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 samples (F for O nominal molar substitution ranging from 5 to 15 at%) synthesized by flame spray pyrolysis in single step. The so obtained photocatalysts were characterized by BET, XRD, XPS, UV-vis absorption analyses and in situ X-ray Absorption Spectroscopy (XAS) at the Pt L3-edge. XPS analysis confirmed the presence of both surface and bulk fluorine. XAS revealed that Pt, mostly in oxidized form in the as prepared samples, readily reduced to the metallic form under gas-phase photocatalytic reaction conditions. Photocatalytic hydrogen production tests showed that 5 at% F for O substitution in TiO2 led to an increase in the H2 and CO2 production rates, whereas above this value the H2 production rate decreased linearly with increasing the nominal F loading in the photocatalyst. A significant increase of selectivity to CO2, from 35% to 45%, was obtained over the best performing fluorinated photocatalyst with respect to the flame-made F-free one. Thus, TiO2 fluorination favors the indirect methanol photo-oxidation mechanism by increasing surface hydroxyl radical formation with a consequent increase of selectivity toward CO2 production. On the other hand, excessive fluorination introduces structural defects, acting as electron-hole recombination centers, and may hinder interface electron transfer, because of the F-induced surface electronegativity. Both effects concur to decrease photo-activity.