Shuguo Ma

Date of Award

5-2003

Level of Access Assigned by Author

Open-Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor

Brian G. Frederick

Second Committee Member

François G. Amar

Third Committee Member

Alice E. Bruce

Abstract

The adsorption, diffusion, reactions, and desorption of water, alcohols, ethers, and an organophosphonate were studied using calibrated thermal desorption spectroscopy (CTDS) on thin film WO3(OOl) surfaces grown epitaxially on a single crystal α- Al2O3(li02) (sapphire) substrate. The studies were conducted on oxidized and reduced surfaces, which were characterized by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The desorption spectra for molecular desorption of all of these molecules shifted to lower temperature with increasing coverage, and had overlapping tails on the high temperature side. Monte Carlo simulations show that this typical desorption shape can be characterized by first-order or pseudo-first-order desorption kinetics in which rapid diffusion allows the molecules to find and desorb from the most strongly bound sites of a heterogeneous surface. For methanol, UPS revealed molecular adsorption on the oxidized surface and dissociative adsorption on the reduced surface, but only molecular desorption of methanol was detected. For ethanol and isopropanol, competition between molecular desorption and dehydroxylation at lower temperature controls the alkoxy coverage remaining and consequently, the selectivity toward alkenes. The selectivity was coverage dependent, but was not significantly different on the oxidized and reduced WO3(001) surfaces. The rate limiting step in the dehydration of the alkoxy species to ethylene or propylene was identified as C-0 bond scission. Dimethoxymethane showed some decomposition to methanol on both surfaces, while only molecular desorption was found for dimethyl ether. Dimethyl methyl phosphonate (DMMP) decomposed on both the reduced and oxidized surfaces through loss of methoxy groups. Methanol and dimethyl ether were detected on the oxidized surface, while only dimethyl ether was observed on the reduced surface. Comparison of the dimethyl ether production with spectra following adsorption of dimethyl ether suggests that the rate limiting step is a surface catalyzed, intramolecular coupling reaction between methoxy groups. The C1s and P2p XPS features were consistent with a methyl phosphate-like species remaining after DMMP decomposition.

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