Periodic Alkali Metal Modulation of Carbon Nitride for Volcano-Type Photocatalytic H₂O₂ Production

Alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) are systematically doped into graphitic carbon nitride (g-C₃N₄, CN) to investigate the periodic regulation of photocatalytic H₂O₂ production. Using ethanol as a sacrificial agent under visible-light irradiation, the H₂O₂ generation rate exhibits a pronounced volcano-type trend along the Li-Na-K-Rb-Cs series, with K⁺-doped CN achieving the highest activity of 1795 µmol g^-1 h^-1, representing a 2.1-fold enhancement over pristine CN. Mechanistic studies reveal that alkali metal ions incorporation modulates the electronic structure of CN through orbital interactions, enhances π-conjugation, promotes charge separation, and facilitates interfacial charge transport. Density functional theory (DFT) calculations further indicate that the introduction of alkali metal ions lowers the energy barriers of key 2e^- oxygen reduction reaction steps and establishes a kinetically efficient pathway for the selective formation of H₂O₂. This work presents a periodic law-guided orbital modulation strategy and provides a rational design principle for high-performance CN-based photocatalysts.