Different adsorbate binding mechanisms of hydrocarbons:: Theoretical studies for Cu(111)-C2H2 and Cu(111)-C2H4

Experimental results for acetylene and ethylene adsorption on metal surfaces indicate qualitatively different adsorbate binding mechanisms, depending on the adsorbate and substrate material. Experiments on Cu(111)-C2H2 identify a strongly distorted adsorbate while the adsorption energy is small. In contrast, recent experiments on Cu(111)-C2H4 have identified a weakly physisorbed adsorbate without noticeable structural changes. The qualitatively different binding behavior between C2H2 and C2H4 with Cu(111) has been examined by ab-initio density functional theory (DFT) cluster studies. Restricted geometry optimizations yield potential energy curves E(z) which exhibit two minima for both adsorbates. The outer minimum refers to an undistorted adsorbate (indicating a physisorbed state) while the inner minimum yields a strongly distorted adsorbate (confirming a competitive binding state). Qualitative differences between the inner and outer potential minima of the two systems can explain their different behavior. The E(z) curves can also rationalize why in experiments for Cu(111)-C2H2 and Cu(111)-C2H4 only one of the two adsorbate states has been verified so far. However, the present calculations suggest to further look for experiments where both adsorbate states can be prepared. (C) 1998 Elsevier Science B.V. All rights reserved.