Date of Award

Winter 2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular Science and Nanotechnology

First Advisor

Gergana Nestorova

Abstract

Halloysite nanotubes are a versatile nanomaterial that can be used in a wide variety of applications. They have a unique structure which could be described as a flat material that consists of silica on one side and alumina on the other; this structure is rolled up in a way naturally forming an internal 10-15 nm lumen and interlayer spacing. This could lead to many potential applications for example incorporating halloysite as a template material or as a support structure. They are an inexpensive clay material that is available in large quantities (thousands of tons), so they may be practically used in industrial applications. In this work, they were used as a support structure for catalysts, gas sensors, as well as lithium sulfur batteries.

In catalysis it is essential to minimize aggregation of catalytic nanoparticles as aggregation leads to a reduction of surface area used for conducting chemical reactions. Support structures are used to minimize this aggregation. Halloysite nanotubes were used as a support structure to chemically attach two different catalytic metal nanoparticles: cobalt and ruthenium. There are a variety of techniques to synthesize catalytic nanoparticles onto/inside halloysite. To enhance inner-wall and lumen metal cluster formation for ruthenium, we intercalated the tube with furfuraldehyde and then converted it to tetradentate ligands which have shown specific binding to Ru3+ ions from ethanol solution at elevated temperatures. One of the purposes of the ligands is to increase the amount of ruthenium loaded inside the halloysite. The other purpose is to firmly hold the ruthenium inside the halloysite to prevent leakage of Ru. Metal particles of 2-5 nm diameters were formed both in the central lumen and in the interlayer spaces of the tube walls. This core-shell engineered catalyst was tested in hydrogenation of aromatics. Maximum turnover frequency (TOF) achieved was 17282 h-1 in terms of hydrogen uptake per surface area of Ru-atoms. For halloysite-based ruthenium catalysts some metal leaching was observed after the 1st reaction cycle, which may be attributed to poorly retained nanoparticles located outside of the nanotubes, but these Ru-core-shell nanocatalysts were recycled up to ten times without loss of catalytic activity.

Halloysite was also used to load cobalt for catalytic purposes. Active cobalt mesocatalysts were prepared using four main methods: Wet impregnation of cobalt chloride into halloysite, adsorption of cobalt (II, III) oxide onto halloysite, and cobalt chloride linkage onto/into halloysite using azines as well as APTES. It turned out that the most efficient catalyst was the halloysite loaded with cobalt chloride using the azine acetone as a ligand with an R value (L/min x g(cat)) of three, which is higher than other previously made cobalt mesocatalysts which typically range from 0.9-2.9.

In addition to supporting metal catalytic nanoparticles, halloysite was used to load zirconia nanoparticles for gas sensor applications. We synthesized Y2O3-ZrO2 on halloysite clay nanotubes, an example of an external metal oxide – internal ceramic coreshell system. This produced 5-10 nm diameter particles on the nanotubes. This system should limit the particle agglomeration at working temperatures of 500 ºC.

Halloysite nanotubes were also used as a nanoconfinement structure to load sulfur particles for increasing lithium-sulfur battery efficiency. The available nanoscale space in the lumen of halloysites nanotubes and between assembled halloysites clusters help suppress the dissolution and migration of polysulfides in liquid electrolyte solution. The halloysite-sulfur cathode composite was incorporated into coin batteries, and the halloysite/sulfur composites successfully improve the cycling stability, retaining ~84% of the starting capacity for over 250 cycles.

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