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

Abstract

Understanding Pd effects on NOx storage and release is crucial for designing passive NOx adsorber (PNA) to control NOx emissions during diesel cold-starts. Herein, we report two oxidation states of Pd species on CexZr1-xO2 regulated by metal-support interaction. Pdδ+ (0 < δ < 2) in Pd/Ce0.25Zr0.75O2 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/Ce0.75Zr0.25O2 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/Ce0.25Zr0.75O2 is recommended due to its proper NOx release temperature as well as better water and SO2 resistance. Pd-exchanged zeolites are widely studied as passive NOx adsorber materials for emission control for combustion engines. Here, Yue Peng and colleagues reveal that a relatively strong metal-support interaction between Pd and Ce0.25Zr0.75O2 leads to two oxidation states of Pd optimizing NOx storage efficiency and NOx release temperature.