Reactivity and stability of Zr-doped CeO2 for solar thermochemical H2O splitting in combination with partial oxidation of methane via isothermal cycles

Ceria-based oxides have attracted a lot of attention as an attractive redox material because of their large capacity for storing and releasing oxygen in the solar-driven thermochemical water splitting (STWS) process. Nevertheless, the extremely high temperatures and low oxygen partial pressure required to achieve deep degrees of reduction and large temper-ature swing in a redox cycle introduce challenges in the practical implementation. These above challenges can be addressed in a unique way by integrating partial oxidation of methane into the reduction step. The STWS can therefore operate isothermally at signif-icantly lower temperatures. In this work, the CeO2-ZrO2 solid solutions (Ce1-xZrxO2) are synthesized, characterized, and assessed for thermochemical water splitting in combina-tion with partial oxidation of methane. Up to 160 consecutive redox cycles are also con-ducted in a bench-scale fixed bed. At an operating temperature of 900 degrees C, methane successfully promotes the reduction of Ce1-xZrxO2 to produce the synthesis gas with a 2:1H2/CO ratio and 87.86% selectivity. When compared to CeO2, the thermodynamic fuel generation capability of CeO2 with Zr4+ doping is three times greater in the partial oxida-tion of methane step and water splitting step. Ce0.8Zr0.2O2 (C8Z2) demonstrates the best redox activity in terms of CO and H2 production in a redox cycle among the various Zr4+ doping levels. After 160 consecutive redox cycles, C8Z2 is also very robust, maintaining its redox activity. The C8Z2 composite redox solid solution thus exhibits excellent redox activity and long-term redox stability, potentially making it appropriate for STWS in combination with partial oxidation of methane. (c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.