Nonredox Electron Relay via Strain-Engineered Cobalt Sites Confined in TiO2 Nanotubes for Superior Peroxide Activation.

The catalytic efficiency of heterogeneous advanced oxidation processes is intrinsically constrained by the kinetic limitations and instability of transition metal redox cycles. Herein, we demonstrate a strategy to transcend this fundamental limitation by spatially confining cobalt single atoms within the compressive lattice strain field of TiO2 nanotubes (Co/TiO2 NTs). This engineered local environment electronically modulates the cobalt centers, shifting their function from classical redox mediators to static electron relays. Spectroscopic and computational analyses reveal that the compressive strain elevates the d-band center of the internally confined cobalt atoms, thereby enhancing the adsorption of peroxides and facilitating direct electron transfer from the photoexcited TiO2 substrate. This nonredox activation mechanism bypasses the conventional Co(II)/Co(III) cycle, enabling unprecedented activation efficiencies for three predominant peroxides (peroxymonosulfate, peroxodisulfate, and hydrogen peroxide) with hydroxyl radical conversion yields reaching 85%. The nanotube's inherent nanoconfinement further ensures operational robustness by imparting molecular sieving capabilities that exclude macromolecular interferents while concentrating target pollutants and reactive species. The practical viability of this concept is underscored by scaling the catalyst into a 3D-printed monolithic flow-through reactor, which sustained complete contaminant removal over 1000 h of continuous operation treating real wastewater, with no detectable cobalt leaching. This work establishes spatial confinement as a generative tool for designing nonredox catalytic pathways, offering a generalizable blueprint for next-generation high-efficiency, stable environmental catalysis.