Photocatalytic production of hydrogen from carbohydrates

The photocatalytic production of H2 from water is seen as a promising way for the storage of solar light into a clean energy vector. The direct water splitting is poorly efficient and a common solution to improve hydrogen productivity is to add an organic sacrificial agent, which may be oxidised more effectively than water. In this work we have selected as organic sacrificial agents some model carbohydrates that can be obtained by hydrolysis of cellulose or lignocellulosic material. The process conditions have been at first optimised using a 1.3 L photoreactor and changing the operating conditions, including pressure, temperature, catalyst amount and pH using glucose as model reactant and commercial TiO2 (P25 by Evonik) as catalyst. The temperature showed an important effect on hydrogen productivity, since at T  60°C high glucose conversion was observed, but with negligible hydrogen yield. By contrast, at higher temperature (80°C), the consecutive reaction paths to hydrogen were fulfilled leading to appreciable productivity of H2, CO and CO2 in gas phase. In some cases, also some ethane and ethylene were observed. Then, the screening of different catalytic materials was carried out under the best reaction conditions (P = 4 bar, T = 80°C, 0.25 g/L catalyst, 5 g/L glucose, pH = 5.5). The commercial titania was compared with one prepared by precipitation (TiO2) and one prepared by flame pyrolysis (FP). The samples were added with CuO in different loading (0.2 or 1 wt%), added by impregnation (I), deposition precipitation (D) or using citrate ions as complexing agents (C). The highest hydrogen productivity was achieved with 0.2 wt% CuO prepared by impregnation over the FP-prepared titania, which led to 5 mol H2 per hour per kg of catalyst. By looking at the performance during time, a constant productivity was observed for the whole duration of the test, without any evidence of deactivation. Acnowledgements The financial contribution of MIUR through the PRIN2015 grant (20153T4REF) is gratefully acknowledged. I. Rossetti and E. Bahadori are grateful to Fondazione Cariplo and Regione Lombardia for financial support (2016-0858 – “Up-Unconventional Photoreactors”).