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 (NO 3 – ) 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 NO 3 – 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 NO 3 – could be activated by NO molecules, arising from the significant overlap of the 2p orbitals between the N in NO and the O in NO 3 – . Then, photogenerated electrons (e – ) captured by NO drive the transformation of NO 3 – under light irradiation via the NO 3 – + NO – → 2NO 2 – route. Additionally, although photogenerated holes (h + ) and hydroxyl radicals (•OH) could oxidize NO into NO 3 –, the rate of production of NO 3 – 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 NO 3 – . This work clarifies the influence of reactants on surface NO 3 – conversion during photocatalytic NOx oxidation, providing a comprehensive mechanism for the photochemical NO 3 – conversion pathway.