Bonding and Adsorbed Species

Bonding and Adsorbed Species.—Broden et al. have investigated geometric and electronic effects in the chemisorption and reaction of acetylene, ethylene, and benzene on Ir(lOO) surfaces. Though not a paper on catalysis, the study represents primary information on adsorption processes for molecules which may or may not be involved in catalytic changes. The physical techniques used were LEED, AES, and UPS. 

When acetylene was adsorbed on Ir, which exposed the (5x1) structure, no ordered adsorbed species could be detected the original (5x1) LEED pattern persisted even after adsorption of acetylene to saturation coverage. This was unexpected, for the reconstructed Pt(lOO) surface is destroyed by adsorption of acetylene or ethylene. Adsorption on Ir(100)-(1 x 1) of acetylene gave results similar to those for Ir(100)-(5 x 1), viz. there was no structural change in the surface. 

From an examination of the tt, 3cTg, 2cr orbitals for gaseous and adsorbed acetylene at 132 K on the (5x1) surface it appears that the perturbation of the acetylene is small and restricted to the It- orbital. The orbital identification for ethylene, gaseous and adsorbed, is more complex, but it appears that the molecule is relatively unperturbed upon adsorption except for the 77-like bin orbital. 

The spectral interpretation which the authors employ depends on observations from organometallic chemistry in which olefins are bonded to a metal atom. The general view of lengthening of C—C bonds and the bending of molecules under these conditions is used in their interpretation. By bending is meant the phenomenon in which the phenyl groups are bent by 40° out of the Pt—C—C plane. Thus the authors believe that acetylene stretches and probably bends when adsorbed at room temperature on Ir(100)-(1 x l)and(5 x 1). On 

Of catalytic importance is the observation that preadsorption of hydrogen leads to C—C scission of acetylene chemisorbed at room temperature on Ir(lOO)-(5 X 1). 

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