Molecular beam sampling mass-spectrometric study of high-temperature benzene oxidation

The oxidation of benzene has been investigated with a high-temperature alumina flow reactor in the temperature range of 950–1150 K at residence times of ca. 1 ms and pressures of ca. 400 mbar. Analysis of the reaction products at various stages of the reaction was carried out by direct expansion of the reacting gas yielding a molecular beam that was analyzed by a mass spectrometer. Product identification studies were made by comparing the results from the oxidations of C₆H₆ and C₆D₆. Besides the products found in previous studies a number of new oxygenated intermediate species were identified, namely benzoquinone, cyclopentadienone, acrolein, and a C₄H₄O species. In addition, some higher hydrocarbons have been found even at high oxygen excess. The role of the intermediates within the current ideas of the reaction mechanism is discussed.     

Conversion of natural gas to C2 hydrocarbons through dielectric-barrier discharge plasma catalysis

The experiments are carried out in the system of continuous flow reactors with dielectric-barrier discharge (DBD) for studies on the conversion of natural gas to C₂ hydrocarbons through plasma catalysis under the atmosphere pressure and room temperature. The influence of discharge frequency, structure of electrode, discharge voltage, number of electrode, ratio of H₂/CH₄, flow rate and catalyst on conversion of methane and selectivity of C₂ hydrocarbons are investigated. At the same time, the reaction process is investigated. Higher conversion of methane and selectivity of C₂ hydrocarbons are achieved and deposited carbons are eliminated by proper choice of parameters. The appropriate operation parameters in dielectric-barrier discharge plasma field are that the supply voltage is 20–40 kV (8.4–40 W), the frequency of power supply is 20 kHz, the structure of (b) electrode is suitable, and the flow of methane is 20–60 mL · min⁻¹. The conversion of methane can reach 45%, the selectivity of C₂ hydrocarbons is 76%, and the total selectivity of C₂ hydrocarbons and C₃ hydrocarbons is nearly 100%. The conversion of methane increases with the increase of voltage and decreases with the flow of methane increase; the selectivity of C₂ hydrocarbons decreases with the increase of voltage and increases with the flow of methane increase. The selectivity of C₂ hydrocarbons is improved with catalyst for conversion of natural gas to C₂ hydrocarbons in plasma field. Methane molecule collision with radicals is mainly responsible for product formation.     

Water splitting in low-temperature ac plasmas at atmospheric pressure

Plasma-induced water splitting at atmospheric pressure has been studied with a novel fan-type Pt reactor and several tubular-type reactors: an all-quartz reactor, a glass reactor, and three metal reactors with Pt. Ni, and Fe as electrodes. Reaction products have been analyzed by using GC (gas chromatography) and Q-MS (quadrupole mass spectrometry). Optical emission spectroscopic studies of the process have been carried out by employing a CCD (charge-coupled device) detector. Water splitting with tubular quartz and glass reactors is probably non-catalytic. However, a heterogeneous catalytic function of surface of metal electrodes has been observed. The variation of hydrogen yield (Y_H) and energy efficiency (E_e) with operational parameters such as input voltages (U_in), flow rates of carrier gas (F_He), and concentrations of water (C_W) has been examined. Plasma-induced water splitting can be described with a kinetic equation of-dC_w/dt = kC_W ^0.2. The rate constants at 3.25 kV are 2.8 × 10⁻⁴, 3.5 × 10⁻³, and 3.4 × 10⁻² mol^0.8L^−0.8 min⁻¹ for tubular glass reactor, a tubular Pt reactor, and a fan-type Pt reactor, respectively. A CSTR (continuous-stirred tank reactor) and PFR (piston-flow reactor) model have been applied to a fan-type reactor and tubular reactor, respectively. A mechanism on the basis of optical emission spectroscopic data has been obtained comprising the energy transfer from excited carrier gas species to water molecules, which split via radicals of HO^·, O^·, and H^· to form H₂ and O₂. The fan-type Pt reactors exhibit highest activity and energy efficiency among the reactors tested. Higher yields of hydrogen are achieved at higher input voltages, low flow rates, and low concentrations of water (Y_H = 78 % at U_in of 3.75 kV, F_He of 20 mL/min, and C_W of 0.86 %). The energy efficiency exhibits an opposite trend (E_e = 6.1 % at U_in of 1.25 kV, F_He of 60 mL/min and C_W of 3.1 %).     

激光溅射金属等离子体与醇类分子束反应的研究：产物离子对分子束状态的依赖性

The gas phase reactions of metal plasma with alcohol clusters were studied by time of flight mass spectrometry （TOFMS） using laser ablation-molecular beam （LAMB） method. The significant dependence of the product cluster ions on the molecular beam conditions was observed. When the plasma acted on the low density parts of the pulsed molecular beam, the metal-alcohol complexes M^＋An （M=Cu, Al, Mg, Ni and A=C2H5OH, CH3OH） were the dominant products, and the sizes of product ion clusters were smaller. While the plasma acted on the high density part of the beam, however, the main products turned to be protonated alcohol clusters H^＋An and, as the reactions of plasma with methanol were concerned, the protonated water-methanol complexes H3O^＋（CH3OH）n with a larger size （n≤12 for ethanol and n≤24 for methanol）. Similarly, as the pressure of the carrier helium gas was varied from 1 × 10^5 to 5 × 10^5 Pa, the main products were changed from M^＋An to H^＋An and the sizes of the clusters also increased. The changes in the product clusters were attributed to the different formation mechanism of the output ions, that is, the M^＋An ions came from the reaction of metal ion with alcohol clusters, while H^＋An mainly from collisional reaction of electron with alcohol clusters.     

Modeling the thermal DENOx process in flow reactors. Surface effects and Nitrous Oxide formation

We have investigated the impact of surface reactions such as NH₃ decomposition and radical adsorption on quartz flow reactor data for Thermal DeNO_x using a model that accounts for surface chemistry as well as molecular transport. Our calculations support experimental observations that surface effects are not important for experiments carried out in low surface to volume quartz reactors. The reaction mechanism for Thermal DeNO_x has been revised in order to reflect recent experimental results. Among the important changes are a smaller chain branching ratio for the NH₂ + NO reaction and a shorter NNH lifetime than previously used in modeling. The revised mechanism has been tested against a range of experimental flow reactor data for Thermal DeNO_x with reasonable results. The formation of N₂O in Thermal DeNO_x has been modelled and calculations show good agreement with experimental data. The important reactions in formation and destruction of N₂O have been identified. Our calculations indicate that N₂O is formed primarily from the reaction between NH and NO, even though the NH₂ + NO₂ reaction possibly contributes at lower temperatures. At higher temperatures N₂O concentrations are limited by thermal dissociation of N₂O and by reaction with radicals, primarily OH. © 1994 John Wiley & Sons, Inc.     

Reaction of F atoms with dichloromethane by modulated molecular beam mass spectrometry

The reaction of atomic fluorine with dichloromethane has been studied by the diffusion cloud in a flow technique. Fluorine atoms were generated through F₂ dissociation in a high-frequency discharge. The reaction products were detected mass spectrometrically, applying the technique of focusing the paramagnetic component of the molecular beam in an inhomogeneous magnetic field to detect radical species. Cl atoms and CHCl₂ and CF₃ free radicals have been identified among the reaction products. The initial step was shown to be hydrogen atom abstraction. The room temperature rate constant of this reaction was found to be k₀ = (1.51 ± 0.28) X 10^?11 cm³/s. The rate constant of the secondary reaction of fluorine atoms with dichloromethyl radicals, which is suggested to produce mainly HCl, was evaluated as 3 X 10^?10 cm³/s.     

Analysis of Nascent Rotational Energy Distributions and Reaction Mechanisms of the Gas‐Phase Radical–Radical Reaction O(3P)+(CH3)2CH→C3H6+OH

This paper reports on the gas‐phase radical–radical dynamics of the reaction of ground‐state atomic oxygen [O(³P), from the photodissociation of NO₂] with secondary isopropyl radicals [(CH₃)₂CH, from the supersonic flash pyrolysis of isopropyl bromide]. The major reaction channel, O(³P)+(CH₃)₂CH→C₃H₆ (propene)+OH, is examined by high‐resolution laser‐induced fluorescence spectroscopy in crossed‐beam configuration. Population analysis shows bimodal nascent rotational distributions of OH (X²Π) products with low‐ and high‐N′′ components in a ratio of 1.25:1. No significant spin–orbit or Λ‐doublet propensities are exhibited in the ground vibrational state. Ab initio computations at the CBS‐QB3 theory level and comparison with prior theory show that the statistical method is not suitable for describing the main reaction channel at the molecular level. Two competing mechanisms are predicted to exist on the lowest doublet potential‐energy surface: direct abstraction, giving the dominant low‐N′′ components, and formation of short‐lived addition complexes that result in hot rotational distributions, giving the high‐N′′ components. The observed competing mechanisms contrast with previous bulk kinetic experiments conducted in a fast‐flow system with photoionization mass spectrometry, which suggested a single abstraction pathway. In addition, comparison of the reactions of O(³P) with primary and tertiary hydrocarbon radicals allows molecular‐level discussion of the reactivity and mechanism of the title reaction.     

Applications of catalytic inorganic membrane reactors to refinery products

Catalytic membrane reactors are reviewed as applied to opportunities and applications within petroleum refineries. Since so many inorganic membranes take advantage of H₂ permselectivity and H₂ demands are increasing in a refinery, there are a number of interesting process applications being considered. H₂ production can be enhanced by using Pd based membranes for dehydrogenation, oxydehydrogenation, and decomposition reactions. Permselective H₂ membranes could be used for carrying out selective hydrogenations of organic substrates and coupled reactions. These membranes have been also considered for enhancing steam reforming reactions for the production of bulk H₂, the water gas shift reaction, and the conversion of natural gas to syngas and liquid fuels. Dense oxide membranes are also being developed for the selective oxidation of CH₄ to syngas. For many of these processes, the formation of carbon during steam reforming or dehydrogenation reactions will always be a huge hurdle towards any successful commercial application of Pd membranes to such processes. In any of these applications one has to understand production problems associated with the metal membranes, the refinery demands for high purity H₂, and the reactor fabrication hurdles; these will be evaluated with recent examples. For all these applications, the critical issues that need to be resolved for the commercial use of catalytic membrane reactors will be discussed.     

Evaluation of HX (X = Cl,Br, O) product distributions from chemiluminescence. I. H atom reactions

The HX product state distributions from the H+Cl₂, Br₂, NO₂Cl, PCl₃, and NO₂ reactions have been studied by the infrared chemiluminescence technique in two different laboratories with two types of reactors; a fast-flow system with = 1 Torr of Ar buffer gas and a low-pressure, cold-wall system (usually called the cold-wall arrested-relaxation method). The same Einstein coefficients were used in both laboratories to convert intensities to populations and emphasis is placed upon evaluation of the reliability of the resulting vibrational-rotational HX distributions. Good agreement was found between the HX distributions from the cold-wall reactors from the two laboratories and for both types of reactors for all of the reactions, except PCl₃. For the H+Cl₂, Br₂ and NO₂ reactions, our general results are in good accord with presently accepted data; but, our experiments provide somewhat more detail than in the literature. The NO₂Cl results are new and <f_v(HCl) > = 0.40 and <f_R(HCl) > = 0.01. The H+PCl₃ reaction appears to proceed by two channels and the HCl chemiluminescence cannot be assigned only to HCl formation via direct Cl atom abstraction.     

Generation,analysis, and deposition of silicon nanocrystals up to 10 nm in diameter

Silicon clusters have been generated by CO₂- laser-induced decomposition of SiH₄ in a flow reactor. By introducing a conical nozzle into the reaction zone, the clusters are extracted into a molecular beam apparatus and analyzed with a time-of-flight mass spectrometer. Besides small clusters, the mass spectra show also very large aggregates containing a few thousand of silicon atoms. A velocity analysis of the neutral cluster beam reveals that the particle velocity decreases with increasing mass. Thus it is possible to perform a size selection of the neutrals by introducing a velocity selector into the cluster beam. The silicon clusters have been deposited at low energy, and TEM micrographs of size-selectively prepared films are presented. A quantitative analysis of two different deposits reveals mean particle diameters of 3.5 and 6.5 nm. The results demonstrate the possibility of fabricating thin films of nanostructured materials with a mean particle diameter controlled by combining the cluster beam deposition technique with a velocity selector.     

Performance of trickle-bed bioreactors for converting synthesis gas to methane

Carbon monoxide, H₂, and CO₂ in synthesis gas can be converted to CH₄ by employing a triculture ofRhodospirillum rubrum, Methanosarcina barken, andMethanobacterium formicicum. Trickle-bed reactors have been found to be effective for this conversion because of their high mass-transfer coefficients. This paper compares results obtained for the conversion of synthesis gas to CH₄ in 5-cm- and 16.5-cm-diameter trickle-bed reactors. Mass-transfer and scale-up parameters are defined, and light requirements forR. rubrum are considered in bioreactor design.

     

Determination of the rates of formation of gaseous products from the pyrolysis of octahydro-1, 3, 5, 7-tetranitro-1, 3, 5, 7-tetrazocine (hmx) by simultaneous thermogravimetric modulated beam mass spectrometry

The method for determining the rates of formation of gaseous pyrolysis products during thermal decompositions by simultaneous thermogravimetric modulated beam mass spectrometry is presented. The analysis procedure that handles both molecular and continuum flow from the reaction cell is described. The technique is illustrated with the isothermal decomposition of HMX. The temporal behaviors of the rates of formation of the pyrolysis products, H₂O, HCN, CO, CH₂O, NO, N₂O, methylformamide, C₂H₆N₂O, and octahydro-1-nitroso-3, 5, 7-trinitro-1, 3, 5, 7-tetrazocene, formed during the isothermal decomposition of HMX at 211°C, are presented. The results show that a complex condensed-phase reaction mechanism controls the decomposition.     

Kinetics of the reaction of C2H5O2 with NO at 295 K

The reaction of C₂H₅O₂ with NO in helium carrier gas at 295 K with [He] = 1.6 × 10¹⁷ cm^?3 has been studied using a gas flow reactor sampled by a mass spectrometer. Because no parent molecular ion or suitable fragment ion produced by C₂H₅O₂ could be detected, the reaction was followed by measuring the formation of NO₂. In so doing, account had to be taken of the small amount of HO₂ known to be present in the reaction mixture, which also leads to NO₂ on reaction with NO. The rate coefficient for the total reaction of C₂H₅O₂ with NO was found to be (8.9 ± 3.0) × 10^?12 cm³/s, and the path which produces NO₂ was found to account for at least 80% of all C₂H₅O₂.     

Neutral molecular ZnX (X=O, OH, N) compounds in a molecular beam

Neutral ZnO and ZnOH molecules could be produced in a molecular beam by expansion of laser ablated zinc together with H₂O, O₂ or N₂O seeded in a rare gas (Ar, Ne, He). Due to the characteristic Zn isotope distribution, the zinc containing compounds, ionized with a 100 fs laser pulse, could unambiguously be identified with a TOF mass spectrometer. The abundance of ZnOH produced in our experiments exceeds the one of ZnO and ZnN by orders of magnitude if H₂O is present in the system. Small quantities of (ZnO)₂H and Zn₂(OH)₃ compounds could also be observed. To our knowledge this is the first evidence for the occurrence of neutral ZnO and ZnOH molecules in a molecular beam.     

Thermal effects in collision-free infrared multiphoton absorption by SF6 and CF3Br

An opto-thermal molecular beam study has been carried out to investigate the multiple-photon laser excitation of SF₆ and CF₃Br. The molecular beam was produced by means of a supersonic expansion through a nozzle at variable temperature. The opto-thermal signal was measured by means of a high-sensitivity superconducting bolometer. The multiple-photon excitation of SF₆ has been measured as a function of the initial ro-vibrational population of the molecule. The experimental results have been compared with both previously published data of molecular beam and gas cell experiments and theoretical calculations. A satisfactory agreement has been found between some of our experimental results and the theoretical spectra obtained by means of the heat-bath feed-back model.     

Sensitivity enhancement using chemically reactive gas cluster ion beams in secondary ion mass spectrometry (SIMS)

We report for the first time on significant molecular secondary ion yield increases by modifying the chemistry of a water cluster primary ion beam. This was demonstrated using 70-keV ion beams of 0.15 eV/amu. For the neutral drug Bezafibrate, secondary ion yield enhancements ×5–10 were observed when replacing the Ar carrier gas in a water gas cluster ion beam (GCIB) source with a mixture containing 12% CO₂ and 2% O₂ in Ar. For the cationic drug Ranitidine, the ion yield enhancements using the CO₂-containing carrier gas were up to ×20–50 in positive mode and ×2–4 in negative mode. The extent of molecular fragmentation was very similar from both cluster beams. We conclude that additional chemically reactive species are present in the impact zone using the (H₂O/CO₂)_n projectile, which promote the formation of secondary ions of both polarity through projectile impact-induced chemical reactions. This methodology can be applied to further extend the capabilities of high-resolution 3-dimensional mass spectral imaging using reactive GCIB-SIMS.     

Two‐Dimensional Spatial Resolution of Concentration Profiles in Catalytic Reactors by Planar Laser‐Induced Fluorescence: NO Reduction over Diesel Oxidation Catalysts

Planar laser‐induced fluorescence (PLIF) enables noninvasive in situ investigations of catalytic flow reactors. The method is based on the selective detection of two‐dimensional absolute concentration maps of conversion‐relevant species in the surrounding gas phase inside a catalytic channel. Exemplarily, the catalytic reduction of NO with hydrogen (2 NO+5 H₂→2 H₂O+2 NH₃) is investigated over a Pt/Al₂O₃ coated diesel oxidation catalyst by NO PLIF inside an optically accessible channel reactor. Quenching‐corrected 2D concentration maps of the NO fluorescence above the catalytic surface are obtained under both, nonreactive and reactive conditions. The impact of varying feed concentration, temperature, and flow velocities on NO concentration profiles are investigated in steady state. The technique presented has a high potential for a better understanding of interactions of mass transfer and surface kinetics in heterogeneously catalyzed gas‐phase reactions.     

The hetero‐homogeneous pyrolysis of propane,in the presence or in the absence of dihydrogen,and the measurement of uptake coefficients of hydrogen atoms

Propane pyrolysis is studied in the presence and the absence of dihydrogen between 743 and 803 K, in the propane pressure range 10–100 Torr, and at 20–254 Torr dihydrogen pressure. In unpacked Pyrex reactors, dihydrogen accelerates propane dehydrogenation and demethanation. The reaction is modeled by a conventional homogeneous free‐radical chain mechanism. Propane pyrolysis is strongly inhibited by the walls of reactors packed with stainless steel, zirconium, or palladium foils. Adding dihydrogen to propane still increases the rates of product formation. The reaction in these packed reactors is modeled by the kinetic scheme proposed for the homogeneous reaction and by the heterogeneous process H. ⇄ ½H₂ (w₂)(−w₂) of chain termination and initiation. In the absence of dihydrogen, step (−w₂) is negligible and precise values of uptake coefficients of hydrogen atoms are obtained at 773 K: 0.31 for stainless steel 0.10 for zirconium 0.05 for palladium In the presence of dihydrogen, steps (w₂) and (−:w₂) are instantaneously at equilibrium. The latter system should be useful to study any reaction of hydrogen atoms in the temperature range. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 340–364, 2000     

Statu quo sur la méthanation du dioxyde de carbone : une revue de la littérature

This review summarizes the statu quo and the perspectives of chemical methanation. CO₂ methanation, including catalyst deactivation, reactors, mechanisms, and thermodynamics are presented. This reaction serves as a test bed for our fundamental understanding of heterogeneous catalysis and is used in various industrial processes, including the removal of oxo-compounds (CO_x) in the feed gas for the ammonia synthesis, in connection with the gasification of coal, where it can be used to produce methane from synthesis gas, and in relation to Fischer–Tropsch's synthesis. Moreover, CO₂ methanation became of interest as a renewable energy storage system based on a “power-to-gas” conversion process by SNG (synthetic natural gas) production integrating water electrolysis and CO₂ methanation as a highly effective way to store the energy produced by renewables sources. The effectiveness and efficiency of the “power-to-gas” plants strongly depends on the CO₂-methanation process.     

Conformational relaxation of S-(+)-carvone and R-(+)-limonene studied by microwave Fourier transform spectroscopy and quantum chemical calculations

S-(+)-carvone (C₁₀H₁₄O, 5-isopropenyl-2-methylcyclohex-2-en-1-one) and R-(+)-limonene (C₁₀H₁₆, 4-isopropenyl-1-methylcyclohexene) have been characterized in the gas phase using a Fourier transform microwave spectrometer coupled to a supersonic molecular beam. Two conformers—with the isopropenyl group in the equatorial position—have been detected for each compound and described by a set of molecular parameters including the principal rotational constants and the quartic centrifugal distortion parameters. Quantum chemical calculations indicate that a third conformer might not be observed due to relaxation processes in the jet. The gas phase results are compared with the liquid phase IR-Raman-VCD spectra.