Synthesis Gas Production from Natural Gas on Supported Pt Catalysts

Availableonlineatwww.sciencedirect.comdbJournal of Natural Gas Chemistry 15(2006)21-27SCIENCE PRESSArticleSynthesis Gas Production from Natural Gason Supported Pt CatalystsMariana M.v. M. Souza, 2*, Octavio R. Macedo Neto, Martin SchmallNUCAT/PEQ/COPPE, Universidade Federal do rio de Janeiro, C P. 68502, 21945-970, Rio de Janeiro, brazil2. Escola de Quimica, Universidade Federal do rio de janeiro, C P. 68542, 21940-900, Rio de Janeiro, BrazilManuscript received November 9, 2005; revised December 5, 2005]Abstract: Auto-thermal reforming of methane, combining partial oxidation and reforming of methanewith COz or steam, was carried out with Pt/Al2O3, Pt/ZrO2 and Pt/CeO2 catalysts, in a temperaturerange of 300-900 C. The auto-thermal reforming occurs in two simultaneous stages, namely, total combus-tion of methane and reforming of the unconverted methane with steam and CO2, with the O2 conversionf 100% starting from 450C For combination with CO2 reforming, the Pt/CeO2 catalyst showed thelowest initial activity at 800C, and the highest stability over 40 h on-stream. This catalyst also presentedthe best performance for the reaction with steam at 800C. The higher resistance to coke formation of thecatalyst supported on ceria is due to the metal-support interactions and the higher mobility of oxygen inthe oxide latticeKey words: methane; steam reforming; CO2 reforming; partial oxidation; auto-thermal reformingplatinum catalyst; ceria; zir1. Introductionbecause of the highly endothermic property of the reaction and the H2/CO ratio obtained (about 3 )is onlyRenewed attention in both academic and indus- suitable for processes requiring a H2-rich feed(such astrial research has recently been focused on alterna- ammonia synthesis and petroleum refining process)tive routes for conversion of natural gas(methane)to but it is too high for fuel synthesis via Fischer-Tropschsynthesis gas, a mixture of Co and H2, which can be reaction (2, 3. Moreover, steam reformers are inade-used to produce chemical products with high added quate for hydrogen generation in fuel cell electric vehi-values, such as hydrocarbons and oxygenated com- cles because of their high thermal inertia for frequentpounds. In GTL (gas-to-liquid) plants, where natural start-up and shut-down operation condition (4, 5]gas is first converted to synthesis gas, which is theReforming of methane with CO2 is an alternatifeedstock for Fischer-Tropsch synthesis of hydrocar- route for synthesis gas generation, with important adbons, where, above 60%-70% of the cost of the overall vantages for some applicationsince the H2/c(process is associated with the production of synthesis uct ratio(about 1)is more adequate for synthesis ofas[1, 2. Therefore, reduction in synthesis gas gener- oxoalcohols, acetic acid and dimethyl ether [6, 7]. It isation costs would have a large and direct influence on also environmental friendly process because it reducesthe overall economics of these downstream industrial CO 2 emissions, which is the main source of greenhouseeffeSteam reforming of methane(SRM)is the mostimportant industrial process for production of syn. ofH中国煤化工CNMHTR), a combination-tion reactions, is anthesis gas. SRM is a very energy-intensive process advantageous route for synthesis gas piCorresponding author. Fax:(5521)25627598; E-mail: mmattos @eq ufrj. brMariana M. V M. Souza et al. Journal of Natural Gas Chemistry Vol. 15 No. 1 2006both economical and technical reasons. It has low- by H2 and CO chemisorption and TPR (19energy requirements due to the opposite contributionof the exothermic methane oxidation and endother- 2.2. Catalyst testingmic steam reforming. The combination of these re-actions can improve the reactor temperature controlThe reaction was carried out in a fixed-bed flowand reduce the formation of hot spots, avoiding cat- type quartz reactor loaded with 20 mg of catalyst,alyst deactivation by sintering or carbon deposition. under atmospheric pressure. The total feed flow rateMoreover, ATR allows the production of synthesis gas was held constant at 200 cm/min(WHSV=160h-1ywith a wider range of H2/Co ratios by manipulat- with flowing He. The steam was added to the systeming the relative concentrations of CO2/H2O and Oby a saturator with temperature control. The activityin the feed [8-10]. All these advantages indicate that tests were performed at different temperatures, rang-atR should be the technology of choice for large-scale ing from 300 to 900"C in steps of 50C that were keptGTL plants [11]. In addition, a fuel processor based for 30 min at each temperature. The loss in catalyston auto-thermal reforming of methane could provide activity at 800C was monitored up to 40 h on streama low cost and compact system, with fast start-up and The reaction products were analyzed by on-line gascapability to follow load variations, more adequate for chromatograph(CHROMPACK CP9001), equippedfuel cell electric vehicles [12, 13with a Hayesep d column and a thermal conductivityIt is well-known that the support has a significantdetectoreffect on the overall catalytic behaviors and the useof reducible oxides, like ZrO2 and CeO2, can result 2.3. Carbon deposition measurementsin additional process benefits when compared to irre-The amount of coke formed over the catalystsducible oxides, such as Al2 O3 or SiO2[14-16). The after deactivation tests at 800C was examined byreducibility and oxygen transfer capacity of zroz thermogravimetric analysis(TGA), using a RIGAKUand CeO2 have shown to be fundamental in keep-ermoanalyzer(model TAS 100). The samples wereing the active phase surface free of carbon deposits15, 17, 18. We have previously reported that Pt/zrOz heated at a rate of 10/min to 800"C in a flow ofand Pt/CeO2 are effective formulations for CO2 re- 15%02/N2(50 cm/min)forming and partial oxidation of methane [19].Theaim of this work is to investigate the coupling between3. Results and discussionthe steam or CO2 reforming with partial oxidation ofmethane, over Pt/ZrO2 and Pt/CeO2 catalysts, com- 3.1. Coupling between CO2 reforming and par-aring with the catalytic behavior of Pt/Al2O3tial oxidation of methane2. ExperimentalThe comparison of catalyst activities for combined CO2 reforming and partial oxidation of methane2.1. Catalyst preparationis displayed in Figure 1, in terms of CHa conversionand H2/Co product ratio, respectively. The O2 con-Al2 O3, ZrO2 and Ceo z supports were prepared version is 100% starting from 450 C. All three cata-by calcination of y-alumina(Engelhard Corrporationsts presented similar activities, with Pt/ZrO2 beingCatalyst), zirconium hydroxide(MEL Chemicals)and slightly less active in the mid-temperature range(450-cerium ammonium nitrate(Aldrich)at 550"C for 2 h 600"C) and the most active at temperatures higherunder flowing aithan 700CThe Pt catalysts were prepared by incipient wetThe activity was almost constant in temperatureness impregnation of the supports with an aqueous range between 450 and 600C, mainly for Pt/Al203solution of chloroplatinic acid (H2PtCl6, Aldrich), fol- and Pt/ZrO2 catalysts, which can be related to thelowed by drying at 120"C for 16 h and calcination in combustion of methane to CO2 and H2O, a veryair at 550C for 2 h. All samples contained abouteXo中国煤化工1 wt% of platinum, which was determined by X-rayfluorescenceThese catalysts have already been characterizedCNMH52+H20(1)△H29K=-802kJ/molJournal of Natural Gas Chemistry Vol. 15 No. 1 2006(1)Temperature(℃)Figure 1. Catalytic activities in terms of CHa conversion(a) and H2/Co product ratio(b)of Pt catalysts forcombined COz reforming and partial oxidation of methane as a function of temperature.Reaction conditions: O2/CH4=0. 25, CO2/CH4=0.5(1)Pt/Al2O3,(2)Pt/ZrO2, (3)Pt/CeO2With increasing temperature, CH4 and CO2 con- shown in Figure 2 for the Pt/CeO2 catalyst. The O2versions increase while the H2/CO ratio decreases. At conversion is 100% starting from 450C. The produc-low temperatures, the H2/Co ratio is most influenced tion of steam starts at 450C and begins to decreaseby the partial oxidation of methane and the reverse at 550C, remaining almost constant until 800Cwater-gas shift(WGS)reaction, and can be illustrated This profile also confirms that methane combustionoccurred to a greater extent at lower temperaturesCH4+1/202÷→CO+2H2△H29sK=-36kJ/mol(2) which explains the H2O production and decreasedCO2 conversionCO2+H2+→CO+H2O△H298K=41kJ/molAt high temperatures, the H2/Co ratio is practicallyonly due to CO2 reforming of methane illustrated asCO,CH4+CO2←→CO+H2AH298 K=247 kJ/molUsing the same feed ratio, Ruckenstein andlu 20]obtained a H2/CO ratio of 1. 3 over nickel cat-alysts, at 790.C. These catalysts probably have higheractivity to CHA oxidation than the Pt catalysts stud-ied hereSeveral authors have proposedanism for the partial oxidation of methane. In thefirst step combustion of methane takes place, producing CO2 and H2O: in the second step, synthesis gas isproduced via CO 2 and steam reforming reactions ofun-reacted methane[[21-23]中国煤化工)Figurfor combinedThis indirect mechanism is also clearly evidencedCN Gtial oxidation offor combined CO2 reforming and partial oxidation offunction of temperatmethane, as can be seen by the composition profilesover Pt/CeO2 catalyst. Reaction condi-tions: O2/CH4=0. 25, Co2/CHa=0.5Mariana M. V M. Souza et al. Journal of Natural Gas Chemistry Vol. 15 No. 1 20063.2. Coupling between steam reforming and CH4 conversion, maintaining this conversion up topartial oxidation of methane550C. The Pt/CeO2 was the most active catalystover the mid-temperature range (450-600'C)while atFigures 3(a)and 3(b) display a comparison of cat- higher temperatures Pt/ZrO2 exhibited slightly betalyst activities for combined steam reforming and par- ter activity than Pt/Al2O3 and Pt/CeO2. The contial oxidation of methane, in terms of CH4 conversion stant conversion in intermediate temperatures(up toand H2/ Co product ratio, respectively. The Pt/ZrO2 650C)can also be associated to the total combustionshowed the highest activity at 400oC, with 20% of of methane(reaction 1)60Figure 3. Catalytic activities in terms of CHa conversion(a) and Ha/Co product ratio(b)of Pt catalysts forcombined steam reforming and partial oxidation of methane as a function of temperatureReaction conditions: O2/CH4=0.25, H2O/CH4=0.5(1)Pt/Al2 O3,(2)Pt/ZrO2,(3)Pt/CeO2The observed high H2/Co ratio (6.0)at low conversion at this temperature is 97%. The Pt/CeOrlowest initial(73%ofgests that water-gas shift reaction occurs to a great conversion) but with higher stability over 40 h on-extent with reforming of methane, as already reported stream(63% of final conversion). The Pt/Al2O3 andin the literature[9, 24. At the same time, the decrease Pt/ZrO2 deactivated very fast during 39 h on streamin H2/Co ratio with increasing reaction temperature with a deactivation rate of about 0.9 and 1.0%/h,re-is consistent with the fact that WGS reaction is ther- spectively, while the Pt/CeO2 deactivated in the firstmodynamically unfavorable at higher temperatures. 6 h of reaction, remaining stable for over 30 h, withAt high temperatures, the H2/Co ratio lower a deactivation rate of only 0.4%/h. The amount ofthan 2.0(this ratio is 1.4 for Pt/ZrO2 and 1.6 for coking on these catalysts after the deactivation testPt/CeO2 at 800C)indicates that the reverse of WGs at 800C was quantified by TGA measurements, car-reaction takes place simultaneously with reforming of ried out in an oxygen-containing atmosphere(Figuremethane, which can be illustrated as4(b)). The stability of the Pt/CeO2 catalyst is reallyassociated with the observation of little coke forma-CH4+H20+Co+3H2△H298K=206kJ/mol(5) tion during the reaction. On the other hand, TGAexperiment showed a weight loss of about 9% and 7%on treating the Pt/Al2O3 and Pt/ZrO 2 catalysts in3.3. Deactivation testsoxygen: the weight loss at low temperatures(<100"C)IS中国煤化工 ation of support andComparison of catalyst stabilities for combined at hiCNMHG) is related to cokeO2 reforming and partial oxidation of methane at oxida800C is shown in Figure 4(a). The equilibrium CHa deposition during 40 h on streamJournal of Natural Gas Chemistry Vol. 15 No. 1 2006-1510152025303540Temperature(℃)Figure 4.(a) Deactivation test for combined CO2 and partial oxidation of methane in terms of CHa conversiona function ofstream at 800C Deactivation test conditions: O2/CHa=0.25, CO2 /CH4=0.5(b)TGA of Ptsts after deactivation test at 800C. TGA test conditions: 15%O2/N2, 10C/minand feed flow(1)Pt/Al2O3,(2)Pt/ZrO2,(3)Pt/CeO2Figure 5(a)displays the comparison of catalyst ity, but deactivated at a rate of 0.86%/h during 28 hstabilities for combined steam reforming and partial on stream. Pt/Al] O3 was the least active catalyst andoxidation of methane at 800C. The equilibrium CHa the deactivation rate was 0.42%/h. The Pt/CeO 2 cat-conversion at this temperature is 95%. The catalysts alyst exhibited the best performance, with CH4 con-exhibited different initial activities and deactivation version decreasing from 77% to 71% during 30 h ofrates. The Pt/ZrO2 showed the highest initial activ- reaction( deactivation rate of 0. 27%/h)是a-10-150100200300400500600700800Time (hTemperature(℃)Figure 5.(a) Deactivation test for combined steam and partial oxidation of methane in terms of CH4 conversionas a function of time on stream at 800C. Deactivation test conditions: O2/CH4=0.25, H20/CH4=0.5(b)TGA of Pt catalysts after deactivation test at 800 C. TGA test conditions: 15%O2/N2, 10C/minand feed flow rate=50 cm/min(1)Pt/Al2O3,(2)Pt/ZrO2,(3)Pt/CeOz中国煤化工The deactivation is related to the deposition of in- metiCN MH Gso in this case. theactive carbon over the active surface and the amount stability of the Pt/ceO2 catalyst is associated withof coke on these catalysts quantified by thermogravi- the observation of little coke formation during the re-Mariana M. V. M. Souza et al. /Journal of Natural Gas Chemistry Vol. 15 No I 2006action. a weight loss of about 17% was observed inin which CO2 or steam reforming and partial oxi-TGA experiments for Pt/Al2O3 and Pt/ZrO2 cata- dation take place simultaneously. The compositionlysts, indicating a significant amount of carbon depo- profiles showed that the reaction proceeds via a two-sition during 30 h on stream.step mechanism with the total combustion of methaneThe deposition of inactive carbon during methane followed by reforming of un-reacted methane withreforming can be originated from either methane de- CO2 and H2O, and the heat released by combustioncomposition (reaction 6)or CO disproportionation favors the reforming reaction. The H2/ Co product(Boudouard reaction 7), which are thermodynami- ratio can be manipulated according to the addition ofcally favorable below 900C [3, 25], can be illustrated CO2 or steam to auto-thermal reforming and it is pos-sible to achieve the optimum ratio for gtl processesCH4←→C+2H2(H2/co=2) by coupling steam reforming and partial△H29K=75kJ/moloxidation of methane2C0←→C+COThe Pt/ZrO2 catalyst showed the highest initial△H29K=-172kJ/mol(7) activity but deactivated very fast, due to deposition ofresidual carbon. The higher stability of the Pt/CeO2Thermodynamic calculations showed that thecatalyst is closely related to its coking resistance ductent of carbon deposition during reforming decreases to Pt-support interactions with formation of interfaat higher reaction temperatures, in agreement with cial sites which promote CO2dissociation,inhibit-several experimental observation [7, 26). These resultsg carbon formation by Boudouard reaction, and thesuggest that co disproportionation is the main con- higher oxygen mobility in the ceria latticetributor to carbon deposition because it is exothermand the equilibrium constant decreases with increas-Referencesing temperaturetclosely related to their coking resistivity which has Rostrup- Nielsen J R. Catal Today, 1994, 21: 257een attributed to the strong interaction of platinum2 Pena M A, Gomez J P, Fierro J L G. Appl Catal A1996,144:7with cationic sites Ce "t on the support surface. The3] Tsang S C, Claridge J B, Green M L H. Catal Today,interfacial sites on Pt-support promote CO2 dissocia-1995,23:3tion, improving the catalyst stability by shifting the (4) Pino L, Recupero V, Beninati S, Shukla A K, HegdeBoudouard reaction (reaction 7). Our previous FM S, Bera P. Appl Catal A, 2002, 225: 63IR analysis [18] showed that the Pt-Zrnt interface is (5) Pino L, Vita A, Cordaro M, Recupero v, Hegde msactive for CO and CO2 adsorption, with a decreaseApplin the Pt-CO bond strength, inhibiting C-o bond (6) Aparicio L M J Catal, 1997, 165: 262breaking and consequently producing less carbon for- [7 Bradford M C J, Vannice M A. Catal Reu-Sci Engmation on the catalyst surface. This same kind of199,41(1):interaction could be responsible for the stability of [8 Liu S, Xiong G, Dong H, Yang W. Appl Catal A, 2000,Pt/CeO2, which presents higher reduction degree and202:141dispersion than Pt/ZrO2, as shown in our previous 19) Liu Z-W, Jun K-W, Roh H-S, Park S-E. J Powerresults of TPR and CO and H2 chemisorption [19]Sources,2002,111:283Moreover, ceria is a well-known oxygen supplier, and [10 Ayabe S, Omoto H, Utaka T, Kikuchi R, Sasaki K,ts oxygen mobility is greater than that of zirconiaTeraoka Y, eguchi K. 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