Photocatalytic NOx mitigation under relevant conditions using carbon nanotube-modified titania

Abstract

Although a majority of photocatalysts exhibit improved NO conversion to NO2, the performance in the oxidation of NO2, the more toxic form of NOx, 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 effective transformation of NOx into nitrates. Using the objective figure of merit for NOx abatement, DeNOx index, the catalyst performance in a laminar-flow reactor was evaluated under simulated conditions that are relevant in abating NOx. The conditions probed include relative humidity (RH), initial NOx 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 limits NO conversion when the reactor headspace is too large (>3 mm), due to limited diffusion of NOx 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 NOx, but which dictates the optimal catalyst configuration for flue gas treatment. The remarkable DeNOx activity of CNT-TiO2 over a wide range of RH levels is rationalized based on the ratio of physisorbed-to-chemisorbed water on the photocatalyst surface.