Oxides, perovskite

In the Lai.,CsxMn03 catalyst, the T decreases with an increase of x value and shows an almost constant value upon substitution of x>0.3. It is thought that the oxygen vacancy sites of perovskite oxide increase with an increase of amount of Cs and the oxidation activity also increases. This result is also verified by the TPR result of these catalysts(Fig. 3). As shown in Fig. 3, the reduction peak appears at low temperature with an increase of x value and no change is shown at more than x=0.3. It can thus be concluded that the catalytic performance of these oxides increases as the amount of Cs in the crystal lattice increases. However, the substitution of Cs to more than x=0.3 leads to excess Cs, which is present on the surface of mixed oxides might have no effect on the catalytic activity... 

Double Substitution In such processes, two substitutions take place simultaneously. For example, in perovskite oxides, La may be replaced by Sr at the same time as Co is replaced by Fe to give solid solutions Lai Sr Coi yFey03 5. These materials exhibit mixed ionic and electronic conduction at high temperatures and have been used in a number of applications, including solid oxide fuel cells and oxygen separation. 

Acceptor doping in perovskite oxides gives materials with a vacancy population that can act as proton conductors in moist atmospheres (Section 6.9). In addition, the doped materials are generally p-type semiconductors. This means that in moist atmospheres there is the possibility of mixed conductivity involving three charge carriers (H+, O2-, and h ) or four if electrons, e, are included. 

The perovskite oxides used for SOFC cathodes can react with other fuel cell components especially with yttria-zirconia electrolyte and chromium-containing interconnect materials at high temperatures. However, the relative reactivity of the cathodes at a particular temperature and the formation of different phases in the fuel cell atmosphere... 

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... 

Voorhoeve, RJH Johnson, DWJr Remeika, JP Gallagher, PK. Perovskite oxides materials science in catalysis. Science, 1997, Voulme 195, 827-833. 

Crespin, M. and Hall, W. K. The surface chemistry of some perovskite oxides. J. Catal, 1981, Volume 69, Issue 2, 359-370. 

Alifanti, M Auer, R Kirchnerova, J Thyrion, F Grange, P Delmon, B. Activity in methane combustion and sensitivity to sulfur poisoning of LauxCexMni.yCoyOs perovskite oxides. Appl. Catal, B Environmental, 2003, Volmne 41, Issues 1-2, 71-81. 

Equation 13 can be solved numerically for Tc as a function of the proton-lattice coupling. The parameters are chosen so as to fit the experimental value of Tc for KDP. For C = 21 732 K/A and g2ygAyf close to those used for perovskite oxides, Tc Ikdp = 115 K. In Fig. 3 Tc is shown as a function of C with all other parameters fixed. Including the deuteration effects (Table 2), Ter = C Idkdp/C Ikdp 1 2. With this estimate TcIdkdp = 168 K. C itself depends only weakly on /, g2y g4 but a strong dependence on/ is observed, which is the coupling between the PO4 shells and the K" " ions. This, on the other hand, should not be dependent on deuteration. 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

One of the best-characterized perovskite oxides with ordering of anion vacancies is the brownmillerite stmctme exhibited by Ca2Fe205 and Ca2FeA105 (Grenier et al, 1981). The compositions could be considered as anion-deficient perovskites with one-sixth of anion sites being vacant. The orthorhombic unit cell of the brownmillerite structure (a = 5.425, b = 5.598 and c = 14.768 A for Ca2Fe205) arises because of vacancy-ordering and is related to the cubic perovskite as a - c-... 

We have listed important perovskite oxides containing B-site transition-metal atoms... 

Figure 6.18 Perovskite oxides containing transition-metal ions in different spin configurations. Oxides are grouped into various regions on the basis of transfer energy b. (Following Goodenough, 1971.)...

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