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 Fe 2 TiO 5 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 CO 2 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 ( E a = 1.64 eV) than for K ( E a = 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 Fe 2 TiO 5 -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.