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Palladium-supported alumina aerogels were prepared by two different supercritical drying methods. In one method, an alumina wet gel was dried under supercritical conditions of ethanol in an autoclave. In the other, the aerogel was supercritically dried by extracting ethanol using carbon dioxide in an extractor. The Pd-supported alumina aerogel prepared in the autoclave exhibited a high specific surface area of 112.8 m 2 /g after firing at 1200 °C for 5 h, while the other had a specific surface area of only 5.2 m 2 /g due to ff-alumina transformation. Their catalytic properties for methane combustion were measured. The Pd-supported alumina aerogel prepared in the autoclave combusts methane perfectly at 50-60 °C lower temperature than the other. Palladium particles on the alumina aerogel prepared in the autoclave contained palladium oxide, while those prepared in the CO 2 extractor contained only palladium metal.

I. INTRODUCTION Several catalyst-supported alumina aerogels have previously been studied, mainly due to their high surface area and pore volume.1"5 The present authors have also prepared alumina aerogels and reported that the alumina aerogels maintained a porous structure at higher temperatures over 1000 °C.6'7 In the present study, Pdsupported alumina aerogels are prepared by two different methods. 78 In one, Pd-supported alumina aerogels are prepared in an autoclave at 270 °C and 26.5 MPa, while in the other, the solvent of the alumina gel is supercritically extracted using carbon dioxide (CO2) in a CO 2 extractor at 80 °C and 15.7 MPa.7 The alumina aerogel catalysts were fired at 1200 °C for 5 h, and the activities for methane combustion were evaluated. Heat resistance of the catalyst is very important in combustion reactions.9"11

[Al(OBu sec ) 3 : H 2 O = 1:3], and the Al(OBusec)2-etac complex was hydrolyzed to an alumina sol. Palladium chloride (PdCl2) was added to the hydrolyzed alumina sol along with pyridine, making palladium chloride and pyridine complex in the sol.8 This complex prevented precipitation of Pd metal during the aging process. Moreover, the pyridine caused the acceleration of the gelation of the alumina sol and the structure of the

Ai[O(CH3)CHC2H5]3

—

Ethyl acetoacetate (EAA) Ethanol

AI[O(CH3)CHC2H5]2-EAA

— HzO (hydrolysis)

II. EXPERIMENTAL Figure 1 shows the flow chart of procedure to synthesize the Pd-supported alumina aerogels. Aluminum sec-butoxide [Al(OBusec)3] and ethyl acetoacetate (etac) were mixed, with the reaction yielding an Al(OBu sec ) 2 etac complex. The preparation was conducted at room temperature to control the speed of hydrolysis [Al(OBu sec ) 3 : etac = 1:1]. 12 Ethanol was added as a solvent. Water diluted by ethanol was gradually added

Alumina sol

— PdCl2, Pyridine Gellation and aging (60^;, 7 days)

Pd-supported alumina wet gel

Supercritical drying in autoclave ( 2 7 0 1 , 26.5 MPa)

Supercritical extraction byCO2 (80 ^C, 15.7 MPa)

a)

Present address: Nagoya Division, Superconductivity Research Laboratory, International Superconductivity Technology C