Role of hydrogen in benzene formation from benzenethiol on the Ni(111) surface

The role of surface hydrogen in the desulfurization of benzenethiol to form benzene on the Ni(111) surface has been explored in hydrogen pressures ranging between UHV and up to 7 x 10(-3) Torr. Hydrogen availability plays a significant role in the reaction of adsorbed benzenethiol, as demonstrated over a wide range of reaction conditions. Adsorbed hydrogen and phenylthiolate, formed from sulfur-hydrogen bond scission of the benzenethiol upon adsorption at 100 K, react with increasing temperature to produce the primary desorbing products benzene and hydrogen. The dominant benzene formation channel at high coverage occurs at 260 K, with a smaller benzene peak occurring at 290 K and a final benzene peak near 400 K, Increasing the availability of surface hydrogen does not significantly affect either the benzene yield or the benzene formation temperature for high benzenethiol coverage. The same reaction temperature is observed even for hydrogen pressures up to 7 x 10(-3) Ton, as indicated by in situ temperature programmed fluorescence yield near edge spectroscopy (TP-FYNES) experiments. For low coverages of phenylthiolate on the clean Ni(111) surface, benzene formation decreases substantially and the higher temperature processes become more important. Hydrogen preadsorption in the low benzenethiol coverage case dramatically increases the amount of benzene formed at 260 K and decreases the high-temperature peak. Deuterium incorporation following reaction with coadsorbed deuterium clearly indicates three distinct levels of isotope incorporation for the benzene: peaks at 260, 290, and 400 K. Incorporation of a single deuterium dominates for the peak at 260 K, indicating an adsorbed phenylthiolate intermediate dominates for this low-temperature process. Multiple incorporation dominates for the peak at 240 K, indicating a dehydrogenated intermediate of the ''benzyne'' type, which is also observed by vibrational spectroscopy. Benzene formed above 400 K includes even more extensive deuterium incorporation, suggesting an even more extensively dehydrogenated intermediate.