Bridging Oxygen Sites Research Articles

Mechanisms of Proton Formation from Interaction of H2 with E‘ and Oxygen Vacancy Centers in SiO2: Cluster Model Calculations

We performed first-principles quantum-mechanical calculations on the mechanisms of proton formation in SiO2. Cluster models of various size were used to represent two oxygen deficient centers in silica, the E‘ center, a Si dangling bond, and the neutral oxygen vacancy. The dissociation of H2, the formation of protons, and the mechanism and energetics of proton migration were investigated. We found that the E‘ center is an active site for hydrogen cracking and proton generation; the reaction of the E‘ center with H2 should lead to a Si dangling bond near a Si−H group and a mobile proton. The proton is stabilized at bridging oxygen sites with formation of oxonium ions, (≡Si−)2OH+, where the oxygen atom is three-coordinated. Another mechanism of proton formation is from the reaction of atomic H with a neutral O vacancy. The process leads to a free electron and a stable proton bridge structure where the H+ ion is midway between two Si atoms.

Ab initio calculations of H/sup +/ energetics in SiO/sub 2/: Implications for transport

The process of H/sup +/ diffusion in SiO/sub 2/ is studied using first-principles density-functional theory. This is done by examining stable bonding sites, migration pathways, and barrier heights of H/sup +/ in /spl alpha/-quartz. The H/sup +/ is found to bind to the bridging oxygen site in a variety of ways, but always displaying an attraction to neighboring oxygen atoms. This attraction provides a mechanism for understanding the diffusion pathways and barrier heights for this system. The relative stability of H/sup +/ in SiO/sub 2/ is also discussed in the context of transport.

Effect of embedding and cluster size on the ab initio study of potassium adsorption at rutile(110)

The results of Hartree–Fock ab initio model calculations for the adsorption of a single potassium atom on a bridging oxygen site of the rutile(110) surface are presented. The surface is simulated with small clusters Ti2O4 and Ti4O8, either embedded in a finite array of total ion potentials and point charges or saturated with H, OH, and H2O. The effect of the different embedding techniques on physical aspects of the adsorption process is discussed in terms of electronic structure and energetics. Comparison of available experimental data and previous theoretical studies with the present results show that medium-sized model clusters give a reasonable description of the adsorption process, independent of the embedding method. For very small models, even the qualitative picture of the K—TiO2 interaction changes when different embedding techniques are used. These results can be generalized to develop rules for designing cluster models for metal adsorption on rutile surfaces. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 127–132, 1999

Molecular cluster models of aluminum oxide and aluminum hydroxide surfaces

Ab initio, molecular orbital calculations for two different Hartree-Fock basis levels were performed on clusters in the system Al-O-H, and tested by comparing derived vibrational frequencies to the measured values for aluminum oxides and aluminum oxyhydroxide minerals. Models were chosen to reflect surface groups that may be present on aluminous minerals such as α-Al2O3 (corundum) and Al(OH)3 (gibbsite). Protonation and deprotonation reactions on bridging and terminal oxygen or hydroxyl sites were modeled to investigate the effects of solution pH on the structure of surface groups. Relative deprotonation energies, including solvation effects, were calculated for each site using the self-consistent isodensity polarized continuum model of Keith and Frisch (1994), and then used to predict the order in which each particular surface group protonates.

Oxygen Speciation in Hydrous Silicate Glasses: An Oxygen-17 NMR Study

The structural roles of H2O and OH in hydrous Na2Si4O9 (sodium tetrasilcate, NTS) and NaAlSi3O8 (“albite”) glasses have been investigated with 17O, one-dimensional fast magic-angle spinning (MAS), static, and cross-polarization (CP), and two-dimensional triple quantum MAS (3QMAS) NMR techniques. Silica gel and crystalline analcime (NaAlSi2O6·H2O) were also studied as model compounds. Peaks for molecular H2O are clearly identified in the analcime and in the hydrous glasses in one-dimensional, partial saturation experiments. Bridging and nonbridging oxygen sites are clearly resolvable in the 3QMAS spectra, as is a feature related to SiOH in hydrous NTS glass. This feature may also be present in the hydrous albite glass, although, lacking further characterization of other site types (e.g., NaOH, AlOH, Si−OH−Al), this assignment remains tentative. 1H−17O CP experiments with short contact times reveal a peak that is similar in both hydrous glasses and silica gel, again possibly related to SiOH groups.

Analysis of Diffuse Scattering from Single Crystals via the Reverse Monte Carlo Technique. II. The Defect Structure of Calcium-Stabilized Zirconia

The results of an attempt to use reverse Monte Carlo (RMC) refinement to fit a model structure to the diffuse X-ray and neutron scattering of the complex defect structure of calcium-stabilized cubic zirconia (Ca-CSZ), composition Zr0.85Ca0.15O1.85, are reported. This is the first attempt to use the RMC method to analyse single-crystals diffuse scattering of a disordered system containing both chemical disorder and associated displacement disorder. The possibilities and problems of the RMC method for this kind of disordered system are discussed and the method is compared with techniques used in previous studies of the diffuse scattering of CSZs. The study has revealed that the application of RMC to single-crystal data needs to take into account two seemingly incompatible requirements. When the model crystal is large, relatively low-noise calculated diffraction patterns can be obtained but the model contains so many degrees of freedom that the fit obtained does not properly reflect the short-range properties inherent in the data. On the other hand, when the model crystal is small the calculated diffraction patterns are very noisy and the efficacy of the fit is again compromised. Despite these limitations the results obtained in the present study do appear to present a qualitative picture of the local ordering consistent with previous studies. The present RMC refinements of Ca-CSZ support the previously postulated oxygen-vacancy ordering scheme, i.e. vacancy pairs separated by ½ 〈111〉 along the body diagonal of oxygen cubes which contain a cation. The cations are disturbed from their average positions in such a way that if either of the two bridging oxygen sites between a pair of cations separated by ½ 〈111〉 is vacant, then the cations are found to have moved further apart than the average distance, whereas when both bridging oxygen sites are occupied a closer then average distance is observed. A new result from this RMC study is that a negative occupancy correlation exists between Ca neighbours separated by ½ 〈111〉 and 〈100〉, i.e. there is a tendency to avoid Ca–Ca nearest or next-nearest neighbours.

The interconversion of SnO 2(110) reduced phases by reaction with formic acid

Angle-resolved photoemission (ARPES), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) are used to study modifications to the surface electronic structure and symmetry of reduced SnO 2(110)1 × 1 and 1 × 2 surfaces, following interaction with formic acid. Formic acid adsorbs molecularly on both surfaces at 105 K and is desorbed by annealing to 375 K, leaving no carbon residues on the surface. LEED shows that while the surface order of the 1 × 1 reduced surface remains unchanged by this treatment the half order beams from the 1 × 2 surface are completely removed. ARPES spectra from the 1 × 1 surface are unaffected by formic acid adsorption and annealing, but for the 1 × 2 surface change to resemble the 1 × 1 surface data. We speculate that the interaction either removes or replaces bridging oxygen sites on the 1 × 2 surface.

Short-Range Order in KPO3 Glass Studied by Neutron and X-Ray Diffraction

The short-range order of KPO3 glass has been studied by diffraction methods in order to make evident the different behaviour of the P-O bonds within the PO4 tetrahedron. The oxygen sites are devided into bridging and terminal (non-bridging) oxygen sites, corresponding to two P-O bond lengths, the difference of which amounts to 14.5 pm. Previous conclusions about the changes of the P-O bond lengths under the influence of modifier cations of different electric field strength are corroborated. The K-O environments reveal two apparently different distances rKO with equal contributions to the total K-O coordination number of about 6.7. To explain this phenomenon, the K+ cations are suggested to be located in non-spherical cavities

Cation Distribution in Mixed Alkali Disilicate Glasses

We have investigated the structure of five disilicate glass compositionsK2O·2SiO2, (Na0.25K0.75 )2O·2SiO2, (Na0.50K0.50 )2O·2SiO2, (Na0.75K0.25)2O·2SiO2, and Na2O·2SiO2using 2D 17O Dynamic Angle Spinning (DAS) Nuclear Magnetic Resonance (NMR). The nonbridging and bridging oxygen sites can be resolved in the isotropic 17O DAS spectrum. The bridging oxygen isotropic resonance shifts from 16.1 ppm in K2O·2SiO2 to 11.8 ppm in Na2O·2SiO2 and remains almost identical in width. The nonbridging oxygen isotropic line shapes in the mixed compositions are broader with a maximum intensity that continuously shifts from the position found in the pure potassium composition to the position found in the pure sodium composition. The evolution of the nonbridging oxygen isotropic line shape with changing alkali composition is consistent with a model of the glass structure where four alkali cations are distributed in random combinations around each nonbridging oxygen.

The structure of the stoichiometric and reduced SnO 2(110) surface

First-principles calculations based on density functional theory (DFT) and the pseudopotential method have been used to study the stoichiometric and reduced SnO 2(110) surface. The ionic relaxations are found to be moderate for both the stoichiometric and reduced surfaces, and are very similar to those found in recent DFT-pseudopotential work on TiO 2. Removal of neutral oxygen leaves two electrons per oxygen on the surface, which are distributed in channels passing through bridging oxygen sites. The associated electron density can be attributed to reduction of tin from Sn 4+ to Sn 2+, but only if the charge distribution on Sn 2+ is recognized to be highly asymmetric. Reduction of the surface gives rise to a broad distribution of gap states, in qualitative agreement with spectroscopic measurements.

Creation of variable concentrations of defects on TiO 2(110) using low-density electron beams

Low density (~ μA cm 2 ) 0.48 and 1.0 keV electron beams have been used to create surface defects on a TiO 2(110) surface. These electron-beam induced defects were examined primarily by X-ray photoelectron spectroscopy (XPS) with supporting ultraviolet photoemission spectroscopy (UPS). Glancing and normal emission XPS spectra of nearly defect-free surfaces revealed that Ti atoms on the surface were similar to the bulk Ti, while some surface oxygen atoms were different from the bulk oxygen. XPS of Ti 2p 3 2 was used to quantify the defect concentration and to examine the defect electronic structure. Based on our calculation of defect concentrations and the comparison of our results with results and models from the literature, we conclude that oxygen vacancies induced by electron beams in the current study are mostly from the bridging oxygen sites, in agreement with the previous work. A range of defect concentrations with similar electronic structure, mainly composed of Ti 3+, have been induced by low-density electron beams. Beam energy and exposure were the experimental variables. The rates of defect formation at low beam exposure were beam-energy dependent, with a faster growth rate at 0.48 keV than at 1.0 keV. These defects were similar to those by thermal annealing in vacuum, but a higher concentration of defects could be obtained with longer beam exposure. However, the e-beam induced defects were different from those produced by Ar + ion bombardment since both this and previous studies have found defects produced by Ar + ion bombardment to be complex, with a variety of different local environments where oxygen and titanium surface atoms coexist.

Ab initio quantum mechanical studies of the kinetics and mechanisms of silicate dissolution: H+(H3O+) catalysis

It is generally believed that the hydrolysis of Si —O —Si or Si —O —Al bonds is the key step in the dissolution processes of most silicates and aluminosilicates. High level ab initio molecular orbital calculations have been carried out to investigate the atomic processes of mineral dissolution by studying the following reactions: Si —O —Si + H2O → Si —OH + Si —OH, Si —O —Al + H2O → Si —OH + Si —OH, Si —O —A1 + H3O+ → Si —OH + Si —OH2+, Si —O —A1 + H3O+ → Si —OH + Si —OH2+, Our ab initio results provide detailed molecular mechanisms, which can be used to explain the pH dependence of mineral dissolution rates, the formation of leached layers during feldspar dissolution, and recent experimental data on the kinetic isotope effect associated with quartz and feldspar dissolution. The hydrolysis reactions written above were studied by truncating the mineral surface so that a finite cluster of atoms could be used in the ab initio calculations. The potential energies involved in the adsorption of various aqueous species, as well as the reaction pathways leading to the formation of the transition state in each of the reactions, were then studied from first principles. The ab initio results indicate that: 1.(1) The attack of H+ (H3O+) onto the bridging oxygen site significantly weakens the Si — O — Si and Si — O — Al bridging bonds,2.(2) The calculated activation energies for the hydrolysis of Si — O — Si and Si —O —Al bonds by H2O are 29 (32) kcal/mol and 26 (27) kcal/mol, respectively, while under H2O+ catalysis, they are 24 (22) kcal /mol and 16 kcal /mol (at the MP2/6-31G∗ level). Therefore, our ab initio results provide direct evidence of both H3O+ catalysis and of the preferred hydrolysis of Si — O — Al linkages over Si — O — Si linkages, and3.(3) Calculations show that pure H2O hydrolysis leads to a rather big kinetic isotope effect (KH2OKD2O = 2−4) while under H3O+ catalysis KH3O+KD3O+ = 0.7−1.1. The latter results are in good agreement with recent experimental data for quartz and feldspar dissolution at low pH KH3O+KD3O+ = 1.1−1.3).

Ab initio and semiempirical cluster studies on the reactivity of the vanadium pentoxide (010) surface

The reactivity of vanadium pentoxide (010) surface sites with respect to adsorption of hydrogen and desorption of OH (forming reaction steps in the nucleophilic oxidation of hydrocarbons) is studied by cluster models of different size. Whereas semiempirical INDO methods are used to examine the cluster size dependence in larger substrate clusters up to V6O20, ab initio Hartree-Fock techniques are employed to study quantitative details of the chemisorptive H- and OH-surface interaction in smaller V2O9H and V2O8OH clusters. Three different oxygen adsorption sites, the vanadyl site and two bridging oxygen sites, are considered. It is found that hydrogen binds to the surface oxygen at the vanadyl site quite strongly leading to formation of an OH surface group which itself is bound strongly to the surface vanadium. At the bridging oxygen sites hydrogen can also form surface OH but this species is bound rather weakly to the substrate and can desorb from the surface. These results suggest for the nucleophilic oxidation of hydrocarbons that bridging oxygen sites rather than vanadyl sites at the V2O5 surface are involved in the reaction process.

Molecular Structure and Binding Sites of Cobalt(II) Surface Complexes on Kaolinite from X-Ray Absorption Spectroscopy

AbstractX-ray absorption spectroscopy (XAS) was used to determine the local molecular environment of Co(II) surface complexes sorbed on three different kaolinites at ambient temperature and pressure in contact with an aqueous solution. Interatomic distances and types and numbers of backscattering atoms have been derived from analysis of the extended X-ray absorption fine structure (EXAFS). These data show that, at the lowest amounts of Co uptake on kaolinite (0.20–0.32 µmol m−2), Co is surrounded by ≈6 O atoms at 2.04–2.08 Å and a small number or Al or Si atoms (N = 0.6–1.5) at two distinct distances, 2.67–2.72 Å and 3.38–3.43 Å. These results indicate that Co bonds to the kaolinite surface as octahedrally coordinated, bidentate inner-sphere mononuclear complexes at low surface coverages, confirming indirect evidence from solution studies that a fraction of sorbed Co forms strongly bound complexes on kaolinite. In addition to inner-sphere complexes identified by EXAFS spectroscopy, solution studies provide evidence for the presence of weakly bound, outer-sphere Co complexes that cannot be detected directly by EXAFS. One orientation for inner-sphere complexes indicated by XAS is bidentate bonding of Co to oxygen atoms at two Al-O-Si edge sites or an Al-O-Si and Al-OH (inner hydroxyl) edge site, i.e., corner-sharing between Co octahedra and Al and Si polyhedra. At slightly higher surface sorption densities (0.51–0.57/ µmol m−2), the presence of a small number of second-neighbor Co atoms (average NCo < 1) at 3.10–3.13 Å indicates the formation of oxy- or hydroxy-bridged, multinuclear surface complexes in addition to mononuclear complexes. At these surface coverages, Co-Co and Co-Al/Si distances derived from EXAFS are consistent with edge-sharing between Co and Al octahedra on either edges or (001) faces of the aluminol sheet in kaolinite. Multinuclear complexes form on kaolinite at low surface sorption densities equivalent to <5% coverage by a monolayer of oxygen-ligated Co octahedra over the N2-BET surface area. These spectroscopic results have several implications for macroscopic modeling of metal ion uptake on kaolinite: 1) Primary binding sites on the kaolinite surface at low uptake are edge, non-bridging Al-OH inner hydroxyl sites and edge Al-O-Si bridging oxygen sites, not Si-OH sites typically assumed in sorption models; 2) specific adsorption of Co is via bidentate, inner-sphere complexation; and 3) at slightly higher uptake but still a small fraction of monolayer coverage, formation of Co multinuclear complexes, primarily edge-sharing with Al-OH octahedra, begins to dominate sorption.

27Al and 29Si solid-state NMR studies of dealuminated mordenite

27 Al and 29Si MAS and CP MAS NMR have been used to monitor structural modifications of mordenite subjected to hydrothermal dealumination, acid leaching and calcination. Six, four- and possibly five-coordinate extraframework aluminium species are formed upon expulsion of aluminium from the framework. 1H–29Si CP MAS NMR monitors the creation of Si—OH defect sites. Calcination of dealuminated mordenite greatly reduces the number of such defects, indicating that they are substantially ‘healed’ by silicon atoms from other parts of the crystal, and also via dehydration of hydroxy groups to form bridging oxygen sites. The occupancies of crystallographically inequivalent Si(0AI) sites calculated from 29Si MAS NMR spectra of dealuminated mordenite show that aluminium atoms occupy some of the T2 tetrahedral sites in the parent material.

Ab initio molecular orbital cluster studies of the zeolite ZSM-5. 1. Proton affinities

In this study we carried out ab initio molecular orbital calculations using the STO-3G, 3-21G, and 6-31G basis sets on aluminosilicate clusters representing ZSM-5 containing up to 46 tetrahedrally coordinated tetravalent atoms and including (SiO)[sub n] rings. The proton affinity and OH stretching frequency of the central bridging oxygen site (O(24)site) are examined as a function of cluster size and shape. The proton affinity is found to depend significantly (5-15 kcal/mol) on the shape of the cluster. Calculation of electrostatic potential in the region of the central bridging oxygen site for the clusters indicates that this dependence is largely due to electrostatic effects. The effects of geometry relaxation are studied with a constant-volume relaxation method in which a part of the cluster is optimized with the remaining being fixed in its crystal structure. Results indicate that relaxation increases the proton affinity by about 10-15 kcal/mol over the value from the rigid experimental structure. The final calculated proton affinity for the O(24) site in ZSM-5, including correction for zero-point effects, O:Si ratio, and higher level theory, is consistent with experiment. The OH stretching vibrational frequency is in good agreement with the experimental value. 53 refs., 13 figs., 6 tabs.

Hydrogen adsorption and OH desorption at vanadium pentoxide surfaces: ab initio cluster model studies

The adsorption of hydrogen and subsequent removal of OH from the vanadium pentoxide (010) surface (both processes are selected reaction steps in the nucleophilic oxidation of hydrocarbons) are studied by V 2O 9, V 2O 9H, and V 2O 8OH cluster models using the ab initio Hartree—Fock method. Three different oxygen adsorption sites are considered: the vanadyl site, the bridging oxygen site between two surface vanadyl groups, and the bridging oxygen site between two surface vanadium centers. The results suggest that for the vanadyl sites, hydrogen binds to the surface oxygen with a fairly strong bond (3.3 eV from the present calculations) leading to the formation of stable OH surface groups which are bound to the surface vanadium atoms. In contrast, hydrogen approaching the bridging oxygen sites forms an even stronger surface OH bond but that particular surface oxygen becomes mobile so that the surface OH species can desorb with only a small or no activation barrier leaving an oxygen vacancy behind. This suggests that for the nucleophilic oxidation of hydrocarbons at vanadium pentoxide surfaces, it is the bridging oxygen sites rather than the vanadyl sites that are involved in the process of hydrogen abstraction from the organic molecule, the formation of mobile surface OH species, and the subsequent incorporation into the hydrocarbon molecule near the surface.

The Partial Oxidation of Methane to Formaldehyde: Role of Different Crystal Planes of MoO 3

Partial oxidation of methane to formaldehyde was studied over MoO 3 samples which exposed different relative amounts of basal (010) and side (100) plane area. Characterization studies included X-ray diffraction, laser Raman spectroscopy, in situ laser Raman spectroscopy combined with isotopic labeling, scanning electron microscopy, three-dimensional imaging technique, X-ray photoelectron spectroscopy, BET surface area measurement, and temperature-programmed reduction. These characterization results were used in conjunction with steady-state reaction studies using methane, formaldehyde, CO, and methanol as the feed over molybdenum trioxide samples to investigate the catalytic anisotropy previously observed over these catalysts for other reaction systems. These studies suggested that the MoO sites located on the side plane could be promoting the formation of formaldehyde, while the bridging oxygen sites located on the basal planes were more likely to lead to complete oxidation.

Reactions on modified silica surfaces

Molecular dynamic computer simulations have been used to model adsorbate interactions at silica surfaces. Deposition of alkali ions into vSiO 2 and SiO 2− x F x surfaces provided a means to study the effect of varying surface composition on the behavior of adsorbates. Silica surfaces with a range in the concentration of non-bridging oxygen and silica surfaces doped with varying concentrations of fluorine were exposed to the alkali adsorbates. The fluorinated surfaces were less reactive towards the alkali than were the vitreous silica surfaces. Reactivity was also reduced when the non-bridging oxygen concentration was decreased on the silica surfaces and the fluorine concentration was decreased on the fluorine doped surfaces. Non-bridging oxygen sites on the silica surfaces were energetically the most favorable for bonding, followed by fluorine sites on the fluorinated surfaces and bridging oxygen sites on both types of surfaces.

The formation of leached layers on albite surfaces during dissolution under hydrothermal conditions

Hydrothermally altered (225°C) albite was compositionally depth-profiled using X-ray photoelectron spectroscopy (XPS)coupled with calibrated Ar ion sputtering. Solution data were collected during dissolution runs for the same crystals which were spectroscopically analyzed. We found that leached zones depleted in Na, Al, and O develop during the initial, incongruent phase of dissolution. Angle resolved XPS (ARXPS) demonstrated that Na and Al are significantly depleted from the upper few monolayers. Depths of leaching, which range from 10 to 900 Å, decrease with increasing pH in theacid region and increase with pH in the basic region. Based on calculated dissolution rates the depth of leaching can be roughly correlated with the release rate of Si. From the observation that the equivalents of H + consumed always exceed the equivalents of Na + and Al 3+ released, hydrolysis cannot be considered to be a simple ion exchange process. The XPS spectra also revealed the presence of Cl − over the entire leaching depth for samples run at pH < pH zpc (zero point of charge) and Ba 2+ at pH > pH zpc, suggesting electrostatic adsorption of aqueous species at charged sites within the leached layer. The presence of Cl − and Ba 2+ also show that preferential leaching creates a porous and open structure which allows for the large-scale influx of solvent molecules. Preliminary evaluations of diffusion transport rates through leached layers suggest that dissolution is not rate limited by diffusion. Instead, the kinetics of dissolution seem to be related to the intrinsic rate of structural hydrolysis. Using the XPS and solution data in conjunction with theoretical and experimental studies in the literature, we propose a dissolution mechanism based on initial ion exchange followed by the hydrolysis of Al and Si, which is modeled as the breakdown of activated complexes formed at bridging oxygen (O br) sites. Elemental mass balances based on comparisons between the XPS and solution data suggest that dissolution occurs non-uniformly and is probably preferentially constrained to dislocations and macroscopic defects within the structure.