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Doping titanium dioxide (anatase) with elements carrying an extra electron, such as Nb and F, or with their mixtures leads to n-type materials showing peculiar properties with respect to the pristine oxide. Niobium and fluorine are present in the lattice in the form of Nb5+ and F– ions (detected by XPS), and the extra electrons carried by the dopants are stabilized on titanium ions, which become EPR-visible as Ti3+ ions homogeneously dispersed in the bulk of the crystals. Under such conditions, the optical band gap transition is slightly red-shifted (by a few tenths of an electronvolt) for all samples containing fluorine, and the Fermi level lies, depending on the material, at the boundary or even in the lower region of the conduction band. The typical Ti3+(I) centers generated by valence induction are responsible for the already reported conductivity properties of the system. The presence of these centers also influences the process of electron injection in the solid, favoring the dilution of additional ...

In the present paper, the thermal deactivation of a commercial de-NO x V 2 O 5 –WO 3 /TiO 2 catalyst is investigated. In order to simulate long-term operation in gas firing samples of the catalyst are calcined at different temperatures from 773 to 1173 K. The structural, morphological and chemico-physical changes caused by calcination at high temperatures are investigated by XRD, BET and Hg porosimetry measurements, FT-IR, FT-Raman and EPR spectroscopies, and the activity in the reduction of NO with ammonia, the direct oxidation of ammonia and the oxidation of SO 2 is measured over the different catalyst samples. It is shown that sintering of the TiO 2 support leads to aggregation of isolated vanadium ions; this favours the selective catalytic reduction (SCR) reaction at low temperatures, the oxidation of ammonia at high temperatures and the undesired oxidation of SO 2 as well. Thus, it is expected that catalysts operated for long-term in gas firing at high temperatures are no longer appropriate for commercial use in the SCR process, because in spite of the greater activity in the de-NO x reaction, the temperature window is too narrow and the activity in the oxidation of SO 2 is too high.

In this work we have studied computationally, at the Hartree–Fock and density functional levels, the interaction of O 2 and N 2 molecules with paramagnetic oxygen vacancy ( F S + centers ) located at the (1 0 0) MgO surface. We have employed the molecular cluster approach, which describes the adsorption site by means of a limited number of ions embedded in a large array of point charges, as well as the more sophisticated perturbed cluster method, where the selected cluster is self-consistently embedded in the quantum-chemical description of the regular host crystal. We found that, at short distances from the surface, an electron transfer from the surface to the admolecules occurs, with formation of charge-transfer complexes F S 2+ /N 2 − or F S 2+ /O 2 − that are stabilized by the electrostatic interaction between the charged fragments. However, F S 2+ /O 2 − is stable with respect to the dissociation into F S + +O 2 fragments, while F S 2+ /N 2 − results to be a metastable product.

Upon nitrogen adsorption at low temperature onto the surface of magnesium oxide and calcium oxide containing FS+ centers (single electron trapped in a suitable surface vacancy), electron transfer occurs from the solid to the adsorbed molecule. About 90% of the total electron density is localized on the adsorbed molecule. The 11-electron N2- radical anion so formed has been detected by electron paramagnetic resonance for both 14N2 and 15N2. The electron transfer is reversible and, when the pressure is lowered or the temperature increased, the N2 molecules desorb, regenerating the original FS+ center. The diatomic radical ion lies parallel to the surface, and the electron density is mainly confined in the πy* orbital. Theoretical calculations at the DFT level indicate that a small energy barrier separates the unbound FS+/N2 state from the bound FS2+/N2- state. This explains the facile reversibility of the electron-transfer process. The calculated spin densities are in excellent agreement with those derived ...

The local structure of V oxide/ZSM-5 catalysts prepared by the solid-state reaction of the ZSM-5 zeolites having different Si/Al ratios have been investigated by means of in situ photoluminescence spectroscopy in combination with other spectroscopic techniques such as UV−Vis, solid-state NMR, and ESR. It was found that vanadyl ions are present as tetrahedrally coordinated V5+ species bonded to terminal Si−OH groups and square pyramidally coordinated VO2+ species at the zeolite cationic sites for zeolites having a low Si/Al ratio (Si/Al = 20). On the other hand, for a high Si/Al ratio (Si/Al = 950), only the tetrahedrally coordinated V5+ species can be observed. UV irradiation of the V oxide/ZSM-5 catalyst in the presence of NO was found to lead to the photocatalytic decomposition of NO into N2, O2, and N2O. The increase of efficiency was found to be more remarkable on the V oxide/ZSM-5 catalyst with a high Si/Al ratio than with a low Si/Al ratio. The photoluminescence spectrum of the V oxide/ZSM-5 catalys...

Ab initio calculations on a linear –OTiOTiO– chain embedded in an envelope of (SiO4) tetrahedra, mimicking the structure of Na2TiSi5O13 molecular sieve (ETS-10), confirm that the peculiar optical properties of the solid are associated with the presence of –OTiOTiO– linear chains behaving as quantum wires. The optical [in the UV-Vis (ultraviolet-range)] and the magnetic [(ESR) electron spin resonance] properties of these chains can be modified by adsorbing Na vapors. The sodium atoms diffusing into the channels undergo a ionization process with formation of Na+ (localized in the main channels) and Ti3+ (in the –OTiOTiO– chain, which so becomes a nonstoichiometric wire) characterized by Ti/Na ratios in the 2–4 range. Successive adsorption of oxygen at room temperature leads to the partial (Ti/Na∼2) or total (Ti/Na∼4) restoration of the chain stoichiometry and to the predominant formation of sodium oxide. The formation of a minor fraction of superoxide negative ions whose magnetic properties are revealed by ESR spectroscopy is also observed. Total restoration of the optical properties of the original samples is always obtained when the oxygen adsorption is made at 473 K. The sample keeps its structural integrity during the reduction and successive oxidation process.

Electron paramagnetic resonance was employed to follow the isotopic exchange reaction between gaseous hydrogen and surface deuteroxyl groups at the surface of CaO. This was possible because of the superhyperfine interaction between the H (D) nucleus of surface OH (OD) group and a nearby paramagnetic centre constituted, in one case, by a surface trapped single electron and, in a second case, by a surface-adsorbed superoxide ion. Copyright © 2000 John Wiley & Sons, Ltd.

TiO2-supported V2O5–MoO3 catalysts, having V and Mo loadings representative of commercial SCR catalysts, are considered in this study. These catalysts are constituted by TiO2 anatase supporting the V and Mo active components. MoO3 acts as a “structural” promoter preventing the TiO2 matrix from sintering upon vanadia addition. The Mo and V oxide are present on the catalyst surface in the form of molybdenylic and vanadylic species, respectively, and the presence of polymeric MoxOy species cannot be excluded. The features of the V and Mo surface oxide species closely resemble those observed over the binary V2O5/TiO2 and MoO3/TiO2 catalysts, thus pointing out the vibrational independence of the V and Mo surface vanadyl and molybdenyl oxide species. However, in spite of their structural and vibrational independence, the presence of electronic interactions between the TiO2-supported V and Mo oxides is also apparent. These interactions may operate via the TiO2 support or may involve mixed V–Mo surface oxide species who were, however, not observed. The catalyst surface is characterized by strong acidity, probed by NH3-TPD and FT-IR. Ammonia is coordinatively held over Lewis acid sites (associated with Ti, V, and Mo surface cation species) and is protonated as NH4+ ions over Mo–OH or V–OH Bronsted sites. The addition of Mo and V causes the formation of Bronsted sites and of stronger Lewis acid sites, if compared to TiO2. The V2O5–MoO3/TiO2 catalysts are very active in the reduction of NO by NH3, and exhibit a higher reactivity with respect to the corresponding binary V2O5/TiO2 and MoO3/TiO2 samples. Calculations show that the reactivity of V and/or Mo in the ternary catalysts is higher than that measured over V2O5/TiO2 and MoO3/TiO2 having the same V and Mo loading: hence it is suggested that a synergism operates in the SCR reaction between the V and Mo surface oxide species. Accordingly in these catalysts molybdenum also acts as a “chemical” promoter for the SCR reaction. On the basis of the characterization data, it is suggested that the observed synergism in the SCR reaction is related to the existence of the V–Mo electronic interactions. This picture closely resembles that obtained in the case of the analogous V2O5–WO3/TiO2 system and indicates that the effects of the addition of WO3 and MoO3 to V2O5/TiO2 are similar, both oxides acting as “chemical” promoters besides playing a “structural” function as well. However the V2O5–MoO3/TiO2 samples show higher formation of N2O and lower NO conversions at high temperatures: these differences are possibly associated with the different electronic characteristics of Mo compared to W and to their higher reactivity in the ammonia oxidation reactions. It is found that water addition in the feed improves the catalyst performance in that it preserves high NO conversions and high N2 selectivities at high temperatures: this is due to its strong inhibiting effect on the ammonia oxidation reactions occurring simultaneously with the SCR reactions.

An embedded-cluster Hartree−Fock approximation is adopted for simulating the formation of Fs(H) color centers at the (001) surface of magnesium oxide. This process is assumed to take place in two steps at an isolated surface anion vacancy: first, a hydrogen molecule is adsorbed dissociatively at the defect; second, following UV irradiation, a neutral hydrogen atom is removed and an electron remains trapped at the vacancy with a hydroxyl group nearby. According to the present calculations, the activation energy for the dissociation is appreciable (about 25 kcal/mol) and the products (a proton bound to a low-coordinated oxygen and a hydride ion above the vacancy) are considerably less stable than the reactants. The excitation of the adsorbed species owing to the UV irradiation is simulated by considering a singlet−triplet transition of the hydride−vacancy complex, which then dissociates into an H atom and a trapped lone electron. The electronic structure and the EPR parameters of the resulting paramagnetic state are explored. The theoretical results agree in many respects with the experimental data as concerns one of the forms of heterolitically dissociated hydrogen which are found at the defective MgO surface. However, from the viewpoint of the energetics, this model is untenable because that species is known to form irreversibly at room temperature with low activation energy.

EPR and ENDOR spectroscopy have been used to investigate a variety of trapped electron centers on the surface of polycrystalline MgO. The oxide was dehydrated under vacuum at different temperatures...

In order to investigate the molecular mechanism whereby iron in asbestos and in other iron containing fibres plays a crucial role in fibre induced carcinogenicity, two Fe2+-exchanged zeolites (Y and H-ZSM-5) have been prepared and used as model solids. Fe-Y was active both in free radical generation via hydrogen abstraction and in single strand DNA damage, but lost both these properties when oxidized by hydrogen peroxide. Fe-ZSM-5, on the other hand, was nearly inactive. Iron chelators and reductants (ascorbate, citrate and EDTA) enhance the activity in DNA single strand breaks. Moreover, they reactivate oxidized Fe-Y and activate Fe-ZSM-5. Iron is mobilized and Fe3+ is partially reduced to Fe2+. The most pronounced effect was found when citrate was present together with ascorbate. H-abstraction turns out to be a rather general reaction, occurring also with peptides as target molecules. In this case EPR spectra show that more than one type of radical moiety is generated.

The superoxide radical anion O2- is both an important intermediate in heterogeneous catalytic oxidation and a useful probe for positive charges in ionic solids, such as metal oxides and zeolites. The paper illustrates the main circumstances under which stable superoxide anions are formed at surfaces: (i) direct surface–oxygen electron transfer; (ii) photoinduced electron transfer; (iii) surface intermolecular electron transfer; (iv) decomposition of hydrogen peroxide.

TiO2-supported MoO3 catalysts, with MoO3 loading similar to that of commercial DeNOx-SCR catalysts (in the range 0–11% w/w) have been prepared, characterized and tested in the NO reduction by NH3. The structural and morphological characteristics of the anatase-TiO2 support are not significantly changed upon MoO3 addition. For MoO3 loading below or near 9% (w/w) (corresponding to the formation of the theoretical monolayer), evidence for the presence of isolated molybdenyl surface species have been collected by FT-IR, Laser Raman and EPR spectroscopies. Above this loading, the presence of crystalline MoO3 is observed. The MoO3/TiO2 catalysts are active in the reduction of NO by NH3: the reactivity of the catalysts increases on increasing the MoO3 loading, whereas the N2 selectivity decreases due to the formation of undesired N2O. The formation of N2O is primarily ascribed to a reaction between NH3 and NO, and not to the ammonia oxidation reaction. A comparison with a WO3/TiO2 catalyst having similar molar composition indicates that the WO3- and MoO3-containing samples exhibit similar structural and morphological characteristics, but different reactivity: the WO3/TiO2 sample is less active but more selective in the SCR reaction. On the basis of EPR data, the different catalytic behavior has been tentatively ascribed to the different redox characteristics of the samples.

The basic steps of catalytic oxidation processes at the surface of metal oxides consist in redox interactions involving the solid and one or more reactants (in particular molecular oxygen). The general redox properties of a metal oxide can be investigated by means of the electron paramagnetic resonance (EPR) technique which is capable of detecting the paramagnetic centers often formed upon one electron redox processes. The principles of the EPR technique are briefly described in the first part of this paper. In the second part routine methods to measure the electron donor and the electron acceptor ability of a surface (using particular probe molecules) are described. Finally the paper deals with the interactions of molecular oxygen with the surfaces of oxides leading to electron transfer towards the adsorbed molecule. Various examples of electron transfer are discussed in relation to the nature and activation conditions of the solid. In conclusion the particular case of surface intermolecular electron transfer (SIET) is briefly illustrated.

Electron spin resonance (ESR) or Electron paramagnetic resonance (EPR) spectroscopy is an experimental technique for detecting and characterizing chemical systems bearing one or more unpaired electrons. The application of the technique, the background of which is similar to the more diffused nuclear magnetic resonance (NMR) spectroscopy, therefore concerns (i) organic and inorganic free radicals, (ii) trapped radicals in various matrices, including irradiated solids (radiation chemistry), frozen inert matrices and solid surfaces, (iii) transition metal ion compounds in classical inorganic systems or in biological systems, (iv) excited paramagnetic states (triplets) and several other systems of relevant scientific interest. The aim of this article is to provide an overall survey on the basic principles and on the various applications of the technique. This is done by describing, first, the physical basis of the electron resonance phenomenon (microwave absorption when the system is in the line of force of a strong external magnetic field) and, subsequently, the main types of interactions that the unpaired electron undergoes in the chemical system to which it belongs and which determines the features of the experimental spectra. These are essentially (i) the magnetic interaction of the electron with nuclei of nonzero nuclear spin (hyperfine interaction) which determines the multiline structure of the spectrum (hyperfine structure) and (ii) the interaction of the electron spin with the electron orbital angular momentum, occurring through the so-called spin–orbit coupling. This latter interaction causes the dependence of the resonance on the orientation of the radical in the external magnetic field. Particular emphasis will be given to the different types of spectra observed according to the physical state of the investigated sample (liquid solutions, single solid crystals or microcrystalline powdered solids). In the last case (powders), the essential methodology for understanding the often complex profiles of the experimental spectra is described. Advanced electron resonance techniques, essentially ENDOR (electron–nuclear double resonance) and time-resolved EPR will also be briefly mentioned. The final part of the article is devoted to some analytical applications of the technique, including spin trapping (an essential tool for the detection and quantitative evaluation of reactive short-lived radicals), radiation dosimetry (with applications to medical problems and irradiated foodstuff analysis) and dating of geologically relevant systems.

ZrO₂-TiO₂ mixed oxides of various composition, with the molar fraction of TiO₂ ranging from 0.1% to 15%, have been prepared via sol-gel synthesis and then calcined at 1273 K to check both their thermal stability and physicochemical properties. These solids are usually employed in photocatalytic processes and as active phase supports in heterogeneous catalysis. As indicated by X-ray diffraction and Raman spectroscopy, solid solutions based on Ti ions diluted in the ZrO₂ matrix are formed in the whole range of Ti molar fraction examined. Materials with low Ti loading (0.1%–1%) are basically constituted by the monoclinic phase of ZrO₂ while the tetragonal phase becomes prevalent at 15% of TiO₂ molar fraction. The presence of Ti ions modify the electronic structure of the solid as revealed by investigation of the optical properties. The typical band gap transition of ZrO₂ undergoes, in fact, a red shift roughly proportional to the Ti loading which reach the remarkable value of 1.6 eV for the sample with 10% of molar Ti concentration. Comparing chemical analysis of the solids with XPS data it has been put into evidence that the titanium ions distribution into the solid is not uniform and the concentration of Ti⁴⁺ tend tomore » be higher in subsurface layers than in the crystal bulk. The introduction of titanium ions in the structure increases the reducibility of the solid. Annealing under vacuum at various temperatures causes oxygen depletion with consequent reduction of the solid which shows up mainly in terms of formation of Ti³⁺ reduced centres which are characterized by a typical EPR signal. Ti³⁺ defects forms, as also forecast by theoretical modelling of the solid, as their energy is lower than that of other possible reduced defective centers. The reduced solids are able to transfer electrons to adsorbed oxygen molecules in mild condition resulting in the formation of surface superoxide anions (O₂⁻) which are stabilized on surface Zr⁴⁺ or, alternatively, on Ti⁴⁺ ions according to the sample composition. - Graphical abstract: The presence of Ti in ZrO₂ modifies UV–vis and electronic properties of mixed oxides. Highlights: • ZrO₂–TiO₂ mixed oxides have been prepared via sol–gel synthesis. • Solids with low Ti loading are constituted by the monoclinic phase of ZrO₂. • The presence of Ti ions modify the electronic structure of the solid. • The introduction of titanium ions increases the reducibility of the solid.« less

Electron Paramagnetic Resonance (EPR) techniques have been employed to investigate charge carrier trapping in the two main TiO2 polymorphs, anatase and rutile, with particular attention to the features of electron trapping sites (formally Ti(3+) ions). The classic CW-EPR technique in this case provides signals based on the g tensor only. Nevertheless a systematic analysis of the signals obtained in the various cases (anatase and rutile, surface and bulk centers, regular and defective sites) has been performed providing useful guidelines on a field affected by some confusion. The problem of the localization of the electron spin density has been tackled by means of Pulse-EPR hyperfine techniques on samples appositely enriched with (17)O. This approach has led to evidence of a substantial difference, in terms of wavefunction localization between anatase (electrons trapped in regular lattice sites exhibiting delocalized electron density) and rutile (interstitial sites showing localized electron density).

The surface reactivity of metal oxides has been studied by continuous wave electron paramagnetic resonance spectroscopy and is discussed by means of selected examples concerning magnesium oxide. In particular, the influence of external physical or chemical treatments experienced by the solid on its surface structure and reactivity is illustrated. Examples are given of the effects of progressive dehydration of the solid, doping with transition metal ions and, particularly, addition of alkali metal vapours.

We have investigated the interaction of O 2 − with Mg 2+ ions at terraces, edges and corners and with the F s centers of the MgO (100) surface by means of ab i3itio cluster model wavefunctions. While O 2 is unbound or weakly bound at surface cations, O 2 − is electrostatically bound with adsorption energies from 2 to 4 eV depending on the local coordination. O 2 interacts with the electrons trapped at the F s centers to form O 2 − and F s + . In a similar way, O 2 becomes O 2 − on F s + centers with the formation of F s 2+ O 2 − surface complexes. The high cost for the ionization of the F s centers is compensated by the electrostatic attraction between O 2 − and the charged vacancies. The effect of the local electric field on the g -tensor is discussed.

A peroxy radical species is generated in the framework of Na+/Y zeolites, upon treatment with a hydrogen peroxide solution, drying and evacuation of the solid at 420 K. The amount of the species increases upon increasing the temperature of the treatment under vacuum up 570 K. The EPR spectrum of the species at 77 K is characterized by anisotropicg tensor (g 1=2.059,g 2=2.010,g 3=2.007). Upon increasing the recording temperature the spectrum undergoes a dramatic evolution indicating the onset of motional phenomena. This dynamic behavior together with the values of theg tensor components and the high thermal stability of the species allow one to distinguish the observed species from the more common superoxide radical ion produced by oxygen adsorption on Na+/Y zeolites treated with metallic sodium.

The application of IR, Raman, UV–visible, EPR, XANES, EXAFS, and TPR techniques to the determination of the Fe 3+ local environment in Fe-silicalite is described and discussed. These methods give information concerning the structural changes of the zeolitic structure, upon thermal treatments and interaction with adsorbates. In particular the effect of template burning at 773 K or at 973 K on the local Fe 3+ environment has been discussed in detail. The reactivity of the zeolite towards NH 3 has also been considered.

The interaction of hydrogen peroxide with various metal oxides has been followed by EPR in the aim of investigating the paramagnetic species formed along the complex processes of H2O2 decomposition and interaction with the oxide. Data are reported about three systems (MgO, ZrO2 and Me2+/Y zeolite) which are representative of three classes of oxides (insulators, semiconductors and aluminosilicate molecular sieves). Several radical species have been put into evidence in the various cases. The superoxide O2− ion has been observed in all cases while in the case of MgO also the OH radical and the O− radical ion have been identified. In the case of Zn2+/Y zeolite, beside the superoxide radical anion, a new signal is observed which shows up upon treating the sample at 640 K and which is preliminarily assigned to a species in the triplet state in the zeolitic cavity.

The reactivity of copper-on-alumina catalysts in the reduction of NO by NH{sub 3} + O{sub 2} (SCR reaction) and oxidation of NH{sub 3} to N{sub 2} (NH{sub 3}ox reaction) as a function of the copper loading shows that (i) the specific rate of the SCR reaction per mole of copper decreases with increasing copper loading, whereas the NH{sub 3}ox reaction rate is nearly independent of the copper loading and (ii) the rate of the NH{sub 3}ox reaction is significantly higher in the absence of NO than that of the same reaction occurring during the SCR reaction. The catalysts were characterized by electron spin resonance (ESR), X-ray diffraction analysis, and UV-visible diffuse reflectance. The nature of the NO and NH{sub 3} adsorbed species was studied using ESR and infrared (IR) spectroscopies, and IR studies of the reactive coadsorption of NO and NH{sub 3} were carried out. The results obtained indicate that the above catalytic effects are due to the formation of nitrate species which block the reactivity of copper sites. The transformation of NO to N{sub 2} via the intermediate formation of ammonium nitrate was shown, but the data suggest that this is only a secondary pathway of reaction responsible formore » the formation of N{sub 2}O as a byproduct. The nature of active copper species on the surface and the presence of competitive pathways of transformation of NO to N{sub 2} are also discussed.« less

Vanadium and titanium bimetallic AlPO-5 molecular sieves have been synthesized and characterized by means of Electron Spin Echo detected EPR and Hyperfine Sublevel Correlation (HYSCORE) spectroscopy. Direct evidence for framework substitution of redox-active Ti ions and VO(2+) units at Al sites is provided through the detection of large (31)P hyperfine couplings.

The generation of surface charge carriers in N-doped TiO2 under various types of irradiation has been investigated by electron paramagnetic resonance using an approach consisting in scavenging surface migrated electrons and holes using oxygen and hydrogen, respectively. N-doped TiO2, which is moderately active in photocatalytic processes under visible light, forms surface electrons and, at lower extent, surface holes due to the synergistic effect of visible components (around 400 nm) and near-infrared ones. The visible radiation excites electrons from intra band gap NOx– states to the conduction band, while NIR frequencies excite electrons from the valence band to the NOx– centers. The limited concentration of such centers explains the moderate efficiency of the whole process and, consequently, of the photocatalytic activity of N-TiO2 in visible light with respect to the case of UV light irradiation. Despite the mentioned limits this material remains a fundamental starting point for a new generation of ph...

Two series of doped titanium dioxide samples (S-TiO(2) and F-TiO(2)) were prepared by the sol-gel method in the presence of different amounts of dopant source (thiourea and NH(4)F, respectively), followed by calcination at 500, 600 or 700 °C, and characterised by BET, UV-vis absorption, XPS, HRTEM, XRD and EPR analyses. Reference undoped materials were prepared by the same synthetic procedure. Their photocatalytic activity under visible light was investigated employing the photocatalytic degradation of formic acid in aqueous suspension as test reaction. S-doped TiO(2) showed a photocatalytic activity quite similar to that of undoped materials. In this regard, the insertion of S, characterised by a relatively large ionic radius, into the TiO(2) crystalline structure appears rather difficult, as confirmed by XPS analysis. On the contrary, moderate F doping was beneficial in increasing the rate of formic acid photocatalytic degradation, especially for photocatalysts calcined at high temperature, consisting of highly crystalline pure anatase, in which the rate of detrimental charge carrier recombination was reduced. For both series of doped materials, high doping levels appear to limit the semiconductor photoactivity, probably due to the formation of a progressively increasing number of charge recombination centres. The EPR characterisation of the investigated doped TiO(2) samples evidenced the presence of nitrogen containing species (nitric oxide radical encapsulated in micro-void, with no photoactivity, and N(b)˙ species, active in visible light sensitisation) and of titanium reduced centres Ti(3+), due to charge imbalance consequent to dopant introduction in the TiO(2) lattice either in anionic (F(-)) or in cationic form (S(6+)).

Aromatase (CYP19A1), is a microsomal cytochrome P450 catalysing the conversion of androgens to estrogens. Non-steroidal inhibitors, such as anastrozole, are important drugs in breast cancer therapy. Using hyperfine sublevel correlation (HYSCORE) spectroscopy we provide the first experimental evidence of the binding of anastrozole to the iron heme of human aromatase.

Three Mg–Nb–O compounds were synthesized in order to understand the role of oxides as promoters/catalysts for hydrogen storage in Mg/MgH2 systems. Ternary Mg–Nb–O compounds were prepared by solid-state reactions. The effect of calcination temperature on phase formation and chemical composition of the solid-state products were studied by X-ray diffraction. Gradual reaction of precursor components and simultaneous formation of solid-state products were found to be strongly dependent on calcination conditions. Hydrogen sorption properties were investigated by Mass Spectrometry. MgNb2O6 and Mg4Nb2O9 phases were rather inactive with respect to hydrogen. On the contrary, Mg3Nb6O11 compound showed remarkable uptake and release of molecular hydrogen. The reversible hydrogen interaction with this phase is discussed with respect to octahedral niobium clusters present in the oxide structure.

Reduced states in TiO(2) : (17)O hyperfine sublevel correlation spectroscopy was used to monitor the local environment of stable Ti(3+) ions generated in a (17)O-enriched polycrystalline TiO(2) (rutile) sample. A hyperfine interaction of about 8 MHz is found, which is analogous to that observed for molecular Ti(3+) aqua complex cations and suggests a localized nature of the unpaired electron wave function for these centers at 4 K.

Hydrogen storage in MgH 2 is strongly limited by kinetic constraints, so that several additives have been considered in order to improve the rate of hydrogen desorption. The present work is devoted to investigate the effect of BaRuO 3 addition on hydrogen absorption and desorption properties of MgH 2 . BaRuO 3 was prepared by solid-state synthesis of BaO and Ru in oxidizing environment. The interaction of as-prepared BaRuO 3 with hydrogen at room temperature was examined by thermal desorption and X-ray diffraction experiments, showing that BaRuO 3 has the ability to uptake hydrogen in the bulk. The hydrogen interaction is discussed with respect to octahedral ruthenium local environment present in the oxide structure. Addition of 1 mol.% BaRuO 3 to MgH 2 was performed by ball milling. Hydrogen desorption properties were investigated by Sievert and DSC, connected with a H 2 detector. The H 2 desorbed from of the nano-structured materials is lower than the theoretical content in MgH 2 , due to the presence of a non-reactive MgO layer. A remarkable increase of the H 2 desorption kinetics in nano-structured MgH 2 was observed with respect to bare MgH 2 . The activation energy for hydrogen desorption from MgH 2 /BaRuO 3 mixture was estimated by Arrhenius and Kissinger methods and a value around 130 kJ mol −1 and 90 kJ mol −1 was obtained, respectively.

Oxygen adsorption on the surface of MgO, previously submitted to magnesium metal vapour, gives rise to the formation of at least five distinct species of adsorbed superoxide (O 2 − ) characterized by different g zz values. Besides species previously observed on MgO, where O 2 − is generated by other methods, two new species are observed, having g zz = 2.091 and g zz = 2.064 respectively. The latter species is tightly bound to the surface and is assigned to O 2 − adsorbed on top of an array of five Mg 2+ ions exposed because of the presence of an anion vacancy. The former is assigned to O 2 − weakly adsorbed on top of a five-coordinated Mg 2+ ion at the (100) planar face of the oxide. This species differs from the other species present at the surface in that it is mobile and, as the temperature is raised from 77 to 298 K, exhibits rotation around the axis perpendicular to the surface. Spectra obtained by employing 17 O-enriched O 2 indicate that both superoxide species are symmetrically adsorbed on the surface.

Three distinct radical species are formed and stabilized in the solid matrix upon the treatment of magnesium oxide with hydrogen peroxide in aqueous solution leading to the transformation of the solid into magnesium peroxide. The species in question are the O 2 - superoxide radical ion, the O - radical ion, and the OH hydroxyl radical. The latter radical is characterized by orthorhombic g and A tensors whose principal values, due to the ionicity of the trapping matrix, are slightly different from those previously observed for OH in irradiated ice or similar systems. In comparison to the OH radical trapped in ice, the hydroxyl radical observed in the present case (as well as the two companion radicals) is stable up to 473 K

A novel paramagnetic ionic cluster containing three lithium ions (likely Li 3 2+ ) has been obtained upon treatment of the dehydrated MgO surface with vapors of lithium. The corresponding EPR spectrum is characterized by g=2.0011 and a hyperfine splitting of 8.26 G (1 G=0.1 mT).

Nitrogen boron co-doped TiO2 prepared via sol-gel synthesis and active under visible light, contains two types of paramagnetic extrinsic defects, both exhibiting a well resolved EPR spectrum. The first center is the well characterized [NiO](center dot) species (i = interstitial) also present in N-doped TiO2, while the second one involves both N and B. This latter center (labeled [NOB](center dot)) exhibits well resolved EPR spectra obtained using either 14 N or 15 N which show a high spin density in a N 2p orbital. The structure of the [NOB](center dot) species is different from that previously proposed in the literature and is actually based on the presence of interstitial N and B atoms both bound to the same lattice oxygen ion. The interstitial B is also linked to two other lattice oxygen ions reproducing the trigonal planar structure typical of boron compounds. The energy level of the [NOB](center dot) center lies near the edge of the valence band of TiO2 and, as such, does not contribute to the visible light absorption. However, [NOB](center dot) can easily trap one electron generating the [NOB](center dot) diamagnetic center which introduces a gap state at about 0.4 eV above the top of the valence band. This latter species can contribute to the visible light activity.

V2O5-WO3/TiO2 samples with compositions similar to those of commercial de-NOx catalysts (WO3 ∼ 9% w/w, V2O5, < 3% w/w) are characterised by XRD, surface area and pore size distribution, Fourier transform infrared, laser Raman, UV-vis diffuse reflectance, electron paramagnetic resonance spectroscopies, and catalytic tests in the reduction of NOx by NH3. The V2O5-WO3/TiO2 catalysts exhibit higher reactivity than the binary V2O5/TiO2 and WO3/TiO2 samples with the same metal loading, and the temperature window for the SCR reaction is greatly widened. The catalysts consist of anatase TiO2 and their morphological properties closely resemble that of WO3/TiO2 for (V + W) surface coverages lower than one. Monomeric vanadyls and wolframyls and polymeric WwOy groups are observed in the samples with low vanadia loadings that are apparently similar to those present on the surface of the binary oxide systems with comparable metal loadings. On increasing the vanadium loading, polyvanadate species are also formed. EPR, FTIR, FT-Raman, and UV-vis techniques indicate a strong electronic interaction between V and W oxide species at the surface of the TiO2 support. This interaction leads to a higher reducibility of the ternary sample with respect to the corresponding binary ones. A synergism between V and W oxide surface species is suggested, which accounts for the high reactivity of the ternary samples in the SCR reaction. It is suggested that the higher reducibility of the samples, due to the electronic interactions between V and W and the TiO2 support, is responsible for the higher reactivity of the ternary catalysts, particularly at low temperatures.

The superoxide radical anion O2- is both an important intermediate in heterogeneous catalytic oxidation and a useful probe for positive charges in ionic solids, such as metal oxides and zeolites. The paper illustrates the main circumstances under which stable superoxide anions are formed at surfaces: (i) direct surface–oxygen electron transfer; (ii) photoinduced electron transfer; (iii) surface intermolecular electron transfer; (iv) decomposition of hydrogen peroxide.

Physisorption of N2 gas onto the surface of a metal oxide (MgO or CaO), containing paramagnetic trapped electron centers (FS + color centers), leads to the formation of a paramagnetic species that, on the basis of its EPR spectrum and of the related spin-Hamiltonian parameters, is identified as a N2 - radical anion. The species in fact contains two nitrogen atoms and its g and A tensors are in agreement with what observed for the N2 - radical trapped in irradiated crystal of various azides. The surface N2 - species is formed by surface-tomolecule one-electron transfer, and its stability strictly parallels the stability of the physisorbed layer, the species formation being completely reversible and pressure dependent. When the N2 adlayer is desorbed, in fact, the N2 - spectrum vanishes and the original Fs + spectrum is restored. Ab initio quantum chemical calculations on an embedded MgO cluster fully confirm the observed phenomenon indicating, in agreement with EPR analysis, the electron transfer of a large fraction of electron density into the ð orbitals of the admolecule.

The formation of O(2) (-) radical anions by contact of O(2) molecules with a Na pre-covered MgO surface is studied by a combined EPR and quantum chemical approach. Na atoms deposited on polycrystalline MgO samples are brought into contact with O(2). The typical EPR signal of isolated Na atoms disappears when the reaction with O(2) takes place and new paramagnetic species are observed, which are attributed to different surface-stabilised O(2) (-) radicals. Hyperfine sublevel correlation (HYSCORE) spectroscopy allows the superhyperfine interaction tensor of O(2) (-)Na(+) species to be determined, demonstrating the direct coordination of the O(2) (-) adsorbate to surface Na(+) cations. DFT calculations enable the structural details of the formed species to be determined. Matrix-isolated alkali superoxides are used as a standard to enable comparison of the formed species, revealing important and unexpected contributions of the MgO matrix in determining the electronic structure of the surface-stabilised Na(+)-O(2) (-) complexes.

Electron bombardment of single-crystalline MgO(001) films has been shown to result in the formation of surface color centers. In this report we present an alternative way to produce surface color centers on single-crystalline MgO(001) films using additive Mg coloring. Electron paramagnetic resonance (EPR) spectroscopy was used to prove the formation of surface paramagnetic color centers upon deposition of small amounts of Mg on MgO(001) films at 50 K. With increasing coverage the number of Mg induced color centers rises up to a coverage of about 0.03 monolayers and thereafter decreases with increasing Mg coverage. At high coverage and low temperatures metallic Mg clusters nucleate on the surface of the MgO(001) film as shown by infrared (IR) spectroscopy using CO as a probe molecule.

In the present work, different ZrO2−TiO2 mixed oxides, which are usually employed in photocatalytic processes and as catalytic support in heterogeneous catalysis, have been prepared via sol−gel synthesis or by wet impregnation. In the former case, as indicated by X-ray diffraction and Raman spectroscopy, solid solutions based on Ti ions diluted in the ZrO2 matrix are formed in the range of Ti molar fraction from 0.01 to 0.15. These solids have interesting optical properties as the typical band gap transition of ZrO2 undergoes a red shift proportional to the Ti loading which reach the remarkable value of 1.3 eV for the highest Ti concentration. Annealing under vacuum at various temperatures causes oxygen depletion with consequent reduction of the solid which shows up mainly in terms of formation of Ti3+ reduced centers which are characterized by a typical broad EPR signal similar to that observed in reduced TiO2 (anatase). The ability of the mixed oxides in stabilizing surface superoxide anions (O2•−) has ...

66-3 FIELD Section Title:Surface Chemistry and Colloids Laboratoire Reactivite de Surface, UPMC, University Paris 06, Paris, Fr. FIELD URL: written in English; The adsorption geometries, energies, and vibrational frequencies of methanol on MgO defective surfaces have been calcd. by periodic DFT simulations. The results are very comparable with those obtained with water and are also in very good accordance with microcalorimetry and IR expts. At low coverage, the dissocn. is obsd. on all defects involving ions in low coordinations. Over and above the coordination no. of surface ions, the adsorption energy is strongly governed by the surface topol.: dissocn. on confined sites gives rise to methoxy groups highly stabilized by bridging two or even three cations. The occurrence of such very strong sites on MgO powder is confirmed by microcalorimetry. The dissocn. ability depends on the methanol coverage because it modifies the surface relaxation and the network of H bonds, resulting, for a given defect, in similar adsorption energies for mol. and dissocd. species at high coverage. This explains why there are more strong sites (quantified by microcalorimetry) than dissocg. sites (quantified by IR). [on SciFinder(R)]

Sulphur contamination of alkaline-earth oxide surfaces shows important consequences in many chemistry fields such as surface science and catalysis. We used the Electron Paramagnetic Resonance (EPR) technique to study the interaction of SO2 molecules with the bare and electron enriched surfaces of MgO and CaO. Two paramagnetic products were identified via EPR thanks to this interaction, the SO2 − and S2O − radicals whose abundance depends on the surface oxide properties. In particular, higher basicity and higher number of defects in the case of CaO lead to a higher amount of these two radical species.

Some cases of formation and stabilization of charge carriers (electron and holes) at the surface of solid oxides are discussed. Charge carriers can be simultaneously or independently stabilized at the surface metal oxides. In the former case, they usually derive from a process of charge separation in the solid triggered by above band gap irradiation. In the second case, the charge carrier isolation is the result of a chemical alteration of the stoichiometric equilibrium of the solid either by matter addition or by effect of chemical impurities (valence induction). Electron paramagnetic resonance (EPR) is highly suited to monitor the process of charge separation and to characterize electron or hole centers stabilized in the solid or at its surface. In this paper examples of trapped electron and/or trapped hole centers as detected by continuous-wave EPR at the surface of simple binary oxides are discussed with particular emphasis to the formation mechanism, the EPR parameters and the chemical reactivity.

Oxidation of magnesium nitride, Mg3N2, leads to segregated nitrogen impurities within the bulk of MgO. Electron paramagnetic resonance experiments in conjunction with quantum chemical calculations allow us to identify these paramagnetic centers as N2- radical anions trapped in anion vacancies in the bulk of MgO. The characteristics of the defect center, derived from the spin Hamiltonian parameters, exhibit similarities but also some interesting differences with respect to the classical N2- species in KCl, a sort of prototype of molecular anions trapped in ionic crystals. © 2010 American Chemical Society.