Photocatalytic Production of Hydrogen

The photocatalytic production of hydrogen from water solutions has been thoroughly investigated in recent years, either employing a recirculation laboratory scale apparatus with photocatalysts immobilized on porous supports and continuously fed with vapor, or in a two compartments cell, which allows the separate production of hydrogen and oxygen from water photosplitting. A series of TiO2-based photocatalysts, also modified by noble metals and synthesized either in single step by flame spray pyrolysis, or by photodeposition, or by deposition of preformed noble metal nanoparticles on TiO2, has been systematically tested in the first apparatus, usually employing methanol as sacrificial agent, i.e. in a low temperature steam reforming reaction. In the attempt to extend the photocatalytic response into the visible region, a series of NH4F-doped, noble metal-modified TiO2 photocatalysts, exhibiting good performances in the photocatalytic oxidation of organics, has been tested also in the photocatalytic production of hydrogen from water solutions. Hydrogen was always found to evolve at constant rate, which significantly increased upon noble metals addition, Pt being the most effective co-catalyst, especially if deposited by flame spray pyrolysis in one step, followed by gold and silver, according to the difference between their work function values and that of titania. A kinetic model for the oxidation processes occurring in parallel to hydrogen production has been proposed,6 based on formaldehyde, formic acid and CO2 production rate profiles as a function of methanol partial pressure. The photoactive material in the recently set up two compartments cell consisted in a thin titanium dioxide layer deposited on the irradiated face of a titanium disk in contact with a basic solution. Oxygen evolved at constant rate from the illuminated TiO2 surface, whereas hydrogen evolution occurred on the opposite side of the disk, which was modified by Pt deposition. The already encouraging rates of pure H2 production attained employing aqueous solutions in the two compartments have been increased in the presence of electron donors more efficient than water in contact with the illuminated photoanode. Recent efforts have been focused on the optimization of the cell operation conditions and on the development of more efficient photocatalytic thin layers possibly extending photoactivity to a larger portion of the solar spectrum.