Fast surface diffusion is the cornerstone of catalytic reactions, crystal growth, and many other chemical processes. An atom on a clean surface diffuses quickly by hopping over low energy barriers from binding site to binding site. But when the density of atoms increases or when two different species are on the surface, as is often the case in industrial processes, diffusion becomes more complicated. Intuitively, one expects higher density and the presence of other particles to slow down diffusion as the adsorbed species get in each other’s way. Using high-speed, variable-temperature scanning tunneling microscopy (STM), Joost Wintterlin of the Ludwig-Maximilians University Munich in Germany and his colleagues observed a previously unidentified mechanism that allows surprisingly quick diffusion of atoms on a surface crowded with molecules.
The researchers found that an oxygen atom (red in figure) on a ruthenium surface (gray) prefers to sit in hollow sites at the center of three Ru atoms, whereas carbon monoxide molecules (blue) occupy lattice sites directly on top of Ru atoms. When an O atom finds itself trapped inside a hexagonal ring of CO molecules, it jumps among the three available sites at the center of the hexagon, as shown in the left panel of the figure. But a second process, in which the O atom and CO molecules exchange positions (right panel), frees the O from the CO cage and places it in the center of a newly arranged ring of CO molecules. By repeatedly swapping places with CO molecules, the O atom can make its way across the surface.
The experimental time resolution wasn’t fast enough to show whether the O and CO move simultaneously, the O moves first, or the CO moves first. Density functional theory calculations revealed that the activation energy was high for concerted motion and the O-initiated scenario, whereas low-energy-barrier CO density fluctuations easily opened the door for the O atom to escape. That fast diffusion mechanism happens at a rate comparable to the diffusion of O on a bare surface. Wintterlin and his colleagues were surprised by the result, given that high surface coverage, common in the high pressures required by industrial processes, is expected to lead to orders-of-magnitude reduction in mobility. (A.-K. Henss et al., Science 363, 715, 2019.)
