Optimization of the NO photooxidation and the role of relative humidity

Photocatalysis was recognised as a suitable process for the photoabatement of atmospheric pollutants. The photooxidation mechanism on TiO₂ has been widely studied. However, recent studies demonstrated that the very often-assumed photooxidation intermediated by the hydroxyl radical cannot explain all the experimental observations. Indeed, this study contributes for a new understanding of NO photooxidation. First, the adsorption equilibrium isotherms of NO, NO₂ and H₂O on the photocatalyst, Aeroxide^® P25 from Evonik Industries, were obtained. Also, the concentration of hydroxyl radicals was determined by photoluminescence. A comprehensive design of experiments was then followed; NO conversion and selectivity were obtained as a function of the relative humidity, irradiance, NO inlet concentration and residence time, following a response surface methodology (RSM). These results were then used to discuss the photooxidation mechanism of NO.