Trends in the phase stability and thermochemical oxygen exchange of ceria doped with potentially tetravalent metals

Abstract

Ceria is among the most prominent materials for generating clean fuels through solar thermochemical CO₂ reduction and water splitting. The main optimization parameter for ceria in solar reactors is the oxygen exchange capacity (OEC, Δδ), which can be notably improved through various dopant types. Among them, tetravalent dopants excel through the formation of active vacancies which lead to particularly high OEC values. We thus performed a comprehensive screening evaluation of all dopants in the periodic table which have been reported to adopt an oxidation state of +IV. All thermally stable doped ceria samples, M_0.1Ce_0.9O_2−δ (M = Si, Ti, V, Cr, Zr, Nb, Rh, Hf, Ta, Nb, V, Pr, and Tb), were first analyzed for Δδ improvement with thermogravimetric analysis (TGA). Dopant solubility limits and behavior in the ceria host lattice was evaluated with scanning electron microscopy (SEM-EDX) and powder X-ray diffraction techniques. No indications for carbonate side product formation were found. Hf-, Zr-, and Ta-doped ceria display higher OEC than pristine ceria, and Raman spectroscopy indicated that their improved performance is accompanied by a higher versatility in the underlying vacancy formation processes. Furthermore, the effective dopant radii are close to an optimal dopant radius around 0.8 Å for maximum Δδ according to TGA cycling experiments. These experimentally derived trends for doped ceria were supported by density functional theory (DFT) calculations which analogously correlate Δδ with the partial electronic charge of the metal dopants.