Ethyne cyclization to benzene over Cu(l 10) 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(lll). Isotope tracing experiments show that cyclization occurs by an associative mechanism, and the use ofcis-l,2-dichlorocyclobutene indicates that C4H4ÍS the key reaction intermediate, as it is on Pd(lll). Additional data, including results of experiments with C8H8 and with C4H4CI2 + 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) -CeHeCa) + C2H2(a) is ruled out. The mechanism 2C2H2(a) -C4H4(a); C4H4(a) + 02 2( ) -CeHeCa) is established, and it is shown that the first step is rate limiting overall.
High-resolution electron energy loss spectroscopy and
angle-resolved ultraviolet photoelectron spectroscopy data indicate that ethyne adopts a low symmetry (most likely
C1) adsorption geometry on Cu(110).
Detailed ab
initio Hartree−Fock cluster
calculations identify a minimum on the potential energy
surface
for ethyne in a C1 adsorption geometry. This structure
also provides the best agreement between the
experimental and calculated vibrational frequencies of the geometries
investigated. In addition, the
calculations show that the internal structure of the ethyne molecule is
relatively insensitive to the adsorption
site and that the adsorbed molecule is essentially sp2
hybridized.
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