Catalytic Nonthermal Plasma Reactor for Dry Reforming of Methane

Article pubs.acs.org/EF

Catalytic Nonthermal Plasma Reactor for Dry Reforming of Methane Sk. Mahammadunnisa, P. Manoj Kumar Reddy, B. Ramaraju, and Ch. Subrahmanyam* Energy and Environmental Research Laboratory, Department of Chemistry, Indian Institute of Technology Hyderabad, Andhra Pradesh 502205, India ABSTRACT: Co-processing of two greenhouse gases, methane and carbon dioxide, was carried out in a dielectric barrier plasma reactor. The influence of feed gas proportion on the performance of the plasma reactor was investigated, especially with an objective to increase the conversion of the reactants and selectivity to syngas. To understand the influence of the catalyst on dry reforming, 10−30% NiO/Al2O3 catalysts were prepared by a single-step combustion synthesis and various physicochemical techniques confirmed the formation of nanosized Ni particles. A total of 10% of the discharge volume was packed with Ni/Al2O3 catalysts in a packed-bed configuration. An interesting observation is the increased syngas selectivity on the addition of catalyst to plasma, and among the catalysts tested, 20% Ni showed a H2/CO ratio of 2.25 against 1.2 with a plasma reactor alone.

1. INTRODUCTION The negative effect of greenhouse gases, especially carbon dioxide (CO2) and methane (CH4) has been well-established, and there is an immediate need to control the emissions of these gases.1 Even though the concentration of these gases is far less than that of water vapor, they reflect infrared radiation, thereby increasing the global temperature.1−5 A traditional approach to regulate CO2 includes sequestration, whereas methane has been converted to hydrogen in steam methane reforming (SMR), followed by a water-gas shift reaction. Even though hydrogen production by this approach is a fledged technique, it may not be environmentally benign because of simultaneous evolution of CO2 along with hydrogen, whereas CO2 sequestration has a limited scope. During the past few decades, CO2 reforming of CH4, also known as dry methane reforming (DMR), has been attracting considerable attention from both industrial and environmental perspectives, especially as an alternative for the production of synthesis gas (H2 + CO).6 Syngas is one of the most important industrial feed stocks for the production of a variety of chemicals in so-called Fischer−Tropsch processes.7−9 Even though several thermal, catalytic, plasma, and plasma catalytic processes were proposed for dry reforming,1,10−18 upon a closer look, dry reforming demands 20% higher energy than SMR. In addition, optimization of conditions for a desired H2/CO ratio has been a challenge. Dry reforming conditions imply temperatures in the range of 800−1100 °C and a pressure well above ambient conditions, where the catalyst may not be very active and undergoes deactivation.10−12 In recent years, industrial applications of nonthermal plasma (NTP) generated under ambient conditions have been receiving considerable attention.8,18−21 NTP generated by electrical discharges under ambient conditions has specific advantages, such as mild operating conditions mainly composed of electrons, ions, excited atoms and molecules, radicals, and meta stable ions. These highly energetic electrons initiate the chemical reactions in the gas phase. In addition, because of a low degree of ionization, most of the energy in NTP will be used for accelerating electrons. Hence, with a suitable © 2013 American Chemical Society

configuration of the reactor and catalyst, dry reforming by the NTP approach may be made energetically feasible.19−21 The combination of plasma and heterogeneous catalysis for syngas production by hydrocarbon reforming has been recently tested, and it may be concluded that both chemical and physical properties of the plasma and catalyst may be modified by integration of the catalyst in the plasma discharge zone.10,22,23 The presence of the catalyst in plasma may show a synergistic effect to maximize the process performance.22 It has been shown that metal-based catalysts on different supports (Al2O3, SiO2, TiO2, ZrO2, CeO2, and zeolites) showed significant activity in dry reforming.24−28 Especially, Ni-based catalysts have been widely tested for the dry reforming reaction because of their low cost and high activity and selectivity.24,28−30 Many studies have been focused on the selection of promoters, supports, and preparation methods to increase the activity of Ni-based catalysts for dry reforming.29,30 It was reported that Ni-based catalysts prepared by the impregnation method deactivate quickly.6 In this context, the solution combustion (SC) method is a rapid and simple approach for the preparation of catalysts.31,32 During the present study, SC was employed to prepare Ni− Al2O3 catalysts with varied Ni contents from 10 to 30 wt %. The as-synthesized catalysts were characterized by N 2 adsorption−desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), and hydrogen temperatureprogrammed reduction (H2-TPR). Dry reforming of methane was tested by operating the plasma in dielectric barrier discharge (DBD) configuration and with Ni/Al2O3-integrated plasma. The plasma reactor was optimized as well as various parameters, such as applied voltages, composition of the gas mixture, and catalyst combination to achieve the best performance. Special Issue: Accelerating Fossil Energy Technology Development through Integrated Computation and Experiment Received: December 31, 2012 Revised: February 25, 2013 Published: February 26, 2013 4441

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Figure 1. Schematic representation of the catalyst-packed dielectric barrier discharge reactor for dry reforming of methane. capacitance multiplied by its capacitance corresponds to charge accumulated in the reactor. The plasma input energy per cycle is equal to the area enclosed by the charge−voltage curve in the V−Q Lissajous figure.20 The specific input energy (SIE) was calculated by dividing the power with the flow rate of the gas as expressed below.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Desired amounts of Ni (NO3)2·6H2O and Al (NO3)3·9H2O were dissolved in a minimum amount of water, and a required amount of citric acid to maintain oxidant/fuel = 1 was added under vigorous stirring. The resulting solution after sonication at room temperature for 2 h was transferred to a quartz beaker and introduced into the preheated muffle furnace at 450 °C to achieve spontaneous combustion. The prepared Ni−Al2O3 catalysts were denoted as x% Ni−Al (where x = 10, 20, and 30 wt % Ni on Al2O3 in the catalyst). For comparison, Al2O3 were also prepared under the same conditions. Synthesized metal oxides were reduced to the metallic state in a H2 atmosphere (100 mL min−1) for 6 h at 600 °C. After reduction, the Ni-based catalyst was cooled to room temperature in H2 flow to prevent bulk oxidation of the Ni nanoparticles.34 2.2. Catalyst Characterization. Nitrogen adsorption−desorption isotherms were obtained with a Quantachrome autosorb automated gas sorption analyzer (NOVA 2200e). Prior to adsorption, the samples were degassed under vacuum at 200 °C for 4 h. Specific surface areas were determined by the Brunauer−Emmett−Teller (BET) method. Powder XRD patterns were obtained on a PANalytical X’pert pro Xray diffractometer using Cu Kα (λ = 1.541 Å radiation, 30 mA, and 40 kV). The formation of the nanosized catalyst was confirmed by TEM performed on a FEI model TECNAI G 220 S-Twin instrument. The catalyst powders were dispersed in ethanol by ultrasonication, and the suspension was then dropped onto a carbon-coated copper grid. Inductively coupled plasma−optical emission spectroscopy (ICP− OES) on a Teledyne prodigy high-dispersion analysis was performed to identify the amount of Ni present. The nickel dispersion (calculated from H2 chemisorption after 6 h of reduction at 600 °C) and H2-TPR measurements were carried out in a flow system Quantachrome autosorb-IQ (automated gas sorption analyzer) equipped with a thermal conductivity detector (TPR−TCD). Prior to the H2-TPR measurements, 50 mg of the sample placed in a quartz U-tube reactor was pretreated in argon stream at 300 °C for 0.5 h and then cooled to room temperature. The TPR profile was recorded by increasing the temperature of the catalyst bed at 30 to 950 °C in 10% H2 flow at 50 mL min−1. 2.3. Plasma Reactor. The experimental setup is shown in Figure 1. Briefly, NTP was generated in a cylindrical quartz tube. The outer surface of the quartz tube wrapped with copper wire for 15 cm acts as an outer electrode, whereas a stainless-steel rod placed at the center of the tube was the inner electrode. The discharge gap was 3.5 mm, which corresponds to a discharge volume of 27.2 cm3. A total of 10% of this active volume between electrodes was packed with a Ni/Al2O3 catalyst placed in between quartz wool plugs toward the outlet of the reactor. The flow rate of the gas mixture was kept constant at 40 mL/ min, and a required amount of high pure argon was used as the diluent to achieve the desired concentration. The plasma reactor was powered by a high-voltage alternating current (AC) generator (Yaskawa varispeed F7 AC inverter up to 40 kV and 50 Hz). The voltage (V) and charge (Q) waveforms were recorded with an oscilloscope Tektronix (TDS 2014B) using a 1000:1 high-voltage probe (Agilent 34136A HV). The voltage across the

SIE (J/L) = power (W)/gas flow rate (L/s) The gas mixture was controlled by a set of mass flow controllers (Aalborg, Orangeburg, NY). CH4 (10 vol % diluted in Ar) and CO2 (5 vol % diluted in Ar) gases were introduced into the reactor. At the reactor outlet, gas analysis was made with an online gas chromatograph (Varian 450-GC), whereas online gas chromatography−mass spectrometry (GC−MS, Thermo Fischer) was used to identify the byproducts. CH4 and CO2 conversions, CO and H2 selectivities, H2/ CO ratio, and H2 yield are calculated as given below.

CH4 conversion (%) = ((moles of CH4 inlet − moles of CH4 outlet) /(moles of CH4 in inlet)) × 100 CO2 conversion (%) = ((moles of CO2 in inlet − moles of CO2 outlet) /(moles of CO2 in inlet)) × 100

H 2 selectivity (%) = (moles of H 2 produced/2 × moles of CH4 converted) × 100 CO selectivity (%) = (moles of CO produced /(moles of CH4 converted + moles of CO2 converted)) × 100

H 2 /CO ratio = moles of H 2 produced/moles of CO produced

H 2 yield = CH4 conversion × H 2 selectivity/100

3. RESULTS AND DISCUSSION 3.1. XRD. XRD patterns of the reduced samples shown in Figure 2 confirmed the characteristic patterns of γ-Al2O3, NiAl2O4, and Ni phases. These XRD patterns also revealed that γ-Al2O3 and Ni had low crystallinity, as evidenced by the broad peaks because of the presence of γ-Al2O3 and NiAl2O4, respectively.6 The designed catalyst exhibited high specific surface area (SA) (SA of 0, 10, 20, and 30% Ni/Al2O3 is 250, 240, 230, and 195 m2 g−1, respectively). With an increasing nickel loading, the main diffraction peak of γ-Al2O3 shifted to lower “d” spacing because of diffusion of NiO particles into the support structure to form the NiAl2O4 phase.31 The peaks for 4442

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Figure 3. Typical TEM of the reduced 20% Ni/Al catalyst.

Figure 2. XRD patterns of reduced Ni/Al2O3 catalysts and Al2O3 support.

the defect spinel phase may be observed at values 2θ of 37°, 43.5°, and 66.7°, whereas relatively higher 2θ values indicated that Ni2+ in the spinel phase has been reduced to Ni0. From the ICP−OES experiment, it is confirmed that the Ni percentage in the catalysts at 10, 20 and 30% Ni/Al is very close to the expected nickel loading (shown in Table 1). The estimated metal dispersion for 10, 20, and 30% Ni/Al2O3 catalysts are shown in Table 1, and it may be concluded that a higher nickel dispersion is observed for 20% Ni/Al. 3.2. TEM. The TEM micrographs of the reduced 20% Ni/ Al2O3 catalysts are presented in Figure 3. As seen in Figure 3, Ni nanoparticles with a typical diameter of 2:1 for CH4/CO2. It was observed that the H2/CO ratio was only 1.2 with the plasma reactor alone. To improve the performance of the DBD reactor, Ni catalysts were prepared by combustion synthesis and 10% of the active discharge volume was packed with catalysts. Typical results indicated the improved performance of the reactor for dilute mixtures and addition of Ni/Al2O3 catalysts. Among the catalysts studied, 20% Ni/Al2O3 showed the best conversion and highest H2/CO ratio of 2.25 against 1.2 with the plasma reactor alone. The adsorption, desorption, and reaction of the Ni catalyst under plasmas is still new. However, the better catalytic behavior of the 20% Ni/Al2O3 catalyst in the dry reforming of methane may be due to the formation of nanocrystalline NiAl2O4, highly dispersed Ni nanoparticles, and/or high reducibility compared to a higher or lower weight percentage of Ni.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +91-40-23016050. Fax: +91-40-23016032. E-mail: [email protected]. Notes

Figure 9. Formation of CO2 on different Ni/Al2O3 catalytic systems after 1 h of reaction.

The authors declare no competing financial interest. 4445

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ACKNOWLEDGMENTS The authors thank the Council of Scientific and Industrial Research (CSIR) India for financial assistance and Dr. T. P. Radhakrishnan for TEM analysis.

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