Exploring the photocatalytic conversion mechanism of gaseous formaldehyde degradation on TiO2-x-OV surface

Xinwei Li; Haiwei Li; Yu Huang; Junji Cao; Tingting Huang; Rong Li; Qian Zhang; Shun-Cheng Lee; Wingkei Ho

doi:10.1016/j.jhazmat.2021.127217

Exploring the photocatalytic conversion mechanism of gaseous formaldehyde degradation on TiO_2-x-OV surface

J Hazard Mater. 2022 Feb 15;424(Pt A):127217. doi: 10.1016/j.jhazmat.2021.127217. Epub 2021 Sep 14.

Authors

Xinwei Li¹, Haiwei Li², Yu Huang³, Junji Cao³, Tingting Huang⁴, Rong Li⁴, Qian Zhang⁵, Shun-Cheng Lee⁶, Wingkei Ho⁷

Affiliations

¹ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China; The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China.
² Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China; The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China; Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control (AEMPC), Collaborative Innovation Center of Atmospheric Environment and Equipment Technology (CIC-AEET), School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, China.
³ State Key Laboratory of Loess and Quaternary Geology (SKLLQG) and Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China.
⁴ The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China; Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong, China.
⁵ Department of Science and Environmental Studies, The Education University of Hong Kong, Hong Kong, China.
⁶ Department of Civil and Environmental Engineering, The Hong Kong Polytechnic University, Hong Kong, China; The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China. Electronic address: ceslee@polyu.edu.hk.
⁷ The Hong Kong Polytechnic University Shenzhen Research Institute, Shenzhen 518057, China; State Key Laboratory of Loess and Quaternary Geology (SKLLQG) and Key Laboratory of Aerosol Chemistry and Physics, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710061, China; State Key Laboratory of Marine Pollution, The City University of Hong Kong, Hong Kong, China. Electronic address: keithho@eduhk.hk.

PMID: 34879541
DOI: 10.1016/j.jhazmat.2021.127217

Abstract

To understand the conversion mechanism of photocatalytic gaseous formaldehyde (HCHO) degradation, strontium (Sr)-doped TiO_2-x-OV catalysts was designed and synthesized in this study, with comparable HCHO removal performance. Our results proved that foreign-element doping reduced Ti⁴⁺ to the lower oxidation state Ti^{(4- x)+}, and that the internal charge kinetics was largely facilitated by the unbalanced electron distribution. Oxygen vacancies (OVs) were developed spontaneously to realize an electron-localized phenomenon in TiO_2-x-OV, thereby boosting O₂ adsorption and activation for the enhanced generation of reactive oxygen species (ROS). At the chemisorption stage, in-situ DRIFTS spectra and density functional theory calculation results revealed that surface adsorbed O₂ (O_ads) and lattice O (O_lat) engaged in the isomerisation of HCHO to dioxymethylene (DOM) on TiO_2-x-OV and TiO₂, respectively. Time-resolved DRIFTS spectra under light irradiation revealed that the DOM was then converted to formate and thoroughly oxidized to CO₂ and H₂O in TiO_2-x-OV. While bicarbonate byproducts were detected from DOM hydroxylation or possible side conversion of CO₂ in TiO₂, owing to insufficient consumption of surface hydroxyl. Our study enhances the understanding on the photocatalytic oxidation of HCHO, thereby promoting the practical application in indoor air purification.

Keywords: Formaldehyde; Intermediates; Oxygen vacancies; Photocatalytic degradation; Quantum efficiency.