The (2 1 0) surface of intermetallic B20 compound GaPd as a selective hydrogenation catalyst: A DFT study

Abstract

Recently, it has been demonstrated that intermetallic compounds composed of transition (Pd and Co) and simple (Ga and Al) metals used as catalysts provide excellent efficiency and selectivity for the hydrogenation of acetylene to ethylene. Motivated by experimental work on GaPd catalysts, we have performed a detailed density-functional study of the hydrogenation of acetylene catalyzed by the B20-type GaPd compound. We have identified the pseudo-fivefold (210) surface of the B20 structure as a possible catalytically active surface termination. The atomic structure of the (210) surface can be described by a triangle-rectangle tiling. An atomistic scenario has been constructed for the hydrogenation process catalyzed by this surface. We demonstrate that the active sites for the hydrogenation of acetylene to vinyl and further to ethylene are triangular arrangements of two Ga and one Pd atom atoms. Acetylene is bound to bridge sites formed by two Ga atoms. Vinyl formed in a first hydrogenation step is strongly attached to a Ga atom through its CH group, while the CH2 group shifts toward a neighboring Pd atom. In contrast, ethylene is bound only on top of the Pd atom most strongly protruding from the surface. The activation energies for all steps of the reaction (dissociative adsorption and diffusion of hydrogen, adsorption of all hydrocarbon species, hydrogenation of acetylene to vinyl, of vinyl to ethylene, and of ethylene to ethyl, desorption of ethylene) have been calculated. The activation energies for the rate-controlling steps of 60–70kJ/mol are comparable to those on other catalytically active compounds AlPd and Al13Co4 investigated in previous studies and also comparable or even lower than for a pure Pd catalyst. The desorption energy for ethylene of 45kJ/mol is at least by 14kJ/mol lower than the activation energy of ethylene to ethyl and provides thus excellent selectivity of GaPd. We show that the decisive factors for the activity and selectivity of the catalyst (corrugated surface with slightly protruding transition-metal atoms forming together with two neighboring Al/Ga atoms a triangular arrangement) are essentially the same on all investigated intermetallic compounds GaPd, AlPd, and Al13Co4.