Comparison of Nanocrystalline and Polycrystalline Oxide Supports for Catalytic Oxidation of Methane
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Darmstadt University of Technology, FB 21-Materials Science Department, FG Thin Films, Petersenstr. 23, 64287 Darmstadt, Germany 2
Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA, fax: 001-352-392-9513
ABSTRACT Nanocrystalline and polycrystalline ZrO2, CeO2 and Mn304 without and with 5 wt% Pd have been tested for methane oxidation. The nanocrystalline catalysts perform better than the polycrystalline catalysts on a weight basis. Nanocrystalline (n)-CeO2 performs much better than nZrO2, and both perform much better than polycrystalline (p)-CeO2 and p-ZrO2. 5 wt% Pd/n-ZrO2 is the best catalyst tested, but 5 wt% Pd/n-CeO2 performs nearly as well. A 20 wt% Ag/p-ZrO2 and a 20 wt% Ag/n-ZrO2 catalyst were also tested. The Ag yields a significant improvement over the bare supports, but not as much so as Pd. 1.
INTRODUCTION
Methane is a powerful greenhouse gas and must be removed from the exhaust streams of natural-gas engines, but it is also the most difficult paraffimic hydrocarbon to oxidize catalytically due to the absence of C-C bonds. In vehicles which operate using natural gas, this removal can be accomplished using a catalytic converter equipped with the proper catalyst material. The use of Pd/A12 0 3 catalysts in CH4 oxidation has received the most attention, but they require high operating temperatures and often exhibit an unacceptable decay rate. Some studies have tested Pdcontaining catalysts on supports other than A120 3 (1-4). The results of these studies indicate that it may be possible to develop improved methane oxidation catalysts through the use of other supports such as ZrO2 (3,4) CeO 2 , Mn 3 0 4 , SnO2 and Co 3 0 4 (4). Based on these findings, Pd supported on polycrystalline (p-) ZrO 2 , p-Mn3 0 4 and p-CeO 2 have been tested for CH 4 oxidation. Furthermore, due to the recent advancements in the preparation of nanocrystalline materials, their use as catalyst supports is also of interest and could lead to higher activities in catalytic processes due to their unique chemical and electronic properties. Therefore, Pd was also supported on nanocrystalline (n-) ZrO 2 , n-Mn 30 4 and n-CeO 2 supports and each was tested for CH4 oxidation also. Ag supported on p- and n-ZrO2 were also examined in order to determine if Ag could be used rather than the more costly Pd.
35 Mat. Res. Soc. Symp. Proc. Vol. 497 © 1998 Materials Research Society
2. EXPERIMENTAL Catalyst Supports The nanostructured oxides used in this study were synthesized using the gas condensation method. In this process metals, suboxides or oxides are evaporated from a Joule-heated refractory crucible in a He atmosphere of 5-20 mbar. Nanoparticles, which are formed by homogeneous nucleation in the supersaturated vapor phase close to the crucible, are transported by a convective gas flow toward a LN 2 -cooled stainless-steel plate where they are collected by thermophoretic forces. To prepare the nanocrystalline Mn 3 0 4 , ZrO2 and CeO 2 ; Mn metal, ZrO and CeO2 were evaporated respectively. The powders we
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