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Particle attachment and neck formation inside TiO2 nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO2 nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O2 adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

ConspectusEven in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal-support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO

Site isolated vanadium ions supported on zeolites are central in many catalytic processes. However, an atomistic description of the species formed is often hampered by the heterogeneous nature of zeolites and the random distribution of active species. In this contribution we applied pulsed electron paramagnetic resonance spectroscopy on 17O exchanged ZSM-5 zeolite loaded with V4+ ions, via gas phase reaction with VCl4 vapors. From the magnetic parameters relative to 1H, 17O, 27Al and 51V isotopes we prove that a single, structurally well-defined VO(OH)+ species is formed at zeolite Al framework sites and derive a detailed atomistic model inclusive of bond lengths and angles. Comparison is set to vanadyl aquo complexes, highlighting analogies between surface and solution coordination chemistry. The results showcase the power of EPR spectroscopy and 17O isotopic labelling in the geometrical and electronic characterisation of inorganic complexes formed at the surface of oxide materials. Pulsed EPR on disorder systems can therefore compete in structural insight with more well-established X-ray techniques.

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.

The influence of bromide ions on systems containing highly reactive radical species is of great interest for environmental remediation, atmospheric chemistry, and the synthesis of high-added-value ...

Even in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal–support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO2) oxides and are among the most relevant catalysts for a wealth of key processes of industrial and environmental relevance, from the abatement of NOxto the selective oxidation of hydrocarbons and the conversion of methane to methanol. There exist several spectroscopic techniques able to inform on the geometric and electronic structure of isolated single metal ion sites, but either they yield information averaged over the bulk or they lack description of the intimate features of chemical bonding, which include covalency, ionicity, electron and spin delocalization. All of these can be recovered at once by measuring the magnetic interactions between open-shell metals and the surrounding nuclei with Electron Paramagnetic Resonance (EPR) spectroscopy. In the case of oxides, this entails the synthesis of17O isotopically enriched materials. We have established17O EPR as a unique source of information about the local binding environment around oxygen of magnetic atoms or ions on different oxidic supports to rationalize structure–property relationships. Here, we will describe strategies for17O surface enrichments and approaches to monitor the state of charge and spin delocalization of atoms or ions from K to Zn dispersed on oxide surfaces characterized by different chemical properties (i.e., basicity or reducibility). Emphasis is placed on chemical insight at the atomic-scale level achieved by17O EPR, which is a crucial step in understanding the structure–property relationships of single metal atom catalysts and in enabling efficient design of future materials for a range of end uses.

The photochemical activity of the recently proposed CeO2–ZnO photocatalytic material active under visible light has been improved by means of significant modifications of its morphology. A polymeric templating agent (Pluronic) has been used in the synthesis obtaining a particle morphology based on hollow spheres that is better defined in the case of high template concentration. The charge separation ability and the light-induced surface electron transfer under irradiation with visible polychromatic light in various ranges of wavelengths has been investigated by electron paramagnetic resonance. The reactivity of the photogenerated holes has been monitored by the spin trapping technique in the presence of DMPO. The hollow spheres morphology achieved through the synthesis here reported leads to systems with a higher photoactivity under visible irradiation than the same system displaying the classic platelets morphology. A parallel increase of the photocatalytic activity of this novel system in pollution remediation reactions is therefore predictable.

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective 17 O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.

Engineering defects in semiconducting metal oxides is a challenge that remains at the forefront of materials chemistry research. Nitrogen has emerged as one of the most attractive elements able to tune the photochemical and photocatalytic properties of semiconducting oxides, boosting visible-light harvesting and charge separation events, key elements in promoting solar driven chemical reactions. Doping with nitrogen is also a strategy suggested to obtain p-type conduction properties in oxides showing n-type features in their pristine state and to impart collective magnetic properties to the same systems. Here, we review the evolution in the understanding of the role of nitrogen doping in modifying the photochemical and electronic properties of the most common semiconducting oxides used in mentioned applications including: TiO2, ZnO, SnO2 and zirconium titanates. With an emphasis on polycrystalline materials, we highlight the unique role of Electron Paramagnetic Resonance (EPR) spectroscopy in the direct detection of open-shell N-based defects and in the definition of their structural and electronic properties. Synthetic strategies for the insertion of nitrogen defects in the various matrices are also discussed, along with the influence of the corresponding low-lying energy states on the general electronic properties of the doped solids., Engineered nitrogen defects in semiconducting oxides imparts appealing photophysical and photochemical properties to the materials which can be selectively addressed through EPR spectroscopy.

International audience; Surface reactive oxygen species play a fundamental role in selective oxidation and combustion reactions catalyzed by metal oxides. Unravelling the nature of these transient species, and understanding the details of both their electronic structure and reactivity has been for decades one of the prime research areas of surface and catalytic chemistry. The longstanding activity of Michel Che in this area has been a source of inspiration for generation of researchers, and constitute his outstanding and enduring scientific legacy. Present review provides a survey of the surface oxygen species on oxide materials, which moving on from the Che's pioneering works includes noteworthy results obtained during the years by other researchers. In particular, applications of two powerful techniques such as electron paramagnetic resonance (EPR) and photoluminescence (PL) for detailed characterization of paramagnetic (O-, O2-, O3-) and diamagnetic surface oxygen varieties (O 2surf), respectively, are addressed here. Classification and insights into the formation pathways of these species on various oxide surfaces, as well as their involvement in model and real gas/solid and liquid/solid reactions of catalytic and photocatalytic relevance is also considered. Finally, the relationship between the thermodynamic and kinetic Bronsted basicity of low coordinated O 2-Lc anions is discussed, on the basis of Che's fundamental studies and on recent developments.

International audience; This special Issue of the Journal of Catalysis pays tribute to Professor Michel Che. In addition to the Editorial Preface, colleagues share their recollections of the vigorous endeavour he displayed throughout his scientific life to promote efficient international collaborations, as well as to bridge fundamental science and its applications. His human qualities are unanimously valued, which has left a trail marked with friendship.

Abstract Some relevant aspects of Electron Paramagnetic Resonance (EPR) applied to the fields of surface chemistry and heterogeneous catalysis are illustrated in this perspective paper that aims to show the potential of these techniques in describing critical features of surface structures and reactivity. Selected examples are employed covering distinct aspects of catalytic science from morphological analysis of surfaces to detailed descriptions of chemical bonding and catalytic sites topology. In conclusions the pros and cons related to the acquisition of EPR instrumentations in an advanced laboratory of surface chemistry and heterogeneous catalysis are briefly considered. Graphic Abstract

In this work, the activity enhancement of TiO(2)photocatalysts by alkaline treatment has been investigated. Commercial (BDH, U.K., anatase phase) TiO(2)samples, treated in alkaline solutions of NaOH at different concentrations and temperatures, were tested for the photocatalytic partial oxidation of 3-pyridinemethanol in water under UVA irradiation to form vitamin B-3. Photocatalyst characterization has been carried out by XRD, BET, TEM, TGA, photoluminescence, FTIR, Raman, DRS, EPR and photocurrent measurements. The alkaline-treated samples showed an increased activity of up to 7 times (by considering first order rate constant values) compared to untreated BDH and the superior performance was maintained even after a thermal treatment at 400 degrees C, while the selectivity values were affected only marginally by the treatment (beingca.40% and 15% for 3-pyridinemethanal and vitamin B-3, respectively). The effect of NaOH concentration and treatment duration on the photocatalytic activity has also been studied extensively. The alkaline treatment has also proven to be effective in modifying the surface properties of another commercial catalyst,i.e., Merck, anatase, improving its reactivity in the conversions of both 4-methoxybenzyl alcohol and 3-pyridinemethanol. The NaOH treatment boosted the porosity and surface area, while lowering the percent crystallinity of the catalysts, due to a higher density of surface hydroxyl groups. It also enhanced the adsorption capability towards the reacting substrate, as shown by FTIR analysis. Despite the oxygen vacancies (or total abundance of Ti(3+)centres) not being changed after the NaOH treatment, the process of hole trapping under irradiation is drastically affected due to the different water adsorption capabilities of the samples, as highlighted by EPR. The proposed simple treatment of commercial photocatalysts has the potential to support the development of effective photocatalysts, which do not require complex surface modification or doping to achieve high yields in reactions relevant to the chemical industry.

The preparation of smart Ln:ZrO2@SiO2 nanoplatforms with grafted photosensitizer (Rose Bengal) which couple optical imaging with photo-dynamic therapy (PDT) is presented. A careful control of the lanthanide dopant loading is considered to enhance the photoemission properties of the lanthanide ions (Er, Pr, Yb) inside the ZrO2 crystal structure. The nanosystem with the lowest lanthanide loading maintains the size, phase and morphology of pristine ZrO2 nanoparticles and exhibit the best performances in term of the overall luminescence properties. Upon functionalization with a silica shell to covalently bound Rose Bengal, a theranostic platform is prepared which is very efficient in singlet oxygen generation, as demonstrated by EPR and UV–vis spectroscopy studies. Preliminary cell viability tests show that while both pristine and Ln doped ZrO2 nanoparticles do not exert cytotoxicity, neither upon illumination nor in dark condition, Rose Bengal grafted samples are able to significantly reduce cell viability under light exposure, thus confirming the high potential of these nanoparticles as PDT tools.

Evidence of the existence of nitrate radical in irradiated aqueous TiO2 suspensions in the presence of nitrate ions are reported for the first time. The joint use of UV/Vis and EPR spectroscopy showed that nitrate radicals are formed by hole induced oxidation of nitrate ions. Photocatalytic degradation of a model alkene compound allowed to highlight the presence of an intermediate organic nitrate deriving from nitrate radical attack to the double bond of the substrate. These results not only allow deeper understanding of photocatalytic processes, but open the route to new green photocatalytic syntheses initiated by nitrate radicals and to new insights in the field of atmospheric chemistry.

In the present work, two H2 evolution photocatalysts were prepared by employing two different oxides, TiO2 and zirconium titanate (ZrTiO4), as the support of various copper phases. For both the supports the same Cu loading (0.5% w/w) was adopted, but two different impregnation procedures have been followed, leading to different forms of Cu in the final composite material that are: (i) Cu(II) species dispersed on the oxide surface and (ii) Cu2O particles dispersed on the oxide surface. The present paper based on the parallel use of photocatalytic test and spectroscopic analysis performed in catalytic conditions illustrates the evolution of photocatalytic systems occurring during the H2 evolution reaction tests, pointing out that the as-prepared materials represent a pre-catalyst and they are modified during irradiation leading to the real working systems different from the starting ones. The herein presented spectroscopic analysis aims to contribute to the living debate on the oxidation state of copper in mixed Cu/oxide materials and on its role in hydrogen evolution under photocatalytic conditions.

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.

Lutetium-doped titanium dioxide (Lu-TiO2) was synthesized using a sol–gel method. The obtained compounds were characterized by X-ray diffraction and electron paramagnetic resonance. The photocatalytic decomposition of adenosine 5′-triphosphate (ATP) under UV irradiation was investigated to estimate the effect of the doped lutetium. Our results showed that the Lu-TiO2 nanoparticles have higher photoactivity than an undoped TiO2. Furthermore, the role of lutetium in Lu-TiO2, the structure of the photoreaction site on Lu-TiO2 and the reaction mechanism are also discussed.

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.

Nitrogen doping represents an important strategy to modulate the optical, magnetic and photochemical properties of oxides for applications spanning from photocatalysis to optoelectronics and spintronics. In this work for the first time zirconium titanate, a material that exhibits many attractive properties, including an excellent dielectric constant, high corrosion resistance, high permittivity at microwave frequencies and excellent temperature stability, has been doped with nitrogen via a wet chemistry method. A detailed description of the geometrical and the electronic structure of the dopant centre has been obtained coupling Electron Paramagnetic Resonance (EPR) spectroscopy and density functional theory (DFT) calculations. Insertion of nitrogen impurities into the ZrTiO4 lattice modifies the optical properties causing an appreciable absorption in the visible range. The joint analysis of the EPR evidence and the DFT elaboration indicates that the nitrogen impurities preferentially occupy an interstitial position of the lattice generating intra-band gap states about 1 eV above the valence band edge that are responsible for the visible light absorption. The majority of the intra-band gap states are diamagnetic (N−) while a minor fraction is paramagnetic (N˙). These centres are photosensitive in that the ratio between N− and N˙ is modified, upon irradiation, because of electron excitation from the intra-band gap states to the conduction band.

The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.

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.

The interaction of oxygen with Ce-ZrO2 (Ce ions diluted in the zirconia matrix) is a key aspect for many applications of this material. In this work, for the first time, an unusual reversible interaction of the O2 molecule with the surface of this solid is reported. The joint use of electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations has allowed a full description of the reversible O2 adsorption phenomenon that involves a full electron transfer from Ce3+ centers to the adsorbed O2 forming the superoxide anion (O2â¢-). The process can be completely reversed by removing the O2 gas from the atmosphere as confirmed by the calculated values of both the adsorption energy (0.45 eV) and the activation energy (negligible) of the process. This behavior is typical of a solid oxygen carrier. However, at variance of the reported cases of molecular and solid oxygen carriers that usually form Ε1 end-on adducts, in the present case a side-on, Ε2 Ce-O2 surface complex is observed. Comparison of structural data and DFT results also indicates that Ce doping occurs in the lattice of tetragonal zirconia while this is not observed in the monoclinic phase.

© 2015 Elsevier B.V. A conserved cysteine located in the signature motif of the catalytic center (H-cluster) of [FeFe]-hydrogenases functions in proton transfer. This residue corresponds to C298 in Clostridium acetobutylicum CaHydA. Despite the chemical and structural difference, the mutant C298D retains fast catalytic activity, while replacement with any other amino acid causes significant activity loss. Given the proximity of C298 to the H-cluster, the effect of the C298D mutation on the catalytic center was studied by continuous wave (CW) and pulse electron paramagnetic resonance (EPR) and by Fourier transform infrared (FTIR) spectroscopies. Comparison of the C298D mutant with the wild type CaHydA by CW and pulse EPR showed that the electronic structure of the center is not altered. FTIR spectroscopy confirmed that absorption peak values observed in the mutant are virtually identical to those observed in the wild type, indicating that the H-cluster is not generally affected by the mutation. Significant differences were observed only in the inhibited state Hox-CO: the vibrational modes assigned to the COexo and Fed-CO in this state are shifted to lower values in C298D, suggesting different interaction of these ligands with the protein moiety when C298 is changed to D298. More relevant to the catalytic cycle, the redox equilibrium between the Hox and Hred states is modified by the mutation, causing a prevalence of the oxidized state. This work highlights how the interactions between the protein environment and the H-cluster, a dynamic closely interconnected system, can be engineered and studied in the perspective of designing bio-inspired catalysts and mimics.

The photochemical and photophysical properties of a defective zinc oxide prepared by precipitation have been investigated using mainly electron paramagnetic resonance (EPR) and photoluminescence (PL) spectroscopy. As already reported in the literature the band gap of the oxide contains intra band gap states related to the presence of point defects (mainly cation vacancies) in the structure. The concentration and the energy levels of such defects, a fraction of them being paramagnetic, are the basis of a mechanisms of double and multiple excitations allowing visible photons with energy lower than the band gap value (hν Eg. Remarkably, some of the paramagnetic defects present in the bulk of the as prepared materials derive from excitation of VB electrons induced by ambient light illumination. The electron excitation from VB to CB has been observed upon irradiation under vacuum at 77 K with polychromatic light and applying four different filters to the emitted radiation with cut-off in the range between 400 nm and 495 nm. The process has been monitored by EPR as the photoexcitation ends up in the formation of trapped electron centers (signal at g = 1.96) and trapped hole centers (signal in the range 2.021 > g > 2.003) both clearly detected by this technique. A scheme of the energy state distribution in the band gap of the defective oxide, obtained coupling EPR and photoluminescence results, is proposed. Finally, a non-negligible fraction of the visible light generated carriers reach the surface of the nanocrystals, entailing the typical redox reactions of photocatalysis as indicated by the detection of hydroxyl radicals in solution performed by spin-trapping. For this reason, the wet chemistry prepared zinc oxide can be considered a visible light active (VLA) system.

In the present work, the activity of Ce and Er-doped ZrO2 nanopowders for sun-driven photocatalytic water oxidation has been investigated. ZrO2 powders with tunable amounts of tetragonal, monoclinic and cubic polymorphs have been synthesized by introducing Ce and Er (from 0.5 to 10 mol % on an oxide basis) through hydrothermal method. The aim of this work is to investigate the role of rare earth (RE) ions rich of electrons (Er3+) and with entirely empty levels (Ce4+) in the ZrO2 matrix for the sun-driven photocatalytic water oxidation reaction. The samples have been characterized by means of UV-Vis spectroscopy, X-Ray diffraction (XRD), N2 adsorption, X-ray photoelectron spectrophotometry (XPS) and transmission electronic microscopy (TEM) with energy dispersive spectroscopy (EDS). With respect to the bare ZrO2 mainly containing monoclinic (m-) phase, an increasing amount of rare-earth (RE) dopant was found to improve the specific BET surface area and to stabilize the tetragonal (t-) or cubic (c-) polymorphs of ZrO2 at room temperature. XRD data confirmed that dopants were mainly inserted in the t-ZrO2 phase. The photocatalytic O2 evolution from water under AM 1.5 G simulated sunlight illumination of the prepared samples have been correlated with their optical, structural and chemical properties. The effect of the dopant concentration on the chemical-physical and photocatalytic properties of the Er- and Ce-doped ZrO2 materials was elucidated. The samples with 5% of RE oxide were the most active, i.e., three times more than pure zirconia. Their superior photocatalytic activity was found to be mainly correlated to two factors: (i) an optimal surface concentration of RE ions of about 3.7%, which increased charge carriers separation in the photocatalysts surface due more superficial defects of the t-ZrO2 and a higher surface area, thus enhancing the reaction kinetics, (ii) a controlled amount of monoclinic vs. tetragonal (or cubic) polymorphs of zirconia with an optimum ratio of about 70/30 of t-ZrO2/m-ZrO2. Instead, the increased ability of the RE-doped ZrO2 to harvest visible light was found to have a secondary role on the photocatalytic activity of the Ce-doped ZrO2 material.

In the present work, two H

Evidence of the existence of nitrate radical in irradiated aqueous TiO

Bare ZrO2, Ce doped ZrO2 and Er doped ZrO2 samples have been prepared by a hydrothermal process and have been used as photocatalysts for 2-propanol oxidation reaction in gas solid regime. Moreover, some preliminary tests have been carried out for CO2 reduction. The samples were physico-chemically characterized and both bare and doped ZrO2 based materials resulted active for oxidation and reduction reactions by using UV and solar irradiation. The reactivity results have been correlated with the compositional, structural and morphological features of the photocatalysts.

The present paper describes the role of electron paramagnetic resonance (EPR) in the investigation of fundamental phenomena occurring in heterogeneous photocatalysis, namely, the light-induced charge carrier separation, the stabilization of the carriers and their consequent surface reactivity. We will describe the behavior of a series of photoactive oxides (TiO2, ZrO2, ZrTiO4 and ZnO) with different band gaps which exhibit electron and hole stabilization in all cases. The EPR technique reveals the nature of the different stabilization sites (e.g. Ti4+, Zr4+, Zn2+) for the electrons and monitors the hole trapping in terms of the formation of O·– centers. The electron transfer reactivity at the surface of the photogenerated carriers is then monitored using specific scavengers admitted in the gas phase over the solid, specifically, molecular oxygen and molecular hydrogen for electron scavenging and hole scavenging, respectively. The method illustrated herein is particularly useful as a preliminary means of checking the features of novel photocatalytic materials.

Mixed solid system involving cerium and zirconium titanate (ZrTiO4) have been prepared using the sol-gel technique. Both X-ray diffraction and DFT calculations firmly indicate that, till a doping level of 10 mol%, cerium ions are dissolved in the titanate matrix (which has the scrutynite structure, analogous to those of the main TiO2 polymorphs) occupying the cationic sites and progressively altering its cell parameters. Cerium is hosted in the matrix both in the form of Ce4+ and Ce3+ ions (XPS results). The trivalent state seems to be favoured even though the state of the dopant depends on the treatment undergone by the material. DFT calculations describe the intra-band gap states formed in both cases and the strong localisation of the single electron in the case of Ce3+ (4f1). Differently from the case of Ce doped ZrO2, that shows photoactivity in the visible light because of the presence of cerium, the doped titanate is inactive in the same conditions. Under UV–vis illumination charge separation occurs (EPR results) and the low-loading doped systems (0.5%, 1%) form OH radicals, detected by spin trapping, more efficiently than the pristine matrix. The absence of photoactivity in the visible range is interpreted in terms of the detrimental role (charge recombination) played by both the occupied intra-band gap states associated to Ce3+ and the corresponding oxygen vacancies formed in the lattice by charge compensation.

The synthesis of a mixed CeO2–ZnO oxide results in a photocatalyst active under visible light. The characterization of the new material shows that Ce does not enter as a dopant in ZnO but rather forms isolated CeO2 nanoparticles supported on the surface of larger particles of the more abundant zinc oxide phase. The as obtained material exhibits a band gap corresponding to UV light (∼3.3 eV), but nevertheless. it shows a relevant photoactivity under irradiation with photons with λ > 420 nm (visible light). The working hypothesis is that visible light irradiation leads to a charge separation and stabilization of a fraction of the carriers connected with the formation of the CeO2/ZnO interface. This phenomenon has been investigated by means of several methods. A specific EPR-based approach allowed to monitor and quantify the charge separation following the formation of holes in the valence band (VB) of the two materials. More complex is detecting the nature of the excited electrons, as this involves the formation of EPR invisible Ce3+ ions by trapping the electrons into localized 4f states of Ce ions at the interface between the two oxides. DFT calculations provide a rational for some of the observed phenomena and a basis for the discussion of the band alignment of the two systems as a consequence of the formation of a heterojunction. The theoretical results show that indeed electrons can be excited at the interface from the VB of the two oxides into the Ce 4f states with photons of 2.3 eV, thus justifying the occurrence of a visible-light activity despite the higher band gap of the two materials.

A series of novel BiyXz-TiO2/SrTiO3 composites were prepared by multistep synthesis route. The as-prepared photocatalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), Fourier transform infrared (FT-IR), Raman spectra and BET analysis. The photocatalytic activity test was performed in aqueous solution of phenol under the irradiation of visible light range (λ ≥ 420 nm). Obtained results revealed that the BiOI_TiO2/SrTiO3 sample exhibit the highest photocatalytic activity under visible irradiation (0.6 μmol/dm3/min). Thus, it was demonstrated that modification of the TiO2/SrTiO3 microspheres by flowers-like structure made of bismuth oxyiodide resulted in enhancement of photocatalytic activity under visible light. The role of active species during the decomposition process of organic compound was investigated using different types of active species scavengers as well as electron paramagnetic resonance analysis (EPR). The study showed that in the BiOI_TiO2/SrTiO3/Vis system the holes (h+) plays relevant role in phenol decomposition. Furthermore, the stability and recyclable properties of obtained BiOI_TiO2/SrTiO3 sample were confirmed during three consecutive processes.

Because of the tremendous structural diversity of ionic liquids (ILs), simple transfer of observations performed for one IL used for IL-TiO2 preparation on different samples is not possible. Therefore, four ionic liquids, all containing distinct nitrogen-bearing organic cations (pyridinium, pyrrolidinium, ammonium, imidazolium), were used for the first time for the preparation of IL-TiO2 composites. The role of the individual IL cation in the synthesis of TiO2 microspheres, as well as the effect of the IL structure on the mechanism of the visible-light (Vis)-induced photoactivity of IL-TiO2 was presented and discussed in regard to structure, morphology, absorption properties, elemental composition, and reactive species involved in the photocatalytic reaction of phenol degradation. The successful modification of the TiO2 with organic IL species including possible interactions between IL and TiO2 surface, as well as the TiO2 matrix (doping with N), were confirmed. The sample that exhibited the highest photo...

Dehydroxylated, hydroxylated and hydrated anatase TiO2 samples have been exposed to acetylene at room temperature. The interaction leads to the formation of polycyclic aromatic hydrocarbons (PAHs) and is accompanied by the appearance of Ti3+ ions, as shown by electron paramagnetic resonance (EPR) spectra. Fully or partly dehydroxylated samples show higher reactivity, whereas the hydrated samples are chemically inert. The experimental results point towards a crucial role of the more reactive (001) facets of anatase nanoparticles. Density functional theory calculations show that acetylene physisorbs on the anatase (101) surface without activation of the C-H bond. The reduced (101) surface (O vacancies) leads to acetylene activation but not to dissociative adsorption. In contrast, the dehydroxylated (001) anatase surface is very active and leads to the spontaneous splitting of the C-H bond with formation of Ti-C2H and OH groups. This is followed by subsequent additions of C2H2 molecules with formation of PAHs. During the dissociation of C2H2, radical species do not form and electrons are not transferred to the surface because direct Ti-C covalent bonds form on the surface. However, the ring closure in the formation of the aromatic compounds leaves behind hydrogen atoms that donate their valence electrons to the oxide. This results in the appearance of EPR-active Ti3+ centres. On the surface of things: Acetylene oligomerisation occurs on the surface of anatase TiO2 leading to the formation of polycyclic aromatic hydrocarbons and a reduced titania surface. The mechanism of the reaction has been elucidated by EPR measurements and DFT calculations (see figure).

The typical activation of a fourth generation ZieglerNatta catalyst TiCl4/MgCl2/phthalate with triethyl aluminum generates Ti3+ centers that are investigated by multi-frequency continuous wave and pulse EPR methods. Two families of isolated, molecule-like Ti3+ species have been identified. A comparison of the experimentally derived g tensors and 35,37Cl hyperfine and nuclear-quadrupole tensors with DFT-computed values suggests that the dominant EPR-active Ti3+ species is located on MgCl2(110) surfaces (or equivalent MgCl2 terminations with tetra-coordinated Mg). O2 reactivity tests show that a fraction of these Ti sites is chemically accessible, an important result in view of the search for the true catalyst active site in olefin polymerization.

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 presence 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.

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.

We report direct evidence for quintuplet spin states in a particular kind of reduced TiO2 anatase obtained by the mild oxidation of TiB2 under hydrothermal conditions. Continuous-wave and pulse EPR spectroscopy at X and Q band frequencies provide compelling evidence for the presence of S=2 states, stable in a wide range of temperatures up to room temperature. A tentative model, corroborated by spin-polarized DFT calculations, is proposed, which consists of four ferromagnetically interacting Ti3+ ions with distances ranging from 0.5 nm to 0.8 nm and tetrahedral arrangement.