alpha-Fe2O3 Nanoparticles as Oxygen Carriers for Chemical Looping Combustion: An Integrated Materials Characterization Approach to Understanding Oxygen Carrier Performance, Reduction Mechanism, and Particle Size Effects

Through continuous flow reactor experiments, materials characterization, and theoretical calculations, we provide new insights into the reduction of hematite (alpha-Fe2O3) nanoparticles by methane (CH4) during chemical looping combustion (CLC). Across CLC-relevant temperatures (500-800 degrees C) and gas flow rates (2.5-250 h(-1)), decreasing alpha-Fe2O3 particle size (from 350 to 3 nm) increased the duration over which CH4 was completely converted to CO2 (i.e., 100% yield). We attribute this size-dependent performance trend to the greater availability of lattice oxygen atoms in the near-surface region of smaller particles with higher surface area-to-volume ratios. All particle sizes then exhibited a relatively rapid rate of reactivity loss that was size- and temperature-independent, reflecting a greater role for magnetite (Fe3O4), the primary alpha-Fe2O3 reduction product, in CH4 oxidation. Bulk (X-ray diffraction, XRD) and surface (X-ray photoelectron spectroscopy, XPS) analysis revealed that oxygen carrier reduction proceeds via a two-stage solid-state mechanism; alpha-Fe2O3 reduction to Fe3O4 followed the unreacted shrinking core model (USCM) while subsequent reduction of Fe3O4 to wfistite (FeO) and FeO to iron metal (Fe) followed the nucleation and nuclei growth model (NNGM). Atomistic thermodynamics modeling based on density functional theory supports that reduction initiates via the USCM, as partially reduced alpha-Fe2O3 surfaces exhibited a wide range of stability relative to bulk Fe3O4. Reduction and reoxidation cycling experiments were also performed to explore more practical aspects related to the long-term performance of unsupported alpha-Fe2O3 nanoparticles as oxygen carriers for CLC.