Selective Remediation and Reduction of Cr(VI) to Cr(II) Using Carbon-Supported Co, Ni, and Zn Single-Atom Catalysts:...

Hexavalent chromium (Cr-(VI)) is a prevalent heavy metal contaminant in soil, exhibiting high biotoxicity and posing significant threats to both ecosystems and public health. Reducing Cr-(VI) to lower oxidation states without introducing an additional reducing agent is an effective strategy to mitigate its toxicity. However, the selective remediation and reduction of highly hazardous Cr-(VI) have been hampered by the absence of an appropriate catalyst with desired activity and a comprehensive molecular-level understanding of the underlying mechanisms. Herein, a two-dimensional N-doped graphene (N₄G) with a 3d transition metal as an active site was systemically investigated for reduction of Cr-(VI) to Cr-(II) using density functional theory (DFT) calculations. The results show that Co-N₄G, Ni-N₄G, and Zn-N₄G catalysts exhibit pronounced selective adsorption toward the Cr-(VI) and humic acid (HA) in comparison to H₂O, O₂, and N₂ under soil environments. The reaction pathways and associated dynamic barriers for each reduction step of Cr reduction were explored in detail, and the involved mechanisms of Cr-(VI) reduction to Cr-(IV) and Cr-(IV) to Cr-(II) was elucidated. Notably, the Co-N₄G and Ni-N₄G catalysts exhibit low E _bar of 0.59 and 0.49 eV during the reduction of Cr-(VI) to Cr-(VI) over other single-atom catalysts. In contrast, the Zn-N₄G catalyst exhibits higher catalytic activity toward the Cr-(IV) to Cr-(II) with an E _bar of 1.35 eV. These findings offer an atom-scale insight for the rational design of advanced catalysts to enhance the selective remediation of heavy metal contamination in soil.