In Situ Raman Studying the Microstructure and Function of FeIV Species in Advanced Oxidation Process

Abstract

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 a 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 FeⅣ 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 than 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 FeⅣ species, providing valuable guidance for the design of more efficient Fe-SACs in AOPs.