Statistical thermodynamics of non-stoichiometric ceria and ceria zirconia solid solutions

Abstract

The thermodynamic redox properties of ceria and ceria zirconia solid solutions are analysed with a new methodology for modelling such systems based on the statistical mechanics of lattice configurations. Experimental thermogravimetric equilibrium data obtained for small non-stoichiometry measurements are combined with literature data to cover a large range of non-stoichiometry (CeO2-δ, δ = 0.001-0.32), temperature (1073-1773 K) and oxygen partial pressure (1-10(-13) bar). A dilute species model of defect clusters , obeying the law of mass action, was sufficient to describe the system over the whole range of conditions, leading to a simple analytical equation of state for the system. This offers new physical insight into the redox properties of ceria based materials, and the theoretical methods developed should also be of great interest for other materials which exhibit continuous oxygen non-stoichiometry similar to ceria, such as perovskite oxides.

Ceria: Recent Results on Dopant-Induced Surface Phenomena

“…The enhancement of reducibility implies an alteration of the respective redox thermodynamics. In comparison to un-doped ceria the redox reaction of Zr doped ceria reveals a lower enthalpy and entropy [6,[9][10][11][12].…”

“…The reducibility of ceria can be improved by doping with zirconia due to an increase of the equilibrium p O 2 at a given temperature [5][6][7][8][9][10][11][12]. The enhancement of reducibility implies an alteration of the respective redox thermodynamics.…”

Redox chemistry of CaMnO3 and Ca0.8Sr0.2MnO3 oxygen storage perovskites

“…In some cases, it is also necessary to include a dependence of Dh d on d and temperature, 17,19 but as we shall see later, a constant value offers a very good t for the materials studied here.…”

“…The congurational entropy is taken from the previous derivation by the authors for ceria, 17 and is given by…”

“…Moreover, there are DFT studies which suggest that defect clusters prefer the next nearest neighbour ceria sites, 26,27 leading to a lot of possible congurations, which would explain the much larger entropy of defect formation seen in ceria compared to perovskites. 17 This is a fundamental limitation of the crystal motifs of these materials, and thus perovskite oxides, regardless of composition, will likely have a lower entropy of vacancy formation than ceria. This has important implications for designing materials for thermochemical fuel production cycles, where perovskites 28,29 have been suggested as a lower temperature alternative to the ceria cycle.…”