Vittorio Loddo

Nanostructured Rutile TiO2 for Selective Photocatalytic Oxidation of Aromatic Alcohols to Aldehydes in Water

Selective photocatalytic oxidation of aromatic alcohols to aldehydes was performed in water in the presence of TiO2 rutile photocatalysts that exhibited a low degree of crystallinity. The nanostructured rutile samples, prepared ex TiCl4 at very low temperature, ensured a selectivity toward the aldehyde 3 to 4-fold higher than the commercial rutile tested (Sigma-Aldrich).

Photocatalytic oxidation of toluene on irradiated TiO2: comparison of degradation performance in humidified air, in water and in water containing a zwitterionic surfactant

Photocatalytic degradation of toluene was carried out both in gas–solid and in liquid–solid regime by using polycrystalline samples of TiO2 Merck and TiO2 Degussa P25. For the gas–solid regime two types of continuous photoreactor were used, a fixed bed one of cylindrical shape and a Carberry type photoreactor, both irradiated by near-UV light. The inlet reacting mixture consisted of air containing toluene and water vapours. The influence of the gas flow rate and the presence of water vapour on the photocatalytic process was investigated. CO2 and benzaldehyde were the toluene degradation products detected in the gas phase by using TiO2 Merck. In the presence of water vapour this catalyst exhibited a stable activity, which greatly decreased in the absence of water vapour. On the contrary, TiO2 Degussa P25 produced CO2 and traces of benzaldehyde but it continuously deactivated even in the presence of water vapour. For the liquid–solid regime a batch photoreactor with immersed lamp was used. In order to increase the reaction rate, a zwitterionic surfactant, i.e. tetradecyldimethylamino-oxide, was added to the reacting mixture. A complete photo-oxidation of toluene was achieved after few hours of irradiation in the presence of both types of photocatalysts; longer irradiation times produced the photodegradation of surfactant. The main intermediates of toluene degradation were p-cresol and benzaldehyde while traces of pyrogallol and benzyl alcohol were also found. Benzoic acid, hydroquinone and trans, trans-muconic acid were detected only with TiO2 Merck. The reaction rate was higher in the presence of the surfactant suggesting that this compound acts as a sequestration agent. An FTIR study gave information on the role played by superficial hydroxyl groups both on the onset of activity and on the deactivation process. On the basis of photoreactivity results and of FTIR investigation the differences of activity and distribution and nature of toluene degradation products are critically discussed for the three reacting systems used.

The role of H2O in the photocatalytic oxidation of toluene in vapour phase on anatase TiO2 catalyst: A FTIR study

Photocatalytic oxidation of toluene has been carried out in a gas–solid regime by using polycrystalline anatase TiO2 in a fixed-bed continuous reactor. Air containing toluene and water vapours in various molar ratios was fed to the photoreactor irradiated by a medium pressure Hg lamp. Toluene was mainly photo-oxidised to benzaldehyde, and small amount of benzene, benzyl alcohol and traces of benzoic acid and phenol were also detected. In the presence of water, no decrease of photoreactivity was observed at steady-state conditions. By removing water vapour from the feed, the conversion of toluene to benzaldehyde was almost completely inhibited, and an irreversible deactivation of the catalyst occurred. FTIR investigations indicated that benzaldehyde is photoproduced on the TiO2 surface even in the absence of water vapour, but exposure of the catalyst to the UV light in a dry atmosphere results in an irreversible consumption of surface hydroxyl groups. As these species play a key role in the photoreactive process, this dehydroxylation should be the reason of the catalyst deactivation observed in the catalytic runs carried out in the absence of water vapour.

Selective photocatalytic oxidation of 4-substituted aromatic alcohols in water with rutile TiO2 prepared at room temperature

Home-prepared (HP) rutile TiO2catalysts were prepared at room temperature by using H2O and TiCl4 in different ratios and without addition of additives. The catalysts were used for carrying out the selective photocatalytic oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde in aqueous suspension, free from any organic co-solvent. The selectivities showed by the home prepared catalysts were in the 45–74% range, up to four times higher than that of a commercial rutile TiO2 sample, the reaction rates being comparable. By using the most selective photocatalyst, the oxidation of benzyl, 4-methylbenzyl, and 4-nitrobenzyl alcohols was also carried out in order to investigate the influence of the substituent group on the oxidation rate and selectivity. The presence of an –OCH3group positively influenced the selectivity whereas a –NO2group showed to have a detrimental effect. The Hammett relationship effectively describes the influence of substituent group on the kinetic constant of partial oxidation of aromatic alcohols to aldehydes.

Oxidation of Aromatic Alcohols in Irradiated Aqueous Suspensions of Commercial and Home-prepared Rutile TiO2: A Selectivity Study

The photocatalytic oxidation of benzyl alcohol (BA) and 4-methoxybenzyl alcohol (MBA) has been performed in pure water by using commercial TiO(2) samples (Sigma-Aldrich, Merck, Degussa P25) and rutile TiO(2) prepared from TiCl(4) at low temperature. Particular attention has been devoted to the identification of the produced aromatic compounds along with the formed CO(2). Oxidation products such as the corresponding aromatic aldehyde and acid, as well as mono- and dihydroxylated aldehydes have been detected. The home-prepared rutile sample showed a marked selectivity towards the formation of the aromatic aldehyde (38 and 60 % for BA and MBA, respectively), resulting in a three- to sevenfold improvement relative to commercial samples, with the only byproduct being CO(2). This catalyst was found to be the most selective in the formation of aldehyde in water. By using the commercial or the calcined home-prepared samples, many hydroxylated aromatic compounds were detected besides the aldehyde and the acid. This finding points to a higher selectivity performance of the home-prepared rutile relative to the commercial TiO(2) samples. Some of the home-prepared samples were also dialysed to check the influence of the presence of Cl(-) species on catalyst reactivity and selectivity. We have attempted to explain the different reaction rate and selectivity observed for MBA and BA.

Ozone enhanced activity of aqueous titanium dioxide suspensions for photocatalytic oxidation of free cyanide ions

The ozonization and the heterogeneous photocatalytic method are contemporarily applied for the oxidation of free cyanide ions. A batch photoreactor with immersed lamp was used for treating aqueous suspensions containing polycrystalline TiO2 powders irradiated in the near-UV region. Air or ozone–air mixtures were used as oxidation reactants. At the used experimental conditions, the photoreaction proceeds at a measurable rate until low values of cyanide concentration. In the presence of ozone (O3) several parallel reaction routes are contributing to the overall cyanide degradation: (i) homogeneous oxidation by ozone; (ii) heterogeneous catalytic reaction by ozone; (iii) photocatalytic degradation with oxygen; and (iv) photocatalytic degradation with ozone. The rate of cyanide oxidation by heterogeneous photocatalytic mechanism is enhanced by ozone with respect to that observed in the presence of neat oxygen. A kinetic model based on Langmuir–Hinshelwood equations describes adequately the photoreactivity results and provides the values of the kinetic constants and equilibrium adsorption constants for the catalytic and photocatalytic reactions contributing to cyanide oxidation. In order to gain further information on the interaction of ozone with TiO2 surfaces, either in the dark or under UV-irradiation, an ESR study has been also carried out. The results indicate that ozone strongly interacts with TiO2, leading mainly to the formation of ozonide radicals.

Photocatalytic oxidation of cyanide in aqueous TiO2 suspensions irradiated by sunlight in mild and strong oxidant conditions

Abstract The photocatalytic oxidation of free cyanide ions was carried out in aqueous aerated suspensions containing polycrystalline TiO2 (anatase) irradiated by sunlight. The influence of the presence of an organic compound (phenol) or of a strong oxidant (H2O2) on the photoprocess was also studied. The dependence of cyanide photo-oxidation rate on the following parameters: (1) cyanide concentration; (2) catalyst amount; and (3) phenol concentration was investigated. At the used experimental conditions, the kinetics of cyanide photo-oxidation is independent of the initial cyanide concentration and of the catalyst amount while it is affected by the phenol concentration and by the presence of H2O2. The Langmuir–Hinshelwood kinetic model has been used for phenomenologically describing the photoreactivity results. The reaction pathway was also investigated: cyanate, nitrite, nitrate and carbonate were found to be the main oxidation products. The mass balance of nitrogen was achieved only in strongly oxidant conditions; this insight suggests that some volatile nitrogen-containing species are formed at mild oxidation conditions.

Nanochemistry aspects of titania in dye -sensitized solar cells

We analyze the main nanochemistry factors affecting photovoltaic performance in TiO2 employed as wide bandgap semiconductor in dye-sensitized solar cells (DSCs). What is the best morphology of the oxide? Which processes yield the required structures? Finally, putting the discussion in the context of the rapid evolution of photovoltaic technologies, we argue that new titania nanostructures will form the basic component of second-generation solar modules based on dye solar cells.

Preparation and characterisation of TiO2 (anatase) supported on TiO2 (rutile) catalysts employed for 4-nitrophenol photodegradation in aqueous medium and comparison with TiO2 (anatase) supported on Al2O3

Abstract In this paper the preparation of two sets of polycrystalline photocatalysts prepared by supporting TiO2 (anatase) on TiO2 (rutile) or Al2O3 is reported. The powders were prepared by a wet impregnation method using titanium(IV) isopropylate. The impregnation was followed by thermal treatment and this procedure was repeated several times in order to obtain samples with progressively increasing amounts of TiO2 (anatase). The photocatalytic activity of all of the specimens was tested by carrying out the 4-nitrophenol photodegradation, used as a “probe” reaction, in aqueous medium and in different kinds of batch photoreactors. The solids were characterised by X-ray diffraction (XRD), specific surface area (BET) and porosity determination, scanning electron microscopy observation (SEM) coupled with energy dispersive X-ray analysis (EDX), visible–ultraviolet diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR) monitoring of surface acidity. The characterisation results indicate an increase of TiO2 anatase phase with the number of impregnations. The porosity and the surface area of all the photocatalysts prepared by supporting TiO2 (anatase) on TiO2 (rutile) were always higher than that of the bare support, while those of the samples prepared by supporting TiO2 (anatase) on Al2O3 were lower than that of bare Al2O3. The FTIR measurements indicated that Lewis acid sites (Ti4+ or both Ti4+ and Al3+) were present on the surface of the two sets of supported samples, while no Bronsted acid sites were detected. The preparation method used allowed to obtain samples quite resistant to disaggregation when they were mixed in water. Moreover they showed to be easily decantable after the occurrence of the runs. Both sets of TiO2 (anatase) supported samples resulted photoactive and the photoactivity increased by increasing the content of the anatase phase, although it was less significant than that of a pure home prepared and Degussa P25 TiO2 samples, tested for the sake of comparison.

Preparation and characterization of Al2O3 supported TiO2 catalysts employed for 4-nitrophenol photodegradation in aqueous medium

Abstract In this paper the preparation and some bulk and surface characterizations of set of Al2O3 supported TiO2 photocatalysts are reported. The powders were home prepared by a wet impregnation method using titanium (IV) isopropylate. The impregnation was followed by thermal treatments and this procedure was repeated several times in order to obtain samples with different amounts of TiO2. The solids were characterized by X-ray diffraction, specific surface area determination, scanning electron microscopy coupled with an electron microprobe used in an energy-dispersive mode and diffuse reflectance spectroscopy. Moreover, all of them were tested as photocatalysts by using different kinds of batch photoreactors for 4-nitrophenol photodegradation in aqueous medium chosen as a ‘probe’ reaction. The characterization results indicate that a layer of anatase TiO2 formed on the surface of Al2O particles, its extent depending on the number of impregnations. The photoactivity of the powders increased by increasing the number of impregnations up to seven times. For samples obtained by impregnating seven times, the photoreactivity was higher, but not much different from that observed by using samples impregnated five times. The photoactivities of the samples obtained with five and seven impregnations were found to be not much lower than that of a pure home prepared TiO2 sample, but quite lower than that of a Degussa P25 TiO2 sample. Finally, it is worth noting that the alumina supported TiO2 catalysts are easily separated from water by decantation and no significant deactivation due to disaggregation of the particles and/or erosion of their surface was shown to occur in some of the used experimental conditions, i.e. when photoreactors coupled with an electric impeller were used.