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

Urea-selective catalytic reduction (SCR) catalysts are regarded as the leading NO_x aftertreatment technology to meet the 2010 NO_x emission standards for on-highway vehicles running on heavy-duty diesel engines. However, issues such as low NO_x 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 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 SCR catalyst. The corresponding kinetic models were then validated on temperature ramp tests that showed good match with the test data.