Effect of surface modification on the structure and reactivity of cyclohexane on Mo(110)

In this study we report the effect of atomic sulfur on the surface chemistry of cyclohexane on Mo(110). At low coverages (0.05 monolayer), cyclohexane adsorbs predominantly in a strongly perturbed state that exhibits pronounced C-H vibrational ''mode-softening'' and undergoes facile dehydrogenation at similar to 195 K. We find that the dehydrogenation occurs primarily from cyclohexane molecules exhibiting mode softening and that the 1, 3 and 5 carbons of this strongly perturbed cyclohexane state lie in the same plane, parallel to the surface, based on infrared data. In contrast, at higher coverages a second, relatively unperturbed cyclohexane state is preferentially populated, wherein the ring is tilted with respect to the surface plane and leads predominantly to molecular desorption. A continuous increase in the frequency of the weakened state and concomitant decrease in the dehydrogenation probability occurs, with increase in the concentration of the coadsorbed sulfur. Furthermore, the effect is more pronounced for oxygen than for sulfur for a coverage of similar to 0.1 monolayers, indicating that the electronegativity of the coadsorbate is the key parameter in these systems. These studies demonstrate conclusively that the weakening arises primarily from donation of electron density from the metal to the C-H antibonding orbitals of cyclohexane.