Sulfur-Engineered Second-Shell in Fe-N Catalysts: Dual Active Sites Harmonized Activity and Stability in Real Water Environment.

Single-atom catalysts (SACs) exhibit outstanding catalytic activity, yet their application in real complex environments is constrained by the single active sites and instabilities that are susceptible to inactivation. Extensive efforts have been made to regulate the metal coordination environment, but the catalytic role of nonmetal dopants, especially beyond the first shell, remains underexplored. Herein, S-engineered second-shell Fe single-atom catalysts (FeNSC) are reported, in which S sites not only function as additional nonmetallic active sites separated from Fe but also reinforce the stability of the catalysts. A dual-site mechanism maintains atomically dispersed single-atom sites in multi-substrate catalysis consistently, where second-shell S dopants catalyze Cd(II) via deep d-p orbital hybridization, while Fe centers target Pb(II) through frontier orbital hybridization. Such dual-site yields exceptional electrocatalytic performance, achieving sensitivities of 83.27 µA µm for Pb(II) and 63.53 µA µm for Cd(II) in real mining irrigation water samples, compared to prior electrocatalysts tested under ideal laboratory conditions. It also offers better resistance to interfering ions, at concentrations two orders of magnitude higher than the target ions. This study highlights the dual functionality of second-shell nonmetal dopants, establishing a versatile design platform for stable single-atom catalysts applicable to complex environmental applications.