Spectroscopic identification of surface phenyl thiolate and benzyne on Mo(110)

Abstract

Chemisorbed phenyl thiolate (C6H5S) and surface benzyne (C6H4) formed during the temperature programmed reaction of benzenethiol on Mo(110) have been characterized using x-ray photoelectron and high resolution electron energy loss spectroscopies. Electron energy loss spectroscopy demonstrates that at all exposures the S–H bond of benzenethiol breaks upon adsorption at 120 K, while x-ray photoelectron spectroscopy confirms that the C–S bond is intact. The intermediate formed upon adsorption is assigned as a chemisorbed phenyl thiolate. At high exposures, approximately 40% of the phenyl thiolate reacts by way of hydrogenolysis at 350 K to form gaseous benzene and atomic sulfur, while 60% undergoes dehydrogenation at ≊370 K to form surface benzyne and atomic sulfur. Electron energy loss spectroscopy reveals that surface benzyne is aromatic in nature, and x-ray photoelectron spectroscopy confirms that the C–S bond is no longer intact. Surface benzyne is unusually stable on the sulfided Mo(110), decomposing at 680 K to gaseous dihydrogen and atomic carbon, with an activation energy of 42 kcal/mol in the limit of high coverage. At low coverages, the phenyl thiolate decomposes on Mo(110) to surface carbon, surface sulfur, and gaseous dihydrogen, with decomposition complete below 600 K. At low coverages, no formation of gaseous benzene or surface benzyne occurs. The coverage dependence of the reaction kinetics of the phenyl thiolate on Mo(110) are attributed, in part, to a change in the structure of the adsorbed thiolate at high coverages.