Korean, German researchers find metal catalysts act at a distance

The science ministry said Thursday that a joint Korean-German research team has found that the atomic structure of a metal catalyst surface can influence chemical bonding several nanometers away from the point where a molecule attaches.

The work, published in the journal Science, was carried out by the Center for Quantum Nanoscience at the Institute for Basic Science in Korea and the Max Planck Institute for Multidisciplinary Sciences in Germany.

Metal catalysts speed up chemical reactions and underpin a broad swath of industry. The reaction begins when molecules of a reactant stick to the metal surface, and how strongly they bind shapes how smoothly the subsequent chemistry proceeds. Until now, that bond was explained mainly through interaction with the atoms a molecule touches directly; longer-range electronic effects had been observed, but mostly at very low temperatures, and were thought to have little bearing on bond strength in working catalysts.

The team tested the assumption on platinum surfaces built as stepped terraces of atoms, preparing samples in which the distance between atomic steps differed, then measured how strongly ammonia, carbon monoxide and nitrogen monoxide bound to the metal. The molecules bound with different strengths depending on their distance from a neighboring step, even where the local structure at the attachment point was similar. The difference in binding energy reached about 0.2 electron volts, a margin large enough to change how stable the bond is, and the effect extended roughly 4 to 7 nanometers, or the width of several dozen atoms laid side by side.

The researchers attributed the effect to the sea of electrons that spreads across a metal rather than staying with particular atoms. When a molecule attaches, its electrons interact with that sea, leaving the bond exposed to the influence of atomic structure far from the contact point. To explain why the reach differs by molecule, the team proposed a model combining molecular orbitals from chemistry with electronic bands from physics, showing that each molecule's electronic character pairs with different electron states in the metal.

"Understanding surface chemistry properly requires integrating the chemist's view centered on molecules with the physicist's view of the electronic properties of metals," said Florian Nitz of the Max Planck Institute for Multidisciplinary Sciences, the paper's first author. "Catalyst development once relied on long experimentation and trial and error, but it is now moving toward predicting and designing desired properties from the principles of chemical reactions," said Dmitriy Borodin, a group leader at the IBS center and a co-corresponding author.