Poly(triazine imide) Crystals for Efficient CO₂ Photoreduction: Surface Pyridine Nitrogen Dominates the Performance

Polymeric carbon nitrides (PCNs), usually the melon phase, have been extensively applied as photocatalysts for CO₂ reduction; however, their performance is still unsatisfactory. The condensed allotrope, namely, poly­(triazine imide) (PTI) with extended conjugation and a crystallized structure, indeed holds more favorable compositional and structural advantages for photocatalytic CO₂ reduction but remains to be fully exploited. Herein, hexagonal prism-shaped PTI crystals were synthesized and developed as a high-performance photocatalyst for CO₂ reduction. With Co­(bpy)₃²⁺ as a cocatalyst, the PTI crystals exhibit a CO evolution rate of 44 μmol h^–1 (i.e., 1467 μmol g^–1 h^–1) with 93% selectivity, markedly superior to that of the melon counterpart. Moreover, PTI crystals manifest an apparent quantum efficiency of 12.9% at 365 nm, representing the state-of-the-art value by PCN photocatalysts for CO₂-to-CO reduction without using noble metals. The surface pyridine N species of PTI are exposed as active sites to dominate CO₂ activation and conversion, which, together with the high crystallinity to facilitate charge separation and transport, endows high CO₂ reduction efficiency. In situ diffuse reflectance infrared Fourier transform spectroscopy determines the key intermediates during the CO₂ reduction reaction and, consequently, constructs the possible reaction mechanism.