Data di Pubblicazione:
2018
Citazione:
Development of innovative photoreactors and photocatalytic processes for hydrogen production / I. Rossetti, E. Bahadori, G. Ramis. ((Intervento presentato al convegno European Hydrogen Energy Conference tenutosi a Malaga nel 2018.
Abstract:
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 agents [1], such as organic compounds obtained through the photoreduction of CO2 or the photoreforming of organic solutions, e.g. carbohydrate containing hydrolysed substrates.
The attention was mainly focused on photoreactor design and the relative process, i.e. on the development of suitable devices that could be rather easily scaled up and that can maximize the hydrogen productivity. In particular, for the photoreduction of CO2 a fully innovative photoreactor was realized, able to operate up to 20 bar pressure. This boosted the solubility of CO2 and, thus, its conversion to regenerated fuels and hydrogen. This approach allowed us to explore high pressure and high temperature operating conditions, which are unconventional for photocatalysis. On the other hand, the optimization of a photocatalytic process based on highly non-ideal, concentrated, sugar-based solutions is non trivial. At last, calculations were done to size a full scale reactor able to harvest solar light for hydrogen production in different geographic zones.
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%), Ag and Pd (0.1 mol%). Photocatalytic testing was carried out on a bench scale reactor 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 in a dedicated 1.2 L reactor, 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 group [2-4]. 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 was 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 organic products accumulate until Na2SO3 is present, then, organics start to convert through photoreforming generating H2, possibly CO and CO2 [2,3]. 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
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
I. Rossetti, E. Bahadori, G. Ramis
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