Photocatalytic H2 production over flame made fluorinated Pt/TiO2

The photocatalytic production of hydrogen from methanol steam reforming was investigated over a series of fluorinated Pt/TiO2 (nominal O molar substitution with F ranging from 0% to 15%) synthesised in continuous and single step by flame spray pyrolysis. All photocatalysts were mostly composed of anatase with low amounts of rutile and possessed a surface area of ca. 110 m2/g which increased up to ca. 140 m2/g in the fluorinated samples. XPS analysis confirmed both the surface and bulk presence of fluorine. Moreover, the spectra in the O 1s region revealed an exceptional OH surface coverage in the samples with lower F content. In situ XRD showed no particular difference in the thermal stability upon annealing in He stream, anatase to rutile phase transition occurring above 600\ub0C for both the undoped and the F-doped samples. However, a significant reduced volume of the crystal unit cell of both the anatase and the rutile phases was observed for the F-doped samples as obtained by Rietveld refinement of the XRD patterns. This result can be explained considering the higher electronegativity of F compared to O, causing a slight shrinking of the Ti-F bond length. XAS revealed that Pt was mostly in the oxidised form in the as prepared samples. However, in situ XAS showed that Pt readily reduced to the metallic form under photocatalytic 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. Hydrogen evolved at constant rate, which significantly increased upon noble metal addition. 5% nominal O substitution with F led to an increase in the H2 and CO2 production rates. Above this values the H2 production rate decreased linearly with increasing the nominal F content. A significant increase of selectivity to CO2, from 35% to 45%, was obtained over the best performing fluorinated photocatalyst with respect to the F-free one. Two reaction paths were shown to be involved in methanol oxidation during the photo-steam reforming reaction, i.e. a direct valence band hole mediated path and indirect hydroxyl radical mediated path, which is more selective toward CO2 production. TiO2 fluorination seems to favour the indirect mechanism by increasing the surface hydroxyl radicals formation, whereas excess fluorination introduces an excess of bulk structural defects, acting as electron-hole recombination centres and leading to a reaction rate decrease