Document Type

Thesis - University Access Only

Award Date

2001

Degree Name

Doctor of Philosophy (PhD)

Department / School

Chemistry and Biochemistry

Abstract

Aluminum containing materials such as aluminas, clays, and zeolites are of interest due to their interesting chemical and catalytic properties. Previous research has included the use of infrared spectroscopy to study the surface of these materials. The results from this research has allowed for the postulation of the surface structure. Since for a majority of these materials it is difficult to form single crystals, most structural information is based on refinement of x-ray powder patterns. The observed diffraction patterns can be quite broad due to the surface morphology of the materials. This lack of resolution can result in ambiguous assignments of the structural components present. The use of solid-state 27 Al NMR spectroscopy allows for the observation of the local structure of these materials. NMR spectroscopy is sensitive to interactions within 10A, hence differences in the coordination of the aluminum nuclei are easily distinguishable. Aluminum is a quadrupolar nucleus, hence multiple transitions are present and broad resonances are observed reducing the spectral resolution. However, recent advances in the field has allowed for increased resolution of quadrupolar nuclei. Two-dimensional and double resonance NMR techniques are being increasingly used to study inorganic solids. With these more sophisticated techniques, increasingly specific and detailed information pertaining to an atom's local environment can be obtained and related to crystal structure, bonding, motion, and disorder on the atomic scale. For example, the recently developed multiple-quantum magic-angle spinning (MQMAS) has improved the resolution of NMR spectra of quadrupolar systems by an order of magnitude over magic-angle spinning (MAS). Since in these systems the surface of the material is of interest, I used I H-27 Al cross-polarization to study those aluminum sites that are located on the surface. With 1H-27Al cross-polarization, magnetization is transferred from surface protons to adjacent aluminum sites. These measurements have resulted in the first observation of a five-coordinate site in the transition aluminas studied. The use of heteronuclear correlation (HETCOR) spectroscopy allows for the observation of connectivities between the proton sites and their associated aluminum sites. These techniques, however, are still limited in resolution due to the broad lines that are still present. Previously, in order to increase resolution, MQMAS was implemented. Because of the low population of surface sites, standard MQMAS is unable to distinguish these sites from those in the bulk. To increase the sensitivity of the surface sites, cross-polarization was incorporated into the MQMAS pulse sequence. The resulting CPMQMAS experiment has enabled the observation of distinct surface sites.

Library of Congress Subject Headings

Aluminum oxide -- Structure
Aluminum oxide -- Surfaces
Nuclear magnetic resonance spectroscopy

Format

application/pdf

Number of Pages

187

Publisher

South Dakota State University

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