In situ Raman studying the microstructure and function of FeIV species in advanced oxidation processes†

Due to the efficient and stable pollutant degradation properties, Fe^IV species from Fe-based single-atom catalysts (Fe-SACs) have garnered significant interest in advanced oxidation processes (AOPs). However, the microstructure and function of Fe^IV species in these processes remain contentious. In this study, we developed Au@SiO₂@Fe-SACs and utilized a combination of in situ surface-enhanced Raman spectroscopy, theoretical calculations, and synchrotron radiation techniques to elucidate the structure and functional mechanisms of Fe^IV species during AOPs. Our findings demonstrated that Fe-SACs with an Fe^IIN₄ structure were loaded on Au@SiO₂ to obtain Au@SiO₂@Fe-SACs. During PMS oxidation, a Raman peak associated with the Fe–O bonds appeared at 837 cm⁻¹ along with blue-shifts of Fe–N bonds from 183 cm⁻¹ and 322 cm⁻¹ to 191 cm⁻¹ and 335 cm⁻¹, proving the generation of Fe^IV species. Specifically, the elongation of the Fe–O bond displaced the Fe atom from the NC plane, resulting in an extension of the Fe–N bond length from 1.88 Å to 1.93 Å. Furthermore, the Fe^IV species directly oxidized typical pollutant phenol through a direct oxidation transformation pathway (DOTP) within a wide pH range of 3 to 9. They exhibited a significant increase in removal efficiency of phenol compared to the hydroxyl radicals (·OH) from activated H₂O₂ and effective reduction of total organic carbon (TOC). This study offers critical insights into the structural and functional attributes of Fe^IV species, providing valuable guidance for the design of more efficient Fe-SACs in AOPs.