Main SCR Reactions

Since the NOx emissions from the diesel engine consist of approximately 90 % NO and only 5-10 % NO2, reaction (Rl) will be the main reaction. 

If aqueous urea solution is used as reductant, the urea droplets decompose to ammonia in the hot exhaust gases. This proceeds through first evaporating water in the urea droplets. After this, the urea decomposes thermally via the reaction (R4) forming one molecule of NH3. 

After this, isocyanic acid, HNCO can undergo hydrolysis via reaction (R5) giving another NH3 molecule. 

---