| Abstract
| - Acetylene trimerizes to benzene on the (111) face of copper, as it does on the (100) and (110) planes. However,Cu(111) also yields butadiene and cyclooctatetraene, the latter never previously found with Cu or any othermaterial. No coverage threshold is observed for the onset of these coupling reactions, implying high adsorbatemobility: gaseous benzene is formed by a surface reaction rate-limited process, whereas butadiene andcyclooctatetraene are formed by desorption rate-limited processes. H/D isotope tracing shows that benzeneformation proceeds via a statistically random associative mechanism, whereas butadiene formation is associatedwith strong kinetic isotope effects, probably associated with C−H cleavage. A pericyclic mechanism involvingdimerization of C4H4 metallocycles is proposed to account for the formation of cyclooctatetraene. We alsofound that ∼45 nm α-alumina supported copper particles operated under catalytic conditions at atmosphericpressure yield the same principal reaction products as those found with Cu(111) under vacuum conditions. Ittherefore seems likely that the elementary reaction steps that describe the surface chemistry of the modelsystem are also important under practical conditions. Comparison of the structure, bonding, and reactivity ofacetylene on Cu(111) and Pd(111) indicates that the effectiveness of copper in promoting C−H cleavage inadsorbed acetylene is associated with greater rehybridization of the C−C bond with concomitant weakeningof the C−H bond.
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