Intermetallic Compounds as Selective Heterogeneous Catalysts: Insights from DFT

During the last decade, it has been demonstrated that the use of intermetallic compounds composed of simple and transition metals as catalysts leads to improved activity and selectivity for a number of industrially important processes. Herein, we discuss the insight gained from density functional theory (DFT) calculations for the example of the semihydrogenation of acetylene to ethylene catalyzed by compounds of Pd with Ga or Al and of Co or Fe with Al. Investigations of the potentially active surfaces demonstrate that the chemical composition of nonstoichiometric surfaces depends strongly on the preparation conditions. Ga‐ or Al‐terminated surfaces are catalytically inactive because they cannot dissociate hydrogen, but Pd‐terminated surfaces are also unfavorable because the activation energy for the rate‐controlling step is higher than the desorption energy of acetylene. Active sites promoting a selective semihydrogenation are triangular groups consisting of two simple‐metal (Al, Ga) atoms and one transition‐metal (Pd, Co) atom. Acetylene is strongly bound to simple‐metal atoms in a di‐σ configuration, whereas ethylene is weakly π bonded on top of a transition‐metal atom. The change from strong to weak bonding is of great importance for the selectivity of the process. The novel point elucidated by the DFT calculations is the strong reactivity of the non‐transition‐metal atoms arising from the strong metallocovalent bonding in the compounds, which also contributes to their stability.