Plasma Catalysis Research Articles

Insight into the synergistic effect between plasma and oxygen vacancy in direct oxidation of CH4 to CH3OH by molecular O2 at mild condition

The direct conversion of methane into the value-added chemical methanol using molecular oxygen was investigated via plasma catalysis under mild condition. An efficient synergistic effect between plasma and catalyst was achieved using an oxygen vacancy-rich ZnO/Al2O3 catalyst. A high methanol selectivity of 48.5 % and a high synergy factor of 2.1 could be achieved simultaneously over ZnO/Al2O3 R300 catalyst. The presence of oxygen vacancies played a crucial role in methanol formation. A close relationship between oxygen vacancy content and methanol selectivity was established. In situ DRIFT and optical emission spectra were employed to identify key intermediates, and a plausible reaction mechanism was proposed. The CH3 and CH3O species were the key intermediates on the catalyst surface. The oxygen vacancy could efficiently adsorb the O radicals and accelerate the formation of CH3O species. This work offers insights into the rational engineering of efficient catalyst for plasma catalysis process.

Mechanism and kinetic study of n-undecane degradation on Co-CeO2: A dual approach combining thermal and plasma catalysis

The catalytic degradation of long-chain alkanes presents a formidable challenge in environmental chemistry. n-Undecane (C11), a quintessential long-chain alkane, predominates as a component of volatile organic compounds (VOCs) emitted from the printing industry and diesel-powered vehicles. The uncontrolled release of these compounds into the atmosphere necessitates stringent measures. This study reports on the plasma-catalytic degradation of n-undecane over the CeO2 based catalysts cooperated with cobalt (Co), manganese (Mn), aluminum (Al), nickel (Ni), iron (Fe), or magnesium (Mg) via aerosol pyrolysis method. Notably, the Co-doped CeO2 catalyst with a Co:Ce atom ratio of 4:1, in conjunction with plasma, achieves an optimal degradation performance. Under the reaction conditions of 90 °C and an energy density of 40 J/L, C11 conversion and COx selectivity reached impressive levels of 100 % and 97 %, respectively. Characterization findings indicate that elevated ratios of Ce3+/(Ce3++Ce4+) and Co3+/(Co2++Co3+) are conducive to the catalytic degradation of C11. Kinetic models have been developed to elucidate the thermal and plasma catalytic degradation mechanisms

Novel methods for determination and manipulation of surface charges performed on an atmospheric DBD microplasma

Abstract In photo- and electrocatalysis it is already well-established that surface charges can alter chemical adsorption and reaction paths of the catalyst. However, the novel realm of plasma catalysis lacks experimental exploration regarding the impact of plasma-induced surface charges on catalytic processes. This paper paves the way by introducing a straightforward method for precisely charging the dielectric (catalyst) in a dielectric barrier discharge microplasma at atmospheric pressure. It additionally enables the determination of this surface charge avoiding artifacts or volume charge interference. Through pre-charging of the dielectric, the charges’ impacts on re-ignition and forming of charge equilibria during the discharge are investigated. Moreover, a laser-assisted operando technique is introduced to generate up to 15% of either positive or negative additional surface charges (compared to the default operation) within 3.5 μs after irradiation and potential for even higher yields. The charges’ origin and the plasma’s response to these laser-induced surface charges are explored.

Oxygen atom activated ZIF-67/carbon cloth in plasma system for CO2 reduction

ZIF-67 was grown in situ on carbon cloth (CC) using a simple one-step method. The prepared ZIF-67/CC electrodes exhibited excellent CO2 reduction reaction (CO2RR) performance in a dielectric barrier discharge plasma reactor. The highest concentrations of produced formic acid and formaldehyde were 9.16 and 0.068 mmol L−1 at a reaction time of 1 h, respectively. The high performance is related to the unique high aspect ratio structure and pad-like cavity of ZIF-67, which results not only in an increase in the specific surface area for CO2 adsorption but also in the hydrophobicity of the electrode. Unexpectedly, the superoxide radical (·O2−) greatly affects the reduction performance of the electrode. In addition, the ZIF-67/CC electrode maintained good CO2RR performance in the presence of different pollutants, and the production of formic acid and formaldehyde increased to 10.81 and 0.11 mmol L−1 at 1 h with the addition of 10 mg L−1 phenol. This research provides new directions in the field of plasma catalysis.

Hybrid plasma catalysis-thermal system for non-oxidative coupling of methane to ethylene and hydrogen

We report a two-stage hybrid plasma catalysis-thermal system for non-oxidative coupling of methane (NOCM) to ethylene (C2H4) and hydrogen (H2), achieving 66 % C2H4 selectivity and 60 % H2 selectivity with 28 % CH4 conversion. This corresponds to a C2H4 yield of 18 %, which is one of the highest reported in literature. The system consists of the first plasma catalysis stage (stage 1) and the second thermal cracking stage (stage 2). Comprehensive analyses using in-situ mass spectrometry (MS) and gas chromatography (GC) reveal that the combination of plasma and Pt/ZrO2 catalyst in stage 1 predominantly facilitates the conversion of methane to ethane. Characterizations employing X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy-scanning transmission electron microscopy-energy dispersive X-ray (HRTEM-STEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR), indicate that the coordinatively unsaturated site of Pt could be the active sites for C–H bond cracking to generate abundant CH3 radicals (CH3·), enhancing the C2 selectivity by inhibiting the non-selective formation of coke and higher hydrocarbons. Subsequently, the products from stage 1 (especially C2H6) undergo pyrolysis reactions at 880 °C in stage 2, which further boosts the C2H4 selectivity and yield. Based on the reaction performance, catalyst characterization, optical emission spectroscopy (OES), and in-situ Fourier transform infrared spectroscopy (FTIR) results, we reveal the reaction mechanism within the hybrid two-stage plasma catalysis-thermal system for converting CH4 to C2H4 and H2.

Unveiling the Mechanism of Plasma-Catalytic Low-Temperature Water-Gas Shift Reaction over Cu/γ-Al2O3 Catalysts.

The water-gas shift (WGS) reaction is a crucial process for hydrogen production. Unfortunately, achieving high reaction rates and yields for the WGS reaction at low temperatures remains a challenge due to kinetic limitations. Here, nonthermal plasma coupled to Cu/γ-Al2O3 catalysts was employed to enable the WGS reaction at considerably lower temperatures (up to 140 °C). For comparison, thermal-catalytic WGS reactions using the same catalysts were conducted at 140-300 °C. The best performance (72.1% CO conversion and 67.4% H2 yield) was achieved using an 8 wt % Cu/γ-Al2O3 catalyst in plasma catalysis at ∼140 °C, with 8.74 MJ mol-1 energy consumption and 8.5% H2 fuel production efficiency. Notably, conventional thermal catalysis proved to be ineffective at such low temperatures. Density functional theory calculations, coupled with in situ diffuse reflectance infrared Fourier transform spectroscopy, revealed that the plasma-generated OH radicals significantly enhanced the WGS reaction by influencing both the redox and carboxyl reaction pathways.

Non-thermal plasma-catalytic processes for CO2 conversion toward circular economy: fundamentals, current status, and future challenges.

Practical and energy-efficient carbon dioxide (CO2) conversion to value-added and fuel-graded products and transitioning from fossil fuels are promising ways to cope with climate change and to enable the circular economy. The carbon circular economy aims to capture, utilize, and minimize CO2 emissions as much as possible. To cope with the thermodynamic stability and highly endothermic nature of CO2 conversion via conventional thermochemical process, the potential application of non-thermal plasma (NTP) with the catalyst, i.e., the hybrid plasma catalysis process to achieve the synergistic effects, in most cases, seems to promise alternatives under non-equilibrium conditions. This review focuses on the NTP fundamentals and comparison with conventional technologies. A critical review has been conducted on the CO2 reduction with water (H2O), methane (CH4) reduction with CO2 to syngas (CO + H2), CO2 dissociation to carbon monoxide (CO), CO2 hydrogenation, CO2 conversion to organic acids, and one-step CO2-CH4 reforming to the liquid chemicals. Finally, future challenges are discussed comprehensively, indicating that plasma catalysis has immense investigative areas.

Sustainable H2O2 production via solution plasma catalysis

Clean production of hydrogen peroxide (H2O2) with water, oxygen, and renewable energy is considered an important green synthesis route, offering a valuable substitute for the traditional anthraquinone method. Currently, renewable energy-driven production of H2O2 mostly relies on soluble additives, such as electrolytes and sacrificial agents, inevitably compromising the purity and sustainability of H2O2. Herein, we develop a solution plasma catalysis technique that eliminates the need for soluble additives, enabling eco-friendly production of concentrated H2O2 directly from water and O2. Screening over 40 catalysts demonstrates the superior catalytic performance of carbon nitride interacting with discharge plasma in water. High-throughput density functional theory calculations for 68 models, along with machine learning using 29 descriptors, identify cyano carbon nitride (CCN) as the most efficient catalyst. Solution plasma catalysis with the CCN achieves concentrated H2O2 of 20 mmol L-1, two orders of magnitude higher than photocatalysis by the same catalyst. Plasma diagnostics, isotope labeling, and COMSOL simulations collectively validate that the interplay of solution plasma and the CCN accounts for the significantly increased production of singlet oxygen and H2O2 thereafter. Our findings offer an efficient and sustainable pathway for H2O2 production, promising wide-ranging applications across the chemical industry, public health, and environmental remediation.

Oxidation of lean methane by dielectric barrier discharge plasma and Mn–Ce catalyst

The synergistic dielectric barrier discharge plasma and Mn-Ce catalysts are employed to facilitate the low temperature and highly efficient oxidation of lean methane. The effect of the catalyst alone, plasma alone, and their synergistic effect on CH4 conversion are experimentally analyzed. It shows that the CH4 conversion rate under plasma catalysis is more than three times that of catalyst alone and plasma alone, with a maximum of 66.7% at 375 °C and 21 W, revealing a significant synergistic effect of plasma catalysis. The influence of Mn/Ce mass ratios (11Mn/Ce and 45Mn/Ce) and operational conditions (CH4/O2 molar ratio, preheating temperature, and discharge power) on CH4 conversion by plasma catalysis are also investigated. The CH4 conversion rate increases with the decrease of the CH4/O2 molar ratio and the increase in preheating temperature and discharge power. Meanwhile, the Mn/Ce mass ratio affects the CH4 oxidation and the synergistic effect of plasma catalysis. The oxidation activity of the 11Mn/Ce catalyst is mostly affected by the power at the preheating temperature below 325 °C. However, the 45Mn/Ce catalyst is affected by both preheating temperature and power. In addition, as the discharge power increases, the CH4 conversion rate exhibits a linear increase for 11Mn/Ce but exponential for 45Mn/Ce.

Efficient removal of tetracycline antibiotic by nonthermal plasma-catalysis combination process

This study presents an innovative approach to plasma catalysis by employing nonthermal plasma type gliding arc with various iron oxides -Fe(II), Fe(III) and Ferrate Fe(VI)- for the removal of tetracycline antibiotic (TC). The experimental results demonstrated significant improvements in degradation rates, with plasma-Fe(VI) achieving complete degradation after 15 min of treatment and the plasma-Fe(II) achieving 97.28% removal after 30 min. The effect of TC initial concentration and catalyst dose was investigated, revealing that lower TC concentration favored the degradation and shortened treatment times with even a minimum amount of catalyst doses enhancing degradation results. Higher total organic carbon (TOC) removal was achieved in the combination systems (83.01%; 82.07%; and 42.82% for plasma/Fe(VI); plasma/Fe(II); and plasma /Fe(III) respectively) compared to plasma alone barely achieved 25.67% of mineralization. Additionally, the study examined the role of various radical scavengers-targeting <sup>•</sup>OH, superoxide, and free electrons- in the degradation process. TC intermediates were also identified, degradation pathways were proposed, and the intermediates toxicity was assessed. This research contributes to a deeper understanding of organic compounds treatment using nonthermal plasma, combination with the ferrate process or other catalysts.

Dielectric barrier discharge plasma catalysis for diesel particulate matter oxidation: Optimization and synergistic differences in transition metal catalysts

Dielectric barrier discharge plasma catalysis for diesel particulate matter oxidation: Optimization and synergistic differences in transition metal catalysts

Plasma Catalysis Modeling: How Ideal Is Atomic Hydrogen for Eley-Rideal?

Plasma catalysis is an emerging technology, but a lot of questions about the underlying surface mechanisms remain unanswered. One of these questions is how important Eley-Rideal (ER) reactions are, next to Langmuir-Hinshelwood reactions. Most plasma catalysis kinetic models predict ER reactions to be important and sometimes even vital for the surface chemistry. In this work, we take a critical look at how ER reactions involving H radicals are incorporated in kinetic models describing CO2 hydrogenation and NH3 synthesis. To this end, we construct potential energy surface (PES) intersections, similar to elbow plots constructed for dissociative chemisorption. The results of the PES intersections are in agreement with ab initio molecular dynamics (AIMD) findings in literature while being computationally much cheaper. We find that, for the reactions studied here, adsorption is more probable than a reaction via the hot atom (HA) mechanism, which in turn is more probable than a reaction via the ER mechanism. We also conclude that kinetic models of plasma-catalytic systems tend to overestimate the importance of ER reactions. Furthermore, as opposed to what is often assumed in kinetic models, the choice of catalyst will influence the ER reaction probability. Overall, the description of ER reactions is too much "ideal" in models. Based on our findings, we make a number of recommendations on how to incorporate ER reactions in kinetic models to avoid overestimation of their importance.

Plasma catalysis. A study of the influence of oxysulfur, SOx• species through the degradation of sulfonated dyes by non-thermal plasma

This work studied the degradation reaction of sulfonated dyes, indigo carmine, phenol red, and their mixtures by non-thermal plasma (NTP). Interestingly, the degradation rate constant showed a faster process and lower activation energy (Ea) for the dye mixtures than for the degradation reaction of the individual dyes. This unexpected result opened up new opportunities for understanding plasma chemistry and the interaction between reactive species formed by the plasma and the target molecule. As no catalyst or chemical additive was added to the reactor, the decrease in Ea came from a self-synergistic effect (SSE), through the dye molecules fragmentation, which resulted in plasma catalysis. The hypothesis proposed in this work is that oxysulfur (SOx) species are formed by the desulfonation reaction of dyes. The sulfonic groups (SO3) present in the chemical structures of dyes can function as precursors for forming several SOx•– species. Studies based on oxygenated sulfonated species such as SO3•–, SO4•– and SO5•– have been widely applied in advanced oxidative and reductive processes due to their satisfactory efficiency and low cost. Among them, SO4•– is the key reactive species with the best performance in the degradation of pollutants due to its high oxidation potential (E° = 2.60 V). In addition, it is an alternative source of HO• in aqueous media, improving the oxidation reaction. In order to elucidate the SSE, the kinetic process was followed by UV–Vis analysis, and the reactive species, such as alkyl, hydroxyl, and oxy-sulfur radicals were identified by Electron Paramagnetic Resonance. The by-products of the NTP degradation reaction were analyzed by ultrafast liquid chromatography coupled with a mass spectrometer, and a fragmentation route was proposed.

Post‐microwave plasma catalysis: Current developments and future implications

AbstractPost‐microwave plasma catalysis combines plasma preactivation of the gas phase followed by a fixed bed catalyst. This process has the potential to be energy efficient, modular, versatile and to generate high numbers of active species compared with other plasma generation technologies. Specific applications of these post‐microwave plasma systems include the dissociation and activation of stable molecules and the modification of catalyst materials. This concept article will address the current findings in microwave plasma catalysis research and propose future questions that need to be addressed to further develop the technology.

Efficient simultaneous removal of diesel particulate matter and hydrocarbons from diesel exhaust gas at low temperatures over Cu–CeO2/Al2O3 coupling with dielectric barrier discharge plasma

Diesel particulate matter (DPM) and hydrocarbons (HCs) emitted from diesel engines have a negative affect on air quality and human health. Catalysts for oxidative removal of DPM and HCs are currently used universally but their low removal efficiency at low temperatures is a problem. In this study, Cu-doped CeO2 loaded on Al2O3 coupled with plasma was used to enhance low-temperature oxidation of DPM and HCs. Removals of DPM and HCs at 200 °C using the catalyst were as high as 90% with plasma but below 30% without plasma. Operando plasma diffuse reflectance infrared Fourier transform spectroscopy coupled with mass spectrometry was conducted to reveal the functional mechanism of the oxygen species in the DPM oxidation process. It was found that Cu–CeO2 can promote the formation of adsorbed oxygen ( – ) and terminal oxygen (M=O), which can react with DPM to form carbonates that are easily converted to gaseous CO2. Our results provide a practical plasma catalysis technology to obtain simultaneous removals of DPM and HCs at low temperatures.

Plasma Catalysis for Hydrogen Production: A Bright Future for Decarbonization.

Thermal approaches have played a dominant role in driving chemical reactions within the chemicals and fuels industries, benefiting from ongoing enhancements in efficiency via heat integration, catalyst development, and process intensification. Nevertheless, these traditional thermal approaches remain heavily reliant on fossil fuels, and there exists an urgent demand for the implementation of renewable energy technologies to synthesize fuels, commodity chemicals, and specialty chemicals. Nonthermal plasmas have gained considerable attention in recent years as a promising solution, and the prospects of combining plasmas with suitable catalysts have become even more appealing. Moreover, the evolution of nonthermal plasma catalysis approaches for the generation of clean hydrogen could be transformative in reducing greenhouse gas emissions. This comprehensive review highlights the influential contributions in plasma catalysis for hydrogen production, discusses recent advancements, and provides future prospects for researchers aiming to advance the production of clean hydrogen.

Enhancements of electric field and afterglow of non-equilibrium plasma by Pb(ZrxTi1−x)O3 ferroelectric electrode

Manipulating surface charge, electric field, and plasma afterglow in a non-equilibrium plasma is critical to control plasma-surface interaction for plasma catalysis and manufacturing. Here, we show enhancements of surface charge, electric field during breakdown, and afterglow by ferroelectric barrier discharge. The results show that the ferroelectrics manifest spontaneous electric polarization to increase the surface charge by two orders of magnitude compared to discharge with an alumina barrier. Time-resolved in-situ electric field measurements reveal that the fast polarization of ferroelectrics enhances the electric field during the breakdown in streamer discharge and doubles the electric field compared to the dielectric barrier discharge. Moreover, due to the existence of surface charge, the ferroelectric electrode extends the afterglow time and makes discharge sustained longer when alternating the external electric field polarity. The present results show that ferroelectric barrier discharge offers a promising technique to tune plasma properties for efficient plasma catalysis and electrified manufacturing.

Progress in Green Ammonia Synthesis Technology: Catalytic Behavior of Ammonia Synthesis Catalysts

AbstractAmmonia as a green energy source has attracted a lot of attention in recent years. Despite its industrial intensity, the Haber‐Bosch process remains a primary ammonia source, emitting significant CO2 (≈2.9 tons per ton of ammonia). Future ammonia synthesis methods aim to surpass the Haber‐Bosch process by operating under milder conditions. These methods encompass chemical looping, thermal catalysis, electrochemical catalysis, photocatalysis, and plasma catalysis, albeit with inherent limitations. Although thermal catalysis has reduced conditions to ≈5 MPa, innovative catalysts are still scarce. Electrochemical catalysis produces hydrogen via water electrolysis but encounters challenges in Faraday efficiency and ammonia yield. Photocatalytic synthesis, while energy‐efficient, suffers from sluggish reaction rates. Plasma synthesis, while achieving low temperatures and pressures, faces difficulties in ammonia yield amidst competitive reactions. Chemical looping synthesis, enabling independent nitrogen fixation and hydrogenation, lacks efficient nitrogen transport catalysts. Effective catalysts are a common requirement across these methods. This review explores recent advancements, elucidating reaction mechanisms, nitrogen activation, and catalyst performance, while discussing the strengths, weaknesses, and future prospects of ammonia synthesis technologies to foster further innovation in the field.

Bi-reforming with a ratio of CH4/CO2/H2O = 3/1/2 by gliding arc plasma catalysis for power to fuels

With the aim of directly producing a high-quality syngas with a ratio of H2/CO = 2, bi-reforming of CH4 with the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2 is carried out in a gliding arc-based warm plasma catalytic reactor. The gliding arc plasma is a typical warm plasma (WP), which provides favorable conditions for CO2 activation, and it is found that the highest conversions are obtained in the case of reaction using the WP alone. A comparison of reactions using the WP alone (the WP case), the conventional catalyst alone (the CC case), and the WP plus catalyst (the WPC case) reveals that the WPC case can overcome the disadvantages of both the WP and CC cases. In the WPC case, CH4, CO2, and H2O react at the ideal stoichiometric ratio of CH4/CO2/H2O = 3/1/2. In addition, higher reactant conversions and energy efficiencies are obtained in the WPC case than in the WP case. A high-quality syngas with H2/CO = 2 is obtained, with similar conversions of (89 ± 1)% for all of CH4, CO2, and H2O and an energy efficiency of 71%.

Ammonia synthesis by plasma catalysis in an atmospheric RF helium plasma

The in-plasma-catalytic synthesis of ammonia from nitrogen and hydrogen admixed to a helium RF plasma is studied with infrared absorption spectroscopy, optical emission spectroscopy, and through chemical kinetics modeling. Sandblasted glass is used as support for iron, platinum, and copper catalysts up to a surface temperature of 150 ∘C . It is shown that the optimum ammonia production occurs at very small N2/(N2+H2) ratios with an increase of concentration with plasma power. The global kinetic modelling agrees well with the data for a variation of the N2+H2 admixture and the absorbed plasma power. The introduction of the catalyst enhances ammonia production by up to a factor of 2. Based on the comparison with the modelling, this is linked to a change in the electron kinetics due to the presence of the catalyst. It is postulated that introducing the catalyst increases the reduced electric field because it reduces the secondary electron emission coefficient. As a result, the dissociation of N2 is stimulated, thereby enhancing the NH3 formation. These experiments show that the impact of the catalyst on the plasma performance in noble gas-diluted RF plasmas can be more important than the impact of the plasma on any catalytic surface process.