The construction of air purification system using metal and non-metal doped titanium dioxide as photocatalyst against gaseous formaldehyde

Abstract

The demand for the proper management of indoor air quality has been growing with the rising standards of living quality and health consciousness. As many sources are contributing to indoor air pollution, the type of airborne pollutants is also diverse enough to include volatile organic compounds (VOCs). VOCs pose a serious threat to the ecological environment and human health as most VOCs are highly toxic, carcinogenic, and hazardous. In terms of human health, most VOCs are a significant contributor to sick-building syndrome. Among VOCs, formaldehyde (FA) has received intensive attention due to its harmfulness to humans in indoor air. As a carcinogen, FA is also reported to be associated with diverse symptoms such as nasopharyngeal carcinoma, lung damage, and leukemia. For these reasons, it is highly desirable to develop efficient technologies for its removal in air. In this context, photocatalytic degradation is considered as an optimal option for indoor-level contaminant abatement due to its low cost, excellent stability, and non-toxicity. Especially, titanium dioxide (TiO2) is greatly recognized in removing the gaseous FA by the photocatalytic system. Unfortunately, the main drawback of bare TiO2 is wide band gap energy and high recombination rate to restrict their application under real-world conditions. To improve the photocatalytic efficiency of bare TiO2, significant efforts have been made by metal (e.g., Pt, Pd, Au, Ag, and Ni), non-metal (e.g., B, S, C, F, N, Cl, and Br), and co-doping system. Metal components create the dopant state near the conduction band. In contrast, non-metal components form the new state closet to the valence band that expands the TiO2 light absorption with the lowering of the band gap energy. Moreover, the synergistic effects between metal and non-metal doping in co-doped TiO2 can improve electron and hole separation. Among many options available for the modification of TiO2, the use of Pt as dopant can effectively enhance the photocatalytic activity of TiO2. Pt nanoparticles produce the highest Schottky barrier, therefore facilitating electron capture and hindering the recombination rate between electrons and hole pairs. Furthermore, N doping is a simple, low-cost, non-toxic, and effective approach for enhancing the photocatalytic activity of TiO2. To obtain the optimum photocatalytic efficiency, it is essential to explore an ideal amount of dopant into TiO2 to increase the separation of charge carriers while inhibiting the recombination rate. Additionally, appropriate synthesizing and coating methods are also needed to improve the catalyst features for the real application toward air purification. In chapter 2, a mini-scale photocatalytic air purifier (AP) system was built by impregnating nitrogen-doped titanium dioxide (N-TiO2) photocatalyst into a porous honeycomb (HC) filter based on a dip-coating method. The photocatalytic performance of the AP-HC-N-TiO2 filter against formaldehyde (FA: 0.5 – 5 ppm) vapor was investigated in relation to N/Ti molar ratios and loading amount of N-TiO2 in a closed chamber (17 L) under UV-LED irradiation (1 watt; λ=370 nm). The optical features of N-TiO2 nanocatalyst improved greatly with an increased N/Ti molar ratio from 1 to 10 as reflected by a decrease in the bandgap energy (Eg) from 3.25 to 3.10 eV. At N/Ti =10, the photocatalytic degradation efficiency of FA (5 ppm) was optimized (95%) at the maximum loading amount of N-TiO2 on the filter (20 mg mL-1) under dry conditions while its performance declined under the humid and low flow rate conditions. Based on the in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis, the photocatalytic degradation of FA followed its complete mineralization through the generation of HCOO-, H2O, and CO2. This study is expected to offer useful insights into the construction of an optimized TiO2-based photocatalytic mineralization system against aldehyde VOCs in the ambient environment. In chapter 3, the advanced nanocomposite materials-based photocatalytic technique has been applied extensively for the oxidation/mineralization of emerging air pollutants such as volatile organic compounds (e.g., formaldehyde (FA)). In this study, a nitrogen-doped-TiO2 (N-TiO2) nanocomposite catalyst has been synthesized by sol-gel method, coated over ceramic ball through dip-coating method, and applied for photocatalytic oxidation (PCO) of FA. The N-TiO2 composite catalyst was applied for catalytic degradation of FA under UV light illumination (32 W and λ=352 nm) through the control of process variables. Accordingly, the N-TiO2 catalyst exhibited 100% removal of FA with quantum yield (QY) = 2.90E-04 molec photo-1, space time yield (STY) = 2.20E-05 molec photo-1, and figure of merit (FOM) = 1.08E-12 L mol mg-1 J-1 h-1). According to the in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis, photocatalytic degradation pathway of FA molecules appeared as: FA  dioxymethylene (DOM)  HCOO-  H2O + CO2. This study is expected to offer deeper insights into the utility of N-TiO2 for the photocatalytic degradation of FA in dynamic system. In chapter 4, the photocatalytic oxidation technology has displayed great potential for the treatment of volatile organic compounds (e.g., formaldehyde (FA)) in air. In this study, metal/non-metal co-doped TiO2 (i.e., 1% platinum/nitrogen doped TiO2: Pt/N-TiO2) photocatalyst is developed to degrade gaseous FA. The Pt/N-TiO2 catalyst is prepared by sol-gel method and immobilized over ceramic ball through dip-coating method. The removal efficiency of FA molecules by Pt/N-TiO2, was assessed in reference to N-TiO2 and pristine TiO2 under UV irradiation (8×4=32 W; λ=352 nm) through the control of process variables (e.g., FA concentration, flow rate, relative humidity, and O2 contents). Additionally, in-situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) analysis indicated that the co-existence of moisture would play an important role for photocatalytic conversion of FA molecules with the degradation pathway of FA  DOM  HCOO-  H2O + CO2. The superior PCO potential of Pt/N-TiO2 catalyst is attributed to better light absorption, inhibition of charge carrier recombination, and n-type Schottky junction with lower PL intensity and higher average life time of charge carriers. This study is expected to provide the real application of Pt/N-TiO2 coated onto ceramic balls based air-purification option for room temperature thermo/photocatalytic FA degradation. Finally, this dissertation provides readers with valuable insights into the feasibility of photocatalytic degradation of FA. Metal, non-metal, and co-doping into TiO2 improved the photocatalytic activity relative to bare TiO2 and P25. For the real application of air purification, prepared TiO2 was immobilized onto the honeycomb filter and ceramic ball. Furthermore, the photocatalytic degradation efficiency of FA was evaluated by the various environmental effects (e.g., FA concentration, flow rate, humidity, O2 contents, and reusability) under the static and dynamic systems. The mechanism of photocatalytic degradation of FA was explored by using in-situ DRIFTS analysis under the step-by-step photocatalytic decomposition of FA from the dark condition (adsorption) to the UV condition (photocatalytic oxidation).