ADSORPTION AND THERMAL-DECOMPOSITION OF BENZENE ON NI(110) STUDIED BY CHEMICAL, SPECTROSCOPIC, AND COMPUTATIONAL METHODS

The chemisorption and reactions of benzene on Ni(110) have been studied by temperature-programmed desorption (TPD) including isotopic labeling, X-ray photoelectron spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), and low-energy electron diffraction (LEED) as a function of coverage and adsorption temperature between 100 and 300 K. At saturation, 70-80% of the benzene is irreversibly chemisorbed, and C-H bond scission commences at 320 K. For high exposures, molecular desorption competes with decomposition. A c(4 x 2) LEED pattern is observed at saturation coverage of chemisorbed benzene (0.2 monolayer by XPS). HREEL spectroscopy indicates that the benzene ring lies parallel to the surface. Semiempirical molecular orbital calculations have been made and predict the most likely adsorption site for benzene chemisorption to be the atop site at a height of about 1.75 angstrom or the short bridge site at 1.90 angstrom. Upon annealing above 300 K, the benzene decomposes, evolving H-2 and forming a surface carbide. Additionally, a species forms which ultimately desorbs as benzene at 460 K but also undergoes H-D exchange with benzene-d6. An unambiguous identification of this fragment has not been made, but the vibrational spectroscopy and isotopic exchange data are consistent with the assignment of a phenyl or benzyne group. The major effects of coadsorbed sulfur and oxygen are to inhibit dissociation and to weaken the interaction between the benzene and the surface.