Regulation of Oxygen Migration and Catalytic Performance in Fe ₂ TiO ₅ via Alkali Metal Doping for Methane Chemical Looping Reforming

This work examines how alkali modification with sodium and potassium tunes oxygen transport and catalysis in Fe2TiO5 oxygen carriers for methane chemical-looping reforming. Density-functional theory maps oxygen-vacancy formation and migration along multiple surface and subsurface routes and, together with microkinetic modeling, quantifies turnover frequencies and rate-limiting contributions for CO and CO2 formation. Potassium consistently outperforms sodium: the representative surface-to-bulk oxygen hop has a lower migration barrier with K than with Na, yielding higher turnover frequency (TOF) that peak around 800–900 K and a broader high-activity window. Deep bulk K incorporation further enhances oxygen mobility and reduces the kinetic weight of oxygen migration in the overall rate control, while also favoring selective CO formation. Sodium exerts a more localized effect: once incorporated, bulk diffusion becomes easier for Na (Ea = 1.64 eV) than for K (Ea = 2.03 eV), but the net improvements in TOF and oxygen-transport control are smaller and confined to a narrower temperature span. The combined DFT-microkinetic analysis identifies optimal dopant identity and placement, establishes oxygen migration as the dominant bottleneck in Fe2TiO5-based carriers, and provides actionable design rules: prioritize bulk K doping to maximize oxygen mobility, expand the 800–900 K activity plateau, and promote CO selectivity, with Na offering incremental stabilization where excessive lattice distortion is a concern.