Dynamic Crystal-Structure and Active-Site of Defective ZnAl-Catalysts During CO₂ Photoreduction

Understanding of surface defects of a catalyst during the reaction process is critical to establish an accurate structure-activity relationship. Herein, combining operando X-ray diffraction/photoelectron spectroscopy with scanning probe microscope, we have first established the correlations between CO₂ photoreduction activities and crystal-structure/active-site of defective ZnAl layered double hydroxides (ZnAl-LDH). Specifically, the introduction of oxygen vacancies in ZnAl-LDH could effectively promote the selective adsorption of both CO₂ and H₂O molecules on surface Al and Zn active sites, respectively, leading to opposite transition states and the evident shrinking of crystalline structures. Under light irradiation, the adsorbed CO₂ molecules transformed into *COOH intermediate on surface Al active sites, while the H₂O molecules dissociated into OH group on Zn sites to provide proton, simultaneously leading to the expansion of crystalline structures and increase of layer spacing. Accordingly, these defect-dependent evolutions of surface active sites and crystalline structure contributed to the significant improvement of CO₂ reduction to CO activity (17.2 µmol g^-1 h^-1), much higher than that of pristine ZnAl-LDH (6.3 µmol g^-1 h^-1). This work provides new insights for in-depth understanding of the electronic and crystalline changes of defective photocatalysts during the reaction process.