Si Bridges Research Articles

Study on CuO-CeO2/SiC catalysts in the sulfur-iodine cycle for hydrogen production

The structure and activity of ceria–copper oxides supported on nanometer silicon carbide (nano-SiC) particles for sulfuric acid decomposition were studied. The promoting activities and stabilities of CuO–CeO2 complex oxides loaded on nano-SiC grains were prominently higher than those of CuO–CeO2 without carriers, especially at <800 °C. The results of X-ray diffraction (XRD), transmission electron microscopy (TEM), and high resolution transmission electron microscopy (HRTEM) showed that copper-cerium composite oxides grains were well dispersed and fixed on the surface of SiC particle, and the average sizes of CuO–CeO2 complex oxides carried by SiC particles were considerably smaller than those of CuO–CeO2 without carriers. Based on the analysis of HRTEM images, X-ray photoelectron spectroscopy (XPS) spectra, and infrared radiation (IR) pattern, the majority of SiC surfaces were converted to SiO2 layers, in which CeO2 grains immobilized by coordination bonding Ce–O–Si bridges. From the temperature programmed reduction (TPR) patterns, compared with those of CuO–CeO2 without carrier, the reduction temperature of CuO to Cu2O in CuO–CeO2/SiC catalysts was lowered, especially at (Ce + Cu)/SiC atomic ratio of 5 mol.%. The catalysts also showed relatively high activities at 727 °C for 20 h of continuous operation. A mechanism of SO3 decomposition on CuO–CeO2/SiC was proposed according to the characterization results. Copyright © 2016 John Wiley & Sons, Ltd.

Investigation of bromine atom transfer mechanism from an alkyl bromide molecule to an O-bonded alkyl group in a FAU zeolite by the ONIOM method

The details of the bromine atom transfer mechanism from an ethyl bromide molecule to an O-bonded alkyl group, attached to the FAU zeolite wall, were elucidated, by means of Quantum Mechanical and Molecular Mechanical (QM/MM) calculations. The investigation of this mechanism has been established on the 84T model cluster of faujasite zeolite with the help of the ONIOM approach, utilizing three-layer ONIOM3(B3LYP/6-31 + G(d,p):HF/6-31G:UFF), ONIOM3(M06-2X/6-31 + G(d,p):HF/6-31G:UFF) and ONIOM3(MP2(full)/6-31 + G(d,p):HF/6-31G:UFF) schemes. Our results indicate that the reaction proceeds via a ring structure formation path, where the ethyl bromide molecule is strongly attracted to both the alkyl group and the nearby Si bridging lattice O atom. Interaction energies for the O-bonded alkyl (ethyl or isopropyl) group linked to an ethyl bromide molecule in a FAU pore according to the ONIOM3(MP2(full)/6-31+G(d,p):HF/6-31G:UFF) calculation are approximately −18.3 and −19.3 kcal mol−1, respectively. In addition, at the same level of theory, the bromine atom shift from the ethyl bromide molecule to the adjacent O-bonded ethyl or isopropyl group, forming a new alkyl bromide and a new ethyl radical, are characterized by an activation barrier of 25.1 and 17.9 kcal mol−1, respectively, relative to the reagent complex.

Intrinsic flexibility of porous materials; theory, modelling and the flexibility window of the EMT zeolite framework.

Framework materials have structures containing strongly bonded polyhedral groups of atoms connected through their vertices. Typically the energy cost for variations of the inter-polyhedral geometry is much less than the cost of distortions of the polyhedra themselves - as in the case of silicates, where the geometry of the SiO4 tetrahedral group is much more strongly constrained than the Si-O-Si bridging angle. As a result, framework materials frequently display intrinsic flexibility, and their dynamic and static properties are strongly influenced by low-energy collective motions of the polyhedra. Insight into these motions can be obtained in reciprocal space through the `rigid unit mode' (RUM) model, and in real-space through template-based geometric simulations. We briefly review the framework flexibility phenomena in energy-relevant materials, including ionic conductors, perovskites and zeolites. In particular we examine the `flexibility window' phenomenon in zeolites and present novel results on the flexibility window of the EMT framework, which shed light on the role of structure-directing agents. Our key finding is that the crown ether, despite its steric bulk, does not limit the geometric flexibility of the framework.

Direct incorporation of B, Al, and Ga into medium-pore ITH zeolite: Synthesis, acidic, and catalytic properties

Direct crystallization of B-, Al- and Ga-substituted medium-pore ITH zeolites was carried out using hexamethonium hydroxide (HMH) and N,N,N′,N′,-tetramethyl-1,6-hexanediamine (TMHDA) as structure-directing agents. The type of organic SDA was found determining the crystal size and the nature of Brønsted acid sites in isomorphously substituted ITH zeolites. The use of HMH resulted in formation of tiny (length of 0.5–3μm) needle-like ITH crystals having bridging Si(OH+)E(−) (E=B, Al, Ga) groups with acidic strength increasing in the following sequence B<Ga<Al. In contrast, ITH zeolites with bulky (length of 40–50μm) crystals prepared using TMHDA possess loosely bond framework and [(SiO)3AlOH]−H+ and (SiO)2BOH groups (absorption band at 3670cm−1) perturbing upon interaction with pyridine. B-, Al- and Ga-ITH possessing tiny crystals showed improved catalytic performance in tetrahydropyranylation of 1-hexanol in comparison with non-acidic heteroelement-free germanosilicate zeolites and isomorphously substituted ITH zeolites with bulky crystals. The yield of target ether is enhanced with increasing strength of acid sites (e.g. B-<Ga-<Al-ITH) or with growth in their concentration.

Synthesis and physicochemical properties of methoxy-substituted diphenyldihydroacridine and its Si and Ge bridged analogues and corresponding nitroxide radical derivatives

Three dihydroacridine analogues, 2,7-dimethoxy-9,10-dihydro-9,9-diphenylacridine (1-C), 2,8-dimethoxy-10,10-diphenyl-5,10-dihydrophenazasiline (1-Si), and 2,8-dimethoxy-10,10-diphenyl-5,10-dihydrophenazagermine (1-Ge), and corresponding nitroxide radical derivatives, 2,7-dimethoxy-9,10-dihydro-9,9-diphenylacridine-10-oxyl (2-C), 2,8-dimethoxy-10,10-diphenyl-5,10-dihydrophenazasiline-5-oxyl (2-Si), and 2,8-dimethoxy-10,10-diphenyl-5,10-dihydrophenazagermine-5-oxyl (2-Ge) were synthesized in order to study the effect of the Si and Ge bridging atoms on the physicochemical properties of the diaryl nitroxide radical derivatives. These derivatives were stable in non-halogenated solvents and in the solid state. Their structures were crystallographically analyzed and UV–vis, fluorescence, EPR, and cyclic voltammetry measurements were performed to investigate the physicochemical properties of the dihydroacridine analogues and nitroxide radical derivatives.

Structural characterization of heavy metal SiO2–Bi2O3 glasses and glass–ceramics

29Si MAS NMR spectroscopic data obtained for heavy metal SiO2–Bi2O3 glasses and partially crystallized samples of similar composition were used to investigate the changes induced in their local structure as the ratio between Bi3+ and Si4+ cations decreases from 18:1 to 6:7. Progressive increase of SiO2/Bi2O3 ratio contributes to the structural relaxation of vitreous network and enhances the glass stability. In glass samples with high silica content the increasing number of Si–O–Si bridges proves the three-dimensional polymerization of silicate network. The shape of NMR signals for glass–ceramic samples depends on the crystalline phases formed for different SiO2/Bi2O3 ratios. The crystalline phases developed by heat treatment of vitreous matrices were identified by X-ray diffraction as Bi12SiO20, Bi5.6Si0.5O9.4 and Bi2SiO5. Scanning electron microscopy analysis was used to examine the morphology of the crystals developed in the glassy silica rich matrices.

Delocalization of the atom in glass and its liquids

In the developed two-level model, the atom delocalization in silicate glass represents a shift of the bonded oxygen atom in the Si—O—Si bridge resulting in a local low-activation deformation of the siliconoxygen network.

Preparation of SiOC/HfO2 fibers from silicon alkoxides and tetrachloride hafnium by a sol–gel process

This study presents fabrication and characterization of novel SiOC/HfO2 fibers by a sol–gel process. Polyhafnosiloxane (PHfSO) fibers have been drawn from silicon alkoxide solution consisting of tetraethoxylsilane (TEOS) and dimethyldiethoxylsilane (DMDES) using tetrachloride hafnium (HfCl4) as hafnium source. The spinnability of the solution has been optimized by varying the amount of HfCl4 and H2O. The Hf incorporation is confirmed by the formation of Hf–O–Si bridge through co-condensation of HfCl4, TEOS and DEDES. Subsequent pyrolysis treatment of the PHfSO gel fibers yields dense SiOC/HfO2 fibers with high ceramic yield of 81wt%. The SiOC/HfO2 fibers exhibit good thermal stability up to 1500°C and mechanical property with tensile strength of 930MPa.

Theoretical study on crystal structures, elastic stiffness, and intrinsic thermal conductivities of β-, γ-, and δ-Y2Si2O7

Abstract

Crystal structure analyses of two TMA silicates with ordered defects: RUB-20, a layered zeolite precursor, and RUB-22, a microporous framework silicate

Abstract Synthesis experiments in the simple system TMAOH/SiO2/H2O led at 160°C to the crystallization of two new tetramethylammonium (TMA) silicates, RUB-20 and RUB-22. The common feature of the two silicates is the presence of ordered defects within the silicate network, necessary to compensate the charge of the occluded TMA cation. RUB-20, [(CH3)4N]1.7[Si18O35(OH)4], is a layered material with a0 = 10.493(2) Å, b0 = 13.988(2) Å, c0 = 7.411(1) Å, β = 97.80(2)° and space group P21/m. It consists of ferrierite type (fer) silicate layers which possess a significant amount of defects. These defects apply only to one particular Si–O–Si bridge which is hydrolyzed to about 50% to form two additional Si–OH/Si–O- groups which point to the interlayer region of the structure. These silanol groups together with the regular terminal silanol groups of neighbouring fer layers form a system of hydrogen bonds which binds the layers together. In addition, weak ionic bonds exist between the negatively charged silicate layers and the intercalated TMA cations. RUB-22 is a zeolite-like silicate possessing a microporous framework with double-cages, housing the TMA cations. The Rietveld structure analysis revealed a pseudo monoclinic unit cell with a0 = 13.6626(2) Å, b0 = 13.2400(2) Å, c0 = 12.3009(2) Å, α = 90.003(3)°, β = 112.700(1)°, γ = 90.005(3) and space group C 1 ¯ . $C\bar 1.$ RUB-22, [(CH3)4N]4[Si32O60(OH)12], possesses an interrupted silica framework of the same topology as RUB-10, [(CH3)4N]4[Si32B4O72] (zeolite framework type RUT), however, with the difference that defects replace the [BO4]-tetrahedra. The defects can be regarded as “hydroxyl nests” surrounding a vacancy □(OH-Si)3(-O-Si) instead of B(-O-Si)4 groups. Heating the as made materials at 600°C led in the case of RUB-20 to a highly disordered zeolite of the CDO type and in the case of RUB-22 to a distorted and defective RUT-type material.

Brønsted and Lewis acid sites of Sn-beta zeolite, in combination with the borate salt, catalyze the epimerization of glucose: A density functional theory study

Sn-beta zeolite, in combination with borate salts, is a potential inorganic catalyst for sugars epimerization. We investigate, at molecular level, the catalytic mechanism of glucose epimerization to mannose, using density functional theory. Our calculations suggest that the tetrahedral borate ion forms a complex with glucose and inhibits the competitive isomerization reaction. The Lewis-acidic stannanol group of Sn-beta catalyzes glucose ring opening, which is followed by the silanol group (Brønsted acid site) catalyzed enolization. The epimerization then proceeds via an intramolecular 1,2 carbon shift and is found to be the rate-limiting step with an activation enthalpy of 26.3kcal/mol. Catalytic activities of different tetravalent metal centers are compared, and Sn is found to be the most active metal. Additionally, it was found that the proximity of silanol group to the stannanol group, within the zeolitic framework, plays a key role in enhancing the catalytic activity of the silanol group. Hence, it is crucial to perform calculations with the entire ring structure of Sn-beta that opens up due to the hydrolysis of Sn–O–Si bridge.

Raman study of α-quartz-type Ge1−xSixO2 (0 &lt; x ≤ 0.067) single crystals for piezoelectric applications

Raman lines due to Ge–O–Si bridges in α-Ge1−xSixO2 single crystals are identified with the help of polarized Raman and calculations.

Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces

Static friction between amorphous silica surfaces with a varying number of interfacial siloxane (Si–O–Si) bridges was studied using molecular dynamic simulations. Static friction was found to increase linearly with the applied normal pressure, which can be explained in the framework of Prandlt–Tomlinson’s model. Friction force was found to increase with concentration of siloxane bridges, but with a decreasing gradient, with the latter being due to interactions between neighboring siloxane bridges. In addition, we identified atomic-level wear mechanisms of silica. These mechanisms include both transfer of individual atoms accompanied by breaking interfacial siloxane bridges and transfer of atomic cluster initialized by rupturing of surface Si–O bonds. Our simulations showed that small clusters are continually formed and dissolved at the sliding interface, which plays an important role in wear at silica/silica interface.

NMR Evidence for Specific Germanium Siting in IM-12 Zeolite

Ge-rich ITQ-13 and ITQ-22 zeolites were degermanated by acid leaching of as-made—still containing organic template—and calcined forms. These two zeolites could be almost completely degermanated without modifying the framework structure. This behavior is different from IM-12 zeolite, which undergoes framework transformation from UTL to OKO type upon degermanation. All three zeolite types contain double four-ring (D4R) units populated by germanium atoms. The UTL to OKO transformation involves the elimination of Germanate 4 rings (Ge 4R) from double four-ring (D4R) units connecting the layers. The differences of Si and Ge atom siting in D4R units in the three zeolites were probed using 19F MAS NMR after post synthetic incorporation of fluoride and 1H–29Si CP/MAS NMR. In IM-12, NMR suggests that Ge atoms are all located in one of the faces of D4R units and form 4Rs that connect Si-rich layers together. In the degermanation process, the 4 Ge atoms of the Ge 4Rs are dislodged simultaneously, in excellent agreement with earlier EXAFS data. In ITQ-13 and ITQ-22 zeolites, Ge distributions are more complex, leading a progressive degermanation of the framework and to the presence of Si–O–Si bridges that prevents the structure from collapsing upon Ge extraction. Data suggest that IM-12 is unique in the family of Ge-containing zeolites.

Optical and structural studies of CaMoO4:Sm, CaMoO4:Sm@CaMoO4 and CaMoO4:Sm@CaMoO4@SiO2 core–shell nanoparticles

A simple polyol and Stober surface coating process was introduced to synthesize CaMoO4:Sm and their silica-coated CaMoO4:Sm nanoparticles. Powder X-ray diffraction (XRD) and different spectroscopic techniques were employed to investigate the structural and optical properties of as-prepared nanoparticles. The surface of CaMoO4:Sm nanoparticles was modified to alter the solubility of the nanoparticles and to study the influence of surface coating on emission spectra of samarium ions. Optical properties of these nanoparticles showed a strong dependence on the surface modification of nanoparticles, indicating the influence of coating in the enhancement of luminescent intensity. These results were attributed to the formation of chemical bonds between CaMoO4:Sm@ CaMoO4 core and non-crystalline SiO2 shell via Mo–O–Si bridges, which activate the ‘dormant’ Sm3+ ion on the surfaces of nanoparticles. It is, therefore, concluded that good hydrophilicity resulting from active functional groups in solutions and more intense luminescence enable doped core–shell nanoparticles to have great potential to be used as fluorescence biolabels in the future.

Co-catalytic enhancement of H2 production by SiO2 nanoparticles

SiO2 nanoparticles, of varying size 9–55nm, show significant co-catalytic activity for H2-production from HCOOH by the homogenous FeII/P(CH2CH2PPh2)3 catalyst. Particle size and specific surface area are shown to play key-role in this phenomenon. Larger nanoparticles have better co-catalytic effect. Arrhenius analysis reveals that the significant co-catalytic effect can is attributed to a lowering of the activation energy of the rate-limiting step by ΔEa=16–36kJ/mol. This phenomenon is attributed to a thermodynamic promotion of HCOOH deprotonation by the SiO2 nanoparticles, accelerating in this way coordination of HCOO− on the FeII atom of active catalyst, during catalysis. The surface Si–O–Si bridges are shown to be responsible for the promotion of this HCOOH deprotonation, while surface-adsorbed H2O retards the co-catalytic efficiency.

Structural analysis of the interface of silicon nanocrystals embedded in a Si3N4 matrix

The structure and interface states of thin nanocomposite layers containing Si nanocrystals embedded in an amorphous nitride matrix have been analyzed by Raman spectroscopy and x-ray photoelectron spectroscopy (XPS). The Si 2p core-level spectrum of the nanocomposite layer was deconvoluted by using five Gaussian–Lorentzian contributions corresponding to the different nitride states of Si. It was shown that the Si-ncs/Si3N4 interfaces formed during annealing are composed of a high density of Si2+ subnitrides. These subnitrides are more likely to be responsible for the shoulder at 494 cm−1 on the Raman spectra. Considering the proportion of Si2+ subnitrides with respect to Si0 states, an abrupt transition from the crystalline to the amorphous phase due to Si2=N–Si bridge bonds is suggested.

Pseudomorphic replacement of diopside during interaction with (Ni,Mg)Cl2 aqueous solutions: Implications for the Ni-enrichment mechanism in talc- and serpentine-type phases

A hydrothermal experimental study of diopside interaction with (Ni,Mg)Cl2 aqueous solutions has been carried out to clarify the replacement mechanism and pattern of element mobilization and its relevance to peridotite alteration. Three different solution compositions were used with Ni/Mg ratios of 0, 0.5, and 1, respectively. Experiments were carried out in cold-seal pressure-vessels at 300–600°C and 100MPa pressure for 15days in gold capsules. For 600°C NiCl2 experiments, the solutions were also spiked with 50% H218O to study the behavior of isotopic exchange and hence the replacement mechanism. The diopside–pseudomorph interface is compositionally and structurally sharp. After the MgCl2 experiments pure talc and/or serpentine comprised the replacement phases, whereas reaction in the Ni-bearing fluids produced talc–willemseite and/or lizardite–nepouite series compositions. Additionally, a complex rim of Ni-poor and Ni-rich regions developed in the NiCl2 and (Ni,Mg)Cl2 experiments. Raman spectroscopy of the Ni-rich talc from the experiment with 18O-enriched solution showed a shift of ~10±1cm−1 for the SiO(bridging)Si band towards lower wavenumbers relative to unspiked solution experiments, indicating that 18O was incorporated into the silicate framework; thus the reaction progressed via an interface-coupled dissolution-precipitation mechanism. Overgrowths of an olivine-type phase at 500–600°C and serpentine-type phases in 300–400°C experiments were also observed. For a simple binary composition (e.g. Ni–Mg) of the interacting solution, the fluid composition (different Ni/Mg ratio) at the reacting interface and its silica activity has a strong control on the phase precipitated. The sequential formation of a Ni-poor inner phase and Ni-rich outer phase indicates a stepwise increase in Ni incorporation within the neo-formed phase that is representative of a cyclic dissolution-precipitation process during supergene enrichment of Ni in natural ore deposits. Additionally, Ca transport occurs in the opposite direction to Ni during alteration, which may be important as a geochemical tracer for economic Ni-deposits and as a source of Ca for the rodingitization processes that are often associated with peridotite occurrences.

Transformation of silicate gels during heat treatment in air and in argon – Spectroscopic studies

The sol–gel method offers the possibility of obtaining silicate materials with different chemical compositions. When using TEOS or other organic precursor to silica capable of hydrolysis and poly-condensation, it is possible to use inorganic or organic precursors to produce other ingredients.This paper presents results of studying two series of silicate sols with the addition of calcium, in which the molar ratio of calcium to silicon was Ca/Si=x/(100−x), where x was, respectively, 0–40 (x=0–control sample). The resulting gels were subjected to heat treatment, wherein the heating was carried out simultaneously in air or in argon. To study the various stages of transformation of the gels, IR spectroscopy was used as the main research method to observe gradual disappearance of bands associated with bonds typical of organic materials and formation of bands characteristic of Si–O–Si bridging bonds.Due to the amorphous or fine crystalline nature of the resulting material, as confirmed in XRD studies, the different bands on the IR spectra were characterized by large full width at half maximum, hence an attempt was made to decompose the spectra into component bands. The analytic parameters of the resulting bands warranted the conclusion that there had been structural changes caused by the varying synthesis parameters.A comparison of the sol spectra after heat treatment in air and in argon at different temperatures showed a clear decrease in the full width at half maximum in the case of bands of samples with calcium content x⩾30. The resulting spectra were compared with spectra of crystalline tobermorite, quartz and pseudowollastonite, which suggested the possibility of existence of areas with quartz-like ordering in the case of materials with calcium content x⩾20 for the samples heated in argon and areas with pseudowollastonite-like ordering in the case of materials with calcium content x⩾10 for the samples heated in air atmosphere. The conclusions drawn on the basis of infrared spectra were confirmed by XRD – prolonged heating of gels at 700°C allowed us to obtain fine quartz and pseudowollastonite.

Density functional theory investigations into the structure and spectroscopic properties of the Ti4+ species in Ti-MWW zeolite

The location and structure of the framework Ti4+ species in Ti-MWW zeolites were studied by density functional theory (DFT) based on the cluster models mimicking the Ti-MWW zeolite with a Si/Ti ratio of 35. The geometry, spectroscopic properties and energies of substitution for the Ti4+ species at different T sites were investigated. The results indicated that the Ti4+ species in form of Ti(OSi)4 prefers to locate at the T1 and T3 sites, positioning on the edge of 12-membered ring cavity and the intersection of the 10-membered ring intralayer channels in MWW, respectively. In their calculated infrared (IR) spectra, the vibrational mode at 960cm−1 band is actually a collective vibration of the antisymmetric stretching of Ti(–O–Si)4 in a deformed tetrahedral geometry. The totally symmetric Ti(OSi)4 tetrahedron at the Ti4 site is absent from the 960cm−1 band because the vibrational mode is infrared inactive. Hydrolysis of a Ti–O–Si bridge was thermodynamically favorable and resulted in a tripodal defect Ti species with an inversed Ti–OH group and a remaining Si–OH in the adjacent T site. The calculated IR spectra of the tripodal Ti species have a blue shift in the characteristic IR band to 990–1000cm−1. The UV–vis spectra of the Ti4+ species were calculated by the time-dependent DFT method. The calculated IR and UV–vis spectra are in very good agreement with the experimental measurements, and provide microscopic features for the identification of the structure of framework titanium in Ti-MWW zeolites.