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 on the rutile TiO2 surface. This work deals with the preparation and characterization of grafted TiO2-based photocatalytic materials to be employed in hydriogen production from water-methanol vapor mixtures. A series of Cu(II)/TiO2 photocatalysts has been prepared by the impregnation method, with nominal Cu/TiO2 ratios ranging from 0.05 to 0.5 wt.%, and further modified by 0.5 wt.% Pt nanoparticles deposition. The photocatalysts were labeled as Pt/Cu(X)/T, X referring to the nominal weight percent of copper. Diffuse reflection spectra 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 is 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 photo-steam reforming of methanol was investigated in a recirculating apparatus, with a xenon lamp as irradiation source. The rate of hydrogen production and the percent selectivity to CO and to CO2 exhibit a bell-shaped photoactivity trend 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.