Photocatalytic NO_x Mitigation Under Relevant Conditions Using Carbon Nanotube-Modified Titania

Although a majority of photocatalysts exhibit improved NO conversion to NO₂, the performance in the oxidation of NO₂, the more toxic form of NO_x, to nitrate remains a challenge; in addition, the performance of hybrid photocatalysts under practical conditions is unclear. This study demonstrates the use of carbon nanotube-TiO₂ (CNT-TiO₂) photocatalyst films for the effective transformation of NO_x into nitrates. Using the objective figure of merit for NO_x abatement, DeNO_x index, the catalyst performance in a laminar-flow reactor was evaluated under simulated conditions that are relevant in abating NO_x. The conditions probed include relative humidity (RH), initial NO_x concentration, reactor geometry (headspace distance), and state of the catalyst (fresh vs. recycled). Our results reveal CNT-TiO₂ significantly outperforms P25 despite exhibiting comparable NO conversion at low RH. P25 experiences a 66% reduction in DeNOx at high RH compared to low RH while CNT-TiO₂ only incurs a 27% reduction. For recycled photocatalysts, this disparity becomes even more pronounced; CNT-TiO₂ experiences a 49% reduction in DeNOx at high RH, whereas P25 experiences a 134% reduction. In addition, mass transfer from the bulk airflow was found to limit NO conversion when the reactor headspace is too large (> 3 mm), due to limited diffusion of NO_x to the photocatalyst surface. Our findings highlight the importance of headspace distance, a parameter that has mostly been overlooked in reactor design for photocatalytic oxidation of NO_x, but is expected to dictate the optimal catalyst configuration for flue gas treatment. The remarkable DeNO_x activity observed in CNT-TiO₂ over a wide range of RH levels is rationalized based on the ratio of physisorbed-to-chemisorbed water on the photocatalyst surface.