Enhanced photocatalytic activity of Pt deposited on titania nanotube arrays for the hydrogen production with glycerol as a sacrificial agent

Abstract

Photocatalytic activity of Pt-loaded TiO2 nanotube arrays (Pt-TNTA) in the presence of glycerol as a sacrificial agent to produce H2 has been studied. The effects of Pt loading and the methods by which it is deposited on TNTA (chemical reduction and photo-assisted deposition) were carefully examined. Intermediate products were also identified in order to scrutinise the reaction pathways of hydrogen generation in this particular system. FESEM imaging confirmed the formation of nantotubular structures of TiO2 with average inner diameter of 80 nm, wall thickness of 20 nm, and length of approximately 2.5 mm. The generated nanotube arrays were of anatase structure with crystallite size within the range of 22e30 nm. Pt was successfully deposited on the surface of TNTA, as corroborated by EDX spectra, elemental mapping and TEM analysis. Band gap narrowing upon Pt loading was implied by UVeVis DRS analysis, resulting in a band gap value of 2.93 eV, notably lower than a typical value of 3.2 eV associated with anatase. The photocatalyst sample with Pt deposited via a photo-assisted deposition method (Pt-TNTA-PDC) evidently outperformed its bare TNTA counterpart in producing hydrogen by 4.7 times, while that with Pt deposited by chemical reduction could improve H2 production by 3.8 times. During photocatalytic operations, glycerol served an important purpose in suppressing electron-hole recombination by providing holes an oxidative target which is less energy-demanding than water. It is proposed that glycerol underwent dehydrogenation and decarbonylation processes producing ethylene glycol, followed by dehydration and oxidation towards acetic acid, before transforming into H2 and CO2, eventually. We suggest that Pt plays a role not only in the enhancement of H2 photoproduction, but also in governing the direction of reactions, hence the intermediates and final products.