Phase-dependent photocatalytic selective oxidation of cyclohexane over copper vanadates

Abstract

In this work, copper vanadates with different crystal phases were prepared via a simple sol–gel method, and used as photocatalysts for the selective oxidation of cyclohexane to cyclohexanol and cyclohexanone (KA-oil). X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and Raman results confirmed that the as-prepared photocatalysts with major crystal phases of CuV₂O₆, Cu₂V₂O₇ and Cu₅V₂O₁₀, respectively were synthesized successfully by adjusting the molar ratios of Cu/V. Activity tests showed that the three catalysts have different activities in the photocatalytic cyclohexane oxidation and the sample with the Cu₅V₂O₁₀ crystal phase exhibited the highest photocatalytic activity with a cyclohexane conversion rate of 12236.4 μmol h⁻¹ g⁻¹, which was 156.9 and 1.58 times that of the samples with the CuV₂O₆ and Cu₂V₂O₇ phases, respectively. This is mainly due to the fact that Cu₅V₂O₁₀ has a suitable energy band structure, stronger light absorption ability, significantly faster electron transport ability and lower electrochemical impedance compared to CuV₂O₆ and Cu₂V₂O₇. In addition, a radical scavenger experiment was used to clarify the mechanism of photocatalytic oxidation, which confirmed that ˙OH and holes play key roles in the process. The photogenerated electrons on the conduction band (CB) selectively promote the two-electron reduction of O₂ and inhibit the formation of ˙O₂⁻, which effectively depresses the photocatalytic decomposition of cyclohexanone and achieves a high selectivity.