Unraveling Dynamic Interplay on Defective Indium Sites During CO ₂ Photoreduction

ABSTRACT 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 CO ₂ molecules on defective In ₂ S ₃ 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 CO ₂ molecule adsorption, accompanying with the emergence of high oxidation states of surface In sites. Upon light illumination, the adsorbed CO ₂ 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 In ₂ S ₃ 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 In ₂ S ₃ (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.