title: Sintering properties of platinum nanoparticles on different oxide-based substrates
Creators: name: Ott, Susanne Veronika
name: PATENTANMELDUNG, !!!
Publisher: name: Heidelberg University Library
Year: 2020

description: Metal nanoparticles play a significant role in exhaust combustion. They oxidize harmful products
like carbon monoxide and hydrocarbons in order to prevent major environmental and health
issues. In a converter, platinum nanoparticles (Pt NPs) are impregnated in a thin coating of a
porous ceramic oxide. Due to their high surface-to-volume ratio, Pt NPs can provide high catalytic
activities; however, elevated temperatures in the exhaust gas flow lead to thermal deactivation of
the catalyst via sintering, thereby resulting in large losses in efficiency over the catalyst’s lifetime.
In this thesis, the sintering behavior of 5-6 nm sized Pt NPs synthesized via block copolymer
micellar nanolithography on various planar oxide-based substrates is investigated. First, their
coarsening on both crystalline and amorphous silica (SiO2) and alumina (Al2O3) is evaluated in
regard to the mechanisms of Ostwald ripening and particle migration and coalescence. Sinter
studies at 750°C in air reveal an enhanced thermal stability on the amorphous alumina-support
Al2O3(a). Second, key influencing parameters on the sinter resistivity of the Pt NPs are identified.
An increased NP adhesion on the amorphous substrates, a higher roughness and surface potential,
as well as a larger contact angle of water on Al2O3(a) are all found to significantly contribute to
enhanced sinter stability.
Furthermore, the thermal behavior of Pt NPs on dual-structured surfaces is examined at the interface
between Al2O3(a) and SiO2 to study the impact of compositional surface heterogeneities.
The particles favor the high metal interaction Al2O3(a)-side over the low metal interaction SiO2-
side as shown by their diffusion away from the silica. Additionally, structural heterogeneities
on sapphire wafers with varying tilt angles, and thus step edges of different height and size,
contribute to a smaller increase in Pt NP diameter over time on the more tilted substrates when
exposed to 1200°C under vacuum compared to NPs on less tilted substrates. Hereby, larger
sintered particles are observed to preferably align along the step edges. This is due to a locally
increased surface potential at the edges and because these edges function as Ehrlich-Schwoebel
barriers. Thereby they hinder the diffusion of particles on the substrate. Lastly, the sinter stability
of Pt NPs is successfully enhanced via the deposition of an isolating silica or alumina layer by solgel
techniques. These films are shown not to cover the Pt NPs and also prevent the migration of
platinum clusters toward each other during sinter studies at 750°C under atmospheric conditions.
Taken together, this data contributes to a better understanding of the thermal stability of Pt NPs
catalysts with respect to the underlying support. The information gained from these sinter studies
can be harnessed in the design of more thermally stable Pt NP catalysts, which can ultimately
contribute to more environmentally sustainable technologies.