Solid State Phenomena Vol. 94 (2003) pp 181-184
© (2003) Trans Tech Publications, Switzerland
doi:10.4028/www.scientific.net/SSP.94.181
Preparation of Nanocrystalline Iron Carbide by Reaction of Iron with
Methane
W. Arabczyk, W. Konicki, U. Narkiewicz
Technical University of Szczecin, Institute of Chemical and Environment Engineering,
Pulaskiego 10, 70-322 Szczecin, Poland
Keywords : nanocrystalline iron carbide, carburisation, nanocrystalline iron, carbon deposit
Abstract. The process of the formation of nanocrystalline iron carbide in the reaction of
nanocrystalline iron with pure methane, or CH4/H2 (2 :1) gas mixture under atmospheric pressure, at
580OC has been studied. The carburisation process has been controlled by a thermobalance. The
rate of the carburisation process depended on the chemical composition of the iron surface and on
the process conditions. As a result of the process, iron carbide is obtained together with unconverted
a-iron or a carbon deposit. The optimal conditions to produce iron carbide only can be reached
when the reaction rate is lowered, by the dilution of methane with hydrogen, or by a modification of
the surface of the nanocrystalline iron. The samples before and after the carburisation process have
been characterised by XRD. The average size of the iron carbide crystallites produced was 35 nm.
Introduction
The carburisation of iron and steel is a well known method to improve the hardness of these
materials. The iron carbides obtained as a product of iron carburisation are interesting materials,
which can be used, for example, as sensors, magnets, alternative raw materials for the production of
steel [1,2], for the denitrification of liquid steel [3] or as catalysts [4]. Fine or coarse grain iron
carbide can be obtained by carburisation of iron oxides with carbon monoxide [5, 6]; sonochemical
decomposition of Fe(CO)5 [7]; mechanosynthesis of elemental Fe and graphite powders [8]; plasma
enhanced chemical vapour deposition [9]; or reaction of hydrocarbons with iron [10-15]. The
nanocrystalline iron carbide was prepared by a reaction with of methane with nanocrystalline iron at
a relatively low temperatures (below 600 °C).
Experimental
Nanocrystalline iron carbide has been obtained by the carburisation of nanocrystalline iron using
methane. Thee method of preparation of nanocrystalline iron have been patented recently [16]. Pure
nanocrystalline iron is not a suitable raw material for the preparation of iron carbide because of its
tendency to sinter at elevated temperatures. To increase the thermal stability of nanocrystalline iron
against sintering, it is necessary to add some oxide with very little reducibility, such as Al2O3, as a
structural promoter. The use of such structural promoters does not modify the specific catalytic
properties of iron and enables the a stable nanocrystalline structure to be formed.
The samples used in the experiments were obtained by the fusion of magnetite with structural
promoter oxides. The alloy was reduced under hydrogen. The obtained pyrophoric samples were
passivated using nitrogen with the addition of 0.5 % of oxygen. The chemical composition after the
passivation was determined using an inductively coupled plasma atomic emission spectroscopy
(AES-ICP). As well as iron, the samples contained 2.9 wt. % Al2O3, 3.0 wt.%, CaO, 0.3 wt. %,
SiO2 and 1 wt.% of oxides of other metals (Mg, Ni, Cr, Ti, V). The samples were sieved to obtain a
fraction of size 1.2 to 1.5 mm. The average crystallite size of the samples, determined using X-ray
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182
Interfacial Effects and Novel Properties of Nanomaterials
diffraction (XRD) (CoKa1) and calculated using the Scherrer equation, was 17 nm. Samples
modified by the addition of sulphur or of potassium oxide were also examined. Sulphur was
introduced into the samples by impregnation with a sulphuric acid solution, when potassium oxide
was introduced at the stage of the fusion of the nanocrystalline material.The carburisation process
was carried out in a thermobalance. Grains of the sample (1 g) were placed in one monolayer in a
platinum basket hung in the thermobalance. Before carburisation the samples were reduced at a
temperature rising from 20 to 580OC at a rate of 15OC/min. under a hydrogen flow of 0.2 (dm3·g1
·min.-1). After the reduction the carburisation process, using methane or methane/hydrogen
mixtures, under atmospheric pressure, with a space velocity of 1.2 dm3·g-1·min.-1 at a temperature of
580OC was started.
Results and Discussion
Relative increase of mass, [gC/gFe]
According to the equilibrium diagram of the Fe-C system, there are two stable phases in this system,
carbon dissolved in a-Fe and graphite, and the metastable state of Fe-Fe3C [17]. The equilibrium
chemical composition of the binary system Fe-C depends on the temperature as well as on the
composition of the gas phase (CH4/H2 mixture) [18].There are numerous papers by Grabke [19-22 ]
related to the carburisation of iron in the region of carbon solubility in a-Fe and in g-Fe. According
to these studies, the limiting step in the process is the reaction occurring on the gas-metal phase
boundary. This paper deals with the next step of the carburisation process, carried out in the region
where iron carbide starts to form. There is no literature data concerning methane decomposition on
iron at low temperatures (500-600 °C).
The results of the thermo-gravimetric
0.35
measurements are shown in Figure.1
1 2
4
3
0.30
as a relative mass increase (gC/gFe)
during the carburisation process at a
0.25
temperature of 580 OC. The slope of
0.20
the TG curves correspond to the rate
0.15
of the carburisation process. The
dashed horizontal line corresponds to
0.10
0.072
the carbon to iron mass ratio
T=580 C
0.05
equivalent to the stoichiometric
0.00
composition of Fe3C (0.072). The
0
2000
4000
6000
8000
10000
12000
14000
carburisation process in pure
Tim e, [s]
methane occurs quickly (line 1) and
Fig.1. The relative mass increase during carburisation of when the limit of the stoichiometric
the samples: 1 - carburised under CH4, 2 - with addition composition of Fe3C is overcome,
of sulphur, carburised under CH4, 3 - carburised under the carbon deposit starts to form.
the CH4/H2 (2:1), 4 - with addition of potassium, There are two reaction areas: the
first, in which the sample under
carburised under CH4/H2 (2:1).
carburisation contains iron carbide
and the unconverted iron, and the second, in which the sample contains iron carbide and the
carbonaceous deposit. There is no sharp limit between the two areas, but it is possible to optimise
the conditions to obtain only the iron carbide. To achieve this outcome it is necessary to inhibit the
carburisation reaction rate by an increase of the reverse reaction rate; i.e decarburisation due to the
increase of the hydrogen partial pressure in the reaction mixture; or by changes in the surface
structure of the carburised sample. The addition of sulphur (300 ppm) to the iron sample causes a
decrease of the reaction rate (line 2). The dilution of methane with hydrogen (CH4:H2 = 2:1) also
decreases the reaction rate (line 3). The rate of the iron carbide formation on the sample modified by
the addition of potassium oxide (K2O) is lower (line 4) than that of the sample without potassium
O
Solid State Phenomena Vol. 94
1 2 0 0
Non marked peaks are
attributed to Fe3C
Graphite
183
(the same gas mixture was used in
both cases). On the contrary, the
addition of potassium causes an
increase of the carbon deposition.
Intensity
5
The phase composition of the
samples after carburisation was
3
characterised using XRD. The
2
XRD diffraction patterns are
1
shown in Fig.2. In the first
Fe(110)
Fe(200)
pattern, corresponding to the iron
25
35
45
55
65
75
85
sample before the carburisation
2-theta, [deg]
process only the peaks of a-Fe are
Fig. 2. The diffraction patterns of: 1- iron, samples after observed. The additional peaks in
the carburisation at 580°°C up to 6.67 wt% C: 2 - under the diffraction patterns above are
CH4, 3 - sample with potassium, carburised under CH4, 4 attributed to the Fe3C. Under the
- iron carburised under CH4/H2, and 5 - iron carburised experimental conditions the other
under CH4 up to 22 wt% C.
iron carbides were not detected,
as shown in agreement with the
literature data [17]. In the
diffraction pattern (3) of the Fe-K sample, with addition of K2O, carburised under pure methane, the
intensity of the Fe3C peaks is greater than for the sample without potassium carburised to the same
carbon content (6.67 wt.%). This is because in the sample without potassium the reaction occurs
faster and the deposition of carbon starts at lower carbon concentration than in the case of the
sample modified with a potassium addition. In the diffraction pattern of the sample (4) carburised
under the mixture of methane with hydrogen, the intensity of the Fe3C peaks is greater than for the
same sample carburised under the pure methane (2) up to the same carbon content (6.67 wt.%). This
is because the lowering of the reaction rate increases the proportion of carbon deposited. The
diffraction pattern at the top of the figure relates to the sample carburised under pure CH4 up to 22
wt% C. The fine crystalline phase of graphite is observed (2Q=30°) in this case.
4
0
Conversion Fe to Fe3C
3
Fig.3 shows the dependence of the degree of conversion of iron to iron carbide (determined on the
basis of corresponding XRD intensities of the a-Fe and Fe3C peaks) on the carbon content in the
samples (determined using the thermogravimetry method). The three samples are compared here:
(1) - initial iron sample, (2) - iron
1
modified with an addition of sulphur
4
3
(300 ppm), (3) - iron with an addition
0.8
2
of potassium oxide (0.65 wt.% K2O).
1
All three samples were carburised
0.6
with pure methane. The dashed line
(4) corresponds to the formation of
0.4
the stoichiometric Fe3C, obtained in
the reaction with the mixture
T=580 C
0.2
CH4:H2=2:1. In the region where the
curves in this plot have a linear
0
0
2
4
6 6.67
8
10
12
14
16
18
character (at the beginning of the
Content of carbon, [wt %]
process) only Fe3C is formed. The
deviation from linearity corresponds
Fig. 3. The effect of the carbon content in the iron on the
to the formation of the carbonaceous
conversion degree of Fe to Fe3C.
O
184
Interfacial Effects and Novel Properties of Nanomaterials
deposit. The carbon deposit is formed at lower carbon concentration on the unmodified sample, than
on those modified with potassium or sulphur. The formation of the carbon deposit occurs below the
stoichiometric composition of Fe3C (6.67 wt% C).
Taking into account the results presented in Figures.1 and 3, to obtain pure iron carbide at the
temperature of 580°C on unmodified nanocrystalline iron the mixture of methane with hydrogen
(CH4:H2=2:1) should be used (conditions close to equilibrium). The reaction has to be stopped just
before reaching the mass increase corresponding to the formation of Fe3C (6.67 wt.% C), by rapidly
cooling with nitrogen. The sample of iron carbide obtained was characterised using the XRD
method. The average size of the nanoparticles of iron carbide was 35 nm.
Summary
Nanocrystaline iron carbide can be prepared alone or together with a carbonaceous deposit by the
reaction of methane with nanocrystalline iron at temperatures below 600 °C. The composition of the
carburised samples depends on the reaction temperature, on the surface modification of the iron
sample and on the composition of the reaction mixture used for carburisation. The carbon
deposition is retarded when the carburisation reaction occurs slowly, and to inhibit this reaction the
dilution of methane with hydrogen can be used, or alternatively the modification of the
nanocrystalline iron with potassium oxide or with sulphur.
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Interfacial Effects and Novel Properties of Nanomaterials
10.4028/www.scientific.net/SSP.94
Preparation of Nanocrystalline Iron Carbide by Reaction of Iron with Methane
10.4028/www.scientific.net/SSP.94.181
DOI References
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doi:10.1016/0008-6223(68)90003-1