Catalytic properties of Al₁₃Co₄ studied by ab initio methods

Abstract

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, Armbrüster 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-σ 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.

Keywords:

• Al₁₃Co₄ surface
• catalytic reactions
• acetylene hydrogenation

Acknowledgements

The work has been supported by the Austrian Ministry for Education, Science and Art through the Center for Computational Materials Science. MK is also grateful for support from the Grant Agency for Science of Slovakia (No. 2/5096/25) and from the Slovak Research and Development Agency (Grant No. APVV-0413-06, CEX-Nanosmart).