Kinetics and Mechanistic Studies of Hydrocarbon Oxidation and Selective Catalytic Reduction of NOx in Monolith Reactors
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The integrated LNT/SCR system is a promising technology for the reduction of NOx emission from the exhaust of lean-burn or diesel engine vehicles. The LNT/SCR operating concept involves the storage of NOx during the lean phase while unreacted hydrocarbons (C3H6, C2H4 etc.) and generated NH3 exit the LNT during the rich phase. In addition to NH3, olefinic hydrocarbons and/or partially oxidized species that break through the LNT may also adsorb on the SCR catalyst leading to additional NOx reduction by those species. In first part of the work, we performed bench-flow reactor and in-situ DRIFTS experiments to elucidate the propylene + NO + O2 reaction system on Cu–SSZ13 (chabazite) monolithic catalyst. Experiments were conducted under both steady state and transient conditions for application relevant feed conditions. Based on the bench-flow reactor studies and in-situ DRIFTS experiments a phenomenological reaction mechanism is proposed that involves reaction between oxygenates (partially oxidized hydrocarbon species) and NO to form isocyanates species (-NCO), detected by DRIFTS which are further reduced to N2. The experimental work is followed by developing a mechanistic-based kinetic model is developed for selective catalytic reduction of NO with C3H6 on Cu chabazite (Cu-SSZ13) monolithic catalyst based on bench scale flow reactor studies and in-situ DRIFTS measurements. The kinetic model was developed in steps, starting with steady-state CO oxidation, followed by C3H6 oxidation, and then the C3H6 + NO + O2 reaction system. The models for CO+O2, C3H6+O2 and C3H6+NO+O2 were also validated using a new set of steady state experiments. The model provides insight about non-NH3 pathways (hydrocarbons, oxygenates, isocyanates etc.) for NOx reduction across the SCR. Finally, we developed a global kinetic model for co oxidation of CO and C3H6 over Pt/Al2O3 monolithic catalyst. The kinetic model developed is used to elucidate the dynamic and steady state hysteresis behavior observed during oxidation of CO+C3H6 mixture on Pt/Al2O3.