Photocatalytic Hydrogen Production from Aqueous Solutions on Noble Metal-Modified and/or Doped TiO2

The photocatalytic production of hydrogen from water/methanol solutions was studied on titanium dioxide and on a series of NH4F-doped TiO2 photocatalysts prepared by sol-gel synthesis, varying the dopant-loading from 0% up to 12%. The photocatalysts were tested both naked and after deposition of gold and platinum nanoparticles, performed either by the reverse micelles method or by photoreduction of metal precursors. Pt-modified undoped titania with 0-2 wt.% metal loadings was also investigated. The photocatalytic runs were carried out in a recirculation apparatus with the photocatalyst bed continuously fed with methanol/water vapor, employing a xenon lamp as irradiation source. Methanol underwent oxidation up to CO and CO2; formaldehyde and formic acid were identified as oxidation intermediates. For all of the investigated TiO2 samples, the rate of H2 production greatly increased upon Au and Pt nanoparticles deposition, because of their ability to enhance the separation of photoproduced electron-hole pairs. In particular Pt was a better co-catalyst than Au, in agreement with their work function values. No significant increase in the H2 production rate was attained with Pt loadings higher than 0.75% up to 2%. Doping of TiO2 enhanced the hydrogen production rate, with an identical bell-shaped trend with increasing the dopant content for the naked, Au- and Pt-modified titania series, with the 5% NH4F-doped sample always being the best photocatalyst within each series. Thus, the structural properties of bulk doped titania ensure a more effective separation of photoproduced charge carriers. Indeed, doping followed by annealing at 700°C led to the formation of highly crystalline pure anatase, which is more photoactive than rutile. In contrast, over-doping led a decrease of the hydrogen production rate, possibly due to an excess of structure defects.