Photoreactors Design in the Exploitation of Biorefinery Processes: the Case of Hydrogen Production

INTRODUCTION The production of hydrogen through photoreforming of aqueous solutions of organic compounds is considered as a way to exploit solar energy storage in the form of hydrogen. The photocatalytic reforming occurs through the following general reaction: which is promoted by a photocatalyst. In this work, we dealt with the use of different sugars, namely glucose, xylose and arabinose, as well as levulinic acid. They were used as examples of compounds that may be rather easily obtained from the hydrolysis of biomass. Our attention was predominantly focused on the development of innovative reactors, possibly operating under unconventional conditions, with fine tuning of the operation parameters, rather than of materials properties. EXPERIMENTAL/THEORETICAL STUDY The selected photocatalysts were based on TiO2, since the focus was reactor optimization. The materials were prepared by flame spray pyrolysis and compared with commercial samples of nanostructured TiO2 P25 by Evonik. Different metals, such as Cu and Au, with loading ranging from 0.1 to 0.5 wt% were added as co-catalysts. The role of the metals was that of electron sinks, to inhibit the electron-hole recombination and they were also selected due to the formation of a plasmon resonance band which improves visible light absorption. The samples were characterized by N2 adsorption-desorption, X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and temperature programmed reduction/oxidation (TPR/TPO). The photoreforming reaction was carried out in different prototypes of photoreactors specifically developed in our lab. RESULTS AND DISCUSSION In one reactor configuration, an external 200 W lamp was used, with emission wavelengths centred around 365 nm. A first photoreactor was developed with internal capacity ca. 0.3 L, with big head space for gas collection and very efficient mixing of the suspension thanks to an optimized length/diameter ratio (L/D) ca. 2. A drawback was the poor irradiation efficiency of the suspension, which limited the overall productivity, irrespectively or the substrate. The same UVA radiation was tested also in a home designed photoreactor with an immersion lamp (75 or 150 W, coaxial with the reactor). Two reactor sizes were tested, ca. 200 ml or 1.5 L. A significant amount of H2 was obtained with very simple catalyst formulations, e.g. 11 mol kgcat-1 h-1 were obtained at 4 bar, 80 ˚C over commercial TiO2 samples and using glucose as hole scavenger. This result is very remarkable with respect to similar research in conventional photoreactors. CONCLUSION In the present work we developed different prototypes of photoreactors to accomplish hydrogen production from biomass derived organic compounds and for the photoreduction of CO2 for the regeneration of fuels and chemicals. Reactor modelling is in progress for both applications, including the optimization of radiation distribution in the photoreactor. ACKNOWLEDGMENTS Fondazione Cariplo / Regione Lombardia (UP-Unconventional Photoreactors) and MIUR (HERCULES - Heterogeneous robust catalysts to upgrade low value biomass streams) are gratefully acknowledged for financial support.