SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 2002; 34: 418–422Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sia.1330

Study of the oxide/carbide transition on iron surfacesduring catalytic coke formation

F. Bonnet,1,3∗ F. Ropital,1 P. Lecour,1 D. Espinat,1 Y. Huiban,1 L. Gengembre,2 Y. Berthier3

and P. Marcus3

1 Institut Francais du Petrole, 92852 Rueil Malmaison Cedex, France2 Laboratoire de Catalyse de Lille (LCL), CNRS, 59655 Villeneuve d’Ascq Cedex, France3 Laboratoire de Physico-Chimie des Surfaces, CNRS, ENSCP, 75231 Paris Cedex 05, France

Received 16 July 2001; Revised 14 December 2001; Accepted 3 January 2002

The thermodynamic equilibria between metallic iron, iron oxides, iron carbides and a hydrocar-bon/hydrogen mixture were calculated at 600 ◦C. On the basis of metastable Fe–C–O phase diagram,iron oxides can be converted directly into carbides in a reducing and carburizing atmosphere. Experimen-tal results on the rate of oxide/carbide conversion are reported. Thermogravimetric measurements havebeen performed in an iC4H10 –H2 –Ar atmosphere at 600 ◦C on preoxidized iron samples. The kineticsof the oxide layer transformation were studied by sequential exposure experiments. Scanning electronmicroscopy observations and x-ray diffraction analysis have been carried out. The results lead to theconclusion that magnetite (Fe3O4) is transformed into carbide particles, acting as a catalyst for graphiticfilament growth. The initial stages of the oxide/carbide transition were studied by x-ray photoelectronspectroscopy. The results confirm that no metallic iron was formed during the transformation. Copyright 2002 John Wiley & Sons, Ltd.

KEYWORDS: coke deposition; iron oxide; iron carbide; metastable phase diagram; thermogravimetry (ATG); x-rayphotoelectron spectroscopy (XPS)

INTRODUCTION

The formation of carbon filaments, which occurs at carbonactivity ac > 1 in the temperature range 400–700 °C, isa major problem in many chemical and petrochemicalprocesses where hydrocarbons or other strongly carburizingatmospheres are involved. One of the mechanisms has beenelucidated for iron and low-alloy steels: fundamental studieswith transmission electron microscopy (TEM) observationshave shown clearly that the unstable carbide M3C, whichis formed at the iron surface after supersaturation, is anintermediate of the reaction.1,2 The formation of a protectiveiron oxide scale can prevent surface carburization and cokedeposition. Indeed, the solubility of carbon in iron oxidessuch as FeO and Fe3O4 has been measured by Grabke et al.3 tobe <0.01 ppm even at 1000 °C. This low solubility is expectedto prevent any bulk diffusion of carbon in iron oxides andthus any carburization of the metal. However, when the oxidescales are exposed to reducing and carburizing atmospheressuch as hydrocarbon feeds, interfacial reactions between theoxide layer and the hydrocarbon species occur.

Baker et al.4 compared the catalytic reactivity of metalliciron (Fe), wustite (FeO) and haematite (Fe2O3) as precursorsfor the formation of carbon filaments from ethane andacetylene. Their results lead to the conclusion that the order

ŁCorrespondence to: F. Bonnet, Laboratoire de Physico-Chimie desSurfaces, CNRS, ENSCP, 75231 Paris Cedex 05, France.E-mail: frederic.bonnet@irsid.usinor.com

of activity is FeO > Fe ¾ Fe2O3. It was proposed that the highcatalytic activity of FeO can be attributed to the formationof an iron-rich sponge-like product of high surface area.Moreover, small metallic particles can be formed from FeOreduction, which would lead to the rapid production ofgraphitic filaments. Similarly Grabke5 proposed, in the caseof Fe–Cr alloys, that the reduction of spinels of (Fe,Cr)2O3

in a reducing and carburizing atmosphere can induce theformation of catalytic particles.

Tokura et al.6 investigated the reactivity of preoxidizediron samples in CH4 –H2 gas atmospheres at 1000 °C. Theycompared the kinetics of coke deposition and the porosityof the surface samples obtained after complete reductionof the oxide scale. The coke formation rate increased withthe initial oxide layer thickness, which was interpreted bythe formation of an active ‘metallic’ surface generated byreduction of the oxide scale.

To summarize, three mechanisms are currently proposedto explain the participation of iron oxide in catalytic coking:the iron oxides, once reduced, can increase the surface area,which increases the kinetics of the carburization step; theycan be transformed in fine metal particles and act as catalystsfor graphitic filament growth; and iron oxides or partiallyreduced iron can have catalytic activity in hydrocarbondecomposition processes.

In the present study, the metastable Fe–C–O phasediagram was calculated at 600 °C to define the differentreactions that can occur at the oxide/gas interface in a

Copyright 2002 John Wiley & Sons, Ltd.

Oxide/carbide transition on iron surfaces 419

reducing and carburizing atmosphere. The kinetics of cokeformation on preoxidized surfaces of pure iron were inves-tigated with a microbalance and the different steps of thereaction were studied by sequential exposure experiments.Scanning electron microscopy (SEM) observations and x-raydiffraction (XRD) analysis were performed to investigate theiron oxide transformation. X-ray photoelectron spectroscopy(XPS) analyses were carried out to study the initial stages ofthe reaction. The objective of this work was to elucidate therole of iron oxides in the mechanism of coke deposition.

EXPERIMENTAL

ThermogravimetryThermogravimetric measurements were carried out onpreoxidized pure iron samples exposed to a flowingcarburizing mixture (30% H2, 30% iC4H10, 40% Ar) at 600 °Cunder 1 atm. The mixture and the flow rate (50 ml min�1)were controlled by mass flow meters. The samples usedin this study were polycrystalline iron (10 mm ð 5 mm ð1 mm) from Weber (purity >99.9%). Before inserting thesamples into the furnace, they were ground with 1200 gritSiC, cleaned with ethanol and annealed for 12 h at 700 °C ina hydrogen flow.

In the thermobalance, the samples were heated inan argon flow up to 600 °C. The oxygen (3 ppm) andwater (3 ppm) contents of argon (purity ½99.998%) wereused to obtain the oxidized surface state. The argonwas replaced by the gas mixture as soon as the desiredtemperature was reached, and the mass of coke wascontinuously recorded. The iron samples were exposed fordifferent periods of coking. To stop the reaction, the gasmixture was replaced by pure argon (purity >99.9996%) andthe furnace was switched off. The samples remained in thefurnace during cooling.

X-ray photoelectron spectroscopy analysisThe initial stages of oxide transformation were studiedby XPS. Experiments have been carried out using a VGEscalab 220 XL spectrometer with monochromated Al K˛(1486.6 eV) radiation, operating at 15 kV and 300 W. Thesurvey spectrum (10–1000 eV) and the Fe 2p, O 1s and C 1sspectra were recorded under normal emission geometry ata constant pass energy of 30 eV. Charging effects were notdetected on any sample during XPS analysing. The bindingenergy of the O 1s level from oxide at 530.1 eV was used asan internal reference for calibration. The surface treatments,which consisted of oxidation and coking, were carried out inthe preparation chamber of the spectrometer.

Before inserting the samples into the chemical reactor,they were first polished with SiC grit paper and thendiamond polished to 1 µm. They were then washed withethanol and annealed for 12 h at 700 °C in a hydrogenflow. Surface cleaning was completed for ultrahigh vacuum(UHV) experiments by ArC sputtering performed at 2 keVfor ¾10 min.

In the preparation chamber, oxidation was performedwith the oxygen content of the argon (purity >99.9996%)under 1 atm. The samples were heated up to 300 °C in an

argon flow and maintained at this temperature for 15 min.Then argon was replaced by the gas mixture 50% H2 –50%iC4H10 and the sample was heated up to 550 °C and exposedto a carburizing and reducing atmosphere for 30 min. Themixture and the flow rate (30 ml min�1) were controlledby mass flow meters. For both experiments (oxidation andcarburization) the reactions are stopped by pumping the gasmixture before cooling to room temperature.

RESULTS

Thermodynamic calculationsIn a reducing and carburizing atmosphere, the stable phasefor iron is the carbide. Numerous iron carbides are reportedin the metallurgical literature, with compositions rangingfrom Fe3C to Fe2C when the carbon activity increases. Butonly two of them (Fe3C and Fe2C) have been studied undermetastable equilibrium conditions. It is only for these twocompounds that thermodynamic data are available. Thethermodynamic equilibria between Fe, FeO, Fe3O4, Fe2O3,Fe3C and Fe2C were calculated at 600 °C and the metastableFe–C–O phase diagram was constructed to define thedifferent reactions that can occur at the metal/oxide andoxide/gas interfaces (Fig. 1). The thermodynamic data forthe iron oxide and for cementite (Fe3C) were provided byThermodata Bank’98. The values of Browning et al. wereused for Fe2C.7

On the basis of this metastable phase diagram, iron oxidescan be converted directly into carbides in a carburizingand reducing atmosphere. The phases involved in theoxide/carbide transition depend on the carbon activity andon the temperature. In processes involving hydrocarbonssuch as isobutane, the carbon activity can be calculated fromthe following equilibrium

iC4H10 �g� D 4C (g) C 5H2 (g) ac D(

KpiC4H10

pH52

)1/4

with lnK = 30.43 at 600 °C

where K is the equilibrium constant of the chemical reactionand piC4H10 and pH2 are the partial pressures of the gas. The

-4

-3

-2

-1

0

1

2

3

4

-30 -28 -26 -24 -22 -20

log pO2 (atm)

log

a c

Fe0,947O(s)Fe(s)

Fe3O4(s)Fe3C

Fe2C

Figure 1. Metastable Fe–C–O phase diagram at 600 °C.

Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 418–422

420 F. Bonnet et al.

experimental conditions used in this study (30% iC4H10, 30%H2, 40% Ar) establish a carbon activity of ac D 6710 at 600 °C.

Kinetics of oxide/carbide conversionThe mass of coke measured after exposures for differentlengths of time is plotted in Fig. 2 as a function of time. Duringcoking, the change in the iron surface was characterized bySEM (Fig. 3) and the different phases present on the samplesurface were identified by XRD with Co K˛ radiation (Fig. 4).

Before starting the experiments, the nature and theproportions of the iron oxides present on the surfacewhen the temperature reaches 600 °C were characterizedby SEM (Fig. 1(a)) and XRD (Fig. 4). The initial oxide layerwas composed of magnetite (Fe3O4, 75%) and haematite(Fe2O3, 25%), with a thickness of ¾2–3 µm. The thicknesswas measured by SEM observation of metallographiccross-sections. According to the metastable Fe–C–O phasediagram, when the atmosphere becomes reducing withac D 6710, haematite should be reduced in magnetite andmagnetite should be converted into Fe2C.

After 10 min in the gas mixture, the iron oxide layer is stillpresent on the surface, but cementite and graphite are alsodetected. The relative fraction of magnetite (Fe3O4) decreasesrapidly compared with the fraction of haematite (Fe2O3). Thisresult is consistent with thermodynamic data. Haematitemust be reduced in magnetite before its conversion into

0

0.1

0.2

0.3

0.4

0.5

0.6

0 5 10 15 20 25 30 35 40 45 50 55 60time (min)

∆m/A

(m

g/cm

2 )

T = 600 °Cp = 1 atm flow rate = 50 ml/min

Figure 2. Mass gain versus time for preoxidized iron samplesexposed to a carburizing atmosphere of 30% iC4H10, 30% H2

and 40% Ar.

carbide. The iron carbide that is formed is cementite (Fe3C),which is different from the thermodynamic predictions.Metallographic cross-sections show that cementite is alsoformed at the metal/oxide interface. During coking, themagnetite is transformed into carbide particles, which(Fig. 3(c)) act as a catalyst for graphitic filament growth(Fig. 3(d)). However, kinetics factors must be taken intoaccount in the transformation. Indeed, another mechanism

- a : 0 min - b : 10 min

- c : 30 min - d : 50 min

Figure 3. Evolution of the oxidized iron surface after sequential exposure in a carburizing atmosphere of 30% iC4H10, 30% H2 and40% Ar at 600 °C.

Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 418–422

Oxide/carbide transition on iron surfaces 421

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35 40 45 50 55

Time (min)

rela

tive

frac

tion

(%)

Fe3C

Fe2O3

Fe3O4

Figure 4. Evolution of the relative fractions of haematite(Fe2O3), magnetite (Fe3O4) and cementite (Fe3C) present onthe surface of samples exposed to a carburizing mixture of30% iC4H10, 30% H2 and 40% Ar at 600 °C.

different from thermodynamic prediction can occur if therate of oxide/carbide conversion is impeded. In a reducingand carburizing atmosphere, the iron oxides can be reducedprogressively reduced in metallic iron, which is thenconverted into carbide.

Haematite �Fe2O3� ! Magnetite �Fe3O4� ! Wustite (FeO)

! Iron (Fe) ! Cementite �Fe3C�

To determine which cause is followed during the oxide/car-bide transition, XPS analyses were performed to study thefirst step of the conversion. Because of the thickness of theiron oxide, only metallic iron formed by reduction of theoxide can be detected.

Study of the first stages of oxide/carbide transitionby XPSX-ray photoelectron spectroscopy analyses were performedto characterize the initial oxide and the changes afterexposure of the preoxidized iron sample to the gas mixtureof isobutane/hydrogen (ac D 2887) for 30 min at 550 °C.

Figure 5 shows the XPS spectra and the curve fitting of Fe 2p,O 1s and C 1s for the oxidized (a) and coked (b) iron surface.Shirley (non-linear) background subtraction8 was used here.The parameters of the fit and the results are listed in Table 1.The Fe 2p spectrum was fitted over the range 702–739 eVand the fitted spectrum consists of doublets for both Fe2C

(710.3 eV) and Fe3C (711.5 eV) species, with doublets forFe2C and Fe3C shake-up satellites. The binding energies ofthe shake-up satellites of Fe2C(2p3/2) and Fe3C(2p3/2) are714.5 eV and 719.8 eV, respectively, which is consistent withexperimental data obtained by others authors.9,10 Separationbetween peaks and the peak height ratio are fixed in thecurve fitting. The binding energy (BE) and full width at halfmaximum (FWHM) values for both Fe2C and Fe3C peaksshow that the initial oxide layer is composed of a mixtureof Fe3O4 and Fe2O3, with some FeOOH. The O 1s spectrumis composed of three peaks that correspond to the signalsfrom oxygen in oxide (O2�) at 530.1 eV, oxygen in hydroxylgroups (OH�) at 531.4 eV and water (H2O) at 532.2 eV.Carbon contamination is observed on the oxide scale.

After coking, the intensities of the iron oxide peaksdecrease and the formation of a carbide species at 708.5 eV isdetected in the Fe 2p spectrum, which is in good agreementwith literature values.11,12 The C 1s spectrum is composed offive peaks that correspond to the signals from iron carbide(283.8 eV), carbon in graphite at 284.6 eV, hydrocarbon C–Cbonds at 285.4 eV and carbon–oxygen bonds with –C–Osingle bond (286.4 eV) and –C O double bond (289 eV).

As indicated above, iron carbide is detected on the surfacesample after 30 min in the isobutane/hydrogen mixture. Nometallic iron was detected, which indicates that iron carbideis present in the oxide layer. This gives evidence that the ironoxides present initially on the surface sample are directlyconverted into carbide. Metallic iron is not an intermediaryspecies in the conversion.

CONCLUSION

The thermodynamic equilibria between iron, iron oxidesand iron carbides were calculated at 600 °C. According to

Table 1. Curve-fitting parameters for Fe 2p, O 1s and C 1s XPS spectra recorded after oxidation and subsequent coking of the ironsurface

Fe 2p3/2 Fe 2p3/2 (satellites) Fe 2p1/2 Fe 2p1/2 (satellites)

Fe 2p Carbide Fe2C Fe3C Fe2C Fe3C Carbide Fe2C Fe3C Fe2C Fe3C

BE 708.5 710.3 711.5 714.6 719.8 722 723.6 725.1 728.1 733.2FWHM 1.8 2 3.5 5.2 5.2 1.8 2 3.5 5.2 5.2Height (oxidized) — 22 745 31 843 19 750 15 231 — 13 514 18 920 11 876 9150Height (coked) 8330 19 908 28 797 17 249 13 822 5613 13 416 15 816 11 710 7424

O 1s C 1s

O2� C O H2O Carbide Graphite C–C –C–O –C O

BE 530.1 531.4 532.2 283.8 284.6 285.4 286.4 289FWHM 1.3 1.3 1.3 1.3 1.2 1.2 1.4 1.35Height (oxidized) 30 671 2434 1281 — 2254 2058 485 2765Height (coked) 27 570 3648 1300 11 797 7775 12 212 3418 7309

Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 418–422

422 F. Bonnet et al.

(a) Oxidized iron surface (b) Coked iron surface

O1s O1s

Fe2p Fe2p

OH-

O2-

Sat. Fe3+

Sat. Fe2+

Fe3+

Fe2+

Sat. Fe3+

Sat. Fe2+

Fe3+

Fe2+

H2O H2OOH-

O2-

C1s C1s

−C = O

−C−O

C−C

Graphite

Carbide

−C = OGraphite

C−C

−C−O

20

40

60

80

100

735 730 725 720 715 710 705

Binding Energy (eV)

533534 532 531 530 529 528 527

Binding Energy (eV)

290 288 286 284 282 280

Binding Energy (eV)

KC

PS

0

50

100

150

KC

PS

6000

6500

7000

7500

CP

S

290 288 286 284 282 280

Binding Energy (eV)

6000

6500

7000

7500

CP

S

533534 532 531 530 529 528 527

Binding Energy (eV)

0

50

100

150

KC

PS

20

40

60

80

100

735 730 725 720 715 710 705

Binding Energy (eV)

KC

PS

Carbide

Figure 5. The XPS spectra of Fe 2p, O 1s and C 1s obtained on the oxidized (a) and coked (b) iron surface.

the metastable Fe–C–O phase diagram, the iron oxidescan be directly converted into carbide in a reducing andcarburizing atmosphere. The chemical species involved inthe oxide/carbide transition depend on the temperature andcarbon activity. The kinetics of the conversion were studiedby thermogravimetry in an isobutane/hydrogen/argonmixture with ac D 6710 at 600 °C. Magnetite (Fe3O4) israpidly converted into carbide particles (Fe3C), whichcatalyse the formation of graphitic filaments. The resultsare consistent with thermodynamic data except for theiron carbide involved in the conversion (Fe3C insteadof Fe2C). The first stages of the oxide/carbide transitionwere studied by XPS. The results confirm the directconversion of iron oxide into carbide without the formationof metallic iron.

REFERENCES

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74.5. Grabke HJ. Mater. Corros. 1999; 50: 673.6. Tokura S, Otsuka N, Kudo T. Corrosion 1993; 49: 561.7. Browning LC, De Witt TW, Emmett PH. J. Am. Chem. Soc. 1950;

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10. Graat PCJ, Somers MAJ. Appl. Surf. Sci. 1996; 100/101: 36.11. Shabanova N, Trapeznikov VA. J. Electron Spectrosc. 1975; 6: 297.12. Kuivila CS, Butt JB, Stair PC. Appl. Surf. Sci. 1988; 99/121: 99.

Copyright 2002 John Wiley & Sons, Ltd. Surf. Interface Anal. 2002; 34: 418–422