Effects of particle size on oxygen surface exchange kinetics determined by pulse isotope exchange

• LSF64 and 8-YSZ oxygen exchange rates at 2% O 2 was determined. • PIE is a powerful technique; however, sufficiently small sample particles are essential. • Diffusion may affect kinetics even under proposed surface-limited regimes. Methods based on heterogeneous isotope exchange between the gas-phase and oxide surfaces have become popular experimental approaches for enhancing the fundamental understanding of rate-limiting elementary processes of the surface kinetics and developing materials with increased electrocatalytic activity towards oxygen reduction. This contribution explores the limitations of the pulse isotope exchange (PIE) technique, focusing on how particle size of the sample affects the reliability of derived kinetic parameters. Dense powders of varying particle sizes were evaluated for two materials with fundamentally different defect chemistries and transport properties: La 0.6 Sr 0.4 FeO 3-δ (LSF64) and (Y 2 O 3 ) 0.04 (ZrO 2 ) 0.96 (8-YSZ). The findings show that the ratio between the oxygen exchange and diffusion coefficients is crucial to determine the appropriate range of particle sizes that maintains the system within a kinetic regime governed by surface kinetics, adhering to the PIE approach prerequisites. If particles are too large, homoexchange and bulk diffusion can affect the measurements resulting in erroneous kinetic parameters.