CH4 Plasma Research Articles

Plasma-driven non-oxidative coupling of methane to ethylene and hydrogen at mild temperature over CuxO/CeO2 catalyst

We report one-step non-oxidative coupling of methane (CH4) to ethylene (C2H4) at atmospheric pressure and mild temperature (ca. 180–190 °C), by a combination of non-thermal plasma and a CuOx/CeO2 catalyst. The C2H4 selectivity gradually increases during an induction period. The corresponding spent catalysts at different stages were systematically characterized to disclose the evolution of the CuOx/CeO2 catalyst. During the induction period, the CuO/CeO2 catalyst was partially reduced to generate Cu+, Ce3+ and Ov species, which accompany the formation of Cu+-Ov-Ce3+ sites, as proven by XRD, HRTEM, XPS, Raman, EPR and H2-TPR. In addition, the C2H4 selectivity is proportional to the fraction of Cu+, Ce3+, Ov and Cu-O-Ce species, which indicates that Cu+-Ov-Ce3+ is the active site for non-oxidative coupling of CH4 to C2H4. Furthermore, in-situ FTIR results indicate that the Cu+-Ov-Ce3+ interface sites can promote dehydrogenation of CH3* (from CH4 plasma) to produce CH2* on the catalyst surface, which is the basic reason why CuOx/CeO2 acts as a catalyst in speeding up the non-oxidative coupling of CH4 for C2H4 production.

Nonthermal Plasma-Stimulated C-N Coupling from CH4 and N2 Depends on the Presence of Surface CHx and Plasma-Phase CN Species.

Formation of C-N containing compounds from plasma-catalytic coupling of CH4 and N2 over various transition metals (Ni, Pd, Cu, Ag, and Au) is investigated using a multimodal spectroscopic approach, combining polarization-modulation infrared reflection-absorption spectroscopy (PM-IRAS) and optical emission spectroscopy (OES). Through sequential experiments utilizing CH4 and N2 nonthermal plasmas, we minimize plasma-phase reactions and identify key intermediates for C-N coupling on metal surfaces. Results show that simultaneous CH4 and N2 exposure with plasma stimulation produces surface C-N species. However, N2-CH4 sequential exposure does not lead to C-N species formation, while CH4-N2 sequential exposure reveals the presence of CHx surface species and CN radical species as key precursors to C-N species formation. From further analysis using X-ray photoelectron spectroscopy and liquid chromatography-mass spectrometry, the influence of exposure conditions on the degree of nitrogen incorporation and the nature of C-N species formed were revealed. The work highlights the importance of surface chemistry and exposure conditions in surface C-N coupling with plasma stimulation.

Remote plasma chemical vapor deposition of silicon oxycarbide film with a styrene-containing precursor and in situ O2 plasma treatment

In this paper, a study was conducted on silicon oxycarbide thin-film deposition using remote plasma chemical vapor deposition. A di-methyldimethoxysilane)2styrene (MDMS)2styrene precursor and CH4 plasma were used in the deposition process, and in situ oxygen plasma treatment was performed to control the carbon content and properties of the thin film. During the deposition process, the thin-film growth rate was found to be almost constant regardless of in situ plasma treatment. This means that in situ oxygen plasma treatment had no significant effect on thin-film growth itself. Auger electron spectroscopy analysis revealed that the carbon content of the deposited thin film was 43 % when only deposition was performed, but could be lowered to 3 % by in situ oxygen plasma treatment, which demonstrated that the carbon content can be controlled in a relatively wide range. X-ray photoelectron spectroscopy showed that it was possible to preserve SiC bonds even as the oxygen content increased with the number of in situ plasma treatments. The refractive index increased from 1.33 to 1.49, indicating a decrease in the number of pores and an increase in density. Meanwhile, the dielectric constant increased with the oxygen content, but did not exceed 3.9 due to the SiC bond structure. As the pore size decreased, the leakage current tended to decrease to 9.51 × 10−9 A/cm2 in an electric field of 1 MV/cm, and the breakdown voltage increased. Finally, it was possible to improve the etching characteristic through plasma treatment by lowering the wet etch rate from 78 to 19 Å/min.

Nanostructured Carbon Films Obtained by CH4 Plasma Deposition and Annealing at High Temperatures: Structural Features and Their Influence on the Electrical and Optoelectronic Properties

Nanostructured Carbon Films Obtained by CH4 Plasma Deposition and Annealing at High Temperatures: Structural Features and Their Influence on the Electrical and Optoelectronic Properties

Modelling the dynamics of hydrogen synthesis from methane in nanosecond‐pulsed plasmas

AbstractA chemical kinetics model was developed to characterise the gas‐phase dynamics of H2 production in nanosecond‐pulsed CH4 plasmas. Pulsed behaviour was observed in the calculated electric field, electron temperature and species densities at all pressures. The model agrees reasonably with experimental results, showing CH4 conversion at 30% and C2H2 and H2 as major products. The underlying mechanisms in CH4 dissociation and H2 formation were analysed, highlighting the large contribution of vibrationally excited CH4 and H2 to coupling energy from the plasma into gas‐phase heating, and revealing that H2 synthesis is not affected by applied pressure, with selectivity remaining unchanged at ~42% in the 1–5 bar range.

Insights into the role of H radicals in a plasma-enabled hydrogenation of heavy oil model compound

Plasma hydrogenation technology represents a sustainable and environment-friendly solution to upgrade heavy oil, but the underlying mechanisms of plasma hydrogenation for heavy oil or its model compounds remain insufficiently understood. Here we demonstrate the utilization of hydrogen-rich plasma for the hydrogenation of heavy oil model compound ethylbenzene under ambient conditions without catalysts, highlighting the crucial role of free radicals in this process. Simulation results show that H2 plasma yields a higher density (1.98 × 1014vs 1.06 × 1014 cm−3) and slower consumption rate (1.0 × 1016vs 2.5 × 1018 cm−3 s−1) of H radicals, and a higher average H number (1.8–4.3 vs 0.5–1.7) added to the aromatic ring, leading to a more efficient hydrogenation of aromatic rings compared to CH4 plasma. Moreover, an increase in H radical density may result in the cleavage of hydrogenated products due to elevated high-energy electron levels, hence potentially undermining the overall effectiveness. Experimental results further reveal that radical recombination is the dominant reaction mechanism for H-involved reactions, since almost no hydrogenated products were observed after CH4 plasma treatment. These novel insights into plasma-enabled aromatic ring hydrogenation provide a new and green approach for future upgrading of heavy oils.

Nanostructured carbon films obtained by CH4 plasma deposition and annealing at high temperature: structural features and their effects on electrical and optoelectronic properties

This article is dedicated to the study of the structural, electrical, and optoelectronic properties of nanostructured carbon films obtained by methane plasma deposition, followed by annealing at high temperatures (650–800 °C). The conditions for obtaining the films affected the final physicochemical parameters. We studied the film morphology using atomic force microscopy, scanning electron microscopy, Raman spectroscopy, X-ray energy-dispersive analysis, and analysis of the current voltage (C-V) characteristics. The film thickness ranged from 20 to 150 nm, with a C/O ratio of 4:1. Structural studies have shown that the resulting nanostructured carbon films consist mainly of nanographite flakes, the lateral dimensions of which lie in the lateral size (La) range of 5 to 12 nm, and contain different fractional concentrations of sp3/sp2 crystalline phases of carbon. We have established that with an increase in the annealing temperature, the defectiveness of the carbon film structure increases; however, at the same time, the degree of graphitization increases, as indicated by the Raman spectroscopy data and the calculated values of layer resistances from the C-V characteristics. The values of photocurrents were calculated, from which it was found that the samples exhibited photosensitivity in the temperature range of room temperature to –173 °C, based on the temperature dependences of the C-V. The obtained results can be useful in creating day and night light sensors as well as temperature sensors suitable for use at low temperatures.

Methane coupling by a parallel-electrodes pulsed gliding plasma: A strong correlation between conversion ability and the plasma mobility

The coupling of methane using a highly mobile nanosecond pulsed plasma is examined. A stroboscopic imaging system is used to determine the drift velocity of the plasma column (vd). When vd varies within a narrow range of 1.2–2.2 m/s in CO2 + CH4 plasma, the energy efficiency increases from 35% to over 60% showing very sensitive correlation. The reactants conversion rates also increase linearly with vd, ranging from 40 to 60%. Yields of ethylene and ethane monotonically increases with vd when pure methane is introduced. A mathematical model predicts a high spatial overlap of successive plasma pulses, with an overlap of 78% at the highest vd obtained here. This overlap, inversely related to vd, negatively affects on the process due to energy dissipation by products and high temperatures in the overlap zone. To address the problem of low conversion rates or efficiencies of plasmas, using non-overlapping plasma reactors may be a viable solution.

Enhancing surface adhesion of polytetrafluoroethylene induced by two-step in-situ treatment with radiofrequency capacitively coupled Ar/Ar+CH4+NH3 plasma

Although some progress in plasma modification of the polytetrafluoroethylene (PTFE) surface has been made recently, its adhesion strength still needs to be further improved. In this work, the surface of a PTFE sample was treated with a two-step in-situ method. Firstly, the PTFE surface was treated with capacitively coupled Ar plasma to improve its mechanical interlocking performance; then, Ar+NH3+CH4 plasma was used to deposit an a-CN x :H cross-linking layer on the PTFE surface to improve the molecular bonding ability. After treatment, a high specific surface area of 2.20 and a low F/C ratio of 0.32 were achieved on the PTFE surface. Its surface free energy was increased significantly and its maximum adhesion strength reached 77.1 N·10 mm−1, which is 56% higher than that of the single-step Ar plasma-treated sample and 32% higher than that of the single-step Ar+CH4+NH3 plasma-treated sample.

Plasma assisted dry reforming of methane: Syngas and hydrocarbons formation mechanisms

Plasma reactions of CO2 + CH4 mixtures have been proposed as a suitable process for the dry reforming of methane. Without specific catalysts, most studies report the formation of CO and H2 as main reaction products and arise the question whether CHx radicals coming from CH4 may interact with intermediate species formed by electron impact dissociation of CO2, a critical step for the formation of high added value oxygenated compounds. We have addressed this question studying the CO2 + CH4 plasma reaction in a ferroelectric-moderated packed-bed reactor varying the reactants ratio. Analysis of the reaction products by mass spectrometry and the plasma reaction intermediates by optical emission spectroscopy suggest that few direct cross-link interactions exist between intermediate plasma species issued from CH4 or CO2. This preliminary evidence is corroborated by experiments using 13CO2 instead 12CO2 as reactant. The isotope labeling procedure has proved that plasma reaction mechanisms of CO2 and CH4 molecules proceed almost independently, with the formation of small amounts of water and the removal of carbon deposits resulting CH4 plasma decomposition as sole evidences of cross reactions. These results highlight the need of using catalysts to promote specific surface reactions for a better control of the selectivity of the process.

Generation and decay of N2(A3Σu +) molecules in reacting CO2 and CH4 plasmas

Time-resolved number densities of N2(A3Σu +, v = 0, 1) molecules in diffuse ns pulse discharge plasmas in N2, N2–H2, N2–CH4, N2–CO2, and N2–CO2–CH4 are measured by tunable diode laser absorption spectroscopy (TDLAS). The first series of measurements is made in the discharge pulse bursts at a relatively low pulse repetition rate (3 kHz), when the N2(A3Σu +) generation and decay after individual discharge pulses is fully resolved. The second set of data is taken during a sequence of two pulse bursts generated at a higher pulse repetition rate (100 kHz), for different delay times between the first and second bursts. This approach is used to determine the effect of accumulation and decay of reacting species generated in the plasma, including N, H, and O atoms, CO molecules, and C2 hydrocarbon product species, on the rate of N2(A3Σu +) production and quenching. The effect of these species can be isolated since the rates of N2(A3Σu +) quenching by the initial reactant species (H2, CH4, and CO2) are slow. Comparison of the measurement results with the kinetic modeling predictions is used to obtain insight into the plasma chemical reaction kinetics. The results complement the measurements of N, H, O, and CO in high-pressure reacting plasmas, and help quantify the plasma chemical processes driven by the electron impact dissociation, electronic excitation, and reactive quenching of the excited electronic states. The present results may be used for the development and validation of higher fidelity kinetic models of reacting plasmas, incorporating state-specific electronic and vibrational energy transfer and chemical reactions.

Effects of deposition precursors of hydrogenated amorphous carbon films on the plasma etching resistance based on mass spectrometer measurements and machine learning analysis

In the growth of hydrogenated amorphous carbon (a-C:H) films by plasma enhanced chemical vapor deposition (PECVD) with H2, CH4 and C3H6 plasma, the gas phase reaction of active species and their contribution to properties of deposited a-C:H film were quantitatively investigated using appearance quadrupole mass spectrometry (QMS) and machine learning. The QMS measurement indicated that two-types of neutral radicals were generated by the dissociation and polymerization of the raw material gasses. A random forest regression model was employed as a prediction model to analyze the correlation between the neutral species and etching rates by molecular oxygen (O2) plasma without any stage bias, which were measured by in-situ ellipsometry. The coefficient of determination (R2), an indicator of the degree of prediction accuracy, was 0.906 and 0.584 for the training and test data, respectively. SHapley Additive exPlanations (SHAP), interpreting the random forest model, were used to quantitatively indicate the contribution of radicals to the etching rate, including their synergistic and secondary effects. They indicated that carbon-rich radicals, such as C3H3 and C5H5, contributed to a decrease in the etching rate, whereas hydrogen-rich radicals, such as CHx (x=1,2,3,4), C2H5, C4H9, and C5H9,11 induced an increase in the etching rate by the O2 plasma. (200 words)

Carbon coated nickel cobalt phosphide with sea urchin-like structure by low temperature plasma processing for hydrogen evolution and urea oxidation

Hydrogen production from organic wastewater splitting is a promising approach to reduce electrical energy consumption and remove organic pollutants simultaneously. The key challenge is to develop an effective electrocatalyst with low-cost, high-performance and high-corrosion resistance in harsh environment. Here, a self-supported carbon coated nickel cobalt phosphide grown on nickel foam (CoNiP@C/NF) electrode is synthesized by combining hydrothermal synthesis and plasma processing. Surface coating by graphene-like nanostructure and phosphorization are achieved at mild temperature by low temperature CH4 plasma and Ar-H2-P plasma, respectively, enabling retention of the sea urchin-like structure obtained by hydrothermal reaction. Benefiting from the modulated built-in electric field, the self-supporting CoNiP@C/NF electrode exhibits highly efficient bifunctional catalytic activity for hydrogen evolution reaction (HER) and urea oxidation reaction (UOR), featured for the low voltage of 1.43 V at 20 mA cm−2 of a HER||UOR electrolysis cell. Whether in alkaline solution or organic wastewater, CoNiP@C/NF electrodes could generate H2 continuously and stably for more than 60 h. DFT calculation indicates that Ni doping modifies the surface charge density, which optimizes the adsorption/desorption of intermediates and facilitates the water dissociation during HER. Moreover, CoNiP possesses strong binding with the C layer, thus promoting the structural stability and charge transfer efficiency of the catalyst.

Growth of diamond-like carbon films with significant nanocrystalline phases in a low-pressure high-density CH4 plasma in ICP-CVD: Effect of negative dc substrate bias

Working at relatively low-temperature and low-pressure, the development and optimization of diamond-like carbon (DLC) films with significant nanodiamond components on inexpensive glass substrates, without conventional pre-treatment by diamond powders, is the main objective of the present work. Using the most familiar CH4-precursors diluted by Ar in a high-density plasma triggered at high rf power (900 W) in a planar inductively-coupled plasma chemical vapour deposition (ICP-CVD) system, DLC thin-films are prepared via optimization in the flow rate of the precursor gas at 30 mTorr (∼4 Pa) pressure and 450 °C substrate temperature and, further by applying negative dc-bias to the substrates. The optimized samples obtained with negative substrate-bias possess good crystalline properties. From Raman results, the most crystalline sample is obtained with characteristic minimum ID/IG ratio reduced from 0.75 to 0.61, maximum IDia/ID enhanced from 0.94 to 1.09, and the sp3 hybridized CC bonds increased from 44% to ∼64% at a substrate-bias of –40 V. The residual stress, and the optical band gap were calculated from Raman analysis. The optimized DLC film exhibits a high optical band gap (3.71 eV) as calculated from Tauc’s plot analysis of the UV–Vis data. The well-identified 〈111〉 crystallographic planes in the TEM micrograph demonstrate a significant fraction of nanocrystalline diamond component in the optimum DLC film.

Hydrophobization of paper intended for packaging

Superhydrophobic surfaces are highly desired for several applications due to their exceptional properties such as self-cleaning, anti-icing, anti-friction and others. Such surfaces can be prepared via numerous methods including plasma technology [1-7]. Among plasma technology methods used to prepare these surfaces, the plasma enhanced chemical vapor deposition method, which provides the advantages of low cost, simple processing, and easy to form micro-nano structure. In this work, a treatment of surface paper for improving hydrophobicity using a PECVD technique was realized, paper substrates was treated by CH4 plasma , the substrates were held on a grounded substrate, with time variation of 5, 10, 15 and 20 min, while pressure and power have been kept constant at 8.10-2 and 100 W respectively. After deposition we proceeded to carry out structural and morphological characterization of the treated surfaces, by (SEM), AFM, FTIR and then through contact angle measurements. It is found that all layers are hydrophobic and super-hydrophobic. Except the layers treated for 10 minutes which are hydrophobic with a contact angle equal to 137.7° , the layers treated for 05, 15 and 20 minutes show superhydrophobic surfaces with a contact angles equal to 153.8°, , 149.2° and 156◦ respectively.

Diamond-Like Carbon Thin Films from Low-Pressure and High-Density CH4 Plasma

DLC thin films were grown using a low-pressure high-density CH4/Ar plasma on glass substrates in a planar RF (13.56 MHz) inductively coupled plasma (ICP) CVD system. Using a plasma triggered by RF power to a mixture of 20 sccm CH4 and 50 sccm Ar gas maintained at a low pressure of ∼30 mTorr, a series of samples were prepared by varying the substrate temperature and then changing the RF power. The optimised DLC films, obtained at 500 °C and 900 W demonstrated a high optical transmission of ∼97 % corresponding to awideoptical band gap of ∼3.58 eV; and a minimum of ID/IG ∼0.61 along witha maximum ofIDia/ID ∼1.32 in Raman analysis established the maximum sp3/sp2 content in the carbon network.

COx-free co-cracking of n-decane and CH4 to hydrogen and acetylene using pulsed spark plasma

Heavy hydrocarbons pyrolysis is a high energy consumption and high carbon emission process. The non-thermal plasma is a promising technology to convert heavy oil into value-add chemicals without COx emission. In this paper, n-decane was used as a model compound of the heavy hydrocarbons, and its co-cracking characteristics with CH4 by micro-second pulsed spark plasma were investigated in terms of carrier gas species and pulse frequencies. The maximum hydrogen and acetylene flow rates (48.7 mL/min and 29.9 mL/min) with energy consumption of 6.4 kW·h/m3H2 and 10.4 kW·h/m3C2H2 were achieved under the condition of 1 kHz pulse frequency. Compared with those of the Ar atmosphere, H2 and C2H2 production improved prominently by 41.4% and 133.6%. It indicates that the H radicals from CH4 plasma played a dominating role in the n-decane cracking, which was verified via controlled experiments in H2 atmosphere. The evolution of the optical emission spectroscopy (OES) revealed that the rotational temperature rose rapidly to 3700±300 K in hundreds of nanoseconds, during which the n-decane was cracked into small particles, such as H, C2, CH and so on. Based on these results, a possible reaction mechanism of the co-cracking was also discussed. This work may provide an effective COx-free method for fossil utilization.

Hexagonal Phase Ni3Fe Nanosheets toward High‐Performance Water Splitting by a Room‐Temperature Methane Plasma Method

AbstractIn addition to tuning the size and shape, exploring highly efficient and cost‐effective catalysts with active metastable phase represents an alternative strategy to improve electrocatalytic water splitting performance. However, it still remains a big challenge to synthesize unconventional crystal structures under mild conditions. Herein, a facile CH4 plasma assisted strategy to synthesize hexagonal close‐packed (hcp) Ni3Fe/C porous nanosheets by in situ topology reduction of Ni3Fe‐LDH nanosheets is successfully developed. Compared with its face‐centered cubic phase counterpart (fcc‐Ni3Fe/C), the resulting hcp‐Ni3Fe/C catalyst exhibits much enhanced activity with overpotentials of 70 and 201 mV at a current density of 10 mA cm−2 for the hydrogen evolution reaction and oxygen evolution reaction, respectively. Moreover, a small cell voltage of 1.54 V is realized to drive overall water splitting. Theoretical calculations further reveal that hcp‐Ni3Fe allows energetically favorable adsorption and dissociation of H2O molecules. This work presents a new strategy for designing advanced metastable phase electrocatalysts with high performance for energy and environmentally relevant reactions.

Liquid-phase methane bubble plasma discharge for heavy oil processing: Insights into free radicals-induced hydrogenation

Hydrogenation, an alternative to conventional cracking, is poised to revolutionize heavy oil upgrading by raising the hydrogen-to-carbon ratio at lower process pressures and temperatures. The emerging low-temperature plasma-enabled hydrogenation is an effective, fast, and environment-friendly process; however, the conversion rate and energy efficiency are still insufficient. To address these limitations, here we propose an innovative bubble-assisted methane plasma-liquid process for heavy oil processing (using ethylbenzene as a model compound) and offer new insights into radical-assisted hydrogenation. Results from the plasma kinetics modeling and density functional theory calculations indicate that ·H and ·CH3 radicals generated by CH4 plasma are two main drivers for ethylbenzene hydrogenation, and their chemical reaction rates with ethylbenzene molecules strongly depend on their spatial distribution in bubbles, which further govern the reaction direction towards ethylbenzene hydrogenation or free radical recombination. Moreover, it is found that by controlling the bubble numbers and plasma parameters, the H density can increase by an order of magnitude from 1.4 × 1020 to 3.88 × 1021 m−3, while the number of hydrogenated aromatic rings increases by ∼ 58%, which further confirms the excellences of the proposed plasma-bubble technology in chemical regulation and feasible processing. In addition, the feasibility of the plasma-enabled hydrogenation has been verified by the experiment, which highlighted the competition between the radical recombination and radical-aromatic ring interactions. Overall, the revealed interaction mechanisms between the plasma-generated radicals and ethylbenzene provide new insights and guiding principles for the future upgrading of heavy oils and other industrial hydrocarbon products.

Spectral study of argon-methane mixture plasma jet generated by a DC plasmatron

We present the results of studying optical emission spectra of Ar:CH4 plasma produced on a DC plasmatron for graphene synthesis. We have identified the basic set of spectral lines and bands in the obtained spectra and shown that H lines and C2 bands appear due to direct excitation by an electron strike of corresponding neutral particles. C2 molecular bands were also identified in the spectra with intensity considerably lower compared to previous studies where He: C2H2 mixture was used as plasma-forming gas.