title: Mechanism of Zn Particle Oxidation by H2O and CO2in the Presence of ZnO.
abstract: In this work we investigate the mechanism of Zn oxidation with CO2and/or H2O to produce solar derived fuels (CO and/or H2) as part of the Zn/ZnO thermochemical redox cycle. It has been observed that the ZnO contamination of Zn produced by solar thermal reduction of ZnO (solar Zn) facilitates oxidation of the metallic Zn by CO2and H2O, allowing for nearly complete conversion at temperatures as low as 350 °C. Reaching the same reaction extent starting with pure Zn requires considerably higher temperatures which imposes use of unconventional hard-to-operate reaction configurations utilizing Zn as vapor. The mechanism of this enhancement is investigated by studying the oxidation of solid Zn diluted with ZnO or Al2O3at 350-400 °C utilizing thermogravimetry. It is found that ZnO acts as the site for the oxidation of Zn originating from the vapor phase, thereby serving as a sink for Zn vapor and maintaining the driving force for sustainable Zn sublimation. As this Zn sublimation competes with the growth of an impervious ZnO scale over the surface of the remaining solid Zn, the presence of the ZnO increases the reaction extent according to the magnitude of its surface area. This mechanism is supported by energy-dispersive X-ray (EDX) spectroscopy, revealing a substantial deposition of produced ZnO over the surface of the ZnO-seeded Al2O3diluent.
authors.affiliation: Department of Mechanical and Process Engineering, ETH Zurich , Zurich 8092, Switzerland.
authors.forename: David
authors.initials: D
authors.lastname: Weibel
authors.affiliation: Department of Mechanical and Process Engineering, ETH Zurich , Zurich 8092, Switzerland.
authors.forename: Zoran R
authors.initials: ZR
authors.lastname: Jovanovic
authors.affiliation: Department of Mechanical and Process Engineering, ETH Zurich , Zurich 8092, Switzerland.
authors.forename: Elena
authors.initials: E
authors.lastname: Gálvez
authors.affiliation: Department of Mechanical and Process Engineering, ETH Zurich , Zurich 8092, Switzerland ; Solar Technology Laboratory, Paul Scherrer Institute , Villigen 5232, Switzerland.
authors.forename: Aldo
authors.initials: A
authors.lastname: Steinfeld