Fluorite-type oxides

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

In order to improve the resistance of Ni/Al203-based catalysts to sintering and coke formation, some workers have proposed the use of cerium compounds [36]. Ceria, a stable fluorite-type oxide, has been studied for various reactions due to its redox properties [37]. Zhu and Flytzani-Stephanopoulos [38] studied Ni/ceria catalysts for the POX of methane, finding that the presence of ceria, coupled with a high nickel dispersion, allows more stability and resistance to coke deposition. The synergistic effect of the highly dispersed nickel/ceria system is attributed to the facile transfer of oxygen from ceria to the nickel interface with oxidation of any carbon species produced from methane dissociation on nickel. 

Note s.s., solid solution with La or Zr oxide Cl and C2, cubic fluorite-type oxide. 

In generalizing these results, we can apply them to other solid electrolytes as well, for example, to other fluorite type oxides (e.g., Hf02, CeOz) that have been doped with heterovalent cations (e.g., SrO, BaO, Y203, La203). 

Fig. 7. Activation energy dependence of the anion vacancy concentration in various fluorite type oxide solid solutions. From ref. [14],...

Since Roberts has discussed nonstoichiometric fluorite-type oxides in several papers, the thermodynamics need be only briefly summarized. Below 1123° C., the distinct nature of the nonstoichiometric U02 and U409 phases, and the more highly ordered nature of the latter, are shown by the differences in partial molar enthalpy of oxygen in the two phases, and by the considerable decrease in entropy accompanying the U02+a. phase reaction (Table II). 

Fluorite type oxides are particularly prone to nonstoichio-metric effects. This most commonly occurs in the form of cation nonstoichiometry induced by partial reduction of the cation or by replacement of a portion of the oxide by flnoride. Anion excess phases can occur as a result of cation oxidation or by replacement with higher valence impurities. The dominant defect in this structure involves the migration of oxygen to the large cuboidal interstice resulting in the formation of a vacancy at a normal lattice site. A vacancy of this type is called a Frenkel defect. 

Ceria-based catalysts are intensively used because of their high chemical and physical stability, high oxygen mobility and high oxygen vacancy concentrations, which are characteristic of fluorite-type oxides. The possibility of cycling easily between reduced and oxidized states (Ce Ce" ) permits the reversible addition... 

Vargas, J.C., Libs, S., Roger, A.-C., and Kiennemann, A. Study of Ce-Zr-Co fluorite-type oxide as catalysts for hydrogen production by steam reforming of bioethanol. Catalysis Today, 2005, 107-108, 417. 

The ionic transport properties of fluorite-type oxide phases (see Chapter 2), another important family of solid electrolytes, are also discussed in subsequent chapters. Briefly, for the well-known zirconia electrolytes, Zr itself is too small to sustain the fluorite structure at moderate temperatures doping with divalent (Ca + ) or trivalent (e.g., Y " ", St " ", Yb " ") cations stabilizes the high-temperature polymorph with the cubic fluorite-type structure. Due to the electroneutrahty condition, anion vacancies are formed ... 

In this section, systematic results on the electronic structures and chemical bonding of rare-earth sesquioxides and fluorite-type oxides are introduced briefly. In the subsection 4-1, systematic results on the electronic structures and chemical bonding of sesquioxides are described and the description on rare-earth fluorite-type oxides is mainly focused on cerium dioxide in the subsection 4-2. 

In this section, fluorite-type oxides, for example, the cerium dioxide (Ce02, ceria) is discussed. It has been very widely investigated because of the high chemical stability with high melting point 2730 °C. It belongs to the cubic system... 

In a fluorite-type oxide, whose chemical formula is expressed as MO2, the structure type is named afler Cap2, in which the cation/anion radius ratio is such that the anions achieve a simple cubic packing with the cations occupying half the available sites with 8-fold coordination. The packing of the anions is not as compact as those found in other oxides such as spinel or corundum-type structures, which have closest packing of oxide ions. 

The fluorite-type oxides are not the only possible 0 ion conductors. Perovskite-t e oxides wit anion acancies can be formed by substituting M for M or M for M. The ionic... 

Yttria-stabilized Z1O2, discovered by Nemst [39], is still one of the state-of-the-art SOFC electrolyte materials which was used to demonstrate the first SOFC (and the first solid electrolyte fuel cell) in 1937 at ETH-Ziirich [40]. Electronic defect concentrations are negligibly low [41]. As can be observed in Fig. 6.1, the ionic conductivity of fluorite-type oxides stabilized with hypovalent elements exhibits a maximum at a certain dopant concentration above which defect interactions occur [42-45]. As shown in Fig. 6.2, it is known that the peak value of the ionic... 

Pyrochlores and Other Fluorite-Type Oxides (Y, Nb, Zr)02S Pyrochlore-type oxides have the general formula A2 B2 07 in which A is a rare-earth element such as Gd or Y, and B is Ti or Zr. Gd2Zr207 is the typical composition of pyrochlore-type oxides and can be considered as fluorite-type in which ionic defects are regularly arranged. The defect structure and the mixed conductivity can be controlled by the value of x in, for example, Gd2(ZrxTii x)20 . which is abbreviated as GZT [69]. When the ionic radius of rare-earth elements for the A site is larger than that of Gd, the structure changes from highly defective fluorite to pyrochlore [70, 71]. 

Lattice Oxygen Transfer in Fluorite-Type Oxides Containing Ce, Pr, and/or Tb, Z.C. Kang and L. Eyring, J. Solid State Chem., 155, 129-137 (2000). 

Figures 1.4(a) and 1.4(b) show parts of the baddeleyite-type structure of zirconium dioxide ZrO. This structure belongs to the monoclinic space group P2i/c and has a distorted fluorite-type structure. The coordination number (CN) of the Zr cations is seven, which is smaller than that of the fluorite-type oxide (CN = 8). This smaller value (CN = 7) is attributable to the smaller size of Zr cations ( (Zr + s) = 0.84 A) compared to the criterion deduced from Pauling s first rule (r( >8) 1.079 A) for A cations with a CN of eight...

A characteristic feature of perovskite-type oxide ion conductors is that they are often accompanied with p-type electronic conduction under an oxidizing atmosphere such as air at elevated temperatures. As described in Section 3.2, the contribution of electronic conduction depends on P02 in the atmosphere and temperature. As a typical example, Fig. 3.4 shows the P02 dependence of conductivities of CaTiOs- and SrTiOs-based solid solutions at 800°C [16]. P-type electronic conduction appears in the region of high P02 and n-type one under low oxygen partial pressure, i.e., a reducing atmosphere. The shape of the curve Incr lnPo2 is essentially the same as that shown schematically in Fig. 3.1. In many fluorite-type oxide ion conductors such as stabilized zirconias and... 

In many perovskite-type oxide ion conductors, the equilibrium constant so large that hole conduction appears even at a relatively weak oxidizing atmosphere such as air, whereas in the case of fluorite-type oxides such as stabilized zirconias, the equilibrium constant Kj is too small to cause p-type electronic conduction in air at elevated temperatures. 

---