Methylcyclopentane conversion over platinum single crystal surfaces: Evidence for the cyclic mechanism of n-hexane isomerization

Abstract

The surface structure and temperature dependence of methylcyclopentane conversion was investigated near atmospheric pressure at 540–650 K over four platinum single crystal surfaces with flat (100) and (111), and stepped (332) and (557) orientations. The atomic structure and surface composition of the active catalysts were determined before and after reactions using low energy electron diffraction and Auger electron spectroscopy. Selective ring opening to produce 2- and 3-methylpentanes readily occurred on both flat surfaces studied, the (111) and the (100). Comparing results obtained here with previous work done on n-hexane conversion indicates that 2- and 3-methylpentane formation from n-hexane occurs through a methylcyclopentane-like surface species on these platinum species. The rate of formation of methylpentanes was found to be nearly two times faster on the (100) surface than on the (111) surface. The presence of steps on the platinum surface had no significant effect on the rates of ring opening or aromatization since these line defects were quickly covered with carbonaceous deposits as shown by carbon monoxide titration of the active sites. The formation of benzene from methylcyclopentane was found to be much slower than it is from n-hexane on these platinum single crystals. This suggests that platinum alone cannot catalyze the ring expansion of methylcyclopentane. Additional sites, like those found on alumina-supported platinum, are required for this reaction to occur.