Stability perovskite oxides

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

Stability of the K2NiF4 structure in oxides. Just as for perovskite oxides, a tolerance factor t may be defined for A2B04 oxides as... 

Oxygen (O2-) anion conductors - stabilized zir-conia, stabilized - bismuth oxide, - BIMEVOX, doped cerium dioxide, numerous perovskite-type - solid solutions derived from Ln(A)B (B") 03 (A = Ca, Sr, Ba B = Ga, Al, In B" = Mg, Ni, Co, Fe), La2Mo207 and its derivatives, pyrochlores based on Ln2Ti07. 

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. 

For FT SOFCs, perovskite oxides (La,Sr)(Co,Fe)O3 5 have attracted particular attention [50]. Due to chemical reactivity with yttria-stabilized zirconia (YSZ), the use of a protective interlayer between the cathode and electrolyte is required to increase the system s stability during long-term operation [51-53]. 

The ample diversity of properties that these compounds exhibit, is derived from the fact that over 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite oxide structure and also from the possibility of synthesis of multicomponent perovskites by partial substitution of cations in positions A and B giving rise to compounds of formula (AjfA i- )(ByB i-J,)03. This accounts for the variety of reactions in which they have been used as catalysts. Other interesting characteristics of perovskites are related to the stability of mixed oxidation states or unusual oxidation states in the structure. In this respect, the studies of Michel et al. (12) on a new metallic Cu2+-Cu3+ mixed-valence Ba-La-Cu oxide greatly favored the development of perovskites exhibiting superconductivity above liquid N2 temperature (13). In addition, these isomorphic compounds, because of their controllable physical and chemical properties, were used as model systems for basic research (14). 

The ease of reduction increases, therefore, from Cr3+ to Ni3+ in the series of LaM03 perovskites. The same trend has been found for the simple oxides of Fe, Co, and Ni (115). On the other hand, the mixed oxides LaM03 (M = Fe, Co, Ni) were found to be more stable in a H2 atmosphere than the simple oxides NiO, Fe203, and Co304 (92) this shows the increased stability of transition-metal cations in a perovskite structure. The TPR experiments also show that the stability of perovskite oxides increases with increasing size of the A ion. 

Reduction of NO with CO or H2 was found to be an interesting example of intrafacial catalytic process (30). If this reaction is conducted over a transition-metal oxide, the reaction rate appears to be related primarily to the thermodynamic stability of the oxygen vacancies adjacent to a transition metal ion. Associative as well as dissociative adsorption of NO have been reported on perovskite oxides (14, 22, 80, 174) (see also Section VI,B) the adsorption on the reduced oxides is stronger than in the oxidic compounds. Dissociative adsorption takes place at moderate temperatures as in NO reduction over Lao.gsBao.isCoOs at 100°C with the subsequent formation of N2 and N20 (73). 

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