Photoactive Metal Oxide Nanoparticles for the Removal of Indoor Air Pollutants

Abstract

Nowadays, we spend up to 90% of our time indoors: at home, at the office, or traveling in a vehicle. The coronavirus pandemic has made it clear that healthy indoor environments are essential for human health. However, not only viruses that pollute the air: volatile organic compounds (VOCs) also play a role. These molecules are released during everyday activities such as cooking, from new furniture, or simply by living together in enclosed spaces. Aside from causing unpleasant odors, they can lead to headaches, fatigue, and eye irritation for example. Breaking down VOCs is therefore important for human health. Decomposition of molecules requires an energy source, such as heat, electricity, or light. In this thesis, we investigated light-activated materials (photocatalysts). These materials can break down harmful molecules and could, for example, be incorporated into wall paint to create air-purifying walls. Chapter 1 provides an introduction to VOCs, photocatalysis, and its possible applications. In Chapter 2, we investigated photocatalysts (Co3O4/TiO2) for the decolorization of indigo carmine, a dye often found in wastewater, using sunlight and LED light. We found that a 1.4 wt% loading of Co3O4 on TiO2 resulted in the highest decolorization activity. Moreover, we demonstrated that decolorization is not the same as complete degradation, even though they are often interpreted as such. We hypothesize that indigo carmine aids in its own degradation mechanism, where a small amount of Co3O4 increases the photo-activity, while an increased Co3O4-content limits the activity due to photocatalyst surface blockage. In Chapter 3, we moved to gas-phase VOCs and investigated acetone degradation under UV light using TiO2/adsorbent hybrid photocatalysts. Here, the adsorbent is zeolite ZSM-5 with various silicon-to-aluminum ratios. We varied different reaction conditions, such as light intensity, relative air humidity, O2 concentration in the air, and zeolite silicon-to-aluminum ratio. We found that the degradation reaction was mainly influenced by the UV-light intensity. Chapter 4 describes the design and development of a setup based on gas chromatography to detect VOCs in very low concentrations. A method to analyze the acetone photo-degradation reaction in a batch-type reactor, mimicking a closed-off indoor space, was developed. Using this method, acetone could be detected at a concentration as low as 1 ppm in the specialized combined reactor-analytical device setup. Finally, Chapter 5 describes the most important findings, and looks ahead to future research directions. Additionally the testing of photocatalysts in wall paint is described. Here, we found that the activity for materials shown to be active for acetone photo-degradation in previous chapters was lacking. This emphasizes the need for testing photocatalysts in their final application context.