Metal Bonds Reach Further Than Expected: Atomic Steps Influence Molecules Several Nanometers Away

Start with a platinum surface cut so that its atoms form regular steps, like a tiny staircase. Molecules tend to bind especially strongly at those steps. A team from the Max Planck Institute for Multidisciplinary Sciences (MPI-NAT) and the Center for Quantum Nanoscience (QNS) at the Institute for Basic Science (IBS) used such surfaces to ask a simple question: does a molecule sitting at one step care where the next step is?

It does. The researchers report in Science that the strength of a molecule's bond at a step changed when a neighboring step was moved as far as roughly 10 nanometers away. The size of the change depended on the molecule, and adsorption energies shifted by up to 0.2 electron volts.

The usual picture of chemical bonding is local: a molecule lands on a surface and bonds with one or a few nearby atoms. For many purposes that works well. Metals complicate it, because their electrons occupy states that extend throughout the solid. When a localized molecular orbital interacts with those delocalized states, the electronic structure involved in the bond can reach well beyond the adsorption site. The source offers an analogy: pressing a finger into sand disturbs only the grains touched, while pressing a stretched drum skin deforms a wide area because the surface is connected. In a metal, the delocalized electronic structure plays the role of that connection.

Long-range electronic disturbances around atoms and molecules on metal surfaces had been seen before, especially in low-temperature studies. But they were generally not thought to contribute much to bond strengths under conditions relevant to catalysis. This work shows directly that the strength of the molecule-metal bond itself stays sensitive to structure many atomic spacings away.

To measure it, the team prepared platinum surfaces with different distances between neighboring steps, in effect using the platinum lattice as an atomic-scale ruler. They measured the adsorption energies of ammonia (NH3), carbon monoxide (CO) and nitric oxide (NO) with Velocity-Resolved Kinetics, a technique that tracks how quickly molecules leave a surface and uses that to extract binding energies. Changing the step spacing changed the bond strength at a given step even when the neighbor was several nanometers off. The characteristic reach was about 7 nanometers for NH3, 6 for CO and 4 for NO.

To explain why molecules differ, the researchers built a Frontier Orbital-Band Model, which joins the molecular-orbital view from chemistry with the electronic-band view from solid-state physics. It indicates that different molecules couple to different electronic states in the metal, so each responds differently to structural changes elsewhere. Comparing platinum with palladium, the team found the long-range effect much weaker in palladium, consistent with the idea that it depends on how strongly electronic states extend through the metal.

The finding matters most for nanoscale catalysts. Catalytically active platinum nanoparticles are often only a few nanometers across, similar to the distances the study found. A molecule on such a particle may be influenced by structural features across a substantial fraction of the whole particle, not just its immediate neighbors. Models of catalytic activity that consider only the local arrangement of atoms may therefore be incomplete for metals with strongly delocalized bands.

The work came from a collaboration between MPI-NAT, where the high-precision measurement methods were developed, and QNS, which contributed a surface-physics perspective. Dmitriy Borodin, who co-developed the methods during his doctoral research at MPI-NAT before moving to QNS, helped link the two. Florian Nitz, a PhD student at the University of Göttingen and lead author, led development of the model that explained the results. What to watch next is whether theoretical models of nanoscale catalysts begin to include this wider electronic influence.