J.A. Navío

Photocatalytic properties of iron-doped titania semiconductors

An exhaustive review on the photochemical properties of iron-doped TiO2 semiconductors is presented. Photocatalytic reactions (reductions and oxidations) using Fe-containing TiO2 on different organic and inorganic substrates are reported. Different aspects relating to structural, surface and photophysical properties of these photocatalysts are extensively discussed. The origin of the photoactivity of this kind of mixed oxides is considered with regards to previously proposed physical and chemical processes and on the role of the iron content.

TiO2 activation by using activated carbon as a support: Part I. Surface characterisation and decantability study

In the present work, the effect of mixing TiO2 with different portions of activated carbon (AC) has been studied. Transmission electron microscopy (TEM), scanning electron micrograph (SEM), laser scattering, FTIR and UV spectrophotometric analysis have shown changes of some of the TiO2 physico-chemical characteristics in catalysts containing an AC concentration lower than 15% in weight. Changes in the catalyst colour, the vibration bands of the hydroxylic groups from FTIR studies and the UV absorption spectrum have been observed. It seems that these changes correspond with TiO2 acid–base changes. SEM and TEM studies show a perfect TiO2 particles distribution on the AC surface in catalysts with lower AC contents, that also yields a homogeneous particle size distribution. One of the most interesting features of the resulting catalysts is their fast decantability in comparison with that of TiO2. This way one of the most important drawbacks of photocatalysis, the catalyst separation from the solution, may be overcome. The present study shows that the AC not only exerts a synergistic effect when is combined with TiO2 as some authors suggest, but it also modifies the catalyst characteristics.

Photocatalytic deactivation of commercial TiO2 samples during simultaneous photoreduction of Cr(VI) and photooxidation of salicylic acid

Abstract TiO 2 Degussa P25 and Hombikat UV-100 photocatalysts have been widely characterized. Certain structural and morphological differences have been found. Photocatalytic behaviour has been studied for Cr(VI) reduction and for salicylic acid oxidation, in photoreactions with single and mixed substrates. In the photoreaction with mixed substrates, higher conversion values have been found in comparison to those observed for single substrates. However, clear deactivation process can be noticed. This photocatalytic deactivation is higher for Degussa P25 photocatalyst than for Hombikat UV-100 and can be explained in terms of the surface features exhibited for both TiO 2 during the photoprocesses.

TiO2 activation by using activated carbon as a support: Part II. Photoreactivity and FTIR study

Abstract It has been demonstrated that the modification of the TiO2 acid–base properties by the presence of activated carbon (AC) considerably modify the interaction of some molecules with the catalyst surface and hence its photocatalytic characteristics. FTIR studies have shown that while phenol interacts with TiO2 yielding a phenoxide, in the catalyst low AC containing catalysts mixtures the surfacial hydroxylic groups are inserted in the aromatic ring in the symmetric position to the –OH group. Additionally, the 4-aminophenol study shows that the molecule interacts with the TiO2 surface by means of the amino group, while in the catalyst containing AC the interaction takes place through the hydroxylic group. These results have confirmed the catalyst acid–base properties changes determined by the characterisation studies already performed. Also, it has been observed that catalysts with the lowest AC content show better catalytic behaviour than the untreated TiO2 and those with higher AC load. Another interesting result is the efficiency of these catalysts under solar irradiation.

Photocatalytic properties of ZrO2 and Fe/ZrO2 semiconductors prepared by a sol-gel technique

The photocatalytic efficiency of pure and 0.5‐20 wt.% Fe-containing ZrO 2 samples, prepared by a sol‐gel technique, was tested in the transformation of environmentally important substrates, such as nitrite, EDTA and Cr(VI) and compared with that of TiO2 (Degussa P-25). All samples were active, although the efficiency was lower than that of TiO 2. Nevertheless, the presence of iron increased the activity for nitrite photooxidation, with the maximum efficiency at 5 wt.% Fe content. At higher Fe content, the efficiency decreased but it was always higher than that of the undoped sample. For EDTA oxidation and Cr(VI) reduction, the same trend was observed. Remarkably, Cr(VI) reduction in the presence of EDTA was strongly accelerated, all zirconia samples being as active as P-25, although initial rates were lower and somewhat detrimentally affected by the presence of iron. The photocatalytic activity depends on the structural, surface and optical properties of the sample, on the preparation conditions and on the nature of the photocatalytic reaction. ©1999 Elsevier Science S.A. All rights reserved.

Bulk and surface characterization of powder iron-doped titania photocatalysts

Some additional contributions to structural and surface features of powder iron-doped titania photocatalysts have been investigated by applying a variety of complementary techniques: temperature programmed desorption-mass spectrometry analysis, non-isothermal thermogravimetric analysis, differential thermoanalysis, infrared spectroscopy, scanning electron microscopy and X-ray diffraction analysis. The effects of iron concentration and pretreatment temperature, over the polymorphic transformation anatase-rutile, were analysed. The results obtained were used in an attempt to correlate structural and surface aspects of catalysts, with their reported activity versus inactivity to photoreduce dinitrogen to ammonia.

Comparison of the photocatalytic efficiency of TiO2, iron oxides and mixed Ti(IV)-Fe(III) oxides : photodegradation of oligocarboxylic acids

Abstract Comparative photo-oxidations of three oligocarboxylic acids, i.e. oxalic acid, EDTA and malonic acid, were performed with different catalysts, i.e. TiO 2 (Degussa P-25), α-Fe 2 O 3 , γ-Fe 3 O 4 and two samples of mixed Fe(III)Ti(IV) oxides containing 0.5 wt.% and 5 wt.% Fe respectively. Different irradiation wavelengths were used. It was found that the photocatalytic efficiency depends mainly on the physicochemical properties of the catalyst, the irradiation wavelength, and the redox potential and adsorption ability of the substrate. The effect of surface complexes formed between the oxide and the organic compound was analysed. TiO 2 was found to be the best catalyst. The relative utility and stability of iron oxides and mixed FeTi oxides is discussed.

A novel preparation of high surface area TiO2 nanoparticles from alkoxide precursor and using active carbon as additive

Abstract TiO 2 nanoparticles have been prepared by a novel alkoxide sol–gel precipitation. The presence of active carbon in different percentages could act as an interesting template. Upon calcination, carbon is eliminated leaving surface features significantly different from TiO 2 prepared in the absence of carbon. Wide surface and structural characterisation of samples have been carried out. Correlations with carbon percentage is pointed out from this characterisation. Interesting spherical aggregates of nanosized TiO 2 are observed from TEM images probably stabilised by the presence of carbon. Physicochemical correlations made will be very useful in further application of these TiO 2 to be used as potential high surface area photocatalyst.

Novel Bi2WO6–TiO2 heterostructures for Rhodamine B degradation under sunlike irradiation

Highly efficient Bi(2)WO(6)-TiO(2) heterostructure is synthesized by means of a hydrothermal method having highly photoactivity for the degradation of Rhodamine B under sunlike irradiation. From the structural characterization it has been demonstrated that TiO(2) is incorporated on the Aurivillius structure. Interesting synergetic effect between TiO(2) and Bi(2)WO(6) leads to an improved charge carrier separation mechanism, causing the excellent photocatalytic performance under sunlike irradiation. The photocatalytic performance of Bi(2)WO(6) and Bi(2)WO(6)-TiO(2) was compared under different irradiation conditions and using increasing Rhodamine B concentration up to 25 ppm. After the photocatalytic analysis of both systems, the mineralization efficiency of the heterostructure appears significantly higher with respect to Bi(2)WO(6).

Formation of zirconium titanate powder from a sol-gel prepared reactive precursor

A hydroxoperoxo compound of Zr and Ti (HXPZT) was prepared as a reactive powder by a sol-gel method. X-ray diffraction and SEM studies showed it to be formed from small (ca. 10–15 μm) amorphous particles. The effects of progressive thermal treatment were determined by differential thermal analysis, thermogravimetric analysis, temperature programmed desorption, X-ray diffraction, energy-dispersive spectrometry and SEM. The features observed have been attributed to the removal of physically and chemically bonded water and of some nitrogen-containing compound. Crystallization of zirconium titanate occurs at low temperature (650 °C), giving rise to spherical (ca. 2 μm dia.) regular particles.