Catalytic properties of Al13Co4 studied by ab initio methods

While the surfaces of ordinary crystals provide only a few inequivalent adsorption sites, the complex landscape of the surfaces of quasicrystals and their approximants provides a rich variety of different adsorption sites. Recently, Armbruster et al. reported that Al13Co4, whose structure is closely related to decagonal Al-Ni-Co quasicrystals, is an efficient and selective hydrogenation catalyst for alkynes. In the present work, the hydrogenation of acetylene to ethylene on the (100) surface of Al13Co4 has been studied using ab initio density-functional simulations. Surprisingly, the stable cleavage surface of Al13Co4 is strongly corrugated. The surface is covered by zig-zag chains of edge-sharing Al pentagons, each centered on a Co atom and separated from neighboring chains by wide troughs. The binding energies for adsorption and co-adsorption of H2 and C2H2 molecules at various surface sites have been calculated. Surprisingly, in the energetically most favorable configuration, acetylene is bound in a di-sigma configuration to two Al atoms, not to the Co atom. We have searched for the optimal reaction pathway for the dissociatible adsorption of hydrogen and for the hydrogenation of acetylene to vinyl, ethylene by a Langmuir-Hinshelwood mechanism. The energetic barriers for all reaction steps were calculated by the nudged-elastic-band method. It was found that the energetic barrier of any reaction step does not exceed 0.65 eV (63 kJ/mol). This value is lower than the activation energies for acetylene to ethylene hydrogenation over a Pd catalyst where barriers of 78 kJ/mol and 85 kJ/mol were reported for the rate-determining steps.