Applied Catalysis A: General, Vol.575, 120-131, 2019
Heteroatom-induced band-reconstruction of metal vanadates for photocatalytic cyclohexane oxidation towards KA-oil selectivity
Structure-function correlation constitutes a central theme in the selective photocatalytic oxidation of cyclohexane with molecular oxygen, as a result of precise tailoring band-structure towards KA-oil selectivity. Herein, we comparatively and systematically investigate the correlation among optical properties, electronic structure, and photocatalytic performance of two d degrees-vanadate photocatalysts (BiVO4 and Cu3V2O8) using theoretical calculations and experimental techniques. Results show that BiVO4 promotes the selective oxidation pathway under visible-light irradiation with higher efficiency (TOF = 7.5 h(-1), based on the molar amount of V sites) and chemoselectivity (cyclohexanone/cyclohexanol molar ratio = 3.0) than the Cu3V2O8. Meanwhile, the photo catalytic performance of BiVO4 demonstrates high photocatalytic selectivity towards KA-oil (95%), while Cu3V2O8 seems to facilitate the accommodation of chlorine promoters, leading to very different chlorocyclohexane selectivity (46%). Such the distinct photocatalytic performances of BiVO4 and Cu3V2O8 can be ascribed to their unique electronic structures to facilitate the e(-)-h(+) charge carriers and generation of reactive species during photoirradiation. The Bi3+ 6p states of BiVO4 are predominant components of the conduction band minimum (CBM) and hybridize with Bi3+ 6s, and O2- 2p and V5+ 3d orbitals at the valence band maximum (VBM); whereas the Cu2+ 3d and V5+ 3d orbitals contribute to the CBM of Cu3V2O8, and Cu2+ 3d, O2- 2p and V5+ 3d orbitals at the VBM. So BiVO4 with s - p matrix elements is more effective than the Cu3V2O8 in producing (OH)-O-center dot, h(+) and H2O2, which can rapidly oxidize the C - Cl band cleavage for chlorocyclohexane intermediate to regenerate newly Cl-center dot radicals in the next cycle reactions. Heteroatom-tuning of photocatalytic performance is further supported by universal evidences from experimental results by replacing Bi or Cu of the above vanadates with Ag, La, Sr, Fe or Ti atom and the study of the electrons, holes and radicals mediated reaction pathway by scavengers, respectively.