Modeling the effect of spill-over on oxygen exchange kinetics of oxide ceramics covered by catalytically active surface particles

The oxygen exchange kinetics of ceramic oxides, covered by catalytically active surface particles, has been investigated by finite element modeling. Two different particle shapes are applied, viz. spherical (ellipsoidal shape with equal axes parallel to the surface) and square-shaped particles. Additionally, the size distribution of square-shaped particles is taken into account by systematically varying the sizes of one larger and 12 smaller particles. The effect of spill-over of reactive adsorbed species on the oxygen exchange kinetics is simulated for various surface diffusion coefficients. The finite element model has been used to calculate relaxation curves for the total amount of exchanged oxygen from which apparent values for the chemical diffusion coefficient and the chemical surface exchange coefficient could be extracted. The modeling results have been interpreted in terms of the triple-phase boundary length of the catalytic particles. Apart from a tremendous increase of the surface reaction kinetics owing to catalyzed oxygen exchange at the triple phase boundaries, an additional enhancement is observed in case of spill-over, if the surface diffusion coefficient exceeds the bulk diffusion coefficient by 2–3 orders of magnitude and the surface coverage is sufficiently low.