Performance and Reaction Mechanisms of an Indoor Air Decontamination System Combining Non-thermal Plasma and Photocatalyst for VOC Removal

An indoor air decontamination system combining a dielectric barrier discharge (DBD), UV-C lamps, and a TiO₂ photocatalyst, is investigated for the simultaneous removal of two volatile organic compounds (VOCs) — formaldehyde and methanol — at a high flow rate of 300 L·min⁻¹. The contributions of each component are evaluated independently and in combination, showing a strong synergistic effect between DBD and UV-C, particularly for formaldehyde removal, reaching up to 50% efficiency in a single pass at 60% relative humidity — even in the absence of TiO₂. The photocatalyst is found to significantly improve the removal efficiency of methanol, which is detrimental to formaldehyde removal efficiency. The effect of specific input energy (SIE) is examined, showing that combining DBD with UV-C at equivalent SIE outperforms the discharge alone by a factor of two or more. The underlying degradation mechanisms are discussed in detail, identifying oxygen atoms, OH radicals, and several indirect OH reservoirs as key contributors. A notable observation is the release of NO₂ upon UV-C ignition following the discharge operation, attributed to the photodesorption of surface nitrates accumulated on TiO₂ during plasma exposure, which temporarily scavenges OH radicals and reduces removal efficiency.