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 center dot OH radicals, possibly stemmed from H2O oxidation by the photoexcited holes. Synchrotron-radiation vacuum ultraviolet photoionization mass spectroscopy and (H2O)-O-18 isotope-labeling experiment directly evidence the formed center dot 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 center dot 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.