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

Abstract

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.

Figure 1

N₂ adsorption–desorption isotherm profiles of the heterostructured CeO₂-based nanocatalysts with associated pore size distribution profiles (as inset).

Figure 2

XRD patterns of the heterostructured CeO₂-based metal oxide nanocatalysts.

Figure 3

(a) UV–vis/NIR spectra and (b) Tauc plots of the heterostructure CeO₂-based metal oxide interfaced nanocatalysts.

Figure 4

Photoluminescence spectra of various heterostructured CeO₂-based photocatalysts.

Figure 5

SEM images of heterostructured CeO₂ interfaced nanocatalysts.

Figure 6

TEM/HRTEM images and SAED patterns of (a) CeO₂, (b) Cu–CeO₂, (c) Co–CeO₂, (d) Ni–CeO₂, (e) Mn–CeO₂, and (f) Fe–CeO₂ heterostructured nanocatalysts.

Figure 7

XPS spectra of (a) CeO₂, (b) Cu–CeO₂, (c) Co–CeO₂, (d) Ni–CeO₂, (e) Mn–CeO₂, and (f) Fe–CeO₂ heterostructured nanocatalysts.

Figure 8

EPR spectra of the heterostructure CeO₂ interfaced nanocatalysts.

Figure 9

Screening of the photocatalytic activity of the heterostructure CeO₂ catalysts in pinene oxidation. Reaction conditions: pinene (10 mmol; 1.36 g), catalyst (0.10 g), acetonitrile (125 mL), T = 25 °C, and t = 5 h.

Scheme 1. Plausible Reaction Pathways Involved in the Oxidation of Pinene

Figure 10

Plausible electron transfer mechanism of heterostructured CeO₂-based metal oxide interfaced nanocatalysts under visible-light irradiation: (a) CeO₂, (b) Cu–CeO₂, (c) Co–CeO₂, (d) Ni–CeO₂, (e) Mn–CeO₂, and (f) Fe–CeO₂.

Scheme 2. Plausible Reaction Pathways Involved in the Catalytic Oxidation of Pinene to Pinene Oxide and its Subsequent Isomerization Products

Figure 11

Screening the photoactivity of the heterostructured CeO₂ catalysts in the pinene hydroperoxidation reaction. Reaction conditions: pinene (10 mmol; 1.36 g), catalyst (0.10 g), 30% H₂O₂ (30 mmol), acetonitrile (12.5 mL), T = 65 °C, and t = 5 h.

Figure 12

Recyclability performance tests of the Fe–CeO₂ catalyst in the pinene photooxidation reaction. Reaction conditions: pinene (10 mmol; 1.36 g), catalyst (0.10 g), acetonitrile (125 mL), T = 25 °C, and t = 3 h.