Unravelling the pathway determining the CO2 selectivity in photocatalytic toluene oxidation on TiO2 with different particle size

The rapid intermediate species accumulation on TiO2 induces the deactivation and is a long-standing unsolved issue in the photocatalytic oxidation of volatile organic compounds (VOCs). Exploring efficient pathway to achieve deep oxidation is of crucial significance. In this study, we observed that TiO2 with small particle size (S-TiO2) had high toluene conversion but low CO2 mineralization, while TiO2 with large particle size (L-TiO2) had low toluene conversion but high CO2 mineralization. The characterization results showed that S-TiO2 had a larger specific surface area and more surface hydroxyl groups than L-TiO2, which accounted for its higher toluene conversion. Additionally, low toluene adsorption but a larger quantity of reactive oxygen species (ROS) was observed on L-TiO2. Our findings further revealed that toluene on S-TiO2 was initially oxidized to benzene-ring-containing intermediate species by ROS. However, the accumulation of these species suppressed the ROS production and blocked the ring-opening reaction, resulting in low CO2 selectivity. In contrast, nearly no intermediate species was accumulated on L-TiO2 due to the low toluene adsorption and abundant ROS production, which allowed the photocatalytic oxidation of toluene on L-TiO2 to follow the direct ring-opening pathway and achieve higher CO2 selectivity. This study provides new insight into the mechanism of the photocatalytic oxidation of VOCs.