Samsung Advanced Institute of Technology

Humidity induces significant and inconsistent fluctuations in the photocatalytic efficiency during the practical use of photocatalysts to remove volatile organic compounds (VOCs). In this study, operando spectroscopy was used to elucidate the correlation between humidity levels and VOC degradation using TiO₂ catalysts for real-world applications. We conducted both lab- and pilot-scale experiments to degrade acetaldehyde and observed an increase in activity as the humidity levels decreased. Humidity fluctuations had a more pronounced impact on the flow system than on the batch system. Time-resolved operando spectroscopy using an ¹³C isotope of acetaldehyde revealed that humidity plays a distinct role in dark adsorption and photocatalytic reactions. Under low-humidity conditions, there was an increase in the aldol condensation reaction, leading to a higher production of crotonaldehyde. Moreover, increased formate and CO₂ production from acetate via photocatalytic oxidation were observed. The humidity produced from aldol condensation is sufficient for radical production; therefore, excessive humidity plays a negative role in methane production from acetate photodegradation. This study elucidates the role of humidity in practical applications and offers valuable guidance for realistic air purification processes. Furthermore, it underscores the significance of identifying the key intermediates that enhance the selectivity of gaseous pollutant oxidation reactions.