Abstract:
The invention proposes a sensing element which serves to determine the concentration of oxidizable constituents in a gas mixture, in particular of hydrocarbons, NO, and NH 3 . Electrically conductive spinels of the general formula ABB′O 4 , or electrically conductive pseudobrookites of the general formula ABB′O 5 , serve in this context as the material for the measurement electrode.

Description:
BACKGROUND INFORMATION 
     The present invention is based on a sensing element for determining the concentration of oxidizable constituents in a gas mixture, in particular for the determination of saturated and unsaturated hydrocarbons, nitrogen oxides, and ammonia. 
     Sensing elements which contain sensor materials of the general formula A 2−x A′ x BO 4  are known from German Patent 23 34 044 C3. These are rare-earth compounds of the K 2 MgF 4  structural type, for the detection of oxidizable gases. It is also known from German Patent 42 44 723 A1 to use rare-earth cuprates of the formula A 2−x L x CuO 4  for the detection of oxygen in gas mixtures, in particular in exhaust gases of internal combustion engines and combustion facilities. In addition, the article by H. Meixner and U. Lampe in Sensors and Actuators B 1996, 33, pp. 198-202 describes a plurality of metal oxides for the determination of various gas components. For some time, however, it has proven difficult to find suitable materials with high selectivities for, e.g., the determination of saturated and unsaturated hydrocarbons and of ammonia or nitrogen oxides. This was attributable, among other factors, to the poor corrosion stability of the electrode materials used, which often have a strong tendency toward undesirable sulfate formation on the electrode surface. 
     SUMMARY OF THE INVENTION 
     In contrast to the known existing art, the sensing element according to the present invention having the material used for the measurement electrode, which contains either an electrically conductive spinel of the general formula ABB′O 4  or an electrically conductive pseudobrookite of the general formula ABB′O 5 , exhibits excellent corrosion resistance at high temperatures, a low tendency to form sulfates, and a high selectivity for oxidizable gaseous compounds. It is thus possible, in a simple and advantageous manner, to determine corrosive ammonia without having the measurement electrode chemically attacked. As a result of the possible structural variations of these two classes of compounds, i.e. the spinels and the pseudobrookites, it is possible to make available a variety of structures for the determination of various gases. Because of their structure, spinels possess a particularly high degree of spatial occupancy, in which one-eighth of the tetrahedral vacancies and half the octahedral vacancies of the oxygen sublattice, which forms approximately a cubic close-packed (ccp) configuration, are occupied by cations. This dense structure, which is also exhibited by the pseudobrookites, inhibits or prevents any diffusion of the metal cations which contaminate the electrode material, derived for example from oxides of exhaust systems, into the metal oxide electrode; this would be associated with poisoning of the electrode and thus with a change in signal. 
     The features set forth in the dependent claims make possible advantageous developments of and improvements to the sensing element recited in the principal claim. 
     In a preferred embodiment, what is used as the spinel is a so-called 2,3 spinel, A representing a divalent transition metal cation and B and B′ a trivalent transition metal cation. A can, for example, represent the divalent cations of cobalt, nickel, or copper, B the trivalent cations of chromium, iron, and manganese, and B′ the trivalent cations of chromium and manganese. 2,3 spinels, for example NiFeMnO 4  or CoCr 2 O 4  or CoCrMnO 4 , have a high sensitivity in particular for unsaturated hydrocarbons. The oxygen content of the gas mixture, as long as it exceeds 1% in, for example, exhaust gases of internal combustion engines, has no great effect on the selectivity or sensitivity of the measurement signal; the same is true for other gas constituents. The measurement electrode has a thickness of 5-100 μm, preferably 20-30 μm. 
     In a further advantageous embodiment, what is used as the spinel is a 4,2 spinel, A representing a tetravalent transition metal cation and B and B′ a divalent transition metal cation. A can be, for example, a tetravalent cation of titanium or zirconium, but niobium is also possible. B and B′ can represent, for example, the divalent cations of cobalt and nickel. One possible combination is, for example, TiCo 2 O 4 , so that particularly high sensitivities for nitrogen oxides can be attained with this metal oxide electrode. The sensitivity of these 4,2 spinels for nitrogen oxides is so high that the other constituents of the exhaust gas exhibit no cross-sensitivities. The oxygen content of the lean exhaust gas also has no great influence on the sensitivity of the sensor signal. 
     A further advantageous embodiment of the sensing element according to the present invention consists in the use of so-called 6,1 spinels, A representing a hexavalent transition metal cation and B and B′ a univalent metal cation. A can be, for example, the hexavalent cation of tungsten, molybdenum, or chromium, and B and B′ represent, for example, the univalent cations of the coinage metals and of elements of the first main group, for example gold, silver, copper, potassium, lithium, and sodium. Depending on their composition, 6,1 spinels exhibit high sensitivity for a variety of gases, so that with suitable combinations of 6,1 spinels both hydrocarbons and nitrogen oxides, or also ammonia, can be determined. 
     In a further preferred embodiment, a pseudobrookite of the formula ABB′O 5 , composed of metallic transition elements, can be used as the sensor material. In this context A represents a tetravalent transition metal cation, and B and B′ a trivalent transition metal cation. In particular A represents, for example, the tetravalent cations of titanium and zirconium, niobium also being possible. B and B′ represent the trivalent cations of chromium, iron, and manganese. Pseudobrookites according to the present invention have a high sensitivity for saturated and unsaturated hydrocarbons; once again, no cross-sensitivities to the other constituents in a gas mixture occur. The sensing element is also characterized in that occupancy of the positions B and B′ is accomplished stoichiometrically or nonstoichiometrically, and correspond to the general empirical formulas AB x B′ 2−x O 4  and/or AB x B′ 2−x O 5 , where 0&lt;x&lt;2. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a section through a sensor according to the present invention, and 
     FIG. 2 shows a section through a further sensor according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 depicts a sensing element  6  according to the present invention in section. A layer made of a porous or dense solid electrolyte  2 , for example comprising yttrium oxide-stabilized zirconium dioxide (YSZ) or other usual solid electrolyte compounds, bears on the one large surface a reference electrode  3  which is made, for example, of platinum or a similar metal, and on the other large surface an electrode  1  which contains a metal oxide according to the present invention. Reference electrode  3  is exposed to the exhaust gas directly, or to the exhaust gas through the porous solid electrolyte or through a lateral diffusion layer or a diffusion layer embedded into the solid electrolyte; this is not depicted in the drawing. Reference electrode  3  can also be exposed to air. 
     FIG. 2 shows a further exemplary embodiment of a sensing element  7  according to the present invention, in section. The configuration is identical to the one depicted in FIG. 1, except that a second reference electrode  4  is additionally applied. Here again reference electrodes  3 ,  4  are exposed to the exhaust gas directly, or to the exhaust gas through the porous solid electrolyte or through a lateral diffusion layer (not depicted) or a diffusion layer embedded into the solid electrolyte. One of the two reference electrodes can also be exposed to air. 
     Manufacture of the metal oxides according to the present invention, both the spinels and the pseudobrookites, is accomplished using commonly known methods, for example mixing stoichiometric quantities of the corresponding oxides followed by sintering, or using hydrothermal methods. 
     To determine the concentration of oxidizable constituents in exhaust gases, the sensor is heated by way of a heating apparatus (not depicted in the drawings) to a temperature between 300 and 1000° C., advantageously to approximately 600° C. In order to improve the electrochemical properties, it is furthermore advisable to add some of the spinels or pseudobrookites according to the present invention to the solid electrolyte, in order to increase the reactive surface at the triple-phase boundary. 
     Hydrocarbons, for example, are electrochemically oxidized on the metal oxide electrode in a manner similar to that in a fuel cell, according to the following semireaction: 
     
       
         C 3 H 6 +9O 2− →3CO 2 +3H 2 O+18e − . 
       
     
     The situation is similar for ammonia and nitric oxide, which can be electrochemically oxidized in the presence of oxygen: 
     
       
         2NH 3 +3O 2− →N 2 +3H 2 O+6e − . 
       
     
     
       
         NO+O 2− →NO 2 +2e − . 
       
     
     Electrochemical oxidation of the gas constituents takes place in the simultaneous presence of oxygen. The reaction rate necessary for heterogeneous oxidation of hydrocarbons, ammonia, and also nitric oxide at the electrode surface is particularly low in the case of the metal oxides according to the present invention. As a result, hydrocarbons, ammonia, and nitric oxide are effectively oxidized electrochemically in the presence of oxygen, and can be determined separately. 
     Exemplary embodiments of metal oxide electrodes according to the present invention are set forth in Tables 1 and 2. For example, NiFeMnO, as a representative of the 2,3 spinels, can be used for the determination of hydrocarbons. So can CoCr 2 O 4  or CoCrMnO 4 , which have equally good sensitivity for unsaturated hydrocarbons. The high sensitivity of 2,3 spinels in particular for unsaturated hydrocarbons is the result of adsorptive interactions of the π electrons of the double bonds of the corresponding hydrocarbon with electrophilic acceptor locations on the (1,1,0) or (1,1,1) spinel face. Further examples according to the present invention are MnCr 2 O 4 , which can be used for nitrogen oxides. 
     A 4,2 spinel, for example having the formula TiCo 2 O 4 , possesses outstanding sensitivity for nitric oxide. Pseudobrookites, for example having the formula TiCr 2 O 5 , also have good sensitivity for hydrocarbons and also, in particular, for ammonia. 
     A manufacturing method for a sensor according to the present invention is described by the example which follows: NiFeMnO 4 , for example manufactured by mixing the oxides or by hydrothermal synthesis, is printed using a commonly known thick-film technique onto a substrate which bears a reference electrode, made for example of platinum, and above that a solid electrolyte layer comprising, for example, stabilized zirconium dioxide. A heating apparatus is applied on the opposite side of the substrate. The sensor is sintered at 1200° C. for 90 minutes, with a heating/cooling ramp rate of 300° C. per hour. After sintering, the solid electrolyte has pores on the order of 10 μm to 100 μm in size. Once the platinum conductor paths, which contact only the measurement electrode, have been attached and pressed on, the sensor is then ready to operate. 
     Table 1 shows the correlation between sensor current and O 2  concentration. It is clearly evident that the sensitivity of the sensor for unsaturated hydrocarbons is only slightly influenced by the O 2  concentration. 
     Table 2 below lists several examples of a sensor element according to the present invention for determining various gaseous compounds in gas mixtures. The examples serve only for explanation, and do not in any way limit the invention. As is evident from the table, the spinels or pseudobrookites exhibit almost no cross-sensitivity for other gas components in a gas mixture besides the components being determined. It is also evident that the oxygen content of the gas mixture, exemplified by the behavior of NiFeMnO 4  in lean exhaust gas, exerts no influence on the signal being measured. This applies to all the spinels and pseudobrookites according to the present invention. 
     
       
         
               
             
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Influence of O 2  concentration on sensitivity and 
               
               
                 selectivity of measurement signal for the sensing of 
               
               
                 unsaturated hydrocarbons (HC). 
               
               
                 NiFeMnO 4  measurement electrode 
               
               
                 O 2 -Gehalt.. = O 2  concentration (vol %) 
               
               
                 Sensorstrom.. = Sensor current (μA) at an HC 
               
               
                 concentration of: 
               
             
          
           
               
                 0 2   
                   
               
               
                 Concentration 
                 Sensor current (μA) at an HC concentration of: 
               
             
          
           
               
                 (vol %) 
                 0 ppm 
                 125 ppm 
                 250 ppm 
                 500 ppm 
                 1000 ppm 
               
               
                   
               
               
                  0,675 
                 −0,35 
                 3,5 
                 7,9 
                 14,7 
                 22,75 
               
               
                  1,25 
                 −0,35 
                 3,5 
                 7,7 
                 14 7 
                 24,5  
               
               
                 2,5 
                 −0,7  
                  3,15 
                  7,35 
                 14,0 
                 24,5  
               
               
                 5,0 
                 −1,05 
                  3,15 
                 7,0 
                 13,3 
                 22,75 
               
               
                 10,0 
                 −1,4  
                 2,8 
                 6,3 
                 12,6 
                 22,05 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
             
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Examples of sensing elements according to the present 
               
               
                 invention having a measurement electrode made of spinel and/or 
               
               
                 pseudobrookite; operating temperature of the sensing element is 
               
               
                 850° C. in each case. 
               
             
          
           
               
                   
                   
                 Sensor current (μA) at a gas 
               
               
                   
                   
                 concentration of: 
               
             
          
           
               
                   
                 O 2   
                 430 
                 440 
                 440 
                 900 
                 200 
               
               
                   
                 Concentration 
                 ppm 
                 ppm 
                 ppm 
                 ppm 
                 ppm 
               
               
                 Measurement 
                 (vol %) 
                 c 3 H 6   
                 H 2   
                 CO 
                 NO 
                 NH 3   
               
               
                   
               
               
                 CoCR 2 O 4   
                 10 
                 2, 3 
                 0 
                 0 
                 0, 2 
                 1, 0 
               
               
                 CoCrMnO 4   
                 10 
                 &gt;31 
                 1, 2 
                 1, 9 
                 1, 6 
                 22 
               
               
                 TiCr 2 O 5   
                 10 
                 1, 9 
                 0, 28 
                 0, 56 
                 0, 4 
                 2, 2 
               
               
                 MnCr 2 O 4   
                 10 
                 0, 5 
                 0, 3 
                 0, 4 
                 1, 4 
                 0, 6 
               
               
                 TiCo 2 O 4   
                 10 
                 1, 5 
                 0, 5 
                 0, 6 
                 3, 0 
                 1, 2 
               
               
                   
               
               
                 Meβelektrodenmaterial = Measurement electrode material  
               
               
                 O2-Gehalt.. = O 2  concentration (vol %)  
               
               
                 Sensorstrom.. = Sensor current (μA) at a gas concentration of: