EFFECT OF STRONGLY BOUND OXYGEN ON THE DEHYDROGENATION AND HYDROGENATION ACTIVITY AND SELECTIVITY OF PLATINUM SINGLE-CRYSTAL SURFACES

The dehydrogenation of cyclohexene and cyclohexane, and the hydrogenation of cyclohexene were studied on the clean and preoxidized surfaces of three platinum single crystals-a Pt(111), a stepped Pt(S)-[6(111) × (100)] and a kinked Pt(S)-[7(111) × (310)]-at low pressure (10-5 to 10-5 Torr total pressure), at 150 °C. Oxygen coverages were monitored by Auger electron spectroscopy (AES); the surface structures of the clean and oxidized platinum crystals were determined by low-energy electron diffraction (LEED). Low coverages of strongly bound oxygen enhanced the rates of the dehydrogenation and hydrogenation reactions, and changed the selectivity of cyclohexene dehydrogenation to benzene over hydrogenation to cyclohexane. These effects of preoxidation on catalytic rates and selectivity were found to be sensitive to the structure of the platinum surface, kink sites playing a particularly active role in the enhancement of dehydrogenation and hydrogenation activity by strongly bound oxygen. Three models are discussed which relate the oxidation of platinum surfaces to the observed effects on catalytic activity and the structure sensitivity. A change in the electronic structure of the platinum surface through oxidation provides the best general model for explaining the oxygen effects, though complex formation involving the adsorbed oxygen or surface reconstruction during oxidation may also be important. © 1979.