Enhanced solar photoreduction of CO2 to liquid fuel over rGO grafted NiO-CeO2 heterostructure nanocomposite

Intrinsic oxygen vacancies at CeO2 surface are known to activate thermodynamically stable CO2 molecules, enhancing the reaction rate and reducing reduction energy. However, charge recombination at the ceria-based cathode surface suppresses the multi-electron transfer process required for a complete reduction of CO2 molecules to generate useful hydrocarbons. To suppress this charge recombination and facilitate the multi-electron transfer process, p-type NiO and reduced graphene oxide (rGO) were hybridized with CeO2 to form rGO-grafted NiO-CeO2 photocatalyst, which can convert CO2 to formaldehyde at a rate of 421.09 μmol g−1 h−1; about 4 times higher than that of pristine CeO2. Formation of photo-induced oxygen vacancy of CeO2 photocatalyst resulted in a change of Ce-O bond length at ceria surface were monitored in-situ by X-ray absorption near edge structure (XANES), and X-ray absorption fine structure (EXAFS) spectroscopy. Tracking the formation of CO2 anion radical (CO2•-) and its subsequent protonation with in-situ electron paramagnetic resonance spectroscopy and attenuated total reflection-infrared (ATR-IR) spectroscopy, mechanism and reaction pathway of CO2 reduction into formaldehyde formation have been elucidated.