Photocatalysis is regarded as a promising method for removal of indoor gas-phase volatile organic compounds such as formaldehyde (HCHO), which could cause health problems. Conventional studies on the HCHO photocatalytic degradation primarily focused on the role of hydroxyl radical(*OH) as a major reactive oxygen species (ROS) which is generated from the oxidation of H2O. This perspective may not be suitable for all gaseous HCHO photodegradation in various conditions including dry conditions. In atmospheric conditions, singlet oxygen (1O2) derived from O2 is well known as a powerful oxidant due to its longer lifetime and diffusion distance. Here, we report the BiOI/TiO2 composite photocatalyst, which is mainly operated by 1O2 for efficient HCHO photocatalytic degradation. BiOI was coated on TiO2 nanoparticles by electrostatic interaction during facile one-pot synthesis and formed sufficient p-n heterojunctions. The introduction of p-n heterojunctions causes electron transfer promoting the generation of mobile 1O2 as a major ROS instead of *OH in dry conditions. In a single-pass reactor, the BiOI/TiO2 photocatalysts exhibited a 1.4 times higher HCHO-to-CO2 conversion rate constant compared to TiO2 in dry conditions. Through spectroscopic analyses, we verified enhanced photocatalytic activity of BiOI/TiO2 photocatalyst by charge separation of composites. Moreover, we investigated mechanistic studies to unravel 1O2 driven HCHO photocatalysis pathways based on radical monitoring and band alignment. The radical detection methods proved that interfacial charge transfer through the p-n junction in BiOI/TiO2 photocatalyst depressed *OH generation but enhanced mobile 1O2 generation for gas-phase HCHO oxidation. Also the in-situ DRIFTS study confirmed the accelerated oxidation of HCHO and intermediates on BiOI/TiO2 photocatalyst, even in the dry conditions due to the generation 1O2 from the O2. Thus, we propose that the 1O2 played a major ROS in the photocatalytic elimination of gaseous HCHO on p-n junction photocatalyst in dry conditions. Furthermore, we demonstrated the photocatalytic filter by applying BiOI/TiO2 to the porous honeycomb type diesel particulate filter. The photocatalytic filter successfully converted HCHO to CO2 for 150 h with a conversion efficiency of 87% in a single-pass reactor in dry conditions.

Singlet oxygen driven photocatalytic degradation of gaseous formaldehyde via p-n junction photocatalyst and its mechanistic study

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