Geometrical and electronic effects in the conversion of acetylene to benzene on Au(111)/Pd and Au/Pd surface alloys

In contrast with Pd(111), Au(111) is totally inert towards acetylene cyclisation. However, Pd overlayers on Au(111) efficiently catalyse the trimerisation of acetylene to benzene, even at submonolayer thickness. The extent of acetylene conversion and the binding energy of the resulting benzene depend on the atomic composition, morphology and electronic properties of the surface. Thin, un-annealed films exhibit maximum benzene yield at ∼ 3 ML thickness. Annealing leads initially to increased ordering of the Pd overlayer and ultimately to Pd/Au alloy formation. For a given Pd film thickness, limited annealing leads to increased benzene formation. Rougher, thicker Pd films are less efficient catalysts than smoother, thinner ones and appear to give rise to a new binding site for the reactively formed benzene. For a given benzene yield, Au/Pd alloy formation induced by annealing has a major effect on the desorption temperature of reactively-formed benzene. The results are discussed in terms of geometric and electronic effects on the reaction mechanism.