Mechanism of ozone catalysis by transition metal hydroxyl oxides: From reactive oxygen species to surface structural hydroxyl

Transition metal hydroxyl oxides have been used widely as ozone catalysts for water purification. However, the fabrication methods selected and the pollutants treated vary, making it difficult to compare their performance. In this study, ZnOOH, FeOOH, CoOOH, and MnOOH were synthesized by the same method to remove multiple structurally similar targets. Results showed that the ZnOOH performed the best, removing over 90% of all targets within 40 min, and presenting the highest ozone utilization rate of 0.081 min -1 . Multiple ROS quantification and contribution experiments jointly demonstrated that ZnOOH had an advantage over the other catalysts only in ·OH generation, but the ·OH contributed 93% of target removal in the ZnOOH/O 3 process. Structure analysis of ZnOOH suggested that its ·OH generation had little connection with the contents of surface metal and adsorbed water, but was dependent on the surface structural hydroxyl that would undergo loss at 100-300℃. This concentration of ZnOOH was 10.1 mM/g, which was 1.7, 5.3, and 6.3 times higher than that of FeOOH, CoOOH, and MnOOH, respectively. Heat treatment further revealed that these surface hydroxyl mainly originated from the hydroxyl ions from water dissociation and could regenerate timely during reuse. Finally, a possible catalysis pathway was proposed. • The dominant catalytic ozonation mechanism of a series of TMHOs was explored. • All the target removal followed the same sequence of ZnOOH > FeOOH > CoOOH > MnOOH. • ·OH was the main contributor to the target removal in all catalytic systems. • The yield of ·OH was dependent on the surface hydroxyl content of each catalyst. • The surface hydroxyl of TMHOs could regenerate timely from water dissociation.