H2 production by photoreforming of glucose

H2 production by photoreforming of glucose F. Contea, G. Casalinia, A. Tripodia, G. Ramisb and I. Rossettia a Chemical Plants and Industrial Chemistry Group, Dip. Chimica, Università degli Studi di Milano, CNR-ISTM and INSTM Unit Milano-Università, via C. Golgi 19, 20133 Milan; b Dip. Ing. Chimica, Civile ed Ambientale, Università degli Studi di Genova and INSTM Unit Genova, via all’Opera Pia 15A, 16145 Genoa A possible technology to valorise the sugars-rich processed liquor of pulp industry is the photoreforming of carbohydrates, i.e. water splitting for H2 production assisted by organic molecules acting as hole scavengers (HS) in substitution of the slow oxygen oxidation reaction. In this way electron-hole separation is prompted under illumination, improving H2 evolution productivity. The aim of this work was to perform catalysts screening to assess the best performance among different surface decorated samples of nanosized titania. The effect of thermal treatments to induce structural modifications and defects formation was also explored. The catalysts were prepared using either commercial P25 titania by Evonik or a home-made nanostructured titania prepared by flame spray pyrolysis. Metal decorated samples were prepared by adding 0.1-1% noble metals (Pt, Pd, Au, Ag) by impregnation or sol immobilisation. Samples containing 0.1% Pt were subject to specific reduction/calcination treatments for 2-8 h at temperature 400-700°C either in H2 or N2 to check the effect of morphologic, structural and textural modifications or the possible partial reduction of Ti4+ to Ti3+ with formation of oxygen vacancies. Activity tests were carried out in an AISI 316 head-flanged micro-pilot batch photo-reactor with cylindrical shape. UV radiation was provided by a 125 W double bulbed medium pressure Hg vapour lamp, located vertically in the axis of the reactor. The desired temperatures were reached by flowing heating/cooling water into the reactor jacket from an heating/cooling bath. Operating conditions were: Temperature 80°C, Pressure 4 bar of N2, HS concentration 5-15 g/L, Photocatalyst concentration 0,25 g/L, Testing time 5h. pH was always kept neutral or naturally established. The highest H2 productivity and glucose conversion achieved among all the catalysts was obtained with a 0.1% mol Pt/P25 catalyst that underwent a post-activation thermal annealing at 400 °C in N2 for 4h. H2 productivity was 4.3 mol kgcat-1 hirr-1, in accordance with literature reported results published until now regarding glucose photoreforming (maximum value reported for metal loaded catalysts was ca. 5.7 mol kgcat-1 hirr-1), while glucose conversion was 13 % after 5 h irradiation. Simulated processed black Kraft liquors were taken as an example of spent biomass derived waste water that can be a possible substrate for photoreforming, to produce a valuable energy vector while depurating water. The liquors were photoreformed using 1,0% wt Au6Pt4/P25 and 0,1% mol Pt/P25, two of the best performing catalysts with model carbohydrates. In both cases H2 productivity was lower than with pure glucose, while CO2 and ethane were the main products (2.7 and 3.4 mol kgcat-1 hirr-1 respectively for 1.0 wt% Au6Pt4/P25, and 3.1 and 4.6 mol kgcat-1 hirr-1 for 0.1% mol Pt/P25). This may be due to the combined effect of photo-Kolbe decarboxylation, α-carbon radical dimerization, dicarboxylic acid intermediates dehydration and hydrogenation and finally pHPZC H+ shielding effect. Carbon conversions to gaseous products were both higher than 15%, suggesting easier degradation pathways for hydroxy-dicarboxylic acid derived from sugar degradation. Simulated processed sulfite spent liquors were photoreformed on 0.1% wt Pt/P25. H2 and by-products productivities were instead very low. This can be ascribed to sugar surface crowding effect due to too high HS concentration, which could cause active site blocking. Moreover, since liquor pH is acid and lower than pHPZC of TiO2, an H+ repulsive shielding effect prevents H2 evolution. Catalysts characterisation confirmed that the coexistence of a double anatase/rutile phase was determinant to achieve sufficiently high activity, together with a significant role of titania reduction, with formation of oxygen vacancies.