Urea is the precursor of Ammonia

Ammonia likewise is harmful and thus not easy to store or handle. Therefore it is much safer to use nontoxic urea as an ideal precursor to form ammonia. When a urea/water solution is injected in the hot exhaust gas the urea molecules will decompose in two steps to form ammonia and carbon dioxide (CO2).

(NH2)2CO --> NH3 + HNCO (thermolysis)
HNCO + H2O --> NH3 + CO2 (hydrolysis)

The following illustration outlines the first of the chemical reactions shown above. It is easy to recognize that the atoms are rearranged to form new molecules.