Development of TWC and PGM Free Catalyst Combination as Gasoline Exhaust Aftertreatment

摘要

Reduction of the amount of platinum group metals (PGM: Pt, Pd, Rh) utilized in three-way catalysts (TWC) has been required from a point of resource shortage and cost effectiveness. A conventional TWC system is composed of a close-coupled (CC) catalyst and an underfloor (UF) catalyst, both PGM-based. The CC-TWC promotes HC/CO oxidation and NOx reduction by CO. The UF-TWC mainly facilitates further NOx reduction by CO. In this study, a TWC system comprising a CC catalyst with PGM and an UF catalyst without PGM has been described. The newly developed system, performing reasonably well with a conventional stoichiometric gasoline combustion engine, offers an opportunity to reduce PGM usage. In this system, the UF-non-PGM catalyst is composed of a Ni/CeO_2 bottom layer which functions as a deNOx catalyst with CO-NO reaction and a zeolite based top layer which works as a deNOx catalyst with passive NH_3-SCR reaction. The CC catalyst operates as a conventional TWC but also generates a certain amount of NH_3 under rich conditions, and the generated NH_3 is used as the reductant for the SCR reaction on the UF-non-PGM catalyst. The NH_3 generation on the CC-TWC and the specific deNOx concept in UF-non-PGM catalyst were validated by model gas reactor test, and the performance of the CC-TWC + UF-non-PGM catalyst system was evaluated by chassis test. It was found that such a system showed promising performance in LA4 and US06 evaluations. The NOx conversions of the UF-non-PGM catalyst were 85% and 40% in LA4 and US06 modes, respectively.