H2 production by methanol photo-steam reforming on flame-made Cu and Cu-Pt modified TiO2 materials

In this work, we studied the effect of Cu or Cu-Pt modification on TiO2 photocatalysts prepared by both flame spray pyrolysis [1] and impregnation techniques (grafting). Two different series of samples were prepared: i) titanium dioxide modified with Cu (ranging from 0.05 up to 0.5 wt.%) and ii) titanium dioxide modified with both Cu (from 0 to 0.5 wt.%) and Pt (fixed at 0.5 wt.%). The two series were labelled as TCX and TPCX, respectively, where X stands for the nominal percent Cu amount. These photocatalysts were characterised by UV-vis DRS, XRPD and BET analyses. Their photocatalytic activity was investigated in a thermodynamic up-hill reaction for solar fuels productions, i.e. hydrogen production from methanol photo-steam reforming [2], according to the reaction CH3OH + H2O → 3H2 + CO2. As shown in the table, the use of copper-modified TiO2 materials led to a ca. doubled hydrogen production rate rH2 with respect to bare titanium dioxide (T), despite their absorption properties, phase composition and specific surface area did not changed significantly. Such photoactivity enhancement was obtained for the whole series of Cu-modified materials. Furthermore, the presence of Pt in the TPCX series caused a significant photoactivity increase with respect to the Cu-only-modified materials (TCX series), confirming the high efficiency of Pt nanoparticles in promoting electron/hole separation by “capturing” conduction band electrons. The co-presence of small amounts of Cu together with Pt (see TPC0.05 vs. TPC0.0) led to a ca. 25% increase of H2 production rate. By contrast, a linearly decreasing photoactivity trend was observed for higher Cu loadings in the TPCX series, probably due the formation of a Pt-Cu alloy with consequently reduced photopromoted electron trapping efficiency [3]. On the other hand, the remarkable photoactivity enhancement observed in the presence of small Cu amounts in the TPCX series could be explained by the ability of Cu of accepting electrons both from the TiO2 conduction band and by interfacial charge transfer [4]. This results in an increased oxidative power of the photocatalyst, with consequent reduced selectivity to CO. Samples obtained by wet-chemistry technique provided higher H2 production rates (up to 30 mmol·h-1·gcat-1) than those produced by flame spray pyrolysis (see table), probably due to the higher Cu dispersion onto the semiconductor oxide obtainable by the impregnation method, and also because no Pt-Cu alloy could be formed in this case, the two metals being deposited separately.