Palladium-assisted NOx storage and release on CexZr1-xO2 for passive NOx adsorber in diesel exhaust aftertreatment

Understanding Pd effects on NO_x storage and release is crucial for designing passive NO_x adsorber (PNA) to control NO_x emissions during diesel cold-starts. Herein, we report two oxidation states of Pd species on Ce_xZr_1-xO₂ regulated by metal-support interaction. Pd^δ+ (0 < δ < 2) in Pd/Ce_0.25Zr_0.75O₂ exhibits a high affinity for O₂ adsorption, which promotes the oxidation of adsorbed NO to nitrates at 100 °C. These nitrates are thermally unstable due to electron transfer from the Pd atom to the N-O bond, facilitating the decomposition of nitrates to NO₂ above 200 °C. In contrast, Pd²⁺ in Pd/Ce_0.75Zr_0.25O₂ prefer to NO adsorption. A large amount of adsorbed NO and nitrites accumulate on Pd²⁺ and Ce⁴⁺ results in high levels of NO release below 200 °C. For the potential application in PNA, Pd/Ce_0.25Zr_0.75O₂ is recommended due to its proper NO_x release temperature as well as better water and SO₂ resistance.