Photocatalytic hydrogen generation by TiO2/Pt/biochar composite catalyst with maize stovers as sacrificial agents

Abstract: A new photocatalyst of TiO2/Pt/biochar composite was prepared by a photodeposition and hydrothermal synthesis with the maize stover suspension as a sacrificial agent. The photocatalytic performance of compound photocatalyst was investigated for hydrogen production from water. An analysis was made on the effects of sacrificial agent concentrations in the maize stover suspension on the photocatalytic performance of hydrogen production. The microstructure of the composites was characterized by a Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectra, UV-Vis DRS (UV-Vis DRS), fluorescence spectroscopy (PL), and Electrochemical Impedance Spectroscopy (EIS). The results showed that the hydrogen production of photocatalytic system increased first and then decreased, with the increase of the mass fraction of sacrificial agent in the maize stover suspension. Once the mass fraction of maize stover suspension was 30%, the TiO2/Pt/biochar composite showed the best photocatalytic performance of hydrogen production. Specifically, the hydrogen production rate reached 225 μmol/(h?g), which was 11.2 times higher than that of pure TiO2/Pt. The reason was that there were a large number of aldehyde groups in the molecular composition of maize stover. Furthermore, the reducing active group was excited under the irradiation of simulated sunlight, and then was oxidized with some holes, further to consume the photogenerated holes. The recombination rate of photogenerated carriers and holes was thus reduced to improve the efficiency of photocatalytic hydrogen production. As such, the surface-active functional group -CHO in the maize stover absorbed the energy to activate the functional group, where the sacrificial agent was oxidized with some holes, thus decreasing the composite efficiency rate of photoelectron and hole pair. Meanwhile, the internal hydrogen bond of maize stover was broken to release a large number of hydroxyl radicals and electrons, which indirectly improved the efficiency of photocatalytic hydrogen production. The produced CO2 also proved the presence of the composite reaction in the maize stover as the sacrificial agent. The free group -CHO on the surface of straw was excited to obtain the active CHO· by the simulated solar energy, which reacted with the photogenic holes to generate the COOH intermediates at the same time. This group was extremely unstable and then continued to oxidize with some holes, eventually generating CO2 and H2O with the releasing energy of 220 kJ/mol. When the mass fraction of the sacrificial agent was more than 50% of the maize stover suspension, the hydrogen production of the photocatalytic system was lower than that of TiO2/Pt/biochar, indicating that the maize straw sacrifice consumed some holes during the composite reaction. However, the excessive supported maize stover tended to weaken the light absorption intensity of TiO2 photocatalyst, thus leading to the reduction of electron-hole pair generation. In addition, the photocatalytic hydrogen production of TiO2/Pt/biochar was still 900 μmol/g after five times of reuse, indicating a higher photostability than that of pure TiO2/Pt. Consequently, it was of great significance and feasibility for the practical application and industrial production of photocatalyst. The maize stover suspension can be widely expected to serve as the sacrificial agent and derivative biochar in the photocatalytic decomposition of water to produce hydrogen. The finding can provide a new idea to construct the cost saving and green development of biomass photocatalytic system.