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 one of the most effective options for the remediation of aromatic volatile organic compounds (VOCs) in indoor air. To achieve this, a titanium dioxide (TiO₂)-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 TiO₂. The PCD efficiency (X_T) of this Pd/Na-TiO₂ against toluene (4 ppm at 20 % relative humidity (RH) and a gas hourly space velocity of 3 h^-1) is 75.3 % (CO₂ yield of 44.5 %). This represents 1.2- and 1.4-fold increases compared to Pd/TiO₂ and TiO₂, respectively. Pd/Na-TiO₂ 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 TiO₂ (0.084 %) and Pd/TiO₂ (0.097 %). 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 (O₂)/water (H₂O) activation, Pd acts as the toluene adsorption site, and OVs donate electrons to facilitate photocatalytic degradation under humid conditions. This study addresses the challenge of eliminating persistent aromatic air pollutants by engineering a photocatalytic system where tuned surface defects (OVs) and sodium-promoted palladium sites work in concert. The integration of these synergistic interactions is demonstrated to be the key to unlocking high-efficiency VOC destruction, paving the way for advanced air purification technologies.