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

Abstract

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 NOx 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 NOx adsorbents presented excellent NOx 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 NOx adsorption ability were investigated. The amount of active oxygen species and the NOx adsorption performance at 150 °C were highly correlated. However, the NOx 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 NOx at 250 °C by directly trapping it. Overall, the introduced K2CO3 enhanced the NOx adsorption performance in two ways: (1) as a surface modifier to generate active oxygen species and (2) via the direct adsorption of NOx at high temperature.