Investigation of an Alternative Reaction Pathway in the Cyclization of Ethyne to Benzene on Palladium: Cyclooctatetraene on Pd(111)

Cyclooctatetraene (COT) to benzene conversion can be induced on Pd(111) under conditions of (low) temperature and overlayer density similar to those that obtain during the trimerization of ethyne to benzene. Coadsorbed NO inhibits COT decomposition, but does not increase its reactivity; HREELS shows that COT drastically affects NO site occupancy. Coadsorbed C ₆ D ₆ both inhibits decomposition and strongly enhances the reactivity of COT: almost 100% COT → benzene selectivity can be achieved. A consistent interpretation of the vibrational spectra and reactivity is given: flat-lying COT undergoes decomposition, μ ⁴ tub-COT is unreactive, chair-COT μ ⁴ is reactive. This explanation takes account of the effects of metal-mediated charge transfer on the geometry of adsorbed COT. C ₄ H ₄ cannot be induced to undergo coupling to COT, even in the presence of C ₆ D ₆ or NO, both of which strongly promote ethyne-to-benzene conversion. It is therefore argued that C ₂ H ₂ → C ₄ H ₄ → C ₈ H ₈ → C ₆ B ₆ (+C ₂ H ₂ ) cannot be a major pathway in ethyne cyclization.