Electronic Structure Engineering of δ-MnO₂ via Multi-Cation Co-Doping for Efficient Peroxymonosulfate Activation

The efficiency of peroxymonosulfate (PMS)-based advanced oxidation processes is largely governed by interfacial electron transfer dynamics between the catalyst and oxidant. In this study, we introduce a multi-cation co-doping strategy incorporating Fe, Ni, and Cu into δ-MnO₂ lattices to modulate its electronic configuration and enhance PMS activation performance. The resulting ternary-doped nanocatalyst exhibits substantially improved catalytic activity, achieving over 90% degradation of various recalcitrant organic pollutants. Structural characterization using synchrotron X-ray diffraction (SXRD) reveals that multi-cation incorporation preserves the layered δ-MnO₂ framework while inducing lattice distortions that modify the local coordination environment. Density functional theory (DFT) calculations further demonstrate that Fe, Ni, and Cu co-doping optimizes the d-band center of Mn sites and creates electron-deficient regions that facilitate PMS adsorption and activation. In situ Raman spectroscopy coupled with electrochemical impedance analysis confirms that the engineered electronic structure promotes bidirectional electron transfer at the catalyst-PMS interface, enabling both non-radical pathways involving singlet oxygen (¹O₂) generation and radical-mediated oxidation processes. Quenching experiments corroborate these findings, showing that multiple reactive species contribute synergistically to pollutant removal across different water matrices. The catalyst also maintains excellent structural stability and catalytic performance over multiple reuse cycles, indicating its potential for practical applications. This work provides a fundamental understanding of how multi-metal synergistic effects govern electron transfer processes in PMS activation systems and offers a rational design framework for developing high-performance heterogeneous catalysts for sustainable water remediation technologies.