Sedat Yurdakal

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).

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.

Heterogeneous Photocatalysis and Photoelectrocatalysis: From Unselective Abatement of Noxious Species to Selective Production of High-Value Chemicals

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.

Self-assembled titania–silica–sepiolite based nanocomposites for water decontamination

This work describes the preparation of microchannel structured monolithic pieces by the “ice-segregation induced self-assembly” (ISISA) process. The monoliths exhibit a hierarchical structure composed of homogeneously mixed colloidal silica resulting from hydrolysis and condensation of an alkoxysilane precursor (e.g., TEOS) and TiO2 polycrystalline nanoparticles adsorbed onto sepiolite fibres. The combination of such a different species into a single macroporous structure provides a multifunctional material capable of water decontamination by pollutant removal from aqueous media (via adsorption on sepiolite) and subsequent elimination by UV irradiation (via photocatalytic oxidation on TiO2nanoparticles). The performance of the resulting materials has been studied using two organic compounds often present in wastewater such as p-nitrophenol (PNP) and methylene blue (MB).

Enhancing selectivity in photocatalytic formation of p-anisaldehyde in aqueous suspension under solar light irradiation via TiO2 N-doping

The photocatalytic partial oxidation of 4-methoxybenzyl alcohol to the corresponding aldehyde (p-anisaldehyde) was performed under simulated solar irradiation by using home prepared N-doped TiO2 catalysts. The photocatalysts were prepared by a sol–gel method, using TiCl4 as TiO2 precursor and NH4Cl, urea or NH4OH as N-doping sources. A commercial TiO2 (Degussa P25) was also used for comparison aims. The prepared catalysts were characterized by BET specific surface area, XRD, ESEM and UV-vis spectroscopy. The reactivity results show that (i) the doped catalysts are predominantly amorphous, and they show selectivity values far higher than those of the corresponding undoped ones and of well crystallized catalysts – even if the last ones show a higher activity – and (ii) exploitation of solar light significantly increases the reaction selectivity. In addition, different light sources were also used in order to investigate the effect of radiation wavelength ranges on the reactivity and selectivity to aldehyde.

Selective Photocatalytic Oxidation of 4-Methoxybenzyl Alcohol to p-Anisaldehyde in Organic-Free Water in a Continuous Annular Fixed Bed Reactor

Photocatalytic oxidation of 4-methoxybenzyl alcohol to p-anisaldehyde was performed in water organic-free solutions by using a fixed bed continuous photoreactor containing Pyrex beads on which a TiO2 home prepared photocatalyst was supported. The influence of liquid flow rate, inlet alcohol concentration and catalyst amount on the photoprocess was studied. The highest selectivity to p-anisaldehyde was about 47% being CO2, the other main oxidation product; traces of 4-methoxybenzoic acid were also detected. The radiation field inside the photoreactor has been modelled by applying the Monte Carlo method thus allowing the determination of the local volumetric rate of photon absorption (LVRPA). It was found that the radiation intensity profile sharply decreases inside the bed so that an important aliquot of the bed is not active for the photoreaction occurrence. This finding indicates that the reactivity results, obtained by measuring the concentration values of reagents and products at the exit of photoreactor, can not be used jointly with the radiation modelling ones for determining the dependence of reaction kinetics on light intensity. The Langmuir-Hinshelwood approach has been satisfactorily applied for modelling the photoreactivity results and the values of all the model parameters have been determined.

Partial oxidation of aromatic alcohols via TiO2 photocatalysis: the influence of substituent groups on the activity and selectivity

Aromatic alcohols with substituent groups in different positions have been partially oxidised to the corresponding aldehydes in a photocatalytic system in order to investigate the influence of the substituents on reactivity and selectivity to aldehyde. To this aim benzyl alcohol, 2-methoxybenzyl alcohol, 3-methoxybenzyl alcohol, 4-methoxybenzyl alcohol, 2,4-dimethoxybenzyl alcohol, 4-hydroxybenzyl alcohol and 4-hydroxy-3-methoxybenzyl alcohol have been photocatalytically oxidised to their corresponding aldehydes in aqueous TiO2 suspensions under near-UV irradiation. Home-made and commercial rutile TiO2 samples were used as photocatalysts. The catalysts were characterized by XRD, BET, SEM, TEM and TGA measurements. For all the used substrates the main oxidation products were the corresponding aldehydes and CO2. The aromatic alcohols showed selectivity values decreasing with the substituent position on the aromatic ring according the following order: para > ortho > meta. In the presence of two substituent groups, the overall oxidation rate increased while the selectivity decreased. The home-made catalyst generally showed selectivity higher but activity lower than those of the commercial one. The results showed that the reaction rate and selectivity were dependent not only on the catalyst properties such as crystallinity and hydrophilicity but also on the kind and position of the substituent groups of the aromatic alcohols.

Determination of Photoadsorption Capacity of Polychrystalline TiO2 Catalyst in Irradiated Slurry

Abstract In the field of heterogeneous catalysis all reactions involve the chemical adsorption of one or more of the reactants onto the catalyst surface as an intermediate step in the overall reaction. The quantification, identification, and knowledge of the behavior of chemisorbed species are the object of extensive investigation being this information used for understanding the actual catalytic mechanisms. In principle heterogeneous photocatalysis follows the same pattern with the difference that the chemical adsorption is initiated by light absorbed by the photocatalyst surface (photoadsorption). In the practice, however, while in heterogeneous catalysis the investigation of chemisorption does not exhibit insurmountable difficulties, in photocatalysis this investigation is hindered by the fact that photoadsorption and photoreaction simultaneously start under irradiation of the catalyst then preventing the investigation of photoadsorption phenomenon per se. This chapter presents a quantitative method able to determine the amount of substrate photoadsorbed onto the catalyst surface in liquid–solid systems under reaction conditions. The method is applied to the following photocatalytic processes carried out in aqueous suspensions: (i) oxidation of phenol in the presence of a commercial TiO 2 catalyst (Degussa P25) and (ii) oxidation of benzyl alcohol in the presence of a home-prepared TiO 2 catalyst. The influence of substrate concentration, catalyst amount, and irradiation power on photoadsorption is investigated. The equilibrium data of photoadsorption capacity is fitted vs. the liquid-phase substrate concentration by using the following adsorption isotherms: Langmuir, Freundlich, and Redlich–Peterson. By taking into account the photoadsorption phenomena, the kinetic modeling of the photooxidation processes are carried out for each of the adsorption isotherms and the parameters of the models are determined. In the case of benzyl alcohol, the results indicate that the Langmuir adsorption isotherm best fits the photoadsorption phenomenon while for phenol the Freundlich adsorption isotherm gives a better fit. In both cases all the parameters of kinetic model are favorably affected by an increase of the photon flow absorbed per unit mass of catalyst.

Electrochemical Determination of Bisphenol A with Pencil Graphite Electrodes Modified with Co(II), Ni(II), Cu(II) and Fe(II) Phthalocyaninetetrasulfonates

Pencil graphite electrodes modified with Co(II), Ni(II), Cu(II) and Fe(II) metallophthalocyaninetetrasulfonates (MePcTSs) were investigated for an electrochemical determination of bisphenol A (BPA). The electrochemical performances of the modified electrodes for different pH values in phosphate and the Britton-Robinson buffers were determined by cyclic voltammetry; the electrode performances were better in the Britton-Robinson buffer. NiPcTS and CoPcTS modifications of the electrodes had remarkable enhancements on their performances. The differential pulse voltammetry parameters for the electrodes were optimized, and we found that the electrochemical response versus the concentration of BPA is linear from 5.0 × 10(-7) to 1.0 × 10(-5) M for the NiPcTS and CoPcTS modified electrodes. The detection limits of these modified electrodes are 2.9 × 10(-7) and 4.3 × 10(-7) M, respectively, and the effects of interfering species are less than 5%. The results show that NiPcTS and CoPcTS modified pencil graphite electrodes could be used for electrochemical determinations of BPA for analytical purposes.