Probing the Surface of a Heterogeneous Catalyst: Double Resonance of Carbon Monoxide Chemisorbed on Highly Dispersed Platinum
Makowka, Claus Dieter
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https://hdl.handle.net/2142/77347
Description
Title
Probing the Surface of a Heterogeneous Catalyst: Double Resonance of Carbon Monoxide Chemisorbed on Highly Dispersed Platinum
Author(s)
Makowka, Claus Dieter
Issue Date
1982
Department of Study
Physics
Discipline
Physics
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Physics, Condensed Matter
Language
eng
Abstract
Spin Echo Double Resonance (SEDOR) is observed at 77 K for small Pt particles on which ('13)CO was chemisorbed. The samples are Pt supported on eta-alumina with Pt dispersions (fraction of the atoms that are on the surface) of 26% and 76%. After cleaning, the samples are exposed to CO enriched to 90% ('13)C. The ('13)C NMR is observed indirectly through its effect on the ('195)Pt spin echo. SEDOR allows the NMR of the surface ('195)Pt nuclei to be observed underneath the resonance of the nonsurface nuclei from which the surface nuclei are ordinarily unresolved. The position (1.096 kG/MHz)a and width (500 G at 80 kG) of the ('195)Pt SEDOR signal confirms the assignment by Rhodes et al. of a peak in the ('195)Pt NMR line shapes to the surface layer of platinum atoms. No surface nuclei are detected at any other locations in the ('195)Pt NMR line shape. Though the ('195)Pt NMR line shapes for the two dispersions are very different, the SEDOR line shapes are identical. SEDOR shows that the bonding of the CO is to the clean Pt surface through the carbon and that the Pt-C J coupling is similar to that in Pt carbonyls.
A phenomenological theory of the Knight shift in small platinum particles is developed to understand the Pt NMR line shapes. The theory successfully reproduces all the major features of the line shape data over the full range of dispersions studied by Rhodes et al. The theory has two principal assumptions. The first is that the 5d electron local density of states at the Fermi energy is reduced at the particle surface, healing exponentially to the bulk density of states on moving away from the surface. The second is treatment of the 6s electrons as free electrons confined to a spherical box, giving oscillations in the Knight shift. Surface nuclei give a distinct peak in agreement with the SEDOR results.
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