Morphology, Properties and Reactivity of Nanostructures

Garrett M Mitchell, Purdue University

Abstract

Metal nanoparticles have long been of paramount importance in many areas such as: emission reduction in cars, hydrogen production via the water-gas shift reaction, and lithium-ion storage in batteries. For these purposes, the size and shape of the nanoparticles have been shown to play a crucial role in improving nanoparticle performance. For characterization of nanostructures, the use of transmission electron microscopy (TEM) has been shown to be extremely useful. Via a TEM instrument, one can learn about nanoparticle properties such as: particle size, 3D morphology, chemical composition, fine structure, crystallography, even to atomic resolution. No other technique boasts such ability at such a high xyz resolution. This work includes TEM work for many different applications within catalysis and energy storage fields. In catalytic applications, the <1 nm particle sizes often sought after generally lead to higher activity per unit mass of the>catalyst, but also have the tendency to sinter due to concomitant increases in the surface free energy, leading to catalyst deactivation especially at elevated temperatures. To investigate the sintering, (Pt,Au)-iron oxide heterodimer nanoparticles were heated in the microscope with simultaneous imaging. For that purpose, the sample was irradiated with a 532 nm pulsed laser, with laser powers of 4-25 mW within a TEM microscope to investigate particle sintering as it happens. The Au and Pt phases were both found to wet over the Fe3O4 phase, a behavior opposite to the Strong Metal Support Interactions (SMSI - caused by oxide wetting the metal) which were expected from well-known literature reports. This new behavior demonstrates that not only nanoparticle size, but also the support particle size can affect catalytic properties. This is shown by the fact that the size of the support oxide in these heterodimer nanoparticles is only 3 times the diameter of the active metal nanoparticles, compared to a greater than 20 times size difference for a standard metal oxide supported nanoparticle system. Nanoparticle metal catalysts can also undergo significant catalytic improvement via the addition of promoting metals. Kinetics were measured on a series of Pt/Co on carbon nanotube support catalysts, and addition of Co was seen to improve the turnover frequency by 10 times. Leaching of the bulk Co phases, while preserving PtCo alloy structures, reduced activity by more than 18 times demonstrating the need for a Pt/CoOxHy interface for catalytic promotion, and showing that PtCo alloying did not produce the promotion effect. Although, for the PtCo catalysts for WGS, the formation of a Co-oxyhydroxide phase was proved to be vital, nanoparticle alloying is also well-known to improve dehydrogenation kinetics. This was shown for a series of PtM catalysts with core/shell structures, which were found to be highly selective for propane dehydrogenation as a result of the PtM intermetallic phase. XAS studies of these materials led to the discovery that formation of a continuous PtM alloy surface layer that is 2–3 atomic layers thick was sufficient to obtain identical catalytic properties between those of the core–shell and full alloy catalysts. TEM characterization was also performed to determine the core/shell nature of these catalysts.

Degree

Ph.D.

Advisors

Ribeiro, Purdue University.

Subject Area

Analytical chemistry|Chemistry|Materials science|Nanotechnology|Optics

COinS