Devices and methods for the production of ammonia as a reducing agent for the selective catalytic reduction (SCR) of nitrogen oxides in the exhaust gas of a combustion source, especially of an internal combustion engine. The ammonia is admixed with the exhaust gas in the direction of flow in front of a SCR-catalytic converter. In so doing, nitrogen oxide is produced in a nitrogen oxide production unit separated from the combustion source and its exhaust gas duct and is stored in at least one storage catalytic converter or a nitrogen oxide storage and a subsequently connected catalytic converter. Furthermore, hydrogen and/or a hydrogen-carbon monoxide-mixture is produced in a hydrogen production unit separated from the combustion source and its exhaust gas duct and delivered to a storage catalytic converter. Hydrogen and the stored nitrogen oxide are converted catalytically to ammonia in the storage catalytic converter or in the catalytic converter.