Effects of Pd/CeO2 on the Abatement of Unburnt Hydrocarbon Emissions from Gasoline Vehicles during the Cold-Start Period.

Abstract

Unburnt hydrocarbon emissions from gasoline vehicles contribute to air pollution and pose health risks, necessitating effective removal strategies. Conventional three-way catalysts (TWCs) effectively oxidize hydrocarbons to CO₂ at temperatures above 250 °C. However, the increasing adoption of turbocharged gasoline engines, which emit exhaust gases at significantly lower temperatures (∼300 °C versus ∼ 800 °C in conventional vehicles), has renewed concerns over hydrocarbon emissions. The primary challenge is the prolonged warm-up time of catalytic converters, during which TWCs remain largely ineffective. To address this, we developed a dual-function Pd/CeO₂ catalyst capable of retaining unburnt hydrocarbons through adsorption during cold start and promoting their oxidation at elevated temperatures. Propylene (C₃H₆), a representative short-chain olefin in gasoline exhaust, was used as a probe molecule to evaluate the catalyst's hydrocarbon abatement performance under realistic conditions. Periodic density functional theory calculations were conducted to investigate and compare C₃H₆ oxidation pathways on pure Pd and the Pd-CeO₂ interface. O₂ dissociation was identified as the most kinetically hindered step on pure Pd, whereas OH* formation was the rate-limiting step at the Pd-CeO₂ interface. Consequently, the Pd surface exhibits sluggish oxidation activity, whereas the interface is prone to O-poisoning, which suppresses catalytic turnover. Interestingly, the coexistence of pure Pd and Pd-CeO₂ sites induces a synergistic effect mediated by intermediate spillover, which significantly enhances C₃H₆ oxidation. Considering the simultaneous emission of C₃H₆ and NO in gasoline exhaust, we also investigated their interaction and found a mutual inhibitory effect, with NO more strongly suppressing C₃H₆ oxidation. This suppression is linked to high NO _x * coverage at the Pd-CeO₂ interface, which hinders O* diffusion to the Pd surface. Our work reveals the mechanistic basis of Pd/CeO₂'s dual functionality and offers a framework for designing TWCs effective at hydrocarbon abatement during cold-start in modern gasoline engines.