Pseudo-2-Fold Surface of the Al₁₃Co₄ Catalyst: Structure, Stability, and Hydrogen Adsorption

A few low-order approximants to decagonal quasicrystals have been shown to provide excellent activity and selectivity for the hydrogenation of alkenes and alkynes. It is the case for the Al₁₃Co₄ compound, for which the catalytic properties of the pseudo-2-fold orientation have been revealed to be among the best. A combination of surface science studies, including surface X-ray diffraction, and calculations based on density functional theory is used here to derive an atomistic model for the pseudo-2-fold o-Al₁₃Co₄ surface, whose faceted and columnar structure is found very similar to the one of the 2-fold surface of the d-Al-Ni-Co quasicrystal. Facets substantially stabilize the system, with energies in the range 1.19-1.31 J/m², i.e., much smaller than the ones of the pseudo-10-fold (1.49-1.68 J/m²) and pseudo-2-fold (1.66 J/m²) surfaces. Faceting is also a main factor at the origin of the Al₁₃Co₄ catalytic performances, as illustrated by the comparison of the pseudo-10-fold, pseudo-2-fold and facet potential energy maps for hydrogen adsorption. This work gives insights toward the design of complex intermetallic catalysts through surface nanostructuration for optimized catalytic performances.