Nuclear magnetic resonance study of promoted catalysis
Norcross, James Andrew
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https://hdl.handle.net/2142/18890
Description
Title
Nuclear magnetic resonance study of promoted catalysis
Author(s)
Norcross, James Andrew
Issue Date
1991
Doctoral Committee Chair(s)
Slichter, C.P.
Department of Study
Physics
Discipline
Physics
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Nuclear magnetic resonance (NMR)
promoted catalysts
alkali metals
Pt
Language
en
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.
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