A. Sclafani

Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media

Abstract The influence of deposited silver upon the photocatalytic activity of titania in the rutile and anatase allotropic forms has been studied in three different reactions. The common feature found for these three reactions was an electron transfer from illuminated TiO 2 to silver particles. In the first reaction (platinum photodeposition). it has been shown that the initial presence of metallic silver orientates the localization of subsequent Pt photodeposits with, in particular, a 100% selectivity to Pt deposition on top of silver particles or agglomerates previously deposited on anatase. In the second reaction (2-propanol oxidation), Ag deposit was found beneficial for the activity of rutile and detrimental for that of anatase. For rutile which is less active, silver helps for the electron-hole pair dissociation. By contrast, for anatase, the negatively charged Ag particles preferentially attract photoholes and become recombination centers, thus decreasing the photocatalytic activity. For the third reaction (dehydrogenation of 2-propanol), Ag/TiO 2 catalysts were found very poorly active, as expected for a group I-B metal. However, in the additional presence of platinum deposits, Pt/Ag/TiO 2 catalysts are as active as their Pt/TiO 2 homologues, thus confirming that Pt is deposited on top of silver with good electronic contacts between both metals. The three photocatalytic reactions have common electronic processes based on the photoelectron generation on titania and the subsequent electron transfer to the metal (s).

Photocatalytic degradation of phenol in aqueous titanium dioxide dispersions

The results of a study of photocatalytic degradation of phenol using aqueous oxygenated TiO2 (anatase) suspensions in a batch Pyrex photoreactor are reported. The influence on the photodegradation rate of various parameters as pH, phenol and TiO2 content, oxygen partial pressure, anions present in the dispersions was investigated. A complete oxidation of phenol was observed. Intermediate compounds, catechol and quinone, were detected. It was observed that the photodegradation also proceeded with sunlight radiation. A mechanistic and kinetic model, which accounts for the results obtained, is given. Likely reasons for inactivity of the rutile modification for this reaction are also given.

Photocatalytic degradation of nitrophenols in aqueous titanium dioxide dispersion

Abstract The results are reported of an investigation into the photodegradation of nitrophenol isomers carried out in aqueous titania (anatase) dispersions under various experimental conditions. We investigated the influence of various parameters on the photodegradation rate, such as the concentrations of the catalyst, dissolved oxygen and organic compound, and the presence of various anions in the dispersion. A plausible explanation is proposed for the influence of these parameters, both mechanistic and kinetic factors being accounted for. Gas chromatography and mass spectrometry were used as analytical methods in order to determine intermediate products. Spectroscopic methods (UV-vis and IR) were used to obtain information on the fate of adsorbed and intermediate species formed during the reaction.

Effect of silver deposits on the photocatalytic activity of titanium dioxide samples for the dehydrogenation or oxidation of 2-propanol

Abstract Until now, the effect of the deposition of metal particles onto a powdered semiconductor oxide on its photocatalytic activity has been determined for group VIII transition metals only. In this study, AGTiO2 samples were prepared by photodeposition, characterized by transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) and tested, under band-gap illumination, using the oxidation of 2-propanol (neat liquid phase or mixed with water) in the absence or presence of molecular oxygen. The silver deposits were found to be metallic both before and after use, but the size of the particles and their distribution on the support were markedly influenced by the type of TiO2 sample. A small amount of silver caused the catalytic oxygen-free oxidation of 2-propanol, although the rate was much lower than that with platinum which possesses dehydrogenating properties. This effect is thought to be due to the electron-attracting role of the metal particles. For oxidation of 2-propanol by molecular oxygen, an optimum silver content was observed (corresponding to a rate improvement of approximately 50%). This was interpreted to be due to the beneficial effects of silver on the trapping of electrons and/or the reduction of oxygen. At higher than optimum silver contents there was a detrimental increase in the probability of recombination of charges at the numerous silver particles. The optimum was shifted to lower values in the presence of water, which is tentatively explained by competitive hole trapping by 2-propanol, water and silver.

Photocatalytic degradaton of phenol in aqueous polycrystalline TiO2 dispersions: the influence of Fe3+, Fe2+ and Ag+ on the reaction rate

The influence of Fe3+, Fe2+ and Ag+ on phenol photodegradation performed in aqueous polycrystalline TiO2 (anatase) and TiO2 (rutile) dispersions was investigated. These ions can react very easily with peroxo species produced on the catalyst surface and/or in the solution. Experimental conditions for the continuous photoproduction of Fenton reactive can be achieved in this way. Maximum photoactivity was observed for TiO2 (anatase) in the presence of oxygen and [Fe3+] = 5 × 10−4 M. The behaviour of Fe2+ was similar to Fe3+ for the same experimental conditions. Anatase photoactivity was beneficially influenced in the presence of oxygen and [Ag+] = 1 × 10−4 M.

Photocatalytic reduction of CO2 with H2O on TiO2 and Cu/TiO2 catalysts

Photoinduced reduction of CO2 by H2O to produce CH4 and CH3OH has been investigated on wellcharacterized standard TiO2 catalysts and on a Cu2+ loaded TiO2 catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H2O to CO2. Anatase TiO2, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH4 formation. Photogenerated Ti3+ ions, H and CH3 radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO2 catalyst (Cu/TiO2) brings about additional formation of CH3OH. XPS studies suggest that Cu+ plays a significant role in CH3OH formation.

Influence of hydrogen peroxide on the kinetics of phenol photodegradation in aqueous titanium dioxide dispersion

Abstract The influence of hydrogen peroxide on the phenol photodegradation rate under near UV radiation was investigated in homogeneous and heterogeneous systems: the heterogeneous systems were aqueous dispersions of titanium dioxide (anatase). The hydrogen peroxide photodecomposition reaction was also investigated for all the systems used. The highest rate of phenol photodegradation was observed in the heterogeneous system for the contemporary presence of hydrogen peroxide and oxygen. The rate equations for phenol photodegradation and hydrogen peroxide photodecomposition for both systems were obtained. A mechanistic model able to explain the main experimental results is also proposed.

Influence of tungsten oxide on structural and surface properties of sol–gel prepared TiO2 employed for 4-nitrophenol photodegradation

A set of tungsten oxide/TiO2 polycrystalline samples (W/Ti) prepared by the sol–gel method has been characterised using several techniques, namely thermogravimetric analysis, gas-chromatography, mass spectrometry, thermogravimetric analysis combined with mass spectrometry or with gas chromatography/mass spectrometry, X-ray diffraction, X-ray photoelectron spectroscopy, determination of specific surface area, scanning electron microscopy, Fourier-transform infrared spectroscopy and monitorisation of pyridine adsorption for surface acidity. Moreover the samples have been empolyed as catalysts for 4-nitrophenol photodegradation in aqueous suspensions used as a ‘probe’ reaction. Characterisation results indicate that the surface of the W/Ti particles is enriched with homogeneously dispersed tungsten as well as microcrystalline or amorphous species. Maximum photoactivity for 4-nitrophenol photodegradation was achieved for a sample containing 1.7 mol W per 100 mol Ti.

Surface properties of iron-titania photocatalysts employed for 4-nitrophenol photodegradation in aqueous TiO2 dispersion

Iron(III) doped specimens (0.01–5% atomic Fe ∶ Ti) have been prepared by impregnating polycrystalline “home prepared” TiO2 (mainly anatase) and have been studied for photocatalytic degradation of 4-nitrophenol in aqueous dispersions. Some structural and surface properties have been studied by X-ray diffraction, specific surface area and porosity measurements and FTIR monitoring of pyridine, ammonia and methanoic acid adsorption for surface acidity and basicity. Their surface properties were compared with a corresponding series of photocatalysts prepared by a coprecipitation method. Adsorption of pyridine and ammonia indicates the presence of surface acid Lewis sites, a low concentration of surface Brønsted sites being present only in the samples prepared by impregnation. The photocatalytic experiments indicated that the presence of iron ions does not influence or is detrimental for the occurrence of the 4-nitrophenol photodegradation.

Photocatalytic oxidation of phenol on TiO2 powders. A Fourier transform infrared study

Abstract In the present paper a detailed investigation of a well known photoreaction, i.e. phenol photodegradation in the presence of TiO 2 , has been carried out. The Fourier transform infrared (FT-IR) technique has been used to characterize the catalyst surface and to follow the photoprocess by “in situ” measurements in a gas—solid regime, simulating the aqueous liquid—solid regime in which this photodegradation reaction has been usually carried out. The influence on the photoprocess of the surface hydroxylation of the catalyst has been investigated and the presence of some reaction intermediates has been revealed by FT-IR. The experimental results have demonstrated that the chosen gas—solid system in the presence of water vapour is an acceptable simulation of the aqueous liquid—solid regime.