Structure activity relationships in catalysis studied using model catalysts

Andrew D Smeltz, Purdue University

Abstract

Fundamental understanding of interactions between chemical species and catalytically active surfaces is important to the development of the science of heterogeneous catalysis, which in turn is critical to the U.S. and global economies since about 90% of chemical manufacturing processes and more than 20% of all industrial products in the U.S. employ underlying catalytic steps. This work utilizes model catalysts which have well defined surface geometries and the ability to be directly accessed under reaction conditions using surface sensitive techniques to better understand the apparent structure activity relationship observed for the platinum catalyzed oxidation of NO to NO 2. Initial activities of Pt(321) and Pt(111), surfaces which are representative of small and large particles respectively, were the same to within a factor of two demonstrating that that NO oxidation is not a structure sensitive reaction. Global kinetic parameters for both surfaces were found to be functions of NO2:NO. Apparent activation energy and reaction orders for O 2 and NO increased with NO2:NO whereas the reaction order in NO2 decreased. Reaction orders for NO and NO2 at high NO2:NO indicate that O2 adsorption is occurring in a direct, dissociative fashion. Ex situ AES and XPS after reaction as well as in situ XPS experiments were used to determine the chemical identity and concentrations of reactive intermediate species. Atomic oxygen, [O*], was found to be the most abundant surface intermediate and controlled by the ratio of NO2:NO. Catalyst deactivation during long term stability studies on Pt(100) and Pt(111) was correlated to [O*] up to 1.4 ML, suggesting gradual platinum oxide formation was responsible for deactivation. This confirms that platinum is susceptible to oxidation during reaction and is responsible for the apparent structure activity relationship for NO oxidation. A discreet particle model was developed which utilizes results from in situ XAS, TEM, and kinetic studies on a wide variety of catalysts to better understand how particle size and support influences platinum oxidation under NO oxidation conditions. The model quantitatively predicts the turnover rate to with in a factor of two for moderate and high activity catalysts and which catalysts will have little or no activity.

Degree

Ph.D.

Advisors

Delgass, Purdue University.

Subject Area

Chemical engineering

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