Data di Pubblicazione:
2018
Citazione:
Photoreactors design in the exploitation of biorefinery processes / I.G. Rossetti, E. Bahadori, G. Ramis. ((Intervento presentato al 4. convegno International Conference on Catalysis for Biorefineries tenutosi a Lione nel 2017.
Abstract:
1. Introduction
The production of renewable fuels can be achieved by photocatalysis in the frame of an integrated biorefinery. In this context, we deal with two main processes. At first, 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. It can be used either as energy vector or as a reactant in the biorefinery itself, such as for hydrotreating processes, for the Fischer Tropsch synthesis of higher hydrocarbons, or for much less mature, still under development processes, such as those aiming at lignin valorization.
The photocatalytic reforming occurs through the following general reaction1:
which is promoted by a photocatalyst, i.e. a semiconductor, which can drive the H+ ion reduction thanks to an electron which is photopromoted from the valence to the conduction band of the material upon light absorption. To close the circuit, the hole which is formed in the valence band should be filled with electrons coming from different reactive species, that is, holes oxidise other reactants. In principle holes may oxidise water, but the reaction is very slow and not competitive with the very fast relaxation phenomena occurring in the semiconductor. Therefore, is more readily oxidisable compounds are available in solution, the may consume holes, making hydrogen production faster. Many efforts have been devoted to the research of suitable materials for the photocatalytic H2 production and to the screening of various organic sacrificial agents, predominantly focusing on simple molecules. 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.
In the second example, we investigated the photoreduction of CO2 as a mean to regenerate fuels in form of methane and methanol, depending on the photocatalyst and operating conditions adopted2. Interestingly, the liquid phase products of this reaction include different amounts of HCOOH, HCHO and CH3OH, which may undergo photoreforming themselves, thus allowing the formation in gas phase of a substantial amount of H2. This makes the two reactions very tightly correlated.
For both reactions 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.
2. Experimental
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 Pt, Pd, Ag and Au, with loading ranging from 0.1 to 0.5 wt% were either added during the FP synthesis or added by post synthesis impregnation. The role of the metals was that of electron sinks, to inhibit the electron-hole recombination. Some of them were also selected due to the formation of a plasmon resonance band which improves visible light absorption (Ag, Au), other due to their specific affinity for H2. 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, as described in the next section.
3. Results and discussion
As for photoreforming, in every case, an external 200 W lamp was used, with emission wavelengths centred around 365 nm. A first photoreactor was developed with internal capacity ca. 0
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
I.G. Rossetti, E. Bahadori, G. Ramis
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