José A. Ayllón

TiO2 deactivation during gas-phase photocatalytic oxidation of ethanol

The deactivation of TiO2 Degussa P25 during the gas-phase photocatalytic oxidation of ethanol has been studied. Water vapor plays a clear competitive role for surface sites adsorption, thus hampering the ethanol photo-oxidation. Dark adsorption of ethanol on a fresh catalyst shows a Langmuirian behavior with the formation of a monolayer of adsorbate. Dark adsorption in a TiO2 surface that has been used in consecutive photocatalytic experiments of ethanol degradation gives non-Langmuirian isotherms, indicating the existence of noticeable changes of the catalyst surface structure. After several irradiations the catalyst activity decreases. Such deactivation has been investigated, observing that the rate constant of ethanol and acetaldehyde (its main degradation product) oxidation decreases with irradiation time. Several surface treatments have been studied in order to find suitable procedures for catalytic activity recovery, but regular decay of activity is always observed after every treatment.

Photocatalytic degradation of short-chain organic diacids

Abstract The heterogeneous photocatalytic oxidation of fumaric, maleic and oxalic acids over TiO 2 has been investigated. For aqueous suspensions at pH lower than the point of zero charge (pzc) of TiO 2 , the photocatalytic degradation of the three studied diacids follows the Langmuir–Hinshelwood kinetic model, with the rate constant of the process decreasing in the order oxalic acid > maleic acid ≅ fumaric acid. At these low pH media, the adsorption of the organic diacids onto TiO 2 particles is a key feature for their degradation, which is initiated by a photo-Kolbe process. For fumaric and maleic acids, a cis – trans isomerisation induced by the interaction between adsorbed molecule and semiconductor surface occurs. At pH’s higher than the pzc of TiO 2 the rate of oxalic acid oxidation decreases noticeably, while fumaric and maleic acids are both efficiently degraded in homogeneous phase by reacting with OH radicals photochemically generated on the TiO 2 surface, giving rise to a significant increment of both isomers degradation rate with increasing pH. At these pH’s higher than the pzc of the TiO 2 , the three studied diacids show a very low degree of adsorption onto the semiconductor surface and no evidence of cis – trans isomerisation for both maleic and fumaric acids is detected. In accordance with the observed pH effects on degradation rate and over detected intermediates, a different mineralisation pathway is proposed as function of initial pH.

TiO2 thin film deposition from solution using microwave heating

Abstract A new method has been devised for the deposition of TiO2 thin films on conducting glass using a microwave reactor. The substrates are immersed in a diluted homogeneous aqueous solution which was prepared by mixing equal volumes of a fluorine-complexed titanium(IV) solution ([Ti]=3.4×10−2 M) and 6.8×10−2 M boric acid solution. Low microwave power and short deposition time have been used. The TiO2 layers obtained are well-adhered, homogenous, with good specularity and colored by interference of reflected light. Their thickness is in the range of 100–500 nm. SEM experiments denote that films are formed by small crystallites having linear dimensions under 100 nm. Crystal dimensions depend on microwave power and deposition time. The layers show a high degree of crystallinity and the observed crystal phase is anatase. Microwave heating has proved to be an efficient and inexpensive method for solution growth of TiO2 films; it should also be of importance for other materials layers grown from solution.

Synthesis and photocatalytic activity of mesoporous anatase prepared from tetrabutylammonium-titania composites

Abstract The synthesis of new titania-based materials obtained from the hydrolysis of titanium(IV) isopropoxide (TIP) in the presence of tetrabutylammonium hydroxide (TBAOH) was investigated. Hybrid composites constituted by tetrabutylammonium (TBA) cations embedded into a titania matrix were produced in a first stage. Powder X-ray diffraction (XRD) patterns of these composites exhibited broad low-angle reflections, which indicates the existence of some degree of ordering on the scale of mesopores. Porous titania with BET surface areas up to 110 m 2 g −1 and diverse distributions of pores in the range of mesopores were obtained after different treatments devoted to the removal of TBA from these composites. The photocatalytic activity of the pure titania final products was tested by the gas phase oxidation of formaldehyde.

Comparative study of nanocrystalline TiO2 photoelectrodes based on characteristics of nanopowder used

TiO2 photoelectrodes are made using the “dip-coating” technique with colloidal suspensions of TiO2 nanopowders. Two different commercial TiO2 powders are selected with nanocrystals having different porosity and somewhat different crystal structure. Photoelectrodes are characterized using a practical two-electrode photoelectrochemical cell arrangement. Differences in open-circuit photovoltage and short-circuit photocurrent are reported and explained based on initial powder characteristics. Differences in time response found are also explained.

1,2-Diolates of titanium as suitable precursors for the preparation of photoactive high surface titania

Diolates of titanium of stoichiometric formula (Ti(OCHRCH2O)2 )( R = H, CH3, CH2CH3) were prepared by reacting a titanium source (titanium(IV) isopropoxide (TIP) or titanium(IV) chloride) with an excess of the respective 1,2- diol. The diol was employed in a great excess; no other solvent was present in the synthesis procedure. The titanium diolates obtained have a white powder appearance and show a high degree of crystallinity as deduced from powder X-ray diffraction. It has been observed that their stability towards moisture is determined by the R group (R = H, CH3, CH2CH3). Hydrolysis of these titanium 1,2-diolates in water was a simple way to produce high surface porous titania with BET surface area up to 300 m 2 g 1 . These as-synthesised titania contain anatase crystallites and show photocatalytic activity versus formaldehyde degradation prior to calcination. Nevertheless, the decomposition rate of formaldehyde increased notably when titania samples were annealed at 773 K. This thermal treatment increases their crystallinity at the expense of a BET surface area decrease down to 75 m 2 g 1 or less. The photocatalytic performance of the annealed samples is superior than that of commercial TiO2 (Degussa P-25, Hombikat UV-100). ©1999 Elsevier Science B.V. All rights reserved.

Titanium(IV) oxide thin films obtained by a two-step soft-solution method

Abstract TiO 2 films were deposited on either glass or Si substrates by using a two-step soft-solution method. A submonolayer of anatase TiO 2 nanocrystals was first deposited on the substrate by a drain-coating process that was performed at 333 K from an aqueous TiO 2 colloidal solution. The substrate partially covered with the TiO 2 nanocrystals was immersed in an aqueous solution, containing a titania precursor [fluorine-complexed Ti(IV)], and the whole was treated with microwave irradiation. The nanocrystals deposited on the substrate acted as growth seeds in the subsequent formation of the TiO 2 film. The obtained TiO 2 films showed a high degree of crystallinity (anatase), even without further thermal treatment; however, they did not show photocatalytic activity. Thickness of the films was varied as a function of microwave power and irradiation time.

Preparation of anatase powders from fluorine-complexed titanium(IV) aqueous solution using microwave irradiation

The microwave radiation activated precipitation of titanium dioxide from homogeneous aqueous solutions of titanium–fluoro complexes and boric acid has been carried out in batch reactors. Crystalline anatase powders with narrow monodisperse size distribution have been obtained in a short time, at low temperature and atmospheric pressure. Microwave power, irradiation time and precursor concentration determine the properties of the obtained materials, i.e. crystal size, powder aggregate morphology and BET surface area. The obtained materials do not show photocatalytic activity, probably due to a considerable fluoride content that is in the range of 2–5%.

A novel B–H⇀Ru agostic bond. Crystal structure of [RuCl{7,8-µ-S(CH2CH2)S-C2B9H10}(PPh3)2]·Me2CO

The complex [RuCl{7,8-µ-S(CH2CH2)S-C2B9H10}(PPh3)2]·Me2CO 1 contains octahedral ruthenium(II), the sixth coordination site being occupied by an agostic B–H⇀Ru in the first example of B(3)–H⇀Ru coordination in the solid state; solution NMR studies show the presence of B(3)–Ru–H bonding.

Electrochemically assisted deposition of titanium dioxide on aluminium cathodes

Adherent, uniform, and porous TiO2 films have been obtained at ambient temperature by a novel electrochemical process involving the simultaneous cathodic electrodeposition of a Ti(IV)–peroxo complex and electrophoretic deposition of nanocrystalline TiO2 particles on an Al cathode. Electrodepositions were performed using an electrolytic suspension containing a water-soluble titanium complex stabilized with a peroxo ligand and a commercial nanocrystalline TiO2 powder (Degussa P25), and by applying a constant cell voltage between 2.0 and 4.5 V. The reduction process favours the decomposition of the soluble titanium complex to yield an amorphous TiO2 precipitate on the cathode. This amorphous precipitate also acts as a binder to the nanocrystalline TiO2 particles. The whole assembly shows photocatalytic activity for the degradation of salicylic acid without the need of thermal post-treatment.