Photoelectron diffraction study of a catalytically active overlayer:: C2H2 on Pd{111}

A quantitative structure determination of a newly discovered (2 x2) adsorption phase of acetylene chemisorbed on Pd{111}has been performed by scanned-energy mode photoelectron diffraction: this phase corresponds to the threshold coverage for the catalytic conversion of acetylene to benzene. The carbon atoms in the C2H2 molecule are located almost over bridge sites with a C-C bond length of 1.34+0.10 Angstrom. The centre of the molecule being positioned almost over a hollow site. Of the two hollow sites the hcp site (directly above a second layer Pd atom) is favoured, particularly by a subset of the data most sensitive to this aspect of the structure. bur the full analysis indicates that the fee site (above a third layer Pd atom) cannot formally be excluded. The adsorption site adopted by acetylene in the higher coverage (root 3x root 3)R30 degrees phase on Pd{111} is essentially identical. This is the dominant structure in the coverage regime which is catalitically active for the conversion of acetylene to benzene. The implications of these findings for acetylene coupling reactions over Pd{111} are discussed. (C) 1998 Elsevier Science B.V.