Photocatalytic Hydrogen Evolution Over Pt/Co-TiO2 Photocatalysts

Abstract

In this study, the photocatalytic hydrogen evolution reaction from aqueous methanol was investigated upon simulated solar light using platinum loaded on cobalt doped TiO2 (Pt/Co-TiO2) composites. Controversial results of cobalt-based composites create doubts about their photocatalytic activity. Thus, cobalt doped TiO2 composites were synthesized differently, and the photocatalytic activity was examined for the photocatalytic hydrogen generation. The current study aims to investigate the influence of cobalt doping and platinum loading on the photocatalytic activities of TiO2 nanoparticles for the photocatalytic H2 generation. The 0.5 wt.% Co-TiO2 and bare TiO2 photocatalysts were synthesized using two different methods, namely, reflux and hydrothermal synthesis. Additionally, the Pt deposition on the prepared Co-TiO2 and TiO2 catalysts (1 wt.% Pt) was performed using a photo-platinization method. The as-prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy/ energy dispersive X-ray analysis (SEM/EDX), transmission electron microscopy (TEM), ultraviolet- visible spectroscopy (UV-vis), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and electrochemical impedance spectroscopy (EIS). The XRD and EPR studies clearly indicated that the Co was incorporated into the titanium dioxide lattice. The EIS results suggested that the reduction of protons over Co-TiO2 and bare TiO2 materials was possible from a thermodynamic point of view. However, the photocatalytic results revealed that the formed amount of H2 was extremely low and close to the detection limit. The evolution of H2 from aqueous methanol (10 vol%) showed higher rates when employing 1 wt.% Pt loaded on 0.5 wt.% Co-TiO2 photocatalysts under simulated solar light irradiation. A maximum of 317 ± 44 μmol.h-1 was observed over the Pt/Co-TiO2-HT photocatalyst. EPR results confirmed that the cobalt ions were introduced into the TiO2 lattice by trapping the photogenerated conduction band electrons and decreasing the defects in the crystal cell. The Mott−Schottky analysis of electrochemical impedance measurements showed that all catalysts were ntype semiconductors and that cobalt doping induces impurity level within the band gap of TiO2. The experimental results of photocatalytic H2 generation from methanol-reforming demonstrated that no significant impact of Co-doping on the photocatalytic H2 formation was observed neither for bare TiO2 samples nor for the platinized materials. Based on these experimental findings, a possible mechanism for the continuous photocatalytic activity of Pt/Co-TiO2 photocatalysts under simulated solar light was proposed.