Distinguishing the Catalytic Roles of In-Plane Versus Edge Sulfur Vacancies in MoS2: Selective Lactic Acid Coupling Driven by Undercoordinated Edge Sites

Understanding the distinct functions of different defect sites on two-dimensional materials is crucial for catalyst design. While sulfur vacancies (Vs) on MoS2 are recognized as active sites, the specific roles of in-plane versus edge Vs remain ambiguous. Herein, we systematically engineer and differentiate in-plane and edge Vs on MoS2 to probe their catalytic roles in the lactic acid coupling reaction. Through a combination of Raman spectroscopy, in situ O2 adsorption IR, and electron paramagnetic resonance (EPR) analyses, we confirm the successful creation and stabilization of these two vacancy types. Catalytic performance evaluation reveals that MoS2 rich in edge Vs exhibits significantly higher activity and superior selectivity toward the desired coupling product, in stark contrast to samples dominated by in-plane Vs, which show negligible activity. Density functional theory (DFT) calculations elucidate the origin of this disparity: edge Vs provide undercoordinated Mo sites that facilitate the optimal adsorption and activation of lactic acid molecules, lowering the energy barrier for the initial deprotonation step. In contrast, in-plane Vs present a less favorable electronic structure for reactant interaction. At similar conversion levels, MoS2 enriched with edge sulfur vacancies achieved over 80% selectivity toward the coupling reaction, while the selectivity remained below 10% for samples dominated by in-plane sulfur vacancies. This work provides direct experimental and theoretical evidence distinguishing the catalytic contributions of sulfur vacancy location, highlighting the predominance of undercoordinated edge sites in selective lactic acid conversion reactions. It offers fundamental insights for defect engineering in transition metal dichalcogenide catalysts.