Application of in-plasma catalysis and post-plasma catalysis for methane partial oxidation to methan

Available online at www.sciencedirect.com| Joumal ofScienceDirect7I Natural GasChemistryEL SEVIERJournal of Natural Gas Chemistry 19(2010)628 -637www.elsevier.com/locate/jngcApplication of in-plasma catalysis and post-plasma catalysis for methanepartial oxidation to methanol over a Fe2O3-CuO/y-Al2O3 catalystLin Chen+2，Xingwang Zhang'，Liang Huang'，Lecheng Lei I 1I. Institute of Industrial Ecology and Emvironment, Zhejiang Universirty, Hangzhou 310027, Zhejiang, China;2. Institute of Environmental Science and Engineering, College of Materials and Environmental Engineering,Hangzhou Dianzi University, Hangzhou 310018, Zhejiang, ChinaI Manuseript reived April 20, 2010; revised August 17, 2010 ]JumadAbstractMethane partial oxidation to methanol (MPOM) using dielectric barrier discharge over a Fe2O3-CuO//-Al2O3 catalyst was performed. Themulticomponent catalyst was combined with plasma two different configurations, i.e., in-plasma catalysis (IPC) and post-plasma catalysis(PPC). It was found that the catalytic performance , the cataly s fbr MPOM as strongly dependent on the hybrid configuration. A bettersynergistic performance of plasma and catalysis was chieved in the IPC configuation, but the catalysts packed in the discharge zone showedlower sabilit than those connected to the discharge 2zbne In sequer 14. AintGies, such as ozone, atomic oxygen and methyl radicals, weproduced from the plasma-catalysis process, and mal a major cthanol sythesis. These active species were identified bymeans of in situ optical emission spectra, ozone measu ement and FT-TEISMaAnrmed that the amount of active species in the IF敏url Eassystem was greater than that in the PPC system. The n sults of TQI XRP an N2 ads oflor-desorption revealed that carbon deposition on the'spent catalyst surface was responsible for the catalyst deactivatiorKey wordsmethane partial oxidation to methanol; plasma catalfe comb nation; synergistic performance; catalytic stability1. Introductionand high density, which are able to initiate plasma methanereforming reactions [4- 6]. Unfortunately, plasma does notRecently due to the rapid depletion of oil reserves andusually allow for achieving high levels in both methane con-strong volatility in the oil price, alternative methods for syn-version and aimed product selectivity. The processes involvqthesizing hydrocarbon fuels have again received considerableplasma are always less selective than catalytic processes, leadattention. The world's seafloors contain abundant reserves ofing to greatly complex product distributions. A possible wnatural gas in gas or solid form called natural gas hydrates.to overcome this problem might be to exploit the inherent syn-These natural gas hydrates contain methane, whose potentialergetic potential between plasma mediated reactions and het-transformation into more valuable methanol has acquired sig-erogeneous catalysts [7,8]. This combination can be operatednificant interest. However, due to the high stability of theC-Hin two configurations: in-plasma catalysis (IPC) with the cata-bond, the direct conversion of methane into methanol typi-lyst inside the reactor, or post-plasma catalysis (PPC) with thecally requires extreme reaction conditions, such as high tem-catalyst downstream of the plasma unit. The role of the plasmaperature (400- 1000 °C) and high pressure (> 10 atm), whichis highly dependent on the two configurations [9,10]. In a PPClimits its industrial application [1,2]. Consequently, the appli-configuration, plasma do not directly interact with catalysiscation of non-thermal plasma such as a dielectric barrier dis-because reactive species produced in the discharge zone havecharge (DBD) appears to be an important alternative way tosuch short lifetimes that they disappear before reaching theproduce chemicals under ambient conditions [3]. The DBD iscatalyst surface. The plasma can provide chemically reactivean efficient source of high energetic eletrons with 1-10eVspecies 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 Scienceion of China (No. Y5080192)，the Project of Science and Technology Department of Zhejianghing (2007013061), MOST Project ofChina (No. 2007AA062339, No. 2008BAC32B06 and No. 2007AA06A409) and the Open Project Progr中国煤化工Soures PlutionControl, the Ministry of Agriculure of the People's Republic of China and Science Foundation of ChinesfYHCNM HGCopyrightO2010, Dalian Institute of Chemical Physics, the Chinese Academy of Sciences. All rights reserved.doi:10. 1016/S 1003-9953(09)60129-8Journal of Natural Gas Chemistry VoL. 19 No.62010629more easily converted products to accelerate the catalysis.2. ExperimentalWhile in an IPC configuration, both plasma and catalysis takeplace simultaneously and interact with each other. The perfor-2.1. Experiment systemmance enhancement mechanisms are rather complicated. Inour previous work [1 1], there was a synergistic effect between .cold plasma and a ceramic supported Fe2O3 or a Fe2O3-CuOA schematic diagram of the experimental setup is showncomposite catalyst for MPOM in the PPC process. However,in Figure 1. The DBD reactor was a corundum tube withwhen the catalyst was introduced to the IPC configuration, lit an inner diameter of 16 mm, a wall thickness of 2mm andtle synergistic performance was observed. It is well knowna length of 320 mm. A stainless steel rod with a diameterthat supports play an important role in determining the natureof 12 mm located in the center of the corundum tube actedand number of active sites, and consequently in the activity ofas a high voltage electrode, leading to an annular dischargethe catalysts. For the IPC configuration, if the catalyst has agap distance of 2 mm. The outer wall of the corundum tubelarge specific surface area, it may provide a large adsorptionwas coated with a wire netting, serving as the outer electrode.capacity for reactant molecules, which in turn would increaseThe plasma was generated by a high-voltage AC power sup-the collision probability between the reactant molecules andply (CTP-2000k) that has a maximum voltage and frequencyshort-lived excited species [12]. Liu et al. [13] conductedof 10.0 kV and 10 kHz, respectively. The input power was cal-plasma catalytic methane conversion in the IPC configuration.culated according to the product of input voltage and current.They used zeolite X and zeolite A to improve the methaneAn oscilloscope (Tektronix TDS 1012) was used to record theconversion and product selectivity. Ni/-Al2O3 catalyst pre-voltage and current through a high voltage probe (Tektronixsented an exciting synergetic effect with the plasma, on whichP6015A), and a current probe (Tektronix TCP 202). PPC re-the conversion of reactants at 573 K reached the same resultsactions were conducted in a two-stage systemia the sequenceas pure catalytic reaction at 1073 K [14].of a gas phase discharge reactor without catatyst in the firstAccordingly, these previous studies stimulated us to fur-stage and a packed bed reactor containing a catalystof 6g inther consider the role of adsorption in the plasma catalysisthe second stage. The catalyst bed was heated to a moderatehybrid system, and think of that it may be beneficial to usetemperature (<250 °C) and the plasma zone was kept at roomporous material such as 7-Al2O3 instead of non-porous ce-temperature. IPC reactions were constructed by packing theramic pellet as the support. In this work, 7-Al2O3 supportedcatalyst pellets of 6 g within the discharge gap and in this wayFe2O3-CuO composite catalysts were prepared and employedthe catalyst was partially overlapped with the plasma zone.in the DBD reactor. Their catalytic performance for methaneThe temperature of the reactor was controlled by a heated fur-conversion and methanol yield in the two different plasma-nace and measured using a thermocouple affixed to the outercatalysis modes (PPC and IPC) was studied. The amount ofreactor surface.active species formed in the two modes was monitored andAll the experiments were carried out under atmosphericmeasured using in situ optical emission spectra, ozone mea-pressure. The feed gases, CH4 (purity>99.99%) and air weresurement and FT-IR spectra. Furthermore, the stability of theintroduced into the reactor via mass flow controllers (Seven-MPOM reactions over the hybrid system was examined.star D08-2F). The effluent gas from the reactor was analyzed(aAC powersupply]↓ElectrodeCorundumFurnaceCH4 FF2talysts十toGCAir tFC(bAC powe_supplyIFurmaceH4 FMFCAir」MIFCCatalysts中国煤化工Figure 1. Scheme diagram of plasma calysis system. (a) PPC configuratio.MYHCNMHG .Cat alvsts630 .Lin Chen et al./ Journal of Natural Gas Chemistry Vol. 19 No.6 2010a flame ionization detector (FID), a mechanizer and a 2 mTDX-01 column which can separate CH4, CO and CO2. An-CH4 conversion(%)Mole of CH4 in feed: 100%other gas chromatograph (FULL-9790) with a FID and a2m .(1)porapak Q column was used to analyze the concentration ofmethanol in the liquid fraction collected by a cold trap. TheYield of product(%) =Mole of productozone 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 optic3. Results and discussionspectrometer) equipped with 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 instead3.1. Methane partial oxidation in the PPC combinationof the corundum to obtain clear spectra. The OES measure-ment was carried out in the wavelength range of 200-1 100 nmThere is no activity for MPOM over the M/Al2O3 cata-with the resolution of 0.07-0. 10 nm. The optical emission sig-lyst alone and no methane conversion and methanol formationnal was collected via a fiber located at approximately 5 mmcould be detected. However, methane conversion of 33.2%away from the reactor wall.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 with2.2. Catalyst preparationthe combination of the plasma with 7-Al2O3 or M/Al2O3 areshown in Figure 2. The CH4 conversion over 7-Al2O3 orFe2O3-CuO/-Al2O3 catalyst with 5 wt% Fe and 5 wt%M/Al2O3 was almost the same as that without any catalystCu loading was prepared by incipient wetness impregnationin Figure 2(a), implying that the two materials did not pos-of a porous 7-alumina with desirable aqueous solutions ofsess obvious activity for methane conversion at low temper-Fe(NO3)3 9H2O and Cu(NO3)2 3H2O. After impregnation foratures. As shown in Figure 2(b), methanol yield over the12 h, the samples were dried overnight at 110 °C, and thencatalyst and blank carrier increased between 50 and 150 °C,calcined at 500 °C in air for 4 h. The prepared Fe2O3-CuO/y-and then decreased slightly beyond that temperature range.Al2O3 is labeled as M/Al2O3. The Fe2O3-CuO/-Al2O3 cat-Some methanol coming from the plasma unit was adsorbedon the surface of the porous materials, resulting in the lowalysts after 10 h PPC reaction and IPC reaction are designatedmethanol yield in lower temperature range (< 100°C), whileas M/Al2O3-P and M/Al2O3-I, respectively.at higher temperatures (> 200 °C), most adsorbed methanolmay be converted to carbon dioxide, which is consistent with2.3. Catalyst characterizationthe high CO2 yield beyond 200 °C (Figure 2d). The methanolyield reached a peak of 1.3% at 150 °C by the combinationThe Fourier Transform-Infrared Spectroscopy (FT-IR)of the plasma and M/Al2O3, which was higher by 21.2% thanthat of the blank carrier at the same temperature.spectra were recorded using an FT-IR spectrometer (NicoletAs shown in Figure 2(c), the CO concentration was560) at room temperature. Self-supported wafers were formedslightly lower in the plasma-catalysis system than that in theby pressing the catalyst powder scratched from the catalyst.solely plasma system. The decrease in CO yield may be eitherThermogravimetry (TG) analysis was conducted on aascribed to the total oxidation to CO2 (Equation 3) or to COSDT Q600 V8.2 Build 100 instrument with a heating rate of conversion to methanol by the reaction with H2 generated in10 °C/min.the reactions over the effective catalysts (Equation 4).X-ray powder diffraction (XRD) patterns were recorded2CO + O2Culys, , 2CO2.(3on a D/Max-2550pc X-ray diffractometer using Cu- K。radi-ation operated at 40 kV and 250 mA.co+ 2H2 Calyty + CH3OH(4)The Brunauer- Emmett-Teller (BET) suface area, totalpore volume and average pore size of the samples were mea-sured by nitrogen adsorption-desorption at - 196°C with a3.2. Methane partial oxidation in the IPC combinationBELSORP-mini surface area analyzer. All samples were de-gassed under vacum at 200。C for 4 h prior to measurement.Figure 3 shows the experimental results in the IPC sys-tem. As seen from Figure 3(a). both the M/AlO3 and the car-2.4. Reaction performance evaluationrier 7-Al2O3 show中国煤化工e conversion at300 °C, reaching .YH| CNMH Gely. This resultThe parameters investigated in this paper are defined asmay be partly ascaul uiai ii Iractants adsorbedfollows:on the porous 7-Al2O3 surface have longer residence time in39Journal of Natural Gas Chemistry VoL. 19 No.62010631the discharge than that in the non- packed reactor. Further-ace. As4 the plasma discharge region expanded, the concen-more, compared with the non-packed plasma reactor, the av-trations of active species increased due to the high collisionerage electric field in a packed-bed reactor would be enhancedprobability between electrons and gas molecules, leading tobecause of the short distance in the adjacency of contact pointsthe acceleration of plasma chemical reactions. Therefore, the[11,15]. Van Durme et al. [16] indicated that the packing pel-packed-bed DBDs exhibit more effective energy efficiencylets were helpful for expanding the discharge region becausethan thle hnon-packed ones, resulting in a higher methane con-the microdischarges were apt to propagate along the solid sur-vesion in the former configuration.34.4F-◆Plasma and y-Al,O,a)- Plasma and MAIO,: Plasma onlyAmbienttemperature号1.033活33mbient .号0.8◆Plasma and y-ALO30.6Fo- Plasma and M/AI,O2Plasma only32000152005010020250 .Catalyst temperature (C)2.7 --●Plasma and y-Al,O,c)(d)Plasma and M/AI,O,5(2508 2.258 2457气- Plasma and y-Al2O,.1 F2.3F-0- Plasma and M/AI2O,. Plasima onlyz4Figure 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, 6gAs shown in Figure 3(b), relatively low methanol selec-influence on the dispersion and activity of active componentstivity was obtained over 7-Al2O3 despite the high methane[1分]. In our experiments, Fe2O3 and CuO were deposited onto装?conversion (Figure 3a). It seems that the strong adsorptive ca-th装surt年ce of high surface area materials to ensure a high de-pacity of methane molecule and intermediates on the porousgree of contact between the reactants and the catalysts. Formaterial surface prolonged the residence time in the discharge,th? IPC configuration, the formation of short-lived oxidizingleading to a shift of the reaction selectivity towards methanespecies in the gas- phase discharge could diffuse into the pores入total oxidation to carbon dioxide rather than partial oxidationofAhe catalyst, resulting in high reaction efficiency [18]. Into methanol. This viewpoint is further evidenced by the highthis' way, both high methane conversion and high methanolLCO2 yield in the presence of 7-Al2O3 (Figure 3d). It alsosectivity could be achieved. Similar results were reported incan be seen from Figure 3(b) that the methanol yield overRets. [19,20]. Therefore, the proper use of a support could en-M/Al2O3 increased with the increase of reaction temperaturehance the catalytic activity of metal oxides in plasma-catalysisfrom 100°C to 200°C and then a slow decrease occurred.combination.A maximum CH3OH yield of 1.6% was achieved at 200 °C,Figure 3(c) ilustrates the variation of CO yield with tem-which was about 67.4% higher than that of the blank carrierperatufe.' The introduction of r/-Al2O3 increased the CO yield,γ-Al2O3 at the same temperature. Therefore, it is clear thatresulting from the中国煤化工f methane overalumina supported metal oxide catalyst with well-developed porous y-aluminaYHCN MH G over MAl2O3porosity exhibited better catalytic activity for MPOM in thedecreased with th..iu. As stated previ-IPC process than in the PPC process. For supported catalysts,ously, the CO consumption may be due to the total oxidation2.0 both texture and surface properties of support have a greatand methanol synthesis.150Catalyst temperature(。9Catal! yst tenperature(。q50 632Lin Chen et al./ Journal of Natural Gas Chenitury Vol. 19 No.6 201050■Plasma and-Al,O,+ Plasma and r-Al,O,(b)- Plasma and MAL,O,45F- 一 Plasma only▲Plasma only4.5 F苦1.04(150200010251Temperature (C)十Plasma and r-AlO,392.8” Plasma and -Al2O;(c.oF-●Plasma and M/AI,O,d)一 Plasma onlyPlasma and MAIO3-▲Plasma only.6 F8 3.03C22 F3002.0 L.oLTemperature(C)Figure 3. Conversion of (a) methane and yields of (b) methanol, (C) CO and (d) CO2 as a fyngtion 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 gAs seen from Figure 3(d), the selectivity to CO2 wasB'Ig) at 337.1 nm, 357.6 nm and 380.4 nm. The molecu-much higher in the reactor with the porous packing materi-lar ions produced after ionization were excited and identifiedals (/-Al2O3, M/Al2O3) than in the empty reactor. Especiallyfrom the strong emission bands of the first negative system2.8 when the temperature was higher than 250 °C, the methane(B Euz:x2Zg+) at 391.4 nm and 427.8 nm. The strong nitro-total oxidation may overwhelm partial oxidation, resulting ingen emission line showed that active N2 plays an importanta decrease in methanol yield. (Figure 3b). Hence, a moderaterole in the effective transformation of energy, producing elec-temperature is beneficial for methanol synthesis.trons through the Penning ionization reaction or highly ener-getic species [21,22].^“ In,adition, the weak peak at the position of 431.4 nm is2.6 3.3. Chemical species formed in the PPC and IPC combina-featured'as the CH emission (A2A-x2I1 (0, 0)), showing thetionsdogomposition of methane in the DBD system (Equation 5)..3.I. Identification of active species using in situ opticalCH4 +e-→CHx+(4-x)H+e(5emission spectraNThe pptical emission spectrum of the plasma in oxygen is .。24The in situ optical emission spectra were recorded topr2sented in Figure 4(b). Beside the nitrogen emission bands,identify active components, i.e. atoms or molecules in ex-the atomic oxygen line at 777.4 nm (3'P- +35S) has been .cited electronic, vibrational and rotational states, in the plasmaidentified. Oxygen dissociation can produce a large numberof highly active species which make a major contribution tophase.Figure 4(a) displays the optical emission spectrum ofthe methanol formation [23].25CH4 plasma in the range of wavelength from 250 to 600nm, which contains the most emission lines. Since the ex-中国煤化工periment was carried out at atmospheric pressure, it was ex-The presenceCNMHGeaseintheratepected there were many impurities to be identified. The ni-of direct electronu11 iCaliull. On the othertrogen presents bands of the second positive system (C'Iu-hand, because of the presence of excited molecules of N2, an-200250100Temperature(。9Teperature(。94000440 0NJournal of Natural Gas Chemistry Vol. 19No. 6201063342004000，other additional pathway of O2 dissociation takes place and ure 4c), since the above excited species may rapidly partici-cannot be neglected [24]嵩60pate, inchemical reactions in the gas phase [11,21]:e+N2 - 蓉4A32 t)+e(7)CA4+0-→CHr + OH +(3-x)H-→ CH3OH (9N2(A3 Et)+O22→N2+20(P) " (8The contribution of this pathway to the O atom generationAs mentioned in section 3.2, introducing catalysts into theis comparable to O2 dissociationby elesgtron sollisigzn.plasmoaodischarge may affect the type of discharge or can in-However, the emission,Jlines of CH and O were not ob-duce a shift in the distribution of the accelerated electrons,1000 served in the optical emission spectra cortfiy:m plidsma (Fig-which again influences the production of excited species [16]. .004400 E4000 g4200 E4200 (a803000 ()0FH1400 E40，50200020wuh W12007008001000 E宣1500428430432454361000Wavelength (nm)6005000400200250 300 350 400 450 500 550 600304070(Cd)4000 t60002000 t00 70Wavelengh (nm)10000e)800060Figure 4. Optical emission spectra of DBD at input power of 120 W and discharge frequency of 7kHz.中国煤化工150 scm; (b)O2plasma, O2 flow rate of 150 sccm; (c) CH4/air pA00, feed flow rate of 300 sccm, CH4/air ratio of 1/1 inIYHCNMHGyrateof300scm,CH4/air ratio of 1/1 in feed, plasma reactor packed with 7-Al2O3 (6 g); (e) CH4/air plasma, feed flowri- 1/1 in feed, plasmareactor packed with M-Al2O3 (6g)20300Vave | ength( nm)634Lin Chen et al./ Journal of Natual Gas Chemistry Vol. 19 No.62010As expected, when 7-Al2O3 or M/Al2O3 was introduced intothe discharge zone, the intensity of almost all lines increased03+Z0-→Z+ 202remarkably (Figure 4d and e). Increasing the emission inten-sity, which is related to the density of excited species, may .where, z is an active site on a catalytic surface. During ozoneresult in enhanced methane conversion and methanol yielddecomposition, active oxygen species (such as O(P), O(D),1080O, and possibly O3 ) may be formed and show strong oxidiz-上LU上V1上上, u V上bility.3.3.2. Ozone formation in the nwo combinations3.3.3. FT-IR analysisIt is well known that DB业WyA coniwining gas canproduce high levels of ozone [25]. The formation of O3The FTIR spectra of catalysts before and after the reac-molecules in DBDinly owing to the following reaction:tion are presented in Figure 6. Except the absorption bands ofx7the adsorbed water at 3500 cm-1 and 1630 cm -1, the absorp-oR93) o了o。I,V上上10)tion bands at 2952, 2925, 2864 and 2349 cm-11 occurred in the .Due to its long lifetime, ozone is of importance in bothspectra of the used catalysts. The band at 2349 cm-1 originat-REand IPC combinations. The ozone cncntrations at theing from gaseous CO2 appeared with appreciable intensity.were measured and the resuts are presented in Figure 5.The appearance of the other peaks correspond to VUas(CH3)(at 2952 cm: ), Uas(CH2) (at 2925 cm ), and v(CH2) (at2864 cm~ ) implied that some organicposited or adsorbed on the catalyst surface. These infrared1000二已Plasma-only system四PPC systembands were more intense in M/AI2O3-I than in M/AI2O3-P,88 IPC systemsuggesting that a higher methane conversion could be ob-tained due to the IPC synergistic effect.600 f10040090s 8070y-Al2O、MAI,O,rAI2O M/AILO260Figure s. Ozoe poduction in pure air with dferet plles in the IPC or50. M/AI,O,geue.e Recte cndtiori input pwer 120W dschage fre. M/AIL,O2-P40sccm: CH/air ratio in fe Ii:cataiystamoumprure teed tlowrate 300MAILO,-14000 3500 30002500 2000 1500 1000400 is noteworthy that the combination of r-Al2O3 orM/Al2O3 with the plasma in the IPC configuration consid-erably enhanced ozone production. As stated earlier, packed-Figure 6. FT-IR spectra of the catalystsbed plasma reactors, constructed by packing catalytic or non-catalytic diclectric pellets inside non- thermal plasma reactors,3.4.Catalytic stability in the Iwo plasma catalysiscan improve the energy efficiency effectively [26]. The in-combinationscrease! 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-It has been experimentally demonstrated that the combi-composition of ozone than that of blank carrier with thenation of the plasma and M/Al2O3 with the two configurations200 in the IPC system. In terms of the reactivity towardresults in a synergistic efect, i.e., the performance obtained bymethane, te oxygen atom is chemically more active speciesplasma-catalysis is better than the summation con of plasma-than ozone. The introduction of M/AI2O3 to the dischargeand catalysis-alolone, indicating that plasma-catalysis presentszone produced a larger amount of active oxygen species,the promising application on MPOM. To evaluate the reac-which further promoted the methanol generation.tion stabilit, the extended MPOM reactions over M/Al2O3Adding 7-Al2O3 or M/Al2O3 to the post-plasma position,were carried out in the IPC or the PPC reactor, where the cat-resulted in a sligight reductionn of the outlet concentration ofalyst temperature was maintained at 200°C.1e time depen-ozone. The heterogeneous catalytic decomposition of ozonedency of the meth中国煤化工7. The experi-takes place according to the following mechanism [27]:ment demonstrateJHCNMHGnceinthePPCconfiguration charguroug，aiu iiaiiud the initial ac-O3+Z-→Z0+O2(11) tivity for 10h, showing that almost no catalyst deactivationy- A1203 M A1203y-A120MV A1203Journal of Natural Gas Chemistry VoL. 19 No.620106351 coyld be detected during the reaction period. Nevertheless in| th@ IPC case, the methanol yield maintained stable with 1.6%plasma reactor wall, but it did not disrupt the discharge. In thein the first 2 h and then dropped gradually to 1.4% after run-second catalytic stage, no significant amount of carbon wasning the reaction for 10 h, which was lower than that withattached to the catalyst surface. The low reaction temperaturethe discharge alone (1.5%). Therefore, it appeared that the(50-250 °C) is beneficial to limiting coke deposition result-M/Al2O3 catalyst exhibited less stability during the IPC pro-ing from CH4 decomposition. In consequence, the M/Al2O3cess. This result may be due to the fact that the catalyst used catalyst in the PPC configuration maintains a better stabilityin the IPC configuration easily suffers from the deactivationthan in the IPC configuration.caused by carbon deposition. According to thermodynamicTo prove the above viewpoint, the characterization of thecalculations, the origin of inactive carbon during the methaneM/Al2O3 catalyst was carried out before/after the reaction byreforming might occur via CH4 decomposition (Equation 13)TG, XRD, and N2 adsorption-desorption. The used catalystsand CO disproportionation (Equation 14) [28].were taken after running the reaction for 10h in the PPC orIPC configuration.CH4-→C+ 2H2(13)TG analysis was conducted to test the carbon deposition1.6on the catalysts. The M/Al2O3-P catalyst did not show any2C0一+C+ CO2(14)weight loss in its TG analysis, indicating that the coke de-position on it was not obvious. However, a weight loss of1.83.8 wt% occurred at 200 °C over the MAl2O3-I catalyst, in-dicating that there was carbon deposit on the catalyst surface.-●PPCTherefore, it is reasonable to suggest that the obvious deacti-装-o- IPC1.6-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-勺1.4fore and after the reaction. The M/Al2O3 displayed only the①characteristic peaks of Al2O3 and Fe2O3 with no separationof CuO phase. The absence of CuO diffraction peaks sug-1.2。文1.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 found1.0L墨0for the used catalysts in both the PPC and IPC configurations,Reaction time (h)indicating that there was no change in Al2O3 and Fe2O3 crys-Figure 7. CH3OH yield v8. reaction time in the IPC or the PPC process usingtallite structure after the plasma reaction. It was noteworthyM/Al2O3. Reaction conditions: input power, 120 W; discharge frequency, 7that a weak peak at 20= 26° was observed for the M/Al2O3-IkHz; discharge gap distance, 2.0 mm; atmosphere pressure; catalyst temper-sample, which was related to the plane (0 0 2) of the graphiticature, 200 °C; feed flowrate, 300 sccm; CH4/air ratio in feed, 1/1; catalystweight, 6g .structure, showing the formation of coke on the catalyst.CH4 decomposition is endothermic, and its equilibriumconstant increases with increasing temperature. Conversely,CO disproportionation is exothermic, and its equilibrium con-Fe2O31 stiant decreases with the increase of temperature. ComparedIwfth CH4, CO is more difficult to be dissociated via the colli-sion with electrons. The CO dissociation energyof 11.1eV isMwwMIALO,higher than the CHx (x= 1-4) dissociation energy of 4.9 eVww[29]. Accordingly, CH4 decomposition will make the main。M/AIL,O;-Pcontribution to the coke deposition in the POM reaction.As CH4 decomposition requires high temperature to producewwwwwn.solid coke, carbon formation is favored at high temperatureMAl2O-1(973 K