Date of Award

2016

Document Type

Open Access Dissertation

Department

Chemistry and Biochemistry

Sub-Department

College of Arts and Sciences

First Advisor

Donna A. Chen

Abstract

For decades a fundamental understanding of heterogeneous catalysts has been pursued for rational catalyst design using model systems under ultrahigh vacuum (UHV) conditions; however, there exist stark differences between the simplified models investigated under UHV and the industrial catalysts used at high pressures. To bridge these gaps, it becomes essential to utilize progressively more complex materials and to correlate their surface structure and activity using incrementally higher pressure techniques. In this work, both model Pt-Re catalysts and powdered metal-organic frameworks (MOFs) were studied using a suite of traditional UHV surface science techniques in addition to ambient pressure X-ray photoelectron spectroscopy (APXPS) and UHV-coupled ambient pressure cells. CO oxidation and methanol oxidation were investigated by APXPS on Pt(111), Re films on Pt(111), and on Pt−Re alloy model surfaces. Pt-Re alloy surfaces were found to dissociate oxygen more readily than Pt surfaces, and CO was found to desorb at lower temperatures from Pt−Re alloy surfaces than from Pt. Pt and Pt-Re surfaces were found to have similar product formation with Pt-Re alloys having higher activity and maintaining greater selectivity than Pt to CO2 formation. Model Pt-Re systems were also studied for methanol oxidation in a UHV-coupled microreactor where products were determined via GC-TCD, and pre- and post-reaction surfaces were characterized by XPS without exposure to air. The Pt-Re alloy surface initially showed less activity than Pt; but over extended time periods, the alloy maintained higher activity than Pt, which deactivated due to accumulation of atomic carbon. Re films were unstable since they form volatile Re2O7, but alloying Re with Pt made it less susceptible to sublimation. Water-gas shift was also performed in the microreactor on TiO2(110)-supported Pt, Re, and Pt-Re bimetallic clusters. Surface ReOx appears to block Pt active sites, but Re underneath Pt shows higher activity than Pt alone as Re modifies Pt. A separate UHV XPS-coupled high pressure cell was used to study the generation of mixed valence Cu+1/+2 sites in a powdered MOF before and after extensive heating and exposure to H2, O2, CO, and air, which corresponded to changes in the valence band, indicating tunability of the MOF’s electronic properties.

Rights

© 2016, Audrey S. Duke

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