Urea Decomposition and Implication for NOx Reduction with Cu‐Zeolite and Vanadia‐Selective Catalytic Reduction

Understanding urea decomposition is critical to achieve highly efficient selective catalytic reduction (SCR). The urea decomposition process in an exhaust pipe and in Cu‐zeolite and vanadia‐SCR (V‐SCR) was studied in engine test cells. The unconverted urea at the SCR inlet can be substantial at lower temperatures. HNCO and NH3 are two dominant products at the SCR inlet. Urea and HNCO continue to decompose in SCR catalysts, with a rate much faster than in the homogeneous stream. The HNCO hydrolysis process is extremely efficient in Cu‐zeolite SCR and the abundant NH3 from urea overdosing can improve the NOx conversion efficiency. While for V‐SCR, the HNCO hydrolysis reaction can become the rate‐limiting step (especially after aging), abundant urea at low temperatures impairs NOx reduction. HNCO and NH3 were found as two dominant decomposition products at the selective catalytic reduction (SCR) inlet, almost always at a molar ratio of 1:1. HNCO hydrolysis in the catalyst in Cu‐zeolite SCR is extremely efficient, whereas it can become the rate‐limiting step for V‐SCR, especially for aged catalysts. It is recommended to adopt different low‐temperature dosing strategies for different SCR catalysts.