Materials Transactions, Vol. 45, No. 6 (2004) pp. 1911 to 1914 #2004 The Japan Institute of Metals
Materialographic Investigation on the Mechanism of Hydrogen Production through the Reaction between Iron Carbide and Steam at a Temperature of 673 K Masaaki Hisa1 , Atsushi Tsutsumi2 and Tomohiro Akiyama1; * 1 2
Department of Chemical Engineering, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan Department of Chemical System Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
Materialographic investigation was applied to the microstructure evolution of iron carbide brought by the reaction with steam at 673 K in order to clarify the mechanism of hydrogen production through the iron carbide–steam reaction. Optical and scanning electron microscopy in combination with X-ray diffractometry revealed that a layer of iron oxide, mainly magnetite, forms on the external and internal surfaces of porous particles of iron carbide as a result of the direct oxidation of iron carbide. The oxide phase contains nano-sized graphite particles that are highly crystallized and dispersed uniformly in the oxide layer. The oxidation of iron carbide by steam was confirmed to be responsible for the hydrogen production at 673 K. (Received January 23, 2004; Accepted April 23, 2004) Keywords: iron carbide, hydrogen production, microscopy, X-ray diffractometry, microstructure, nano-sized graphite
1.
Introduction
Most of hydrogen for commercial use is currently produced by the methane-steam reforming process. The product of the process is usually a mixture of hydrogen and carbon monoxide, which is finally converted into carbon dioxide through the shift reaction. Purification of the product gas is, therefore, inevitable in order to obtain hydrogen of high purity for applications such as polymer-electrode fuel cells in order to protect platinum used as catalyst in the cells from CO poisoning. The steam-iron method is an alternative way of hydrogen production, and highly purified hydrogen can be obtained because hydrogen is the only gas product in the process. Nevertheless, the steam-iron method requires high temperature (1073 K or above) and high pressure to maintain the reaction, and thus this is the major drawback that prevents extended commercial uses of the process. Otsuka et al. have conquered this problem by developing an innovative way of hydrogen production, using iron that is alloyed with aluminum, chromium, titanium, zirconium, gallium and vanadium.1–3) The new process is fundamentally the same as the steam-iron method, but it is operated at temperatures below 573 K without applying high pressure. On the other hand, Akiyama et al. and Ito et al. have recently presented another possibility to obtain hydrogen. They put their eyes on the simple reaction between iron carbide (IC) and water and pointed out that hydrogen with high purity can be collected at temperatures around 573 K through the reaction.4,5) The situation was maintained up to about 823 K, beyond which the concentration of carbon monoxide in the product gas increased rapidly. Although the hydrogen production in the process was presumed to result from either adsorption of oxygen or oxidation of iron, details of the IC– steam reaction at such low temperatures are yet to be understood. In the present study, therefore, an attempt was made to clarify the mechanism of hydrogen production in the temperature range by means of materialography. Optical *Corresponding
author. E-mail:
[email protected]
microscopy, scanning electron microscopy in conjunction with composition analysis by spectroscopy and X-ray diffraction were employed to investigate the microstructure evolution brought into IC by the reaction with steam at 673 K. 2.
Experimental Procedure
The IC used in the present study was prepared from Carajas iron ore by Qualitech Steel Corporation with a twostep fluidized bed process. Chemical analysis revealed that the phase purity of Fe3 C in the IC was around 90 mass%, and metallic Fe, Fe3 O4 and Fe2 O3 were included as impurities. After sieved mechanically with a 300 mm stainless steel sieve, the IC powder was used for the experiment. Around 150 mg of the IC specimen was heat treated with a simultaneous DSC/TGA thermal analyzer (TA Instruments SDT2960). The specimens were kept at 393 K for 3.6 ks for drying purpose prior to the heat treatment, and then kept at 673 K for 86.4 ks, followed by furnace cooling. Heating rate was set at 0.167 Ks1 throughout the experiment, and all of the procedures were carried out under a flowing argon gas atmosphere that contained 4.6 ppm of water. The flow rate of Ar was set at 8:3 107 m3 s1 . Heat treatment of the IC was also carried out in a vacuum condition in order to eliminate the effect of water. Particles of the IC, the total weight of which was about 1 g, were sealed in quartz tubes under vacuum with a pressure lower than 104 Pa. The encapsuled specimens were heat treated at 673 K for 86.4 ks in a tube furnace and then air-cooled. The temperature variation during the heat treatment was kept within 2 K. Phase identification of the heat treated specimens was performed by powder X-ray diffractometry (XRD) with a Rigaku RINT-1500 diffractometer equipped with a rotational Cu target (tube voltage = 50 kV, tube current = 300 mA) and a graphite monochromator. Specimens for microstructure investigation were prepared with a conventional polishing technique after the heat treated ICs were molded in resin. They were examined with an Olympus BX51M optical microscope and a JEOL JEM-6700F scanning electron
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M. Hisa, A. Tsutsumi and T. Akiyama Stage I
Stage II
Stage III
(a) Fe3C Fe Fe3O4 Fe2O3 C
102
100 99 98 97 96 95 373
573
773 973 Temperature, T / K
1173
1373
Fig. 1 TG curve of the iron carbide specimen heated continuously at 0.167 Ks1 in an Ar atmosphere containing 4.6 ppm of water.
microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS). In order to prevent charge-up by incident electrons, specimens for SEM examination were coated with vacuum-evaporated aluminum prior to observation. 3.
Relative Intensity (Arbitrary Unit)
Relative Mass (%)
101
(b)
26°
27°
Results and Discussion
The reaction between the IC and water proceeded via three stages during continuous heating up to 1373 K, as shown in Fig. 1. The behavior is similar to the result presented by Akiyama et al. previously,4) except the temperature range of each stage. Stage I, in which hydrogen is predominantly generated over other gas species, commenced at a temperature around 473 K and ceased at about 813 K. These temperatures are respectively 100 K and 60 K lower than those reported by Akiyama et al. for Stage I. In addition, Stage II continued up to 1173 K in the present investigation, leading to a large extension of Stage II. Although the reason causing these variations remains unclear, it might be attributed to a low concentration of water in the atmosphere, compared to that used in the previous investigation, i.e. 3.14 vol%. In order to enhance the reaction between IC and steam, the temperature for the following heat treatment was set at 673 K. The heat treatment was terminated after 21.6 ks because the early stage of the IC–steam reaction was of interest in the present investigation. The X-ray spectrum of the IC specimen heat treated at 673 K is shown in Fig. 2, together with the spectrum of a raw specimen. The heat treatment at 673 K decreased the intensity of reflections from Fe3 C and Fe, whereas several peaks were intensified by the heat treatment. The intensified peaks were identified as reflections of iron oxides, Fe3 O4 and Fe2 O3 . This result suggests that the weight increase observed in Stage I is associated with the formation of iron oxides and thus the hydrogen generation in the stage is mainly attributable to the oxidation of Fe in the IC. XRD analysis also revealed an interesting fact on the nature of carbon in the IC. The reflection occurring at around 26 was identified to arise from graphite, which was the only carbon-related phase, other than Fe3 C, observed in the IC. Comparison with the PDF data on graphite indicated that the
20°
40°
60°
80°
100°
120°
Scattering Angle, 2θ Fig. 2 XRD spectra of the iron carbide specimens: (a) raw, and (b) heat treated at 673 K for 21.6 ks. The inset of (b) is the 0002 reflection of graphite indicating a split between Cu K–L3 and Cu K–L2 .
reflection, which was indexed as 0002 of graphite, shifted from 2 ¼ 26:4 to 26.7 , implying that the lattice of the graphite that formed in the IC contracts possibly by 1% along its c-axis direction. Furthermore, the reflection was found to show fairly narrow width and splitting between Cu K–L3 and Cu K–L2 is clearly observed on the reflection, as shown in the inset of Fig. 2(b). The occurrence of splitting between K–L3 and K–L2 at such low scattering angles is rather surprising and may suggest that the graphite in the IC is perhaps composed of a lattice with high degree of crystallinity. The microstructure of the IC specimen was characterized by its porous nature, as shown in Fig. 3. In a raw condition, particles of the IC contained fine cracks and pores, most of which were in the order of 106 m or less. Some cracks, in particular relatively large ones, were aligned with three-fold symmetry. Each of the symmetry variants was found to be parallel to the outer edges of the particle shown in Fig. 3(a), probably reflecting a crystallographic feature of the IC. After heat treatment at 673 K, a layer of a grayish phase was found to develop around the edges of IC particles, and the same phase also grew on the surface of cracks and pores inside the particles, making their microstructure less porous. The newly formed phase was supposed to be the iron oxide FeOx that was detected by XRD analysis. The microstructure of the heat treated IC specimen was further examined in detail by SEM observation coupled with
H2 Production through the Reaction between Fe3 C and H2 O
(a)
(c)
(b)
(d)
1913
Fig. 3 Optical micrographs showing typical cross-sectional microstructures of the iron carbide: (a) and (b) raw, and (c) and (d) heat treated at 673 K for 21.6 ks. The scale bars show 50 mm.
EDS analysis. Backscattered electron (BE) imaging revealed the microstructure to include two phases, which appear bright and dark in Fig. 4(a). The dark phase was primarily located at the surface of IC particles or adjacent to internal pores in the particles. EDS spectra taken from those bright and dark phases indicated that Fe, C and O were the major elements composing both the phases, and the dark phase was proved to contain higher concentration of oxygen than the bright phase, as shown in Fig. 4(b). This observation coincides the results obtained by XRD and optical microscopy and therefore the bright and dark phases were identified as Fe3 C and FeOx respectively, although the C/Fe ratios for the carbide phase are slightly higher than the stoichiometric composition (C/Fe = 0.33). Interestingly, the oxide regions included fine black dots (encircled in the micrograph) that were less than 1 mm in size and dispersed uniformly over the oxide phase. Since the BE image in Fig. 4(a) provides a compositional contrast depending upon average atomic numbers, the areas appearing in dark are presumably rich in light elements, i.e. C in this case. Considering the result of XRD that showed the presence of graphite in the heat treated IC specimen, the fine black dots were thought to be nano-sized particles of graphite. Consequently, the microstructure of the IC heat treated at 673 K can be described as a multi-phase structure that is composed of Fe3 C, FeOx (mainly Fe3 O4 ) and C (graphite) nanoparticles embedded in the oxide regions. The C/Fe ratios measured for the oxide areas tended to be lower than those for the carbide areas and thus part of carbon in the IC might have gasified via the reaction with steam during the heat
treatment.5) Based on the above-mentioned experimental results, a possible reaction equation for the IC–steam reaction that occurs in Stage I is likely to be described as follows. Fe3 C þ 4H2 O(g) ! Fe3 O4 þ 4H2 (g) þ C
ð1Þ
On the other hand, it is well known that Fe3 C is a thermodynamically metastable phase in the Fe–C binary system. According to thermodynamical calculation using the HSC Chemistry 5 software,6) the Gibbs energy for Fe3 C is indeed higher around 20 kJmol1 than that for the thermodynamicall stable state, i.e. 3Fe + C, at 298 K (see Fig. 5). It means that Fe3 C can decompose into Fe and C during heat treatment in Stage I. If this is the case, the reaction between the IC and steam possibly proceeds in the following manner: Fe3 C ! 3Fe þ C; and then 3Fe þ 4H2 O(g) ! Fe3 O4 þ 4H2 (g):
ð2aÞ ð2bÞ
Therefore, in order to investigate whether the decomposition of Fe3 C precedes the oxidation of Fe, it is essential to separate the reaction (2a), if it occurs, from the reaction (2b). To achieve this purpose, heat treatment was performed in vacuo to prevent the reaction (2b) taking place, and specimens after heat treatment were examined by XRD for phase identification. Figure 6 shows the XRD spectrum of the specimen that was heat treated at 673 K for 86.4 ks. It was found that the spectrum was almost identical to that obtained from the raw specimen shown in Fig. 2(a) and therefore that the decomposition of Fe3 C in the IC rarely proceeded at the
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M. Hisa, A. Tsutsumi and T. Akiyama
Graphite
Fe3C
Intensity (Arbitrary Unit)
(a)
Carbide Oxide
Fe Fe3O4 C
40°
Pore
(b)
O / Fe Ratio
Oxide (Solid) Carbide (Solid) Carbide (Porous)
1.0
0.0 0.0
0.2
0.4
0.6
0.8
1.0
1.2
C / Fe Ratio Fig. 4 Microstructure of the iron carbide heat treated at 673 K for 21.6 ks examined by the SEM–EDS technique: (a) backscattered electron micrograph and (b) summary of composition analysis by EDS. EDS data were collected from three types of microstructure: carbide in solid areas, carbide in porous areas and oxide in solid areas. The relatively high values of O/Fe ratio probably result from the presence of Al2 O3 that formed on the specimen surface as a result of partial oxidation of the conductive coating of Al.
Gibbs Energy Change, ∆G˚ / kJmol-1
100°
120°
Fig. 6 XRD spectrum of the iron carbide specimen heat treated at 673 K for 86.4 ks in vacuum with a pressure lower than 104 Pa.
2.0
0.5
Fe3 C þ 4H2 O(g) ! Fe3 O4 þ 4H2 (g) þ C:
The authors wish to thank Professors Munetake Satoh and Hiroshi Tsuda of Osaka Prefecture University for their assistance in specimen preparation for microstructural observation. A part of the present study was financially supported by a grant aid ‘‘The Project of Fundamental Technology Development for Energy Conservation’’ that was provided by the New Energy and Industrial Technology Development Organization of Japan.
0 -5 -10 -15 -20 -25 273
materialographic and diffraction techniques in order to clarify the mechanism of hydrogen generation through the IC–steam reaction at 673 K. Microstructure investigation by optical microscopy and SEM–EDS, combined with phase identification by XRD, revealed that the oxidation of Fe3 C by steam proceeds at the temperature when the IC is heated in a flowing Ar atmosphere containing 4.6 ppm of water. The oxidation initiates at the edges of IC particles and on the surfaces of cracks and pores inside the particles. The oxide phase formed is mainly Fe3 O4 and contains nano-sized graphite particles that are highly crystallized and dispersed uniformly in the oxide layer. The formation of the oxide is the cause of the weight increase observed in the first stage of the IC–steam reaction that occurs at temperatures below 813 K. The oxidation of Fe3 C is responsible for the hydrogen production in the temperature range, and the reaction equation is accordingly determined as
Acknowledgements
5
Fe3C
473
673 873 Temperature, T / K
3Fe + C
1073
1273
Gibbs energy change for the decomposition of Fe3 C into Fe and C.
temperature. Hydrogen generation occurring in Stage I is thus attributable to the direct oxidation of Fe3 C in the IC by steam, and the eq. (1) represents the reaction proceeding in the stage. 4.
80°
Scattering Angle, 2 θ
2 µm
1.5
Fig. 5
60°
Summary
The microstructure evolution of iron carbide brought by the reaction with steam was investigated by way of
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