Combination of glow-discharge and arc plasmas for CF4 abatement

Decomposition of CF₄ by glow-discharge and arc plasmas was studied using a tubular quartz reactor, a disk type, and a T-type quartz reactor. The effects of different metal electrodes, input voltage, and reactor type on the efficiency of CF₄ total destruction (DRE) were studied. The T-shape reactor was more efficient in CF₄ destruction than either the disk or tubular type due to a combined effect of glow discharge and arc plasmas. Several hydrogen and oxygen sources, such as H₂O, H₂, O₂, and CH₄, were used to convert CF₄. Using H₂ and O₂ as the hydrogen and oxygen sources presented better DRE than using H₂O. The effect of different hydrogen and oxygen sources on the conversion of CF₄ followed the trend: (H₂ + O₂) > (CH₄ + O₂) > H₂O. The maximum DRE of 95% was observed with 0.5% CF₄ using H₂ and O₂. A mass spectrometer and an emission spectroscope equipped with a charge-coupled detector (CCD) were used to characterize the products and intermediates. Mass spectrometric studies indicated that the reaction products were HF, CO₂, and trace amounts of NO. N₂ first negative and second positive emission lines were observed in the glow discharge plasmas as well as in the arc plasmas of N₂. However, C and F intermediates were observed only in arc plasmas of CF₄. Reactions occurring in the glow discharge plasmas and arcs seem to follow different mechanisms.     

非平衡等离子体放电模式及反应器结构对氨气分解制氢的影响

在常压下研究了不同等离子体放电模式及反应器结构对氨分解制氢反应的影响.实验中调节反应器结构分别产生了介质阻挡放电和交流弧放电两种放电模式.通过对两种放电模式的放电图像、电压-电流波形和氨分解过程中等离子体区活性物种的发射光谱(OES)研究发现,与介质阻挡放电相比,交流弧放电为局部强放电,具有更高的电源效率和电子密度.因此,在介质阻挡放电中氨气分子大部分通过生成电子激发态物种NH3*,再与载能电子碰撞断裂N―H键进行氨分解反应;而在交流弧放电中载能电子具有更高的平均电子能量,可直接断裂氨气分子的N―H键生成NH2和NH等高活性物种,促进氨分解反应的进行.结果表明,交流弧放电的氨分解效果要明显优于介质阻挡放电.在交流弧放电模式下不同类型反应器对氨气分解转化率由高到低的顺序为:管-管管-板针-板板-板.在输入功率为30 W,气隙间距为6 mm时,管-管交流弧放电的氨气转化率达到60%左右,而板-板介质阻挡放电的氨气转化率仅为4%.     

吡啶萃取对烟煤热解过程焦油生成特性的影响

煤中含有以非共价键结合的可萃取物，煤的萃取物和萃余物热解反应性不同。本研究首先用醋酸消除煤中静电作用力，再以吡啶萃取消除氢键作用力，通过热重和固定床研究了煤萃取物和萃余物的热解特性。相对于原煤，萃取物（E1）的H/C原子比较高，而萃余物（R1）比原煤的孔径有所增大。热重实验表明，萃取物热分解温度低，失重率大；萃余物在485℃之前失重大于原煤，温度高于485℃小于原煤。固定床氮气热解表明，萃取物（E1）的焦油产率和气体比原煤高；萃余物（R1）的焦油产率低于原煤焦油产率。而氢气气氛下，萃取残渣的焦油产率明显高于原煤，这是由于吡啶萃余物具有更开放的孔结构，有利于加氢热解过程氢向孔内扩散，减少了缩聚反应。     

Optical emission diagnostics in cascade arc plasma polymerization and surface modification processes

Optical Emission Spectroscopy (OES) was used to identify reactive species and their excitation states in low-temperature cascade arc plasmas of N₂, CF₄, C₂F₄, CH₄, and CH₃OH. In a cascade arc plasma, the plasma gas (argon or helium) was excited in the cascade arc generator and injected into a reactor in vacuum. A reactive gas was injected into the cascade arc torch (CAT) that was expanding in the reactor. What kind of species of a reactive gas, for example, nitrogen, are created in the reactor is dependent on the electronic energy levels of the plasma gas in the cascade arc plasma jet. OES revealed that no ion of nitrogen was found when argon was used as the plasma gas of which metastable species had energy less than the ionization energy of nitrogen. When helium was used, ions of nitrogen were found. While OES is a powerful tool to identify the products of the cascade arc generation (activation process), it is less useful to identify the reactive species that are responsible for surface modification of polymers and also for plasma polymerization. The plasma surface modification and plasma polymerization are deactivation processes that cannot be identified by photoemission, which is also a deactivation process. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1583–1592, 1998     

The first example of tungsten-based carbene generation from WCl6 and atomic carbon and its use in olefin metathesis

We describe a new route for the synthesis of tungsten-based carbenes generated by the reaction of WCl₆ with atomic carbon in a carbon arc reactor. The active species formed under these conditions, [W] = CCl₂, was found to catalyze olefin metathesis reactions of 1-octene, 2-octene and 1-heptene. We also evaluated the mechanism of formation of [W] = CCl₂ within the WCl₆/C system at the DFT level.     

Plasma Nitrogen Oxides Synthesis in a Milli-Scale Gliding Arc Reactor: Investigating the Electrical and Process Parameters

Nitrogen fixed in the form of nitrogen oxides is essential to produce fertilizers and many other chemical products, which is vital to sustain life. The performance of a milli-scale gliding arc reactor operated under atmospheric pressure has been studied for nitrogen oxides synthesis. In this work, the electrical and process parameters of the gliding arc reactor, such as frequency, pulse width, amplitude and feed ratio were investigated respectively. The experiments were performed at 1 L/min in a gliding arc discharge regime. The highest concentration of NO_x was found to be ~1 % at energy consumption of 10 kWh/kg of NO_x. Increase in frequency, pulse width and amplitude resulted in an increased specific energy input and NO_x concentration. The feed ratio (N₂/O₂) affected the amount of NO and NO₂ produced, which gives possibility to independently obtain the desired ratio of NO/NO₂ by tuning the electrical and process parameters.     

H2-free Semi-hydrogenation of Butadiene by the Atomic Sieving Effect of Pd Membrane with Tree-like Pd Dendrites Array

H₂-free semi-hydrogenation at room temperature shows great advantage for replacing the thermocatalytic process in industry owing to the high energy and resource saving, however, remains great challenges. Herein, a tree-like Pd dendrites array decorated Pd membrane was constructed as the core device in an electrochemistry assisted gas-fed membrane reactor for butadiene semi-hydrogenation. It reveals that hydrogen atomic sieving effect of this Pd-based membrane under electrochemical condition was the key for semi-hydrogenation. The configuration study of Pd nanostructured membrane demonstrates that the penetration of hydrogen atoms through Pd membrane from electrochemical side to chemical side is affected by the consumption of hydrogen atom in semi-hydrogenation step. Such atomic sieving property of nanostructured Pd membrane with 5.1 times increase in catalytic active surface area brings above 14 times higher in butadiene conversion than that of bare Pd foil, with ≈90 % of butenes selectivity at butadiene conversion ≈98 % over 300 h of H₂-free reaction under 15 mA cm⁻².     

Characterization of an argon-hydrogen microwave discharge used as an atomic hydrogen source. Effect of hydrogen dilution on the atomic hydrogen production

This work is devoted to the study of an argon-hydrogen microwave plasma used as an atomic hydrogen source. Our attention has focused on the effect of the hydrogen dilution in argon on atomic hydrogen production. Diagnostics are performed either in the discharge or in the post-discharge using emission spectroscopy (actinometry) and mass spectrometry. The agreement between actinometry and mass spectrometry diagnostics proves that actinometry on the Ha(656.3 nm) and Hβ(486.1 nm) hydrogen Balmer lines can be used to measure the relative atomic hydrogen density within the microwave discharge. Results show that the atomic hydrogen density is maximum for a gas mixture corresponding to the partial pressure ratioP _{H 2}/P _Ar range between 1.5 and 2. The variation of atomic hydrogen density can be explained by a change of the dominant reactive mechanisms. At a low hydrogen partial pressure the dominant processes are the charge transfers with recombinations between Ar⁺ and H₂ which lead to ArH⁺ and H ₂ ⁺ ion formation. Both ions are dissociated in dissociative electron attachment processes. At a low argon partial pressure the electron temperature and the electron density decrease with increasing partial pressure ratio. The dominant mechanisms become direct reactions between charged particles (e, H⁺, H ₂ ⁺ , and H ₃ ⁺ ) or excited species H(n=2) with H₂ producing H atoms.     

Simulation study on the catalytic decomposition of hydrogen iodide in a membrane reactor with a silica membrane for the thermochemical water-splitting IS process

Catalytic decomposition of hydrogen iodide in a membrane reactor was investigated theoretically for the application to the hydrogen production step in the thermochemical iodine–sulfur (IS) process. Characteristics of the membrane reactor were evaluated using observed permeances of H₂ and HI in a homemade silica membrane that was prepared by chemical vapor deposition (CVD) method (selectivity of H₂/HI: 650). The effect of the H₂/I₂ selectivity on the performance of the membrane reactor was evaluated by simulation since I₂ permeance through the homemade silica membrane could not be determined so far because of the difficulty of the measurements. It was found from the simulation study that the conversion of over 0.9 would be attainable using the membrane reactor with the homemade silica membrane. Design criterion of the membrane reactor was discussed using the relationships between the ratio of reaction zone volume to the membrane surface area, the dimensionless reactor length and the conversion.     

The chemical reduction of small inorganic gases in an electrothermal plasma reactor

Chemical reduction of small inorganic gases is accomplished using an electro-thermal plasma reactor. This benchscale reactor maintains a highly reactive plasma zone in a fluidized bed of carbon particles through which an electrical current is discharged. The carbon particles function as current-controlling media, heat sinks and reaction sites. Chemicals introduced into this medium are subjected to a variety of energy sources and chemically reactive species, which result in chemical reduction. It is shown that the inorganic gases, H₂O, CO₂, NO, NO₂ and SO₂, are chemically reduced in the plasma zone of the reactor. The end products consist of hydrogen, carbon monoxide and nitrogen gases. Additionally, elemental sulfur is deposited onto the carbon particles.     

New In-situ Sampling and Analysis of the Production of CeO<Subscript>2</Subscript> Powders from Liquid Precursors using a Novel Wet Collection System in a rf Inductively Coupled Thermal Plasma Reactor. Part 1: Reactor System and Sampling Probe

A new reactor and a novel in-situ sampling technique were developed for the study of the synthesis of CeO₂ powders produced from dissolved cerium nitrate salts. The conical reactor minimized particle recirculation and provided a highly symmetrical and undisturbed plasma flow suitable for the analysis of the phenomena affecting the formation of CeO₂ powders. Both a calorimetric study of the reactor and a thermodynamic analysis of CeO₂ formation were conducted. The sampling probe is described and near-isokinetic sampling was achieved. The sampled particles were collected using a miniature wet collection system, i.e. a mist atomizer and a custom-made spray chamber. A numerical simulation of the velocity and temperature fields of the plasma gas in the reactor was done using Fluent. A comprehensive droplet-to-particle formation mechanism presented elsewhere is revisited and expanded based on calorimetry, thermodynamics of CeO₂ formation, numerical simulations and collected particles. No traces of other oxidation states other than CeO₂ were found.     

Abnormally high heat generation by transition metals interacting with hydrogen and oxygen molecules

Abnormally high heats, exceeding 2000 kJ/mol (20 eV) per molecule of O₂, are generated by interaction of the oxygen with the hydrogen absorbed on palladium, gold and nickel particles at 25 °C to 220 °C. The highest heats were observed when the metals were treated with micromole quantities of argon, prior to absorption of hydrogen, as well as its interactions with metal particles reaching nanometer size. In the latter case the heat evolutions due to the interactions with hydrogen were approaching 5000 kJ/mol. The interactions with oxygen in inert gas environments, such as that of argon, yielded higher heat evolutions than those given by pure O₂ pulses injected into nitrogen carrier gas. The results revealed an important role of argon in increasing the intensity of atomic hydrogen-oxygen reactions to a level several times higher than the heat of water formation from molecular hydrogen and oxygen.     

Catalytic properties of Ru nanoparticles embedded on ordered mesoporous carbon with different pore size in Fischer-Tropsch synthesis

A series of 3 wt% Ru embedded on ordered mesoporous carbon (OMC) catalysts with different pore sizes were prepared by autoreduction between ruthenium precursors and carbon sources at 1123 K. Ru nanoparticles were embedded on the carbon walls of OMC. Characterization technologies including power X-ray diffraction (XRD), nitrogen adsorption-desorption, transmission electron microscopy (TEM), and hydrogen temperature-programmed reduction (H₂-TPR) were used to scrutinize the catalysts. The catalyst activity for Fischer-Tropsch synthesis (FTS) was measured in a fixed bed reactor. It was revealed that 3 wt% Ru-OMC catalysts exhibited highly ordered mesoporous structure and large surface area. Compared with the catalysts with smaller pores, the catalysts with larger pores were inclined to form larger Ru particles. These 3 wt% Ru-OMC catalysts with different pore sizes were more stable than 3 wt% Ru/AC catalyst during the FTS reactions because Ru particles were embedded on the carbon walls, suppressing particles aggregation, movement and oxidation. The catalytic activity and C₅₊ selectivity were found to increase with the increasing pore size, however, CH₄ selectivity showed the opposite trend. These changes may be explained in terms of the special environment of the active Ru sites and the diffusion of products in the pores of the catalysts, suggesting that the activity and hydrocarbon selectivity are more dependent on the pore size of OMC than on the Ru particle size.     

Electroreduction of carbon dioxide: Part III. Adsorption and reduction of CO2 on platinum

The main properties of reductional adsorption of CO₂ on the platinum metals are studied. Chemisorbed particles are found to be produced only on platinum and rhodium. Electroreduction of CO₂ on these metals proceeds as a result of the interaction of CO₂ molecules activated in the course of adsorption on the metal surface with chemisorbed hydrogen. As a result, strongly chemisorbed particles are obtained on the surface which are the products of more profound reduction of CO₂ than to formic acid. The further reduction of these chemisorbed particles, accompanied by their desorption into the solution, is very slow due to very strong coupling of sorbed particles with the surface and to very fast backward adsorption of the reduction products. Neither reductional chemisorption of CO₂ nor interactions of CO₂ with adsorbed hydrogen were observed for iridium, palladium, osmium or ruthenium.     

On the mechanism of thermal oxidation of methane

Using the method of freezing radicals in conjunction with ESR spectroscopic measurements, the kinetics of the thermal oxidation of methane has been studied under atmospheric pressure depending on the temperature, composition of the mixture, and nature of the surface of the reaction vessel. It has been shown that in a reactor treated with boric acid, the intermediates methylhydroperoxide and hydrogen peroxide are responsible for chain branching. It has been established that the leading active centers of the reaction are the HO₂ radicals, while chain branching occurs as a result of the decomposition of peroxy compounds—methylhydroperoxide and hydrogen peroxide. In reactors treated with potassium bromide, the concentrations of radicals and peroxy compounds were found to be lower than the sensitivity of the method of measurement. Computations were performed for the scheme of methane oxidation at 738 K for a reactor treated with boric acid. Satisfactory agreement was found between the experimental and computed kinetic curves of accumulation of main intermediates CH₂O, H₂O₂, CH₃OOH. The influence of their addition on the kinetics of the reaction has been considered. It has been shown that the addition of formaldehyde does not lead to chain branching, however; it contributes to the formation of those peroxy compounds that bring about chain branching. Mathematical modeling confirmed conclusions made on the basis of experimental data concerning the nature of the leading active centers and the products that are responsible for the degenerate chain branching.     

Eliminated Phototoxicity of TiO2 Particles by an Atomic‐Layer‐Deposited Al2O3 Coating Layer for UV‐Protection Applications

We demonstrate the conformal coating of an ultrathin Al₂O₃ layer on TiO₂ nanoparticles through atomic layer deposition by using a specifically designed rotary reactor to eliminate the phototoxicity of the particles for cosmetic use. The ALD reactor is modified to improve the coating efficiency as well as the agitation of the particles for conformal coating. Elemental and microstructural analyses show that ultrathin Al₂O₃ layers are conformally deposited on the TiO₂ nanoparticles with a controlled thickness. Rhodamine B dye molecules on Al₂O₃‐coated TiO₂ exhibited a long life time under UV irradiation, that is, more than 2 h, compared to that on bare TiO₂, that is, 8 min, indicating mitigation of photocatalytic activity by the coated layer. The effect of carbon impurities in the film resulting from various deposition temperatures and thicknesses of the Al₂O₃ layer on the photocatalytic activity are also thoroughly investigated with controlled experimental condition by using dye molecules on the surface. Our results reveal that an increased carbon impurity resulting from a low processing temperature provides a charge conduction path and generates reactive oxygen species causing the degradation of dye molecule. A thin coated layer, that is, less than 3 nm, also induced the tunneling of electrons and holes to the surface, hence oxidizing dye molecules. Furthermore, the introduction of an Al₂O₃ layer on TiO₂ improves the light trapping thus, enhances the UV absorption.     

Syngas Production from Propane Using Atmospheric Non-thermal Plasma

Propane steam reforming using a sliding discharge reactor was investigated under atmospheric pressure and low temperature (420 K). Non-thermal plasma steam reforming proceeded efficiently and hydrogen was formed as a main product (H₂ concentration up to 50%). By-products (C₂-hydrocarbons, methane, carbon dioxide) were measured with concentrations lower than 6%. The mean electrical power injected in the discharge is less than 2 kW. The process efficiency is described in terms of propane conversion rate, steam reforming and cracking selectivity, as well as by-products production. Chemical processes modelling based on classical thermodynamic equilibrium reactor is also proposed. Calculated data fit quiet well experimental results and indicate that the improvement of C₃H₈ conversion and then H₂ production can be achieved by increasing the gas fraction through the discharge. By improving the reactor design, the non-thermal plasma has a potential for being an effective way for supplying hydrogen or synthesis gas.     

Selective oxidation of CO under conditions of catalyst surface ignition

The oxidation of CO in the presence of an excess of hydrogen and to 20% CO₂ and H₂O in the starting mixture was studied in flow reactors with high and low rates of heat removal. The ignition of the catalyst surface was observed in the reactor with a low rate of heat removal; catalyst surface ignition initially occurred at a “hot” spot (section) of the catalyst bed and gradually propagated along the bed. Experimental data on the relaxation dynamics of residual CO concentration and temperature in a catalyst bed under conditions of small heater temperature disturbances near and at the critical temperature of ignition and the effect of oxygen concentration in the starting mixture on this process are reported. It was found exprimentally that the ignition regime in the tested cases was more favorable for the selective oxidation of CO in an excess of hydrogen than the reaction in an isothermal reactor; this was likely due to the more favorable temperature distribution over the length of the catalyst bed.     

Generation of Iron Atoms in the Gas Phase by Microwave‐Induced Plasma Afterglows

A fast‐flow reactor technique is described by which Fe atoms can be produced in the gas phase in the afterglow of microwave‐induced plasmas in hydrogen/argon and hydrogen/helium mixtures. When the iron salt FeCl₃(s) was brought into the gas phase by thermal sublimation at temperatures between 360 and 405 K, it was partly converted to Fe atoms by reaction of the gaseous compounds Fe_xCl_3x(g) with hydrogen atoms. The Fe atoms were detected by atomic absorption spectroscopy (AAS). It was shown that sublimation of the salt is the rate‐determining step of the overall plasma‐afterglow atomisation process. Experimental conditions for the generation of Fe atoms suited to kinetic studies start at a temperature of 303 K. In the downstream region the concentration of Fe atoms decays due to diffusion to the reactor wall. Binary diffusion coefficients D_Fe/Ar and D_Fe/He of 231.5±6.6 and 370.0±15.5 cm² s^?1 Torr at 303 K, respectively, were determined.