Schottky junction- and oxygen vacancy-driven charge separation for enhanced photocatalytic degradation of toluene over sodium- and palladium-modified titanium dioxide

The development of photocatalytic degradation (PCD) system is critically important for the effective remediation of aromatic volatile organic compounds (VOCs) in indoor air. To achieve this, a titanium dioxide (TiO 2 )-based photocatalyst has been engineered through co-modification with sodium (Na) and palladium (Pd). Its efficacy in the PCD reaction is achieved through a Schottky junction formed between Pd nanoparticles (NPs) and Na-doped TiO 2 . The PCD efficiency (X T ) of Na-Pd/TiO 2 against toluene (4 ppm at 20 % relative humidity (RH) and a gas hourly space velocity of 3 h −1 ) is 75.3 % (CO 2 yield of 44.5 %). This represents a 1.20- and 1.38-fold times increase compared to Pd/TiO 2 and TiO 2 , respectively. Na-Pd/TiO 2 achieves the highest dynamic clean air delivery rate (D-CADR) of 40.2 L h −1 g −1 . Its PCD performance, evaluated in terms of apparent quantum yield (AQY), reaches 0.113 %, which is significantly higher than that of TiO 2 (0.084 %) and Pd/TiO 2 (0.097 %), respectively. The Schottky junction with oxygen vacancies (OVs) facilitates the transfer of photogenerated electrons to the Pd NPs, enhances the separation efficiency of electron–hole pairs, and promotes the generation of reactive oxygen species (i.e., superoxide anion and hydroxyl radicals). Density functional theory calculations reveal that Na promotes molecular oxygen (O 2 )/water (H 2 O) activation, Pd acts as the toluene adsorption site, and OVs donate electrons to facilitate photocatalytic degradation under humid conditions. This study sheds light on the crucial interactions between surface OVs and Na/Pd NPs for the efficient removal of aromatic VOCs to design an advanced photocatalytic system through the tuning of surface defects and metal sites.