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|Title:||Nuclear Magnetic Resonance of Platinum Catalysts: The Discovery of The Surface Resonance|
|Author(s):||Rhodes, Howard Edgar, Jr.|
|Department / Program:||Physics|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Subject(s):||Physics, Condensed Matter|
|Abstract:||We have observed ('195)Pt NMR at 74 MHz in three samples of 10% platinum-on-alumina catalysts at 77 and 4.2(DEGREES)K. The NMR absorption lines for each sample extend from the resonance position of ('195)Pt in bulk Pt metal past the resonance position in diamagnetic Pt compounds to positive Knight shifts. The lineshapes differ dramatically among the samples, displaying several pronounced peaks. Using lineshape and relaxation time measurements coupled with other physical measurements, we can associate a low field peak in the lineshape structure with a single layer of platinum nuclei on the surface of the platinum particles, bonded to adsorbed atoms.
The samples, prepared by Dr. J. H. Sinfelt, were characterized using hydrogen chemisorption. The particle size distribution was determined by electron microscopy to obey a log normal distribution having peaks at 9, 27, and 39 Angstroms for the three samples, respectively.
Relaxation times T(,1) and T(,2) were measured on each sample as a function of position along the line and of temperature. Both relaxation times obeyed a universal curve independent of particle size but strongly dependent on Knight shift. The T(,1) relaxation time displayed an anomolous increase at the low field peak in the lineshape structure. At any point along the lineshape two distinct states were observed: (1)those with long T(,1)'s and (2)those with short T(,1)'s. However, both states had the same T(,2). We present a theory that explains the temperature dependence of T(,2) and the relation of T(,2) to T(,1). We find the "slow beat" phenomenon in the negative Knight shift region of the line for all samples. We give a theory for the slow beats in atomic clusters which yielded information on the form of the Korringa oscillations in the interior of the clusters.
We performed three surface treatments on one of the samples. We found the lineshape, position, and T(,1) of the low field peak to be strongly dependent on surface conditions, consistent with our assignment of this peak as the platinum "surface resonance."
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1981.
|Date Available in IDEALS:||2015-05-13|