Raman scattering studies of the condensed phase of the HIx feed of the sulfur-iodine thermochemical cycle

Abstract

SUMMARYThe species present in the condensed phase of the HI x , hydrogen producing, feed in the water-splitting, sulfur-iodine thermochemical cycle have been investigated using spontaneous Raman scattering. Measurements of I 2 -containing species in the low Raman-shift region from 50 to 400 cm À1 in samples of the two aqueous binary systems, I 2 /H 2 O and HI/H 2 O, and the ternary system HI/I 2 /H 2 O with and without the addition of H 2 SO 4 have provided a consistent picture of the aqueous iodine and polyiodide chemistry. Samples were contained in sealed silica ampoules and were heated to temperatures in the range 20-3001C. The results, which cover a wide range of I 2 and HI mole fractions, and x I2 =x HI mole ratios, in the HI/I 2 /H 2 O system, reveal the co-occurrence of, and H 1 -I À (I 2 ) 2 solvated species in the condensed phase of HI x . Thus, while the first is mostly evident by its strong fundamental band with Raman shift in the range from 110 to 115 cm À1, the other two species appear convoluted in a broad prominent band whose Raman shift ranges between 153 and 172 cm À1 depending on the x I2 =x HI mole ratio. These well characterized Raman features are proposed as an in situ diagnostic for process control of the cycle.

First Observation of Defined Structural Motifs in the Sulfur‐Iodine Thermochemical Cycle and Their Role in Hydrogen Production

“…At first glance the assignment of the overtone series might seem dubious, since it does not seem to fit with the anharmonic oscillator model. However, we have recently demonstrated the attenuation of these two overtones changing the excitation frequency off resonance to 1064 nm [14]. Furthermore, Kiefer and Bernstein [43] also reported similar band positions for CsI 3 in aqueous solution, which are in close agreement with our assignment.…”

“…However, the most remarkable aspect of this system is the displacement of the band at ca 166 cm −1 , this varies from 161 to 170 cm −1 , depending on the sample's composition. This is clearly shown in Figure 13(a), the band is related to the I − (I 2 ) n species earlier discussed in Section 1.2 (a fuller discussion of this band and its displacement can be found elsewhere [14]).…”

“…in Figure 2, were investigated over a range of different temperatures [14]. The Raman activity of the I 2 -containing species present in the system made it possible to rationalise the observations in terms of the following hydrated species:…”

“…More useful SRS spectra were acquired using the NExT Filter at 514.5 nm and these are shown in Figure 8. An SRS study of these systems in this region has recently been reported and discussed in detail by our research group and they are only briefly summarised here [14].…”

Raman and inelastic neutron scattering spectra of (NH4)2SO3, an intermediate for solar hydrogen production

“…Since the Raman cross section of sulfur is particularly favourable, sulfites and sulfates have been investigated in solution, glassy state and solid state, often accompanied by infrared spectroscopy [18][19][20][21][22][23][24][25]. Raman spectroscopy has been also used to study other sulfur family cycle, the S-I cycle, to gain further insights into molecular dynamics of the solutions involved and chemical species occurring within [26][27][28]. A complementary technique, inelastic neutron scattering (INS) spectroscopy, exhibits a remarkable hydrogen cross section that allows to study either hydrogen bonding, hydrogenated species, adsorbed molecular hydrogen, and even proton conduction [29,30].…”