Temperature programmed desorption of co-adsorbed hydrogen and acetylene on Pd(lll)

Co-adsorbed acetylene and hydrogen react on Pd(l l l ) to form ethylene and butene. Benzene is also formed and it is found that the low temperature (~ 280 K) desorption state observed on the clean surface is virtually unaffected by the presence of hydrogen whereas the higher temperature (~ 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 is therefore ascribed to its influence on the chemistry of acetylene rather than being due to the effect of hydrogen on the benzene that has been formed. The reaction between hydrogen and acetylene to form ethylene is shown to be first order in hydrogen coverage and to proceed with an activation energy of ~ 23 kJ/mol. The trailing edge of the ethylene desorption profile is proposed to be due to the loss of acetylene because of its conversion to a vinylidene species. A fit to the desorption profile using this model gives an activation energy for this reaction of ~ 16 kJ/mol with a reaction pre-factor of ~ 600 s-1.