Surface hydroxyl groups of synthetic α-FeOOH in promoting {radical dot}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 ({radical dot}OH) generation from aqueous ozone. Nitrobenzene was used as an ozone-resistant probe to quantify {radical dot}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 {radical dot}OH generation. Compared with other synthetic oxo-hydroxides such as β-FeOOH, γ-FeOOH and γ-AlOOH, α-FeOOH achieved a highest R_c value (i.e., 1.11 × 10^-7, molar concentration ratio of {radical dot}OH to O₃) in catalytic ozonation. No correlation could be established between the surface hydroxyl density and the {radical dot}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 {radical dot}OH generation from aqueous ozone. The surface hydroxyl-ozone interaction was thus proposed for the catalyzed {radical dot}OH generation, which can explain why neutral surface hydroxyl species of α-FeOOH was more active than protonated or deprotonated species.