Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: A

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Review

Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: a Review Prateek Mehta, Patrick Barboun, David B. Go, Jason C. Hicks, and William F. Schneider ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.9b00263 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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Catalysis enabled by plasma activation of strong chemical bonds: a review Prateek Mehta,† Patrick Barboun,† David B. Go,∗,‡,† Jason C. Hicks,∗,† and William F. Schneider∗,† †Department of Chemical and Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States ‡Department of Aerospace and Mechanical Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States E-mail: [email protected]; [email protected]; [email protected]

Abstract Non-thermal plasma-driven catalysis is an emerging subfield of heterogeneous catalysis that is particularly promising for the chemical transformation of hard-to-activate molecules (e.g. N2 , CO2 , CH4 ). In this review, we illustrate this promise of plasmaenhanced catalysis, focusing on the ammonia synthesis and methane dry reforming reactions, two reactions that have received wide attention and that illustrate the potential for plasma excitations to mitigate kinetic and thermodynamic obstacles to chemical conversions. We highlight how plasma-activation of reactants can provide access to overall reaction rates, conversions, product yields, and/or product distributions unattainable by thermal catalysis at similar temperatures and pressures. Particular emphasis is given to efforts aimed at discerning the underlying mechanisms at play in these systems. We discuss opportunities for and challenges to the advancement of the field.

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Many of the most important technological processes related to energy and the environment involve the chemical transformation of molecules that are intrinsically unreactive. For example, the production of ammonia-based fertilizers that sustain the world’s population requires the activation of the robust dinitrogen triple bond, accomplished at large scale via the Haber-Bosch process for nitrogen fixation (N2 + 3 H2 − )− −* − 2 NH3 ). 1,2 The hydrogen in this process is most commonly supplied by steam reforming of methane (CH4 + H2 O − )− −* − CO + 3 H2 ), another molecule that is notoriously difficult to activate. 3–5 While not practiced at scale, dry reforming of methane with CO2 (CO2 + CH4 − )− −* − 2 CO + 2 H2 ) has also received significant attention, as it produces industrially important syngas while simultaneously converting two greenhouse gases. 6,7 In general, the conversion of CO2 to more valuable fuels and chemicals is limited by its high thermodynamic stability and kinetic inertness, restricting climate change mitigation efforts. 8 Methanol synthesis (CO2 + 3 H2 − )− −* − CH3 OH + H2 O), and the related reverse water gas shift reaction (CO2 + H2 − )− −* − CO + H2 O), are the only technical scale processes that consume CO2 . A mixture of H2 , CO, and CO2 is used as feedstock for methanol synthesis, because the higher C/O ratio in the gas mixture improves the overall thermodynamics for methanol production. Within the current infrastructure, the above conversions are driven thermally and heterogeneously catalyzed, typically under extreme conditions. For instance, the Haber-Bosch process requires temperatures of around 700 K and pressures in the range of 100–200 atm. 2,9,10 Similarly, practical methane steam reforming reactors can operate at over 1000 K 3 and methanol synthesis requires temperatures

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around 500–600 K and pressures of 50–100 atm. 11 The severe reaction conditions required in the above transformations are connected to the kinetic and thermodynamic challenges associated with activating reactants and achieving satisfactory yields of products. Equilibrium thermodynamics dictates that the latter is only possible under conditions where the reaction is sufficiently exergonic. Methane reforming reactions, for example, require high temperatures because they are highly endergonic and thus equilibrium-limited at low temperatures. On the other hand, ammonia synthesis is exergonic at near ambient temperatures and pressures, but conventional metal-based catalysts are not kinetically active at these conditions. Practical reaction rates are only achievable at higher temperatures (700–800 K), and a simultaneous increase in pressure (100–200 atm) is required to shift the equilibrium towards products. 2,9,10,12 The catalyst inactivity for ammonia synthesis (and for many other reactions) at mild temperatures and pressures is now understood to be a consequence of correlations between the binding energies of surface-bound intermediates and transition states on conventional catalysts. 13–17 These correlations constrain the parameter space available for catalyst optimization and thereby limit the maximum achievable rates—by changing the catalyst material it is not possible to manipulate the energetics of any single elementary step without also influencing those of other steps in the reaction pathway. In a similar fashion, the performance of all temperature-driven reactions is limited by the fact that thermal energy is intrinsically non-selective, i.e. it by definition equally influences all degrees of freedom of all reaction intermediates across all elementary steps. Greater flexibility in the design of catalytic systems may be provided by coupling traditional catalysts with extrinsic non-thermal stimuli that direct energy into the reaction coordinate of targeted elementary steps without significantly affecting others. An example of this approach is photocatalysis, in which photo-generated and non-thermal electrons and holes participate in surface reactions. Plasmonic nanostructures 18–23 similarly are believed to channel visible-light energy into specific states of adsorbed molecules. Population of these

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metastable states can promote dissociation or other reactions at rates greater than thermal, allowing chemical transformations to proceed at conditions milder than are possible by nonselective heating alone. Non-thermal plasma-enabled catalysis is an emerging field that plays on similar principles. A non-thermal plasma is a partially ionized gas, typically generated by an electric discharge. 24 Free electrons in the plasma collide with heavier gas particles to generate a rich mix of reactive species, including vibrationally or electronically excited molecules, radicals, atoms, and ions. The key characteristic of such plasmas is that thermal equilibrium is not maintained between all degrees of freedom within the plasma. These non-equilibrium conditions arise both because the electric discharge excites only certain degrees of freedom and because energy transfer between various degrees of freedom are not uniformly fast. Typically, the bulk gas temperature (i.e. the translational temperature of the heavy particles) of a nonthermal plasma remains close to ambient (300–1000 K), whereas the effective temperature of the electrons can be of the order of 104 –105 K. Other degrees of freedom of the gas have temperatures between these two extremes—for example, the temperature of the vibrational degrees of freedom may be in the range of a few thousand K. The non-thermal nature of these plasmas make them attractive for use alongside traditional catalytic operation, especially for the activation of unreactive molecules. For instance, plasma-excited molecules may experience lowered barriers for dissociation on catalyst surfaces. Additionally, the plasma may create ions, radicals, etc. that participate in alternative reaction pathways towards products both in the homogeneous plasma phase and on the catalyst surface. Further, the plasma environment may alter the rates of surface-catalyzed reactions, for instance through electric field or photochemical effects. Finally, because the plasmas are non-thermal and not characterized by a single temperature, conversions towards products may not necessarily be bound by thermodynamic equilibrium constraints of the bulk gas temperature and pressure. 25 In this contribution, we present a review of the literature and discuss the potential of non-thermal plasma-enabled catalysis to perform difficult chemical transformations under

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mild conditions. To frame the discussion, we primarily focus on ammonia synthesis and the dry reforming of methane. These two reactions have been most widely investigated and probe the advantages offered by plasma-catalysis in different chemical regimes. As noted above, ammonia synthesis is kinetically limited at low temperatures and pressures by the high barrier for N2 dissociation on conventional metal catalysts. Plasma excitation of N2 offers a route to activate the strong N – N bond and enable lower T and P reduction to NH3 . Dry reforming of methane, on the other hand, is severely equilibrium limited and is possible only at temperatures greater than ≈700 K. As we will discuss below, non-equilibrium plasmas offer additional kinetic benefits at high temperatures, and importantly, bypass thermodynamic limitations to enable the reaction at temperatures as low as room temperature. Furthermore, in plasma catalytic dry reforming, a much wider product distribution is accessible, allowing for the direct conversion of the reactants to value-added chemicals and liquid fuels. While work in the field has successfully demonstrated the above ideas, significant improvements are necessary before such technologies can be considered for practical applications. A recent perspective from Bogaerts and Neyts provides an overview of the highlights of the state-of-the-art of plasma technology as applied to N2 and CO2 conversions, noting the energy efficiencies achieved so far, and steps needed for further improvement. 26 It is clear from their discussion, and from our own survey of the literature, that one of the key bottlenecks for progress in plasma-enhanced catalysis is limited understanding of the fundamental mechanisms at play. Our objective here is to present an evaluation of the proposed mechanisms and attempts made to discern them. Accordingly, we will only briefly overview macroscopiclevel details and their influence on yields and energy efficiencies, noting that more detailed reviews of plasmas, reactor configurations, and operational parameters used for both ammonia synthesis 27–29 and dry-reforming 30–32 are available. We expect that looking at the literature as a whole, especially looking at both ammonia synthesis and dry reforming in parallel, will lead to new insights transferable to many reactions.

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General concepts. Different non-thermal plasma classes have been considered for plasmaenhanced catalysis, but the most widely studied is the dielectric barrier discharge (DBD). 33 DBDs operate at ambient or near-ambient pressures and temperatures, facilitating their use alongside catalysts, while other classes of plasmas typically operate at low pressures (e.g. glow and radio-frequency discharges) and/or have gas temperatures of 1000 K or greater (e.g. “warm” plasmas such as microwave or gliding-arc plasma). These differences in operating conditions translate to differences in the dominant reactive species present in DBDs and these other types of plasmas. Thus, comparisons across experiments employing plasmas of different types must be made with considerable caution, and for internal consistency, we restrict the scope of this review to DBD plasmas. Other plasmas do offer some potential advantages over DBD discharges in non-catalytic applications, including higher energy efficiency and different distributions of excitation energy, and could potentially be attractive for use in plasma-enabled catalysis with the appropriate reactor design. 26 A dielectric barrier discharge is typically generated by applying high voltage across two electrodes, one or both of which is coated by a dielectric material. In a typical reactor setup, shown in Figure 1a, two concentric electrodes are used to generate the plasma. In the absence of any packing, a DBD generally operates in the filamentary mode. 24 Microdischarge filaments extend across the discharge gap between the electrodes and have a duration of around 10–100 ns. These filaments are characterized by a number of current pulses per halfcycle of the applied voltage, as shown in Figure 1b. 34 Catalyst packing may alter the discharge characteristics, as both the plasma volume and the discharge gap are reduced. 24 The exact influence of the packing on the discharge properties is sensitive to the volume packed, as well as the identity and physical characteristics of the packing material (e.g. particle size, shape, dielectric constant, etc.) 35–37 Experiments and simulations reveal that when the entire reactor volume is packed, the plasma discharge transitions to a predominantly surface discharge mode accompanied by weak, spatially limited discharges in the void spaces of the packing. 35–39 These weaker discharges are characterized by more diffuse and lower amplitude current

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pulses. Filamentary behavior dominates these other modes in a partially packed reactor. 39 Thus, the physical characteristics of the plasma/catalyst bed can have a significant impact on plasma characteristics and the coupling between plasma and catalyst chemistry.

Figure 1: (a) A schematic representation of a DBD reactor. (b) Typical electrical signals in a DBD reactor without packing (Reproduced from Ref. 34 ).

The bulk gas temperature in DBDs is typically 100–200 K above room temperature and is a function of the supplied electrical energy. This temperature increase arises from collisional energy transfer between high-energy plasma species and the bulk gas. It is common to express the electrical energy as the specific energy input (SEI), or the energy deposited per

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mole of gas, SEI =

plasma power molar flow rate

(1)

In practical applications, the bulk gas temperature is often further controlled through an external thermal energy source. In plasma-enabled catalysis, the SEI and thermal energy are thus two independent and tunable control variables. Plasma energy transfer is initiated by collisions between energetic electrons and gas molecules and can be deposited in various internal degrees of freedom or result in molecular ionization or fragmentation. The activation energy thresholds of the four most significant electron impact activation channels typically follow the order vibrational excitation < electronic excitation < bond dissociation < ionization. Rotational state excitations have the lowest excitation thresholds but these excitations typically do not contribute to further plasma chemistry and are not discussed here. Threshold energies of different activation channels of the molecules discussed in this review are tabulated in Table 1. Bond dissociation energies are included for comparison. Electron impact dissociation typically proceeds by excitation of the molecule into a repulsive electronic state above the bond dissociation energy limit. The excess energy is thus lost to other degrees of freedom. 40 The rates of the electron impact reactions are a function of the average electron energy, or, equivalently, the electron temperature. The mean electron energy, in turn, is correlated with the reduced electric field of the plasma, defined as the ratio between the electric field, Table 1: Bond dissociation energies, 41 energies of relevant vibrationally 42 and first electronically excited states, 42,43 ionization energies, 41 and electron impact dissociation energy thresholds 42,44 of N2 , CH4 , and CO2 , all in eV. For CO2 and CH4 , the energy to dissociate one of the equivalent bonds in each molecule is reported as the bond dissociation energy. Note that different values for electron impact dissociation thresholds can be found in different databases. 45 Molecule N2 CH4 CO2

Ediss 9.8 4.5 5.5

Evib 0.29 (σg+ ) 0.37 (t2 ) 0.29 (σu+ )

Eelect 6.2 (3 Σ+ u) 8.8 (3 T2 ) 7.0 (1 B2 )

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Eion 15.6 13.0 13.8


e−impact Ediss 13.0 9.0 11.5

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E, and the concentration of neutral molecules, n. As an example, in a N2 plasma, a reduced electric field of 100 Td (Td = Townsend; 1 Td = 10−21 V m2 ) corresponds to an average electron energy of approximately 2 eV. 26 Figure 2(a-b) show the fraction of electron energy deposited into relevant excitation, dissociation, or ionization channels of N2 and CO2 as a function of E/n, calculated from the cross sections of the electron impact reactions. 26,46 It is clear from Figure 2(a-b) that significant fractions of the electron energy are deposited into rotational and vibrational excitations at low reduced electric fields, while the higher energy electronic excitation, ionization, and dissociation channels become significant at reduced electric fields of 100 Td or higher. Reduced electric fields of this magnitude are typical of DBDs—dashed vertical lines in Figure 2 indicate E/n values demarking the onset of the DBD regime. All four activation channels may thus be important for plasma-phase and plasma-catalytic conversion of N2 and CO2 using DBDs. We could not find similar results for CH4 plasmas in the literature, but we expect its behavior to be similar to N2 and CO2 .

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Figure 2: Electron energy loss channels in (a) N2 plasmas (adapted from Ref. 26 ) and (b) CO2 plasmas (reproduced from Ref., 46 published by The Royal Society of Chemistry) as a function the reduced electric field. The vertical dashed line indicates the start of the DBD regime. While Figure 2 provides some insight into which activated species may be present at a given reduced electric field, it is not fully indicative of the importance of any particular activation channel in plasma-phase or plasma-catalytic reaction kinetics. For example, because the energy thresholds of the different channels of electron impact activation vary significantly (Table 1) the relative concentrations of species in vibrationally or electronically excited, ionized or dissociated states cannot be inferred directly by the proportion of electron energy deposited into the different activation channels. The activated species may also return to the ground state without reaction, often by losing energy to translational degrees of freedom. 10


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Electronic and ionic states are known to have very short lifetimes at atmospheric pressure and it has been suggested that they may not play a significant role in DBD-driven catalysis. 47 Energy from electron impact may also be deposited into vibrational modes that are orthogonal to the relevant reaction coordinate. Thermal collisions between molecules may also lead to excitations that promote the reaction (e.g. vibrational-vibrational ‘ladder-climbing’), or relaxations of excited states (e.g. vibrational-translational relaxations), as discussed in the following paragraph. Due to their low excitation thresholds, vibrational kinetics have particular relevance in non-equilibrium plasma chemistry. 24 Electron impact excites low energy vibrational modes, which may lead to vibrational temperatures significantly higher than the bulk gas temperature (1000–4000 K). The vibrationally excited molecules participate in collisional energy exchange between themselves, involving vibrational-vibrational (V-V) relaxations that tend to populate highly excited vibrational states, as well as vibrational-translational (V-T) relaxations which depopulate the excited states. In particular, when the bulk gas temperature is low, V-V relaxations are significantly faster than V-T relaxations, resulting in vibrational distributions that have population of high energy vibrational states that far exceed those given by a Boltzmann distribution. 24,48 Two-temperature distributions parametrized on both the vibrational temperature and the bulk gas temperature, such as the Treanor distribution, are sometimes used to describe the vibrational overpopulation. 24 Dissociation rates of vibrationally excited molecules are known to be higher both for homogeneous reactions 24 and for surface chemisorption. 49–55 Plasma-catalytic kinetics are thus a convolution of both homogeneous plasma-phase chemistry and heterogeneous surface chemistry, the latter potentially modified by the actions of the plasma. We focus this review primarily on the impact of plasma-excitation of hard-to-activate molecules on catalysis. Other modes of synergy between plasmas and catalysts have also been identified, including surface charging, electric field effects, hot spot formation, morphological changes of the catalyst, and changes in discharge characteristics

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due to the properties of the catalyst. The reader is referred to other reviews for a more detailed discussion of these effects. 47,56 Thermal NH3 synthesis. The thermal ammonia synthesis reaction, N2 + 3 H2 − )− −* − 2 NH3 , is one of the most widely studied reactions in catalysis. As noted in the Introduction, the reaction is exergonic at 298 K and 1 atm pressure (∆G◦ = −32.8 kJ mol−1 ). 2 The adsorbatefree surfaces of metals such as Fe and Ru can dissociate N2 at these conditions, but higher temperatures are necessary to promote further hydrogenation and ultimately liberation of NH3 . Figure 3 plots the equilibrium mole fraction of ammonia as a function of temperature at three different pressures. 2 At high temperatures, equilibrium is shifted towards the reactants, and elevated pressures are necessary to counterbalance this effect and shift the equilibrium back towards NH3 . 12,57 The industrial Haber-Bosch process for ammonia synthesis is thus carried out at high temperatures and pressures. One of the challenges in ammonia synthesis catalysis is to design a lower temperature and pressure catalytic process with activity comparable to the industrial process. 9

Figure 3: Ammonia synthesis equilibrium curves as a function of temperature and pressure at stoichiometric gas composition. Reproduced from Ref 2 with permission from John Wiley and Sons. On metal catalysts at high temperatures and pressures, the reaction is well described by

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the elementary steps, 12,58,59

where

∗

∗ N2 + 2 ∗ − )− −* − 2N

(2)

∗ −− H2 + 2 ∗ ) −* − 2H

(3)

∗ ∗ −− N∗ + H ∗ ) −* − NH +

(4)

∗ ∗ NH∗ + H∗ − )− −* − NH2 +

(5)

∗ NH2 ∗ + H∗ − )− −* − NH3 + 2

(6)

indicates a vacant surface site and X∗ indicates surface adsorbed X.

The net rate of the reaction over transition metal catalysts is observed to obey the Sabatier principle, 60,61 in which optimal rates are a balance between the ability of a catalytic surface to activate and bind reactants and to liberate products. First-principles calculations based on density functional theory (DFT) provide more insight into this behavior. Figure 4a shows the experimentally determined ammonia synthesis rates as a function of the DFTcalculated N adsorption energy. 12,62,63 For comparison, Figure 4b plots ammonia synthesis turnover frequences (TOFs), i.e. the rate per metal step site (the sites known to be most active for N2 dissociation 64,65 ), calculated using DFT-based microkinetic modeling. 12,63 On the right leg of these so-called volcano curves (i.e. metals with low N binding energies), slow N2 dissociation limits the rate of NH3 production. On the left leg of the volcano, net rates are limited by the lack of availability of free surface sites due to the slow rate of generation and desorption of products. The volcano maximum occurs at intermediate binding energies. Ru and Fe straddle the top of the volcano, and Fe catalysts are commonly used in the industrial process owing to their lower cost. The rates in Figure 4(a-b) can be well-described by a single parameter, the N adsorption energy, because the energies of all N-containing intermediates and transition states in the reaction mechanism are linearly correlated on metal catalysts. 13–15 An example of such a correlation for the N adsorption energy and the transition state energy for N2 activation on 13



Figure 4: (a) Experimental ammonia synthesis rates on metal catalysts as a function of the DFT-calculated N adsorption energy. (b) Ammonia synthesis turnover frequencies on FCC/HCP metal step sites as a function of the N adsorption energy obtained by microkinetic modeling. (c) Calculated ammonia synthesis rate as a function of the N adsorption energy (EN ) and the N2 dissociation barrier (EN−N ). The dashed line represents the scaling relation between EN and EN−N on metal steps. Reproduced from Ref. 63 with permission from Elsevier. transition metal step sites is shown by the dashed line in Figure 4c. 16,63 When these linear correlations hold, the theoretical maximum achievable reaction rate at a give reaction condition is set by the maximum of the volcano curve. However, rates much higher than the volcano maximum at a given condition (or equivalent rates at less severe reaction conditions) may be possible if the activation energy of N2 dissociation could be varied independently of the remaining energies (Figure 4c). This recognition has motivated research towards the design of materials that deviate from the linear correlations characteristic of metals. 9,16 Activation of N2 by external stimulation, such as a non-thermal plasma, provides an alternative route towards the same end. Plasma-catalytic NH3 synthesis. Plasmas have been considered for N2 transformations for over a century. In fact, the thermal plasma-based Birkeland-Eyde 27,66,67 process for N2 oxidation with O2 was the first commercial approach to N2 fixation, before the emergence of the Haber-Bosch process. Since then numerous efforts have been made to develop plasmachemical N2 fixation (oxidation and reduction) processes that can compete with Haber-Bosch (see the review of Patil et al. for a detailed history of these developments 27 ). Early studies of non-thermal plasma driven ammonia synthesis were performed using low pressure dis14


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charges, including glow discharges, 68–72 radio-frequency discharges, 73–76 and microwave discharges. 74,77 More recent studies, covering the past twenty-five years, however, have almost exclusively been performed using the atmospheric-pressure dielectric barrier discharge. 78–94 Key results and mechanisms reported using DBD-based plasma-catalytic systems are discussed below. Investigations of ammonia synthesis using plasma-assisted catalysis have been largely exploratory—a wide range of catalysts have been employed in a variety of reactor configurations and operating conditions. The reports consistently show that ammonia is produced in a DBD plasma applied to N2 and H2 operated at atmospheric pressure and mild temperatures (< 200 °C), even in the absence of a catalyst. Ammonia yields are typically amplified when a catalyst is used in tandem with the plasma. Ru-based catalysts, 80,85–87,90 have been explored most extensively, drawing on the knowledge that Ru is the best known thermal catalyst. Less expensive metals such as Ni and Co, 91,92 as well as metal oxides (e.g. Al2 O3 80,88 or MgO 78,82 ), and carbon-based 88 materials have also been tried. Three groups have compared the performance of a series of metal catalysts. 81,93,94 Alkali metals, which are known to promote the thermal ammonia synthesis reaction, 95–97 were added during plasma-catalytic operation in some studies. 85,87,90 Ferroelectric materials, such as BaTiO3 or PZT (lead zirconate titanate) 83,84,91,92 used primarily to manipulate the properties of the discharge, have been reported to have some catalytic activity on their own. The reactor walls have also been proposed to have a catalytic function, either on their own or when coated with an appropriate material. 81,89 Additionally, electrodes used to generate the plasma may also function as catalysts, as reported by Iwamoto and coworkers who used wool-like electrodes of several metals as inner electrodes in their DBD reactor. 89,93 The DBD plasma-catalytic reactors have been constructed in various configurations. Cylindrical reactors with concentric electrodes are most frequently used, while some researchers have also used parallel plate electrodes, where it is easier to vary the discharge gap. 83,84 The catalysts have been introduced into the reactor in various forms, including powders, 86,87,90,94 pellets, 82–85,91 spheres 88 and membrane-like

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tubes of supporting material coated with catalyst. 80,81 The effects of N2 :H2 gas composition on ammonia yields 78–81,85,89,91,93,94 and energy efficiencies (gNH3 /kWh) 83,87,90 have received wide attention. Ammonia yields are generally found to be highest in non-stoichiometric, N2 -rich feeds, 78,85,87,89,90,93,94 an observation that has been attributed to a higher fraction of plasma energy being deposited in N2 . This interpretation appears to be consistent with reports from Bai et al., who determined the optimal N2 :H2 ratio as a function of gas flow rate. At higher gas flow rates, and thus decreasing SEI (equation (1)), the optimal gas ratio is observed to shift towards higher N2 concentrations. However, the effect of gas composition on performance appears to be sensitive to the operating conditions and catalyst. 80,81,83,91 For instance, Gomez-Ramirez et al. report that energy efficiency is maximized in stoichiometric N2 :H2 mixtures over a PZT ferroelectric but in 1:1 mixtures over BaTiO3 . 83 Similarly, Mizushima et al. 80,81 reported NH3 yields to be highest at the stoichiometric gas composition over Fe and Ru catalysts, while yields are higher over Ni and Pt catalysts in excess N2 feeds. In contrast, yields were highest for H2 excess feeds in the plasma reactor in the absence of the catalyst. Ammonia production and energy efficiency are also observed to be a function of the energy supplied to the plasma (commonly reported as power, voltage, or SEI). 79–81,83,85–87,89–91 Ammonia production is generally observed to increase with supplied electrical energy, at least at low conversion. Kim et al. 85 report that NH3 production plateaus or even decreases at high SEI, especially if the gas temperature is also increased by heating. The authors speculate that the reversal reflects contributions of NH3 decomposition at high conversion and temperature. The relationship between the energy efficiency (amount of ammonia produced per unit energy) and the SEI is more complex and system-dependent. Energy efficiency is reported to monotonically decrease with increasing SEI 80 or to rise and then decrease. 83,86,87,90 While it is clear that plasma treatment of N2 /H2 mixtures enable higher rates of ammonia synthesis than possible thermally, direct evidence for any particular reaction mechanism remains elusive. However, the literature is in agreement that the primary reason for the

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increased activity is the plasma-driven activation of N2 molecules. As noted above, the four basic N2 activation channels in a non-thermal plasma are direct dissociation, ionization, electronic excitation, and vibrational excitation. All four activation channels operate in non-thermal plasmas, and all four have been proposed to influence ammonia synthesis rates. However, there is no general agreement on the relative importance of any particular activation channel. For example, in early reports of DBD-only ammonia synthesis, Bai and coworkers 78,79 hypothesized that the reaction is initiated by electron impact ionization of N2 to N2 + , which combines with H or H2 to form NH2 + . The NH2 + dissociates upon electron collision to form an NH radical, which undergoes further hydrogenation to form NH3 . Accordingly, they suggested that the most suitable plasmas for ammonia production are ones in which a significant fraction of electrons have energies greater than the N2 ionization potential of 15.63 eV, corresponding to plasmas with E/n ≥ 300 Td. Gomez-Ramirez et al. 83,84 similarly proposed N2 + ions to be initiators of both gas-phase and heterogeneous ammonia synthesis channels, based on the observation of N2 + ions in the ferroelectric-packed DBD reactor using optical emission spectroscopy (OES). On the other hand, Akay et al. 91 speculated that excited neutral N2 molecules or dissociated N atoms, which require lower electron energies to produce than N2 + (Table 1), are responsible for ammonia production. Vibrational excitation is known to increase N2 dissociation rates on surfaces 49,51,52,54,55 as well as in the gas phase 98,99 and has accordingly been considered important by some groups. 88,94 N2 DBD plasmas fall in the E/n range shown in Figure 2a, where all four activation channels may be active. The influence of any particular activation channel on the ammonia synthesis mechanism is thus expected to be strongly sensitive to the plasma conditions in any particular experiment, perhaps explaining the lack of consensus in the literature about the primary activated nitrogen species. Moreover, the importance of these activation channels may also differ from plasma-only to plasma-catalytic operation. For instance, excitation of the lowest-energy vibrational states may be insufficient to initiate homogeneous plasma phase reactions, but such excited molecules may only experience small dissociation barriers

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on a catalyst surface. It is thus plausible that the plasma-phase activity is due to the strongly activated nitrogen species (e.g. N atoms, ions, or highly excited vibrational states), and the amplification of reaction rates upon introduction of the catalyst is from reactions of less active (e.g. low lying vibrational states) N2 on the surface. Several groups have used optical emission spectroscopy to identify excited intermediates in the plasma phase. A representative spatially and temporally averaged OES (from Iwamoto and coworkers 89,93 ) is shown in Figure 5. OES reported by other groups show similar features. 83,84,94 Because of differences in transition probabilities, the relative concentrations of excited species cannot be directly inferred from emission intensities. Two groups have attempted to correlate peak intensities to ammonia yield 83 or production rate 89 as a function of operating voltage. While both groups report that ammonia production increases with increasing voltage, they offer different interpretations of the OES. Gomez-Ramirez et al. report that the relative intensity of the N2 + emission decreases with increasing voltage, while those of NH and N increase. These authors propose that N2 + is a key reaction intermediate, and that the decrease in its peak intensity at higher voltages is due to its consumption in NH3 formation. In contrast, Aihara et al. concluded that N2 (ex) is the active intermediate, based on the concurrent increase in the ammonia synthesis rate and the N2 (ex) peak intensity with operating voltage. It is yet unclear whether either the ionic or the electronically excited forms of N2 play a significant role in plasma-enhanced catalysis—Whitehead mentions that such species typically have very short radiative lifetimes or are rapidly quenched at atmospheric pressure. 47 One exception is the metastable A3 Σ+ u state, which has an energy of 6.2 eV above the ground state and a radiative lifetime of around 2 s. Because only electronic transitions are observable using OES, information about vibrational excitation is extracted indirectly by fitting observed spectra to modeled ones, taking vibrational, electronic, and rotational temperatures as fitting parameters. Temperatures extracted from such fits typically have uncertainties of 50–100 K. 100–102 N2 vibrational temperatures in the range of 2500–4000 K have been reported for DBD-only reactors, 35,94 and

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Figure 5: A representative plasma-activated N2 optical emission spectra. Note that N2 ∗ indicates the excited molecular state of N2 in the figure, but is denoted as N2 (ex) in the text to avoid confusion with a surface adsorbed species. Reproduced from Ref. 93 for DBDs packed with titania, 35 alumina, 88,94 alumina-supported metal, 94 or carbon-based 88 catalysts. The bulk gas temperature is typically observed to be only 400–500 K, indicating slow energy transfer between vibrational and translational degrees of freedom. Some indications that vibrational excitation plays an important role in plasma catalysis are provided by the experiments of Kim et al., 85 who found pulsed discharges, which are known to be more efficient at promoting vibrational excitation, to produce higher NH3 yields than AC discharges at the same specific energy input. Comparison of plasma-catalytic performance across a series of metal-based catalysts has helped connect and contrast plasma catalysis to the well-established concepts of thermal ammonia synthesis catalysis. The first such investigation was carried out by Mizushima et al. 81 using a DBD reactor equipped with bare and metal-loaded (Ru, Fe, Ni, Pt) membranelike alumina tubes as catalysts. While some ammonia was produced in the unpacked reactor and the reactor packed only with Al2 O3 , ammonia yields were highest when packed with supported metal catalysts. The ordering of catalytic activity (in terms of NH3 yields) was found to be sensitive to the voltage applied and the N2 :H2 ratio used. At the highest voltage tested (4.5 kV) and at a stoichiometric inlet gas composition, ammonia yields varied as Ru/Al2 O3 > Ni/Al2 O3 ≈ Pt/Al2 O3 > Fe/Al2 O3 > Al2 O3 . The authors noted that the relatively high yields of NH3 obtained for Ni and Pt, metals not active for the thermal 19



reaction, suggested mechanistic departures from conventional heterogeneous catalysis. For the Ru/Al2 O3 and bare-Al2 O3 systems, Mizushima et al. attempted to characterize surface bound nitrogen species using temperature programmed desorption (TPD) and infrared spectroscopy (IR). 81 The two materials were treated with N2 plasma for 5 minutes under ambient conditions and the tube was evacuated prior to TPD. The TPD spectra indicated the presence of strongly bound N species on Ru/Al2 O3 and bare-Al2 O3 —similar amounts of N2 desorbed from both materials in the temperature range of 500–800 K. In contrast, no N2 desorption was detected when the materials were exposed to N2 without plasma treatment. N-N stretching modes, corresponding to adsorbed molecular N2 , were observed on both materials using IR spectroscopy at ambient conditions. However, these IR bands disappeared at room temperature under the evacuated conditions used during TPD, leading the authors to infer that N2 desorbed at higher temperatures originated from strongly bound N atoms on the surface. In addition, the researchers performed an isotope exchange reaction using a 1:1 discharge of reactor.

14

14

N2 and

15

N2 in an unpacked, Al2 O3 -packed, and Ru/Al2 O3 -packed

N 15N was observed in all three cases, with its amount doubling in the Al2 O3 -

packed and Ru/Al2 O3 -packed reactors compared to empty reactor. The authors concluded that N2 dissociation took place in the plasma phase (and/or on exposed reactor surfaces), and was promoted further by the presence of a catalyst. Recently, Iwamoto et al. used wool-like metal wires (Al, Ti, Fe, Ni, Co, Mo, Pd, Ag, W, Pt, Au) with identical surface areas as inner electrodes in their DBD-reactor. 93 A copper net wrapped around the quartz reactor served as the outer electrode. Plasma-catalytic experiments were performed at constant applied voltage (5 kV) and frequency (50 kHz) with no external heating. Ammonia production rates were measured five times for each metal— the results from the initial run are reproduced in Figure 6. The x -axis on Figure 6 plots the N adsorption energy on each metal calculated using DFT. From the correlation between ammonia synthesis rate and the N binding energy, the authors inferred that metals that bind N weakest have highest catalytic activity. The authors found that some of the electrodes

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degraded during the experiments and metal was deposited onto the inner walls of the reactor. This degradation led to differences the reaction rates and the order of metal activity from measurement to measurement. Because the metals are also one electrode of the DBD, it is difficult to separate their catalytic and electric effects. For instance, the power consumption varied across metals, which is expected to influence the plasma properties and the rates of the plasma-phase reactions. The bulk temperature (not reported in the study), which influences the rates of the surface reactions, could also vary depending on the electrode.

Figure 6: Measured ammonia synthesis rates in a DBD-reactor employing metal wool-like electrode catalysts as a function of the DFT-calculated N adsorption energy. Reproduced from Ref. 93

To avoid some of the above issues, Mehta et al. considered NH3 synthesis rates in a DBD reactor over alumina-supported metals (Fe, Ru, Co, Ni, Pt). 94 The reactor comprised of a quartz tube with a tungsten rod inserted through the center serving as the ground electrode. A stainless steel mesh was used as the outer electrode. Reaction rate measurements were performed at a constant bulk temperature (438 K) and a constant power deposition (10 W). Under these conditions, the macroscopic plasma characteristics (e.g. capacitance, average number and lifetime of filaments, time- and space-averaged vibrational and rotational temperatures) were found to be independent of the catalyst used. 103 In addition, all catalysts 21



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were found to be thermally inactive under these conditions. Rates were measured on the metal-on-alumina catalysts and the alumina support as a function of residence time, and were subsequently extrapolated to initial rates (i.e. in the limit of zero-residence time). The intrinsic activity of the metal catalysts were compared as site-time yields, defined as the 0 difference between the initial rates on the alumina supported metal catalysts (rM/Al ) 2 O3 +DBD

and the initial rates of the background reactions (i.e. rates of reactions in the plasma-phase 0 ) normalized by the total number of catalytic sites (nsites ), as or on the support, rAl 2 O3 +DBD

accessed by CO chemisorption:

STY =

0 0 − rAl rM/Al 2 O3 +DBD 2 O3 +DBD

nsites

(7)

All metals were found to have positive STY, providing evidence that plasma-catalytic interactions enhance the plasma-only ammonia-synthesis rates. The catalyst activity was found to vary as Co > Ni = Ru > Pt > Fe. In conjunction with the above experiments, Mehta et al. proposed a microkinetic model to rationalize and predict plasma-enhanced NH3 synthesis rates. This model incorporates N2 dissociation rate enhancements arising from vibrational excitation, 50–55,104,105 into predicted NH3 synthesis rates. The model begins from DFT-based thermal rate predictions as a function of metal and facet. Predicted thermal NH3 synthesis rates, computed by solving the reaction network in equations (2) to (6) at 1 atm, 473 K, and 1% conversion are shown as “plasma-off” as a function of N binding energy across a series of common catalytic metals in Figure 7(b-c). The plasma-enhanced model modifies the N2 dissociation rate to be an explicit function of N2 vibrational state. Vibrational-state-specific rate constants are written as kv = Ea −αEv A exp − kB T for state v, so that the dissociation activation energy is deprecated by the vibrational energy Ev scaled by an efficiency factor, α, that decreases with decreasing Ea 24 (Figure 7a). N2 vibrational populations were taken to be at steady state and were described by a truncated Treanor distribution at the OES-extracted vibrational temperature of 3000 K. Ensemble averaging over population-weighted vibrational states yielded the “plasma-on” 22


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ACS Energy Letters

rates shown in Figure 7(b-c). Vibrational excitation was predicted to enhance site-specific rates by several orders of magnitude over thermal rates in some cases. Enhancements were predicted to be greater on catalytic sites that bind N weaker than those most active for thermal catalysis (Ru steps), since they have smaller barriers for the hydrogenation of NHx (x = 0, 1, 2) intermediates. Co and Ni steps were predicted to have the highest rates at the modeled conditions (Figure 7b). Further, the models also predict that terrace sites, which have N2 dissociation barriers considerably larger than steps in thermal catalysis, may also become active if N2 is sufficiently excited by the plasma (Figure 7c).

Figure 7: (a) Schematic reaction coordinate comparing activation energies for N2 dissociation starting from ground (blue) or second vibrationally excited state. The dashed green and orange curves correspond to a vibrational efficiency (α) of unity and less than unity, respectively. (b-c) Predicted N2 vibrational-distribution-weighted (plasma-on) and thermal (plasma-off) ammonia synthesis rates on (b) step and (c) terrace sites. The dashed lines indicate the maximum rates possible for the hydrogenation step. Reaction conditions: 1 atm, 473 K, N2 :H2 = 1:3, conversion=1%. Reproduced from Ref. 94 .

The model provides a conceptual framework for the rational design and optimization of plasma-catalytic systems. It shows that by decoupling N2 activation from the binding ener23



gies of adsorbed fragments, plasma-enhanced catalysis can circumvent the kinetic limitations imposed by scaling relations in thermal ammonia synthesis, allowing for much higher rates than thermal catalysis. Furthermore, the model shows that it is not optimal to simply use the best thermal catalyst in plasma-enhanced catalysis—the greatest possible rate enhancements are in the region of materials space far removed from what is optimal for thermal operation. The model predicted trends and shift in optimal material were found to be consistent with experimentally measured site-time yields. At the same time, the simple model has limitations, as it considers only one type of plasma excitation (vibrational) coupling to the catalyst surface, neglects time-evolution of the vibrational distribution associated with micro-discharges, ignores homogeneous reaction pathways, and possible effects of the plasma on surface reactions (e.g. surface charging, electric fields, etc.). Finally, we briefly mention that a zero-dimensional kinetic model of ammonia synthesis in an atmospheric pressure plasma reactor employing Fe catalysts was recently constructed by Hong et al., 106 building upon related models of the reaction in low pressure plasmas. 107,108 It is unclear whether the model parameters (e.g. reduced electric field of 30-50 Td) are representative of a DBD plasma, so we do not discuss the results here. However, we do note that models such as these are quite complex and it is difficult to interpret the results without a careful evaluation of the modeling choices and assessment of the uncertainties in the kinetic parameters (which typically have to be collected from multiple distinct sources). −− Thermal dry reforming of methane. Dry reforming of methane (DRM), CO2 + CH4 ) −* − 2 CO + 2 H2 , is an attractive reaction from an industrial and environmental perspective, as it converts two greenhouse gases into more valuable products. 6–8,109 The reaction (∆H ◦ (298 K) = 247 kJ mol−1 ) is the most endothermic of the methane reforming reactions, i.e. compared to steam reforming (SRM) or partial oxidation. 3 High temperatures are required to achieve significant conversion, as shown by the equilibrium curves in Figure 8. DRM typically proceeds with the simultaneous occurrence of the reverse water gas shift reaction (RWGS), −− CO2 + H2 ) −* − CO + H2 O, the thermodynamics of which are included in both Figure 8(a-

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b). Coke formation, via either the complete decomposition of CH4 or the Boudard reaction (2 CO − )− −* − CO2 + C) is a significant obstacle in all methane reforming reactions, as it leads to catalyst deactivation. Figure 8(a-b) compares DRM equilibrium (a) without and (b) with carbon formation included. The figure indicates that temperatures above 900 ◦C are required to completely prevent C formation, although lower temperatures may be used on catalysts that are resistant to coking. On metal catalysts, promotion with potassium is often employed to reduce the rate of coking. 3 Catalysts have very high activities at the temperatures where the reaction is thermodynamically favorable, making it challenging to make kinetic assessments that are uncontaminated by transport effects and catalyst sintering, loss of surface area, and deactivation. Consequently, there is significant debate in the literature about the catalytic mechanisms of methane reforming (both DRM and SRM). Several rate expressions have been proposed with different reaction steps considered to be rate limiting. See Refs. 6,7,110,111 for a detailed account of the proposed mechanisms. Taking care to avoid kinetic artifacts due to the aforementioned issues, Iglesia and co-workers performed kinetic and isotopic measurements for methane conversion reactions (SRM, DRM, and methane decomposition) on a series of metal catalysts (Ir, Rh, Ru, Pt, Pd, Ni). 110,112–117 At the conditions investigated (550–750 ◦C, 100–1500 kPa total pressure), SRM, DRM, and methane decomposition were found to be mechanistically equivalent on these catalysts, with C – H bond activation being the only kinetically relevant step. Accordingly, the measured turnover frequencies were first-order in the methane partial pressure and independent of the co-reactants: rate = kapp PCH4

(8)

Their proposed mechanism is illustrated in Figure 9. CH4 decomposed to elemental carbon −− in a series of sequential H-abstraction steps, the first of which, CH4 + * ) −* − CH3 * + H* , was the most difficult. CO2 activation and the reverse water gas shift reaction were quasiequilibrated. They found that the reaction was structure-sensitive, i.e. the TOFs increased 25



Figure 8: Thermodynamic equilibrium amounts of dry reforming products and reactants as a function of temperature assuming 1 atm total pressure and an inlet composition of 1:1 CO2 :CH4 , assuming (a) no carbon formation occurs, and (b) carbon formation occurs. Contributions from the water gas shift reaction are included in both (a) and (b). Reproduced from Ref. 7 with permission from The Royal Society of Chemistry.

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with increasing metal dispersion, indicating that methane activation occurred on the steps and kinks of the metal nanoparticles. At similar metal dispersions, the rates were found to be unaffected by the identity of the support (ZrO2 , γ-Al2 O3 , CeO2 , or MgO). Subsequent microkinetic models by Maestri et al. 111 parametrized on experimentally measured SRM and DRM data by Donazzi et al., 118 were largely consistent with results from the Iglesia group, although it was suggested that the effective reaction order with respect to products is dependent on temperature.

Figure 9: Elementary steps involved in CH4 reforming and water gas shift reactions proquasi-equilibrated posed by Wei et al. on Ni-based catalysts ( −−→ irreversible step, − − * step, )−− reversible step, ki and Ki detote rate and equilibrium constants respectively). Reproduced from Ref. 110 with permission from Elsevier.

Microkinetic models suggest that the rate-limiting catalytic step is sensitive to reaction conditions. 119–122 For example, DFT-based models from Fan et al. predict that both CH4 activation and CHx oxidation are slow steps for DRM on Ni catalysts, with the former rate limiting at low pressures and the latter becoming more kinetically relevant at higher pressures. 121 Similarly, in their models of SRM, Jones et al. predict that CH4 activation is rate determining at high temperatures and on more reactive metals (e.g. Ru), while CO 27



formation controls the rate at low temperatures and on less reactive metals 120 (e.g. Ni). The exact ordering of metal activity has also varied across research groups, although the measured rates on typically investigated metal catalysts (Ir, Rh, Ru, Pt, Pd, Ni) are within an order of magnitude. 110,112,114–117,120 Noble metals (e.g. Pt, Ru, Rh) are generally more resistant to deactivation due to coke-formation compared to metals like Ni, although Ni catalysts are widely used in methane reforming reactions owing to their lower cost. 3,7 Plasma-catalytic dry reforming of methane. Early experiments for the tandem conversion of CO2 and CH4 were performed in the absence of catalysts using thermal plasma arcs, RF plasmas, corona discharges, or gliding arc discharges. 24,32,123 Investigations on plasmacatalytic dry reforming have appeared in the literature over the last twenty years, and like plasma-catalytic ammonia synthesis, most of these have been carried out in DBD reactors. 36,38,39,124–141 These experiments reveal behavior significantly different from thermallydriven dry reforming. For instance, plasma-only and plasma-catalytic dry reforming yield products at both high and low bulk temperatures. The possibility of low-temperature operation makes non-thermal plasmas an intriguing complement to thermal dry reforming, where the constraints of equilibrium thermodynamics do not allow any conversion below ≈700 K (see Figure 8). Furthermore, in addition to the conventional dry reforming products (CO and H2 ), C2+ hydrocarbons, as well as high value liquid oxygenates (e.g. acetic acid, methanol, ethanol, formaldehyde) are formed during plasma-driven operation. To illustrate the unique temperature-dependent behavior in plasma-catalytic dry reforming, we reproduce temperature-sweep experiments from Kim et al. 137 in Figure 10. These authors compared CH4 conversions and H2 yields (Figure 10b) in reactors operating in thermal (20 wt % Ni/Al2 O3 ), plasma-only (unpacked DBD reactor), plasma-support (DBD reactor packed with Al2 O3 ), and plasma-catalyst (DBD reactor packed with 20 wt % Ni/Al2 O3 ) modes. Figure 10b shows that the CH4 conversion profile is characterized by high and low temperature regimes (above and below approximately 600 K) that show distinct behavior. At high temperatures, the trends in plasma-catalytic conversion closely resemble those for

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thermal catalysis, but higher conversions are observed at the same bulk temperature in plasma-catalysis. In the low temperature regime, CH4 conversion is not possible thermally, but all plasma-based experiments show significant conversion. The low temperature regime is characterized by a wide range of products, 36,38,39,124,126–128,130,132–134,140,142,143 and it has been suggested that catalysts may be used to manipulate the reaction selectivity in this regime. We discuss the high and low temperature regimes separately in more detail below.

Figure 10: (a) Reaction environments used for dry reforming of methane: thermal reaction using 100 mg of Al2 O3 or 20 wt % Ni on Al2 O3 (20Ni); DBD-only reaction; plasma-catalytic reaction (DBD + 100 mg of Al2 O3 or 20Ni). (b) CH4 conversions (XCH4 ) and H2 yields (YH2 ) in the different reaction environments as a function of bulk temperature. Reaction conditions: CH4 :He:CO2 = 1:2:1; total flow rate of 20 mL min−1 ; 1 atm; 10 W. Reprinted from Ref. 137

High temperature plasma-catalytic DRM. The temperature-sweep experiments reported by Kim et al. 137 (Figure 10) indicate that conversions of CH4 in a DBD-only environment or in a DBD reactor packed with an Al2 O3 support approach zero at bulk temperatures above 650 K. Prior reports on plasma-only methane reforming 144,145 or methane partial oxidation 145 also demonstrated a similar lack of plasma reactivity at elevated temperatures. 29



It was suggested in these reports that the lack of activity at high temperatures was the result of the discharge transitioning from a filamentary mode to a more diffuse mode. Kim et al. observed a similar mode transition at high temperature in their plasma-only experiments, but filamentary behavior was retained during the DBD-Al2 O3 runs. It was thus concluded that retention of filamentary behavior does not promote plasma-phase reactions high temperature. The authors speculated the radicals and ions that were considered to drive low temperature CH4 conversion in the plasma-phase relaxed or were deactivated at higher temperatures. Figure 10, as well as similar experiments by Wang et al. 142,146 and Zhang et al., 131 show that Ni/Al2 O3 catalysts are active thermally at temperatures above 630 K, and addition of a plasma further increases the CH4 conversion. Kim et al. proposed two hypotheses to explain the enhancement in conversion: (1) local heating of the catalyst surface by the plasma, or (2) reaction of plasma-excited species on the catalyst surface. To verify whether the surface temperature increased during plasma-catalytic operation, the authors performed a series of melting point experiments with antimony. It was postulated that antimony would melt at lower bulk temperatures than observed in the thermal catalysis experiments if the plasma created higher surface temperatures. These experiments indicated that the plasma increased the bulk temperature by less than 10 K, considered insufficient to result in the observed increase in conversion. Enhancements in conversion were thus attributed to the promotion of the rate-limiting step for thermal catalysis, CH4 dissociative adsorption, by plasma excitation. Vibrational excitation was proposed to be particularly important, as it is known to enhance CH4 sticking on surfaces in molecular beam experiments. 50,55,147 Vibrationally excited methane was also postulated to play a significant role in steam reforming of methane by Nozaki et al., 31,148 on the basis on one-dimensional plasma kinetic simulations of pure methane, which indicated that vibrationally excited methane molecules are the most long-lived and abundant of the plasma excited species. In a follow-up study, 139 Kim et al. investigated the kinetics of high-temperature (790–

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890 K) rate enhancements in plasma-catalytic DRM using the same Ni/Al2 O3 catalysts. Care was taken to ensure that the measurements were performed in the reaction-limited regime, i.e. at low conversion and free from transport effects. Thermal-catalytic rates were found to be first order in the methane partial pressure and independent of the partial pressure of CO2 , in good agreement with the established rate orders in thermal catalysis (equation (8)). However, the plasma-catalytic rates increased linearly only at low methane partial pressures before plateauing as PCH4 was increased. These rates, after subtraction of background contributions from the plasma and the support, were found to be well described by the expression:

rate =

kplasma−cat PCH4 (1 + KCH4 PCH4 )2

(9)

The plasma-catalytic rates also decreased slightly with increasing CO2 partial pressure, an effect that was considered to be too negligible to include in equation (9). Based on the above results, the authors proposed that the surface reaction mechanisms in plasma-catalysis were similar to the Langmuir-Hinshelwood type mechanisms established for thermal catalysis, with chemisorption of CH4 promoted by plasma excitation. This faster adsorption of reactants was suggested to result in a saturation of surface sites, and consequently the plateau in plasma-catalytic rates at higher methane partial pressures. Further, the plasma-catalytic rate constant, kplasma−cat , did not follow Arrhenius behavior with the bulk temperature. Rather, the authors found that the rate constants increased with increasing SEI (defined in equation (1)). Based on this correlation, the authors proposed an alternative Arrhenius-like expression to describe this dependence:

kplasma−cat =

0 kplasma−cat

Ea exp − SEI

(10)

Arrhenius-like fits of the plasma-catalytic rate constants to equation (10) are reproduced in Figure 11(a-c). The activation energy, Ea , in Equation (10) was interpreted as the specific energy required to initiate the plasma-phase reaction. The different intercepts of the lines

31



0 in Figure 11b indicate that the pre-factor, kplasma−cat , is dependent on the gas flowrate. A

positive correlation between conversion and input power was similarly observed in the high temperature regime by Wang et al. 146

Figure 11: (a) Logarithmic reaction rate constant post plasma-phase correction (ln (kplasma−cat )) vs reciprocal powerDBD at different bulk reaction temperatures at a total flow rate of 120 mL min−1 . (b-c) ln (kplasma−cat ) vs reciprocal powerDBD at 790 K (b) and 890 K (c). Reproduced from Ref. 139 with permission from The Royal Society of Chemistry.

We note that Hegemann et al. previously identified a relationship analogous to equation (10) during plasma polymerization of acrylic acid in the low to moderate SEI regime. 149,150 The authors proposed that in this linear regime, the rate limiting step was the activation of acrylic acid monomers by the plasma and the subsequent polymerization steps on the surface were quasi-equilibrated. 151 This semi-empirical expression sparked an interesting discussion within the plasma community, see Refs. 151–157 for further details. A robust microscopic interpretation of the activation energy and the pre-exponential factor in equation (10) is currently lacking, and remains an opportunity for future research. In addition to enhancing the rate of dry reforming, there is some evidence that plasma excitation may reduce catalyst coking. Kameshima et al. performed dry reforming experiments in a DBD reactor using a pulsed supply of CH4 . 138,141,158 In these experiments, CO2 was continuously fed through the DBD reactor packed with Ni/Al2 O3 catalyst and methane was only supplied in intermittent pulses. When both reactants were flowed into the reactor 32


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(DRM phase) at 550 ◦C, CO, H2 , H2 O and solid C were formed. When the methane supply was interrupted, leaving CO2 as the only reactant (de-coking phase), the deposited carbon was oxidized and desorbed as CO. Carbon oxidation was attributed to the reverse-Boudard reaction, CO2 + C − )− −* − 2 CO. The authors indicate that switching off the CH4 supply during the de-coking phase is necessary because the DRM reaction and coke formation due to the decomposition of adsorbed CHx are faster that the reverse-Boudard reaction in the DRM phase. 159 Plasma-excitation of CO2 was proposed to increase the rate of the reverse-Boudard reaction above the thermal rate. Low temperature plasma-catalytic DRM. Plasma-catalytic DRM has been studied more extensively at bulk gas temperatures (

Catalysis Enabled by Plasma Activation of Strong Chemical Bonds: A

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