(126a) 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 NO_x from exhaust in the presence of excess oxygen. Due to the harmful effects of NO_x, EPA rules have become stringent regarding the NO_x emissions from lean burn engines.  Combined LNT-SCR systems are emerging as promising technology for NO_x 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 NO_x during the lean phase while unreacted hydrocarbons (C₃H_6, 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 NO_x 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 NO_x 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.