Optimizing the lean hydrocarbon NOx trap: Sequential and dual-layer configurations

Abstract

Vehicular emission control catalysts are ineffective in eliminating CO, hydrocarbons, and NOx during engine cold-start when exhaust temperatures are below 200 °C. In this study the performance of coupled low temperature NOx, n-C12H26 (C12), and C3H6 trapping, release and conversion for a series of model Lean Hydrocarbon NOx Trap (LHCNT) catalysts are examined. Pd and Pt supported on small-pore (SSZ-13) and large-pore (BEA) zeolites are selected based on the performance during transient NO and C12 uptake, release and conversion experiments. These catalysts are combined into sequential (Pt + Pd/BEA → Pd/SSZ-13; Pd/SSZ-13 → Pt + Pd/BEA) and dual-layer (Pt + Pd/BEA top, Pd/SSZ-13 bottom) configurations in an attempt to improve the trapping and conversion performance. While all three configurations trap between 75 and 100 μmolNOx/g-cat, the Pd/SSZ-13 → Pt + Pd/BEA sequential configuration is most effective in simultaneously trapping C12 and NO in the presence of H2O, resulting in excellent NO and C12 storage below 100 °C with release and/or conversion at or above 200 °C. For each configuration, C12 oxidation lights-off below 300 °C and NO oxidation achieves ∼35 % conversion in the absence of C12. Neither the presence of C12 nor the order of the sequential configuration has a significant impact on NO uptake. C12 significantly delays NO and NO2 desorption to temperatures exceeding 300 °C. The more compact dual-layer catalyst is most effective in forming NO2 as the release temperature lines up with the maximum NO conversion temperature but traps less C12 than the sequential configurations. The addition of C3H6 in the feed on the dual-layer catalyst leads to further delay in the NOx desorption as well as increased NO and C12 conversion at high temperatures. The overall findings provide guidance in the optimizing LHCNT configuration for realistic feeds.