The electrochemical sensor, which is designed more especially for monitoring the correct function of exhaust gas cleaning means in exhaust gas systems, has a solid electrolyte and a diffusion barrier of porous material and is arranged downstream from the exhaust gas cleaning means in the exhaust gas pipe. The sensor operates in a temperature range of -50.degree. C. to 500.degree. C., throughout which it functions substantially independently of temperature. Preferably nitrogen oxides are used as representative components for monitoring the efficacy of the exhaust gas cleaning means and a signal is produced when the signal from the sensor exceeds a given value.

BACKGROUND OF THE INVENTION 
The invention relates to an electrochemical sensor for the detection of 
nitrogen oxides. 
The German unexamined specification No. 2,304,464 describes a sensor for 
monitoring the function of catalysts in the exhaust gas cleaning systems 
of internal combustion engines. The sensor consists of an oxygen or 
.lambda. probe. If the catalyst is poisoned or if for some other reason, 
such as aging, the catalyst is not capable of functioning fully to oxidize 
or reduce, respectively, hydrocarbons, carbon monoxide, or nitrogen 
oxides, a higher oxygen concentration will appear downstream for the 
catalyst, since the oxygen will then not be consumed catalytically. In the 
said known arrangement the sensor is arranged downstream from the catalyst 
and measures the oxygen coming from the catalyst. On reaching a certain, 
preset value, a signal is then produced which indicates that the catalyst 
is no longer, or no longer sufficiently, active. 
The ability of a catalyst to convert hydrocarbons, carbon monoxide and 
oxides of nitrogen into less harmful gases by oxidation is not ascertained 
directly in the known device, but rather indirectly by measuring the 
oxygen still appearing downstream from the catalyst. This method is very 
inaccurate and is unsuitable for practical application, since the oxygen 
concentration downstream from the catalyst is not only dependent on the 
function of the catalyst by itself but furthermore on the manner of 
operation of the engine in the various load ranges. Moreover the 
measurement of the oxygen concentration downstream from the catalyst may 
not be used for evaluating the function of the catalyst, because such data 
do not represent the nitrogen oxide concentration downstream from the 
catalyst. Accordingly, the known device is not able to reliably monitor 
the function of a catalyst. 
The German unexamined specification No. 2,335,402 describes a sensor for 
the electrochemical determination of the content of nitrogen oxides in 
exhaust gases, which amongst other things is used to monitor the function 
of an exhaust gas catalyst. Such a sensor is arranged in an exhaust pipe 
shunt and the maximum permitted temperature of the known sensor is 
70.degree. C. owing to its design and for instance owing to the use of 
Teflon as a diffusion barrier. 
The disadvantage of this device is more especially that the nitrogen oxide 
fraction in the exhaust gas is not measured downstream from the catalyst 
but in the shunt or bypass. Downstream from the catalyst there is a 
temperature of at least 400 degrees in the exhaust system, i. e. in the 
exhaust gas. The known sensor is in no way able to withstand this 
temperature. Accordingly the sensor in the shunt in addition has to be 
cooled, something creating added complexity. It is furthermore possible 
for the shunt to be shut off intentionally or unintentionally and as a 
result no reliable monitoring of the catalyst will be possible. 
Although there are methods of physical analysis by which nitrogen oxides, 
hydrocarbons and/or carbon monoxide may be measured at temperatures of 
400.degree. C., flame ionisation detectors, infrared analyzers and/or 
chemoluminescence measuring instruments are used for this purpose, which 
without exception represent complicated items of apparatus and are in no 
way suitable for use in exhaust systems both on account of the expense and 
also as regards the amount of space required, mechanical stability and 
other factors. 
SUMMARY OF THE INVENTION 
Accordingly the object of the invention is to devise an electrochemical 
sensor, and more particularly a sensor for nitrogen oxides as 
representative components of exhaust gases, and also for carbon monoxide 
and/or hydrocarbons, which operates satisfactorily even at maximum 
temperatures, which both as regards its dimensions and also as regards 
mechanical stability, that is to say compactness and resistance to 
vibrations, is suitable for incorporation in motor vehicles, may be 
produced at a low price, operates reliably in a temperature range of 
approximately -50.degree. C. to +500.degree. C. and is essentially 
independent of the engine load and the degree of combustion therein. 
This object is attained by arranging the sensor in the exhaust duct on the 
output side of the catalyst relative to the flow of the gas and providing 
a solid electrolyte and a diffusion barrier of a porous metal. 
Owing to the fact that in the electrochemical sensor of the present 
invention there is a solid electrolyte and the diffusion barrier consists 
of porous metal, and owing to the possibility of arranging the sensor 
downstream from the catalyst in the exhaust pipe, there is reliable 
determination of an exhaust gas component not reacted by the catalyst. The 
representative component is normally the nitrogen oxides, but by using the 
sensor in accordance with the invention it is also however possible to 
determine the amount of hydrocarbons or the carbon monoxide in order to 
assure a reliable monitoring of the function of the catalyst. The result 
is then a continuous monitoring of the catalyst, which is more 
particularly independent of the amount of oxygen in the exhaust gases. 
Furthermore, the determination of the exhaust gas component is generally 
not dependent on the varying load of the engine, which changes in rapid 
succession, as for instance on speeding up starting from idling, then 
overtaking, or if when on a highway the engine has to develop full power. 
More particularly the sensor in accordance with the invention is 
mechanically very robust, as is necessary either in a motor vehicle 
exhaust gas system, owing to the vibration of the vehicle and pulsations 
in the exhaust gas, or indeed in exhaust gas systems for stationary plant. 
Moreover, sensors in accordance with the invention are simple in structure 
and manner of manufacture and accordingly are low in price. A further 
advantage of the sensors in keeping with the invention is that they are 
not able to be poisoned by water vapor and gasoline. If a sensor in 
keeping with the invention is employed for the determination of the amount 
of nitrogen oxides, it will not have any cross-sensitivity or induced 
sensitivity to carbon monoxide, carbon dioxide, sulfur oxides or hydrogen. 
In keeping with a preferred form of the present invention the diffusion 
barrier is constructed in the form of a porous layer of the material 
forming the operating electrode. This is useful inasfar as the adhesion, 
by way of a solid layer of the same material, of the porous layer on the 
substrate, as for instance a ceramic plate or a suitably pre-treated metal 
plate, is substantially superior to the adhesion of the porous layer 
directly on the substrate. 
Furthermore the connection of the electrical lead is simpler and more 
secure. 
It is preferably possible for the diffusion barrier consisting of porous 
material to be able to function as the operating electrode. The gas 
molecules passing through the diffusion barrier with a definite volume 
interact with the electrochemical system in the three phase space thereof. 
In accordance with the specific conditions, requirements and the selection 
of the materials, electrolyte etc., it is also possible to provide an 
additional reference electrode. In this case the operating and counter 
electrode, that is to say the cathode and the anode may be made of the 
same material. If in a further possible, alternative form of the 
invention, the counter electrode simultaneously serves as a reference 
electrode, then the counter electrode will be made of a material different 
to that of the operating electrode. 
The solid electrolyte is preferably a compound or a mixture of compounds, 
which throughout the working temperature range of -50.degree. C. to 
+500.degree. C. will have a conductivity of at least 10-11 ohm/cm in order 
to ensure that the output signal of the sensor is large enough for further 
processing. 
Preferably compounds or mixtures of compounds are utilized which do not 
undergo any change in phase and more particularly do not undergo any 
change in the crystalline phase and do not decompose in the said operating 
temperature range. This is to ensure the thermal stability of the solid 
electrolyte throughout the temperature range. 
Particularly advantageous solid electrolytes are spinels, more especially 
suitable members of this group of compounds being zirconium phosphate, 
khibinskite, wadeites, titisicon, nasicon, stibic acid and uranyl 
arsenide. 
Further particularly suitable materials for use as solid electrolytes are 
heteropolyacids, as for example hydrogen uranyl phosphate, 
tungstatophsophoric acid and molybdatophosphoric acid. The spinels and 
heteropolyacids are more particularly suitable on account of their thermal 
stability and their constant conductivity over a large temperature range. 
As regards the selection of the material for the solid electrolyte it is 
preferred to use one which is essentially inert with respect to the 
electrode material so that the working life of sensors may be extended. 
The operating electrode is preferably made of gold and the diffusion 
barrier is pefrerably made of porous gold. In certain circumstances it is 
possible to utililize other materials for the operating electrode. 
The counter electrode consists of at least one of the materials from the 
group of platinum, palladium, rhodium, silver and ruthenium oxide, same 
being more particularly advantageous. 
In the event of an additional reference electrode being employed, it is 
particularly expedient if it is made of AgNO.sub.3 with a counter 
electrode of silver. If a two electrode system is used in which the 
counter electrode is simultaneously the reference electrode, it is 
furthermore particularly advantageous if the latter is formed from at 
least one of the mixtures including Pd/PdO, Ni/NiO, Cu/Cu.sub.2 O, Ag/AgC1 
and Pd/PdH.sub.x . Such electrodes have a particularly low polarisibility 
and maintain a sufficiently constant potential even when a current is 
flowing. The essential criterion for the selection of such systems is that 
they should operate reversibly in the respective temperature range. In the 
event of mixtures of oxides having a high electrical conductivity, they 
may also be used for the electrodes, despite a small phase width. 
The sensors in keeping with the invention have a high selectivity as 
regards the exhaust gas component to be determined and are highly 
resistant to poisoning. Furthermore the output signals are highly 
reproducible. The range of temperature in which they may be employed 
extends from -50.degree. C. to 500.degree. C. 
In keeping with a preferred form of the invention the sensor in accordance 
with the invention is produced by a process in which the diffusion barrier 
consisting of porous metal is formed as a metallic paste, mixed with 
finely divided material such as polysterene or other propellants capable 
of evaporating at raised temperatures. When the metallic paste is fully 
fired, the added material will evaporate with the intentional production 
of cavities resulting in porosity. Preferably the metallic paste is 
applied by screen printing in this method of production. It is also an 
advantage if the diffusion barrier is formed by sputtering, the pore size 
or the channel size being dependent on the sputtering temperature and/or 
the level of vacuum in the sputtering chamber so that they may be adjusted 
by varying same. This technique is known per se. 
The sensor signal is preferably processed in such a way that a signal in 
some form, as for example as an optical or acoustic signal, is produced 
when the sensor signal exceeds an adjustable and/or preset signal level. 
This means that if a higher concentration of the exhaust gas component, 
more especially the nitrogen oxide component, to be determined occurs 
downstream from the catalyst, an indication will be provided of the 
failure of the catalyst to function fully, as a result for example of a 
sudden contamination or of gradual aging of the catalyst. The user of the 
motor vehicle will so be made aware of the matter and informed that he is 
to replace the catalyst. 
In accordance with a further, preferred form of the invention, the 
processing of the probe signal may be undertaken in a microprocessor, for 
example one forming a part of the electronic system of the vehicle and 
present in any case.

DETAILED DESCRIPTION OF THE INVENTION 
FIGS. 1 and 2 show a substrate 1, as for example in the form of a ceramic 
plate, an aluminum oxide plate or a pre-treated metal plate, on which a 
cathode 2 functioning as the operating electrode has been applied and, at 
some distance therefrom the counter electrode 3 as an anode. The operating 
electrode 2 consists at least in part of a porous metal, as for instance 
one of porous gold, acting as a diffusion barrier 7 and which has been 
applied to the substrate 1 in the manner noted. The exhaust gas component 
to be determined, as for instance a nitrogen oxide, diffuses through the 
operating electrode 2 acting as the diffusion barrier 7, in the vicinity 
of the surface of the operating electrode 2, same being in contact with a 
solid electrolyte 4 and being separated by an insulating layer 8 from the 
operating electrode 2. The electrons freed on the reduction of the 
nitrogen oxides make their way through the conducting electrolyte 4 to the 
counter electrode 3 so that the resulting current may be measured by a 
measuring device 6. The current signal produced on such measurement is 
used as an output signal when the nitrogen oxide concentration exceeds a 
certain level. 
The entire structure is covered by an insulating guard layer 5. 
For special applications it is particularly advantageous if the sensor as 
described is placed in the same form of arrangement on the other side of 
the substrate 1 in the same manner, i. e. as applied to an imaginary 
folded sheet so as to be symmetrical to the fold thereof. When used the 
one side is exposed to the gas to be determined and the other, identically 
formed side, is exposed to a reference gas. 
The sensor in keeping with the invention may be produced in a simple, 
low-cost manner by a brushing or a thick layer technique. The result is 
then a sensor with dimensions making it suitable also for incorporation in 
pre-existing exhaust gas systems. Furthermore, the sensor produced in this 
manner is highly insensitive to vibrations. 
Although the invention has only been described with reference to one 
working example thereof, a man versed in the art will be aware of numerous 
variations and developments of the invention. As an example, it is 
possible to incorporate a unit with the sensor as such with an integrated 
processing circuit--for instance one produced in hybrid technology and 
possibly separated by thermal insulation--so that the result is a compact 
integral processing component with small dimensions, which may be fitted 
or used as a replacement in a simple manner, as for instance to a 
connector formed on the exhaust gas pipe downstream from the catalyst, by 
means of a screw connection, whenever this is required.