We report here two different simple, one‐pot, and low cost chemical synthetic routes for the preparation of Cu2O nanocrystals: (a) thermal decomposition of copper–organic precursors copper(II) acetate or copper(II) acetylacetonate in long chain organic solvents oleyl alcohol and trioctylamine, respectively, at 170 °C and (b) a surfactant‐free solvothermal approach involving the reaction of copper(II) acetylacetonate in acetone at 140 °C. The structure and morphology of the nanocrystals have been characterized in detail by XRD, FTIR spectroscopy, Raman spectroscopy, and high‐resolution transmission electron microscopy (HRTEM). The optical properties of the nanocrystals have been explored by diffuse‐reflectance spectroscopy (DRS) and a blueshift of the optical band gap of the nanocrystals is observed owing to size effects. Based on the FTIR, GC–MS, and 13C{1H} NMR studies of post‐reaction solutions, different formation mechanisms for the Cu2O nanocrystals, which depend on the synthetic approach, have been proposed. Oleyl alcohol and trioctylamine play dual roles as solvents and mild reductants and reduce CuII species to CuI species during the course of the thermal decomposition reactions. The solvothermal reaction of copper(II) acetylacetonate in acetone possibly proceeds by acetylacetone‐mediated reduction of Cu2+ to Cu+ in the absence of any reducing agent. The potential of Cu2O nanocrystals as photocatalytic materials for hydrogen generation from water/methanol (2:1) mixtures under UV/Vis irradiation has also been evaluated. The results show that all the nanocystalline Cu2O samples generate H2.

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Summary 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 (Ea) 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. Ea 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.

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