Hydrocarbon Effect on a Fe-zeolite Urea-SCR Catalyst: An Experimental and Modeling Study 2010-01-1171

Synergies between various catalytic converters such as SCR and DPF are vital to the success of an integrated aftertreatment system for simultaneous NO

and particulate matter control in diesel engines. Several issues such as hydrocarbon poisoning, thermal aging and other coupled aftertreatment dynamics need to be addressed to develop an effective emission control system. This work is significant especially in an integrated DPF-SCR aftertreatment scenario where the SCR catalyst on the filter substrate is exposed to un-burnt diesel hydrocarbons during active regeneration of the particulate filter. This paper reports an experimental and modeling study to understand the effect of hydrocarbons on a Fe-zeolite urea-SCR catalyst. Several bench-reactor tests to understand the inhibition of NO oxidation, to characterize hydrocarbon storage and to investigate the impact of hydrocarbons on SCR reactions were conducted. Toluene was chosen as a representative hydrocarbon in diesel exhaust and various tests using toluene reveal its inhibition of NO oxidation at low temperatures and its oxidation to CO and CO₂ at high temperatures. Surface isotherm tests were conducted to characterize the adsorption-desorption equilibrium of toluene through Langmuir isotherms. Using the rate parameters, a toluene storage model was developed and validated in simulation. With toluene in the stream, controlled SCR tests were run on the reactor and performance metrics such as NO

conversion and NH₃ slip were compared to a set of previously run tests with no toluene in the stream. Tests indicate a significant effect of toluene on NO

and NH₃ conversion efficiencies even at temperatures greater than 300°C. A kinetic model to address the toluene inhibition during NO oxidation reaction was developed and is reported in the paper.