Abstract:The cold start phase of diesel engines represents a critical stage for nitrogen oxides (NO)emissions, which pose significant challenges to environmental sustainability and public health. Duringthis phase, the exhaust gas temperature remains low, making conventional selective catalytic reduction(SCR) systems ineffective. To address this issue, passive NO adsorbers (PNAs) have emerged as aneffective solution for achieving ultra-low NO emissions, serving as pivotal components in next-generation diesel exhaust aftertreatment systems. These materials are designed to capture NO at lowtemperatures and subsequently release them when the exhaust reaches higher temperatures, allowingefficient mitigation through downstream SCR catalysts. This review systematically explores recentadvancements in PNA technology, focusing on materials design strategies, reaction mechanisms, andperformance evaluation. Transition metal oxides (e.g., CeO, CoO, and Ru/CeO) and zeolites (e.g.,Pd/SSZ-13, Pd/BEA, and Co/SSZ-13) are two typical types of materials currently being studied in thePNAs; they exhibit unique advantages and are discussed in terms of their adsorption/desorptionperformance, oxidizing activity, and thermal stability. Transition metal oxides exhibit superior catalyticoxidation abilities for CO and hydrocarbons (HCs), and good NO storage, with CeO achievingadsorption 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 maintainingEnergy Environmental Protectionperformance after 1 000 ℃ hydrothermal aging). Further discussion on the role of precious metals andthe potential of non-precious metal-based zeolites in PNA application is presented. Considering thedistinct physicochemical properties of the above materials, a detailed discussion of the NO adsorption-desorption mechanisms is proposed. The first mechanism, the NO oxidative storage mechanism,primarily occurs on metal oxide surfaces and involves NO storage as nitrite/nitrate, accompanied byNO oxidation by active oxygen species. NO release results from nitrite/nitrate decomposition at hightemperature. The second mechanism, the metal-ion-complex mechanism, corresponds to NO adsorptionas metal ionic complexes, widely found in zeolite-type PNA materials. Furthermore, the effects ofexhaust components in practical application, such as HO, CO, and HC, on the NO adsorption-desorption behavior, dynamic structural transformation, and reaction species are discussed. This reviewalso summarizes the impact of hydrothermal aging and chemical poisoning (phosphorus and sulfur) onadsorption sites and related solutions regarding protection and regeneration of PNA materials. Despitesignificant advancements in PNA materials, several challenges remain, including the need to optimizehigh-surface-area oxides for hydrothermal resilience, prevent Pd ion agglomeration, and elucidate adeeper understanding of the HC interaction mechanism. Future research efforts should prioritizemultifunctional materials with enhanced poison resistance and scalable synthesis methods to meetevolving global emission regulation. Close-