Date of Award

2013

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Graduate Group

Chemical and Biomolecular Engineering

First Advisor

John M. Vohs

Abstract

Recent advancements in fuel cells, known for their high efficiency in converting chemical energy into electrical energy with zero greenhouse gas emissions, prompts the need for efficient methods of hydrogen production, as most fuel cell systems require pure hydrogen as the fuel. Reforming of methanol and ethanol is one attractive route since, at least in theory, this pathway can be considered carbon neutral. This, in return, has motivated a study focusing on the development of highly active and selective catalysts for the steam reforming of alcohols. In this thesis, we discuss fundamental studies on two different steam reforming catalysts - Co-based catalysts for steam reforming of ethanol (SRE) and PtZn bimetallic catalysts for steam reforming of methanol (SRM). Within the scope of Co-based catalysts, we have investigated the structure and reactivity of model catalysts consisting of Co films and particles supported on various metal oxides, such as ZnO(0001), YSZ(100), and CeO2/YSZ(100), to elucidate the mechanism of the SRE reaction on cobalt catalysts and identify the support effects. The results obtained demonstrate that partially oxidized cobalt is active for the partial oxidation of adsorbed ethoxide groups to produce acetaldehyde, which is thought to be a key intermediate in the SRE reaction. Comparison of the results obtained for the reducible (ZnO and CeO2) and non-reducible (ZrO2) supports shows that the facile transfer of oxygen from reducible supports may play a role in maintaining the cobalt in a partially oxidized state, and removing the deposited carbon on Co that is formed via unselective decomposition of ethanol. On the PtZn catalysts for SRM application, we have carried out a surface science study utilizing model catalysts of vapor deposited Zn on a Pt(111) single crystal to characterize the structural effect of incorporating Zn into Pt surfaces and its correlation towards methanol reactivity. It was discovered that the addition of Zn results in the stabilization of chemisorbed methanol allowing it to undergo an OH bond scission to form methoxy on the catalyst surface. These methoxy groups that are bound on Zn sites are found to undergo dehydrogenation to form formaldehyde, which is stable up to 400 K. Therefore, it is proposed that Zn effects are not limited only in electronically altering the properties of Pt, but also providing sites where the aldehyde intermediates are bound preventing them from undergoing further dehydrogenation to CO.

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