Photocatalytic synthesis of vanillin using N-doped carbon nanotubes/ZnO catalysts under UV-LED irradiation

ZnO synthesized by solid-state thermal process was combined with different contents of nitrogen-doped carbon nanotubes (N-CNT). The materials were characterized by several techniques including thermogravimetric analysis, N-2 adsorption-desorption isotherms, scanning and transmission electron microscopies, and diffuse reflectance UV-vis and photoluminescence spectroscopies. The performance of neat ZnO and N-CNT/ZnO composite materials was evaluated in the selective photocatalytic oxidation of vanillyl alcohol into vanillin under UV-LED irradiation. The presence of the carbon phase in the composite materials (from 5.0 to 10 wt.%) revealed to be crucial for increasing the performance of the photocatalysts. The best performance for vanillyl alcohol oxidation was obtained using the composite containing 5% of carbon phase (5.0%N-CNT/ZnO), with an increase of 22% in vanillin concentration comparing to neat ZnO after 2 h of reaction. This enhancement in the efficiency of ZnO by the introduction of the carbon phase is attributed to the action of N-CNT as effective electron scavengers for ZnO, as revealed by the photoluminescence quenching, inhibiting the recombination of electrons and holes.