In the last decades photo-catalysis studies have been increased for their potential applications in energy production by renewable and non-polluting sources. The interest in photo-catalysis became more concrete when the direct conversion of solar energy into chemical energy via photochemical routes has been found to be more effective in heterogeneous systems. The industrial application of these photochemical reactions, however, requires not only the scientific knowledge of the photochemistry of the reaction, but also a rational understanding of the engineering aspects of the photochemical process. The design of a photo-reactor is based on a careful evaluation of the factors which can influence the reactor performance: light source, geometrical configuration, material of construction, heat exchange and mixing and flow characteristics. In conventional reactors the conversion for a given reactor volume is independent of geometry, if the effects due to flow characteristics are neglected. However, for photo-reactors, reactor geometry and the spatial relation between reactor and light source are most important. The geometrical configuration of a photo-reactor is usually chosen in a way to attain maximum benefit from the pattern of irradiation. In this work a modular photochemical system was developed in order to evaluate the photo-catalytic performance of nano-structured titania (TiO2) thin films, prepared by anodization, in hydrogen evolution by water splitting. The photo-reactor was designed by taking into account the particular heterogeneous essence of the process: photo-catalytic substrates, prepared as thin films, should have well designed characteristics, such as good absorption of photons of right energy, a long lifetime of activated states, a good adsorption of reactants, and a relatively easy desorption of products; but in order to maximize the photo-catalyst activity, a correct design of the photo-reactor should also be adopted, in terms of geometrical configuration and operating conditions. The photo-reactor (150 ml), constructed in Pirex, was equipped with a side quartz window in the way of UV-visible rays, coming from an external lamp (300 W Xe-arc lamp), reaching on radial direction the photo-catalytic-film hanging from the reactor head and immersed into the water. The highly ordered nano-structured titania thin films were prepared by controlled anodic oxidation of titanium foils. The essence of method may be described as a reconstruction of a thin titania layer (formed initially by oxidation of a titanium foil) which occurs under the application of a differential potential which creates also strong local electrical fields at the surface. The 1D nano-structured array obtained, in shapes of nanotubes or nanocoils, may significantly modify the adsorption of reactants and stabilization of transition state complexes. The possibility to control of the catalyst multi-functionality through the ability of controlling the nano-architecture represents an interesting opportunity to bridge the material gap in catalysis between ordered crystals used in surface studies and real catalysts. The surface characterization was made by different techniques, such as FESEM, TEM, GAXRD, UV-Vis diffuse reflectance and current-time transients during the anodization process. Characterization results showed that the nano-structured thin films reveal amorphous titania after anodization, the anatase phase appearing only after annealing up to 500°C, and that the voltage applied during the anodization (5-20 V) influences the mean dimension of these nanostructures, their wall thickness and also their degree of packing. Photo-catalytic results showed that hydrogen evolution strongly depends on the quantity of titania anodized on the titanium substrate. Work is in progress to synthesize a thicker or a multi-layer nano-structured film by anodizing at higher voltage (60-100 V) and for a longer period of time. Moreover a new experimental setup has been developed for water photo-electrolysis measurements, with the titania nano-structured arrays used as photo-anode. This configuration permits the separation of hydrogen and oxygen, contemplating also the in-situ measurements of the generated current.

Nano-engineered materials for H2 production by water photo-electrolysis

AMPELLI, Claudio;PASSALACQUA, Rosalba;PERATHONER, Siglinda;CENTI, Gabriele
2009-01-01

Abstract

In the last decades photo-catalysis studies have been increased for their potential applications in energy production by renewable and non-polluting sources. The interest in photo-catalysis became more concrete when the direct conversion of solar energy into chemical energy via photochemical routes has been found to be more effective in heterogeneous systems. The industrial application of these photochemical reactions, however, requires not only the scientific knowledge of the photochemistry of the reaction, but also a rational understanding of the engineering aspects of the photochemical process. The design of a photo-reactor is based on a careful evaluation of the factors which can influence the reactor performance: light source, geometrical configuration, material of construction, heat exchange and mixing and flow characteristics. In conventional reactors the conversion for a given reactor volume is independent of geometry, if the effects due to flow characteristics are neglected. However, for photo-reactors, reactor geometry and the spatial relation between reactor and light source are most important. The geometrical configuration of a photo-reactor is usually chosen in a way to attain maximum benefit from the pattern of irradiation. In this work a modular photochemical system was developed in order to evaluate the photo-catalytic performance of nano-structured titania (TiO2) thin films, prepared by anodization, in hydrogen evolution by water splitting. The photo-reactor was designed by taking into account the particular heterogeneous essence of the process: photo-catalytic substrates, prepared as thin films, should have well designed characteristics, such as good absorption of photons of right energy, a long lifetime of activated states, a good adsorption of reactants, and a relatively easy desorption of products; but in order to maximize the photo-catalyst activity, a correct design of the photo-reactor should also be adopted, in terms of geometrical configuration and operating conditions. The photo-reactor (150 ml), constructed in Pirex, was equipped with a side quartz window in the way of UV-visible rays, coming from an external lamp (300 W Xe-arc lamp), reaching on radial direction the photo-catalytic-film hanging from the reactor head and immersed into the water. The highly ordered nano-structured titania thin films were prepared by controlled anodic oxidation of titanium foils. The essence of method may be described as a reconstruction of a thin titania layer (formed initially by oxidation of a titanium foil) which occurs under the application of a differential potential which creates also strong local electrical fields at the surface. The 1D nano-structured array obtained, in shapes of nanotubes or nanocoils, may significantly modify the adsorption of reactants and stabilization of transition state complexes. The possibility to control of the catalyst multi-functionality through the ability of controlling the nano-architecture represents an interesting opportunity to bridge the material gap in catalysis between ordered crystals used in surface studies and real catalysts. The surface characterization was made by different techniques, such as FESEM, TEM, GAXRD, UV-Vis diffuse reflectance and current-time transients during the anodization process. Characterization results showed that the nano-structured thin films reveal amorphous titania after anodization, the anatase phase appearing only after annealing up to 500°C, and that the voltage applied during the anodization (5-20 V) influences the mean dimension of these nanostructures, their wall thickness and also their degree of packing. Photo-catalytic results showed that hydrogen evolution strongly depends on the quantity of titania anodized on the titanium substrate. Work is in progress to synthesize a thicker or a multi-layer nano-structured film by anodizing at higher voltage (60-100 V) and for a longer period of time. Moreover a new experimental setup has been developed for water photo-electrolysis measurements, with the titania nano-structured arrays used as photo-anode. This configuration permits the separation of hydrogen and oxygen, contemplating also the in-situ measurements of the generated current.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11570/2628976
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