f–f Transition in Er(OH) ₃ Promotes Photosynthesis of H ₂ O ₂ Coupling With Oxidation of Benzylamine Over Sodium Poly(heptazine imides)

ABSTRACT f–f electronic transitions of rare‐earth elements typically exhibit unique physicochemical properties that benefit the photocatalytic performance of the catalysts containing the elements. Although conventional rare‐earth metal oxides or single‐atom doping in semiconductors have been extensively studied, the construction of heterojunction catalysts incorporating rare‐metal hydroxides remains unexplored. In this work, Er(OH) 3 ‐modified crystalline sodium poly(heptazine imides) (Na‐PHI) photocatalysts were prepared for the first time by a facile one‐step NaCl‐assisted thermal polymerization process. The 4 H 11/2 → 4 I 15/2 electronic transition in Er(OH) 3 can be absorbed by Na‐PHI, which improves the utilization of light ( λ ≥ 500 nm) and enhances the separation efficiency of photogenerated charge carriers. The obtained Er(OH) 3 /Na‐PHI catalyst (Er‐Na‐PHI) exhibits high photocatalytic activity for the reduction of O 2 to H 2 O 2 via a direct one‐step two‐electron pathway, with simultaneous oxidation of benzylamine (BA) to N‐benzylbenzaldimine (BBAD). The optimized Er‐Na‐PHI exhibits the highest H 2 O 2 production rate of 0.30 mmol g −1 h −1 in an aqueous solution of BA under visible light (800 nm ≥ λ ≥ 400 nm), nearly 1.7 times that of Na‐PHI (0.18 mmol g −1 h −1 ). Additionally, Er‐Na‐PHI demonstrates a high selectivity (c.a. 100%) and a high yield (83%) for BBAD. Under blue light illumination, the H 2 O 2 production rate over Er‐Na‐PHI reaches 0.14 mmol g −1 h −1 , whereas no H 2 O 2 is generated over pure Na‐PHI or Er(OH) 3 . This study demonstrates that the rare‐earth composite cocatalysts are favorable for H 2 O 2 generation with synergistic organic oxidation.