Recent Results on Dopant influenced Chemical Expansion and Surface Alteration of Ceria

Abstract

The utilization of clean and sustainable energy becomes important for solving global environmental problems and securing future energy supply. A possible method for producing CO for fuels is a two-step CO2 splitting reaction in a solar thermo-chemical cycle. Especially doped CeO2 are considered as promising redox materials for thermochemical purposes [1]. Redox studies on dense Ce-Zr-O, Ce-La-O, Ce-Yb-O, Ce-Zr–La-O and Ce-Zr–Yb-O ceramics were carried out by thermogravimetric investigations and dilatometry. The reduction state determined by both methods correspond to each other up to 1500 K under low pressure (p= 2x10-5 mbar). At higher temperatures, the thermogravimetry deviates from the actual degree of reduction, since the mass loss is not only due to reduction but also due to the selective evaporation of CeO2, whose vapour pressure is higher than that of the doping ions [2]. As a result, there is an accumulation of doping ions, which leads to a porous surface zone. Surface enrichment of zirconia is detrimental for reoxidazion kinetic of CO2 or H2O splitting reaction since re-oxidation temperatures of (Ce,Zr)O2−δ are known to be shifted towards lower temperatures with increasing ZrO2 content [3-5]. Additional doping with trivalent doping ions such as La3+ and Yb3+ can influence this selective evaporation. The evaporation zone in the form of a porous surface layer is much smaller with Ce-Zr-O pellets additionally doped with La3+ and Yb3+. Samples co-doped with La3+ and Yb3+ exhibited also high oxidation kinetics that were much faster than 10mol% Zr-doped ceria. Additionally, the extrinsic oxygen vacancies introduced by the trivalent dopants influence the chemical expansion/contraction of the ceria crystal lattice during the redox reaction. Thus, the chemical expansion during reduction is higher for trivalent doped ceria than for tetravalent doped ceria. Since the chemical expansion/contraction is influenced by the mobility and preferential localization of the intrinsic oxygen vacancies in addition to the ionic radii [6-10], it can be assumed that additional extrinsic vacancies of introduced trivalent dopant ions promote the delocalization of the intrinsic oxygen vacancies, while smaller tetravalent ions such as Zr4+ counteract this. Thus, doped cerium oxides with trivalent and tetravalent ions show almost no change in chemical expansion, as the opposite effects compensate each other.