Unraveling Dynamic Interplay on Defective Indium Sites During CO₂ Photoreduction

Defect engineering is a promising strategy in heterogeneous catalysis, while understanding how the defective sites affect reactivity still remains ambiguous. Herein, we demonstrated an in-depth experimental study of dynamic interaction of CO₂ molecules on defective In₂S₃ surfaces by integrating operando X-ray photoelectron spectroscopy with infrared spectroscopy. More specifically, the formation of S vacancy could promote the electron enrichment on adjacent In sites for facilitating CO₂ molecule adsorption, accompanying with the emergence of high oxidation states of surface In sites. Upon light illumination, the adsorbed CO₂ molecules undergo a sequential transformation into *COOH and *CO intermediates. However, an unexpected upside-down inversion of *CO species on defective In sites from In-*CO to In-*OC has been firstly observed, severely restraining desorption of CO product from In₂S₃ surfaces. Further tailoring the electronic structure of defective In sites by substituting residual S atom with O atom for inhibiting *CO inversion, a significant improvement of CO evolution activity has been achieved (18.3 µmol g^-1 h^-1), nearly five times higher than that of pristine In₂S₃ (3.77 µmol g^-1 h^-1). This work provides the new insights on dynamic roles of defect-derived active sites in determining catalytic activity, which might offer opportunities for fine control of catalytic process.