CYCLOHEXANETHIOL ADSORPTION AND REACTION ON THE NI(111) SURFACE

The surface reactions of cyclohexanethiol (CHT) have been characterize d on the Ni(111) surface as part of a larger study of C-S bond activat ion in thiols. The C-S bond is broken by direct interaction with the n ickel surface, and hydrogen is primarily involved in subsequent hydrog enation 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 desorpt ion. Addition of coadsorbed hydrogen does not modify the temperature o r extent of desulfurization. A comparison of desulfurization in CHT an d benzenethiol indicates that the energetics for C-S bond activation c orrelate with bond strength, suggesting a radical mechanism. Only a sm all fraction of the cyclohexane desorbs at 240 K. Most of the gas-phas e desulfurized products desert, in a second process starting at 300 K. Cyclohexane is the first product in this second process. Stepwise deh ydrogenation of C-6 surface intermediate(s) results in desorption of i ncreasingly dehydrogenated C-6 hydrocarbons with increasing temperatur e, culminating in benzene desorption at 315 K. Increasing dehydrogenat ion with increasing temperature is correlated with free hydrogen desor ption in this same temperature range. A final disproportionation proce ss in the 450 K range produces a small amount of benzene. With increas ing temperature no further gas-phase organics are produced. Overall, a bout half of the CHT in a monolayer undergoes complete dehydrogenation to form adsorbed carbon and sulfur. For small coverages (<0.07 ML) mo st 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 C-6 hydroca rbons are formed in the 300-315 K temperature region. It is interestin g to note that a small fraction of CHT remains molecular following ads orption at low temperature, producing a CHT peak at 180 K even for low coverages.