Perovskites composition

Simner SP, Anderson MD, Templeton JW, and Stevenson JW. Silver-perovskite composite SOFC cathodes processed via mechanofusion. J. Power Sources 2007 168 236-239. 

Clearly, not all these perovskite compositions are useful for oxygen delivery applications. For example, ceramics based on Laj.xAxCrOj (x == Sr, Ba, Ca), Lai, rxCri yMny03.5 and Lai xCaxCri yCOy03 have been proposed for use as interconnection material (separator) in solid oxide fuel cells (SOFC), and therefore should be dense and impermeable in order to prevent burning off of the fuel without generating electricity [137,138]. 

Selected perovskite compositions are also targeted in basic SOFC research for use as potential electrode material for the cathodic reduction of oxygen. The most promising cathode materials to date are the manganites Lai rxMn03 [137,138]. The composition with x = 0.15 scarcely permeates oxygen up to 900°C, as was measured by feeding air emd helium to opposite sides of a dense sintered membrane of 1 mm thickness [136]. The observed behaviour is consis-... 

The precise perovskite composition may be tailored for a specific application. To obtain a high performance membrane, however, many technical and material problems remain to be solved. This final section will focus on several issues, which are not yet well understood, but are thought to be of importance for further development of the membrane devices. 

Perovskites of the type LnMO, (Ln = lanthanide and M = transition metal) may offer interesting features as precursors for supported metal catalysts. For example, careful reduction can be carried out in order to produce a finely dispersed transition metal over the sesquioxides Ln,0,. Also, the flexibility in the perovskite composition allows the preparation of compounds of the type LnM ., M 0, (M and M = different transition metals) or Ln., A,MO, (A - for example an alkaline or alkaline earth metal), which show the unique possibility as precursor of producing well dispersed bimetallic catalyst or doped metal catalyst. 

Perovskite composition Surface area (m2g- ) CH4 conversion (%) CO selectivity (%) H2/CO ratio... 

A series of perovskite compositions were synthesized using oxides and carbonates of the cations by conventional ceramic process. The synthesized powders were characterized using powder x-ray diffraction technique to ensure phase purity. Conductivity measurements were made in H2-H2O atmosphere to determine proton conductity. As the perovskite compositions are inherently mixed conducting, the transference numbers for proton and electron conduction were also determined by varying the partial pressures of hydrogen and steam across the membrane. 

Two compositions of the composite materials were evaluated for stability in syngas. Sintered pellets of the composite were crushed to powder form to increase the surface area of the material. The powdered composites were exposed to syngas at 900°C for 4 hours. A baseline perovskite composition was also exposed to similar conditions. X-ray diffraction analysis of the materials is shown in Fig. 4.5. As can be seen, while the baseline perovskite readily formed a carbonate phase, the composite materials showed significantly improved stability in syngas. 

Here the narrow prescription of Chapter 1 is widened to deal with more chemically complex phases, in which the materials may contain mixtures of A, B and X ions as well as chemical defects. In these cases, using an ionic model, it is only necessary that the nominal charges balance to obtain a viable perovskite composition. In many instances these ions are distributed at random over the available sites, but for some simple ratios they can order to form phases with double or triple perovskite-type unit cells. The distribution and valence of these ordered or partly ordered cations and anions are often not totally apparent from difEraction studies, and they are often clarified by use of the bond valence sums derived from experimentally determined bond distances. Information on the bond valence method is given in Appendix A for readers unfamiliar with it Point defects also become significant in these materials. The standard Kroger- fink notation, used for labelling these defects, is outlined in Appendix B. 

With optimized perovskite compositions even much higher fluxes in the range of 6 to 7 cm /cm min could be obtained recently [306]. The oxygen flux is not a linear inverse relationship to membrane thickness, because for thin membranes (about 0.3 mm) surface reactions become rate limiting. But even... 

To this end, I invited an international team of highly expert scientists from the field of membrane science and technology to write about the state-of-the-art of the various kinds of membranes (polymeric, Pd- and non-Pd-based, carbon, zeolite, perovskite, composite, ceramic and so on) used in membrane reactors, modelling aspects related to all kinds of membrane reactors, the various applications of membrane reactors and, finally, economic aspects. 

Efforts to optimize the electrochemical performance of the cathode at reduced temperatures (500-800 C) have led to the exploration of alternative perovskite compositions,... 

Table 14.3 compiles a selection of published results on O2 dififusion and surface exchange coefficients in perovskites and perovskite membranes. The last two rows of the table include H2 diffusion and surface exchange coefficients. As can be inferred from Table 14.3, the vacancy diffusion in perovskites falls into the range of 2 X 10 -2 x 10 m /s, whereas the surface exchange coefficients show a broad variety of values depending on the perovskite composition and on the presence of secondary phases. 

As already mentioned, one of the principal aims of using perovskite-type oxides as oxidation catalysts is to avoid the use of expensive and low thermally stable noble-metal-supported catalysts (e.g., Pt/Al203). Thus, by modifying the perovskite composition using alkaline earth metal or metal transition cations (A- and/ or B-substituted materials) and by improving their textural and defective properties during the synthesis process, it is possible to obtain perovskite-based catalysts as active as noble-metal-supported ones for the low-temperature CO oxidation reaction. Nevertheless, some authors have recently claimed the... 

There is clearly an influence of the perovskite composition (x) on the reduc-ibility and the linear increase of hydrogen consumption with during the first... 

FIGURE 4-11 Mechanism of methane internal reforming and H electrooxidation at the YSZ electrolyte-Ni-ABOj perovskite composite anode interface. 

Selected perovskite compositions are also targeted in basic SOFC research for nse as potential electrode material for the cathodic reduction of oxygen. The most promising cathode materials to date are the manganites Laj The composition with x= 0.15... 

Since the initial observations by Teraoka et al., " a considerable number of studies have appeared. Selected perovskite compositions have been reexamined, while a few others have... 

Another possibility is to use perovskite compositions. The most promising candidate at this time is LaGaOj doped with Sr and Mg. Other possible perovskites are Ba2 In205 doped with Ce or La. Other ionic conducting oxides have also been found including La ioSi6026 composition. Proton conductors such as SrCeOs, SrZrOs or BaCeOs doped with Y or Yb may also be effective electrolytes, but reaction with CO2 has to be resolved first. 

The perovskite compositions contain several dopants to obtain an optimum for electronic and ionic conductivity and thermochemical stability at operational conditions. 

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