ETHYNE CYCLIZATION TO BENZENE OVER CU(110)

Ethyne cyclization to benzene over Cu(110) is an efficient reaction th at proceeds at low temperatures with close to 100% efficiency. On the clean surface, C2H2 adsorbs into islands, there is no threshold covera ge 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 pr ocess. In each of these four respects the behavior is very different f rom that found on Pd(111). Isotope tracing experiments show that cycli zation occurs by an associative mechanism, and the use of cis-1,2-dich lorocyclobutene 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 schem e 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); C4 H4(a) + C2H2(a) --> C6H6(a) is established, and it is shown that the f irst step is rate Limiting overall.