Kinetic Modeling Study of NO_x Conversion Based on Physicochemical Characteristics of Hydrothermally Aged SCR/DPF Catalyst

摘要

Diesel engines have better fuel economy over comparable gasoline engines and are useful for the reduction of CO_2 emissions. However, to meet stringent emission standards, the technology for reducing NO_x and particulate matter (PM) in diesel engine exhaust needs to be improved. A conventional selective catalytic reduction (SCR) system consists of a diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and urea-SCR catalyst. Recently, more stringent regulations have led to the development of SCR systems with a larger volume and increased the cost of such systems. In order to solve these problems, an SCR catalyst-coated DPF (SCR7DPF) is proposed. An SCR/DPF system has lower volume and cost compared to the conventional SCR system. The SCR/DPF catalyst has two functions: combustion of PM and reduction of NO_x emissions. As PM is removed from the DPF at high temperatures (>650°C), the SCR/DPF system is exposed to higher temperatures as compared with those in the conventional SCR system. In this study, we investigated the NO_x reduction performance and the properties of a hydrothermally aged SCR/DPF catalyst. Using these data, a model that can predict the NO_x conversion of the hydrothermally aged SCR/DPF catalyst was constructed. A commercial copper-exchanged zeolite catalyst, Cu-ZSM-5, was used and aged in synthetic air with 10% water over the temperature range 650-750°C. The effects of hydrothermal aging on the catalysts were investigated using a synthetic gas bench, and a detailed analysis of the structure of the hydrothermally aged catalyst was performed. Using the experimental data, we succeeded in constructing a hydrothermally aged SCR/DPF model for predicting the NO_x conversion based on changes in the physicochemical characteristics of the catalysts with changes in the hydrothermal aging conditions. This work is the first step toward bridging the gap between a lab-simulated performance model and the global reactivity observed under real-world conditions.