Global Kinetics for Ammonia Formation and Oxidation Reactions in a Commercial Three-Way Catalyst

Abstract

Lean-burn spark ignition direct injection (SIDI) engines offer significant potential for improving engine efficiency and reducing greenhouse gas emissions. However, NO x reduction in lean-burn SIDI engines presents significant challenges. One of the exhaust architectures that is currently being investigated and developed for lean-gasoline applications is the passive ammonia-SCR (three-way catalystÀselective catalyst reduction) system. It involves the use of the closed-coupled three-way catalyst (TWC) to generate NH 3 (during fuel-rich conditions) for use by the downstream NH 3 ÀSCR for NO x reduction. However, the NH 3 formed in TWC has to be controlled so that there is enough NH 3 for NO x reduction in SCR, and at the same time, there is no NH 3 slip in the tailpipe. A mathematical model for the TWC, which can predict the net NH 3 coming out of the TWC, will be very useful for control algorithm development and for understanding and optimizing the exhaust architecture. The focus of this work is on the kinetic modeling of NH 3 formation and oxidation reactions in a three way catalyst (TWC). Controlled steady-state and transient test cell experiments were performed at different air-to-fuel ratios and different engine conditions. This data was used to expand the existing TWC global reaction to include three additional global reactions that can account for NH 3 formation and oxidation. A part of the experimental data was used to estimate the kinetic parameters of the above reactions (using a one-dimensional mathematical model for the TWC) through optimization techniques. The estimated kinetic parameters are able to predict the rest of the steady-state and controlled transient experimental data reasonably well. ' INTRODUCTIONLean-burn spark ignition direct injection (SIDI) engines offer significant benefit in improving fuel economy and reducing greenhouse gas emissions. 1,2 However, NO x reduction in leanburn SIDI engines presents significant challenges. Active NO x abatement systems include selective catalytic reduction for NO x by unburnt hydrocarbons (HC) present in engine exhaust (HC-SCR) or externally injected ammonia (urea) in the engine exhaust (NH 3 ÀSCR), and storing of NO x in the catalyst during lean engine operation and reducing it during short duration rich excursions (LNT, that is, lean NO x trap). Urea-based SCR systems require a secondary fluid tank with an injection system resulting in added cost and complexity. LNT catalysts suffer from higher platinum group metal (PGM) costs and also from sulfur poisoning. At General Motors (GM), one of the exhaust architectures that are currently being investigated and developed for lean-gasoline applications is the urea-less passive ammonia-SCR (three-way catalystÀselective catalytic reduction catalyst) system. 3,4 It involves the use of the closed-coupled three-way catalyst (TWC), which apart from oxidizing the carbon monoxides, unburnt and partially burnt hydrocarbons, is used to generate NH 3 periodically (during fuel-rich conditions) 5 for use by the downstrea...

Modeling the Catalytic Performance of Coated Gasoline Particulate Filters under Various Operating Conditions

“…In order to fit the kinetic parameters to the measurement results, parameters from several sources ,,, are used as initial values. The kinetic parameters are then adjusted in three steps.…”

“…We group the reaction pathways into oxidation (R1–R4), NO reduction (R7–R11), ceria sites (R12–R21), and WGS or SR (R5–R6) reactions. It would be possible to add reactions for NH 3 ,, or CH 4 . For the interested reader, the choice of model species and reaction pathways is further illustrated in the Supporting Information.…”

“…Holder et al, 9 Ramanathan and co-workers, 11,14 Weilenmann, 15 Della Torre et al, 12 and Okajima et al 45 Mitsouridis et al 27 control oriented real-time oxygen storage dynamics Auckenthaler et al, 28 Kumar et al, 46 Bickel et al, 29 Guardiola et al, 47 and Schurholz et al 30 Arunachalam et al 48 a…”

“…Depending on the aim, some authors utilized only light-off measurements with simplified inlet feeds because their goal was, for example, to distinguish the effects of aging . Other authors fitted their kinetics with measurements of the total vehicle, , but the low-pass deformations of the measurements influence the result . Thus, kinetic parameters are usually calibrated with synthetic gas test benches which produce a steady inlet feed for the catalyst with a higher accuracy.…”

Lean NOx Reduction by In-Situ-Formed NH3 under Periodic Lean/Rich Conditions over Rhodium-Loaded Al-Rich Beta Zeolites

“…Thus, a drawback of the passive SCR is the requirement of strict N 2 /N 2 O/NH 3 selectivity control in the TWC over a wide range of conditions coupled with a complicated mathematical model for lean/rich cyclic engine operations. 23 Previous studies on NO + H 2 , NO + CO + (H 2 O), TWC, and the rich period of NSR show that the N 2 /N 2 O/NH 3 selectivity is significantly affected by the reaction conditions and the nature of the catalytic sites. 26−39 The NO reduction using the conventional PGM particle-based TWC and NSR catalysts generally affords a mixture of N 2 /N 2 O/NH 3 .…”