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The catalytic decomposition of hydrogen iodide in the IS thermochemical cycle
Chu-Sik Park, Jung-Min KIM, Kyoung-Soo KANG, Gab-Jin Hwang, Ki-Kwang Bae
Hydrogen Energy Research Group, Korea Institute of Energy Research,71-2 Jangdong Yuseong-gu Daejeon
305-343 Korea, [email protected]
ABSTRACT:
The catalytic decomposition tests of hydrogen iodide(HI) for the IS thermochemical cycle were performed.
In order to make the pure HI gas, the hydriodic acid was evaporated and carried by Argon gas and then
introduced into the moisture removal device filled with phosphoric acid. The pure HI gas was flowed in the
decomposition reactor mounted with a certain quantity of the decomposition catalysts.
The test conditions of the decomposition reaction were varied by the decomposition temperature
(300~500℃) and three kinds of the decomposition catalysts ; Pt/Activated carbon(10wt.%, Aldrich chemical
Co.), Pt/Alumina(5wt.%, Aldrich chemical Co.), and only Activated carbon.
To investigate the morphology change of the decomposition catalysts, SEM, XRD, EDAX and BET analyses
were preformed before and after the decomposition reaction.
The HI conversion of all the catalysts increased with the increasing decompositon temperature and the
morphology change of the decomposition catalysts were transformed respectively.
KEYWORDS : Hydrogen Iodide, catalytic decomposition, IS thermochemical cycle, conversion
1. Introduction
Hydrogen is an environmentally attractive alternative to displace fossils fuels and has three advantages of
zero emissions, the endless supply and the applications in a variety of energy source, including renewable
[1]. However, currently hydrogen is produced from fossils fuels as a raw material. Among the many methods
for the hydrogen production, thermochemical water splitting, a process that accomplishes the decomposition
of water into hydrogen and oxygen, is a promising candidate for thermochemical hydrogen production
without fossils fuels.
The study of thermochemical hydrogen production started in about 1964-1966 by Funk and in the following
few years was further studied and emphasized as the best use of high temperature nuclear heat for nonelectric application.
For a start of works and papers of Marchetti in about 1970, this renewal of the studies on hydrogen from
water using nuclear heat was followed by an increase of interest in many country and laboratories. Much
pioneering work has been carried out in the research into efficient and workable thermochemical cycles. The
energy crisis emphasized the need for these studies, together with the need to find clean fuels which will
reduce pollution problems.
The IS(Iodine-Sulfur) thermochemical water splitting process has been taken note of a large-scale method
for hydrogen production lately.
The IS process consists of three chemical reactions that sum to the dissociation of water.
2H2O + SO2 + I2 → H2SO4 + 2HI (~120℃), Exothermic (1)
H2SO4 → H2O + SO2 + 1/2O2 (~830-900℃), Endothermic (2)
2HI → H2 + I2 (300-450℃), Endothermic (3)
H2O → H2 + 1/2 O2 (4)
The IS process cycle is divided into three sections as depicted in Fig. 1[2,3,4].
Among the IS processes, HI decomposition process(Eq.(3)) has an significant influence on the thermal
efficiency of the IS thermochemical water-splitting cycle, because HIx solution(HI-H2O-I2) supplied from
Busen reaction(Eq.(1)) is distilled and then becomes an azeotrope(HI-H2O, molar ratio of H2O/HI=5).
The distillation of the azeotrope and the low-equilibrium decomposition ratio of HI(about 20%) requires a
large thermal burden, so that decrease the thermal efficiency of IS cycle. To avoid the problems, an
extractive distillation and a reactive distillation have so far been proposed by General Atomics[5].
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Fig. 1. The Iodine-sulfur process cycle
As an alternative, membrane technologies (electro-electrodialysis, membrane-reactor) to improve the HI
decomposition efficiency has been studied by Japan Atomic Energy Research Institute[6].
Prior to the decomposition test, the thermodynamic state of HI decomposition was calculated and then the
HI conversion was computed with Gibbs Energy Minimization method using MALT2. The results were
respectively shown in Fig. 2 and Fig. 3.
It should be noticed that the HI(gas phase) decomposition is an irreversible reaction and the HI conversion
is below 0.2 under 1atm at 650K.
80
Energy Requirment(kJ/mol)
60
2HI(g) = I2(sl) + H2(g)
40
∆G
20
2HI(g) = I2(g) + H2(g)
0
-20
-40
-60
200
∆H
300
400
500
600
700
800
900
1000
Temperature(K)
Fig. 2. Thermodynamic state diagram of HI decomposition.
In this study, we are making a basic experiment on the properties of Platinum and activated carbon
catalysts before and after HI decomposition reaction and pursuing to seek and screen an appropriate
condition of HI decomposition catalyst to improve the thermal efficiency of HI decomposition reaction.
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0.35
40 atm
0.30
HI Conversion
30 atm
0.25
20 atm
0.20
0.15
1 atm
Condensation Temperature of I 2
0.10
400
450
500
550
600
650
700
750
800
850
900
Temperature(K)
Fig. 3. The diagram of the calculated HI conversion.
2. Experimental
Fig. 2. shows the schematic diagram of the apparatus for the decomposition of hydrogen iodide. The
reactor tube was made of quartz with a 20mm inside diameter. The other parts of the apparatus were mostly
made of Pyrex glass.
Fig. 4. Schematic diagram of the apparatus for the decomposition reaction of hydrogen iodide.
The starting material, hydriodic acid(56wt.%, Kanto chemical Co.), was evaporated and carried by Argon
gas (30ml/min) at 150℃ and then introduced into the water removal filled with phosphoric acid. After
checking the constant HI gas supply, HI gas was flowed in the decomposition reactor mounted with a certain
quantity of the decomposition catalysts(1 or 2g). All the products, except for hydrogen, were finally trapped
in cold trap(0℃).
The conversion of hydrogen iodide was measured with a gas chromatography equipped with thermal
conductivity using a Molecular Sieves 5A column. A gas chromatography was calibrated to accurately detect
the decomposed hydrogen with pure H2 before decomposition reaction. The initial stage of the catalytic
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decomposition in the present experiments was unstable. So measurements of the HI conversion were
executed in the steady state.
The decomposition reaction was carried out at atmospheric pressure. The test conditions of the
decomposition reaction were varied by the decomposition temperature(300~500℃) and the various kinds of
the decomposition catalysts.
In this experiment, three kinds of catalysts, Pt/Activated carbon(10wt.%, Aldrich chemical Co.),
Pt/Alumina(5wt.%, Aldrich chemical Co.), and Activated carbon(hereafter HAC), were used for the
decomposition of hydrogen iodide.
To investigate the morphology change of the decomposition catalysts, SEM, XRD, EDAX and BET analyses
were preformed before and after the decomposition reaction.
3. Results
3.1. The physical properties of the HI decomposition catalysts
3.1.1. SEM analyses
From the results of the SEM micrographs in Fig. 5, the crystal morphology of Pt/Alumina was not clear with
the increasing decomposition temperature. The reason for the dim crystal morphology is speculated to be
related to the fact that the agglomerative crystal due to the high thermal effect.
The crystal morphology of Pt/Carbon and Activated carbon showed a lot of pores like the catalytic loss on
their surface.
500℃
300℃
as-received
Pt/Activated carbon
Pt/Alumina
HAC
Fig. 5. Comparison of SEM micrographs of Pt catalysts at a different decomposition temperature.
3.1.2. XRD analyses
The results of X-ray diffraction patterns of the HI decomposition catalysts were given in Fig.6.
The intensity of the platinum of the tested platinum-supported catalysts was strong with the increasing
decomposition temperature. But the intensity of the supported ingredients of the decomposition catalysts
decreased gradually with the increasing decomposition temperature.
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Fig. 6. Comparison of XRD patterns of Pt catalysts at the different decomposition temperature ;
a) Pt/Activated carbon , b) Pt/Alumina.
3.1.3. BET and EDAX analyses
BET surface areas and EDAX analyses of the HI decomposition catalysts are listed in Table1.
BET surface areas of the HI decomposition cataysts as the raw materials were analized. From the BET
measurements, the surface area of HAC was relatively large.
EDAX analyses of the HI decomposition cataysts tested at 500℃ were performed. It is observed from the
EDAX measurements that the iodine content of the HI decomposition catalysts included activated carbon
was relatively high.
Table 1. The reasults of BET and EDAX of theHI decomposition cataysts.
Analysis
Catalyst
EDAX(Wt.%)
2
BET(m /g)
Pt
Al
C
I2
Pt/Activated carbon
287.18
10.22
-
79.76
10.01
Pt/Alumina
119.94
13.10
84.43
-
2.48
HAC
558.39
-
-
90.72
9.28
3.2. The HI decomposition properties of the catalysts
Figure 1 show the experimental results of the HI decomposition where the conversions of hydrogen Iodide
are plotted against the decompositon temperature.
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25
Pt/Activated carbon
Pt/Alumina
HAC
HI conversion(%)/cat.-g
20
15
10
5
0
250
300
350
400
450
500
550
Decomposition Temperature(℃)
Fig. 7. Conversion of hydrogen iodide over the HI decomposition cataysts
with the decompositon temperature.
The HI conversion of all the tested catalysts increased with the increasing decompositon temperature.
Comparision of the HI conversion reveals that the conversion of the HI decomposition catalysts included
platinum was higher than that of non-platinum catalyst.
3. Conclusion
From the results of the XRD patterns, the intensity of the tested platinum-supported catalyst was strong with
the increasing decomposition temperature.
From the results of the SEM micrographs, the crystal morphology of Pt/Alumina was not clear, but that of
Pt/Carbon and Activated carbon showed a lot of pores on their surface.
2
From the BET measurements, the surface area of Activated carbon(HAC) was found to be 558.39 m /g
and was relatively large.
Comparing with the HI conversion of the HI decomposition catalysts, the HI conversion of all the tested
catalysts increased with the increasing decompositon temperature and the HI conversion of platinumsupported catalysts was higher than that of only Activated carbon.
4. Acknowledgements
This research has been done under Nuclear Hydrogen Production Technology Development and
Demonstration (NHDD) project and we are grateful to MOST for financial help.
References:
st
[1] Brenda Johnston, Michael C. mayo and Anshuman Khare, Hydrogen : the energy source for 21 century,
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Energy, Vol. 4, p 499, 1979.
[3] Paul M. Mathias, Applied thermodynamics in chemical technology : current practice and future challenges,
Fluid Phase Equilibria, Vol.228-229, p.49, 2005.
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[4] Wu Xinxin, Onuki Kaoru, Thermochemical water splitting for hydrogen production utilizing nuclear heat
from an HTGR, Tsinghua Science and Technology, Vol.10, p. 270, 2005.
[5] D.O’Keefe, C. Allen, G.Besenbruch, Preliminary results from bench-scale testing of a sulfur-iodine
thermochemical water-splitting cycle, 1981
[6] Mikihiro Nomura, Seiji Kasahara, Hiroyuki Okuda, Shin-ichi Nakao, Evaluation of IS process featuring
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