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APPLIED CHEMISTRY In Situ Fourier Transform Infrared (FTIR) Study of Nonthermal-Plasma-Assisted Methane Oxidative Conversion S. A. Nair,* Tomohiro Nozaki, and Ken Okazaki Department of Mechanical & Control Engineering, Tokyo Institute of Technology, 2-12-1 O-Okayama, Meguro-ku, 152-8552 Tokyo, Japan
Nonthermal plasma-assisted methane conversion has been widely investigated as a potential low-temperature process. The desired end objective is synthesis gas production, one-step production of liquid oxygenates, or coupling products. Either oxygen or steam or both is used as a second reactant or reactants, to provide additional O atoms and initiate plasma-assisted radical processing. The present paper intends to investigate the various possible products/reactive species that are formed during plasma processing. In situ Fourier transform infrared (FTIR) absorption spectrometry is used to monitor the products/reactive groups. Experiments are performed at room temperature and at low energy inputs (50 kJ/(mol CH4)) in a gas mixture of CH4/O2/N2/H2O (atmospheric humidity). In the absence of oxygen, alkane, alkene, and alkyne groups are formed as the products, which indicates termination reactions. With increasing oxygen concentration (11, 15, and 33%) a gradual shift from alkane to a -CHO group is observed. In addition, alcohol group formation is detected with oxygen input, which indicates coupling between the CH4 dissociation products and the O radicals as a primary step. The effect of higher energy input and the presence of catalytic surfaces such as platinum also are investigated. 1. Introduction The stability of the C-H bond necessitates the need to have high temperatures for CH4 conversion, either by oxidative processes (such as those in partial oxidation (T > 600 K)) or by steam reforming (T > 800 K). In either case, the end product, based on the economics, is synthesis gas, which further acts as a building block for many bulk chemicals such as methanol or other liquid oxygenates. Various catalysts have also been investigated and alternative chemistries have been proposed for a one-step production of liquid oxygenates.1 The advantage of a single-step process is elimination of the need for expensive reforming and the high-temperature process for the production of synthesis gas. However, the high temperatures needed to overcome the activation barrier for CH4 conversion, combined with higher reactivity of the product, lead to lower selectivity and yield for oxygenates.2 This often leads to a tradeoff between the achievable CH4 conversion (at lower temperatures) and the obtained selectivity for oxygenates, from an economical point of view. Nonthermal plasma (e.g., dielectric barrier discharges) has the inherent advantage of initiating reactions via energetic collisions with electrons (∼4-5 eV).3 Therefore, conversion can be initiated irrespective of temperature, with the limitation being the electrical energy cost. Over the past decade, research has been conducted in the area of nonthermal plasma-assisted NOx/SOx removal from the exhaust of power plants4,5 and for diesel engine exhaust,6 volatile organic compounds (VOC) removal,7 heavy hydrocarbon re* To whom correspondence should be addressed. Current Address: Aditya Birla Science and Technology Company, Ltd., Aditya Birla Group, Plot No. 1 & 1-A/1, MIDC Taloja, New Mumbai 410208, Maharashtra, India. E-mail address:
[email protected].
moval (tar) removal from biomass gasification gas,8 and water treatment.9 Some of the applications are on the verge of bridging the gap between laboratory and industry10,11 The investigations focus on understanding physical processes inside a nonthermal plasma reactor12 and, furthermore, the engineering of reactors for various processing conditions.5,8 However, from the process optimization point of view, the chemical kinetics and the reaction pathways13,14 provide the key information in addressing the energy costs and obtaining the relevant process parameters. The use of nonthermal plasma for bulk-phase conversion has been done in the areas of plasma-assisted oxidative methane conversion15,16 and plasma-assisted methane reforming,17 as well as the CO2 reforming of methane.18 Experimental results have indicated that the conversion is dependent on both the energy input and the temperature of the process.19 Plasma-assisted catalytic processes have been studied as well, to exploit synergetic effects. However, apart from conversion, selectivity for a desired product is another key question for process viability. The selectivity of the products formed in plasma is dependent on the temperature, radical density, and radical reaction kinetics. Plasma combined with catalysts has been investigated to increase selectivity for the desired products (e.g., liquid oxygenates) with varying degree of success.20 However, to design a process, fundamental information related to the various aspects of nonthermal plasma-assisted processing (plasma generation, radical density, reaction kinetics and pathways, etc) is needed. Recent investigations in the area of methane conversion have focused on achievable conversions at various temperatures in the presence of various catalysts. Investigations have also been conducted to study the associated physicochemical processes (e.g., using emission spectroscopy to obtain the real gas
10.1021/ie0606688 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/24/2007
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Figure 1. Fourier transform infrared (FTIR) gas cell.
temperature20 and the initial radical density during nonthermal plasma-assisted methane conversion22). The present paper intends to highlight the reaction pathways in plasma-assisted methane conversion, as well as during oxidative conversion. FTIR spectroscopy is used to monitor the progress of the reaction and identify the various intermediate species that are formed during the course of the reaction. 2. Experimental Setup The experimental setup consists of a plasma reactor that is placed inside a demountable gas cell with a cell path length of 50 mm. The dimension of the setup and the electrode arrangement is as shown in Figure 1. The plasma reactor is a dielectric barrier discharge (DBD) reactor with a plate-to-plate configuration. The high-voltage (HV) electrode is an aluminum plate (with dimensions of 22 mm × 27 mm × 5 mm) with a thickness of 5 mm energized by a high-voltage amplifier (TREK, model 20/20C high-voltage amplifier). A glass plate of appropriate dimensions (27 mm × 32 mm × 2 mm) is used as the dielectric material. The ground electrode (an aluminum plate, with dimensions of 24 mm × 27 mm × 5 mm) is fixed to the glass plate by means of adhesive tape. The gap distance between the HV electrode and the glass dielectric for plasma generation is fixed at ∼4-5 mm. The edges of the electrodes are rounded off to prevent high electric fields which lead to localized plasma
generation. The gas cell is closed on the ends by means of infrared (IR) transparent windows (KRS-5, φ 40) with sealing, to prevent leakages. [KRS-5 (thallium bromoiodide) is used as an IR window material, because of its low solubility in water.] The gas cell is mountable on the FTIR (JASCO-FTIR 660 plus) sample holder and, therefore, can be positioned accurately for every experiment. The path length for the FTIR apparatus is ∼20 cm, and the reactor length is ∼6.5 cm. The cell has provisions for a gas inlet and outlet through which the highvoltage and the ground connections are made. The reactant gas is fed through one of the inlet port, and the other port is connected to the exhaust. Several holes are drilled on the HV electrode, to allow for gas flow into the plasma zone. A voltage of ∼4-6 kV at a frequency of ∼700-1200 Hz is applied to have uniform plasma generation. The input power at the applied voltage is in the range of ∼500-800 mW. 2.1. Experimental Procedure. The experiments are performed in gas mixtures that consist of CH4 and O2 at various ratios. CH4 gas is in the form of a mixture of CH4/N2 (90% CH4), with the nitrogen (N2) being added as an inert gas. The reactant gases or the mixture under investigation is flushed through the cell for a period of 30 min to achieve steady-state conditions. The FTIR spectrum for the unreacted reactant gases is taken as the background spectrum.
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Figure 2. Absorption spectrum during nonthermal plasma processing in a CH4/N2 gas mixture. Each spectrum is recorded at a resolution of 4 cm-1, averaged over 32 scans, with an interval of ∼2-3 min. Energy input is 0.5 W (or 70 kJ/(mol CH4)). Table 1. Experimental Plan Flow Rate (sccm) stage I II III IV V VI VII
surface
CH4/N2
O2
CH4/O2 ratio
energy density (kJ/(mol CH4))
platinum platinum platinum
10 10 10 10 10 10 10
0 1.3 1.8 5.5 1.8 5.5 5.5
7 5 1.8 5 1.8 1.8
70 70 70 70 70 70 150
Furthermore, with the application of a plasma (∼70-150 kJ/ (mol CH4)), spectra are recorded with a resolution of 4 cm-1, averaged over 32 scans. The total experimentation time (i.e., the time for plasma application) is ∼20 min, to attain steadystate conditions, in terms of concentration, and thereby obtain reproducible spectral data. In addition, the effects of a lowtemperature oxidation catalyst, such as platinum, also have been investigated, to ascertain the reaction pathways in a plasma catalytic combination. A complete overview of the experiments performed within this study is listed in Table 1. The objective of the experiments is to observe the changes during the course of the reaction. With the conversion in the reactor being limited because of the low energy density, the concentration of the formed reactants would be relatively small and cannot be easily detected in the presence of an overlapping spectrum, because of the CH4/N2 atmosphere. Hence, to detect the presence of the intermediates formed, and to distinguish the product spectrum from the unreacted gases, a spectrum of the background consisting of the unreacted gas is taken. Furthermore, conversion is limited because of the limited energy input into the reactor. Given that the experiment is
performed in the sampling space of the FTIR instrument, electrical interferences at higher energy density with the electronics of the device (FTIR) are possible, and these interactions are undesirable. 3. Results and Discussion 3.1. CH4/N2 Gas Mixture. Optical emission spectroscopy in discharges with pure CH4 has identified species such as CH and C2, but the nature of these species cannot be reliably obtained.18 IR absorption spectroscopy could be used to identify the nature of the various groups that are formed. Thus, prior to investigating the mixture compositions, experiments are performed in a CH4 discharge. The CH4/N2 gas mixture is flushed through the FTIR gas cell at a flow rate of 10 sccm for a period of 30 min to attain steady-state conditions. [Here, “sccm” is standard cubic centimeters per minute.] A background spectrum is recorded with this gas composition, to identify the groups that are formed during plasma processing. A nonthermal plasma is created in this gas composition by applying an energy of 0.5 W (500 mW). The energy density can be estimated to be ∼70 kJ/mol CH4. N2 can be assumed to be an inert gas mixture at this relatively low energy density and due to its relatively low concentration (10%). [Although vibrationally excited N2 states may exist in a nonthermal plasma, its influence on the product distribution still must be ascertained.] Figure 2 shows the absorption spectrum recorded during the creation of the plasma. Each spectrum is recorded after an interval of ∼2-3 min at a resolution of 4 cm-1 and an average of ∼32 scans. Each spectrum has been further smoothened by fast Fourier transform (FFT) to reduce noise (with ∼15 points; however, the real spectrum is shown simultaneously in the background).
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Figure 3. (A) Absorption spectra during nonthermal plasma in a CH4/N2 gas mixture at various time intervals. The shaded region indicates the change in absorption intensity due to CH4 conversion. The peaks resulting from the product or the species formation during plasma formation are indicated at the respective wavenumber (cm-1). The spectra indicated in the figure is after FFT smoothening (with 15 points); the real signal is shown in the background (light gray trace). (For peak identification, refer to the text.) (B) Product distribution at the outlet of the plasma reactor by gas chromatography (GC) analysis (CH4/N2 gas mixture, T ) 403 K, E ) 80-150 kJ/(mol CH4)).
A growth of peaks can be seen in the vicinity of 3000, 952, and 730 cm-1. To identify the peaks distinctively, the first and last spectrum (shown in Figure 2) (the spectrum obtained just after plasma application and the spectrum obtained toward the end of the plasma run) are replotted in Figure 3A. The peaks formed in the negative side on the absorption axis reflect a decrease in the concentration of CH4, because of its conversion in the plasma discharge. The following peaks can be identified: (a) peaks in the range of ∼3000-2700 cm-1 (these are assigned to CH3 stretch vibration);
(b) peaks in the range of ∼3760-3580 cm-1 (these are assigned to the symmetric and asymmetric stretch vibrations that are associated with H2O molecules); (c) a peak in the range of 1640 cm-1 (this is assigned to the bending mode vibration of H2O molecules); (d) a peak at 726 cm-1 (this is assigned to CH deformation vibration (with is associated with alkynes); (e) a peak at 946 cm-1 (this is assigned to CH out-of-plane vibrations (associated with alkenes)
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(f) a peak at 1033 cm-1 (this is assigned to the C-O stretching vibration (associated with primary alcohols); and (g) a peak at 2340 cm-1, which is assigned to the CO2 (atmospheric, because, during the experiment, the sampling section of the device is open and exposed to the room environment). The presence of peaks that are associated with water molecules in the gas phase can be associated with atmospheric humidity. The complete reactor system, although it does not have any leakages, is not completely vacuum-tight; hence, during the course of the experiments, there is always a change in the concentration of water molecules. However, this does not change, nor does it lead to a dual interpretation of the results obtained. The peaks from the CH3 stretch vibration and the CH out-of-plane and deformation vibrations are mainly from the associated alkane, alkene, and alkyne groups.
CH4 f CH3 + CH2 + CH The relative concentration of CH3, CH2, and CH can be expressed as follows:
CH3 ) R[CH4] CH2 ) β[CH4] CH ) γ[CH4] The parameters R, β, and γ are functions of the electron density, electron energy, and gas composition. However, these parameters have been estimated with kinetic models using a numerical fitting with experimental results from a DBD reactor.18 The estimated values for R, β, and γ are 0.3, 0.0015, and 0.003, respectively. Based on these investigations, the CH3 concentration can be ∼2 orders higher than the corresponding CH2 and CH fragments. GC analysis of the products for similar experiments at the end of the reactor has revealed the products to be a mixture that consists of C2H6, C2H4, and C2H2. The detected peaks in the FTIR spectrum at ∼3000-2700 cm-1, 946 cm-1, and 726 cm-1 can therefore be assigned to the products C2H6, C2H4, and C2H2, respectively. Thus, in the absence of additional reactants, the produced fragments undergo termination reactions to form stable products. The added presence of water molecules in the reactor leads to the formation of a small amount of oxygenated products; however, GC analysis has indicated the concentration to be very small. At higher temperatures (T > 100 °C), however, the produced fragments undergo further reaction to produce higher-order hydrocarbons, such as C3 (see Figure 3B).23 The objective of the investigation is to present qualitative information about the intermediate products that are formed during the plasma-assisted conversion process, which is difficult to obtain otherwise. Hence, the observations are reported to highlight the following: (1) the intermediates formed and (2) the effect of the changing conditions on the chemical reaction pathways. The quantitative information obtained or presented in the manuscript is obtained based on experiments in a different experimental setup, with a cylindrical reactor, in a plug-flowbased DBD configuration.23 Furthermore, for nonthermal plasma reactor configurations, it is well-known that the conversion is a function of the input energy in the reactor, and this is true for the case of relatively lower energy density. 3.2. CH4/N2/O2 Gas Mixture. The O2 concentration was varied to obtain a range of gas mixtures, from fuel-rich to a
stoichiometric composition, to analyze the reaction pathways in oxidative methane conversion. Figure 4A shows the absorption spectra recorded at various O2 concentrations: 0%, 11.5%, 15.25%, and 33.33% (CH4/O2 ratios of 7, 5, and 1.8). Figure 4B indicates the product distribution measured (by GC analysis) at the outlet of a DBD reactor for two distinct compositions, viz. CH4/O2 ) 2 and 24. The experimental data indicate an increase in the concentration of C2 intermediates (C2H4, C2H6) with a decrease in O2 concentration. For relatively low energy densities (
