Photocatalytic Generation of Reactive Oxygen Species on Fe and Mn Oxide Minerals: Mechanistic Pathways and Influence of Semiconducting Properties

Iron and manganese oxide minerals are potent environmental catalysts with many biogeochemical functions in diverse applications, wherein they serve as photocatalysts or electrocatalysts. Many of these applications involve the generation of highly reactive oxygen species (ROS), such as •OH, H2O2, and O2–•, as intermediates on mineral surfaces that further catalyze chemical and structural transformations. However, the specificity of these minerals for the type of ROS generated and the factors that control the specificity are not known or well explored. This study evaluated the nature of ROS generated under photocatalytic excitation of several Fe and Mn oxides/hydroxides and compared their behavior with well-known photocatalysts such as anatase (TiO2) and clay mineral. The results show that Fe and Mn oxides have contrasting properties. Fe minerals, especially FeIIFeIII oxyhydroxide (green rust), have the highest activity for the formation of H2O2, but only a low to negligible activity for •OH, while Mn minerals (triclinic birnessite) have the highest activity for •OH and a low to moderate activity for H2O2. The highest concentrations of ROS seen with these minerals are nearly 5 to 7 times higher than that seen with photoactive TiO2. The catalytic specificity is related to the unique electronic band alignments of birnessite and green rust minerals relative to those of other common semiconductors. These results shown here can provide a better understanding of ROS-induced chemical transformations seen in environmental processes.