The structure and reactivity of chemisorbed aromatics: Spectroscopic studies of benzene on Mo(110)

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Abstract
The reactions of benzene on Mo(110) have been investigated under ultrahigh vacuum conditions using temperature programmed reaction spectroscopy (TPRS), x-ray photoelectron spectroscopy (XPS), and near-edge x-ray absorption fine structure (NEXAFS) measurements. Benzene undergoes competing decomposition and desorption processes during temperature programmed reaction; saturation exposures of C6H6 made at a crystal temperature of 120 K yield a small amount of benzene desorption at 360 K as well as substantial H2 evolution at 400 and 530 K. Additionally, a trio of molecular benzene desorptions attributed to weakly bound and multilayer states is observed below 200 K. Isotopic exchange experiments demonstrate that the 360 K molecular desorption arises from intact chemisorbed benzene. For surface temperatures up to 300 K, carbon 1s photoelectron spectra show only a single photoemission peak at a binding energy of 284.0 eV, attributed to molecular benzene. Above 300 K, increasing amounts of atomic carbon are observed at a binding energy of 282.8 eV, indicating that the kinetics for C–C and C–H bond activation in this system are similar. Decomposition to surface C and gaseous H2 is complete by 700 K. At 200 K, near-edge x-ray absorption fine structure measurements confirm the presence of molecular benzene in an orientation with the C6 plane parallel to the surface. After annealing to 450 K, the NEXAFS is complicated by the admixture of absorption structures from a surface intermediate and atomic carbon. Comparison of this NEXAFS with that for surface benzyne (C6H4) produced by annealing benzenethiol (C6H5SH) adsorbed on Mo(110) to 600 K suggests that the intermediate formed from benzene is C6H4 as well.