Effects of Catalysts in Carbon Dioxide Reforming of Methane via

Energy & Fuels 2006, 20, 1033-1038

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Effects of Catalysts in Carbon Dioxide Reforming of Methane via Corona Plasma Reactions Ming-Wei Li,*,† Cui-Ping Liu,† Yi-Ling Tian,† Gen-Hui Xu,‡ Feng-Cai Zhang,† and Ya-Quan Wang‡ Department of Chemistry and School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, P. R. China ReceiVed July 9, 2005. ReVised Manuscript ReceiVed February 14, 2006

The effects of two kinds of representative catalysts, Ni/Al2O3 and HZSM-5 zeolite, were investigated in the CO2 reforming of CH4 via corona plasma reactions at atmospheric pressure. The catalysts showed little effect on the conversions of CH4 and CO2, and we propose that rapid free-radical processes play a main role during the plasma reactions. However, although a high-voltage electric field was used to initiate the corona discharges and many discharged particles were present within the plasma, which possibly affected the properties of the solid catalysts, Ni/Al2O3 and HZSM-5 zeolite exhibited catalytic selectivity and activity in the reactions. When the catalysts were used in the reactions, the H2/CO ratios in the products were modified, and more hydrocarbons and oxygenates were formed. It was found that a small amount of phenol was directly synthesized in the plasma reactions when HZSM-5 zeolite was used. Thermogravimetric analyses revealed that the coke depositing over the different catalysts was of different types. It is proposed that the coke over Ni/Al2O3 mainly originated from CH4 decomposition, even though a comparative portion of the coke originated from CO disproportionation. More coke deposited over HZSM-5 zeolite than over Ni/Al2O3, and it contained more hydrocarbons and oxygenates, simply reflecting the characteristics of the catalyst.

Introduction In recent years, the carbon dioxide reforming of methane, or the so-called dry reforming of methane, has attracted much attention in academia and industry because of its advantages over the steam reforming or partial oxidation of methane.1-3 CO2 reforming of CH4 not only converts both greenhouse gases of CO2 and CH4, but also yields syngas with a lower H2/CO molar ratio, which is a preferable feedstock for the synthesis of liquid hydrocarbons or oxygenates. We have investigated the CO2 reforming of CH4 via corona plasma reactions whereby electricity provides energy for this intensely endothermic process.4 In the plasma reactions, higher conversions of CH4 and CO2 are easily achieved, and the H2/CO ratios in products are conveniently controlled by adjusting the CH4/CO2 ratios in the feeds. Additionally, the corona plasma has comparatively low gas temperature, which decreases the coke deposition during the reactions. However, the energy efficiency of the plasma reactions is low (∼10%) at present, and much work needs to be done to increase the energy efficiency before the nonequilibrium plasma process can become a competitive alternative to conventional catalytic methods. It is very interesting to apply appropriate catalysts in plasma reactions. There have been reports that catalysts can increase the conversions of reactants or energy efficiency in plasma * Corresponding author. E-mail: [email protected]. † Department of Chemistry. ‡ School of Chemical Engineering and Technology. (1) Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, P. D. F. Nature 1991, 352, 225-226. (2) Edwards, J. H.; Maitra, A. M. Fuel Process. Technol. 1995, 42, 269289. (3) Bradford, M. C. J.; Vannice, M. A. Catal. ReV.-Sci. Eng. 1999, 41, 1-42. (4) Li, M.-W.; Xu, G.-H.; Tian, Y.-L.; Chen, L.; Fu, H.-F. J. Phys. Chem. A 2004, 108, 1687-1693.

reactions.5-7 A previous study using a Langmuir probe indicated that the use of zeolite markedly increased the electron temperature of plasma and clearly reduced the discharge power, so a mechanism of catalyst-enhanced nonequilibrium of plasma was proposed.8 On the other hand, it is also attractive that highvalue chemicals can be directly catalytically synthesized from CO2 and CH4 via nonequilibrium plasma technology. It has been reported that some hydrocarbons and oxygenates can be directly formed in this way.9-11 However, there is a concern that the electric field used to initiate discharge and the charged particles existing within the plasma might modify the properties of the solid catalysts. Such modification possibly results from the changes of binding energies of chemisorbed species. Consequently, it is necessary to study the effects of catalysts in the CO2 reforming of CH4 under plasma conditions. In this article, we report the performance in plasma reactions using CO2 and CH4 as reactants of two kinds of typical catalysts: Ni/Al2O3, which is a representative catalyst for the CO2 reforming of methane, and HZSM-5 zeolite, which is a representative catalyst for the synthesis of hydrocarbons from syngas. The products were analyzed by means of gas chromatography and quadrupole mass spectrometry. The characteristics of coke deposition over the catalysts were investigated by (5) Kizling, M. B.; Ja¨rås, S. G. Appl. Catal., A 1996, 147, 1-21. (6) Suib, S. L.; Brock, S. L.; Marquez, M.; Luo, J.; Matsumoto, H.; Hayashi, Y. J. Phys. Chem. B 1998, 102, 9661-9666. (7) Brock, S. L.; Marquez, M.; Suib, S. L.; Hayashi, Y.; Matsumoto, H. J. Catal. 1998, 180, 225-233. (8) Liu, C.-J.; Wang, J.-X.; Yu, K.-L.; Eliasson, B.; Xia, Q.; Xue, B.; Zhang Y.-H. J. Electrostat. 2002, 54, 149-158. (9) Liu, C.-J.; Mallinson, R.; Lobban, L. Appl. Catal., A 1999, 178, 1727. (10) Eliasson, B.; Liu, C.-J.; Kogelschatz, U. Ind. Eng. Chem. Res. 2000, 39, 1221-1227. (11) Zhang, K.; Eliasson, B.; Kogelschatz, U. Ind. Eng. Chem. Res. 2002, 41, 1462-1468.

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Figure 1. Schematic diagram of the process of CO2 reforming via corona plasma using a catalyst: A, high-voltage dc source; B, wire electrode; C, plate electrode; D, quartz tube; E, quadrupole mass spectrometer; F, cold trap; G, gas chromatograph; H, flowmeter; I, corona discharge; J, catalyst granules.

Li et al. mass flowmeters. The reactants were well-mixed and flowed through the reactor at room temperature and atmospheric pressure. However, the temperatures of the reactants increased as they passed through the plasma area as a result of the conversion of electric energy into heat energy, but they did not exceed 420 K as measured by a method described in the literature.12 Under each set of conditions, 60 min was allowed for stabilization before product analyses. Product Analyses and Calculations. The reactor effluent was first qualitatively analyzed by an on-line quadrupole mass spectrometer (QMS, Balzers MSC 200). The measurement range of the QMS is between 0 and 200 amu. The compounds in the effluent were detected by monitoring the signals of their peaks. In this work, we focused on the signals of hydrocarbons and oxygenates. The main peaks of benzene (C6H6), toluene (C7H8), and phenol (C6H6O) are at 78, 91, and 94 amu, respectively. Then, the effluent was introduced into a cold trap to remove the liquid products. Finally, the gas products were quantitatively analyzed by an on-line gas chromatograph, and their flow rates were measured by a soap-film flowmeter in order to carry out balance calculations of the elements. The conversions (X) of CH4 and CO2, selectivities (S) of H2 and CO, and balance calculation (B) of carbon are defined as follows: XCH4 )

thermogravimetric analyses, and the formation mechanisms of coke are discussed.

XCO2 )

Experimental Section Catalyst Preparation. The Ni/Al2O3 samples were prepared by an impregnation method. γ-Al2O3 granules with a particle fraction of 40-60 mesh were immersed in an aqueous solution of Ni(NO3)2‚ 6H2O (0.5 M), and the nickel oxide loading of the catalyst was 10 wt %. The suspension thus obtained was stirred for 1 h at room temperature and evaporated to dryness. Then, the particles were calcined in an oven at 923 K (heating rate, 3 K/min) for 24 h to decompose the metal precursor. For each experiment, a Ni/Al2O3 sample was placed in the reactor and was reduced in situ at 1073 K for 2 h in a dilute hydrogen flow at atmospheric pressure before use. Samples of HZSM-5 zeolite (purchased from the Chemical Plant of Nankai University, SiO2/Al2O3 ) 38) were pressed into tablets, crushed, and sieved to obtain a particle fraction of 40-60 mesh. Then, they were calcined at 723 K for 2 h to remove the water adsorbed in the channels of the zeolite before use. Catalyst Test. The catalysts were tested in a fixed-bed continuous-flow quartz reactor with an inner diameter of 13.2 mm. The corona plasma apparatus, shown in Figure 1, is similar to that employed previously.4 The main differences are that a catalyst was applied and a new high-voltage power supply was used in this case. The quartz tubular reactor consists of a wire-plate stainless steel electrode configuration. Catalyst granules (0.2 g) were put on a sheet of stainless steel wire mesh (200 mesh) at the plate electrode to avoid their leaking. The upper wire electrode was positioned with its top 10 mm above the catalyst. A pulsed dc power supply with a high-voltage transformer of 0-100 kV rms (root-meansquare) was used to initiate corona discharges. It was found that coke formed in the plasma reactions without catalysts when the CH4/CO2 molar ratio was higher than 2/1 and the coke tended to deposit onto the cathode in a dc electric field.4 To decrease the possible formation of coke over the catalyst lying on the plate electrode, reactants with a lower CH4/CO2 ratio (i.e., 1/2) were used in this study. Additionally, negative corona discharge was applied, i.e., the plate electrode was grounded as the anode, and the wire electrode was at a negative potential as the cathode. Experimentally, there was no obvious coke formation at the wire cathode. The typical breakdown voltage was 5-6 kV, and the discharge power was measured by electronically integrating the product of voltage and current. The flow rates of the two reactants, CH4 (>99.9%) and CO2 (>99.5%) in a molar ratio of CH4/CO2 ) 1/2, were controlled by

SH2 ) SCO )

[CH4]in - [CH4]out [CH4]in [CO2]in - [CO2]out [CO2]in

× 100%

× 100%

[H2]out 1 × 100% 2 [CH4]in - [CH4]out [CO]out

[CH4]in - [CH4]out + [CO2]in - [CO2]out

(

BC ) 1 -

)

[CH4]out + [CO2]out + [CO]out [CH4]in + [CO2]in

× 100%

× 100%

where [CH4]in and [CO2]in are the flow rates of the introduced reactants and [CH4]out, [CO2]out, [H2]out, and [CO]out are the flow rates of the corresponding compositions in the effluents. Characterization of Coke and Catalysts. The coke deposition on the catalysts was analyzed using a thermal gravimeter (TG, Shimadzu TG-50) with a workstation (TA-50WSI) for collecting data. Experiments were performed in an air flow of 50 mL/min at atmospheric pressure. About 10 mg of catalyst that had been used in the plasma reaction for 2 h was put into a silica pan and was heated from room temperature to 950 K at a heating rate of 15 K/min. The differential thermogravimetric (DTG) data were obtained directly from the TG data using the workstation software. The crystal structures of the fresh and used Ni/Al2O3 samples were characterized by X-ray diffraction (XRD, Rigaku D/Max) with a Cu KR irradiation source.

Results and Discussion General Situation. For a better understanding of the effects of catalysts and discharge power on the conversions of reactants under plasma conditions, we performed experiments at different discharge powers using Ni/Al2O3 and HZSM-5 zeolite as catalysts. As shown in Figure 2, regardless of whether a catalyst was used, the conversions of CH4 and CO2 increased with increasing discharge power from 18 to 42 W. The conversion of CH4 changed from 38.1% and 69.5%, and that of CO2 changed from (12) Marafee, A.; Liu, C.; Xu, G.; Mallinson, R.; Lobban, L. Ind. Eng. Chem. Res. 1997, 36, 632-637.

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Figure 2. Effects of catalysts and discharge power on the conversions of (a) CH4 and (b) CO2 in the plasma CO2-reforming reaction. Flow rate, 60 mL/min; CH4/CO2 ratio, 1/2; reaction time, 1 h.

31.7% to 58.6%. The conversions of the reactants did not change markedly when the catalysts were used, similarly to results reported for catalytic dielectric-barrier discharges with zeolites.10,11 We speculate that these results reflect the characteristics of plasma reactions. Within plasma exist many activated species, including free electrons, free radicals, ions, activated molecules, etc. It is supposed that the activation and conversion of reactants is mainly realized through rapid free-radical reactions. The activity of catalysts is concealed because of the comparatively slower progress of heterogeneous catalytic processes, which mainly occur on the catalyst surface and consist of several steps: diffusion, chemisorption, surface reactions, and desorption. It was found that the selectivity of H2 decreased with increasing discharge power (Figure 3a), whereas the selectivity of CO exhibited the opposite behavior (Figure 3b). Here, a simplified reaction mechanism is proposed to explain the difference in the change in selectivities with discharge power between H2 and CO. The initial step of the free-radical reactions is the dissociation of the reactant molecules. Generally, the free electron energy within a corona plasma lies at around 5 eV13 and increases with increasing discharge power. CH4 and CO2 molecules, whose dissociation energies are 4.5 and 5.5 eV, respectively, dissociate as a result of electron collisions (i.e., CH4 + e- f CH3 + H + e- and CO2 + e- f CO + O + e-). Because of the lower dissociation energy of CH4 compared to CO2, the conversion of CH4 was higher than that of CO2 at the same discharge power even though the CH4/CO2 ratio was 1/2, as shown in Figure 2. H2 could form via the recombination of hydrogen atoms (i.e., H + H f H2) and other free-radical (13) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19, 1063-1077.

Figure 3. Effects of catalysts and discharge power on (a) the selectivity of H2, (b) the selectivity of CO, and (c) the H2/CO ratio in products obtained by the plasma CO2-reforming reaction. Flow rate, 60 mL/ min; CH4/CO2 ratio, 1/2; reaction time, 1 h.

reactions, such as H + CHx f H2 + CHx-1 (x ) 1-4). When the discharge power was increased, more CO2 and CH4 molecules dissociated. However, CO molecules have a higher dissociation energy of 11.1 eV and are difficult to dissociate, so the selectivity and yield of CO increased with increasing discharge power. On the other hand, the oxygen atoms (O), which mainly originate from the dissociation of CO2 molecules, could react with H2 or H and form H2O molecules, inducing the selectivity of H2 to decrease with increasing discharge power. Catalytic Performance. Experimentally, the catalysts did play a role in the plasma reactions. As depicted in Figure 3a and b, when Ni/Al2O3 was used as a catalyst, the selectivities of H2 were lower than those measured when no catalyst or when HZSM-5 zeolite was used, but the selectivities of CO were higher. In contrast, there were lower selectivities of H2 and higher selectivities of CO when HZSM-5 zeolite was used as a

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Table 1. Effects of the Catalysts on the Reactions Occurring during the CO2 Reforming of CH4 via Corona Plasmaa sample

H2/CO ratio

XCH4 (%)

XCO2 (%)

SH2 (%)

SCO (%)

BC (%)

no catalyst Ni/Al2O3 HZSM-5

0.63 0.59 0.65

60.4 59.2 58.9

48.0 50.9 50.1

74.2 73.2 77.0

90.6 91.5 87.3

4.9 4.6 6.8

a Flow rate, 60 mL/min; CH /CO ratio, 1/2; discharge power, 30 W; 4 2 reaction time, 1 h.

catalyst. These differences caused the H2/CO ratios in the products to increase in the order Ni/Al2O3 < no catalyst < HZSM-5, as shown in Figure 3c. Some typical results are listed in Table 1. The balance calculations for carbon (BC) indicated that some carbon formed carbon-containing compounds other than CO. There was no obvious coke deposition, as a CH4/CO2 ratio of 1/2 was used in the reactions without a catalyst, so the carbon-containing products other than CO should be in the gas phase and mainly consisted of C2-C6 hydrocarbons, which were detected by QMS analyses. However, obvious coke deposited over Ni/Al2O3 and HZSM-5 zeolite after 2 h of reaction, indicating that solid and/ or liquid carbon-containing products formed when the catalysts were used. According to the carbon balance calculations, it was estimated that no more than 7% of the carbon was transformed into hydrocarbons and oxygenates in our work, which is similar to the result reported for dielectric-barrier discharge with catalysts.11 Some water also formed in the plasma reactions according to the hydrogen and oxygen balance calculations and QMS analyses. We speculated that reactions producing water occurred in the reaction system, such as CH4 + 2CO2 f H2 + 3CO + H2O. From Figure 3 and Table 1, it can be seen that Ni/Al2O3 used in the reactions increased the selectivity of CO and decreased the amounts of carbon-containing compounds other than CO. These results arise from the effects of both the catalyst and the nonequilibrium plasma. It has long been known that Ni-based catalysts exhibit a high initial activity for the conversion of CH4 and CO2.14-18 However, Ni-based catalysts usually suffer from rapid deactivation as a result of coke deposition, which hinders their use in industrial applications. It has been accepted by most researchers that CO2 molecules are readily chemisorbed and dissociated into CO on the surface of Ni-based catalysts. Consequently, compared to the case of not using a catalyst, reactions using Ni/Al2O3 showed a higher selectivity of CO in this work. According to thermodynamic calculations,19 the origin of inactive carbon during the CO2 reforming of CH4 might occur via CH4 decomposition (i.e., CH4 f C + 2H2) or CO disproportionation (i.e., 2CO f C + CO2). CH4 decomposition is endothermic, and its equilibrium constant increases with increasing temperature. Conversely, CO disproportionation is exothermic, and its equilibrium constant decreases with increasing temperature. Nonequilibrium plasma has a low-temperature characteristic that is a benefit in limiting coke deposition arising from CH4 decomposition. Additionally, there is indirect evidence for the low-temperature character of corona plasma. It has been reported that γ-Al2O3 is unstable at higher temperatures (>973 (14) Gadalla, A. M.; Bower, B. Chem. Eng. Sci. 1988, 43, 3049-3062. (15) Cheng, Z.; Wu, Q.; Li, J.; Zhu, Q. Catal. Today 1996, 30, 147155. (16) Wang, S.; Lu, G. Q. Appl. Catal., A 1998, 169, 271-280. (17) Choi, J.-S.; Moon, K.-I.; Kim, Y. G.; Lee, J. S.; Kim, C.-H.; Trimm, D. L. Catal. Lett. 1998, 52, 43-47. (18) Wang, S.; Lu, G. Q. Ind. Eng. Chem. Res. 1999, 38, 2615-2625. (19) Reitmeier, R. E.; Atwood, K.; Bennett, H. A., Jr.; Baugh, H. M. Ind. Eng. Chem. 1948, 40, 620-626.

K) and should transform into R-Al2O3.20,21 In our work, XRD analyses were used to characterize the crystal structures of the fresh and used Ni/Al2O3 catalyst. It was found that the used γ-Al2O3 did not transform into R-Al2O3 after being subjected to the plasma reaction for 2 h, implying that the corona plasma exhibited low-temperature characteristics. Therefore, the application of Ni/Al2O3 catalyst and nonequilibrium plasma increased CO2 dissociation and limited coke deposition arising from CH4 decomposition. However, because of the lower dissociation energy of CH4 compared to that of CO2 and because free-radical processes dominate plasma reactions, the conversions of CH4 were also higher than those of CO2, as shown in Figure 3 and Table 1. Therefore, it is suggested that the coke over Ni/Al2O3 still mainly originated from CH4 decomposition and formed on the Ni/Al2O3 surface following a mechanism discussed shortly. As for HZSM-5 zeolite, it is proposed that HZSM-5 zeolite not only enhanced the conversion of CH4 and produced more H2, but also promoted the continuous conversion of H2 and CO and formation of more hydrocarbons and oxygenates. HZSM-5 zeolite is a conventional catalyst used for the synthesis of hydrocarbons from methanol, and the acid sites on HZSM-5 zeolite play an important role in its catalysis.22,23 Much H2 and CO was present within the plasma system, and previous studies have reported that higher hydrocarbons are directly formed within CO2 and CH4 plasma systems when zeolites are used as catalysts.10,11 Therefore, it is likely that HZSM-5 zeolite promoted the direct synthesis of hydrocarbons and oxygenates in our plasma system and that the coke depositing over the zeolite contained more kinds hydrocarbons and oxygenates. Subsequent QMS and TG analyses were used to test this hypothesis. QMS Analyses of the Products. To clarify the roles played by the catalysts, QMS spectra of the reactor effluents were recorded, as shown in Figure 4. The spectra reflect the results of three plasma reactions, including use of no catalyst (Figure 4a), use of Ni/Al2O3 (Figure 4b), and use of HZSM-5 zeolite (Figure 4c). Hydrocarbons and other compounds formed in all three plasma systems, including benzene (78 amu) and toluene (91 amu). The amounts of benzene and toluene increased with increasing discharge power from 18 to 42 W. Peaks were also present at 63, 67, and 74 amu that probably are the peaks of C5 and C6 hydrocarbons. For example, the main peak of pentyne (C5H8) is at 67 amu, and the peaks of 1,5-hexadiyne (C6H6) include 63, 74, and 78 amu. Different compounds were also detected in the three systems. Compared to the case in which no catalyst was used, more peaks appeared in the QMS spectra when Ni/Al2O3 was used, implying more kinds of compounds forming. It is notable that there is a peak at 84 amu for a discharge power of 42 W, which is considered to be the main peak of cyclohexane (C6H12). The spectra of HZSM-5 zeolite catalytic system also showed more peaks, implying that more kinds of compounds formed. The peak at 106 amu is proposed as the weak peak of xylene (C8H10), whose main peak is at 91 amu. New peaks appeared at 70 and 72 amu in the spectra compared to the other two cases. Notably, some phenol, whose main and weak peaks are at 94 (20) Roh, H.-S.; Jun, K.-W.; Baek, S.-C.; Park, S.-E. Catal. Lett. 2002, 81, 147-151. (21) Zhang, Y.; Xiong, G.; Sheng, S.; Yang, W. Catal. Today 2000, 63, 517-522. (22) Haw, J. F.; Song, W.; Marcus, D. M.; Nicholas, J. B. Acc. Chem. Res. 2003, 36, 317-326. (23) Wang, W.; Buchholz, A.; Seiler, M.; Hunger, M. J. Am. Chem. Soc. 2003, 125, 15260-15267.

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Figure 5. TG curves (s) and DTG curves (- - -) during heating under air of (a) used Ni/Al2O3 and (b) used HZSM-5 zeolite. Flow rate, 60 mL/min; CH4/CO2 ratio, 1/2; discharge power, 42 W; reaction time, 2 h.

Figure 4. QMS spectra of the reactor effluents with (a) no catalyst, (b) Ni/Al2O3 catalyst, and (c) HZSM-5 zeolite catalyst at different discharge powers. Flow rate, 60 mL/min; CH4/CO2 ratio, 1/2; reaction time, 1 h.

and 66 amu, respectively, was detected and identified by comparison with a pure phenol sample. The present results are qualitative but indicate that HZSM-5 zeolite played a catalytic role in the synthesis of phenol in the plasma reaction, which should be an interesting finding for researchers pursuing the study of phenol synthesis. Phenol is one of the most important intermediates for the chemical industry. Presently, almost all of the industrial production of phenol is performed via the complicated three-step oxidation of benzene. Much acetone is produced as a byproduct of this process, and the demand for acetone is now decreasing.24 Therefore, it is highly desirable to replace the current process with a one-step, byproduct-free

process, and it is interesting that phenol formed during the plasma reactions in this study. The main peak of phenol at 94 amu decreased with increasing discharge power, whereas the main peak of benzene at 78 amu increased at the same time, indicating that lower discharge powers are advantageous for the synthesis of phenol. One explanation for this finding is that the dissociation energy of C6H5-OH (4.64 eV) is lower, so it easily decomposed when the discharge power increased. Alternatively, the oxidation of benzene is exothermic, and the equilibrium constant decreases with increasing temperature, so a lower discharge power or lower gas temperature is advantageous for the synthesis of phenol. The results encourage us as a new method for the direct synthesis of phenol via oxidation of benzene using HZSM-5 zeolite as a catalyst. Additionally, the synthesis of phenol indicated that HZSM-5 zeolite could play a catalytic role in the corona plasma initiated by a high-voltage electric field. TG Analyses of the Catalysts. It is well-known that coke deposition occurs widely during the catalytic conversion of hydrocarbons and is the main cause of catalyst deactivation in such processes. TG analysis is a means of characterizing coke. Figure 5a and b shows the TG-DTG curves of used Ni/Al2O3 and used HZSM-5 zeolite, respectively. These plots reflect different characteristics of coke. As shown in Figure 5a, the TG curve indicates that the coke deposited on Ni/Al2O3 was mainly oxidized between 650 and 870 K, suggesting that it is a low-temperature type. The sample (24) Ren, Y. L.; Wang, L.; Zhang, X. W. Prog. Chem. 2003, 15, 420426.

1038 Energy & Fuels, Vol. 20, No. 3, 2006

weight increased at around 630 K as a result of oxidation of nickel. Three peaks appear in the DTG curve, indicating three types of carbon: CR, Cβ, and Cγ.25-27 The first peak at 330 K probably originates from a highly reactive surface carbon species (called CR). The second peak at 480 K reflects a less reactive surface carbon species (Cβ), which possibly formed through polymerization and rearrangement of CR. The third peak at about 740 K reflects a poorly reactive carbon (Cγ), possibly a graphitelike phase. Unlike the Ni/Al2O3 sample, more coke deposited on the HZSM-5 zeolite, as depicted in Figure 5b. This result is consistent with the carbon balance calculations reported in Table 1. The DTG curve includes two peaks at around 350 and 900 K. It is proposed that the peak at 350 K is caused by the loss of water absorbed in the zeolite channels, and the other peak at 900 K reflects the oxidation of graphite-like coke, which mainly occurred in the temperature range between 740 and 920 K. Mechanism of Coke Deposition. The QMS spectra and TGDTG curves reflect different characteristics of the products and coke formed in the different reactions and imply different mechanisms of coke deposition. As mentioned earlier, the application of Ni/Al2O3 catalyst increased CO2 dissociation. The highly reactive surface carbon species (CR) is suggested to originate mainly from CO disproportionation on the nickel surface. However, the dissociation energy of CH4 is lower than that of CO2, and we propose that the free-radical processes are the main mechanisms in the plasma reactions. Therefore, the main coke originates from CH4 decomposition, and the poorly reactive type of carbon (Cγ) forms via dehydrogenation of aromatics. It is speculated that the free electrons in the plasma collide with the CH4 molecules physisorbed on the Ni surface. The collisions distort the CH4 molecules from their tetrahedral configuration, thereby lowering the barrier to dissociate them into adsorbed methyl (CH3) species and hydrogen atoms. The adsorbed CH3 dissociates into CH, and the CH recombines to form adsorbed C2H2 via a mechanism discussed in the literature.28,29 The adsorbed C2H2 species can react with adsorbed CH, and they also could dimerize or trimerize into a series of hydrocarbons including pentyne (C5H8), cyclohexane (C6H12), (25) Koerts, T.; Deelen, M. J. A. G.; van Santen, R. A. J. Catal. 1992, 138, 101-114. (26) Trimm, D. L. Catal. Today 1997, 37, 233-238. (27) Yan, Z.-F.; Ding, R.-G.; Song, L.-H.; Qian, L. Energy Fuels 1998, 12, 1114-1120. (28) Yang, Q. Y.; Johnson, A. D.; Maynard, K. J.; Ceyer, S. T. J. Am. Chem. Soc. 1989, 111, 8748-8749. (29) Choudhary, T. V.; Aksoylu, E.; Goodman, D. W. Catal. ReV. 2003, 45, 151-203.

Li et al.

and benzene (C6H6), whose main peaks in QMS spectra are at 67, 84, and 78 amu, respectively. Unlike the case for Ni/Al2O3, for HZSM-5 zeolite, it is suggested that the coke contains not only a graphite-like phase, but also various solid and liquid hydrocarbons and oxygenates formed under its catalytic action. Many CO and H2 molecules exist in the plasma and can be used to synthesize further hydrocarbons and oxygenates. The TG curve shown in Figure 5b is smoother than that shown in Figure 5a, and it is speculated that there are two causes for this. One is that various compounds have different oxidation temperatures, and the other is that the coke mainly deposits in the channels of HZSM-5 zeolite, so the rates of coke oxygenation and sample weight loss are mainly controlled by the diffusion of gases into the channels. Within the plasma system, an electric field and many charged particles exist, and they possibly affect the character of the catalysts or the movement of particles. For example, we found previously that coke mainly deposits on the cathode during dc corona plasma reactions.4 In this study, we confirmed that the electric field does not completely keep the catalysts from playing a catalytic role. However, a detailed understanding of the catalytic behavior of Ni/Al2O3 and HZSM-5 zeolite in CH4 and CO2 mixtures under plasma conditions requires further study. Conclusions In this work, the roles of catalysts in the CO2 reforming of CH4 via plasma reactions were investigated. Although freeradical reactions play a main role during the plasma reactions, the representative catalysts, Ni/Al2O3 and HZSM-5 zeolite, showed their particular catalytic effects. The application of a catalyst made the selectivities of H2 and CO change, which induced a modification of the H2/CO ratios in the products. It is notable that HZSM-5 zeolite showed a catalytic effect for the direct synthesis of phenol within the plasma CO2-reforming system. TG-DTG analyses revealed that the coke deposits on Ni/Al2O3 and HZSM-5 are of different types. It is proposed that the coke deposited over Ni/Al2O3 originates from both CH4 decomposition and CO disproportionation. More coke formed on HZSM-5 zeolite, and this coke included more hydrocarbons and oxygenates synthesized by the catalytic effect of HZSM-5 zeolite. This study makes it clear that an appropriate catalyst could play a catalytic role in plasma reactions, even in the presence of a high-voltage electric field used to initiate corona discharge. Acknowledgment. The support of the Natural Science Foundation of China (Project 20476071) is greatly appreciated. EF050207J