Direct Observation of Surface Reactions of Acetylene on Pd(111) with Scanning Tunneling Microscopy

The cyclotrimerization of acetylene to benzene on a Pd(111) surface has been studied by scanning tunneling microscopy (STM). The formation of benzene becomes visible upon increasing the acetylene coverage at 140 K. Initially, the acetylene molecules adsorb in a (2 × 2) ordered layer, which is compressed into an ordered (√3 × √3) R 30° structure when more acetylene is added. The cyclotrimerization reaction is observed just before saturation of the (√3 × √3) R 30° structure and stops when the saturation coverage is reached. Further exposure to acetylene does not result in a reaction, indicating that the cyclotrimerization reaction involves a transient adsorption state, different from that in the (√3 × √3) R 30° layer. The (√3 × √3) R 30° layer itself is stable up to 230 K, even when a background pressure of acetylene is present. At 230 K, the (√3 × √3) R 30° layer decays, a process which is related to the isomerization and further decomposition of acetylene. The results are consistent with the available spectroscopic data and indicate that the limited conversion of acetylene to benzene is determined by the relative rates of (√3 × √3) R 30° domain formation and cyclotrimerization.