The Mechanism of Hydrocarbon Catalysis on Platinum Crystal Surfaces

Abstract

In the past several years we have studied the atomic structure of platinum crystal surfaces and the structure of adsorbed hydrocarbons by low-energy electron diffraction and the surface composition by Auger electron spectroscopy. Catalytic reactions of low reaction probability (dehydrocyclization, dehydrogenation) have been studied on one face of a single crystal less than 1 cm² in area by mass spectrometry at low pressures (∼10^-4 Torr) and by gas chromatography at high pressures (∼10³ Torr).

The atomic structure of high-Miller-Index platinum surfaces is characterized by atomic height steps arranged periodically and separated by atomic terraces of low-Miller-Index orientation [(111) or (100)]. Experiments that compare the reactivity of crystal surfaces and supported platinum particles indicate that the atomic structure of polydispersed catalyst particles can be reproduced on these stepped crystal surfaces. The structure of adsorbed hydrocarbons has been studied on both low- and high-Miller-Index platinum surfaces. Atomic steps play a controlling role in dehydrogenating the hydrocarbon molecules and in dissociating hydrogen and other diatomic molecules of large binding energy. n-Heptane may undergo isomerization, hydrogenolysis, and dehydrocyclization on the various platinum surfaces. The atomic structure of the stepped crystal surfaces appears to control the product distribution for these competing reactions.