NOX storage and reduction over a perovskite-based lean NOX trap catalyst

Abstract

Typical lean NOX trap (LNT) catalysts contain Pt, which catalyzes NO oxidation and NOX reduction, and an alkali or alkaline earth material for NOX storage via nitrate formation. The catalyst is operated in a cyclic mode, with one phase of the cycle under oxidizing conditions where NOX is trapped, and a second phase, which is reductant-rich relative to O2, where stored NOX is reduced to N2. A recently developed catalyst uses a perovskite material as part of the LNT formulation for the oxidation reactions thereby eliminating the need for Pt. Pd and Rh are still added, to accommodate hydrocarbon oxidation and NO reduction, respectively. NO oxidation kinetics over the fully formulated and bare perovskite material were determined, with NO, O2 and NO2 orders being at or near 1, 1 and −1, respectively for both samples. The fully formulated sample, which contains Ba supported on the perovskite, was evaluated in terms of NOX trapping ability and NOX reduction as a function of temperature and reduction phase properties. Trapping and overall performance increased with temperature to 375°C, primarily due to improved NO oxidation, as NO2 is more readily trapped, or better diffusion of nitrates away from the initial trapping sites or into the Ba particles. At higher temperatures nitrate stability decreased, thus decreasing the trapping ability. At these higher temperatures, a more significant amount of unreduced NOX formed during the reduction phase, primarily due to nitrate instability and decomposition and the relative rates of the NOX and oxygen storage (OS) component reduction reactions. Most of the chemistry observed was similar to that observed over Pt-based LNT catalysts. However, there were some distinct differences, including a stronger nitrate diffusion resistance at low temperature, a more significant reductant-induced nitrate decomposition reaction and nitrate inhibition of OSC reduction at the onset of the regeneration phase.