Industrial catalysts often have complex surface structures in which the active sites are under-coordinated surface atoms: species that have fewer nearest neighbours than other surface atoms. Stepped single-crystal surfaces — well-defined surfaces with atomically flat terraces connected by numerous atomic steps — have been used as model systems to study these catalysts, typically in ultrahigh vacuum environments. However, real catalysts must operate at pressures and temperatures much higher than these model experiments normally probe. Miquel Salmeron, Gabor Somorjai and colleagues at Lawrence Berkeley National Laboratory and the University of California, Berkeley have now examined stepped platinum surfaces under realistic reaction conditions and found that they undergo significant and reversible restructuring.
The researchers used scanning tunnelling microscopy and X·ray photoelectron spectroscopy to examine the adsorption of carbon monoxide (CO) — a reactant in many important commercial reactions — at near ambient pressures and at room temperature. As the CO surface coverage approached a complete monolayer, the platinum surfaces were found to break-up and form nanoscale clusters. When the CO was removed, the original surface structure returned. Density functional theory calculations indicate that the restructuring occurs to relieve CO–CO repulsion in the compressed high-coverage adsorbate layer.