Probing reaction pathways for H₂O-mediated HCHO photooxidation at room temperature

Photooxidation provides a promising strategy for removing the dominant indoor pollutant of HCHO, while the underlying photooxidation mechanism is still unclear, especially the exact role of H₂O molecules. Herein, we utilize in-situ spectral techniques to unveil the H₂O-mediated HCHO photooxidation mechanism. As an example, the synthetic defective Bi₂WO₆ ultrathin sheets realize high-rate HCHO photooxidation with the assistance of H₂O at room temperature. In-situ electron paramagnetic resonance spectroscopy demonstrates the existence of •OH radicals, possibly stemmed from H₂O oxidation by the photoexcited holes. Synchrotron-radiation vacuum ultraviolet photoionization mass spectroscopy and H₂¹⁸O isotope-labeling experiment directly evidence the formed •OH radicals as the source of oxygen atoms, trigger HCHO photooxidation to produce CO₂, while in-situ Fourier transform infrared spectroscopy discloses the HCOO* radical is the main photooxidation intermediate. Density-functional-theory calculations further reveal the •OH formation process is the rate-limiting step, strongly verifying the critical role of H₂O in promoting HCHO photooxidation. This work first clearly uncovers the H₂O-mediated HCHO photooxidation mechanism, holding promise for high-efficiency indoor HCHO removal at ambient conditions.