Lewis Acid–Base Dual Sites Enabled by Mg ²⁺ Doping in CdS Quantum Dots for Selective Photocatalytic CO ₂ Reduction to CH ₄

Abstract Photocatalytic CO ₂ reduction using semiconductor quantum dots (QDs) holds great promise for solar‐to‐fuel conversion, yet challenges in selectivity and efficiency remain barriers to applications. Herein, a Mg ²⁺ ‐doped CdS (Mg:CdS) QDs is presented that enables highly selective CH ₄ production via a dual‐site catalytic mechanism. Experimental and theoretical investigations reveal that Mg ²⁺ doping induces profound electronic and structural modifications in CdS QDs. Notably, Mg ²⁺ doping introduces distinct Lewis acid–base dual sites on the QD surface, effectively promoting CO ₂ adsorption and activation through a Cd⋯C═O⋯Mg interaction. This unique activation pathway, elucidated via in situ infrared spectroscopy and density functional theory calculations, facilitates the formation of *CHO intermediates by reducing the O─C─O bond angle and weakening C═O bonds, thereby steering the reaction selectivity toward CH ₄ . As a result, the Mg:CdS QDs achieve an exceptional CH ₄ selectivity of 88.7% with a production rate of 45.8 µmol g ⁻¹ h ⁻¹ under simulated solar irradiation, significantly surpassing undoped CdS. This study not only unveils a novel mechanistic paradigm for alkaline earth metal doping in CO ₂ photoreduction but also provides a strategy for engineering highly selective QD photocatalysts for solar‐driven CO ₂ conversion.