H₂O Promoted the Photocatalytic Toluene Oxidation at Ambient Humidity on Zn-Doped K-OMS-2: Mechanism Study

The poor stability of the MnO2 catalyst limits its application in photocatalytic toluene oxidation, especially at ambient humidity. Herein, the doped Zn2+ attached to the tunnel of K-OMS-2 by replacing K+ (Zn-KM photocatalyst) with an appropriate hydrophilicity surprisingly exhibits better photocatalytic performance and stability for the toluene oxidation at ambient humidity than that of KM. Especially, at a higher RH of 42%, Zn-KM shows approximately double the times of toluene removal (80%) than KM, with an excellent stability for at least 8 h. Comprehensive characterizations and calculations suggest that on the Zn-KM photocatalyst, the H2O molecule significantly reduces the adsorption energy of toluene (from −3.38 to −3.72 eV) via the hydrogen bond–mediated intermediate; simultaneously, its enhanced average Mn–O bond (from 2.27 to 2.20 Å) forms the intensive built-in electric field and promotes faster electron transfer, leading to more oxygen radical generation, richer key ring-opening intermediate of alkoxide species, and an increased stability. However, the strongly hydrophilic KM photocatalyst has easier H2O adsorption (−1.19 eV), inhibiting toluene adsorption and oxidation. This work provides the economic viability of MnO2-based photocatalysts for toluene removal.