Ambient NO₂ adsorption removal by Mg–Al layered double hydroxides and derived mixed metal oxides

NO₂ is a potent air pollutant because of its deleterious effects on human beings and other organisms. The state-of-the-art catalysis-based deNO_x techniques (e.g., selective catalytic/non-catalytic reduction) are incapable of ambient NO₂ abatement due to their low efficiency at temperatures below 300 °C. It is thus conceivable to directly capture NO₂ from the atmosphere by selective adsorption on porous materials. This work reports the rational development and demonstration of the Mg–Al layered double hydroxides (LDHs) and their derived mixed metal oxides (MMO), using environmentally benign solvents, as high-capacity adsorbents for ambient NO₂ abatement. By boosting the densities of accessible basic sites using layer delamination strategies, the highest NO₂ adsorption capacity of 8.52 mmol/g was achieved by the delaminated LDH material (LDH-AM), which was substantially higher than other popular and robust adsorbents, such as zeolites (0.36–3 mmol/g) and carbon-based adsorbents (2–6 mmol/g). Using Fourier transform infrared spectroscopy and powder X-ray diffraction, it was revealed that NO₂ adsorption occurs on the surface M-OH basic sites and within the layers by simultaneously replacing the interlayer CO₃²⁻ ions of LDH. This work affords not only promising, durable, and scalable adsorbents for ambient NO₂ removal but also a strategy to develop adsorbents with high density of basic sites for capture of other pollutant acid gases from the environment.