Oxide perovskite-type

Perovskite-type oxides are well known oxidation catalysts in the gas phase [259]. In 1970, Meadowcroft suggested that LaCo03 doped with strontium was a less expensive alternative to platinum for air cathodes in alk- 

In a series of papers, Matsumoto et al. [86, 280, 371-374] reported the results of extensive studies of the electrocatalysis of oxygen reduction on LaNi03 and related perovskite oxide electrodes from the viewpoint of their bulk solid state structural and electronic properties. The main conclusions of their studies are 

Electrocatalytic activity, as reflected in j0, decreased with increase in the oxide resistivity and a relation between ja and the resistivity has been derived theoretically 

The study of the electrochemical behavior of peroxide has indicated two possible routes of elimination at the LaNi03-aqueous interface [81] electrochemical dismutation at low overvoltage and electroreduction at high overpotentials. Table 4 shows a comparison of electrokinetic data for several perovskite oxides. 

Kinetic data for the reduction of oxygen on various perovskite oxide electrodes 

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Properties and Applications of Perovskite-Type Oxides, edited by L. G. Tejuca and J. L. G. Fierro... 

Catalytic removal of diesel soot particulates over LaMnOs perovskite-type oxides... 

Catalytic combustion of diesel soot particulates over LaMnOs perovskite-type oxides prepared by malic acid method has been studied. In the LaMn03 catalyst, the partial substitution of alkali metal ions into A site enhanced the catalytic activity in the combustion of diesel soot particulates and the activity was shown in following order Cs>K>Na. In the LarxCs MnOj catalyst, the catalytic activity increased with an increase of X value and showed constant activity at the substitution of x>0.3... 

Several researchers have focused their attention on the application of oxide materials to lower the oxidation temperature of soot particulates. It was reported that active soot oxidation catalysts are PbO, C03O4, V2O5, M0O3, CuO, and perovskite type oxides[3]. 

In this paper, we prepared LaMnOa perovskite-type oxides using the malic acid method and investigated their physical properties. It has been also investigated the effect of partial substitution of metal iorrs into La and Mn sites and the reaction conditions on the activity for the combustion of soot particulates. 

The preparation method of perovskite-type oxides was taken from the previous paper[4]. Malic acid was added into mixed aqueous solution of metal nitrates in a desired proportion so as for the molar ratio of malic... 

Table 1. Perovskite-type oxides prepared by malic acid method and their catalytic performances...

Fig. 1. TG spectra of carbon particulates with Fig. 2. TPR profiles measured for various Lao.gCso MnOj catalyst heating rate=l K/min. perovskite type oxides heating rate=10 K/min,...

Fig. 2 shows the temperature as a function of irradiation time of Cu based material under microwave irradiation. CuO reached 792 K, whereas La2Cu04, CuTa20e and Cu-MOR gave only 325, 299 and 312 K, respectively. The performances of the perovskite type oxides were not very significant compared to the expectation from the paper reported by Will et al. [5]. This is probably because we used a single mode microwave oven whereas Will et al. employed multi-mode one. The multi-mode microwave oven is sometimes not very sensitive to sample s physical properties, such as electronic conductivity, crystal sizes. From the results by electric fixmace heating in Fig. 1, at least 400 K is necessary for NH3 removal. So, CuO was employed in the further experiments although other materials still reserve the possibility as active catalysts when we employ a multi-mode microwave oven. 

Kaliaguine, S., Van Neste, A., Szabo, V. et al. (2001) Perovskite-type oxides synthesized by reactive grinding Part I. Preparation and characterization, Appl. Catal. A 209, 345. 

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Square Concentration Pulses in Oxidation Catalysis over Perovskite-type Oxides of Manganese and Iron... 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

The activities of the perovskite-type oxides are strongly dependent on pretreatment in reducing or oxidizing atmospheres at a.600°C. This was found for other perovskite catalysts as well (1). Reducing pretreatments lead to more active catalysts (Figures 5 and 6). The reason for this is not known, but better binding of CO to the reduced surface is a possible explanation. 

The enthalpies of formation of selected perovskite-type oxides are given as a function of the tolerance factor in Figure 7.17. Perovskites where the A atom is a Group 2 element and B is a d or / element that readily takes a tetravalent state [19, 20] show a regular variation with the tolerance factor. Empirically, it is suggested that the cations that give t close to 1 have the most exothermic enthalpies of formation. When t is reduced, the crystal structure becomes distorted from cubic symmetry and this also appears to reduce the thermodynamic stability of the... 

Perovskites, 27 358 band structure, 38 131-132 crystal structure, 38 123-125 Perovskite-type oxides see also specific lanthanum-based catalysts actinide storage in radioactive waste, 36 315-316... 

Fereshteh, R., Caroline, S., James, A. F., 2002. Sphalerite activation and surface Pb ion concentration. Inter. J. Miner. Process, 67 43 - 58 Fierro, R. E., Tryk, D., Scherson, D., Yeager, E., 1988. Perovskite-type oxides oxygen electrocatalysis and bulk structure. Journal of Power Sources, 22 (3 - 4) 387 - 398... 

Figure 5. Time-averaged structure of a protonic defect in perovskite-type oxides (cubic case), showing the eight orientations of the centrai hydroxide ion stabiiized by a hydrogen-bond interaction with the eight next-nearest oxygen neighbors. ...

From the thermodynamics of such dynamical hydrogen bonds , one may actually expect an activation enthalpy of long-range proton diffusion of not more than 0.15 eV, provided that the configuration O—H "0 is linear, for which the proton-transfer barrier vanishes at 0/0 distances of less than 250 pm. However, the mobility of protonic defects in cubic perovskite-type oxides has activation enthalpies on the order of 0.4—0.6 eV. This raises the question as to which interactions control the activation enthalpy of proton transfer. 

Figure 16. Self-diffusion coefficient of oxide ion vacancies in different perovskite-type oxides,which equals the chemical water diffusion coefficient in the fully hydrated state (see text). (Figure reproduced with the kind permission from Elsevier.)...

The special electric, magnetic, optical, superconductive and catalytic properties of perovskite-typed oxides make this group of materials attracting and widely used. Perovskites were named according to the similarity of their structure with the CaTiOs compoimd. The... 

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