Hydrothermal-treatment-based facile one-step preparation of K-promoted NO_x adsorbents derived from hydrotalcite-like compounds

K-promoted mixed metal oxides were prepared from K₂CO₃-promoted Co-Mg-Al hydrotalcite-like compounds using a simple hydrothermal synthesis method. The introduced K₂CO₃ largely increased the NO_x uptake by generating active oxygen species; however, the presence of an excessive amount of K₂CO₃ in the aggregated bulk phase suppressed the formation of active oxygen species. The developed NO_x adsorbents presented excellent NO_x uptake at low temperatures (0.566 and 2.63 mmol g⁻¹ at 150 and 250 °C, respectively). In addition, the effects of active oxygen species on the NO oxidation and NO_x adsorption ability were investigated. The amount of active oxygen species and the NO_x adsorption performance at 150 °C were highly correlated. However, the NO_x uptake at 250 °C gradually increased with the amount of K₂CO₃, although the K₂CO₃ species in the aggregated bulk phase inhibited the formation of active oxygen species. It was revealed that the K₂CO₃ in the bulk phase promoted the adsorption of NO_x at 250 °C by directly trapping it. Overall, the introduced K₂CO₃ enhanced the NO_x adsorption performance in two ways: (1) as a surface modifier to generate active oxygen species and (2) via the direct adsorption of NO_x at high temperature.