Pd-doped β-Bi2O3/Bi2Sn2O7 hybrid nanocomposites for photocatalytic fluorene oxidation: A green approach for the synthesis of fluorenone/fluorenol mixture

• Hetero-junction β-Bi2O3/Sn2Bi2O7 supported Pd was synthesized via hydrothermal method.
• It promoted fluorene partial oxidation by 100% yield into fluorenone/fluorenol products.
• The activity has been affected by the quantum yield, mesoporosity and electron transfer.

Bismuth oxide, tin oxide and Pd metal (Pd/SnBi3SG) hybrids, synthesized via sol–gel technique while employing polyethylene glycol template at a ratio of 3 (Bi/Sn = 3) were tested toward the photocatalytic oxidation of fluorene under ultraviolet and visible light irradiations in comparison with Pd/BiSG and Pd free SnBi3SG photocatalysts. These catalysts were characterized using X-ray diffraction (XRD), UV–vis diffuse reflectance (DRUV–vis), N2 sorptiometry, Raman spectroscopy, transmission electron microscopy (TEM) and GC–MS technique, which used for analyzing the photo-oxidation products. The actual photocatalyst exhibited the highest activity (100% conversion, TOF ≈ 9.4 × 10−6 s−1, fluorenone/fluorenol = 3/1) following oxygen flushing for 30 min (35 ml/min) before UV irradiation. This was mainly due to the close proximity between β-Bi2O3 and Bi2Sn2O7 heterojunction as well as increasing the mesoporosity margin comparatively. On the other hand, the Pd/BiSG catalyst that exhibited smaller crystallite size (20 nm vs. 44 nm) and higher surface area (21.0 vs. 12.0 m2/g) than Pd/SnBi3SG indicated lower activity (Pd/BiSG, 72% conv.). This highlights the importance of the modified electronic structure of Pd/SnBi3SG in designing efficient charge separation as well as high quantum yield value (Φ ∼ 0.1 ± 0.05) exceeding that of Pd/BiSG (3 × 10−2) and SnBi3SG (10−2) photocatalysts. The catalytic behavior and mechanism, reactivity–structure relationship and recyclable use of the hybrid photocatalysts have been thoroughly examined. An indirect chemical probe method using active species scavengers elucidated that the photo-oxidation mechanism was proceeded via holes and O2− moieties rather than singlet oxygen moieties.

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