Photocatalytic hydrogen production
In addition to biological process, photocatalytic oxidation of ethanol provides alternative interesting approach to generate hydrogen. Similar to photo-fermentation where enzyme is used to catalyze the conversion, solar energy is again utilized to offer sufficient power to produce hydrogen from ethanol under the facilitation of inorganic catalyst. Among many catalysts documented in the literature, TiO2 [8-10] is the most commonly used catalyst base due to its excellent photoreactivity which has a suitable band gap for efficient light photon absorption. Upon radiation, the electron contained in a semiconductor such as TiO2 will be excited and transferred from valence band to conduction band, resulting in the creation of an electron-hole pair and in turn providing an active site for redox reaction. As shown in Figure 1, reaction (1) is a typical redox reaction where H2O serves as the oxidant to oxidize ethanol while itself being reduced to H2. The adsorbed ethanol and water species will react with each other on the surface of the active sites of the synthesized photocatalyst to produce H2. Usually, certain amount of active metal (noble metal or transition metal) will be loaded to the TiO2 support to promote its photoactivity. According to the publications, Cu, Ni, V, Pt, Pd, Rh, Au, Ir, and Ru have been tested [11-14], among which Pt doped TiO2 exhibits the highest photoactivity toward hydrogen production from bioethanol. Various synthesis methods have been successfully demonstrated to get TiO2 supported catalyst with desirable particle size and morphology for hydrogen generation maximization. Besides TiO2 supported catalyst, there are multiple other novel semiconductors being developed recently for effective hydrogen production including CdS , VO2 , WO3 , and ZnSn(OH)6 . Nevertheless, the hydrogen production efficiency from catalytic ethanol oxidation still remains at very low level probably due to two facts: the fast recombination rate of the created electron-hole pairs and the low photon absorption efficiency at visible light range. Although hydrogen evolution rate of 21 mmol/gcat/h has been reported and is the fastest rate claimed so far in the literature , it is still significantly lower than that obtained from thermochemical ethanol conversion. Therefore, the technical breakthrough is required in the field of photocatalysis before the commercialization of this technique can be seriously considered.
Fig. 1. The schematic diagram of photocatalytic ethanol reforming