Heterostructured CeO₂–M (M = Co, Cu, Mn, Fe, Ni) Oxide Nanocatalysts for the Visible-Light Photooxidation of Pinene to Aroma Oxygenates

Herein, we report the enhanced photocatalytic activity of heterostructured CeO₂ nanocatalysts interfaced with Cu, Co, Ni, Mn, and Fe metal oxides. The CeO₂ catalysts exhibited an enhanced red shift in the visible-light response compared to CeO₂. This improved absorption range effectively suppressed electron (e^–)/hole (⁺h) recombination by forming localized energy bands associated with defect oxygen vacancies (V_o) induced by the Mⁿ⁺ ions incorporated in CeO₂. Under visible-light irradiation, CeO₂ catalysts are active for α-pinene oxidation to the aroma oxygenates, pinene oxide, verbenol, and verbenone. Both Fe₂O₃–CeO₂ and NiO–CeO₂ gave the highest pinene conversions of 71.3 and 53.1%, respectively, with corresponding pinene oxide selectivities of 57.3 and 58.2%. The enhanced photocatalytic performance of the heterostructured CeO₂ catalysts compared to CeO₂ is attributed to their enhanced visible-light absorption range and efficient suppression of e^–/⁺h recombination. The Fe₂O₃–CeO₂ catalyst was highly recyclable and did not show any significant loss of its photoactivity.