Vacancy-rich structure inducing efficient persulfate activation for tetracycline degradation over Ni-Fe layered double hydroxide nanosheets

Layered double hydroxide (LDH)-based catalysts have been corroborated to be effective for persulfate (PS) activation towards wastewater remediation. However, the effect of vacancies especially the role of cation vacancies in LDH has not been well illustrated. In this study, NiFe-LDH with Ni(II) vacancies (NiFe-LDH-VNi) or Fe(III) vacancies (NiFe-LDH-VFe) were fabricated via defect-engineering to tune catalytic activity on PS activation for tetracycline (TC) degradation. Characterizations of X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy confirmed cation vacancies were successfully implanted in NiFe-LDH catalyst. Experiments demonstrated that cation vacancies enhance the catalytic performance while NiFe-LDH-VNi exhibited better activation performance since vacancies could facilitate the electron transfer, thereby forming more reactive oxygen species. Meanwhile, theoretical calculations disclosed that the NiFe-LDH with Ni(II) or Fe(III) vacancies possessed larger surface energy, lower Fermi energy and more positive Mulliken atomic charges than bare NiFe-LDH. About 80% of 30 mg·L−1 TC could be mineralized through the catalysis of NiFe-LDH-VNi/PS system in 90 min. Furthermore, quenching experiments confirmed that the SO4− and OH generated by interface reaction accounted for the TC degradation. 1O2 and O2− species were also identified in the presence of oxygen vacancies through nitrogen gas cleaning experiments. Ultimately, a possible mechanism of TC degradation was proposed in NiFe-LDH/PS system with Ni(II) or Fe(III) vacancies. This study provides a possible guideline to discover the appropriate cation vacancies that contribute to the activation process and also provides a strategy to rationally design high-activity catalysts based on defect engineering.