Modeling Species Inhibition of NO Oxidation in Urea-SCR Catalysts for Diesel Engine $NOx$ Control

Urea-selective catalytic reduction (SCR) catalysts are regarded as the leading $NOx$ aftertreatment technology to meet the 2010 $NOx$ emission standards for on-highway vehicles running on heavy duty diesel engines. However, issues such as low $NOx$ conversion at low temperature conditions still exist due to various factors, including incomplete urea thermolysis, inhibition of SCR reactions by hydrocarbons, and $H2O$⁠. We have observed a noticeable reduction in the standard SCR reaction efficiency at low temperature with increasing water content. We observed a similar effect when hydrocarbons are present in the stream. This effect is absent under fast SCR conditions where $NO∼NO2$ in the feed gas. As a first step in understanding the effects of such inhibition on SCR reaction steps, kinetic models that predict the inhibition behavior of $H2O$ and hydrocarbons on NO oxidation are presented in the paper. A one-dimensional SCR model was developed based on the conservation of species equations and was coded as a C-language S-function and implemented in MATLAB/SIMULINK environment. NO oxidation and $NO2$ dissociation kinetics were defined as a function of the respective adsorbate’s storage in the Fe–zeolite SCR catalyst. The corresponding kinetic models were then validated on temperature ramp tests that showed good match with the test data. Such inhibition models will improve the accuracy of the model based control design for integrated diesel particulate filter-SCR aftertreatment systems.