Perovskite-type materials

The changes in non-stoichiometry and point defects of solid perovskite (BaTiOs) at 900°C can be observed with Raman spectroscopy (51). The method is believed to be more sensitive than the neutron scattering technique and has become the standard in determining stoichiometric information for solid materials. The interest in perovskite-type materials stems from their use in solid-state capacitors. 

A combustion catalyst must thus simultaneously fulfill requirements of high activity at combustor inlet conditions and high stability at the maximum temperatures occurring in the catalytic combustor. Unfortunately, these are contradictory demands This was demonstrated by McCarty and Wise [551. Figure 6, taken from their study, shows the relationship between methane oxidation activity and the stability of various perovskite-type materials (LaM03) The trade-off between activity and stability is clear. 

For a substituted perovskite-type material with oxygen vacancy-type defect structure, the overall formula can be written as Aj A Bi where A, A denotes... 

In particular, perovskites-type materials with composition (AgF)(MFy)j , were reported to be active catalysts in ethylene and propylene oxyfluorination [3]. However, the synthesis of the perovskite-type mixed fluoride [4,5] leads to the formation of other compounds, such as metal oxides or reduced metals (i.e., Ag°). For this reason, we tried to improve previous results with the aim of optimizing the catalyst synthesis. 

Table 12 shows the conversion of methane and selectivity to C2 for the layered perovskite-type materials using a co-fed mode. K LaaTisOio and K.2Pr2Ti30io presented the highest C2 yields, comparable to many of the results reported for active OCM catalysts. 

Xu, C. Kondo, T. Sakakura, H. Kumata, K. Takahashi, Y. Ito. R. Optical third harmonic generation in layered perovskite-type material (CioH2iNH3)2Pbl4. Solid State Cornmun. 1991, 79, 245. 

It is well known that if a solute differs in its atomic size by more than 15% from the host, then limited or no solubility is likely (Hume-Rothery, 1955). Perovskite-type materials crystallize with close-packed structures, whose main feature is a presence of frameworks of tilted and distorted octahedra. Even minor substitutions on the A-site would lead to strained octahedral frameworks, and further increase may result in its destruction. Therefore, in perovskites, the conventional 15% limit initially established for metals should be reduced to the level of 10% or even less. This agrees well with the observed value Ar 0.11 A. The analysis of the critical contents via the perovskite cell deformation reveals radius limitations at 1.213, 1.212,1.210,1.208, and 1.209 A corresponding to x = 0.06, 0.06, 0.04, 0.03, and 0.02 for Er, Ho, Y, Dy, and Tb-containing Lai j Rj Ga03, respectively. Observed radii corresponding to limitations in solubility for different Lai cR cGa03 (R = Tb-Er and Y) are very close to each other. 

Skinner, S.J. (2001). Recent Advances in Perovskite-type Materials for Solid Oxide Fuel Cell Cathodes. International Journal of Inorganic Materials, Vol. 3, (March 2001), pp. 113-121, ISSN 1466-6049... 

One of the extensively studied perovskite-type materials, LacSri cMnOs (LSM), is of special interest due to numerous applications, particularly as the cathode for solid oxide fuel cells [750]. 

To conclude, it is no doubt that HTMW method is a genuine technique for low temperatures and short reaction times however, it demands at the same time a multiparameter optimization studies to obtain a perovskite-type material with some desired properties. 

Reducibility of the perovskite-type materials correlates very well with the presence of the defects. For SrCoO, the substitution of Sr by lanthanides results in a better defined bimodal H2 consumption for the stepwise reduction of Co(III) to Co(II) at low temperatures and the formation of metallic Co(0) at high temperatures, which is also observed for the partially substituted perovskites and confirmed by quantitative analysis [16]. The chemical nature of the cation controls this process. During the first reduction step, the perovskite lattice is preserved. 

The following section describes the utilization of perovskite-type materials as CO oxidation catalysts, particularly emphasizing the effect of the materials composition on their catalytic efficiency. Aspects such as the participating oxygen species (adsorbed oxygen and/or bulk-mobile 0 species), surface redox properties, and the CO affinity of the considered catalysts will be discussed. 

In this section, the applications of perovskite-type materials in automotive exhaust catalysis are shortly presented. The latest advances in the field are included, with particular emphasis on structure-activity relationship. The section is devoted to two separate parts (a) application of perovkite oxides in model reactions related to three-way catalysis and (b) application of perovskite oxides under simulated or real exhaust conditions. 

Application of Perovskites in Bdiaust Emission Control S75 Table 25.4 Catalytic performance of perovskite-type materials in NO + CO reaction [64]. 

Skinner SJ (2001) Recent advances in Perovskite-type materials for solid oxide fuel cell cathodes. Int J Inorg Mater 3 113-121... 

Perovskite-type materials have also been investigated as cathodes for SOFCs. Lanthanide-based perovskites showed a high conductivity and a high catalytic activity for oxygen reduction. Applying a thin porous layer of YSZ particles on LSM electrodes also increased the performance as the polarization resistance is reduced. Especially for operating at lower temperatures (650-700 °C), it is important to have an efficient cathode [36]. 

The oxide ion conductor (Lao.8Sro.2)(Gao.8Mgo.i5Coo.o5)02.8. which has an ideal perovskite-type structure, exhibits diffusion paths along the [110], [110], [011], [Oil], [101] and [101] directions to form a three-dimensional network of equivalent diffusion pathways (Fig. 6.5(b)) [10]. In contrast, in the present double perovskite-type material Lao.64(Tio.92Nbo.o8)02.99, a two-dimensional diffusion pathway, by which 03 atoms migrate along the [110] and [110] directions (Fig. 6.7(b)), is present. This two-dimensional feature is attributable to the layered structure of the material, which consists of La-occupied Lal-Ol,... 

Fig. 8 Selected polarisation data for nickelate cathodes with YSZ and LSGM electrolytes higlighting the competitive performance compared to conventional perovskite type materials [2]. Reproduced here with kind permission from Elsevier 2011...

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