Photochemical Transformation Pathways of Nitrates from Photocatalytic NOx Oxidation: Implications for Controlling Secondary Pollutants

Photocatalytic NOx abatement is crucial for the atmospheric environment. Nonetheless, the possible pathway for the conversion of the product (NO3–) from photocatalytic NOx oxidation and its impact on sustained NOx removal have been overlooked, which is related to the final environmental benefit. Herein, the elusive NO3– conversion mechanism is revealed via the synergetic application of in situ characterization and theoretical calculation technologies. It is found that the N–O bond in surface NO3– could be activated by NO molecules, arising from the significant overlap of the 2p orbitals between the N in NO and the O in NO3–. Then, photogenerated electrons (e–) captured by NO drive the transformation of NO3– under light irradiation via the NO3– + NO– → 2NO2– route. Additionally, although photogenerated holes (h+) and hydroxyl radicals (•OH) could oxidize NO into NO3–, the rate of production of NO3– is much slower than that of photochemical transformation by NO–. The results of control experiments show that NO– is the key species to trigger the decomposition of surface NO3–. This work clarifies the influence of reactants on surface NO3– conversion during photocatalytic NOx oxidation, providing a comprehensive mechanism for the photochemical NO3– conversion pathway.