Synthesis of an improved hierarchical carbon-fiber composite as a catalyst support for platinum and its application in electrocatalysis

A hierarchical carbon-fiber composite was synthesized based on carbon cloth (CC) modified with primary carbon microfibers (CMF) and subsequently secondary carbon nanotubes (CNT), thus forming a three-dimensional hierarchical structure with high BET surface area. The primary CMFs and the secondary CNTs are grown with electrodeposited iron nanoparticles as catalysts from methane and ethylene, respectively. After deposition of Pt nanoparticles by chemical vapor deposition from (trimethyl)cyclopentadienylplatinum, the resulting hierarchical composite was used as catalyst in the electrocatalytic oxygen reduction (oxygen reduction reaction, ORR) as specific test reaction. The modification of the CC with CMFs and CNTs improved the electrochemical properties of the carbon composite as revealed by electrochemical impedance measurements evidencing a low charge transfer resistance for redox mediators at the modified CC. X-ray photoelectron spectroscopy measurements were carried out to identify the chemical state and the surface atomic concentration of the Pt catalysts deposited on the hierarchical carbon composites. The ORR activity of Pt supported on different composites was investigated using rotating disk electrode measurements and scanning electrochemical microscopy. These electrochemical studies revealed that the obtained structured catalyst support is very promising for electrochemical applications, e.g. fuel cells.     

Optimal oxygen concentration strategy through an isothermal oxidative coupling of methane plug flow reactor to obtain a high yield of C2 hydrocarbons

An optimal oxygen concentration trajectory in an isothermal OCM plug flow reactor for maximizing C₂ production was determined by the algorithm of piecewise linear continuous optimal control by iterative dynamic programming (PLCOCIDP). The best performance of the reactor was obtained at 1,085 K with a yield of 53.9%; while, at its maximum value, it only reached 12.7% in case of having no control on the oxygen concentration along the reactor. Also, the effects of different parameters such as reactor temperature, contact time, and dilution ratio (N₂/CH₄) on the yield of C₂ hydrocarbons and corresponding optimal profile of oxygen concentration were studied. The results showed an improvement of C₂ production at higher contact times or lower dilution ratios. Furthermore, in the process of oxidative coupling of methane, controlling oxygen concentration along the reactor was more important than controlling the reactor temperature. In addition, oxygen feeding strategy had almost no effect on the optimum temperature of the reactor. Finally, using the optimal oxygen strategy along the reactor has more effect on ethylene selectivity compared to ethane.     

二氧化碳和富氧空气对甲烷与乙烯燃烧的影响

利用碳氢燃烧实验台,研究了二氧化碳和富氧空气对甲烷、乙烯层流燃烧火焰的特性影响,分析了不同气氛下火焰结构特性和温度分布规律. 结果表明,随氧浓度从21%增加到50%(j),甲烷、乙烯火焰高度下降70%,火焰温度和亮度同时增加,且发光区域呈向下收缩趋势;相同氧浓度下,乙烯火焰亮度高于甲烷. 随CO2浓度由0增加到20%(j),火焰高度增加28%,各高度处火焰边缘温度平均下降290℃,中心温度平均下降132℃. 火焰亮度降低,由黄色变为暗黄色,底部亮度更低,CO2浓度超过20%(j)后,火焰出现悬浮状态,最终被吹熄.     

An experimental study of oxidative coupling of methane in a solid oxide fuel cell with 1 wt%Sr/La2O3-Bi2O3-Ag-YSZ membrane

A solid oxide fuel cell with 1 wt%Sr/La₂O₃-Bi₂O₃-Ag-YSZ membrane was applied to oxidative coupling of methane. Membrane composition had a great effect on the reaction and current generated. An increase in the current generated was accompanied by a decrease in C₂ selectivity and an increase in CH₄ conversion. There is an optimal temperature for C₂-selectivity. CH₄ conversion decreased, C₂-selectivity increased and current generated decreased slightly with a rise in total flow rates. CH₄ conversion and the current generated increased with a rise in oxygen concentration. If only C₂-selectivity and current were concerned, the higher the methane concentration, the more favourable for the cogeneration of electrical energy and ethane and ethylene. Stability of the membrane was also tested.     

The efficient synthesis of carbon nano-onions using chemical vapor deposition on an unsupported Ni–Fe alloy catalyst

Well graphitized carbon nano-onions (CNOs) with large yield have been synthesized by the catalytic decomposition of methane over an unsupported Ni–Fe catalyst at 850 °C. The unsupported Ni–Fe catalyst was prepared by a reduction-substitution method. In the Ni–Fe alloy particle, α-Fe(Ni) phase (kamacite) transforms to γ-Fe–Ni phase at the high temperature for hydrogen reduction and chemical vapor deposition. The synthesized CNOs contain either a Fe_0.64Ni_0.36 particle or a hollow core with thick graphitic layers and a polyhedral shape. Based on the characterization, we believe that the catalyst involved in the synthesis of carbon products is Fe–Ni–C austenite rather than the γ-Fe–Ni phase (Fe_0.64Ni_0.36). A growth mechanism for the CNOs is proposed.     

Oxidative coupling of methane with microwave and RF plasma catalytic reaction over transitional metals loaded on ZSM-5

In the microwave and radio frequency (RF) plasma catalytic reaction at room temperature, the oxidative coupling of methane (OCM) over transitional metals loaded on ZSM-5 has been carried out. The transitional metals, Fe, Ni, Co and Cu (1B family element), loaded on ZSM-5 have been tested for the OCM using the plasma catalytic reaction. In this work, the conversion of methane to C2 products has been improved without the carbon deposition when the oxygen as a co-reactant gas was supplied. The order of the catalysts of higher conversion to C2 products in the microwave plasma reaction with plasma power of 120 W is Co–ZSM-5>Fe–ZSM-5Cu–ZSM-5>Ni–ZSM-5. Selectivity to ethylene is as high as 29.8%, to ethane 10% and to acetylene 49.9% and conversion of methane was as high as about 54.9% with pressure of 5–10 Torr, total flow rate of 125 ml/min and a methane/oxygen ratio of 4:1 over Co–ZSM-5. It is suggested that the catalyst may provide an active site for combination of radicals. It was found that yield of C2 products has been enhanced in the microwave plasma catalytic reaction, as microwave plasma system of high frequency has better energy efficiency than RF plasma system of low frequency.     

Determination of optimal temperature profile in an OCM plug flow reactor for the maximizing of ethylene production

An optimal temperature profile for achieving the highest amount of ethylene production in a fixed bed plug flow reactor of oxidative coupling of methane (OCM) was determined and analyzed from chemical reactions point of view. A performance index was defined to solve the problem and the optimal temperature profile was found by means of “piecewise linear continuous optimal control by dynamic programming” (PLCOCIDP) algorithm. The performance obtained from the optimal temperature profile was compared with the best isothermal performance of the reactor, gained from optimization calculations. It was concluded that the quantity of ethylene production by applying the optimal temperature profile is about 42% greater than that of the best isothermal performance of the reactor at 1096.3 K. It was also observed that maximization of ethylene production and minimization of carbon dioxide production are interrelated in this system. The effects of total pressure and initial oxygen concentration were studied as well. The optimal temperature profile spread out and its minimum point rose a little with increasing the concentration of oxygen and/or decreasing the total pressure. Moreover, when the total pressure or initial oxygen mole fraction was elevated, the ethylene molar flux increased, however the selectivity of ethylene reduced in case of increasing the oxygen molar flux.     

Reactivity of a spray-dried NiO/NiAl2O4 oxygen carrier for chemical-looping combustion

Kinetic data of a promising oxygen carrier of NiO/NiAl₂O₄ have been established from experiments in a small fluidized bed batch reactor using methane. The particles were prepared by spray-drying using commercially available raw material and selected as the best candidates from an earlier screening study. The particles clearly showed high reactivity, with a maximum gas yield between 86% and 93% in the temperature interval 750 °C to 950 °C when using a bed mass and a gas flow corresponding to only 6 kg/MW_fuel. A comparison of the reactivity with data from TGA experiments showed that the reactivity generally was faster in the batch fluidized bed in the investigated temperature interval. A simple reactor model using kinetic data from the batch fluidized bed reactor and the TGA predicted a minimum mass of 9–24 kg/MW_fuel of oxygen carrier particles for full gas yield of methane to carbon dioxide in the fuel reactor. Comparison with experiments performed in a 10 and 120 kW CLC reactor with the same type of oxygen carrier showed that even when employing 13 to 50 times the amount of oxygen carrier theoretically needed for complete gas conversion, full gas yield was not obtained in the circulating systems. Hence it is of great importance to consider the fluid dynamics and gas-solid contact when modeling the fuel reactor of a chemical-looping combustor.     

Carbon deposition and catalytic deactivation during CO2 reforming of CH4 over Co/MgO catalyst

The deactivation mechanism of Co/MgO catalyst for the reforming of methane with carbon dioxide was investigated. The conversion of CH₄ displayed a significant decrease in the initial stage caused by carbon deposition. There were two types of cokes, carbon nanotubes (CNTs) and carbon nano-onions (CNOs). The number of the CNO layers that coated on the surface of Co nanoparticles (NPs) increased rapidly in the initial reforming time, which was responsible for the deactivation of the Co/MgO catalyst. The deposition of CNOs was attributed to the oxidation of Co NPs. Therefore, the deactivation of the Co/MgO catalyst was originated from the first oxidization of the Co NPs into Co₃O₄ by O species (OH intermediate, CO₂, H₂O) during the reforming reaction, which accelerates the formation of coke that blocked the active metal, thus led to catalyst deactivation.     

Gas-phase flame synthesis and properties of magnetic iron oxide nanoparticles with reduced oxidation state

Iron oxide nanoparticles of reduced oxidation state, mainly in the form of magnetite, have been synthesized utilizing a new continuous, gas-phase, nonpremixed flame method using hydrocarbon fuels. This method takes advantage of the characteristics of the inverse flame, which is produced by injection of oxidizer into a surrounding flow of fuel. Unlike traditional flame methods, this configuration allows for the iron particle formation to be maintained in a more reducing environment. The effects of flame temperature, oxygen-enrichment and fuel dilution (i.e. the stoichiometric mixture fraction), and fuel composition on particle size, Fe oxidation state, and magnetic properties are evaluated and discussed. The crystallite size, Fe(II) fraction, and saturation magnetization were all found to increase with flame temperature. Flames of methane and ethylene were used, and the use of ethylene resulted in particles containing metallic Fe(0), in addition to magnetite, while no Fe(0) was present in samples synthesized using methane.     

Hydrogen production from methane in a dielectric barrier discharge using oxide zinc and chromium as catalyst

The hydrogen fuel cell is a promising option as a future energy resource; however, the nature of the gas is such that the conversion process of other fuels to hydrogen on board is necessary. Among the raw fuel resources, methane could be the best candidate as it is plentiful. In this experiment, the possibility of producing hydrogen with less carbon formation from methane by a dielectric barrier discharge (DBD) was investigated. Without the addition of a catalyst, the formation of hydrogen reached between 30% and 35% at methane residence time of 0.22 min and supplied powers in the range of 60-130 W. The hydrogen selectivity increased at higher supplied power, but the process efficiency, defined as a ratio of the produced hydrogen to the supplied power, decreased slightly. In order to boost the hydrogen production with less carbon formation, a mixed oxide catalyst of zinc and chromium was added to the reactor. It was shown that the production of hydrogen was ca. 40% higher than the non-catalytic plasma process.     

A direct catalytic conversion of natural gas to C2+ hydrocarbons by microwave plasma

Methane, the major constituent of natural gas, was converted to higher hydrocarbons by a microwave plasma. The yield of C2+ products increased from 29.2 % to 42.2% with increasing plasma power and decreasing flow rate of methane. When catalysts were used in the plasma reactor, the selectivities of ethylene and acetylene increased, while the yield of C2+ remained constant. Among various catalysts used, Fe catalyst showed the highest ethylene selectivity of 30 %. And when the actual natural gas was introduced, more C2+ products were obtained (46%). This is due to the ethane and propane in the natural gas. Applying electric field inductance for evolving the high plasma, we obtained high C2+ products of 63.7 % when Pd-Ni bimetal catalyst was used.     

Oxidative coupling of methane to C2-hydrocarbons over lithium–cerium-promoted MgO and MgO–CaO catalysts

The oxidative coupling of methane to ethylene and ethane was studied over lithium–cerium-promoted MgO and MgO–CaO catalysts in the presence of molecular oxygen at 730°C and at atmospheric pressure in a continuous flow, fixed bed quartz reactor. The catalysts were prepared by an impregnation method and finally calcined at 900°C. The surface area, pore size distribution and pore volume of the catalysts were determined. The feed consisted of only methane and oxygen in the molar ratio of 2:1. The results obtained over the catalyst systems, viz. (i) lithium–cerium-promoted MgO and (ii) lithium–cerium-promoted MgO–CaO, have been compared. A relatively high C₂-selectivity has been obtained with Li–Ce-promoted MgO–CaO catalysts. The optimum yield and selectivity for C₂-hydrocarbons were found to be 21·5% and 76·8% respectively at a methane conversion of 28% over Li (7 wt%)–Ce (2 wt%)-doped MgO–CaO (3:1 wt ratio) catalyst. The various factors governing the activity and the selectivity of the catalyst systems have been discussed.     

固定床反应器中乙烯环氧化模拟计算及分析

采用二维拟均相反应器数学模型对国产 YS 型银催化剂的反应性能和反应器运行状况进行了模拟计算和操作分析,并与工业实际数据及相关实验研究结果进行了比较。计算结果和分析表明,相同反应操作条件下,提高乙烯和氧的浓度可加速反应的进行,增加环氧乙烷的产量,其中氧浓度的影响更大,其浓度提高可明显增加 EO 的选择性;二氧化碳对主、副反应均有抑制作用,其浓度提高对生产不利;适当增加抑制剂 EDC 的量有利于提高催化剂的选择性;提高操作空速有利于改善反应状况和提高 EO 产量。分析表明,甲烷致稳较氮气致稳具有明显的优越性,有利于改善催化剂床层的温度分布、提高反应器运行的热稳定性和 EO 生成的选择性。     

Oxidative coupling of methane over strontium-doped neodymium oxide: Parametric evaluations

Since its discovery in 1982, oxidative coupling of methane (OCM) has been considered one of the most promising approaches for the on-purpose synthesis of ethylene. The development of more selective catalysts is essential to improve process economics. In this work, undoped neodymium oxide as well as neodymium oxide doped with high (20%) and low (2.5%) levels of strontium were tested in a high-throughput fashion covering a wide range of operating conditions. The catalysts were shown to be able to achieve greater than 18% C₂+ yield. Space velocity was shown to play a significant role in C₂+ selectivity. For a methane to oxygen feed ratio of 3.5, selectivity increased with increasing space velocity, reaching a maximum of 62% at a methane conversion of 30% at an optimal space velocity of ~250,000 ml/h/g. The difference in activity between the three samples was linked to the contribution of different oxygen centers.     

Hydrogen production by decomposition of ethane-containing methane over carbon black catalysts

Mixtures of methane and small amounts of ethane were decomposed in the presence of carbon black (CB) catalysts at 1,073–1,223 K for hydrogen production. Although most of the added ethane was first decomposed to ethylene and hydrogen predominantly by non-catalytic reaction, subsequent decomposition of ethylene was effectively facilitated by the CB catalysts. Because some methane was produced from ethane, the net methane conversion decreased as the added ethane increased. The rate of hydrogen production from methane was decreased by the added ethane. A reason for this is that adsorption of methane on the active sites is inhibited by more easily adsorbing ethylene. In spite of this, the hydrogen yield increased with an increase of the added ethane because the contribution of ethane and ethylene decomposition to the hydrogen production was dominant over methane decomposition. A higher hydrogen yield was obtained in the presence of a higher-surface-area CB catalyst.     

SOFC中低浓度干甲烷在Ni/YSZ阳极上的反应

固体氧化物燃料电池(SOFC)趋向于直接使用甲烷天然气为燃料,确定甲烷在固体氧化物燃料电池阳极发生的化学与电化学反应非常重要.以Ni/YSZ为阳极、YSZ板做电解质、LSM为阴极,用涂浆法制作电解质支撑的电池,研究低浓度干甲烷在固体氧化物燃料电池中的反应.改变甲烷浓度、电池工作温度、电解质厚度,用在线色谱测量不同电流密度下,阳极出口气体产生速率.根据阳极出口气体产生速率变化,分析干甲烷在阳极的反应变化.通过氧消耗计算和转移电子数的分析,说明甲烷在电池阳极发生不同类型的反应.电流密度小时,甲烷发生部分氧化反应.电流密度大时,发生氢氧化和CO氧化,部分甲烷发生总反应为完全氧化的反应.部分甲烷发生完全氧化反应的同时,部分甲烷仍发生部分氧化反应,但其反应速率随电流密度增加逐渐降低.甲烷浓度和试验温度增加,甲烷开始发生完全氧化的电流密度增加.     

A study of the hydrogen production from a small plasma converter

In this paper, the hydrogen production characteristics of a purpose-designed and built small scale plasma converter for small engines was investigated. Hydrogen was produced through the reformation of ionised hydrocarbon fuel and air mixture by means of a spark discharge.The experimental results showed that a suitable size of the reaction chamber can increase the concentration of the produced hydrogen and that under a given methane supply rate, a low O₂/C ratio resulted in high hydrogen production concentration as well as high hydrogen volume flow rate. As the methane supply rate was increased, however, the hydrogen concentration reduced while the hydrogen volume flow rate increased. Hydrogen concentration was also improved when the intake gas mixture resided longer in the reaction channel, although the hydrogen volume flow rate revealed an inversed trend. It was also shown that by combining an intake air swirl with an appropriate O₂/C ratio and methane supply rate, both the methane conversion efficiency and the hydrogen production concentration were improved. In general, under the optimal operating condition, the plasma converter produced a maximum hydrogen concentration of 48% and a hydrogen volume flow rate of 70 mL/min.     

The simultaneous activation of methane and carbon dioxide to C2 hydrocarbons under pulse corona plasma over La2O3/γ-Al2O3 catalyst

The pulse corona plasma has been used as an activation method for reaction of methane and carbon dioxide, the product was C₂ hydrocarbons and by-products were CO and H₂. Methane conversion and the yield of C₂ hydrocarbons were affected by the carbon dioxide concentration in the feed. The conversion of methane increased with increasing carbon dioxide concentration in the feed whereas the yield of C₂ hydrocarbons decreased. The synergism of La₂O₃/γ-Al₂O₃ and plasma gave methane conversion of 24.9% and C₂ hydrocarbons yield of 18.1% were obtained at the power input of plasma was 30 W. The distribution of C₂ hydrocarbons changed by using Pd-La₂O₃/γ-Al₂O₃ catalyst, the major C₂ product was ethylene.     

N,N′-双(对甲苯磺酰基)-2-哌嗪羧酸甲酯的合成

由乙二胺和对甲苯磺酰氯反应制得N,N′-双(对甲苯磺酰基)乙二胺。此步最佳工艺条件为:n(对甲苯磺酰氯)∶n(乙二胺)=2.2∶1,反应溶剂为苯,反应温度为40～45℃,反应时间为6h,收率80%。由丙烯酸甲酯和溴反应制得α,β-二溴丙酸甲酯,收率为88%。再由上述二种中间体反应合成标题化合物,此步省略了N,N′-双(对甲苯磺酰基)乙二胺的二钠盐制备,收率为73%。