The reactivity of low index [(111) and (100)] and stepped platinum single crystal surfaces

Abstract

Several high Miller index crystal surfaces of platinum have been shown to consist of low index (111) or (100) terraces of constant width, linked by steps of monatomic height. The surface structures that form in the presence of diatomic molecules (H 2 , O 2 , CO, NO), aliphatic and aromatic hydrocarbons on the (111), (100) and on these stepped platinum surfaces were studied by low energy diffraction. Several catalytic reactions (H 2 + D 2 ; H 2 + O 2 ; dehydrocyclization of n -heptane) that take place on the various platinum crystal surfaces at low pressures were monitored by means of a mass spectrometer and the surface composition by Auger electron spectroscopy. The chemisorption characteristics of diatomic molecules on stepped platinum surfaces are markedly different from those on low index [(111) and (100)] platinum surfaces. Organic molecules of different types form ordered surface structures on low index faces of platinum, but decompose rapidly on stepped surfaces under identical experimental conditions. Surface chemical reactions of diatomic molecules that were studied take place only on stepped surfaces at a detectable rate. The dehydrocyclization of n -heptane to toluene occurs slowly on the Pt(lll) face. The rate of decomposition of n -heptane on stepped platinum surfaces is rapid and the carbon deposit that forms prevents dehydrocyclization. In the presence of hydrogen, however, dehydrocyclization occurs at a rapid rate on stepped surfaces with atomic terraces of (111) orientation, while at a slower rate on stepped surfaces with atomic terraces of (100) orientation. It appears that catalytic reactions can readily be studied using one face of a single crystal of small surface area. Dissociation of diatomic molecules and breaking of C—C and C—H bonds occur preferentially at atomic steps, while the structure of the atomic terraces plays an important role in the more complex dehydrocyclization reaction.