A unique approach to the vapour phase of the HIx feed of the sulfur iodine thermochemical cycle: A Raman spectroscopy study

Abstract

The Sulfur Iodine cycle has great potential for large scale hydrogen production from water. The HIx processing stage (Section III) of the cycle exhibits the major challenges. Among them, experimental vapour liquid equilibrium data is scarce, mainly due to inherent hurdles to monitor the system under operating conditions of this section, i.e. temperature, acidity, and corrosion. Until now, UV–Visible and FTIR spectroscopies are the online monitoring techniques of choice, however due their selection rules, there is no experimental spectroscopic evidence of H2 during direct decomposition of HIx. Bearing this in mind, here we demonstrate the feasibility of exploiting two different Raman spectroscopic techniques as in situ monitoring tools, to gain substantial knowledge on the mechanisms of hydrogen production in Section III. Whereas resonance Raman spectra revealed the occurrence of HI·(H2O)3 in the vapour phase; coherent anti-Stokes Raman spectra of H2 allowed us to estimate kinetic data such as the activation energy for HI decomposition of a given hyper-azeotropic ternary mixture (i.e. xHI: 0.144; xI2:0.308; xH2O:0.548) is 149.15 kJ/mol.