Kinetic Model of NOx SCR Using Urea on Commercial Cu−Zeolite Catalyst

Abstract

Analysis of catalyst performance data over a commercial Cu−zeolite SCR catalyst shows that NH3 oxidation leads to NO formation at temperatures above 500 °C. This contributes to the reduction in NO conversion efficiency at high temperatures. It is shown that the conversion of NH3 to NO should be second order in NH3 concentration. A kinetic model incorporating this reaction is developed for a commercial SCR catalyst. The kinetic parameters are estimated using NO and NH3 conversion data collected over varying temperatures, flow rates, NH3 to NOx ratios, and NO2 to NOx ratios using actual engine exhaust. The optimized pre-exponential factors and activation energy are reported. The model is validated by comparing predictions with catalyst performance data over other engine operating conditions. The model predictions are in reasonable agreement with the measurements except at low temperatures and flow rates. The R2 value for NO conversion efficiency predictions is 0.88 and for NH3 slip is 0.76 excluding the predictions at low temperature and flow. Substoichiometric and superstoichiometric consumption of ammonia is observed while using urea for NOx SCR. It is proposed that lower ammonia consumption is due to direct reduction of NOx by unconverted urea or intermediates formed during urea to ammonia conversion. More than the stoichiometric amount of NH3 consumption at low temperature cannot be attributed to ammonia oxidation. The model shows that the predictions for this data can be significantly improved by allowing for incomplete conversion of urea to ammonia.