Cooperative Catalysis Over a Potassium Phosphotungstate/Carbon Nitride Nanosheet S-Scheme Heterojunction for Boosted Photocatalytic CO2 Reduction.

Carbon nitride is a promising photocatalyst for CO2 reduction; however, its performance is limited by rapid charge-carrier recombination and insufficient CO2 adsorption and activation. Herein, a potassium phosphotungstate/carbon nitride nanosheet (KPW/CNS) heterojunction was constructed through a facile electrostatic assembly strategy for photocatalytic CO2 reduction. The optimized photocatalyst (5KPW/CNS) exhibited CH4 and CO evolution rates of 6.3 and 9.3 μmol g-1 h-1, respectively, corresponding to a CH4 selectivity of 40% and excellent photostability. In comparison, pristine CNS produced only 0.4 μmol g-1 h-1 of CH4, and showed a much lower CH4 selectivity of 7.0%. Combined X-ray photoelectron spectroscopy, Kelvin probe force microscopy and density functional theory calculations revealed the formation of built-in electric field at the KPW/CNS interface, which drives an S-scheme charge-transfer pathway and promotes efficient spatial separation of photogenerated charge carriers. More importantly, mechanistic investigations demonstrated a cooperative catalytic pathway involving both components of the heterojunction. CNS primarily acts as the electron-enriched site for CO2 adsorption and activation to form *CO intermediates, whereas KPW facilitates the subsequent hydrogenation of *CO toward *CHO, *CH2O, and ultimately CH4. This synergistic interaction promotes the CO2-to-CH4 conversion pathway and accounts for the enhanced CH4 selectivity of the KPW/CNS heterojunction.