Process for checking the operatability of a nitrogen oxide storage catalyst

#The present invention relates to a process for checking the operatability of a nitrogen oxide storage catalyst which is used to remove the nitrogen oxides contained in the exhaust gas stream of a lean burn engine and contains at least a nitrogen oxide storage material, a catalytically active component and optionally an oxygen storage material, wherein the lean burn engine is operated with cyclic alternation of the air/fuel mixture from lean to rich and the nitrogen oxides contained in the exhaust gas are stored by the nitrogen oxide storage material in the presence of lean exhaust gas (storage phase) and desorbed and converted in the presence of rich exhaust gas (regeneration phase).

#Nitrogen oxide storage catalysts were developed specifically for the treatment of exhaust gases from lean operated internal combustion engines. Diesel engines and lean burn gasoline engines belong to the group of lean operated internal combustion engines. Both types of engines are called lean burn engines in the following. Lean burn engines, in particular gasoline engines with a direct fuel injection system, are being used to an increasing extent in vehicle construction because they enable theoretical fuel savings of up to 25%, as compared with stoichiometrically operated internal combustion engines.

#Nitrogen oxide storage catalysts have the ability to store nitrogen oxides over a wide temperature range under oxidizing exhaust gas conditions, that is during lean operation. This operating stage is therefore also called the storage phase in the following description.

#The invention is now explained in more detail with reference to FIGS. 1 to 6, wherein

#FIG. 1 is a graph measurements of the oxidation equilibrium in accordance with reaction equation(1);

#FIG. 2 is a graph of nitrogen oxide storage capacity of a storage catalyst in the freshly prepared state and after various types of damage as a function of temperature;

#To check the nitrogen oxide storage capacity, various types of sensors are used. Either nitrogen oxide sensors or lambda linear sensors, lambda jump sensors, hydrocarbon sensors, carbon monoxide sensors or hydrogen sensors may be used. Furthermore, it is possible to determine the nitrogen oxide storage capacity by means of the change in a physical property of the storage catalyst which depends on the amount of stored nitrogen oxides. The complex electrical impedance of the catalyst is particularly suitable for this purpose. A process for determining the nitrogen oxide storage capacity of a storage catalyst by means of its complex electrical impedance is described, for example, in EP 0 936 348 A2.

#A nitrogen oxide sensor is able to measure directly the nitrogen oxide concentration in the exhaust gas. Due to their structure, currently available nitrogen oxide sensors also simultaneously produce a linear and a jump-type signal for the oxygen content of the exhaust gas (lambda signal).

#A lambda linear sensor provides a sensor signal which rises linearly with the oxygen content of the exhaust gas. A lambda jump sensor, however, provides a sensor voltage which has only two states. On exceeding or undershooting a threshold value for the oxygen concentration in the exhaust gas the sensor voltage jumps from one state to the other. Under lean conditions, the sensor voltage is virtually zero. Under rich conditions, the sensor shows an output voltage which is typically between 700 and 900 mV. The sensor voltage has a steep gradient in the transition region around lambda=1. With the aid of this type of sensor, breakthrough of the rich exhaust gas at the end of the regeneration phase can easily be detected. The modes of operation of lambda sensors which are suitable for the process are described in Kraftfahrttechnischen Taschenbuch from Bosch, VDI-Verlag, 20th edition dated 1995, pages 490 to 492.