Ethyne Cyclization to Benzene over Cu(110)

Ethyne cyclization to benzene over Cu(110) is an efficient reaction that proceeds at low temperatures with close to 100% efficiency. On the clean surface, C₂H₂ adsorbs into islands, there is no threshold coverage for the onset of reaction, and benzene evolution into the gas phase occurs in a single TPR peak due to a surface reaction rate limited process. In each of these four respects the behavior is very different from that found on Pd(111). Isotope tracing experiments show that cyclization occurs by an associative mechanism, and the use of cis-1,2-dichlorocyclobutene indicates that C₄H₄ is the key reaction intermediate, as it is on Pd(111). Additional data, including results of experiments with C₈H₈ and with C₄H₄Cl₂ + C₂D₂, demonstrate that cyclooctatetraene is not a reaction intermediate in this system, and the possible scheme 2C₂H₂(a) → C₄H₄(a); 2C₄H₄(a) → C₈H₈(a); C₈H₈(a) → C₆H₆(a) + C₂H₂(a) is ruled out. The mechanism 2C₂H₂(a) → C₄H₄(a); C₄H₄(a) + C₂H₂(a) → C₆H₆(a) is established, and it is shown that the first step is rate limiting overall.