One step flame made fluorinated Pt/TiO2 photocatalysts for hydrogen production

The photocatalytic production of hydrogen from methanol steam reforming was investigated over a series of fluorinated Pt/TiO2 photocatalysts, labelled as Pt/TiO2-F(X), with X indicating the nominal O molar substitution with F ranging from 5% to 15%, which were synthesised in continuous and single step by flame spray pyrolysis. The so obtained photocatalysts were characterized by BET, XRD, XPS, UV-vis analyses and in situ X-ray Absorption Spectroscopy (XAS) at the Pt L3-edge at the XAS beamline of the ANKA synchrotron. All of the flame made photocatalysts were mostly composed of anatase (ca 95%) with low amounts of rutile and possessed a surface area of ca 110 m2 g-1 which increased up to ca 140 m2 g-1 in the fluorinated samples. XPS analysis confirmed the bulk substitution of oxygen with fluorine. XAS revealed that Pt was mostly in oxidised form in the as prepared samples. However, in situ XAS showed that Pt readily reduced to the metallic form under reaction conditions. The photocatalytic tests were performed in a closed recirculation apparatus with the photocatalyst bed continuously fed with methanol/water vapours. Methanol underwent oxidation up to CO2 through the formation of formaldehyde and formic acid as intermediate species. Carbon monoxide, methane, methyl formate, acetaldehyde and dimethyl ether were identified as side products. Hydrogen and all of the other products evolved at constant rate, which significantly increased upon noble metal addition. As shown in the Figure, 5% O substitution with F led to an increase in the H2 and CO2 production rates. Above this value the H2 production rate decreased linearly with increasing the nominal F loading. A significant increase of selectivity to CO2, from 35% to 45%, was obtained over the most performing fluorinated photocatalyst with respect to the F-free one. Two different reaction paths were shown to be involved in methanol oxidation during the photo-steam reaction: i) a direct, valence band hole mediated path and ii) an indirect hydroxyl radical mediated path, which is more selective toward CO2 production. Thus, present results suggest that TiO2 fluorination favours the indirect mechanism by increasing the surface hydroxyl radical formation. On the other hand, an excess of fluorination introduces structural defects, acting as electron-hole recombination centres and leading to a decreasing hydrogen production.