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International audience; The mechanisms of H 2 dissociation on three stoichiometric anatase TiO 2 terminations, (001), (100) and (101), have been studied by means of density functional theory (PBE+U) calculations. A two-step process was considered: first, H 2 dissociation into H + and H − pair, and second the H − species migrates to a neighboring O site, transferring the electrons to the substrate. On (001), it shows the lowest activation barriers for hydrogen dissociation, 0.37 eV, whereas the highest value was found on (101), 0.98 eV. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.10 to 2.37 eV), which indicates the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. Electronic structure, vibrational frequency analysis as well as temperature effects are studied to characterize the reactivity. The relationship between electronic structure, geometry and reactivity is analyzed by means of reactivity descriptors, and the results are compared with ceria and rutile TiO 2 facets.

Copper and manganese modified zeolites play key roles in selective oxidation catalytic reactions. Combining Density Functional Perturbation Theory (DFPT) implemented in VASP, and infrared spectroscopy experiments, we have studied and elucidated the incorporation of Cu2+ and Mn2+ in the framework of SiBEA zeolite. Six isomers are investigated for both models CuSiBEA and MnSiBEA. The energy stability study identified the most stable isomer in each case, which correspond to M-O2 bent moieties in the vicinity of hydroxyl groups. The isomers are then characterized by experimental and theoretical IR spectra. Both experimental and calculated IR spectra are in good agreement highlighting the presence of active sites (Cu-OH and Mn-OH) in the obtained models. The accurate molecular description of the nature of the active sites presented in this work will be important for understanding the catalysis going on and for developing novel catalytic processes.

Much of our current understanding of catalytic activity is derived from materials with well-defined structures, either molecular (i.e., homogeneous catalysts) or extended, ordered solids (i.e., heterogenous catalysts and surface science), and active sites in real catalysts are often assumed to closely resemble structures found in such ordered materials, at least locally. Yet many real catalysts involve amorphous materials and are much more active than their well-ordered counterparts would suggest. This implies that at least some active sites in amorphous materials are intrinsically different. One of these important and versatile amorphous materials is silica. The materials properties are based in many cases on the way molecules interact with the silica surface. Since several years we investigate silica and its role as support of transition metal oxide catalysts, but also the interaction with bio-organic systems, and this at different levels: from the phenomenological description of the interaction at the interface substrate-adsorbate (adsorption and self-assembling of biomolecules) to technologic applications (catalysis, ionic liquids), pharmaceutical (controlled delivery of drug molecules) and their plausible role in different scenarios in the origin of life. For this, we use the tools of quantum chemistry combined with experimental back up with the aim to improve the understanding of the complex chemical behavior of these silica based materials.

The nature of isolated tungsten oxide (WOx) sites in the dispersed phase on the surface of a β-cristobalite (β-SiO2) support in undoped and Na- or Mn-doped WOx/SiO2 model catalysts used for the oxi...

International audience; Fukui functions (FF) are chemical descriptors useful to explain the reactivity of systems towards electron transfer. Whereas they have been widely employed for molecules, their application to extended systems is scarce. One of the reasons for the limited devel-1 opment of such analysis in solids is the improper evaluation of FF in the usual computational approaches based on density functional theory and periodic boundary conditions. In this work we compare the available approaches and propose a new method, based on the interpolation of partially charged systems, that mitigates some of the problems encountered. We discuss the reactivity of alkaline earth oxides (MgO, CaO, SrO and BaO) in terms of the FF analysis, providing a robust way to account for the higher reactivity of surface oxygen sites compared to bulk sites.

The most accurate theoretical method to describe excitons is the solution of the Bethe-Salpeter equation in the GW approximation (GW-BSE). However, because of its computation cost, time-dependent density functional theory (TDDFT) is becoming the alternative approach to GW-BSE to describe excitons in solids. Nowadays, the most efficient strategy to describe optical spectra of solids in TDDFT is to use long-range corrected exchange-correlation kernels on top of GW or scissor-corrected energies. In recent years, a different strategy based on range-separated hybrid functionals started to be developed in the framework of time-dependent generalised Kohn-Sham density functional theory (TDGKSDFT). Here, we compare the performance of long-range corrected kernels with range-separated hybrid functionals for the description of excitons in solids. This comparison has the purpose to weight the pros and cons of using range-separated hybrid functionals, giving new perspectives for theoretical developments of these functionals. We illustrate the comparison for the case of Si and LiF, representative of solid state excitons., 25 pages, 11 figures

Titanium dioxide is a key material in many fields, including technological, industrial and biomedical applications. Many of these applications are related to the surface reactivity of TiO

Titanium dioxide nanoparticles have risen concerns about their possible toxicity and the European Food Safety Authority recently banned the use of TiO2 nano-additive in food products. Following the intent of relating nanomaterials atomic structure with their toxicity without having to conduct large scale experiments on living organisms, we investigate the aggregation of titanium dioxide nanoparticles using a multi-scale technique: starting from ab initio Density Functional Theory to get an accurate determination of the energetics and electronic structure, we switch to classical Molecular Dynamics simulations to calculate the Potential of Mean Force for the connection of two identical nanoparticles in water; the fitting of the latter by a set of mathematical equations is the key for the upscale. Lastly, we perform Brownian Dynamics simulations where each nanoparticle is a spherical bead. This coarsening strategy allows studying the aggregation of a few thousand nanoparticles. Applying this novel procedure, we find three new molecular descriptors, namely, the aggregation free energy and two numerical parameters used to correct the observed deviation from the aggregation kinetic described by the Smoluchowski theory. Molecular descriptors can be fed into QSAR models to predict the toxicity of a material knowing its physicochemical properties, without having to conduct large scale experiments on living organisms.

International audience; Titanium oxide (TiO2) is an oxide with a wide range of technological and industrial applications. The interaction of hydrogen with TiO2 surface is an important step in many catalytic processes, such as spillover or water photosplitting. In the last years, the role of the subsurface hydrogen has been invoked many times to explain different phenomena, from black titania to reversible storage. In this work, we systematically investigate the paths for hydrogen diffusion from surface into subsurface focusing on the surface topology, the thermal and isotopic effects, and the degree of reduction of the substrate, by means of state of the art periodic DFT calculations. We find differences in the behavior of the rutile TiO2 surfaces (001), (100) and (110) slightly exothermic reaction energies from-0.02 eV to-0.15 eV, and activation energies from 0.39 eV to 1.00 eV. Deuterium diffusion from rutile (110) surface to the bulk, as well as thermal effects, were found to affect less than 0.04 eV the energy profile. On the contrary, the degree of reduction of the rutile (110) was found to noticeably decrease the activation barrier down to 0.8 eV for the (110) slab. We analyze structural, electronic and reactivity parameters that affect the kinetic barriers for the surface to subsurface H diffusion, and conclude that topology and reduction degree are valid strategies to tune the surface-tobulk migration process.

International audience; Polymorphism commonly exists in the preparation of cocrystals and has attracted widespread attention from both the pharmaceutical industry and academia. However, few studies have examined how to discover polymorphic cocrystals and their potential formation mechanism. In this study, we report the novel discovery of salicylic acid: 3-nitrobenzamide (SA-3NBZ) polymorphic cocrystals by thermal methods. The formation mechanism is elucidated based on theoretical calculations. SA-3NBZ polymorphic cocrystals with molar ratio of 1:1 and 2:2 were discovered using the combination of differential scanning calorimetry and hot stage microscopy. Single-crystal X-ray diffraction analysis confirmed this discovery. Density functional theory calculations corrected with dispersion were conducted to illustrate the energetic stabilization of SA polymorphic cocrystals. Compared with the starting materials, formation of the cocrystals at 1:1 and 2:2 present a weak stabilization with overall energy reduction of −0.01 and −0.05 eV/molecule, respectively. The calculated noncovalent interactions index further suggests that intralayer hydrogen bonds and van der Waals forces contribute to these weak interactions. The density functional theory calculations are in good agreement with the X-ray diffraction data. Hence, thermal analysis is a simple and reliable method to discover polymorphic cocrystals.

International audience; Valence Shell Electron Pair Repulsion (VSEPR) theory constitutes one of the pillars of theoretical predictive chemistry. It was proposed even before the advent of the “spin” concept, and it is still a very useful tool in chemistry. This article proposes an extension of VSEPR theory to understand the core structure and predict core polarization in main group elements. We show from first principles (Electron Localization Function analysis) how the inner and outer core shells are organized. In particular, electrons in those regions are organized following the shape of the dual polyhedron of the valence shell (3rd period) or the equivalent one (4th and 5th periods). We interpret these results in terms of the “hard” and “soft” core character. All studied systems follow this trend, providing a predictive framework for predicting electron distribution in the core. We also show that lone pairs behave as “standard ligands” in terms of core polarization. The predictive character of the model was tested by proposing the core polarization of different systems not included in the original set (such as XeF4 and Fe(CN)63‐) and checking the hypothesis by means of a posteriori calculations. From the experimental point of view, the extension of VSEPR to the core region has consequences for timely crystallography. In particular, it enables to explain the core polarization proposed in the resolution of accurate X‐ray structures. This result is illustrated for the particular case of a‐silicon.

International audience; The adsorption of ammonia–borane (AB) over the (001) surface of TiC and ZrC, has been studied systematically by means of periodic-boundary density functional calculations using slab models. We present evidence that a surface with an appreciable degree of polarity such as those used in this research, can simultaneously activate the N-H and B-H bonds of the AB molecule and its dimer AB-AB. The molecule is highly activated by both supports, and the N-H and B-H bonds are stretched long enough to release one or two hydrogens. Additionally, the bond distance B-N is shortened by 0.04 and 0.06 Å by TiC and ZrC supports respectively, which results in the strengthening of the bond, what seems convenient to avoid unwanted by-products (NH3, BH3, etc.). A systematic study for the adsorption of BH2NH2, which is isoelectronic with ethylene, was also done. The adsorption leads to a major elongation of the B-N bond with respect to the calculated value in the gas phase (0.17 and 0.18 Å for TiC and ZrC respectively). Both supports can activate the molecule, with the biggest impact being on the BH2 fragment. On the other hand, the migration of hydrogen atoms on both supports is a thermodynamically favorable process. On the surface H prefers bonding to a C, which is in agreement with other studies for dissociative adsorption of H2 on metallic carbides. Nevertheless, we show that the hydrogen molecule readily forms when both atoms sit on the transition metal of the surface. On the basis of this theoretical study, both supports are proposed as potential catalysts for the dehydrogenation of ammonia–borane.

International audience; Synthesis of 1D TiO2nanorods is promising nowadays owing to their broad applications in photocatalytic devices. A groundbreaking synthesis procedure of one-step producing large-scale, free-standing and heterogeneous phase (anatase and rutile) 1D TiO2 nanorod array which efficiently enhances the photocatalytic activity has been previously proposed. However, the detailed growth mechanism of this catalyst remains unclear, mainly because the synthesis takes place in a closed autoclave. This study presents a complementary approach to understand the growth mechanism using computer modeling based on density functional theory (DFT). The interaction of the precursor (titanium butoxide in hydrochloric acid media, modelled by TiCl4) and solvent (water) with preexisting rutile titania is computed to derive the thermodynamic trends to grow additional TiO2 layers. Five different termination models of the rutile surfaces were built: (001), (100), (101), (110) and (111), and their interaction with precursor and water was computed. Molecular and dissociative adsorption are considered as well as coadsorption. Our results point to a preferential growth of the (001) termination of rutile TiO2 through intermediates involving TiCl3+, Cl− and hydroxyl groups. The results obtained show how computational chemistry can be used to resolve experimental growth information. The protocol used i.e. computing different terminations and their explicit interaction with both the precursor and the solvent, can be extended to other materials in which the growth takes place following well-defined preferential directions.

International audience; A detailed density functional theory study is presented to clarify the mechanistic aspects of the methanol, CH 3 OH, dehydrogenation process to yield hydrogen, H 2 , and formaldehyde, CH 2 O. A gas-phase vanadium oxide cluster is used as a model system to represent reduced V (III) oxides supported on TiO 2 catalyst. The theoretical results provide a complete scenario, involving several reaction pathways in which different methanol adsorption sites are considered, with presence of hydride and methoxide intermediates. Methanol dissociative adsorption process is both kinetically and thermodynamically feasible on V-O-Ti and V=O sites, and it might lead to form hydride species with interesting catalytic reactivity. The formation of H 2 and CH 2 O on reduced vanadium sites, V(III), is found to be more favorable than for oxidized vanadium species V(V), taking place along energy barriers of 29.9 kcal/mol and 41.0 kcal/mol respectively.

A four-step reaction mechanism is proposed for the H2 dissociation over pure ceria and Gallium promoted mixed oxide materials, in a combined experimental and computational investigation. Two samples of cerium-gallium mixed oxides with Ce/Ga atomic ratios equal to 90/10 and 80/20 were studied by time-resolved diffuse reflectance infrared spectroscopy under H2 (D2) flow at isothermal condition in the range of 523?623 K. X-ray photoelectron spectrometry allowed to conclude that only Ce4+ is reduced to Ce3+ (Ga3+ is not reduced), in agreement with density functional theory (DFT) results. The time evolution profiles of gallium hydride ðGaAHÞ species, hydroxyl groups (OH) and Ce3+ infrared signals were analyzed and kinetic rate parameters for each step were obtained by mathematical modeling. The values for activation energies were in agreement with those calculated by DFT, for the different elementary pathways. A small activation energy (4 kcal/mol) was found for H2 dissociation found on GaOCe sites assuming that the heterolytic cleavage of the HAH bond is the rate determining step. On pure ceria, the experimental activation energy is 23 kcal/mol, showing that the addition of Ga3+ cations boosts the splitting of H2. Interestingly, the reduction step of pure CeO2 surface domains seems to proceed via a CeH/OH pair intermediate, according to DFT calculations. Moreover, 71Ga NMR experiments indicatethe possible presence of gallia nanodomains. It is proposed that the generation of Ga OACe sites in the perimeter of such surface gallia nanodomains is responsible for the enhanced reactivity of the mixed materials. The key role of this new type of sites to improve the efficiency of relevant catalytic reactions such as selective alkyne hydrogenation and light alkane dehydrogenation is then analyzed. Fil: Vecchietti, María Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Baltanas, Miguel Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Gervais, Christel. Universite de Paris VI; Francia Fil: Collins, Sebastián Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Blanco. Ginesa. Universidad de Cádiz; España Fil: Matz, Olivier. Universite de Paris VI; Francia Fil: Calatayud, Monica. Universite de Paris VI; Francia Fil: Bonivardi, Adrian Lionel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina

International audience; Bottom-up and top-down derived nanoparticle structures refined by accurate ab initio calculations are used to investigate the size dependent emergence of crystallinity in titania from the monomer upwards. Global optimisation and data mining are used to provide a series of (TiO 2) N global minima candidates in the range N = 1–38, where our approach provides many new low energy structures for N > 10. A range of nanocrystal cuts from the anatase crystal structure are also considered up to a size of over 250 atoms. All nanocrystals considered are predicted to be metastable with respect to non-crystalline nanoclusters, which has implications with respect to the limitations of the cluster approach to modelling large titania nanosystems. Extrapolating both data sets using a generalised expansion of a top-down derived energy expression for nanoparticles, we obtain an estimate of the non-crystalline to crystalline crossover size for titania. Our results compare well with the available experimental results and imply that anatase-like crys-tallinity emerges in titania nanoparticles of approximately 2–3 nm diameter.

Iridium oxide (IrOx) is one of the best water splitting electrocatalysts, but its active site details are not well known. As with all heterogeneous catalysts, a strategy for counting the number of active sites is not clear, and understanding their nature and structure is remarkably difficult. In this work, we performed a combined study using optical spectroscopy, magnetic resonance and electrochemistry to characterize the interaction of IrOx nanoparticles (NPs) with a probe molecule, catechol. The catalyst is heterogeneous given that the substrate is in a different phase, but behaves as a homogeneous catalyst from the point of view of electrochemistry since it remains in colloidal suspension. We find two types of binding sites: centers A which bind catechol irreversibly making up 21% of the surface, and centers B which bind catechol reversibly making up 79% of the surface. UV-vis absorption spectroscopy shows that the A sites are responsible for the characteristic blue color of the NPs. Electrochemical experiments indicate that the B sites are catalytically active and we give the number of active sites per nanoparticle. We conclude by performing a survey of ligands used in solar cell architectures and show which ones bind well to the surface and which ones inhibit the catalytic activity when doing so, presenting quantitative guidelines for the correct handling of IrOx nanoparticles during their incorporation into multifunctional solar energy harvesting architectures. We suggest ligands binding on the surface oxygen atoms allow for large bound ligand densities with no detrimental effect on the catalytic activity.

International audience; "Click" chemistry, used to promote nanoparticle assemblies, is a powerful strategy which has emerged very recently to control the spatial arrangement of nanoparticles onto surfaces. Such a strategy may be of high interest for applications such as magnetic recording media or magnetic sensors which are based on the fine control of the collective properties of nanoparticles. Nevertheless, self-assembly driven by clickable functional groups still remains to be understood. Mixed self-assembled monolayers (SAMs) of alkane−thiol molecules were used to control the spatial arrangement of nanoparticles onto gold substrates. This approach was combined with click chemistry in order to control the immobilization of nanoparticles on selective areas through specific copper catalyzed alkyne−azide cycloaddition (CuAAC) reaction. Mixed SAMs consist of co-adsorbed 11-(undec-1-ynyl)thiol (S-CC) and 12-(dodecane)thiol (S-CH 3) molecules. The variation of the molar ratio between both molecules resulted in significant modulation of the structure of nanoparticle assemblies. The spatial arrangement of nanoparticles revealed the very complex structure of alkyne/methylene terminated mixed SAMs. Alkyne terminal groups could not be only studied by the usual characterization surface techniques such as PM-IRRAS and XPS. Therefore, azido-terminated nanoparticles acted as probing agents to determine the spatial distribution of alkyne groups at the surface of mixed SAMs. This approach was combined with scanning tunneling microscopy (STM) and DFT calculations to get a deeper insight into the structure of mixed SAMs of S-CC and S-CH 3 molecules. Gold substrate topography, chemical affinity of molecules, intermolecular interactions and length of alkyl chains were found to be critical parameters that rule the SAM structure.

International audience; A detailed CCSD(T)//B3LYP study is presented to unravel the gas-phase reductive hydrogenation process of dioxides MO 2 (M= Si, Ti, Zr, Sn, Hf, Ir Ce) according to the following reaction MO 2 + H 2 → M(OH) 2. For the reductive hydrogenation process a heterolytic H-H bond cleavage is considered via hydride intermediates OMH(OH). A discussion concerning the effects of the reducibility of the metal centers and the structural aspects of the dioxides is presented. The results show that the activation of molecular hydrogen is directly related to the capability of the oxide to polarize the H 2 molecule prior to the H-H bond cleavage, although the formation of hydride intermediates do not necessarily guarantee further reduction of the metal center. The activation of the reduction reaction to form M(OH) 2 is found to be significantly larger than the activation to form the OMH(OH) intermediate. This gas-phase study aims to enhance the fundamental understanding of elementary steps in reductive hydrogenation processes of metal oxides.

H2 dissociation on ceria (CeO2) has attracted much attention in the last years because of its potential application in catalysis for hydrogenation reactions, as well as for the stabilization of hydride bulk and surface species. The ability of ceria to split hydrogen is strongly dependent on the surface morphology. However, to the best of our knowledge, the reactivity of the cerium sesquioxide Ce2O3 A-type (hexagonal structure with space group $$P\bar{3}m1$$ ) has not been previously addressed. In the present study, we investigate (i) the formation of oxygen vacancies in A-Ce2O3 bulk and (ii) the effect of the surface topology in the H2 dissociation and in the oxygen vacancy formation for the four most stable surfaces of A-Ce2O3: (0001), (01− 11), (11− 20) and (11− 21). Our results indicate a significant decrease of the energetic barrier for the hydrogen dissociation compared to stoichiometric CeO2, with an activation energy of ~ 0.1 eV. Interestingly, Ce2O3 surfaces lead to a heterolytic product with hydride species more stable than the homolytic product, which is the opposite behavior found in CeO2. These results suggest a better performance of Ce2O3 than CeO2 for H2 dissociation and provide insight in the nature of hydride-Ce2O3 interfaces that could be important intermediates in the formation of CeHx phases from cerium oxide.

Catechol on TiO2 is a model system for a class of molecules that bind and interact very strongly with metal oxides. This interaction gives rise to a marked charge-transfer absorption band that can be used to sensitize the complex to visible light. In solar cells, these are called type II sensitizers in contrast with type I sensitizers where an excitation of the molecule with subsequent charge injection is the main mechanism for placing an electron in the conduction band of the semiconductor. The adsorption geometry of these molecules is critical in their functioning. Nuclear magnetic resonance (NMR) spectroscopic methods can be used to elucidate structural information about the local geometry at the substrate–molecule interface. NMR methods coupled with density functional theory (DFT) allow for the detailed characterization of molecular binding modes. In the present work, we report a solid-state NMR and DFT study of catechol on TiO2. DFT-GIPAW chemical shift predictions for the 13C CP-MAS experiments unam...

The ethanol steam reforming reaction, together with the adsorption and decomposition of ethanol was studied on CeO2 and gallium-doped ceria (CeGaOx) by a combined experimental and theoretical approach using infrared spectroscopy (IR), mass spectrometry (MS) and density functional theory (DFT) calculations. At 100°C, different types of monodentate ethoxy species were identified as standing-up (SU) on Ce4+ and lying-down (LD) on Ce4+ and Ga3+, with the alkyl chain more perpendicular or parallel to the surface, respectively. It is suggested that the incorporation of Ga into the ceria lattice changes the decomposition pathway of LD species, which converts to acetate instead of ethylene, attributed to the increased lattice oxygen lability in the CeOGa interface upon doping and the propensity to form GaH surface species. Under ethanol steam reforming conditions, Ga doping of ceria-based materials has a drastic effect by improving the H2:CO2 ratio, changing the product distribution and reducing coke formation. Fil: Vecchietti, María Julia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina Fil: Fornero, Esteban Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Calatayud, Mónica. Sorbonne Université, Laboratoire de Chimie Théorique; Francia Fil: Collins, Sebastián Enrique. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Mohr, Susanne. Friedrich-alexander-universität Erlangen-nürnberg; Alemania Fil: Ganduglia-Pirovano, M. Verónica. Instituto de Catálisis y Petroleoquímica, Csic; España Fil: Libuda, Jörg. Friedrich-alexander-universität Erlangen-nürnberg; Alemania Fil: Bonivardi, Adrian L.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina

Nanoscale titania (TiO

Oxygen vacancies are related to specific optical, conductivity and magnetic properties in macroscopic SiO2 and TiO2 compounds. As such, the ease with which oxygen vacancies form often determines the application potential of these materials in many technological fields. However, little is known about the role of oxygen vacancies in nanosized materials. In this work we compute the energies to create oxygen vacancies in highly stable nanoclusters of (TiO2)N, (SiO2)N, and mixed (TixSi1−xO2)N for sizes between N = 2 and N = 24 units. Contrary to the results for bulk and surfaces, we predict that removing an oxygen atom from global minima silica clusters is energetically more favorable than from the respective titania species. This unexpected chemical behavior is clearly linked to the inherent presence of terminal unsaturated oxygens at these nanoscale systems. In order to fully characterize our findings, we provide an extensive set of descriptors (oxygen vacancy formation energy, electron localization, density of states, relaxation energy, and geometry) that can be used to compare our results with those for other compositions and sizes. Our results will help in the search of novel nanomaterials for technological and scientific applications such as heterogeneous catalysis, electronics, and cluster chemistry.

The structure of the iron oxide γ-Fe2O3 surfaces along (001) direction has been modeled for the first time using periodic DFT. Four different types of surfaces were analyzed depending on the upper layer composition (presence of tetrahedral or octahedral Fe) and the presence of iron vacancy locations. The inclusion of a Hubbard (DFT + U) correction allows a good agreement between calculated and experimental bulk properties. Relaxation of surface layers results in important surface reconstructions, electronic charge transfer with respect to the bulk, and a decrease in the dipole moment. Surfaces with tetrahedral Fe atoms on the top layer resulted to be the most stable ones. An almost linear correlation between the work function and the electronic charge transfer to O atoms was found. An analysis of the density of states shows that the Fermi level in tetrahedral Fe-terminated surfaces is far from the valence band whereas the reverse occurs with octahedral Fe-terminated atoms. The tetrahedral Fe-terminated surface has a more basic character than the octahedral ones. In addition, the presence of a subsurface Fe vacancy stabilizes the system and seems to be adequate for bond activation by electron transfer from the surface to the adsorbate, such as H2.

Nanoscale titania (TiO2) and silica (SiO2) are massively produced technologically important nanomaterials used in a wide range of technological applications where nano-titania is the active component (e.g. water splitting, pollution remediation, self-cleaning coatings). Generally, these applications entail contact with water and a degree of hydration of these nano-oxides. Although the hydration of nano-silica has been fairly well studied, the corresponding level of microscopic understanding for nano-titania is severely lacking. Here, using accurate electronic structure calculations we perform a detailed and comprehensive study of the hydration of titania nanoclusters. Firstly, using global optimisation, we establish the most energetically stable structures of a set of (TiO2)M(H2O)N nanoclusters with sizes ranging through M = 4-16 and with N/M ratios of ≤ 1.0. Using this extensive dataset we investigate how the structures, energy gaps, and thermodynamic stabilities of these species depend on size, temperature and water vapour pressure. To provide a broader chemical context for our study we also provide this full set of data for the respective set of (SiO2)M(H2O)N nanoclusters which we use to compare and contrast their properties with those of nano-titania. Our broad systematic study thus provides a comparative and foundational reference study for a thorough understanding of how hydration affects the structure, energetics and properties of both nano-SiO2 and nano-TiO2.

Nanostructured titanosilicate materials based upon interfacing nano-TiO2 with nano-SiO2 have drawn much attention due to their huge potential for applications in a diverse range of important fields including gas sensing, (photo)catalysis, solar cells, photonics/optical components, tailored multi-(bio)functional supports and self-cleaning coatings. In each case it is the specific mixed combination of the two SiO2 and TiO2 nanophases that determines the unique properties of the final nanomaterial. In the bulk, stoichiometric mixing of TiO2 with SiO2 is limited by formation of segregated TiO2 nanoparticles or metastable glassy phases and more controlled disperse crystalline mixings only occur at small fractions of TiO2 (

International audience; It is well established that enhanced atomic mobility exists at ceria (CeO 2) surfaces in high vacuum (HV) conditions within the TEM, owing to beam-induced reduction effects. Controlling the atmosphere surrounding the specimen within an aberration-corrected environmental TEM (ETEM) allows to monitor the atomic mobility at {100} surfaces, and to directly visualize the projected atomic structure of the top surface of ceria nanocubes through high resolution TEM (HRTEM) imaging [1]. Therefore, it is pertinent to investigate the current capabilities of the ETEM to validate atomistic models of adsorbed molecules at the surface of materials. However, it is only possible to conclude about the nature of the contrasts at the surface by comparing the experimental HRTEM images to image simulations.

A detailed density functional theory study is presented to clarify the mechanistic aspects of the methanol (CH

A comparative theoretical study applying Dual Descriptor has been performed using Water Gas Shift reaction, catalyzed by copper supported on monoclinic and tetragonal zirconia. The Dual Descriptor and thermodynamic considerations were used to understand the structural effects that occur in the reaction when it is catalyzed by C u on both zirconia phases. It was found that the formation of carbonates in the C u supported on tetragonal zirconia dehydrogenates consistently with experimental results.

International audience; IrO2 is a key material for photocatalytic applications as water oxidation catalyst. Despite its increasing interest, little is known about its molecular structure and reactivity. In this study, the surface properties of stoichiometric rutile IrO2 are investigated by means of periodic density functional theory (DFT), including the structural, energetic, electronic properties, and chemical reactivity toward catechol, a probe molecule mimicking photocatalytic linkers. Our results show that the (110)-IrO2 rutile termination is the most stable, and we discuss the role of the number and type of surface sites in the relative stability compared with (100), (001), and (101) terminations. Regarding the reactivity of the surfaces with catechol, our results show that the molecule dissociates and binds in bidentate, chelate, and monodentate modes. Interestingly, we find the chelated mode selectively favored over the (001) termination with the highest adsorption energy, −3.93 eV, being unstable on other terminations. The bidentate mode is preferred on (110) −3.83 eV, (100) −3.24 eV, and (101) −2.23 eV. The selective stabilization of the chelated mode, suggested in the literature to be responsible for the optical absorption in TiO2 nanoparticles, could guide in the search of tailored iridia-based interfaces for photocatalytic applications.

International audience; The reducibility of bulk metal oxides in which the cation is in its highest oxidation state (MgO, Sc2O3, Y2O3, TiO2, m-ZrO2, m-HfO2, CeO2, V2O5, Nb2O5, Ta2O5, WO3, CrO3, Al2O3, β-Ga2O3, SiO2, SnO2 and ZnO) has been studied by standard periodic density functional theory. We have defined and calculated descriptors able to describe and quantify semi-quantitatively the extent of reduction: electronic band gap, oxygen vacancy formation energy and electronic localization. We find that there is no single criterion for characterizing the reducibility. We discuss the advantages and limitations of each method, and we apply them to classify the materials with the PBE+U and B3LYP functionals. Typical irreducible oxides such as MgO show a large band gap, high oxygen vacancy formation energy and electronic localization of the reduction electrons forming and F-center, with a diamagnetic singlet electronic state. Reducible oxides such as TiO2 present small band gaps, small oxygen vacancy formation energy and electron localization of the reduction electrons in the cations, decreasing their oxidation state and presenting open-shell electronic states. Intermediate or ambivalent behavior is found for ZrO2, HfO2, β-Ga2O3, ZnO and SnO2.

The dopamine-TiO2 system shows a specific spectroscopic response, surface enhanced Raman scattering (SERS), whose mechanism is not fully understood. In this study, the goal is to reveal the key role of the molecule–nanoparticle interface in the electronic structure by means of ab initio modeling. The dopamine adsorption energy on anatase surfaces is computed and related to changes in the electronic structure. Two features are observed: the appearance of a state in the material band gap, and charge transfer between molecule and surface upon electronic excitation. The analysis of the energetics of the systems would point to a selective adsorption of dopamine on the (001) and (100) terminations, with much less affinity for the (101) plane.

International audience; Small adsorbed molecules play a key role in the morphology of inorganic nanoparticles. The presence of in situ 11 generated hydrogen during the synthesis of platinum nanoparticles is found to drive the growth of cubic nanocrystals, but little is 12 known about the processes occurring at the molecular level. In this paper, we use standard ab initio calculations to show that 13 hydrogen preferentially adsorbs on (100) Pt facets compared to (111) stabilizing the cubic morphology. Moreover, we provide 14 experimental and theoretical evidence that moderate partial pressure of hydrogen is needed to obtain nanocubes. In the absence 15 of hydrogen, or for low partial pressures, small nanoparticles with undefined shape are formed; however, longer exposure to 16 hydrogen pressure around 1 atm leads to the formation of cubes. Finally, this theoretical result allows presenting an experimental 17 protocol to be used to obtain platinum nanocubes with different degree of truncation. 18 ■ INTRODUCTION 19 Many physical and chemical properties depend on the shape of 20 the particles forming the material, 1 hence the challenge of 21 controlling the crystal morphology in a wide range of scientific 22 and technological applications. The chemical route is largely used 23 to control the shape of inorganic nanocrystals, although there is 24 still no theoretical drawback of the main forces driving it. Despite 25 many studies found in the literature, the theoretical and 26 computer simulation of nanoparticles' synthesis and growth are 27 still in the initial stage of development, and most of these works 28 do not take into account the complexity of the chemical 29 synthesis. It is still paramount to explain the mechanisms at the 30 origin of the uniformity of shape. Indeed, the field is still wide 31 open to future research aiming at identifying the conditions to 32 control the nanomorphology. 33 In the chemical synthesis of the nanocrystals, the chemical 34 bath possesses a complex composition containing various 35 reactants, solvents, surfactants, (counter)ions, and impurities. 36 Moreover, during the reaction, byproducts can be formed, which 37 could play a role in the nuclei formation and the following 38 nanoparticles' growth. 2 For instance, the role of dissolved gas H 2 39 on the shape control of platinum nanocrystals has recently been 40 demonstrated experimentally. 3 Some authors also report the role 41 of halide, ligands, or adsorbed CO on the nanomorphology. 4−6 42 Besides, the role of the initial shape of the nanocrystals and the 43 growth kinetics have been emphasized. 7,8 Beyond the exper-44 imental recipe, a theoretical approach is needed to rationalize and 45 predict the stability of the nano-objects. In the present paper, the 46 goal is to emphasize the role of a byproduct generated during the 47 chemical synthesis in the final nanoparticle morphology. We 48 focus here on the specific case of platinum. We use ab initio 49 calculations to explain how the presence of in situ generated H 2 50 drives the formation of cubic nanoparticles. We also explain the 51 impact of the experimental conditions, namely, the partial

Interfacial charge transfer has been an area of intense interest because of its relevance in molecular electronics, dye-sensitized solar cells, surface-enhanced Raman scattering (SERS), and photocatalysis. Although the chemical natures of both the contact and the linker have been shown to play important roles in determining the properties of hybrid dye/molecule–metal oxide complexes, little is known about the nature of the charge-transfer pathways. In this work, we explore in detail the idea that Raman enhancement and charge transfer are intimately related. To this end, we analyze the vibrational modes of molecules exhibiting the maximum enhancement of the Raman activities when they are adsorbed on semiconducting metal oxide nanoparticles. Our analysis of the potential energy distributions of these modes in the hybrid complexes indicates the significant involvement of bending and torsional modes of atoms deep within the metal oxide nanoparticle. Whereas the individual contribution of each of these oxide b...

Titanium oxide (TiO2) has been widely used in many fields, such as photocatalysis, photovoltaics, catalysis, and sensors, where its interaction with molecular H2 with TiO2 surface plays an important role. However, the activation of hydrogen over rutile TiO2 surfaces has not been systematically studied regarding the surface termination dependence. In this work, we use density functional theory (PBE+U) to identify the pathways for two processes: the heterolytic dissociation of H2 as a hydride&ndash, proton pair, and the subsequent H transfer from Ti to near O accompanied by reduction of the Ti sites. Four stoichiometric surface orientations were considered: (001), (100), (110), and (101). The lowest activation barriers are found for hydrogen dissociation on (001) and (110), with energies of 0.56 eV and 0.50 eV, respectively. The highest activation barriers are found on (100) and (101), with energies of 1.08 eV and 0.79 eV, respectively. For hydrogen transfer from Ti to near O, the activation barriers are higher (from 1.40 to 1.86 eV). Our results indicate that the dissociation step is kinetically more favorable than the H transfer process, although the latter is thermodynamically more favorable. We discuss the implications in the stability of the hydride&ndash, proton pair, and provide structures, electronic structure, vibrational analysis, and temperature effects to characterize the reactivity of the four TiO2 orientations.

In order to obtain a hydrogen storage system that is cost-effective, safe and energy-viable, in this work we studied the adsorption process of different sites and configurations of the hydrogen molecule in both MOF HKUST-1 and FeBTC structure. For the computational study was used the Ab initio simulation package from Vienna (VASP) with the GGA PBE functional in an SBU of 672 atoms. The exposed metal sites were found to be the best interaction site for the hydrogen molecule. In this work showed adsorption energy of -0.114 eV and -0.93 eV for HKUST-1 and FeBTC, respectively. This work also evidences the modifications of the atomic positions when the hydrogen molecule interacts with the MOF and the network energy.

We obtain an explicit solution for the stationary state populations of a dissipative Fano model, where a discrete excited state is coupled to a continumm set of states; both excited set of states are reachable by photo-excitation from the ground state. The dissipative dynamic is described by a Liouville equation in Lindblad form and the field intensity can take arbitrary values within the model. We show that the continuum states population as a function of laser frequency can always be expressed as a Fano profile plus a Lorentzian function with effective parameters whose explicit expressions are given in the case of a closed system coupled to a bath as well as for the original Fano scattering framework. Although the solution is intricate, it can be elegantly expressed as a linear transformation of the kernel of a $4\times 4$ matrix which has the meaning of an effective Liouvillian. We unveil key notable processes related to the optical nonlinearity and which had not been reported to date: electromagnetic induced-transparency, population inversions, power narrowing and broadening, as well as an effective reduction of the Fano asymmetry parameter., Comment: appendix added, corrected some typos

The interaction of molecules with titanium oxide substrates may lead to substantial modifications of their optical properties, in particular a red shift of the absorption spectrum compared to that of bare titania. In the present paper we discuss the role of the interface between two molecules, catechol and dopamine, with gas-phase (TiO2)N clusters (N = 2, 4, 6). We studied, for the interface, the bidentate modes (the molecule bonded to two Ti sites via its two oxygen sites), which was the most energetically favorable, followed by the chelated modes (the molecule bonded to one Ti site via its two oxygen sites), and the monodentate mode (the molecule bonded to one Ti site via one oxygen site). The absorption spectra were calculated with time-dependent functional-theory with CAM-B3LYP for the description of charge-transfer excitations. We observe a red shift of the molecule/cluster systems with respect to the molecules and clusters alone. Moreover, the chelated mode was found to present bands at lower energies than the other modes, making it the most interesting mode to tune the absorption edge of these systems.

International audience; The molecular and dissociative adsorption of butanethiol (C4H9SH) on regular Ag (111) surfaces has been studied by means of periodic ab initio density functional techniques. In molecular form, butanethiol is bound to the surface only by weak polarization-induced forces with the C–S axis tilted by 38° relative to the normal surface. The S atom occupies a position between a hollow fcc and a bridge site. In the dissociative adsorption process, the S–H bond breaks leading to butanethiolate. The S atom of the thiolate also occupies a threefold position, slightly displaced to a hollow fcc site compared to the thiol adsorption case. The C–S axis of the thiolate is tilted by about 37°. The calculated adsorption energies show that the butanethiol and butanethiolate have similar adsorption ability. The computed reaction pathway for the S–H dissociation gives an activation energy of 0.98 eV indicating that the thiolate formation from thiol, although not spontaneous at room temperature, might be feasible on silver surfaces. The dissociation process induces both adsorbate and surface polarization with a significant charge transfer from the substrate to the adsorbate.

Density Functional Theory (DFT) calculations have been performed for ethane, ethylene, propane, and propylene adsorption on the rutile VSbO4 structure to uncover the reaction mechanism during both ODH and ammoxidation reactions. This study is complementary to a previous paper in which the adsorption of ammonia on this structure was deeply analyzed, and in addition, experimental activity results for both reactions are shown to complete the theoretical DFT calculations. These results show that ammonia, ethane, and ethylene compete for the same active sites, the former adsorbing strongly. This explains why this catalytic system is not active for ethane ammoxidation; the presence of ammonia blocks the other molecule adsorption sites and prevents ethane oxidative dehydrogenation to ethylene. Such competition does not occur with propane or propene since the adsorption of both ammonia and hydrocarbon is possible at different sites, explaining why this catalytic system is active for propane ODH reaction. During a...

ZrO2 is known in chemistry to be a stable material. However, some technological applications point to specific redox reactivity related to surface oxygen composition. In this paper, ab initio calculations are used to characterize structure–reactivity relationships on monoclinic zirconia surfaces as regards thermodynamic stability, electronic structure, and chemical reactivity in reducing conditions. It is shown that the formation of different types of oxygen vacancies is possible on the surfaces of monoclinic zirconia. 2-Fold vacancies are found to induce an important structural relaxation. Moreover, the presence of O vacancies affects the surface electronic structure in two ways: either zirconium sites are reduced or F-centers are formed; the final state depends on the local geometry around the vacancy. Finally, some oxygen vacancy sites are found to be highly reactive toward dihydrogen, leading to its spontaneous dissociation and formation of dihydride species. These results reveal a rich and complex su...

The mechanisms of adsorption of hydrogen on the anatase TiO2(101) surface and of its diffusion in the bulk are investigated with DFT calculations and compared with similar results obtained for the diffusion of hydrogen on the rutile (110) surface. Because of the different oxygen environments in anatase and rutile surfaces, the H binding energy on the anatase surface is 0.2-0.3 eV smaller than in rutile. Various processes for H diffusion are investigated using the climbing nudged-elastic-band (cNEB) approach. We have identified three main diffusion mechanisms, leading to migration of H on the surface, diffusion into the bulk, and desorption of H-2 molecule. Our calculated activation barrier (E-act) shows that migration of H into the bulk is the kinetically most favorable process.