Melissa Santala thinks about small things in a very big way.
Pointing to an image on her computer, which some might mistake for modern art, she explains the dark streaks and lighter dots. “That spacing between these dots is about two-tenths of a nanometer, so 0.2 billionths of a meter,” she said. “You’re seeing columns of atoms in the material.”
Santala, an assistant professor of materials science at Oregon State University, and her research group use transmission electron microscopy, or TEM, and other atomic-level characterization techniques to examine the microstructure behavior of materials and learn how to improve chemical efficiencies.
She studies the interfaces between dissimilar materials, such as the metals and oxides that are used in catalysis, the process of increasing the rate of a chemical reaction by introducing a different substance into the mix that acts as a “marriage broker” for the reactants.
This work is of particular interest to the automotive and petroleum refining industries, where gamma alumina, a transition phase of aluminum oxide, is commonly used as a support for catalytically active metal nanoparticles in the production of fuels. Some metals used for catalysis, such as platinum, are expensive. Since the reactions occur on the surface, only a very thin layer of catalytic metal, just a few atoms thick, is needed. Nanoparticles are optimal, because they maximize the surface area where reactions occur. The alumina support keeps the platinum particles distributed. This prevents them from bunching up, which would result in less available surface area and require the use of more platinum.
The activity of these catalytic nanoparticles, which are used to reduce the harmful byproducts of engine combustion or fuel processing, is strongly affected by metal and metal oxide interactions. Alumina-supported metal catalysts have been widely studied, but the interfaces — the area of contact between the metal and aluminum oxide — have not been structurally characterized at the atomic level.
“We want to know what the chemistry is at the interface,” Santala said. “Does the platinum bond to the oxygen or does it bond to the aluminum, and does that change if you’re processing at different temperatures or different atmospheres?” Santala also hopes to learn whether interactions at the interface make a difference in the behavior of the system as a catalyst.
In more recent experiments, one goal has been to quantitatively characterize the atomic structure of the interfaces between platinum nanoparticles in gamma alumina using TEM. Santala’s group will then use this data to validate their computer models that explain the processes occurring in platinum-alumina interfaces. Santala is collaborating with Líney Árnadóttír, associate professor of chemical engineering at Oregon State, who is leading the computational aspects of the project.
The results of this collaboration will advance the fundamental knowledge of the connection between structure and thermodynamic properties of metal/transition-alumina interfaces and will further the understanding of catalysts used for chemical processing.
Another collaborator, Ari Clauser, a third-year doctoral student in materials science advised by Santala, was selected for the Department of Energy’s Office of Science Graduate Student Research program, which provides supplemental funds for doctoral students to conduct part of their thesis research at a host DOE laboratory. Over the summer of 2019, Clauser worked at the National Center for Electron Microscopy at the Lawrence Berkeley National Laboratory in Berkeley, California. There she imaged platinum and gamma alumina atoms at the interfaces using aberration-corrected scanning transmission electron microscopy, a technique that will produce images with extremely high spatial resolution.
“Looking at atoms is the coolest thing in the world,” Clauser said. “Not many people get to visually regard the building blocks of the universe and the beautiful symmetry and order atoms adopt. It feels like being let in on some kind of unwritten universal secret.”