Gradient migration hydrogenation of adsorbed hydrogen modulated by anasymmetric configuration of S vacancies with atomic Zn dopant for photocatalytic CO2 methanation

Abstract

It remains a challenge to modulate kinetic and thermodynamic factors synergistically by regulating the geometrical configuration of active sites for selective photocatalytic CO₂ reduction due to multiple processes relating to multiple protons coupled with multi-electron transfer. Herein, an asymmetric configuration of self-adapting S vacancies (V_S) induced by atomic Zn substitution confined in In₂S₃ nanosheets was prepared and applied to photocatalytic CO₂ reduction. Experimental results and theoretical simulations uncover that the asymmetric configuration of Zn substituting tetra-coordinated In atoms and V_S exhibit a synergistic effect for regulating regional charge separation. Three In atoms are exposed when V_S decrease the energy barrier for H₂O activation, supplying more surface adsorbed hydrogen for CO₂ hydrogenation. An adsorbed intermediate in CO₂ conversion increases the energy barrier for H₂ formation to inhibit H₂ generation. In particular, an asymmetric geometrical configuration (Zn–V_S–In) not only lowers the energy barrier of CO₂ activation and intermediate conversion thermodynamically, but also enhances photothermal effects to drive surface adsorbed hydrogen migration among gradient S sites formed by V_S to participate in CO₂ hydrogenation kinetically. The optimized Zn doped In₂S₃ nanosheets achieve efficient CO₂ methanation (50.7 μmol g⁻¹ h⁻¹) in pure water, which is 6.8 times higher than that of In₂S₃ (7.5 μmol g⁻¹ h⁻¹), and the selectivity is up to 92.3%. Ultimately, a selective photocatalytic CO₂ methanation mechanism is proposed for the first time, including H₂O activation, surface hydrogenation, migration hydrogenation involving adsorbed hydrogen and CO₂ conversion by considering thermodynamic and kinetic factors synergistically. This work not only provides new insights into CO₂ reduction by a synergistically kinetic and thermodynamic mechanism but also provides a promising route to CO₂ methanation.