백금계 촉매상에서 산화질소(NO)의 산화반응속도에 관한 실험 및 모델링 연구

Abstract

To improve the NOX conversion over a SCR (selective catalytic reduction) catalyst, the DOC (diesel oxidation catalyst) is usually placed upstream of the SCR catalyst to enhance the fast SCR reaction (4NH3 + 2NO + 2NO2 → 4N2 + 6H2O) using equimolar amounts of NO and NO2. Here, a ratio of NO2/NOX above 50% should be avoided, because the reaction with NO2 only (4NH3 + 4NO + O2 → 4N2 + 6H2O) is slower than the standard SCR reaction (4NH3 + 4NO + O2 → 4N2 + 6H2O). In order to accurately predict the performance characteristics of SCR catalysts, it is therefore desired to develop a more simple and reliable mathematical and kinetic models on the oxidation kinetics of nitric oxide over a DOC. In the present work, the prediction accuracy and limit of three different chemical reaction kinetics models are presented to describe the chemicophysical characteristics and conversion performance of DOCs. Steady-state experiments with DOCs mounted on a light-duty four-cylinder 2.0-L turbocharged diesel engine then are performed, using an engine-dynamometer system to calibrate the kinetic parameters such as activation energies and preexponential factors of heterogeneous reactions. The reaction kinetics for NO oxidation over Pt-based catalysts is determined in conjunction with a transient one-dimensional (1D) heterogeneous plug flow reactor (PFR) model with diesel exhaust gas temperatures in the range of 115~525°C and space velocities in the range of (0.4~6.5)⨉105 h-1.