Date

2016

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Chemical Engineering

First Adviser

Snyder, Mark A.

Other advisers/committee members

Wachs, Israel E.; Kiely, Christopher J.; Caram, Hugo S.

Abstract

Metal oxides are utilized in diverse applications due to their ability to form single component crystals, multi-component mixed oxide systems, and supported oxide materials. During the preparation of a supported oxide catalyst, low surface energies enable increased atomic surface mobility and permit oxides to spread more readily than metals on the surface of an underlying support. These properties facilitate the synthesis of high surface area catalysts, allow widespread surface coverage, afford the development of monolayers, and offer synergetic catalytic effects between a surface oxide and its support. However, high surface mobility also makes an oxide susceptible to methods of reconfiguration including sintering, pore collapse, phase transformation, and catalytic deactivation at elevated temperatures. Therefore, it remains imperative to develop methods of promoting oxide stability when designing catalysts for operation at high temperatures. This work provides a conceptual framework for the stabilization of metastable oxides via the incorporation of a second, supported oxide, along the surface of the metastable support. This concept of stabilization via surface modification is first demonstrated in experiments which utilize colloidal silica (SiO2), in a hard templating approach, to enhance the surface area of zirconia (ZrO2) and titania (TiO2). The presence of residual SiO2 in these templated materials is found to promote stabilization of the metastable phase (i.e. tetragonal ZrO2 and anatase TiO2). This method of surface stabilization is then validated as a general phenomenon through a series of studies investigating model supported oxides consisting of SiO2/ZrO2, SiO2/TiO2, and thin TiO2 films deposited on silicon wafers. Finally, this technique is utilized to engineer stabilized catalysts by depositing various surface oxides (e.g. molybdenum oxide, silica, tungsten oxide, and ceria) along crystalline and hydroxide supports.The selection of an appropriate surface oxide is shown to ballast the underlying support while preserving its surface area and crystal structure at elevated temperatures (T > 500 ˚C). This concept can be employed in the synthesis of traditional catalytic materials produced via incipient wetness impregnation (IWI) on either commercial crystalline supports or their hydroxide precursors; in doing so, one can simultaneously stabilize a catalyst and promote unique surface chemistry. A combination of mechanisms is established for oxide stabilization; a surface oxide can act to: increase the cohesive energy of the metastable oxide, limit strain on the supporting oxide, fill surface defects, induce localized surface doping, and generate distorted terminal surface species with unique acidity. The insight provided by this work allows a rational approach for stabilizing supported oxides, limits their high temperature reconstruction, and results in the design of robust catalysts with improved performance.

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