Microkinetic study of NO oxidation, standard and fast NH3-SCR on CeWOx at low temperatures

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A comprehensive microkinetic model was established to describe NO oxidation, standard SCR and fast SCR on CeWOx at low temperatures for the first time, providing an in-depth understanding of the NH3-SCR mechanism. The model divided the NH3 adsorption sites into two groups, including sites that could prohibit NO adsorption and that could not affect NO adsorption. Different rate-limiting steps during NO oxidation, standard and fast SCR were identified, and different reaction orders toward NO, NH3 and NO2 were obtained in different reactions. The mechanisms of standard and fast SCR were quite similar. Both the Eley-Rideal and Langmuir-Hinshelwood mechanisms were present, and the reaction between nitrites and adsorbed NH3 made the main contribution to the NOx conversion. The apparent activation energy was affected by the reaction between nitrites and adsorbed NH3 and the reaction between NO and NH4NO3, and the activation energy of the latter reaction was higher. The activation energy of fast SCR was higher than that of standard SCR due to a greater contribution by the reaction between NO and NH4NO3, but larger amounts of nitrites and NH4NO3 caused higher activity for fast SCR than standard SCR. The sites for NO adsorption to produce nitrites were low in number but highly active, which might be a reason why CeWOx is highly active for standard SCR under high gas hourly space velocity, and increasing the sites for NO adsorption might be an effective way to accelerate NH3-SCR. This work provides guiding information for the development of new and effective NH3-SCR catalysts.