Stepwise Solar Methane Reforming and Water‐Splitting via Lattice Oxygen Transfer in Iron and Cerium Oxides

Overview & Synopsis

Chemical looping reforming of methane (CLRM) involves lattice oxygen transfer in metal oxides. This study aims to compare iron (Fe 2 O 3) and cerium (CeO 2) oxides as oxygen carrier materials for isothermal solar-driven stepwise CH 4 reforming and H 2 O splitting. Experiments were conducted in a directly-irradiated lab-scale solar reactor heated by concentrated sunlight in the temperature range 950-1150 °C. Using solar energy for process heat reduces the dependence on fossil energy resources and avoids CO 2 emissions, while converting solar energy into chemical fuels. The performance of the oxygen carrier materials was compared and evaluated by determining the averaged amount of oxygen transferred, methane conversion, syngas yield, and thermochemical cycling stability. As a result, iron oxide reduction with methane strongly depended on temperature and displayed relatively lower reaction rate than CeO 2. The reduced iron (Fe) was not completely re-oxidized to iron oxide (Fe 3 O 4) after water-splitting step because of its low thermal stability resulting in strong 2 sintering and agglomeration, thereby decreasing syngas yield and eventually leading to material deactivation. In contrast, cerium oxide exhibited faster reaction rate and stable syngas yield with H 2 /CO molar ratio approaching two over repeated cycles. Stable patterns in the averaged oxygen nonstoichiometry (=0.35-0.38), methane conversion (X CH₄ =46.9-60.9%), and total syngas yield (5.67-6.80 mmol/g CeO₂ for reduction and 2.38-2.57 mmol/g CeO₂ for oxidation) over twelve successive cycles for ceria reticulated foam demonstrated excellent thermal cycling stability. Thus, employing Fe 2 O 3 as oxygen carrier was not suitable for solar CLRM, but iron oxide reduction with methane could be a promising option for solar metallurgy aiming at producing both metallic iron and syngas.