NOVELTY - The method for storing and delivering ammonia for vehicles, comprises supplying a first ammonia storage material (1a) capable of reversibly adsorbing or absorbing and desorbing ammonia having a first ammonia vapor pressure into fluid communication with a second ammonia storage material (2a) capable of reversibly adsorbing or absorbing and desorbing ammonia having a second ammonia vapor pressure lower than the first ammonia vapor pressure, which is partially saturated with or completely void of ammonia, when the first ammonia vapor pressure is at or higher than a pressure threshold.    USE - The method is useful for storing and delivering ammonia for vehicles.    ADVANTAGE - The method ensures safe and reversible storing and delivering of ammonia from storage material.    DETAILED DESCRIPTION - The method for storing and delivering ammonia for vehicles, comprises supplying a first ammonia storage material (1a) capable of reversibly adsorbing or absorbing and desorbing ammonia having a first ammonia vapor pressure into fluid communication with a second ammonia storage material (2a) capable of reversibly adsorbing or absorbing and desorbing ammonia having a second ammonia vapor pressure lower than the first ammonia vapor pressure, which is partially saturated with or completely void of ammonia, when the first ammonia vapor pressure is at or higher than a pressure threshold. The first and second materials are contained in different containers in fluid communication, which is interrupted and re-established. The first material is at a higher temperature than the second material and is used as an ammonia source in a system having an ammonia consumption unit in operation. The ammonia consumption unit is a catalytic device for splitting ammonia into nitrogen and hydrogen or a fuel cell. The first material is brought into fluid contact with the second material at or above the pressure threshold, during the operation of the ammonia consumption unit. The pressure threshold is at or higher than 5 bars and is assumed to be exceeded as soon as a signal from a secondary safety system signals an abnormality such as a fire or a crash-like situation. The ammonia partial pressure of the second material having the second ammonia pressure is below 0.1-1 bar at 25 degrees C. The first material has a higher ammonia partial pressure in its ammonia-saturated state than the second material in its ammonia-saturated state at a given temperature. The storage materials are the same materials, when in the same ammonia saturated state and the first material has a higher degree of ammonia saturation than the second material. One of the first and second ammonia storage materials is an ammonia absorbing and desorbing material. The second material is in proximity to a material that can dissipate the heat of absorption generated by the second material when adsorbing or absorbing ammonia. The second storage material is heated to desorb ammonia, when the saturation with ammonia of the second material is higher than an upper value at which the second material effectively absorbs ammonia released from the first ammonia storage material at the pressure threshold. The second ammonia storage material is used to supplement ammonia released from the first material during operation of the ammonia consumption unit to such a degree that the saturation with ammonia of the second material continuously remains below an upper value at which the second material absorbs ammonia released from the first material at the pressure threshold. INDEPENDENT CLAIMS are included for:    (1) an apparatus for storing and delivering ammonia; and    (2) automotive nitrogen oxide (NOx) treatment system.    DESCRIPTION OF DRAWING(S) - The figure shows an ammonia storage and delivery apparatus.    Storage tank (1)    Storage chamber (2)    Ammonia storage materials (1a, 2a)    Heating means (3b)    Buffer container. (5)