A characteristic length linking oxygen transport and product selectivity in chemical looping methane anaerobic oxidation

Oxygen redox kinetics is crucial to the efficacy of redox catalysts for the chemical looping anaerobic oxidation of methane. However, the entanglement of oxygen surface exchange and bulk diffusion complicates the optimization of redox catalysts. This paper describes a characteristic length linking the oxygen transport and product selectivity in the chemical looping anaerobic oxidation of methane. The characteristic length L is defined as the ratio of the effective diffusion length (le) of lattice oxygen to the characteristic thickness (lc) of redox catalysts using xNiO/CeAlO4-δ (x = 0, 0.2, 0.5, 1, 2.5, 5) as a model redox catalyst. An in-depth kinetic analysis and structural characterization reveal that product selectivity depends on oxygen transfer kinetics. From which three distinct regions can be derived, i.e., surface exchange control with L smaller than 0.1 (x = 0–0.2), mixed control with L of 0.1–10 (x = 0.5–1) and bulk diffusion control with L larger than 10 (x = 2.5–5), which correspondingly tend to produce CO2, syngas, and carbon deposition, respectively. The established characteristic length L offers a universal metric for optimizing other redox catalysts for chemical looping anaerobic oxidation of methane. Oxygen redox kinetics play a key role in determining the performance of redox catalysts in the chemical looping anaerobic oxidation of methane (CLAOM), however, the intertwined effects of oxygen surface exchange and bulk diffusion make optimizing these catalysts challenging. Here the authors identify a characteristic length linking oxygen transport and product selectivity in CLAOM.