The C−H···M Interaction and Reactivity Differences of n-Octane on the (1×1) and (5×20) Surfaces of Pt(100)

The interaction and reactivity of n -octane adsorbed on the (1×1) and (5×20) surfaces of Pt(100) have been examined by reflection−absorption infrared (RAIRS), temperature-programmed reaction (TPRS), and Auger electron (AES) spectroscopies and by low-energy electron diffraction (LEED). A strong C−H···M interaction between the adsorbate and the metal was present on both surfaces at 100 K, as evidenced by the presence of “softened modes” in the C−H stretching region of the vibrational spectra centered at ∼2630 and 2750 cm - 1 for the (1×1) and (5×20) surfaces, respectively. The softened modes observed for n -octane on the (5×20) surface of Pt(100) are reminiscent of those seen when this molecule is adsorbed on Pt(111). On both of these surfaces (Pt(100)-(5×20) and Pt(111)) the molecule adopts an all-trans conformation and is adsorbed so as to align the plane of the C−C−C framework parallel to that of the surface. This organization leads to a series of bands appearing in the 2500−2840-cm - 1 region which result from the high-symmetry C−H···M contacts occurring between the n -alkane overlayer and the underlying hexagonal symmetry surfaces. The softened modes observed for an overlayer of octane on the unreconstructed (1×1) surface at low temperature, however, were broad and featureless. Relative to the case for the Pt(100)-(5×20) surface, the activity for the dehydrogenation of an overlayer of n -octane was much greater on Pt(100)-(1×1). These reactivity differences appear to be weakly correlated with the nature of the mode-softening seen in the low-temperature vibrational spectra.