Polycrystalline ZnO/TiO2 solids have been prepared with four different methods using home prepared TiO2 (anatase) or TiO2 (rutile) as supports and Zn(NO3)2·6H2O or Zn(CH3COO)2·2H2O as precursors for ZnO. The bulk and surface properties of the samples were investigated by means of TG-DTA, XRD, TEM, SEM-EDAX, XPS, BET surface area determination, and porosity measurements. XRD and TEM results indicate that no significant defect structures exist in any of the samples. The ZnO crystallinity and its enrichment on the surface of TiO2 particles were dependent on the preparation method. The surface areas generally decrease by increasing the amount of ZnO except when ZnO from Zn(CH3COO)2·2H2O was supported on TiO2 (rutile). The samples prepared from Zn(CH3COO)2·2H2O were more porous than those prepared from Zn(NO3)2·6H2O. This was confirmed by BET surface area determinations and SEM observations. XPS spectra indicate that the atomic ratio between OH- and O2- on the particles surface is similar for samples with the same ZnO content independent of the precursor used for the samples' preparation. Moreover a much higher segregation of ZnO was found for samples obtained by using the acetate precursor.

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Electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring of pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining surface acidity and basicity, respectively, were used to carry out further characterization of mixed ZnO/TiO 2 polycrystalline solids prepared by different methods. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium. ESR results indicated the presence of signals attributable to Zn + species in ZnO/TiO 2 (anatase) solids, while in ZnO/TiO 2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti 3+ centers. FT-IR spectra showed no significant differences of the surface hydroxylation degree of the various photocatalysts, whereas the surface acidic properties generally decrease by increasing the amount of ZnO. Coupling of ZnO and TiO 2 semiconducting powders was not so beneficial, as expected on the basis of the intrinsic electronic properties, to enhance the photoreactivity for the studied reaction, although some of the powders showed photoactivities slightly higher than those of bare TiO 2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO 2 (rutile)) appear promising from an application point of view because no filtration was needed after the occurrence of the photoreactivity tests to separate them from the solution because they decanted easily. The mineralization of 4-nitrophenol was checked by determining the total organic carbon (TOC) and a complete photooxidation occurred in few hours in the presence of some of the samples.

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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.

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.

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