Int. J. Hydrogen Energy Vol. 9, No. 8. pp. 695-700, 1984. Printed in Great Britain.

360-3199/84 $3.00 + 0.00 Pergamon Press Ltd.

© 1984 International Association for Hydrogen Energy.

KINETICS OF THE CATALYTIC DECOMPOSITION OF HYDROGEN IODIDE IN THE THERMOCHEMICAL HYDROGEN PRODUCTION

Y. SHINDO, N. ITO, K. HARAYA, T. HAKUTA and H. YOSHITOME

Process Research and Development Division, National Chemical Laboratory for Industry, Tsukuba, Ibaraki, 305 Japan

(Received for publication 26 May 1983)

Abstract--The decomposition rates of hydrogen iodide over platinum supported y-alumina were measured in the range from 480 to 700 K by the use of a flow method. It was found that Pt/y-alumina catalyst was effective for the decomposition of hydrogen iodide. According to the Langmuir-Hinshelwood model, an overall rate equation was obtained on the basis of the mechanism where the rate-determining step was a surface reaction. Experimental data were well correlated with the rate equation.

a

aq b C t: AG g K.2

KH20

K.I

KI: gp k

k'

kf

kr

PH2

P.2o PHI

PI2

phi R S T t W X

Xe

NOMENCLATURE

Adsorption constant defined by equation (7) Aqueous solution Constant defined by equation (26) Constant defined by equation (28) Total flow rate, mol s -t Free energy change, kJ mol -t Gas phase Adsorption equilibrium constant of hydrogen, kPa-1 Adsorption equilibrium constant of water vapor, kPa-1 Adsorption equilibrium constant of hydrogen iodide, kPa -1 Adsorption equilibrium constant of iodine, kPa -~ Equilibrium constant Rate constant defined by equation (17), mol kg S-1

Rate constant defined by equation (18), mol kg -t S-1

Forward reaction rate constant on the catalyst sur- face, kPa mol kg -~ s -1 Reverse reaction rate constant on the catalyst sur- face, kPa mol kg -1 s -a Partial pressure of hydrogen, kPa Partial pressure of water vapor, kPa Partial pressure of hydrogen iodide, kPa Partial pressure of iodine, kPa Initial partial pressure of hydrogen iodide, kPa Gas constant, J K -~ tool -~ Solid phase Absolute temperature, K Reaction time, kg s mo1-1 Catalyst weight, kg Conversion of hydrogen iodide Equilibrium conversion of hydrogen iodide

INTRODUCTION

Hydrogen has been widely proposed as a highly flex- ible and non-polluting energy carrier of the future, and thermochemical water decomposition has been shown to be one of the most attractive methods for large-scale production of hydrogen. Of the cycles proposed so far,

the magnesium-iodine cycle [1-3] has been presented by our laboratory, and has been investigated by the present authors [4-7]. The four reactions making up this cycle are as follows:

100-15&C 6 MgO(s) + 6 Iz(s) , Mg(IO3)2(s)

+ 5 MgI2 (aq), (1)

60~C Mg(IO3)2(s) , MgO(s) + I2(g) + ~O2(g), (2)

400°C 5(MgI2.6H20)(s) ,5 MgO(s)

+ 25 HzO(g) + 10 HI(g), (3)

10 HI(g) <,o¢c 5 H2(g) + 5 I2(g). (4)

The fourth reaction, i.e. decomposition of hydrogen iodide, is one of the most significant reactions not only in this cycle but also in other thermochemical water- splitting cycles using iodine [8-11]. It is desirable to carry out the reaction around or below 700 K from the standpoint of the construction materials and the thermal efficiency of the whole cycle. However, the rate of the reaction is a homogeneous gas-phase reaction is known to be very low in the temperature range below 700 K [12-14]. Thus, it is desirable to use a catalyst to increase the reaction rate.

There have been few reports on the kinetics studies of the catalytic decomposition of hydrogen iodide. Hin- shelwood and Burk [15] studied tt',e catalytic decom- position of hydrogen iodide over platinum wire in the temperature range between 712 and 934 K by trapping the iodine formed by ice. They reported that the reac- tion proceeded according to the following rate equation:

dpHl_ k, (5) dt

in which pHI is the partial pressure of hydrogen iodide and k is a rate constant. Iida [16] investigated the decomposition of hydrogen iodide over platinum sup- ported on Teflon in a closed circulation system with the

695

696 Y. SHINDO et al.

continuous removal of the iodine formed with ice. He reported that the reaction proceeded according to the following rate equation:

dpHI = pm (6) dt 14.76pn: + 7.49pm + 208

where pH2 is the partial pressure of hydrogen. These rate equations are unapplicable to a practical reaction process for thermochemical hydrogen production, because in both of their experiments the reaction sys- tems were simplified by the continuous removal of the iodine formed.

Recently, from a view point of a thermochemical cycle, Oosawa et al. [17] have proposed a Langmuir- Hinshelwood type rate equation of the decomposition of hydrogen iodide over platinum-supported active car- bon catalyst expressed as follows:

dx _ k(1 - x) k'x dt 1 + ax (1 + ax) z' (7)

where x is the conversion of hydrogen iodide. Equation (7) explained well their experimental results. However, the constants k, k' and a are somewhat complicated. They reported that k and k' were mainly concerned with the reaction rate, and a was mainly concerned with the adsorption of iodine. Oosawa et al. [18] also reported the results of a screening test of various metal catalysts on various supports. Valuable information with respect to the catalysts for the decomposition of hydrogen iodide was given.

As described above, there have been only a few fundamental and practical studies on the catalytic decomposition of hydrogen iodide. In the present work Pt/y-alumina was chosen and examined for the decom- position of hydrogen iodide from the point of view of a practical thermochemical water-splitting cycle. A kinetic analysis of the reaction was also made. A new rate equation for the decomposition of hydrogen iodide was derived on the basis of the Langmuir-Hinshelwood mechanism.

EXPERIMENTAL

Experimental apparatus and procedure

A tubular-flow reactor operated in an integral manner was used in order to investigate the decomposition of

hydrogen iodide. Figure 1 shows the schematic diagram of the apparatus. The reactor tube was made of quartz with a 20 mm inside diameter. The other parts of the apparatus were mostly made of Pyrex glass. The starting material, hydriodic acid, was evaporated at 250°C, then introduced into the decomposition reactor. The reaction was carried out at atmospheric pressure. The reaction time was varied by changing the feed rate of the reac- tants or the amount of the catalyst. All the products, except for hydrogen, were trapped in the spiral con- denser. The flow rate of the resultant hydrogen was measured with a soap-film flowmeter. The conversion of hydrogen iodide was calculated from the flow rate of hydrogen.

Hydriodic acid and Pt/y-alumina catalyst

As a source of hydrogen iodide, commercially avail- able hydriodic acid (7.5 mol 1-1, Nakarai Co., Sp. Gr.) was used in order to set the reactant compositions similar to those in the decomposition of hydrogen iodide for the magnesium-iodine cycle, where a certain amount of water vapor accompanies the hydrogen iodide [3,6]. Pt/y-alumina catalyst (0.5 wt%, Nippon Engelhard Co., Lot No. 8101) was purchased for the present experiments. The catalyst shape is a cylindrical pellet 3 mm in diameter and 3.5 mm in length. The Pt/y-alumina catalyst used in the present experiments showed a high and unstable activity in the initial stages of reaction. Therefore, measurements of the reaction rates were carried out in the steady state.

RESULTS

(1) Figures 2--4 show the experimental results of the decomposition of hydrogen iodide in the range from 480-700 K, where the conversions of hydrogen iodide are plotted against the reaction time. The reaction time t is defined with the catalyst weight W and the feed rate of reactants F,

t= W/F. (8)

(2) In the actual thermochemical cycle the unreacted hydrogen iodide and untrapped iodine would be recycled. Thus, effects of an excess of iodine on the decomposition rate of hydrogen iodide must also be

spiral ~ H2 condenser scrubber ['-I purge

heater heater ~ ~ 13 Hi solution II1

,

micro feeder (evaporatorT!C ' ( (Catalyst T!C. ~ NaOH HzO soap_~lfilm Ireactor Iz,HI and flowmeter

H20 trap

Fig. 1. Schematic diagram of apparatus.

KINETICS OF THE CATALYTIC DECOMPOSITION OF HYDROGEN IODIDE

°° I

=

0~ 500K

0 50 100 140 React ion time, t (kg.s/mol)

Fig. 2. Conversion of hydrogen iodide over Pt/y-alumina catalyst with reaction time.

0.25

697

_ ~ ~ 700K 020 ' i . - , ~ ' ' ~ ~.~ ~ 6 6 0 K

0

0.05

[.catalyst weight: 4g I

0 to 20 30 Reac to in time, t (kg . s /mol )

Fig. 4. Conversion of hydrogen iodide over Pt/7-alumina catalyst with reaction time.

examined. Figures 5 and 6 show the effects of the presence of iodine contained in the starting hydrogen iodide solution on the decomposition rate. Addition of even a small amount of iodine was observed to decrease the reaction rate considerably.

DISCUSSION

Reverse reaction

The equil ibrium constant Kp and the equil ibrium con-

version x¢ of the decomposition of hydrogen iodide are shown in Table 1. They were calculated by the free energy values cited from the J A N A F [19], by means of the following equations:

Kp = exp ( ~ T G ) , (9)

_ xJ2 ( i 0 ) Kp 1 - Xe

The free energy changes of the reaction are positive in

O.20

7 • lo :~5205 g

O.05

L I a t-eight:I.gl 0 20 40 60

React ion time, t (kg.slmol)

Fig. 3. Conversion of hydrogen iodide over Pt/y-alumina catalyst with reaction time.

020 j

l Initial molar ratio, I2 /H! o :0 ~:0.161 ~:0.053 e :0.205

0,is ~ :0.107 • :0.270

~0.I0 O

0.05

[ catatyst we ight :20g 0 I

50 100 140 React ion time, t (kg.slmol)

Fig. 5. Effect of iodine on the rate of the decomposition of hydrogen iodide.

698 Y. SHINDO et al.

~¢ 0.I0

0

3. Initial molar ratio,121Hl

o :0 ® :0.150 :0.052 o::0.199 / / 9 .

Q :0.099

0.15 2 I - 0~.~ I2 + 2o , (14)

H20 + o ~ H20 - o, (15)

where e represents an adsorption site on the catalyst surface. According to the Langmuir-Hinshelwood model, an overall rate equation for the decomposition of hydrogen iodide is obtained on the basis of the reaction mechanism described above.

dpHI

dt

= kfKnlpm - k,X/(Kt2KH2pUpH2) [ lplX/(KI2p,2) + X/(K.~p.:) + Kr~ip.i + K.~op.~o] 2

(16)

in whichpu andpH2o are the partial pressures of iodine and water vapor respectively; KHI, K~2, KH2 and KH2O are adsorption equilibrium constants. Here, defining k

0 0 10 20 30

Reaction time, t ( k g . s l m o l )

Fig. 6. Effect of iodine on the rate of the decomposition of hydrogen iodide.

the temperature range of the present experiments, so that the equilibrium constants Kp are not high enough. The calculated values presented in Table 1 suggests that the reverse reaction, i.e. formation of hydrogen iodide, would not be negligibly small.

React ion s cheme

The present authors derived a new rate equation in a procedure based on the following assumptions:

(1) Adsorptions of hydrogen iodide and water vapor are molecular. Those of hydrogen and iodine are dissociative.

(2) The rate-determining step is the decomposition of hydrogen iodide on the catalyst surface.

On the basis of the above assumptions, the mech- anism of the reaction can be written as follows.

HI + a ~ HI - a, (11)

H I - a + a ~ H - a + I - a , (12)

2 H - o ~ H 2 + 20, (13)

Table 1. Thermodynamical properties of decomposition of HI

Temperature Free energy T change AG Equilibrium Equilibrium

(K) (kJ/mol) constant Kp conversion x~

480 9.9 0.084 0.143 500 10.1 0.088 0.150 550 10.5 0.100 0.167 600 11.0 0.111 0.182 650 11.4 0.122 0.197 700 11.8 0.133 0.210 750 12.1 0.143 0.222

and k' as the forms k = k fKm, (17)

k ' = krX/(K~:Km), (18)

and using the following relation in terms of thermodynamics.

k K o = ~ , (19)

equation (16) is rewritten dpm

dt p m - X/(pi2pn2)/Kp

= k [1 + x/ri2p,3 + X/(KH:pH~) + K.ip.~ + r . ~ p . ~ ] 2"

(20) The denominator of equation (20) is rather complicated. Oosawa et al. [17] suggested that none of the compo- nents except for iodine were considered to be dominant in the adsorption on the surface of platinum catalyst. Taking this into account, the authors eliminated the adsorption terms, except for iodine's, from equation (20) as negligible and thus obtained the following equation:

_ dpHl= k P n i - x/(p~2p.2)/Kp (21) dt [1 + X/(Ki2px2)l 2 '

In the case that the feed gas includes only iodine except for inert gases, equation (21) is readily solved as below. The partial pressures, pHI, p12 andpHz, are written in terms of conversion x:

pHI = p0HI(1 -- X), (22)

pI2 = pOIx/2, (23)

pH~ = p° ix /2 , (24)

where p°l is the initial pressure of hydrogen iodide. Inserting equations (22)-(24) into equation (21), we obtain

dx k (1 - x) - x /2Kp (25) d--'t = (1 + bV 'x ) 2 '

KINETICS OF THE CATALYTIC DECOMPOSITION OF HYDROGEN IODIDE 699

where

b = VJ(Khp~ti/2). (26)

Integrating equation (25) under the initial condition, t = 0; x = 0, yields

kt = - b (x + 4X/x) + C)

C 2

2b x In (1 - cx) + c--~c In (~

where 1

c = 1 + - - 2 Kp"

Equation (27) is valid in the case that no iodine is present initially.

Equation (21) was applied to the experimental results shown in Figs 2--6. The curves in these figures were drawn by the least-squares fitting method on the basis of equation (21). One may easily find that there are remarkable agreements between observed and calcu- lated values. Almost the same values of k andKl~ were obtained by varying the initial molar ratios (IffHI) shown in Figs 5 and 6. The viability of equation (21) was also confirmed under experimental conditions where excess iodine was present in the reaction system. Although more characterizations or verifications of the reaction mechanism would be desirable, the experi- mental data were explained by equation (21) so far as kinetics are concerned.

2.0

Activation energy and heat o f adsorption

Figures 7 and 8 show the plots of In k and In K~2 against the reciprocal absolute temperature. The

-2,5

-3.0

-3,5

-411

-4.5

-5.0 1.4 1.6

I0001T

+ x/(c~)~ -x/(CX)]' (27) 2~ o

(28)

\ 1.8 2.0

(K'b

-211

-411

Fig. 7. Arrhenius plot for the decomposition of hydrogen iodide over Pt/y-alumina catalyst.

J U IA

°S I3

S

1,6 1.8 2.O

IO001T (K -I)

Fig. 8. Relation between K12 and reciprocal absolute tem- perature.

straight lines in the figures were obtained by least- squares fitting. The values of k and Kl2 are expressed in the following equations as functions of temperature:

( -27.6~ (29) k = 7 . 6 4 e x p \ RT ] '

lO_ 9 [95.8\ Kt2 = 1.24 x exp (30)

The apparent activation energy of the decomposition of hydrogen iodide over Pt/y-alumina catalyst is 27.6 kJ mo1-1 and heat of adsorption of iodine on the catalyst is 95.8 kJ mol -~ as presented in equations (29) and (30). The apparent activation energy derived in the present experiments is smaller than those obtained by previous investigators. Oosawa et al. obtained 53.2 kJ tool -~ over Pt/active-carbon catalyst and 34.4 kJ tool -1 over the active carbon catalyst [171. Hinshelwood and Burk obtained 58.6 kJ mol -~ over the Pt wire [15].

CONCLUSION

The decomposition rates of hydrogen iodide over platinum-supported ~,-alumina catalyst were measured under various conditions. It was shown that hydrogen iodide was decomposed rapidly by the use of the Pt/ ~,-alumina catalyst. A new rate equation was presented for practical use in thermochemical hydrogen pro- duction. The rate equation was formulated on the basis of the Langmuir-Hinshelwood model and on the assumption that the rate-determining step was a surface reaction. The rate data were well interpreted by the proposed rate equation. The presence of iodine was found to reduce the decomposition rate of hydrogen iodide. It is necessary to remove the iodine in the feed

700 Y. SHINDO et al.

gas prior to the reaction so as to obtain sufficient con- version of hydrogen iodide.

Acknowledgements--We wish to thank Professor H. Inoue, University of Tokyo, for his valuable discussions. We also would like to express our appreciation to Dr. S. Mizuta and Dr. Y. Oosawa of the Energy Chemistry Division of our laboratory for their helpful discussions.

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