Semihydrogenation of Acetylene on the (010) Surface of GaPd2: Ga Enrichment Improves Selectivity

Ab-initio density-functional calculations have been used to investigate the structure and stability of the (010) surfaces of the intermetallic compound GaPd₂ and their activity and selectivity for the catalytic semihydrogenation of acetylene to ethylene. The calculations of the surface energies show that Ga-enrichment of the surface is energetically favored under Ga-rich preparation conditions. The bulk-terminated stoichiometric GaPd₂(010) surface is catalytically active but does not exhibit the desired selectivity. The high Pd concentration in the surface leads to the formation of Pd₃ triplets favoring a strong binding of ethylene and its further hydrogenation to ethyl. In contrast Ga-enriched GaPd₂(010) surfaces provide an excellent selectivity for the formation of ethylene. Selectivity increases with increasing number of Ga atoms in the vicinity of the active Pd atoms. The atomistic scenarios for the dissociative adsorption of hydrogen, the diffusion of atomic hydrogen, and the hydrogenation reactions of acetylene to vinyl and further to ethylene demonstrate that on the selective surfaces the catalytically active centers are triangular configurations of one Pd and two Ga atoms binding acetylene through di-σ bonds in Ga–Ga bridge sites. Ethylene, on the other hand is only weakly π-bonded on top of the Pd atom such that desorption is easier than further hydrogenation to ethyl. It is remarkable that the atomic structure of one of the Ga-enriched GaPd₂(010) surfaces is very similar to that of the (001) surface of the Ga₃Pd₅ compound and to the structure of the (210) surface of GaPd. The structural similarity of the surfaces results in comparable catalytic properties. The comparison with the results for the semihydrogenation of acetylene to ethylene on the (210) surfaces of GaPd and AlPd and on the (100) surface of Al₁₃Co₄ suggests that the concept of catalytically active centers consisting of transition-metal–sp-bonded-metal complexes has general validity.