Date

2015

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemistry

First Adviser

Moore, David T.

Other advisers/committee members

Ferguson, Gregory; Roberts, James; Snyder, Mark

Abstract

This dissertation focuses on the development of Interfacial Matrix Stabilization Spectroscopy (IMSS) for study of the mechanism of catalytic oxidation of CO over gold nanoparticles (AuNPs) supported on titanium dioxide (TiO2). In this unique application of matrix isolation, reactant molecules isolated in a cryogenic matrix are deposited over a thin-film of catalyst at 4 K to 20 K, in which subsequent annealing up to 40 K promotes diffusion to the catalytic interface and formation of reactive complexes. Energy-dissipating conditions trap species, and the evolution of vibrational bands during annealing is tracked with Fourier Transform Infrared (FT-IR) Spectroscopy to obtain structural information of reactive intermediates. In particular, this work demonstrates the value of IMSS as a spectroscopic tool to identify short-lived, hard to detect species.Films of Au/TiO2 powder were prepared via thermal de-wetting of Au film over TiO2 NPs in vacuo at 500 K. Direct adsorption studies at 40 K under high vacuum conditions (10-8 to 10-5 Torr) of CO deposited in helium over Au/TiO2 and TiO2, in the absence of the cryogenic matrix, reveal CO vibrational bands in the range of 2090 cm-1 - 2106 cm-1 attributed to binding at Au sites, dependent on de-wetting conditions, as well as pressure/temperature conditions for CO binding. Comparison of these results to binding under equilibrium pressure (1 Torr CO) allows for differentiation between weakly-bound species present in dynamic equilibrium with the gas phase, and those existing in stable potential wells. These studies in the absence of the cryogenic matrix serve as foundational work towards understanding the adsorption of species without added stabilization of the matrix. The ability of IMSS to control the delivery of reactant molecules to the catalyst interface is first demonstrated through deposition of CO isolated an argon matrix with a krypton over-layer, followed by incremental annealing steps between 25 K and 40 K. Results reveal a band at 2180 cm-1 only after annealing to sufficiently high temperatures, in which this band indicates CO chemisorption at the support. Additional experiments demonstrate that, following pre-saturation of CO binding sites during a direct adsorption step, O2 deposition in an argon matrix over-layer reveals a new band at 2112 cm-1 upon annealing. This band is most pronounced for samples containing gold and with O2 present in the matrix, demonstrating the isolation of transient complexes at the surface previously unobserved for CO and O2 direct adsorption experiments in the absence of the matrix.

Included in

Chemistry Commons

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