Simple Model for Calculating the Rate Constants of Plasma-Chemical Reactions in Low-Pressure DC Magnetron Discharges

A simple model which allows one to calculate the rate coefficients of plasma-chemical reactions in low-pressure DC magnetron discharges is presented. In this model, the electron cyclotron frequency is assumed to be much greater than any electron collision frequency. We also assume that plasma-chemical reactions take place in the near-cathode bright region where the magnetic field, the electric field, the electron density, and the electron energy are maximum. The collision probabilities have been calculated for an electron moving in crossed E × B fields by averaging the cross-sections of plasma-chemical reactions along its trajectory and over all its possible initial pitch angles. Based on this model we calculated the rate constants of the plasma-chemical reactions taking place in DC magnetron reactive sputtering in argon–oxygen gas mixtures.     

高温Shilov甲烷转化反应的机理和反应动力学研究

传统的Shilov反应是以PtCl2作为催化剂在水溶液中实现甲烷转化的,该反应的条件温和,在低至80°C时即可将甲烷中非常稳定的C–H键活化.然而,如果将反应温度提高达100°C以上,催化剂Pt(II)则非常容易发生歧化反应转化为Pt(0)或者Pt(IV),其中Pt(0)将会以沉淀的形式存在于反应溶液中.所以该反应只能在较低的温度进行, Shilov体系也只能得到较低的甲烷转化率,因此如何避免高温时催化剂因沉淀失活成为了提高反应转化率的研究重点.本文重点考察了高温条件下Shilov体系的反应机理和反应动力学,从而寻求提高催化体系活性和稳定性的途径.我们在特殊设计的金管反应器中进行了一系列的H/D置换实验,通过GC根据产物不同的分子量来分析检测.实验中,利用特殊设计的金管反应器可将反应压力增加到25.5 MPa,此时甲烷的溶解度与常温条件下(~60°C)相比可被提高1000倍以上,因此甲烷的转化率大大提高.在高温(~200°C)条件下的Shilov体系的水溶液中添加了CD3COOD, F3COOD, D2SO4, DCl和一系列阳离子为[1mim]+的离子液体来考察它们对催化剂沉淀的抑制作用,结果发现,在140°C时添加30%CD3COOD可在少量催化剂存在的条件下就能够明显促进H/D交换,与Shilov的结论吻合.这可能是由于CD3COO基团的螯合作用造成的,但将反应温度升到150°C时则不可避免的生成了Pt(0)沉淀.而F3COOD却在较多催化剂的条件下仍未表现出明显作用,可能是因为F较强的亲电子性使得F3COO基团的螯合作用变弱所致.在140°C时, D2SO4和DCl均能有效抑制Pt(0)沉淀的生成,尤其是DCl,在185°C反应24 h后仍能够稳定水溶液中的Pt基催化剂,但是在该条件下D2SO4却并没有作用.我们还发现, Cl–的浓度与沉淀的抑制直接相关,浓度越高对Pt基催化剂的稳定作用越强,但质子浓度的增加则对沉淀现象没有太大影响,我们推断原因是大量的Cl-能够在[PtCl6]2–的共同作用下将Pt(0)重新转化为了[PtCl6]2–.在140°C进行反应时,各类离子液体的添加能够使Pt(0)沉淀得到抑制,但是对H/D交换率却没有影响,可能是因为离子液体与Pt基催化剂螯合形成了Pt-离子液复合物而削弱了催化活性.在此基础上,我们特别考察了Cl–浓度对催化剂沉淀的影响,发现在200°C时将Cl-浓度提高到一定程度,就能够完全抑制Pt(0)的生成,但Pt基催化剂的活性也会被同时削弱.由于高压金管反应器的应用和高浓度Cl–的添加,使得甲烷的转化率达到90%以上,因此,我们设计了H/D同位素交换实验来考察反应的活性和选择性,从而针对高温Shilov体系的反应动力学进行研究.反应在200°C时进行,催化剂为K2PtCl4,反应介质为30% CD3COOD和DCl的水溶液,实验产物中检测到了CH3D, CH2D2, CHD3和CD4四种甲烷的多重氘代同位素体,说明了交换反应中有多个C–H键被活化.在此基础上,为了对甲烷活化过程进行全面描述,我们建立了涵盖所有连锁反应在内的综合反应网络,其中包含了H/D交换过程中涉及到的一系列平行的一级反应,基于实验数据通过阿伦尼乌斯方程计算得到了全部反应的频率因子、活化能和化学计量系数等反应动力学参数.结果证明,由于甲烷中所有的C–H键均相同,因此多重氘代产物的生成在甲烷转化过程中是不可避免的.其中,甲烷的单一氘代反应活化能为29.9 kcal/mol,双重氘代反应活化能为29.8 kcal/mol,两者十分相近,因此甲烷活化后的单一氘代产物的选择性最高不会超过50%.     

Destruction of sulfonol in its aqueous solutions by contact glow discharge treatment: 1. Product formation kinetics

Degradation of the anionic surfactant Sulfonol (sodium alkylbenzenesulfonate) in its aqueous solutions acting as a liquid cathode of atmospheric-pressure dc discharge in air has been studied. The products of plasma-chemical decomposition of Sulfonol in the liquid phase have been identified. The kinetic characteristics of the formation of products of plasma-chemical reactions at a substrate concentration of 5 × 10^?3 g/L (0.014 mmol/L) and a discharge current of 40 mA have been determined.     

A High-Efficiency Reactor for the Pulsed Plasma Conversion of Methane

The authors recently developed a high-frequency pulsed plasma process for methane conversion to acetylene and hydrogen using a co-axial cylindrical (CAC) type of reactor. The energy efficiency represented by methane conversion rate per unit input energy has been improved so that such a pulsed plasma has potential for commercial acetylene production. A pulsed plasma consists of a pulsed corona discharge and a pulsed spark discharge. Most of energy is injected over the duration of the pulsed spark discharge. Methane conversion using this kind of pulsed plasma is a kind of pyrolysis enhanced by the pulsed spark discharge. In this study, a point-to-point (PTP) type of reactor that can produce a discharge channel over the duration of a pulse discharge was used for the pulsed plasma conversion of methane. The energy efficiency and carbon formation on electrodes have been improved. The influences of pulse frequency and pulse voltage on methane conversion rate and product selectivity were investigated. The features of methane conversion using PTP and CAC reactors were discussed.     

Mathematical Modeling of the Plasma-Chemical Pyrolysis of Methane

A mathematical model was developed for the plasma-chemical pyrolysis of methane, which includes the latest data on the mechanism and kinetics of chemical processes of hydrocarbon pyrolysis and mixing of methane jets with hydrogen heated in an arc plasma torch. The results of calculations on methane conversion and the synthesis of acetylene and its homologues satisfactorily agree with experimental data over a wide range of parameters of the process. It was shown that the methane conversion is initiated via interaction with atomic hydrogen, acetylene is produced through the dissociation of intermediate products involving radicals, and the consumption of acetylene is due to the synthesis of its homologues involving vinylidenecarbene and methylenecarbene in the ground and excited states.     

Quantum chemical simulation of the reaction of methane with gold(I) acetylacetonate and aquaacetylacetonate complexes

The interaction between methane and gold(I) acetylacetonate via electrophilic substitution (reaction (I)) and oxidative addition (reaction (II)) is simulated. In both cases, the formation of the products is thermodynamically favorable: the decrease in energy is 31 kcal/mol for reaction (I) and 26 kcal/mol for reaction (II). The product of reaction (II) is additionally stabilized by Au-H interaction. Both reactions have a low activation barrier and proceed via the formation of structurally different methane complexes reducing the energy of the system by 9.3 kcal/mol for reaction (I) and by 10.9 kcal/mol for reaction (II). The complex [Au(H₂O)(acac)] is also capable of forming methane complexes. These complexes result from a thermally neutral reaction and turn into products after overcoming a low energy barrier. The structure of the complex activating methane in the gold-rutin system is deduced from the data obtained.     

Kinetic Modeling of Secondary Methane Formation and 1‐Olefin Hydrogenation in Fischer–Tropsch Synthesis over a Cobalt Catalyst

A detailed kinetic model of Fischer–Tropsch synthesis (FTS) product formation, including secondary methane formation and 1‐olefin hydrogenation, has been developed. Methane formation in FTS over the cobalt‐based catalyst is well known to be higher‐than‐expected compared to other n‐paraffin products under typical reaction conditions. A novel model proposes secondary methane formation on a different type of active site, which is not active in forming C₂₊ products, to explain this anomalous methane behavior. In addition, a model of secondary 1‐olefin hydrogenation has also been developed. Secondary 1‐olefin hydrogenation is related to secondary methane formation with both reactions happening on the same type of active sites. The model parameters were estimated from experimental data obtained with Co/Re/γ‐Al₂O₃ catalyst in a slurry‐phase stirred tank reactor over a range of conditions (T = 478, 493, and 503 K, P = 1.5 and 2.5 MPa, H₂/CO feed ratio = 1.4 and 2.1, and X _CO = 16–62%). The proposed model including secondary methane formation and 1‐olefin hydrogenation is shown to provide an improved quantitative and qualitative prediction of experimentally observed behavior compared to the detailed model with only primary reactions.     

HEAT: High accuracy extrapolated ab initio thermochemistry

A theoretical model chemistry designed to achieve high accuracy for enthalpies of formation of atoms and small molecules is described. This approach is entirely independent of experimental data and contains no empirical scaling factors, and includes a treatment of electron correlation up to the full coupled-cluster singles, doubles, triples and quadruples approach. Energies are further augmented by anharmonic zero-point vibrational energies, a scalar relativistic correction, first-order spin-orbit coupling, and the diagonal Born-Oppenheimer correction. The accuracy of the approach is assessed by several means. Enthalpies of formation (at 0 K) calculated for a test suite of 31 atoms and molecules via direct calculation of the corresponding elemental formation reactions are within 1 kJ mol(-1) to experiment in all cases. Given the quite different bonding environments in the product and reactant sides of these reactions, the results strongly indicate that even greater accuracy may be expected in reactions that preserve (either exactly or approximately) the number and types of chemical bonds.     

Uses and analyses of curtin-hammett/winstein-holness systems involving second order reactions

The Curtin-Hammett (C-H) principle and the Winstein-Holness (W-H) equation approximate the product ratio and overall rate constant of reaction for systems involving a starting material which exists in two forms, each of which reacts via first-order kinetics to give a different product. The C-H/W-H approximations are valid when the rates of isomer interconversion are significantly faster than the rates of product formation. The present treatment encompasses non-first-order reactions to product. A numerical predictor-corrector technique is used to show (1) that relative reagent concentration can affect both the product ratio and the observed rates of product formation; (2) that the absolute concentration of reagent and substrate can affect the kinetics; and (3) that factors (1) and (2) above can affect the validity of the C-H/W-H approximations for non-first-order C-H/W-H schemes.     

Electron–proton and electron–methyl exchanges in pyrolysis of polyacetylene and polymethylacetylene

Pyrolysis of polyacetylene is marked by high yields of proton-enriched products methane, ethane, ethylene, propane, polypylene, butadiene, cyclopentadiene, 1,3-pentadiene, and toluene in total amounts exceeding benzene. The activation energies for their formation are low. Polyacetylene doped with AsF₅ and iodine produced these products in even higher yields of two to 17 times of undoped polymers. The dominant mechanism is thought to be random-chain scission followed by electron–proton exchange reactions. Polymethylacetylene is thermally less stable than polyacetylene. Pyrolysis gave mesitylene as the expected main product. However, as in the case of polyacetylene, large amounts of proton-enriched products were formed with moderate activation energies. (The yields of methane, propylene, and propane are nearly the same in the pyrolysis of polymethylacetylene as compared to that of polyacetylene at 923°K referenced to mesitylene and benzene, respectively.) By analogy, mechanisms involving both electron-proton and electron–methyl exchange reactions were proposed to account for the formation of all the pyrolyzates of polymethylacetylene. These reactions, not observed in the pyrolysis of polypropylene and polyisoprene, are attributable to the conjugated backbone permitting facile migrations of electrons, protons, and methyl groups.     

Plasma-Chemical Conversion of Lower Alkanes with Stimulated Condensation of Incomplete Oxidation Products

Oxidative conversion of a mixture of natural gas and oxygen in a barrier-discharge plasma-chemical reaction was investigated experimentally. The process was conducted at atmospheric pressure and room temperature. The discharge was initiated by high-voltage pulses of 50–100 s duration at a repetition frequency of up to 2 kHz. The principal feature of the process was that in the plasma-chemical reactor conditions were created which stimulated the condensation of the products of incomplete oxidation of methane that resulted in the formation of aerosol even from nonsaturated vapor. The removal of intermediate reagents from the gaseous phases into the aerosol prevented them from further oxidation. Depending on the experimental conditions, the mass percentage of the components of the condensate formed varied within the following limits: formic acid from 20 to 40%, methanol from 8 to 15%, methylformate from 4 to 8%, and water from 40 to 60%. The conversion process has been realized on a laboratory setup of average power up to 1 kW. In the single-pass mode, a 57% degree of conversion of the mixture has been achieved. The energy value of the condensate is 15–20 kWh/kg.     

Beyond-thermal-equilibrium" conversion of methane to acetylene and hydrogen under pulsed corona discharge

At ambient temperature and pressure, C2H2 and H2 are the dominating products from pure methane conversion under pulsed corona discharge (PCD). When the energy density of 194-1788 kJ/mol was applied, 7%-30% of C2H2 yield and 6%-35% of H2 yield per pass have been obtained. These results are higher than the maximum thermodynamic yield of C2H2 (5.1%) and H2 (3.8%) at 100 kPa and 1100 K, respectively. Thereby, pulsed corona discharge is a very effective tool for "beyond-thermal-equilibrium" conversion of methane to C2H2 and H2 at ambient temperature and pressure. In the PCD energy density range of 339-822 kJ/mol, the carbon distribution of the methane conversion products is found to be: C2H2 86%-89%, C2H6 4%-6%, C2H4 4%-6%, C3 -2%, C4 -1%. Through comparison of the product from pure methane, ethane and ethylene conversion at the same discharge conditions, it can be concluded that three pathways may be responsible for the C2H2 formation via CHx radicals produced from the collisions of CH4 molecules with energi     

Kinetics and mechanism of the thermal reactions of endo- and exo-5-cyanobicyclo[2.2.2]oct-2-ene and methyl-substituted derivatives in the gas phase

The thermal reactions of endo- and exo-5-cyanobicyclo-[2.2.2]oct-2-ene and their trans- and cis-6-methyl-substituted derivatives have been investigated in the gas phase between 518 and 630 K. Each product decomposes by two parallel first-order retro-Diels-Alder reactions, a main one with formation of cyclohexa-1,3-diene and a minor one with elimination of ethene. Slight isomerizations are also observed. The kinetic results can be explained in terms of a biradical mechanism. The rate-determining step is shown to depend on the amount of resonance energy in the biradical. Heats of formation and entropies of the bicyclo[2.2.2]oct-2-enes studied are estimated.     

Alkane bromination revisited: "reproportionation" in gas-phase methane bromination leads to higher selectivity for CH3Br at moderate temperatures

The reaction of methane and bromine is a mildly exothermic and exergonic example of free radical alkane activation. We show here that the reaction of methane and bromine (CH4:Br2 > or = 1) may yield either a kinetically or a thermodynamically determined bromomethane product distribution and proceeds in two main phases between 450 and 550 degrees C under ambient pressure on the laboratory time scale. This is in contrast to the highly exothermic methane fluorination or chlorination reactions, which give kinetic product distributions, and to the endergonic iodination of methane, which yields an equilibrium distribution of iodomethanes. The first phase of reaction between methane and bromine is a relatively rapid consumption of bromine to yield a kinetic methane bromination product distribution characterized by low methane conversion, low methyl bromide selectivity, and higher polybromomethane selectivity. In the second slower phase CHxBr(4-x) reproportionation leads to significantly higher methane conversion and higher methyl bromide selectivity. For methane bromination at 525 degrees C, CH4 conversion and CH3Br selectivity reach 73.5% and 69.5%, respectively, after ample (60 s) time for reproportionation. The high selectivity and simple configuration make this pathway an attractive candidate for scale-up in halogen-mediated methane partial oxidation processes.     

Removal of Model Pollutants in Aqueous Solution by Gliding Arc Discharge. Part II: Modeling and Simulation Study

Plasma Chemistry and Plasma Processing - The aim of this work is the modeling of plasma-chemical reactions taking place between highly oxidizing gaseous species (·OH, ·NO and...     

Intensification of methane and hydrogen storage in clathrate hydrate and future prospect

Gas hydrate is a new technology for energy gas (methane/hydrogen) storage due to its large capacity of gas storage and safe. But industrial application of hydrate storage process was hindered by some problems. For methane, the main problems are low formation rate and storage capacity, which can be solved by strengthening mass and heat transfer, such as adding additives, stirring, bubbling, etc. One kind of additives can change the equilibrium curve to reduce the formation pressure of methane hydrate, and the other kind of additives is surfactant, which can form micelle with water and increase the interface of water-gas. Dry water has the similar effects on the methane hydrate as surfactant. Additionally, stirring, bubbling, and spraying can increase formation rate and storage capacity due to mass transfer strengthened. Inserting internal or external heat exchange also can improve formation rate because of good heat transfer. For hydrogen, the main difficulties are very high pressure for hydrate formed. Tetrahydrofuran (THF), tetrabutylammonium bromide (TBAB) and tetrabutylammonium fluoride (TBAF) have been proved to be able to decrease the hydrogen hydrate formation pressure significantly.     

Spectrofluorimetric study of the degradation of alpha-amino beta-lactam antibiotics catalysed by metal ions in methanol.

The alpha-aminopenicillins ampicillin and amoxicillin and a cephalosporin, cephalothin, give rise to a fluorescent product when their methanolic solutions are incubated for prolonged time periods. The process also occurs in the presence of the metal ions Cd2+, Co2+ and Zn2+. The effects of the different ions on the emission and excitation wavelengths and the appearance rate of the fluorophore were studied. The appearance of the fluorescent product was zero order for ampicillin and amoxicillin in metal ion-free solution and solutions with Cd2+ and Zn2+, whereas in the presence of Co2+ ion it was first order under the experimental conditions used; for cephalothin it was first order in all cases. Apparent fluorescent compound formation rates were calculated in the zero-order reactions and rate constants in the first-order reactions. The activation energy of the formation reaction of the fluorescent products of amoxicillin and ampicillin was calculated from a study of the reactions at four temperatures; all the values recorded were between 34 and 118 kJ mol-1. As a possible mechanism for the formation of these products, cyclization of the penamaldic derivative of the antibiotic, which is formed in the first stage of the methanolytic reaction, is proposed.     

Methane formation route in the conversion of methanol to hydrocarbons

The influence factors and paths of methane formation during methanol to hydrocarbons （MTH） reaction were studied experimentally and thermodynamically. The fixed-bed reaction results show that the formation of methane was favored by not only high temperature, but also high feed velocity, low pressure, as well as weak acid sites dominated on deactivated catalyst. The thermodynamic analysis results indicate that methane would be formed via the decomposition reactions of methanol and DME, and the hydrogenolysis reactions of methanol and DME. The decomposition reactions are thermal chemistry processes and easily occurred at high temperature. However, they are influenced by catalyst and reaction conditions through DME intermediate. By contrast, the hydrogenolysis reactions belong to catalytic processes. Parallel experiments suggest that, in real MTH reactions, the hydrogenolysis reactions should be mainly enabled by surface active H atom which might come from hydrogen transfer reactions such as aromatization. But H2 will be involved if the catalyst has active components like NiO.     

Reaction of analyte ions with neutral chemical ionization gas

Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, USA Ion-molecule reactions of neutral methane with analyte ions under normal methane chemical ionization conditions are discussed. Reactant ions can be generated by direct electron ionization (EI) fragmentation, chemical ionization (CI) fragmentation, or collision-induced dissociation (CID). Examples in which products of such reactions appear in mass spectra in both conventional CI sources in “beam” instruments and low pressure CI in a quadrupole ion trap are presented. Also shown is an example in which MS/MS product ions react with neutral methane used for CI in an ion trap. It is shown that it is relatively straightforward to recognize such reactions in a quadrupole ion trap and in certain cases to minimize or preclude them. Effects of various operating parameters have been investigated and are discussed.