Vacancy Dynamics Govern Selectivity in Chemical Looping Methane Oxidation over NiO

Abstract Chemical looping methane oxidation over NiO is governed by oxygen-diffusion-controlled vacancy dynamics rather than by a single fixed reaction pathway. Density functional theory calculations show that the preferred mechanism and its rate-determining step change with the oxygen-vacancy population on NiO(100), shifting from a CH3O-mediated route on the pristine surface (101.7 kJ/mol) to dehydrogenation-dominated and oxygen-assisted routes limited by CO (82.9 kJ/mol) and CH2* (91.3 kJ/mol) oxidation as vacancies accumulate. Subsurface-to-surface oxygen migration, with barriers of 213–243 kJ/mol, dictates how rapidly consumed lattice oxygen is replenished, and a diffusion-coupled microkinetic model predicts a temperature-dependent transition from a surface-limited regime favoring incomplete oxidation to a subsurface-limited regime sustaining complete combustion. Bubbling fluidized-bed experiments on a spray-dried NiO oxygen carrier (≈85 μm particles, ≈13 wt % reversible oxygen capacity) confirm this picture: CH4 breakthrough with concurrent CO and H2 evolution occurs progressively earlier as temperature is lowered (190, 310, and 750 s at 600, 700, and 800 °C), whereas complete oxidation to CO2 is maintained throughout the run at 900 °C. These results identify lattice-oxygen diffusion as a key operando descriptor of selectivity and suggest a design principle for oxygen carriers and operating windows in chemical looping methane conversion.