In situ formation and hydrolysis of Zn nanoparticles for H-2 production by the 2-step ZnO/Zn water-splitting thermochemical cycle

The production of solar hydrogen via the Zn/ZnO water-splitting thermochemical cycle consists of a 1st-step solar endothermic dissociation of ZnO and a 2nd-step non-solar exothermic hydrolysis of Zn. We report on a novel combined process for the efficient execution of the second step that encompasses the formation of Zn nanoparticles followed by their in situ hydrolysis for H-2 generation. The advantages of using Zn-nanoparticles are three-fold: (1) their inherent high specific surface area augments the reaction kinetics, heat transfer, and mass transfer; (2) their large surface to volume ratio favors complete or nearly complete oxidation; and (3) their entrainment in a gas flow allows for simple, continuous, and controllable feeding of reactants and removal of products. This combined process is experimentally demonstrated using a tubular aerosol flow reactor featuring Zn-evaporation, steam-quenching, and Zn/H2O-reaction zones. When the Zn-evaporation zone was operated at 1023 K and the Zn/H2O reaction zone was operated continuously just below the Zn(g) saturation temperature, Zn particles of 69 nm average crystallite size were formed and in situ hydrolyzed by up to 83% degree of chemical conversion, while the H-2 yield reached up to 70% after a single pass of H2O of 0.85 s residence time. (c) 2005 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.