Process for operating an exhaust gas treatment unit containing a sulfur trap and a nitrogen oxides storage catalyst

A process for operating an exhaust gas treatment unit for an internal combustion engine which is operated during most of the operating period with lean air/fuel ratios, wherein the exhaust gas treatment unit contains:

• a nitrogen oxides storage catalyst with an activity window .DELTA.T.sub.NOX between the temperatures T.sub.K,1 and T.sub.K,2 for the storage of nitrogen oxides at normalized air/fuel ratios greater than 1 and release of the nitrogen oxides at normalized air/fuel ratios less than or equal to 1 and a sulfur desorption temperature T.sub.K,DeSOx, above which the sulfates stored on the catalyst are decomposed at normalized air/fuel ratios less than or equal to 1 and
• a sulfur trap which is upstream of the nitrogen oxides storage catalyst and located at a distance from this, with a sulfur desorption temperature T.sub.S,DeSOx above which sulfates stored on the sulfur trap are decomposed at normalized air/fuel ratios less than or equal to 1,
• wherein there is a temperature difference .DELTA.T.sub.S,K between the sulfur trap and the storage catalyst, between the exhaust gas temperature T.sub.S just upstream of the sulfur trap and the exhaust gas temperature T.sub.K just upstream of the storage catalyst, and the process comprises:
• a) storing of the nitrogen oxides contained in the exhaust gas on the nitrogen oxides storage catalyst and storing the sulfur oxides on the sulfur trap at normalized air/fuel ratios greater than 1 and with exhaust gas temperatures T.sub.K within the activity window .DELTA.T.sub.NOX, wherein at the same time the exhaust gas temperature T.sub.S is less than the sulfur desorption temperature T.sub.S,DeSOx, and cyclically lowering the normalized air/fuel ratio in the exhaust gas to less than 1 to release the stored nitrogen oxides;
• and
• b) removing sulfur from the sulfur trap after each predetermined number N.sub.1 of nitrogen oxides storage cycles by raising the exhaust gas temperature T.sub.S above the sulfur desorption temperature T.sub.S,DeSOx of the sulfur trap and lowering the normalized air/fuel ratio in the exhaust gas to below 1.