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. ■ INTRODUCTION The hydrogenation of acetylene to ethylene, C2H2+H2 → C2H4, is a chemical reaction of industrial importance. Ethylene produced by a steam cracking process contains a small fraction of acetylene. In the ethylene feedstock used for the production of polyethylene, any contamination by acetylene has to be removed to avoid poisoning of the polymerization catalyst. Usually Pd-based hydrogenation catalysts are used for the hydrogenation of acetylene to ethylene, and because further hydrogenation of ethylene to ethane is undesired, the selectivity of the catalyst plays a significant role. A typical hydrogenation catalyst contains metallic palladium dispersed on an inert oxidic support, e.g., Al2O3. Dispersed palladium exhibits high activity but unsatisfactory selectivity. The properties of Pd as catalyst for the hydrogenation of alkynes to alkenes have been studied experimentally and theoretically for many years. It has been demonstrated that pure Pd catalysts are selective if the reaction is performed under conditions where subsurface carbon is formed. However, the subsurface hydrogen promotes further hydrogenation to an alkane and hence reduces selectivity. It was also found that alloying Pd with other metals such as Ag, Au, Pb, or Ga can significantly improve selectivity. Osswald et al. compared the performance of various catalysts for acetylene hydrogenation under laboratory conditions. The relevant parameters are the rate of conversion (expressing the concentration of acetylene before and after the reaction) and the selectivity (measuring the concentration of the desired product, ethylene, after the reaction). For pure Pd on an Al2O3 support, a conversion rate of 43% and a low selectivity of only 17% were reported. For a Pd20Ag80 alloy catalyst, the conversion rate increased to 83% and the selectivity to 49%. Alloying Pd with Ag can be considered as the current industrial solution for acetylene hydrogenation. Recently, a novel concept for the design of selective and stable catalysts for the hydrogenation of alkynes has been announced by Kovnir et al.: the isolation of the active sites on the surface of a complex intermetallic compound. On the surface of pure metals, every atom is a potential active site. As Pd atoms react strongly with alkynes and alkenes, the reactants are fully hydrogenated to alkanes and the catalysts have poor selectivity. The coordination of Pd atoms by less reactive atoms such as Ag in a Pd−Ag alloy catalyst reduces the adsorption energies of alkenes and favors their desorption over further hydrogenation to alkanes. Detailed ab initio density functional calculations, combined with kinetic Monte Carlo studies of Neurock et al. have demonstrated that electronic effects (changes in the local electronic density of states) are far less important than geometric effects (the number of Ag atoms occupying nearest neighbor sites around a Pd atom). In substitutional alloys, both components are randomly distributed, and the optimal local coordination is realized only around a fraction of the Pd atoms present on the alloy surface. In contrast, the surfaces of intermetallic compounds are ordered, Received: December 21, 2011 Revised: February 15, 2012 Published: February 21, 2012 Article