Lanthanum perovskites

The desorption of CO (m/e=28) and CO2 (m/e=44) was recorded by MS during the CO-TPD experiments for La(Co, Mn, Fe)i i(Cu, Pd)i03 samples. A quantitative analysis of the various carbonaceous gases desorbed from perovskites is summarized in Table 7. Both CO and CO2 desorptions were observed indicating that the oxidation of CO into CO2 occurs over those lanthanum perovskites during CO-TPD experiments. 

Perovskite-type phases can form when small cations and large cations combine in a mixed oxide. The surface, however, appears to be largely dominated by the large cations and basic oxide anions, as in the cases of BaTi03 [173], SrTi03 [171] and several lanthanum perovskites [174]. 

The frequency-swept technique has also been used to determine the " La NMR spectra of the lanthanum perovskite compounds LaCrOj, LaMnOa and LaCoOs (Bastow 1994). LaCr03 has an orthorhombic structure with a La NMR spectrum showing a well-defined second-order quadrupolar lineshape (Figure 10.22B). By contrast, the La NMR spectrum of orthorhombic LaMnOs (Figure 10.22C) shows an... 

Doping the lanthanum perovskites with a divalent metal cation such as Sr produces... 

This paper deals with the characterization and catalytic activity of cerium substituted lanthanum perovskites Lai-xCexBOs (B = Ti, Cr, Mn, Fe, Ni, Co),x, varying from 0 to 0.6 [10]. 

Mn. Ni andFe are attractive choices as their respective lanthanum perovskites show CH4 conversion at lower temperatures than LaMnOs [54]. Also, Ni is used as the oxidation catalyst in traditional Ni/YSZ anodes. Although large amounts of Co, Fe, or Ni substitution are unstable [56, 61], small amounts of Ni (10 mol%) were suggested to be kinetically stable in the LaCrOa lattice [73]. Furthermore, the use of relatively small amounts of Co, Fe, or Ni, even if exsoluted, is not expected to lead to the mechanical failure that is caused by large amounts of especially Ni, as carbon formation and volume changes upon redox cycling will be limited. 

Up to now, the most promising results were obtained with Co insertion in B sites of lanthanum perovskite materials [11]. While the structural properties are preserved under reaction conditions up to 900 °C under hmnid atmosphere. 

Another application is in tire oxidation of vapour mixtures in a chemical vapour transport reaction, the attempt being to coat materials with a tlrin layer of solid electrolyte. For example, a gas phase mixture consisting of the iodides of zirconium and yttrium is oxidized to form a thin layer of ytnia-stabilized zirconia on the surface of an electrode such as one of the lanthanum-snontium doped transition metal perovskites Lai j.Srj.M03 7, which can transmit oxygen as ions and electrons from an isolated volume of oxygen gas. 

The structure of the perovskite-type lithium ion conductor Li0 29La0 57Ti03 is represented in Fig. 6. The small gray circles depict the lithium ions, the big gray circles the lanthanum ions. These are randomly distributed over the A sites 14 per-... 

Figure 6. Structure of the perovskite-type lithium-ion conductor Li 2yLa057TiO3. The lithium ions (small, gray) and the lanthanum ions (large, gray) are randomly distributed over the A sites, of which 14 percent are vacancies, enabling the lithium ions to be mobile. Titanium forms TiOh octahedra, as shown in yellow. The unit cell is indicated.

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

O Connell M, Norman AK, Huttermann CF, Morris MA (1999) Catalytic oxidation over lanthanum-transition metal perovskite materials. Catal Today 47 123-132... 

Baythoun, M. S. G Sale, F. R. 1982. Production of strontium-substituted lanthanum manganite perovskite powder by the amorphous citrate process. /. Mat. Sci. 17 2757-2769. 

At the other end of the conduction spectrum, many oxides have conductivities dominated by electron and positive hole contributions to the extent that some, such as Re03, SnC>2 and the perovskite LaCrCb have conductivities at the level of metallic conduction. High levels of p-type semiconduction are found in some transition metal perovskites especially those containing alio-valent ions. Thus the lanthanum-based perovskites containing transition metal ions, e.g. LaM03 (M-Cr, Mn, Fe, Co, Ni) have enhanced p-type semiconduction due to the dependence of the transition metal ion valencies on the ambient... 

The superconducting oxides include both perovskites and Ruddlesden-Popper compounds which have an orthorhombic arrangement of cubic cells, alternatively of the perovskite and sodium chloride structures. The common feature of all of these is the presence of copper as a major component. The first ceramic superconductor was a lanthanum-strontium substituted cuprate (Lai Sr Cu04 z), which is a perovskite, but subsequently the inter-oxide compound Y203 2BaO 3CuO, commonly referred to as a 123 compound, was shown to have superior performance. The speculation concerning the conduction mechanism is that this involves either Cu3+-Cu2+ positive hole... 

Oxygen Tracer Diffusion Coefficient (D ), Oxygen Surface Exchange Coefficient (Ar) and Oxygen Ionic Conductivity (a, in air) of Doped Lanthanum Cobalt Ferrite-Based Perovskites... 

Mn activity is especially high when there is deficiency at the A-site in perovskites. The diffusion of Mn into YSZ causes Mn depletion of the LSM and in the case of stoichiometric LSM chemically active La203 is formed, leading to the formation of La2Zr207 at the interface. This is supported by the observation of the formation of pyrochlore in 75%YSZ/25%LSM and not in 20%YSZ/80%LSM mixtures sintered at 1300°C for 2 h [166], Excess of Mn in LSM/YSZ could delay or impede the formation of La203 species at the interface. On the other hand, there is hardly detectable La in the YSZ, indicating very low solubility of lanthanum in YSZ. 

Lanthanum chromite is a p-type conductor so divalent ions, which act as electron acceptors on the trivalent (La3+ or Cr3+) sites, are used to increase the conductivity. As discussed above, the most common dopants are calcium and strontium on the lanthanum site. Although there is considerable scatter in the conductivities reported by different researchers due to differences in microstrucure and morpohology, the increase in conductivity with calcium doping is typically higher than that with strontium doping [4], The increase in conductivity at 700°C in air with calcium additions is shown in Figure 4.1 [1, 2, 28-44], One of the advantages of the perovskite structure is that it... 

Tai TW and Lessing PA. Modified resin-intermediate of perovskite powders. Part II. Processing for fine, nonagglomerated strontium-doped lanthanum chromite powders. J. Mater. Res. 1992 7 511-519. 

To meet the requirements for electronic conductivity in both the SOFC anode and cathode, a metallic electronic conductor, usually nickel, is typically used in the anode, and a conductive perovskite, such as lanthanum strontium manganite (LSM), is typically used in the cathode. Because the electrochemical reactions in fuel cell electrodes can only occur at surfaces where electronic and ionically conductive phases and the gas phase are in contact with each other (Figure 6.1), it is common... 

Single-phase perovskite MIECs such as Sr-doped lanthanum cobaltite (LSC), lanthanum ferrite (LSF), lanthanum cobalt ferrite (LSCF) and samarium cobaltite (SSC), and Ca-doped lanthanum ferrite (LCF) [13] are sometimes used alone in SOFC cathodes, as depicted in the lower right-hand comer of Figure 6.1, but combining an... 

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