Disclosed is a method for the selective catalytic reduction of NOx in waste/exhaust gas by using ammonia provides by heating one or more salts of formula Ma(NH3)nXz, wherein M represents one or more cations selected from alkaline earth metals and transition metals, X represents one or more anions, a represents the number of cations per salt molecule, z represents the number of anions per salt molecule, and n is a number of from 2 to 12, the one or more salts having been compressed to a bulk density above 70% of the skeleton density before use thereof.