Oxygen anions, mobility

Christie et al. (45) and Pendleton and Taylor (46) have recently reported the results of propylene oxidation over bismuth molybdate and mixed oxides of tin and antimony and of uranium and antimony in the presence of gas-phase oxygen-18. Their work indicated that for each catalyst, the lattice was the only direct source of the oxygen in acrolein and that lattice and/or gas-phase oxygen is used in carbon dioxide formation. The oxygen anion mobility appeared to be greater in the bismuth molybdate catalyst than in the other two. 

Both of these vacancies were believed to provide a practical way to increase the OSC of Ce02. This is true if the oxygen anion mobility is considered. Solid Ce02-based electrolytes show indeed an increased ionic mobility when doped with low-valent cations (Tuller and Nowick 1975). However, the doping may lead to a decrease of the total-OSC. 

The occurrence of an almost constant, albeit rather low, activity level, which is reached after a number of pulses, signifies that a certain quasiequilibrium concentration of active sites is maintained by transport of bulk oxygen anions to the surface. Such a mobility of oxygen is particularly observed for bismuth molybdates and some related catalysts (see below). Typical examples of catalysts which completely loose their activity at a low degree of reduction are the antimonates this is primarily caused by the absence of anion mobility. 

An entirely different selectivity principle known as phase equilibrium comes into play in high-temperature ionic conductors. Many important gases dissolve in ionic solids at elevated temperatures. However, the solubility is rather sharply defined for the gas and the solid by the lattice parameters and the size of the gas molecule. The best example is the solubility of oxygen in zirconium dioxide. When Z1O2 is doped with yttrium ions, it exhibits a high mobility for the O anion. The solubility and anion mobility then become the basis for several electrochemical gas sensors, using yttria-stabilized zirconia (YSZ). 

At operating temperature (100-400°C), the oxygen anions have sufficient mobility in the solid electrolyte. The cell voltage ECeii is then related to the partial pressure of oxygen by the Nernst equation written for the concentration cell. 

Partial substitution of A and B ions is allowed, yielding a plethora of compounds while preserving the perovskite structure. This brings about deficiencies of cations at the A-or B-sites or of oxygen anions (e.g. defective perovskites). Introduction of abnormal valency causes a change in electric properties, while the presence of oxide ion vacancies increases the mobility of oxide ions and, therefore, the ionic conductivity. Thus, perovskites have found wide apphcation as electronic and catalytic materials. 

The basic elements of a SOFC are (1) a cathode, typically a rare earth transition metal perovskite oxide, where oxygen from air is reduced to oxide ions, which then migrate through a solid electrolyte (2) into the anode, (3) where they combine electrochemically with to produce water if hydrogen is the fuel or water and carbon dioxide if methane is used. Carbon monoxide may also be used as a fuel. The solid electrolyte is typically a yttrium or calcium stabilized zirconia fast oxide ion conductor. However, in order to achieve acceptable anion mobility, the cell must be operated at about 1000 °C. This requirement is the main drawback to SOFCs. The standard anode is a Nickel-Zirconia cermet. 

One strategy for increasing the population of mobile oxygen anions in a perovskite lattice, so as to potentially increase ionic conductivity, is to enhance the population of lattice oxygen vacancies. This requirement can be realized in the orthorhombic brownmUlerite stmcture [22, 23] which is derived from the perovskite stmcture by removing one-sixth of the unit cell oxygen atoms to give the empirical... 

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