Photocatalytic Hydrogen Production on Cu(II) Pre-grafted Pt/TiO2

Visible light activation of TiO2 in oxidation reactions was recently achieved by grafting transition metal ions including Fe, Cu, Cr and Rh mainly on the rutile TiO2 surface. Aim of the present work is the preparation and characterization of similarly grafted TiO2-based photocatalytic materials, to be employed in hydrogen production from water-methanol vapor mixtures. A series of Cu(II)/TiO2 photocatalysts has been prepared by the impregnation method, starting from commercial P25 TiO2 and Cu(NO3)2 as Cu precursor, with nominal Cu/TiO2 ratios ranging from 0.05 to 0.5 wt.%, and further modified by 0.5 wt.% Pt nanoparticles deposition, by the deposition-precipitation method in the presence of urea. The photocatalysts were labeled as Pt/Cu(X)/T, with X referring to the nominal weight percent of copper. Diffuse reflection spectra of Cu(II)-grafted materials exhibit an absorption at 700-800 nm, related to Cu(II) d-d transitions, and an absorption shoulder at ca. 450 nm, which can be ascribed to electron interfacial charge transfer (IFCT) from the valence band of TiO2 to grafted Cu(II) species. Cu XANES spectra of samples Pt/Cu(0.3)/T and Pt/Cu(0.5)/T are similar to that of CuO, whereas that of Pt/Cu(0.1)/T is similar to that of metallic copper, indicating that copper in mainly present in metallic form in this case. Pt XANES spectra indicate that Pt is present in both reduced and oxidized state. Metal nanoparticles, well dispersed on the TiO2 surface, are evidenced in HR-TEM images and STEM-HAADF analysis confirms that these consist of 1-3 nm Pt nanoparticles, Cu possibly being dispersed in small clusters anchored on the titania surface. The photocatalytic activity of the materials in hydrogen production by photosteam reforming of methanol was investigated in a recirculating apparatus, with a xenon lamp as irradiation source. By considering the rate of hydrogen production and the percent selectivity to CO and to CO2 obtained by on line gas chromatographic analysis with naked TiO2 (T) or reference samples modified by only Cu (Cu(0.1)/T) or Pt deposition (Pt/T), a bell-shaped photoactivity trend was obtained with increasing the Cu content, with a maximum for Cu(0.1)/T and synergistic effects on photoactivity induced by the co-presence of Cu and Pt on the TiO2 surface could be outlined. The origin of such effects is discussed in relation to the oxidation state of the metals and to their dispersion on the TiO2 surface.