Investigations on the hydrolysis step of copper-chlorine thermochemical cycle for hydrogen production

Detailed investigations of CuCl₂ hydrolysis step of Cu–Cl thermochemical cycle were carried out on various aspects: (a) characterization and thermal properties of reactants/products using X-ray diffraction (XRD), thermogravimetry–mass spectrometry (TG-MS), scanning electron microscopy (SEM), temperature-programmed desorption (TPD), and extended X-ray absorption fine structure (EXAFS); (b) performance evaluation of fixed bed hydrolysis; (c) parametric optimization with respect to S/Cu, flow rate (gas hourly space velocity, GHSV), reaction duration, temperature, and particle size; and (d) monitored hydrolysis using isothermal TG experiments at 360°C, 370°C, 380°C, 390°C, and 400°C to derive kinetic parameters rate constant (k) and activation energy (E_a) on the basis of the shrinking-core model. 97% conversion to Cu₂OCl₂ at 17 630 h⁻¹ of GHSV, 400°C was achieved using ball-milled CuCl₂ (BM6), as compared with that of 55% over commercial un–ball-milled reactant, CuCl₂ (UBM). Correspondingly, higher k value of 2.84 h⁻¹ over BM6 as compared with 0.97 h⁻¹ over UBM reactant at 400°C was achieved. E_a for hydrolysis of BM6 was 93 kJ/mol, while it was 106 kJ/mol for UBM as derived from the Arrhenius plot. A probable pathway for CuCl₂ hydrolysis is proposed here. It was found to be diffusion controlled, and the particle size of reactant molecules affects the packing and diffusion length. Based on our investigations, it is very unlikely to get >99% phase pure product (Cu₂OCl₂). Cu₂OCl₂ is labile in nature and tends to transform into structurally similar and stable compounds CuO and CuCl₂.