MODIFYING SURFACE REACTIVITIES BY A CARBIDE OVERLAYER - A VIBRATIONALSTUDY OF THE REACTION-MECHANISMS OF CYCLOHEXENE AND 1,3-CYCLOHEXADIENE ON MO(110) AND (4X4)-C MO(110) SURFACES/

The dehydrogenation and thermal decomposition mechanisms of cyclohexen e and 1,3-cyclohexadiene on clean Mo(110) and carbide-modified (4 x 4) -C/Mo(110) surfaces have been studied using temperature-programmed des orption (TPD) and high-resolution electron energy loss spectroscopy (H REELS). On the clean Mo(110) surface, partial dehydrogenation of a fra ction of the cyclohexene molecules occurs at temperatures as low as 80 K. When the surface is heated to 150 K, the HREEL spectra obtained ar e characteristic of a C6H9 intermediate, as seen by a comparison with HREEL spectra reported for C6H9 On Pt(111).(1,2) At higher temperature s, competing C-C and C-H bond cleavage reactions lead to the formation of surface carbon and the evolution of hydrogen. In contrast, on the carbide-modified surface, the primary reaction pathway for cyclohexene is selective dehydrogenation to form benzene and hydrogen. In the cas e of 1,3-cyclohexadiene, the HREEL results suggest that dehydrogenatio n to form benzene occurs at 80 K on the clean Mo(110) surface, based o n a comparison with the HREEL spectrum for benzene directly dosed onto Mo(110) at 80 K. However, upon heating, most of the benzene decompose s to form surface carbon and hydrogen, as shown by TPD studies. On the carbide-modified surface, the primary reaction pathway for 1,3-cycloh exadiene is selective dehydrogenation to form benzene, which desorbs a t 313 K. Furthermore, the HREEL results also indicate that a competing reaction pathway occurs to form a surface intermediate which most lik ely has an tilted aromatic c-C-6 ring, such as a surface phenyl specie s.