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High surface area TiO2 was synthetized by means of a solvothermal method and it was then used to prepare two photocatalysts based on copper-modified TiO2. Two different preparation approaches have been adopted based on the use of the same amount of Cu2O (0.5% w/w) as a reactant. The spectroscopic characterization shows that the two preparation procedures lead to distinct distributions of Cu2O and other Cu-based species in the final composite materials and that the photoactivity of the solids is strictly related to the nature of the copper species. The joint application of CW-EPR and optical spectroscopy, both employed in catalytic conditions as a sort of “operando” mode, allowed monitoring the evolution of the photocatalytic systems occurring during the H2 evolution reaction (HER). The effect of water coordination on the Cu(II) species at the surface of titania has been revealed by EPR. The successive photoreduction of the system in the early steps of the photocatalytic reaction involves the reduction of Cu2+, the formation of Ti3+ centres in the oxide matrix and that of metallic copper particles that partially inactivate the system.

Metal ion-doped titanium dioxide (TiO2) was synthesized by using the novel sol–gel method, where copper and manganese were used for dopant ions. The obtained Cu–TiO2 and Mn–TiO2 powders were investigated by electron paramagnetic resonance (EPR) spectroscopy before and after calcination treatment. EPR results indicate that the novel method can homogeneously spread out the metal ions into basic TiO2 structure. After calcination, the Cu2+ ion was in an axially distorted octahedral site, while Mn2+ and Mn4+ ions were created.

Paramagnetic species are often involved in catalytic or photocatalytic reactions occurring at the solid-gas interface of heterogeneous catalysts. In this contribution we will provide an overview of the wealth and breadth of information that can be obtained from EPR in the characterization of paramagnetic species in such systems, illustrating the advantages that modern pulsed EPR methodologies can offer in monitoring the elementary processes occurring within the coordination sphere of surface transition-metal ions. To do so we selected three representative systems, where titanium ions in low oxidation states act as active catalytic sites, trying to outline the methodological approaches which characterize the application of EPR techniques and the questions that can be answered and addressed relative to the characterization of heterogeneous catalytic materials.

In the present study, we investigated the effect of cerium and erbium doping of the zirconium dioxide matrix. We synthesized doped samples using hydrothermal process. The amounts of dopant used were 0.5%, 1% and 5% molar (rare earth oxide over zirconium dioxide) respectively. The samples have been studied via X-ray Diffraction measurements for the structural characterization. UV visible diffuse reflectance was used for the optical analysis, Brunauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally the samples have been analysed via electron paramagnetic resonance (EPR) for the electronic characterization. Then we tested the new synthetized materials to determine their photocatalytic activity in the reaction of degradation of methylene blue performed under irradiation by diodes (LEDs) emitting exclusively visible light.

Zirconium titanate (ZrTiO4) modified via cerium ions insertion in the oxide lattice has been successfully prepared via Sol-Gel synthesis forming solid solutions in the molar range between 0 and 10%. Cerium ions are hosted both as Ce4+ and Ce3+ ions whose ratio depends on the treatment undergone by the solid, with trivalent cerium always abundantly present even in strong oxidative conditions. Surface exposed Ce3+ ions are capable to adsorb O2 in the range of temperature between 273 K and 77 K, as superoxide anion (O2 −), having side-on structure and characterized by complete surface-to-molecule electron transfer (about 98% of spin density on O2 molecule). Surprisingly such abundant adsorption is pressure dependent and fully reversible opening the perspective of applications of this system in the field of oxygen separation from gas mixtures.

TiO2 and Ti-W mixed oxide photocatalysts, with W/Ti molar ratios in the 0–5% range, were prepared through a simple sol-gel method, followed by annealing at 500 or 700 °C, and their photoactivity was tested in the photo-oxidation of formic acid in the aqueous phase under ambient aerobic conditions. XRPD analysis evidenced that in the presence of tungsten the anatase phase was stable even after calcination at 700 °C, with a progressively larger surface area and smaller particle dimensions with increasing tungsten content. Tungsten can both enter the titania lattice, as demonstrated by HAADF-STEM analysis, and also segregate as amorphous WO3 on the photocatalysts surface, as suggested by XPS analysis. The best performing Ti/W oxide photocatalyst is that containing 1.0 mol% W/Ti, mainly due to the tungsten-induced stabilization effect of the anatase phase, whereas electron transfer from TiO2 to WO3, though compatible with the here performed EPR measurements, appears to have no beneficial effect in the investigated reaction, likely due to the low energy level of the conduction band of WO3, from which electrons cannot efficiently transfer to adsorbed dioxygen.

A series of Pt/Cu/TiO2 photocatalysts, showing very high performance in photocatalytic hydrogen production from methanol/water vapour mixtures, were prepared under mild conditions by Cu(II) grafting on commercial P25 TiO2, with nominal Cu/TiO2 ratios ranging from 0.05 to 0.5 wt.%, followed by 0.5 wt.% Pt nanoparticles deposition by the deposition-precipitation method in the presence of urea. The structural features of the so obtained materials were fully characterized by X-ray absorption spectroscopy, which provided information on the oxidation state of the two metals and on the metal-metal and metalTiO(2) interactions, and by EPR analysis, which evidenced electron transfer phenomena involving copper under irradiation. The photocatalysts showed a volcano-shaped photoactivity trend in hydrogen production with increasing nominal Cu content, the maximum rate of H2 evolution (27.2 mmol h-1 gcat-1) being attained with the photocatalyst containing 0.1 wt.% of copper. In this sample CuO nanoclusters appear to be intimately coordinated with surface Ti atoms in a surface structure that partially stabilizes pre grafted copper in metallic form, possibly acting as an electron-transfer bridge at the interface between CuO nanoclusters and TiO2. Synergistic effects in H-2 photocatalytic production are clearly induced by the co-presence of grafted Cu nanoclusters and Pt nanoparticles on the TiO2 surface, with the copper oxidation state switching under UV-vis irradiation, facilitating electron transfer to adsorbed protons. (C) 2017 Elsevier B.V. All rights reserved.

In the present study, we investigated the effect of cerium doping in zinc oxide matrix, used as photoactive material. Cerium ions into the matrix of ZnO can act like a 'trap' for the electrons, lowering the e -/h + recombination rate and so increasing the photocatalytic efficiency of the ZnO. We synthesised doped samples using a simple precipitation route. The amount of dopant used was, 1 and 10% molar. The samples have been studied via x-ray diffraction measurements for the structural characterisation; UV-visible diffuse reflectance was used for the optical analysis; Branauer-Emmett-Teller (BET) model for the measurement of the surface area. Finally, the samples have been analysed via electron paramagnetic resonance (EPR) spectroscopy for the electronic characterisation and for testing their photoactivity. The spin trapping technique was also use to measure the amount of stable radical adducts formed via reaction of OH• radicals with molecules of the DMPO (5,5-dimethyl-1-pyrroline-N-oxide) spin probe.

Binary materials composed of the oxides SiO2, TiO2 and N-doped TiO2 and the Keggin heteropolyacid (PW12) were prepared and physicochemically characterized. They were used as catalysts and photocatalysts for the hydration of propene to 2-propanol. The characterization of the samples, particularly the electron paramagnetic resonance (EPR) spectroscopy results and the acidity properties, were useful to explain the key role played by the PW12 in the composite materials in the thermal and photoassisted catalytic processes. The simultaneous pres-ence of heat and UV light improved the activity of PW12 in the thermal process, and the binary materials showed better (photo)catalytic activities than that of the bare PW12 in almost all cases. For the first time, this work evidenced through EPR spectroscopy that the increase of reactivity under irradiation could be attributed to the ability of photoexcited PW12 to trap electrons, particularly if the PW12 is supported. Moreover, the effect of water on the reactivity was also studied.

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).

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.

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.

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...

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.

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

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.

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.

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.

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).

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.

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.

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...

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.

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.

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.

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.

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.

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.

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

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.

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 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

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+)).

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.

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.