Palladium-alumina cryogel was prepared from palladium nitrate and aluminum sec-butoxide through a sol-gel processing and subsequent freeze drying. The cryogel showed higher thermal stability of palladium and catalytic CO oxidation activity than the corresponding xerogel and impregnation catalysts. The superior stability and activity were ascribed to the uniform distribution of palladium ions in the boehmite gel by the one-pot preparation of palladium-boehmite co-gel, and also to the suppression of transfer and aggregation of the metal during the subsequent freeze drying. As a result palladium ions were finely distributed and stabilized throughout the alumina cryogel support, which consequently suppressed the sintering of palladium at elevated temperatures.
In order to improve thermal stability and catalytic oxidation activity of platinum-alumina cryogels prepared by one-pot sol-gel reactions and subsequent freeze drying, molar ratio of aluminum sec-butoxide, nitric acid, and water used for the sol-gel processing was optimized. It was found that the sintering of platinum at high temperatures was more remarkably suppressed at ASB:HNO 3 :H 2 O = 1:0.17:87, at the ratio of which higher CO oxidation activity was also observed. It was suggested that the suppression of sintering of the metal was attributed to smaller sizes of mesopores obtained at this ratio, retarding transfer and aggregation of platinum.
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