Cyclohexanethiol Adsorption and Reaction on the Ni(111) Surface

Abstract

The surface reactions of cyclohexanethiol (CHT) have been characterized on the Ni(111) surface as part of a larger study of C−S bond activation in thiols. The C−S bond is broken by direct interaction with the nickel surface, and hydrogen is primarily involved in subsequent hydrogenation of desulfurized hydrocarbon intermediates. CHT adsorbs at 120 K primarily as cyclohexylthiolate. With heating, C−S bond scission is observed at 240 K evidenced by both sulfur XPS and cyclohexane desorption. Addition of coadsorbed hydrogen does not modify the temperature or extent of desulfurization. A comparison of desulfurization in CHT and benzenethiol indicates that the energetics for C−S bond activation correlate with bond strength, suggesting a radical mechanism. Only a small fraction of the cyclohexane desorbs at 240 K. Most of the gas-phase desulfurized products desorb in a second process starting at 300 K. Cyclohexane is the first product in this second process. Stepwise dehydrogenation of C6 surface intermediate(s) results in desorption of increasingly dehydrogenated C6 hydrocarbons with increasing temperature, culminating in benzene desorption at 315 K. Increasing dehydrogenation with increasing temperature is correlated with free hydrogen desorption in this same temperature range. A final disproportionation process in the 450 K range produces a small amount of benzene. With increasing temperature no further gas-phase organics are produced. Overall, about half of the CHT in a monolayer undergoes complete dehydrogenation to form adsorbed carbon and sulfur. For small coverages (<0.07 ML) most of the CHT undergoes complete decomposition on the Ni(111) surface. For these small coverages, no cyclohexane is observed at 240 K, even in the presence of coadsorbed hydrogen. Smaller amounts of C6 hydrocarbons are formed in the 300−315 K temperature region. It is interesting to note that a small fraction of CHT remains molecular following adsorption at low temperature, producing a CHT peak at 180 K even for low coverages.