Valentina Gombac

Surface Phases and Photocatalytic Activity Correlation of Bi2O3/Bi2O4-x Nanocomposite

UV-visible irradiation induces surface alteration of Bi2O3 leading to Bi2O3/Bi2O4-x nanocomposites with excellent photocatalytic activity.

Embedded Phases: A Way to Active and Stable Catalysts

Industrial catalysts are typically made of nanosized metal particles, carried by a solid support. The extremely small size of the particles maximizes the surface area exposed to the reactant, leading to higher reactivity. Moreover, the higher the number of metal atoms in contact with the support, the better the catalyst performance. In addition, peculiar properties have been observed for some metal/metal oxide particles of critical sizes. However, thermal stability of these nanostructures is limited by their size; smaller the particle size, the lower the thermal stability. The ability to fabricate and control the structure of nanoparticles allows to influence the resulting properties and, ultimately, to design stable catalysts with the desired characteristics. Tuning particle sizes provides the possibility to modulate the catalytic activity. Unique and unexpected properties have been observed by confining/embedding metal nanoparticles in inorganic channels or cavities, which indeed offers new opportunities for the design of advanced catalytic systems. Innovation in catalyst design is a powerful tool in realizing the goals of more green, efficient and sustainable industrial processes. The present Review focuses on the catalytic performance of noble metal- and non precious metal-based embedded catalysts with respect to traditional impregnated systems. Emphasis is dedicated to the improved thermal stability of these nanostructures compared to conventional systems.

The Potential of Supported Cu2O and CuO Nanosystems in Photocatalytic H2 Production

Hy wire: Supported Cu(2)O nanosystems and CuO nanowires obtained by chemical vapor deposition were used in the photocatalytic splitting of methanol/water solutions to produce hydrogen. The results obtained with these systems open appealing perspectives for the clean conversion of sunlight into storable chemical energy.

CuOx−TiO2 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 CuO(x)/TiO(2) photocatalysts. Embedded CuO(x)@TiO(2) 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 TiO(2), and an unavoidable oxidation of copper. Two reference samples were prepared by classical wet impregnation of preformed TiO(2) with different ratios of anatase, rutile, and brookite polymorphs. The two supports were prepared by sol-gel (TiO(2)-SG) and microemulsion (TiO(2)-ME) 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 CuO(x)/TiO(2) catalysts was also studied with respect to carbonaceous deposits and copper leaching.

Nanostructured Cu/TiO2 Photocatalysts for H2 Production from Ethanol and Glycerol Aqueous Solutions.

The sustainable development of the human society requires an increasing use of renewable raw materials and energy sources. In this context, photocatalysis represents a promising and necessary way to produce solar fuels and chemicals. The photocatalytic hydrogen production of renewable oxygenated compounds from aqueous solutions could represent an important alternative 6] to the more complex water splitting, or to conventional thermal reforming processes. Noble and base metals, including Pt, Au, Pd, Ni, Cu, and Ag, have been reported to be very efficient at increasing the production of H2 in TiO2 photocatalysis. [11] Increasing attention has been devoted to Cu2O and CuO as photocatalysts. [12, 13] Both copper oxides are abundant natural p-type semiconductors and are attractive owing to their virtual non-toxicity. Their bandgaps (2.1 eV for Cu2O and 1.2 eV for CuO) are suitable for photosplitting water to produce hydrogen by using visible light. However, these materials have been shown to be unstable in electrolytic solutions owing to their facile photooxidation. Siripala et al. prepared a Cu2O/TiO2 heterojunction by electrodeposition of copper oxide; the device was shown to perform photoelectrolysis of water, the TiO2 layer providing protection against photocorrosion. Herein, a cheap and active photocatalysts based on Cu nanoparticles dispersed on TiO2 supports, which are capable of operating under solar radiation, are investigated for the production of hydrogen from ethanol and glycerol. Second generation ethanol and sugars, extracted from lignocellulosic parts of vegetables, and glycerol, produced as a by-product of bio-diesel, are attractive and largely available sacrificial agents. 16] Two different TiO2 supports were prepared, the former from titanium isopropoxide (TiO2-SG) [17] by a sol–gel method, the latter from titanyl sulphate (TiO2-PS) by precipitation, [18] followed in both cases by calcination at 450 8C for 6 h. TiO2-SG (surface area 69 mg ) was composed of a mixture of polymorphs (Figure S1 in the Supporting Information). The analysis of its XRD pattern, following the work of Zhang et al. , evidenced the presence of anatase (64 wt %), rutile (8 wt %) and brookite (28 wt %). Mean crystallite sizes of 11, 32, and 11 nm were calculated for anatase, rutile, and brookite, respectively. TiO2-PS possessed a slightly higher surface area (104 m 2 g ). Its powder XRD pattern showed the presence of a pure anatase phase (Figure S1 in the Supporting Information), with a mean crystallite size of 8 nm. The photodeposition of Cu on the surface of both TiO2 supports was performed by using UV/Vis irradiation (2 h) in the presence of copper nitrate and CH3OH as a hole scavenger (Figure S2). XRD analysis of the Cu/TiO2 nanocomposites did not allow the identification of Cu-related phases probably because of their low amount and/or high dispersion. Therefore, the Cu phases photodeposited on TiO2 supports were characterized by using X-ray absorption near-edge structure (XANES) or extended X-ray absorption fine structure (EXAFS) spectroscopy. Immediately after photodeposition, XANES spectra at the Cu K edge was in good agreement with that of Cu foil used as reference standard (Figure 1), thus indicating that copper is deposited in the form of zero-valent copper. The results of the analysis of the EXAFS spectra are summarized in Table 1.

F-Doped Co3O4 Photocatalysts for Sustainable H2 Generation from Water/Ethanol

p-Type Co(3)O(4) nanostructured films are synthesized by a plasma-assisted process and tested in the photocatalytic production of H(2) from water/ethanol solutions under both near-UV and solar irradiation. It is demonstrated that the introduction of fluorine into p-type Co(3)O(4) results in a remarkable performance improvement with respect to the corresponding undoped oxide, highlighting F-doped Co(3)O(4) films as highly promising systems for hydrogen generation. Notably, the obtained yields were among the best ever reported for similar semiconductor-based photocatalytic processes.

Bimetallic Au–Pt/TiO2 photocatalysts active under UV-A and simulated sunlight for H2 production from ethanol

A new class of bimetallic Au–Pt/TiO2 photocatalysts were developed and employed in H2 photoassisted production using ethanol as a sacrificial reagent both under UV-A and simulated sunlight irradiation. Remarkably, preliminary experiments show promising hydrogen evolution under visible light using Au0.5–Pt0.5/TiO2 with methanol. The presence of bimetallic Au–Pt nanoparticles and the TiO2 visible light absorption, induced by the presence of oxygen vacancies and/or Ti3+, are the two parameters accounting for the difference in activity.

H2 Production by Renewables Photoreforming on Pt–Au/TiO2 Catalysts Activated by Reduction

Bimetallic Pt-Au nanoparticles supported on reduced anatase nanocrystals represent a new class of promising photocatalysts with high activity in hydrogen production by photoreforming of aqueous solution of renewable feedstock, such as ethanol and glycerol. The catalysts are easily obtained by metal impregnation of commercial TiO₂, followed by a reductive treatment. Remarkably, deeper catalyst pre-reduction results in enhanced photoactivity. When ethanol is used as sacrificial agent, under both UV-A or simulated sunlight irradiation, H₂ is the most abundant product in the gas stream whereas, in the case of glycerol, significant amounts of CO₂ have also been detected, indicating a more efficient oxidation of the organic sacrificial agent. The presence of bimetallic Pt-Au nanoparticles and of Ti³⁺ sites/O²⁻ vacancies in the bulk structure of titania are two key parameters to maximize light absorption and feedstock activation, finally resulting in good photocatalytic performances.

Vertically oriented CuO/ZnO nanorod arrays: from plasma-assisted synthesis to photocatalytic H2 Production

1D CuO/ZnO nanocomposites were grown on Si(100) substrates by means of an original two-step synthetic strategy. ZnO nanorod (NR) arrays were initially deposited by plasma enhanced-chemical vapor deposition (PE-CVD) from an Ar–O2 atmosphere. Subsequently, tailored amounts of CuO were dispersed over zinc oxide matrices by radio frequency (RF)-sputtering of Cu from Ar plasmas, followed by thermal treatment in air. A thorough characterization of the obtained systems was carried out by X-ray photoelectron and X-ray excited-Auger electron spectroscopies (XPS and XE-AES), glancing incidence X-ray diffraction (GIXRD), field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDXS), atomic force microscopy (AFM), transmission electron microscopy (TEM), electron diffraction (ED) and energy filtered-TEM (EF-TEM). Pure and highly oriented CuO/ZnO NR arrays, free from ternary Zn–Cu–O phases and characterized by a copper(II) oxide content controllable as a function of the adopted RF-power, were successfully obtained. Interestingly, the structural relationships between the two oxides at the CuO/ZnO interface were found to depend on the overall CuO loading. The obtained nanocomposites displayed promising photocatalytic performances in H2 production by reforming of ethanol–water solutions under simulated solar illumination, paving the way to the sustainable conversion of solar light into chemical energy.

Photocatalytic decolourization of dyes on NiO–ZnO nano-composites

NiO-ZnO nanocomposite photocatalysts were synthesized by co-precipitation/co-gel formation techniques using potassium hydroxide (KOH), sodium carbonate (Na(2)CO(3)) and oxalic acid (H(2)C(2)O(4)) as precipitating agents. X-Ray diffraction (XRD) analysis was employed for the determination of crystallite size and phase purity, while BET surface areas were estimated by nitrogen physisorption analysis. The photocatalytic activities of the NiO-ZnO nanocomposites, evaluated using the photo-decolourization of two structurally different dyes, methyl orange and methylene blue, were correlated with the variation in the crystallite size of the constituents of the composites. All the three synthesized nanocomposites showed appreciable activity for the decolourization of dyes. NiO-ZnO nanocomposite derived by KOH exhibited maximum activity for the decolourization of both the dyes and the decolourization rate of methylene blue was found to be comparable with that of Degussa P-25 TiO(2). Finally, the activity of the most active NiO-ZnO composite with relatively large crystallite sizes was compared with the pure components and their physical mixture, synthesized by the identical synthetic route, in the same molar ratio as that of nanocomposite.