Ensemble effects in the coupling of acetylene to benzene on a bimetallic surface: A study with Pd{lll}/Au

Acetylene coupling to benzene on the Pd{lll} 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 C2H₂ on Pd{lll}. The data can be quantitatively rationalized in terms of a simple model in which catalytic activity is dominated by PdeAu and Pd? surface ensembles, allowance being made for the known degree to which pure Pd{ 111} decomposes the reactant and product molecules.