Surface controlled reduction kinetics of nominally undoped polycrystalline CeO₂

Abstract

Ceria is an interesting material for high temperature redox applications like solar-thermal splitting of CO2 and H2O. Technical implementation and reactor design for solar-thermal redox-based fuel generation requires reliable data for the chemical surface exchange coefficient and the chemical diffusivity of oxygen. The results of thermogravimetric relaxation experiments and equilibrium oxygen isotope exchange experiments with subsequent depth profiling analysis suggest that the reduction reaction of even dense samples of pure ceria (1 mm thickness, 93% of theoretical density) with a grain size of about 20 μm is surface reaction controlled. The chemical surface exchange coefficient exhibits a negative apparent activation energy (-64 kJ mol(-1)). This finding is corroborated by similar data from literature for the tracer surface exchange coefficient. The structure of the derived expression for the apparent activation energy further suggests that the chemical surface exchange coefficient should show only a very weak dependence on temperature for ceria doped with lower valence cations.

Splitting CO2 with a ceria‐based redox cycle in a solar‐driven thermogravimetric analyzer

“…Due to its favorable oxidation thermodynamics, rapid reaction kinetics, and morphological stability over a wide range of temperatures, ceria (CeO 2 ) is currently considered the benchmark redox material 2, 3, 4, 5, 6, 7. The two‐step thermochemical cycle is then represented by:…”

Ceria: Recent Results on Dopant-Induced Surface Phenomena

“…Such a behaviour would clearly contradict Equation (A2) which postulates that K O depends on 0 O only in all kinetic regimes. In our earlier joint work on nominally un-doped ceria [33] we also observed a vanishing apparent activation energy of the chemical oxygen surface exchange coefficient K O in the surface exchange controlled regime.…”

“…In our earlier joint work on nominally un-doped ceria [33] we also observed a vanishing apparent activation energy of the chemical oxygen surface exchange coefficient O K  in the surface exchange controlled regime. Summarising one has the following experimental contradictions to the consequences of Equations (A1) and (A2):…”

“…One direct way to follow the kinetics of oxygen uptake or release in situ at high temperatures is a gravimetric experiment [33]. For plate-like samples with thickness h and with both large faces in contact with the atmosphere the mass change, ∆m(t), if normalized to the mass change at equilibrium, ∆m(t eq ), is given by Crank [54] (page 60)…”

“…In the framework of the current investigation the standard phenomenological treatment could recently be extended [33,34]. Because of the open questions on the nature of the K-D correlation we will only consider publications which present data for both K and D and will not discuss other, though interesting, studies like electrical conductivity measurements where electrodes are used and which, at best, supply the oxygen ion conductivity (which would yield the oxygen self-diffusivity) but no information on the surface exchange coefficient (see, e.g., [35]) nor, respectively, electrical conductivity relaxation studies which yielded only surface exchange coefficients but no diffusion coefficients (see, e.g., [36]).…”