A kinetic and DRIFTS study of supported Pt catalysts for NO oxidation
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Catalysis Letters Vol. 110, Nos. 1–2, August 2006 (Ó 2006) DOI: 10.1007/s10562-006-0100-4
A kinetic and DRIFTS study of supported Pt catalysts for NO oxidation Yaying Ji,a Todd J. Toops,b Uschi M. Graham,a Gary Jacobs,a and Mark Crockera,* a Center for Applied Energy Research, University of Kentucky, 2540, Research Park Drive, Lexington, KY 40511, USA Fuels, Engines and Emissions Research Center, Oak Ridge National Laboratory, 2360, Cherahala Blvd., Knoxville, TN 37932-1563, USA
b
Received 8 March 2006; accepted 23 May 2006
NO oxidation was studied over Pt/CeO2 and Pt/SiO2 catalysts. Apparent activation energies (Ea) of 31.4 and 40.6 kJ/mole were determined for Pt/CeO2 and Pt/SiO2, respectively, while reaction orders for NO and O2 were fractional and positive for both catalysts. Pre-treatment of the catalysts with SO2 caused a decrease in the Ea values, while the reaction orders were only slightly changed. In situ DRIFTS measurements indicated that high concentrations of nitrate species were formed on the surface of Pt/CeO2 during NO oxidation, while almost no surface species could be detected on Pt/SiO2. The addition of SO2 resulted in the formation of a highly stable sulfate at the expense of nitrate species and caused an irreversible loss of catalytic activity for Pt/CeO2. KEY WORDS: nitric oxide; oxidation; platinum; ceria; silica.
1. Introduction The oxidation of NO to NO2 over supported Pt catalysts plays a key role in a number of environmental processes, including NOx abatement using NOx storagereduction (NSR) catalysts [1], the selective catalytic reduction (SCR) of NOx using ammonia as the reductant [2], and the oxidation of soot in catalyzed diesel particulate filters (DPFs) [3]. While Pt is the metal of choice due to its unparalleled activity, the nature of the support material and the Pt dispersion also significantly affect catalyst performance. Studies comparing the properties of Pt on different supports show a clear trend with respect to catalyst activity in NO oxidation,SiO2 Al2 O3 ZrO2 [4–6]. This ordering can be rationalized on the basis that NO is adsorbed principally on the support and then migrates to Pt where it is oxidized [5]. Given that NO is weakly adsorbed on silica, and that NO2 is not stored as nitrates, the use of silica as support favors rapid migration of adsorbed NO to Pt, followed by oxidation and facile desorption of NO2. A complicating factor is the dispersion of the supported Pt. Several studies have shown that the rate of NO oxidation on Pt/SiO2 [4,5,7] and Pt/Al2O3 [5,8,9] shows a marked particle size dependency, larger Pt particles exhibiting higher specific activity than smaller ones. Hence, catalyst activity for NO oxidation is likely to be a function of both these effects. To date the majority of kinetic and mechanistic studies have focused on Pt/Al2O3, Pt/BaO/Al2O3 and Pt/SiO2 catalysts [4–17], based on their potential use in automotive applications. In contrast, there have been few reports concerning the properties of Pt supported on *To whom correspondence should be addre
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