Structural, Electronic, and Vibrational Properties of the Ti−O−Ti Quantum Wires in the Titanosilicate ETS-10

Ti-Based Catalysts and Photocatalysts: Characterization and Modeling

This perspective article aims to underline how cutting-edge synchrotron radiation spectroscopies such as extended X-ray absorption spectroscopy (EXAFS), X-ray absorption near edge structure (XANES), high resolution fluorescence detected (HRFD) XANES, X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS) have played a key role in the structural and electronic characterization of Ti-based catalysts and photocatalysts, representing an important additional value to the outcomes of conventional laboratory spectroscopies (UV-Vis, IR, Raman, EPR, NMR etc.). Selected examples are taken from the authors research activity in the last two decades, covering both band-gap and shape engineered TiO₂ materials and microporous titanosilicates (ETS-10, TS-1 and Ti-AlPO-5). The relevance of the state of the art simulation techniques as a support for experiments interpretation is underlined for all the reported examples.

Imaging the Atomic Position of Light Cations in a Porous Network and the Europium(III) Ion Exchange Capability by Aberration-Corrected Electron Microscopy

In the present work, ETS-10 microporous titanosilicate has been synthesized and its structure characterized by means of powder XRD and aberration corrected scanning transmission electron microscopy (C_s -corrected STEM). For the first time, sodium ions have been imaged sitting inside the 7-membered rings. The ion-exchange capability has been tested by the inclusion of rare earth metals (Eu, Tb and Gd) to produce a luminescent material which has been studied by atomic-resolution C_s -corrected STEM. The data produced has allowed unambiguous imaging of light atoms in a microporous framework as well as determining the cationic metal positions for the first time, providing evidence of the importance of advanced electron microscopy methods for the study of the local environment of metals within zeolitic supports providing unique information of both systems (guest and support) at the same time.

Probing the surfaces of heterogeneous catalysts by in situ IR spectroscopy

This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with "in situ" controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H(2), CO, NO and C(2)H(4) are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some "key studies" and trying to evidence similarities and differences among surfaces and probes (572 references).

Selective photocatalytic transformations on microporous titanosilicate ETS-10 driven by size and polarity of molecules

Photocatalytic activity of microporous titanosilicate ETS-10 has been studied in water. The photoactivated ETS-10 shows catalytic activity driven by size and polarity of substrates. ETS-10 efficiently catalyzes a conversion of substrates with a size larger than the pore diameter of ETS-10. In contrast, the reactivity of small substrates depends strongly on substrate polarity; less polar substrates show higher reactivity on ETS-10. Electron spin resonance analysis reveals that large substrates or less polar substrates scarcely diffuse inside the highly polarized micropores of ETS-10 and, hence, react efficiently with hydroxyl radicals (*OH) formed on titanol (Ti-OH) groups exposed on the external surface of ETS-10. In contrast, small polar substrates diffuse easily inside the micropores of ETS-10 and scarcely react with *OH, resulting in low reactivity. The photocatalytic activity of ETS-10 is successfully applicable to selective transformations of large reactants or less polar reactants to small polar products, enabling highly selective dehalogenation and hydroxylation of aromatics.

Enhanced photocatalytic degradation and selective removal of nitrophenols by using surface molecular imprinted titania

Poor selectivity of titania (TiO2) photocatalysis is unfavorable to photocatalytic removal of highlytoxic low-level organic pollutants in polluted waters in the presence of other less toxic high-level pollutants. A new strategy of increasing this selectivity is the surface modification of TiO2 via coating a thin layer of molecular imprinted polymer (MIP), which provides molecular recognition ability toward the template molecules. By using 2-nitrophenol and 4-nitrophenol as target pollutants, MIP-coated TiO2 photocatalysts were prepared via surface molecular imprinting and were observed to have high activity and selectivity toward the photodegradation of the targets. In the presence of bisphenol A (50 mg L(-1)) as a nontarget pollutant, the apparent rate constant for the photodegradation of the target 2-nitrophenol and 4-nitrophenol (1.8 mg L(-1)) over the corresponding MIP-coated TiO2 was 10.73 x 10(-3) and 7.06 x 10(-3) min(-1), being 2.46 and 4.61 times of that (4.36 x 10(-3) and 1.53 x 10(-3) min(-1)) over neat TiO2, respectively. The enhanced photocatalytic selectivity was increased when the concentration of the target was decreased and/or when the difference in both the chemical structure and molecule size between the target and nontarget molecules was increased. The increased selectivity was mainly attributed to the special interaction between the target molecules and the footprints polymer via the functional groups (-OH and -NO2).

Photoelectrocatalytic activity of highly ordered TiO2 nanotube arrays electrode for azo dye degradation

A highly ordered titanium dioxide nanotube arrays (HOTDNA) electrode was prepared in hydrofluoric acid solution by electrochemical anodization technique on a pure titanium sheet. The HOTDNA electrode was characterized by FE-SEM microscopy, X-ray diffraction, and UV-vis spectra. The linear-sweep photovoltammetry response on the HOTDNA electrode is presented in this work. The photogenerated current of 0.3 mA/cm2 was observed significantly upon illumination with applied potential of 0.5 V in the UV regions at the central wavelength of 253.7 nm. Photoelectrocatalytic (PEC) and photocatalytic (PC) activities of the HOTDNA electrode were evaluated in the degradation of methyl orange (MeO) in aqueous solution. A set of optimized conditions such as anodic potential, calcinations temperature, and MeO concentration on the PEC activity was investigated. The PEC and PC activities of HOTDNA electrode were compared. We concluded that the HOTDNA electrode was an effective photoelectrode for achieving an enhanced MeO degradation.

Selective catalysis and nanoscience: an inseparable pair

Selective catalysts can be considered as nanomachines designed to perform the synthesis of molecules with high reaction activity and high selectivity. These properties arise from a precise control of the structure of the active sites, of the three-dimensional environment and of their relationship. In both homogeneous and heterogeneous catalysts the active site three-dimensional environment ensemble is always a complex structure resembling the tuneable structure of enzymes, which are the most efficient catalysts optimized by nature over billions of years. To illustrate this concept the structure of a few homogeneous and heterogeneous catalysts for alkenes hydrogenation and for olefin polymerization are chosen and discussed as examples.

(I2)n Encapsulation inside TiO2: A Way To Tune Photoactivity in the Visible Region

We report on the synthesis of nanovoid-structured TiO(2) material via a sol-gel route using titanium isopropoxide as precursor. The nanovoids are formed during the thermal treatment in air at 773 K. The surfaces of internal cavities are populated by the partial oxidation products of the organic part of the Ti precursor (CO(2), hydrogen carbonates, and residual isopropoxide groups). The thermal treatment in air at 773 K allows the maintainence, in the internal voids, of the encapsulated species. Addition of iodine in the synthesis procedure results in a new nanovoid-structured titanium oxide able to absorb light in the whole visible part of the electromagnetic spectrum. The origin of this absorption is attributed to the presence of (I(2))(n) adducts encapsulated in the nanocavities. These species coexist with partial combustion products of isopropoxide groups. Due to the protection of the TiO(2) walls, the (I(2))(n) adducts are not destroyed by thermal treatments in air. We have investigated whether the electron promoted in the excited state of the dye molecule (upon absorption of visible light from the (I(2))(n) adducts) can be injected into either the TiO(2) conduction band or some titanium-localized acceptor, followed by migration of the injected electron to the surface where it reduces adsorbed organic molecules. Preliminarily experiments conducted with sunlight show that the surface-specific efficiency of this process, tested by following the degradation of methylene blue, is about 10 times higher than that of the P25 commercial TiO(2) photocatalyst.

Reactivity of Ti(iv) species hosted in TS-1 towards H2O2–H2O solutions investigated by ab initio cluster and periodic approaches combined with experimental XANES and EXAFS data: a review and new highlights

This work is intended to underline how the most-advanced experimental and theoretical physical chemistry tools must be used synergistically to understand the reactivity of Ti-silicalite-1 (TS-1) in an important number of low-temperature oxidation reactions with aqueous H(2)O(2) as the oxidant. Literature results are carefully reviewed and accompanied with new, unpublished highlights of both experimental and computational origin. The first part of this work is devoted to a discussion of the defective nature of the material and to the synergic role played by Si vacancies and Ti insertion in the framework. A summary of the experimental Ti-K-edge EXAFS and XANES literature concerning the activated material in vacuo conditions is then presented and compared to the corresponding Ti geometries obtained by ab initio calculations. From such a comparison, the excellent agreement between experiment and theory is evident. A very good agreement is also obtained for the interaction with water and ammonia. For both H(2)O and NH(3), the agreement is due to the possibility to perform experiments in which the probe molecule is dosed from the gas phase, thus allowing to reach the 1 : 1 (or 1 : 2) ratio between the adsorbing Ti sites and the adsorbed molecule. Then, interaction with hydrogen peroxide is discussed, underlining the problems faced in reaching a common view between experimental and theoretical results, owing to the difficulties both in performing experiments where anhydrous H(2)O(2) is dosed on TS-1, and in taking into account the role played by the aqueous medium in the reactivity of Ti(iv) centres toward H(2)O(2) using ab initio calculations. Only once such difficulties have been overcome, by increasing the complexities of both experiments and ab initio models, is a joint-view finally obtained. Where needed, comparison with other experimental results (X-ray and neutron diffraction, NMR, IR, Raman, UV-vis and resonant Raman) is made.

On the influence of ion exchange on the local structure of the titanosilicate ETS-10

The effect of ion exchange with different monovalent cations (NH(4)(+), K(+), Na(+) and Cs(+)) on the local structure of the titanosilicate ETS-10 has been studied by (29)Si MAS NMR and Raman spectroscopy. Although X-ray diffraction shows no significant influence of ion exchange on the long range order, ammonium exchange is found to result in substantial damage to the local structure. Ion exchange experiments with alkali cations under significantly more acidic conditions clearly show that the structural damage brought about by ammonium exchange is not caused by the low pH of the exchange solution. The exchange with potassium and caesium ions also leads to significant changes in the (29)Si NMR and Raman spectra. However, these changes can largely be reversed by sodium back-exchange.

Electronic and Geometric Properties of ETS-10: QM/MM Studies of Cluster Models

Hybrid DFT/MM methods have been used to investigate the electronic and geometric properties of the microporous titanosilicate ETS-10. A comparison of finite length and periodic models demonstrates that band gap energies for ETS-10 can be well represented with relatively small cluster models. Optimization of finite clusters leads to different local geometries for bulk and end sites, where the local bulk TiO6 geometry is in good agreement with recent experimental results. Geometry optimizations reveal that any asymmetry within the axial O-Ti-O chain is negligible. The band gap in the optimized model corresponds to a O(2p) --> Tibulk(3d) transition. The results suggest that the three Ti atom, single chain, symmetric, finite cluster is an effective model for the geometric and electronic properties of bulk and end TiO6 groups in ETS-10.

EXAFS and XANES Investigation of the ETS-10 Microporous Titanosilicate

In this work, we report state-of-the-art analysis of both Ti K-edge high-resolution XANES and EXAFS data collected on the ETS-10 molecular sieve at the GILDA BM8 beamline of the ESRF facility. The interatomic distances and the angles obtained in our EXAFS study are in fair agreement with the single-crystal XRD data of Wang and Jacobson (Chem. Commun. 1999, 973) and with the recent ab initio periodic study of Damin et al. (J. Phys. Chem. B 2004, 108, 1328) Differently from previous EXAFS work (J. Phys. Chem. 1996, 100, 449), our study supports a model of ETS-10 where the Ti atoms are bonded with two equivalent axial oxygen atoms. This model is also able to reproduce the edge and the post-edge region of the XANES spectrum. Conversely, the weak but well-defined pre-edge peak at 4971.3 eV can be explained only by assuming that a fraction of Ti atoms are in a local geometry similar to that of the pentacoordinated Ti sites in the ETS-4 structure. These Ti atoms in ETS-10 should be the terminal of the -Ti-O-Ti-O-Ti- chains, of which the actual number is strongly increased by the high crystal defectivity (Ti vacancies).

Electronic and vibrational properties of a MOF-5 metal–organic framework: ZnO quantum dot behaviour

UV-Vis DRS and photoluminescence (PL) spectroscopy, combined with excitation selective Raman spectroscopy, allow us to understand the main optical and vibrational properties of a metal-organic MOF-5 framework. A O(2-)Zn(2+)[rightward arrow] O(-)Zn(+) ligand to metal charge transfer transition (LMCT) at 350 nm, testifies that the Zn(4)O(13) cluster behaves as a ZnO quantum dot (QD). The organic part acts as a photon antenna able to efficiently transfer the energy to the inorganic ZnO-like QD part, where an intense emission at 525 nm occurs.