System to reduce the amount of NOx in exhaust gases of a vehicle. The system includes a storage space 1 containing an agent, a SCR catalytic converter 5, an injection module 6c to inject the agent upstream of the converter, a heat exchanger 2 containing a porous matrix, a shutter or injector 11 to control the flow rate of the agent to the exchanger, a valve 12 between the storage space and exchanger, to transfer thermal energy to gases during the starting period. The shutter or injector controls the flow of agent into the exchanger during the starting period to raise its temperature, and is closed when gases have reached a certain temperature. The valve regulates exchanger pressure during a period at operating temperature and conveys the agent to storage space when the exchanger pressure is higher than storage space pressure.