Tuning the selectivity of visible light-driven hydroxylation of benzene to phenol by using Cu, Fe and V oxides supported on N-doped TiO2

Abstract

Cu, Fe and V oxides supported on N-TiO₂ (Cu/N-TiO₂, Fe/N-TiO₂, and V/N-TiO₂) were synthesized by an incipient wet impregnation method. The prepared photocatalysts were analyzed by N₂ adsorption at −196 °C to measure the specific surface area (S_BET) values, scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD), X-ray photoelectron spectroscopy (XPS), and Raman, photoluminescence and ultraviolet-visible diffuse reflectance (UV-vis DRS) spectroscopy methods. The prepared photocatalysts were tested in the hydroxylation of benzene to phenol under visible light irradiation in the presence of H₂O₂ as the oxidant. After 360 min of irradiation, Cu/N-TiO₂ achieves a phenol yield equal to 25%, significantly higher than that observed with Fe/N-TiO₂ (2%) and V/N-TiO₂ (2.5%). Moreover, the Cu/N-TiO₂ photocatalyst exhibited a phenol yield higher than that reported in the literature dealing with TiO₂ based photocatalysts for photocatalytic benzene hydroxylation. The better photoactivity of Cu/N-TiO₂ in phenol production was justified by considering both electronic and surface photocatalyst features. In detail, a significant optical absorption in the visible region has been highlighted, due to the intense electronic interactions between CuO and N-TiO₂. Moreover, the surface of the copper oxide component shows low affinity with phenol molecules. Therefore, once photocatalytically generated, phenol easily desorbs from the Cu/N-TiO₂ surface, thus limiting parasitic overoxidation reactions. In fact, after 180 min of visible light irradiation, only 30% of phenol was degraded by Cu/N-TiO₂, while 100% and 81% of it was degraded by Fe/N-TiO₂ and V/N-TiO₂, respectively. The comparison of phenol production kinetic constants, obtained by fitting the experimental data with the least-squares methods, showed that the highest rate of phenol formation (k = 1.41 × 10⁻³ min⁻¹) was obtained by using the Cu/N-TiO₂ photocatalyst. Cu/N-TiO₂ has been recovered from the aqueous solution after a photocatalytic run and reused four times with no reduction in benzene conversion and phenol yield, thus confirming the high stability of the catalytic system.