Novel layered double hydroxide-based passive NOx adsorber: Synergistic effects of Co and Mn on low-temperature NOx storage and regeneration

Passive NOx adsorber (PNA) is an efficient strategy for reducing vehicular NOx emission during cold-start period, and current studies on PNA development are limited to materials containing precious metals (Pt, Pd, and Ag, etc.). Herein, Co–Mn–Mg–Al mixed metal oxides were prepared from layered double hydroxides (LDHs) and utilized as a new type of PNA. Although Co–Mg–Al oxides exhibited excellent NOx adsorption capabilities because Co species promoted the generation of active oxygen species, their NOx adsorption performance deteriorated substantially with decreasing adsorption temperature below 150 °C and they required high temperatures over 400 °C to release the adsorbed NOx. Conversely, Mn–Mg–Al oxides retained their adsorption performance regardless of adsorption temperature and allowed for NOx desorption at relatively low temperatures by decreasing the thermal stability of the adsorbed NOx species. Notably, Co–Mn–Mg–Al mixed oxides exhibited high NOx adsorption performance at low temperatures between 50 and 150 °C and were passively regenerated via thermal treatment within a temperature range of 200–350 °C owing to the synergistic effects of Co and Mn. The developed LDH-based PNA materials presented excellent cyclic NOx adsorption performances for long-term operation, and dynamic tests indicated that NOx adsorbed onto Co–Mn–Mg–Al mixed oxides was effectively released with increasing temperature even in the presence of NO and O2.