Chemistry Research

Nanoscale Chemistry: Every Atom Counts

QUESTION How does chemistry change at the nanoscale? Can a single atom drive a catalytic reaction?

“As the size of particles of materials is reduced to less than 20 nanometers, the physical properties begin to change, and this affects chemistry as well,” says Scott L. Anderson, Distinguished Professor of Chemistry at the U.

For example, the electronic and geometric structure of nanoparticles is different from the bulk structure of materials. Changing geometric structure can include changes in chemical bond lengths and in the shape of surface sites, and both factors affect chemical reactivity.

“Chemistry is all about electrons being shared between reactants, so if the electronic properties of nanoparticles are different, this affects the kinds of chemistry they can undergo,” says Anderson.

WHO Anderson is currently investigating several fundamental questions, all relating to nanoparticle surface chemistry and catalytic reactions.

Catalysts work by selectively lowering the energy barriers that normally inhibit specific chemical reactions, thus reducing the energy and cost involved and improving the selectivity toward desired products, all while minimizing by-products.

One main class of catalysts is “supported catalysts,” where a catalytically active material such as platinum is dispersed in the form of nanoparticles on a support material, like carbon or a metal oxide. This technique maximizes the available surface area of the expensive material, but if the nanoparticles are small enough, the size also begins to “tune” the chemical properties of the catalytic particles, allowing new approaches to catalyst optimization.

“In one case, we are exploring the physical and catalytic properties of small metal clusters containing between one and 30 metal atoms, supported on carbon substrates. In such extremely small clusters, the properties can change dramatically if the cluster size is changed by just one or two atoms,” says Anderson.

For example, in ethanol electro-oxidation, catalyzed by platinum clusters, the activity varies by an order of magnitude for clusters in the one- to 20-atom size range, with Pt4 (a platinum cluster with 4 atoms) and Pt10 being particularly active, and Pt1, Pt7, and Pt8 being almost inert.

“That is of some interest from a practical perspective, but it also provides an opportunity to use size effects to probe the reaction mechanism itself,” says Anderson.

“Working together with a theory collaborator at UCLA, Professor Anastassia Alexandrova, we are preparing and studying alloy clusters where the