Plasma catalysis for gas conversion – Impact of catalyst on the plasma behavior

Plasma catalysis for gas conversion –Impact of catalyst on the plasma behavior M. Ruiz-Martín 1,2 , M. Oliva-Ramírez 1,2 , A. R. González-Elipe 2 and A. Gómez-Ramírez 1,2 How does a catalyst influence plasma behavior? What is the true role of a catalyst in a plasma environment? The observed synergies in plasma–catalysis processes have been usually attributed to conventional surface catalytic interactions. However, recent studies demonstrate that metallic catalysts may induce alterations in plasma behavior that substantially influence the overall process efficiency. This work reviews the latest advances in the field, proposing the term PlasmaCatalysis Promoter (PCP) to designate compounds that facilitate reactions not only at the catalyst surface but also within the plasma phase. It also highlights that advancing in the field requires a holistic approach capable of unraveling the multifunctional role of PCPs in plasma–catalysis processes. Addresses 1 Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Avda. Reina Mercedes, E-41012 Seville, Spain 2 Laboratory of Nanotechnology on Surfaces and Plasma. Instituto de Ciencia de Materiales de Sevilla (CSIC-Universidad de Sevilla), Avda. Américo Vespucio 49, E-41092 Seville, Spain Corresponding author: Gómez-Ramírez,A. ([email protected]) Current Opinion in Green and Sustainable Chemistry 2025, 51:100990 This review comes from a themed issue on Plasma based green chemistry (2025) Edited by Annemie Bogaerts and Yury Gorbanov Available online 7 December 2024 For complete overview of the section, please refer the article collection - Plasma based green chemistry (2025) https://doi.org/10.1016/j.cogsc.2024.100990 2452-2236/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC license (http:// creativecommons.org/licenses/by-nc/4.0/). Keywords Plasma catalysis, Non-thermal plasma, Catalysts, Reaction kinetics, Active phase, Dielectric barrier discharge, Plasma-catalysis promoter. Introduction The incorporation of catalysts in plasma reactors pursuits to enhance the energy efficiency and chemical yield of chemical reactions by creating synergies between plasma gas reactivity and surface processes [1e3]. Unlike traditional catalytic reactors, plasma reactors generally operate under mild conditions and are capable of overcoming thermodynamic restrictions existing at low temperatures and pressures [4]. Although different studies have dealt with these synergies in sustainabilityrelated and energy revalorization reactions, many questions remain open regarding the actual role of catalysts, which sometimes increase the reaction yields and others lead to virtually no improvement [1e3,5,6]. Several types of plasma reactors have been used for plasmaecatalysis processes, including microwave and radiofrequency discharges, as well as gliding arc reactors. However, the most widely used approach to date relies on dielectric barrier discharges (DBD), which are available in both classical and packed-bed (DBD-PB) configurations [7]. The DBD-PB configuration enables a straight integration of the catalyst and its direct contact with the plasma, making this the preferred choice to study plasmaecatalyst interactions [2,3]. When analyzing plasmaecatalysis processes, it is essential to establish the specific role of the catalysts and their possible interactions with the plasma [4]. This requires a thorough characterization of both catalyst (before and after the process) andplasma properties (with andwithout catalyst), as well as a detailed identification of reaction mechanisms on the catalyst surface and in the plasma phase [3,4]. This complex scenario, the large number of physical and chemical parameters involved, and the wide variety of reactor designs and operating conditions avaiable (geometries, sizes, barrier materials, voltage, frequency, type of electrical signal, etc.) make it challenging to conduct comparative studies of results from different laboratories [1,2]. In this context,a debate has arisen about the true role of catalysts in plasma environments and the question of whether reactivity improvements are due to traditional surface catalytic effects and/or to alterations in the electrical properties of the plasma caused by the incorporation of catalysts. Moreover, processes and mechanisms appear tobe highly dependent on the specific reaction under investigation (e.g., they may differ between hydrocarbon reforming and ammonia synthesis) [1,6,8]. In this article, we critically revise the role of catalysts in various plasma-assisted chemical processes reported during the past two years, focusing on the improvements (or lack thereof) found in reaction yield and on the modifications induced in discharge properties and reaction mechanisms by specific plasmaecatalyst interactions. Plasma-catalysis process In DBD-PB reactors, catalysts are either integrated into the barrier material in the form of beads or deposited onto them. Conventional catalysts are formed by a Available online at www.sciencedirect.com ScienceDirect Current Opinion in Green and Sustainable Chemistry www.sciencedirect.com Current Opinion in Green and Sustainable Chemistry 2025, 51:100990 ‘support,’ generally an oxide or similar inert dielectric material, and an‘active phase,’ whose atoms intervene in the chemical reaction and can be a metal, a semiconductor, or another dielectric material. In this review, we will keep these designations borrowed from classical catalysis, although the actual role of the different catalyst components and the dielectric barrier may strongly differ when operating under plasma conditions. Plasmaecatalysis processes are rather complex and may involve a variety of physicochemical processes [1e4]as shown in Figure 1. In classical catalysis (Figure 1a), chemical reactions occur through Langmuire Hinshelwood (L-H) and EleyeRideal (E-R) mechanisms on the surface of the active phase. Conversely, in pure plasma processes (Figure 1b), a series of elemental reactions take place in the gas phase induced by the interaction with plasma electrons. A more complex situation exists in plasma catalysis (Figure 1c), where both plasma and surface reactions may occur, the latter of which occurs not only on the surface of the ‘active phase’ but also on the ‘support’ and even on the barrier material acting as a discharge moderator. Reactions may involve reactant and product molecules, as well as intermediate species produced in the plasma (note that reactions involving product molecules may intervene in reverse reactions producing reactant or other undesired molecules and will therefore contribute to a decrease in the Figure 1 Scheme illustrating the physicochemical processes and mechanisms involved in plasma-catalysis. (a) Classical catalysis. (b) Plasma reactions. (c) Plasma–catalysis processes. In plasma–catalysis processes; reactions can occur not only at the active sites but also on the supports and moderator materials. Furthermore, these reactions can proceed either through neutral species and reactant molecules (i.e., as in conventional catalysis) or through specific processes involving excited species generated in the plasma. The two types of processes can be enhanced by the local heating effects and local electrical fields induced by the plasma. For simplicity, excited species are represented in the scheme with a positive charge, but they can be neutral or negatively charged. 2Plasma based green chemistry (2025) Current Opinion in Green and Sustainable Chemistry 2025, 51:100990 www.sciencedirect.com overall process performance) [1,4,6]. Other factors such as secondary electron emission, enhancement of electrical field at metal moieties, or the effect of support or barrier porosity can be also critical, the latter especially if active phase particles are located in the interior of pores and become either accessible or not to the plasma (e.g., for pore diameters larger or smaller than the plasma Debye length) [9e11]. It is also important to highlight the role of the barrier material itself (e.g., dielectrics, ferroelectrics), whose electrical and surface properties are critical in determining the electrical field distribution and the type of generated discharge [6,10,12,13]. In general, the higher the dielectric constant, the greater the number of microdischarges, while the formation of homogeneous surface discharges is favored for moderators with low dielectric constants; conversely, rough material terminations enhance electrical field ‘tip’ effects and contribute to the formation of such microdischarges. The physicochemical properties of the catalysts are also critical for the control of the reaction; the type of metal used as active phase, degree of support porosity, effective medium dielectric constant and redox and acidebase properties of, respectively, active phase and support define the catalyst’s behavior in the presence of plasma and how these variables may affect plasma behavior [3]. Historically, conventional catalysts employed in thermal processes (e.g., Ru and Fe on an oxide porous support for the case of the NH 3 synthesis) have been utilized as ‘plasma catalyst’, despite their high cost and limited availability in somecases. Moreover, on these conventional catalysts, the highly reactive plasma species produced in the plasma may become irreversibly adsorbed or react with the active phase leading to the poisoning of the catalyst [4]. These and other shortcomings have thrust the research on innovative concepts relying on less expensive materials and the use of disruptive configurations facilitating the adsorption (and desorption) of specific intermediate species [14], reactants, or reaction products, thereby increasing reaction rates even at low operating temperatures [15]. Therefore, when we refer to a ‘plasma catalyst’, we do so in a broad sense, beyond the thermal induced reactivity of conventional catalysts and considering features such as electrical properties, distribution of moderator and catalyst, morphology of coated moderator beads, barrier configuration, etc. Furthermore, materials typically acting as catalyst supports or even as discharge moderators can also act as effective ‘plasma catalysts’. In this line, it has been recently demonstrated that, without discarding the contribution of a certain catalytic activity, the main role of materials supposedly acting as a ‘catalyst’ in various plasma-assisted processes is to modify the discharge properties rather than promote a classical catalytic reaction [5,6]. For example, ‘catalysts,’ including metallic particles as active phase for a given reaction, can promote filamentary discharges, affect their amount and distribution within the reactor, modify their intensity, and provoke other effects, such as lowering the breakdown voltage and inducing localized heating effects [1,2,6]. Other ‘catalysts’ may contribute to promoting surface discharges, which can be beneficial for certain processes. Therefore, in this work, to distinguish these materials from conventional catalysts, we propose the term ‘Plasma-Catalysis Promoter’ (PCP) to refer to components that, through a variety of mechanisms, can promote certain surface reactions and/or vary the intensity of plasmas, enhancing the number and intensity of microdischarges or surface discharges and thus leveraging positive effects in plasmaesolid interactions. Therefore, in the consecutive sections, the acronym PCP will be used to refer to any kind of catalyst material or promotor that contributes to enhance process performance. Whenever possible, their actual role will be also briefly discussed, although in most cases no clear information exists about reaction mechanisms. Recent case examples of plasma-catalysis reactions To date, published works describing the effect of PCP on plasma reactions are scarce and the mechanisms governing catalysteplasma interactions are not yet fully understood. Herein, we discuss the impact of PCPs on reaction performance and plasma properties, focusing on processes that have attracted much interest in the field of plasma chemistry. It is noteworthy though that, to date, there has not been a clear criterion for the selection of PCP and that in most cases conventional catalysts have been chosen for this function. The synthesis of NH₃from N₂and H₂represents one of the most widely studied plasma-catalysis reactions during the last two years [9,10,12e14,16e25]. Recent works have reported an enhancement of NH₃production when utilizing PCP consisting of classical catalyst supports and active phases, including Ni [14], VN [19], La₂O₃[20], NieMg/SBA-15 [23] metal-loaded MgAl double-layer hydroxides [24], Ru [6,22], Ag, Cu, CO [25], and SieAlPhosphate (SAPO) zeolites [9]. However, the underlying mechanisms responsible for the observed improvements in reactivity are not yet fully disclosed, mainly because of the reduced number of works that have focused on the effects of these catalytic materials on plasma behavior and delved into the involved reaction mechanisms. In this regard, it is worth mentioning that Ndayirinde et al. [5] have reported that the incorporation of different Cobased catalysts alters the discharge current curves and affects the consumed power of a DBD-PB reactor, concluding that the impact of the catalytic material on the electrical properties of the discharge was greater than the catalytic effect itself (i.e., these materials were acting as a PCP rather than as a conventional catalyst). De Meyer et al. [1] employed Ni and Co supported on alumina as a catalyst (i.e., PCP in our terminology) and observed that the chemical method employed for its preparation significantly affected the morphology and surface Plasma Catalysis: Impact of Catalyst on the Plasma Ruiz-Martín et al. 3 www.sciencedirect.com Current Opinion in Green and Sustainable Chemistry 2025, 51:100990 distribution of the active phase, which in turn strongly influenced the discharge properties. For example, the application of spray-coating as processing tool increased the plasma volume, while reducing the number of microdischarges. This produced a decrease in the extent of reverse reactions, thus enhancing the overall yield for NH₃production. Navascue ´setal.[6] used a PCP consisting of Ru supported on alumina and found that the incorporation of metal-alumina powders onto high dielectric constant barrier materials (PZT) acting as a moderator was not effective in improving reaction yield, even at elevated temperatures. Moreover, these two latter studies [1,6] report that the incorporation of PCP materials alters the current curves, an effect that disappeared at high temperatures, as mentioned in the study by Navascue ´s et al. [6]. These recent works highlight the importance of barrier materials and the physical properties of PCPs in controlling the discharge behavior. Along the same line, recent studies utilizing artificial neural network procedures demonstrated that discharge power accounts for 26% of the ammonia synthesis rate [16]. Other effects have been also reported by Gershman et al. [18] who, using mesoporous silica structures with and without Au as active phase, foundthat the presence of the metal led to an increase in the rotational temperature of N₂probably due to localized heating effects, which in turn contributed to an increase in the reaction yield at low power levels. Another clear example in this line is the work of Zhang et al. [26] demonstrating that, for a nitrogen fixation process, the presence of a MnO compound increases the number of microdischarges (although with lower intensity), the capacitance, the peak-to-peak exchanged charge, and the N₂vibrational temperature. However, the evidence that PCPs modify the plasma discharge should not oversee the occurrence of surface processes. In the last two years, studies in this area have also been published. Rouwenhorstm and Lefferts [22], using low concentrations of metallic active phase (Ru, Co, Pt, Pd, and Cu) to minimize their impact on the discharge, developed a kinetic model to describe the reaction mechanism and demonstrated a clear plasmacatalysis synergy, where the rate-limiting step was the reaction between N radicals from the plasma and H atoms formed on the catalyst surface. Similarly, the kinetic model proposed by Liu et al. [17] highlights the importance of the reaction between N and H plasma radicals on catalytic surfaces. Ammonia reactions are currently gaining much interest in producing hydrogen from NH 3 [27e32]. Meng et al. [27] have shown that catalysts with higher activity in thermal catalysis are not necessarily the best option in the presence of plasma, highlighting the importance of other effects beyond purely catalytic ones. Wang et al. [30] investigated the effect of the size of Mo₂N moieties, concluding that increasing particle size favored the formation of surface discharges and the transport of reactive species, thereby increasing the ammonia decomposition efficiency. Dry reforming of methane (DRM) has been much investigated during the last two years [3,8,11,33e38]. The selection of catalytic material for this process pursues an effective control of the selectivity towards specific reaction products. Most works on this topic have investigated the effect of catalytic materials on reaction performance, but a few of them have also addressed the effect of the catalyst/PCPon plasma properties. Lenders et al. [2] used a coupled plasma-surface kinetics model to demonstrate that transition metals are not the best option for DRM. De Meyer et al. [1] using a PCP formed by Ni or Co on Al₂O₃demonstrated that it promotes microdischarges and a large plasma volume, contributing to improved reaction performance. Mei et al. [37] employed Ni, Ag, and Pt/ g -Al₂O₃as PCPs and demonstrated that CO₂conversion is related to the basicity of the catalyst support, while plasma discharge and catalyst properties control that of CH 4 . Dou et al. [8] have highlighted the importance of the catalyst/PCP microstructure and its basicity in plasma-induced interfacial catalytic interactions. Meanwhile, Navascue ´s et al. [39] showed that, when using high dielectric constant materials in the absence of a catalyst/PCP, the CO₂and CH₄ reaction pathways are relatively independent. In this line, machine learning studies confirm the minimal impact of catalyst loading on reaction performance [38]. Perspective and conclusions Most recent studies in the field of plasma-catalysis have shown that observed improvements in performance are due to changes in discharge properties induced by the catalytic materials. This suggests that conventional catalysts may not be the optimal choice for these processes. Herein, we introduce the concept of PCP to refer to materials that enhance the efficiency of plasma-catalysis, but not necessarily in the way of conventional catalysts. An optimized design of PCP materials should reduce the extent of reverse reactions, improve energy efficiency, enable operation through a wider temperature range, and promote E-R mechanisms through interaction with plasma species. Additionally, PCPs should optimize electrical discharge properties and plasma parameters. To facilitate the future industrial implementation of DBDPB plasma technology, a paradigm shift is needed: rather than proceeding through unsystematic variations in reactor designs and process parameters, it is essential to figure out how PCP materials promote plasma-assisted catalytic reactions and how they alter plasma behavior. In particular, it is crucial to distinguish between surface reaction contribution from processes taking place in the gas phase, as well as on surfaces of other components present in the reactor. We propose that the effects of PCPs must be considered in a holistic way as schematized in Figure 2.Initwe 4Plasma based green chemistry (2025) Current Opinion in Green and Sustainable Chemistry 2025, 51:100990 www.sciencedirect.com summarize a series of tasks and methodology approaches that should contribute to unraveling key questions about the role played by these materials. The scheme shows that this analysis requires systematic experimental and theoretical works using identical reactor configurations and operating parameters, with the only variable being the presence or absence of a given PCP. Furthermore, PCPs should feature disruptive designs, using new manufacturing techniques for an ad hoc tailoring of their geometric and physicochemical properties. Combining this concept with innovative barrier architectures is expected render the enhancement of efficiency for a given reaction (e.g., designs where the electric field can be changed as needed by eliminating catalyst supports or employing architectures that promote or prevent specific reaction mechanisms). The proper characterization of PCPs demands ‘in situ” studies of adsorbed surface species across a broad range of temperatures. The determination of plasma reaction mechanisms should also be reinforced with new methods (e.g., isotope labeling [39]). This is without dismissing the impact of specific L-H or E-R mechanisms on process performance. The properties of the barrier material in DBD-PB reactors (e.g., dielectric constant, porosity, roughness) or the applied power should be also taken into consideration. Finally, it is worth mentioning that the use of artificial intelligence (AI) tools appears to be a promising approach in this field as it could provide valuable insights into the variables that control the processes. Funding This publication is part of the project PID2020114270RA-I00, funded by MICIU/AEI/ 10.13039/ 501100011033. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments Authors acknowledge projects PID2020-114270RA-I00, PID2023-147916NAI00, and PID2020-112620GB-I00 funded by MICIU/AEI/10.13039/501100011033 and project TED2021-130124 A-I00 funded by AEI/ 10.13039/501100011033/Unio ´n Europea Next Generation EU/PRTR. M. Oliva-Ramı ´rez acknowledges financial support from Grant IJC2020-045087-I funded by MCIN/AEI/10.13039/501100011033 and the European Union NextGeneration EU/PRTR. Data availability No data was used for the research described in the article. References Papers of particular interest, published within the period of review, have been highlighted as: * of special interest * * of outstanding interest 1 ** .De Meyer R, et al.: Importance of plasma discharge characteristics in plasma catalysis: dry reforming of methane vs. ammonia synthesis.Chem Eng J 2024, 488. This work analyzes the effect of incorporating two catalysts -Co and Ni supported on aluminaon plasma behavior during the plasma-assisted ammonia synthesis and dry methane reforming. Authors examine two distinct synthesis techniques (spray-coating and wet impregnation), which lead to significant differences in the morphology and distribution of the catalyst on the surface of the moderator material, thereby affecting the plasma properties in different ways. Authors stress the importance of conducting comprehensive comparative studies to confirm that the catalytic mechanisms are responsible for specific changes in reaction performance. 2 ** .Loenders B, Michiels R, Bogaerts A: Is a catalyst always beneficial in plasma catalysis? Insights from the many physical and chemical interactions.JEnergyChem2023, 85:501. In this review paper, the authors list a series of effects and mechanisms that may be relevant in plasma catalysis processes, particularly during the DRM reaction. 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It emphasizes the significance of well-characterized active sites, along with porosity, Figure 2 Scheme about the holistic approach required to advance in the field of plasma-catalysis and the roles of PCPs as activators of this type of DBD-PB reactions. Without discarding other possible approaches, the scheme highlights methodologies that would likely be required to properly design and characterize optimal PCPs for a given reaction. Plasma Catalysis: Impact of Catalyst on the Plasma Ruiz-Martín et al. 5 www.sciencedirect.com Current Opinion in Green and Sustainable Chemistry 2025, 51:100990 meticulous temperature control, and the characterization of catalysts both before and after the process. 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