A Kinetic Model for the Reactions of Co and H2 to CH4 and C2H2 in a Flow Microwave Discharge Reactor

A kinetic model was developed to describe the reactions of CO and H2 to CH4 and C2H2in a microwave plasma. The experimental system consisted of a 24 mm I.D. tubular quartz reactor which passed through a microwave cavity. A variable-incident power waveguide system could supply up to 800 watts of incident microwave power to the cavity. The reactant gas mixture of H2 and CO flowed through the reactor, where a plasma was maintained under pressures of 20 - 100 mm Hg. The reactor effluent was analyzed by IR spectroscopy for CH4 and C2H2. Conversions of up to 5.3% CO to C2H2 and 7.2% CO to CH4 were observed. A 26-reaction kinetic model was developed and fitted to the experimental data. The plasma reactor was modeled in two zones: a discharge zone where electron-impact dissociations produce H, C, and O, and a downstream recombination zone where the atomic species from the discharge recombine. The discharge zone was modeled as a well-mixed reactor, and the recombination zone was modeled as a plug-flow reactor. The model was able to explain the asymptotic shape of the observed conversion versus residence time data; the effect is due to a kinetic limitation. This also explains why the conversions obtained in the plasma cannot be predicted by thermodynamic equilibrium.     

Dielectric barrier discharge plasma torch treatment of pyrolysis fuel oil in presence of methane and ethane

For the first time, atmospheric-pressure low-temperature plasma treatment of pyrolysis fuel oil (PFO) was investigated in dielectric barrier discharge (DBD) plasma torch reactor. Main parameters including working gas flow rate and Ar/CH₄ ratio along with the effects of separate Ar/C₂H₆ on the cracking of PFO were studied. By increasing the flow of argon and methane, the production rate of hydrocarbons containing ethane, ethylene, acetylene, propane, propylene, C₄, and C₅ increased from 1.72 to 10.48 ml/min for 4000 ml/min argon plus 400 ml/min methane as the working gas. In this case, the production rate of hydrogen increases from 10.58 to 56.86. The production rate of hydrocarbons increased from 3.53 to 13.5 ml/min by decreasing Ar/CH₄ ratio from 40 to 5.6. By changing the type of working gas from methane to ethane, the production of hydrocarbons considerably increased from 6.47 to 17.75 ml/min.     

常压等离子体作用下CO2氧化CH4转化反应的光谱分析

采用光谱原位诊断技术在200～900nm范围内采集并标识了不同CO2添加量时CH4等离子体光谱。确定了常压下不同CO2添加量时CH4等离子体的活性物种。CO2添加量不同活性物种谱峰相对强度变化趋势不同,O活性物种谱峰相对强度随CO2添加量增加迅速增加。C2活性物种谱峰相对强度随CO2添加量增加逐渐降低。反应体系产生的活性O对CH4转化反应产物具有明显影响,CO2添加量不同,CH4转化反应机理不同。当CO2添加量小于30%时,CH4转化以偶联反应为主。CO2添加量大于30%时,CH4转化以重整反应为主。     

Zur plasmachemischen Umsetzung von CO/H2-Gemischen zu leichten Kohlenwasserstoffen in einer Glimmentladung

Mixtures of CO und H₂ can be partially converted into hydrocarbons (Hc), especially methane, by nonthermal activation in a glow discharge. The degree of conversion α CO → Hc and the partial pressure of methane reached values of 12% and 3% resp. The CO-conversion depends on the discharge parameters, the discharge time the energy input resp., and the rate of CO/H₂-mixture. The Hc formed are intermediate products which react further to higher Hc including solid Hc. According to mass spectrometrical analysis of plasma ions a reaction scheme was developed which is considered to be valid for the Hc formation based on an ion as well on neutral particle reactions.     

Catalytic ignition of light hydrocarbons over Rh/Al2O3 studied in a stagnation-point flow reactor

The ignition (light-off) temperatures of catalytic oxidation reactions provide very useful information for understanding their surface reaction mechanism. In this study, the ignition behavior of the oxidation of hydrogen (H₂), carbon monoxide (CO), methane (CH₄), ethane (C₂H₆), and propane (C₃H₈) over Rh/alumina catalysts is examined in a stagnation-point flow reactor. The light-off temperatures are identified by means of the sudden increase of the catalyst temperature when linearly heating the catalyst for various fuel/oxygen ratios. For hydrogen and all hydrocarbons studied, the results show a rise of ignition temperature with increasing fuel/oxygen ratio, whereas the opposite trend is observed for the light-off of CO oxidation. Hydrogen oxidation, however, shows an opposite trend compared to previous investigations, performed on platinum [1], [2].     

火花放电电极结构及其等离子体反应器

在阐述火花放电机制与等离子体特性基础上,着重探讨了火花放电的电极结构与等离子体反应器。新研制的电极旋转的新型 kHz 交流火花放电反应器,在甲烷裂解制乙炔和甲烷与二氧化碳重整制合成气应用研究中,其放电稳定性、反应物转化率、产物浓度和能量效率等指标,均明显优于其它放电反应器。     

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Based upon demonstrating the discharge mechanism and plasma characteristic of spark discharge, electrode configuration and plasma reactor of spark discharge were discussed especially. A novel kHz AC spark discharge reactor with rotating electrode, developed in our group, was applied in methane pyrolysis to produce acetylene and reforming of methane with CO₂ to produce syngas. Its discharge stability, reactant conversion, product concentration and energy efficiency are higher obviously than those of other discharge reactors.     

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在微波化学气相沉积装置上采用微波激发氢气甲烷体系等离子体,通过光学多道分析仪采集等离子的发射光谱.实验表明,甲烷在等离子体中的裂解产物主要以CH,CH-,C2基团的形式存在.这些基团的发射光谱强度主要受放电压强和放电功率的影响.随着微波功率的增加甲烷基团发射光谱强度呈增长的趋势;而随着放电压强的增加则是先增大,后减小.这些实验结果对于理解微波等离子体化学气相沉积(MPCVD)中各种反应过程,调整薄膜制备工艺提供了参考.     

石墨、菱铁矿与超临界水反应的实验研究

 为研究地球深部无机成烃的机制，在金刚石压腔（DAC，温度为800~1 500 ℃，压力略大于1 GPa）中进行了石墨和菱铁矿分别与超临界水反应的实验研究。用气相色谱法分析了气体产物的组成，发现其中均有大量的甲烷生成，并伴有CO₂和CO；此外还有少量其它烃类。上述结果意味着在地球深部高温高压条件下，含碳物质与超临界水反应可能是一种新的、重要的成烃机制。     

On the reaction mechanism of CO2 reforming of methane over a bed of coal char

CO₂ reforming of methane was studied over a bed of coal char in a fixed bed reactor at temperatures between 1073 and 1223 K and atmospheric pressure with a feed composition of CH₄/CO₂/N₂ in the ratio of 1:1:8. Experimental results showed that the char was an effective catalyst for the production of syngas with a maximum H₂/CO ratio of one. It was also found that high H₂/CO ratios were favoured by low pressures and moderate to high temperatures. These results are supported by thermodynamic calculations. A mechanism of seven overall reactions was studied and three catalytic reactions of CH₄ decomposition, char gasification and the Boudouard reaction was identified as being of major importance. The first reaction produces carbon and H₂, the second consumes carbon, and the third (the Boudouard reaction) converts CO₂ to CO while consuming carbon. Equilibrium calculations and experimental results showed that any water present reacts to form H₂ and carbon oxides in the range of temperatures and pressures studied. Carbon deposition over the char bed is the major cause of deactivation. The rate of carbon formation depends on the kinetic balance between the surface reaction of the adsorbed hydrocarbons with oxygen containing species and the further dissociation of the hydrocarbon.     

Investigation of products of thermal methane conversion in hydrocarbon mixtures by mass spectrometry

The influence of ethylene/acetylene admixture on catalytic methane pyrolysis is investigated. Mass spectrometric analysis of the products of reactions in synthesis of carbon structures by thermal chemical gas-phase deposition is carried out for different temperatures, gas mixture compositions, and catalyst concentrations. It is shown that in the presence of ethylene and acetylene, there occurs partial methane decomposition at lower temperatures.     

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在常温常压下采用新型旋转电极等离子体反应器，对辉光等离子体作用下的甲烷偶联反应制C2烃进行了研究。结果表明，甲烷偶联反应的主要产物为C2H2，占C2烃的80%以上，能量效率在5.6%～11.2%之间；增加H2含量可以提高CH4转化率和C2烃收率；在500～2200kJ•mol−1的能量密度范围内，CH4转化率随能量密度的增大而线性增加，C2烃收率随着能量密度的增加呈峰形变化趋势。     

Flame studies of C2 hydrocarbons

The oxidation characteristics of C₂ hydrocarbons were revisited in flames established in the counterflow configuration. Laminar flame speeds of ethane/air, ethylene/air, and acetylene/oxygen/nitrogen mixtures as well as extinction strain rates of non-premixed ethane/air flames were measured using digital particle image velocimetry. The experiments were modeled using three different kinetic models. While the experimental and computed laminar flame speeds agreed closely for all C₂ hydrocarbons under fuel-lean conditions, notable discrepancies were identified under fuel-rich conditions. Using the computed flame structures, insight was provided into the controlling mechanisms that could be responsible for the observed discrepancies. More specifically, the uncertainties associated with the kinetics of the thermal decomposition of the ethyl radical were found to be a potential source of the observed discrepancies for ethane flames. It was shown also by using alternative rate constants for the ethyl radical decomposition, the rate of flame propagation and the extinction propensity are affected notably. Furthermore, the values of the branching ratio of acetylene consumption reactions involving atomic oxygen were found to have a significant effect on the propagation of rich acetylene flames.     

Interactions of NiO particles with polycyclic aromatic hydrocarbons and acetylene generated from the pyrolysis of a model fuel

Experiments have been conducted to study the effects of NiO, a prevalent form of nickel in combustion-generated ash particles, on polycyclic aromatic hydrocarbons (PAH) and other hydrocarbons in a fuel product mixture. The fuel product mixture is generated from the gas-phase pyrolysis, in N₂, of the model fuel catechol in a quartz tube reactor at 1100 K and 5 s, a condition that ensures full conversion of the catechol and produces a mixture—of PAH, hydrocarbon gases, and oxygen-containing species—that is representative of the products of practical liquid and solid fuels in pyrolysis and fuel-rich combustion environments. Once formed, the product mixture is passed, at the same temperature, through a bed of ultrafine NiO particles held in place by quartz wool, in the contact zone of the reactor. The products exiting the reactor are quenched, collected, and analyzed by non-dispersive infrared analyzers, gas chromatography, and high-pressure liquid chromatography with diode-array ultraviolet–visible absorbance detection. The results from the experiments at 1100 K show that—compared to the case of no inorganics in the contact zone—when NiO is present: PAH yields are reduced 86% (from 10.8% to 1.48% fed carbon); all of the highly mutagenic 5- and 6-ring PAH are eliminated; and all of the acetylene, the highest-yield hydrocarbon product when NiO is absent, and other hydrocarbons with carbon–carbon triple bonds are eliminated from the gas phase. Most of the surface-bound carbon is released as CO. Similar experiments at 1275 K show that—except for the release of the surface-bound carbon as CO—the selective surface effects of NiO bring about similar results at higher temperature: 89% reduction in PAH yield, elimination of mutagenic 5- and 6-ring PAH, removal of acetylene and acetylenic species, as well as a decrease in the production of solid carbon (not formed at 1100 K).     

Sonolysis of organic liquid: effect of vapour pressure and evaporation rate

Various kinds of organic liquids, such as hydrocarbons, ethers, ketones and alcohols, were subjected to ultrasonic irradiation and the effects of vapour pressure and evaporation rate of the liquids on decomposition rates and the distribution of decomposition products were investigated. The main decomposition products from hydrocarbons were hydrogen, methane, ethylene and acetylene, and hydrogen, methane, ethylene, carbon monoxide and aldehydes from alcohols. The decomposition rates of organic liquids were generally faster than that of water, and the reaction would proceed via gas-phase chain reactions in the high temperature site by comparison of the product with pyrolysis data in the literature and by considering the results of DPPH experiments. In the relationship between decomposition rate and vapour pressure, different features were observed in alcohols and other liquids. The hydrocarbons most efficiently decomposed under conditions in which their vapour pressures ranged from 0.1 to 0.5 Torr. On the other hand, the most efficient vapour pressure for alcohol sonolysis was about 15 Torr. The deviations became smaller when the evaporation rate was employed instead of vapour pressure, and as the reactive index of sonolysis of organic liquids, evaporation rate may be a better probe than vapour pressure, which is often chosen as the index.     

Surface chemistry studied by in situ X-ray photoelectron spectroscopy

Time-dependent X-ray photoelectron spectroscopy is used to study the kinetics and dynamics of simple surface reactions. Combining high-resolution core level spectroscopy with a supersonic molecular beam in one experimental setup, processes such as the dissociative adsorption of methane on both Pt(111) and Ni(111), the coadsorption of water and CO on Pt(111), and the oxidation of CO on Pt(111) have been studied. In the case of methane, the observed vibrational fine structure in C 1s spectra is used to identify the adsorbed species (CH₃) and further thermal dehydrogenation steps. While simple dehydrogenation via CH is observed on Pt(111), a C–C coupling reaction to acetylene is found on Ni(111). In the coadsorbate phase, CO is found to be able to replace predosed water from the bilayer into multilayers. Water, in turn, leads to a site change of the CO molecules, which are preferably adsorbed at bridge sites in the presence of water, as opposed to on-top adsorption on clean Pt(111). For the truly bimolecular surface reaction, the CO oxidation on Pt(111), the ability of the molecular beam to create a relatively high CO pressure was found essential to study the kinetics of the basic step (CO+OCO₂) without influence of adsorption or diffusion rate. An activation energy of 0.53 eV and a preexponential factor of 5×10⁶ s^-1 are found. PACS 68.43.Mn; 79.60.Dp; 82.20.Pm     

Temperature programmed desorption study of acetylene on a clean,H-covered,and O-covered Pt(111) surface

The reactions of acetylene on a clean, a H-covered and an O-covered Pt(111) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH₄, C₂H₆, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO₂, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.     

Reactions of azomethane on a clean,H-covered,and O-covered Pt(111) surface

The reactions of azomethane were studied on a clean, H-covered, and O-covered Pt(111) surface at different coverages of azomethane, and ratios of hydrogen or oxygen to azomethane, by temperature programmed desorption. On a clean surface, dehydrogenetion, together with breaking of the NN bond to produce HCN and H₂ were the major reactions. Small amounts of methylamine and cyanogen were also observed. On an H-covered surface at high hydrogen coverage, methane was the dominant product, in addition to H₂. This adsorbed hydrogen promoted breaking of the CN bond. In addition, the dehydrogenation product HCN was also observed, as was methylamine. On the O-covered surface, CO, CO₂, H₂O, and small amounts of NO were observed together with the products typical for a clean surface. Comparison of the results with those for acetylene, ethylene, and diazomethane is made.     

Properties of diamondlike films obtained in a barrier discharge at atmospheric pressure

Diamondlike films are synthesized from gaseous hydrocarbons in a barrier discharge at atmospheric pressure. The films were investigated using transmission electron microscopy, electron diffraction, and infrared spectroscopy. A technique for determining the quantitative characteristics of the films (hydrogen content, ratio of different types of carbon-carbon bonds and hydrocarbon groups) using standard samples is described. The highest-quality films were obtained from methane (ratio of hydrogen to carbon atoms H/C=1.04, fraction of diamondlike to graphitelike bonds sp ³: sp ²=100%: 0%) and from a mixture of acetylene and hydrogen in the ratio 1:19 (H/C=0.73, sp ³: sp ²=68%: 32%). Zh. Tekh. Fiz. 67, 100–104 (August 1997)     

The effects of power ultrasound (24 kHz) on the electrochemical reduction of CO2 on polycrystalline copper electrodes

The electrochemical CO₂ reduction reaction (CO2RR) on polycrystalline copper (Cu) electrode was performed in a CO₂-saturated 0.10 M Na₂CO₃ aqueous solution at 278 K in the absence and presence of low-frequency high-power ultrasound (f = 24 kHz, P_T ~ 1.23 kW/dm³) in a specially and well-characterized sonoelectrochemical reactor. It was found that in the presence of ultrasound, the cathodic current (I_c) for CO₂ reduction increased significantly when compared to that in the absence of ultrasound (silent conditions). It was observed that ultrasound increased the faradaic efficiency of carbon monoxide (CO), methane (CH₄) and ethylene (C₂H₄) formation and decreased the faradaic efficiency of molecular hydrogen (H₂). Under ultrasonication, a ca. 40% increase in faradaic efficiency was obtained for methane formation through the CO2RR. In addition, and interestingly, water-soluble CO₂ reduction products such as formic acid and ethanol were found under ultrasonic conditions whereas under silent conditions, these expected electrochemical CO2RR products were absent. It was also found that power ultrasound increases the formation of smaller hydrocarbons through the CO2RR and may initiate new chemical reaction pathways through the sonolytic di-hydrogen splitting yielding other products, and simultaneously reducing the overall molecular hydrogen gas formation.