Catalytic autothermal reforming of methane and propane over supported metal catalysts

Applied Catalysis A: General 241 (2003) 261–269

Catalytic autothermal reforming of methane and propaneover supported metal catalysts

S. Ayabeb, H. Omotoa, T. Utakaa, R. Kikuchia, K. Sasakib, Y. Teraokab, K. Eguchia,∗a Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering,

Kyoto University, Sakyo-ku, Kyoto 606-8501, Japanb Department of Molecular and Material Sciences, Graduate School of Engineering Sciences,

Kyushu University, Kasugakoen, Kasuga 816-8580, Japan

Received 24 June 2002; received in revised form 19 August 2002; accepted 22 August 2002

Abstract

Catalytic autothermal reforming of methane and propane over supported metal catalysts has been investigated in thepresent study. The carbon deposition region and the heat balance of the reaction have been determined from the equilibriumcalculations. The sequence of the activities of the 2 wt.% metal on alumina support for autothermal reforming of methane wasRh > Pd > Ni > Pt > Co. The catalytic activity of 10 wt.% Ni/Al2O3 was higher than that of the 2 wt.% Rh/Al2O3. Theactivity of Ni was significantly lowered by the preferential oxidation of the catalyst in the reactant gas at low temperatures.Although little carbon deposition was observed for the autothermal reforming of methane in the deposition-free region expectedfrom the equilibrium, a large amount of carbon deposition was observed for the propane autothermal reforming even in thesteam-rich conditions. The deposited carbon possessed fibrous morphology. The catalytic autothermal reforming appears tobe initiated by decomposition of hydrocarbon at the inlet zone; then the reforming reaction subsequently proceeded in thecatalyst bed.© 2002 Elsevier Science B.V. All rights reserved.

Keywords: Autothermal reforming; Steam reforming; Partial oxidation; Methane; Propane; Hydrogen; Carbon formation

1. Introduction

Hydrogen has been attracting great interest as a fu-ture clean fuel for combustion engines and fuel cells[1,2]. The efficient and compact production processof hydrogen should be developed for such applicationpurposes. Electrolytic production of hydrogen withelectricity originating from solar cells or hydropowerhas been regarded as the most clean and desirablemethod, but these processes do not supply enough

∗ Corresponding author. Tel.:+81-75-753-5682;fax: +81-75-753-3352.E-mail address: [email protected] (K. Eguchi).

hydrogen in the present stage. Multi-step processingof hydrocarbon-based fuels is the most practical ap-proach in deriving hydrogen efficiently[3,4]. Steamreforming of hydrocarbon has been employed fre-quently in chemical industries for production of hy-drogen. The recent development of fuel cell researchactivity, however, has triggered the processing offuel for compact and portable systems. Researchershave investigated hydrogen production for fuel cellsfrom various fuels, such as methane, DME, severalkinds of hydrocarbons, and ethanol as biogenic fuel[5–12]. Several types of design for efficient hydro-gen production have also been proposed for the fuelcell applications[3,5,7,8,12–14]. Partial oxidation of

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.PII: S0926-860X(02)00471-4

262 S. Ayabe et al. / Applied Catalysis A: General 241 (2003) 261–269

hydrocarbons or steam reforming of hydrocarbonsis needed at the first process step. Desulfurizationprocess should be carried out for removing sulfurcontent to an allowable low level. The decompositionof hydrocarbon structure is followed by a water gasshift reaction for inter-conversion of fuel componentfrom CO- to H2-rich fuel. This process should becarried out at significantly lower temperature thansteam reforming, due to the exothermic nature of thereaction. For the fuel cell application, the final step offuel processing should be CO removal by preferentialoxidation.

The partial oxidation is an exothermic reaction withlarge heat production, whereas the steam reforming isan endothermic reaction. The partial oxidation couldtherefore, easily start up on ignition even without anaid of catalyst. However, conversion efficiency is low-ered as the released heat is wasted. The products arediluted with nitrogen when air is used as an oxidant.On the other hand, the efficiency could be gained whensteam reforming of hydrocarbons is combined, sincethe reaction absorbs thermal energy from the surround-ings. The disadvantage of steam reforming should bea large extent of heating for the reactor and steam gen-erator. Autothermal reforming is the combination ofthe reforming and partial oxidation reforming, wheresteam reforming of hydrocarbons is carried out in thepresence of oxygen. Exothermic, endothermic, andthermo-neutral condition can be selected by choos-ing an appropriate ratio of hydrocarbon:oxygen:steam[7,8,15].

Carbon deposition may cause serious damage forthe stable operation and high conversion of thesethree reactions[16,17]. Although the reaction con-dition for carbon deposition can be estimated fromthermodynamic equilibrium, the real deposition con-dition is far more complicated. The catalytic actionin the steam reforming of hydrocarbon has beenextensively investigated, but the catalytic partial ox-idation and autothermal reforming have not beeninvestigated well and need further research for ef-ficient hydrogen production in small scales. Thepurpose of the present study is to determine thegeneral behavior of the autothermal reforming usingsupported metal catalysts with methane and propaneas fuel. The differences in reactivities of these twohydrocarbons are also of interest in the presentstudy.

2. Experimental

2.1. Catalyst preparation

The supported metal catalysts were prepared by theimpregnation method. Commercial alumina granules(Sumitomo Chemical, NKHD-B914) were immersedin the solution containing metal components. For thepreparation of Ni/Al2O3, 10 g of the powder was sus-pended in the 0.0945 mol l−1 solution of Ni(NO3)2(Kishida Chemical). The suspension thus obtained wasstirred for 2 h at 80◦C and, then evaporated to dryness.The powder was dried in an oven at 110◦C for 6 h;then, the metal precursor was decomposed at 450◦Cin air. The catalyst was finally heated at 850◦C for 2 hin H2 atmosphere.

The metal sources used for other supported metalcatalysts were Rh(NO3)3 (NE Chemcat), H2PtCl6(NE Chemcat), Pd(NO3)2 (Kishida Chemical), andCo(NO3)3 (Kishida Chemical). They were dissolvedin water and the catalysts were prepared by the sameprocedure as used for Ni/Al2O3. Every sample wasreduced with H2 at 850◦C for 2 h. For Rh/Al2O3,Pt/Al2O3, and Pd/Al2O3, each sample was heated inair at 850◦C prior to the reduction in H2. Nickel cat-alysts were also prepared by using SiO2 (Fuji Silicia)and ZrO2 (Dai-ichi Kigenso) as support oxides.

2.2. Catalytic reaction and characterization

A fixed bed flow reactor of quartz tubing was usedfor the catalytic autothermal reforming. The catalyst(1 cm3) was reduced with H2 at 600◦C for 1.5 h priorto the reaction. A gaseous mixture of 16.7% CH4or 11.1% C3H8, 0–50.0% H2O, 1.7–16.7% O2, andN2 (balance) was fed to the catalyst reactor at spacevelocity (SV) of 7200 h−1. A water pump (Shimadzu,LC-10ADvp) was used for the evaporator heated at200◦C to control the steam to carbon (S/C) ratio inthe reaction gas mixture. Products were analyzed byon-line gas chromatography specially designed for theanalysis of light hydrocarbon reformate (Shimadzu,GC-20B-3S). The reaction results were generallyobtained 1.5 h after the setting of the reaction con-dition. The reaction products were monitored withnon-dispersive infrared detector during temperaturescanning. The surface area of the catalysts was ana-lyzed by the BET method using nitrogen adsorption.

S. Ayabe et al. / Applied Catalysis A: General 241 (2003) 261–269 263

Microstructure of the catalysts and carbon depositedon them were observed by high-resolution scanningelectron microscopy (FE-SEM, JEOL 6340F).

3. Results and discussion

3.1. Thermodynamic consideration of reaction

Thermodynamic calculations will be essential forthe evaluation of partial oxidation and autothermaland steam reforming of hydrocarbons. The equilib-rium conversion could often be attained using activecatalysts. Even when the equilibrium could be at-tained, the catalytic reaction sometimes suffered fromcarbon deposition, which may result in pore closureor surface coverage. Carbon deposition condition,as represented inFig. 1, can be estimated by takinginto account the thermodynamic equilibrium of thesteam reforming (1), water gas shift (2), and carbondeposition (3) reactions.

CH4 + H2O → 3H2 + CO (1)

H2O + CO → H2 + CO2 (2)

CH4 → C + 2H2 (3)

The solid lines inFig. 1 represent the boundaryof carbon deposition determined from the thermody-namic calculation. Carbon formation is expected tooccur in the carbon-rich region beyond the line at the

Fig. 1. Carbon deposition region in C–H–O diagram and thereaction conditions employed in the present study.

respective temperatures. However, one should be care-ful because the deposition is sometimes observed inthe actual system even when the reaction is carriedout in the region expected from the equilibrium to becarbon-free. The fragmented species formed by hy-drocarbon scission reaction are apt to grow into thecarbon chain before reacting with oxygenated com-pounds. The carbon deposition region expected fromthe equilibrium and the reaction condition employedin the present investigation are plotted in the C–H–Odiagram. The reaction was carried out mostly in thecarbon-free region in the present investigation: au-tothermal reforming of methane was carried out in thecarbon-free region, whereas several experiments forpropane autothermal reforming were done in the car-bon deposition conditions to investigate the effect ofwater vapor on carbon formation.

The thermal evaluation is extremely important inthe autothermal reforming. The mixing ratio of steamand oxygen is determined by considering the exo- andendothermic extent of the reaction for the practicaloperating. The enthalpies of the reaction at differentwater and oxygen contents were plotted as a functionof temperature as shown inFig. 2. Methane steam re-forming (1) accompanies large absorption of heat, i.e.at the standard condition�H 0

298 = 206.2 kJ mol−1.

Fig. 2. Enthalpy of the reaction at different water and oxygencontents as a function of temperature:x = 2[O2]/[CH4], as shownin reaction (5).


Partial oxidation of methane for stoichiometric forma-tion of synthetic gas can be expressed as:

CH4 + 12O2 → CO+ 2H2

�H 0298 = −35.6 kJ mol−1 (4)

Autothermal reforming of methane can be definedas the combination of these two reactions:

CH4 + x

2O2 + (1 − x)H2O → CO+ (3 − x)H2

�H 0298 = 206.2–241.8x kJ mol−1 (5)

The autothermal reforming of methane definedin the present investigation is in the composi-tion range of 0 < x < 1. Thus, the mixture ofmethane–steam–oxygen was supplied for the reac-tion. These three stoichiometric reactions are locatedon the boundary of deposition of carbon, as shownin Fig. 1. The reaction has been generally carriedout in excess steam and/or oxygen conditions. Thethermo-neutral condition appeared in thex range from0.05 at 400◦C to 0.90 at 900◦C. Thus thex valuecan be selected by considering the system efficiency,carbon deposition, ease for the start-up, availability ofsteam, etc. The present investigation has been carriedout in a small-scale micro-reactor with a large supplyof external heat. Thus, the endo-/exothermic natureof the chemical reaction does not affect the reactionresults significantly.

3.2. Autothermal reforming of methane overvarious supported metal catalysts

The catalytic activity of 10 wt.% Ni/Al2O3 formethane autothermal reforming reaction was tested asa function of temperature in heating and cooling pro-cesses, as shown inFig. 3. In the present investigation,a mixture of methane–oxygen–water was supplied tothe catalysts in the autothermal reforming reaction. Inthe heating process from 300◦C to 850◦C, the activ-ity was quite low and the CH4 conversion was<10%in the whole temperature range. On the other hand,the activity of methane autothermal reforming in thecooling schedule from 850◦C was high and nearlyfollowed the equilibrium conversion. The low activityin the heating step could be ascribed to oxidation ofNi with gaseous oxygen. The activation of oxygenproceeded in the lower temperature range than the

Fig. 3. Temperature dependence of conversion for autothermal re-forming of methane over Ni/Al2O3 catalyst in heating and cool-ing processes. Reaction conditions: CH4, 16.7%; O2, 1.7%; H2O,41.6%; N2 (balance); S/C= 2.5; SV = 7200 h−1; (- - - ) equilib-rium conversion.

catalytic oxidation of methane in the autothermal con-dition. Thus, in the autothermal operation, oxidationof Ni catalyst should be avoided. The noble metalcatalysts did not show any activity difference betweenheating and cooling processes, since oxidation of thecatalyst components did not proceed. Based on thisresult, the following activity tests were carried out inthe cooling process.

With a decrease in temperature, the activity of Nicatalyst abruptly dropped at 300◦C to almost zero.The conversion in the low temperature region dis-agreed with the equilibrium one and with that extrap-olated from the activity curve at higher temperatureseven in the cooling process. A high temperature isnecessary for the activation of methane, since methanemolecules are the most inactive hydrocarbon. Thus,lowering of the reaction temperature results in theextinction of the rich-fuel combustion reaction. Sim-ilar behavior was observed in the scanned operationof the reaction temperature, as shown inFig. 4. Inthis operation, the Ni/Al2O3 catalyst was cooled at3.3◦C min−1 in methane–oxygen–water mixture un-der monitoring of products with a non-dispersiveinfrared (NDIR) detector. The conversion almost


Fig. 4. Temperature dependence of conversion for autothermal re-forming of methane over Ni/Al2O3 catalyst in programmed cool-ing schedule measured by NDIR detector. Reaction conditions:CH4, 16.7%; O2, 1.7%; H2O, 41.6%; N2 (balance); S/C= 2.5;SV = 7200 h−1; cooling rate= 200◦C h−1; (- - -) equilibriumconversion.

followed the equilibrium line at 300◦C or highertemperatures and sharply dropped at 280◦C, due tothe decrease in the reaction temperature.

Autothermal reforming of methane was tested overvarious metal-supported catalysts (Fig. 5). The metalloading was fixed at 2 wt.% on the alumina support.The activity depends upon the kind of metal. The sup-ported Rh catalyst attained the highest activity. Thesequence of the activity was

Rh > Pd> Ni > Pt > Co. (6)

This activity sequence generally agreed with thereported activities of the metal catalysts for steamreforming. The small amount of oxygen did not sig-nificantly change the activity patterns of metals.

The activity of the Rh catalyst was the highestamong the 2 wt.% metal catalysts and more or lessattained equilibrium at 600◦C or higher temperatures.The activity of 10 wt.% Ni catalyst is also plotted inthe figure. The activity of 2 wt.% Ni/Al2O3 was lowerthan that of Rh; indeed, from the practical point ofview, cheap Ni metal can be loaded in greater amounts

Fig. 5. Temperature dependence of conversion for autothermal re-forming of methane over various metal catalysts. Reaction con-ditions: CH4, 16.7%; O2, 1.7%; H2O, 41.6%; N2 (balance);S/C= 2.5; SV = 7200 h−1; (- - -) equilibrium conversion.

than precious metal catalysts. Thus, the overall activ-ity of the 10 wt.% Ni catalyst was higher than that ofthe Rh catalyst.

3.3. Autothermal reforming of methane oversupported Ni catalysts

Nickel has been most often employed as the cata-lyst for steam reforming reaction. The effect of sup-port oxide on the catalytic activity for autothermalreforming was investigated (Fig. 6). The activity of10 wt.% Ni/ZrO2 was almost the same as that of theNi/Al 2O3 catalyst. The activity of Ni/SiO2 was sig-nificantly lower than those of the other two Ni cata-lysts. This low activity of Ni/SiO2 may be ascribed todissolution of Ni oxide in the silica matrix during thepreparation process.

The temperature dependence of methane autother-mal reforming was measured at different O2/CH4 ra-tios, as shown inFig. 7. Little carbon deposition hasbeen observed for the conditions listed in the figure.The H2/(CO+ CO2) ratio increased in the low tem-perature region below 500◦C in accordance with theincrease in the equilibrium conversion of the steamreforming reaction. The H2 ratio decreased slightly


Fig. 6. Temperature dependence of methane conversion for au-tothermal reforming of methane over supported Ni catalysts: (�)10 wt.% Ni/Al2O3, (�) 10 wt.% Ni/ZrO2, (�) 10 wt.% Ni/SiO2;(- - -) equilibrium conversion. Reaction conditions: CH4, 16.7%;O2, 1.7%; H2O, 41.6%; N2 (balance); S/C= 2.5; SV = 7200 h−1.

Fig. 7. H2/(CO+CO2) ratio for autothermal reforming of methaneat different O2/CH4 ratios. Reaction conditions: CH4, 16.7%; O2,1.7, 5.0, 8.3 or 16.7%; H2O, 41.6%; N2 (balance); S/C= 2.5;SV = 7200 h−1; (- - -) equilibrium ratio of H2/(CO+ CO2).

Fig. 8. Time course of (a) H2 concentration and (b) carbon depo-sition for propane autothermal reforming over Ni/Al2O3 catalyst.Reaction conditions: C3H8, 11.1%; O2, 5.56%; H2O, 0, 16.7, 33.7,or 50.0%; N2 (balance); S/C= 0–1.5; SV= 7200 h−1.

as the temperature was raised up to 500◦C or highertemperatures. The observed ratio closely followed theequilibrium conversion.

3.4. Autothermal reforming of propane oversupported Ni catalysts

Reforming or autothermal reforming of higher hy-drocarbons is quite attractive for the practical applica-


tion of a variety of fuels. As a first step of investiga-tion of the fuel flexibility of autothermal reforming tohigher hydrocarbons, propane autothermal reformingwas examined. As the carbon number becomes higher,carbon deposition is more significant.

The time course of H2 concentration for differentsteam/carbon ratio is as shown inFig. 8(a) where theO2/C3H8 ratio was fixed at 2. In this reaction test, theconversion of C3H8 was always 100%. At high steamcontents, the conversion followed the equilibrium line.

Fig. 9. SEM image of carbon deposited on Ni/Al2O3 catalyst after reaction at 600◦C. Reaction conditions: (a) C3H8, 11.1%; O2, 5.56%;N2 (balance); SV= 7200 h−1. (b) C3H8, 11.1%; O2, 5.56%; H2O, 16.7%; N2 (balance); S/C= 0.5; SV = 7200 h−1.

The equilibrium H2 concentration was lowered as thesteam to carbon (S/C) ratio was reduced. The observedH2 concentration was lowered further than the valueexpected from the equilibrium. The H2 concentrationdecreased with an elapse of time for the low S/C ratioconditions, indicating carbon deposition.

The amount of carbon deposited in the course ofthe reaction was estimated from the carbon balance,as shown inFig. 8(b). Significant carbon deposi-tion was observed for the dry condition. For the wet


Table 1Effect of space velocity on the product distribution for propane autothermal reforming over Ni/Al2O3 catalyst at 800◦C

SV (h−1) C3H8

conversion (%)CO selectivity(%)

CO2

selectivity (%)CH4

selectivity (%)C2H4

selectivity (%)Carbonselectivity (%)

H2

concentration (%)

7200 100 54.9 37.8 0.05 0.00 7.20 53.314400 100 54.5 39.1 0.09 0.00 6.34 53.428800 100 54.0 39.2 0.19 0.00 6.62 53.157600 100 52.6 37.3 1.34 0.44 8.69 52.5

Reaction conditions: C3H8, 11.1%; O2, 5.56%; H2O, 50.0%; N2 (balance); S/C= 1.5.

condition of S/C= 1.5, conversion to carbon was es-timated to be ca. 10%, though carbon deposition wasnot expected from the equilibrium. Thus, it is moredifficult to avoid carbon deposition from propane fuelthan from methane fuel.

Deposited carbon was observed by high resolutionSEM, as illustrated inFig. 9. Deposited carbon wasclearly observed for the dry condition of partial oxida-tion. The morphology of the deposited carbon was fi-brous. Such morphology was often recognized for thesupported Ni catalysts, where fine Ni deposits servedas catalysts for growth of carbon fiber[18,19]. Thecatalyst surface in wet condition of S/C= 0.5 is asshown inFig. 9(b). Although the low S/C ratio wasemployed to accelerate the carbon deposition, onlya slight amount of carbon fiber was deposited. Thisclearly indicates that addition of steam is effective insuppressing deposition of carbon.

The reaction process of propane was investigatedby changing the space velocity (SV) from 7200–57,600 h−1 for the catalytic reaction. The flow rate ofthe reaction gas mixture containing C3H8–H2O–O2was unchanged, whereas the amount of the Ni/Al2O3catalyst was taken as a variable. Several reactions,such as oxidation, reforming, and carbon deposition,may competitively proceed in the catalyst bed. Thisexperiment has been carried out to estimate the reac-tion occurring at the front zone of the catalyst bed.The gas composition measured at the reactor outlet issummarized inTable 1. The decrease in the amountof catalyst obviously increased the amount of CH4 inthe high SV condition. Further decrease in the amountof catalyst increased the CH4 selectivity and theC2H4 selectivity. The selectivity to carbon was almostunchanged with the reaction amount of catalysts.

The reaction was also carried out at 800◦C in the ab-sence of Ni catalyst as compiled inTable 2. In the pres-ence of catalyst, the gas species observed at the outlet

Table 2Gas compositions at reactor outlet for propane autothermal re-forming in the absence of Ni/Al2O3 catalyst at 800◦C

C3H8 conversion (%) 98.2CO selectivity (%) 14.9CO2 selectivity (%) 0.9CH4 selectivity (%) 24.5Carbon selectivity (%) 17.8H2 concentration (%) 9.6C2H4 selectivity (%) 32.3C2H6 selectivity (%) 1.4C6

+ selectivity (%) 6.0C3H6 selectivity (%) 0.5i-C5H12 selectivity (%) 1.6

Reaction conditions: C3H8, 11.1%; O2, 5.56%; H2O, 50.0%; N2

(balance); S/C= 1.5.

were C2H4, CH4, CO, CO2, H2, and H2O. The efflu-ent species for the thermal reaction without catalystwere C2H6, C3H6, C5H12, and a small amount of oilycompounds in addition to the previously mentionedspecies observed for the catalytic reaction. Althoughthe conversion of propane was 100% even without theNi catalyst, the reaction behavior was completely dif-ferent. The production of H2 was extremely low forthe thermal reaction. This means reforming reactionproceeded mostly on the Ni surface.

Therefore, it is considered that the conversion ofpropane is initiated by the decomposition into carbonnuclear and lower hydrocarbons at the inlet zone ofthe catalyst bed, then the steam reforming of lowerhydrocarbons proceeded in the rear zone of the catalystbed.

4. Conclusions

Autothermal reforming of methane and hydro-carbons has been regarded as an important process


for the first step in the catalytic process for pro-duction of hydrogen. The Ni catalysts as well asnoble metal catalysts demonstrated high activity forthis reaction. High activity was achieved by loadingsuitable amounts of Ni, whereas, the activity at thesame loading was higher for the Rh-based catalysts.The oxidation of Ni by gaseous oxygen may causedeactivation of the catalyst, since the activation ofoxygen proceeds at lower temperature than that ofmethane. Although methane autothermal reformingdid not suffer from carbon deposition, use of propanealways gave rise to carbon deposition even in the re-gion expected from the equilibrium to be deposition-free.

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Catalytic autothermal reforming of methane and propane over supported metal catalysts