Direct synthesis of vertically oriented Co-Mg-Al layered double hydroxide on spherical γ-Al2O3 for passive NOx adsorber

Abstract

Co-Mg-Al layered double hydroxides (LDHs) are directly synthesized on spherical γ-Al2O3 using a hydrothermal treatment method with urea for low-temperature NOx adsorber, and the formation mechanism of LDHs is studied. γ-Al2O3 acts as an Al3+ source and a support, and urea provides CO32– and OH– during hydrothermal treatment. During the hydrothermal reaction, a gibbsite-like amorphous aluminum hydroxide (AOH) phase is formed on the γ-Al2O3 surface, and Co2+ and Mg2+ are dissolved into AOH layers to form LDHs. The urea concentration is a critical factor for nucleation and particle growth during LDH synthesis, affecting the properties and NOx storage/release performance of LDH-derived mixed metal oxides. Sintering of LDH-derived mixed metal oxide particles is prevented during calcination and regeneration at elevated temperatures owing to chemical bonding between the LDHs and the AOH layer, thus enhancing NOx adsorption performance.