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 2C(2)H(2)(a) --> C4H4(a); 2C(4)H(4)(a) --> C8H8(a); C8H8(a) --> C6H6(a) + C2H2(a) is ruled out. The mechanism 2C(2)H(2)(a) --> C4H4(a); C4H4(a) + C2H2(a) --> C6H6(a) is established, and it is shown that the first step is rate Limiting overall.