Constructing robust Cu and P Lewis acid-base pair sites on tubular g-C3N4 for promoting photocatalytic reduction of CO2 to CH4

The CO₂ photoreduction to high-value-added products is accompanied by a complex activation and dissociation process, and the construction of multiple active sites on photocatalysts for both CO₂ reduction and H₂O dissociation simultaneously is still a daunting challenge. Herein, Cu as Lewis acid (LA) sites and P as Lewis base (LB) sites were successfully modified on the surface of tubular g-C₃N₄ (P/Cu-TCN) to improve the performance of CO₂ photoreduction to CH₄ with H₂O as a proton donor. The production of CH₄ is as high as 63.95 μmol g⁻¹ h⁻¹ over optimal P/Cu-TCN photocatalyst with an outstanding selectivity. The performance is much higher than those of the reported g–C₃N₄–based photocatalytic systems. Experimental results combined with theoretical simulation results show that the electrophilic Cu (Lewis acid) centers induce the activation of the CO bond in CO₂, while electron-enriched P (Lewis base) sites enhance the adsorption of pure water molecules and subsequent proton transfer processes. The incorporation of Cu and P acid-base pairs not only mitigates the elevated Gibbs free energy barrier associated with CO bond cleavage during CO₂ photoreduction and facilitates the overall proton-coupled electron transfer kinetics, but also suppresses the recombination of photogenerated charge carriers while enhancing charge transport efficiency. All of these together improve the catalytic efficiency of the CO₂-to-CH₄ transformation. This study offers research suggestions for the modification of g-C₃N₄ to enhance CO₂ reduction.