Molecular-Scale Modulation of Surface Hydrogen Dynamics for Enhanced Photocatalytic Syngas Production with Adjustable H ₂ /CO Ratios over CdS Nanoparticles

Solar-driven CO 2 conversion into syngas over metal sulfides offers a promising route for fuel synthesis. However, improvements in photocatalytic activity and H 2 /CO ratio control are constrained by a poor understanding of surface hydrogen dynamics. This study demonstrates that introducing sulfur vacancies (S v ) into CdS nanoparticles enables efficient, tunable syngas production by coupling photocatalytic CO 2 reduction with benzyl alcohol oxidation. By regulating the S v concentration, the H 2 /CO ratio was systematically varied from 4.5:1 to 6.0:1 within the constructive modulation regime, with the CdS photocatalyst with medium S v occupancy delivering an eightfold higher syngas yield than pristine CdS. In situ spectroscopy and theoretical calculations reveal that a higher S v concentration enriches electron density at adjacent S sites. This enrichment increases the antibonding orbital occupancy, destabilizes S–H bonds, and accelerates H 2 evolution kinetics, leading to an elevated H 2 /CO ratio. Concurrently, these weakened bonds induce partial spillover of *H species toward S v sites, supplying additional *H for *CO 2 and *COOH protonation, which moderately boosts CO evolution. Results confirm that S v -induced S–H bond weakening drives the H 2 /CO ratio shift and enhances photocatalytic activity, highlighting an effective strategy for tuning photocatalytic syngas production.