Suppression of NO ₂ Generation During One‐Step Deep Oxidation of NO at a Semi‐Artificial Photoenzyme Based on Cytochrome <i>c</i>

Solar-driven catalysis holds promise in molecular oxygen activation and low-concentration NO_x elimination but achieving highly selective conversion of NO to nontoxic nitrate (NO₃ ^-), while suppressing toxic NO₂ formation remains challenging. Here, we report a semi-artificial photoenzyme catalyst constructed by immobilizing mono-heme cytochrome c (cyt c) on carbon nitride (CN) to refine reactive species generation for deep NO oxidation. The hybrid photoenzyme catalyst establishes localized internal electric fields (IEF) between cyt c and CN that promote exciton dissociation, and creates polarized active sites on cyt c that preferentially adsorb O₂, and activate O₂ to active peroxides (•O₂ ^- and *O₂ ^2-) via an electron transfer pathway with inhibiting ¹O₂ formation. The one-step NO oxidation to NO₃ ^- is boosted over cyt c/CN, showing exceptional 97.4% NO conversion with ultralow NO₂ selectivity (1.5%, 6.5 ppb) and long-term stability (98.1% after 300 min) at a weight hourly space velocity (WHSV) of 850 L g^-1·h^-1 across wide humidity ranges. The undesirable NO₂ generation is significantly lower than that of bare CN (17.0%, 104.7 ppb) and reported catalysts. The catalyst exhibits high activity in direct NO₂ removal (92.9%) and rapidly reduces NO levels to below the safe concentration (52 ppb) in a simulated environment chamber.