9
Syngas production by autothermal reforming of methane on supported platinum catalysts Juan A.C. Ruiz a, Fabio B. Passos b, Jose M.C. Bueno c, Eduardo F. Souza-Aguiar d , Lisiane V. Mattos a, Fabio B. Noronha a,* a Lahoratario de Cattilise. /nstituto Nacional de Tecnologia. Av. Venezuela. 82/5/8 Centro. 2/08/-3/2 Rio de Janeiro. RJ. Brazil b Departamento de EngenhariaQufmica e de Petroleo. Universidade Federal Fluminense. Rua Passo da Ptitria /56. CEP 242/0-240. Niteroi. RJ. Brazil C Universidade Federal de Slio Carlos-UFSCar. Slio Carlos. SP, Brazil d Universidade Federal do Rio de Janeiro. CENPESIPETROBRAS. Rio de Janeiro. RJ. Brazil Received27 June 2007; received inrevisedform 8 October 2007; accepted 10October 2007 Availableonline 18 October 2007 The performance ofsupported platinum catalysts on the autothermal reforming of methane was evaluated. The effect of the calcination temperature of the CeZrO z support and of the reaction conditions(reaction temperature, presence of CO z in the feedstock, and H z O/Cf4 molar ratio) wasstudied. The catalystswere characterized by BET, XRD, and OSC analyses and the reaction mechanism was determined by TPSR experiments. The TPSR analyses indicate that autothermal reforming of methane proceeds through a two-step mechanism (indirect mechanism) over all catalysts studied. The PtlCeO.75ZrO.Z50Z catalyst presented the best stability, which depends not only on the amount of oxygen vacancies of the support but also on the metal particle size. The higher reducibility and oxygen storage/release capacity of PtICeO.75ZrO.Z50Z catalyst promote the mechanismofcontinuous removal of carbonaceous deposits from the active sites, which takes place at the metal-support interfacial perimeter. The water alsoparticipatesin this mechanism, favouring the carbon removal of metalparticle. Furthermore, the reaction conditionsinfluenced significantly the behaviour of PtICeO.75ZrO.Z50Z catalysts. The increase of H z O/CH 4 molar ratio had a beneficial effect on the methane conversion and on the Hz/CO molar ratio. However, the increase of the reaction temperature had an opposite effect. Both the methane conversion and Hz/CO molar ratio decreased with the increasing of reaction temperature. Moreover, the addition of CO z to feedstock increased the initial methane conversion. but decreased the stabilityof the catalyst. Recently, the need to reduce the dependence on petroleum as feedstock and the continuous increase of natural gas proven reserves has generated interest in the extensive use of this natural resource. Unfortunately, the majority of world gas reserves are located either in small fields or in remote regions which cannot be monetized using conventional technologies (pipelines or LNG). In addition, some stranded gas reserves are associated to oil. The increase of environmental restrictions to gas flaring or ventilation makes difficult the development of these fields. * Corresponding author. E-mail address:fabiobel@int.gov.br (EB. Noronha). The conversion of natural gas to liquid fuels, the so-called gas-to-liquids (GTL) technology, is a promising alternative to the monetization of stranded gas reserves and to fulfil the environmental restrictions. However,syngas production com- prises almost half of the capital cost of the GTL plant [1]. Therefore, there is a strong interest in reducing the costs of syngas production technologies. The main technologies for producing syngas from natural gas are steam reforming (SR) and partial oxidation (POX). However, neither SRnor POX produces a syngas having the Hz/ CO ratio suited to GTL plants. An alternative is to use both technologies in parallel to generate syngas streamswhich will produce the desired composition when mixed. This is the base of the Shell's GTL plant in Bintulu, Malaysia [2]. Another approach is the autothermal reforming (ATR), which combines SR and non-catalytic partial oxidation in one reactor.

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Syngas production by autothermal reforming of methaneon supported platinum catalysts

Juan A.C. Ruiz a, Fabio B. Passos b, Jose M.C. Bueno c, Eduardo F. Souza-Aguiard,

Lisiane V. Mattos a, Fabio B. Noronha a,*

a Lahoratario de Cattilise. /nstituto Nacional de Tecnologia. Av. Venezuela. 82/5/8 Centro. 2/08/-3/2 Rio de Janeiro. RJ. BrazilbDepartamento de Engenharia Qufmica e de Petroleo. Universidade Federal Fluminense. Rua Passo da Ptitria /56. CEP 242/0-240. Niteroi. RJ. Brazil

C Universidade Federal de Slio Carlos-UFSCar. Slio Carlos. SP, Brazild Universidade Federal do Rio de Janeiro. CENPESIPETROBRAS. Rio de Janeiro. RJ. Brazil

Received 27 June 2007; received in revised form 8 October 2007; accepted 10 October 2007Available online 18 October 2007

The performance of supported platinum catalysts on the autothermal reforming of methane was evaluated. The effect of the calcinationtemperature of the CeZrOz support and of the reaction conditions (reaction temperature, presence of COz in the feedstock, and HzO/Cf4 molarratio) was studied. The catalysts were characterized by BET, XRD, and OSC analyses and the reaction mechanism was determined by TPSRexperiments. The TPSR analyses indicate that autothermal reforming of methane proceeds through a two-step mechanism (indirect mechanism)over all catalysts studied. The PtlCeO.75ZrO.Z50Zcatalyst presented the best stability, which depends not only on the amount of oxygen vacancies ofthe support but also on the metal particle size. The higher reducibility and oxygen storage/release capacity of PtICeO.75ZrO.Z50Zcatalyst promote themechanism of continuous removal of carbonaceous deposits from the active sites, which takes place at the metal-support interfacial perimeter. Thewater also participates in this mechanism, favouring the carbon removal of metal particle. Furthermore, the reaction conditions influencedsignificantly the behaviour of PtICeO.75ZrO.Z50Zcatalysts. The increase of HzO/CH4 molar ratio had a beneficial effect on the methane conversionand on the Hz/CO molar ratio. However, the increase of the reaction temperature had an opposite effect. Both the methane conversion and Hz/COmolar ratio decreased with the increasing of reaction temperature. Moreover, the addition of COz to feedstock increased the initial methaneconversion. but decreased the stability of the catalyst.

Recently, the need to reduce the dependence on petroleum asfeedstock and the continuous increase of natural gas provenreserves has generated interest in the extensive use of thisnatural resource. Unfortunately, the majority of world gasreserves are located either in small fields or in remote regionswhich cannot be monetized using conventional technologies(pipelines or LNG). In addition, some stranded gas reserves areassociated to oil. The increase of environmental restrictions togas flaring or ventilation makes difficult the development ofthese fields.

* Corresponding author.E-mail address:[email protected] (EB. Noronha).

The conversion of natural gas to liquid fuels, the so-calledgas-to-liquids (GTL) technology, is a promising alternative tothe monetization of stranded gas reserves and to fulfil theenvironmental restrictions. However, syngas production com-prises almost half of the capital cost of the GTL plant [1].Therefore, there is a strong interest in reducing the costs ofsyngas production technologies.

The main technologies for producing syngas from naturalgas are steam reforming (SR) and partial oxidation (POX).However, neither SR nor POX produces a syngas having the Hz/CO ratio suited to GTL plants. An alternative is to use bothtechnologies in parallel to generate syngas streams which willproduce the desired composition when mixed. This is the baseof the Shell's GTL plant in Bintulu, Malaysia [2]. Anotherapproach is the autothermal reforming (ATR), which combinesSR and non-catalytic partial oxidation in one reactor.

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Autothermal reforming (ATR) technology is the preferredand most cost effective technology for natural gas conversioninto synthesis gas to GTL plants. The Oryx GTL plant in Qataris based on ATR technology at low steam/carbon ratio (S/C = 0.6). ATR process produces a synthesis gas with a Hz/COratio equal to 2, which is suitable for the subsequent use in theFischer-Tropsch synthesis [3]. This Hz/CO ratio can beachieved through recirculation of COz or a COz rich off gas aswell as reducing the amount of steam in the feed.

Operation at low steam to carbon (S/C) ratio improves thesyngas composition and reduces the COz recycle, whichdecreases the investment and energy consumption. However,the reduction of SIC ratio favours soot formation in the ATRreactor and carbon formation in the pre-reformer [I].

Then, there is a great interest in determining appropriatedreaction conditions and designing catalysts that are able tooperate under severe conditions. Commercial ATR catalyst isa SR nickel catalyst adapted to the ATR conditions. Recently,there was a renewed interest in catalyst development toautothermal reforming for hydrogen production to fuel cell[4-10]. The majority of these studies is based on Ni supportedover different supports [4-9]. These studies revealed thatthe support plays an important role when the stability of theNi catalysts under autothermal reforming conditions isconcerned.

However, only a few reports dealt with noble metal-supported catalysts [5, 10], which would be more appropriate tointensified processes using micro-reactors. Previous papershave reported that the PtlCeZrOz catalysts exhibited a highactivity and stability on dry reforming and partial oxidation ofmethane [11,12]. The enhanced performance of these materialswas attributed to the presence of oxygen vacancies in thesupport, which proved to be fundamental in keeping metalparticles free from coke deposition.

The aim of this work was to evaluate the performance of theplatinum-supported catalysts on autothermal reforming ofmethane. The effect of the calcination temperature of theCeZrOz support and the reaction conditions (reaction tem-perature, presence of COz in the feedstock, HzO/Cf4 molarratio and space velocity) on catalyst activity, selectivity, andstability was studied.

Ah03, CeOz, zrOz, and Ceo.14ZrO.860Zwere prepared bycalcination of 'Y-Alz03, (NH4hCe(N03)6, Zr(OH)4, andCeO.14ZrO.860Z(Magnesium Elektron Inc.), respectively at800 °C for 1 h. CeOz (prec) and CexZrl_xOz supports (x = 0.50and 0.75) were prepared by the precipitation method describedby Hori et al. [13]. An aqueous solution of cerium(IV)ammonium nitrate and zirconium nitrate (Aldrich) wasprepared with the desired Ce/Zr ratio. Then, the ceria andzirconium hydroxides were co-precipitated by the addition ofan excess of ammonium hydroxide. Finally, the precipitate waswashed with distilled water and calcined at 1073 K for 1 h in a

muffle. The CeO.7SZrO.ZsOzsupport was also calcined at 1273 Kfor 1 h. The catalysts were prepared by an incipient wetnessimpregnation technique using an HzPtCI6·6HzO aqueoussolution. After impregnation, the samples were dried at393 K and calcined under air (50 cm3/min) at 673 K for 2 h.All samples contained 1.5 wt.% of platinum. Then, thefollowing catalysts were prepared: PtlAh03, PtlCeOz, PtICeOz (prec), PtlZrOz, Pt/CeO.14ZrO.860Z,PtlCeo.soZro.soOz,PtICeo.7sZro.zsOz-1073 (CeO.7SZrO.ZsOzcalcined at 1073 K) andPtlCeo.7sZro.zsOrI273 (CeO.7SZrO.ZsOzcalcined at 1273 K).

2.2.1. BET surface areaThe BET surface areas of catalysts were measured using a

Micromeritics ASAP 2000 analyzer by nitrogen adsorption atliquid nitrogen temperature.

2.2.2. X-ray diffraction (XRD)X-ray diffraction measurements were carried out using a

Philips PW3710 diffractometer with a Cu Ka radiation(A. = 1.5406 A.). After calcination at 1073 or 1273 K ofsupports, the XRD data were collected between 28 = 25° and75° (0.04°/step; 1 s/step). Crystallite sizes were estimated fromthe integral width of the highest intensity line with using theScherrer equation.

2.2.3. Oxygen storage capacity (OSC)Oxygen storage capacity (OSC) measurements were carried

out in a micro-reactor coupled to a quadrupole mass spectro-meter (Omnistar, Balzers). Prior to OSC analysis, the sampleswere reduced under Hz at 773 K for 1 h and heated to 1073 Kinflowing He. Then, the samples were cooled to 723 K andremained at this temperature during the analysis. The massspectrometer was used to measure the composition of thereactor effluent as a function of time while a 5% Oz/He mixturewas passed through the catalyst. Oxygen consumption wascalculated from the curve corresponding to m/e = 32 taking intoaccount a previous calibration of the equipment.

2.2.4. Cyclohexane dehydrogenationPlatinum dispersion was estimated using the reaction of

cyclohexane dehydrogenation, a structure insensitive reaction[14]. Since Hz and CO adsorption occurs over both CeOz andCeZrOz supports, platinum dispersion could not be determinedfrom chemisorption of both gases [15]. Then, in order toestimate the dispersion of PtlCeZrOz catalysts, a correlationbetween the rate of cyclohexane dehydrogenation and platinumdispersion measured by hydrogen chemisorption establishedfor PtlAlz03 catalysts was employed. This methodology wasused successfully before [16].

Cyclohexane dehydrogenation was performed in a fixed-bedreactor at atmospheric pressure. The catalysts were reduced at773 K for 1 h and the reaction was carried out at 543 K andWHSV = 170 h-I. The reactants were fed to the reactor bybubbling Hz through a saturator containing cyclohexane at285 K (Hz/HC = 13.6). The exit gases were analyzed using a

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gas chromatograph (HP5890) equipped with a HP-INNOWAXcolumn.

2.2.5. Temperature-programmed surface reaction (TPSR)Temperature-programmed surface reaction (TPSR) experi-

ments were performed in the same apparatus used for OSCanalysis. After the activation protocol, the sample (300 mg) waspurged in He at 1073 K for 30 min, and cooled to roomtemperature. Then the sample was submitted to a flow of CUt!H20/02IHe (4:2: 1:25) at 30 cm3/min while the temperature wasraised up to 1073 K at heating rate of 20 Klmin.

The autothermal reforming of methane was carried out in aquartz reactor at atmospheric pressure. Prior to reaction, thecatalyst was reduced at 773 K for 1 h under H2. Then, thecatalyst was heated until reaction temperature 0073 or 1273 K)is reached, under N2. The reaction was performed at a spacevelocity (WHSV) equal to 260 h-I, using a 02/CH4 molar ratioof 0.5, a H20/CH4 molar ratio of 0.2, 0.37, or 0.6. The effect ofCO2 addition to feedstock was evaluated using a CO2/CH4 ratioof 0.01. The reactants were fed to the reactor by bubbling CHJO2 or CHJ02/C02 mixtures (total flow rate = 100 cm3/min)through a saturator containing water at the temperature requiredto obtain the desired H20/CH4 molar ratio. In order to avoid hotspot formation, catalyst samples were diluted with inert SiC(SiC mass/catalyst mass = 2.0) with the bed height beingaround 3 mm. The reaction products were analyzed using a gaschromatograph (Agilent 6890) equipped with a TCD andCarboxen 1010 column (Supelco).

Table 1 shows BET surface areas obtained for all supports.Ah03 exhibited the higher BET surface area. For the othersupports calcined at 1073 K, the addition of zr02 to Ce02increased the surface area from 9 to 43 m2/g, depending on thesupport composition. The enhancement of the surface area withthe addition of zirconia to ceria was also observed by Hori et al.[13], which is attributed to the ability of zirconia to improve thethermal stability of ceria [17]. For CeO.7SZrO.2S02support, the

Table IBET surface area and crystallite size calculated by DRX analysis for allsupports

Surfacearea (m2/g)

Particlesize (nm)

Ah03

Ce02 (prec)Ce02zr02

CeO.l~rO.8602Ceo.soZrO.S002CeO.7SZrO.2s0r1073CeO.7SZrO.2s0r1273

40 so29 (0)

increase of calcination temperature from 1073 to 1273 K led tothe decrease of surface area, that is in agreement with theliterature [13,18].

The XRD results obtained for Ce02, Ce02 (prec), and zr02supports are presented in Fig. 1. A cubic phase (JCPDS: 4-0593) for Ce02 and Ce02 (prec) supports was observed. zr02support exhibited a monoclinic phase (JCPDS: 13-307).

The XRD patterns of CeO.14ZrO.8602, Ceo.soZro.so02>Ceo.7sZro.2s02-1073, and CeO.7SZrO.2s0r1273 supports arepresented in Fig. 2.

For CeO.14ZrO.8602support, the peak at 28 = 30.00 isassociated to the formation of a tetragonal zirconia phase.On the other hand, the doublet at 28 = 34.40 and 35.00 may be

Fig. 2. X-ray diffraction patterns of Ceo.7SZro.2s0r 1073, Ceo.7sZro.2s0rI273,Ceo.SoZr0.5002.and CeO.14ZrO.8602supports. The solid vertical lines are char-acteristic of cubic ceria and the broken vertical lines are characteristic oftetragonal zirconia.

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assigned to the tetragonal ceria-zirconia solid solution. Thepresence of small amount of Ce4+ in the zirconia lattice mayhave shifted the signal to smaller angles. According to theliterature [13], these results indicate the formation of atetragonal Ce-Zr solid solution and a separated zirconia phaseon CeO.14ZrO.860Zsupport.

By adding 50% of zrOz, a shift of ceria peaks from2()= 28.6° to 29.3° and from 2()= 33.1° to 33.8° was observed.These results show that a single phase (Zr-doped cubic ceria)was formed. Nevertheless, the occurrence of another crystallinephase (Ce-doped tetragonal zirconia) cannot be disregarded[13].

The XRD patterns of Ceo.7sZro.zsOz-1073 andCeo.7sZro.zsOz-1273 supports are similar, indicating that theincrease in calcination temperature of the support did not affectthe crystalline structure of the samples. Moreover, the additionof 25% of zrOz to CeOz did not result in a separate zirconiaphase. However, the ceria peaks shifted from 2()= 28.6° to29.0° and from 2()= 33.2° to 33.5°, which reveals the formationof a CeOz-zrOz solid solution with a cubic symmetry [13].

The crystallite sizes determined by XRD analyses for CeOz,CeOz (prec), zrOz, Ceo.I4ZrO.860Z, Ceo.soZro.soOz,Ceo.7sZro.zsOz-1073, and Ceo.7sZro.zsOz-1273 catalysts areshown in Table 1.The crystallite sizes obtained for CeOz, CeOz(prec), and zrOz supports were similar (21-26 nm). Theaddition of zrOz to CeOz caused a decrease in crystallite size.These results agree with the BET surface area. According to theliterature [17], the presence of zrOz improves the thermalstability of CeOz, avoiding the sintering process.

Among the CeZrOz supports calcined at 1073 K,CeO.14ZrO.860Zsupport showed the largest crystallite size. Thisresult can be attributed to the formation of a non-homogeneoussolid solution on Ceo.14ZrO.860Zsupport [13,17], as describedabove. Furthermore, for Ceo.7sZro.zsOz-1073and Ceo.soZro.soOzsupports, a similar crystallite size (8-9 nm) was obtained. Sincethese materials were prepared by the same method (co-precipitation), this result suggests that the composition of thesolid solution did not affect the particle size. Comparing theresults obtained for Ceo.7sZro.zsOz-1073 and CeO.7SZrO.ZsOz-1273 supports, it was observed that the increase in calcinationtemperature led to an increase in crystallite size. These results arein agreement with BET surface area measurements.

Table 2 shows the oxygen uptakes measured for the catalysts.PtIA1z03 catalyst did not present any oxygen consumption, while

Table 2O2 uptakes and metallic dispersion measured for platinum catalysts

Catalysts O2 uptake PI dispersion(",mol/gca,) (%)

PtlAI20J 0 48PtlCe02 (prec) 18 15PtlCe02 194 48PtIZr02 9 29PtlCeo.l~rO.8602 220 34PtlCeo.soZrO.S002 696 14PtlCeO.7SZrO.2s0rI073 626 22PtlCeQ.7SZrO.2s0r1273 726 II

pt/CeOz (prec) and pt/zrOz catalysts exhibited low oxygenuptakes. PtlCeOz and Pt/CeO.14ZrO.860Zcatalysts showed similaroxygen storage capacity, which is higher than that obtained forpt/CeOz (prec) and PtlzrOz catalysts. The oxygen consumptionobserved for PtlCeo.soZro.soOz, PtlCeo.7sZro.zsOz-1073, and PtICeo.7sZro.zsOz-1273 catalysts is considerably higher than thatobtained for the other catalysts. In addition, the increase of zrOzmolar content from 25 to 50% has not caused a significant changeon OSC values.

The values of Oz consumption on Pt/CeZrOz catalysts agreewith those reported by Fally et al. [19] on CeOz-zrOz systems.The OSC values obtained for these mixed oxides with azirconium content between 20 and 50% are very close.Fornasiero et al. [20] also obtained the maximum OSC valuesfor intermediate ZrOz contents (25-50%).

Several studies [17,18] reported that cerium oxide has anoxygen exchange capacity, which is associated to the ability ofcerium to act as an oxygen buffer by storing/releasing Oz due tothe Ce4+/Ce3+ redox couple [18]. The incorporation of zrOzinto CeOz lattice promotes the CeOz redox properties. Thepresence ofZrOz strongly increases the oxygen vacancies of thesupport, increasing its reducibility. The enhancement of oxygenvacancies is due to the high oxygen mobility of the solidsolution formed, which was identified by our XRD data.

The Pt dispersion of the catalysts calculated by means ofcyclohexane dehydrogenation is listed in Table 2. The disper-sion increased in the order PtlCeo.7sZro.zsOz-1273 < PtlCeOz(prec) rv Pt/Ceo.soZro.soOz < Pt/Ceo.7sZro.zsOz-1073 < Pt/zrOz rv PtlCeO.14ZrO.860Z < PtlCeOz = PtlAlz03 catalyst. Theresults obtained for PtlCeo.7sZro.zs0z-1273 and PtICeo.7sZro.zsOz-1073 showed that increasing calcination tem-perature of the support decreased Pt dispersion.

Fig. 3 shows the TPSR profile obtained for Pt/Ceo.7sZro.zsOz-1073catalyst from 400 to 1073 K. All catalystsstudied showed similar profiles. In order to follow in detail thechanges in the signals corresponding to C~, Oz, Hz. and COz•the TPSR profile in the region between 640 and 780 K areshown separately in Fig. 4. At 643 K, the consumption of CH4

Hz

CH.~ Hp=~

°21 cO2.;;; CO

SOO 600 700 800 900 1000 1100

Temperature (K)

Fig. 3. Temperature-programmed surface reaction profile obtained for PtlCeO.7SZro.2s0r 1073 catalyst from 400 to 1073 K.

Page 5: Atr Selectivity

CH~

-;~ H2c;§,';ji

CO2

°640 660 680 700 720 740 760 780

temperature (K)

Fig. 4. Temperature-programmed surface reaction profile obtained for PtlCeO.7SZrO.2s0r1073 catalyst from 670 to 770 K.

and O2was observed along with the formation of H20 and CO2.Above 723 K, large C~ consumption was detected but now, H2and CO are also formed. At temperatures above 873 K, the H2and CO formation kept growing while CO2 and H20 productiondecreased. These results indicate that the autothermal reform-ing of methane is carried out via a two-step mechanism(indirect mechanism). According to this mechanism, the firststep comprises C~ combustion, producing CO2 and H20. Inthe second step, syngas is produced through CO2 and steamreforming of unreacted CH4. This result is agreement with thework of Souza and Schmal, who studied the autothermalreforming of methane on PtlZr02/A1203 catalysts [10].Actually, this result is similar to the two-step reactionmechanism proposed to the partial oxidation of methane[21-23].

3.3.1. Stability of the catalystsFig. 5 shows the methane conversion obtained on

autothermal reforming of methane (ATR of methane) for all

100

~ 7Se....~Cil so~

><= 2S

0100

~~ 7S.,a so..s.,X52S

00 4

-0- PtlCe•...,;zro.,,0,-I073

-e- PtlCe•.."Zr=O.-1273-.0.- PtlCe..",Zro.,p,

-6.- PtlCe•..••Zr•..I6O,

-o-PtiCeO,- •••-PtlzrO,-e- PtlAl,03

-.- PtlCeO,(prec)

8 12 16 20 24time (h)

Fig. 5. Methane conversion (Xmethane) vs. time on stream obtained on auto-thermal reforming of methane for all catalysts (T",acl;on = 1073 K, H201CH4 = 0.2, 02/C~ = 0.5, and WHSV = 260 h-1).

catalysts studied (reaction conditions: 02/C~ and H20/CH4ratio of 0.5 and 0.2, respectively; reaction tempera-ture = 1073 K and WHSV = 260 h-I).

PtlCe02 and PtlCe02 (prec) catalysts showed the higher andthe lower initial activity on ATR of methane, respectively.Furthermore, the PtlA1203, Ptlzr02, PtlCe02 (prec), and PtlCeo.soZrO.S002catalysts deactivated during the reaction. Aslight deactivation is observed on PtlCe02 and PtICeO.14ZrO.8602catalysts. The PtlCeo.7sZro.2s0rI073 and PtICeo.7sZro.2s02-1273 catalysts practically did not lose theiractivity after 24 h time on stream (TOS). This result indicatesthat the calcination temperature of the CeO.7SZrO.2S02supportdid not affect the stability of the catalyst.

Concerning CO and CO2 production (not shown), the COformation decreased, while the production of CO2 increasedduring TOS. This effect is less significant for PtlCeO.7SZrO.2s0r1073, PtlCeo.7sZro.2s02-1273, and PtlCeO.14ZrO.8602catalysts.

In the literature, it has been reported that the support plays animportant role on the stability of supported metallic catalysts onthe CO2 reforming and partial oxidation of methane [24-34].Recently, we have studied the performance of PtlA1203, PtIzr02, and PtlCexZrl_x02 catalysts on CO2 reforming andpartial oxidation of methane [11,30-34]. PtlCeO.7SZrO.2S02catalyst proved to be more stable than PtlAh03 and PtlZr02catalysts on both reactions. The results showed that the stabilityofthis material is due to the high oxygen storage capacity ofthesupport. According to the two-path reaction mechanismproposed to CO2 reforming of methane [11,30,31], thedecomposition of methane takes place on metal surface,resulting in the formation of hydrogen and carbon, which reactswith oxygen from the support near to metal particle to produceCO and oxygen vacancies. The second path involves thedissociation of CO2, The CO2 adsorbs on the support and, nearthe metal particle, it dissociates to form CO and 0. The oxygenformed during the dissociation can then reoxidize the support toprovide a redox mechanism for continuous cleaning. Thebalance between the rate of methane decomposition and the rateof cleaning determines the overall stability of the catalyst on theCO2 reforming of methane. In the absence of a reducible oxide,carbon will deposit around the metal particle, thereby inhibitingthe CO2 dissociation on the support. This effect on the CO2reforming of methane affects the partial oxidation of methanewhich comprises two steps: (i) the combustion of methane and(ii) the steam and CO2 reforming of unreacted methane. In thecase of autothermal reforming of methane, it must be borne inmind that water may also undergo reaction with carbon depositsformed on the metallic particle via CH4 decomposition, hencepromoting the carbon removal [35,36]. According to Wei andIglesia [36], CH4 decomposes to C* in series of elementary H-abstraction steps. C* and H* formed are removed via eitherdesorption or reaction with 0* derived from H20 reactant. Inaddition, water may also replenish oxygen vacancies of thesupport and then indirectly contributes to the mechanism ofcarbon removal from the metal surface [37].

The metal dispersion also affected the performance of PtICeO.7SZrO.2S02,PtlCe0.5oZrO.S002'and PtlCeO.2SZrO.7S02cata-lysts on partial oxidation of methane [12]. PtlCe0.5oZr0.5002

Page 6: Atr Selectivity

catalyst presented the lower stability during this reaction, inspite of its higher amount of oxygen vacancies. The behaviourof this material was assigned to its low metal dispersion. Theincrease of metal particle size decreases the metal-supportinterfacial area, reducing the effectiveness of the cleaningmechanism of metal particle. These results suggest that thestability of the catalysts on partial oxidation of methane isassociated to a proper balance between oxygen transfer abilityof the support and metal dispersion.

In this work, the TPSR experiment revealed that theautothermal reforming of methane takes place through a two-step mechanism, which comprises the total combustion ofmethane followed by the steam and CO2 reforming of methane.Actually, this mechanism is similar to the one proposed to thepartial oxidation of methane [31-33]. Therefore, the supportreducibility also plays an important role on the catalyst stabilityon autothermal reforming, as previously discussed for the CO2reforming and partial oxidation of methane.

The deactivation observed for PtlA1203 catalyst onautothermal reforming of methane can be attributed to thecarbon deposition on the metal surface, since A1203 supportdoes not exhibit OSC. For Pt/zr02 and PtlCe02 (prec) catalysts,the deactivation can be related to their low OSC values(Table 2), which favoured the coke deposition. Temperature-programmed oxidation (TPO) was carried out after reaction onPtlZr02 catalyst in order to study the nature of catalystdeactivation. TPO profile of the PtlZr02 catalyst after 24 h TOSis shown in Fig. 6. The CO2 curve obtained during TPO of thePt/zr02 catalyst exhibited one peak at 668 K indicating theformation of coke during reaction. Recently, we studied thestability of PtlZr02, PtlCe02, and PtlCe-Zr02 catalysts on theCO2 reforming of methane and its relationship with the natureand location of carbon deposits [30,34]. XPS and TPO analysisshowed that the different peaks observed in TPO profile are dueto carbon in different locations on the catalyst surface. The TPOpeak at 668 K could be ascribed to carbon deposited over themetal particle since the catalyst deactivated. Souza and Schmal[10] also observed a strong deactivation of PtlA1203 and Ptlzr02 catalysts on the autothermal reforming of methane after30 h TOS. The deactivation was related to the deposition of

600 800Temperature (K)

Fig. 6. CO2 signal during TPO analysis after autothermal reforming of methaneon PtlZr02 catalyst (Treaction= 1073 K, H20/CH4 = 0.2, 02/CH4 = 0.5, andWHSV = 260 h-1).

inactive carbon over the active surface and the amount of cokeon these catalysts was quantified by thermogravimetricanalysis.

In the case of Pt/Ce0.5oZr0.5002catalyst, that exhibited highoxygen storage capacity, the deactivation is likely due to its lowdispersion, which decreases the metal-support interfacial area.Both the low OSC value and the large metal particle size reducethe effectiveness of the mechanism of carbon removal andincrease the carbon formation on the metal particle. The carbondeposited around the metal particle affects the CO2 dissociationon the support as previously reported on the CO2 reforming ofmethane [11]. Since the second step of the autothermalreforming reaction is CO2 reforming of methane, as revealed byTPSR experiments (Figs. 3 and 4), this step is inhibited. Thisfact can explain the increase of CO2 production as methaneconversion decreased. On the other hand, the improved stabilityof the PtlCeO.75ZrO.250r1073, PtlCeO.75ZrO.250r1273, Pt/Ce02, and PtlCea.t4ZrO.8602 catalysts can be attributed to aproper balance between oxygen transfer capacity of the supportand metal dispersion, which avoids carbon deposition. Thethermo gravimetric analysis (not shown) of PtlCeO.75ZrO.2502-1073 catalyst after catalytic run did not reveal significantcarbon formation.

The stability of the supported Pt catalysts on autothermalreforming of methane depends on the amount of oxygenvacancies of the support as well as the metal particle size.

The PtlCeO.75ZrO.250r1073 and PtlCea.75ZrO.250r1273catalysts were studied in more detail, since these catalysts exhi-bited the highest stability on autothermal reforming of methane.

3.3.2. The effect of the reaction temperatureThe effect of the reaction temperature was evaluated for Pt/

CeO.75ZrO.250r1273 catalyst (H20/Cf4 ratio = 0.2, 02/Cf4ratio = 0.5). At reaction temperature of 1073 K, PtlCeO.75ZrO.250r1273 catalyst was quite stable after a smallinitial transient period. When the reaction temperature wasincreased from 1073 to 1273 K, the catalyst exhibited a strongdeactivation during 4 h TOS (Fig. 7). After that, it becamepractically stable. This result showed that the increase ofreaction temperature favoured the deactivation of the catalyst.

Furthermore, besides the production of H2, CO, CO2, andH20, it was observed the formation of heavy hydrocarbons suchas pyrene and tetracene at the higher reaction temperature.These compounds are the precursors for coke formation, whichcan explain the strong deactivation of PtlCeo.75Zro.250r1273catalyst at 1273 K.

Concerning CO and CO2 formation at reaction temperatureof 1073 K, it was observed an increase of CO selectivity and adecrease of CO2 formation at the beginning of the reaction. Thiseffect was followed by an increase of the CH4 conversion. After2 h TOS, the CO and CO2 formation were stable. However, at1273 K, the selectivity to CO and CO2 presented an oppositebehaviour. During 4 h TOS, the CO formation decreased andCO2 production increased as CC4 conversion decreased. Afterthat, the CO and CO2 selectivity remained stable.

The H2/CO molar ratio followed the same trend of methaneconversion (Fig. 8). At 1073 K, H2/CO ratio was stable at

Page 7: Atr Selectivity

10 15

time (h)

Fig. 7. Methane conversion vs. time on stream obtained on autothermalreforming of methane for PtlCeO.75ZrO.250r1273 catalyst at 1073 and1273 K (H20/C~ molar = 0.2. 02/CH4 = 0.5. and WHSV = 284 h-1).

around 1.60, after 2 h TOS. Nevertheless, at 1273 K, the Hz/COratio strongly decreased during 4 h TOS, becoming stable at alower value (,,-,0.7). This result is similar to that obtained byAasberg-Petersen et al. [3]. They reported that the increase ofreaction temperature from 1223 to 1323 K led to a decrease inHz/CO molar ratio.

As previously discussed, the COz dissociation was affectedby the coke deposition around the metal particle. Since theamount of coke formed was larger at 1273 K, the COzreforming step of the autothermal reforming reaction wasinhibited. This fact can explain the increase in COz production,when methane conversion decreased at the higher reactiontemperature.

Then, the results showed that, at the conditions studied(HzO/C~ ratio = 0.2, Oz/C~ ratio = 0.5), the performance ofPt/Ceo.7sZro.zs0z-1273 catalyst on autothermal reforming ofmethane was better at temperature of reaction equal to 1073 K.

3.3.3. The effect of the CO2 addition to the feedstockThe effect of the COz addition to the feedstock was studied

for Pt/Ceo.7sZro.zsOz-1073 catalyst at reaction temperature of

-o-T . = t073K..-nn-A- T•••••ion = tZ73K

0+--.---,...-..- .....•........,...-..•....,-.---, ....-..-""T'" ......•-T"""-lo 4 8 12 16 20 24

Time/h

Fig. 8. The Hz/CO molar ratio vs. time on stream obtained on autothermalreforming of methane for pt/Ceo.75Zro.250rI273 catalyst at 1073 and 1273 K(H20/CH4 molar = 0.2. 02/CH4 = 0.5. and WHSV = 284 h-').

100

#. 0

C0.~ 60C)>c0u 40'":r:

V20

00

-e-with CO2

-e- without cq•••••......::.........•......._ ...•.

" ••In::a:::-

8 12 t6Time/h

Fig. 9. Methane conversion vs. time on stream obtained on autothermalreforming of methane for PtlCeO.75ZrO.250r1073 catalyst with or withoutCO2 in the feedstock (Treaet;on = 1073 K. H20/C~ = 0.2. 02/CH4 = 0.5.C02/C~ = 0.01. and WHSV = 284 h-').

1073 K, using a HzO/CH4 and Oz/CH4 molar ratio of 0.2 and0.5, respectively.

Fig. 9 presents the methane conversion obtained with andwithout COz. The results showed that the presence of COz in thefeedstock increased the initial methane conversion. However, itdecreased the catalyst stability during 24 h TOS.

The higher initial methane conversion was associated to theoccurrence of dry reforming of methane in addition to steamreforming and partial oxidation reactions. The deactivation wasalso related to the COz reforming of methane, which favours thecarbon deposition on the metal particle [26].

Furthermore, the addition of COz to the feedstock did notaffect the Hz/CO molar ratio. According to the literature [38],the presence of CO2 in the ATR process (COZ/CH4 "-' 0.1-2.0)can lead to a higher CO formation due to the occurrence ofreverse water gas shift reaction, reducing the Hz/CO molarratio. In this work, this effect was not observed probably due tothe low amount of COz added (COZ/CH4 = 0.01).

Therefore, the addition of COz to the feedstock did not havea positive effect on the performance of Pt/Ceo.7sZro.zsOz-1073catalyst on autothermal reforming of methane at temperature ofreaction equal to 1073 K (HzO/C~ ratio = 0.2, Oz/CH4

ratio = 0.5).

3.3.4. The effect of H20/CH4 ratioThe effect of HzO/C~ molar ratio was evaluated for Pt/

Ceo.7sZro.zsOz-1073 at Treaction= 1073 K, using a Oz/CH4

molar ratio of 0.5.Increasing HzO/C~ molar ratio increased the methane

conversion (Fig. lOA). This fact can be attributed to theoccurrence of steam reforming of methane reaction. Accordingto the reaction mechanism proposed for the ATR, the secondstep involves the COz and steam reforming of the unreactedmethane. The increase of HzO/C~ molar ratio will favour thesteam reforming of methane and then the overall reaction.

The CO and COz selectivities are shown in Fig. lOB. For allHzO/CH4 ratios studied, the COz formation slightly increasedand the CO production slightly decreased during 24 h TOS.

Page 8: Atr Selectivity

-.- HPICH. =0.2-e- H,0/CH. = 0.37

-6- HPICH. =0.6

10 15 20 25time/h

(B) 100

80'<f.-0:~ 60'0"<;<I>

0' 40U"'"c'"0U

20

10 15 20 25

time/h

Fig. 10. (A) Methane conversion and (B) CO and CO2 production vs. time on stream obtained on autothermal reforming of methane for Pt/Ceo.7sZro.2s0rI073catalyst, using different H20ICH.. molar ratios (Treaction = 1073 K, 02/CH.. = 0.5, and WHSV = 260 h-1

).

.g 1.5~~a 1.0

~:r::" 0.5

0.37

H,0ICH. molar ratio

Fig. II. The HiCO molar ratio obtained on autothermal reforming of methanefor Pt/Ceo.7sZro.2s0r 1073 catalyst, using different H20/CH.. molar ratios(Tr.action = 1073 K, 02/CH4 = 0.5, and WHSV = 260 h-1).

Moreover, increasing H20/CH4 ratio increased the H2/COratio (Fig. 11). The addition of H20 favours the occurrence ofsteam reforming of methane, which leads to a higher H2/COratio. Some authors [3] observed a similar behaviour of H2/COratio on auto thermal reforming of methane over Ni-basedcatalysts at high pressure, using H20/CH4 molar ratio between0.21 and 0.60.

The results obtained in this work also showed that theappropriated H2/CO ratio for GTL technology was reachedonly for the H20/C~ ratio of 0.6.

The TPSR analyses consistently indicate that the auto-thermal reforming of methane proceeds through a two-stepmechanism (indirect mechanism) over all catalysts studied.Furthermore, PtlCeO.75ZrO.2502catalyst exhibited the beststability on the autothermal reforming of methane at 1073 K.This result was attributed to a proper balance between oxygentransfer ability of the support and metal dispersion. The higher

rate of oxygen transfer keeps the metal surface free of carbon.This mechanism of carbon removal takes place at the metal-support interfacial perimeter. Water also participates of thismechanism, favouring the carbon removal of metal particle .

It was also observed that the calcination temperature of theCeO.7SZrO.2S02support did not affect the performance of Pt/CeO.75ZrO.2S02catalyst on this reaction. However, the behaviourof this catalyst was influenced by the reactions conditions. Anincrease in the reaction temperature decreased both themethane conversion and H2/CO molar ratio. Moreover, theaddition of CO2 to feedstock increased the initial methaneconversion, but decreased the catalyst stability. On the otherhand, increasing H20/C~ molar ratio increased methaneconversion and H2/CO molar ratio.

Taking into account all results obtained, the best perfor-mance of PtlCeO.7SZrO.2S02catalyst on autothermal reformingof methane was obtained at 1073 K, using WHSV = 260 h- t ,

H20/C~ = 0.6 and 0iCH4 = 0.5. At these conditions, thiscatalyst exhibited a good stability and a H2/CO molar ratio('" 2.0) appropriated for GTL technology was obtained.

The authors wish to acknowledge the financial support of thePETROBRAS (0050.0007696.04.2).

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