Unveiling the effects of facet-dependent oxygen vacancy on CeO₂ for electron structure and surface intermediates in CO₂ photoreduction reaction

CO2 photoreduction over CeO2 surfaces has been studied for a few years, and many efforts have been devoted to understanding the effect of exposed crystal facets and oxygen vacancies (OVs) on the catalytic activity. In this work, the formation of OVs over different CeO2 surfaces is investigated through X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron paramagnetic resonance spectroscopy (EPR), revealing the facet-dependent OVs in CeO2. The experimental results and DFT calculations show that the OVs formed in (1 1 1)CeO2 and (1 0 0)CeO2 possess faster charge transfer and a more negative conduction band potential than (1 1 0)CeO2. The in-situ Fourier transform infrared spectroscopy (FTIR) and DFT results revealed the CO2 absorption, activation, and reduction processes over the three samples. Although (1 1 0)CeO2 surface shows the best adsorption capability for CO2, its catalytic activity is still limited by a high barrier (1.95 eV) in the formation of CO species from COOH* intermediate. Through the H2O-assisted CO2 activation process, (1 1 1)CeO2 and (1 0 0)CeO2 show better reduction activity for CO2, resulting in the better activity for CO2 photoreduction. This study demonstrates that the band structure and CO2 activation influenced by OVs show a strong crystal facet dependence in CeO2.