Effective Toluene Ozonation over δ-MnO 2 : Oxygen Vacancy-Induced Reactive Oxygen Species.

To improve the reactivity and lifetime of catalysts in the catalytic ozonation of toluene, a simple strategy was provided to regulate the morphology and microstructure of δ-MnO 2 via the hydrothermal reaction temperature. The effects of the reaction temperature and the ozone to toluene concentration ratio on the catalyst performance were investigated. The optimized MnO 2 -260 catalyst prepared at the limiting hydrothermal temperature (260 °C) showed high catalytic activity ( X Tol = 95%) and excellent stability (1200 min) at the approximately ambient temperature of 40 °C, which was superior to the results in previous studies. The structure and morphology of δ-MnO 2 were characterized by extended X-ray absorption fine structure, X-ray diffraction, scanning electron microscopy, positron annihilation lifetime spectroscopy, electron spin resonance, and other techniques. Experimental results and density functional theory calculations were in agreement that surface oxygen vacancy clusters, especially surface oxygen dimer vacancies, are critical in ozone activation. Oxygen vacancies can facilitate the adsorption and activation of O 3 to generate reactive oxygen species (ROS, including 1 O 2 , O 2 - , and • OH), leading to superior ozonation activity to degrade toluene and intermediates. Meanwhile, free radical detection and scavenger tests indicated that • OH is the primary ROS during toluene ozonation rather than 1 O 2 or O 2 - .