Using XANES to obtain mechanistic information for the hydrolysis of CuCl2 and the decomposition of Cu2OCl2 in the thermochemical Cu–Cl cycle for H2 production

The Advanced Photon Source (APS) at Argonne National Laboratory was used to investigate the progress of two of the reactions of the copper–chlorine cycle for production of hydrogen in situ by studying the evolution of the solid Cu species, using X-ray absorption near edge structure (XANES) spectroscopy. The hydrolysis of CuCl2 (2 CuCl2 + H2O → Cu2OCl2 + 2 HCl) was studied under low and high steam-to-copper ratios from 423 to 725 K, and the decomposition of Cu2OCl2 (Cu2OCl2 → 2 CuCl + ½ O2) in dry and humidified nitrogen up to 750 K. This study showed that the formation of Cu2OCl2 by hydrolysis of CuCl2 is more favorable under low steam-to-copper mole ratios and it reaches a maximum around 675 K. Over this limit, the formation of CuO and Cl2 as reaction byproducts starts to be noticeable. The same reaction byproducts were observed to form under all of the other experimental conditions and at temperatures as low as 635 K. The results from the decomposition studied by XANES are in very good agreement with calorimetric studies (TG/DSC) and they confirm that the formation of Cl2 takes place in the early stages of the decomposition of Cu2OCl2. To the best of our knowledge, this is the first time that the XANES spectrum of a Cu2OCl2 standard has been reported, since in previous studies Cu2OCl2 was always a reaction intermediate.