Does order or disorder determine catalysis?

Hardly any chemical process today occurs without catalysis. The vast majority of all manufacturing and conversion processes in the chemical industry are catalytic. A catalyst is a substance whose participation makes the chemical reaction possible or economically feasible. An important catalytic process is the conversion of atmospheric oxygen to water. This reaction is used, among other things, in sensor technology or corrosion technology. It is especially significant for the technology of chemical energy storage and energy conversion in batteries and fuel cells. The most effective catalysts for this reaction are made from alloys of the expensive and rare element platinum. For years, it has been known that platinum alloys, whose surfaces maintain an exactly defined structural order of atoms, are highly active catalysts.

"To the amazement of science, recent research also shows that structurally completely disordered variants of such platinum alloys, formed through wear and aging of the catalysts, exhibit equally high catalytic activity," explains Prof. Dr. Peter Strasser, head of the "Electrochemical Catalysis and Materials" department at TU Berlin. "However, these two observations seemed incompatible."

Now, the team from TU Berlin led by Peter Strasser, in close collaboration with French colleagues from the University of Grenoble Alpes and the CNRS (Centre national de la recherche scientifique), Swiss colleagues from ETH Zurich and the Paul Scherrer Institute, as well as German colleagues from TU Dresden, has succeeded in developing a more comprehensive description of these two types of catalysts and their reactions, which can resolve the apparent contradictions.

An important role is played by the distortion of the surface structure, a structural parameter that describes the disorder in the arrangement of individual atoms on the platinum surface and includes both catalyst types. The strictly uniform arrangement of metal atoms provides the best conditions for high catalytic reactivity for this reaction. However, structural disorder offers the greatest atomic diversity on the surface, resulting in many different atomic arrangements, some of which can also be highly active. This also leads to high catalytic reactivity overall. A simple analogy would be comparing it to throwing a basketball into a hoop: a professional player gets one attempt and scores. This corresponds to a small number of defined atomic patterns on the catalyst surface. In comparison, a whole school class throwing basketballs simultaneously has a statistically similar chance of scoring at least once as the professional with one throw. This corresponds to a large number of different atomic patterns on the catalyst surface.

The experiments now published in Nature Materials demonstrate: Increasing surface distortion is the key to understanding the aging processes of initially well-ordered, active catalysts, which transform into disordered but still active catalysts.

Finally, the unified description of the two catalyst types not only provides a deeper understanding of how known catalysts work but also offers a way to predict new, even more efficient catalysts for future energy storage and conversion technologies.

Raphaël Chattot, Olivier Le Bacq, Vera Beermann, Stefanie Kühl, Juan Herranz, Sebastian Henning, Laura Kühn, Tristan Asset, Laure Gütaz, Gilles Renou, Jakub Drnec, Pierre Bordet, Alain Pasturel, Alexander Eychmüller, Thomas J. Schmidt, Peter Strasser, Laetitia Dubau, Frédéric Maillard
Surface distortion as a unifying concept and descriptor in oxygen reduction reaction electrocatalysis
Nature Materials. 2018, DOI: 10.1038/s41563-018-0133-2