filamentous carbon formation caused by catalytic metal particles from iron oxide

* F. RopitalInstitut Francais du Petrole,92852 Rueil Malmaison cedex (France)

F. BonnetInstitut Francais du Petrole,92852 Rueil Malmaison cedex (France),Laboratoire de Physico-Chimie des Surfaces,CNRS (UMR 7045),Ecole Nationale Supereure de Chimie de Paris,75231 Paris cedex 05 (France),present address:IRSID,57283 Maizieres-les-Metz (France)

Y. Berthier, P. MarcusLaboratoire de Physico-Chimie des Surfaces,CNRS (UMR 7045),Ecole Nationale Superieure de Chimie de Paris,75231 Paris cedex 05 (France)

Filamentous carbon formation caused bycatalytic metal particles from iron oxide

F. Bonnet, F. Ropital*, Y. Berthier and P. Marcus

The thermodynamic equilibrium between metallic iron, iron oxi-des, iron carbides and an hydrocarbon/hydrogen mixture was cal-culated at 600 8C. On the basis of the metastable Fe-C-O phase dia-gram, both metallic iron and iron oxides can be directly convertedinto carbides in reducing and carburizing atmosphere. Thermogra-vimetric (ATG) measurements have been performed in iC4H10-H2-Ar atmosphere at 600 8C on reduced and pre-oxidised iron samples.The kinetic of coke formation was studied on both surface states bysequential exposure experiments. The initial stages of the transfor-mation were characterised by scanning electron microscopy (SEM)observations and X-ray diffraction (XRD) analysis. On a reducedsurface, the results are consistent with the mechanism currently pro-posed to explain catalytic coke formation. Cementite (Fe3C) isformed on the iron surface after carbon supersaturation (ac > 1).

The graphite deposition on its surface (ac ¼ 1) induces its decom-position. Iron atoms from cementite diffuse through the graphiteand agglomerate to small particles that act as catalysts for furthercarbon deposition. A new mechanism of catalytic particle forma-tion is proposed when an oxide scale initially covers the iron sur-face. In the carburizing and reducing atmosphere, magnetite(Fe3O4) can be directly converted into cementite (Fe3C). XPS ana-lysis confirm that, in this process, metallic iron is not an intermedi-ary specie of the oxide/carbide reaction. At the same time, graphitedeposition occurs at the metal/oxide interface through the crackspresent in the oxide scale. Iron carbide in contact with graphiteis partially decomposed and acts as catalyst for graphitic filamentsgrowth.

1 Introduction

The formation of carbon filaments which occurs at carbonactivities ac > 1 in a range of temperatures 450–700 8C is amajor problem in many chemical and petrochemical processeswhere hydrocarbons or other strongly carburizing atmo-spheres are involved. The carbon deposition on reactor wallsinduces localised disruption in the process such as heat-trans-fer reduction and pressure drops. An excessive carbon deposi-tion causes deterioration of the furnace alloys and high clean-ing cost. The iron catalysed formation of filamentous carbonhas been studied from carbon monoxide and a variety of sa-turated and unsaturated hydrocarbons. Several investigationsperformed by Hochman [1, 2] and Grabke [3–5] provide agood understanding of the thermodynamic, the kinetic andthe mechanism of coke deposition. One of the mechanisms

has been proposed for iron and low-alloyed steels. Coke for-mation involves reactions at the gas/metal interface and car-bon diffusion within the metal. Fundamental studies withTEM observations [6, 7] have clearly shown that the unstablecarbide M3C, which is formed at the iron surface after super-saturation, is an intermediate of the reaction. The formation ofcatalytic particle on iron and on low-alloy steels is generallyexplained by the following reactions mechanism (Fig. 1):(1) Decomposition of hydrocarbons at the metal surface (ad-

sorption and dissociation of hydrocarbons molecules) andsupersaturation of the metallic phase with dissolved car-bon by transfer from the carburizing gas mixture.

(2) When the carbon activity in the metal is higher than thecarbon activity for cementite (Fe3C) formation (ac > aFe/Fe3C) (Fig. 2), the supersaturation of carbon in the metalcauses nucleation and growth of cementite at the metalsurface and at the grain boundaries.

(3) The carbon diffusion is very slow through the cementite inthe range of temperature 400–700 8C [8]. Consequently,the cementite layer acts as a barrier for further carbontransfer from the gas phase to the metal.

(4) The precipitation of graphite at the cementite surface leadsto a carbon activity of ac ¼ 1 at the graphite/cementite in-terface. Cementite becomes unstable and starts to decom-pose according to the following reaction: Fe3C !3Fe þ C.

(5) Carbon atoms resulting from Fe3C decomposition are at-tached to the basal planes of graphite that grow into thecementite [9, 10]. Because of the concentration gradient,iron atoms diffuse through the graphite to the outer surfaceand agglomerate to small particles (about 20 nm dia-meter). Formation of metal particles is energeticallymore favourable than complete layers.

(6) These particles act as catalyst for further carbon deposi-tion and growth of graphitic filaments.

It was accepted relatively early that the mechanism of carbonfilament growth is based on the catalytic decomposition of

870 Bonnet, Ropital, Berthier and Marcus Materials and Corrosion 2003, 54, No. 11

DOI: 10.1002/maco.200303742 F 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

hydrocarbons by particles present at the filament tips. Graphi-tic filaments are formed when carbon deposition occurs fromthe gas phase on one side of the active catalyst particle, fol-lowed by diffusion through the catalytic particle, and finallyprecipitation as graphite at the metal-carbon filaments inter-face. The driving force for the diffusion process would be thecarbon activity gradient [11, 12].

The nature of the catalytic particle has been discussed forthe iron catalysed filament growth. Measurements of carbondeposition rates were done by Baker et al. [13, 14] in con-trolled atmosphere electron microscopy (CAEM). Theyshowed that the activation energy associated with the growthof filaments is closed to the activation energy of carbon diffu-sion in the corresponding metal. These experiments were per-formed with different metals such as Ni, a-Fe, c-Fe, Co, V, Moand Cr. They concluded that the carbon diffusion through thecatalytic particle is the rate-limiting step. A surface diffusionmechanism was also postulated. These results suggest that thecatalytic particle is the reduced metal. Other studies have con-cluded that bulk carbides [15] or surface carbides are the ac-tive catalyst. Bianchini et al. [16] proposed that when carbonactivity in the gas phase is above the thermodynamic limit forthe formation of carbide, nucleation of this phase occurs on

the reactive face of the catalytic particle. This surface carbideseparates the metallic bulk of the catalyst from direct contactwith the surrounding atmosphere.

In spite of the disagreement on the nature of catalytic par-ticles (metal or carbide), there is some agreement that protec-tive oxide scales, including iron oxides, can prevent surfacecarburisation and coke deposition. In the industrial crackingfurnace tubes where pyrolysis reactions take place, variousoxide scales are found at the metal surface. The formationof these oxides occurs during the start of the process or duringthe “decoking” operations in which coke is burnt in an oxidis-ing gas atmosphere such as steam or air. The solubility of car-bon in the iron oxides such as FeO and Fe3O4 has been mea-sured byWolf et al. [17] to be below 0.01 ppm even at 1000 8C.This low solubility is expected to prevent any bulk diffusion ofcarbon in iron oxides and thus carburisation of the metal.However, when the oxide scales are exposed to reducingand carburizing atmospheres, such as hydrocarbon feeds, in-terfacial reactions between the oxide layer and the hydrocar-bon species can occur.

Baker et al. [18] compared the catalytic reactivity of me-tallic iron (Fe), wustite (FeO) and hematite (Fe2O3) as precur-sors for the formation of carbon filaments from ethane andacetylene. Their results lead to the conclusion that the orderof activity is FeO> Fe� Fe2O3. It was proposed that the highcatalytic activity of FeO could be attributed to the formationof an iron-rich sponge-like product of high surface area. More-over, small metallic particles can be formed from FeO reduc-tion, which would lead to the rapid production of graphiticfilaments. Similarly Grabke [19] proposed, in the case ofFe-Cr alloys, that the reduction of mixed oxides (Fe,Cr)2O3in a reducing and carburizing atmosphere can induce the for-mation of catalytic particles.

Tokura et al. [20] investigated the reactivity of pre-oxidisediron samples in CH4-H2 gas atmospheres at 1000 8C. Theycompared the kinetic of the coke deposition for differentpre-oxidised surface states. The coke formation rate increasedwith the initial oxide layer thickness. This was interpreted bythe formation of an active “metallic” surface generated by thereduction of the oxide scale.

To summarise, three mechanisms are currently proposed toexplain the participation of iron oxide in catalytic coking:(i) Iron oxides, once reduced, increase the surface area

which increases the kinetic of the carburisation step,(ii) Iron oxides are reduced in fine metallic particles and act

as catalysts for graphitic filament growth. Finally,

Fig. 1. Mechanism currently proposed to explain the catalytic par-ticle formation on iron and low alloy steels (adapted from [1–5])

Fig. 2. Influence of the temperature on the carbon activity for ce-mentite formation in a-iron

Materials and Corrosion 2003, 54, No. 11 Filamentous carbon formation 871

(iii) Iron oxides or partially reduced iron have a catalytic ac-tivity in the hydrocarbon decomposition processes.

In the present study, the metastable Fe-C-O phase diagramwas calculated in the range of temperature where catalyticcoke is formed, to define the different reactions that can occurat the oxide/gas interface in a reducing and carburizing atmo-sphere. The kinetic of coke formation on reduced and pre-oxi-dised iron surfaces was investigated with a thermobalance(ATG) and the different steps of the reaction were studiedby sequential exposure experiments. Scanning electron micro-scopy (SEM) observations and X-ray diffraction (XRD) ana-lysis were performed to investigate the iron oxide behaviourand to identify the phase transformations. The objective of thiswork is to elucidate the role of iron oxides in the mechanism ofcoke deposition.

2 Thermodynamic calculations

To define the different reactions that can occur at the metal/gas interface and oxide/gas interface in isobutane/hydrogenmixtures, the Fe-C-O phase diagram has been calculated ac-cording to the oxygen partial pressure and carbon activity. In acarburizing atmosphere, iron carbides are thermodynamicallyunstable with respect to graphite. However, the formation rateof graphite is so slow at 600 8C that cementite is generallyformed as a metastable phase. Therefore, the Fe-C-O phasediagram was calculated (Fig. 3 + 4) for stable and metastableconditions. Numerous iron carbides are reported in the metal-lurgical literature with compositions ranging from Fe3C toFe2C when the carbon activity in the gas phase increases. Ce-mentite is the most commonly formed iron carbide phase, butother metastable iron carbides (Fe7C3 and Fe5C2) have alsobeen identified in association with the formation of filamen-tous carbon [21, 22]. From a thermodynamic point of view,

only two of them (Fe3C and Fe2C) have been studied undermetastable equilibrium conditions. Thermodynamic data areavailable only for these two compounds. The thermodynamicequilibrium between metallic iron (Fe), iron oxides (FeO,Fe3O4 and Fe2O3), iron carbides (Fe3C, Fe2C) and isobu-tane/hydrogen mixture were calculated at 600 8C. Thermody-namic data for the iron oxide and cementite were provided byThermodata (THERMODATA, 1001 Avenue Centrale – BP66 Grenoble Campus – 38402 Saint Martin d’Heres cedex,france). The values of Browning et al. were used for theHagg carbide Fe2C [23] (Table 1).

The discussion will be developed in terms of the thermo-dynamic carbon activity ac. Formally, ac is the ratio of carbonfugacity as it exists in the reaction environment to the fugacityof carbon in its standard state (graphite) at the same tempera-ture. In processes involving hydrocarbon and hydrogen, car-bon activity can be calculated from the following equilibrium:

iC4H10ðgÞ¼4CðgraphiteÞþ5H2ðgÞ ac ¼KðpiC4H10Þ

p5H2

� �1=4

with ln ðKÞ ¼ 30:43 at 600 8C ð1Þ

K is the equilibrium constant of the chemical reaction andpiC4H10, pH2 are the partial pressures of the gas iC4H10and H2 respectively The experimental conditions used inthis study (30% iC4H10, 30%H2, 40%Ar) establish a carbonactivity of ac ¼ 6710 at 600 8C.

On the basis of the metastable diagram, iron oxides can bedirectly converted into carbides in carburizing and reducingatmospheres. The phases involved in the oxide/carbide transi-tion depend on the carbon activity and on the temperature. At600 8C Fe3C can be produced from Fe3O4 for carbon activitieslarger than 3.6 (Fig. 4). Intermediate oxicarbide speciesFexCyOz may be formed during this reaction.

Table 1. Variation of the Gibbs energy during the formation of the different phases in the Fe-C-O system

Reactions DfG8 ¼ A þ BT(T in 8K) cal/mol

Uncertainty on DfG8cal/mol

Range of temperature(8K)

0.947Fe(s) þ 1/2O2(g) ¼ Fe0.947O(s) � 63222 þ 15.80T � 1000 843–16503Fe(s) þ 2O2(g) ¼ Fe3O4(s) � 265772 þ 78.40T � 2000 473–18702Fe(s) þ 3/2O2(g) ¼ Fe2O3(s) 195479 þ 61.24T � 3000 473–17353Fe(s) þ C(gr) ¼ Fe3C(s) 6926.9–6.78T � 100 473–12732Fe3C(s) þ C(gr) ¼ 3Fe2C(s) 4850–2.5T – 443–1296

Fig. 3. Stable Fe-C-O phase diagram at 600 8C Fig. 4. Metastable Fe-C-O phase diagram at 600 8C


This transformation is well known [24] for chromium oxideCr2O3 and occurs when the alloys is covered with a graphitelayer and heated to temperatures > 1050 8C. The atmosphereat the oxide/metal interface has a carbon activity ac ¼ 1 (equi-librium with graphite) and its oxygen activity decreases be-cause of the reducing gas mixture. These conditions inducea shift of the equilibrium Cr2O3/Cr3C2 in favour of the carbide.Our calculations show that this oxide/carbide transition alsoexists for iron oxide.

However, kinetic factors must be taken into account in thetransformation. Indeed, another path, different from the ther-modynamic prediction, can occur if the rate of the oxide/car-bide conversion is impeded. The iron oxides can be progres-sively reduced in iron and then converted into carbide:

Fe2O3 ! Fe3O4 ! FexO ! Fe ! Fe3C ð2Þ

Previous studies [25] have been performed by the authors toidentify the first stages of the oxide/carbide transformation instrongly carburizing atmosphere. Iron samples with pre-oxi-dised surfaces were maintained in contact with an isobutane/hydrogen gas mixture (ac ¼ 2887) at 550 8C. The oxide/car-bide transition was characterised by X-ray photoelectronspectroscopy. The results show that iron carbide is obtainedon the sample surface after 30 min in the reducing and carbur-izing gas mixture (Fig. 5). No metallic iron was detected,which indicates that iron carbide is present in the oxide layer.This gives evidence that the iron oxides present initially on the

surface sample can be directly converted into iron carbidewithout the formation of metallic iron. Experimental worksperformed by Nakagawa et al. [26] confirm these results:Fe3O4 (face-centred cubic) may be directly transformed inFe3C (hexagonal) in CO-CO2 mixture at 550 8C.

Thermodynamic calculations show that iron carbide Fe3C,once formed, can also participate to the reduction of Fe2O3[27]. The reduction reaction is possible at temperatures above230 8C (Fig. 6).

To ensure that the oxide/carbide transformation occurs inthe overall temperature range where catalytic coke is formed(450 8C–700 8C), we studied the influence of the temperatureon the different equilibria that occur in the Fe-O-C system(Fig. 7). The oxide/carbide transition occurs in the entire do-main where catalytic coke is formed. Only the nature of thechemical species involved in the transformation varies withtemperature. The triple point Fe3C-Fe0.947O-Fe3O4 is shiftedtoward the high temperatures when carbon activity increases.

3 Experimental procedure

Thermogravimetric analyses (TGA) were performed onpolycrystalline iron samples (10 mm � 5 mm � 1 mm) pro-vided by Weber (purity > 99.9%). The chemical analysis isgiven in Table 3. Before inserting the samples into the fur-nace, they were first mechanically polished to 1200 gritSiC, cleaned with ethanol and annealed during 12 h at

Table 2. Stability fields of iron, iron carbide, iron oxide and graphite

Chemical equilibrium Stability fields

0.947Fe(s) þ 1/2O2(g) ¼ Fe0.947O(s) lnPo2 ¼2DfG8Fe0;947O

RT

3Fe(s) þ 2O2(g) ¼ Fe3O4(s) lnPo2 ¼DfG8Fe3O4

2RT

3Fe0.947O(s) þ 0.394O2(g) ¼ 0.947Fe3O4(s) lnPo2 ¼0:947DfG8Fe3O4

� 3DfG8Fe0:947o0:394RT

2Fe3O4(s) þ 1/2O2(g) ¼ 3Fe2O3(s) lnPo2 ¼6DfG8Fe2O3

� 4DfG8Fe3O4

RT

3Fe0.947O(s) þ 0.947C(gr) ¼ 0.947Fe3C(s) þ 3/2O2(g) lnPo2 ¼6DfG8Fe0:947O � 1:894DfG8Fe3C

3RTþ 2

3lna0:947c

Fe3O4(s) þ C(gr) ¼ Fe3C(s) þ 2O2(g) lnPo2 ¼DfG8Fe3O4

� DfG8Fe3C2RT

þ 1

2lnac

2FeO4(s) þ 3C(gr) ¼ 3Fe2C(s) þ 4O2(g) lnPo2 ¼2DfG8Fe3O4

� 3DfG8Fe2C4RT

þ 3

4lnac

3Fe(s) þ C(gr) ¼ Fe3C(s) lnac ¼DfG8Fe3C

RT

2Fe3C(s) þ C(gr) ¼ 3Fe2C(s) lnac ¼3DfG8Fe2C � 2DfG8Fe3C

RT


700 8C in hydrogen flow. In the thermobalance, the sampleswere suspended with a quartz filament and exposed to a flow-ing carburizing atmosphere (30% H2, 30% iC4H10, 40% Ar) at600 8C under 1 atm. Themixture and the flow rate (50 ml/min)were controlled by mass flow meters. All the experimentswere performed with a “Setaram TG-DTA 92” apparatus.

In the thermobalance, the samples were first heated in a hy-drogen flow up to 600 8C. Hydrogen was replaced by the gasmixture as soon as the temperature was reached and the massof coke was continuously recorded. The iron samples wereexposed during different time periods of coking. To stopthe reaction, the gas mixture was replaced by pure argon (pur-ity > 99.9996%) and the furnace was switched off. The sam-ples remained in the furnace during cooling (30 min).

The evolution of the iron surface and the initial stages of thecoke formation were characterised by scanning electron mi-croscopy (SEM). The different phases present on the carbur-ized sample were identified par X-ray diffraction with Co-Karadiation (XRD).

The experiments were performed with two different initialsurface states of iron: reduced and oxidised. To prepare theoxidised surface state, the samples could be initially heatedin an oxidising gas flow up to 600 8C before inserting the cok-ing gas mixture. However the evolution of the carbon activityat the beginning of the experiment will be different from thatobtained with hydrogen flow and the kinetics of the coking

Fig. 5. XPS spectra of Fe2p region obtained on the oxidised (a) and coked (b) iron surface after 30 min in 50%2-50%iC4H10 gas mixture at550 8C [25]

Fig. 6. Equilibrium constant dependency on temperature for ironoxide reduction by iron carbide: 1- Fe3O4 þ Fe3C ¼ 3FeO þ3Fe þ CO; 2- FeO þ Fe3C ¼ 4Fe þ CO; 3- 3Fe2O3 þ Fe3C ¼2Fe3O4 þ 3Fe þ CO

Fig. 7. Influence of the tempera-ture, carbon activity and oxygenpartial pressure on the stability ofiron oxides and iron carbides

Table 3. Analysis of the iron samples (contents in mass %)

C Mn S P Cu N

0.003 0.04 0.002 0.004 0.008 0.005


reaction could not be directly compared. So, the protocol usedto prepare both surface states should not modify the evolutionof carbon activity during the first step of the coking reaction.The oxygen content of the isobutane was used to prepare theinitial surface states of iron. The higher oxygen content of iso-butane N25 (Table 4) was used to obtain the oxidised surfacestate. As the activities of both carbon and oxygen increase dur-ing the introduction of the gas mixture (isobutane þ hydro-gen), the iron oxide Fe3O4 and Fe2O3 are momentarily stableuntil the gas mixture becomes reducing and carburizing. Thisprotocol allows us to obtain a reproductible oxidised iron sur-face. The isobutane N35 was used to prepare the reduced sur-face state. In both cases, when the hydrogen is replaced by thegas mixture at 600 8C, the carbon activity evolution in thethermogravimetric reactor is the same for both studied surfacestates.

4 Results

4.1 Characterisation of the initial surface states

In the reactor, the samples were exposed to the hydrogen/isobutane mixture during five minutes before stopping the re-action by replacing the gas mixture by pure argon. The surfacestate of the samples was characterised by SEM (Fig. 8)whereas the chemical composition and the thickness of oxidescales were given by XRD (Fig. 9) and electronic microprobeanalyses. After 5 minutes of contact with N25 isobutane, theiron sample is covered by an oxide layer of 1 lm composed ofFe2O3 (25 � 10%) and Fe3O4 (75 � 10%). According to theFe-C-O phase diagram, when the atmosphere becomes redu-cing with ac ¼ 6710, hematite should be reduced in magnetite,and magnetite should be converted into Fe3C. When N35 iso-butane is used no iron oxides are detected on the surface sam-ples with the same techniques.

4.2 Kinetics of the catalytic coke formation

The mass of coke was measured after exposures for differ-ent lengths of time. The different stages of the reaction on thereduced and pre-oxidised iron samples were characterised bySEM and the nature of the reaction products was identified byX-ray diffraction with Co Ka radiation.

4.2.1 Coke formation on the reduced surface state

The mass of coke versus time is plotted in Fig. 10 and theSEM images are shown in Fig. 11. Nucleation and growth ofgraphite needles are observed on the iron surface after expo-sure of 10 min to isobutane. Nucleation occurs first at thegrain boundaries. After 50 min, 95% of the surface is coveredby carbon and small particles start to appear on the graphiticsurface. The maximum size of the particles is about 0.1 lm.The number of particles increases with time and the particlesinduce the growth of filaments. Graphitic filaments appear onthe surface after � 70 min.

After 50 min in the gas mixture, metallographic cross-sec-tions show that the graphite layer has an average thickness of1.5 lm. The size of the carbide precipitates present at the me-tal/graphite interface is about 0.1 lm to 3 lm. These observa-tions are consistent with the mechanism currently proposed byGrabke et al. The different steps of the mechanism are shownin Fig. 11.

Table 4. Composition of the isobutane grade provided by “Air li-quide” and used in the coking experiments (in ppm) to obtain thereduced and oxidised surface states

Isobutane purity H2O O2 CO2 Surfacestate

N25 > 99.5% – 100 Oxidisedstate

N35 > 99.95% 5 10 10 Reducedstate

Fig. 8. SEM characterisationof the iron surface after 5 minin contact with iC4H10-H2-Argas mixture at 600 8C (totalpressure¼ 1 atm); (a) initial re-duced state, (b) initial pre-oxi-dised state

Fig. 9. XRD analysis of the iron surface state after 5 min in contactwith iC4H10-H2-Ar gas mixture at 600 8C (total pressure ¼ 1 atm) –(a) initial reduced state, (b) initial oxidised state


4.2.2 Coke formation on the pre-oxidised surface state

Thermogravimetric measurements show that the rate ofcoke deposition is higher when the iron surface is initiallypre-oxidised (Fig. 12). After 10 min in the gas mixture, theiron oxide layer is still present on the surface, but cementiteand graphite are also detected (Fig. 14). Another mechanismof catalytic particle formation is observed when the surface isinitially pre-oxidised. In the carburizing and reducing atmo-sphere, fragmentation of the oxide layer occurs and metallicparticles with a size range of 50 nm to 400 nm are formed(Fig. 14). At the same time, graphite nucleation occurs onthe metal surface. The size of metallic particles decreases

with time. In contact with graphite, they are progressivelytransformed into particles that catalyse the graphitic filamentsformation (Fig. 14). The first filaments appear after 20 minand their density increases rapidly with time. After 45 min,the iron surface is completely covered by graphitic filaments,whereas many hours are necessary on the reduced surface.

A time of 30 min is required to eliminate the iron oxidesfrom the surface. The relative fraction of magnetite (Fe3O4)decreases rapidly compared with the fraction of hematite(Fe2O3). This result is consistent with thermodynamic data.Hematite must be reduced in magnetite before its conversioninto carbide. Wustite (FexO) which is formed during the re-duction of magnetite in metallic iron is not detected duringcoke formation. This result is consistent with a direct conver-sion of iron oxide into carbide without the formation of me-tallic iron. The iron carbide that is formed is cementite (Fe3C),which is different from thermodynamic predictions. Metallo-graphic cross-sections show that cementite is also formed atthe metal/oxide interface. The average size and the distribu-tion of the precipitates are similar to those observed when theiron surface is initially reduced.

4.2.3 Characterisation of the catalytic particles

Iron samples with an initial reduced and pre-oxidised sur-face state are coked in the gas mixture 30%iC4H10-30%H2-40%Ar during 11 h at 600 8C. After this treatment, thecoke deposition is removed from the surface samples, andthe chemical nature of the catalytic particles is analysed usingX-ray diffraction. Catalytic particles are composed of metalliciron (Fe) and iron carbides (Fe3C and Fe2C) (Fig. 15). Theseresults are consistent with the observations previously per-formed by the authors and carried out by transmission electron

Fig. 10. Mass gain versus time for iron samples (reduced surface)carburized in 30%iC4H10-30%H2-40%Ar gas mixture at 600 8C (to-tal pressure ¼ 1 atm)

Fig. 11. Evolution of the re-duced iron surface after sequen-tial exposure in iC4H10-H2-Aratmosphere at 600 8C


Fig. 12. Mass gain versus time for iron (pre-oxidised and reducedsurface state) samples carburized in 30%iC4H10-30%H2-40%Ar gasmixture at 600 8C (total pressure ¼ 1 atm)

Fig. 13. Evolution with time of the relative fractions of hematite(Fe2O3), magnetite (Fe3O4) and cementite (Fe3C) present on thesamples surfaces (initially pre-oxidised) exposed to a mixture of30%iC4H10-30%H2-40%Ar at 600 8C

Fig. 14. Evolution of the oxi-dised iron surface after sequen-tial exposure in 30%iC4H10-30%H2-40%Ar atmosphere at600 8C


microscopy (TEM) [28]. No difference was observed betweenthe reduced or pre-oxidised surface states.

Thermodynamic calculations show that metallic iron is un-stable with respect to the carburizing atmosphere. Moreover,catalytic particles cannot be composed of pure iron carbide.Indeed, the carbon diffusion trough cementite is very slow.The presence of cementite bulk should induce graphite nu-cleation around the particles and the deactivation of their cat-alytic properties. As already proposed by Bianchini et al. [16],the catalytic particles are probably composed of a mixture ofmetallic iron and iron carbides. A carbide layer on the leadingfaces of the catalytic particles probably separates the metallicbulk of the catalyst from direct contact with the carburizingatmosphere. The nature of the carbide layer depends on thecarbon activity in the gas phase. More and more carbon-rich phases from Fe3C (25 at.%C) via Fe5C2 (28.6 at.%C)

to Fe2C (33 at.%C) are formed with increasing carbon contentin the iron. The different interfaces associated with the parti-cles (gas/carbides/iron/graphite) induce a carbon activity gra-dient (Fig. 16). The carbon from the gas phase can diffusethrough the catalytic particles to feed filament growth.

5 Discussion

5.1 Formation of catalytic particles on reduced ironsurfaces

The observations performed on the initially reduced ironsurfaces are consistent with the mechanism currently pro-posed to explain the formation of catalytic particles oniron. The different steps of this mechanism are summarisedin Fig. 17: (1) carburisation of the iron surface, (2) graphitenucleation on cementite precipitates, (3) decomposition ofiron carbide in contact with graphite and formation of metallicparticles after diffusion of iron atoms through the graphitelayer, (4) carburisation of metallic particles, (5) formationof a carbide layer on the leading face of the catalytic particles,(6) filament growth.

5.2 Formation of catalytic particles on pre-oxidised ironsurfaces

In reducing and carburizing atmosphere, two mechanismsare proposed to explain the transformation of iron oxides. Ironoxides can be progressively reduced in metallic iron which iscarburized to give cementite (reaction 3). Moreover the oxidescan be directly converted into carbides probably through theformation of oxicarbide species (reactions 4 and 5). In the fol-lowing reactions, the different steps and the carbon activityranges are given for a temperature of 600 8C:

ac > 1:6Fe2O3 ! Fe3O4 ! Fe1�dO ! Fe ! Fe3C ð3Þ

1:6 < ac < 3:6Fe2O3 ! Fe3O4 ! Fe1�dO ! FexOyCz ! Fe3C ð4Þ

3:6 < ac < 31:6Fe2O3 ! Fe3O4 ! FexOyCz ! Fe3C ð5Þ

The reduction of hematite pellets (Fe2O3) occurs by thegrowth of successive iron oxide layers as the oxygen content

Fig. 15. XRD analysis of the coke deposition present on an ironsample after exposure of 11 h in 30%iC4H10-30%H2-40%Ar atmo-sphere at 600 8C

Fig. 16. Carbon activity gradient at 600 8C through the catalyticparticles

Fig. 17. Mechanism currently pro-posed to explain the formation ofcatalytic particles on initially re-duced iron surfaces


decreases. The reduction reactions take place at the interfaceof the different oxide layers. The different ways of transfor-mation proposed are described in the schematic diagram ofFig. 18.

The thermodynamic calculations (Fig. 4) and the previousresults obtained by the authors by X-ray photoelectron spec-troscopy [25] showed that in strongly carburizing atmosphere,the oxide/carbide transition occurs without the formation ofmetallic iron. Consequently, only mechanisms (4) and (5)can occur. The fragmentation of the iron oxide layer that oc-curs in the early stage of the reaction can be explained by thedifference of the molar volume between the different iron oxi-des and the cementite (Table 5). For the Fe3O4/Fe3C transfor-mation, the molar volume is divided by a factor of about two.

A new mechanism is postulated to account for the transfor-mation of the iron oxide in catalytic particles for carbon fila-ment growth. This mechanism can be divided into five steps(Fig. 19):(1) In contact to reducing and carburizing atmosphere, mag-

netite is transformed into cementite probably via an “oxi-carbide” species FexOyCz. The difference of molar volumebetween these different phases induces a fragmentation ofthe oxide scale. During the transformation, a cementitelayer encompasses the magnetite grains.

(2) In the same time, through the cracks in Fe3O4, hydrocar-bon molecules reach the metal surface and their decom-position at the gas/metal interface (adsorption and disso-ciation of hydrocarbons molecules) induces carbon super-

saturation of the metallic phase. When the carbon activityin the metal is higher than the carbon activity for cemen-tite formation, the supersaturation of carbon in the metalcauses nucleation and growth of cementite at the metalsurface and at the grain boundaries.

(3) The carbon diffusion is very slow through the cementite inthe range of temperatures 400–700 8C [8]. Consequently,the cementite layer acts as a barrier for further carbontransfer and the precipitation of graphite occurs on the ce-mentite precipitates.

(4) Cementite becomes unstable in contact to graphite andstarts to decompose according to the following reaction:

Cementite (Fe3C) ! Graphite (C) þ Supersaturated solidsolution (Fe þ C(diss.)).

The solid solution is formed at the graphite/cementite inter-face and induces the decomposition of cementite. At thisstep, the catalytic particles are composed of a mixture of ce-mentite and iron supersaturated with carbon. The differentlayer induced a carbon activity gradient through the catalyticparticles.(5) Carbon from the gas phase can diffuse through the cata-

lytic particles via the cementite decomposition and preci-pitate in graphite inducing a separation of the catalyticparticles from the graphite surface. The particles act ascatalyst for further carbon deposition and the continuouscarbon diffusion induced the graphitic filament growth.

The mechanism currently proposed by Grabke et al., whichoccurs on the reduced surface state, can then take place atthe same time after the fragmentation of the oxide scale.

6 Conclusion

The thermodynamic equilibria between iron, iron oxidesand iron carbides were calculated at 600 8C. According tothe metastable Fe-C-O phase diagram, iron oxides can be di-rectly converted into carbide in a reducing and carburizing

Fig. 18. Schematic representation of mechanisms of transforma-tion of hematite pellets in a reducing and carburizing atmosphereat 600 8C for different carbon activities; (1): ac> 1.6� (2): 1.6< ac< 3.6 � (3): 3.6 < ac < 31.6

Table 5. Volumetric mass and molar volume of hematite, magnetite, wustite, iron and cementite. These values are given for stoechiometriccompounds

phase Fe2O3 Fe3O4 FeO Fe Fe3C

Volumetric mass (� 103) (kg/m3) 5.24 5.18 5.87 7.88 7.67Molar volume (� 105) (m3/mol) 3.03 4.45 1.22 0.71 2.34

Fig. 19. New mechanism pro-posed to explain the formation ofcatalytic particles on initially pre-oxidised iron surfaces (transforma-tion of magnetite (Fe3O4) in cataly-tic particles)


atmosphere without the formation of metallic iron. The che-mical species involved in the oxide/carbide transition dependon the temperature and carbon activity. In the range of tem-perature where catalytic coking occurs, hematite (Fe2O3) mustbe reduced into magnetite (Fe3O4) before its conversion intocarbide. The kinetics of the coke formation was studied bythermogravimetry in isobutane/hydrogen/argon mixturewith ac ¼ 6710. A comparison was made between iron sur-faces that were initially reduced or pre-oxidised. The obser-vations performed on the initially reduced surface are consis-tent with the mechanism currently proposed to explain the for-mation of catalytic particles on iron. A newmechanism is pro-posed to explain the formation of catalytic particles on ironsurfaces initially oxidised. In carburizing and reducing atmo-sphere, iron oxides are progressively transformed into carbide.This reaction induces a fragmentation of the oxide scale andthe formation of graphite on the iron surface. In contact tographite, iron carbides become unstable and a carbon activitygradient occurs through carbides which can act as catalyst forthe graphitic filaments growth. This mechanism of catalyticparticle formation increases the rate of coke deposition com-pared to an initially reduced iron surface.

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(Received: June 4, 2003) W 3742


filamentous carbon formation caused by catalytic metal particles from iron oxide

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