TEMPERATURE-PROGRAMMED DESORPTION OF COADSORBED HYDROGEN AND ACETYLENE ON PD(111)

Co-adsorbed acetylene and hydrogen react on Pd(111) to form ethylene a nd butene. Benzene is also formed and it is found that the low tempera ture (similar to 280 K) desorption state observed on the clean surface is virtually unaffected by the presence of hydrogen whereas the highe r temperature (similar to 520 K) state is modified. However, adsorbing benzene onto a hydrogen pre-covered surface just affects the sticking coefficient but not its desorption properties. The effect of hydrogen on the high-temperature, acetylene-derived benzene desorption state i s therefore ascribed to its influence on the chemistry of acetylene ra ther than being due to the effect of hydrogen on the benzene that has been formed. The reaction between hydrogen and acetylene to form ethyl ene is shown to be first order. in hydrogen coverage and to proceed wi th an activation energy of similar to 23 kJ/mol. The trailing edge of the ethylene desorption profile is proposed to be due to the lass of a cetylene because of its conversion to a vinylidene species. A fit to t he desorption profile using this model gives an activation energy for this reaction of similar to 16 kJ/mol with a reaction pre-factor of si milar to 600 s(-1).