Metal surface bonds reach far beyond the atoms a molecule touches

A molecule landing on a metal surface does not simply bond to the one or two atoms it touches. A team at the Max Planck Institute for Multidisciplinary Sciences and the Center for Quantum Nanoscience within the Institute for Basic Science has shown that the strength of a chemical bond formed on platinum can be influenced by structural features of the surface as far as roughly 10 nanometers away.

The question the researchers set out to answer was deceptively simple: how much of a metal surface actually participates in determining the strength of a chemical bond formed on it? Many processes that sustain modern society depend on molecules binding to metal surfaces. The production of synthetic fertilizer, for example, relies on heterogeneous catalysis, in which gases react on the surface of a solid catalyst. For many purposes that picture works well, but it turns out to be incomplete in a fundamental way.

Bonding to a metal is fundamentally different from bonding between isolated atoms or molecules. The electrons in a metal occupy electronic states that extend throughout the solid. When a localized molecular orbital interacts with these delocalized metal states, the electronic structure involved in the bond can extend far beyond the point of contact. The researchers offer an intuitive analogy: pressing a finger into sand disturbs only the grains being touched, whereas pressing it against a stretched drum skin deforms a much larger area, because the surface is mechanically connected. In a metal, it is the delocalized electronic structure that provides this connection, so a molecule binding at one spot can perturb, and be perturbed by, regions of the surface far from its binding site.

Long-range electronic disturbances around atoms and molecules on metal surfaces have been observed before, particularly in low-temperature studies. Until now, however, such effects were generally not considered to make a substantial contribution to chemical bond strengths under conditions relevant to catalysis.

To measure the effect, the team used precisely structured platinum surfaces containing regularly spaced atomic steps, which are sites where molecules often bind particularly strongly. By preparing surfaces with different distances between neighboring steps, they effectively used the platinum lattice as an atomic-scale ruler. They then measured the adsorption energies of three small molecules, ammonia (NH3), carbon monoxide (CO), and nitric oxide (NO), using Velocity-Resolved Kinetics, a technique that determines how rapidly molecules leave a surface and uses that information to extract their binding energies. This combination of controlled surface geometry and quantitative desorption measurements allowed the researchers to correlate bond strength with step separation in a systematic way.

The results showed that changing the distance between neighboring steps changed the strength of the bond at a given step, even when the neighboring step was several nanometers away. The characteristic distance over which the effect persisted depended on the molecule, reaching approximately 7 nanometers for ammonia, 6 nanometers for carbon monoxide, and 4 nanometers for nitric oxide. In experiments where a second step was moved as far as about 10 nanometers from the binding step, the bond strength still changed.

To explain why the effect differed from molecule to molecule, the researchers developed a Frontier Orbital-Band Model that combines the molecular-orbital view of chemistry with the electronic-band view of solid-state physics. The model showed that different molecules couple to different electronic states in the metal, causing each one to respond differently to structural changes elsewhere on the surface. The team also compared platinum with palladium and found that the long-range effect was much weaker in palladium, supporting the idea that the phenomenon depends on how strongly electronic states extend through the metal. Platinum’s electronic structure, it appears, allows bonding information to propagate across the surface far more effectively than palladium’s.

The findings may be particularly important for nanoscale catalysts. Catalytically active platinum nanoparticles often measure only a few nanometers across, comparable to the length scale over which the researchers found that bond strengths remain sensitive to surface structure. On a particle that small, an adsorbed molecule may be influenced by structural features across a substantial fraction of the entire nanoparticle rather than only by the atoms immediately surrounding its binding site.

The work suggests that models of catalytic activity based only on the local arrangement of atoms around an adsorption site may provide an incomplete description for metals with strongly delocalized electronic bands. A more accurate picture must also consider how the surrounding solid modifies the electronic states involved in the bond. Rather than treating a catalytic bond as an interaction confined to a molecule and a few neighboring atoms, the study shows that the wider electronic structure of the metal can influence bond strength over distances of several nanometers.

The result grew out of a collaboration between the Max Planck Institute for Multidisciplinary Sciences, where the high-precision measurement methods were developed, and the Center for Quantum Nanoscience, which brought a surface-physics perspective to the problem. Dmitriy Borodin, who co-developed the measurement methods during his doctoral research at Max Planck before joining QNS, helped bring the two perspectives together. Florian Nitz, a PhD student at the University of Göttingen and lead author of the study, led the development of the Frontier Orbital-Band Model that explained the experimental findings. Together, these contributions produced a unified picture of how localized molecular orbitals interact with delocalized electronic states in a metal, and a reminder that on a metal surface, no molecule bonds alone.

Subject of Research: Long-range chemical bonding and adsorption energies at catalytic metal surfaces

Article Title: Chemical bonding on metals involves surprisingly many atoms

Article References: Chemical bonding on metals involves surprisingly many atoms. Original publication (Bibliographic details unavailable.)

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DOI: Not provided

Keywords: chemical bonding, platinum surfaces, heterogeneous catalysis, adsorption energy, delocalized electrons, Velocity-Resolved Kinetics, Frontier Orbital-Band Model, nanoparticle catalysts, surface science, ammonia, carbon monoxide, nitric oxide