Fe-N-C materials are very promising for single-atom catalysis (SAC), displaying excellent activities in oxidation reactions. The systematic investigation of the iron coordination mode revealed that Fe-N4C catalysts are the most active for C-H bond oxidation. However, these catalysts are commonly synthesized through an uncontrollable pyrolysis process, which leads to poorly organized structures with heterogeneous composition, rarely presenting only atomically dispersed Fe−N−C as active sites. Herein, a reliable and easily reproducible method to obtain highly active Fe SACs (atomically dispersed) with Fe-N4 sites is presented, using a one-step simple cation exchange method. By exchanging sodium cations from high crystalline Na-PHI, stable Fe single-atoms between the heptazine rings of the PHI structure were obtained. These Fe-N4 sites displayed an excellent selectivity in the photocatalytic oxidation of C-H bonds, with a remarkable catalytic activity under visible light irradiation, due to the promotion of highly active Fe(IV)/Fe(V)-oxo species by the holes originated from the photoactive support (PHI).