Modelling excited species and their role on kinetic pathways in the non-oxidative coupling of methane by dielectric barrier discharge

Reactivity of the Ethenium Cation (C2H5+) with Ethyne (C2H2): A Combined Experimental and Theoretical Study

The gas-phase reaction between the ethyl cation (C2H5+) and ethyne (C2H2) is re-investigated by measuring absolute reactive cross sections (CSs) and branching ratios (BRs) as a function of collision energy, in the thermal and hyperthermal energy range, via tandem-guided ion beam mass spectrometry under single collision conditions. Dissociative photoionization of C2H5Br using tuneable VUV radiation in the range 10.5-14.0 eV is employed to generate C2H5+, which has also allowed us to explore the impact of increasing (vibrational) excitation on the reactivity. Reactivity experiments are complemented by theoretical calculations, at the G4 level of theory, of the relative energies and structures of the most relevant stationary points on the reactive potential energy hypersurface (PES) and by mass-analyzed ion kinetic energy (MIKE) spectrometry experiments to probe the metastable decomposition from the [C4H7]+ PES and elucidate the underlying reaction mechanisms. Two main product channels have been identified at a centre-of-mass collision energy of ∼0.1 eV: (a) C3H3++CH4, with BR = 0.76±0.05 and (b) C4H5++H2, with BR = 0.22±0.02. A third channel giving C2H3+ in association with C2H4 is shown to emerge at both high internal excitation of C2H5+ and high collision energies. From CS measurements, energy-dependent total rate constants in the range 4.3×10-11-5.2×10-10 cm3·molecule-1·s-1 have been obtained. Theoretical calculations indicate that both channels stem from a common covalently bound intermediate, CH3CH2CHCH+, from which barrierless and exothermic pathways exist for the production of both cyclic c-C3H3+ and linear H2CCCH+ isomers of the main product channel. For the minor C4H5+ product, two isomers are energetically accessible: the three-member cyclic isomer c-C3H2(CH3)+ and the higher energy linear structure CH2CHCCH2+, but their formation requires multiple isomerization steps and passages via transition states lying only 0.11 eV below the reagents' energy, thus explaining the smaller BR. Results have implications for the modeling of hydrocarbon chemistry in the interstellar medium and the atmospheres of planets and satellites as well as in laboratory plasmas (e.g., plasma-enhanced chemical vapor deposition of carbon nanotubes and diamond-like carbon films).

Conversion mechanism of thermal plasma-enhanced CH4-CO2 reforming system to syngas under the non-catalytic conditions

Thermal plasma activation of CH₄-CO₂ reforming (CRM) to syngas under non-catalytic conditions is an efficient and clean technology for the large-scale utilization of hydrocarbon resources and the conversion of greenhouse gases. This study investigates the equilibrium state and transformation mechanism of a CRM reaction system activated by thermal plasma through experimental, thermodynamic, and kinetic analyses. The experimental results illustrated that the CO₂ conversion rate and H₂ selectivity showed a downward trend with an increase in the CO₂/CH₄ molar ratio, whereas the CH₄ conversion rate and CO selectivity showed the opposite trend. When CO₂/CH₄ molar ratio was 6/4, the selectivity for CO and H₂ increased to 87.0 % and 80.8 %, respectively. Excess CO₂ promotes the partial oxidation of CH₄ to eliminate carbon deposition, resulting in an H₂/CO molar ratio value closer to 1. Thermodynamic results show that the thermal-plasma-initiated CRM reaction can reach thermodynamic equilibrium more easily than the conventional catalyzed reactions, achieving much higher feedstock gas conversion without carbon deposition. The kinetic results obtained from the PSR model revealed that CH₄ and CO₂ were cleaved to form free radicals at the instant of contact with the plasma flame. O, H, and other particles generated in the form of free radicals rapidly collided with each other and transformed into CO and H₂, accelerating the reaction process. The results presented in this study will help reveal the transformation mechanism of the CRM reaction activated by thermal plasma under non-catalytic conditions and provide a new perspective for studying CRM reactions.

Plasma-Catalysis of Nonoxidative Methane Coupling: A Dynamic Investigation of Plasma and Surface Microkinetics over Ni(111)

A heterogeneous catalytic microkinetic model is developed and implemented in a zero-dimensional (0D) plasma model for the dynamic study of methane nonoxidative coupling over Ni(111) at residence times and power densities consistent with experimental reactors. The microkinetic model is thermodynamically consistent and is parameterized based on the heats of chemisorption of surface species on Ni(111). The surface network explicitly accounts for the interactions of plasma species, namely, molecules, radicals, and vibrationally excited states, with the catalyst active sites via adsorption and Eley-Rideal reactions. The Fridman-Macheret model is used to describe the enhancement of the rate of the dissociative adsorption of vibrationally excited CH₄, H₂, and C₂H₆. In combination with a previously developed detailed kinetic scheme for nonthermal methane plasma, 0D simulation results bring insights into the complex dynamic interactions between the plasma phase and the catalyst during methane nonoxidative coupling. Differential turnover frequencies achieved by plasma-catalysis are higher than those of equivalent plasma-only and catalysis-only simulations combined; however, this performance can only be sustained momentarily. Hydrogen produced from dehydrogenation of ethane via electron collisions within the plasma is found to quickly saturate the surface and even promote the conversion of surface CH₃* back to methane.

Nonoxidative Coupling of Methane over Ceria-Supported Single-Atom Pt Catalysts in DBD Plasma

Plasma-catalytic direct nonoxidative coupling of methane (NCM) into C₂ hydrocarbons was investigated over ceria-supported atomically dispersed Pt (Pt/CeO₂-SAC) and nanoparticle Pt (Pt/CeO₂-NP) catalysts in dielectric barrier discharge (DBD) plasma. Nonthermal plasma facilitated C-H bond dissociation in CH₄ at low temperatures (<150 °C) and atmospheric pressure. The presence of Pt/CeO₂ catalysts in plasma further enhanced CH₄ conversion and C₂ hydrocarbon selectivity by enabling the conversion of vibrationally excited methane species with high internal energy on active Pt sites. Noticeably, the Pt/CeO₂-SAC catalyst displayed a more remarkable performance, with a CH₄ conversion of 39% and a C₂ selectivity of 54% at 54 W. The enhanced CH₄ conversion was attributed to abundant coordinatively unsaturated Pt sites in Pt/CeO₂-SAC, which were more active for C-H bond scission. Meanwhile, isolated Pt atoms in Pt/CeO₂-SAC promoted C₂ hydrocarbon formation by hindering the unselective formation of coke from deep dehydrogenation of CH_x^• intermediates and higher hydrocarbons from oligomerization reactions.