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To meet the strict emission regulations for diesel engines, an advanced processing device such as a Urea-SCR (selective catalytic reduction) system is used to reduce NOx emissions. The Real Driving Emissions (RDE) test, which is implemented in the European Union, will expand the range of conditions under which the engine has to operate [1], which will lead to the construction of a Urea-SCR system capable of reducing NOx emissions at lower and higher temperature conditions, and at higher space velocity conditions than existing systems. Simulations are useful in improving the performance of the urea-SCR system. However, it is necessary to construct a reliable NOx reduction model to use for system design, which covers the expanded engine operation conditions. In the urea-SCR system, the mechanism of ammonia (NH3) formation from injected aqueous urea solution is not clear. Thus, it is important to clarify this mechanism to improve the NOx reduction model.

For developing complicated after-treatment equipment for diesel-engine vehicles, such as urea-selective catalytic reduction (urea-SCR) systems, construction of a reaction model that can accurately predict ammonia (NH3) formation from urea is required. Hydrolysis of isocyanic acid (HNCO) is an important intermediate reaction in NH3 formation from urea. In our previous studies [1], a new rate constant for HNCO hydrolysis over fresh Cu-ZSM5 was derived using the measurements of the reaction rate of HNCO hydrolysis with high-purity HNCO formed from cyanuric acid. In this study, the reaction rates of the HNCO hydrolysis and NH3-SCR reactions were measured over a hydrothermally aged Cu-ZSM5 catalyst. A steady-state flow reactor equipped with a Fourier transform infrared spectrometer (FTIR) was employed to obtain the reaction rate of the HNCO hydrolysis and NH3-SCR reactions.