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

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 (SBET) 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.