Process for the desulphurisation of a NOX storage catalyst

A lean burn automotive exhaust system has a nitric oxide storage catalytic converter from which accumulated nitric oxide and sulphur oxides. The removal of sulphur is preceded by an increase in the catalytic temperature under lean burn conditions to that point where the nitric oxides are desorbed, thereby triggering use of a richer air/fuel mixture and consequent sulphur removal. The temperature of the catalytic converter is increased either by an external heater or by further heating of exhaust gases by injection of additional fuel, postponement of the ignition timing or a multi-stage combustion process. During the sulphate expulsion phase the stoichometric ratio is lowered to a value between 0.8 and 0.99 for a period of 1-10 sec. During the sulphur expulsion phase the stoichiometric fuel/air ratio modulates above and below the value of 1 at a frequency of 1-5 Hz. During the warm-up phase prior to expulsion of nitric oxides, the concentration of discharging nitric oxide is monitored by a probe located downstream from the catalytic converter.