Abstract

To improve the <TEX>$NO_X$</TEX> 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 (<TEX>$4NH_3+2NO+2NO_2{\rightarrow}4N_2+6H_2O$</TEX>) using equimolar amounts of NO and <TEX>$NO_2$</TEX>. Here, a ratio of <TEX>$NO_2/NO_X$</TEX> above 50% should be avoided, because the reaction with <TEX>$NO_2$</TEX> only (<TEX>$4NH_3+4NO+O_2{\rightarrow}4N_2+6H_2O$</TEX>) is slower than the standard SCR reaction (<TEX>$4NH_3+4NO+O_2{\rightarrow}4N_2+6H_2O$</TEX>). 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~<TEX>$525^{\circ}C$</TEX> and space velocities in the range of <TEX>$(0.4{\sim}6.5){\times}10^5\;h^{-1}$</TEX>.

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