Studies on Reaction Products, Byproducts, and Intermediates in Thermal Steps of the Cu–Cl Thermochemical Cycle for Hydrogen Generation

The copper–chlorine thermochemical cycle coupled with a nuclear/solar source is a potential renewable process for large-scale hydrogen production. In our earlier publications [Singh, R. V.; J. Therm. Anal. Calorim. 2022, 147, 7063−7076; Singh, R. V.; Int. J. Energy Res. 2020, 44(4), 2845–2863], the limitations in achieving a 100% phase-pure product, Cu₂OCl₂, in the hydrolysis step was emphasized. The present study investigated (i) the effect of the impurity phase accompanying the hydrolysis product on the kinetics of the subsequent O₂ evolution step, (ii) comparison of the thermal decomposition pathway of the hydrolysis product obtained in fixed-bed/fluidized-bed CuCl₂ hydrolysis with various simulated feeds using thermogravimetric mass spectrometry (TG-MS), (iii) mechanistic aspects involved in thermal decomposition of an equimolar CuO + CuCl₂ mixture (referred to as the substitute material, SM) from room temperature (RT) to 500 °C using in situ X-ray absorption spectroscopy (XAS), and (iv) the phase identification at intermediate temperatures during heating of the SM under an argon flow by ex situ high-temperature X-ray diffraction (HT-XRD). Main findings were that the thermal decomposition behavior of the SM was identical to the actual phase-pure hydrolysis product, Cu₂OCl₂. In the presence of the impurity phase, the decomposition temperature decreased by 30–40 °C. It was observed that moles of O₂ evolved from impure feeds are limited by x, where x corresponds to the maximum moles of Cu₂OCl₂ possible in the impure composition. Thermal decomposition of SM revealed the in situ formation of Cu₂OCl₂ via the Cu(OH)Cl phase at 300–350 °C. Further heating of Cu₂OCl₂ evolved O₂ at 450 °C as identified by MS and decomposed into single-phase CuCl at 500–550 °C. Our studies support Serban’s approach [Serban, M.; In Kinetic Study of the Hydrogen and Oxygen Production Reactions in the Copper-Chlorine Thermochemical Cycle, AIChe 2004 Spring National Meeting, April 25–29, 2004, New Orleans, LA, 2004] and conveyed that abstraction of Cl₂ by CuO may be facilitated via Cu₂OCl₂ formation. The in situ X-ray absorption near-edge structure (XANES) transformed from Cu²⁺ to Cu¹⁺ with an increase in temperature. The Cu–O, Cu–Cl, and Cu–Cu bond distances and coordination number provided evidence of in situ formation of Cu₂OCl₂ at 300–350 °C and Cu–Cl at 500 °C. Thus, in situ formation of Cu₂OCl₂ from the SM has strengthened the beneficial supposition and provided the basis for considering equimolar CuO + CuCl₂ (SM) as a surrogate material for Cu₂OCl₂. Also, a methodology to predict the amount of O₂ evolved from impure feeds in the decomposition step is suggested.