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), andAuger electron (AES) spectroscopies andby low-energy electrondiffraction (LEED). A strongC-H‚‚‚M interaction between the adsorbate and the metal was present on both surfaces at 100 K, as evidenced by the presence of “softenedmodes” in the C-Hstretching 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 isadsorbedsoas toalign theplaneof theC-C-Cframeworkparallel 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.