CO₂Dissociation and Upgrading from Two-Step Solar Thermochemical Processes Based on ZnO/Zn and SnO₂/SnO Redox Pairs

This study addresses the solar thermochemical CO 2 splitting via ZnO/Zn and SnO 2 /SnO two-step cycles. This process targets the recycling and upgrading of CO 2 emissions for the production of solar fuels. Reactive Zn- and SnO-rich nanopowders were first synthesized in a high-temperature solar chemical reactor. Their reactivity was then investigated to demonstrate that the produced nanoparticles react efficiently with CO 2, which generates CO and the initial metal oxide that can be recycled. While thermodynamics predict the formation of carbon at low temperatures and a narrow temperature range, in which CO formation is favored, experimental studies were performed in both thermogravimetry and a fixed-bed reactor to demonstrate that the CO 2 -splitting reaction is able to produce CO with high chemical conversions. The Zn oxidation with CO 2 was almost complete and fast from 360 °C, whereas the SnO oxidation required both higher temperatures (around 800 °C) and reaction duration to reach significant conversion (about 85%). Complete SnO conversion was achieved during dynamic thermogravimetric runs up to 1100 °C. The reaction rates increased with the inlet CO 2 mole fraction. The fixed-bed configuration appeared suitable for efficient conversion of the solar-produced Zn metallic powders.