Improving Ozone Decomposition in Humid Conditions via Side-On Adsorption to Break Hydrogen Bond Network on Fe–N ₄

The double role (inhibition or enhancement) of relative humidity (RH) on the performance of catalytic O3 decomposition is determined by the active-site microenvironment. However, the modulation mechanism remains highly ambiguous, hindering the design of catalysts for high RH conditions. In this study, two single-atom catalysts (Fe–N4 and Fe–C3) are prepared to investigate the relationship between the active-site microenvironment and the effect of RH. Under dry conditions, both catalysts follow an identical O3 decomposition pathway with *O2 desorption as the rate-determined step, and the associated energy barriers are 1.44 eV for Fe–N4 and 1.56 eV for Fe–C3. Under highly humid conditions, different bond cleavages of the key intermediate (OOOH) lead to divergent O3 decomposition performances. Specifically, the electrostatic effect between the electron-rich N site and OOOH species, mediated by the Hδ+ atom, facilitates the formation of the Fe–O–OOOH intermediate, generating Fe–OOH species over Fe–N4 with a lower barrier energy (0.32 eV). However, the Fe–O–OOOH intermediate over Fe–C3, which lacks such an electrostatic effect, undergoes a conventional high-barrier pathway (1.59 eV) to convert into Fe–O species. This study demonstrates that the O3 cleavage pathway, mediated by the electrostatic interaction between the OOOH intermediate and the coordination atom, dictates the humidity response divergence. It provides a valuable microenvironment-engineering example for developing humidity-adaptive catalysts across fluctuating RH conditions.