Torr H2 Research Articles

Turnover Rate and Kinetic Mechanism for the Reaction of Hydrodechlorination of 1,1-dichlorotetrafluoroethane (CF3-CFCl2) over a Polycrystalline Pd Foil

The kinetics (turnover rate, activation energy, reaction order) of catalytic hydrodechlorination of 1,1-dichlorotetrafluoroethane or CFC 114a (CF3-CFCl2) were determined on a ~ 0.5 cm2 palladium polycrystalline foil as a catalyst. The reaction rates were measured in the temperature range of 80-200°C at a total pressure of 770 Torr. Using a reaction cell that was connected to an ultrahigh vacuum surface characterization chamber the composition of the metal surface was monitored before and after reaction. The products formed in a parallel reaction network and the turnover rates at 150°C, 50 Torr CFC 114a, 100 Torr H2, and 0.1 Torr HCl were 2.1 s-1 for CF3-CFH2, 3.0× 10-1 s-1 for CF3-CFClH, and 6.4×10-2 s-1 for CF3-CH3. For the two most abundant products, the reaction order is 1 in CF3-CFCl2, 0.5 in H2 and -1 in the reaction product HCl. These results suggest that the rate determining step for the reaction is the associative chemisorption of CF3-CFCl2 on the palladium surface. The palladium surface was free of adsorbates after the reaction with the exception of sulfur impurity that may accumulate during reaction.

Double Graded-Gap Hydrogenated Amorphous Silicon Carbide Thin-Film Light-Emitting Diode with Composition-Graded N Layer and Carbon-Increasing P Layer

An hydrogenated amorphous silicon carbide (a-SiC:H) p-i-n thin-film light-emitting diode (TFLED), in which a composition-graded n layer and carbon-increasing p layer respectively replace the constant optical-gap p-a-SiC:H layer and n-a-SiC:H layer employed in the previously reported double graded-gap (DG) TFLED and which contains dopant-graded p-i and i-n junctions, was successfully fabricated to improve the electroluminescence (EL) of TFLED. This device had a brightness of 400 cd/m2 at an injection current density of 600 mA/cm2 and its EL threshold voltage (V th) was only 9.9 V. The device EL spectrum showed a peak at 586 nm wavelength and emitted yellowish-orange light. The proposed TFLED had good stability of EL intensity and the EL spectrum during normal operation. The optimum conditions of rapid thermal annealing (RTA), which improved the ohmic contact between the amorphous layer and external electrode of the device, were 5 min at 300°C in a 250 Torr H2 ambient.

A simple, quantitative method for measuring pulsed soft x rays

A simple quantitative image processing system for pulsed soft x rays with time and spatial resolution is described. The system consists of easily obtained commercial components including a microchannel plate–phosphor screen combination, a charge coupled device camera, an image memory board, and a personal computer. To make a quantitative measurement possible, the image processing is used in conjunction with measurement of the ‘‘phosphor screen current.’’ Performance of the system is examined using a soft x-ray source generated with a plasma focus device. The order of intensity, ∼1012 photons/s mm2 mrad2 0.01% BW of Ar K lines is observed for about 20 ns from the pinched plasma in which 3% of Ar is added to the 6 Torr H2.

A model of Pt-Rh catalyst prepared by electro-chemical deposition of Rh ions on Pt(100) surface

When Pt-Rh(100) alloy surface was exposed to NO or O2 at temperatures higher than 400 K, a characteristic p(3 × 1) LEED pattern appeared with segregation of Rh atoms on the surface. It was shown that when a Rh-deposited Pt(100) surface is heated in NO or O2 the same 5 × 10−8 Torr of H2 at room temperature, the p(3 × 1) pattern disappears but is readily recovered 5 × 10−8 Torr of H2 at room temperature, the p(3 × 1) pattern disappears but is readily recovered by exposing to O2 of 1 × 10−7 Torr at room temperature. The growth of a Rh-O overlayer on Pt layer is responsible for the formation of the p(3 × 1) structure on either the Pt-Rh(100) alloy or Pt(100) surface, and this peculiar surface may correspond to the active surface of the Pt-Rh catalyst for NOx reduction

Time-resolved electric-field measurements in 30 kHz hydrogen discharges by optical emission Stark spectroscopy

The temporal behavior of the cathode sheath in 30 kHz 0.4–1 Torr H2 discharges has been investigated by optical emission spectroscopy. Analysis of the Stark splitting of plasma-induced H Balmer delta emission was used to measure the electric field with spatial and temporal resolution in the instantaneous cathode sheath. The location of the plasma/sheath boundary was determined from the position of the maximum of the H2 d 3Πu→a 3Σg+ (0,0) Q1 emission at 622.5 nm. Both methods showed that the sheath width increases as the cathode voltage becomes more negative, whereas the width remains constant as the applied voltage drops off. Analysis of the electric-field profile provided information on the time evolution of the ion density close to the electrode during the cathode half-cycle, in agreement with recent numerical calculations. At the beginning of the anodic half-cycle an intense flash of plasma-induced emission was observed, localized within 3 mm from the electrode.

Theoretical interpretation of the kinetics and mechanisms of the HNO + HNO and HNO + 2NO reactions with a unified model

AbstractPreviously measured decay rates of HNO in the presence of NO have been kinetically modeled on the basis of thermochemical data calculated with the BAC‐MP4 technique. The results of this modeling, aided by TST‐RRKM calculations for the association of HNO and the isomerization, decomposition, and stabilization of the many dimers of HNO, reveal that the decay of HNO under NO‐lean conditions occurs primarily by association forming cis‐ and trans‐(HNO)2 at temperatures below 420 K. N2O, which is a relatively minor product, is believed to be formed by H2O elimination from cis‐HON  NOH, a product of succesive isomerization reactions: trans‐(HNO)2† → HN(OH)NO† → HN(O)NOH† → cis‐HON NOH†. The calculated rate constants, which fit experimental data quantitatively, can be represented by k = 1016.2 × T−2.40e−590/T cm3/mol sec for the HNO recombination reaction and k = 10−2.44T3.98e−600/T cm3/mol sec for N2O formation in the temperature range 80–420 K, at a total pressure of 710 torr H2 or He.Under NO‐rich conditions, HNO reacts predominantly by the exothermic termolecular reaction, HNO + 2NO → HN(NO)ONO → HN NO + NO2, with a rate contant of (6 ± 1) × 109 cm6/mol2 sec at room temperature, based on both HNO decay and NO2 production. All existing thermal kinetic data on HNO + HNO and HNO + 2NO processes can be satisfactorily rationalized with a unified model based on the thermochemical data obtained by BAC‐MP4 calculations.

Role of hydrogen in synchrotron-radiation-stimulated evaporation of amorphous SiO2 and microcrystalline Si

We report on the photostimulated evaporation of amorphous SiO2 and microcrystalline Si by synchrotron radiation in ultrahigh vacuum and in H2 ambients up to 0.08 Torr. For a-SiO2, the evaporation was slowed by introduction of H2, which suggests that the highly efficient decomposition process is hindered by hydrogen termination (-OH species formation). For μc-Si, the evaporation rate in ultrahigh vacuum was strongly affected by the degree of crystallization. The evaporation rate doubled with introduction of 0.08 Torr H2. Hydrogenation of the surface (SiHx species formation) slightly reduces the activation barrier for evaporation.

Spectroscopic evidence for the formation of CHx species in the hydrogenation of carbidic carbon on Ni(100)

The CH x species formed on Ni(100) by hydrogenation of carbidic carbon have been detected using high resolution electron energy-loss spectroscopy (HREELS). Exposures of carbidic carbon to 1×10−7 Torr H2 and D2 at 313 K for 20 min produce CH x and CD x species, respectively. These species are identified by two energy-loss peaks for CH x at 2970 and 1380 cm−1 and only one peak for CD x at 1980 cm−1. Because of the existence of the intense peak at 1380 cm−1, in the range of a scissors mode for CH2 and a symmetric deformation mode for CH3, the CH x species are tentatively ascribed to CH2 and/or CH3. The CD x species undergo decomposition at 330–370 K in UHV as well as in hydrogen below 10−7 Torr. No stable CH x species are observed above 400 K, which is lower than the normal reaction temperature of the methanation reaction (500 K).

Energy requirements for the dissociative adsorption of hydrogen on Cu(110)

The activation energy for the dissociative adsorption of H2 on Cu(110) is determined to be 14.3±1.4 kcal/mol (95% confidence limit). This value represents a true Arrhenius activation energy from Boltzmann distributions of impinging H2 gas at ∼623 K. The results were obtained by titrating atomic oxygen from a Cu(110) surface under conditions where the rate of titration was equal to the rate of dissociative hydrogen adsorption and where the oxygen coverage did not affect the rate. The hot copper surface was exposed to ∼10−4 Torr H2 in a microreactor whose walls were at 300 K. The degree to which the H2 translational, rotational, and vibrational populations approach the surface temperature was controlled by adding N2 as an energy transfer medium. The dissociative adsorption probability at 623 K increased from ∼5×10−7 to ∼10−5 when the N2 pressure was increased from 0 to ∼80 Torr. About 70% of this increase occurred below 2 Torr. Known gas‐phase energy transfer rates suggest that the translational temperature of H2 hitting the Cu surface was already at the surface temperature after addition of only 2 Torr N2, but that the v=1 vibrational population was still unchanged. It approached the population of the surface temperature only at 80–300 Torr N2. These results together with beam experiments suggest either translational or vibrational energy can dominate in overcoming the activation barrier to dissociation of an H2 molecule.

Zeolite encapsulated Pd(salen), a selective hydrogenation catalyst

Pd(salen) encapsulated in zeolites X and Y is shown to be a catalyst for the selective hydrogenation of hex-1-ene in the presence of cyclohexene at 25 °C and 400 Torr H2.

Chemical and electronic properties of Pd/SnO2(110) model gas sensors

AbstractA model gas sensor system, Pd/SnO2(110), has been investigated using XPS, UPS, ISS and 4‐point conductivity measurements. The growth of Pd and its chemical interaction with the SnO2 surface were studied in the 0.1–10.0 monolayer equivalent (ML) regime. It was found that the Pd overlayer grows by clustering at 300 K and that the deposited Pd is primarily metallic by 3 ML. Heating Pd‐covered surfaces up to 800 K causes a decrease in the XPS Pd/Sn 3d intensity ratio, possibly due to further clustering of the overlayer above room temperature. Annealing did not, however, change the local metallic nature of the deposited Pd. The gas sensing behavior of these model interfaces was investigated by measuring the sheet conductance at different hydrogen pressures. The increase in conductance for a 3 ML Pd/SnO2(110) surface was 16 times greater than that for the Pd‐free surface at 10−5 Torr H2. This enhancement by Pd is duscussed in terms of a ‘spillover’ effect.

Reflection extended x‐ray absorption fine‐structure measurements on Ni/C and NixSiy/C multilayered reflection coatings

A new method is described to measure bond lengths in the interface between thin layers, by using multilayered reflection coatings both as a dispersive element in an x‐ray beam, and as an object of measurement. Extended x‐ray absorption fine‐structure (EXAFS) oscillations can be observed in the reflected spectrum. From multilayered Ni/C and NixSiy/C reflection coatings mounted in a double‐crystal monochromator we determined, from these oscillations above the Ni LII,III edge, bond lengths equal to 1.8, 2.1, and 2.3 A, which can be assigned to Ni–C, Ni–Si, and Ni–Ni, respectively. NixSiy/C coatings deposited in a 10−6 Torr H2 atmosphere show better resolution than those without. The difference is ascribed to H2 preventing the formation of silicides and the diffusion of the Ni into the carbon; this results in more amorphous layers. A new and simple method is described for obtaining, in the EXAFS analysis, a high‐resolution radial distribution function (RDF) from a small k‐vector space chi function. This metho...

Infrared Fourier transform spectroscopy of XeH

The infrared emission spectrum of the Rydberg molecule XeH was recorded with the Kitt Peak Fourier transform spectrometer. XeH was made in a hollow cathode discharge of 2·2 torr H2 and 0·1 torr Xe. The 0-0 bands of the C 2Π-B 2Σ+ and the D 2Σ+-C 2Π transitions were observed near 3250 cm−1 and 4420 cm−1, respectively. A rotational analysis provides spectroscopic constants for these states.

Kinetic analysis of surface reduction in transition metal oxide single crystals

Abstract A mathematical treatment of the kinetics of metal oxide reduction is presented that is particularly applicable to single crystal substrates under conditions which promote slow, orderly removal of lattice oxygen at the crystal surface. The crystal is modeled as a semi-infinite plane with oxygen removal at the surface only and with Fickian diffusion from subsurface oxide into the depleted surface region. Removal of oxygen at the surface has been related to oxide surface concentration, as well as to oxygen vacancy concentration in an attempt to model the commonly observed induction period to reduction onset. Solutions to the models are given in closed analytic form for zeroth and first-order oxygen surface concentration dependencies and as an infinite, but converging, series in reaction time for the first and higher reaction orders. A numerical method is also used to extend the fit to longer times for which the infinite series method converges too slowly to be practically useful. As an example of the relevance of the kinetic analysis, the model is applied to data obtained on a NiO(100) substrate reduced under 1.0×10−7 to 1.3×10−6 Torr H2.

Beam properties of a high-perveance single-aperture dc extraction system

In continuous operation a positive hydrogen ion beam up to 240 mA has been extracted at 50 kV out of a single aperture, corresponding to a perveance of 2.1×10−8 A/V3/2. The effective mass of the extracted ion beam was measured to be about 1.8 amu. The current density distribution of the extracted ion beam was measured with a small calorimetric probe. It was found that the beam distribution strongly depends on the matching between the plasma current density and the extraction voltage. At overmatched conditions the ion beam may even become hollow. The distribution of the extracted ion beam depends on the pressure along the beam path. At pressures above 3×10−4 Torr H2, the distribution was observed to be Gaussian. At a pressure 2×10−5 Torr H2, the intensity in the wings of the beam is relatively high with respect to the intensity in the beam center. The beam divergence decreases with the square root of the extraction voltage. The space-charge potential of the neutralized beam is estimated to be about 5.6 V at 2×10−5 Torr decreasing to 1.6 V at 10−3 Torr independent of the beam energy.

Reduction of arsenic oxide contamination in molecular beam epitaxy vacuum chambers

Molecular beam epitaxial films of GaAs and AlxGa1−xAs are degraded by the presence of arsenic oxide in the background gas spectrum. This memorandum discusses a technique for reducing the arsenic oxide contamination by baking the vacuum chamber under flow of H2. This technique is useful for an MBE system whose arsenic deposits have been oxidized from exposure to atmosphere. In a chamber which has been exposed to atmosphere for greater than 72 h, a 12 h bakeout under a 6 l/min flow of N2/4% H2 at 1 atm decreased the partial pressure of the arsenic oxides greater than 10× more than did an 18 h vacuum bakeout. Initial results on the improvement of films grown in chambers with high arsenic oxide backgrounds through the use of 5×10−7 Torr of H2 is reported.

The ν3 vibrational spectrum of the free ammonium ion (NH4+)

The ν3 band of the ammonium ion was measured near 3.0 μm with a color center laser spectrometer using the technique of velocity modulated absorption spectroscopy. NH4+ was produced in an ac discharge containing 0.1 Torr of nH3 and 1.5 Torr of H2. Seventy individual transitions were measured with an accuracy of 1×10−6 and assigned to q0, P+, and R− branches of a tetrahedral molecule. The constants derived in a preliminary analysis are: νeff0=3342.540 2(97) cm−1, B0+B1−2B1χ3=11.110 8(35) cm−1, B1−B0=−0.050 71(45) cm−2u1, and D=8.5(2.6)×10−5 cm−1. (AIP)

HF rotational lasers: Enhancement of V→R multiquantum energy transfer by CO and CO2

The effects of CO, CO2, H2, D2, and HC1 on rotational laser emission from HF have been examined in the HF photoelimination lasers initiated by flash photolysis of vinyl fluoride and 1,1 difluoroethene. Whereas only 1.4 Torr of H2, D2, or HCl act to quench or reduce the gain of those transitions observed in the presence of 50 Torr Ar, a few Torr of either CO or CO2 enhance both the number of transitions observed and their gain. With added CO (to both ethenes), a total of 58 transitions emit in the v = 0 to 5 vibrational manifolds, extending from J = 8→7 to 31→30. For CO2, the total number of transitions reaches 52. The patterns of laser enhancement identify energy transfer pathways that are active, of which VHF→RHF is most easily discerned. Multiquantum transfers involving ΔvHF = 3, ΔJ = 20 and ΔE<kT entering into translation are observed. Some specific emissions that are quenched or reduced in gain by CO2 suggest that RHF→VHFVM is also active. Undoubtedly partcipating, but less easily recognized are VHF→RHFVM transfers involving Δv = 1 or 2 for M = CO. The contrasting quenching behavior of H2, D2, and HC1 is attributed to removal of rotational energy from HF by RHF→RM processes that are most efficient for HF when the collision partner is also a diatomic hydride. The implications for our understanding of vibrational and rotational relaxation and the behavior of HF rotational lasers are discussed.

Oxidation and reduction of Ni–Cr alloys

The oxidation and subsequent reduction of Ni–20 Cr was studied by scanning Auger microanalysis and x‐ray photoelectron spectroscopy (XPS) with argon ion sputter profiling. Oxidation at 800 °C produced a 5000 A layer of NiO over a 4000 A layer of primarily Cr2O3 on the alloy grain faces. Near the alloy grain boundaries a band of Cr2O3 formed which was 10 000 A wide and 2000 A thick. The rate of reduction of the oxide films was found to depend on the H2 pressure. After 2 h at 500 °C and 0.02 Torr of H2 a <100 A Ni metal film formed over the NiO while at 1 atm of H2 the NiO layer was completely reduced to Ni metal. The surface and subscale layers of Cr2O3 were not reduced under either condition.

Axial laser heating of three meter theta pinch plasma columns

A 3 m long plasma column formed and confined by a fast rising solenoidal field was irradiated from one end by a powerful pulsed CO2 laser. It was found that beam trapping density minima could be maintained for the length of the laser pulse if the plasma diameter exceeded about 1.5 cm. The erosion of the density minimum was governed by classical diffusion processes. Three meter long plasmas in 2.6 cm bore plasma tubes could be fairly uniformly heated by 3.0 kJ of CO2 laser irradiation. Best results were obtained when heating began before or during the theta pinch implosion phase and the plasma fill pressure exceeded 1.0 Torr H2. Plasma line energies of about 1 kJ/m could be obtained in a magnetic field rising to 6 T in 4.7 μsec.