate the same module components as in the previous figures. Prior to the heat or sintering treatment, the composite body made up of catalytic layer  12 , solid electrolyte layer  14 , and electrodes  16 ,  18  is laminated together with an additional composite body of the same type, both composite bodies having shared additional electrodes  18 . This overall structure, made up of two composite systems, is then subjected to the heat or sintering treatment. The result is the catalytic converter according to a second exemplary embodiment shown in  FIG. 2 . If the overall structure made up of two composite systems is subjected to a drawing process prior to the heat or sintering treatment, then a catalytic converter according to a third exemplary embodiment, as shown in  FIG. 3 , results after the sintering. 
         [0023]    Furthermore, prior to the final heat or sintering treatment, the catalytic converters according to the first to third exemplary embodiments may be rolled up about an axis of rotation that is parallel to a longitudinal axis of the composite system, so that in particular, a cylindrical, three-dimensional catalytic converter body  40  is produced. Following the final heat or sintering treatment, for example, it is provided with electrode contacts by partially grinding the resulting, in particular, cylindrical catalytic converter body  40  at the front end, for example, in each case, and by providing it at the front end with a network of a nickel or silver wire in order to establish contact to first or second electrodes  16 ,  18 . 
         [0024]    The method of using catalytic converter  10 ,  40  is based on the fact that in conventional catalytic converters according to the related art, the oxygen released during the thermal decomposition of nitrogen oxides, for example, deposits at the surface of the catalytic converter material and thereby reduces the catalytic activity. To prevent this, a voltage is applied to electrodes  16 ,  18  of catalytic converter  10 ,  40  during operation, which, as pumping voltage of sufficient magnitude, causes oxygen in the form of oxygen ions to be transported from the at least one first electrode  16  to the at least one further electrode  18 . If an exhaust gas atmosphere is present at the large surface formed by catalytic layer  12 , then the surface of catalytic layer  12  becomes poorer in gaseous oxygen, thereby facilitating the decomposition of oxygen-containing components of a combustion exhaust gas. 
         [0025]    In addition, a deactivation of the catalytically active components provided in catalytic layer  12  is prevented. It is advantageous in this context if the at least one additional electrode  18  acting as anode has an open-pore design, so that the oxygen pumped thereto is able to escape in a simple manner. 
         [0026]    The afore-described catalytic converter according to the exemplary embodiments described is particularly suitable for converting oxygen-containing gas components of a combustion exhaust gas, such as nitrogen oxides or sulfur oxides. It may be utilized both for the exhaust gas aftertreatment of combustion exhaust gases of combustion engines and for the exhaust gas treatment of heating systems or for power plant applications.