Urea-related reactions and their active sites over Cu-SAPO-34: Formation of NH3 and conversion of HNCO

Abstract

Urea-SCR is an efficient method to reduce NOx emission from diesel vehicles, but the active sites of Cu-SAPO-34 for urea-related reactions have not been well identified. In the present work, a series of Cu-SAPO-34 catalysts with different copper species were prepared and tested for urea thermolysis, urea hydrolysis and urea-SCR. Temperature-programmed experiments and in-situ diffuse reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) were used to determine isolated Cu2+ ions as the main active sites for these reactions. HNCO (isocyanic acid) and NH3 are formed via urea thermolysis. HNCO is less active than NH3 to interact with NO, and most of HNCO is hydrolyzed to produce NH3 in urea-SCR. In the presence of NO2, the reactivity of HNCO as a reducing agent is promoted, but both HNCO-SCR and HNCO hydrolysis would be inhibited by the deposition of NH4NO3. These findings provide a foundation for improving deNOx efficiency of Cu-SAPO-34 catalysts on diesel vehicles.