High-density single-atomic Ni–N4 sites for efficient Fenton-like reactions.

Maximizing the number of exposed active sites or regulating the coordination environment of catalysts is important for advanced oxidation processes to produce highly reactive radicals and destroy organic contaminants. Single-atom catalysts (SACs) have great potential as Fenton-like catalysts owing to a high utilization rate of atoms and their unique features bridging the gap between homogeneous and heterogeneous catalysis. Here, single-atom Ni dispersed on N-doped nanoporous carbon (Ni–NC) with a relatively high Ni loading of 9.3 wt% was prepared by a cascade anchoring strategy. Isolated Ni–N₄ sites are fully exposed in Ni–NC, which display excellent catalytic activity by activating H₂O₂ and generating sufficient OH in Fenton-like catalytic oxidation of organic contaminants. Taking the degradation of methylene blue for an example, the degradation rate constant of Ni–NC is up to 0.767 min⁻¹, higher than that of Ni₃N/C with an Ni–N structure in interstitial sites of Ni₆N₂ octahedra (0.226 min⁻¹) and NiO/C with an Ni–O structure (0.016 min⁻¹). Combined with density functional theory (DFT) calculations, the high-density Ni–N₄ moiety obtained from high single-atom Ni loading enhances charge transfer at the reaction interface and reduces the free energy barrier for H₂O₂ activation, thereby enabling a remarkable rapid degradation of contaminants. Moreover, the degradation reactor designed with Ni–NC realizes approximately 100% MB removal during 6 h continuous operation. This work highlights the effect of structure and loading for metal centers on catalytic oxidation reactions. [ABSTRACT FROM AUTHOR]