Impact of Crystal Types of AgFeO₂ Nanoparticles on the Peroxymonosulfate Activation in the Water

A simple co-precipitation method was developed to synthesize AgFeO₂ nanoparticles (NPs) with hexagonal 2H and 3R polytypes coexistence. The ratio of 2H and 3R types in AgFeO₂ NPs were regulated by controlling the calcination temperature (300, 400, and 500 °C). Such AgFeO₂ NPs were used as heterogeneous catalysts to activate peroxymonosulfate (PMS) for the removal of Orange I (OI) in the water. External water conditions effects and the stability of AgFeO₂ NPs were investigated. The catalytic performance of AgFeO₂ NPs was found to be significantly enhanced with the increasing content of 2H-AgFeO₂. ¹O₂, O₂•-, SO₄•-, and •OH were identified as the dominating reactive oxygen species (ROSs) participated in the catalytic process. The electron transfer of Ag⁰/Ag⁺ and Fe²⁺/Fe³⁺ cycles facilitated the decomposition of PMS to generate ROSs. The surface hydroxyl groups (-OH) were regarded as the catalytic active sites. The higher 2H-AgFeO₂ content in AgFeO₂ NPs promoted the concentration of surface hydroxyl groups ( C-OH) and the reactivity of AgFeO₂ NPs for PMS activation. Based on theoretical calculations, the 2H-AgFeO₂ (004) plane with more Fe sites was more conducive to binding with the -OH compared to the 3R-AgFeO₂ (012) plane, ascribed to the stronger adsorption energy and shorter Fe-O bond length between 2H-AgFeO₂ and -OH.