Surface hydroxyl groups of synthetic α-FeOOH in promoting OH generation from aqueous ozone: Property and activity relationship

This work investigated the relationship between the property of the surface hydroxyl groups of hydroxylated synthetic α-FeOOH and their catalytic activity in promoting hydroxyl radical (OH) generation from aqueous ozone. Nitrobenzene was used as an ozone-resistant probe to quantify OH generation. ATR-FTIR analysis reveals that sulfate and phosphate suppressed the catalytic activity of α-FeOOH through substituting its surface hydroxyl groups, which implies that the catalyst surface hydroxyl groups were active sites for promoting OH generation. Compared with other synthetic oxo-hydroxides such as β-FeOOH, γ-FeOOH and γ-AlOOH, α-FeOOH achieved a highest Rc value (i.e., 1.11 × 10−7, molar concentration ratio of OH to O3) in catalytic ozonation. No correlation could be established between the surface hydroxyl density and the OH-promoting activity of the oxo-hydroxides. In contrast, their catalytic activity was found to be reversely related to the IR stretching frequencies of surface hydroxyl groups. The results indicate that not all surface hydroxyl groups of the oxo-hydroxides possessed the same high catalytic activity, but the weak surface MeO–H bonds were favorable sites for promoting OH generation from aqueous ozone. The surface hydroxyl–ozone interaction was thus proposed for the catalyzed OH generation, which can explain why neutral surface hydroxyl species of α-FeOOH was more active than protonated or deprotonated species.