Function Of Gas Phase Composition Research Articles

Light gas adsorption of all-silica DDR- and MFI-type zeolite: computational and experimental investigation

Grand canonical Monte Carlo simulations (GCMC) of adsorption of CO2 and N2 were carried out using the software Materials Studio 4.1 on all-silica DDR- and MFI-type zeolite. The simulated values of sorption capacity, isosteric heat of adsorption and Henry's constant for all-silica DDR- and MFI-type zeolite showed good agreement with experimental data. Accurate pure and binary adsorption data were measured for all-silica DDR-type zeolite crystals. Furthermore, the GCMC simulation was used to assess the adsorption properties and selectivity of CO2/N2 mixture gas as a function of gas phase composition and pressure. Thermodynamic properties and binary mixtures of adsorption showed good agreement with our simulation and experiments.

A transient in situ FTIR and XANES study of CO oxidation over Pt/Al 2O 3 catalysts

We report experimental results for the oxidation of CO over supported Pt/Al 2O 3 catalysts operating in oxygen excess at atmospheric pressure. To study the reaction kinetics under transient conditions we have employed step changes of the O 2 concentration by intermittently switching off the O 2 supply at various temperatures ranging from 523 to 623 K. Detailed in situ FTIR and XANES data for CO coverage and the chemical state of Pt, respectively, are presented together with the CO conversion, which in both cases was monitored by mass spectrometry. A red-shift of the vibrational frequency of linearly bonded CO which correlates with a blue-shift of the Pt L III binding energy indicates that the Pt catalyst initially is partially oxidised and gradually reduced when the O 2 supply is switched off. Control experiments with a NO 2 oxidised Pt/Al 2O 3 catalyst support these findings. A hysteresis in the catalytic activity due to the different rates whereby Pt is oxidised and reduced as a function of gas-phase composition is observed. The activation energy for the Pt oxide reduction (decomposition) process is estimated to be about 50 kJ/mol. The results further emphasise that the conventional three-step Langmuir–Hinshelwood (LH) scheme used to interpret CO oxidation on Pt surfaces must be complemented by a Pt oxidation and reduction mechanism during transient conditions. Moreover, FTIR data suggest that during the extinction, the partially oxidised platinum surface is reduced by chemisorbed CO which should be explicitly accounted for in the modeling of the reaction mechanism.

Structural and optical properties of nitrogen and oxygen doped a-C:H coatings

The deposition and characterization of hydrogenated amorphous carbon films (a-C:H) obtained by radio-frequency plasma enhanced chemical vapor deposition on silicon and on polycarbonate are reported. Small amounts of N 2 and O 2 were added to a CH 4–He gas mixture in order to promote the doping of the films. A systematic study of the coating structure and optical and mechanical properties as a function of gas phase composition and applied radio-frequency power is also reported. X-ray photoelectron spectroscopy (XPS) was used to quantify the amount of nitrogen and oxygen incorporation, while the characterization of sp 2 carbon bonding was performed by Raman spectroscopy. Ellipsometry and UV–Vis–NIR transmission measurements were exploited to relate the refractive index and absorption of light in the visible region to the film composition and the deposition process parameters. The associated variation of the coating hardness was checked by nanoindentation measurements. The results obtained show that oxygen incorporation allows the achievement of an increased film transparency in the visible spectral range, while nitrogen dilution of the methane precursor in the plasma is responsible for the deposition of films that have a higher absorbance in the visible, due to an increased size of graphitic domains. The addition of both doping elements in a small percentage allows the preservation of the characteristic high hardness of a-C:H coatings.

AC impedance study of Ni–YSZ cermet anodes in methane-fuelled internal reforming YSZ fuel cells

The impedance characteristics of the Ni–YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic overpotential in methane-fuelled internal reforming YSZ fuel cells. AC impedance measurements carried out under open-circuit conditions in the temperature range 800–900 °C showed the presence of at least two rate-limiting processes which were influenced by temperature and gas phase composition. The apparent polarization resistance, R app, of the Ni–YSZ cermet anode/YSZ interface was found to increase with increasing methane partial pressure and decreasing H 2O partial pressure. The apparent capacitance, C app, calculated from AC impedance data under open-circuit conditions was found to decrease with increasing methane partial pressure. AC impedance measurements carried out under closed-circuit conditions at 900 °C and four different P H 2O showed the appearance of a low-frequency inductive loop at high anodic overpotentials, in addition to the features observed under open-circuit conditions. The effect of gas phase composition on R app was similar to the one observed under open-circuit conditions. The present results are compared with results presented in literature and discussed in the frame of the mechanism of the electrochemical and catalytic reactions taking place over the anode.

Polarization behavior of Ni-YSZ cermet anodes in YSZ fuel cells running on methane under internal reforming conditions

The polarization characteristics of the Ni-YSZ cermet anode/YSZ interface were studied as a function of gas phase composition, temperature and anodic overpotential in methane fuelled internal reforming YSZ fuel cells. AC impedance measurements carried out under open circuit conditions in the temperature range 800 to 900°C showed the presence of at least two rate-limiting processes that were influenced by temperature and gas phase composition. The apparent polarization resistance, Rapp, of the Ni-YSZ cermet anode/YSZ interface was found to increase with increasing methane partial pressure and decreasing H2O partial pressure. The activation energy related to the apparent polarization conductance R app −1 varied between 0.7 and 1.1 eV, mainly due to the variation of PH 2 O. The order of magnitude of the apparent capacitance, Capp, values calculated from AC impedance data under open circuit conditions indicate that the observed capacitance corresponds to the double layer capacitance of the Ni/YSZ interface and/or a pseudo-capacitance related to adsorption of species on the Ni-YSZ cermet electrode accompanied by charge transfer. Capp was found to decrease with increasing methane partial pressure. AC impedance measurements carried out under closed circuit conditions at 900 °C and four different PH 2 O showed the appearance of a low-frequency inductive loop at high anodic over-potentials, in addition to the features observed under open circuit conditions. The effect of gas phase composition on Rapp was similar to the one observed under open circuit conditions. In addition an almost exponential decrease of Rapp with increasing anodic overpotential was observed. The present results are compared with results presented in literature and discussed in the frame of the mechanism of the electrochemical and catalytic reactions taking place over the anode.

Adsorption of CH4−CF4 Mixtures in Silicalite: Simulation, Experiment, and Theory

Grand canonical Monte Carlo (GCMC) simulations of binary Lennard-Jones mixtures in the zeolite silicalite have been used to predict the adsorption of CH4 and CF4 mixtures as a function of gas phase composition, total pressure, and temperature. For single components and mixtures, predictions of adsorption isotherms and isosteric heats are in good agreement with experiment at room temperature. Within the experimental pressure range of 0 to 17 bar, the mixtures are well described by the ideal adsorbed solution (IAS) theory. For very high loading, deviations from IAS theory appear. The configurations generated in the simulation were used to calculate sorbate−zeolite interaction energy distributions for different types of siting locations within the zeolite pores. These distributions display a pore shape related energetic heterogeneity in different regions of silicalite. Near saturation at a total loading of 12 molecules per unit cell, the shape of the observed energy distribution is relatively independent of ...

Adsorption equilibria of methane and tetrafluoromethane and their binary mixtures on silicalite

Binary mixture adsorption isotherms of CH 4 and CF 4 on silicalite have been measured for different constant gas compositions at 298 K in the pressure range up to 1.7 MPa using a high-pressure microbalance. Additionally, the single-component adsorption isotherms were determined at 273, 298 and 323 K in the same pressure range. The gravimetric method proposed by Van Ness has been applied to obtain the compositions of the adsorbed phase as a function of gas-phase composition and pressure. The experimental mixture adsorption equilibria are compared with predictions of the ideal adsorbed solution (IAS) theory and the multisite Langmuir (MSL) model based on the single-component adsorption isotherms only. Despite the fact that the molecules of CH 4 and CF 4 differ greatly in size, ideal behaviour of their adsorbate mixtures on silicalite is found over a large pressure range. This result is due to the high surface homogeneity as well as the channel-like pore geometry of silicalite, where interactions between the adsorbate molecules can only occur to a small extent. Thus, both the IAS theory and the MSL model achieve a good description of the experimental data.

Effect of competitive adsorption on polar stratospheric cloud reactions

Existing laboratory data for reaction rates on ice and nitric acid trihydrate are interpreted in terms of a simple, speculative mechanism for these reactions. The mechanism assumes that the solid phase is either pure H2O or pure HNO3(H2O)3 and that the state of the surface depends largely on the gas phase composition. This dependence is described by Langmuir adsorption isotherms. The adsorption of a nonreactive species can affect the rate of reaction by inhibiting the adsorption of reactive species. In particular, differences in reaction rates observed on ice and HNO3(H2O)3 are attributed to the adsorption of gas phase HNO3 produced by the decomposition of HNO3(H2O)3. The interpretation of heterogeneous reaction rates requires detailed measurements of the rates as a function of gas phase composition.

In SituFTIR Investigations Of Polymer Surface Modification In Downstream Microwave Plasma Etching

AbstractIn situFourier transform infrared (FTIR) reflection-absorption spectroscopy investigations of etching of thin polyimide and poly(methyl methacrylate) films (200–1500Å) in downstream microwave NF3/O2/Ar plasmas are reported. Etch rates and surface chemistry are monitored as a function of gas phase composition, plasma treatment conditions and time. NF3/Ar plasma treatment leads to significant surface fluorination characterized by the formation of aliphatic fluorine compounds (CFx), acyl fluorides, benzoyl fluoride, and polyfluorinated benzene. Addition of oxygen to the etching gas reduces the degree of surface fluorination and modifies the chemical structure. The absorption bands due to CFxstructures decrease gradually while polyfluorinated benzene rings and benzoyl fluoride are absent for NF3/O2mixtures with more than 20% oxygen. The effect of humidity on the plasma-modified polymers is studied by comparing infrared spectra collectedin situwith those after air exposure. For NF3/O2plasma-treated polyimides significant changes are observed while samples fluorinated in NF3show no changes after exposure to air overnight. The FTIR data are supplemented by XPS analysis.

In Situ Ftir Investigations of Polymer Surface Modification in Downstream Microwave Plasma Etching

AbstractIn situ Fourier transform infrared (FTIR) reflection-absorption spectroscopy investigations of etching of thin polyimide and poly(methyl methacrylate) films (200-1500Å) in downstream microwave NF3/O2/Ar plasmas are reported. Etch rates and surface chemistry are monitored as a function of gas phase composition, plasma treatment conditions and time. NF3/Ar plasma treatment leads to significant surface fluorination characterized by the formation of aliphatic fluorine compounds (CFx), acyl fluorides, benzoyl fluoride, and polyfluorinated benzene. Addition of oxygen to the etching gas reduces the degree of surface fluorination and modifies the chemical structure. The absorption bands due to CFx structures decrease gradually while polyfluorinated benzene rings and benzoyl fluoride are absent for NF3/O2 mixtures with more than 20% oxygen. The effect of humidity on the plasma-modified polymers is studied by comparing infrared spectra collected in situ with those after air exposure. For NF3/O2 plasma-treated polyimides significant changes are observed while samples fluorinated in NF3 show no changes after exposure to air overnight. The FTIR data are supplemented by XPS analysis.

The characterisation of Ga 1− xIn xAs,Al 1− xIn xAs and InP epitaxial layers prepared by metal organic chemical vapour deposition

Epitaxial layers of Ga 1− x In x As, Al 1− x In x As and InP have been prepared in an atmospheric pressure metal organic chemical vapour deposition (MOCVD) system, using trimethyl group III sources and group V hydrides. The characteristics of the layers have been studied as a function of gas phase composition and growth temperature. Elemental analyses of the layers have been carried out using Auger bevel profiling and secondary ion mass spectrometry (SIMS) sputter profiling. Alloy composition and crystal quality have been studied using X-ray diffraction. The optical properties of the layers have been assessed by photoluminescence and their electrical transport characteristics by C - V measurements, electrochemical profiling and Hall Van der Pauw measurements.

The growth and characterization of uniform Ga1-xInxAs (X ≤.25) by Organometallic VPE

Compositionally uniform Ga1-xInxAs epitaxial layers with 0 ≤ x ≤ 0.2 5 have been grown by organometallic VPE on ◃100▹ GaAs substrates. Compositional uniformities of ±0.25 InAs percentage and ±5% thickness over a 3-cm long wafer have been achieved and are essentially independent of small changes in reactor geometry such as the angle of the susceptor tilt or wafer position on the susceptor. Growth has also been demonstrated at x = 0.52. The Ga1-xInxAs is grown using trimethylgallium (TMGa), triethylindium (TEIn) or trimethylindium (TMIn), and trimethylarsenic (TMAs). The use of TMAs eliminates the roomtemperature gas-phase reaction between AsH3 and either TEIn or TMIn, and allows one atmospnere pressure growth conditions to be used without any special mixing arrangements in the reactor. The comparative effects of using TEIn or TMIn as the In source are discussed in terms of crystal quality. Data on crystal composition as a function of gas phase composition and growth rate as a function of composition are presented, and n doping and carrier mobilities and p doping of Ga.80In.20As ars characterized. The vapor pressure of TMIn at 0°C is determined to be 0.21 mmHg.

The Nucleation of CVD Silicon on SiO2 and Si3 N 4 Substrates: I . The System at High Temperatures

The grain size in polycrystalline silicon layers is subject to various external conditions among which the initial nucleation of silicon on the substrate may play a decisive role. This article describes experiments with silicon on and substrates in the system for temperatures between 925° and 1200°C. The saturation cluster densities are determined as a function of gas phase composition and temperature and could be varied between 104–1011 cm−2. The experimental results are compared with predictions of existing nucleation theories using analysis of the monomer silicon adatom concentration. In this way the nucleation can be described satisfactorily and approximate values for the size of the critical cluster can be derived.

Crystal growth of boron monophosphide using a B 2H 6-PH 3-H 2 system

Boron monosphide (BP) with (100) orientation can be epitaxially grown on Si substrates with (100) orientation by thermal decomposition of B 2H 6 and PH 3 in hydrogen. In a horizontal CVD system, the growth rate was studied as a function of gas phase composition and temperature. The growth rate is independent of the PH 3 partial pressure in the region where the PH 3 is in excess. For low values of the B 2H 6 partial pressure the growth rate is proportional to the B 2H 6 partial pressure (a linear region) with an activation energy of 1.8 eV, and for high values of the B 2H 6 partial pressure the growth rate becomes constant (a saturation region) with an acivation energy of 3.0 eV. A simple adsorption-reaction model can be proposed to explain the experimental growth kinetics. The conductivity of the as-grown layer is determined by the PH 3 partial pressure. n-type BP can be obtained for high values of the PH 3 partial pressure and p-type BP for low values. Si doping during the growth and phosphorus anti-site donors are two possibilities to explain the results.

Hydrogen transfer steps in the catalytic hydrogenolysis of cyclohexane

The rate of the isotopic exchange reaction: cyclo- ∗C 6H 12(g) + C 6(g) → cyclo-C 6H 12(g) + ∗C 6(g) (1) was studied over metal and metal oxide catalysts. C 6 included: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. The catalysts tested were: Pt supported on acidified and nonacidified Al 2O 3, Cr 2O 3, Re-Cr 2O 3, Cr 2O 3-Al 2O 3 and Re-Al 2O 3,. The temperature range was 230 to 427 °C. The rate of reaction (1) was studied as a function of the ratio p c 6 p CHA in the range 3 × 10 −2 to 1 × 10 2. Since the reaction steps underlying reaction (1) include hydrogen transfer and rearrangement of the molecular structure, the experimental results have been analyzed in terms of these steps and information on their rate and thermodynamic and stoichiometric characteristics has been derived. The analysis suggested a mechanism for the transfer step that is influenced by catalyst acidity and hydro-carbon structure. Models for the reactive surface layer in metal and metal oxide catalysts are put forward and employed to indicate possibilities for the active sites at the surface. The models are also useful to interpret the observed phenomena of inversion in surface reactivity as a function of gas-phase composition. Finally, the experimental results and theoretical deductions are viewed in the framework of present information on the hydrogenolysis of cycloalkanes and isomerization of C 6 paraffins. The advantages of reaction (1) for fundamental studies on the hydroconversion of cycloalkanes are pointed out.