Sonochemical Defect Engineering for Sustainable CO ₂ Methanation over Ni/CeO ₂ Catalysts with Mechanistic Insights from <i>Operando</i> Studies

Abstract The conversion of CO 2 into synthetic fuels represents a sustainable strategy to mitigate climate change and close the carbon cycle. In this work, we report the sonochemically‐assisted synthesis of Ni/CeO 2 catalysts via co‐precipitation, enabling precise control of Ni dispersion, Ce 3+ /Ce 4+ redox dynamics, and oxygen vacancy density, which are key parameters for efficient CO 2 methanation. The NiCe0 sample, prepared without ultrasound, achieved 15% CO 2 conversion, whereas the NiCe9 sample, subjected to 90 min of sonochemical treatment, showed significantly higher CO 2 conversion. By systematically tuning ultrasound exposure time, we achieved defect engineering at the CeO 2 surface, which was directly correlated with catalytic performance and selectivity. Operando Raman spectroscopy provided real‐time mechanistic insights, revealing that samples with higher vacancy densities favored the formate‐mediated pathway toward CH 4 formation, while lower‐defect samples led to increased CO production. These spectroscopic findings were further supported by XPS and H 2 ‐TPR analyses. Our results establish a clear link between synthesis parameters, structural defects, and catalytic pathways, offering a sustainable and mechanistically guided route for designing efficient CO 2 methanation catalysts.