Enhancing C−H Bond Activation of Methane via Temperature-Controlled, Catalyst−Plasma Interactions Jongsik Kim,† Marshall S. Abbott,† David B. Go,†,‡ and Jason C. Hicks*,† †
Department of Chemical and Biomolecular Engineering, University of Notre Dame, 182 Fitzpatrick Hall, Notre Dame, Indiana 46556, United States ‡ Department of Aerospace and Mechanical Engineering, University of Notre Dame, 372 Fitzpatrick Hall, Notre Dame, Indiana 46556, United States S Supporting Information *
ABSTRACT: Recent shale gas discoveries and advances in plasma chemistry provide the basis to exploit metal surface−plasma interactions to precisely control C−H bond activation on catalytic surfaces, leading to improved reaction efficiencies. Although the exact determination of plasma−catalyst interactions remains a topic of continuing research, this Letter provides evidence that plasma− catalyst interactions exist and can be used to significantly enhance the activation of C−H bonds at temperatures >630 K, probed by the catalytic dry reforming of methane with carbon dioxide using Ni/ Al2O3. We systematically varied bulk temperature and plasma power to determine Ni−plasma interactions. In contrast to reactions at low temperatures (8 fold increase). Other competing contributors, such as gas-phase plasma reactions, charge confinement, and plasma-driven enhanced bulk gas temperatures, played minor roles when operating at temperatures >630 K.
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species via radical formation, ionization, vibrational excitation, and rotational excitation (e.g., ·CH3 and CH4* in Figure 1a).2,23,25,29−31 Excitation in these ways can lower the barrier required to activate C−H bonds and/or change the reaction pathway, thereby facilitating hydrocarbon transformation.2,25,29 Moreover, the benefits of DBD plasma for C−H activation reactions can be further enhanced through the addition of transition-metal catalysts on dielectric supports (e.g., Ni,22,24,29 Pt,23 or Cu/ZnO20 on Al2O324 in Figure 1b) and/or dielectric materials with high relative permittivity (e.g., BaTiO325,32,33). Several plausible hypotheses have been suggested in the literature to account for the enhanced C−H activation using plasma−catalyst combinations: (1) gas-phase electron impactregulated hydrocarbon dissociation caused by the plasma,19,22−25 (2) the generation of increased temperature catalytic sites,25,34−36 (3) the direct interaction between active metal surfaces and the plasma,23,25,26,37−41 and (4) packed-bed effects due to enhanced electric fields from the use of porous
ith the recent discovery of worldwide shale gas reserves, the interest in directly converting light hydrocarbons to fuels and chemicals has substantially increased. These nontrivial chemical transformations of hydrocarbons1,2 require precise control of C−H bond activation on the catalyst surface through oxidative3−5 or nonoxidative6−8 dehydrogenation, coupling,9−11 partial oxidation,12−14 or carbonylation15,16 for selective and efficient conversion to more valuable products. This catalytic activation, however, remains a formidable task primarily because of the chemical inertness of C−H bonds.1,2,17−21 Therefore, substantial energy input through external heating is required to overcome the activation barrier during conventional catalytic conversions.3,22−24 Nonthermal plasmas, in contrast, have been employed as highly promising reaction media for converting a wide range of saturated hydrocarbons to syngas, alcohols, and unsaturated analogues under ambient pressure and low temperature (1 eV) under an applied electric field.20,23,25−28 These can subsequently excite hydrocarbon © XXXX American Chemical Society
Received: April 7, 2016 Accepted: May 5, 2016
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DOI: 10.1021/acsenergylett.6b00051 ACS Energy Lett. 2016, 1, 94−99
Letter
http://pubs.acs.org/journal/aelccp
Letter
ACS Energy Letters
Figure 1. Schematic representation of dry CH4 reforming on or near the surface of an Al2O3-supported metal catalyst in the presence of plasma: (a) excitation of CH4 species by gas-phase electron impact, (b) transition-metal catalyst supported on dielectric support (e.g., Al2O3) present inside plasma discharge zone, and (c) chemical equation of dry CH4 reforming.
dielectric materials.25,28,32,42−45 However, although these postulations have been previously suggested, there are only a few reports that have explored the relative contributions of these phenomena to plasma-driven C−H activation assisted by catalysts.2,23,25,31 Hence, because of the complexity of plasma-assisted catalytic processes, our focus in this study was to determine the sources that cause C−H activation enhancements by performing controlled experiments in a catalyst−DBD plasma reactor using the dry reforming of CH4 with CO2 as a probe reaction (Figure 1c). Ni supported on γ-Al2O3 (Ni/Al2O3) was chosen as a catalyst because of its high activity for this reaction.22−24,26,39 Additionally, to systematically investigate C−H activation, we controlled the CH4 reforming by varying the reaction environment, bulk temperature, and/or DBD plasma power. This Letter highlights the experimental results obtained by performing a series of Ni/Al2O3−DBD plasma-enhanced CH4 reforming reaction runs, which can have significant impact on designing new plasma-assisted catalytic processes. For this study, we first synthesized two Ni/Al2O3 catalysts with different nominal quantities of Ni (i.e., 1 wt % for 1Ni; 20 wt % for 20Ni) (synthesis procedures and characterization can be found in the Supporting Information). The 20 wt % Ni was selected based on previous studies that showed this to be an optimal loading in DBD plasma-assisted CH4 reforming,22 whereas the 1 wt % of Ni loading was selected for the kinetic evaluation discussed later.4,46−48 We operated a series of CH4 reforming reactions under the presence of a DBD plasma in a controlled reaction environment. The objective of this study was to determine the major contributor for C−H activation by evaluating CH4 conversions, H2 yields, and H2/CO selectivities. Therefore, the bulk reaction temperature was first controlled from 380 to 900 K using a furnace for external heating under various reaction environments, while maintaining a DBD power of 10 W (Figures 2 and S3). The use of 20Ni with DBD plasma (20Ni−DBD) showed catalytic performance similar to that of the DBD only analogue (DBD) below 630 K. In contrast, at reaction temperatures of 630 K or greater, 20Ni−DBD significantly outperformed the DBD only and thermally catalyzed 20Ni (20Ni-thermal) experiments. These temperature control runs, therefore,
Figure 2. (a) Reaction environments for the CH4 reforming and (b) profiles of CH4 conversions (XCH4) and H2 yields (YH2) at various reaction environments obtained via bulk temperature controls. Reaction conditions: 100 mg of 20Ni (or Al2O3); CH4:He:CO2 = 1:2:1; total flow rate of 20 mL min−1; 1 atm; 10 W.
suggested that the catalytic role of the 20Ni was significantly enhanced by the DBD plasma at elevated temperatures compared to the thermal only counterpart. We then investigated four potential contributors to this observed reaction enhancement by controlling the reaction environment: (1) plasma-driven electron impact activation in the gas-phase (lacking porous packing materials), (2) plasmadriven gas-phase electron impact from confinement of excited charge species in the presence of porous diluents and/or porous Ni/Al2O3 catalysts, (3) elevated surface temperatures due to the plasma, and (4) interactions between active Ni sites and the plasma. The DBD only control run was employed to determine the effects of gas-phase electron impact on the activation of CH4, possibly leading to the dissociation of C−H bonds at high temperatures.19,22−25 Prior studies on CH4 reforming have indicated that the DBD undergoes a mode transition from typical filamentary behavior (also called microdischarge behavior) at ∼673 K to a more diffuse mode when elevated to ∼773 K at 10 W.49,50 This mode transition can be observed by Lissajous curves, which plot the amount of charge accumulated versus the applied voltage across the DBD plasma.28,51 The presence of a mode transition is characterized by a change in the curve shape from a parallelogram to an oval in the Lissajous plot.49,50 We observed such behavior in the DBD only run, in which the Lissajous curve showed a dramatic geometric change to an oval shape at 890 K (Figure S4).49,50 Moreover, under these reaction conditions (DBD only), no CH4 was reacted at 690 K or greater (DBD; red curves shown in Figure 2b). In conjunction with previous literature,49,50 this indicated that high reaction temperatures can suppress the dissociation of CH4 by the DBD plasma itself.49,50 Thus, DBD plasma-driven electron impact activation was found to play a minor role in activating C−H bonds of CH4 at elevated temperatures. 95
DOI: 10.1021/acsenergylett.6b00051 ACS Energy Lett. 2016, 1, 94−99
Letter
ACS Energy Letters
of C2H6 byproducts (Figure S3c), the increase in the DBD powers from 1 to 10 W resulted in higher CH4 conversions and H2 yields at >670 K (Figure 2b and S3a). Therefore, by altering the DBD plasma power, the extent of the Ni−DBD plasma interaction can be altered and used to subsequently control the C−H activation. Indeed, the Ni−DBD plasma interaction was even more evident given the results of a series of controlled time-onstream experiments using 20Ni catalyst, in which DBD plasma power (10 W) was turned on and off at predetermined times while maintaining a bulk reaction temperature of 790 K (Figure 3a). A temperature of 790 K was selected because of the
Additionally, we evaluated the effect of porous dielectric materials (such as the γ-Al2O3 support or SiO2 diluent) on the activation of CH4 at elevated temperatures.24,27,30,38−41 It has been detailed in previous reports that porous dielectric materials can enhance the electric field in a DBD via the confinement of charged species within the pores and could thereby facilitate various chemical conversions.25,28,32,42−45 Thus, we performed an additional control run using only the porous support subjected to a calcination and reduction condition identical to that used to generate the 20Ni. In contrast to the DBD only run, no geometric change in the Lissajous curves to the oval shape was observed throughout these experiments (Figure S4). That is, the porous Al2O3 inhibited the mode transition we observed at 890 K when performing the DBD only run and retained the filamentary discharge conditions present at low temperatures, which has not been observed and/or reported previously. However, despite retention of a filamentary environment, the Al2O3− DBD run showed
