| Summary: | K and Na have been widely used in photocatalytic H<sub>2</sub>O<sub>2</sub> production. However, Rb and Cs have rarely been studied for their photocatalytic potentials. In addition, the mechanism regulating H<sub>2</sub>O<sub>2</sub> 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 H<sub>2</sub>O<sub>2</sub> 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, H<sub>2</sub>O<sub>2</sub> can also be activated at these sites, which speeds up H<sub>2</sub>O<sub>2</sub> degradation. CN-Rb—with its acceptable metallic character, excellent oxygen adsorption capacity, and reduced H<sub>2</sub>O<sub>2</sub> degradation—has the best photocatalytic H<sub>2</sub>O<sub>2</sub> yield.
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