Study of Catalytic Methane Oxidation Over Pd Supported on Nanocrystalline CeO 2 : Effects of Calcination and Pd Loading
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STUDY OF CATALYTIC METHANE OXIDATION OVER Pd SUPPORTED ON NANOCRYSTALLINE CeO2: EFFECTS OF CALCINATION AND Pd LOADING SEUNG H. OH*, MICHAEL L. EVERETT*, AND GAR B. HOFLUND*, JOHANNES SEYDEL**, HORST W. HAHN** * Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA ** Darmstadt University of Technology, FB 21-Materials Science Department, Thin Film Division, Petersenstrasse 23, 64287 Darmstadt, GERMANY ABSTRACT The catalytic oxidation of methane was studied over palladium supported on nanocrystalline ceria. Three palladium weight loadings (1, 5, and 10 wt%) were tested after calcining at 500 °C, at 280 °C and after no calcination at all. For the 5 and 10 wt% loadings, the 280 °C-calcined and non-calcined catalysts exhibit enhanced activity after several heating and cooling cycles. Calcining these same catalysts at 500 °C results in a systematic decline in activity. For all pretreatments the 1 wt% Pd catalyst exhibits decreasing activity. For the 5 and 10 wt% Pd loadings, the non-calcined catalysts are more active than the 280 °C-calcined catalysts, which are more active than the 500 °C-calcined catalysts. For the 1 wt% Pd catalyst, the opposite is true. The catalyst activity improves as the Pd loading is increased. INTRODUCTION Natural gas has been a large focus in the search for alternative energy resources due to its abundance, environmental advantages, and its technological potential. Natural gas is more plentiful than oil, results in low NOx and SOx emissions and has high fuel efficiency. Also, recent federal emissions regulations have driven the automobile manufacturers to develop natural-gas-based engines. Natural gas does, however, consist primarily of methane, a more powerful greenhouse gas than carbon dioxide (1). It is therefore of interest to minimize the amount of methane present in the tail-gas stream by catalytic oxidation. The methane molecule is difficult to oxidize because it has stable H-C bonds with a bond energy of 439 kJ/mol and it is resistant to many reactants (2). Catalysis plays an important role in converting methane into the less harmful carbon dioxide. Conventional three-way catalysts, which have shown over 95% conversion of nitric oxide and carbon monoxide, exhibit a poor 15% conversion with methane. The reason for this is that the air/fuel ratio for methane oxidation is different than that used for gasoline (3, 4). Simulated methane exhaust conditions show that palladium/platinum catalysts give superior conversion compared to conventional three-way catalysts (5). Burch and Loader (6) have reported that Pd catalysts are superior for CH4 oxidation under oxygen-rich conditions, whereas Pt catalysts are better suited at stoichiometric or lower ratios (i.e. O2:CH4 = 2:1). Most methane oxidation catalysts that have been studied in the past used Al2O3 supports. Recently other support materials have been investigated, including ZrO2, Co3O4, Mn3O4, CeO2, and TiO2 (7, 8). Therefore, it is of interest to understand the various support effects on catalytic methane oxi
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