Humidity-Independent Photocatalytic Toluene Mineralization Benefits from the Utilization of Edge Hydroxyls in Layered Double Hydroxides (LDHs): A Combined Operando and Theoretical Investigation

Water vapor (humidity) has been known as one of the important enhancement factors in the photocatalytic degradation of volatile organic compounds (VOCs). In this study, a composite structure of TiO2/Mg–Al layered double hydroxides (LDHs) has been constructed, which displays unique humidity-independent photocatalytic activity. Optimized structure has shown toluene degradation efficiency values as high as 74.3% under a completely dry atmosphere and the degradation efficiency ranges between 73.2% to 84.3% as the humidity increases from 0 to 90%. The origin of humidity-independent photocatalytic activity is investigated by in situ attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectroscopy, which can distinguish H2O and hydroxyl via different adsorption modes. We found that the OH– would lose electrons and be oxidized into hydroxyl radicals at the edge of LDH to participate in toluene mineralization reaction under light irradiation. The surface structure of the sample can be recovered when the surface hydroxyl is consumed, because of the existence of water vapor. Density functional theory (DFT) theoretical calculations verify that H2O molecules are more likely to dissociate and adsorb through the isolated hydrogen-terminated hydroxyl group (OHT) on LDH surface to generate hydroxyl radicals to participate in toluene photocatalytic degradation under harsh conditions, compared with the case on TiO2 surface. This work introduces a new strategy to promote the efficiency of the photocatalytic degradation of VOCs under a wide range of relative humidity.