Thermodynamic Evaluation of Solid-State Reactions in which a Volatile Product is Formed

The use of equilibrium thermodynamics in describing interfacial reactions between non-ionic inorganic solids is demonstrated using examples of high-temperature interactions in the Ti–Si–N and Mo–Si–N systems. In the case of a diffusion-controlled process, solid-state reactions can be interpreted with chemical potential (activity) diagrams. The role of volatile reaction products formed during interaction in developing the diffusion zone morphology is analysed. The interfacial phenomena in systems based on dense Si₃N₄ and non-nitride forming metals can be explained by assuming a nitrogen pressure build-up at the contact surface. This pressure determines the chemical potential of Si at the interface and, hence, the reaction products in the diffusion zone.     

Hydridosilazanes hydrolysis-condensation reactions studied by <Superscript>1</Superscript>H and <Superscript>29</Superscript>Si liquid NMR spectroscopy

Hydridosilazane compounds containing Si–N and Si–H bonds can be used as precursors of SiO_x materials. The hydrolysis-condensation reactions of tetramethyldisilazane, as a polyhydridosilazane model compound, were investigated by ¹H and ²⁹Si liquid NMR spectroscopy. These reactions were carried out at room temperature for up to 120 min in presence of water. The identified products are short linear siloxane species (hydride terminated polydimethylsiloxanes M^HD_xM^H) and cyclosiloxanes. Silicon hydride persistence in the reactional mixture suggested that silazane group is more sensitive to hydrolysis reaction than silicon hydride group. Moreover, additional experiments evidenced that the low steric hindrance of the silicon hydride influences the silazane hydrolysis kinetic. Hence the presence of ammonia released during silazane hydrolysis reaction was demonstrated to be a catalyst of the silicon hydride hydrolysis reaction.     

Two-Temperature Two-Dimensional Non Chemical Equilibrium Modeling of Ar–CO<Subscript>2</Subscript>–H<Subscript>2</Subscript> Induction Thermal Plasmas at Atmospheric Pressure

Here the authors developed a two-dimensional two-temperature chemical non-equilibrium (2T-NCE) model of Ar–CO₂–H₂ inductively coupled thermal plasmas (ICTP) around atmospheric pressure (760 torr). Assuming 22 different particles in this model and by solving mass conservation equations for each particle, considering diffusion, convection and net production terms resulting from 198 chemical reactions, chemical non-equilibrium effects were taken into account. Species density of each particle or simply particle composition was also derived from the mass conservation equation of each one taking the non chemical equilibrium effect into account. Transport and thermodynamic properties of Ar–CO₂–H₂ thermal plasmas were self-consistently calculated using the first order approximation of the Chapman–Enskog method at each iteration point implementing the local particle composition and temperature. Calculations at reduced pressure (500 and 300 torr) were also done to investigate the effect of pressure on non-equilibrium condition. Results obtained by the present model were compared with results from one temperature chemical equilibrium (1T-CE) model, one-temperature chemically non equilibrium (1T-NCE) model and finally with 2T-NCE model of Ar–N₂–H₂ plasmas. Investigation shows that consideration of non-chemical equilibrium causes the plasma volume radially wider than CE model due to particle diffusion. At low pressure with same input power, presence of diffusion is relatively stronger than at high pressure. Comparison of present reactive model with non-reactive Ar–N₂–H₂ plasmas shows that maximum temperature reaches higher in reactive C–H–O molecular system than non-reactive plasmas due to extra contribution of reaction heat.     

Quantitative nitrogen analysis by Auger electron spectrometry and glow discharge optical emission spectrometry

Nitrides of refractory metals are investigated as diffusion barriers for Cu metallization. The composition, thermal stability and inter diffusion in layered systems are characterized by depth profile analysis. For the quantification of depth profiles determination of sensitivity factors is essential. For nitrogen and other light elements matrix specific standards are often not available and compound standards are used for calibration. We have investigated the systems Ta–N and Ta–Si–N and for comparison Cr–N by means of Auger electron spectrometry (AES) and glow discharge optical emission spectrometry (GDOES). A non-linear calibration curve for the N/Cr intensity ratio was observed with GDOES in the Cr–N-system, probably caused by self-absorption of the Cr line.     

Impact of Transition Metal Substituents on Polysilane Properties: Iron versus Ruthenium

Summary. The previously unknown ruthenio disilanes Rp–Si₂Me₄–C₆H₄X (Rp = η⁵-C₅H₅Ru(CO)₂; X = H, Br, –CHO, CH=C(CN)₂) were synthesized from ClSi₂Me₄C₆H₄X (X = H, Br) and Rp⁻ using conventional chemical methods. Trends in the UV/Vis absorption spectra indicate strong electronic coupling within the Rp–Si–Si–C_aryl fragment and, therefore, closely resemble the ones observed for the corresponding iron complexes. The four compounds however, were shown to be less sensitive towards UV irradiation. The crystal structure of Rp–Si₂Me₄–C₆H₄CH=C(CN)₂ was determined by X-ray diffraction and exhibits an all-trans-array of the Ru–Si–Si–C_aryl moiety, what is a basic requirement for optimal through-bond interaction.     

Structural and thermal properties of monolithic silica–phosphate (SiO<Subscript>2</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>) gel glasses prepared by sol–gel technique

A sol–gel process for producing monolithic silica–phosphate (SiO₂–P₂O₅) system different concentrations of P₂O₅, starting with tetra-ethoxysilane TEOS, and triethyl-phosphate as sources of SiO₂ and P₂O₅ was performed. The gels were heat-treated at temperatures ranging from 100 up to 900 °C. The structural and chemical analyses of the samples were determined by using X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). It was found from the XRD that the existence of phosphorus enhances the crystallization of silica gel, while the FTIR indicated the main functional groups of silica–phosphate. It is important to study the effect of hydroxyl in silica–phosphate glass. The results obtained are promising to use the prepared samples in a variety of applications, ranging from traditional application such as lighting products) to the modern application (such as optical fibers. Optical studies were measured by using the spectrophotometer in wavelength range 0.2–2.5 μm. The refractive index (n) was calculated for the prepared samples, it was found to be strongly affected by structural rearrangement resulting from the elimination of the solvent and the Si–OH, Si–O–Si and Si–O–OH bonding by phosphate and aluminum and it increases by increasing phosphate concentrations. The weight losses have investigated for prepared samples.     

Ab initio study on the mechanism of cycloaddition reaction between H<Subscript>2</Subscript>Si=Si: and formaldehyde

The mechanism of the cycloaddition reaction between singlet H₂Si=Si: and formaldehyde has been investigated with the CCSD(T)//MP2/6-31G* method. From the potential energy profile, it could be predicted that the reaction has three competitive dominant reaction pathways. The reaction rules presented is that the 3p unoccupied orbital of the Si: atom in H₂Si=Si: inserts the π orbital of formaldehyde from the oxygen side, resulting in the formation of an intermediate. Isomerization of the intermediate further generates a four-membered ring silylene (the H₂Si–O in the opposite position). In addition, the [2+2] cycloaddition reaction of the two π-bonds in H₂Si=Si: and formaldehyde also generates another four-membered ring silylene (the H₂Si–O in the syn-position). Because of the unsaturated property of the Si: atom in the two four-membered ring silylenes, the two four-membered ring silylenes could further react with formaldehyde, generating two silicic bis-heterocyclic compounds. Simultaneously, the ring strain of the four-membered ring silylene (the H₂Si–O in the syn-position) makes it isomerize to a twisted four-membered ring product.     

Non‐catalytic,gas‐solid reversible reaction mechanism implementing diffuse interface model

Gas‐solid reactions in chemical and metallurgical industries often involve solid pellets and a gaseous reactant. The progress of chemical reaction is measured by the movement of zones within the pellet and has been explained in terms of diffusion and chemical reaction processes. Earlier models identified a single reaction zone, in addition to product layer and unreacted core. In the present article, two reaction zones are envisaged as a more plausible explanation of the movement of the zones as the reaction proceeds. Earlier models for reversible reactions have assumed that conditions at the interface between the reaction zone and the unreacted core correspond to equilibrium at the prevailing temperature. The gaseous concentrations were assumed to permeate the core at the interfacial values so that no reaction occured in the core. More realistically, the present article envisages an additional zone within which the gaseous concentrations fall from the equilibrium values to zero. It is assumed that in the reaction zone proper, referred to as zone I, having thickness z_I, the concentration profile is sigmoidal. This agrees with the earlier work of Khan and Bowen [1] and Prasannan and Doraswamy. [2] In zone I and the concentration of the reactant gas varies between [A_i] and [A*]. In the zone II, having thickness z₂, concentration varies linearly between [A*] and zero. This model has been applied successfully to the data of the reduction of hematite [3] at different temperatures. The contribution of different forms of resistance, diffusion in product layer, chemical reaction and diffusion in the reactant core, is assessed as function of time (start to the end of reaction). The thickness of the zones remain almost constant as the reaction progresses. In particular, the influences of the product and core diffusion coefficients and chemical equilibrium constant on the extant reaction are evaluated. The dependence of concentration profile and zone thickness on equilibrium constant, K, velocity constant, k, diffusional coefficients D_C and D_P has been investigated thoroughly. The thickness of both zones has been evaluated for leading variables. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 559–570, 1999     

The potential energy surface of the triazadiphosphole formation

The formal GaCl₃-assisted [3+2] cycloaddition of two (Me₃Si)₂N–N(SiMe₃)–PCl₂ molecules resulting in the formation of a triazadiphosphole has been studied by means of B3LYP/6-31G(d,p) computations. These calculations revealed a stepwise reaction mechanism starting from the disguised 1,3-dipole and dipolarophile (Me₃Si)₂N–N(SiMe₃)–PCl₂. Comparison of the potential energy surface for the formation of a triazadiphosphole in the presence and without a Lewis acid indicate, that addition of a Lewis acid such as GaCl₃ decreases the activation barriers to Me₃Si–Cl elimination, in accord with experiment. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Sol–gel silica hybrid biomaterials for application in biodegradation of toxic compounds

The purpose of the present work is the sol–gel synthesis, structure characterization and potential application of hybrid biomaterials based on silica precursor (MTES) and natural polymers such as gelatin or pectin. The structure formation in the biomaterials was investigated by XRD, FTIR, BET and AFM. The results showed that all studied hybrid biomaterials have an amorphous structure. The FT-IR spectra of the obtained materials with MTES showed chemical bonds at 2,975, 1,255, 880 and 690 cm⁻¹ due to the presence of Si–O–R (CH₃ and C₂H₅) and Si–C bonds. In the samples synthesized with TEOS the inorganic and organic components interact by hydrogen bonding, Van der Waals or electrostatic forces. Surface area of investigated samples decreases with increasing of the natural polymers content. The structure evolution was studied by AFM and roughness analysis. Depending on the chemical composition a different design and size of particles and their aggregates on the surface structure were established. The hybrid biomaterials were used for immobilization of bacterial cells and applied in the biodegradation of the toxic compound 4-chlorobutyronitrile, possible constituent of waste water effluents in a laboratory glass bioreactor. Optimization of the process at different temperatures was carried out.     

Synthesis and Reactivity of Stannyloligosilanes IV [1]: From Hydrido Substituted Stannasilanes Towards Stannasiloxanes

Summary.  Hydrido substituted stannasilanes of the type or (Z = H, Me, Ph; R, R′ = alkyl, Ph) are accessible by reaction of either alkali metal stannides (MSn(Z)R ₂; M = Li, Na) with halogen substituted silanes (; X = F, Cl) or chlorostannanes (R ₂SnCl₂, Ph₃SnCl) and fluorosilanes in the presence of magnesium. Stannasilanes with halogen substituents at the silicon as well as the tin atom are formed by treatment of the hydrido substituted stannasilanes with CHCl₃ or CCl₄. The hydrido substituted stannasilanes decompose in contact with air to distannanes and siloxanes or to the linear ( ^t Bu₂Sn(–O– ^t Bu₂Si–OH)₂) and cyclic ((– ^t Bu₂Sn–O– ⁱ Pr₂Si–O–)₂) stannasiloxanes. Received November 29, 2001. Accepted (revised) January 16, 2002     

The “supercompressed” state of matter as an investigation object in high-pressure chemistry

The prospects of a new line of research in high-pressure chemistry are discussed, which is associated with investigations of chemical reactions accompanied by large volume effects at ambient pressure and, hence, resulting in greater compressibilities of the reaction products. The compounds formed by intercalation of alkali metals in graphite and intermetallic compounds formed between alkali metals and silver or Group V metals (Sb, Bi), which crystallize with the BiF₃ structural type, are considered as examples of “chemical compression”. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1442–1447, August, 1999.     

Synthesis and Structural Characterization of a New Macrocyclic Polysiloxane-immobilized Ligand System

Summary. A new porous solid macrocyclic 1,4,7,11,14-pentaazapentadecane-3,15-dione polysiloxane ligand system of the general formula P–(CH₂)₃–C₁₁H₂₂O₂N₅ (where P represents [Si–O]_n siloxane network) has been prepared by the reaction of polysiloxane-immobilized iminobis(N-(2-aminoethyl)acetamide) with 1,3-dibromopropane. The FTIR and XPS results confirm the introduction of the macrocyclic functional ligand group into the polysiloxane network. The new macrocyclic polysiloxane ligand system exhibits high potential for the uptake of metal ions (Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺ and Zn²⁺).     

In situ FTIRS studies of the electrocatalytic oxidation of ethanol on Pt alloy electrodes

Ethanol oxidation on Pt–Os and Pt–Ru–Os alloy electrodes was investigated by electrochemical and spectroelectrochemical techniques. Cyclic voltammetry and chronoamperometric results showed that the Pt–Os alloy has the highest current density at lower potentials. Linear CO_ads, acetic acid, acetaldehyde, and CO₂ were identified as reaction intermediates and/or products by single potential alteration infrared reflectance spectroscopy and subtractively normalized interfacial Fourier transform infrared reflectance spectroscopy techniques. The in situ Fourier transform infrared spectroscopy results showed that the electrooxidative adsorption of ethanol was dissociative providing CO_ads at low potentials. Dedicated to our friend Professor Francisco Carlos Nart (in memorium), IQSC-USP, Brazil.     

Accessing HgSe<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>S<Subscript><Emphasis Type="Italic">1−x</Emphasis></Subscript> Nanoparticles Using the Single-Source Reagent Me<Subscript>3</Subscript>Si–SeS–SiMe<Subscript>3</Subscript>

Mercury-selenosulfide (HgSe _x S _1-x ) nanoparticles have been synthesized using the single-source reagent Me₃Si–SeS–SiMe₃. The reagent distributes Se²⁻ and S²⁻ to the metal core as the reaction between Me₃Si–SeS–SiMe₃ and mercury acetate occurs via a redox pathway, ultimately giving rise to Se–S bond cleavage. Particles are characterized by EDX, TEM and powder X-ray diffraction analysis in conjunction with UV–Visible absorption spectroscopy. Dedicated to Prof. Dr. Dieter Fenske on the occasion of his 65th birthday.     

Preparation, characterization and application of KNH2/Al2O3 and KF/Al2O3 as strong solid bases

This review describes the preparation, characterization and application of KNH₂ loaded on alumina and KF loaded on alumina. These strong solid bases catalyze a variety of organic reactions in a very selective manner. The reactions include isomerizations of alkenes and alkynes, dimerization of alkynes, Tishchenko reaction, and the reaction of silanes to form of Si–C, Si–N and Si–O bonds.     

Effect of oxygen nonstoichiometry on kinetics of oxygen exchange and diffusion in lanthanum-strontium cobaltites

The method of isotopic exchange was used to study the kinetics of oxygen exchange and diffusion in complex oxides of La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} (x = 0.0, y = 0.0; x = 0.6, y = 0.2, 0.4). The rates of oxygen interfacial exchange and its diffusion coefficient were determined for LaCoO_{3 − δ} at the pressure of 5 torr in the temperature range of 600–850°C and at the temperature of 700°C in the pressure range of 1–70 torr. The contributions of the three exchange types were calculated. The order of the dependence of the interfacial exchange rate on the oxygen pressure was 0.51 ± 0.01. In the case of La_0.4Sr_0.6Co_{1 − y} Fe _y O_{3 − δ} (y = 0.2, 0.4) in the temperature range of 600–900°C at the oxygen pressure of 10 torr, the oxygen exchange rates and diffusion coefficients were determined in the material bulk and in the subsurface region; contributions of the three types of exchange were calculated. The paper considers the mechanism of oxygen exchange and diffusion as compared to nonstoichiometry in the oxygen sublattice of the La_{1 − x} Sr _x Co_{1 − y} Fe _y O_{3 − δ} oxides.     

Synthesis,characterization, and non-isothermal curing kinetics of two silicon-containing arylacetylenic monomers

Vinyltri(phenylethynyl)silane ((ph–C≡C)₃–Si–C=CH₂; VTPES) and phenyltri(phenylethynyl)silane ((ph–C≡C)₃–Si–ph; PTPES) were synthesized by Grignard reaction. Their molecular structures were characterized by means of ¹H NMR, ¹³C NMR, ²⁹Si NMR, and FT-IR spectroscopy. Their nonisothermal thermal curing processes were characterized by DSC, and the corresponding kinetic data, for example activation energy (E), pre-exponential factor (A), and the order of the reaction (n), were obtained by the Kissinger method. The results showed that the melting points of VTPES and PTPES were 84 and 116 °C, respectively. Their curing reaction rates were consistent with first-order kinetic equations. VTPES monomer had a lower activation energy and curing temperature as a result of coordination between reactive groups.     

The role of the negatively charged (Si3−Si−F2)− complexes in interactions of fluorine with the (111) surface of silicon

We used the AM1 quantum chemical and cluster models to study the mechanism of formation of a SiF₂-like layer and dissociation of the Si−Si bond during the interaction of atomic fluorine with the (111) surface of silicon. It is shown that the negatively charged (Si₃−Si−F₂)⁻ complex with the five-coordinated centered silicon atom plays an important part in these processes. The above complex participates in the interaction of atomic fluorine with silicon to form a SiF₂-like layer and break the subsurface Si−Si bonds without penetration of fluorine atoms into the subsurface silicon layers. Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 1, pp. 14–21, January–February, 1996. Translated by I. Izvekova     

Selective extraction of sulfonamides from food by use of silica-coated molecularly imprinted polymer nanospheres

We report the use of nanospheres prepared by coating silica with molecularly imprinted polymer (MIP) for sulfamethoxazole (SMO). The resulting SiO₂–SMO–MIP nanoparticles have highly improved imprinting and adsorption capacity, and can be used for separation and determination of sulfonamides in eggs and milk. In the synthesis, monodispersed SiO₂ nanoparticles (Si–NP) of diameter 80 nm were amino-modified by reaction with 3-aminopropyltriethoxylsilane. The acryloyl monolayer was then grafted onto the amine-modified Si–NP. Finally, the MIP films were coated on to the surface of Si–NP by the copolymerization of the vinyl end groups with functional monomer (acrylamide), cross-linking agent (ethylene glycol dimethacrylate), initiator (azo-bis-isobutyronitrile), and template molecule (sulfamethoxazole). The resulting SiO₂–SMO–MIP–NP were characterized by transmission electron microscopy, scanning electron microscopy, and Fourier transform infrared spectrometry. The adsorption properties were demonstrated by equilibrium rebinding experiments and Scatchard analysis. The results showed that the binding sites of the SiO₂–SMO–MIP were highly accessible, and the maximum adsorption capacity of the SiO₂–SMO–MIP for SMO was 20.21 mg g⁻¹. The selectivity of the SiO₂–SMO–MIP–NP obtained was elucidated by using SMO and structurally related sulfonamides. The results indicated that the SiO₂–SMO–MIP had significant selectivity for SMO. The feasibility of removing SMO and sulfadiazine (SDZ) from food samples was proved by use of spiked milk and eggs. A method for the separation and determination of trace SMO and SDZ in milk and egg samples was developed, with recoveries ranging from 69.8% to 89.1%.