the catalyst material of the nitrogen oxide storage catalytic converter removes nitrogen oxide (NOx) from the exhaust gas, which is at that time oxidizing, to form solid barium nitrate. On account of the associated load imposed on the material, from time to time it is necessary to regenerate the NOx storage catalytic converter. This process, which is known as nitrate regeneration, is effected by operating the internal combustion engine with a rich air/fuel mixture for a certain time. In the process, the barium nitrate, which is unstable in the resulting exhaust gas containing reducing agent, decomposes again to form barium carbonate and to release NOx. The latter is then reduced by the reducing agents (H2, CO and HC) present in the exhaust gas, at the precious metal componentoxides or oxygen stored in it, is preferably selected such that a large part of the stored nitrogen oxides or of the stored oxygen is already reduced, thus avoiding leakage of reducing agent. The selection of predeterminable and preferably fixedly applied values for the duration and the air/fuel ratio in the first regeneration phase takes account of the fact that, after the lean-burn storage phase ends, a minimal amount of nitrogen oxides is stored in the nitrogen oxide storage catalytic converter.
In a further refinement of the invention, the second regeneration mode is ended after a predeterminable second period of time. The second period of time is preferably fixedly applied and selected in such a manner that, taking the storage capacity of the nitrogen oxide storage catalytic converter into