High-reactive and coke-resistant polyhedral NiO/Fe₂O₃ oxygen carrier for enhancing chemical looping CH₄–CO₂ dry reforming

Chemical looping dry reforming of methane (CL-DRM) is a promising technology for syngas production and efficient CH₄ and CO₂ utilization to provide appropriate H₂/CO ratio in the reducer for Fischer-Tropsch synthesis. The crucial issue in CL-DRM is to find a highly reactive oxygen carrier (OC) with good stability. Polyhedral NiO/Fe₂O₃ composite oxygen carriers were successfully fabricated via a hydrothermal and calcination method based on different solubility product constants of nickel carbonate (NiCO₃) and ferrous carbonate (FeCO₃) in aqueous solution. The physicochemical properties of the OCs were characterized by XRD, XRF, SEM, TEM, TG/DTA, XPS, H₂-TPR and Raman spectroscopy. The performance of NiO/Fe₂O₃ OCs was measured in a fixed-bed reactor. The experimental results indicated that the Fe₂O₃-0.6Ni OC has the highest syngas yield up to 90.25% with ideal H₂/CO mole ratio (approximately 2) in the reducer at 750 °C. In addition, in reducing atmospheres, the Ni₃Fe alloy was formed instead of metallic Ni and Fe, thus significantly improves coke resistance. Density functional theory (DFT) calculations demonstrated that the energy barrier of rate determining step of dehydrogenation process (CH₃ releases H to form CH₂) is 2.64 eV with OC of pure Fe₂O₃, which is higher than that of the condition with OC of NiO/Fe₂O₃ composite (2.21 eV). Therefore, the addition of NiO to Fe₂O₃ favors the CH₄ dehydrogenation process on the surface of OCs. This study provides guidance for the design of oxygen carriers for chemical looping dry reforming of methane with the better coke-resistant property.