A Simplified Global Model to Describe the Oxidation of Acetylene Under Nanosecond Pulsed Discharges in a Complex Corona Reactor

The objective of this paper is to analyse the oxidation of acetylene under nanosecond pulsed N₂/O₂ discharges generated in a complex multi-pin-to-plane (MPP) corona reactor in the frame of Yan’s generic chemical kinetic model. We made use of the results obtained from the detailed kinetic model published previously (Redolfi et al. in Plasma Chem Plasma Process 29(3):173–195, 2009) in order to propose a global reactor models based on Yan’s generic chemical model and taking into account the non-homogeneous and non-stationary character of the discharges. This enables us expressing the energy cost in terms of physical and kinetic parameters of the discharge. We checked the model validity by comparing predicted and measured energy cost-values for acetylene in MPP reactor. The methodology presented may be adapted to predict the energy cost in other complex corona reactor provided the model parameters are determined experimentally.     

Influence of Plasma Regimes and Catalysts on Ethanethiol Oxidation

Plasma oxidation of ethanethiol in air was investigated using three plasma regimes: surface dielectric pulsed corona discharge, surface dielectric barrier discharge and pulsed corona discharge (PCD) in the plasma reactor. Catalytic plasma degradation of ethanethiol was also performed on the singular or binary metals doped ?èCAl₂O₃. The ethanethiol removal rate increased with increasing energy density but energy efficiency was first increased and then decreased with increasing energy density under three various types of discharges. PCD plasma required the lowest energy density at the similar ethanethiol removal performance compared with the other two plasma discharges. The main intermediate by-products of ethanethiol oxidation by plasma are CH₃CHO, HCHO, CO and CO₂. The sum of these intermediate products selectivities is 19?C43?%, implying that some other intermediates containing carbon were undetermined. When using PCD plasma combined with catalysts, ethanethiol removal rate and energy efficiency were all evidently improved. The maximum energy efficiency was achieved about 200?g kWh^?1 using Fe?CMn/?èCAl₂O₃ assisted PCD plasma, which was about 4.4 times when using PCD plasma alone. The mechanism of ethanethiol oxidation is also discussed.     

Characteristics of the Decomposition of CO2 in a Dielectric Packed-Bed Plasma Reactor

The decomposition of CO₂ in a dielectric packed-bed plasma reactor has been studied. It was found that the dielectric properties and morphology of packing dielectric pellets play important roles in the reaction due to their influence on the electron energy distribution in the plasma. The acid–base properties of the packing materials also affect the reaction through the chemisorption of CO₂ on basic sites of the materials. Heterogeneous reactions on the solid surfaces of the dielectric materials also play a role in the reaction, which was also confirmed through the investigation of the influence of the discharge length on the reaction. The reverse reaction of CO₂ decomposition, the oxidation of CO, was also investigated to further understand the role of dielectric materials in the plasma and their effect on plasma reactions. Both the decomposition of CO₂ and the oxidation of CO in non-packed or dielectric packed reactors are first-ordered.     

Direct Evidence of Local pH Change and the Role of Alkali Cation during CO2 Electroreduction in Aqueous Media

We report, for the first time, utilizing a rotating ring‐disc electrode (RRDE) assembly for detecting changes in the local pH during aqueous CO₂ reduction reaction (CO₂RR). Using Au as a model catalyst where CO is the only product, we show that the CO oxidation peak shifts by ?86±2 mV/pH during CO₂RR, which can be used to directly quantify the change in the local pH near the catalyst surface during electrolysis. We then applied this methodology to investigate the role of cations in affecting the local pH during CO₂RR and find that during CO₂RR to CO on Au in an MHCO₃ buffer (where M is an alkali metal), the experimentally measured local basicity decreased in the order Li⁺ > Na⁺ > K⁺ > Cs⁺, which agreed with an earlier theoretical prediction by Singh et al. Our results also reveal that the formation of CO is independent of the cation. In summary, RRDE is a versatile tool for detecting local pH change over a diverse range of CO₂RR catalysts. Additionally, using the product itself (i.e. CO) as the local pH probe allows us to investigate CO₂RR without the interference of additional probe molecules introduced to the system. Most importantly, considering that most CO₂RR products have pH‐dependent oxidation, RRDE can be a powerful tool for determining the local pH and correlating the local pH to reaction selectivity.     

"Decoking" of a "coked" zeolite catalyst in a glow discharge

'Decoking' of a 'coked' zeolite catalyst in a glow discharge in oxygen is investigated. The 'decoking' process involves reactions of atomic oxygen (O atoms) with 'coke' and yields gases such as CO, CO₂ as well as other gaseous products that could be easily pumped out.Three different modes of discharge were investigated including a static mode, a flowing-gas mode, and a periodic-purge mode where the oxygen and other gaseous products of the discharge were replaced by fresh O₂ gas after short but regular intervals of time. In some cases, additional heating was also used to provide base temperatures of the order of 100 °C to facilitate penetration of oxygen atoms into the inner layers and cages of the zeolite catalyst.This paper presents some results of spectroscopic analytical techniques used to monitor the atomization of oxygen, oxidation of 'coke', and to confirm the process of 'decoking'. More specifically, radiation emission on the 3 s ⁵S– 3p ⁵P transitions of O around 777.2–777.5 nm were selected for monitoring the atomization of O₂. On the other hand, X-ray photo-electron spectroscopy (XPS) was used to determine the amount of residual carbon and extent of 'decoking'. Furthermore, evolution of CO and CO₂ gases as a function of time was systematically monitored in real time. For CO, the 451.1 nm band head belonging to the B¹ - A¹ bands of the Angstrom system of the CO spectrum was used, while for CO₂, the band head at 353.4 nm belonging to the CO₂⁺ spectrum was used. The rates of evolution of CO and CO₂ were related to the rate of 'decoking' of the catalyst. It is noted that in the periodic-purge mode, about 63% of the total yield of CO from a given sample of the catalyst appears in the first 3-min exposure to discharge whereas it takes up to 15 min to remove nearly 94% of the removable carbon under our experimental conditions.     

Selective CO oxidation on a Ru/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst in the surface ignition regime: 1. Fine purification of hydrogen-containing gases

Selective CO oxidation in a mixture simulating the methanol steam reforming product with an air admixture was studied over Ru/Al₂O₃ catalysts in a quasi-adiabatic reactor. On-line monitoring of the gas temperature in the catalyst bed and of the residual CO concentration at different reaction conditions made it possible to observe the ignition and quenching of the catalyst surface, including transitional regimes. A sharp decrease in the residual CO concentration takes place when the reaction passes to the ignition regime. The evolution of the temperature distribution in the catalyst bed in the ignition regime and the specific features of the steady-state and transitional regimes are considered, including the effect of the sample history. In selective CO oxidation and in H₂ oxidation in the absence of CO, the catalyst is deactivated slowly because of ruthenium oxidation. In both reactions, the deactivated catalyst can be reactivated by short-term treatment with hydrogen. A 0.1% Ru/Al₂O₃ catalyst is suggested. In the surface ignition regime, this catalyst can reduce the residual CO concentration from 0.8 vol % to 10–15 ppm at O₂/CO = 1 even in the presence of H₂O and CO₂ (up to ～20 vol %) at a volumetric flow rate of ～100 1 (g Cat)^?1 h^?1, which is one magnitude higher than the flow rates reported for this process in the literature.     

Carbon monoxide production in silent discharge plasmas of air and air-methane mixtures

CO production in high-voltage alternating current (HVAC) silent discharge plasmas of air and air-methane mixtures at atmospheric pressure has been investigated by matrix isolation FTIR spectroscopy. In pure air, CO is produced by decomposition of CO₂. A steady-state CO/CO₂ ratio was determined by varying the flow rate. CO production was considerably enhanced when methane was added to the plasmas. CO production was observed even at very low oxygen concentrations, and did not noticeably decrease due to secondary oxidation reactions, even when methane was discharged in a pure oxygen carrier. CO production in air-methane mixtures is shown to depend on input power. CO production from CO₂ and hydrocarbons in air appears to be a significant obstacle for development of a plasma-based air purification device.     

Control of selectivity in methane conversion reactions in RF plasma: the influence of reaction conditions

RF plasma excitation of methane has been studied in an effort to optimize the reaction conditions for a selective partial oxidation of methane. The reaction products of RF-excited methane are C2 hydrocarbons such as ethane and acetylene when O₂ is not used. The introduction of a few percent of O₂, however, is found to switch the selectivity in favor of CO while CO₂ formation is suppressed down to a level below a few percent. Interestingly, in the low O₂ ratio regime (0–0.6), the selectivity between CO and C2 hydrocarbons is observed to vary systematically in response to the detailed reaction conditions, including flow rate, pressure and applied RF power, which are explained by the competition between coupling and partial oxidation reactions. Variation in the density and the residence time of the active species in the plasma is suggested to determine the overall reaction pathways. The present results suggest a possibility of a selective production of the partial oxidation products of methane such as CO with a high selectivity and a high conversion efficiency using controlled RF plasma from methane and O₂.     

CO_2 residual concentration of potassium-promoted hydrotalcite for deep CO/CO_2 purification in H_2-rich gas

Elevated-temperature pressure swing adsorption is a promising technique for producing high purity hydrogen and controlling greenhouse gas emissions. Thermodynamic analysis indicated that the CO in H_2-rich gas could be controlled to trace levels of below 10 ppm by in situ reduction of the CO_2 concentration to less than 100 ppm via the aforementioned process. The CO_2 adsorption capacity of potassiumpromoted hydrotalcite at elevated temperatures under different adsorption(mole fraction, working pressure) and desorption(flow rate, desorption time, steam effects) conditions was systematically investigated using a fixed bed reactor. It was found that the CO_2 residual concentration before the breakthrough of CO_2 mainly depended on the total amount of purge gas and the CO_2 mole fraction in the inlet syngas.The residual CO_2 concentration and uptake achieved for the inlet gas comprising CO_2(9.7 mL/min) and He(277.6 mL/min) at a working pressure of 3 MPa after 1 h of Ar purging at 300 mL/min were 12.3 ppm and0.341 mmol/g, respectively. Steam purge could greatly improve the cyclic adsorption working capacity, but had no obvious benefit for the recovery of the residual CO_2 concentration compared to purging with an inert gas. The residual CO_2 concentration obtained with the adsorbent could be reduced to 3.2 ppm after 12 h of temperature swing at 450 °C. A new concept based on an adsorption/desorption process, comprising adsorption, steam rinse, depressurization, steam purge, pressurization, and high-temperature steam purge, was proposed for reducing the steam consumption during CO/CO_2 purification.     

Experimental Investigation of Plasma Oxidization of Diesel Particulate Matter

Particulate matter (PM) from diesel vehicles is harmful to humans and should be removed from the exhaust gases before its emission into the atmosphere. Plasma PM oxidation is an advanced method to be used for oxidative PM removal. Factors influencing plasma PM oxidation include gas temperature, gas composition, PM amount, the geometry of plasma reactors. The PM oxidation in atmospheric air discharges was carried out using a pulsed dielectric barrier discharge reactor at temperatures of 100, 150, and 200 °C. It was found that PM is oxidized to CO and CO₂. CO₂/CO concentration ratio is a function of PM amount in the discharge space. PM removal efficiency (PM amount oxidized per kWh energy injection) increased with increasing air temperature and PM amount in the discharge space. Water promotes PM oxidation, which suggested that oxygen atoms produced in the discharge space react with water to yield hydroxyl free radicals that are of more reactivity than oxygen atoms. The activation energy of plasma PM oxidation was kinetically calculated to be 15.4 kJ/mol.     

Effect of porosity of α-alumina on non-thermal plasma decomposition of ethylene in a dielectric-packed bed reactor

The performances of the porous and nonporous α-alumina (α-Al₂O₃) for the decomposition of ethylene in a dielectric-packed bed plasma reactor were comparatively examined with respect to the decomposition efficiency and the formation of byproducts. The decomposition was mainly controlled by discharge power, oxygen content, and properties of the alumina, such as porosity and surface area. The addition of a small quantity of oxygen led to an increase in the generation of oxidative species which eventually increased the ethylene decomposition efficiency. In the presence of 5 % oxygen, ethylene at an initial concentration of 1,898 ppm was completely oxidized into CO or CO₂ when using the porous α-alumina. On the other hand, the nonporous α-alumina resulted in an incomplete oxidation, producing several carbon-containing byproducts other than CO and CO₂. Moreover, with the other conditions kept constant, the decomposition efficiency obtained with the porous α-alumina was higher than that with the nonporous one, suggesting the adsorption capability of the packing material plays an important role in the decomposition process.     

OH Radicals as Oxidizing Agent for the Abatement of Organic Pollutants in Gas Flows by Dielectric Barrier Discharges

OH radicals play an essential role in various plasma-chemical processes aimed at the abatement of organic and inorganic pollutants from off-air flows. We report about the oxidation of carbon monoxide in nonthermal air and nitrogen plasmas in dependence on CO inlet concentration and flow humidity. Thereby the reaction CO + OH CO₂ + H served as a diagnostic tool for OH radical determination in the dielectric barrier discharge at atmospheric pressure. The results were numerically fitted to the equations of a kinetic model allowing the determination of the average OH production efficiency (G_OH-value) and OH lifetime (T_OH) in dependence on flow humidity. Finally,results on ethyl acetate abatement obtained under similar experimental conditions were modeled by OH radical decomposition.     

Infrared studies of the effect of acetylene, hydrogen, and oxygen on CO adsorption on platinum hydrosols

Infrared spectra of CO-treated platinum hydrosols subsequently treated with acetylene, hydrogen, and oxygen reveal that v(CO)_ads decreases from 2070 cm⁻¹ with increasing gas-treatment time. This has been attributed to a reduction in the coverage of adsorbed CO. In Pt sol/CO/C₂H₂ systems, v(CO)_ads decreases to a limiting value of ca. 2060 cm⁻¹ after exposure to acetylene. In the Pt sol/CO/H₂ systems, v(CO)_ads decreases to ca. 2050 cm⁻¹ after exposure to hydrogen gas. The lower frequency in the Pt sol/CO/H₂ system has been attributed to CO adsorption on more active metal sites formed from the reduction of surface platinum oxides. Exposure of the CO-treated platinum hydrosols to O₂ gas was found to cause the eventual disappearance of the v(CO)_ads band in infrared spectra, which was attributed to oxidation of adsorbed CO to CO₂ by weakly bound surface layers of platinum oxides formed by the oxygen treatment.     

First‐principle calculations on CO oxidation catalyzed by a gold nanoparticle

We have elucidated the mechanism of CO oxidation catalyzed by gold nanoparticles through first‐principle density‐functional theory (DFT) calculations. Calculations on selected model show that the low‐coordinated Au atoms of the Au₂₉ nanoparticle carry slightly negative charges, which enhance the O₂ binding energy compared with the corresponding bulk surfaces. Two reaction pathways of the CO oxidation were considered: the Eley–Rideal (ER) and Langmuir–Hinshelwood (LH). The overall LH reaction O_2(ads) + CO_(gas) → O_2(ads) + CO_(ads) → OOCO_(ads) → O_(ads) + CO_2(gas) is calculated to be exothermic by 3.72 eV; the potential energies of the two transition states ( TS_LH1 and TS_LH2 ) are smaller than the reactants, indicating that no net activation energy is required for this process. The CO oxidation via ER reaction Au₂₉ + O_2(gas) + CO_(gas) → Au₂₉–O_2(ads) + CO_(gas) → Au₂₉–CO_3(ads) → Au₂₉–O_(ads) + CO_2(gas) requires an overall activation barrier of 0.19 eV, and the formation of Au₂₉–CO_3(ads) intermediate possesses high exothermicity of 4.33 eV, indicating that this process may compete with the LH mechanism. Thereafter, a second CO molecule can react with the remaining O atom via the ER mechanism with a very small barrier (0.03 eV). Our calculations suggest that the CO oxidation catalyzed by the Au₂₉ nanoparticle is likely to occur at or even below room temperature. To gain insights into high‐catalytic activity of the gold nanoparticles, the interaction nature between adsorbate and substrate is also analyzed by the detailed electronic analysis. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2010     

大气压下甲烷火花放电制乙炔和合成气

用大气压下火花放电方法和发射光谱原位诊断技术, 对CH₄直接转化制乙炔和间接转化制合成气进行了研究, 并与介质阻挡放电进行了比较。结果表明, 火花放电具有能量效率高的突出优点, 能够高效地将CH₄活化成C原子、H原子和C₂等活泼物种。当CH₄单独进料时, 能得到以C₂H₂为主的烃类产物。当CH₄与CO₂和O₂共进料时, 能得到H₂/CO比值可调的合成气产物。在用火花放电转化CH₄和CO₂制合成气时, 添加O₂能够避免反应器的结炭问题, 反应温度只需225 ℃, 与常规催化法相比具有明显的低温优势。     

The kinetics and the mechanism of the oxidation of carbon monoxide in the presence of hydrogen

The kinetics of the slow oxidation of CO in the presence of H₂ have been studied above the second explosion limit for the mixture 2CO + O₂ + X% H₂ at the temperature range of 530–570°C, pressures from 300 to 530 torr, and hydrogen contents of 1.1, 2.8, and 5.7%. The second explosion limit has been experimentally determined for the mixture of 2CO + O₂ containing 1.0, 3.0, and 5.7% H₂. On the basis of the oxidation scheme of CO in the presence of H₂, which includes the accepted mechanism of oxidation of hydrogen supplemented by the reactions in which CO takes part, the second explosion limit and the profiles of the slow reaction are calculated by computer methods. The agreement found between experimental and calculated values allows one to conclude that the scheme under consideration rather completely described the slow reaction above the second limit and the occurrence of the second explosion limit in the mixture CO–O₂–H₂. The rate constant for the reaction HO₂ + CO → OH + CO₂ was calculated from the experimental data and was found to agree with previous determinations.     

Geometric stability of PtFe/PdFe embedded in graphene and catalytic activity for CO oxidation

The direct methanol fuel cell (DMFC) is considered as a promising power source, because of its abundant fuel source, high energy density and environmental friendliness. Among DMFC anode materials, Pt and Pt group metals are considered to be the best electrocatalysts. The combination of Pt with some specific transition metal can reduce the cost and improve the tolerance toward CO poisoning of pure Pt catalysts. In this paper, the geometric stabilities of PtFe/PdFe atoms anchored in graphene sheet and catalytic CO oxidation properties were investigated using the density functional theory method. The results show that the Pt (Pd) and Fe atoms can replace C atoms in graphene sheet. The CO oxidation reaction by molecular O₂ on PtFe–graphene and PdFe–graphene was studied. The results show that the Eley–Rideal (ER) mechanism is expected over the Langmuir–Hinshelwood mechanism for CO oxidation on both PtFe–graphene and PdFe–graphene. Further, complete CO oxidation on PtFe–graphene and PdFe–graphene proceeds via a two‐step ER reaction: CO(gas) + O₂(ads) → CO₂(ads) + O(ads) and CO(gas) + O(ads) → CO₂(ads). Our results reveal that PtFe/PdFe commonly embedded in graphene can be used as a catalyst for CO oxidation. The microscopic mechanism of the CO oxidation reaction on the atomic catalysts was explored.     

The Role of Interfacial Reactions in Determining Plasma–Liquid Chemistry

In this work, we investigate the production of highly oxidative species in solutions exposed to a self-pulsed corona discharge in air. We examine how the properties of the target solution (pH, conductivity) and the discharge power affect the discharge stability and the production of H₂O₂. Indigo carmine, a common organic dye, is used as an indicator of oxidative strength and in particular, hydroxyl radical (OH^·) production. The observed rate of indigo oxidation in contact with the discharge far exceeds that predicted from reactions based on concentrations of species measured in the bulk solution. The generation of H₂O₂ and the oxidation of indigo carmine indicate a high concentration of highly oxidizing species such as OH^· at the plasma–liquid interface. These results indicate that reactions at the air plasma–liquid interface play a dominant role in species oxidation during direct non-equilibrium atmospheric pressure plasma treatment.     

Silver and Copper Nitride Cooperate for CO Electroreduction to Propanol

The need of carbon sources for the chemical industry, alternative to fossil sources, has pointed to CO₂ as a possible feedstock. While CO₂ electroreduction (CO₂R) allows production of interesting organic compounds, it suffers from large carbon losses, mainly due to carbonate formation. This is why, quite recently, tandem CO₂R, a two-step process, with first CO₂R to CO using a solid oxide electrolysis cell followed by CO electroreduction (COR), has been considered, since no carbon is lost as carbonate in either step. Here we report a novel copper-based catalyst, silver-doped copper nitride, with record selectivity for formation of propanol (Faradaic efficiency: 45 %), an industrially relevant compound, from CO electroreduction in gas-fed flow cells. Selective propanol formation occurs at metallic copper atoms derived from copper nitride and is promoted by silver doping as shown experimentally and computationally. In addition, the selectivity for C₂₊ liquid products (Faradaic efficiency: 80 %) is among the highest reported so far. These findings open new perspectives regarding the design of catalysts for production of C₃ compounds from CO₂.     

Kinetic Modelling of Atmospheric Pressure Corona Discharges in Humid Air

Corona discharge is a self-sustained discharge of gaseous medium in inhomogeneous electric fields, which often occurs on transmission lines and has some adverse effect on the power transmission system. In this paper, a kinetic model of corona discharges is presented to simulate the evolution process of charged particles and neutral species in humid air. To investigate the effect of humidity, our model consists of 69 species and 393 chemical reactions which consider important reactions containing H₂O molecules and hydrates. In addition, CO₂ molecules are also included to improve the integrity of reaction database. A temporal evolution of reduced electric field strengths E/N, which are typical experimental values of corona discharges, is used as input. The simulation results show that H₃O⁺ is one of the dominant positive ions which is in qualitative agreement with previous experimental results. The effect of humidity and pulse width on the plasma chemistry is also discussed. It is found that the humidity affects the maximum density and life time of the specific species. Meanwhile, the plasma chemistry could be affected by different pulse widths of the input electric field.