Tetrachlorosilane Consumption in Radio Frequency Glow Discharge

Time-resolved mass spectrometry was used for analysis of the plasma reactions in radio frequency (RF) SiCl ₄ and SiCl ₄ –O ₂ discharges as functions of starting partial pressure and electrical power. Molecular concentrations of the reactants and products from SiCl ₄ alone and with O ₂ were obtained from the mass spectra and used for plotting the kinetic curves. The SiCl ₄ and O ₂ consumption rates were calculated from the kinetic curves and compared with results of theoretical simulation of the reaction. Direct electron impact decomposition was found to be the main pathway for pure SiCl ₄ conversion. On the contrary, the consumption of SiCl ₄ in the SiCl ₄ +O ₂ mixtures was largely chemical. The experimental macrokinetics are in agreement with a model in which oxidation is caused by the atomic oxygen.     

Effect of CH4/Air Ratio on Selectivity of Partial Oxidation of Methane on V2O5/SiO2 Catalyst

We have studied partial oxidation of methane on V₂O₅/SiO₂ (0.8 mass % V) in a flow-through catalytic fixed-bed reactor. We found that the methane/air ratio in the starting reaction mixture has practically no effect on the selectivity of the process. The dependence of the selectivity on the methane conversion can be described by a model with such reaction parameters as the initial selectivity and the relative reactivity (with respect to methane) of the reaction products.     

Kinetic Factors Determining Specific Features of Solid Phase Formation in Dichlorosilane Oxidation

Experimental data on the kinetic regularities of aerosol SiO₂ formation in the course of dichlorosilane oxidation by oxygen at different initial pressures, compositions of the reaction mixture, and temperatures ranging from 380 to 578 K are presented. It is shown that the regularities of the process, including the specific feature of the transition from the regime of solid phase formation in the form of a film to the regime of aerosol formation can be explained on the basis of the Volmer–Weber–Frenkel–Zeldovich nucleation theory taking into account the branched chain nature of the reaction. The conditions for the transition of chain combustion into the regime of chain–thermal explosion almost coincide with the conditions of intensive formation of aerosol. The SF₆ additives inhibit the process and thereby increase the dispersity of aerosol and the minimal pressure of its formation.     

Hochmolekulare Perchlorsiloxane

High-molecular Perchlorosiloxanes The reaction of SiCl₄ and O₂ in the gas phase does not lead spontaneously to the thermodynamically favored products SiO₂ and Cl₂. At temperatures of about 1300 K a lot of intermediates Si_nO_mCl_4n?2m can be isolated. Massspectrometric investigations show the formation of chlorosiloxanes with n = 2…65 and m/n = 0,5…1,7 and molar masses up to 7000 D. These compounds – even the high molecular ones – show typical molecular behaviour: they are solulable in unpolar solvents like hexane and can be sublimed congruently. No indications could be found between the structures of Si? O-frameworks in the siloxanes and solid SiO₂-modifications.     

51V NMR Study of VOCl3Immobilized on the SiO2and MgCl2Surface

The magnetic shielding tensor and quadrupole interaction parameters, as well as the mutual orientation of tensors for the (SiO)VOCl₂complex obtained by the immobilization of VOCl₃on the SiO₂surface are determined. The state of VOCl₃on the surfaces of MgCl₂and modified SiO₂with all surface OH groups replaced by Cl atoms is studied. To prepare the modified SiO₂, CCl₄and SiCl₄are used as chlorinating agents. The formation of structurally similar pentavalent vanadium complexes on the surface of these supports is shown. A model for the coordination environment of vanadium on the chlorine-containing supports is proposed. The vanadium atom exists in the distorted pentahedral environment and is bound to the support through either two chlorine atoms or chlorine and oxygen atoms. A correlation between the coordination of VOCl₃and catalytic properties of VOCl₃/MgCl₂is assumed.     

A kinetic study of soluble glucose oxidase using a rotating-disc electrode

In order to determine the kinetic parameters of glucose oxidation catalysed by the enzyme glucose oxidase (GO) the initial velocity of hydrogen peroxide formation was measured using a rotating disc electrode (RDE). The major advantage of this method is the possibility of continuous measurement of the increase in hydrogen peroxide concentration. This means that the real initial reaction rate V₀ can be determined, which is required for constructing a double-reciprocal plot. Several combinations of substrate concentrations (glucose and oxygen) were used. The method, in which a platinum black RDE was used, appeared to be very useful. Product inhibition experiments showed that the ping-pong mechanism is valid for GO. The three kinetic parameters of this mechanism were determined by initial velocity experiments.     

Untersuchungen an katalytisch aktiven Oberflächenverbindungen. XIII. Struktur und katalytische Eigenschaften von Molybdän-oxid/SiO2-Katalysatoren

Studies on Catalytically Active Surface Compounds. XIII. Structure and Catalytic Properties of Molybdenum Oxide/SiO₂ Catalysts The catalytic properties of Mo oxide/SiO₂ catalysts in the selective oxidation of methanol to formaldehyde are described. It is shown that, independently on the preparation conditions, all catalysts showed relative high activity and selectivity values which were however, not constant during the reaction time. The high initial activity could be stabilized to a limited extent both by prereduction with methanol (but not with hydrogen) and by decreasing the oxygen concentration of the reaction gas. ESR measurements showed that in dependence on the means of reduction (methanol or hydrogen) two different coordinated Mo⁵⁺ ions were formed. Evidence was given by IR spectroscopy that prereduction in methanol caused the formation of methoxy groups stabilizing catalytically active Mo⁵⁺ ions. UV-Vis, ESCA, and electron microscopic measurements showed however, that further aggregation and formation of microcrystallites of MoO₃ took place during the catalytic reaction which caused the observed decrease of the activity.     

Preparation of CuO/SiO<Subscript>2</Subscript> Hollow Spheres for Catalytic Oxidation of Phenol

Porous CuO/SiO₂ hollow microspheres were synthesized via an impregnation method using pure SiO₂ hollow microspheres as the supporter, and Cu species as the functional material. The hollow microspheres were characterized by X-ray diffraction, BET surface area, temperature-programmed reduction, transmission electron microscopy, and scanning electron microscopy. The catalytic activities of the CuO/SiO₂ hollow microspheres were investigated via the removal of the total chemical oxygen demand (COD) in the oxidation of phenol solution with air as an oxidant. The influence of various reaction parameters such as the reaction temperature, the partial pressure of O₂, and the initial pH of the solution were studied in detail. The coordination, dispersion and aggregation degree of copper species on porous materials play an important role for the COD removal of the phenol aqueous solution.     

Effect of pH on the formation and combustion process of sol-gel auto-combustion derived NiZn ferrite/SiO2 composites

A nitrate-citrate-silica gel was prepared from metallic nitrates, citric acid, and silica powder by sol-gel process, and it was further used to synthesize Ni_0.5Zn_0.5Fe₂O₄/5wt% SiO₂ nanocomposites by auto-combustion. The effect of pH on the formation of NiZn ferrite/SiO₂ and thermal properties of gel precursors was studied by XRD, IR, EPR, TGA and DTA techniques. The results revealed that the ratio of the citrate ion to the nitrate ion is directly related to the pH of the solution. The pH in the starting solution affects the combustion process, and then determines the particle size of the as-synthesized powder. The EPR parameters (peak-to-peak linewidth, g factor, and spin number) increased with increasing pH, whereas the spin-spin relaxation time (T₂) decreased. The thermal stability and enthalpy of the decomposition process in air decreased with increasing pH, whereas the enthalpy in nitrogen increased. Moreover, the activation energy (E_a) of thermo-oxidative degradation of the gel precursor at pH 3 was much lower than the gel precursor at pH 5 and 7, and that increased with increasing pH.     

The influence of Kr, CO2, and iso-C4H8 admixtures on the time of the formation of a stable flame front in mixtures of natural gas and isobutylene with oxygen and hydrogen with air under initiation with a spark discharge

High-speed color filming was used to study laminar spherical flame propagation at the initial stage in preliminarily mixed stoichiometric mixtures of natural gas and isobutylene with oxygen containing krypton and carbon dioxide and in hydrogen-air mixtures at atmospheric pressure in a bomb with a constant volume. Under experimental conditions (T ₀ = 298 K, p ₀ = 100 torr, spark discharge energy E ₀ = 0.91 J), the dilution of mixtures with Kr and CO₂ increased the time of formation of a stable flame front by more than 10 times. The introduction of a small chemically active admixture (1.2% isobutylene) into a stoichiometric mixture of hydrogen and air sharply increased the time of formation of a stable flame front, which was evidence of an important role played by the chemical mechanism of the reaction in the formation of the combustion field.     

Vanadium Nitride Films Formed by Rapid Thermal Processing (RTP): Depth Profiles and Interface Reactions Studied by Complementary Analytical Techniques

The nitridation of vanadium films in molecular nitrogen and ammonia using a RTP‐system was investigated. The V films were deposited on silicon substrates covered by 100 nm thermal SiO₂. For a few experiments sapphire substrates were used. Nitride formation at high temperatures (900 and 1100 °C) and interface reactions and diffusion of oxygen out of the SiO₂‐layer into the metal lattice at moderate temperatures (600 and 700 °C) were studied. For characterisation complementary analytical methods were used: X‐ray diffraction (XRD) for phase analysis, secondary neutral mass spectrometry (SNMS) and Rutherford Backscattering (RBS) for acquisition of depth profiles of V, N, O, C and Si, transmission electron microscopy (TEM) in combination with electron energy filtering for imaging element distributions (EFTEM) and recording electron energy loss spectra (EELS) to obtain detailed information about the initial stages of nitride, oxide and oxynitride formation, respectively, and the microstructure and element distributions of the films. In these experiments the SiO₂‐layer acts as diffusion barrier for nitrogen and source for oxygen causing the formation of substoichiometric vanadium oxides and oxynitrides near the V/SiO₂‐interface primarily at temperatures ≤ 900 °C. At a temperature of 1100 °C just a small amount of oxynitride forms near the interface because rapid diffusion of nitrogen and fast formation of VN (diffusion barrier for oxygen) inhibit the outdiffusion of oxygen into the metal layer. In the 600 °C regime, in argon atmosphere oxynitride phases observed in the surface region of these films originate from reaction of residual oxygen in the argon gas, whereas NH₃ as process gas does not lead to oxide or oxynitride formation at the surface (apart from the oxidation caused by storage). NH₃ seems to support the diffusion of oxygen out of the SiO₂‐layer. During the decomposition of ammonia at higher temperatures hydrogen is formed, which could attack the SiO₂. In contrast, sapphire substrates do not act as oxygen source in the 600 °C regime and change the nitridation behaviour of the vanadium films.     

Quantum chemical simulation of silicon tetrachloride hydrogenation

The quantum chemical calculation of the activation parameters of the reduction of a SiCl₄ molecule with molecular hydrogen, atomic hydrogen, and atomic chlorine was performed. The energy parameters were determined within the scope of the density functional theory (DFT) with the complete geometry optimization by the unrestricted UB3LYP/6-311+G(d) method. The calculated activation energies allow one to exclude the participation of molecular hydrogen in processes of dehalogenation of chlorosilanes.     

In-situ UV-Raman study on soot combustion over TiO2 or ZrO2-supported vanadium oxide catalysts

UV-Raman spectroscopy was used to study the molecular structures of TiO₂ or ZrO₂-supported vanadium oxide catalysts. The real time reaction status of soot combustion over these catalysts was detected by in-situ UV-Raman spectroscopy. The results indicate that TiO₂ undergoes a crystalline phase transformation from anatase to rutile phase with the increasing of reaction temperature. However, no obvious phase transformation process is observed for ZrO₂ support. The structures of supported vanadium oxides also depend on the V loading. The vanadium oxide species supported on TiO₂ or ZrO₂ attain monolayer saturation when V loading is equal to 4 (4 is the number of V atoms per 100 support metal ions). Interestingly, this loading ratio (V₄/TiO₂ and V₄/ZrO₂) gave the best catalytic activities for soot combustion reaction on both supports (TiO₂ and ZrO₂). The formation of surface oxygen complexes (SOC) is verified by in-situ UV Raman spectroscopy and the SOC mainly exist as carboxyl groups during soot combustion. The presence of NO in the reaction gas stream can promote the production of SOC.     

Guaiacol hydrodeoxygenation kinetics with catalyst deactivation taken into consideration

The hydrodeoxygenation of guaiacol, a compound modeling bio-oil, has been investigated. Experiments have been carried out in an autoclave at 280, 320, and 360°C and an initial hydrogen pressure of about 17 MPa using a Ni-Cu/SiO₂-ZrO₂-La₂O₃ catalyst. A formal kinetic model of the first stage of guaiacol hydrodeoxygenation is suggested. It is assumed that the reaction is first-order with respect to both guaiacol and hydrogen. The model takes into account the deactivation of the catalyst and the decrease in the concentration of the deactivating compounds due to their hydrogenation. A possible cause of deactivation is the formation of coke and heavy intermediates, as well as the competitive adsorption of intermediate products of guaiacol conversion on the active sites of the catalyst. The parameters with which the model satisfactorily fits the observed dependence of the guaiacol concentration on the reaction time have been determined.     

Gas-phase polymerization of propylene: Reaction kinetics and molecular weight distribution

Gas-phase polymerizations have been executed at different temperatures, pressures, and hydrogen concentrations using Me₂Si[Ind]₂ZrCl₂ / methylaluminoxane / SiO₂(Pennsylvania Quarts) as a catalyst. The reaction rate curves have been described by a kinetic model, which takes into account the initially increasing polymerization rate. The monomer concentration in the polymer has been calculated with the Flory–Huggins equation. The kinetic parameters have been determined by fitting the reaction rate curves with the model. At high temperatures, pressures, and hydrogen concentrations a runaway on particle scale may occur leading to reduced polymer yields. The molecular weight and molecular weight distribution of the polymer samples could be described by a “two-site model.” At constant temperature the chain-transfer probability of sites 1 and 2 depends only on the hydrogen concentration divided by the monomer concentration. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 500–513, 2001     

Participation of molecular hydrogen-originated protonic acid sites in acid-catalyzed reactions

The formation of protons from molecular hydrogen was observed by IR of adsorbed pyridine for Pt/SO₄ ²⁻−ZrO₂ when heated in the presence of hydrogen. The promotive effects of hydrogen on skeletal isomerization of alkanes and cumene cracking are rationalized by the formation of protons from hydrogen. The formation of protons and the promotive effects of hydrogen were also observed for other catalysts such as Co.Mo/SiO₂−Al₂O₃ and a physical mixture of Pt/SiO₂ and H-ZSM-5. The concept “molecular hydrogen-originated protonic acid site” is proposed as an important and widely applicable concept in acid-catalyzed reactions.     

Thermoanalytical study of the solid-state reactions in MgO-Cr2O3 and ZnO-Cr2O3 systems

The solid-state reactions in MgO-Cr₂O₃ and ZnO-Cr₂O₃ systems under atmospheres of oxygen, air and nitrogen were investigated by means of DTA and isothermal kinetic techniques. It was shown that the application of DTA to the solid-state reactions affords useful information with respect to the initial reaction stage: oxidation of Cr₂O₃ to CrO₃ in the presence of MgO or ZnO, followed by the formation of a thin layer of spinel, which consisted of an imperfect lattice, on the surface of the MgO or ZnO grains.     

Palladium species in Pd/H-ZSM-5 zeolite catalysts for CH4-SCR

The active state of palladium for NO reduction with methane (CH₄-SCR) was investigated by comparing the catalytic activity of Pd/H-ZSM-5 with that of PdO/SiO₂. High catalytic activity for CH₄-SCR was given by Pd/H-ZSM-5 in the temperature range of 300–500 °C. PdO/SiO₂ catalyzed the reaction between NO₂ and CH₄ in the absence of oxygen, which retarded the reaction by consuming CH₄ in combustion. CH₄ combustion occurred on either zeolite-supported or silica-supported catalyst, while NO preferentially retarded the combustion on Pd/H-ZSM-5. NO was found to be chemisorbed on the palladium sites in zeolite, while it was hardly chemisorbed on PdO/SiO₂. NaCl titration showed that the palladium species in zeolite are Pd²⁺ cations content, on which NO is strongly chemisorbed resulting in high selectivity for CH₄-SCR.     

Effect of CO<Subscript>2</Subscript> dilution on propane–air isothermal catalytic combustion on platinum

The present work aims to investigate several kinetic aspects of catalytic combustion of the stoichiometric propane–air mixture on a platinum wire in the presence of progressive CO₂ dilution, using a micro-calorimetric method. The method allowed the determination of the induction periods and of the reaction rates during transient and steady-state regimes at different initial pressures P ₀ (10–101 kPa), wire temperatures T _w (470–575 K) and gas-phase composition. The kinetic parameters were evaluated from regression analysis using empirical and mechanistic models. The dilution with CO₂ of the stoichiometric propane–air mixture (4.02% propane) was 10, 20, 30, 40, 50%. For additions equal to or greater than 23%, the gaseous system is outside the flammability limits reported in the literature. The kinetic characteristics of the catalytic combustion remain similar for the whole range, even for 50% CO₂ addition when the propane concentration in mixture diminishes to 2%, a concentration smaller than the lower explosion limit in air in normal conditions.     

The Chemistry of C6H6O radical cations: A study of rearrangement reactions of halogen substituted ethyl phenyl ethers

New experimental data on the rearrangement reaction of various phenoxyethyl halides to give [C₆H₆O]^+˙ are presented and compared with previous studies so that a coherent picture of this process can be developed. By examining the metastable kinetic energy release for low energy decomposing molecular ions of the phenoxyethyl halides, it has been concluded that formation of [C₆H₆O] occurs by competitive 1,2 and 1,3 hydrogen shifts from the alkyl carbons to oxygen followed by a rate determining C? O bond cleavage. This is substantiated by the absence of a primary hydrogen isotope effect. For more highly activated molecular ions, a new mechanism comes into play as evidenced by the appearance of a small hydrogen isotope effect. It is postulated that this third mechanism involves transfer of the alkyl hydrogen to the ortho position of the ring by a rate determining 1,5 shift, followed by a 1,3 hydrogen shift from the ortho methylene group to oxygen and rapid C? O bond cleavage. This 1,3 hydrogen shift to oxygen appears to be ‘catalysed’ by the halogen atoms yielding phenol ions. No indications have been found for the formation of tautomeric 2,4-cyclohexadienone ions. Furthermore, highly activated molecular ions produce [C₆H₆O]^+˙ which can undergo metastable decomposition to lose carbon monoxide. Kinetic energy release measurements for the latter reaction show that the majority of these [C₆H₆O]^+˙ions have been formed as phenol ions as well. These arguments are supported by energetic measurements and by comparisons with previous ion cyclotron resonance and collisional activation studies.