Kinetics and Mechanistic Studies of Selective Catalytic Reduction of NO_x with C_3H_6 on Cu based Zeolite Monolith Catalyst

摘要

Lean burn engines are more fuel efficient than the traditional stoichiometric engines. Themajor drawback of lean burn engines is removal of NO_x from exhaust in the presence of excessoxygen. Due to the harmful effects of NO_x, EPA rules have become stringent regarding the NO_xemissions from lean burn engines. Combined LNT-SCR systems are emerging as promisingtechnology for NO_x reduction. Unlike ammonia-based SCR, this approach utilizes the fuel as thereductant which avoids the need for an ammonia supply system. The operating concept involvesthe storage of NO_x during the lean phase while unreacted hydrocarbons (C_3H_6, C_2H_4 etc.) andgenerated NH3 are released during rich phase. The ammonia produced during the regeneration isstored downstream SCR where it reacts with NO_x that leaves the upstream LNT unreacted. Theadded complexity is the storage of olefinic hydrocarbons in the SCR catalyst and their role as coreductantsof the NO_x coming out from LNT during lean phase. In this study we conductsystematic experiments on the Cu-SSZ13 (chabazite) in bench flow to elucidate the mechanisticsteps during reduction of NO_x with C_3H_6. These experiments included C_3H_6 and NO_x uptake andtemperature-programmed desorption (TPD), NO and C_3H_6 oxidation, standard SCR (NO +C_3H_6), NO and NO_2 SCR (NO + NO_2 + C_3H_6), and NO_2 SCR (NO_2 + C_3H_6).Results reveal an inhibiting effect of C_3H_6 on the SCR reaction due to C_3H_6 blockingsites required for SCR at low temperatures as well as C_3H_6 consumption (oxidation) at highertemperatures. An inhibition effect of NO on C_3H_6 oxidation was also observed at highertemperatures. The inhibition was not caused by competitive adsorption between C_3H_6 and NObut by inert carbonaceous species formed by the reaction between NO and partially oxidizedC_3H_6-derived intermediate which then blocks the active sites. This hypothesis was confirmed bytemperature programmed oxidation experiments. No inhibition by NO_2 on C_3H_6 oxidation wasobserved. Differential kinetic studies were performed for C_3H_6 oxidation and standard NO SCRreaction systems in order to determine the reaction orders and activation energies. In the