Computer Simulation Studies of Bulk Reduction and Oxygen Migration in CeO₂−ZrO₂ Solid Solutions

Abstract

Computer simulation techniques have been used to model cubic CeO2−ZrO2 solid solutions in the whole composition range. Aspects related with the oxygen storage capacity of these materials are emphasized. The energetics of the Ce4+/Ce3+ bulk reduction reaction as well as the activation energy for oxygen migration in the lattice are investigated and compared with the corresponding quantities in pure CeO2. It is found that even small additions of ZrO2 decrease the bulk reduction energy of Ce4+ to values comparable to those reported for surface reduction in pure CeO2. Activation energy calculations indicate an almost monotonic increase of oxygen mobility with increasing zirconia content.

Principles of doping ceria for the solar thermochemical redox splitting of H2O and CO2

“…The fact that strained bonds reduces the ceria reduction energy corrects a long held misunderstanding that they compensate for ceria expansion. 33,[35][36][37][38] Additionally, this explanation provides the fundamental understanding for why the geometric relaxation of Zr-CeO 2 during reduction is associated with a decreased Ovacancy formation energy. 59 While the energy released is sizable, it does not decrease the reduction energy to the same extent as the presence of pre-broken bonds or decreased electron promotion energy.…”

“…33,[35][36][37][38] This nding builds on the work of Wang et al, who suggested that structural relaxation is the driver for the decreased reduction energy, 59 by explaining that the energy released during structural relaxation stems from strain release of the Zr-O bonds. The promotion of two electrons into the Ce forbitals and breakage of four O-cation bonds results in an Ovacancy formation energy that is high enough to drive water splitting when the vacancy is lled.…”

“…33,[35][36][37][38] The rational for this hypothesis is explained as follows. When O-vacancies form in ceria during reduction, each removed O 2À anion nominally releases two electrons to the ceria bulk; these electrons localize on two Ce 4+ reducing them to Ce 3+ cations, as shown in Kroger-Vink notation:…”

“…33,37,[39][40][41][42] The presence of the smaller dopant cations, i.e. Zr and Hf, are believed to alleviate the expansive strain caused by the larger Ce 3+ cations via the overlap of tensile and compressive strain elds; simultaneously decreasing the extent of expansion and decreasing the reduction energy.…”

“…Zr and Hf, are believed to alleviate the expansive strain caused by the larger Ce 3+ cations via the overlap of tensile and compressive strain elds; simultaneously decreasing the extent of expansion and decreasing the reduction energy. 33,[35][36][37][38] This is very logical based on the cationic radius explanation for the expansion of ceria upon reduction. However, we have recently shown that the formation of Ce 3+ cations is not responsible for ceria expansion, but rather ceria expands because of outward relaxation from the O-vacancy due to non-counterpoised forces.…”