Cyclohexanethiol adsorption and reaction on the Ni(100) surface

Abstract

The adsorption and reactions of cyclohexanethiol (CHT) have been characterized on the Ni(100) surface in the presence and absence of coadsorbed hydrogen. Cyclohexanethiol adsorbs at 120 K on the Ni(100) surface primarily as cyclohexylthiolate and surface hydrogen. Saturation coverage of CHT on the Ni(100) surface (0.25 ML) is very similar to other thiols on this surface, and indicates that an upright orientation is dominant. CS bond scission was observed at 240 K, as evidenced by sulfur X-ray photoelectron spectroscopy (XPS). However, no desorbing products are observed at this temperature due to strong interactions with the surface. The CHT CS bond scission temperature is 30 K lower than the CS bond breaking temperature for benzenethiol. Therefore, the energetics for CS bond activation correlate with bond strength, suggesting a radical mechanism. The reaction limited cyclohexane peak at 320 K is the first desulfurized hydrocarbon product which desorbs. Increasing dehydrogenation with increasing temperature results in benzene desorption at 360 K. The complex hydrogen desorption profile over the 320 to 360 K temperature range indicates significant stepwise dehydrogenation of surface intermediates. No carbon containing compounds desorb after the 360 K benzene peak. The amount of hydrogen desorbed indicates that approximately 60% of the CHT undergoes complete dehydrogenation for a saturated monolayer. Preadsorbed hydrogen does not influence CS bond activation, since free hydrogen desorbs below the CS bond scission temperature. However, additional coadsorbed hydrogen decreases the total product yield by blocking adsorption of the thiol. For small CHT coverages, complete decomposition dominates on the Ni(100) surface, yielding only gas phase hydrogen and surface bound sulfur and carbon. It is interesting to note that a small fraction of the CHT remains molecular and interacts primarily through the cyclohexyl ring, producing a molecular CHT peak at 180 K even at low coverages. The CS bond activation temperature is independent of surface hydrogen concentration, indicating that CS bond activation involves direct interaction of the metal with the CS bond, as observed previously for benzenethiol on this surface.