Ensemble Effects in the Coupling of Acetylene to Benzene on a Bimetallic Surface:  A Study with Pd{111}/Au

Acetylene coupling to benzene on the Pd{111} surface is greatly enhanced by the presence of catalytically inert Au atoms. LEED and Auger spectroscopy show that progressive annealing of Au overlayers on Pd{111} leads to the formation of a series of random surface alloys with continuously varying composition. Cyclization activity is a strong function of surface composition the most efficient catalyst corresponds to a surface of composition ∼85% Pd. CO TPD and HREELS data show that acetylene cyclization activity is not correlated with the availability of singleton Pd atoms, nor just with the presence of 3-fold pure Pd sites the preferred chemisorption site for C 2 H 2 on Pd{111}. The data can be quantitatively rationalized in terms of a simple model in which catalytic activity is dominated by Pd 6 Au and Pd 7 surface ensembles, allowance being made for the known degree to which pure Pd{111} decomposes the reactant and product molecules.