Research Progress on Passive NOx Adsorbers (PNAs) for Diesel Vehicles

The cold start phase of diesel engines represents a critical stage for nitrogen oxides (NOx) emissions, which pose significant challenges to environmental sustainability and public health. During this phase, the exhaust gas temperature remains low, making conventional selective catalytic reduction (SCR) systems ineffective. To address this issue, passive NOx adsorbers (PNAs) have emerged as an effective solution for achieving ultra-low NOx emissions, serving as pivotal components in next-generation diesel exhaust aftertreatment systems. These materials are designed to capture NOx at low temperatures and subsequently release them when the exhaust reaches higher temperatures, allowing efficient mitigation through downstream SCR catalysts. This review systematically explores recent advancements in PNA technology, focusing on materials design strategies, reaction mechanisms, and performance evaluation. Transition metal oxides (e.g., CeO2, Co3O4, and Ru/CeO2) and zeolites (e.g., Pd/SSZ-13, Pd/BEA, and Co/SSZ-13) are two typical types of materials currently being studied in the PNAs; they exhibit unique advantages and are discussed in terms of their adsorption/desorption performance, oxidizing activity, and thermal stability. Transition metal oxides exhibit superior catalytic oxidation abilities for CO and hydrocarbons (HCs), and good NOx storage, with CeO2 achieving adsorption capacities up to 0.32 mmol/g at 30 ℃. In contrast, zeolites possess unique pore structure, large specific surface areas, and exceptional hydrothermal stability (e.g., Pd/SSZ-13 maintaining performance after 1000 ℃ hydrothermal aging). Further discussion on the role of precious metals and the potential of non-precious metal-based zeolites in PNA application is presented. Considering the distinct physicochemical properties of the above materials, a detailed discussion of the NOx adsorption-desorption mechanisms is proposed. The first mechanism, the NOx oxidative storage mechanism, primarily occurs on metal oxide surfaces and involves NOx storage as nitrite/nitrate, accompanied by NO oxidation by active oxygen species. NOx release results from nitrite/nitrate decomposition at high temperature. The second mechanism, the metal-ion-complex mechanism, corresponds to NOx adsorption as metal ionic complexes, widely found in zeolite-type PNA materials. Furthermore, the effects of exhaust components in practical application, such as H2O, CO, and HC, on the NOx adsorption-desorption behavior, dynamic structural transformation, and reaction species are discussed. This review also summarizes the impact of hydrothermal aging and chemical poisoning (phosphorus and sulfur) on adsorption sites and related solutions regarding protection and regeneration of PNA materials. Despite significant advancements in PNA materials, several challenges remain, including the need to optimize high-surface-area oxides for hydrothermal resilience, prevent Pd ion agglomeration, and elucidate a deeper understanding of the HC interaction mechanism. Future research efforts should prioritize multifunctional materials with enhanced poison resistance and scalable synthesis methods to meet evolving global emission regulation.