Highly active photocatalytic TiO2 samples were synthesized by thermohydrolysis of TiCl4 in water at 100 °C. Rutile, binary mixtures of anatase and rutile or anatase and brookite or ternary mixtures of anatase, brookite, and rutile were obtained depending on the TiCl4/H2O ratio. Rietveld refinements were employed to evaluate the crystalline phases and composition of the mixtures. The effect of the aging time on the phase composition was also studied. The band gap values of the samples were obtained by the diffuse reflectance spectra. The position of the flat band potentials of anatase, brookite, and rutile was determined measuring the photovoltage as a function of the suspension pH. From these data, the relative positions of the energy bands of the three semiconductors were estimated. 4-Nitrophenol photodegradation was used to evaluate the photoactivity of the various samples. Some powders were more active than Degussa P25. The most efficient samples consisted of a ternary mixture of anatase, brookite, and rutile. The high photocatalytic activity was explained by the presence of junctions among different polymorphic TiO2 phases that enhance the separation of the photogenerated electron−hole pairs, hindering their recombination.

This review deals with the parameters that influence heterogeneous photocatalysis (PC) for selective synthesis of high-value chemicals by itself or in combination with other technologies. In particular, the parameters related to the photocatalysts, such as crystallinity degree, type of polymorph, surface acid–base properties, exposure of particular crystalline facets, coupling of different semiconductors, position of the valence and conduction band edge, addition of doping agents, and those related to the reaction system, such as setup configuration and reactor geometry, type of solvent, type and amount of photocatalyst, affecting the selectivity toward specific products are described and discussed. The presented results highlight that a precise evaluation of the efficiency of the process is a challenging but necessary task to be approached in order to allow real applications of photocatalytic processes.

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Oxygenated aqueous suspensions of home-prepared (HP) and commercial TiO 2 catalysts were used in a batch photoreactor for carrying out the oxidation of benzyl alcohol (BA) and 4methoxybenzyl alcohol (MBA) under different operative conditions. HP catalysts were synthesized from TiCl 4 and underwent a hydrolysis treatment of different times under mild conditions. The textural characterisation of catalysts was carried out with XRD, SEM observations, BET surface area and porosity measurements. For both alcohols the main oxidation products were the corresponding aromatic aldehydes and CO 2 . The HP catalysts exhibited selectivity values towards the aldehyde production up to 28 % (BA conversion: 50 %) and 41 % (MBA conversion: 65 %), about four times higher than those of commercial TiO 2 . The addition of an aliphatic alcohol (methanol, ethanol, 2-propanol or tert-butanol) in small amounts with respect to water decreased the overall oxidation rate of aromatic alcohols but enhanced the selectivity for aldehyde formation up to 1.5 times. The reactivity results suggest that: (i) the aromatic alcohol molecules interact with the TiO 2 surface in different ways that eventually determine two parallel reaction pathways (partial oxidation or mineralization); and (ii) the aliphatic alcohols preferentially compete with aromatic alcohols for the mineralizing pathway.

ZnO-SnO 2 nanocomposites were synthesized by a facile sol-gel synthesis route and characterized through X-ray diffraction, BET specific surface area analysis, UV-vis diffuse reflectance spectroscopy, photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy. The photocatalytic activity of the samples was tested using the degradation of 4-nitrophenol under UV light as model reaction. The ZnO/SnO 2 molar ratio was varied in order to study its influence on the photoefficiency of the samples. The ZnO-SnO 2 nanocomposites showed higher photoactivity than the pure oxides and in particular the sample with ZnO/SnO 2 molar ratio equal to 1/0.05 resulted the best one among the tested powders. The high activity of the mixed samples was attributed to the presence of heterojunctions between the two oxides, which allows an improved charge separation of the photogenerated electron-hole pairs, due to the differences between the energy levels of the conduction and valence bands of ZnO and SnO 2. Photovoltage measurements were performed to determine the energy band structure of the ZnO-SnO 2 heterojunction.

As reported in an accompanying contribution [1], home-prepared (HP) TiO 2 powders were used for carrying out the photocatalytic oxidation of benzyl alcohol (BA) in water. The addition of small amounts of ethanol decreases the oxidation rate of BA but it enhances the selectivity for aldehyde formation. The catalysts textural characterization, carried out with XRD, SEM observations, BET surface area and porosity measurements, has been elsewhere reported [1]. In this paper some intrinsic electronic properties were investigated by Diffuse Reflectance Spectroscopy and quasi-Fermi Level measurements. The values of band gap, valence band and conduction band edges are almost identical for all the HP samples in which anatase phase is predominant, whereas appreciable differences can be noticed for an HP sample containing high amount of rutile phase. A comparative ATR-FTIR study of the HP catalyst showing the highest selectivity and the commercial titania showing the highest activity towards BA oxidation (Degussa P25 TiO 2) was carried out. The ATR-FTIR results indicate that HP and Degussa P25 surfaces show a very dissimilar hydrophilicity and different ability for adsorbing the organic compounds deriving from benzyl alcohol photocatalytic oxidation. Results show moreover that the improved selectivity to aldehyde by adding ethanol is due to a competition between the substrate and the ethanol for adsorption on reactive sites.

Limonene epoxide (1,2-limonene oxide) readily reacts with carbon dioxide inserted in a ring-opening copolymerization reaction and forms polycarbonates of exceptional chemical and physical properties. Both poly(limonene carbonate) and poly(limonene dicarbonate) can be synthesized using low-cost Zn or Al homogeneous catalysts. This study addresses selected relevant questions concerning the technical and economic feasibility of limonene and carbon dioxide polymers en route to the bioeconomy.

Pickering emulsions provide a new way to enhance the efficiency of photocatalytic degradation of water-insoluble pollutants. Indeed, the semiconductor solid particles dually act as the photocatalyst and stabilizer of the emulsion droplets whose size dramatically affects the photocatalytic reaction. The present work aims at the validation of this concept by using bare TiO2 without any surface modification. Nanostructured TiO2 has been prepared by a simple sol–gel process and characterized by X-ray diffraction, specific surface area analysis, scanning electron microscopy, and diffuse reflectance spectroscopy. The emulsions were prepared by using 1-methylnaphthalene (1-MN) as a model organic contaminant scarcely soluble in water and bare TiO2 as the photocatalyst/stabilizer. The emulsions have been characterized by electrical conductivity, optical microscopy, and light-scattering analyses. The photocatalytic degradation of 1-MN was 50 times faster in stable Pickering emulsions with respect to the case of biphasic liquid systems containing TiO2. This finding allows us to propose Pickering emulsions stabilized by TiO2 nanoparticles as an effective and novel way to intensify the photocatalytic degradation of water-insoluble organic pollutants.