Insights into the fundamental role of Mo doping in facilitating the activation of peroxydisulfate by iron-based catalysts: Accelerating the generation of sulfate radicals

Electron-rich Fe sites in iron-based catalysts provide sufficient electrons to activate peroxydisulfate (PDS) for organic pollutants degradation, whereas catalyst activity is restricted by the inert Fe(III)/Fe(II) conversion. Herein, we synthesized Fe and Mo co-doped carbon (Fe/Mo@C) catalysts to explore the effect of Mo doping on Fe(III)/Fe(II) cycling during PDS activation process. Compared to the sole Fe loaded carbon (Fe@C), the introduced Mo converted the iron to a lower valence state (Fe0 and Fe(II)) during the catalysts preparation. Bisphenol A (BPA) was completely degraded within 10 min in Fe/Mo@C-PDS system with the degradation rate of 0.70 min−1, which was 26 times higher than that in Fe@C-PDS system. Mechanism studies indicated that the introduction of Mo promoted the cycling of Fe(II)/Fe(III) in catalytic-oxidation system and accelerated the generation of more SO4•−, resulting in a significant improvement of the catalytic activity. Density functional theory (DFT) calculations revealed that Mo doping facilitated PDS adsorption onto Fe/Mo@C and accelerated the O-O breaking in PDS to produce more SO4•− for BPA degradation. Besides, the intermediates formed during the BPA degradation in Fe/Mo@C-PDS system exhibited lower bio-toxicity than the pristine BPA and those in Fe@C-PDS system. This work elucidates the mechanism of Mo doping to improve the catalytic activity of iron-based catalysts in sulfate-based advanced oxidation processes, providing a new strategy for solving the blockage of Fe(II)/Fe(III) cycle.