Photochemical Oxidation of Ambient Methane for Air Purification and Greenhouse Gas Mitigation

Methane (CH₄) is a potent greenhouse gas with a global warming potential approximately 80 times greater than that of carbon dioxide over a 20-year time horizon, making its atmospheric removal an urgent environmental priority. Photochemical oxidation (PO) represents a promising strategy for indoor air purification and outdoor pollutant abatement by enabling the degradation of climate- and health-relevant trace gases under ultraviolet irradiation. In this work, we investigate CH₄ photochemical oxidation under UV-C light, with particular emphasis on the role of ozone and hydroxyl (OH) radical formation in enhancing methane conversion. Experiments were conducted using UV-C sources at 200 nm (ozone creation) and 254 nm (ozone-free) under dry and humid conditions, with and without the use of a photocatalyst, while varying gas residence time. Irradiation at 200 nm was found to generate ozone in situ, which subsequently promotes the formation of highly reactive OH radicals in the presence of water vapor. This radical-driven chemistry significantly enhances methane oxidation, with humid conditions yielding up to a 30% increase in CH₄ conversion compared to dry conditions. Following an extended catalyst screening, the addition of an optimal photocatalyst provided a modest further improvement of up to 8%, indicating a limited but measurable catalytic contribution under the investigated conditions. Increased residence time consistently resulted in higher methane conversion, underscoring the importance of flow dynamics in PO reactor optimization. These findings highlight the synergistic roles of UV-driven ozone production, OH radical chemistry, and photocatalysis in methane oxidation, and provide key insights for the design of efficient UV-based photochemical systems for air purification and greenhouse gas mitigation.