The existing NO_x post-treatment techniques of diesel vehicle exhaust have a poor performance during the cold start, passive NO_x adsorbent (PNA) is thus proposed. In this paper, Pd/ZSM-5 PNA was obtained by loading 1% Pd onto ZSM-5 zeolite in Si/Al ratio of 11.5 via incipient wetness impregnation and ion exchange, respectively. The effect of reaction atmosphere (with or without O₂ and H₂O) on the adsorbed-released NO_x performance of PNA with two Pd loading methods was investigated. The mechanism of the factors affecting the structure properties, Pd species state and acid properties of Pd/ZSM-5 was analyzed by X-ray diffraction, N₂ adsorption-desorption isotherm, transmission tlectron microscopy, in-situ diffuse reflectance infrared Fourier transform spectroscopy and NH₃-temperature programmed desorption, respectively. The O₂ effect in the pretreatment atmosphere was further evaluated. The results show that there are more NO active adsorption sites with O₂ in pretreatment and reaction atmosphere during incipient wetness impregnation, thus improving the NO adsorption capacity of Pd/ZSM-5. In addition, O₂ promotes the release of NO_x at a lower temperature (~200 ℃), and an incipient wetness impregnation method reduces the release temperature to 430 ℃, which is conducive to the material regeneration. H₂O inhibits NO adsorption, but promotes NO_x release capacity at a higher temperature. This work can provide a reference for the design of high-performance Pd/zeolites. High adsorption-release capacity is the important prerequisite for the development of high-performance PNA.

Catalytic oxidation of toluene over noble metal catalysts is a representative reaction for elimination of volatile organic compounds (VOCs). However, to fully understand the activation of molecular oxygen and the role of active oxygen species generated in this reaction is still a challenging target. Herein, MgO nanosheets and single-atom Pt loaded MgO (Pt SA/MgO) nanosheets were synthesized and used as catalysts in toluene oxidation. The activation process of molecular oxygen and oxidation performance on the two catalysts were contrastively investigated. The Pt SA/MgO exhibited significantly enhanced catalytic activity compared to MgO. The oxygen vacancies can be easily generated on the Pt SA/MgO surface, which facilitate the activation of molecular oxygen and the formation of active oxygen species. Based on the experimental data and theoretical calculations, an active oxygen species promoted oxidation mechanism for toluene was proposed. In the presence of H₂O, the molecular oxygen is more favorable to be dissociated to generate ^•OH on the oxygen vacancies of the Pt SA/MgO surface, which is the dominant active oxygen species. We anticipate that this work may shed light on further investigation of the oxidation mechanism of toluene and other VOCs over noble metal catalysts.