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“Inhibiting recombination to improve the performance of plasma-based CO2 conversion”. Wang K, Ceulemans S, Zhang H, Tsonev I, Zhang Y, Long Y, Fang M, Li X, Yan J, Bogaerts A, Chemical Engineering Journal 481, 148684 (2024). http://doi.org/10.1016/j.cej.2024.148684
Abstract: Warm plasma offers a promising route for CO2 splitting into valuable CO, yet recombination reactions of CO with oxygen, forming again CO2, have recently emerged as critical limitation. This study combines experiments and fluid dynamics + chemical kinetics modelling to comprehensively analyse the recombination reactions upon CO2 splitting in an atmospheric plasmatron. We introduce an innovative in-situ gas sampling technique, enabling 2D spatial mapping of gas product compositions and temperatures, experimentally confirming for the first time the substantial limiting effect of CO recombination reactions in the afterglow region. Our results show that the CO mole fraction at a 5 L/min flow rate drops significantly from 11.9 % at a vertical distance of z = 20 mm in the afterglow region to 8.6 % at z = 40 mm. We constructed a comprehensive 2D model that allows for spatial reaction rates analysis incorporating crucial reactions, and we validated it to kinetically elucidate this phenomenon. CO2 +M⇌O+CO+M and CO2 +O⇌CO+O2 are the dominant reactions, with the forward reactions prevailing in the plasma region and the backward reactions becoming prominent in the afterglow region. These results allow us to propose an afterglow quenching strategy for performance enhancement, which is further demonstrated through a meticulously developed plasmatron reactor with two-stage cooling. Our approach substantially increases the CO2 conversion (e.g., from 6.6 % to 19.5 % at 3 L/min flow rate) and energy efficiency (from 13.5 % to 28.5 %, again at 3 L/min) and significantly shortens the startup time (from ~ 150 s to 25 s). Our study underscores the critical role of inhibiting recombination reactions in plasma-based CO2 conversion and offers new avenues for performance enhancement.
Keywords: A1 Journal Article; Plasma-based CO2 splitting Recombination reactions In-situ gas sampling Fluid dynamics modeling Kinetics modeling Afterglow quenching; Plasma, laser ablation and surface modeling Antwerp (PLASMANT) ;
Impact Factor: 15.1
DOI: 10.1016/j.cej.2024.148684
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“Importance of plasma discharge characteristics in plasma catalysis: Dry reforming of methane vs. ammonia synthesis”. De Meyer R, Gorbanev Y, Ciocarlan R-G, Cool P, Bals S, Bogaerts A, Chemical engineering journal 488, 150838 (2024). http://doi.org/10.1016/j.cej.2024.150838
Abstract: Plasma catalysis is a rapidly growing field, often employing a packed-bed dielectric barrier discharge plasma reactor. Such dielectric barrier discharges are complex, especially when a packing material (e.g., a catalyst) is introduced in the discharge volume. Catalysts are known to affect the plasma discharge, though the underlying mechanisms influencing the plasma physics are not fully understood. Moreover, the effect of the catalysts on the plasma discharge and its subsequent effect on the overall performance is often overlooked. In this work, we deliberately design and synthesize catalysts to affect the plasma discharge in different ways. These Ni or Co alumina-based catalysts are used in plasma-catalytic dry reforming of methane and ammonia synthesis. Our work shows that introducing a metal to the dielectric packing can affect the plasma discharge, and that the distribution of the metal is crucial in this regard. Further, the altered discharge can greatly influence the overall performance. In an atmospheric pressure dielectric barrier discharge reactor, this apparently more uniform plasma yields a significantly better performance for ammonia synthesis compared to the more conventional filamentary discharge, while it underperforms in dry reforming of methane. This study stresses the importance of analyzing the plasma discharge in plasma catalysis experiments. We hope this work encourages a more critical view on the plasma discharge characteristics when studying various catalysts in a plasma reactor.
Keywords: A1 Journal article; Electron microscopy for materials research (EMAT); Laboratory of adsorption and catalysis (LADCA); Plasma Lab for Applications in Sustainability and Medicine – Antwerp (PLASMANT)
Impact Factor: 15.1
DOI: 10.1016/j.cej.2024.150838
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“Coupled multi-dimensional modelling of warm plasmas: Application and validation for an atmospheric pressure glow discharge in CO2/CH4/O2”. Maerivoet S, Tsonev I, Slaets J, Reniers F, Bogaerts A, Chemical Engineering Journal 492, 152006 (2024). http://doi.org/10.1016/j.cej.2024.152006
Abstract: To support experimental research into gas conversion by warm plasmas, models should be developed to explain the experimental observations. These models need to describe all physical and chemical plasma properties in a coupled way. In this paper, we present a modelling approach to solve the complete set of assumed relevant equations, including gas flow, heat balance and species transport, coupled with a rather extensive chemistry set, consisting of 21 species, obtained by reduction of a more detailed chemistry set, consisting of 41 species. We apply this model to study the combined CO2 and CH4 conversion in the presence of O2, in a direct current atmospheric pressure glow discharge. Our model can predict the experimental trends, and can explain why higher O2 fractions result in higher CH4 conversion, namely due to the higher gas temperature, rather than just by additional chemical reactions. Indeed, our model predicts that when more O2 is added, the energy required to reach any set temperature (i.e., the enthalpy) drops, allowing the system to reach higher temperatures with similar amounts of energy. This is in turn related to the higher H2O fraction and lower H2 fraction formed in the plasma, as demonstrated by our model. Altogether, our new self-consistent model can capture the main physics and chemistry occurring in this warm plasma, which is an important step towards predictive modelling for plasma-based gas conversion.
Keywords: A1 Journal Article; Plasma, laser ablation and surface modeling Antwerp (PLASMANT) ;
Impact Factor: 15.1
DOI: 10.1016/j.cej.2024.152006
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“Plasma-catalytic direct oxidation of methane to methanol over Cu-MOR: Revealing the zeolite-confined Cu2+ active sites”. Lv H, Meng S, Cui Z, Li S, Li D, Gao X, Guo H, Bogaerts A, Yi Y, Chemical Engineering Journal 496, 154337 (2024). http://doi.org/10.1016/j.cej.2024.154337
Abstract: Efficient methane conversion to methanol remains a significant challenge in chemical industry. This study investigates the direct oxidation of methane to methanol under mild conditions, employing a synergy of nonthermal plasma and Cu-MOR (Copper-Mordenite) catalysts. Catalytic tests demonstrate that the Cu-MOR IE-3 catalyst (i.e., prepared by three cycles of ion exchange) exhibits superior catalytic performance (with 51 % methanol selectivity and 7.9 % methane conversion). Conversely, the Cu-MOR catalysts prepared via wetness impregnation tend to over-oxidize CH4 to CO and CO2. Through systematic catalyst characterizations (XRD, TPR, UV–Vis, HRTEM, XPS), we elucidate that ion exchange mainly leads to the formation of zeolite-confined Cu2+ species, while wetness impregnation predominantly results in CuO particles. Based on the catalytic performance, catalyst characterizations and in-situ FTIR spectra, we conclude that zeolite-confined Cu2+ species serve as the active sites for plasma-catalytic direct oxidation of methane to methanol.
Keywords: A1 Journal Article; Direct oxidation Methanol production Plasma catalysis Copper-mordenite catalysts; Plasma, laser ablation and surface modeling Antwerp (PLASMANT) ;
Impact Factor: 15.1
DOI: 10.1016/j.cej.2024.154337
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“Plasma-catalytic dry reforming of CH4: Effects of plasma-generated species on the surface chemistry”. Sun J, Chen Q, Qin W, Wu H, Liu B, Li S, Bogaerts A, Chemical Engineering Journal 498, 155847 (2024). http://doi.org/10.1016/j.cej.2024.155847
Abstract: By means of steady-state experiments and a global model, we studied the effects of plasma-generated reactive species on the surface chemistry and coking in plasma-catalytic CH4/CO2 reforming at reduced pressure (8–40 kPa). We used a hybrid ZDPlasKin-CHEMKIN model to predict the species densities over time. The detailed plasma-catalytic mechanism consists of the plasma discharge scheme, a gas-phase chemistry set and a surface mechanism. Our experimental results show that the coupling of Ni/SiO2 catalyst with plasma is more effective in CH4/CO2 activation and conversion than unpacked DBD plasma, with syngas being the main products. The
highest total conversion of 16 % was achieved at 8000 V and 473 K, with corresponding CO and H2 yields of 15 % and 12 %, respectively. The reactants conversion and product selectivity are well captured by the kinetic model. Our simulation results suggest that vibrational species and radicals can accelerate the dissociative adsorption and Eley-Rideal (E-R) reactions. Path flux analysis shows that E-R reactions dominate the surface reaction pathways, which differs from thermal catalysis, indicating that the coupling of non-equilibrium plasma and catalysis can effectively shift the formation and consumption pathways of important adsorbates. For instance, our model suggests that HCOO(s) is primarily generated through the E-R reaction CO2(v) + H(s) → HCOO(s), while the hydrogenation reaction HCOO(s) + H → HCOOH(s) is the main source of HCOOH(s). Carbon deposition on the
catalyst surface is primarily formed through the stepwise dehydrogenation of CH4, while the E-R reactions enhanced by plasma-generated H and O atoms dominate the consumption of carbon deposition. This work provides new insights into the effects of reactive species on the surface chemistry in plasma-catalytic CH4/CO2 reforming.
Keywords: A1 Journal Article; Dry reforming of methane Plasma catalysis Plasma-enhanced surface chemistry Path flux and sensitivity analysis Coking kinetics; Plasma, laser ablation and surface modeling Antwerp (PLASMANT) ;
Impact Factor: 15.1
DOI: 10.1016/j.cej.2024.155847
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“Plasma-assisted NH3 cracking in warm plasma reactors for green H2 production”. Fedirchyk I, Tsonev I, Quiroz Marnef R, Bogaerts A, Chemical Engineering Journal 499, 155946 (2024). http://doi.org/10.1016/j.cej.2024.155946
Abstract: renewable energy. Plasma technology is promising for this purpose, as it can crack NH3 without the need for a catalyst and is highly compatible with renewable electricity, reducing the environmental footprint of the cracking process. This work investigates the NH3 cracking performance of four different warm plasma reactors with different configurations and operating in a wide range of conditions. We show that the NH3 conversion in warm plasma reactors is primarily determined by the specific energy input, with the main difference observed in the energy cost (EC) of cracking. The lowest EC obtained is 146 kJ/mol but at a conversion of only 8 %. A more reasonable conversion of around 50 % yields an EC of around 200 kJ/mol in two of the reactors investigated. Plasma reactors operating at higher feed flow rates are more efficient and yield a higher H2 production rate. Our data indicate that NH3 cracking in these warm plasma reactors occurs mainly via thermal chemistry, with nonthermal plasma chemistry playing a less prominent role. NH3 decomposes not only inside the plasma core but also in a hot volume around it, which reduces the EC. Our study shows that warm plasmas are significantly more efficient for NH3 cracking than cold plasmas, even when the latter are combined with catalysts.
Keywords: A1 Journal Article; Plasma-assisted NH3 cracking Plasma reactors Warm plasma H2 production from NH3; Plasma, laser ablation and surface modeling Antwerp (PLASMANT) ;
Impact Factor: 15.1
DOI: 10.1016/j.cej.2024.155946
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