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Energy & Fuels 2008, 22, 3600–3604
Reaction Enhancement Mechanism of the Nonthermal Discharge and Catalyst Hybrid Reaction for Methane Reforming Tomohiro Nozaki,* Wataru Fukui, and Ken Okazaki Department of Mechanical and Control Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro, Tokyo 1528552, Japan ReceiVed June 14, 2008. ReVised Manuscript ReceiVed August 7, 2008
The reaction enhancement mechanism of methane steam reforming (MSR) in the nonthermal discharge and catalyst hybrid reaction is presented. Coke deposited on Al2O3-supported Ni catalyst was investigated using temperature-programmed oxidation (TPO) analysis and micro-Raman spectroscopy. Although methane conversion in the hybrid reaction is greater than that of normal catalytic reforming, the normal reaction deposited 5 times more coke than the hybrid reaction. Raman spectroscopy revealed that the superposition of the nonthermal discharge on catalysts produced less graphitized coke compared to the normal catalytic reaction, by which it is easily removed by H2O during steam reforming. Excited species produced by nonthermal discharge are so reactive that their reaction is completed only on the pellet surface: coke formation in the catalyst pore was only slightly detectable. In contrast, large amounts of coke were detected from the surface and catalyst pores in the normal reaction. In the hybrid reaction, CH4 and H2O are primarily excited by electron impact. Therefore, methane dehydrogenation followed by coke oxidation is promoted, resulting in a small amount of coke formation with greater methane conversion than in the normal reaction. The results are well-correlated with Arrhenius plot analysis of the overall forward rate constant for MSR in the hybrid reaction.
Introduction To promote the chemical reaction at low temperature using a compact reactor, atmospheric pressure nonthermal discharge is combined proactively with a conventional catalytic reaction system. Air pollution control methods, such as NOx decomposition, particulate matter removal, and voltaic organic compound (VOC) decontamination, using a catalytic plasma hybrid reaction have a long history; excellent reviews of them are available.1-4 More recently, fuel reforming using nonthermal discharge has attracted broad attention, with growing concern related to energy and environmental problems.5-8 In these applications, dielectric barrier discharge (DBD) is widely used for constructing hybrid reaction systems. Generally, 1-5 mm spherical dielectric catalyst pellets are packed into a glass or ceramic tube: a highvoltage electrode is embedded inside the pellets. As the applied voltage reaches a critical value, gas breaks down at the pellet contacts where the electric field is concentrated. Subsequently, several transient filamentary microdischarges propagate along the pellet surface with nanosecond duration: high-energy electron impacts produce various excited species, with better * To whom correspondence should be addressed. Telephone: +81-35734-2179. Fax: +81-3-5734-2893. E-mail:
[email protected]. (1) Kim, H.-H. Plasma Process. Polym. 2004, 1, 91–110. (2) Van Durme, J.; Dewulf, J.; Leys, C.; Van Langenhove, H. Appl. Catal., B 2008, 78, 324–333. (3) Mizuno, A. Plasma Phys. Controlled Fusion 2007, 49 (5A), A1– A15. (4) Okubo, M.; Arita, N.; Kuroki, T.; Yoshida, K.; Yamamoto, T. Plasma Chem. Plasma Process. 2008, 28 (2), 173–187. (5) Hammer, T.; Kappes, T.; Schiene, W. Utilization of greenhouse gases. ACS Symp. Ser. 2003, 852, 292–301. (6) Song, H. K.; Choi, J.-W.; Yue, S. H.; Lee, H.; Na, B.-K. Catal. Today 2004, 89, 27–33. (7) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Catal. Today 2004, 89, 57–65. (8) Nozaki, T.; Muto, N.; Kado, S.; Okazaki, K. Catal. Today 2004, 89, 67–74.
interaction with catalyst pellets. Consequently, the chemical reaction occurs much more rapidly than a normal catalytic reaction at a given temperature. In addition to reaction enhancement resulting from radical production, the heat generated by nonthermal discharge is known to promote a catalytic reaction,9 although few studies describe the detailed energy balance and related temperature increase of the catalyst bed. We have been studying low-temperature methane steam reforming (MSR) in DBD and catalyst hybrid reaction for profitable recovery of low-calorific biogas.7-10 Methane conversion in the hybrid reaction is promoted at low temperature in the sense that the reforming reaction takes place much more rapidly than for a normal reaction at a given temperature. On the other hand, methane conversion and resulting reforming gas components are governed strictly by thermodynamic equilibrium with respect to catalyst temperature. In this regard, the catalyst temperature was measured precisely using thermography. The overall rate constant for the methane dehydrogenation reaction was analyzed on the basis of the Arrhenius plot method.10 Different from a normal catalyst reaction, not only methane excitation by electron impact but also water-vapor excitation exhibit remarkable effects on the overall rate constant: excited methane promotes a dehydrogenation reaction on the catalyst, while excited water vapor oxidizes the carbon network efficiently and increases the number of active sites on catalysts. Consequently, the pre-exponential factor for the overall methane conversion rate was enhanced by a factor of 10-50 in the presence of DBD. The overall activation energy remained unchanged. Therefore, the rate-determining step of MSR (C-H activation energy of 100 kJ/mol) remains essentially unchanged (9) Nozaki, T.; Tsukijihara, H.; Okazaki, K. Energy Fuels 2006, 20 (1), 339–345. (10) Nozaki, T.; Tsukijihara, H.; Fukui, W.; Okazaki, K. Energy Fuels 2007, 21, 2525–2530.
10.1021/ef800461k CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
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Figure 2. Positive pulsed voltage and corresponding current waveforms. A similar current waveform is observed for negative pulsed voltage.
Figure 1. (a) Digital image of DBD produced in a catalyst bed. (b) Gas breakdown scheme in dielectric bed media. A more detailed reactor configuration is provided in a previous report.9
even in the presence of nonthermal discharge. In other words, gas-phase plasma chemistry creates no additional reaction pathways, which might decrease the activation energy of the overall MSR rate. In this paper, the reaction enhancement mechanism of MSR by nonthermal discharge is discussed in terms of coke deposited on Al2O3-supported Ni catalyst. MSR was performed with a small steam/carbon ratio (S/C ) 1) and low gaseous hourly space velocity (GHSV ) 7200 h-1). Coke was deposited intentionally with and without nonthermal discharge. Subsequently, the chemical nature and quantity of coke were characterized using temperature-programmed oxidation (TPO) analysis and micro-Raman spectroscopy. The overall reaction pathway can be traced with coke formation. Results reasonably support Arrhenius plot analysis of plasma-enhanced MSR. The reaction scheme described in this paper is beneficial not only for MSR but also for various DBD and catalyst hybrid reaction systems. Experimental Section Details of the experimental setup are described elsewhere.9 Briefly, a schematic diagram is presented in Figure 1a. Spherical Ni/Al2O3 pellets with 3 mm in diameter (ISOP; Su¨d-Chemie Catalysts Japan, Inc.11) were packed in a sapphire tube with a volume of 20 mm (inner diameter) × 50 mm (length): thickness of the sapphire tube was 1.5 mm. Catalyst pellets were reduced in N2/H2 (90:10 cm3/min) flow at 600 °C for 2 h in advance. A highvoltage electrode (stainless-steel rod ) 3 mm) was installed inside the bed; a grounded electrode, having a 10 mm slot as an observation port, was placed outside of the sapphire tube. Bipolar pulsed voltage was applied between the electrodes at 1-5 kHz repetition frequency. Positive pulsed voltage and corresponding current waveforms are presented in Figure 2. Spike-like current pulses of nanosecond duration appear at a rising part of pulsed voltage, implying that typical DBD was generated in the bed cavity. As depicted in Figure 1b, the electric field concentrates at several pellet contacts. Consequently, several filamentary microdischarges propagate along the pellet surface with nanosecond duration. Although the time-averaged image shows that DBD is generated uniformly in the bed cavity, DBD actually includes several filamentary transient microdischarges localized at several pellet (11) http://www.sud-chemie-jp.com/ (accessed on June 1, 2008). (12) Shamsi, A.; Baltrus, J. P.; Spivey, J. J. Appl. Catal., A 2005, 293, 145–152.
Figure 3. Temperature distribution of catalyst bed (a) without DBD in N2 flow and (b) hybrid reaction (endothermic enthalpy ) 42 W, discharge power ) 40 W).
contacts. The initial feedstock consists of N2, CH4, and H2O ) 460, 690, and 690 cm3 min-1, and gaseous products were analyzed using a gas chromatograph [GC-8A (TCD, FID); Shimadzu Corp.]. Solid carbon was detectable, but the carbon balance was obtained within 10% error: a trace amount of solid carbon gradually accumulated on catalysts during 60 min operation. Parts a and b of Figure 3 show temperature distributions of the catalyst bed. The bed temperature was maintained initially at 600 °C using a ceramic radiant heater. In the absence of DBD, the temperature distribution was fairly uniform in the N2 flow (Figure 3a). When a CH4/H2O mixture was supplied, the bed temperature near the reactor inlet decreased remarkably because MSR absorbs heat (Figure 3b). As the gas component approached thermal equilibrium downstream, MSR was slowed. Therefore, the temperature did not decrease. In fact, coke was mainly deposited on catalysts near the reactor inlet, whereas that packed downstream was less contaminated by carbonaceous matter. Therefore, catalyst pellets near the reactor inlet, packed in the first 10 mm, were investigated. It is also important to note that endothermic enthalpy and discharge power in Figure 3b were carefully adjusted so that a synergistic effect attributable to radical injection emerged.9,10 A synergistic effect is not expected if the discharge power is much greater than endothermic enthalpy. In that case, heat generated by nonthermal discharge governs MSR. We also mention that, even without a catalyst in nonthermal discharge, we might have the
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Table 1. Typical Appearance of Catalyst Pellets (3 mm in Diameter)a
a Two pellet types were distinguished in the hybrid reaction. Catalysts were sampled from the reactor inlet after 60 min of operation under the given conditions: S/C, 1; GHSV, 7200 h-1; bed temperature, 600 °C.
Figure 4. (a) TPO profiles for three samples (30 mg each): total amount of coke for type-A catalyst, 0.474 mg; type-B catalyst, 0.102 mg; and type-C catalyst, 2.46 mg. Deconvoluted TPO profiles for (b) type-A catalyst and (c) type-C catalyst.
situation of decreasing gas temperature caused by endothermic (steam-reforming) reactions.12
Results and Discussion Characterization of Coke. As presented in Table 1, catalyst pellets of two types were distinguished in the hybrid reaction. Type-A catalyst is partly covered by carbon film on its surface, where nonthermal discharge occurred intensively (see Figure 1b). The other catalyst (type B) appears to be uniform, and carbon deposition was not visible to the naked eye. Type-C catalyst, obtained from normal catalytic reaction, also appears to be uniform. However, several small black spots are visible on its surface. Three types of catalyst, sampled from the reactor inlet, were characterized by TPO analysis and Raman spectroscopy.
TPO Analysis. For TPO analysis, pellets of three types were pulverized using a mortar. Then, 30 mg of the powder sample was loaded into the vertically supported quartz tube with 4 mm of inner diameter. A helium and 10% oxygen mixture was introduced from the bottom at 20 cm3 min-1, while the sample temperature was increased to 100 °C and maintained for 10 min to remove moisture. Subsequently, the temperature was raised to 700 °C at the constant rate of 10 °C min-1. Carbon dioxide (CO2), which was formed as a result of coke oxidation, was measured quantitatively using a quadrupole mass spectrometer (Prisma; Pfeiffer Vacuum Technology AG). Carbon monoxide (CO) was also formed, but selectivity was less than 5% compared to the CO2 concentration. Figure 4a shows TPO profiles for three samples. Carbonaceous material was oxidized
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Figure 5. Raman spectra of (a) type-A catalyst surface and (b) type-C catalyst surface.
completely at 700 °C; refractory coke, which oxidizes at a higher temperature (>800 °C), is not produced. The total amount of carbon was estimated by integrating TPO profiles with respect to the oxidation time. The results are listed in the Figure 4 caption. Although methane conversion in the hybrid reaction is greater than normal catalytic reforming (see Table 1), the normal reaction produced 5 times more coke than the hybrid reaction. Type-A catalyst was partly covered by carbon film, and carbon precipitation was limited only on the pellet surface. As discussed in relation to results of Raman spectroscopy, the carbonaceous component was only slightly detected from the cross-section of type-A catalyst. A possible explanation is that the excited species are so reactive that methane dehydrogenation (or coke formation) is completed on the pellet surface and the excited species do not diffuse into catalyst pores. Interestingly, a trace amount of coke was detectable with type-B catalyst, where carbon precipitation was not recognized on its surface: type-B catalyst seems not to participate in the steam-reforming reaction. In contrast, coke was deposited in the catalyst pores in the normal reaction because ground-state methane diffuses further into catalyst pores and then dehydrogenates inside pellets. It is noteworthy that some catalysts are cracked or collapsed because of severe coke formation in the catalyst pores, which was not found in the hybrid reaction. Parts b and c of Figure 4 show that TPO profiles for type-A and type-C catalysts were deconvoluted with a Lorentzian profile. In the hybrid reaction (type A), four peaks are identified and two major peaks are separated respectively at 494 and 555 °C. The TPO profile for a normal reaction (type C) also consists of two small peaks and two large peaks at 543 and 591 °C. The peak temperatures for all peaks in the hybrid reaction are 30-50 °C lower than those observed in normal reaction: the hybrid reaction produces reactive coke. Therefore, it is easily removed by water vapor. The total amount of coke is much less than that in a normal reaction. Raman Spectroscopy. The 514 nm component of the Ar+ laser was used for micro-Raman spectroscopy (STR750 laser raman spectrometer; Seki Technotron Corp.). A pellet was crushed; then, both the catalyst surface and cross-section were investigated. The crystalline structure of coke was characterized by a graphite band (G-band: ca. 1580 cm-1), while imperfect graphite structures or an amorphous carbon network can be characterized by the defect band (D-band: ca. 1350 cm-1). Ratios of peak intensity and the full width at half-maximum (fwhm) of D-/G-band peaks are well-correlated with crystallinity of carbonaceous matter, which also have a good correlation with reactivity.13,14 In this analysis, these two peaks are deconvoluted, (13) Koizumi, N.; Urabe, Y.; Inamura, K.; Itoh, T.; Yamada, M. Catal. Today 2005, 106, 211–218.
Figure 6. D/G peak intensity and fwhm ratios: (9) type-A surface, ([) type-C surface, and (]) type-C cross-section. Raman spectra were rarely obtainable from the type-A cross-section.
assuming a Lorentzian profile. Figure 5 shows Raman spectra of coke formed on the catalyst surface of (a) type-A and (b) type-C catalysts. As discussed for TPO analysis, a trace amount of coke was deposited on the type-B catalyst; few Raman scattering spectra with a sufficient signal-to-noise ratio were obtained. In the hybrid reaction, peak intensity and fwhm of the D-band is much greater than that of the G-band, meaning that the coke is less graphitized than coke produced in the normal reaction. Ratios of peak intensity and the fwhm of D-/ G-band peaks are summarized in Figure 6. Spectra were obtained 5 times for each sample. The average value is presented with the error bar. Raman spectra were only slightly detectable in the cross-section of the type-A catalyst. Methane dehydrogenation in the hybrid reaction produces less graphitized carbon because the D/G ratio is greater than that of coke produced in the normal reaction, implying that the hybrid reaction produces more reactive coke. In fact, coke produced in the hybrid reaction is oxidized at a lower temperature in TPO than in the normal reaction. The results imply that MSR is promoted only on the catalyst surface where DBD is generated; catalyst pores do not contribute to the reaction enhancement mechanism. In contrast, a strong Raman signal was obtained from the surface and crosssection of type-C catalyst. The amount of coke and its crystallinity (reactivity) are well-correlated with TPO analysis. Reaction Enhancement Mechanism in the Hybrid Reaction. A tentative reaction mechanism is presented schematically in Figure 7. As discussed previously, MSR is promoted at pellet contacts, where filamentary discharge is formed preferentially. In these active regions, methane conversion might momentarily (14) He, F.; Liu, C.-J.; Eliasson, B.; Xue, B. Surf. Interface Anal. 2001, 32, 198–201.
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Figure 7. Reaction scheme of the hybrid reaction.
exceed chemical equilibrium. However, the excess amounts of H2 and CO/CO2 returned to CH4 and H2O on catalysts, where no discharge takes place because the reverse reaction (exothermic reaction) occurs more rapidly than the forward reaction. Consequently, the gas component at the reactor outlet is eventually governed by chemical equilibrium. There are several publications about the plasma-catalyst hybrid reaction for methane reforming, where gas components does not necessarily represent thermal equilibrium gas mixtures.15,16 However, we point out that catalyst temperature was not well-evaluated in these studies, although relatively large specific energy density (input power divided by total flow rate) was applied: catalyst temperature must be carefully evaluated for better correlation with gas composition. In addition, as far as MSR is concerned, methanation of CO2 (reverse reaction) was sufficiently faster than MSR (forward reaction), which was experimentally demonstrated in the previous study.9 Normal catalyst reactions might occur where filamentary discharges are absent. However, according to Raman spectroscopy and TPO analysis, the carbonaceous component was not detected from a cross-section of type-A and entire type-B catalysts: the methane dehydrogenation reaction via the excited species is completed on the catalyst surface, and they only slightly diffuse into internal catalyst pores. In contrast, methane dehydrogenation occurs at both surface and catalyst pores in the normal reaction. The superposition of nonthermal discharge on catalysts produces reactive coke than the normal catalytic reaction. Therefore, it is easily removed by water vapor. It is well-known that vibrationally excited methane promotes the dehydrogenation reaction on the transition-metal catalyst.10 In the same way, we speculate that vibrationally excited H2O should also play an important role to remove carbonaceous matter deposited on catalyst that eventually increases active sites on the pellet surface. In fact, H2O has many vibrational states, and those excited species are more reactive than the groundstate molecule.17 Furthermore, results of TPO analyses and Raman spectroscopy are well-correlated with kinetic analyses of the overall MSR reaction.10 Arrhenius plots for the normal catalytic reaction and hybrid reaction are presented in Figure 8. For the detailed analytical method, consult ref 10. The result shows that the pre-exponential factor for overall methane dehydrogenation was increased by a factor of 10-50, although the overall activation energy was fundamentally unchanged even (15) Liu, C.-J.; Xue, B.; Eliasson, B.; He, F.; Li, Y.; Xu, G.-H. Plasma Chem. Plasma Process. 2001, 21, 301–310. (16) Babaritskii, A. I.; Baranov, I. E.; Demkin, S. A.; Zhivotov, V. K.; Potapkin, B. V.; Rusanov, V. D.; Ryazantsev, E. I.; Etievan, C. High Energy Chem. 1999, 33, 404–408. (17) Yousfi, M.; Benabdessadok, M. D. J. Appl. Phys. 1996, 80, 6619– 6630.
Figure 8. Arrhenius plot for the forward CH4 reaction rate constant: (O and b) GHSV ) 18 000 h-1, S/C ) 1, (0 and 9) GHSV ) 18 000 h-1, S/C ) 3, and (4 and 2) GHSV ) 10 800 h-1, S/C ) 1.
in the presence of nonthermal discharge. Not only methane dehydrogenation but also coke removal by water vapor is promoted simultaneously on the active sites of the catalyst. However, the rate-determining step (C-H activation energy ) 100 kJ/mol) is not influenced by nonthermal discharge. That fact implies that gas-phase CH4 dehydrogenation by excited H2O or reactive species, such as OH, does not contribute to the overall reaction enhancement in the hybrid reaction. Concluding Remarks The reaction enhancement mechanism of MSR in the nonthermal discharge and catalyst hybrid reaction was investigated. Regarding MSR, the overall reaction pathway can be tracked in terms of coke formation: although such analysis is not always possible, several important aspects are beneficial for DBD/ catalyst hybrid reaction systems of other types. (1) DBD produced in the bed medium might be recognized as “uniform discharge” in the sense that filamentary microdischarges are widely distributed in the bed cavity. However, actual reaction sites are localized at several pellet contacts, where filamentary microdischarges are preferentially generated. (2) Excited species are so reactive that their reaction is completed at the pellet surface: it is unlikely that reactive species diffuse further into catalyst pores. Therefore, coke was only slightly detectable from the cross-section of the type-A catalyst. The type-C catalyst, obtained in the normal reaction, was heavily contaminated by coke at both surface and catalyst pores. (3) Excited methane promotes dehydrogenation on the catalyst, which produces more reactive coke than a normal reaction. The reactive coke is easily removed by ground-state H2O. In addition, vibrationally excited H2O is expected to enhance coke removal, which eventually increases reactive sites on catalysts. (4) Methane conversion might temporarily exceed the chemical equilibrium because of nonthermal discharge; however, excess H2 and CO/CO2 return to CH4 and H2O as a result of a fast reverse reaction: the gas component in the hybrid reaction is ultimately governed by thermodynamic equilibrium with respect to the catalyst temperature. Acknowledgment. This research was supported by the Ministry of Education, Culture, Sports, Science, and Technology and a Grantin-Aid for Exploratory Research (20656038), 2008-2009. EF800461K
