Impact of PtOx formation in diesel oxidation catalyst on NO2 yield during driving cycles

Abstract Operation of a platinum-based diesel oxidation catalyst under lean conditions leads to the partial transformation of metallic Pt sites into platinum oxides (PtO x ) with considerably lower NO oxidation rate. The varying NO 2 yield depending on PtO x coverage significantly influences the performance of other devices following in a diesel exhaust aftertreatment line: particulate filter (soot oxidation) and SCR or LNT catalyst (NO x reduction). In this paper, we present a global kinetic model of a diesel oxidation catalyst, including PtO x formation induced by reactions with O 2 and NO 2 , PtO x reduction by CO, hydrocarbons and NO, and PtO x thermal decomposition, and use it to reveal the extent of NO 2 yield variation in four standard driving cycles for passenger car emission tests: NEDC, FTP, US06 and SC03. During a single driving cycle, the NO 2 yield decreases by 3–10% relative to the original level of the reduced catalyst. The PtO x formation is a slow process and stabilizes only after approximately four repeated driving cycles. The stabilized NO 2 yield is 7–27% (relative) lower than with the reduced catalyst, depending mainly on the history of operating temperatures. The largest variation is observed around 250–300 °C. At lower temperatures, PtO x are partly reduced during CO and hydrocarbon peaks in the engine exhaust during dynamic operation. At higher temperatures, PtO x start to decompose and NO oxidation becomes limited by equilibrium.