Freeze-drying, perovskite preparation

Wachowski et al. (42,43) have compared the surface areas of a series of eight perovskites prepared by different methods ceramic (<2.4 m2/g), coprecipitation as oxalates (4.5-11 m2/g), explosion (21-37 m2/g), and freeze-drying (22-39 m2/g). Again, surface areas clearly depended on the minimum temperature necessary for complete reaction. The greatest losses in surface area by sintering were observed in the temperature range 700-930°C. 

J. Kirchenova, D. Kwana, J. Vaillancourt, J. Chaouki, 1993, Evaluation of some cobalt and nickel based perovskites prepared by freeze-drying as combustion catalysts, Catal. Lett., 21, 1-2,77-87. 

Following is an example for the preparation of NdCoOs perovskite by freeze-drying method [30] 0.005 mol neodymium sesquioxide was first dissolved in 5 rrrl of concentrated nitric acid, and then added to a solution of 0.01 mol of cobalt (II) acetate in 25 ml of water. The pH of the resulting solution was adjusted to 4.5 by addition of dilute ammorria. Finally, the resulting reddish solution were frozen in liquid N2 and transferred into a freeze-dryer operated at a pressure of 7.6 torr. The resulting blue-pink powder was placed into an alurttina boat and pretreated at 400 °C for 2 h under vacuum. The "ashes" were finally groimd and treated for 24 h under a dynamic oxygen atmosphere at 700 and 900 °C. 

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