H2 production by methanol photo-steam reforming on Pt-modified TiO2: effect of Cu(II) grafting pre-treatment

Aiming at producing 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/Ti ratios ranging from 0.05 to 0.5 wt.%. The copper ions release from these hybrid materials, observed during the photocatalytic degradation of formic acid in aqueous suspensions, suggested a better use of such materials for gas phase photocatalytic reactions. Therefore the photocatalytic activity of these Cu(II)/TiO2 systems, further modified by Pt nanoparticles surface deposition (0.5 wt.%), was investigated in hydrogen production from methanol/water vapour mixtures, according to the reaction: CH3OH + H2O  CO2 + 3 H2. The rate of H2 production and the selectivity to CO2 and to CO obtained on the investigated samples (labelled as Pt/Cu(X)/T, with X referring to the nominal Cu/TiO2 wt.%) have been determined. 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. 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. Intriguing 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. 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 studied also in the gas phase acetaldehyde photodegradation under UV-vis irradiation.