Sulfated and Desulfated Lean NOx-trap Characterization for Optimized Management Strategy in Gasoline Applications

Within the framework of the French research program PREDIT, a study was undertaken by ADEME, IFP, LGRE, PSA Peugeot Citroën and Umicore, whose main objective was a better understanding of the NOx storage and reduction phenomena on an aged, sulfated and desulfated NOx-trap. The target of this work was to use the information on catalyst working conditions to optimize catalyst management for a gasoline direct injection engine. The catalysts were characterized on both engine and synthetic gas benches. Aging and poisoning phenomena were studied and a variety of different chemical analytical tools were used. The behavior of two different thermally aged cores was investigated under rich conditions on a synthetic gas test bench. The dependence of the NOx regeneration efficiency of the traps is reported for several operating parameters, including reductant concentrations, durations of the rich pulse and trap loadings. With only CO as the reductant, the CO/NOx molar ratio needs to be greater than 3 to achieve an efficient regeneration of the less aged LNT. The regeneration of the serevely aged trap is more difficult. To obtain an acceptable regeneration efficiency, the ratio should be much higher (greater than 9), which leads to high reductant emissions. A study of CO alone as the reductant showed that for a given mass of CO, it is better to use a longer pulse and a lower concentration. The performance of H₂ and CO is similar for the regeneration efficiency of the traps. By injecting CO, HC and H₂ together, NOx regeneration efficiency is improved with richer conditions for shorter pulse durations. From engine bench results, CO and HC slip rise once the maximum regeneration efficiency is reached. As the sulfation rate and temperature increase, shorter but richer pulses lead to a better NOx conversion efficiency / fuel penalty trade-off. The O₂ sensor's (located after the catalyst) behavior appears to be tightly linked to H₂ concentration produced by water-gas shift at high temperature. This sensor signal might be used for the NOx-trap management during vehicle life to optimize NOx-trap purge frequency and duration in order to limit as far as possible the fuel penalty and HC/CO slip.The analysis of sulfated and desulfated traps shows that sulfur is spread out within the washcoat depth without any gradient. However, sulfur amounts decrease over the length of the catalyst from inlet to outlet. The desulfation process does not remove all the sulfate species and sulfur is mainly associated with barium. PGM sintering and Pt/Pd/Rh alloy formation have been well observed and contribute clearly to the degradation of catalyst global functionality.