Cu–Cl Thermochemical Water Splitting Cycle: Probing Temperature-Dependent CuCl2 Hydrolysis and Thermolysis Reaction Using In Situ XAS

Abstract

Recently, we reported detailed investigations on a hydrolysis step of Cu-Cl thermochemical cycle (Singh et al. in Int J Energy Res 44:2845-2863 where we demonstrated CuCl 2 hydrolysis using a fixed-bed reactor under optimized conditions. Thermolysis of CuCl 2 and oxide formation are associated hindrances in achieving 100% phase-pure hydrolysis product, Cu 2 OCl 2 . With an objective to understand the thermolysis and hydrolysis processes at molecular level, both these reactions were investigated independently in the present study using in situ XAS as a probe. The progress of both the reactions was recorded from RT to 400 °C by monitoring temperature-dependent evolution of Cu species using Cu K-edge XAS measurements at BL-09, Indus-2 SRS, RRCAT, Indore, India. XAS results are also corroborated with ex situ analysis by XRD and TG of samples containing various compositions of CuCl 2 mixed with boron nitride (pellets employed for XAS). Besides LCF, hydrolysis product Cu 2 OCl 2 yield was further supported by chemical titrations. EXAFS revealed that with increasing temperature, coordination of Cu atoms in reactant CuCl 2 progressively decreased during thermolysis. For hydrolysis process, coordination of Cu atoms in Cu-Cu, Cu-O and Cu-Cl linkages approached towards that of Cu 2 OCl 2 and CuO. CuCl 2 diminished and transformed into CuCl (88 mass%) at 350 °C during thermolysis and ~ 60 mass% of Cu 2 OCl 2 and 40 mass% of CuO during hydrolysis reaction. Based on in situ investigations in the present study, the most probable reactions taking place during CuCl 2 hydrolysis at different temperatures are proposed for S/Cu of 14.6.

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“…The Fourier‐transformed k 3 χ(k) of the Cu K ‐edge extended X‐ray absorption fine structure (EXAFS) spectra of Cs 3 Cu 2 Cl 5 displayed two peaks at R = 1.84 and 2.36 Å without phase shifts correction (Figure 2c), ascribed to the CuCl and CuCu scattering paths, respectively. [ 18 ] After the electrochemical reconstruction to form CuClCs, the peak at 2.15 Å was attributed to the CuCu path, indicating the formation of metallic Cu. The coordination numbers of CuCl and CuCu from curve fitting decreased from 1.7 and 1.8 to 1.1 and 1.4, respectively (Figure S5 and Table S3, Supporting Information).…”

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“…The EXAFS fitting results for the sample along with that of standards are tabulated in Table . Cu cations in the Cu 2 OCl 2 structure are surrounded by two chlorine and three oxygen ions at distances of 2.25 and 1.94 Å, respectively, while in the CuO structure, four oxygen ions surround Cu cations at a distance of 1.94 Å. , , Both Cu 2 OCl 2 and CuO have a Cu–Cu distance of around 2.95 Å.…”

“…Finally, the XRD pattern at 400 °C was resolved. The peaks at lower angles along with almost 100% peak at 15 The CuCl phase with the most intense peak at 2θ = 28.5°( marked as @ JCPDS card no. 06-0344) started appearing with appreciable intensity at 400 °C, confirming simultaneous decomposition of Cu 2 OCl 2 into CuCl as per eq 1.…”

“…However, the main aim is to prepare phase-pure Cu 2 OCl 2 from water in the CuCl 2 hydrolysis step of the Cu−Cl process (Figure 1). Singh et al 16 reported fixed-bed CuCl 2 hydrolysis to Cu 2 OCl 2 , and a conversion of 97% was observed at a gas hourly space hydrolysis and thermolysis reaction using in situ X-ray absorption spectroscopy (XAS) in situ XAS and thermogravimetric analysis mechanistic aspects of CuCl 2 thermolysis and hydrolysis using in situ XAS Singh et al 15 2 investigations on the hydrolysis step of the copper−chlorine thermochemical cycle for hydrogen production fixed-bed reactor for hydrolysis reactions, TG-MS, TPD, XAS, XRD, and isothermal TG performance evaluation of CuCl 2 hydrolysis on a fixed-bed reactor as a function of different parameters; effect of ball milling on reaction kinetics; rate constant and activation energy derived for the hydrolysis reaction using isothermal TG; quantification of the fixed-bed hydrolysis product using titrations, TG, TPD, and XANES; and explanation of the hydrolysis mechanism using the shrinking-core model 1). We have already reported 16 the limitations in achieving the phase-pure Cu 2 OCl 2 hydrolysis product.…”

“…More than 200 thermochemical cycles have been devised based on Hess’s law of thermodynamics . Among these, iodine–sulfur (I–S), − hybrid sulfur, and copper–chlorine − thermochemical processes are found to be efficient and promising for large-scale sustainable hydrogen production. The maximum operation temperature of the Cu–Cl thermochemical cycle ( T max = 530 °C) is much lower as compared to sulfur-based cycles ( T max = 850 °C), thus offering advantages in the selection of materials of construction, and also enhances the practicability for the Cu–Cl process to be coupled with a solar energy source .…”