Photocatalytic activity of Cu(II) pre-grafted Pt/TiO2 in hydrogen production

In order to induce visible light activation of TiO2 by surface grafting of transition metal ions , a series of Cu(II)/TiO2 photocatalysts has been prepared by the impregnation method starting from commercial P25 TiO2 and Cu(NO3)2 as precursor, in order to obtain nominal Cu/TiO2 ratios ranging from 0.05 to 0.5 wt.%. Such materials were successfully employed in gas phase photocatalytic reactions, copper ions having been observed to be released in the aqueous phase during the photocatalytic degradation of formic acid in aqueous suspension. The photocatalytic activity of these Cu(II)/TiO2 systems, further modified by Pt nanoparticles surface deposition (0.5 wt.% nominal amount, attained by deposition-precipitation in the presence of urea), was investigated in hydrogen production from methanol/water vapour mixtures, according to the reaction: CH3OH + H2O  CO2 + 3 H2 (photo-steam reforming of methanol). A solar simulator (xenon arc lamp) was employed as irradiation source. The rate of H2 production, rH2, and the percent selectivity to CO2 and to CO, %SCO2 and %SCO, respectively, obtained on the investigated samples (labelled as Pt/Cu(X)/T, with X referring to the nominal Cu/TiO2 wt.%) are reported in the table. Results obtained with naked TiO2 (T) or reference samples modified by only Cu (Cu(0.1)/T) or Pt (Pt/T) are also reported for comparison. Pt/TiO2 systems were more active than the corresponding bare or Cu-modified oxides. This confirms the well-known positive role of the noble metal, which is mainly related to an increased efficiency of electron-hole separation by ‘capturing’ conduction band electrons. Most interestingly, a bell-shaped photoactivity trend is observed for the Pt/Cu-modified titania with increasing the Cu content, with a maximum for sample Pt/Cu(0.1)/T, containing 0.1 wt.% of copper. Synergistic effects on photoactivity induced by the copresence of Cu and Pt on the TiO2 surface can be outlined. In particular, the highest photoactivity was attained for the sample characterized by smaller noble metal nanoparticles, homogeneously distributed on the oxide surface as evidenced by HRTEM analysis, and showing both metals in the almost complete reduced state, as indicated by EXAFS analysis. Thus, visible light sensitization by copper clusters, which was investigated in photocatalytic oxidation reactions so far, was also confirmed in hydrogen production from water solutions. In order to get a better insight into the role played by TiO2 surface modification with Cu and Pt on both reduction and oxidation photocatalytic processes, the here investigated materials are presently tested also in the gas phase acetaldehyde photodegradation under UV-vis irradiation.