Non-thermal Plasma Conditions Research Articles

Tailoring the surface acidity of catalyst to enhance nonthermal plasma-assisted ammonia synthesis rates

We report nonthermal plasma-assisted catalytic ammonia synthesis from N2 and H2 molecules at near ambient conditions in a custom-built coaxial dielectric barrier discharge plasma reactor. Here, we strategically designed catalysts to facilitate dissociation of strong nitrogen bond and desorption of NH3 molecules. This work demonstrates the use of empty orbitals of boron doped in graphitic carbon nitride (BCN) to activate nitrogen molecules, by creating electron-deficient sites, under nonthermal plasma conditions. Transition metals (Co and Fe) further contribute to the ammonia generation by altering the amount of weak acid sites of the BCN catalyst. As a result, at 20.16 kJ L−1, the maximum ammonia synthesis rate of ∼2.48mmolg−1h−1 was achieved by Co-BCN that corresponds to energy efficiency of 2.09 g-NH3 kWh−1, which was ca. 2.89-folds higher than that of plasma only. Evidently, dual mechanisms at play, leading to enhanced ammonia synthesis rates. Our strategy of tailoring catalyst structure provides a roadmap for engineering synergy with nonthermal plasma towards targeted applications.

Selective Recovery of Cathode Materials from Spent Lithium-Ion Battery Material with a Near-Room-Temperature Separation.

Effective separation of cathode materials from the current collector is a critical step in recycling a spent lithium-ion battery (LIB). This typically necessitates the decomposition or dissolution of the organic binder, poly(vinylidene fluoride) (PVDF), to achieve efficient recovery of cathode materials. However, this process requires a high decomposition temperature, typically between 400 and 600 °C, and can lead to side reactions, such as current collector oxidation/brittleness, decomposition of cathode materials, and formation of metal fluorides. In this study, we propose that non-thermal plasma (NTP) treatment can be used to achieve an extremely high separation of cathode materials and aluminum current collector at near room temperature. Instead of relying on PVDF decomposition, which requires high temperatures, PVDF can be deactivated by partially breaking down long molecular chains with appropriate NTP conditions. With a total treatment time of around 2000 s and an environmental temperature of approximately 80 °C, minor side reactions can be avoided. The separation rate can reach up to 95.69%, and high-quality cathode materials can be obtained with only 0.02 wt % aluminum impurity content. This research could potentially offer a new approach toward minimizing recycling steps and reducing energy consumption in the recycling of spent LIBs. It could also be extended to the recovery of a broader range of electronic wastes.

Understanding the Enhanced Catalytic Desulfurization Mechanism: Gas-Phase and Surface Reactions with a CuCeOx Catalyst under Nonthermal Plasma Conditions.

To understand the mechanisms of enhanced catalytic technologies under nonthermal plasma (NTP) conditions, complex surface processes must be assessed. However, the predictive capability of the Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) processes is limited by various factors. The present study aimed to clarify the interaction mechanisms between NTP and catalysts in the enhancement process, explore the specific pathways of the enhancement process based on E-R and L-H model validations, and obtain data to support the rational design of NTP-enhanced catalytic processes. We investigated CuCeOx catalysts and SO2 removal reaction as a probing reaction using two enhancement scheme configurations, combined with gas-phase reaction process simulations. During the gas-phase reaction stage of the enhancement process, no significant differences were observed among the different configurations caused by the generation of radicals that were induced by N2 (A3Σu+)-excited species. However, introducing CuCeOx catalysts altered the enhancement process, and the placement of the catalyst influenced the corresponding desulfurization mechanism.

Improving the Quality of Bio-oil Using the Interaction of Plastics and Biomass through Copyrolysis Coupled with Nonthermal Plasma Processing.

Bio-oil produced from the pyrolysis of biomass is chemically complex, viscous, highly acidic, and highly oxygenated. Copyrolysis of biomass and plastics can enhance oil quality by raising the H/C ratio, leading to improved biofuel properties. In this work, copyrolysis of polystyrene and biomass was passed to a second-stage dielectric barrier discharge nonthermal plasma reactor with the aim to further improve the product bio-oil. Pyrolysis of the polystyrene and biomass produces volatiles that pass to the second stage to undergo cracking and autohydrogenation reactions under nonthermal plasma conditions. There was a synergistic interaction between biomass and polystyrene in terms of overall oil and gas yield and composition even in the absence of the nonthermal plasma. However, the introduction of the nonthermal plasma produced a marked increase in monocyclic aromatic hydrocarbons (e.g., ethylbenzene), whereas polycyclic aromatic compounds decreased in concentration. Most notably, the influence of the plasma markedly reduced the quantity of oxygenated compounds in the product oil. It is suggested that the unique reactive environment produced by the plasma involving high-energy electrons, excited radicals, ions, and intermediates increases the interaction of the polystyrene and biomass pyrolysis volatiles. Increasing input plasma power from 50 to 70 W further enhanced the effects of the nonthermal plasma.

MgAl layered double hydroxide (LDH) for promoting ammonia synthesis in non-thermal plasma: Role of surface oxygen vacancy

Here, the MgAl layered double hydroxide (LDH) supports and relevant metal on LDH catalysts (i.e., Ni, Co, and Ru) were prepared and investigated under non thermal plasma (NTP) conditions to probe the role of surface oxygen vacancy (OV) in NTP-assisted ammonia (NH3) synthesis. The findings show that OV on the LDH carrier is highly beneficial to NH3 formation under NTP conditions, and concentration of OV on the LDH can be regulated by the post-synthesis calcination and hydrogen plasma etching. Additionally, loading of active metal species on the LDH could promote the NH3 synthesis further due to presence of multiple reaction pathways and the synergy between the surface OV and metal sites in such NTP-catalytic systems. As the result, the catalysts developed by this work showed high ammonia synthesis rates of 4.42−4.52 mmol g−1 h−1 and energy efficiencies of 1.67−1.71 gNH3 kWh−1, respectively. The findings of the work pave the way for the rational design and optimization of highly efficient catalysts with dual active sites for intensifying the NTP-catalytic ammonia synthesis.

Mechanochemical Synthesis of Bimetallic NiCo Supported on a CeO2 Catalyst with Less Metal Loading for Non-Thermal Plasma Catalytic CO2 Hydrogenation

Non-thermal plasma (NTP) catalysis is a promising technology for CO2 valorization with renewable H2, in which catalyst design is one of the key aspects to progress the hybrid technology. Herein, bimetallic NiCo supported on CeO2 catalysts, that is, NiCo/CeO2, were developed with less metal loading of ∼2 wt % using mechanochemical synthesis for NTP-catalytic CO2 methanation. During the synthesis, different addition orders of Ni and Co precursors were investigated, and the results show that the NiCo1/CeO2-I catalyst (which was prepared by the simultaneous addition of Ni and Co precursors, protocol I) exhibited the highest CO2 conversion (∼60%) and CH4 selectivity/yield (∼80%/∼50%), whereas the NiCo1/CeO2-II and NiCo1/CeO2-III catalysts (prepared by sequential addition protocols of II and III) showed very poor catalytic performance. Characterization results suggested that in protocol I, Ni and Co prefer to alloy, and concentrated oxygen vacancies on the CeO2 surface and high surface basicity are retained as well. Such properties of NiCo1/CeO2-I were responsible for CO2 activation and hydrogenation under NTP conditions, which was explained by the proposed mechanisms.

Non-Thermal Ammonia Decomposition for Hydrogen Production over Carbon Films under Low-Temperature Plasma-In-Situ FTIR Studies.

Due to easy storage and transportation, liquid hydrogen carriers will play a significant role in diversifying the energy supply pathways by transporting hydrogen on a large scale. Thus, in this study, amorphous carbonaceous materials have been employed for hydrogen production via ammonia decomposition under non-thermal plasma (NTP) conditions. The adsorption and splitting of ammonia over carbons differing in the chemical structure of surface functional groups have been investigated by in situ spectral studies directly under NTP conditions. As a result of NH3 physical and chemical sorption, surface species in the form of ammonium salts, amide and imide structures decompose immediately after switching on the plasma environment, and new functionalities are formed. Carbon catalysts are very active for NH3 splitting. The determined selectivity to H2 is close to 100% on N-doped carbon material. The data obtained indicate that the tested materials possess excellent catalytic ability for economical, COx-free hydrogen production from NH3 at a low temperature.

Cooperative Catalysis of Vibrationally Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation

Using nonthermal plasma (NTP) to promote CO2 hydrogenation is one of the most promising approaches that overcome the limitations of conventional thermal catalysis. However, the catalytic surface reaction dynamics of NTP-activated species are still under debate. The NTP-activated CO2 hydrogenation was investigated in Pd2Ga/SiO2 alloy catalysts and compared to thermal conditions. Although both thermal and NTP conditions showed close to 100% CO selectivity, it is worth emphasizing that when activated by NTP, CO2 conversion not only improves more than 2-fold under thermal conditions but also breaks the thermodynamic equilibrium limitation. Mechanistic insights into NTP-activated species and alloy catalyst surface were investigated by using in situ transmission infrared spectroscopy, where catalyst surface species were identified during NTP irradiation. Moreover, in in situ X-ray absorption fine-structure analysis under reaction conditions, the catalyst under NTP conditions not only did not undergo restructuring affecting CO2 hydrogenation but also could clearly rule out catalyst activation by heating. In situ characterizations of the catalysts during CO2 hydrogenation depict that vibrationally excited CO2 significantly enhances the catalytic reaction. The agreement of approaches combining experimental studies and density functional theory (DFT) calculations substantiates that vibrationally excited CO2 reacts directly with hydrogen adsorbed on Pd sites while accelerating formate formation due to neighboring Ga sites. Moreover, DFT analysis deduces the key reaction pathway that the decomposition of monodentate formate is promoted by plasma-activated hydrogen species. This work enables the high designability of CO2 hydrogenation catalysts toward value-added chemicals based on the electrification of chemical processes via NTP.

Insights into the limitations to vibrational excitation of CO2: validation of a kinetic model with pulsed glow discharge experiments

Vibrational excitation represents an efficient channel to drive the dissociation of CO2 in a non-thermal plasma. Its viability is investigated in low-pressure pulsed discharges, with the intention of selectively exciting the asymmetric stretching mode, leading to stepwise excitation up to the dissociation limit of the molecule. Gas heating is crucial for the attainability of this process, since the efficiency of vibration–translation (V–T) relaxation strongly depends on temperature, creating a feedback mechanism that can ultimately thermalize the discharge. Indeed, recent experiments demonstrated that the timeframe of V–T non-equilibrium is limited to a few milliseconds at ca. 6 mbar, and shrinks to the μs-scale at 100 mbar. With the aim of backtracking the origin of gas heating in pure CO2 plasma, we perform a kinetic study to describe the energy transfers under typical non-thermal plasma conditions. The validation of our kinetic scheme with pulsed glow discharge experiments enables to depict the gas heating dynamics. In particular, we pinpoint the role of vibration–vibration–translation relaxation in redistributing the energy from asymmetric to symmetric levels of CO2, and the importance of collisional quenching of CO2 electronic states in triggering the heating feedback mechanism in the sub-millisecond scale. This latter finding represents a novelty for the modelling of low-pressure pulsed discharges and we suggest that more attention should be paid to it in future studies. Additionally, O atoms convert vibrational energy into heat, speeding up the feedback loop. The efficiency of these heating pathways, even at relatively low gas temperature and pressure, underpins the lifetime of V–T non-equilibrium and suggests a redefinition of the optimal conditions to exploit the ‘ladder-climbing’ mechanism in CO2 discharges.

Reaction mechanism of toluene decomposition in non-thermal plasma: How does it compare with benzene?

Non-thermal plasma (NTP) catalysis is considered one of the most promising technologies to address a wide range of energy and environmental needs, such as carbon dioxide(CO2) conversion, NH3 synthesis, and volatile organic compounds (VOCs) removal. A systematic approach to optimizing NTP systems benefits from understanding VOCs' fundamental NTP destruction behavior and analyzing the correlations between molecular structures and conversion and selectivity. Herein, the mechanical performance of the toluene destruction in NTP is examined and compared with benzene bearing a similar molecular structure. Different experimental and theoretical techniques are applied, including synchrotron vacuum ultraviolet photoionization mass spectrometry(SVUV-PIMS), thermochemistry, and quantum chemistry. Comparatively, toluene is more readily destroyed under the same NTP conditions than benzene. More intriguingly, the distribution of the decomposition species is significantly different. The theoretical calculations reveal that the abundant methyl radicals generated in toluene decomposition mainly lead to the various species distribution. These radicals promote some reactions, such as the decomposition of o-benzoquinone, one of the key intermediates, thus leading to new reaction pathways and products different from benzene. Finally, the critical mechanism steps of toluene decomposition in the current NTP condition are established, including the reactions between toluene and electrons or active radicals, aromatic ring breakup reactions, and reactions of methyl radicals. This study provides a powerful approach to revealing the different fundamental reaction mechanisms resulting from a slight structural difference and brings new insights for probing the underlying plasma chemistry, which is crucial for its various promising environmental and energy applications.

On the growth of Si nanoparticles in non-thermal plasma: physisorption to chemisorption conversion

Non-thermal plasma systems offer unique opportunities in the fields of bio-imaging, drug delivery, photovoltaics, microelectronics manufacturing. Such interests are largely inspired by the fact that hot plasma electrons coexist with neutral species and ions close to room-temperature under non-thermal plasma conditions. Modeling of these systems requires a deep understanding of the atomistic processes underlying the rich chemistry of the various radicals and ions with the nascent nanoparticle (NP) surface. A key parameter for determining the contribution of a certain radical/ion species to the NP surface growth, called sticking coefficient, is computed as a weighted sum from the simulated sticking outcomes with different collision velocities drawn from a Maxwell–Boltzmann distribution at certain temperatures. In this work, the collisions of SiH x (x = 1–4) fragments and silicon cluster (Si4, Si2H6, and Si29H36) surfaces, responsible for the sticking coefficients, are simulated by molecular dynamics with a reactive force field. The dependence of sticking coefficients on temperature, H coverage of both silane fragments and cluster surfaces, and the size of the cluster, are systematically examined. And the mechanism underlying the sticking events, specifically the conversion of physical aggregation to chemisorption is investigated to better understand the complex interplay between factors influencing the surface growth. The detailed and multi-parameter model of sticking coefficients, accompanied by the mechanism study of physisorption to chemisorption conversion, provides a more accurate and robust approximation of surface growth rate using sticking coefficients, and a deeper understanding of surface growth processes, for the wider non-thermal plasma simulation community.

Kinetic Study of Nonthermal Plasma Activated Catalytic CO2 Hydrogenation over Ni Supported on Silica Catalyst

Nonthermal plasma (NTP) is a promising alternative to conventional thermal activation for promoting efficient heterogeneous catalysis under ambient conditions. However, full understanding of the hybrid NTP catalyst system is challenging due to its complexity. Herein, using the catalytic CO2 hydrogenation over the Ni@SiO2 catalyst as the model system, we carried out comparative kinetic studies of the catalysis under both thermal and NTP conditions to assess the reaction dynamics and kinetics, aimed at understanding the interactions between NTP and catalyst surface. Specifically, the performance of process parameters was evaluated by varying temperature, total flow rate, partial pressure of H2, and dielectric barrier discharge (DBD) power. Kinetic studies showed that, under the NTP conditions, the catalysis exhibited non-Arrhenius behavior likely due to the complex interactions between NTP and Ni catalyst, showing a linear correlation between the natural logarithmic turnover frequency (TOF) and the reciprocal DBD power (instead of the bulk reactor temperature measured). Additionally, under the conditions used, the activation energy of the catalysis under the NTP conditions was independent of the total feed flow rate (at ∼29 kJ mol–1, being significantly lower than that of thermal catalysis (∼80 kJ mol–1). Kinetic studies suggested that the dissociated H species due to NTP discharge in the gas phase are ready to interact with Ni surfaces, thus reducing the activation barrier required to enable the surface reactions.

Effect of metal dispersion and support structure of Ni/silicalite-1 catalysts on non-thermal plasma (NTP) activated CO2 hydrogenation

Non-thermal plasma (NTP) activated heterogeneous catalysis is a promising alternative to thermal catalysis for enabling many challenging reactions (e.g. catalytic CO2 hydrogenation) under mild conditions. However, the mechanistic insight into the interaction between highly energetic electrons and vibrationally-exited reactive species with metal catalyst is still lacking. Here, catalytically active Ni nanoparticles supported on silicalite-1 zeolites with different configurations regarding the location of Ni active sites and support pore structures were comparably investigated using catalytic CO2 hydrogenation under the thermal and NTP conditions. Experimental results revealed that the performance of the NTP-catalysis depends on the configuration of the catalysts significantly. Specifically, catalysts with Ni active sites sit on the outer surface of zeolite crystals (i.e. microporous Ni/S1 and Ni/M-S1@Shell with steam-assisted recrystallised micro-meso-porous structure) showed relatively good catalytic performance at a low applied voltage of 6.0 kV. Conversely, the encapsulated catalyst with hierarchical meso-micro-porous structure (i.e. Ni/D-S1) which has relatively small (i.e. average Ni particle sizes of 2.8±0.7 nm) and dispersed Ni nanoparticles (i.e. Ni dispersion of ca. 2.5 %) demonstrated comparatively the best catalytic performance (i.e. CO2 conversion of ca. 75 %) at 7.5 kV. Additionally, under the NTP conditions studied, Ni on carbon-templated mesoporous silicalite-1 (Ni/M-S1) showed the worst selectivity to CH4, which was attributed to the poor accessibility of Ni active sites encapsulated in the enclosed mesopores. This study demonstrated the crucial role of catalyst design in NTP activated catalysis.

Mechanistic study of non-thermal plasma assisted CO2 hydrogenation over Ru supported on MgAl layered double hydroxide

Carbon dioxide (CO2) hydrogenation to value-added molecules is an attractive way to reduce CO2 emission via upgrading. Herein, non-thermal plasma (NTP) activated CO2 hydrogenation over Ru/MgAl layered double hydroxide (LDH) catalysts was performed. The catalysis under the NTP conditions enabled significantly higher CO2 conversions (∼85 %) and CH4 yield (∼84 %) at relatively low temperatures compared with the conventional thermally activated catalysis. Regarding the catalyst preparation, it was found that the reduction temperature can affect the chemical state of the metal and metal-support interaction significantly, and thus altering the activity of the catalysts in NTP-driven catalytic CO2 hydrogenation. A kinetic study revealed that the NTP-catalysis has a lower activation energy (at ∼21 kJ mol−1) than that of the thermal catalysis (ca. 82 kJ mol−1), due to the alternative pathways enabled by NTP, which was confirmed by the comparative in situ diffuse reflectance infrared Fourier (DRIFTS) coupled with mass spectrometry (MS) characterisation of the catalytic systems.

Probing the Role of a Non‐Thermal Plasma (NTP) in the Hybrid NTP Catalytic Oxidation of Methane

AbstractThree recurring hypotheses are often used to explain the effect of non‐thermal plasmas (NTPs) on NTP catalytic hybrid reactions; namely, modification or heating of the catalyst or creation of new reaction pathways by plasma‐produced species. NTP‐assisted methane (CH4) oxidation over Pd/Al2O3 was investigated by direct monitoring of the X‐ray absorption fine structure of the catalyst, coupled with end‐of‐pipe mass spectrometry. This in situ study revealed that the catalyst did not undergo any significant structural changes under NTP conditions. However, the NTP did lead to an increase in the temperature of the Pd nanoparticles; although this temperature rise was insufficient to activate the thermal CH4 oxidation reaction. The contribution of a lower activation barrier alternative reaction pathway involving the formation of CH3(g) from electron impact reactions is proposed.

Probing the Role of a Non‐Thermal Plasma (NTP) in the Hybrid NTP Catalytic Oxidation of Methane

Three recurring hypotheses are often used to explain the effect of non‐thermal plasmas (NTPs) on NTP catalytic hybrid reactions; namely, modification or heating of the catalyst or creation of new reaction pathways by plasma‐produced species. NTP‐assisted methane (CH4) oxidation over Pd/Al2O3 was investigated by direct monitoring of the X‐ray absorption fine structure of the catalyst, coupled with end‐of‐pipe mass spectrometry. This in situ study revealed that the catalyst did not undergo any significant structural changes under NTP conditions. However, the NTP did lead to an increase in the temperature of the Pd nanoparticles; although this temperature rise was insufficient to activate the thermal CH4 oxidation reaction. The contribution of a lower activation barrier alternative reaction pathway involving the formation of CH3(g) from electron impact reactions is proposed.

Effect of Non-thermal Plasma on Physicochemical Properties of Nam Dok Mai Mango

Mango is considered as a popular and economically important tropical fruit around the world because of its attractive color, favorable flavor, and high nutritional quality. Nowadays, more consumers consciously desire not only fresh and nutritious but also safe fruits with low or free of microorganisms and chemical contamination. Non-thermal plasma (NTP) is one of the emerging technologies that could potentially decontaminate the fresh food and food processing surfaces. Recently, NTP emerges as a new sanitizing method in the agro-industrial application, where the qualitative characteristics of the treated product could probably be modified by the reactive species as well as the residues of oxidation processes. This study evaluates the changes of physicochemical indexes of Nam Dok Mai mango treated with several NTP conditions. The NTP process was discharged in 0.05 % sodium bicarbonate solution under different voltage levels (800-1500 V) and various treatment durations (2-10 min). It was found that there were significant decreases (p 0.05). The results demonstrated that NTP had slight effect on the physicochemical properties of mango. Future work is required to investigate more comprehensive effect of NTP on the pesticides residues in the mango.

Electrical Characteristics of Nonthermal Gliding Arc Discharge Reactor in Argon and Nitrogen Gases

Gliding arc discharge (GAD) has the properties of both thermal and nonthermal plasma conditions. GAD plasma in the atmospheric pressure with argon/nitrogen and its characteristics are described. Some experimental results about alternating current gliding arc plasma generator have been obtained. It seems that the current density strongly depends on the gas type, and increased with increasing discharge current and gas flow rate. In addition, the discharge current of GAD in nitrogen gas (N2) is greater than one in argon gas (Ar) because of N2 needs more breakdown voltage than Ar. The intensity of GAD increased with increasing the gas flow rate. The oscillograms of discharge current in each case of Ar and N2 were obtained. The electron temperatures of Ar and N2 plasma were calculated to be 22 800 and 8400 K, respectively. The characteristics of both Ar and N2 gases in atmospheric pressure, such as current density, electron density with flow rates (5, 10, 20, and 40) standard cubic foot per hour, were investigated and all experimental results were classified. An experimental study was carried out through using of GAD device for medical treatment by exposing three human blood samples of leukemia to the nonthermal GAD plasma for different periods.

Atmospheric Pressure Ammonia Synthesis Using Non-thermal Plasma Assisted Catalysis

This paper described a novel and green approach on catalytic ammonia synthesis using non-thermal plasma (NTP). The process studied in this paper involves the synthesis and absorption of ammonia under atmospheric pressure and low temperature. The effects of operational parameters including applied voltage, frequency, gas component and flow rate on ammonia synthesis under NTP conditions are studied in this paper. In addition, different selected catalysts and absorbents were investigated under different conditions of NTP treatment and the ammonia efficiency was reported and analyzed. Ru catalyst with carbon nanotube support, along with Cs promoter and micro porous absorbents including Molecular Sieve 13X and Amberlyst 15 yield the highest ammonia efficiency in this process. Results further indicated that frequency and applied voltage of 10,000 Hz and 6000 V, with N2:H2 feed ratio of 3:1 provided the optimized efficiency of ammonia synthesis of 2.3 gNH3/kWh.

Carbon dioxide dissociation to carbon monoxide by non-thermal plasma

The decomposition of carbon dioxide was investigated in a non-thermal plasma dielectric barrier discharge reactor filled with glass balls. The CO2 conversion was maximum by using a sinusoidal excitation compared to pulsed excitation. The CO2 conversion and the CO selectivity are increased in the presence of helium, particularly with the AC power supply, but with a decrease in energy efficiency. The formation of solid materials composed of carbon but also of silica was highlighted for the first time, proving that carbon was incorporated into the silica network along with CO2 dissociation under non-thermal plasma conditions. The temperature control of the reactor wall has shown that the CO selectivity is favored by a low temperature wall, whereas the CO2 conversion remained constant, suggesting that the carbon balance default is due to the CO decomposition on the reactor wall.