Engineering Economical and Sustainable Solutions for the Abatement of Volatile Organic Compounds

Abstract

Over the last three decades, combined efforts of industries, the EPA, and automakers have helped reduce the emission of many harmful molecules such as carbon monoxide, sulfur dioxide, and particulate matter to improve air quality. However, rapid industrialization and urbanization have contributed to a significant rise in volatile organic compounds (VOCs), one of the primary air pollutants. Prolonged exposure to certain VOCs, even at concentrations as low as 0.25 ppm, is known to be carcinogenic. Therefore, the detrimental health impacts of VOCs and their increasingly stringent environmental regulations warrant continuous research to develop more effective, economical, and sustainable technologies to mitigate their emissions. In most industries, the VOCs are curtailed by combining an adsorption-desorption process using beaded activated carbon (BAC) with subsequent thermal incineration. This hybrid VOC abatement system suffers from two major limitations. First, the occurrence of strongly, or irreversibly adsorbed species in BAC, referred to as “heel”, prevents complete regeneration of the sorbent, decreasing its capacity and lifetime. Second, the massive energy requirement of the thermal incinerator increases the carbon footprint of the operation and overall operational cost. The research presented in this dissertation provides economical and sustainable strategies to address the limitations of the hybrid abatement process described above. Using spectroscopic and thermogravimetric techniques, we aimed at understanding the factors responsible for the heel accumulation in BAC during gas-phase adsorption-desorption operation. Addressing this, the dissertation provides a facile methodology to modify the surface of the BAC through a chemical treatment to impede heel formation. This modification protocol increases the porosity of BAC by up-to 55% without altering its structural integrity. Consequently, the adsorption capacity of the BAC increased by nearly 38% while decreasing the peak desorption temperature by as much as 50°C due to lowered adsorption strength. Furthermore, this thesis explored feasible methods of regenerating the spent BAC to improve the efficacy of the industrial VOC abatement technique. This effort resulted in the invention of a unique vapor-phase regeneration technique. The lab-scale studies demonstrated that dimethyl sulfoxide vapors could be effectively used to recover nearly 82% of the adsorption capacity of the spent BAC without compromising its structural integrity. The second phase of this research investigated the feasibility of using energy-efficient catalytic oxidation to decompose or destroy VOCs into H2O and CO2 at low temperatures. As such, the dissertation provides a roadmap to the synthesis of a novel catalyst architecture of encapsulating catalytically active noble metals in porous TiO2 support. Electron microscopic studies indicated that encapsulation helps maintain a uniform metal particle distribution (2-5 nm) and promotes metal-support interactions by maximizing interfacial sites, thereby improving catalytic activity. In addition, we discovered that subjecting the encapsulated catalyst to a post-synthesis solvothermal treatment step anchors the active metal more strongly to the support, which helps maintain superior activity under repeated uses. Finally, the thesis attempts to push the boundaries of catalytic VOC oxidation reactions via concurrent utilization of thermal energy and visible light to bring down the overall energy requirement of the VOC abatement. By encapsulating plasmonic silver nanostructures in a porous TiO2 shell resembling a core@shell morphology, we created a multifunctional material capable of generating energetic electrons upon visible light illumination. These electrons can be used in tandem with thermal energy to decompose n-butanol at viable rates at significantly reduced temperatures as low as 200°C.