Photocatalytic CO2 reduction vs. hydrogen production from CH3OH/H2O vapours with a series of metal-modified titanias

Although CO2 photoreduction to formaldehyde and methanol in purified water was first reported in 1979, great interest in this reaction has grown only in the last few years. However, the reaction rates are still very low, the best values being around 10 μmol h-1 gcat-1 for the major products (usually methanol, formic acid or formaldehyde for the reaction in the liquid phase, methane for the gas phase reaction). A series of naked or noble metal (Au, Pt)-modified titanium dioxide photocatalysts, synthesised by deposition of preformed noble metal nanoparticles on TiO2 P25 or containing copper, was tested for CO2 photoreduction and for hydrogen production by photosteam reforming of methanol, in a closed recirculation laboratory scale apparatus. The photocatalytic activity of the investigated series of semiconductors was found to follow the same trend in both photocatalytic reactions. TiO2 modified by deposition of 0.5 wt% Pt and 1 wt% Pt on P25 showed a 20-fold and 50-fold photoactivity increase in hydrogen production, respectively, with respect to naked TiO2. CH4 production rate from CO2 photocatalytic reduction almost doubled by increasing the Pt amount from 0.5 wt% to 1 wt%. Platinum confirmed to be a better co-catalyst than gold in both reactions. The addiction of Pt nanoparticles led to a remarkable decrease of selectivity to CO (SCO) in photosteam reforming of methanol, indicating increased full methanol oxidation up to CO2, whereas Pt addition on TiO2 favoured the complete reduction of CO2 up to CH4. By considering the preliminary results obtained with Cu-modified titania, we are presently investigating a series of home made 0.5 wt% Pt-modified TiO2 materials with different Cu content, aiming at shading light on the role of copper and copper oxidation state in both investigated reactions.