Photocatalytic performance of metal poly(heptazine imide) for carbon dioxide reduction

• Poly(heptazine imide) (PHI) containing metal ions and H-PHI are synthesized. • PHI is used as a photocatalyst for CO 2 conversion. • CO 2 conversion efficiency is dependent on the metal ion. • The large distribution of small particles increases the CO 2 amount adsorbed on PHI. • This distribution is a decisive factor in PHI's CO production efficiency. Poly(heptazine imide) (PHI), a carbon nitride polymer, is a highly efficient visible-light-driven photocatalytic material. We aimed to improve its photocatalytic performance for CO 2 conversion. We prepared M-PHIs by encapsulating different metals ( M = K , Li, Rb, and Na) and H-PHIs, in which the metal of each M-PHI was ion-exchanged with a proton. We evaluated their photocatalytic activities for CO 2 conversion and found that Na-PHI and H-PHI, prepared from Na-PHI (H-PHI(NaCl)), showed more than twice the CO production efficiency of melon and other PHIs. The high CO production efficiency of Na-PHI and H-PHI(NaCl) was attributed to their extremely smaller particle size compared with those of the other PHIs. By closely examining the synthesis conditions of Na-PHI, we have identified a method to intentionally synthesize M-PHI with small particle size. These results provide a new strategy for highly efficient CO 2 conversion using PHI.