i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 6 2 1e3 6 2 5

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Iodine speciation studies on Bunsen reaction of SeI cycleusing spectroscopic techniques

Deepak Tyagi, Salil Varma*, K. Bhattacharya, D. Jain, A.K. Tripathi, C.G.S. Pillai,S.R. Bharadwaj

Chemistry Division, Bhabha Atomic Research Centre, 400085 Mumbai, India

a r t i c l e i n f o

Article history:

Received 9 March 2011

Received in revised form

27 April 2011

Accepted 29 April 2011

Available online 12 June 2011

Keywords:

HIx

Raman spectroscopy

UVevisible spectroscopy

Speciation

* Corresponding author. Tel.: þ91 22 2559228E-mail address: svarma@barc.gov.in (S. V

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.04.221

a b s t r a c t

Bunsen reaction is an important step of sulfureiodine cycle for hydrogen production from

thermochemical splitting of water. Polyiodide species generated during the separation

process need to be identified for complete understanding of the mechanism involved.

Speciation of these polyiodide species formed during Bunsen reaction can lead to better

understanding of kinetics of the process. HIx species formed have been analyzed using

UVevisible and Raman spectroscopic techniques. Peak corresponding to HI3 species have

been ascertained and their conversion to higher HI5, HI7 .. species has been observed.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction are explored for their capability to fulfill the future hydrogen

Fossil fuel resources of earth will not be able to meet the

energy needs of future generations, which leads to investi-

gation of many alternate energy sources. Among these

hydrogen seems to be an attractive energy carrier if it can be

produced in a clean and cost effective manner.

Hydrogen is future fuel to bring out shift from carbon based

energy sources to more energy efficient and environmentally

clean systems [1,2]. The development of newer technologies

based on fuel cells and other techniques require production of

hydrogen on large scale and in an economically viable

manner. The two processes which seems to be most prom-

ising for massive hydrogen production are electrolysis [3,4]

and thermochemical cycles [5e7]. In thermochemical cycles

heat can be directly used so they have potential of better

efficiency than alkaline electrolysis. Various thermochemical

cycles like sulfureiodine cycle [7], copper chlorine cycle [8,9]

2; fax: þ91 22 25505151.arma).2011, Hydrogen Energy P

demand.

Sulfureiodine cycle is based on integration of following

three reactions in a loop:

I2ðIÞ þ SO2ðgÞ þ 2H2OðIÞ/2HIðaq:Þ þH2SO4ðaq:Þ (1)

2HIðgÞ/I2ðgÞ þH2ðgÞ (2)

H2SO4ðgÞ/SO2ðgÞ þH2OðgÞ þ 0:5O2ðgÞ (3)

First reaction is known as the Bunsen reaction, which forms

an important step of sulfureiodine cycle for thermochemical

splitting of water. This reaction is carried out in liquid water

media with a large excess of iodine. Excess iodine is added to

avoid side reactions between iodine and sulfur compounds;

also excess iodine facilitates segregation of the two product

acids into two corresponding liquid phases [10e13]. These

phasesaresulfuricacidphase (about 50wt%H2SO4) inwhichHI

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 6 2 1e3 6 2 53622

exhibits low solubility, andhydrogen iodide phase (namely the

“HIx phase”) containing almost all of the excess iodine, with

H2SO4 as a minor impurity. Hence, the reaction stoichiometry

andmass balance of this process can be described as follows:

ðxþ 1ÞI2 þ SO2 þ ðnþ 2ÞH2O/½H2SO4 þ ðn�mÞH2O� þ ½2HIþ ðxÞI2 þmH2O�Sulphuric acid phase HIx phase

(4)

wherexandnare the iodineandwatermolarexcessquantities,

respectively, in the reactant phase andm is themolar quantity

of the excess water that ends up in the product HIx phase.

Due to presence of excess iodine in HIx phase, various pol-

yiodide species are generated during the separation process.

The formation of various polyiodide species can be explained

as follows:

I2 þ I�4I�3 (5)

2I2 þ I�4I�5 (6)

3I2 þ I�4I�7 (7)

These polyiodide species need to be identified for complete

understanding of the mechanism involved and kinetics fol-

lowed in the processes. Lot of research works are reported in

literature for identification of polyiodide species [14e17]. Most

of the work reported in the literature is related to separation

and identification of salts and complexes of these polyiodides

in molten and solid form, only few of them are for aqueous

solutions [18,19]. The methods employed range from vibra-

tional spectroscopy, Mossbauer spectroscopy, NMR, ESR,

ESCA, UV/Visible spectroscopy, electro-analytical techniques

and X ray based methods.

The problem that we attempted to address in the present

study is the identification of different polyiodide species of HIx

phase of Bunsen reaction in absence and presence of sulfuric

acid. The spectroscopic techniques based on molecular absor-

bance inUVevisiblespectroscopyandRamanspectroscopywere

used to exploit change in symmetry for different HIx species.

Fig. 1 e Raman spectra of HI:I2 1:1 solution with

deconvulation showing v1, v2 and v3 peaks.

2. Experimental

2.1. Raman spectroscopy

Laser Raman spectra of the solutions containing varying

amounts of HI and I2 were recorded on a LABRAM-1 spec-

trometer (ISA) in a back-scattering geometry, at a spectral

resolution of 2 cm�1. An Arþ ion laser (488 nm) was used as an

excitation source. Test solutions were prepared by mixing

0.1M stock solutions of HI and iodine in required proportion to

make a final solutions of 50 ml volume.

2.2. UVevisible spectroscopy

The UV/Visible spectra of the solutions containing varying

amounts of HI, KI and I2 were recorded using a JASCO

Spectrophotometer (Model V-650), JAPAN. Test solutions were

prepared bymixing 2� 10�3 M stock solutions of HI and iodine

in required proportion to make a final solutions of 2 � 10�4 M

concentration for100 ml volume.

3. Results and discussion

3.1. Raman spectroscopy

Raman spectra were recorded for 0.1 M HI with iodine

concentration ranging from 0.1 to 0.6 M, i.e. the spectra were

obtained for I2:I� ratio of 1e6. No spectral features were

observed for both pure I� and pure I2 solutions. The solution

with HI:I2 ratio of 1:1 exhibits a strong broad Raman band in

region 90e170 cm�1 peaking at 113 cm�1 (Fig. 1). This is

attributed to presence centrosymmetric (Dih) I�3 species [14,20].

Triiodide species has a Raman active symmetric stretching

band at v1 at 110 cm�1 and two IR active bands at 50e70 cm�1

and 130e140 cm�1 corresponding to doubly degenerate

bending and asymmetric stretch modes, v2 and v3, respec-

tively. Appearance of peak at 60 cm�1 and 130 cm�1 is indic-

ative of change in symmetry of the I�3 species due to

anioneanion or anionesolvent interaction [21]. Presence of

donor I2 adduct has also been reported to play a major role in

appearance of v3 band [22].

Fig. 2 shows overlay of Raman spectra obtained for

increasing concentration of iodine from 0.1 M to 0.6 M in 0.1 M

HI solution. The broad peak obtained for 1:1 HI:I2 solution is

found to decrease as HI:I2 ratio increases to 1:4 and is constant

for HI:I2 ratio upto 1:5 and then again decreases for 1:6 ratio

Fig. 2 e Raman spectra for different HI:I2 ratio solutions.

Fig. 4 e Multiple deconconvulations corresponding to HI:I2ratio of 1:3.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 6 2 1e3 6 2 5 3623

solution. Considering that 113 cm�1 peak corresponds to I�3species this decrease indicates for conversion of this species

into higher polyiodides either by disproportionation or by

adduct formation with iodine as shown below:

2I�34I�5 þ I�

I�3 þ I24I3ðI2Þ�

Fig. 3 e Variation of intensity of v1, v2 and v3 peaks for

different HI:I2 ratio solutions.

For understanding of this variation in the Raman spectra

the change in intensity of three deconvulated peaks as shown

in Fig. 3 was plotted. The peak corresponding to the v1 band

shows drastic decrease with increasing iodine concentration

upto 1:3 HI:I2 ratio and then remains constant for ratio upto 1:5

before decreasing further for 1:6 ratio. Similar trend is

observed for v3 band, too. Compared to these the v2 band

exhibits consistent increase with increasing HI:I2 ratio. Such

a pattern can be explained by taking into consideration the

presence of I�5 species. It is reported by Sharp et al. [20] that

various bands exist at 165, 55, 146 and 114 cm�1 and 157, 90,

143, 110 cm�1 for linear and bent pentaiodide species,

respectively. This system can be understood in terms of

a disproportionation and coupled equilibrium initiated by the

liquidification of a triiodide system as given below [14]:

I3ðI2Þ�þI�42I�3

Fig. 5 e Raman spectra in presence of different

concentrations of sulfuric acid, depicting no effect of its

presence.

Fig. 6 e UVevisible spectra for different KI-I2 solutions. Fig. 8 e UVevisible spectra for different HI:I2 ratio

solutions.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 6 2 1e3 6 2 53624

Taking these information into account attemptwasmade to

deconvulate Raman spectra corresponding to HI:I2 ratio of 1:3

and multiple deconvulations could be obtained for five peaks

as shown in Fig. 4. Though not very conclusive it does indicate

toward presence of higher polyiodides in equilibriumwith the

triiodide for increasing iodine concentration.

To observe the effect of presence of sulfuric acid as well as

effect of its concentration, varying concentrations of sulfuric

acid are added to a 1:1 HI:I2 solution (Fig. 5). It is observed that

the Raman spectrum is free from any interference from

sulfuric acid, as there is no change in position as well the

height of the most prominent peak (i.e. of I�3 ).

3.2. UVevisible spectroscopy

KI solutions of known concentration were mixed with Iodine

solution to obtain final solutions of 2 � 10�4 M in terms of KI

with KI:I2 ratio varying from 1:0 to 1:10. Fig. 6 depicts UVevi-

sible absorption spectra for the above mentioned samples

Fig. 7 e Variation in absorbance value of 358 and 288 nm

peaks with increasing iodine concentration in KI-I2solution.

along with the spectra for iodine solution of 2 � 10�4 M

concentration. Fig. 6c shows spectrum obtained for KI:I2 1:1

solution. It exhibits two peaks with maxima at 288 nm and

358 nm. Peak pertaining to 358 nm is reported to be of I3�

species [17]. This peak is found to decrease with increasing I�/I2 ratio from 1:1 to 1:10. This is attributed to conversion of I�3species to higher polyiodides. However, this spectrum shows

an additional peaks at w288 nm and this peak is absent from

Fig. 9 e Variation in absorbance values of 193 nm and

223 nm peaks with respect to iodine-HI ratio.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 3 6 2 1e3 6 2 5 3625

spectrum as recorded for respective KI and iodine solutions.

As observed in Fig. 7, this peak also follows a pattern of

decrease in absorbance value with increasing I2:KI ratio as

observed for 358 nm peak. It again indicates toward presence

of I�3 species for its genesis.

When similar measurements were carried out for HI-I2solution of 2 � 10�4 M concentration, similar absorbance

patterns are observed, though with a reduced absorbance

values (Fig. 8). For these solutions the lower wavelength peaks

at w200 nm and w225 nm, which were out of scale for KI

based solutions, can be observed clearly. Both these peaks are

different from earlier mentioned peaks at 288 nm and 358 nm,

in the sense that they appear in case of pristine HI (204 nm)

and iodine (227 nm) solutions too, respectively. When this

data is analyzed for change in absorbance value of these peaks

with respect to increasing I2 concentration inHI solution a plot

similar to Job’s plot is obtained (Fig. 9) with maxima for 1:1

HI:I2 concentration. Maxima for 1:1 composition again points

toward assigning of these peaks to I�3 type of species. Due to

their very high absorbance value these peaks can be very

efficiently utilized for detection of triiodide species with high

sensitivity.

4. Conclusion

Presence of I�3 and higher polyiodides in the HIx phase of

Bunsen reaction of sulfur Iodine thermochemical cycle was

confirmed by UVevisible and Raman studies. The new

UVevisible peaks identified for I�3 species at wavelength of 193

and 223 nm with very high absorbance.

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