Mechanism study on TiO₂ inducing O₂^- and OH radicals in O₃/H₂O₂ system for high-efficiency NO oxidation

To achieve high NO oxidation efficiency, excessive O₃ must be used, which would lead to the high cost and escape of ozone. Herein, we adopted low cost and environmental-friendly TiO₂ as the catalyst of low concentration O₃ and H₂O₂ system for high-efficiency NO_x oxidation. The Ti sites on TiO₂ were the deprotonation sites of H₂O₂ and H₂O into Ti-OOH and Ti-OH species, respectively. We found that the surface of rutile phase TiO₂ had a low concentration Ti-OOH component but a large amount of Ti-OH after contacting with H₂O₂ solution, thus lots of ·OH and a few O₂^- radicals formed with introducing O₃ molecules. H₂O₂ solution induced the formation of a large amount of Ti-OOH and Ti-OH species on the anatase phase TiO₂ surface, thus lots of O₂^- generated in the O₃/H₂O₂ system. O₂^- and OH radicals could efficiently oxidize NO, in which O₂^- radicals could oxidize NO to NO₃^- in one step with high selectively. Therefore, anatase TiO₂ had better performance in NO_x oxidation than rutile phase TiO₂. The effect of temperature and SO₂ concentration on NO oxidation was also investigated, the results showed that TiO₂-A/O₃/H₂O₂ system promoted NO oxidation at a low temperature and a low concentration of SO₂.