Surface chemistry at metallic step defect sites

The role of step sites on metal surfaces in promoting chemical reactions is discussed showing experimental data from the author’s laboratory. The adsorption of CO on two stepped Pt surfaces, Pt (112) and Pt (335), is compared using various methods that are sensitive to adsorption site character. The existence of one-dimensional arrays of chemisorbed CO molecules along the step sites is demonstrated, and CO...CO repulsive steric interactions are observed to cause orthogonal tilting of the step-bound CO species as the coverage is changed on the step sites. For Pt (335), the most active site for the oxidation of CO is found to involve a chemisorbed oxygen atom at the step site, reacting with a CO molecule chemisorbed on a terrace site. It is also shown that a new surface phenomenon, electron stimulated migration (ESM), may be spectroscopically observed for CO on Pt (335). The atomic steps act as trap sites for CO molecules that have been excited by 150 eV electrons. It is likely that this ESM phenomenon is a property of thermally nonaccommodated species in the electronic ground state of the chemisorbed CO that have been produced by quenching from an excited electronic state.