Synergy between Ceria Oxygen Vacancies and Cu Nanoparticles\nFacilitates the Catalytic\nConversion of CO<sub>2</sub> to CO under Mild Conditions

The\nperformance of supported metal catalysts can depend on many factors,\nincluding metal particle size and dispersion and metal–support\ninteractions, and differentiation of these effects is challenging\nbecause of their interwoven relationship. Copper/ceria catalysts are\nwell-known redox catalysts studied in the conversion of CO and CO₂ via oxidation and/or reduction pathways. The redox behaviors\nof each species, Cu-CuO and CeO_<i>x</i>-CeO₂, are often suggested to be interlinked, allowing ceria-supported\ncopper domains to outperform copper species on other, nonredox active\nsupports. In this work, the catalytic activity of nanosized Cu supported\non either cerium oxide or mesoporous silica is explored using samples\nwhere the Cu weight loading, particle size, and dispersion of Cu are\nheld constant to highlight the impact of the two supports on catalytic\nperformance without additional influencing factors. The Cu/CeO₂ catalysts are synthesized via a space-confined method to\nlimit the growth of CeO₂ particles and to achieve a high\ndispersion of Cu. Through in situ XRD and XAS, it is shown that the\npresence of Cu nanoparticles on the CeO₂ support lowers\nthe reduction temperature of CeO₂, allowing formation of\noxygen vacancies at low temperatures <300 °C. The Cu/CeO_<i>x</i> catalyst demonstrates 100% CO selectivity\nin the low temperature (300 °C) and ambient pressure conversion\nof CO₂ to CO, even when approaching equilibrium conversion.\nMoreover, this catalyst is approximately 4 times more active than\nthe corresponding Cu/SiO₂ catalyst with otherwise similar\nstructural attributes. The potential reaction pathways are probed\nby in situ FTIR and in situ XAS at various temperatures, identifying\nCu⁺-CO species and oxygen vacancies forming under some\nconditions. The collected experimental evidence also suggests a reaction\nsequence for CO₂ hydrogenation over Cu/CeO<i>_x</i> catalysts, consistent with DFT reports in the literature.