Low pressure studies of dehydrocyclization of n-heptane on platinum crystal surfaces using mass spectrometry, auger electron spectroscopy, and low energy electron diffraction

Abstract

The dehydrocyclization of n-heptane on platinum crystal surfaces of area less than 1 cm 2 was studied in the temperature range of 100–400 °C and at pressures in the 10 −4 Torr range. The toluene formation rate was monitored in a static system by a mass spectrometer, the surface structure by low energy electron diffraction, and the surface composition by Auger electron spectroscopy. The (111) face and two types of stepped platinum crystal faces were used. The stepped surfaces exhibit a surface structure of ordered steps of monatomic height separated by terraces with (111) orientation for one face and by terraces of (100) orientation for the other. The initial rate of toluene formation on the stepped surface with (111) terraces is twice as fast as on the surface with (100) terraces and the reaction can be sustained in the presence of hydrogen for more than an hour. A new (√3 × √3)-R30 ° surface structure which develops in the course of the reaction or a (9 × 9) surface structure which appears upon heat treatment to 850 °C have no detrimental effect on the reactivity. These surface structures can be attributed to the presence of ordered carbon at the platinum surface. Under identical conditions, the toluene yield of the stepped surface with (100) terraces decreases steadily as a function of the number of doses and reaches the detection limit after about an hour. The low index (111) surface appears to be less reactive than the stepped surface with (111) terraces by at least an order of magnitude. Both of the less reactive surfaces become covered by a disordered carbon-containing layer during the reaction. The presence of hydrogen during the reaction appears to reduce the rate of dissociative chemisorption of n-heptane, so that the dehydrocyclization can successfully compete with it.