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TiO2 catalysts, modified with acidic or alkaline solutions and then platinized, were used for the partial photocatalytic oxidation of 3-pyridinemethanol to 3-pyridinemethanal and vitamin B-3 under environmentally friendly conditions. The reaction took place in water under UVA light and air oxygen. Catalysts were characterized by TEM, photoluminescence, DRIFT-IR, Raman, DRS, XPS, and photocurrent measurements. The photocatalytic activity results show that Pt loading of untreated samples leads to a significant activity improvement (hence product yield) as much as acid and alkaline treatments do. Moreover, the alkaline treated TiO2 samples exhibit a further increase in activity after loading with Pt. Pt acts as an electron scavenger promoting electron transfer from the TiO2 conduction band, consequently boosting the photogenerated pair numbers available for the reactive process. Photocurrent measurements show that the TiO2 photocatalysts' active sites increase significantly after platinization and alkaline/acid treatment. The treated and/or Pt loaded catalysts showed good thermal stability (at least up to 400 degrees C).

The influence of pH on the photocatalytic partial oxidation of 4-methoxybenzyl alcohol (MBA) and vanillyl alcohol (VA) to their corresponding aldehydes in aqueous suspension under UVA irradiation was investigated by using poorly crystalline home-prepared and crystalline commercial TiO2 (BDH, Merck and Degussa P25) photocatalysts. The results clearly show as tuning pH can strongly impart selectivity and activity to photocatalytic processes which are often quite unselective in aqueous suspensions. It was found that pH effect on reaction rate and product selectivity strongly depended on TiO2 crystallinity and substrate type. In the case of MBA oxidation, photoreactivity and selectivity were very high at low pH values for all of TiO2 catalysts, and the crystalline samples showed to be more active than the poorly crystalline ones. At pH= 1 the photoactivity of Degussa P25 was the highest one, and 88% selectivity at 50% conversion was determined. At acidic pH values, selectivity and activity were higher in the presence of HCl than H2SO4 or H3PO4. For VA oxidation, high selectivity values were obtained at high pH’s for all of the samples, and the crystalline samples showed higher activity at the alkaline pH values with respect to that observed at the acidic ones. Experiments starting from the obtained products, that are p-anisaldehyde and vanillin, showed that the selectivity depends on the resistance of those compounds to be subjected to further oxidation under the experimental conditions used.

In this work, the activity enhancement of TiO(2)photocatalysts by alkaline treatment has been investigated. Commercial (BDH, U.K., anatase phase) TiO(2)samples, treated in alkaline solutions of NaOH at different concentrations and temperatures, were tested for the photocatalytic partial oxidation of 3-pyridinemethanol in water under UVA irradiation to form vitamin B-3. Photocatalyst characterization has been carried out by XRD, BET, TEM, TGA, photoluminescence, FTIR, Raman, DRS, EPR and photocurrent measurements. The alkaline-treated samples showed an increased activity of up to 7 times (by considering first order rate constant values) compared to untreated BDH and the superior performance was maintained even after a thermal treatment at 400 degrees C, while the selectivity values were affected only marginally by the treatment (beingca.40% and 15% for 3-pyridinemethanal and vitamin B-3, respectively). The effect of NaOH concentration and treatment duration on the photocatalytic activity has also been studied extensively. The alkaline treatment has also proven to be effective in modifying the surface properties of another commercial catalyst,i.e., Merck, anatase, improving its reactivity in the conversions of both 4-methoxybenzyl alcohol and 3-pyridinemethanol. The NaOH treatment boosted the porosity and surface area, while lowering the percent crystallinity of the catalysts, due to a higher density of surface hydroxyl groups. It also enhanced the adsorption capability towards the reacting substrate, as shown by FTIR analysis. Despite the oxygen vacancies (or total abundance of Ti(3+)centres) not being changed after the NaOH treatment, the process of hole trapping under irradiation is drastically affected due to the different water adsorption capabilities of the samples, as highlighted by EPR. The proposed simple treatment of commercial photocatalysts has the potential to support the development of effective photocatalysts, which do not require complex surface modification or doping to achieve high yields in reactions relevant to the chemical industry.

An important factor that governs solar energy transformation into electrical or chemical energy, when using nanoparticles-based devices, is the spatial location of traps limiting electron transport. Evidences have been presented indicating that the electron diffusion is strongly influenced by nanoparticle boundaries, whose characteristics depend on the particles preparation. In the present work we have studied the role of hydrated excess proton structures in the formation of anatase nanoparticles boundaries in samples prepared by TiCl4 hydrolysis at low temperature and strong acidic conditions. The samples, constituted by anatase nanocrystals but, mainly, by amorphous titania, are studied by H-1-MAS NMR, FT-IR and HRTEM. The results indicate that hydrated excess proton species, generated by solvation of bridging hydroxyls protons of highly defective anatase and/or amorphous titania chains, favor the chains bonding to anatase nanoparticles and/or their condensation at the nanoparticles surface. The chains' bonding facilitates the anatase particles aggregation producing oriented self-assemblings. The preparation conditions determine a strong condensation of amorphous titania chains and it should favor anatase transformation into rutile. (C) 2016 Elsevier B.V. All rights reserved.

General definitions of heterogeneous catalysis and photocatalysis are reported together with parameters to assess the catalytic and photocatalytic performances. Some features of catalytic and photocatalytic reactions, the importance of amorphous and crystalline phases of the solid photocatalysts, and reactors and photoreactors with the most important kinetic aspects are presented.

N-doped anatase/rutile photocatalysts were prepared by a sol-gel method, using TiCl4 as TiO2 precursor and urea as N-dopant source. The catalysts required a thermal treatment at temperature higher than 300 degrees C to achieve actual nitrogen doping; this treatment also allowed obtaining crystalline and hydrophobic samples. Moreover, nitrogen doping modified the surface of samples by increasing the anatase to rutile phase ratio and hydrophilicity. The catalysts were characterized by BET specific surface area, XRD, ESEM, TGA, FT-IR, DRS and XPS measurements, with particular attention to the assessment of the N-doping effect. It was found that XPS analysis should be used together with DRS and FT-IR to discriminate whether N had been incorporated in TiO2 lattice or N species had just adsorbed on the TiO2 surface. Moreover, XPS analysis could be also used for determining the crystallinity of TiO2. The photocatalytic activity of the samples was tested for the partial oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde in water under near-UV, visible, UV-visible and simulated solar light, respectively. Commercial (Degussa P25) and undoped home-prepared TiO2 anatase-rutile catalysts were also used for comparison. The results show that N-doping improves absorption of visible light by the catalysts, but the reaction rate does not increase accordingly. Selectivity was higher in the presence of poorly crystallized catalysts, whereas doping seemed to have a slightly improving effect.

Even if many aspects of three-dimensionally ordered macroporous (3DOM) titania have been extensively studied, the interplay between the amorphous and crystalline components of their skeleton wall has not attracted too much attention, although it should strongly influence the properties of these materials. In order to gain new insight on this interplay, we have studied in detail the wall structure of three 3DOM titania samples prepared by heating the 3DOM titania precursor at 673, 773, and 873 K by X-ray diffraction, thermogravimetric analysis, differential thermal analysis, N2 adsorption/desorption, pore-size distribution, high-resolution transmission electron microscopy, and IHmagic-angle-spinning NMR. In addition, their photoreactivity toward the partial oxidation and mineralization of 4-methoxybenzyl alcohol was determined and compared with that of commercial anatase and P25 samples. The results show that when the 3DOM sample heated at 673 K, whose skeleton wall is formed by very small anatase nanoparticles covered by relatively thick layers of amorphous titania, is heated at 773 K, most amorphous titania is eliminated, leaving a very thin amorphous titania layer covering the slightly grown anatase nanoparticles. The photoreactivity results show that the sample heated at 773 K has the highest overall photoreactivity, which was lower than those of commercial anatase and P2S samples; however, its reactivity toward the alcohol partial oxidation was the highest compared with those obtained with all of the other samples. These results indicate that amorphous titania negatively affects the overall photocatalytic activity; however, its particular distribution in the 3DOM samples enhances the partial oxidation reaction.

In a previous study on commercial TiO2 nanoparticles (J. Soria et al.) we had shown that the superficial disordered layers influence their textural and reactivity features. In the present work we used well crystallized anatase particles (commercial TiO2, BDH) as support of different amounts of titania powders, prepared by TiCl4 hydrolysis and constituted by nanocrystals diluted into an amorphous matrix. The reactivity of these photocatalysts has been tested for the selective oxidation of 4-methoxybenzyl alcohol to aldehyde in aqueous suspension. The catalyst characteristics have been studied using adsorption-desorption isotherms, TEM, HRTEM, TGA, PL, DRIFT-IR, and 1H MAS-NMR spectroscopy. The results show that, though amorphous titania species hinder the photocatalyst overall reactivity, the incorporation of a low amount of those species enhances significantly the reactivity towards the partial oxidation. This effect is attributed to the fact that, during the catalyst preparation, associations between chlorine ions and hydrated excess protons structures are produced. These associations act as surfactants, separating the layer of amorphous titania chains from anatase surface so that, under catalyst irradiation, the transport of photogenerated positive charge from anatase bridging oxygens to those of the amorphous layer is favoured.

It is well known that the surface of metal oxide catalysts presents, usually, non-crystalline species containing impurities and low coordinated cations, which are observed in HRTEM images as a disordered layer of amorphous phase. Despite of being these species more accessible and less stable than the crystalline catalysts components, they have been rarely considered when analysing the catalyst activity. In this work, we have studied the effect of a treatment with HCl solution at pH = 0 on the characteristics of the disordered layer in two commercial anatase TiO2 samples, using techniques such as TGA, 1H MAS-NMR and HRTEM, and by determining these sample photocatalytic activity for the 4-nitrophenol degradation in aqueous suspension. The results indicate that associations of chlorine ions in amorphous titania chains and hydrated excess protons structures interact with anatase bridging hydroxyls. This interaction, that breaks Ti-O-Ti bonds between the anatase particles and the disordered layer species, enhances the anatase hydroxyls acidity, favouring the formation of 0-radicals and eventually increasing the photocatalytic activity for 4-nitrophenol degradation under UV irradiation. (C) 2017 Elsevier B.V. All rights reserved.

The treatment by advanced oxidation processes (AOPs) of waters contaminated by organic pollutants and containing also innocuous bromide ions may generate bromate ions as a co-product. In the present work heterogeneous photocatalysis and ozonation have individually been applied and in combination (integrated process) to degrade the organic compounds in water containing also bromide anions. The results show that: i) the sole photocatalysis does not produce bromate ions and in the case of its presence, it is able to reduce bromate to innocuous bromide ions; ii) the integration of photocatalysis and ozonation synergistically enhances the oxidation capabilities; and iii) in the integrated process bromate ions are not produced as long as some oxidizable organics are present. © 2013 Elsevier Ltd.

Piperonal (heliotropine or 3,4-methylenedioxybenzaldehyde) has been synthesized by oxidizing piperonyl alcohol in aqueous UV-irradiated TiO2 suspensions. This compound was identified by GC–MS chromatography, 1H NMR and melting point determination. The other products of the photoprocess were 1,3-bis(3,4-(methylenedioxy)benzyl) ether (found in traces) and CO2, derived from the parallel pathway of photo-mineralization. Commercial and home-prepared TiO2 samples have been tested and the best selectivity (ca. 35%) was obtained by using the home-prepared ones. The reported green process allows to obtain an added value product (piperonal), upon partial oxidation of a cheap reagent.

Heterogeneous photocatalysis and photoelectrocatalysis have been considered as oxidation technologies to abate unselectively noxious species. This article focuses instead on the utilization of these methods for selective syntheses of organic molecules. Some promising reactions have been reported in the presence of various TiO2 samples and the important role played by the amorphous phase has been discussed. The low solubility of most of the organic compounds in water limits the utilization of photocatalysis. Dimethyl carbonate has been proposed as an alternative green organic solvent. The recovery of the products by coupling photocatalysis with pervaporation membrane technology seems to be a solution for future industrial applications. As far as photoelectrocatalysis is concerned, a decrease in recombination of the photogenerated pairs occurs, enhancing the rate of the oxidation reactions and the quantum yield. Another benefit is to avoid reaction(s) between the intermediates and the substrate, as anodic and cathodic reactions take place in different places., S. Yurdakal thanks to TÜ BITAK (Project no: 111T489) for economic support

A two-dimensional model describing the behavior of a batch cylindrical photoreactor has been validated through a series of photocatalytic reactivity runs. The light intensity radiation field produced by 1–6 external UV fluorescent lamps was modeled together with Langmuir–Hinshelwood kinetics and 4-nitrophenol (NP) and Degussa P25 TiO 2 were used as the probe molecule and the catalyst, respectively. The experiments were carried out under different conditions of light intensity, photocatalyst amount and substrate concentration. The model parameters were the kinetic constant of substrate adsorption, desorption and degradation and the exponent of the power law expressing the kinetics dependence on the light intensity. A least-squares best fitting procedure was used to fit the experimental data to the model and find the four required parameters. A satisfactory agreement was obtained between experimental and calculated values of the observed pseudo-first order kinetic constant, with a coefficient of determination R 2 = 0.83. Notably, the present model is of general interest for cylindrical slurry photoreactors, where different molecules can be oxidized in the presence of several photocatalysts and it can be used, for instance, to accurately compare the performance of a set of catalysts.

A batch cylindrical photocatalytic reactor, externally irradiated by 1–6 UV fluorescent lamps and containing a stirred slurry of polycrystalline TiO2, was modeled by coupling a modified Langmuir–Hinshelwood kinetics together with a two-dimensional light intensity field. The radiation field has been determined on the main assumptions of diffuse radiation, isotropic scattering and negligible backward reflected photon flow. The model has been applied to the photocatalytic oxidation of organic substrates which do not undergo homogeneous photochemical degradation. The model is characterized by the following four parameters: the kinetic constants of substrate adsorption, desorption and degradation and the exponent of the power law expressing the kinetics dependence on the light intensity. The model constants may be determined by applying a simple least-squares best fitting procedure.

5-(Hydroxymethyl)-2-furaldehyde (HMF) was selectively oxidized to 2,5-furandicarbaldehyde (FDC) in; aqueous medium by using home prepared (HP) anatase, rutile, and brookite TiO2 nanoparticles. HP samples were prepared via a sol-gel method by using TiCl4 as the TiO2 precursor. Commercial TiO2 catalysts were also used for, comparison. All samples were characterized by BET specific surface area, XRD, TGA, and SEM, and the reactivity results showed that HP catalysts are predominantly amorphous and give rise to selectivities toward FDC more than twice that of commercial and well crystallized catalysts.

It has been recently reported by Palmisano et al. (2015) [1] that the oxidation of 2-methoxybenzyl alcohol (2-MBA) to 2-methoxybenzaldehyde (2-MBAD) proceeds in water under near-UV light with an unexpected catalytic effect of 2-MBAD. In order to investigate the catalytic role of aldehyde in photolytic oxidation of ortho-substituted benzyl alcohols (OSBAs), reactivity runs were carried out with 2-methylbenzyl alcohol (2-MeBA), 2-nitrobenzyl alcohol (2-NBA), 2-hydroxybenzyl alcohol (2-HBA) and 2-chlorobenzyl alcohol (2-CIBA) in the absence and in the presence of their corresponding aldehyde. None of those alcohols showed a measurable oxidation rate even in the presence of their aldehydes but 2-NBA was oxidised very fast, although no corresponding carbonyl product was obtained. The possible catalytic role of 2-MBAD was investigated for the photolytic oxidation of 4-methoxybenzyl alcohol (4-MBA), 4-nitrobenzyl alcohol (4-NBA), 2-CIBA, 2-HBA, 2-MeBA and ferulic acid (FA). The results showed that 2-MBAD acts as a catalyst only for 4-MBA, 2-HBA and FA. The photocatalytic oxidation of OSBAs in the presence of powdered TiO2 has been also carried out in order to investigate the mutual influences, if any, between homogeneous and heterogeneous processes. The reactivity runs were carried out with TiO2 photocatalyst in water and under near-UV irradiation; a home-prepared (HP) TiO2 sample was used along with Degussa P25. HP catalyst showed the best performance: the 2-MBA half-life time was 5 times smaller and the selectivity to aldehyde 13 times higher than in the presence of Degussa P25. (C) 2016 Elsevier B.V. All rights reserved.

The knowledge of the rate equation is fundamental for the assessment of the activity of the photocatalytic material and for the study, design and optimization of photocatalytic reactors. In the case of photocatalytic reactions, the local volumetric rate of photon absorption (LVRPA) affects the reaction rate and its inherently uneven distribution within a “slurry” reactor makes complex a correct kinetic analysis. In the present work it is shown which are the critical aspects of the kinetic analysis in slurry reactors and how to carry out photocatalytic experiments to minimize the misinterpretations of the experimental results. In particular, the influence of the type of illumination and of the operating conditions (recirculation flow rate, optical thickness) are studied. For instance, it is shown that the average values of the reaction rate and of the rate of photon absorption give no direct insight on the intrinsic kinetic if the reactor is not uniformly illuminated and the optical thickness is beyond a given value, which is significantly higher than the one usually suggested in literature for a “safe” kinetic analysis. Furthermore, the estimation of the average rate of photon absorption must take into account the backscattering of radiation from the reactor. In any case, the utilization of a proper mathematical model allows carrying out a sound kinetic analysis also in critical situations as those which can arise with an illumination by LEDs.

The photoproduction of vanillin is studied in aqueous medium starting from trans-ferulic acid, isoeugenol, eugenol or vanillyl alcohol by using both commercial and home prepared TiO2 samples as photocatalysts and batch Pyrex photoreactors. The photo-oxidation at room temperature of these compounds produces vanillin with a selectivity ranging from 1.4 to 21 mol% with respect to the converted substrate. An investigation on the intermediates was performed in the case of trans-ferulic acid; for this substrate the most important intermediates were homovanillic acid, vanillyl mandelic acid, trans-caffeic acid, formic acid, acetic acid, and oxalic acid. The carbon mass balance, including CO2 derived from the mineralization, was made for all of the substrates at irradiation times corresponding to the highest selectivity to vanillin. High closure of the carbon balance was obtained with vanillyl alcohol (ca. 90%) and trans-ferulic acid (ca. 80%).

Heterogeneous photocatalysis by polycrystalline semiconductor oxides is an unconventional technology that has been mainly applied for the degradation of organic and inorganic pollutants in both the vapor and liquid phase. The application of photocatalysis in synthetic routes has also been researched in recent years, demonstrating its viability as an alternative route in organic chemistry. This work illustrates the main findings in the application of heterogeneous photocatalysis to the oxidation of organic compounds dissolved in water, free of any organic co-solvent. Two main aspects of the oxidative process have been studied: the first one is the partial oxidation of various mono-substituted aromatic compounds, investigated to better understand the influence of the substituent group on the reactivity of these species and on the distribution of oxidized products. The second aspect investigated is the possibility of using heterogeneous photocatalysis for the selective oxidation of alcohols to aldehydes and ketones. The partial oxidation of alcohols to produce carbonyl compounds is of great industrial importance; generally, it is performed in environmentally harmful organic solvents at high temperatures and pressures by using stoichiometric oxygen donors that not only are expensive and toxic, but also produce large amounts of dangerous waste

WOS: 000274769500044, Amorphous TiO2, prepared at room temperature through a sol-gel method implementing hydrolysis of TiCl4, has been supported on graphite rods and then annealed at 673 K. In this way graphite was completely covered by a porous anatase TiO2 layer, with an external thickness of about 1 mu m, with graphite pores completely filled by the semiconductor particles. The obtained electrode was structurally characterized by SEM microscopy coupled to EDAX mapping and by Raman spectroscopy. A Pyrex annular reactor was designed in order to test the prepared electrodes for the photoelectrocatalytic degradation of 4-nitrophenol, a target pollutant dissolved in aqueous conductive solution. The continuous reactor worked in total recirculation mode and the degradation runs were carried out by applying near UV-light, bias or both energy sources. The influence of flow rate, initial 4-nitrophenol concentration and applied potential on the degradation rate was studied

WOS: 000266963000002, A method for determining the amount of substrate adsorbed onto the catalyst surface in liquid-solid photocatalytic system under reaction conditions (photoadsorption) is proposed. The method has been applied to the photocatalytic oxidation of benzyl alcohol carried Out in aqueous suspensions of a home prepared TiO2 catalyst. The influence of alcohol concentration, catalyst amount and irradiation power on photoadsorption was investigated. The photoadsorption per unit mass of catalyst increased by increasing concentration of benzyl alcohol and irradiation power but it decreased, increasing the catalyst amount. By taking into account the photoadsorption phenomena, the kinetic modeling of the photooxidation process has been carried Out and the parameters of the model have been determined. The results indicate that all these parameters are favorably affected by an increase of the photon How absorbed per unit mass of catalyst

In this paper some intrinsic electronic properties of home prepared (HP) TiO2 catalysts were investigated by diffuse reflectance spectroscopy and quasi-Fermi level measurements. These powders were used for carrying out the photocatalytic oxidation of benzyl alcohol to benzaldehyde and CO2 in water; the selectivity for aldehyde formation was enhanced by the addition of small amounts of ethanol, a typical hole trap. 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 benzyl alcohol oxidation (Degussa P25 TiO2) 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 competition between the substrate and the hole trap for adsorption on reactive sites.

Oxygenated aqueous suspensions of home-prepared (HP) and commercial TiO2 catalysts were used in a batch photoreactor for carrying out the oxidation of benzyl alcohol (BA) and 4-methoxybenzyl alcohol (MBA) under different operative conditions. HP catalysts were synthesized from TiCl4 and underwent a hydrolysis treatment of different times under mild conditions. The textural characterization 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 CO2. 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 TiO2. 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 TiO2 surface in different ways that eventually determine two parallel reaction pathways (partial oxidation or mineralization); (ii) the aliphatic alcohols preferentially compete with aromatic alcohols for the mineralizing pathway.

WOS: 000246040000026, The photocatalytic oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde (PAA) was performed in water with organic-free suspensions of home-prepared and commercial titanium dioxide (TiO2) catalysts. The nanostructured TiO2 samples were synthesised by boiling aqueous solutions of titanium tetrachloride (TiCl4) under mild conditions, for different times. The crystallinity increased with the boiling time. The 4-methoxybenzyl alcohol oxidation rate followed the same pattern but the highest yield (41.5% mol) to PAA was found for the least crystalline sample, that showed a quantum efficiency of 0.116%. A comparison with two commercial TiO2 samples showed that all the home-prepared catalysts exhibited a PAA yield higher than that of commercial ones. The only by-products present were traces of 4-methoxybenzoic acid and aliphatic products, carbon dioxide being the other main oxidation product.

WOS: 000245150800018, The oxidation of nitrobenzene and phenylamine has been carried out by heterogeneous photocatalysis in aqueous suspensions of commercial TiO2. The photoreactivity results indicate that two reaction pathways occur in parallel with both substrates from the start of irradiation: partial oxidation to monohydroxy derivatives and mineralization. The first pathway involves the entering of radical HO center dot into the aromatic ring giving rise to monohydroxy derivatives, and the second one involves the total oxidation of the substrates to CO2. The partial oxidation of nitrobenzene produces all the three possible monohydroxy derivatives, while that of phenylamine only produces the ortho- and para-isomers. The extent of adsorption in the dark was found to be about 8% mol for nitrobenzene, while the amounts were not detectable for phenylamine. The Langmuir-Hinshelwood approach has been satisfactorily applied for modeling the photoreactivity results, and the values of the kinetic model parameters have been determined

An unusual autocatalytic photooxidation of 2-methoxybenzyl alcohol has been observed under UV irradiation in aqueous medium. The homogeneous oxidation is catalyzed by the corresponding aldehyde that is also the main oxidation product. The trend of alcohol disappearance rate matches the typical shape of an autocatalytic process, where a crucial and ambivalent role is played by the presence of molecular oxygen. Low oxygen concentrations give rise to a zero-order reaction since the beginning of irradiation, while higher amounts of oxygen reduce the alcohol oxidation rate until the aldehyde reaches a concentration high enough to speed up the alcohol's conversion. Experiments performed by varying alcohol, aldehyde and oxygen concentrations in both aqueous and organic media suggest that the alcohol oxidation is initially favored by the presence of oxygen. Once the formed aldehyde competes for photon absorption, the oxidation is driven by aldehydes alpha cleavage leading to reactive oxygenated species in aqueous medium. An excess of oxygen quenches the latter processes thus inhibiting/slowing down the reaction. (C) 2015 Elsevier B.V. All rights reserved. tAn unusual autocatalytic photooxidation of 2-methoxybenzyl alcohol has been observed under UV irradi-ation in aqueous medium. The homogeneous oxidation is catalyzed by the corresponding aldehyde that isalso the main oxidation product. The trend of alcohol disappearance rate matches the typical shape of anautocatalytic process, where a crucial and ambivalent role is played by the presence of molecular oxygen.Low oxygen concentrations give rise to a zero-order reaction since the beginning of irradiation, whilehigher amounts of oxygen reduce the alcohol oxidation rate until the aldehyde reaches a concentrationhigh enough to speed up the alcohol’s conversion. Experiments performed by varying alcohol, aldehydeand oxygen concentrations in both aqueous and organic media suggest that the alcohol oxidation is ini-tially favored by the presence of oxygen. Once the formed aldehyde competes for photon absorption,the oxidation is driven by aldehydes alpha cleavage leading to reactive oxygenated species in aqueousmedium. An excess of oxygen quenches the latter processes thus inhibiting/slowing down the reaction.

In the so-called photocatalytic ozonation (PO) synergistic effects between photocatalysis and ozonation enhance the rate of oxidation of organic compounds, therefore this integrated process is very promising for use in water treatment. In the present work, it is demonstrated that the improvement that can be obtained by the integrated process in comparison with the uncoupled processes depends on the relative weight between photocatalysis and ozonation. In this respect, it has been found that a relatively low rate of photocatalysis (0.3 times the rate of ozonation) is sufficient to get the highest synergy and to maximize the performances in the oxidation of formate, the adopted model organic compound. The benefits of the coupling of photocatalysis to ozonation are not limited to the increase of the oxidation rate, but include also the control of the ozonation by-product of main concern (bromate). The effect of the pH, of the presence of oxidizable organic compounds, of the presence of nitrogen on the formation or on the reduction of bromate are experimentally studied and discussed. The obtained results have relevance for the practical applications of PO, since on these bases it is possible to identify proper operating conditions.

Il programma NON mi ha consentito di scaricare il PDF di questo articolo. Se lo volete ditemi come inviarlo ABSTRACT Photocatalytic degradation of acetonitrile was carried out in both gas–solid and liquid–solid regimes using two commercial TiO2 catalysts (Merck and Degussa P25). For the gas–solid regime, a continuous annular photoreactor was used. The influence on photodegradation kinetics of the gas flow rate and concentrations of acetonitrile, oxygen, and water was investigated. Acetonitrile degradation products detected in the gas phase included carbon dioxide and hydrogen cyanide. The same photoactivity was exhibited in the presence and in the absence of water vapour. The liquid–solid regime used a batch photoreactor with an immersed lamp (the same as for the gas–solid regime). The oxidation products detected in the solution were cyanide, cyanate, nitrite, nitrate, methanoate, and carbonate ions. The Langmuir–Hinshelwood kinetic model fit the photoreactivity data obtained in both regimes and allowed us to determine the rate constant and equilibrium adsorption constant values. The adsorption constant and kinetic constant value were lower in the liquid–solid regime than in the gas–solid regime. The Merck catalyst had higher values of these parameters for both regimes than the Degussa P25 catalyst. An evaluation of the possible competition between acetonitrile and water molecules for the surface sites of the photocatalyst (Ti 4 + ions and hydroxyl groups) revealed that for high H2O/CH3CN ratios, as is typical for the photo-oxidation process carried out in a liquid/solid regime, acetonitrile molecules were not able to provide a specific interaction with the surface sites of TiO2, remaining dissolved in the interface water molecular layers. In contrast, for low H2O/CH3CN ratios, as is typical for the photo-oxidation process carried out in a gas–solid regime, acetonitrile could win the competition with water for surface hydroxyls.  2005 Elsevier Inc. All rights reserved.

The photocatalytic oxidation of acetonitrile (CH3CN) was carried out in aqueous suspensions of polycrystalline TiO2 P25 Degussa irradiated by sunlight. A plug flow photoreactor in a total recycle loop was used for carrying out reactivity experiments in which the concentrations of acetonitrile, of its intermediate oxidation products and of not-purgeable organic carbon (NPOC) were monitored. The influence of the presence of strong oxidant species (H2O2, S2O82−, ClO−) on the process rate was studied. The dependence of acetonitrile photo-oxidation rate on the substrate concentration and on the catalyst amount was also investigated. The photodegradation rate of substrate and NPOC followed first order kinetics with respect to acetonitrile and NPOC concentrations, respectively. The presence of S2O82− and solar irradiation determined the occurrence of homogeneous degradation of acetonitrile and NPOC. In the presence of irradiated catalyst, a significant synergetic effect was observed for NPOC degradation while for the acetonitrile oxidation this effect was not evident. H2O2 did not affect the process while ClO− affected negatively the acetonitrile oxidation rate and it inhibited the NPOC degradation.

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 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.

Thermal and hydrothermal treatments have been applied to an amorphous TiO2 precursor for obtaining nanosized TiO2 particles (P11t and P11h, respectively) of different photocatalytic properties. The activity of these catalysts has been tested by performing the toluene oxidation in gas phase in a continuous photoreactor. A Fourier transform infrared (FTIR) investigation of the catalysts under conditions prevailing during the test photoreaction has also been carried out. The photoreactivity results showed that CO2 was the main oxidation product and benzaldehyde a stable intermediate. Anatase P11t photoactivity was similar to that observed for commercial photocatalysts, while anatase P11h presented a marked improvement. The FTIR study on these samples indicate that P11h has a higher number of hydrogen-bonded hydroxyl groups that are more stable under RT outgassing than P11t. The higher photoactivity of P11h is attributed to the participation of these hydrogen-bonded hydroxyls in the toluene conversion to CO2. FTIR spectra also suggest that benzaldehyde, the minor oxidation product, originates from toluene adsorbed on isolated hydroxyls. Benzaldehyde is more strongly adsorbed on P11h than on P11t; the presence of water vapor in the reacting mixture, however, facilitates its desorption and/or photooxidation.

Nanostructured TiO2 samples with primary particle size in the 4−20 nm range were prepared by either hydrothermal (H) or thermal (T) treatment of an amorphous precursor, and their behavior under UV illumination at 77 K was studied by means of EPR spectroscopy. The samples of the H series present the smallest crystallite size and after irradiation in a vacuum show some Ti3+ centers. In contrast, under these conditions only weak signals associated with oxygenated radicals are observed for the T samples. However, when oxygen is preadsorbed, several oxygenated complexes (O-, O2-, O2H•, and O3-) are photogenerated in proportions that depend on the characteristics of the material. Subsurface O- species are exclusively detected in the case of the samples of the H series, whereas ozonide radicals and surface O- are stabilized on materials with larger crystalline domains. These oxygenated complexes are thermally unstable, and they disappeared after warming to room temperature in the case of the T samples, but they ...

Polycrystalline mixed WO3/WS2 powders have been prepared by different methods. The samples have been characterized by bulk and surface techniques and tested as catalysts for the photodegradation of phenol. The results have indicated that the coupling of WO3 and WS2 leads to an enhanced rate of disappearance of the organic substrate. The effect is explained as the result of the simultaneous electron transfer from WS2 to WO3 and hole transfer from WO3 to WS2. The efficiency of the mixed WO3/WS2 systems strongly depends on the relative amounts of WO3 and WS2 in the various samples. A maximum of photoactivity is obtained when the surface WS2 and WO3 molar fraction is about 0.5.

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.

Mixed WO3/WS2 powders were prepared by oxidation of WS2. The physical properties of the samples were characterized by X-ray diffraction, diffuse reflectance spectroscopy, and scanning electron microscopy; they were strongly dependent upon the time and temperature of oxidation. The chemical state and the elemental distribution of the sample surface were investigated by X-ray photoelectron spectroscopy. The photocatalytic results have showed that the aqueous suspensions of the mixed WO3/WS2 systems have significantly higher activities than pure WS2 and WO3 for the photodegradation of phenol. The enhanced performance can be related to the presence of heterojunctions WO3/WS2 on the single particles. The best results were obtained when the surface WS2 molar fraction was about 0.5.

A continuous photoreactor, working in a total recycle mode and irradiated by a low-pressure Hg lamp, has been used to study the bactericidal effect of a photocatalyst, formed by TiO2 embedded in SiO2 fibres, on Legionella pneumophila and a consortium of common gram-negative aerobic bacteria: (Escherichia coli, Klebsiella sp., Pseudomona sp. and Proteus sp.) in water. The kinetic modeling of the inactivation process, carried out with the measured values of viable bacteria concentration at the outlet of photoreactor, evidenced that for each pass inside the photoreactor the ratio between the outlet and inlet cell concentrations was of order of 0.01 for the inactivation of L. pneumophila. For the other aerobic bacteria, which are usually taken as reference in photocatalytic bacteria inactivation studies, this ratio was of order of 0.3 for the first hour of illumination, while upon prolonged irradiation (up to 9 h) this ratio increased to 0.7. Several factors inducing this latter decrease of efficiency are possible, as e.g. competition for photocatalytic attack between microorganisms and organic compounds released by damaged bacteria or photoinduced alteration of a small fraction of still viable bacteria making them less interactive with the photocatalyst. The inactivation mechanism normally proposed for common bacteria involves an initial attack of the photogenerated radicals to the outer membrane; the consequent membrane dispersion allows the radicals to damage the cytoplasmic membrane. The higher lethality of the photocatalytic method observed towards Legionella: (in comparison to the other aerobic bacteria) is explained considering that the radicals attack the Legionella secretion system, which is adapted for high virulence and would become activated for and through adhesion to the TiO2 surface. This attack would then be able to inactivate L. pneumophila without dispersing the outer membrane. Apart from this, the water flow through the catalyst fibres can facilitate the bacteria transport towards the anatase surface, and additionally the generated shear stress may help adhesion, at least for some bacteria as E. coli, contributing further to improve the photokilling efficiency; both factors may contribute to the efficiency of this photoreactor configuration.

The photocatalytic oxidation of free cyanides in aqueous suspensions containing polycrystalline TiO2(anatase) powders irradiated in the near-UV region has been investigated. The rate of cyanide photooxidation has been studied by varying the following operative parameters: (i) initial cyanide concentration; (ii) catalyst concentration; (iii) initial pH; (iv) power of irradiation; and (v) chloride ion concentration in the reacting mixture. Under the used experimental conditions the photoreaction proceeded at a measurable rate until the complete disappearance of cyanides. The kinetics of cyanide photooxidation is affected by the catalyst concentration, the chloride ion concentration, and the power of irradiation while it is independent of the initial cyanide concentration and the pH. The detrimental effect of chloride ions on cyanide photooxidation rate is not determined by a competition mechanism of chloride ions with cyanide ions or oxygen molecules for adsorption on active sites. Chloride ions affect the photoreaction rate by lowering the concentration of dissolved oxygen to values for which oxygen may become a rate limiting reactant. The Langmuir–Hinshelwood kinetic model well fits all the photoreactivity results. The reaction pathway was also investigated; cyanate, nitrate, and carbonate were found to be the main oxidation products. A mass balance on nitrogen was also successfully carried out. Specific experiments were carried out in a particular setup for measuring both the photon flow absorbed by the reacting suspension and the cyanide photoreaction rate; for these particular conditions the quantum yield value was calculated.

Tungsten chalcogenides powders have been prepared by sulfidation of WO3 with gaseous H2S. Different mixed WS2/WO3 systems have been obtained by varying the amount of initial WO3 powder, the time, o...

Metal oxides structures, formed by ordered self-assembling of crystalline nanoparticles aggregates, are attracting growing attention in fields such as catalysis or solar cells. The properties of these structures are influenced not only by their morphology and nanoparticles composition but also by the assembled particles boundaries. In order to determine the influence of these boundaries on the structures properties, the present investigation has been devoted to study the characteristics of anatase nanoparticles boundaries in titania aggregates prepared by TiCl4 hydrolysis at low temperature. Proton magic angle spinning nuclear magnetic resonance and high resolution transmission electron microscopy show that these samples mainly contain amorphous TiO 2 strongly bound to specific faces of anatase particles. This amorphous titania-mediated attachment induces the ordered self-assembling of the anatase particles, with favored formation of mesopores in the nanoparticles boundaries. Photocatalytic degradation of 4-methoxybenzyl alcohol in water, test reaction used as a method to get information on the boundaries properties, indicates that the mesoporous amorphous TiO2 offers active sites for the alcohol catalytic partial oxidation, but it strongly hampers inter-particles electron transfer. These effects, characteristic of amorphous oxide chain morphology, can be expected when low dispersed dopants or promoters are incorporated to metal oxides. © 2013 Elsevier B.V. All rights reserved., J.Sa. and I.S. thank the Spanish Agency CICYT (project MAT2010-19837-C06-02) and the regional Government (project S-2009/PPQ 1626) for financial support.

The photoelectrocatalytic partial oxidation of 4-methoxybenzyl alcohol in aqueous solution irradiated by near-UV light was carried out in a three-electrode batch reactor. TiO2 films were either deposited by dip-coating of a TiO2 sol onto a Ti foil and subsequent calcination or generated on Ti plates by thermal oxidation in air at 400-700 degrees C. The effects of the anode preparation method and bias potential values on conversion and selectivity to the corresponding aldehyde were investigated. The photoelectrocatalytic results were compared with the photocatalytic and electrocatalytic ones. The results indicated that no reaction occurred during the electrocatalytic experiments, whereas the photocatalytic reactivity was positively influenced by the application of a small bias (0.75 V vs. saturated calomel electrode). By applying the previous bias the highest initial degradation rates, r(0), and selectivities to 4-methoxybenzaldehyde, S, were obtained in photoelectrocatalytic experiments by using Ti plate calcined at 500 degrees C (r(0) = 0.1507 mM h(-1), S=85%) or TiO2 film prepared by dip-coating and calcined at 700 degrees C (r(0) = 0.1339 mM h(-1), S=90%). In order to investigate the influence of the substituent groups on reactivity and selectivity, photoelectrocatalytic runs with benzyl alcohol, 2-methoxybenzyl alcohol, 3-methoxybenzyl alcohol, 2,4-dimethoxybenzyl alcohol, 2,3,4-trimethoxybenzyl alcohol, 4-nitrobenzyl alcohol and 4-hydroxybenzyl alcohol have been performed. The selectivity and reactivity values increased by increasing the electron-donor properties of the substituent groups. This positive effect, however, was obtained only if the electron-donor group is in para or ortho position with respect to the alcoholic group. (C) 2012 Elsevier B.V. All rights reserved.