Kinetic Modeling of Ammonia-SCR and Experimental Studies over Monolithic Cu-ZSM-5 Catalyst

Ammonia-selective catalytic reduction (SCR) systems have been introduced commercially in diesel vehicles, however catalyst systems with higher conversion efficiency and better control characteristics are required to know the actual emissions during operation and the emissions in random test cycles. Computational fluid dynamics (CFD) is an effective approach when applied to SCR catalyst development, and many models have been proposed, but these models need experimental verification and are limited in the situations they apply to. Further, taking account of redox cycle is important to have better accuracy in transient operation, however there are few models considering the cycle. Model development considering the redox reactions in a zeolite catalyst, Cu-ZSM-5, is the object of the research here, and the effects of exhaust gas composition on the SCR reaction and NH₃ oxidation at high temperatures are investigated. The simulations are compared with the experimental results of a surrogate gas, a mixture of nitrogen monoxide (NO), oxygen (O₂), water vapor (H₂O), and nitrogen (N₂), and the accuracy of the developed model is validated. To investigate the effects of O₂ concentration on standard SCR and NH₃ oxidation, the experiments are conducted with the surrogate gas. The results suggest that the O₂ has a larger reaction order (1.25-1.51) to NH₃ oxidation than that to the standard SCR (0.73-0.75). The model considering these reaction orders of O₂ predicts the effect of O2 on NOx and NH₃ conversions well. A transient operation experiment is also conducted with the Cu-ZSM-5 catalyst. The experiment consists of NH₃ adsorption part and NO reduction part, where the re-oxidizing of copper does not occur. The results clearly show that Cu⁺ does not contribute to the SCR reaction. Further, the NH₃-SCR simulation model was greatly improved by considering the redox reaction cycle and the concentrations of oxygen.