Photocatalytic processes for sustainable hydrogen production from renewable sources

2010/2011The importance of hydrogen as an appealing energy vector, due to its high efficiency and environment-friendly use in Fuel Cells, is nowadays well recognized and documented. Nevertheless, in spite of several research activities in this field, the large-scale production of H2 is still a challenging issue in view of the possible transition to an H2-based economy. In this context, the development of materials capable of acting as multi-functional platforms for the sustainable generation, though representing a strategic target, is still far from being completely satisfied. In order to make feasible the dream of utilizing sunlight for sustainable energy production, it is of paramount importance to develop catalytic systems that are not affected by leaching or poisoning phenomena and possess a high photonic efficiency, in particular upon visible activation. Heterogeneous catalysis is a key area that can help solving this issue. Using the tools offered by nanotechnology, the tailored preparation of nanoarchitectures can lead to the obtainment of photocatalytic materials that show remarkably better performance than that currently achievable even with state-of-the-art materials. The main focus of this thesis is the preparation of such tailored photoactive materials and their characterization in order to obtain catalysts that are active and stable for the sustainable photocatalytic hydrogen production by photoreforming of biomass derived compounds as raw materials. Different synthetic approaches are developed in this work to achieve the above mentioned scopes. The materials were prepared either in the form of nanopowders with controlled morphology or of supported nanostructures. Embedding approach, in which preformed metal nanoparticles are encapsulated in porous titania, and photodeposition of metal nanoparticles over preformed tailored supporting titania were investigated for nanopowder materials. Different oxide-based materials were synthesized by Chemical Vapor Deposition (CVD) and Plasma enhanced-CVD for the supported systems. The CVD route is compatible with large-scale production, to prepare metal oxide nanostructures on Si (100), enabling the resulting metal oxide phase composition and nanoscale organization to be controlled by simple variation of the growth temperature. In addition, and more interestingly, the photocatalytic production of hydrogen on the supported catalysts upon irradiation with UV and even visible light proved that the control of the system morphogenesis is crucial to obtain good performances even in the absence of TiO2. The results obtained represent an important step forward in the exploration of new active nanosystems for the conversion of solar light into storable chemical energy. All the findings significantly contributed to the development of photocatalytic materials for hydrogen production.XXIV Ciclo196

Photoassisted H2 production by metal oxide nanomaterials fabricated through CVD-based approaches

The development of efficient photocatalytic nanomaterials is of outstanding importance for the conversion of electromagnetic radiation into chemical energy. Among the various technological options, photoreforming of aqueous media containing suitable organic oxygenates is an attractive strategy towards a sustainable H2 generation. In this regard, the present contribution will provide a survey of our recent research activities on photoactivated H2 production over supported oxide-based nanosystems. Their preparation is carried out by original synthetic routes based on chemical vapor deposition (CVD), either thermal or plasma-enhanced (PE-CVD), in some cases combined with radio frequency (RF) sputtering. In particular, we will present and discuss the synthesis, characterization and functional performances of two material classes: (i) single-phase undoped (CuxO, with x = 1, 2; Co3O4) and anion-doped (F:Co3O4) metal oxides; (ii) oxide-based nanocomposites (Ag/ZnO and CuO/ZnO). Beside highlighting the potential of CVD routes in achieving specific nanosystem properties, the future challenges and perspectives for research advancements in these fields will also be outlined

Photoassisted H2 production by metal oxide nanomaterials fabricated through CVD-based approaches

The development of efficient photocatalytic nanomaterials is of outstanding importance for the conversion of electromagnetic radiation into chemical energy. Among the various technological options, photoreforming of aqueous media containing suitable organic oxygenates is an attractive strategy towards a sustainable H2 generation. In this regard, the present contribution will provide a survey of our recent research activities on photoactivated H2 production over supported oxide-based nanosystems. Their preparation is carried out by original synthetic routes based on chemical vapor deposition (CVD), either thermal or plasma-enhanced (PE-CVD), in some cases combined with radio frequency (RF) sputtering. In particular, we will present and discuss the synthesis, characterization and functional performances of two material classes: (i) single-phase undoped (CuxO, with x = 1, 2; Co3O4) and anion-doped (F:Co3O4) metal oxides; (ii) oxide-based nanocomposites (Ag/ZnO and CuO/ZnO). Beside highlighting the potential of CVD routes in achieving specific nanosystem properties, the future challenges and perspectives for research advancements in these fields will also be outlined

Bi12O17Cl2/(BiO)2CO3 Nanocomposite Materials for Pollutant Adsorption and Degradation: Modulation of the Functional Properties by Composition Tailoring

Bi12O17Cl2/(BiO)2CO3 nanocomposite materials were studied as bifunctional systems for depuration of wastewater. They are able to efficiently adsorb and decompose rhodamine B (RhB) and methyl orange (MO), used as model pollutants. Bi12O17Cl2/(BiO)2CO3 nanocomposites were synthesized at room temperature and ambient pressure by means of controlled hydrolysis of BiCl3 in the presence of a surfactant (Brij 76). Cold treatments of the pristine samples with UV light or thermal annealing at different temperatures (370\u2013500 \ub0C) and atmospheres (air, Ar/30% O2) were adopted to modulate the relative amounts of Bi12O17Cl2/(BiO)2CO3 and hence the morphology, surface area, \u3b6-potential, optical absorption in the visible range, and the adsorption/degradation of pollutants. The best performance was achieved by (BiO)2CO3-rich samples, which adsorbed 80% of MO and decomposed the remaining 20% by visible light photocatalysis. Irrespective of the dye, all of the samples were able to almost complete the adsorption step within 10 min contact time. Bi12O17Cl2-rich composite materials displayed a lower adsorption ability, but thanks to the stronger absorption in the visible range they behaved as more effective photocatalysts. The obtained results evidenced the ability of the employed strategy to modulate sample properties in a wide range, thus pointing out the effectiveness of this approach for the synthesis of multifunctional inorganic materials for environmental remediation

Insights into the Plasma-Assisted Fabrication and Nanoscopic Investigation of Tailored MnO2 Nanomaterials

Among transition metal oxides, MnO2 is of considerable importance for various technological end-uses, from heterogeneous catalysis to gas sensing, owing to its structural flexibility and unique properties at the nanoscale. In this work, we demonstrate the successful fabrication of supported MnO2 nanomaterials by a catalyst-free, plasma-assisted process starting from a fluorinated manganese(II) molecular source in Ar/O2 plasmas. A thorough multitechnique characterization aimed at the systematic investigation of material structure, chemical composition, and morphology revealed the formation of F-doped, oxygen-deficient, MnO2-based nanomaterials, with a fluorine content tunable as a function of growth temperature (TG). Whereas phase-pure \u3b2-MnO2 was obtained for 100 \ub0C 64 TG 64 300 \ub0C, the formation of mixed phase MnO2 + Mn2O3 nanosystems took place at 400 \ub0C. In addition, the system nano-organization could be finely tailored, resulting in a controllable evolution from wheat-ear columnar arrays to high aspect ratio pointed-tip nanorod assemblies. Concomitantly, magnetic force microscopy analyses suggested the formation of spin domains with features dependent on material morphology. Preliminary tests in Vis-light activated photocatalytic degradation of rhodamine B aqueous solutions pave the way to possible applications of the target materials in wastewater purification

Supported F-Doped alpha-Fe2O3 Nanomaterials: Synthesis, Characterization and Photo-Assisted H-2 Production

Supported fluorine-doped alpha-Fe2O3 nanomaterials were synthesized by Plasma Enhanced-Chemical Vapor Deposition (PE-CVD) at temperatures between 300 and 500 degrees C, using a fluorinated iron(II) diketonate-diamine compound as a single-source precursor for both Fe and F. The system structure, morphology and composition were thoroughly investigated by various characterization techniques, highlighting the possibility of controlling the fluorine doping level by varying the sole growth temperature. Photocatalytic H-2 production from water/ethanol solutions under simulated solar irradiation evidenced promising gas evolution rates, candidating the present PE-CVD approach as a valuable strategy to fabricate highly active supported materials

CuOx 12TiO2 Photocatalysts for H2 Production from Ethanol and Glycerol Solutions

Hydrogen production by photocatalytic reforming of aqueous solutions of ethanol and glycerol was studied with the use of impregnated and embedded CuOx/TiO2 photocatalysts. Embedded CuOx@TiO2 was prepared by a water-in-oil microemulsion method, which consists in the formation of Cu nanoparticles in the microemulsion followed by controlled hydrolysis and condensation of tetraisopropyl orthotitanate with the aim of covering the protected metal particles with a surrounding layer of porous titanium oxyhydroxide. Mild calcination leads to the complete removal of the organic residues, the crystallization of TiO2, and an unavoidable oxidation of copper. Two reference samples were prepared by classical wet impregnation of preformed TiO2 with different ratios of anatase, rutile, and brookite polymorphs. The two supports were prepared by sol 12gel (TiO2 12SG) and microemulsion (TiO2 12ME) methods. Superior performances have been observed for the embedded system, which shows higher hydrogen production rates with respect to the impregnated systems using either ethanol or glycerol as sacrificial molecules. Deep structural characterization of the materials has been performed by coupling high resolution transmission electron microscopy (HRTEM), high angle annular dark field-scanning transmission electron microscopy (HAADF-STEM), X-ray absorption fine structure (XAFS), and electron paramagnetic resonance (EPR) techniques. Correlation between copper oxidation state and its dispersion and reactivity has been attempted. Finally, the stability of the CuOx/TiO2 catalysts was also studied with respect to carbonaceous deposits and copper leaching

TiO2-Mesoporous Silica Nanocomposites: cooperative effect in the photocatalytic degradation of dyes and drugs

TiO2-SiO2 composites containing 10 wt.%, 20 wt.%, 30% and 40 wt.% of TiO2, obtained by using preformed mesoporous silica nanoparticles MSNs and titanium isopropoxide as titanium source, have been investigated in detail using a variety of techniques. All the samples were characterized by N2-physisorption, X-ray powder diffraction (XRPD), diffusive reflective UV–vis spectroscopy (DRUV-vis), X-ray photoelectron spectroscopy (XPS) and imaged using transmission electron microscopy (TEM). The TiO2-MSN composites, that exhibited a spherical morphology, high specific surface areas and titania in the anatase phase, owing to their specific chemical-physical properties were studied as catalysts in the photocatalytic degradation of Methylene Blue, Methyl Orange and Paracetamol, as examples of polluted wastewaters. The well-defined porous structures of MSNs may offer a special environment for titania nanoparticles, increasing the specific surface area and the thermal stability of the composite, thus modifying the photocatalytic behavior of the materials. The TiO2 loading, the particle size and the surface characteristics were related to the degree of UV absorption and the measured energy band gap of the nanocomposites. A cooperative effect between the two components (TiO2 and SiO2) could be the key factor at the basis of the good photocatalytic performances: nanostructured TiO2 in intimate contact with MSN provides the sites for generation of OH• radicals by oxidation of water and the SiO2 skeleton is able to adsorb the molecules of cationic dyes and prevent poisoning of the TiO2 surface