Abstract: Hydrogen spillover has been studied by means of benzene hydrogenation reaction on Al_2O_3. The spillover hydrogen is produced from a point source of spillover Pt/Al_2O_3. Observing benzene adsorbed on Al_2O_3 by in situ FTIR, it is found that the adsorption is a physisorption which occurs by intereaction between π electrons of benzene and surface OH groups of Al_2O_3. The adsorbed force existing between benzene and OH groups is weak, and can not make adsorbed benzene being in active state. Therefore, the benzene hydrogenation reaction on Al_2O3 containing a point source of spillover Pt/Al_2O3 is mainly an interface reaction.
The infrared spectra following observation benzene hydrogenation reaction illustrate that there are a few cyclohexane molecules in beam after the reaction proceeds for 15 mimutes at 50 ℃ and 550 Torr of H_2, and its intensities of CH_2 stretching vibration bands increase linearly with time until all benzene molecules in beam are transformed after 5 hours. These data imply that the spillover hydrogen may migrate some distance on Al_2O3 surface under experiment conditions, and its migrate velocity is the rate controlling step for benzene hydrogenation.
Cyclohexadiene and cyclohexene are observed in further temperature progra mmed surface reaction-mass spectroscopy (TPSR-MS) on Pt/Al_2O_3 catalyst. The result clearly reveals that some successive steps are involved in the hydrogenation process, i.e., firstly benzene is hydrogenated into cyclohexadiene, and then it is transformed into cyclohexene, finally cyclohexane is formed and desorbed In view of their examinable concentrations are low, it may be considered that all these steps are fast.