Application of Advanced Functional Materials for Indoor Air Purification: Interplay Between Adsorption and Catalysis

Abstract

Volatile organic compounds (VOCs) represent major hazardous gaseous pollutants present ubiquitously in the outdoor and indoor air. Recent years have witnessed a growing interest in applying advanced functional materials to mitigate VOCs from the atmosphere due to their adverse effects on human health and the environment. As per the World Health Organization (WHO), pollution in ambient air accounts for the yearly deaths of several million people due to lung cancer, heart disease, stroke, chronic and acute respiratory diseases. In addition to the large amounts of VOCs liberated annually by unprocessed industrial effluents to the atmosphere, a plethora of sources in the indoor environment (e.g., furniture, paint, and other products) are identified to contribute to the emissions of VOCs (e.g., formaldehyde (FA)) into the indoor air. As humans in urban areas usually spend about 90% of their lifetime indoors, it is essential to remove indoor pollutants efficiently (e.g., using high-performing air purifiers). In this regard, the utility of advanced functional materials is greatly recognized in removing the gaseous VOCs through adsorption and/or catalytic oxidation-based processes with the aid of their meritful properties (e.g., remarkably high specific surface areas and favorable surface functionalities). Adsorption-based approaches capture the VOC molecules from the gas phase onto the solid material. As a good example, amine (NH2) surface functionalities can enhance the adsorption of most VOCs (particularly aldehydes and ketones). At the same time, open metallic sites in metal-organic frameworks (MOFs) can also boost the overall adsorption capacity. The adsorption type can be classified as physical, chemical, or reactive based on the mechanism. The latter produces less hazardous end-products (formate from FA on metal oxide surfaces). On the other hand, catalytic oxidation can ideally convert VOCs into innocuous end-products (carbon dioxide (CO2) and water) in an oxygen-rich environment. The low/room temperature (RT) oxidation of FA in the dark is of particular interest in light of the high toxicity and ubiquity of this highly volatile (-19°C boiling point) aldehyde. Due to the low boiling point of FA, conventional adsorbents (e.g., activated carbon) cannot capture it effectively. Hence, the direct catalytic oxidation of FA into CO2 at RT in the dark is desirable from a real-world application perspective because such a process can proceed under ambient conditions without requiring additional external energy sources (e.g., heat and light). The present dissertation explores the application of several materials developed during my thesis research work (e.g., MOF/manganese dioxide nanocomposites, mixed-metal oxide/rice-husk biochar composites, and eggshell-waste supported noble metal (platinum and silver) catalysts) for the adsorptive and catalytic removal of VOC (mostly using FA as the model compound) in the dark at low/RT conditions. Particular attention has been paid to the utilization of solid waste (e.g., eggshell and rice husk) to synthesize the aforementioned advanced materials in light of the sustainability and circular economy goals. A Pt3Ti intermetallic compound derived from titanium carbide MXene was also explored for the direct RT oxidation of FA as an advanced high-performing catalyst material. Detailed physicochemical characterizations of the synthesized materials coupled with in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were employed along with density functional theory simulations to better describe the factors and processes (e.g., adsorption and catalytic oxidation mechanisms, surface phenomena, and reaction pathways) controlling the removal of VOCs in air. Overall, the results reported in the present dissertation are expected to pave the way for the construction of high-performing, efficient, and cheap air purifiers to remove multicomponent VOCs under real-world conditions based on the functional materials developed for VOC treatment via combined adsorption-catalysis mechanisms.