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

초록

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 degrees C. However, the increasing adoption of turbocharged gasoline engines, which emit exhaust gases at significantly lower temperatures (similar to 300 degrees C versus similar to 800 degrees 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. O-2 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 NOx* 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.

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