Elena Selli

Pyridine and ammonia as probes for FTIR analysis of solid acid catalysts

Pyridine and ammonia have been used as probe molecules for the quantitative analysis of surface acidity of some solid catalysts by FTIR spectroscopy. For pyridine, a scale of acidity and an evaluation of the relative acid strength at different temperatures (373–773 K) were obtained for both Bronsted- and Lewis-acid sites. Correlation was verified between the concentration of Bronsted sites and the catalytic activity of the catalysts examined for the dehydration of 2-(2-hydroxyethyl)pyridine to 2-vinylpyridine. In contrast, ammonia was a much less reliable probe, mainly due to the overlapping of the resulting IR absorption bands. Moreover, it decomposed even at rather low temperatures, when adsorbed onto the catalysts.

Degradation of organic water pollutants through sonophotocatalysis in the presence of TiO2

The degradation of 2-chlorophenol and of the two azo dyes acid orange 8 and acid red 1 in aqueous solution was investigated kinetically under sonolysis at 20 kHz and under photocatalysis in the presence of titanium dioxide particles, as well as under simultaneous sonolysis and photocatalysis, i.e. sonophotocatalysis. The influence on the degradation and mineralisation rates of the initial substrate concentration and of the photocatalyst amount was systematically investigated to ascertain the origin of the synergistic effect observed between the two degradation techniques. The evolution of hydrogen peroxide during kinetic runs was also monitored. Small amounts of Fe(III) were found to affect both the adsorption equilibria on the semiconductor and the degradation paths. Ultrasound may modify the rate of photocatalytic degradation by promoting the deaggregation of the photocatalyst, by inducing the desorption of organic substrates and degradation intermediates from the photocatalyst surface and, mainly, by favouring the scission of the photocatalytically and sonolytically produced H(2)O(2), with a consequent increase of oxidising species in the aqueous phase.

Surface modification of poly(ethylene terephthalate) fibers induced by radio frequency air plasma treatment

The surface chemical and physical modifications of poly(ethylene terephthalate) (PET) fibers induced by radiofrequency air plasma treatments were correlated with the characteristics of the discharge parameters and the chemical composition of the plasma itself, to identify the plasma-induced surface processes prevailing under different operating conditions. Treated polymer surfaces were characterized by water droplet absorption time measurements and XPS analysis, as a function of the aging time in different media, and by AFM analysis. They exhibited a remarkable increase in hydrophilicity, accompanied by extensive etching and by the implantation of both oxygen- and nitrogen-containing polar groups. Etching was mainly a consequence of ion bombardment, yielding low molecular weight, water soluble oxidation products, while surface chemical modifications were mainly due to the action of neutral species on the plasma-activated polymer surface.

Comparison between the surface acidity of solid catalysts determined by TPD and FTIR analysis of pre-adsorbed pyridine

Abstract A series of both crystalline (Y zeolites, beta zeolites and mordenites) and amorphous alumino-silicates were investigated by means of temperature-programmed desorption with mass spectrometry detection (TPD-MS) and of Fourier transform infra-red (FTIR) analysis of adsorbed pyridine, in order to compare the surface concentration of acid sites potentially involved in a specific catalytic process, as determined through the two techniques. Up to three distributions of acid sites were evidenced in the investigated catalysts by deconvolution of the TPD-MS spectra recorded after pyridine adsorption at different temperatures (150–300°C), depending on the type of catalyst and on its dealumination degree. FTIR analysis showed that pyridine adsorption on acid solids occurs under kinetic control and that equilibrium attainment implies a redistribution of pyridine molecules between Bronsted and Lewis surface sites. The reliability of the two titration techniques was verified by the good match between the values of overall surface acidity determined through them under equilibrium conditions at 150°C.

Effects of gold nanoparticles deposition on the photocatalytic activity of titanium dioxide under visible light

The effects of gold nanoparticles deposited on titanium dioxide on the photocatalytic oxidative degradation of two organic substrates, i.e. formic acid and the azo dye Acid Red 1, and on the parallel O(2) reduction yielding hydrogen peroxide have been investigated under visible light irradiation. The method employed to reduce Au(iii) to metallic gold in the preparation of Au/TiO(2) photocatalysts was found to affect their photoactivity, also by modifying the properties of TiO(2). The presence of gold on TiO(2) facilitates both the electron transfer to O(2) and the mineralization of formic acid, which mainly proceeds through direct interaction with photoproduced valence band holes. The so-formed highly reductant CO(2)*(-) intermediate species may contribute in maintaining gold in metallic form. The controversial results obtained in the photocatalytic degradation of the dye were rationalised by taking into account that with this substrate, which mainly undergoes oxidation through a hydroxyl radical mediated mechanism, the photogenerated holes may partly oxidise gold nanoparticles, which consequently act as recombination centres of photoproduced charge carriers.

Enhanced photocatalytic formation of hydroxyl radicals on fluorinated TiO2.

Direct experimental evidence of the higher concentration of hydroxyl radicals generated on fluorinated titanium dioxide (F-TiO2) under irradiation was obtained by spin-trapping EPR measurements. The faster photoinduced bleaching of the azo dye Acid Red 1 (AR1) observed in the presence of F-TiO2 was explained by the high affinity of the azo double bond towards *OH radicals. Moreover, the pronounced decrease of the AR1 bleaching rate by addition of 2-propanol, as hydroxyl radicals scavenger, on F-TiO2 and not on naked TiO2 demonstrated that on fluorinated titania AR1 is mainly degraded via *OH radical attack.

Sono-photocatalytic degradation of 2-chlorophenol in water: kinetic and energetic comparison with other techniques

The degradation of 2-chlorophenol in water was kinetically investigated using the following different techniques, employed either separately or simultaneously, always with the same experimental set-up: light irradiation (315-400 nm), sonication, photocatalysis with different types of TiO2, photocatalysis with sonication. The influence of the reaction volume and of different gas mixtures, containing Ar and O2, as well as O2/O3, was also investigated. Finally, an energetic comparison among these different techniques was performed, focused on an industrial application of some of them.

Synergistic effects of sonolysis combined with photocatalysis in the degradation of an azo dye

Photocatalysis and sonolysis exhibit synergistic effects in the degradation of organic molecules in aqueous suspension, when low ultrasound frequency (i.e. 20 kHz) is employed. The degradation of the azo dye Acid Orange 8 in aqueous suspensions was systematically evaluated under sonolysis, photocatalysis and sonophotocatalysis as a function of dye concentration, amount and type of photocatalyst (TiO2 or ZnO) and catalase addition, in terms of pseudo-first-order degradation rate constants. The evolution of hydrogen peroxide was also monitored. Synergistic effects of the combination of ultrasound and photocatalysis should mainly involve the aqueous phase and be due to an increased concentration of reactive radicals, consequent to the action of ultrasound on the peroxide species produced by both photocatalysis and sonolysis.

Intrinsic Au Decoration of Growing TiO2 Nanotubes and Formation of a High‐Efficiency Photocatalyst for H2 Production

Electrochemical anodization of low-concentration (0.02-0.2 at% Au) TiAu alloys in a fluoride electrolyte leads to self-organized TiO2 nanotubes that show a controllable, regular in situ decoration with elemental Au nanoclusters of ≈5 nm in diameter. The degree of self-decoration can be adjusted by the Au concentration in the alloy and the anodization time. Such Au particle decorated tubes show a high activity for photocatalytic H2 production from ethanol solutions.

H2O2 evolution during the photocatalytic degradation of organic molecules on fluorinated TiO2

The effect of TiO2 surface fluorination on the hydrogen peroxide evolution occurring in photocatalytic runs was investigated employing the azo dye Acid Red 1 (AR1) and two model organic molecules with acidic properties, i.e. formic acid (FA) and benzoic acid (BA), as substrates of oxidative degradation. While AR1 and BA photocatalytic degradation on fluorinated titanium dioxide (F–TiO2) was markedly faster than on unmodified TiO2, because of enhanced hydroxyl radical formation, H2O2 concentration during the photodegradation of both substrates on F–TiO2 was lower, possibly because of the reduced rate of interfacial electron transfer. By contrast, FA underwent slower photocatalytic degradation on F–TiO2, but, at the same time, hydrogen peroxide concentration was relatively high, while no H2O2 could be detected during FA photodegradation on unmodified TiO2. Photocatalytic runs in the presence of the nitrate anion, able to react with the CO2˙− species produced from FA oxidation, but not with conduction band electrons, demonstrated that CO2˙− plays a relevant role in H2O2 formation during FA degradation on F–TiO2. In fact, surface fluoride, having a shielding effect at the semiconductor–water interface, not only inhibits the photocatalytic decomposition of H2O2, but also favours CO2˙− desorption and reaction with dissolved O2, generating H2O2. By contrast, CO2˙− mainly gives electron transfer to the conduction band of naked TiO2 and surface reduction of the photocatalytically produced H2O2.