Ambient temperature NO2 removal by adsorption on transition metal ion-exchanged chabazite zeolites

NO2 as a toxic air pollutant causes serious health problems and contributes to environmental damages, which needs to be abated to a harmless level urgently. However, the abatement of low-concentration NO2 pollution under ambient temperature is challenging due to the ineffectiveness of the state-of-the-art deNOx technologies which are only efficient at elevated temperatures (>300 °C). Adsorption is herein studied as an alternative approach for ambient temperature NO2 removal, using transition metal (i.e., Ag+, Cu2+, Co2+, Ni2+, Zn2+, In3+, and Ce3+) ion-exchanged chabazite (CHA) zeolites with different Si/Al ratios (Si/Al = 2, 6, and 12). NO2 is dominantly physiosorbed on the as-prepared samples, which enables the reversible adsorption in cyclic adsorption process. Ni2+ and Co2+ as active adsorption sites showed elevated affinity to NO2 molecules, which was attributed to the formation of Pi-back bonding via the donation of the electrons in dz2 orbital of the metal ions to Pi × orbitals of NO2 molecule, as evidenced by the changes in the electron occupancies in the orbitals of both transition metal ions and NO2 molecule upon NO2 adsorption (DFT calculation). Co2+ ion-exchanged CHA with Si/Al ratio of 2 (Co2+-2-L) showed an outstanding NO2 capacity of 4.65 mmol/g, with a stable NO2 removal performance verified by the adsorption-desorption cycles. The NO release amount of our adsorbents (<14%) was lower than other reported materials and can be further reduced by our designed double-bed adsorber. This study provides new fundamental knowledge in developing zeolite-based Pi-back bonding adsorbents for NO2 removal, which not only is of timely significance for developing sought-after technologies for NO2 abatement, but also provides a model adsorbent for the adsorptive removal of other molecules with Pi bonds (e.g., CO, NO, etc.)