Unraveling the Mechanism for H₂O₂ Photogeneration on Polymeric Carbon Nitride with Alkali Metal Modification

K and Na have been widely used in photocatalytic H2O2 production. However, Rb and Cs have rarely been studied for their photocatalytic potentials. In addition, the mechanism regulating H2O2 production from different alkali metal (M)-modified polymeric carbon nitride (PCN) is still unknown. Therefore, M-doped PCN was fabricated using thermal copolymerization in the presence of Li, Na, K, Rb, or Cs. The activity of CN-M was enhanced by the increase in the metallic character of alkali metals. However, CN-Cs’s photocatalytic H2O2 activity is not optimal even though it has the strongest metallic character. A stronger metallic character is anticipated to yield stronger Lewis acidic sites. Although ethanol can be adsorbed and activated at strong Lewis acidic sites, H2O2 can also be activated at these sites, which speeds up H2O2 degradation. CN-Rb—with its acceptable metallic character, excellent oxygen adsorption capacity, and reduced H2O2 degradation—has the best photocatalytic H2O2 yield