Nanostructured materials for the valorization of (waste) organic solutions and CO2 recycle for fuels by photocatalytic reforming

INTRODUCTION The direct use of solar energy is intriguing for H2 production. The direct photocatalytic water splitting (WS) is thermodynamically limited by the high Gibbs free energy (237 kJ/mol) and very low efficiency is reported for direct WS also for kinetic reasons. Sacrificial reagents, such as methanol or EDTA, can improve hydrogen productivity, but they are non renewable. Compared to WS, the photocatalytic reforming (PR) is a valid approach to produce H2 under ambient conditions and using sunlight, the cheapest energy source available on earth. PR is also thermodynamically more feasible than WS. Thus, the attention is here focused on the use of waste organic compounds to be used as sacrificial agents1, such as organic compounds obtained through the photoreduction of CO2 or the photoreforming of organic solutions, e.g. carbohydrate containing hydrolysed substrates. EXPERIMENTAL Different nanostructured materials were used. Commercial nanometric titanium dioxide P25 by Evonik was used as photocatalyst and suspended in water with a concentration of 0.25-0.75 gL-1. Alternative nanostructured materials were prepared by flame spray pyrolysis (FP) or in mesoporous form by wet template synthesis. Different metals were added as cocatalysts, e.g. Au nanoclusters (0.1-0.5 wt%) and Pd (0.1 wt%). Photocatalytic testing was carried out either at ambient pressure, using different carbohydrates and organic model molecules (methanol, glucose, xylose, arabinose, formic acid, methanol, formaldehyde and levulinic acid). These were selected because they represent the basic composition of an acid-hydrolysed fraction from cellulose. On the other hand, CO2 photoreduction was carried out at high pressure (up to 20 bar) and temperature (up to 80°C), in the presence of Na2SO3 as hole scavenger. RESULTS AND DISCUSSION The photoreduction of CO2 investigated at high pressure is a fully new approach proposed by our group2-4. CH4 can be obtained as gas phase product (e.g. with Au/TiO2 catalysts), whereas in liquid phase formic acid, formaldehyde and methanol, can be obtained in variable amount depending on the operating conditions and catalyst used. Unexpectedly, considerable H2 amount in gas phase was also obtained, sometimes as primary product. The hypothesis that it is produced from WS is ruled out by the absence of a corresponding stoichiometric amount of oxygen. Furthermore, by exploring the products distribution as a function of time, we observed that liquid products accumulate until Na2SO3 is present, the, organics start to convert through photoreforming generating H2, possibly CO and CO22,3. Productivity as high as 102 mmol h−1 kgcat−1 for H2, 16537 mmol h−1 kgcat−1 for formaldehyde and 2954 mmol h−1 kgcat−1 for formic acid were achieved when operating at a 7 bar of CO2 over the aqueous solution, 80 °C with 0.5 g L−1 TiO2 by tuning reaction time and pH. On the other hand, different organic compounds have been tested for the photoreforming under ambient conditions in batch mode by using a 0.1 wt% suspension of the photocatalyst and an organic amount of 5-16 wt% in water. Irradiation was achieved with an UV lamp, with maximum emission at 254 nm and measured irradiating power of 113 W/m2. The highest substrate conversion was obtained with 0.1 wt% Au/TiO2-rutile, which however gave a wider spectrum of intermediate products in liquid phase with respect to 0.1 wt% Au/TiO2-P25 and, therefore, lower H2 productivity. The best results obtained with the rutile TiO2 sample were 89 mmol kgcat-1 h-1 of H2, 7 mmol kgcat-1 h-1 of CO, 74 mmol kgcat-1 h-1 of CO2 . H2 productivity increased to 276 mmol kgcat-1 h-1 by using P25, wherease it dropped to 40 mmol kgcat-1 h-1 when using anatase as polymorph for TiO2. CONCLUSION Interesting productivities of regererated fuels and H2 have been obtained by selecting proper nanostructured photocatalysts and operating conditions through photoreduction of CO2 and photoreforming of various organic substrates. REFERENCES 1. I. Rossetti, ISRN Chemical Engineering, Article ID 964936, (2012). doi:10.5402/2012/964936 2. F. Galli et al., Appl. Catal. B: Environmental, 200, 386 (2017) 3. I. Rossetti et al., Catal. Sci & Technol., 5, 4481 (2015) 4. I. Rossetti et al., RSC Adv., 4, 28883 (2014) ACKNOWLEDGMENTS The financial contribution of MIUR through the PRIN2015 grant (20153T4REF) is gratefully acknowledged.