the catalytic decomposition of hydrogen iodide in the is

WHEC 16 / 13-16 June 2006 – Lyon France

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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 non-electric 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.

-60

-40

-20

0

20

40

60

80

200 300 400 500 600 700 800 900 1000

Temperature(K)

Energy Requirment(kJ/mol)

2HI(g) = I2(sl) + H2(g)

2HI(g) = I2(g) + H2(g)

∆H

∆G

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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0.10

0.15

0.20

0.25

0.30

0.35

400 450 500 550 600 650 700 750 800 850 900

Temperature(K)

HI Conversion

40 atm

30 atm

20 atm

1 atm

Condensation Temperature of I 2

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.

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.

Pt/Activated carbon Pt/Alumina HAC

500℃

300℃

as-received


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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. 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.

EDAX(Wt.%) Analysis

Catalyst BET(m

2/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


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0

5

10

15

20

25

250 300 350 400 450 500 550

Decomposition Temperature(℃)

HI conversion(%)/cat.-g

Pt/Activated carbon

Pt/Alumina

HAC

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. From the BET measurements, the surface area of Activated carbon(HAC) was found to be 558.39 m

2/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 platinum-supported 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: [1] Brenda Johnston, Michael C. mayo and Anshuman Khare, Hydrogen : the energy source for 21

st century,

Technovation, Vol.25, p. 569, 2005. [2] G.E.Beghi, Present state and future prospects of thermochemical hydrogen production, Int. J. Hydrogen 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 membrane techniques by total thermal efficiency, International Journal of Hydrogen Energy, 2004.

the catalytic decomposition of hydrogen iodide in the is

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