Chemistry and Chemical Biology ETDs

Publication Date

Fall 12-10-2020

Abstract

Converting alkanes to other, more chemically and economically valuable molecules requires catalysts that can survive elevated temperatures and highly reducing environments. These environments can cause many metal-nanoparticle based catalysts to sinter rapidly, causing a loss of activity. They must also tolerate the coke formation, as well, since coke can restrict access to active sites by gas phase molecules, thus lowering catalytic activity. While there are routes to improve both the sinter and coke resistance of catalysts, an alternative strategy is to develop a protocol for regenerating the activity of the catalyst in question when coke formation or sintering becomes problematic. By accessing a state of where single atoms of the active species are trapped in the support surface under oxidizing conditions, the effects of both sintering and coking can be reversed during regeneration. By ensuring that this state is accessible using only air as the oxidant, it improves the utility of such catalysts so that they can be used in a distributed manufacturing environment, to access some of the otherwise stranded natural gas resources available in shale. The use of ceria as a support for trapping single atoms is established in the literature. When used as a support for propane dehydrogenation catalysts and in comparison with other supports such as silica or alumina, it shifts the major cause of deactivation away from sintering. This does result in the catalyst becoming vulnerable to poisoning by strongly bound carbonaceous species. However, because the catalyst has access to the single atom state, its activity can be regenerated readily and indefinitely. Access to a single atom state requires very strong interactions between the metal species and support. Design of supports and the search for compatible metals is an active area of investigation, and this dissertation reports on one such system, platinum metal supported on magnesium aluminate spinel. Previous work on spinel systems did not show single atoms of platinum because commercially available spinels have a surface overcoating that acts as a nucleation site for platinum particles. This material also displays a transition between the single atom and nanoparticle state upon reduction, which is reversible upon oxidation. Critical to the control over single atom states is the understanding of how they are formed and under what conditions. The final experimental chapter of this dissertation discusses a modification to the normal X-ray Absorption Spectroscopy technique called Modulation Excitation that has helped to identify the probably mechanism of the conversion of ruthenium nanoparticles to single atom species on a ceria support. This transformation is likely to occur via an ablative process, where single atoms of Ru leave the nanoparticle, likely as part of a ruthenium oxide molecule. These molecules are trapped by sites on the ceria surface as they diffuse throughout the system. In totality, this dissertation discusses several implications to stability, activity and regenerability of the access granted to the single atom state on certain metal-support systems.

Language

English

Keywords

Catalysis, Atom Trapping, Alkane Transformation, Single Atom Catalysis, Dehydrogenation

Document Type

Dissertation

Level of Degree

Doctoral

Department Name

Department of Chemistry and Chemical Biology

First Committee Member (Chair)

Abhaya K. Datye

Second Committee Member

Jeffrey Rack

Third Committee Member

Hua Guo

Fourth Committee Member

Fernando Garzon

Fifth Committee Member

Jeffrey T. Miller

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