Bridging Oxygen Sites Research Articles

Deciphering the thermal, physical, structural, and optical characteristics of sodium-doped vanadophosphate glasses

Ionic glasses offer a multitude of prospects for a wide range of electrochemical applications such as solid-state batteries, which eventually require more accessible, secure, and ecologically sound electrolytes and electrode materials. Here, sodium-doped vanadium phosphate (NVP) glasses in the chemical composition, xNa2O.(50-x)V2O5.50P2O5 (where, x = 0, 10, 20, 30, 40, and 50 mol%) were effectively synthesized via the melt-quenching technique, and their amorphous nature was confirmed by the appearance of broad humps in X-ray diffraction technique. Numerous physical parameters, namely, density, molar volume, oxygen packing density (OPD), polaron radius, field strength, etc., were studied as a function of Na2O concentration. Furthermore, the structural alterations of the glass network were examined in relation to the modifications in the FTIR and Raman spectra brought about by varying concentrations of sodium ions, such as the formation of more bridging oxygen sites. The optical characteristics were investigated using UV–vis spectroscopy, and the transmittance in the as-prepared glass series elevated as Na2O content was increased. The presence of three absorption peaks centered at 320, 650, and 864 nm points towards the possibility of V4+ ions in the as-prepared glass samples. They are unequivocally amorphous semiconductors because their Urbach energy values fall between 0.12 and 0.57 eV. Optical band gap energy, refractive index, polarizability, dielectric constant, and additional parameters were also examined.

Studies of luminescence traits and Judd-Ofelt analysis of Sm3+ activated oxyfluoride glasses

Novel Sm3+ activated oxyfluoride glass was synthesized via the melt-quenching approach with subsequent heat treatment. The X-ray diffraction measurement discloses the amorphous nature of prepared glasses. And, FTIR illustrates the molar surplus of Bi2O3 raises the network connectivity by creating bridging oxygen sites in the structure. The Judd-Ofelt (J-O) intensity parameters exhibit Ω4>Ω2>Ω6 trend, witnessing the rigidity of the glass structure and ionic bonding between Sm3+ ions and surrounding ligands. The 4G5/2 → 6H7/2 transition of Sm3+ is the most applicable for reddish-orange lasers due to their higher values of radiative transition probability, branching ratio, and stimulated emission cross-section. The optimum emission intensity is obtained with 30 mol% Bi2O3 and the corresponding decay time is 1.40 ms. Additionally, the F30 glass sample was lit in a reddish-orange region having a CCT value of 1689 K. The thermal luminescence quenching of the prepared glass was studied and the thermal stability parameters activation energy was estimated. Further, the contactless optical thermometry capability of the glass sample was explored within the range of 303 K–603 K using the luminescence intensity ratio (LIR) technique. Thus, the above studies on the glass sample suggest that it can be used as an active medium in lasers, contactless optical temperature sensing, and high-energy scintillator applications.

Introducing Brønsted acid sites to accelerate the bridging-oxygen-assisted deprotonation in acidic water oxidation

Oxygen evolution reaction (OER) consists of four sequential proton-coupled electron transfer steps, which suffer from sluggish kinetics even on state-of-the-art ruthenium dioxide (RuO2) catalysts. Understanding and controlling the proton transfer process could be an effective strategy to improve OER performances. Herein, we present a strategy to accelerate the deprotonation of OER intermediates by introducing strong Brønsted acid sites (e.g. tungsten oxides, WOx) into the RuO2. The Ru-W binary oxide is reported as a stable and active iridium-free acidic OER catalyst that exhibits a low overpotential (235 mV at 10 mA cm−2) and low degradation rate (0.014 mV h−1) over a 550-hour stability test. Electrochemical studies, in-situ near-ambient pressure X-ray photoelectron spectroscopy and density functional theory show that the W-O-Ru Brønsted acid sites are instrumental to facilitate proton transfer from the oxo-intermediate to the neighboring bridging oxygen sites, thus accelerating bridging-oxygen-assisted deprotonation OER steps in acidic electrolytes. The universality of the strategy is demonstrated for other Ru-M binary metal oxides (M = Cr, Mo, Nb, Ta, and Ti).

A simultaneous operando FTIR & Raman study of propane ODH mechanism over V-Zr-O catalysts

Simultaneous operando Raman and transmission FTIR spectroscopic measurements have been combined for the study of the structure-activity relationships of V-Zr-O catalysts during propane oxidative dehydrogenation (ODH). The combination of both spectroscopies during reaction allows the identification of reaction intermediates and yields structural information about the catalyst. The experimental results are compared with available data in the literature to propose a reaction mechanism for propane ODH on V-Zr-O catalytic systems, and to identify the role of different oxygen species bound to vanadium sites. The results are consistent with a Mars-van Krevelen mechanism. Besides, they clearly show higher activity of bridging oxygen sites (V-O-V, V-O-Zr) than terminal VO bonds.

About hopping mechanism for sodium diffusion in silicate liquids with low sodium concentrations: Molecular dynamics simulation

Molecular dynamics simulation is done for Na2O.4SiO2 melt at ambient pressure and 1873 K. The simulation shows that Na atoms are placed in O polyhedrons and not in Si ones. Moreover, 75.86% of total Na are placed in NBO and FO (non-bridging and free oxygen) polyhedrons, the volume of which is 27.36% of simulation box. We propose a new mechanism for sodium diffusion. Accordingly, Na moves collectively with network formers (O and Si) staying in the same polyhedron, and frequently displaces between different polyhedrons. The second motion type gives the essential contribution to sodium diffusivity, and leads to very fast sodium mobility. As NBO ↔ BO happens, Na atoms redistributed between O polyhedrons make the collective motion with network formers. Each BO (bridging oxygen), NBO polyhedron has 1 and 2 sites, respectively. The site energy for Na located in BO site is larger than NBO site, and the transition energy for Na moving from present site to neighboring BO site is larger than the one to NBO site. The hops are strongly correlated with each other at low temperature. Furthermore, the dynamical structure is found to be heterogeneous. During 150 ps, the system comprises two distinct regions occupied by SiO subnet and OO cluster. The region occupied by OO cluster resembles a preferential diffusion pathway for sodium, the volume of which is 20.5% of simulation box.

Diffusion mechanism and dependence of diffusion on sodium silicate compositions

The distribution of sodium and diffusion mechanism in sodium-silicate melt with various compositions are investigated via molecular dynamics simulation. The microstructure and dynamical characteristic have been studied with the help of Voronoi polyhedron, simplex and Si-O subnet, oxygen-cluster. The simulation results reveal that Na atoms tend to be in the O polyhedrons and not in Si ones. Moreover, the Na atoms are mainly located in non-bridging oxygen (NBO) and free oxygen (FO) polyhedrons. The Voronoi volume of bridging oxygen (BO) or NBO weakly depends on the number of Na located in polyhedron which indicates the strong Si-O bond compared to Na-O bond. The structure of sodium silicate melt consists of two separate regions: the Na-poor regions of Si-BO subnets and a Na-rich region of NBO-FO cluster. The density of sodium in the NBO-FO cluster is by 3–6 times larger than the one of Si-O subnets. This largest NBO-FO cluster represents a diffusion channel for sodium in sodium-silicate. Furthermore, we find that each BO polyhedron contains 0 or 1 Na atom meanwhile each NBO polyhedron contains 1 or 2 Na atoms. Thereby, the BO and NBO polyhedron contains 1 and 2 sites, respectively. The energy for Na atom located in BO site is larger than that in NBO site. The transition energy for Na atom moving from present site to neighboring NBO site is smaller than that to neighboring BO site. So, Na atoms move frequently to neighboring polyhedrons of NBO, and rarely to the BO ones.

Operando Reactor-Cell with Simultaneous Transmission FTIR and Raman Characterization (IRRaman) for the Study of Gas-Phase Reactions with Solid Catalysts.

Raman and transmission FTIR spectroscopic techniques have been coupled in a new homemade reactor-cell designed in a joint CSIC-LCS collaboration. The setup is easily adapted to any FTIR and fiber-coupled Raman spectrometers and gas analysis techniques. It allows for simultaneous operando FTIR and Raman spectroscopic measurement, which provide complementary characterization of adsorbed species, reaction intermediates, and structural properties of the catalyst. This system was validated with the study of vanadium-based catalysts during propane oxydehydrogenation (ODH). The combined use of both spectroscopies with gas analysis techniques to measure the activity contributes to the understanding of propane ODH and the identification of the role of different oxygen species bound to vanadium sites. For example, the simultaneous characterization of the catalyst under the same conditions by IR and Raman confirms that the V═O mode has the same frequency in both spectroscopies and that bridging oxygen sites (V-O-V, V-O-Zr) present higher activity than terminal V═O bonds. These results demonstrate the high potential of the new simultaneous transmission IR-Raman operando rig to correlate the activity and the structure of catalysts, thus assisting the rational design of catalytic processes.

On shape dependence of the toxicity of rutile nanoparticles

Using B3LYP method, the nearly spheric structure of [Ti7O28H26]2- , the rod-like structures of [Ti2O10H10]2- and [Ti7O30H10]2- chains and the structures of their neutral complexes with Cu2+ coordinated at various terminal or bridging oxygen sites were optimized in order to assess the toxicity of rutile nanoparticles

Acceleration of wettability switching on TiO2 thin films under ultraviolet irradiation and direct current bias voltage

This study reports the results of combining ultraviolet (UV) light irradiation and direct current (DC) bias voltage on the acceleration of the photoresponsive wettability switching of TiO2 thin films. According to the mechanism of wettability switching reported previously, hydrophilic surface termination is caused by the reaction between the photogenerated holes and bridging oxygen sites on the TiO2 surface. In this study, we investigate the effect of applying DC bias voltage to TiO2 thin films to intentionally polarize holes toward the TiO2 surface for increasing and accelerating the generation of absorption sites. The wettability switching width of the contact angles from the hydrophobic state to hydrophilic state was virtually doubled and the switching time of the contact angles was reduced to approximately 1/10th by combining a DC bias voltage of 0.9 V and UV irradiation, compared to TiO2 thin films processed only with UV irradiation. This photoresponsive wettability switching based on TiO2 photocatalysts is a promising technology for wireless liquid manipulation as amplification of the switching width and acceleration of the switching response of the surfaces are important for practical applications.

Structure, Dynamics, and Mechanical Properties of Cross-Linked Calcium Aluminosilicate Hydrate: A Molecular Dynamics Study

C-A-S-H is the main hydration product of environmental friendly concrete with cement material partially substituted by the industrial waste. The molecular structure of C-A-S-H gel determines the durability of the material. In this study, the cross-linking C-A-S-H models with Al/Si ratios of 0, 0.05, 0.10, 0.15, and 0.2 are constructed, and the structure, reactivity, and mechanical properties of the C-A-S-H gel are investigated by the reactive force field molecular dynamics. The incorporation of aluminate species in the C-A-S-H gel modifies the silicate-aluminate skeleton and interlayer water molecules. On the one hand, the bridging silicate tetrahedron is substituted by the aluminate species that polymerize with defective silicate chains, improve the crystalline order, enhance the Q species connectivity, and transform the layered C-S-H structure to cross-linked branch of C-A-S-H gel. On the other hand, the Al–Si substitution enhances the reactivity of the bridging oxygen sites in Si–O–Al, which contribute...

Monitoring dynamics of different processes on rutile TiO2(110) surface by following work function change

Time evolution of the work function (WF) change of TiO2 (110) rutile surface due to the oxygen adsorption and electron bombardment has been investigated. Oxygen adsorption has been performed on surfaces prepared in different ways (annealing temperature, in UHV or in oxygen atmosphere) which have different amounts of bridging oxygen vacancies (BOVs). The surfaces having low amount of BOVs were used in electron bombardment experiments. WF increases due to the O2 adsorption, and the overall change increases with the amount of BOVs. The result is consistent with the BOV healing by O2 adsorption. Oxygen adsorption results can be correlated with the shape of the Ti 2p photoelectron line: there is a linear dependence between the total WF change and the relative amount of the Ti3+ contribution in the Ti 2p3/2 line up to 0.2 eV when WF change reaches the saturation. Electron bombardment reduces WF due to the well known process of electron stimulated desorption of oxygen from bridging oxygen sites. The total change reaches about 0.5 eV, when the saturation is reached. It was shown that the efficiency for creating BOVs by electron bombardment increases with the electron energy.

A mechanism for the hole-mediated water photooxidation on TiO2 (1 0 1) surfaces

The mechanism of water photooxidation on TiO2 surfaces is still controversial. Here we report a first-principles density functional study based on a hybrid functional method in which an adsorbed water molecule is found to directly interact with a self-trapped hole at a bridging oxygen site and to transform into an OH• radical species through a concerted proton/hole transfer. This study analyzes both the thermodynamics and kinetics of this step of the reaction, which is generally considered to be the rate determining one. The fate of the OH• radical is then investigated in terms of its reactivity with different surface species, with a second OH• radical, or with a second water molecule coming from the environment. We find that OH• radicals can either acquire a hydrogen from surrounding water molecules or, if they meet, couple to form hydrogen peroxide with highly associated energy gain.

Oxygen interstitials and vacancies in LaSrGa3O7-based melilites

The Sr-rich composition of the layered tetrahedral meililite, La0.8Sr1.2Ga3O6.9, was synthesized and a structural investigation on La0.8Sr1.2Ga3O6.9 using neutron powder diffraction revealed a site preference of oxygen vacancies on the bridging oxygen sites of the 4-linked GaO4 tetrahedra. Impedance measurement revealed limited ionic conduction in the oxygen-deficient La0.8Sr1.2Ga3O6.9, presumably associated with oxygen vacancies, which is ~2 orders of magnitude higher than the parent material LaSrGa3O7 but ~3–4 orders of magnitude lower than the interstitial oxide ionic conductivity in La-rich composition, La1.54Sr0.46Ga3O7.27. Low temperature neutron powder diffraction characterization was performed for the oxygen-excess, La-rich composition, La1.54Sr0.46Ga3O7.27, which confirmed the position near the center of the pentagonal tunnels for the oxygen interstitials identified previously using the room temperature data. Solid state 71Ga NMR data collected on these LaSrGa3O7-based materials with stoichometric, excess, and deficient oxygen contents was found not able to distinguish these three compositions. A metastability temperature gap within 850–1280°C was identified for the oxygen interstitial-conducting La1.54Sr0.46Ga3O7.27. The structures of these oxygen excess and deficient gallate melilites further demonstrate the structural flexibility of the LaSrGa3O7-based layer tetrahedral network.

Recombination of Formaldehyde and Hydrogen Atoms on TiO2(110)

Chemical changes on methanol/TiO2(110) interface after UV illumination have been investigated by two-photon photoemission spectroscopy (2PPE) and scanning tunneling microscope (STM). Spontaneous recombination of the photocatalytic products, i.e., formaldehyde and hydrogen atoms at the bridging oxygen sites, has been observed using 2PPE. In addition, tip induced recombination of formaldehyde and hydrogen atoms has also been imaged using STM, which further shows that recombination can occur only when the photocatalytic dissociation products stay in the original sites. The experimental observation that methanol can only be dissociated under external forces such as UV illumination while the reverse reaction can take place spontaneously at 130 K may provide an important system for the theoretical calculations of the energetics of adsorbates/TiO2(110).

Existence of basal oxygen vacancies on the rutileTiO2(110)surface

On a rutile ${\mathrm{TiO}}_{2}\phantom{\rule{0.16em}{0ex}}(110)$ surface, vacancies at the bridging oxygen sites are thought to be major surface point defects and strongly influence the surface phenomena. Using systematic density functional theory (DFT) calculations, however, we show that vacancies at the basal oxygen sites have comparable formation energies to the bridging oxygen vacancies. The correction of self-interaction error for localized $\mathrm{Ti}\phantom{\rule{0.16em}{0ex}}3d$ states in DFT plays an important role for correctly describing the relative stability of the oxygen vacancies as well as the electronic structures. This new type of stable surface oxygen vacancy can be formed with a similar amount of the bridging oxygen vacancy in a relatively dilute condition, which may affect atom deposition and chemical reactions on the surface.

Reaction path analysis of propene selective oxidation over V2O5 and V2O5/TiO2

The selective oxidation of propene on the vanadyl and bridging oxygen sites of the fully oxidized (001) V2O5 surface and of an epitaxial vanadia monolayer supported on (001) TiO2 anatase is analyzed using periodic density functional theory (DFT). The formation of several oxygenated products, that is, allyl alcohol, propene oxide, acrolein, formaldehyde and acetaldehyde, is investigated. Selective oxidation proceeds via a Mars–van Krevelen redox mechanism and its elementary steps on the vanadia surface are identified. Propene activation occurs preferentially on the vanadyl oxygen sites, with allyl CH and CC bond activation being equally favorable. Supporting a vanadia monolayer on titania strongly enhances both bond activations as compared to unsupported V2O5, yielding a lower activation energy. No dominant pathway is found on V2O5, with allyl alcohol, propene oxide, and acrolein formation being almost equally favorable, while acrolein formation is expected on V2O5/TiO2. However, propene oxidation to acrolein is overall less preferable on V2O5/TiO2 than V2O5, due to the reduced activity of the supported catalyst in oxygenation reactions. The latter is linked to the creation of V3+ sites during propene oxidation over V2O5/TiO2. On the other hand, the electrons injected in the empty V 3d states during propene oxidation over V2O5 lead to the creation of V4+ sites.

Role of Water in Methanol Photochemistry on Rutile TiO2(110)

Photochemistry of the molecularly and dissociatively adsorbed forms of methanol on the vacuum-annealed rutile TiO2(110) surface was explored using temperature-programmed desorption (TPD) both with and without coadsorbed water. Methoxy, and not methanol, was confirmed as the photochemically active form of adsorbed methanol on this surface. UV irradiation of methoxy-covered TiO2(110) led to depletion of the methoxy coverage and to formation of formaldehyde and a surface OH group. Coadsorbed water did not promote either molecular methanol photochemistry or thermal decomposition of methanol to methoxy. However, terminal OH groups (OHt), prepared by coadsorption of water and oxygen atoms, thermally converted molecularly adsorbed methanol to methoxy at 120 K thus enabling photoactivity. While chemisorbed water molecules had no influence on methoxy photochemistry, water molecules hydrogen-bonded in the second layer to bridging oxygen (Obr) sites inhibited methoxy photodecomposition to formaldehyde. From this, we...

Observation of Photocatalytic Dissociation of Water on Terminal Ti Sites of TiO2(110)-1 × 1 Surface

The water splitting reaction based on the promising TiO(2) photocatalyst is one of the fundamental processes that bears significant implication in hydrogen energy technology and has been extensively studied. However, a long-standing puzzling question in understanding the reaction sequence of the water splitting is whether the initial reaction step is a photocatalytic process and how it happens. Here, using the low temperature scanning tunneling microscopy (STM) performed at 80 K, we observed the dissociation of individually adsorbed water molecules at the 5-fold coordinated Ti (Ti(5c)) sites of the reduced TiO(2) (110)-1 × 1 surface under the irradiation of UV lights with the wavelength shorter than 400 nm, or to say its energy larger than the band gap of 3.1 eV for the rutile TiO(2). This finding thus clearly suggests the involvement of a photocatalytic dissociation process that produces two kinds of hydroxyl species. One is always present at the adjacent bridging oxygen sites, that is, OH(br), and the other either occurs as OH(t) at Ti(5c) sites away from the original ones or even desorbs from the surface. In comparison, the tip-induced dissociation of the water can only produce OH(t) or oxygen adatoms exactly at the original Ti(5c) sites, without the trace of OH(br). Such a difference clearly indicates that the photocatalytic dissociation of the water undergoes a process that differs significantly from the attachment of electrons injected by the tip. Our results imply that the initial step of the water dissociation under the UV light irradiation may not be reduced by the electrons, but most likely oxidized by the holes generated by the photons.

Reaction path analysis of propane selective oxidation over V2O5 and V2O5/TiO2

The selective oxidation of propane on the vanadyl and bridging oxygen sites of the fully oxidized (001) V2O5 surface and of an epitaxial vanadia monolayer supported on (001) TiO2 anatase is analysed using periodic density functional theory (DFT). Both the oxidative dehydrogenation leading to propene and the formation of oxygenated products, namely n-propanol, i-propanol, propanal and acetone, are investigated. Selective oxidation proceeds via a Mars–van Krevelen redox mechanism, and its elementary steps on the vanadia surface are identified. Propane chemisorption preferentially occurs through a secondary CH bond activation via a direct hydrogen abstraction by a lattice oxygen. Supporting a vanadia monolayer on titania strongly enhances the CH bond activation as compared to unsupported V2O5, yielding a lower activation energy and a more exothermic propane chemisorption. In accordance with experimental observations, the calculations show that the titania support not only modifies the activity of the vanadia monolayer but it also affects the selectivity of the catalyst, favouring the formation of propene compared to the formation of i-propanol and acetone. The vanadyl oxygen is overall the most active site on V2O5 and V2O5/TiO2, while the bridging oxygen is more selective towards propane dehydrogenation.

Atomic-scale structures of interfaces between phyllosilicate edges and water

We report first-principles molecular dynamics (FPMD) studies on the structures of interfaces between phyllosilicate edges and water. Using FPMD, the substrates and solvents are simulated at the same first-principles level, and the thermal motions are sampled via molecular dynamics. Both the neutral and charged silicate frameworks are considered, and for charged cases, the octahedral (Mg for Al) and tetrahedral (Al for Si) substitutions are taken into account. For all frameworks, we focus on the commonly occurring (010)- and (110)-type edge surfaces.With constrained FPMD, we calculated the free energy of the leaving processes of coordinated water of octahedral cations; therefore, the coordination states of those edge cations are determined. For (010)-type edges, both the 5- and 6-fold coordination states of Al are stable and occur with a similar probability, whereas only the 5-fold coordination is stable for Mg cations. For (110)-type edges, only the 6-fold states of Al cations are stable. However, for Mg cations, both coordination states are stable. In the 5-fold case, the solvent water molecules form H-bonds with the bridging oxygen atoms (i.e., MgOSi). The free energy results indicate that there should be a considerable number of 5-fold coordinated octahedral sites (i.e., MgOH/AlOH) at the interfaces. The interfacial structures and acid/base groups were determined by detailed H-bonding analyses. (1) Bridging oxygen sites. For (010) edges, the bridging oxygen atoms are not effective proton-accepting sites because they are inaccessible from the solvent. For (110) edges, the oxygen atoms of neutral and octahedrally substituted frameworks (i.e., MOSi for MAl/Mg) are proton-accepting sites. For T-sheet substituted cases, the bridging oxygen site becomes a proton-donating group (i.e., AlOHSi) through the capture of a proton. (2) T-sheet groups. All T-sheet edge groups are MOH (MSi/Al) and act as both proton donors and acceptors. (3) O-sheet groups. For (010) edges with 6-fold Al, the active surface groups include Al(OH)(H2O) and Al(OH)2, whereas for Mg cations, the edge group is Mg(OH2)2. For 5-fold coordination, the active groups are AlOH. At (110) edges, proton-donating sites include AlOH, AlOH2 and MgOH2 groups. Overall, our results reveal the significant effects of isomorphic substitutions on the interfacial structures, and the models provide a molecular-level basis for understanding relevant interfacial processes.