Theoretical insight into hydroxyl production via H2O2 decomposition over the Fe3O4(311) surface

Fenton's reagent provides a method to produce active hydroxyl radicals (OH) for chemical oxidation by mixing iron oxide and hydrogen peroxide, which divides into homogeneous and heterogeneous Fenton's reagent. Heterogeneous Fenton's reagent is fabricated from H2O2 and various iron oxide solid materials, such as alpha-FeOOH, alpha-Fe2O3, and Fe3O4. Fe3O4 possesses the Fe2+/Fe3+ mixed valence oxidational state and has been reported to have good catalytic activity. However, the reaction mechanism of H2O2 decomposition on Fe3O4 surfaces is still unclear. In this work, we performed DFT calculations to investigate the H2O2 decomposition mechanisms over the Fe3O4(311) surface. There are two iron environments for H2O2 adsorption and decomposition on the Fe3O4(311) surface, a Fe2+/Fe3+ environment and a Fe3+/Fe3+ environment. We found that the H2O2 can adsorb on the Fe2+/Fe3+ environment by molecular adsorption but by dissociative adsorption on the Fe3+/Fe3+ environment. Our results show that both adsorption structures can produce two OH groups on the Fe3O4(311) surface thermodynamically. In addition, based on the electronic property analysis, H2O2 on the Fe2+/Fe3+ environment follows the Haber-Weiss mechanism to form one OH anion and one OH radical. On the other hand, H2O2 on the Fe3+/Fe3+ environment follows the radical mechanism to form two OH radicals. In particular, the OH radical formed on Fe2+/Fe3+ has energy levels on both sides of the Fermi energy level. It can be expected that this OH radical has good redox activity.