Assembly for reducing the nitrogen oxides circulating through an exhaust line of an internal combustion engine, said assembly comprising: - a tank of liquid urea having one or more outlets and an inlet fed by at least one filling pipe , and said tank being associated with a distribution pump , - a first feed pipe connecting said tank to said exhaust line , said first pipe being associated with an injector for injecting at least some of the liquid urea into said exhaust line , - said exhaust line comprising: o a mixer having at least one inlet to which said first pipe is connected, and an outlet connected to ∘ a catalytic converter for the selective catalytic reduction of the nitrogen oxides, said assembly being characterized in that it further comprises: - an electrolysis means for converting at least some of the liquid urea into gaseous ammonia, and - a second feed pipe for introducing said gaseous ammonia at the outlet of said mixer , said second pipe connecting said tank to said exhaust line and comprising: ∘ an absorber means able to absorb and/or to desorb gaseous ammonia, said absorber means having at least one inlet and one outlet , ∘ a first valve situated at said inlet of said absorber means , ∘ a second valve situated at said outlet of said absorber means , ∘ a device for modifying the internal pressure of the absorber means , and ∘ a third valve for venting said absorber means . - a tank of liquid urea having one or more outlets and an inlet fed by at least one filling pipe , and said tank being associated with a distribution pump , - a first feed pipe connecting said tank to said exhaust line , said first pipe being associated with an injector for injecting at least some of the liquid urea into said exhaust line , - said exhaust line comprising: o a mixer having at least one inlet to which said first pipe is connected, and an outlet connected to ∘ a catalytic converter for the selective catalytic reduction of the nitrogen oxides, o a mixer having at least one inlet to which said first pipe is connected, and an outlet connected to ∘ a catalytic converter for the selective catalytic reduction of the nitrogen oxides, - an electrolysis means for converting at least some of the liquid urea into gaseous ammonia, and - a second feed pipe for introducing said gaseous ammonia at the outlet of said mixer , said second pipe connecting said tank to said exhaust line and comprising: ∘ an absorber means able to absorb and/or to desorb gaseous ammonia, said absorber means having at least one inlet and one outlet , ∘ a first valve situated at said inlet of said absorber means , ∘ a second valve situated at said outlet of said absorber means , ∘ a device for modifying the internal pressure of the absorber means , and ∘ a third valve for venting said absorber means . ∘ an absorber means able to absorb and/or to desorb gaseous ammonia, said absorber means having at least one inlet and one outlet , ∘ a first valve situated at said inlet of said absorber means , ∘ a second valve situated at said outlet of said absorber means , ∘ a device for modifying the internal pressure of the absorber means , and ∘ a third valve for venting said absorber means . <EOS>