Environmental catalytic materials for VOC degradation

Abstract

Volatile organic compounds (VOCs) are a class of organic pollutants that pose significant risks to human well-being and ecological systems. Toluene is one of the most common industrial air pollutants, contributing to ground-level ozone and photochemical smog formation. Effective removal and destruction techniques are crucial for protecting human health and the environment. Photothermal catalytic oxidation has gained prominence in recent years as a highly promising approach for the destruction of VOCs. This innovative technique exhibits the capability to oxidize VOCs into carbon dioxide (CO2) and water (H2O). This process relies on the development of a light-driven photothermal catalyst that possesses excellent light absorption and conversion properties. Transition metal oxides have been identified as potential candidates due to their affordability, high activity, and tunable light absorption properties. However, their industrial application has been limited by a lack of stability in the presence of reactant poisons. Therefore, the fabrication of highly stable catalyst materials capable of efficiently utilizing sustainable solar energy remains a significant challenge in the field. The measurement of CO2 yield percentage (CO2 yield %) pivotal metric for evaluating VOC conversion.
To address the challenges associated with VOC oxidation, this study focused on the development of a photothermal catalyst by supporting platinum (Pt) nanoparticles on mesoporous titanium-modified ultrastable Y zeolite (mTiO2/USY) nanocomposites. Electron paramagnetic resonance (EPR) spectroscopy and Temperature-Programmed Desorption (TPD) analysis were employed under both illuminated and non-illuminated conditions to elucidate the roles of oxygen defects, Ti3+ species, and Pt nanoparticles. The results demonstrated a significant enhancement in Pt nanoparticle stability through electronic interactions with defective mTiO2. Additionally, the introduction of Pt nanoparticles increased the surface oxygen content compared to the parent mTiO2/USY catalyst. These modifications, combined with oxygen defects and Ti3+ species, play a pivotal role in activating surface-adsorbed oxygen. Consequently, the efficient separation of photogenerated charges was facilitated, and the Mars-van Krevelen (MvK) mechanism was promoted. Under the simulated solar light intensity of 490 mW/cm2, the 0.9Pt-mTiO2/USY catalyst exhibited exceptional toluene conversion, with the catalyst surface temperature reaching 243 oC, surpassing the performance of the parent mTiO2/USY catalyst. Furthermore, the 0.9Pt-mTiO2/USY nanocomposite demonstrated activity under both UV-vis light and IR heating, indicating its versatility in harnessing different energy sources. The synergistic effects of Pt modulation, surface oxygen activation, and Ti3+ species are crucial for enhancing photothermal activity.
In the pursuit of cost-effective alternatives, this study investigated the influence and performance of plasmonic metal oxides, namely copper oxide (CuO) and tungsten trioxide (WO3), for the photothermal catalytic oxidative removal of toluene. A series of nanocomposites, denoted as yCuOx-WOx/mTiO2-x-USY, were synthesized using a co-impregnation method. These nanocomposites underwent a two-step treatment process, involving calcination under air at 400 oC for 3 h, followed by hydrogenation at 600 oC for 2 h under H2 flow. The experimental findings revealed that the incorporation of CuO metal oxide into the WOx/mTiO2-x-USY catalyst led to a substantial improvement in light absorption capacity and a rapid increase in the surface temperature of the catalyst. Among the catalysts evaluated, the 20CuOx-WOx/mTiO2-x-USY displayed remarkable photothermal catalytic efficiency under humid conditions, achieving a toluene conversion and CO2 yield of 90.4 % and 82.0 % under 500 mW/cm2 full light intensity. The enhanced photothermal performance was attributed to the efficient utilization of absorbed incident light energy, resulting in elevated localized surface temperatures. This confirmed the crucial role of localized heat in initiating the photothermal catalytic process. Moreover, the presence of highly active Cu+/Cu2+↔O2↔W5+/W6+ redox cycle significantly improved the overall performance of the system. Furthermore, in-situ DRIFTS experiments provided valuable insights into a plausible reaction pathway for toluene oxidation.
Lastly, taking advantage of the intrinsic properties of strontium titanate (STO), a bimetallic complex of CuOx-CeO2-x was incorporated on the surface of mesoporous STO-modified USY (mSTO/USY) catalysts through a redox-assisted wet impregnation method, followed by calcination and hydrogenation treatment. The resulting ternary catalyst, CuOx-CeO2-x-STO/USY, demonstrated efficient light-driven photothermal catalytic oxidation of toluene. Preliminary characterization results revealed that the addition of CuO metal oxide enhanced the catalyst light absorption and electron transfer abilities under solar irradiation, while ceria provided the catalyst system with a unique redox cycle that promoted toluene oxidation under irradiation. The exceptional activity of the CuOx-CeO2-x-STO/USY catalyst can be ascribed to its strong light absorption capacity, abundant oxygen defects, and the presence of CeO2-x and CuOx species. Characterization techniques, including XPS, He-TPD, Tol-TPD, and O2-TPD confirm that the incorporation of CuOx-CeO2-x and oxygen vacancies enriches the catalyst surface with active sites, promoting the activation of adsorbed reactants and enhancing catalytic performance. In-situ DRIFTS analysis proposed that the oxidation over the CuOx-CeO2-x-STO/USY surface followed the Mvk mechanism, generating activated oxygen that oxidized toluene to form CO2 and H2O. Simultaneously, oxygen from the gas and bulk phases migrated to the oxygen vacancies to replenish the reduced metal oxide surface, sustaining the cyclic reaction pathway.
Overall, this study demonstrates the development of efficient light-driven photothermal catalysts for toluene oxidation. The incorporation of supported noble nanoparticles, plasmonic metal oxides, and metal oxide-based catalysts showcase promising strategies for improving the performance and stability of these catalyst materials. These findings contribute to the advancement of sustainable and effective approaches for VOC removal and environmental protection.

Date of Award	Nov 2023
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Yong Sun (Supervisor), Jia Hong-Peng (Supervisor) & Collin E. Snape (Supervisor)