In a system and method for the reduction of nitrogen oxides in the exhaust of an internal combustion engine, the exhaust line of the internal combustion engine contains a DeNOx catalytic converter, in which the nitrogen oxides are reduced by means of hydrogen that is produced on-board the vehicle. The DeNOx catalytic converter is incorporated into a temperature-controlled heat exchanger, allowing the DeNOx catalytic converter to be operated within a desired temperature range, thereby improving the level of conversion.