Perovskite surface area

O2 and H2 dissociation kinetics are better at higher temperatures (>400 °C), low-cost electrode structures of high surface area Ni and oxides such as spinels or perovskites to replace the very effective, but costly, Pt catalysts have been sought. 

Three LaCoOs samples (1,11, and 111) with different specific surface areas were prepared by reactive grinding. In the case of LaCoOs (1), only one step of grinding was performed. This step allowed us to obtain a erystalline LaCoOs phase. LaCoOs (11) and LaCoOs (111) were prepared in two grinding steps a first step to obtain perovskite crystallization and a second step with additive to enhanee speeific surface area. The obtained compounds (perovskite + additive) were washed repeatedly (with water or solvent) to free samples from any traee of additive. The physical properties of the three catalysts are presented in Table 10. LaCoOs (1) was designed to present a very low specific surface area for comparison purposes. NaCl used as the additive in the case of LaCoOs (11) led to a lower surface area than ZnO used for LaCoOs (111), even if the crystallite size calculated with the Sherrer equation led to similar values for the three catalysts. The three catalysts prepared were perovskites having specific surface areas between 4.2, 10.9 and 17.2 m /g after calcination at 550 °C. A second milling step was performed in the presence of an additive, yielding an enhanced specific surface area. 

Dl, particle size calculated from the Sherrer equation Sjh, specific surface area calculated assuming cubic particles and a density of LaCo03 equal to 7.29 D2, equivalent cubic particle size calculated from BET surface area. P, perovskite C, cobalt oxide. 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

Two series of catalysts were synthesized for subsequent evaluation as methane dimerization catalysts. The first series was alkali modified zinc oxide (6) and magnesium oxide catalysts (7), which were reported to be active for methane activation, while the second series was ion modified perovskites described by Machida and Enyo (8). The objective of the present study was to determine whether the aerosol technique could provide a wide range of ion substitutions as homogeneous solid solutions, and to determine whether moderately high surface area catalysts could... 

Fig. 7.111. Current density (based on real surface area) for oxygen evolution on perovskites at an overpotential of 0.3 V vs. M—OH bond strength. The transition-metal ions (M) in perovskites are indicated with different symbols. (Reprinted from J. O M. Bock-ris and T. Ottagawa, J. Electrochem. Soc. 131 2965,1984. Reproduced by permission of The Electrochemical Society, Inc.)...

Table 10. Specific surface area of perovskite-type catalysts, prepared by the citrate process and the acetate process (adapted from Ref. 41).

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