A theoretical study of catalytic coupling of propyne on Cu{111}

The coupling mechanism of two propyne molecules on the Cu{111} surface has been studied by means of a DFT cluster model approach. The gas-phase dimerization is highly unfavored because of the energy cost to activate propyne by promoting molecules to the triplet state. However, on the surface, propyne is adsorbed with a geometry very close to that of gas-phase propyne in the triplet state and, therefore, activation of the reacting molecules does not incur any additional energy cost. Moreover, isomerization to vinylcarbene is necessary to allow head-to-tail or head-to-head coupling resulting in 1,4- and 1,3-cyclohexadiene intermediates. Vinylcarbene biradicals are present at the surface because the isomerization process proceeds at practically no (thermodynamic) cost. Both head-to-tail and head-to-head interactions suggested by experiment are possible. Both cyclohexadiene intermediates can dehydrogenate to yield benzene and H-2 with a moderate energy cost. An alternative head-to-head interaction, without interacting tails, yields two C-6 noncyclic intermediates which, upon H-2 addition, can be regarded as being responsible for the 82 amu product observed in the reaction.