Intermetallic Compound AlPd As a Selective Hydrogenation Catalyst: A DFT Study

Recently, it has been demonstrated that intermetallic compounds composed of Pd and Ga or Co and Al provide excellent selectivity for the hydrogenation of acetylene to ethylene. Motivated by experimental works on GaPd catalysts, we have performed a detailed ab initio study of acetylene hydrogenation by the pseudo 5-fold (120) surface of the isostructural and isoelectronic AlPd compound crystallizing in the B20-type structure. The structure of the surface can be described by a triangle-rectangle tiling, and we demonstrate that the most active sites for hydrogenation are triangular arrangements of two Al and one Pd atom. Acetylene is bound to bridge sites between two Al atoms. The most favorable adsorption site for ethylene is on top of the most strongly protruding Pd atom. Activation energies for all steps of the reaction have been calculated. We demonstrate that the activation energies for the rate-controlling steps are comparable to those on reference catalysts (Pd, Pd-Ag and Al13Co4) and that a desorption energy for ethylene that is lower than the activation energy of ethylene to ethyl provides thus good selectivity. We show that the decisive factors for the activity and selectivity of the catalyst are the same on both intermetallic compounds AlPd and Al13Co4.