New insights into the removal of nitric oxide using UiO-66-NH2: Synergistic photooxidation and subsequent adsorption

UiO-66 variants are widely used for photocatalysis and adsorption, but few studies reported their combined effects in one process. This paper reports that the mechanism behind NO removal using UiO-66-NH2 includes photocatalytic oxidation and subsequent adsorption. First, the UiO-66-NH2 was selected out of UiO-66-X variants (X = H, NH2, NO2, OH, Br, Br-2) because of its high NO removal efficiency (i.e., 80.54%), which is 8.55-64.95 times higher than those of other UiO-66 variants. Then, the NO reaction pathway and degradation mechanisms are proposed based on the experimental results and theoretical calculations. In an anhydrous environment, the NO removal efficiency increases from 80.54% at 40-60% of humidity to 96.23%. Moreover, NO2 emissions and catalyst deactivate are not observed. These findings indicate that the photocatalytic NO degradation includes the photocatalytic oxidation of NO into NO2 on the surface of UiO-66-NH2 and subsequent NO2 adsorption in micropores. The reason is that the formation of NO(x)(- )ions without H2O is theoretically impossible according to the principle of electroneutrality. In-situ DRIFTS also confirms the mechanism. Furthermore, density functional theory (DFT) and grand canonical Monte Carlo (GCMC) simulations were carried out to understand the improved NO removal caused by synergistic photocatalysis and adsorption. In summary, this work proposes a new mechanism for NO removal that combines the photocatalytic oxidation and adsorption capability of UiO-66-NH2, the new mechanism provides a new strategy to further improve the NO removal efficiency of UiO-66-NH2 and a new way of inhibiting the deactivation.