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Polycrystalline zirconium titanate, ZrTiO4, has been prepared via sol-gel chemistry and its properties both as a semiconducting oxide and as a photoactive system have been investigated by experimental and computational techniques. The oxide exhibits a structure analogous to those of the main TiO2 polymorphs with a slightly higher band gap of 3.65 eV. Theoretical results explain this small difference in terms of composition of the lower part of the conduction band, which is based on Ti 3d orbitals. Interestingly zirconium titanate is much less reducible than TiO2 upon annealing under a vacuum in the range between room temperature and 673 K. Over this temperature the system releases O2, forming oxygen vacancies and trapping excess electrons on tetravalent Ti ions as firmly indicated by both electron paramagnetic resonance (EPR) and computational results. Irradiating the solid with UV photons a charge separation is observed by EPR and the photogenerated carriers easily reach the surface where they react with gas phase molecules. ZrTiO4 shows, therefore, a photochemical behavior very similar to that of TiO2. Its band potentials, however, slightly differ from those of titania (in particular, the conduction band potential is more negative of about 0.20 eV), making the solid of potential interest for further photocatalytic investigations. (Graph Presented).

Transition-metal ions with open-shell configurations hold promise in the development of novel coordination chemistry and potentially unprecedented redox catalysis. Framework-substituted Ti(3+) ions with tetrahedral coordination are generated by reductive activation of titanium silicalite-1 with triethylaluminum, an indispensable co-catalyst for heterogeneous Ziegler-Natta polymerization catalysts. Continuous-wave and pulse electron paramagnetic resonance methods are applied to unravel details on the local environment of the reduced transition metal-ions, which are shown to be part of the silica framework by detection of (29)Si hyperfine interactions. The chemical accessibility of the reduced sites is probed using ammonia as probe molecule. Evidence is found for the coordination of a single ammonia molecule. Comparison to similar systems, such as TiAlPO-5, reveals clear differences in the coordination chemistry of the reduced Ti sites in the two solids, which may be understood considering the different electronic properties of the solid frameworks.

The interaction of oxygen with the surface of CeO2–TiO2 mixed oxides prepared via sol–gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O2− ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed (“side-on” structure) on the top of a Ce4+ ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner (“end-on” structure). We suggest that the active sites are Ce3+ ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce3+ ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure.

We report direct evidence for quintuplet spin states in a particular kind of reduced TiO

Aromatase catalyses the conversion of androgens into estrogens and is a well-known target for breast cancer therapy. As it has been suggested that its activity is affected by inhibitors of phosphodiesterase-5, this work investigates the potential interaction of sildenafil with aromatase. This is carried out both at molecular level through structural and kinetics assays applied to the purified enzyme, and at cellular level using neuronal and breast cancer cell lines. Sildenafil is found to bind to aromatase with a KD of 0.58 ± 0.05 μM acting as a partial and mixed inhibitor with a maximal inhibition of 35 ± 2%. Hyperfine sublevel correlation spectroscopy and docking studies show that sildenafil binds to the heme iron via its 6th axial water ligand. These results also provide information on the starting molecular scaffold for the development of new generations of drugs designed to inhibit aromatase as well as phosphodiesterase-5, a new emerging target for breast cancer therapy.

A key factor affecting the photo-efficiency of TiO2 is strictly related to the fate of charge carriers, electrons (e−) and holes (h+), generated upon band gap excitation. In the present paper we point our attention to the nature of the hole trapping sites in the anatase polymorph monitored coupling the conventional continuous wave EPR (CW-EPR) technique with pulse electron nuclear double resonance (ENDOR) experiments. The attention is focused on the role of surface adsorbed water (both in molecular and in dissociated form) in the stabilization of photogenerated hole centres. CW-EPR results indicate that two distinct O − hole centres can be identified in Anatase (O −surf. and O −subsurf.) and that the quantitative ratio (measured in terms of spectral intensity) of these two species is markedly conditioned by the presence of surface physisorbed water. For the first time a h+-proton distance, evaluated via ENDOR measurement, is reported.

© 2016 Hydrogen Energy Publications LLC The hybrid systems composed by [FeFe]-hydrogenase anchored to the surface of three distinct types of TiO2 (anatase) have been investigated using Electron Paramagnetic Resonance (EPR) spectroscopy in dark and under illumination. The three supports were bare TiO2 nitrogen doped TiO2 (N-TiO2) and a sub-stoichiometric form of the same oxide (TiO2−x) exhibiting blue color. EPR spectroscopy has shown that the electrons photogenerated by irradiation of the supports are stabilised by the solid forming Ti3+ paramagnetic ions while, in the case of the hybrid systems electrons are scavenged by the anchored protein becoming available for H+ reduction. The ability of the three hybrid systems in hydrogen production under solar light illumination has been compared. The formation of H2 is higher for the system containing N-TiO2 (yellow) with respect to that based on the bare oxide (white) indicating that the visible light absorbed, due to the presence of N states, is actually exploited for hydrogen production. The system containing reduced blue TiO2, in spite of its deep coloration, is less active suggesting that a specific type of visible light absorption is needed to produce photoexcited electrons capable to interact with the anchored protein.

Nitrogen doped tin(iv) oxide (SnO2) materials in the form of nanometric powders have been prepared by precipitation with ammonia. Their properties have been compared with those of undoped materials obtained in a similar way using various physical techniques such as photoelectron spectroscopies (XPS and UPS), UV-Vis-NIR spectroscopy and electron paramagnetic resonance (EPR). Nitrogen doping leads to the formation of various nitrogen containing species, the more relevant of which is a nitride-type ionic species, based on the substitution of a lattice oxygen atom with a nitrogen atom. This species exists in two forms, paramagnetic (hole centre, formally N(2-)) and diamagnetic (N(3-)). The mutual ratio of the two species varies according to the oxidation state of the material. The doped solid, like most of the semiconducting oxides, tends to lose oxygen forming oxygen vacancies upon annealing under vacuum and leaving an excess of electrons in the solid. The stoichiometry of the solid can thus be markedly changed depending on the external conditions. Excess electrons are present both as itinerant electrons in the conduction band and as Sn(ii) states lying close to the valence band maximum. The presence of nitride-type centres, which are low energy states located below the top of the valence band, decreases the energy cost for the formation of oxygen vacancies by O2 release from the lattice. This particular feature of the doped system represents a severe limit to the preparation of a p-type SnO2via nitrogen doping.

Undoped TiO 2 and Fe 3+ doped (0.1, 0.3, 0.6 and 1 wt.%) TiO 2 nanoparticles have been synthesized by the acid-catalyzed sol–gel method. Iron cations are introduced in the initial solution, before gelification, what promotes their lattice localization. The Fe 3+ doped TiO 2 films have been fabricated using a dip-coating technique. The effect of iron content on the crystalline structure, phase transformation and grain growth were determined by X-ray diffraction (XRD), Raman spectroscopy, UV–visible diffused reflectance spectroscopy (DRS) and Electron paramagnetic resonance (EPR) spectroscopy. It has demonstrated that all catalysts are composed of mixed-phase crystals of anatase and brookite with anatase as dominant phase. The crystallinity of the brookite and anatase phases decreased with increasing the iron content. The analysis of EPR result further confirms that Fe 3+ ion are successfully doped in the TiO 2 lattice by substituting Ti 4+ . It was demonstrated that Fe 3+ ion in the TiO 2 films plays a role as the intermediate for the efficient separation of photogenerated hole–electron pairs and increases the photocurrent response of the film under UV light irradiation. The maximum photocurrent is obtained on the Fe 3+ doped TiO 2 film with 0.1% Fe, which is 1.46 times that achieved on undoped TiO 2 film.

A systematic analysis of the reduced states in the titanium dioxide matrix (anatase polymorph) has been performed coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) with advanced pulse-EPR techniques and introducing the 17O magnetic isotope into the solid. Reduced states were originated in various ways including valence induction via aliovalent elements (F, Nb) and reducing treatments of the bare oxide including surface reaction with reducing agents (H, Na) and thermal annealing under vacuum with consequent oxygen depletion. Two main paramagnetic species were identified via EPR both amenable to Ti3+ ions. The former (EPR signal A: axial symmetry with g∥ = 1.962 and g⊥ = 1.992) is observed in all case and has been conclusively assigned to reduced Ti3+ centers in regular lattice sites of the anatase matrix; the second (signal B: broad line centered at g = 1.93) is present only in reduced materials and is assigned, on the basis of the analysis of the hyperfine interaction of the ce...

We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of SO 2 molecules on bare and partially hydroxylated alkali-earth oxide (AEO) surface. When SO 2 is adsorbed on the surfaces of higher basicity (CaO and SrO) disproportionation reaction occurs with the formation of SO 2 − radical. We show also that, when the same reaction is present on partially hydroxylated surfaces a fraction of the electrons exchanged during the redox process are trapped on the oxide surface forming colour centres ((H + )(e − ) centres).

H2 absorption and desorption reactions in MgH2 promoted by ball-milling with 1 mol% MgNb2O6, Mg4Nb2O9 and Mg3Nb6O11 have been investigated. MgH2 was milled with the bare oxides for 12 h under a high purity Ar atmosphere. Absorption and desorption reactions in the ball-milled samples were studied by in situ X-ray diffraction (XRD) in isothermal conditions with Anton Paar XRK 900 reaction chamber. XRD patterns for absorption were recorded at 573 K under hydrogen pressure of 0.9 MPa and for desorption at 623 K in vacuum. Experimental data were analysed according to the Rietveld method. Ball-milled samples showed the presence of a mixture of β and γ allotropes of MgH2, with significantly broadened diffraction peaks due to reduced crystallite size and strain, together with bare additives. The presence of Mg–Nb oxides significantly accelerates the hydrogen absorption and desorption processes. The amount of hydrogen absorbed in the presence of Mg–Nb–O phases is lower than the maximum stoichiometric capacity, because of the presence of a non-reactive MgO layer on the surface of the powders or at the grain boundaries. Experimental results are discussed on the basis of thermodynamic and kinetic arguments.

The (17)O and (1)H hyperfine interactions of water ligands in the Ti(III) aquo complex in a frozen solution were determined using Hyperfine Sublevel Correlation (HYSCORE) and Pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies at 9.5 GHz. The isotropic hyperfine interaction (hfi) constant of the water ligand (17)O was found to be about 7.5 MHz. (1)H Single Matched Resonance Transfer (SMART) HYSCORE spectra allowed resolution of the hfi interactions of the two inequivalent water ligand protons and the relative orientations of their hfi tensors. The magnetic and geometrical parameters extracted from the experiments were compared with the results of DFT computations for different geometrical arrangements of the water ligands around the cation. The theoretical observable properties (g tensor (1)H and (17)O hfi tensors and their orientations) of the [Ti(H(2)O)(6)](3+) complex are in quantitative agreement with the experiments for two slightly different geometrical arrangements associated with D(3d) and C(i) symmetries.

Continuous wave (CW) and pulse electron paramagnetic resonance (EPR) experiments, in conjunction with density functional theory (DFT) calculations, provide a detailed description of defective centres produced upon nitrogen doping of polycrystalline ZnO. Two distinct paramagnetic species are formed upon annealing of ZnO nanoparticles in an NH3 atmosphere, which are characterized by the interaction of the unpaired electron with one and two N nuclei. HYSCORE experiments provide the full hyperfine and quadrupole interaction tensors for the monomeric defect, which, on the basis of quantum chemical calculations, is assigned to a nitrogen ion substituting a lattice oxygen ion. © 2010 The Royal Society of Chemistry.

Mo{sub 1-x}W{sub x}O{sub 3} oxides with different cationic fraction (x=0.2, 0.5 and 0.8) and, for comparison purposes, pure MoO{sub 3} and WO{sub 3} were prepared. Along with textural and structural characterizations, absorbance FT-IR, diffuse reflectance UV-vis-NIR and EPR spectroscopies were employed to study the changes in the electronic properties of these materials passing from Mo{sub 1-x}W{sub x}O{sub 3} in oxidizing atmosphere to Mo{sub 1-x}W{sub x}O{sub 3-{delta}} in reducing conditions. XRD analysis showed that the Mo-W mixed oxides are constituted by two or three crystalline phases, whose abundance and composition are well characterized by structural refinement with the Rietveld method. Only the sample with the highest Mo content (x=0.2) shows a predominant mixed phase and also a superior ability to lose oxygen with respect to the other mixed oxides. The oxygen loss in the reduced oxides induces the formation of defects with electronic levels in the band gap of the material, in particular, electrons trapped in oxygen vacancies and/or at cationic sites (polarons). While the nature of defect sites induced in the mixed and in the pure oxides is similar, the photo-ionization energies, the ratio between surface and bulk defects and the stability of the defects in oxygen at increasing temperaturemore » are peculiar of each mixed oxide. - Graphical abstract: The combined use of different techniques provides new insight into structural, optical and electronic properties of semiconductor Mo{sub 1-x}W{sub x}O{sub 3-{delta}} mixed oxides.« less

The reaction of excess electrons at the surface of MgO with ammonia leads to surface ammoniated electrons analogous to those formed when alkali metals are dissolved in anhydrous ammonia. Surface excess electrons are found to be solvated by up to three ammonia molecules, and well-resolved CW and pulsed EPR spectra allow for a precise description of the unpaired electron spin density distribution over the solvent molecules. The large majority of the electron spin density resides in the first-shell nitrogen fragments. HYSCORE spectra allow obtaining for the first time the full hyperfine interaction of the solvated electron with the ammonia protons, which is consistent with a small and negative spin density in the 1H 1s orbital. Furthermore, the hyperfine and nuclear quadrupole tenors of the second-shell nitrogens could be unravelled.

Nitrogen doped TiO{sub 2} represents one of the most promising material for photocatalitic degradation of environmental pollutants with visible light. However, at present, a great deal of activity is devoted to the anatase polymorph while few data about rutile are available. In the present paper we report an experimental characterization of N doped polycrystalline rutile TiO{sub 2} prepared via sol-gel synthesis. Nitrogen doping does not affect the valence band to conduction band separation but, generates intra band gap localized states which are responsible of the on set of visible light absorption. The intra band gap states correspond to a nitrogen containing defect similar but not coincident with that recently reported for N doped anatase. - Graphical abstract: Nitrogen doped TiO{sub 2} represents one of the most promising material for photocatalitic degradation of environmental pollutants with visible light. However, at present, a great deal of activity is devoted to the anatase polymorph while few data about rutile are available. In the present paper we report an experimental characterization of N doped polycrystalline rutile TiO{sub 2} prepared via sol-gel synthesis.

Nitrogen-doped TiO2 materials were successfully prepared following three different preparation routes (sol−gel, mechanochemistry, and oxidation of TiN) and characterized by X-ray diffraction, electron microscopy, and various spectroscopic techniques. All samples absorb visible light, and the one obtained via sol−gel, showing the anatase structure, is the most active in the decomposition of organic compounds under visible light. Various nitrogen-containing species have been observed in the materials, whose presence and abundances depends on the preparative route. Ammonium NH4+ ions are residual of the synthesis using ammonium salts (sol−gel, mechanochemistry) and are quite easily eliminated, as shown by the parallel behavior of both NMR and XPS spectra. Cyanide CN− ions form at high temperature in parallel with the phase transition of the solid to rutile. Molecular nitric oxide forms in the case of materials exhibiting close porosity. The already reported bulk radical species, Nb•, is the only paramagnetic...

On the basis of experimental evidence and DFT calculations, we propose a simple yet viable way to stabilize and chemically activate gold nanoclusters on MgO. First the MgO surface is functionalized by creation of trapped electrons, (H+)(e-) centers (exposure to atomic H or to H2 under UV light, deposition of low amounts of alkali metals on partially hydroxylated surfaces, etc.); the second step consists in the self-aggregation of gold clusters deposited from the gas phase. The calculations show that the (H+)(e-) centers act both as nucleation and activation sites. The process can lead to thermally stable gold cluster anions whose catalytic activity is enhanced by the presence of an excess electron. © 2008 American Chemical Society.

Various Mg/Nb/O mixed phases have been prepared by solid state reaction in order to mimic similar compounds formed in the ball milled system composed of MgH2 and Nb2O5. The latter oxide has a role as a promoter of the hydrogen uptake and desorption in the Mg/MgH2 system for hydrogen storage. One mixed phase in particular (Mg3Nb6O11) have shown a reversible uptake of molecular hydrogen, confirmed by thermal desorption experiments followed by mass spectroscopy. Comparison of the behaviour of this phase with NbO shows how a reversible interaction with hydrogen may be linked to the presence of octahedral niobium clusters in both oxides. © 2008 International Association for Hydrogen Energy.

Fluorine-doped titanium dioxide was prepared via sol-gel synthesis and subsequent calcination in air. The presence of fluorine in the lattice induces the formation of reduced Ti3+ centers that localize the extra electron needed for charge compensation and are observed by electron paramagnetic resonance. Density functional theory calculations using hybrid functionals are in full agreement with such description. The extra electron is highly localized in a 3d orbital of titanium and lies a few tenths of an electron volt below the bottom of the conduction band. The preparation via sol-gel synthesis using aqueous solutions of fluorides also causes the formation of surface F- ions that substitute surface hydroxyl groups (OH -) without generating reduced centers. © 2008 American Chemical Society.

The adsorption of small amounts of alkali metal atoms (Li, Na, K, Rb, and Cs) on the surface of MgO powders and thin films has been studied by means of EPR spectroscopy and DFT calculations. From a comparison of the measured and computed g values and hyperfine coupling constants (hfccs), a tentative assignment of the preferred adsorption sites is proposed. All atoms bind preferentially to surface oxide anions, but the location of these anions differs as a function of the deposition temperature and alkali metal. Lithium forms relatively strong bonds with MgO and can be stabilized at low temperatures on terrace sites. Potassium interacts very weakly with MgO and is stabilized only at specific sites, such as at reverse corners where it can interact simultaneously with three surface oxygen atoms (rubidium and cesium presumably behave in the same way). Sodium forms bonds of intermediate strength and could, in principle, populate more than a single site when deposited at room temperature. In all cases, large deviations of the hfccs from the gas-phase values are observed. These reductions in the hfccs are due to polarization effects and are not connected to ionization of the alkali metal, which would lead to the formation of an adsorbed cation and a trapped electron. In this respect, hydrogen atoms behave completely differently. Under similar conditions, they form (H(+))(e(-)) pairs. The reasons for this different behavior are discussed.

Nitrogen-doped TiO2, a novel photocatalyst active in the decomposition of organic pollutants using visible light, contains several different types of paramagnetic centers. These are molecular species, such as NO and NO2 radicals and other species, deeply interacting with the TiO2 structure. All or part of these species is related to specific properties of the solid. Electron paramagnetic resonance has been employed to characterize the N-containing paramagnetic species present in N-doped anatase TiO2 powders obtained via sol-gel synthesis. In the present work attention is focused on molecular species generated during the synthesis process and segregated in cavities of the TiO2 structure.

Nb2O5 is known as good promoter for adsorption/desorption kinetics of hydrogen in magnesium hydride. In this article the interaction with hydrogen of bare Nb2O5, the oxidic component of the mixed MgH2/Nb2O5 system, is investigated in various conditions (i.e. employing atomic, molecular and nascent hydrogen). The state of the hydrogen-Nb2O5 system was monitored using various techniques including: X-ray Diffraction, Electron Paramagnetic Resonance, Diffuse Reflectance UV-Vis Spectroscopy, Thermal Desorption Spectroscopy -Mass Spectrometry, Differential Scanning Calorimetry and Thermal Programmed Desorption. Niobium (V) oxide is not at all inert while interacting with hydrogen. This oxide is partially reduced by hydrogen, which is incorporated in the solid and released as both molecular hydrogen and water. This peculiar behaviour, reminiscent of some properties of the bronze family, suggests an active chemical role played by Nb2O5 in the mixed MgH2/Nb2O5 system.

Dehydroxylated, hydroxylated and hydrated anatase TiO

An effective synthesis strategy for the extensive fluorination of the TiO2 anatase lattice has been developed which provides a highly stable blue-colored titania powder. The process also produces a convenient faceted morphology of the doped nanoparticles. Both theoretical and experimental data indicate an ordered atomic structure, in which an exceptionally high amount of fluorine ions substitute oxygen ions in the TiO2 lattice. The extra-electrons borne by fluorine are stabilized by lattice Ti cations via a mechanism of valence induction, eventually leading to a consequent high amount of reduced Ti3+ centers. Such structure, whose general formula can be expressed as Ti4+(1-x)Ti3+xO2–(2-x)F–x, confers an excellent stability to the as-synthesized nanoparticles (in spite of the excess electrons), explaining for example why the blue color is retained even upon storage in ambient atmosphere. But moreover, the high concentration of the relatively shallow Ti3+ generated states also form a sort of sub-band close to the bottom of the conduction band itself. The whole effect results in a tangible decrease (0.2 eV) of the anatase band gap which allows an inspiring upgrading of its UV photocatalytic activity. Our results also suggest that narrowing the band gap is insufficient for a substantial improvement in the visible light harvesting., This work was supported by the Spanish Ministry of Economy and Competitiveness (MINECO) through the projects IPT-2011-1113-310000 (NANOBAC), MAT2013-40722-R (SCOBA), MAT2013-47878-C2-1-R and CTQ2012-30751 and by the Comunidad de Madrid through the project MULTIMAT-CHALLENGE P2013/MIT-2862. T.J. also acknowledges the European Science Foundation (ESF).

A combination of electron paramagnetic resonance (EPR) spectra and density functional theory (DFT) calculations is used to characterize the paramagnetic species in rutile N-doped SnO2 samples synthesized by wet chemistry methods. In particular, the nature of paramagnetic N species, substitutional or interstitial, and their effect on the electronic structure are discussed. Complex EPR spectra generated by the interaction of the unpaired electron with N and Sn nuclei have been accurately simulated to obtain the EPR properties (g and A tensors). The results suggest that the N dopants form a rather symmetric structure with three magnetically equivalent or nearly equivalent Sn atoms surrounding the N impurity. After a careful assessment of an all-electron basis set for Sn atoms, realistic models of substitutional and interstitial N-doped SnO2 structures have been designed, and the corresponding hyperfine coupling constants (hpcc) were computed. The comparison between computed and measured hpcc values leads to the assignment of the paramagnetic centers in N-doped SnO2 to substitutional N dopants that take the position of the O atoms in the lattice. The DFT calculations finally suggest the N impurities induce the formation of localized empty states (electron holes) in the intra band gap region.

The importance of surface paramagnetic species owes much to that of surface phenomena which are involved in numerous areas of chemistry and material science such as heterogeneous catalysis, photochemistry and, in general terms, nano-sciences and technology. In the present contribution the opportunities offered by the use of EPR in the field of heterogeneous catalysis, with emphasis on the application of hyperfine techniques, will be illustrated taking as an example Ti-based catalytic materials. The reductive activation of framework titanium ions in the different materials and their sub-sequent reactivity towards NH3 is followed, highlighting subtle differences in chemical reactivity related to the different matrixes.

The nature of hole centers in a series of MeO2 (TiO2, ZrO2, HfO2) metal oxides doped with trivalent Al or Ga ions has been investigated coupling the classic continuous wave electron paramagnetic resonance (CW-EPR) technique with advanced density functional theory (DFT) calculations. The insertion of an aliovalent ion in the structure of the tetravalent oxides is compensated by the creation of oxygen vacancies leading to diamagnetic defective systems. The hole centers are observed by EPR after irradiation using ultraviolet (UV) frequencies (with consequent formation of an electron-hole pair) and trapping of the photogenerated electron. The distortion imparted by the presence of the dopant stabilizes these centers. This generates a rich superhyperfine structure, since the dopants employed in this investigation (Al and Ga) have a nonzero nuclear spin. The DFT calculations performed on a wide set of possible hole-trapping sites occurring in the solid, allow us to identify (comparing the calculated EPR parameters of various models with the experimental ones) the nature of the observed hole centers in all cases. These are always three-coordinated oxygen ions with one Al (or Ga) ion in the first coordinative sphere. As it has been observed in other cases of holes centers, the spin density associated with the unpaired electron is concentrated in an O p-orbital with a modest delocalization toward the first neighboring ions.

Small amounts of Ce4+ ions dispersed in the bulk of ZrO2 give to this material unexpected properties of photoactivity under visible light. Three different samples have been synthesized following different methods and using different precursors and all show some degree of photoactivity. This has been monitored via EPR spectroscopy in terms of charge separation (electrons and holes formation) under polychromatic light having λ > 420 nm. This behaviour, unimaginable for bare zirconia due to the large band gap of this oxide (around 5 eV), is possible because of the presence in the solid of empty 4f Ce states located in the middle of the band gap which act as intermediate levels in a double excitation mechanism. The doped oxide here described can be considered an example of third generation photoactive material operating under visible light.

Electron Paramagnetic Resonance (EPR) plays a paramount role in understanding the nature and the main features of point defects in the bulk of both ionic and covalent solids. In the present chapter, after a brief introduction to the technique, the application of EPR to unravel the surface counterparts of bulk point defects in the case of metal oxides will be illustrated with particular attention to the defects capable of charge carriers trapping.

Sulfur contamination of alkaline earth oxide surfaces has important consequences in many fields of chemistry, such as surface science and catalysis. We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of CS2 molecules under different pressure and temperature conditions, with the bare and electron-enriched surface of MgO. CS2 reacts on the oxide surface through two distinct reaction paths, one leading to diamagnetic species, the other to paramagnetic surface entities. Both these reaction paths lead to the fragmentation of the CS2 molecule and, in some cases to oligomerization processes. EPR experiments allowed to follow the formation of the unstable CS 2 − radical intermediate and its evolution through formation of several surface radicals like S−, S 3 − and S n − (n ⩾ 3), depending on the reaction conditions.

Photosensitive, germanium-doped multicomponent silicate glasses with different amount of boron oxide and sodium oxide were prepared and investigated by means of electron paramagnetic resonance (EPR) and ultraviolet–visible (UV–Vis) spectroscopy in order to investigate the structural changes occurring in the glasses under UV laser irradiation at 248 nm (KrF excimer UV laser). Spectral data were recorded before and after UV laser irradiation: all the glasses showed EPR signals only after UV laser irradiation, independently from the presence of a UV–Vis absorption band on the as prepared glasses. The main results showed that the presence or the absence of boron and sodium and their amount play a role in the type and the number of defects, their time and thermal decay, under the same UV laser exposure conditions. A clear correlation between the EPR signal and the UV–Vis absorption is observed only for the 240 nm band. It is also demonstrated that the sodium ions decrease the number of paramagnetic defects in these glasses and deeply change the germano-silicate glass structure. The presence, the absence and the co-presence of sodium and boron ions has a role in the structure, the number, the nature and the stability of the paramagnetic defects. It is important to understand the role of sodium and boron in order to increase the photosensitivity of germano-silicate glasses, for their interest in the field of ion-exchangeable glasses for photonic waveguides and Bragg-grating based devices.

The nature of electron traps at the surface of polycrystalline MgO is analyzed for the first time in terms of interaction of the electron spin with the nuclear spin of the O-17 anions of the surface. MgO crystals enriched in the O-17 isotope have been prepared and the corresponding hyperfine coupling constants have been measured in EPR spectra. The trapped electrons are produced by exposure of the samples to H-2 followed by UV irradiation, with consequent production of paramagnetic (H+)(e(-)) centers. The EPR spectrum shows a multiplet structure with two main sextets separated by 51 and 10 G, respectively, due to the interaction of the unpaired electron with two nonequivalent O-17 nuclei. The results are interpreted based on accurate quantum chemical calculations. It is suggested that the paramagnetic centers observed correspond to (H+)(e(-)) pairs formed at step sites of the MgO surface

Several “classic” functional materials, when prepared in nanostructured forms, exhibit unique physical properties that present opportunities for new applications.[1–3] This is the case for mesoporous silicon, a material widely investigated for its peculiar photoluminescence, optical, and electronic properties.[4] These properties have been harnessed for a variety of challenging chemical sensor applications,[5] such as the quantification of reactive pollutant gases (for example, NO2). The present article reports the observation of a dramatic increase in the conduction-electron population in mesoporous silicon, which occurs upon adsorption of ammonia onto the surface. This effect has been monitored by EPR spectroscopy and is interpreted in terms of a reversible change from Langevin to Pauli paramagnetism, which corresponds to an insulator-to-metal transition. Mesoporous silicon (m-PS) is prepared by electrochemically etching B-doped p-type crystalline silicon in HF solutions, where the boron content is typically of the order of 1019–1020 atomscm 3. It possesses a dendritic structure, comprised of Si nanowires with diameters ranging from 5– 12 nm and a specific surface area up to 600 m2cm 3. Despite the number of papers devoted to m-PS in recent years, a number of questions are still unanswered, for example, concerning the hole-based electrical conductivity of B-doped Si, which drops by several orders of magnitude upon the formation of pores, and renders the PS nearly an insulator. This loss of conductivity cannot be ascribed to any preferential removal of boron atoms during etching. The disappearance of the great majority of free holes has been interpreted by various, sometimes conflicting hypotheses.[7–11] There are a few examples of experiments in which the conductivity of porous Si samples increases. For example, contact between porous silicon and polar liquids causes such an increase, and this observation has been interpreted as the

129Xe-NMR has been employed in the characterization of the activation of two iron silicalites, with different iron loadings. The 129Xe chemical shift recorded as a function of the gas pressure is e...

Coadsorption of hydrogen and nitric oxide at the surface of polycrystalline MgO has been studied by means of electron paramagnetic resonance spectroscopy. 14NO and 15NO have been used as probes for monitoring highly exposed sites on the surface in particular as it concerns the role played by the same sites in the heterolytic dissociation of the hydrogen molecule. Two forms of dissociated hydrogen on MgO are known, the former one irreversibly bound at the surface in a wide range of temperatures, the latter desorbing H2 upon lowering the pressure. NO is physisorbed as paramagnetic NO monomers on three distinct types of low-coordinated surface cations while a small fraction (about 2%) of NO forms NO22− species on low-coordinated anions. The irreversible form of H2 does not alter the NO chemistry as monitored by EPR, whereas, coadsorption of the two gases affects the formation of physisorbed NO monomers in a way which depends on the H2–NO ratio. For high H2–NO ratios no adsorbed monomeric NO is observed whereas when lowering the H2–NO ratio down to 1/10 the two NO sites with higher binding energy are still depressed while the weakest one becomes available. These results allow to identify the sites responsible for NO adsorption in monomeric form as connected to those responsible for the reversible dissociation of the hydrogen molecule. On the other hand, the sites involved in the H2 irreversible splitting and those responsible for the adsorption of NO in either chemisorbed or physisorbed form are different one from the other. The present results provide new experimental evidence for modeling the surface active sites on MgO.

O − species obtained as hole centers, generated via X-ray irradiation of polycrystalline MgO, O 2− ( s ) + h ν→ O − ( s ) +( e − ) trapped are compared to the same chemical species generated via bleaching of the surface colour centers with N 2 O, ( e − ) trapped + N 2 O ( g ) → O − ( s ) + N 2( g ) The electron paramagnetic resonance (EPR) spectrum of the former species presents a single, well defined feature which corresponds to one of the several species present in the complex O − spectrum, obtained by reaction with N 2 O. The O − and the trapped electron, formed upon X-ray irradiation, decay simultaneously indicating that a recombination process takes place and that they are not too far apart. EPR parameters have been determined by quantum-mechanical calculations for a variety of possible electron rich sites on which an oxygen atom adsorbs. It is found that both oxygen vacancies and morphological sites on the MgO surface fulfill the spectroscopic characteristics of the electron trapping sites probed by the O − radical ion. A new model for a paramagnetic F s + (H) colour center has been proposed as a possible morphological electron trap where an oxygen atom is adsorbed. Thermochemical measurements for the reaction of (e − ) trapped with N 2 O give an average reaction enthalpy of 450±50 kJ mol −1 , which is in reasonable agreement with the computed values.

The surface basic sites at the surface of calcium oxide have been explored following the interaction of nitric oxide with the solid by EPR. Various paramagnetic species are formed depending on the pressure of the gas contacted with the solid. In the initial steps of the interaction, a small amount of a NO 3 2- species is formed probably because of the presence at the surface of "anomalous" peroxy or peroxo-like groups. For higher NO pressures, four types of NO 2 2- surface species characterized by distinct values of the g and hyperfine tensors are formed by addition of the neutral molecule to basic O 2 - sites of the surface. The process involves a considerable rearrangement of the spin distribution with respect to the NO molecule with a net decrease of the electron spin density on the N atom. The number of basic sites involved in this interaction is very low and amounts to a value in the range of 0.25-0.5% of the whole surface. These active sites are low coordination sites present at morphological defects of the polycrystalline solid. The concentration of the basic sites is 25 times greater than that of the corresponding sites on MgO thus confirming the higher overall basicity of CaO with respect to magnesium oxide. The surface activity shown by MgO and consisting in a low energy interaction of Lewis acid sites with the NO molecule is, on CaO, totally suppressed.

A study was made of the iron species dislodged from the tetrahedral lattice position by thermal treatments in four Fe silicalite samples with Si/Fe ratios of 25, 50, 90, and 150. The last two samples have a sufficiently low iron concentration to be considered true catalysts (i.e., having Fe dilutions as low as that used in the industrial conversion of benzene to phenol (Si/Fe>80)) in partial oxidation reactions. A systematic spectroscopic study is presented in order to obtain a global picture of a system whose complexity can be understood by assuming the formation of a wide variety of transient coordinatively unsaturated iron species, with different oxidation states, which ultimately lead to grafted or anchored isolated and/or clustered species by interaction with residual tetrahedral Si(OH)Fe, defective SiOH groups, or strained SiOSi bridges of the hosting matrix. The structure characterization was obtained by means of the parallel use of IR, X-ray absorption near-edge spectroscopy, and electron paramagnetic resonance techniques, before, during, and after interaction with CO, N2O, and NO probe molecules. This approach was applied to samples characterized by different iron content, activation temperature, and redox treatments. This allowed us to throw light onto structure, oxidation state, and mutual interconversion of a very complex family of extraframework iron species. Oxidation with O2 or N2O led to the formation of strongly bonded oxygen species. The relation between these species and the so-called α-oxygen species, claimed as the active sites for the hydroxylation of benzene to phenol by N2O, is discussed.

The adsorption of molecular oxygen ~enriched with 17O) onto high surface area MgO has been\ud studied by electron paramagnetic resonance ~EPR! spectroscopy. The oxide surface was pretreated\ud in such a way so that surface trapped electron FS\ud 1~H! centers are produced. Subsequent dioxygen\ud adsorption results in an electron transfer reaction from FS\ud 1~H! centers to O2, producing a surface\ud stabilized superoxide (O2\ud 2) anion. The resulting EPR spectrum of the paramagnetic anion is\ud complicated by the simultaneous presence of a high number of ‘‘normal’’ hyperfine lines along the\ud principal axes and also by several off-axis extra features which have complicated previous\ud interpretations of the Ayy and Azz components. By adopting a suitable adsorption procedure which\ud suppresses the superoxide speciation, using a highly crystalline MgO material and controlling the\ud isotopomer composition through appropriate 17O enrichments, the resolution of the EPR spectrum\ud has been dramatically improved. Analysis of the 1H superhyperfine structure (uAHu/beg\ud 5@3.9,2.2,1.3#G), resulting from a dipolar interaction between the adsorbed O2\ud 2 anion and a\ud neighboring OH group, and positions of the extra absorption lines in the spectrum, have provided us\ud with auxiliary sources of information to determine for the first time the complete 17O hyperfine\ud tensor (AO/beg5@276.36,7.18,8.24# G!. The tensor has been analyzed in detail using a localized\ud spin model. The spin density is shared among the 2p p\ud x (0.495), 2px\ud y(20.024) and 2s(0.011)\ud orbitals. The total spin density on O2\ud 2 indicates that a complete surface electron transfer from the\ud FS\ud 1~H! center to dioxygen occurs upon adsorption, in line with recent ab initio calculations.

A variety of surface anion vacancies, or point defects, are created by high-temperature activation of a series of polycrystalline alkaline earth metal oxides (MgO, CaO and SrO). Subsequent UV irradiation of the activated oxide under a hydrogen atmosphere results in the generation of surface colour centres [FS +(H)], by electron trapping at these anion vacancies. The paramagnetic properties of these colour centres were studied by EPR and ENDOR spectroscopy. 1H ENDOR spectroscopy revealed that a well defined heterogeneity of trapped electron species exists on each oxide surface, as characterized by the different superhyperfine couplings between the trapped electron and the nearby proton of the FS + (H) centre. On MgO and CaO two dominant FS + (H) centres were identified (labelled sites I and II) whereas on SrO three FS + (H) species were found (sites I, II and III). The possible surface sites responsible for electron stabilization are discussed, and include a 3Ccornermono-vacancy, a 4Cmono-vacancy and an anion–cation di-vacancy. The results indicate that regardless of the oxide used, a common degree of morphological similarities exists on each oxide.

The interaction of acetylene with the TiO2 surface at room temperature entails a complex set of self-assembly reactions with the formation of products having relatively high molecular weight. In a previous paper by some of us (Jain, S. M.; et al. J. Mater. Chem. A 2014, 2, 12247-12254), the C2H2-TiO2 reaction has been monitored, essentially by Fourier transform infrared spectroscopy, at the surface of P25 (a mixture of anatase and rutile, typical benchmark material in the field of photocatalysis) in order to elucidate the nature of the products of this surface reaction. In the present paper, the same process was followed, for the first time, using electron paramagnetic resonance (EPR) and monitoring by the thermogravimetric analysis the weight loss of the material upon heating in order to further investigate the complex mechanism of the surface reaction. This was done using pure anatase and comparing the EPR results with those concerning both rutile and P25. The self-assembly mechanism occurring at the interface is accompanied by the formation of EPR visible Ti(3+) centers due to electrons injection in the TiO2 substrate. This finding clarifies that at least one of the reaction channels of this complex process (namely, the formation of polycyclic aromatic hydrocarbons) is based on the heterolytic dissociative chemisorption of acetylene, followed by a redox interaction between the adsorbate and the solid, which allows the creation of the building blocks necessary to assemble polyaromatic molecules.

The typical activation of a fourth generation Ziegler-Natta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti(3+) centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti(3+) species have been identified. A comparison of the experimentally derived g tensors and (35,37)Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti(3+) species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

The interaction of oxygen with the surface of CeO2-TiO2 mixed oxides prepared via sol-gel was investigated by means of electron paramagnetic resonance (EPR). Upon admission of molecular oxygen onto the surface of the as prepared materials (which underwent final oxidative calcination) the formation of superoxide O2(-) ions is observed without the need for preliminary annealing in a vacuum and consequent oxygen depletion. The superoxide species is symmetrically adsorbed ("side-on" structure) on the top of a Ce(4+) ion. Surprisingly the electron transfer is fully reversible at room temperature having the typical behavior shown by molecular oxygen carriers, which, however, link to oxygen in a completely different manner ("end-on" structure). We suggest that the active sites are Ce(3+) ions present in the stoichiometric cerium titanate which forms during the synthesis. The features of these Ce(3+) ions must be different from those of the same ions formed in CeO2 by reductive treatments, which show a different reactivity to O2. The observation reported here opens up innovative perspectives in the field of heterogeneous catalysis and in that of sensors as the total reversibility of the electron transfer is observed in a significant range of oxygen pressure.

The dispersion of small amounts of Ce4+ ions in the bulk of ZrO2 leads to a photoactive material sensitive to visible light. This is shown by monitoring with EPR the formation and the reactivity of photogenerated (λ > 420 nm) charge carriers. The effect, as confirmed by DFT calculations, is due to the presence in the solid of empty 4f Ce states at the mid gap, which act as intermediate levels in a double excitation mechanism. This solid can be considered an example of a third-generation photoactive material. © 2014 American Chemical Society.