NO₂ Removal by Adsorption on Transition-Metal-Based Layered Double Hydroxides

The emission of nitrogen dioxide (NO₂) has caused severe air pollution and threatened the safety of the environment and people's health. Various techniques have been intensively explored for the abatement of NO2, mostly on the basis of catalytic reduction at elevated temperature, but few have shown satisfactory NO₂ removal efficiency at ambient conditions. The use of solid porous adsorbents is a promising approach for NO₂ removal due to their high capacity and low energy penalty for regeneration. Here, we report the uncalcined transition-metal-based layered double hydroxides (TM-Al-CO₃ LDHs) as ambient NO₂ adsorbents. The dynamic breakthrough experiments demonstrated that Ni-Al-CO₃ LDH showed a superior NO₂ adsorption capacity above 5.3 mmol g^-1 and the lowest NO generation ratio (∼31.7% of the total NO₂ input) among the four TM-Al-CO₃ adsorbents. The in situ diffuse reflectance infrared Fourier transform spectroscopy disclosed the reactive adsorption mechanism between NO₂ and LDHs via acid-base interaction. The reversibility of active adsorption sites in Ni-Al-CO₃ LDH could maintain over 74% after six adsorption-desorption cycles, suggesting a decent regenerability of Ni-Al-CO₃ as the NO₂ adsorbent.