274773 Kinetics and Mechanistic Studies of Selective Catalytic Reduction of NOx with C3H6 On Cu Based Zeolite Monolith Catalyst
Lean burn engines are more fuel efficient than the traditional stoichiometric engines. The major drawback of lean burn engines is removal of NOx from exhaust in the presence of excess oxygen. Due to the harmful effects of NOx, EPA rules have become stringent regarding the NOx emissions from lean burn engines. Combined LNT-SCR systems are emerging as promising technology for NOx reduction. Unlike ammonia-based SCR, this approach utilizes the fuel as the reductant which avoids the need for an ammonia supply system. The operating concept involves the storage of NOx during the lean phase while unreacted hydrocarbons (C3H6, C2H4 etc.) and generated NH3 are released during rich phase. The ammonia produced during the regeneration is stored downstream SCR where it reacts with NOx that leaves the upstream LNT unreacted. The added complexity is the storage of olefinic hydrocarbons in the SCR catalyst and their role as co- reductants of the NOx coming out from LNT during lean phase. In this study we conduct systematic experiments on the Cu-SSZ13 (chabazite) in bench flow and TAP reactors to elucidate the mechanistic steps during reduction of NOx with C3H6. These experiments included C3H6 and NOx uptake and temperature-programmed desorption (TPD), NO and C3H6 oxidation, standard SCR (NO+ C3H6), NO and NO2 SCR (NO+NO2+ C3H6), and NO2 SCR (NO2+ C3H6).
Results obtained to date reveal an inhibiting effect of C3H6 on the SCR reaction due to C3H6 blocking sites required for SCR at low temperatures as well as C3H6 consumption (oxidation) at higher temperatures. An inhibiting effect of NO and NO2 on the C3H6 oxidation was also observed due to NO2 blocking sites required for C3H6 oxidation. The kinetics of standard NO SCR and NO and NO2 SCR reaction, and C3H6 oxidation were studied in the temperature range of 200 – 300 oC to determine the reaction orders (with respect to individual species taking part in these reactions) and activation energies of these reactions. A proposed reaction mechanism and corresponding kinetic model was developed consistent with the experimental observations. The TAP studies involve pump-probe experiments in which isotopically labeled NH3provide leads on potential NOx reduction pathways.