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Title:Nuclear magnetic resonance study of promoted catalysis
Author(s):Norcross, James Andrew
Doctoral Committee Chair(s):Slichter, C.P.
Department / Program:Physics
Subject(s):Nuclear magnetic resonance (NMR)
promoted catalysts
alkali metals
Abstract:We have used NMR to study the properties of promoted catalysts prepared by adding alkali metals (Na, K, Rb, or Cs) to silica supported Pt clusters. We have also examined the effect of the alkali metals on CO and C2H4 adsorbed on the Pt. The properties of the promoter were studied in samples con t a1. n1. ng 23N a. By 'performing a 23Na- 13C double resonance with adsorbed CO, we determined that at least 22% of the Na in the samples is on the Pt. The 23Na line shows no indication of a Knight shift and the Na spin-lattice relaxation varies as 1/T2 , suggesting that the Na orbitals mix with the Pt conduction band to a much lesser degree than do those of adsorbed CO. Consideration of several results leads us to conclude that the Na forms three dimensional structures on the Pt, most likely Na20 molecules or crystallites. We observed the effect of the promoters Na, K, Rb, and Cs on the NMR properties of adsorbed CO. In all cases the effects are small. The CO line is narrowed and shifted to higher frequency in the presence of the promoter while the CO spin lattice relaxation is slowed. The effect of the promoter is greater in the samples with the heavier alkali metals K, Rb and Cs. In the case of CO with Rb, for which the effect of the alkali metal is greatest, a modest change of about 10-15% in the parameters used to describe the NMR results is adequate to account for the effect of the Rb. We investigated C2H4 adsorbed on Pt both with and without K. By performing 13C-1H double resonance we discovered that the C2H4 exists as a CCH3 species on the surface in both samples. The results of an annealing experiment, conducted to study C-C bond scission, indicate about a 2% reduction in the activation energy for C-C bond scission for C2H4 coadsorbed with K.
Issue Date:1991
Genre:Dissertation / Thesis
Rights Information:1991 James Andrew Norcross
Date Available in IDEALS:2011-04-27
Identifier in Online Catalog:3471844

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