Author: Li, Haiwei
Title: Enhanced degradation of indoor formaldehyde by room-temperature catalytic oxidation
Advisors: Lee, Shun-cheng (CEE)
Degree: Ph.D.
Year: 2019
Subject: Hong Kong Polytechnic University -- Dissertations
Air -- Purification
Indoor air pollution
Department: Department of Civil and Environmental Engineering
Pages: 214 pages : color illustrations
Language: English
Abstract: Formaldehyde (HCHO) is emitted from diverse sources indoors and a long-term exposure incurs severe cancer risks, making it a priority pollutant to eliminate. Considering the toxicity of HCHO at very low concentrations, standards for the emission levels of the pollutant become more stringent in Hong Kong. For the complete conversion of HCHO under ambient conditions, room-temperature catalytic oxidation (RCO) processes have been viewed as a promising strategy to remove HCHO in indoor air, in view of an energy-saving and self-driven process with high catalytic activity and CO₂ selectivity that does not need space-cost equipment. Herein, a series of MnOx-CeO₂ (MCO)-based catalysts were synthesized to enhance the low-temperature and moisture-dependent catalysis for HCHO removal. First, MCO catalysts with different major exposed facets were synthesized using varying morphology-controlling methods. The exposed three major {111}, {110}, and {001} facet-dominated surfaces were systematically examined in their distinct surface-active complexes concerning the formation of oxygen vacancies, catalytically active zones, and active-site behaviors. Given that a distinct hydrophilic structure was observed in the surface chemistry of water interacting with TiO₂ and Pt, thereby the loading treatment of TiO₂ and Pt at MnOx-CeO2 catalyst was administered to explore a distinct metal oxide/water interface, which can remediate the catalytic activity in a relatively dry feed. Two different MnOx-CeO₂-based catalysts, namely MnOx-CeO2 supported TiO₂ and MnOx-CeO₂ supported Pt (denoted as TO/MCO and Pt/MCO, respectively), were modified and characterized to overcome the limitations in low-temperature and moisture-dependent catalysis of pristine MnOx-CeO₂. Compared with that in Pt/MCO, a thinner ordered interface between molecular water and TiO₂ surface was presented within TO/MCO. No large reductions in the removal efficiencies were found under the decreased relative humidity, and the complete oxidation of HCHO at parts-per-billion (ppb) levels into CO₂ approximated 41% in dry air that was comparable with 57% in high humidity at room temperature.
Next, speciation and reaction kinetics of formaldehyde oxides (CH₂OO), often available in the literature on tropospheric oxidation, were studied in the RCO of HCHO. The CH₂OO intermediates were in situ measured through time-resolved vacuum-ultraviolet time-of-flight photoionization mass spectrometry during the HCHO reaction with TO/MCO and Pt/MCO, respectively. Three CH₂OO isomers were determined by the varying photoionization energies, i.e., main formic acid, small dioxirane, and minor CH₂OO Criegee, and validated as the primary intermediates in HCHO oxidation. CH₂OO Criegee is quite reactive, whereas formic acid and dioxirane have longer lifetime. The production, stabilization, and removal of the three intermediates are preferentially performed at high humidity, matching well with the decay rate of CH₂OO at approximately 6.6 × 10³ s⁻¹ in humid feed gas faster than 4.0 × 10³ s⁻¹ in dry feed. The potential adverse effects of catalyst nanoparticles on ambient environment and human health were evaluated through a cytotoxicological study of the as-prepared catalysts toward living yeast cells. All of the fresh MCO, Pt/MCO, and TO/MCO catalysts appeared benign biocompatibility and low cytotoxicity. The cytotoxicity of the spent catalysts toward yeast cells was compared and evaluated to conclude that TO/MCO can suppress toxic off-gas emissions to the ambient. Afterward, the environmental-friendly MCO nanoparticles were scaled-up to function as the filter substrates in a prototype of a household air cleaner. An in-field pilot test of the air cleaner was conducted in 5 days at a newly-decorated office to evaluate the HCHO removal efficiencies in a real indoor environment. After start-up of the air cleaner, the concentration of HCHO dramatically decreased and remained as low as 20 ppb over an 8 h period, which satisfied with the Excellent Class (30 μg·m⁻³, equal with std. 24 ppb of an 8 h average) of the Indoor Air Quality Certification Scheme for Offices and Public Places in Hong Kong. This work supplied a strategy recommendation for translating the laboratory research into tangible products with commercial value to attain the stringent standard protocols of HCHO emissions in indoor air.
Rights: All rights reserved
Access: open access

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