Probing reaction pathways for H 2 O-mediated HCHO photooxidation at room temperature
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i National Laboratory for Physical Sciences at Microscale, National Synchrotron Radiation Laboratory, Hefei Science Center of CAS, University of Science and Technology of China, Hefei 230026 , China 2 Institute of Energy, Hefei Comprehensive National Science Center, Hefei 230031, China © Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020 Received: 31 August 2020 / Revised: 27 September 2020 / Accepted: 15 October 2020
ABSTRACT 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.
KEYWORDS mechanistic study, H2O-mediated, HCHO photooxidation, room temperature
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Introduction
Nowadays, formaldehyde is widely used in plywood, adhesives, fiberboard, permanent-press fabrics and decorative coatings, while it is also easily released from these upholstery materials and household products [1, 2]. In view of the toxicity and volatility, the formaldehyde is generally regarded to be one of the most common and harmful indoor air pollutants [3, 4]. Thus, removal of indoor formaldehyde is particularly important for human health. Significant efforts have been devoted to eliminate the formaldehyde pollution, among which photocatalysis is considered to be an appealing strategy for static indoor formaldehyde removal under ambient conditions, since the photocatalysts can photoconvert formaldehyde into CO2 and H2O at room temperature and atmospheric pressure [5–8]. It is worth noting that the H2O and O2 in indoor air would be inevitably involved in the formaldehyde photooxidation reaction; however, to date, the exact role of H2O and O2 in the catalytic process is still unclear. Only a few studies show that the H2O vapor has positive effects on the oxidation of formaldehyde, lack of valid and direct reasons. Meanwhile, the mechanism of how lattice oxygen or O
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