Journal of Natural Gas Chemistry 19(2010)628–637

Application of in-plasma catalysis and post-plasma catalysis for methanepartial oxidation to methanol over a Fe2O3-CuO/γγγ-Al2O3 catalyst

Lin Chen1,2, Xingwang Zhang1, Liang Huang1, Lecheng Lei1∗

1. Institute of Industrial Ecology and Environment, Zhejiang University, Hangzhou 310027, Zhejiang, China;2. Institute of Environmental Science and Engineering, College of Materials and Environmental Engineering,

Hangzhou Dianzi University, Hangzhou 310018, Zhejiang, China

[Manuscript received April 20, 2010; revised August 17, 2010 ]

AbstractMethane partial oxidation to methanol (MPOM) using dielectric barrier discharge over a Fe2O3-CuO/γ-Al2O3 catalyst was performed. Themulticomponent catalyst was combined with plasma in two different configurations, i.e., in-plasma catalysis (IPC) and post-plasma catalysis(PPC). It was found that the catalytic performance of the catalysts for MPOM was strongly dependent on the hybrid configuration. A bettersynergistic performance of plasma and catalysis was achieved in the IPC configuration, but the catalysts packed in the discharge zone showedlower stability than those connected to the discharge zone in sequence. Active species, such as ozone, atomic oxygen and methyl radicals, wereproduced from the plasma-catalysis process, and made a major contribution to methanol synthesis. These active species were identified by themeans of in situ optical emission spectra, ozone measurement and FT-IR spectra. It was confirmed that the amount of active species in the IPCsystem was greater than that in the PPC system. The results of TG, XRD, and N2 adsorption-desorption revealed that carbon deposition on thespent catalyst surface was responsible for the catalyst deactivation in the IPC configuration.

Key wordsmethane partial oxidation to methanol; plasma catalysis combination; synergistic performance; catalytic stability

1. Introduction

Recently due to the rapid depletion of oil reserves andstrong volatility in the oil price, alternative methods for syn-thesizing hydrocarbon fuels have again received considerableattention. The world’s seafloors contain abundant reserves ofnatural gas in gas or solid form called natural gas hydrates.These natural gas hydrates contain methane, whose potentialtransformation into more valuable methanol has acquired sig-nificant interest. However, due to the high stability of the C–Hbond, the direct conversion of methane into methanol typi-cally requires extreme reaction conditions, such as high tem-perature (400−1000 ◦C) and high pressure (>10 atm), whichlimits its industrial application [1,2]. Consequently, the appli-cation of non-thermal plasma such as a dielectric barrier dis-charge (DBD) appears to be an important alternative way toproduce chemicals under ambient conditions [3]. The DBD isan efficient source of high energetic electrons with 1−10 eV

and high density, which are able to initiate plasma methanereforming reactions [4−6]. Unfortunately, plasma does notusually allow for achieving high levels in both methane con-version and aimed product selectivity. The processes involvedplasma are always less selective than catalytic processes, lead-ing to greatly complex product distributions. A possible wayto overcome this problemmight be to exploit the inherent syn-ergetic potential between plasma mediated reactions and het-erogeneous catalysts [7,8]. This combination can be operatedin two configurations: in-plasma catalysis (IPC) with the cata-lyst inside the reactor, or post-plasma catalysis (PPC) with thecatalyst downstream of the plasma unit. The role of the plasmais highly dependent on the two configurations [9,10]. In a PPCconfiguration, plasma do not directly interact with catalysisbecause reactive species produced in the discharge zone havesuch short lifetimes that they disappear before reaching thecatalyst surface. The plasma can provide chemically reactivespecies for further catalysis or pre-convert the reactants into

∗ Corresponding author. Tel: +86-571-88273090; Fax: +86-571-88273916; E-mail: lclei@zju.edu.cnThis work was supported by the National Natural Science Foundation of China (No. 20836008 and U0633003), the Zhejiang Provincial Natural Science

Foundation of China (No. Y5080192), the Project of Science and Technology Department of Zhejiang Province of China (2007C13061), MOST Project ofChina (No. 2007AA06Z339, No. 2008BAC32B06 and No. 2007AA06A409) and the Open Project Program of Key Laboratory of Non-point Sources PollutionControl, the Ministry of Agriculture of the People’s Republic of China and Science Foundation of Chinese University.

Copyright©2010, Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. All rights reserved.doi:10.1016/S1003-9953(09)60129-8

Journal of Natural Gas Chemistry Vol. 19 No. 6 2010 629

more easily converted products to accelerate the catalysis.While in an IPC configuration, both plasma and catalysis takeplace simultaneously and interact with each other. The perfor-mance enhancement mechanisms are rather complicated. Inour previous work [11], there was a synergistic effect betweencold plasma and a ceramic supported Fe2O3 or a Fe2O3-CuOcomposite catalyst for MPOM in the PPC process. However,when the catalyst was introduced to the IPC configuration, lit-tle synergistic performance was observed. It is well knownthat supports play an important role in determining the natureand number of active sites, and consequently in the activity ofthe catalysts. For the IPC configuration, if the catalyst has alarge specific surface area, it may provide a large adsorptioncapacity for reactant molecules, which in turn would increasethe collision probability between the reactant molecules andshort-lived excited species [12]. Liu et al. [13] conductedplasma catalytic methane conversion in the IPC configuration.They used zeolite X and zeolite A to improve the methaneconversion and product selectivity. Ni/γ-Al2O3 catalyst pre-sented an exciting synergetic effect with the plasma, on whichthe conversion of reactants at 573 K reached the same resultsas pure catalytic reaction at 1073 K [14].

Accordingly, these previous studies stimulated us to fur-ther consider the role of adsorption in the plasma catalysishybrid system, and think of that it may be beneficial to useporous material such as γ-Al2O3 instead of non-porous ce-ramic pellet as the support. In this work, γ-Al2O3 supportedFe2O3-CuO composite catalysts were prepared and employedin the DBD reactor. Their catalytic performance for methaneconversion and methanol yield in the two different plasma-catalysis modes (PPC and IPC) was studied. The amount ofactive species formed in the two modes was monitored andmeasured using in situ optical emission spectra, ozone mea-surement and FT-IR spectra. Furthermore, the stability of theMPOM reactions over the hybrid system was examined.

2. Experimental

2.1. Experiment system

A schematic diagram of the experimental setup is shownin Figure 1. The DBD reactor was a corundum tube withan inner diameter of 16 mm, a wall thickness of 2 mm anda length of 320 mm. A stainless steel rod with a diameterof 12 mm located in the center of the corundum tube actedas a high voltage electrode, leading to an annular dischargegap distance of 2 mm. The outer wall of the corundum tubewas coated with a wire netting, serving as the outer electrode.The plasma was generated by a high-voltage AC power sup-ply (CTP-2000k) that has a maximum voltage and frequencyof 10.0 kV and 10 kHz, respectively. The input power was cal-culated according to the product of input voltage and current.An oscilloscope (Tektronix TDS 1012) was used to record thevoltage and current through a high voltage probe (TektronixP6015A), and a current probe (Tektronix TCP 202). PPC re-actions were conducted in a two-stage system in the sequenceof a gas phase discharge reactor without catalyst in the firststage and a packed bed reactor containing a catalyst of 6 g inthe second stage. The catalyst bed was heated to a moderatetemperature (<250 ◦C) and the plasma zone was kept at roomtemperature. IPC reactions were constructed by packing thecatalyst pellets of 6 g within the discharge gap and in this waythe catalyst was partially overlapped with the plasma zone.The temperature of the reactor was controlled by a heated fur-nace and measured using a thermocouple affixed to the outerreactor surface.

All the experiments were carried out under atmosphericpressure. The feed gases, CH4 (purity>99.99%) and air wereintroduced into the reactor via mass flow controllers (Seven-star D08-2F). The effluent gas from the reactor was analyzed

Figure 1. Scheme diagram of plasma-catalysis system. (a) PPC configuration, (b) IPC configuration

630 Lin Chen et al./ Journal of Natural Gas Chemistry Vol. 19 No. 6 2010

with an online gas chromatograph (FULI-9790) equippedwitha flame ionization detector (FID), a mechanizer and a 2 mTDX-01 column which can separate CH4, CO and CO2. An-other gas chromatograph (FULI-9790) with a FID and a 2 mporapak Q column was used to analyze the concentration ofmethanol in the liquid fraction collected by a cold trap. Theozone concentration in the outlet gas flow was monitored byan ozone analyzer (MOT500-O3, China). In situ plasma op-tical emission spectroscopy (OES) (AvaSpec-2048 fiber opticspectrometer) equippedwith a CCD detector was used to char-acterize the plasma phase. In the OES measurement, a trans-parent quartz tube was used as the barrier of the reactor insteadof the corundum to obtain clear spectra. The OES measure-ment was carried out in the wavelength range of 200–1100 nmwith the resolution of 0.07–0.10 nm. The optical emission sig-nal was collected via a fiber located at approximately 5 mmaway from the reactor wall.

2.2. Catalyst preparation

Fe2O3-CuO/γ-Al2O3 catalyst with 5 wt% Fe and 5 wt%Cu loading was prepared by incipient wetness impregnationof a porous γ-alumina with desirable aqueous solutions ofFe(NO3)3·9H2O and Cu(NO3)2·3H2O. After impregnation for12 h, the samples were dried overnight at 110 ◦C, and thencalcined at 500 ◦C in air for 4 h. The prepared Fe2O3-CuO/γ-Al2O3 is labeled as M/Al2O3. The Fe2O3-CuO/γ-Al2O3 cat-alysts after 10 h PPC reaction and IPC reaction are designatedas M/Al2O3-P and M/Al2O3-I, respectively.

2.3. Catalyst characterization

The Fourier Transform-Infrared Spectroscopy (FT-IR)spectra were recorded using an FT-IR spectrometer (Nicolet560) at room temperature. Self-supportedwafers were formedby pressing the catalyst powder scratched from the catalyst.

Thermogravimetry (TG) analysis was conducted on aSDT Q600 V8.2 Build 100 instrument with a heating rate of10 ◦C/min.

X-ray powder diffraction (XRD) patterns were recordedon a D/Max-2550pc X-ray diffractometer using Cu-Kα radi-ation operated at 40 kV and 250 mA.

The Brunauer-Emmett-Teller (BET) surface area, totalpore volume and average pore size of the samples were mea-sured by nitrogen adsorption-desorption at −196 ◦C with aBELSORP-mini surface area analyzer. All samples were de-gassed under vacuum at 200 ◦C for 4 h prior to measurement.

2.4. Reaction performance evaluation

The parameters investigated in this paper are defined asfollows:

CH4 conversion(%) =Mole of converted CH4Mole of CH4 in feed

×100%(1)

Yield of product(%) =Mole of product

Mole of CH4 in feed×100% (2)

3. Results and discussion

3.1. Methane partial oxidation in the PPC combination

There is no activity for MPOM over the M/Al2O3 cata-lyst alone and no methane conversion and methanol formationcould be detected. However, methane conversion of 33.2%and methanol yield of 1.0% can be obtained when the plasmais used without the M/Al2O3 catalyst. The results of methaneconversion and product yields in the PPC configuration withthe combination of the plasma with γ-Al2O3 or M/Al2O3 areshown in Figure 2. The CH4 conversion over γ-Al2O3 orM/Al2O3 was almost the same as that without any catalystin Figure 2(a), implying that the two materials did not pos-sess obvious activity for methane conversion at low temper-atures. As shown in Figure 2(b), methanol yield over thecatalyst and blank carrier increased between 50 and 150 ◦C,and then decreased slightly beyond that temperature range.Some methanol coming from the plasma unit was adsorbedon the surface of the porous materials, resulting in the lowmethanol yield in lower temperature range (<100 ◦C), whileat higher temperatures (>200 ◦C), most adsorbed methanolmay be converted to carbon dioxide, which is consistent withthe high CO2 yield beyond 200 ◦C (Figure 2d). The methanolyield reached a peak of 1.3% at 150 ◦C by the combinationof the plasma and M/Al2O3, which was higher by 21.2% thanthat of the blank carrier at the same temperature.

As shown in Figure 2(c), the CO concentration wasslightly lower in the plasma-catalysis system than that in thesolely plasma system. The decrease in CO yield may be eitherascribed to the total oxidation to CO2 (Equation 3) or to COconversion to methanol by the reaction with H2 generated inthe reactions over the effective catalysts (Equation 4).

2CO+O2Catalyst−−−−−→ 2CO2 (3)

CO+2H2Catalyst−−−−−→ CH3OH (4)

3.2. Methane partial oxidation in the IPC combination

Figure 3 shows the experimental results in the IPC sys-tem. As seen from Figure 3(a), both the M/Al2O3 and the car-rier γ-Al2O3 showed remarkably high methane conversion at300 ◦C, reaching 46.9% and 43.8%, respectively. This resultmay be partly ascribed to the fact that the reactants adsorbedon the porous γ-Al2O3 surface have longer residence time in

Journal of Natural Gas Chemistry Vol. 19 No. 6 2010 631

the discharge than that in the non-packed reactor. Further-more, compared with the non-packed plasma reactor, the av-erage electric field in a packed-bed reactor would be enhancedbecause of the short distance in the adjacency of contact points[11,15]. Van Durme et al. [16] indicated that the packing pel-lets were helpful for expanding the discharge region becausethe microdischarges were apt to propagate along the solid sur-

face. As the plasma discharge region expanded, the concen-trations of active species increased due to the high collisionprobability between electrons and gas molecules, leading tothe acceleration of plasma chemical reactions. Therefore, thepacked-bed DBDs exhibit more effective energy efficiencythan the non-packed ones, resulting in a higher methane con-version in the former configuration.

Figure 2. Conversion of (a) methane and yields of (b) methanol, (c) CO and (d) CO2 as a function of temperature in the PPC configuration. Reaction conditions:input power, 120 W; discharge frequency, 7 kHz; atmosphere pressure; feed flowrate, 300 sccm; CH4/air ratio in feed, 1/1; catalyst amount, 6 g

As shown in Figure 3(b), relatively low methanol selec-tivity was obtained over γ-Al2O3 despite the high methaneconversion (Figure 3a). It seems that the strong adsorptive ca-pacity of methane molecule and intermediates on the porousmaterial surface prolonged the residence time in the discharge,leading to a shift of the reaction selectivity towards methanetotal oxidation to carbon dioxide rather than partial oxidationto methanol. This viewpoint is further evidenced by the highCO2 yield in the presence of γ-Al2O3 (Figure 3d). It alsocan be seen from Figure 3(b) that the methanol yield overM/Al2O3 increased with the increase of reaction temperaturefrom 100 ◦C to 200 ◦C and then a slow decrease occurred.A maximum CH3OH yield of 1.6% was achieved at 200 ◦C,which was about 67.4% higher than that of the blank carrierγ-Al2O3 at the same temperature. Therefore, it is clear thatalumina supported metal oxide catalyst with well-developedporosity exhibited better catalytic activity for MPOM in theIPC process than in the PPC process. For supported catalysts,both texture and surface properties of support have a great

influence on the dispersion and activity of active components[17]. In our experiments, Fe2O3 and CuO were deposited ontothe surface of high surface area materials to ensure a high de-gree of contact between the reactants and the catalysts. Forthe IPC configuration, the formation of short-lived oxidizingspecies in the gas-phase discharge could diffuse into the poresof the catalyst, resulting in high reaction efficiency [18]. Inthis way, both high methane conversion and high methanolselectivity could be achieved. Similar results were reported inRefs. [19,20]. Therefore, the proper use of a support could en-hance the catalytic activity of metal oxides in plasma-catalysiscombination.

Figure 3(c) illustrates the variation of CO yield with tem-perature. The introduction of γ-Al2O3 increased the CO yield,resulting from the high decomposition rate of methane overporous γ-alumina. However, the yield of CO over M/Al2O3decreased with the increase of temperature. As stated previ-ously, the CO consumption may be due to the total oxidationand methanol synthesis.

632 Lin Chen et al./ Journal of Natural Gas Chemistry Vol. 19 No. 6 2010

Figure 3. Conversion of (a) methane and yields of (b) methanol, (c) CO and (d) CO2 as a function of temperature in the IPC configuration. Reaction conditions:input power, 120 W; discharge frequency, 7 kHz; atmosphere pressure; feed flowrate, 300 sccm; CH4/air ratio in feed, 1/1; catalyst amount, 6 g

As seen from Figure 3(d), the selectivity to CO2 wasmuch higher in the reactor with the porous packing materi-als (γ-Al2O3, M/Al2O3) than in the empty reactor. Especiallywhen the temperature was higher than 250 ◦C, the methanetotal oxidation may overwhelm partial oxidation, resulting ina decrease in methanol yield. (Figure 3b). Hence, a moderatetemperature is beneficial for methanol synthesis.

3.3. Chemical species formed in the PPC and IPC combina-tions

3.3.1. Identif ication of active species using in situ opticalemission spectra

The in situ optical emission spectra were recorded toidentify active components, i.e. atoms or molecules in ex-cited electronic, vibrational and rotational states, in the plasmaphase.

Figure 4(a) displays the optical emission spectrum ofCH4 plasma in the range of wavelength from 250 to 600nm, which contains the most emission lines. Since the ex-periment was carried out at atmospheric pressure, it was ex-pected there were many impurities to be identified. The ni-trogen presents bands of the second positive system (C3 u-

B3 g) at 337.1 nm, 357.6 nm and 380.4 nm. The molecu-lar ions produced after ionization were excited and identifiedfrom the strong emission bands of the first negative system(B2 u+-X2 g+) at 391.4 nm and 427.8 nm. The strong nitro-gen emission line showed that active N2 plays an importantrole in the effective transformation of energy, producing elec-trons through the Penning ionization reaction or highly ener-getic species [21,22].

In addition, the weak peak at the position of 431.4 nm isfeatured as the CH emission (A2 -X2 (0, 0)), showing thedecomposition of methane in the DBD system (Equation 5).

CH4+ e−→ CHx +(4− x)H+ e (5)

The optical emission spectrum of the plasma in oxygen ispresented in Figure 4(b). Beside the nitrogen emission bands,the atomic oxygen line at 777.4 nm (35P−→35S) has beenidentified. Oxygen dissociation can produce a large numberof highly active species which make a major contribution tothe methanol formation [23].

O2+ e−→ 2O+ e (6)

The presence of nitrogen results in a decrease in the rateof direct electron-oxygen collision reaction. On the otherhand, because of the presence of excited molecules of N2, an-

Journal of Natural Gas Chemistry Vol. 19 No. 6 2010 633

other additional pathway of O2 dissociation takes place andcannot be neglected [24]:

e+N2 −→N2(A3 +

u )+ e (7)

N2(A3 +

u )+O2 −→ N2+2O(3P) (8)

The contribution of this pathway to the O atom generationis comparable to O2 dissociation by electron collision.

However, the emission lines of CH and O were not ob-served in the optical emission spectra of CH4/air plasma (Fig-

ure 4c), since the above excited species may rapidly partici-pate in chemical reactions in the gas phase [11,21]:

CH4+O−→ CHx +OH+(3− x)H−→ CH3OH (9)

As mentioned in section 3.2, introducing catalysts into theplasma discharge may affect the type of discharge or can in-duce a shift in the distribution of the accelerated electrons,which again influences the production of excited species [16].

Figure 4. Optical emission spectra of DBD at input power of 120 W and discharge frequency of 7 kHz. (a) CH4 plasma, CH4 flow rate of 150 sccm; (b) O2plasma, O2 flow rate of 150 sccm; (c) CH4/air plasma, feed flow rate of 300 sccm, CH4/air ratio of 1/1 in feed; (d) CH4/air plasma, feed flow rate of 300 sccm,CH4/air ratio of 1/1 in feed, plasma reactor packed with γ-Al2O3 (6 g); (e) CH4/air plasma, feed flowrate of 300 sccm, CH4/air ratio of 1/1 in feed, plasmareactor packed with M-Al2O3 (6 g)

634 Lin Chen et al./ Journal of Natural Gas Chemistry Vol. 19 No. 6 2010

As expected, when γ-Al2O3 or M/Al2O3 was introduced intothe discharge zone, the intensity of almost all lines increasedremarkably (Figure 4d and e). Increasing the emission inten-sity, which is related to the density of excited species, mayresult in enhanced methane conversion and methanol yield(Figure 3).

3.3.2. Ozone formation in the two combinations

It is well known that DBD in oxygen-containing gas canproduce high levels of ozone [25]. The formation of O3molecules in DBD is mainly owing to the following reaction:

O+O2 −→ O3 (10)

Due to its long lifetime, ozone is of importance in bothPPC and IPC combinations. The ozone concentrations at theoutlet were measured and the results are presented in Figure 5.

Figure 5. Ozone production in pure air with different pellets in the IPC orthe PPC system. Reaction conditions: input power, 120 W; discharge fre-quency, 7 kHz; atmosphere pressure; room temperature; feed flowrate, 300sccm; CH4/air ratio in feed, 1/1; catalyst amount, 6 g

It is noteworthy that the combination of γ-Al2O3 orM/Al2O3 with the plasma in the IPC configuration consid-erably enhanced ozone production. As stated earlier, packed-bed plasma reactors, constructed by packing catalytic or non-catalytic dielectric pellets inside non-thermal plasma reactors,can improve the energy efficiency effectively [26]. The in-creased active oxygen species in turn promoted the ozoneformation. Moreover, the combination of M/Al2O3 with theplasma in the IPC system more effectively promoted the de-composition of ozone than that of blank carrier with theplasma in the IPC system. In terms of the reactivity towardmethane, the oxygen atom is chemically more active speciesthan ozone. The introduction of M/Al2O3 to the dischargezone produced a larger amount of active oxygen species,which further promoted the methanol generation.

Adding γ-Al2O3 or M/Al2O3 to the post-plasma position,resulted in a slight reduction of the outlet concentration ofozone. The heterogeneous catalytic decomposition of ozonetakes place according to the following mechanism [27]:

O3+Z−→ ZO+O2 (11)

O3+ZO−→ Z+2O2 (12)

where, Z is an active site on a catalytic surface. During ozonedecomposition, active oxygen species (such as O (3P), O (1D),O, and possibly O−3 ) may be formed and show strong oxidiz-ability.

3.3.3. FT-IR analysis

The FT-IR spectra of catalysts before and after the reac-tion are presented in Figure 6. Except the absorption bands ofthe adsorbed water at 3500 cm−1 and 1630 cm−1, the absorp-tion bands at 2952, 2925, 2864 and 2349 cm−1 occurred in thespectra of the used catalysts. The band at 2349 cm−1 originat-ing from gaseous CO2 appeared with appreciable intensity.The appearance of the other peaks correspond to υas(CH3)(at 2952 cm−1), υas(CH2) (at 2925 cm−1), and υs(CH2) (at2864 cm−1) implied that some organic compounds were de-posited or adsorbed on the catalyst surface. These infraredbands were more intense in M/Al2O3-I than in M/Al2O3-P,suggesting that a higher methane conversion could be ob-tained due to the IPC synergistic effect.

Figure 6. FT-IR spectra of the catalysts

3.4. Catalytic stability in the two plasma-catalysiscombinations

It has been experimentally demonstrated that the combi-nation of the plasma andM/Al2O3 with the two configurationsresults in a synergistic effect, i.e., the performance obtained byplasma-catalysis is better than the summation of plasma-aloneand catalysis-alone, indicating that plasma-catalysis presentsthe promising application on MPOM. To evaluate the reac-tion stability, the extended MPOM reactions over M/Al2O3were carried out in the IPC or the PPC reactor, where the cat-alyst temperature was maintained at 200 ◦C. The time depen-dency of the methanol yield is given in Figure 7. The experi-ment demonstrated that the catalytic performance in the PPCconfiguration changed slightly, and maintained the initial ac-tivity for 10 h, showing that almost no catalyst deactivation

Journal of Natural Gas Chemistry Vol. 19 No. 6 2010 635

could be detected during the reaction period. Nevertheless inthe IPC case, the methanol yield maintained stable with 1.6%in the first 2 h and then dropped gradually to 1.4% after run-ning the reaction for 10 h, which was lower than that withthe discharge alone (1.5%). Therefore, it appeared that theM/Al2O3 catalyst exhibited less stability during the IPC pro-cess. This result may be due to the fact that the catalyst usedin the IPC configuration easily suffers from the deactivationcaused by carbon deposition. According to thermodynamiccalculations, the origin of inactive carbon during the methanereforming might occur via CH4 decomposition (Equation 13)and CO disproportionation (Equation 14) [28].

CH4 −→ C+2H2 (13)

2CO−→ C+CO2 (14)

Figure 7. CH3OH yield vs. reaction time in the IPC or the PPC process usingM/Al2O3. Reaction conditions: input power, 120 W; discharge frequency, 7kHz; discharge gap distance, 2.0 mm; atmosphere pressure; catalyst temper-ature, 200 ◦C; feed flowrate, 300 sccm; CH4/air ratio in feed, 1/1; catalystweight, 6 g

CH4 decomposition is endothermic, and its equilibriumconstant increases with increasing temperature. Conversely,CO disproportionation is exothermic, and its equilibrium con-stant decreases with the increase of temperature. Comparedwith CH4, CO is more difficult to be dissociated via the colli-sion with electrons. The CO dissociation energy of 11.1 eV ishigher than the CHx (x = 1−4) dissociation energy of 4.9 eV[29]. Accordingly, CH4 decomposition will make the maincontribution to the coke deposition in the POM reaction.As CH4 decomposition requires high temperature to producesolid coke, carbon formation is favored at high temperature(973 K<T<1273 K) during thermal reaction of MPOM [30].Although the ions and neutral atoms are often near the am-bient temperature in a non-thermal plasma reaction, the elec-tron temperature is still typically several thousand degree Cel-sius. Because of the large difference in mass, the electronsare much easier and much faster to reach the thermodynamicequilibrium than those coming into equilibrium with the ionsor neutral atoms. Thus, it can be speculated that carbon for-mation is inevitable in the IPC reactor even at ambient temper-ature [31,32]. Using the PPC configuration, a small amount

of carbon deposition was observed on the anode surface or theplasma reactor wall, but it did not disrupt the discharge. In thesecond catalytic stage, no significant amount of carbon wasattached to the catalyst surface. The low reaction temperature(50−250 ◦C) is beneficial to limiting coke deposition result-ing from CH4 decomposition. In consequence, the M/Al2O3catalyst in the PPC configuration maintains a better stabilitythan in the IPC configuration.

To prove the above viewpoint, the characterization of theM/Al2O3 catalyst was carried out before/after the reaction byTG, XRD, and N2 adsorption-desorption. The used catalystswere taken after running the reaction for 10 h in the PPC orIPC configuration.

TG analysis was conducted to test the carbon depositionon the catalysts. The M/Al2O3-P catalyst did not show anyweight loss in its TG analysis, indicating that the coke de-position on it was not obvious. However, a weight loss of3.8 wt% occurred at 200 ◦C over the M/Al2O3-I catalyst, in-dicating that there was carbon deposit on the catalyst surface.Therefore, it is reasonable to suggest that the obvious deacti-vation in the IPC configuration is resulted from the coke for-mation on the catalyst surface.

Figure 8 illustrates the XRD patterns of the catalysts be-fore and after the reaction. The M/Al2O3 displayed only thecharacteristic peaks of Al2O3 and Fe2O3 with no separationof CuO phase. The absence of CuO diffraction peaks sug-gested that CuO species were highly dispersed on the surfaceof the supports in some amorphous forms [33]. No observableshift in diffraction lines of Al2O3 and Fe2O3 could be foundfor the used catalysts in both the PPC and IPC configurations,indicating that there was no change in Al2O3 and Fe2O3 crys-tallite structure after the plasma reaction. It was noteworthythat a weak peak at 2θ = 26o was observed for the M/Al2O3-Isample, which was related to the plane (0 0 2) of the graphiticstructure, showing the formation of coke on the catalyst.

Figure 8. XRD patterns of the catalysts

The surface area and pore size distribution of the catalystsbefore and after the reaction are given in Table 1 and Fig-ure 9, respectively. The γ-Al2O3 has a specific surface area of239.1 m2/g and the pore volume of 0.42 cm3/g. The aluminaused in this work possesses a narrow pore size distribution and

636 Lin Chen et al./ Journal of Natural Gas Chemistry Vol. 19 No. 6 2010

an average pore diameter of about 7.0 nm. After deposition ofactive metal oxides and following calcination, both the BETsurface area and total pore volume decreased. However theaverage pore diameter of M/Al2O3 was larger than that of thealumina support owing to the blocking of the some smallerpores by metal oxides deposition. The catalyst used in thePPC configuration and after running the reaction for 10 h didnot cause any significant change in the specific surface areaor pore size distribution. However, for the catalyst used in theIPC configuration, its specific surface area and the pore vol-ume suffered a slight decline, i.e., from 178.8 to 163.8 m2/gand from 0.39 to 0.37 cm3/g, while the average pore diametershifted to a higher value (from 8.8 to 8.9 nm). Based on theseresults, it was concluded that the M/Al2O3 catalyst exhibiteda superior anti-deactivation ability in the PPC configuration.

Table 1. The textual properties of the catalysts beforeand after the reaction

Surface area Total pore Average poreCatalyst

(m2/g) volume (cm3/g) diameter (nm)γ- Al2O3 239.1 0.42 7.0M/Al2O3 178.8 0.39 8.8M/Al2O3-P 173.0 0.38 8.9M/Al2O3-I 163.8 0.37 8.9

Figure 9. The pore size distribution of the catalysts

4. Conclusions

The synergetic effect of Fe2O3-CuO/γ-Al2O3 catalystwith the plasma for MPOM was investigated in the PPC andIPC configuration. The following conclusions were obtained:

(1) The synergetic effect of plasma and catalysis couldbe observed in both combinations of the PPC and IPCconfiguration. For the IPC configuration, the plasma-catalysiscombination was found to be more effective for MPOM, butthe catalyst was less stable than that in the PPC configuration.

(2) The amount of active species (e.g., O3, O and CH)generated in the IPC system was greater than that in the PPCsystem.

(3) The catalyst deactivation in the IPC configurationcould be mainly ascribed to the coke formation on the cata-lyst surface.

References

[1] Bjorklund M C, Carr R W. Ind Eng Chem Res, 2002, 41(25):6528

[2] Walker G S, Lapszewicz J A, Foulds G A. Catal Today, 1994,21(2-3): 519

[3] Indarto A. IEEE Transactions on Dielectrics and Electrical In-sulation, 2008, 15(4): 1038

[4] Yao S L, Takemoto T, Ouyang F, Nakayama A, Suzuki E,Mizuno A, Okumoto M. Energy Fuels, 2000, 14(2): 459

[5] Okazaki K, Kishida T, Ogawa K, Nozaki T. Energy ConversManage, 2002, 43(9-12): 1459

[6] Zhang Y P, Li Y, Wang Y, Liu C J, Eliasson B. Fuel ProcessTechnol, 2003, 83(1-3): 101

[7] Pietruszka B, Heintze M. Catal Today, 2004, 90(1-2): 151[8] Indarto A. J Chin Inst Chem Eng, 2008, 39(1): 23[9] Kim H H, Oh S M, Ogata A, Futamura S. Appl Catal B, 2005,

56(3): 213[10] Wang Q, Yan B H, Jin Y, Cheng Y. Energy Fuels, 2009, 23(8):

4196[11] Chen L, Zhang X W, Huang L, Lei L C. Chem Eng Process,

2009, 48(8): 1333[12] Chen H L, Lee H M, Chen S H, Chang M B, Yu S J, Li S N.

Environ Sci Technol, 2009, 43(7): 2216[13] Liu C J, Eliasson B, Xue B Z, Li Y, Wang Y Q. React Kinet

Catal Lett, 2001, 74(1): 71[14] Lee H, Song H K, Min B R. Chem Lett, 2006, 35(6): 646[15] Chen H L, Lee HM, Chen S H, Chao Y, Chang M B. Appl Catal

B, 2008, 85(1-2): 1[16] Van Durme J, Dewulf J, Leys C, Van Langenhove H. Appl Catal

B, 2008, 78(3-4): 324[17] Breysse M, Afanasiev P, Geantet C, Vrinat M. Catal Today,

2003, 86(1-4): 5[18] Holzer F, Roland U, Kopinke F D. Appl Catal B, 2002, 38(3):

163[19] Wang S B, Lu G Q M. Appl Catal B, 1998, 16(3): 269[20] Ruckenstein E, Wang H Y. J Catal, 1999, 187(1): 151[21] Wang Y F, Tsai C H, Chang W Y, Kuo Y M. Int J Hydrogen

Energy, 2010, 35(1): 135[22] Topala I, Asandulesa M, Dumitrascu N, Popa G, Durand J. J

Optoelectron Adv Mater, 2008, 10(8): 2028[23] Kato S, Yamamoto Y, Hara T. Energy Convers Manage, 2001,

42(15-17): 1853[24] Chirokov A, Gutsol A, Fridman A. Pure Appl Chem, 2005,

77(2): 487[25] Futamura S, Zhang A, Einaga H, Kabashima H. Catal Today,

2002, 72(3-4): 259[26] Chen H L, Lee H M, Chen S H, Chang M B. Ind Eng Chem Res,

2008, 47(7): 2122[27] Harling A M, Glover D J, Whitehead J C, Zhang K. Appl Catal

B, 2009, 90(1-2): 157[28] Li M W, Liu C P, Tian Y L, Xu G H, Zhang F C, Wang Y Q.

Energy Fuels, 2006, 20(3): 1033

Journal of Natural Gas Chemistry Vol. 19 No. 6 2010 637

[29] Li M W, Xu G H, Tian Y L, Chen L, Fu H F. J Phys Chem A,2004, 108(10): 1687

[30] Nair S A, Nozaki T, Okazaki K. Chem Eng J, 2007, 132(1-3):85

[31] Zhao G B, John S, Zhang J J, Wang L, Muknahallipatna S,Hamann J C, Ackerman J F, Argyle M D, Plumb O A. Chem

Eng J, 2006, 125(2): 67

[32] Le H, Lobban L L, Mallinson R G. Catal Today, 2004, 89(1-2):15

[33] Sirichaiprasert K, Luengnaruemitchai A, Pongstabodee S. Int JHydrogen Energy, 2007, 32(7): 915