  First-principles calculations are performed to study the adsorption and
dehydrogenation of ethylene on PdAu(001) bimetallic surfaces. The activation
energies for C-H bond breaking of ethylene strongly depend on the ensemble
effect. Particularly, ethylene is unlikely to decompose on the most popular
ensembles of PdAu(001), Pd monomers or Pd second neighborhoods, since the Pd-Au
bridges are less attractive towards vinyl and the eliminated H atom than the
Pd-Pd bridges in the transition state. Correlations are found among the
activation energies, reaction energies and adsorption energies, but no obvious
relationship exist between the surface activity and the Pd-d band center.
