Characterizing monolithic Lean NO[x in subscript] Trap catalysts using a bench-flow reactor

by 1973- Kim, Hakyong

The performance of EmeraChem Lean NOx Trap (LNT) catalysts with company proprietary washcoat formulation has been investigated using a bench-flow reactor (BFR). The washcoat composition consists of Pt, the precious metal component (PM), and two NOx storage media of Ba and K, supported on ?-Al2O3 washcoat. A series of isothermal absorption experiments was carried out at different temperatures and gas hourly space velocities (GHSVs) in order to investigate the effect of temperature and GHSV on the nitrogen oxides (NOx) trapping capacity of EmeraChem LNT catalysts. The NOx storage capacity exhibits a “volcano-type” dependence on the temperature, with a maximum storage capacity of catalyst occurring at 350°C. On the other hand the NOx storage capacity of the LNT decreases as the GHSV increases. The effects of lean and rich duration and type of reductants on the regeneration of the LNT catalyst are investigated at an optimum operating temperature of 350°C and a gas hourly space velocity of 50,000 hr-1. The experiments are performed for two different cases. In the first case, the time durations of the lean and rich phases are varied at a fixed concentration of the reductants: carbon monoxide (CO) and hydrogen (H2). In the second case, the reductant concentrations ? CO and H2 ? are varied at constant time duration of the lean and the rich phases. For the first case, a lean/rich cycle of 100s lean and 5s rich is found to be optimum since this cycle would offer the highest NOx conversion as well as the best fuel efficiency in regenerating the LNT catalysts. For the second case, H2 is a slightly better reductant in reducing NOx than CO. iv Two separate direct fuel injection (DFI) experiments are carried out: DFI with and without O2 in the simulated exhaust gases. The results from DFI without O2 show that the amount of NOx slip and NOx excursion decreases and reaches a steady value as the concentration of the reductant increases. The results from DFI with O2 show that the average NOx conversion decreases significantly in the presence of O2 in the regeneration phase. The results suggest that some of the reductant is consumed by directly reacting with oxygen, and thus less is available to participate in the reduction reactions. v